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An adaptive distance protection scheme for power systems with SFCL

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An adaptive distance protection scheme for power systems with SFCL

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Content AN ADAPTIVE DISTANCE PROTECTION SCHEME
FOR POWER SYSTEMS WITH SFCL
by
Jaewoo Eum
A Thesis Presented to the
FACULTY OF THE USC VITERBI SCHOOL OF ENGINEERING
UNIVERSITY OF SOUTHERN CALIFORNIA
In Partial Fulfillment of the
Requirements for the Degree
MASTER OF SCIENCE
(ELECTRICAL ENGINEERING (ELECTRIC POWER))
August 2020
Copyright 2020 Jaewoo Eum

ii

To
Minwoo, Junwon, and Kyuwon

iii
ACKNOWLEDGMENTS
First of all, I would like to thank the company and the people around me for giving me this valuable
educational opportunity. I appreciate all the support that the company has provided. In the near
future, I hope that my learning and this paper will contribute to the development of the company.

I would like to express my gratitude to my advisor Dr. Mohammed Beshir who gave a lot of advice
and feedback in writing this paper. I was very lucky to meet Dr. Mohammed Beshir at USC
graduate school. I was delighted to learn something new from him. I would also like to extend my
thanks to Dr. Edward Maby and Dr. Edmond Jonckheere for taking the time to participate as a
thesis committee member.

Last but not least, I would like to thank my wife, Minwoo from the bottom of my heart. She
encouraged me to study again 13 years after I graduated from college. Without her endless support
and love, I do not think I could have finished this graduate course. I also thank my children, Junwon,
and Kyuwon. I could not spend as much time as they wanted because I had a lot of work to do.
They always made me smile and gave me the power to move forward.

iv
TABLE OF CONTENTS
Acknowledgments.......................................................................................................................... iii
List of Tables ................................................................................................................................. vi
List of Figures ............................................................................................................................... vii
Abbreviations ............................................................................................................................... viii
Abstract .......................................................................................................................................... ix
Chapter 1: Introduction .................................................................................................................. 1
1.1 Overview ................................................................................................................. 1
1.2 Problem definitions.................................................................................................. 2
1.3 Methodology ............................................................................................................ 3
Chapter 2: Literature Review ......................................................................................................... 5
2.1 Fault Current in the Power System .......................................................................... 5
2.1.1 Measures to Reduce Fault Currents .............................................................. 6
2.1.2 Comparison of Mitigation Measures ............................................................ 8
2.2 Superconducting Fault Current Limiter ................................................................... 9
2.2.1 Superconductivity ......................................................................................... 9
2.2.2 Various Types of Superconducting Fault Current Limiters ........................ 11
2.2.3 Comparison of Superconducting Fault Current Limiters............................ 14
2.3 Previously Proposed Protection Schemes.............................................................. 15
2.3.1 Directional Comparison Blocking Protection ............................................. 15
2.3.2 Extended Distance Protection ..................................................................... 16
Chapter 3: Adaptive Distance Protection Scheme ....................................................................... 18
3.1 Design Concepts .................................................................................................... 18
3.2 Analysis of SFCL Configuration and Operating Characteristics .......................... 19
3.2.1 Configuration of Hybrid Resistive Type SFCL .......................................... 19
3.2.2 Sequential Changes in SFCL Configuration ............................................... 20
3.2.3 Analysis of Changing Impedance due to the SFCL Operation ................... 22
3.3 Flow chart and Additional Setting Parameters ...................................................... 25
3.3.1 Flow Chart of the Adaptive Distance Protection Scheme .......................... 25
3.3.2 Additional Setting Parameters .................................................................... 29
Chapter 4: System Modeling ....................................................................................................... 38
4.1 System Description ................................................................................................ 38
4.1.1 Configuration of the Power System ............................................................ 38
4.1.2 System Parameters ...................................................................................... 38
4.2 Distance Protection Relay Modeling ..................................................................... 39
4.2.1 Protection Zones and Operating Time ........................................................ 39
4.2.2 Setting Parameters ...................................................................................... 40

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4.3 SFCL Modeling ..................................................................................................... 41
4.3.1 SFCL Module Design ................................................................................. 41
4.3.2 Sequential Operation of SFCL .................................................................... 41
4.3.3 Parameters of SFCL .................................................................................... 43
4.4 Adaptive Distance Protection Scheme Modeling .................................................. 44
4.4.1 HTS Zone .................................................................................................... 44
4.4.2 Impedance Compensation Factor ................................................................ 46
4.4.3 Fast Zone 1 Trip Function........................................................................... 47
Chapter 5: Results and Discussions ............................................................................................ 48
5.1 Simulations to Analyze the Impact of SFCL ......................................................... 48
5.1.1 Case Study A1: Faults at the Various Locations ........................................ 49
5.1.2 Case Study A2: 3PB Fault at 10% from Substation A................................ 52
5.2 Simulations using the Adaptive Distance Protection Scheme ............................... 58
5.2.1 Case Study B1: Simulations of Phase-to-Phase Faults ............................... 58
5.2.2 Case Study B2: Simulations of Phase-to-Ground Faults ............................ 62
Chapter 6: Conclusions and Future Works ................................................................................. 66
6.1 Conclusions ........................................................................................................... 66
6.2 Future Works ......................................................................................................... 67
References ..................................................................................................................................... 69

vi
LIST OF TABLES
Table 2.1. Comparison of Various Measures.................................................................................. 8
Table 2.2. Comparison of Various Types of SFCL ...................................................................... 14
Table 3.1. Parameters of HTS Zone .............................................................................................. 30
Table 4.1. Parameters of Generators and Transmission Lines ...................................................... 39
Table 4.2. Parameters of Distance Protection Relay ..................................................................... 40
Table 4.3. Parameters of SFCL ..................................................................................................... 43
Table 4.4. Additional Parameters of the Adaptive Protection Scheme......................................... 44
Table 5.1. Parameters of SFCL in Case Study A1 ........................................................................ 49
Table 5.2. Comparison between Fault Currents............................................................................ 51
Table 5.3. Parameters of SFCL in Case Study A2 ........................................................................ 52
Table 5.4. Additional Parameters in Case Study B1 ..................................................................... 58
Table 5.5. Simulation Results of Phase-to-Phase Faults............................................................... 61
Table 5.6. Additional Parameters in Case Study B2 ..................................................................... 62
Table 5.7. Simulation Results of Phase-to-Ground Faults ............................................................ 65

vii
LIST OF FIGURES
Figure 1.1. Trends in Korea’s Electric Power System .................................................................... 1
Figure 2.1. Critical Conditions of Superconductivity ..................................................................... 9
Figure 2.2. Impedance Characteristics of Mercury ....................................................................... 10
Figure 2.3. Configuration of Directional Comparison Blocking Scheme .................................... 15
Figure 2.4. Configuration of Extended Distance Protection Scheme ........................................... 17
Figure 3.1. Configuration of Hybrid Resistive Type SFCL .......................................................... 19
Figure 3.2. Sequential Changes of SFCL Configuration .............................................................. 21
Figure 3.3. Distance Protection Relay at Substation A ................................................................. 22
Figure 3.4. Quadrilateral Characteristics of Distance Protection Relay ....................................... 22
Figure 3.5. Fault Impedance trajectories....................................................................................... 23
Figure 3.6. Flow Chart of an Adaptive Distance Protection Scheme ........................................... 27
Figure 3.7. Flow Chart of a General Distance Protection Scheme ............................................... 28
Figure 3.8. Fault Impedance due to the SFCL Operation ............................................................. 29
Figure 3.9. Sequence Network for Phase-to-Phase Fault ............................................................. 31
Figure 3.10. Sequence Networks for Phase-to-Ground Fault ....................................................... 34
Figure 4.1. One-Line Diagram of the Loop Power System with SFCL ....................................... 38
Figure 4.2. Simplified Quadrilateral Distance Protection ............................................................. 39
Figure 4.3. Configuration of SFCL in PSCAD ............................................................................. 41
Figure 4.4. Logic Diagram of the Sequential Operation of SFCL in PSCAD .............................. 42
Figure 4.5. Operating Time Chart of the Relay and SFCL ........................................................... 42
Figure 4.6. Impedance Characteristics of HTS ............................................................................. 43
Figure 4.7. Parallelogram Shape of HTS Zone ............................................................................. 44
Figure 4.8. Logic Diagram of the HTS Zone Detection in PSCAD ............................................. 45
Figure 4.9. Compensating Module in PSCAD.............................................................................. 46
Figure 4.10. Internal Logic Diagram of the Compensating Module in PSCAD ........................... 46
Figure 4.11. Logic Diagram of Fast Zone 1 Trip Function .......................................................... 47
Figure 5.1. Configuration of Case Study A and B ........................................................................ 48
Figure 5.2. Current Waveforms of Case Study A2 ....................................................................... 53
Figure 5.3. Current Distributions during the Period (c) ................................................................ 53
Figure 5.4. Voltage Waveforms of Case Study A2 ...................................................................... 54
Figure 5.5. Fault Impedance Trajectories according to the Quenching Current ........................... 55
Figure 5.6. Fault Impedance Trajectories according to the HTS impedance................................ 56
Figure 5.7. Fault Impedance Trajectories according to the CLR Impedance ............................... 57
Figure 5.8 Fault Impedance Trajectories of 3PB Fault at 50%.................................................... 59
Figure 5.9. Fault Impedance Trajectories of SLG Fault at 60% ................................................... 63

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ABBREVIATIONS
SFCL: Superconducting Fault Current Limiter
HTS: High Temperature Superconductor
CLR: Current Limiting Reactor
TL: Transmission Line
CB: Circuit Breaker
HVDC: High Voltage Direct Current
21: Distance Protection
CT: Current Transformer
PT: Potential Transformer
MTA: Maximum Torque Angle
DCB: Directional Comparison Blocking Protection
IS: Inner Signal
OS: Outer Signal
RS: Receiving Signal
TS: Transferring Signal
TD: Time Delay
3PB: Three-Phase Balanced Fault
SLG: Single Line-to-Ground Fault
NC: Normally Closed
NO: Normally Open

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ABSTRACT
A large fault current is a significant issue to the power system stability from an operational
standpoint. If a fault occurs in a power system, fault currents larger than normal load currents will
flow from power sources to a fault point. In the meanwhile, large fault impedances decrease fault
currents and small fault impedances increase fault currents. Electric utility companies in South
Korea have been building more power plants and substations to meet the growing demand for
electricity. They have been constructing a double-circuit transmission line to connect electric
facilities and making the loop power system. The trends have helped to enhance the reliability of
the power system, but have gradually reduced fault impedances. As a result, fault currents in some
areas have exceeded the rated short-circuit current of circuit breakers.
This paper will study the characteristics of superconducting fault current limiter (SFCL), a new
method of restricting the fault currents, and analyze the impacts of the SFCL on distance protection
relays. During a power system failure, the SFCL can reduce the fault current by increasing the
impedance of the superconductor immediately. However, the problem is that the increasing
impedance also affects the fault impedance seen by distance protection relays. This can cause the
distance protection relays to malfunction. Therefore, it is important to consider the changing
impedance from a protection perspective. The goal of this paper is to propose an adaptive distance
protection scheme that can cope actively with the changing impedance resulting from the operation
of the SFCL. Accordingly, distance protection relays can protect electric power systems. This
paper will use PSCAD to model a 154kV loop power system containing an SFCL and a distance
protection relay to simulate various faults and to make the SFCL operate. In addition to the
modeling, symmetrical components and sequence networks will be used to analyze the impact of
the SFCL.

1

INTRODUCTION
1.1 Overview
Electricity consumption and generation capacity in South Korea have a new record every
year. As of 2016, the country's total electricity consumption was 497 TWh with an annual average
increase of 3.65 percent over the previous decade. Along with this increase in power demand,
electricity generation capacity also increased 4.9 percent annually to 106 GW as shown in Figure
1.1. Considering the planned power generation plants, it seems that the electricity generation
capacity will continue to increase in the future [1].

Figure 1.1. Trends in Korea’s Electric Power System
For this growing demand and supply of electricity, new substations are built and more
transmission lines are needed to connect the substations. With more power facilities built inside
such a limited land, the power system is likely to become more complex and unstable. Besides,

2
the parallelized power systems tend to reduce the system impedance, resulting in an increasing
fault current. As a result, the fault current could exceed the rated short-circuit current of the existing
circuit breakers and the circuit breakers could fail to cut off the fault current. The increasing fault
current has gradually become a critical issue in Korea’s power system [2].
In the electric power industry, various methods are being proposed and used to restrict the
large fault currents. It is a general method to separate a certain connection in the power systems.
As illustrated in Figure 1.1, Korea’s power system has about 100 places where buses and
transmission lines are disconnected to reduce fault currents. Current limiting reactor (CLR) is also
one of the practical methods in Korea’s power system to increase the system impedance by
providing an additional impedance. In recent years, in addition to the conventional methods,
several studies have been actively conducted on a superconducting fault current limiter (SFCL),
one of the applications that use superconducting technology, to reduce fault current and improve
the stability of the power system through rapid voltage recovery. In 2011, a 22.9kV SFCL was
installed on the secondary side of a power transformer at Icheon substation in South Korea to
evaluate the effect of the SFCL [3], [4], [5].
1.2 Problem definitions
SFCL is one of the promising methods to reduce large fault currents. It is economical
because the SFCL has very low power losses due to the very low impedance under the normal
condition. In contrast, it can reduce fault currents by increasing its impedance drastically when the
power system fails. Accordingly, circuit breakers can cut off the reduced fault current. However,
the problem is that the impedance of the SFCL changes voltages and currents during the fault.
These changes tend to increase the fault impedance seen by the distance protection relay.
Consequently, the distance protection relay recognizes the internal fault as the external fault due

3
to the increased fault impedance. This means that the distance protection relay fails to isolate the
fault from the healthy power system in due course.
This problem becomes more complicated in the loop power system where the fault currents
come in from several directions. In the radial power system, the fault currents flow in one direction,
so only the changing impedance affects the distance protection relays located on one side of the
SFCL. On the other hand, the fault currents flow in various directions in the loop system, so more
distance protection relays are affected by the SFCL. Previous field-applied studies have mainly
verified the impact of the SFCL installed in the radial power system, and have not yet studied in
the loop power system.
It is effective to install SFCLs in the loop system because the power system has a large
number of loop systems and the fault current is usually large in the loop power system rather than
in the radial power system. Therefore, it is essential to solving the problem of malfunctions in the
distance protection relay for the successful application of the SFCL in the loop power system.
1.3 Methodology
In this paper, a loop power system with an SFCL is modeled using Power system computer-
aided design (PSCAD). The power system has six substations connected by five single-circuit
transmission lines. A hybrid resistive type SFCL and a distance protection relay are located in the
same substation to see the impact of the SFCL from a protective point of view.
The distance protection relay constantly measures voltages and currents in real-time and
converts the measured values into impedances. When the impedances enter the predetermined
protection zones, the distance protection relay operates and sends a trip signal to isolate the fault
from the power system. It is necessary to simulate faults in many places because the distance

4
protection relay protects not only its transmission line but also its adjacent transmission lines in
the forward direction [6].
By analyzing the impedance seen by the distance protection relay, this paper will propose
an adaptive distance protection scheme to overcome the impact of the changing impedance due to
the SFCL. Simulations will verify the performance of the proposed protection scheme.

5

LITERATURE REVIEW
2.1 Fault Current in the Power System
The power system consists of various electrical equipment such as generators, transmission
lines, transformers, and circuit breakers. Each equipment has a dielectric strength to isolate the
power system from outside and is required to operate within its rated range of voltage and current.
When there is no fault in the power system, all current flows only through the connected electrical
equipment. However, a fault can occur in the power system for some reason such as a dielectric
breakdown or electrical damage due to overload or overvoltage. As a result, a massive fault current
will flow into the fault point.
In general, it is helpful to connect the power system in order to enhance the reliability of
the power system from an operational perspective. On the other hand, the system impedance tends
to decrease as the power system is connected. Accordingly, the fault current is likely to increase
due to the reduced impedance. This is because the fault current is closely related to the impedance
of the power system.
As the fault current becomes larger and exceeds the rated cut-off capacity of the circuit
breaker, the circuit breaker may not isolate the fault adequately from the power system. Then, the
electrical damage will be worse as the fault persists. The fault area is also expanded. Therefore,
various measures are used to solve the problem of increasing fault current, such as disconnecting
the power system and replacing circuit breakers with a large-capacity.

6
2.1.1 Measures to Reduce Fault Currents
A. Superconducting Fault Current Limiter Installation
The superconducting fault current limiter (SFCL) is one of the applications of
superconducting technologies. The SFCL has been developed to limit the massive fault current
within the rated short-circuit current of circuit breakers. The SFCL consists of the high-
temperature superconductor (HTS), reactor, and cooling system.
The HTS changes its impedance according to the conditions such as the critical current,
temperature, and magnetic field. It is important to maintain three critical conditions for the HTS
to maintain the low impedance. This state is called a superconducting state. On the other hand, if
even one condition is not met, the HTS has a certain impedance, which is called a quenching state.
The impedance of the HTS restricts the initial fault current. The cooling system keeps the
temperature below 77K to meet the requirement of the critical temperature. The reactor provides
a predetermined impedance to reduce the continuous fault current.
The SFCL improves the reliability of the power system since it does not cause changes in
the topology of power systems. Besides, the SFCL is economical due to the low power losses in a
superconducting state. On the other hand, there is a possibility of malfunctioning of the distance
protection relay because of the changing impedance. The cooling system is also required for
keeping the low temperature and it needs additional cost.
B. Power System Separation
The power grids consist of many substations and power plants. A substation is connected
to other substations through two or more transmission lines. According to Ohm’s law, separating
buses or transmission lines could increase impedances and decrease fault currents. It is the most
common measure because power system operators can use this measure immediately when the

7
power system has a large fault current problem in a certain condition. The good thing is that there
is no additional cost. However, it can be the worst measure from a reliability perspective because
it intentionally disconnects various routes linking the power system.
C. Circuit Breaker Replacement
Replacing circuit breakers could reinforce the capacity of circuit breakers. Due to the
development of technology, the capacity of circuit breakers has increased. These circuit breakers
can properly stop high fault currents. This measure is beneficial for power system operators
because there is no need to change any power system configurations.
On the other hand, replacing circuit breakers is less cost-effective. It takes time to replace
every circuit breaker that has a problem with a low cut-off capacity. Besides, it needs to take risks
of unwanted power outages including some planned power outages. It is because the working
process is significantly sensitive and dangerous.
D. Current Limiting Reactor Installation
Installing a current limiting reactor (CLR) in series with a transmission line is a typical
measure to reduce the fault current. The CLR is a very simple electrical equipment that consists
mainly of reactance components and reduces currents by increasing the electrical distance to the
transmission line. In general, the CLR is installed in one of the substations of the transmission line
and the construction cost is not too expensive. On the other hand, once the CLR is in operation, it
affects the impedance of the power system all the time. The power losses increase because of the
additional impedance of the CLR.
E. HVDC Connection
High voltage direct current (HVDC) is one of the power transmission technologies that
convert alternating current (AC) to direct current (DC) and vice versa. HVDC uses power

8
electronics such as thyristor and IGBT. It is effective for long-distance transmission due to low
power losses. There are two types of HVDC links; Back-to-Back, which is installed at one
converter station, and Point-to-Point, which is installed between two converter stations located at
a long distance. Back-to-Back is installed mainly for decreasing the fault current between the AC
power systems. However, HVDC is expensive to construct because it requires the construction of
additional converter stations that include various power electronic equipment such as thyristor
valves and converter transformers. Besides, it is not easy to analyze the effects of the power system
as DC and AC power systems are mixed.
2.1.2 Comparison of Mitigation Measures
There are several measures to reduce the fault current in the power system. Table 2.1 shows
the advantages and disadvantages of the measures.
Table 2.1. Comparison of Various Measures

Measures Advantages Disadvantages
SFCL
Installation
Low power losses
Simple installation
Reliable
Additional cost
Difficulty in operation of relays
Cooling system needed
Power Grid
Separation
No costs
No additional equipment needed
Reduction in the reliability
Temporary measure
Circuit Breaker
Replacement
Simple
Reliable
Additional cost
Planned power outage
CLR
Installation
Simple and effective
Less expensive
Increased in power losses
HVDC
Connection
Fault current cut off between AC
Convenient to control power flows
Additional cost(expensive)
Difficult to analyze power systems

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2.2 Superconducting Fault Current Limiter
Many studies are being conducted on SFCL as a way to mitigate the fault current of the
power system. There are various types of SFCL; the resistive type, hybrid type, inductive type,
and diode-bridge type SFCL. Even though the SFCL has different configurations according to the
combination of the superconductor and electrical equipment, they have in common to use the
superconductivity. Therefore, it is important to understand the characteristics of superconductivity
before the study on the SFCL.
2.2.1 Superconductivity
Since Dutch physicist Kamerlingh Onnes discovered superconductivity in 1911, many
scientists and inventors have developed superconducting technologies and applications. One
important characteristic of superconductivity is that some materials called superconductors have
almost zero impedance within the critical temperature, current, and magnetic field, as illustrated
in Figure 2.1 [7]. This is called a superconducting state.

Figure 2.1. Critical Conditions of Superconductivity

10
On the other hand, if even one of the conditions is not satisfied, the superconductors have
a certain impedance, which is called a quenching state. Figure 2.2 illustrates the change in
impedance of mercury [7]. The impedance is zero under the temperature of 4.2K and increases as
the temperature rises. No impedance means that the power losses could be zero because there are
no electrical energy dissipations. It is economical to use superconductors for electricity delivery.
However, there were several obstacles to using superconductors for commercial purposes. In the
earlier stage, few materials were found as superconductors. Besides, it was too expensive to obtain
liquid helium required for superconductors to keep the superconductivity.

Figure 2.2. Impedance Characteristics of Mercury
Impedance [ Ω]
Temperature [K]

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In 1986, a new crystalline chemical compound, yttrium barium copper oxide (YBCO), was
made. This material could maintain superconductivity at high temperatures (93K) using liquid
nitrogen that has a temperature of 77K. Since liquid nitrogen is less expensive, the new material
has facilitated the development of practical applications for the power system including SFCLs.
2.2.2 Various Types of Superconducting Fault Current Limiters
A. Resistive Type SFCL
The resistive type SFCL is an early form of SFCL that was developed to use
superconducting technology as a current limiter [8]. The superconductor is connected in series
with a transmission line in the power system and uses the resistance of the superconductor itself
to reduce the fault current. Normally, it has zero resistance in a superconducting state, so it can
transmit power without loss. Conversely, if a fault current of the power system exceeds the critical
current, the superconducting state changes to a quenching state. As the resistance of the
superconductor increases, it is possible to reduce the fault current. Since the superconductor is
responsible for detecting the fault and limiting faulty current, it has a simple structure. It also has
the advantage of a small waveform distortion of faulty current. However, additional measures for
thermal dissipation are needed since the fault current through the superconductor creates a lot of
heat. Besides, the cost may increase due to the necessity of a large number of superconductors for
the distribution of the increasing fault current.
B. Hybrid Type SFCL
The hybrid type SFCL is composed of a resistive superconductor and a current limiting
reactor (CLR) in parallel. The impedance of the CLR can be resistance or reactance. An electrical
coil is installed in parallel across the entire circuit, and a switch is installed in series with the
superconductor. The switch is also called a circuit breaker. In general, a load current flows through

12
the superconductor circuit with low resistance. In case of a fault, the switch of the superconductor
circuit is open so that the fault current flows through the CLR. The switch in the superconductor
circuit is operated by the electrical coil excited due to the fault current. The superconductor detects
a fault and reduces early fault current. After the switch is open, the CLR effectively limits the fault
current, which helps reduce the burden of superconductors. However, there is a weakness that the
operation of the SFCL is affected by the operation of the switch. Therefore, it is important to select
the appropriate switch for the correct operation of the hybrid SFCL considering the failure
characteristics of the power system. A fast speed switch is required for rapid switching of the fault
current, as there is usually a delay in switching time.
C. Inductive Type SFCL
The inductive type SFCL is called a shielded-core or transformer type SFCL. Unlike the
resistive type SFCL, the superconductor is not connected directly to the power system. The
primary winding is connected to the power system and the load current flows through the winding.
The superconductor element is connected to the secondary winding through a magnetic core. When
a fault occurs in the power system, the superconductor detects the fault current and generates
resistance, which acts as a switch for reducing the flow of magnetic fields. Then, the fault current
is limited due to the difference in the magnetic field. In general, it is helpful for the cooling system
to keep the low temperature due to low heat without resistance. However, there are technical
difficulties in manufacturing superconductors compared to the resistive type SFCL. Besides, since
the basic structure uses a transformer, the volume can be large, and the SFCL has a distorted
waveform due to the hysteresis loss and eddy current loss.

13
D. Diode Bridge Type SFCL
The diode bridge type SFCL consists of the diodes or thyristors connected using a full-
bridge circuit. A superconducting coil and DC voltage source are connected inside the bridge
circuit. AC current flows to diode bridges in a normal condition. At this time, there is no alternating
current flowing to the superconducting device because the external current does not exceed the
current flowing through the diode. On the other hand, when a fault current occurs, the fault current
exceeds the direct current and the inductance of the superconducting coil limits the fault current.
This SFCL has the advantage of having no power loss because it uses superconductors as coils to
reduce the fault current. However, it is necessary to install DC voltage source and there are
disadvantages of power loss and harmonics due to the diodes.

14
2.2.3 Comparison of Superconducting Fault Current Limiters
It is possible to maintain superconductivity using relatively inexpensive liquid nitrogen due
to the development of high-temperature superconductors. As superconducting technologies have
become more economical, demand for superconducting applications including SFCL is also
increasing in the power sector. Table 2.2 shows the comparison of the SFCL. As discussed, each
SFCL has different configurations and characteristics, so it is important to analyze the power
system before installing SFCL.
Table 2.2. Comparison of Various Types of SFCL

Types Features
Resistive
 Simple configuration and small size
 Self-triggered to a quenching state
 Damage to HTS during a fault
Hybrid
 Fast operation(less than 4ms)
 Self-triggered to a quenching state
 Depend on a fast switch
Inductive
 Physical separation of HTS from grid
 Self-triggered to a quench state
 Large size and heavy weight
Diode Bridge
 No quenching state required
 Use of diodes or thyristors
 Special control required

15
2.3 Previously Proposed Protection Schemes
According to the literature review, there are two protection schemes in order to consider
the changing impedance when a hybrid resistive type SFCL is installed in the power system. One
thing is to install a directional comparison blocking relay in both substations [9]. The other thing
is to use the adjustable distance protection relay that allows the protection zones to be adjusted
according to the operation of the SFCL [10], [11].
2.3.1 Directional Comparison Blocking Protection
In Reference [9], the directional comparison blocking (DCB) protection is used to
determine an internal or external fault in the power system. Figure 2.3 demonstrates the
configuration of DCB protection. If a fault occurs on the transmission line between substations A
and B, each DCB sets the inner signal (IS) on and the outer signal (OS) is off. As a result, circuit
breakers (CB) in both substations are tripped by its DCB protection.

Figure 2.3. Configuration of Directional Comparison Blocking Scheme

16
On the other hand, if it is an external fault at a transmission line behind substation B, DCB
in the substation B sets the IS off and the OS on. Even though the DCB in the substation A sets
the IS on and the OS off, a transferring signal (TS) is sent to the remote DCB relay at substation
A to block tripping a circuit breaker in the substation A. This protection scheme has a simple
configuration and is useful to determine the fault point on a directional basis. However, this
protection scheme can only protect its transmission line where the SFCL is installed. For the same
purpose, the current differential relay, more advanced protection, is widely applied in the power
system as primary protection. Along with the primary protection, backup protection should have
the ability to protect not only its transmission line but also other adjacent transmission lines, such
as the distance protection relay. As backup protection, DCB does not reflect changing impedances
due to the operation of the SFCL, and there is a limitation that it does not protect the power system
in case the failure of adjacent transmission lines continues.
2.3.2 Extended Distance Protection
References [10] and [11] describe the extended distance protection scheme considering the
SFCL installed in the power system in South Korea. According to the references, South Korea has
installed and operated a hybrid resistive type SFCL at Seo-Gochang substation in 2017. The SFCL
is installed in the substation at the end of the radial power system to minimize the impact on the
power system due to the test operation of the SFCL. The distance protection relay has three
protection zones. The distance protection relay has three existing protection zones as group A and
extended protection zones to reflect the effect of the SFCL as group B.
When the HTS changes to the quenching state or the switch of HTS is open, the distance
protection scheme detects the operation of the SFCL and selects Group B to apply the extended
protection zones for the increased impedance. The status of SFCL operations is also shared with

17
other distance protection relays using the communication network as illustrated in Figure 2.4.
However, this protection scheme can cause two problems. First, there may be a short time delay
for switching the setting groups, which may result in failure of the protection function or delay in
removing the fault. Second, since the operational status of the SFCL is shared over the
communication network, it needs additional costs to install communication apparatus and cables.
The more problematic issue is that the operation of the distance protection relay will depend on
communication. The protection relays are necessary to determine the fault independently.

Figure 2.4. Configuration of Extended Distance Protection Scheme

18

ADAPTIVE DISTANCE PROTECTION SCHEME
3.1 Design Concepts
This chapter will propose a new protection scheme to protect the power system where an
SFCL is installed. The protection scheme is called an adaptive distance protection scheme and will
be applied to distance protection relays. The adaptive distance protection scheme needs to meet
the general requirements: reliability, selectivity, speed of operation, simplicity, and economics.
First, the adaptive distance protection scheme will enhance reliability. When an SFCL is
installed in the power system, it can affect remote distance protection relays as well as the local
distance protection relay. That is why the previously proposed protection schemes used
communication devices to share the status of the SFCL with other protection relays. The problem
is that it can be impossible to perform the specified protection functions if the communication
devices fail. Therefore, the adaptive distance protection scheme will be designed for each
protection relay to perform its protective functions independently.
Second, the adaptive distance protection scheme will ensure the same selectivity and
operating speed compared to the existing protection relay. The protection scheme will be designed
to have the same protection settings even if an SFCL is installed in the existing power system.
There is no need to switch the setting groups based on the status of the SFCL.
Lastly, the adaptive distance protection scheme will simplify the circuit configuration and
minimize additional costs. The adaptive distance protection scheme does not require additional
communication or control equipment, making the circuit simple. Besides, the cost can be reduced
by using existing protective devices.

19
3.2 Analysis of SFCL Configuration and Operating Characteristics
This paper aims to propose an adaptive distance protection scheme when a hybrid resistive
type SFCL is installed in the power system. Since SFCLs have different impedance characteristics
due to the configuration, they can affect the power system in a different way. Therefore, it is
important to analyze the fault impedance that varies depending on the configuration and operating
characteristics of the hybrid resistive type SFCL.
3.2.1 Configuration of Hybrid Resistive Type SFCL
Figure 3.1 shows the hybrid resistive type SFCL installed in the substation A. The SFCL
consists of a high-temperature superconductor (HTS), a current limiting reactor (CLR), fast-speed
switches (SW1 and SW2) and a cryostat [12], [13]. The HTS restricts the initial fault current by
switching from a nearly zero impedance in the superconducting state to the specific impedance in
the quenching state. The CLR is used to limit the continuous fault current. The switches determine
the path through which the current flows. Initially, the SW1 is normally open (NO) and the SW2
is normally closed (NC). The switches are operating when the electrical coil is excited. The
cryostat is filled with liquid nitrogen and maintains the critical temperature to keep the
superconducting state.

Figure 3.1. Configuration of Hybrid Resistive Type SFCL

20
3.2.2 Sequential Changes in SFCL Configuration
In the event of a power system failure, from a circuit perspective, the SFCL represents
three major sequential changes from the normal state [14]. As demonstrated in Figure 3.2 (a), the
load current flows through the transmission line and the HTS under normal conditions. The first
change is that a failure of the power system causes a large fault current to flow through the HTS
as shown in Figure 3.2 (b). If the fault current is greater than the critical current, the HTS loses its
superconductivity and has a designed impedance. Although there is no change in the circuit at this
time, internally the HTS increases its impedance, reducing the initial fault current. At the same
time, the small amount of the current flows into the electrical coil and the SW1 is closed. In the
second change, Figure 3.2 (c) shows the fault current flows through the HTS and the CLR both
until the SW2 is open. In the third change, after the SW2 is open, all fault currents flow through
the CLR as shown in Figure 3.2 (d). The fast-speed switches are an important factor to design the
hybrid resistive type SFCL because they change the path of the fault current. If the fault is not
properly removed for some reason, abnormal conditions persist. In general, the fault is cleared
because protection relays operate within the predetermined time. After the fault is removed, the
SW1 and the SW2 return to their original status, and the SFCL waits until the main circuit breakers
are closed.

21

(a) Load Current in the Normal Condition

(b) Fault Current Flows through HTS

(c) Fault Current Flows through HTS and CLR

(d) Fault Current Flows through CLR
Figure 3.2. Sequential Changes of SFCL Configuration

22
3.2.3 Analysis of Changing Impedance due to the SFCL Operation
A simplified SFCL and the power system with several distance protection relays are
illustrated in Figure 3.3. All substations have a distance protection relay (21), which protects the
transmission line. A distance protection relay has three impedance ranges called protection zones.
Each protection zone has a different operating time as shown in Figures 3.3 and 3.4. For instance,
if a fault occurs somewhere in the middle of the transmission line between substation A and B, the
distance protection relay will have the fault impedance by calculating the measured voltage and
current at substation A. Since the fault impedance is located in the zone 1, the distance protection
relay will release a trip signal to the circuit breaker (CB).

Figure 3.3. Distance Protection Relay at Substation A

Figure 3.4. Quadrilateral Characteristics of Distance Protection Relay

23
In addition to the conventional distance protection scheme, it is essential to analyze the
changing impedance of the SFCL that affects the fault impedance, since the distance protection
relay distinguishes internal and external failures with the fault impedance. Figure 3.5 demonstrates
the trajectories of the fault impedance calculated in the distance protection relay at substation A.
First, the dashed black line indicates the fault impedance trajectory when the SFCL is not activated.
Normally, the initial impedance remains at the load point (a). If a fault occurs, the fault current
increases and the voltage decreases, resulting in the fault impedance converging at a certain point
(b) in the zones. This is the general fault impedance trajectory seen by the distance protection relay.
On the other hand, the solid red line indicates the fault impedance trajectory when the SFCL is
activated. The initial impedance is located at the load point (a). However, as the circuit
configuration of SFCL has changed, the fault impedance trajectory seen by the distance relay
moved to another point (b').

Figure 3.5. Fault Impedance trajectories

24
Each point is described as follows.
(a) The initial load impedance
(b) The final fault impedance without the SFCL operation
(b') The final fault impedance with the SFCL operation
(1) The fault impedance moved due to the HTS impedance.
(2) The fault impedance moved due to the HTS and CLR impedance.
(3) The fault impedance moved due to the CLR impedance.
As illustrated in Figure 3.5, once the SFCL is activated, the final fault impedance changes
and there are noticeable differences in the middle of the trajectory. The detailed trajectory is as
follows. After the fault current exceeding the critical current of the HTS, the fault impedance
moves to the point (1) due to the increasing resistive impedance of the HTS. As the SW1 closes,
the impedance of the CLR and the impedance of the HTS affect the original fault impedance and
move to the point (2). After the SW2 is open subsequently, the impedance of the HTS was
eliminated. Therefore, the impedance of the CLR only affects the original fault impedance, and
the fault impedance moves to the point (3). The point (3) is the same position as the point (b’). It
can be seen that the final fault impedance increases from (b) to (b'). This is why the distance
protection relay has problems to determine the exact fault location. Therefore, appropriate
compensation is required to ensure that the distance protection relay has the original fault
impedance. This can minimize the impact of the SFCL.

25
3.3 Flow chart and Additional Setting Parameters
As discussed earlier, the trajectory of the fault impedance seen by the distance protection
relay varies depending on the SFCL operation. The distance protection relay can determine the
operation of the SFCL based on the trajectory and compensate for the increased impedance caused
by the SFCL. By doing this, the relay can protect the power system using the existing protection
settings such as protection zones and operating time.
3.3.1 Flow Chart of the Adaptive Distance Protection Scheme
A flow chart is a good method to describe the adaptive distance protection scheme
depending on the operation of the SFCL. Figure 3.6 demonstrates a flow chart of an adaptive
distance protection scheme and Figure 3.7 shows a flow chart of a general distance protection
scheme. The flow chart in Figure 3.6 contains four main steps added to the general distance
protection scheme to protect the power system considering the SFCL.
The first step is to check if an SFCL is installed that can affect the operation of the distance
protection relay. If there is no SFCL, the distance protection relay will protect the power system
according to the general flow chart without any changes as shown in Figure 3.7. If an SFCL is
installed in the power system, it is necessary to add several new setting parameters related to the
SFCL. These parameters include the magnitude of the quenching current, HTS Zone, impedance
compensation factor, and fast zone 1 trip function. Setting rules of the parameters is covered in
Section 3.3.2.
The second step is to verify that the fault current exceeds the quenching current set in the
first step. If the fault current exceeds the quenching current, the distance protection relay
recognizes the operation of the SFCL and proceeds to the next step. Otherwise, the distance
protection relay determines that the SFCL is not operating and performs the existing protection

26
scheme without applying the impedance compensation function and the fast operating function of
zone 1.
The third step is to find out that the initial fault impedance stays at the predetermined HTS
Zone for more than a certain amount of time. Since the impedance of the HTS has the greatest
effect on the initial fault current, the initial fault impedance can be used to verify the operation of
the SFCL. In contrast, if the fault impedance does not stay in the HTS Zone, this means that the
SFCL is not activated.
The fourth step, lastly, is to apply the impedance compensation and the fast zone 1 trip
function. If the distance protection relay determines that the SFCL is operated in the previous steps,
the distance protection relay will perform the impedance compensation for the fault impedance
increased due to the SFCL and adjust to the original fault impedance. It will also perform the
predetermined fast zone 1 trip function. The fast zone 1 trip function is the high-speed trip function
to reduce the operating time of zone 1. This is because it takes more time to move into the
protection zones while the SFCL is operating.
After the above four steps, the distance protection relay uses the compensated fault
impedance to determine the failure based on the predetermined protection zones and operating
time, like the general protection scheme. At last, the distance protection relay clears the failure.

27

Figure 3.6. Flow Chart of an Adaptive Distance Protection Scheme

28

Figure 3.7. Flow Chart of a General Distance Protection Scheme

29
3.3.2 Additional Setting Parameters
A. Quenching Current
The quenching current of the distance protection relay is set to 90% of the critical current
of the HTS. Consequently, the distance protection relay can be more sensitive. The distance
protection relay can occasionally misjudge that the HTS is in the quenching state when the actual
HTS is not in the quenching state. Nevertheless, there is no problem because the HTS Zone can
check again the status of the HTS and confirm the operation of the SFCL.
B. HTS Zone
As discussed in Section 3.2.3, the fault impedance seen by the distance protection relay has
three major sequential changes. When a fault occurs in the power system, all the initial fault
currents will flow through the HTS. If the fault current exceeds the quenching current, the
impedance increases from zero to the predetermined impedance, and the original fault impedance
enters the HTS Zone and stays at point (1) for a period of time as shown in Figure 3.8.

Figure 3.8. Fault Impedance due to the SFCL Operation

30
The changed fault impedance tends to increase along the x-axis due to the impedance of
the HTS, having the same magnitude of reactance according to the fault location. This is because
the HTS impedance is a resistance component. The HTS Zone should cover Zone 1, 2, and 3,
because the further the fault point is away from the distance protection relay, the greater the fault
impedance included with the HTS impedance. Therefore, the distance protection relay has the HTS
Zone in the shape of a parallelogram. The recommended setting parameters of the HTS Zone are
shown in Table 3.1. When it comes to R2, the HTS impedance is directly used for the three-phase
balanced fault (or phase-to-phase fault). On the other hand, it is necessary to consider the zero
sequence compensation factor for the single line ground fault (or phase-to-ground fault).

Table 3.1. Parameters of HTS Zone
Fault Type R1* R2* X1 X2 Remarks
Phase-to-phase R of Zone 1 HTS + 1 - 1 X of Zone 3 + 1 Unit: Ω
Phase-to-ground R of Zone 1
3×HTS
3+𝑘 ∗∗
+ 1
- 1 X of Zone 3 + 1 Unit: Ω

* Blind of R1 and R2 use the same MTA of the line impedance.
** k: the zero sequence impedance compensation factor.

C. Impedance Compensation Factor
When the SFCL is activated, the final fault impedance seen by the distance protection relay
is shifted from the point (b) to the point (b') as shown in Figure 3.8. This is because the impedance
of the CLR has a major effect on the original fault impedance. (The fault current flows through
CLR only after the SW2 is open.) Because of the increased fault impedance, the distance protection
relay determines that the location of the fault is farther than the original location of the fault. As a

31
result, the distance protection relay could operate late or fail to operate. To solve this problem, it
is necessary to compensate for the fault impedance in consideration of the increased impedance of
the CLR. By using the impedance compensation factor, the distance protection relay can determine
the fault in accordance with the existing protection zones. The following is an analysis of how
much impedance compensation is required through symmetrical components.
• Analysis of Symmetric Components for Phase-to-Phase Fault
The three-phase balanced fault (3PB) is used to calculate the phase-to-phase impedance
since the calculation results are the same. The good thing is that the 3PB fault only has the positive-
sequence network and it is possible to analyze the phase-to-phase fault in a simple way. The
positive-sequence network is shown in Figure 3.9.

Figure 3.9. Sequence Network for Phase-to-Phase Fault
According to the positive-sequence network, it is possible to find out the symmetrical components
from (3.1) to (3.4).

𝑉 𝐴 0
= 𝑉 𝐴 2
= 0
𝑉 𝐴 1
= 𝑟 𝑍 𝐿 1
𝐼 𝐴 1

(3.1)

32
𝑉 𝐴𝑎
= 𝑉 𝐴 0
+ 𝑉 𝐴 1
+ 𝑉 𝐴 2
= 𝑉 𝐴 1

𝑉 𝐴𝑏
= 𝑉 𝐴 0
+ 𝑎 2
𝑉 𝐴 1
+ 𝑎 𝑉 𝐴 2
= 𝑎 2
𝑉 𝐴 1

𝑉 𝐴𝑐
= 𝑉 𝐴 0
+ 𝑎 𝑉 𝐴 1
+ 𝑎 2
𝑉 𝐴 2
= 𝑉 𝐴 1

(3.2)

𝐼 𝐴 0
= 𝐼 𝐴 2
= 0
𝐼 𝐴 1
=
𝐸 𝑍 𝐿 1

(3.3)

𝐼 𝐴𝑎
= 𝐼 𝐴 0
+ 𝐼 𝐴 1
+ 𝐼 𝐴 2
= 𝐼 𝐴 1

𝐼 𝐴𝑏
= 𝐼 𝐴 0
+ 𝑎 2
𝐼 𝐴 1
+ 𝑎 𝐼 𝐴 2
= 𝑎 2
𝐼 𝐴 1

𝐼 𝐴𝑐
= 𝐼 𝐴 0
+ 𝑎 𝐼 𝐴 1
+ 𝑎 2
𝐼 𝐴 2
= 𝐼 𝐴 1

(3.4)

The phase-to-phase fault impedance seen by the distance protection relay can be obtained using
the equation (3.5). The impedance does not reflect the increased impedance of the SFCL

𝑍 𝑝 ℎ−𝑝 ℎ
=
𝑉 𝐴𝑎
− 𝑉 𝐴𝑏
𝐼 𝐴𝑎
− 𝐼 𝐴𝑏
=
𝑉 𝐴 1
− 𝑎 2
𝑉 𝐴 1
𝐼 𝐴 1
− 𝑎 2
𝐼 𝐴 1
=
(1 − 𝑎 2
)𝑉 𝐴 1
(1 − 𝑎 2
)𝐼 𝐴 1
=
𝑉 𝐴 1
𝐼 𝐴 1
=
𝑟 𝑍 𝐿 1
𝐼 𝐴 1
𝐼 𝐴 1

= 𝑟 𝑍 𝐿 1

(3.5)

Next, equation (3.6) through (3.9) represents the symmetrical components considering the
impedance of the SFCL.
𝑉 ′
𝐴 0
= 𝑉 ′
𝐴 2
= 0, 𝑉 ′
𝐴 1
= (𝑟 𝑍 𝐿 1
+ 𝑍 𝑆𝐹𝐶𝐿 1
)𝐼 ′
𝐴 1
(3.6)

𝑉 ′
𝐴𝑎
= 𝑉 ′
𝐴 0
+ 𝑉 ′
𝐴 1
+ 𝑉 ′
𝐴 2
= 𝑉 ′
𝐴 1

𝑉 ′
𝐴𝑏
= 𝑉 ′
𝐴 0
+ 𝑎 2
𝑉 ′
𝐴 1
+ 𝑎 𝑉 ′
𝐴 2
= 𝑎 2
𝑉 ′
𝐴 1

𝑉 ′
𝐴𝑐
= 𝑉 ′
𝐴 0
+ 𝑎 𝑉 ′
𝐴 1
+ 𝑎 2
𝑉 ′
𝐴 2
= 𝑉 ′
𝐴 1

(3.7)

33

𝐼 ′
𝐴 0
= 𝐼 ′
𝐴 2
= 0
𝐼 ′
𝐴 1
=
𝐸 𝑟 𝑍 𝐿 1
+ 𝑍 𝑆𝐹𝐶𝐿 1

(3.8)

𝐼 ′
𝐴𝑎
= 𝐼 ′
𝐴 0
+ 𝐼 ′
𝐴 1
+ 𝐼 ′
𝐴 2
= 𝐼 ′
𝐴 1

𝐼 ′
𝐴𝑏
= 𝐼 ′
𝐴 0
+ 𝑎 2
𝐼 ′
𝐴 1
+ 𝑎 𝐼 ′
𝐴 2
= 𝑎 2
𝐼 ′
𝐴 1

𝐼 ′
𝐴𝑐
= 𝐼 ′
𝐴 0
+ 𝑎 𝐼 ′
𝐴 1
+ 𝑎 2
𝐼 ′
𝐴 2
= 𝐼 ′
𝐴 1

(3.9)

Equation (3.10) and (3.11) indicate the phase-to-phase fault impedance seen by the distance
protection relay when the SFCL is activated. Numerically, it can be calculated that it increases
by the same magnitude of the impedance of the SFCL.

𝑍 ′
𝑝 ℎ−𝑝 ℎ
=
𝑉 ′
𝐴𝑎
− 𝑉 ′
𝐴𝑏
𝐼 ′
𝐴𝑎
− 𝐼 ′
𝐴𝑏
=
𝑉 ′
𝐴 1
− 𝑎 2
𝑉 ′
𝐴 1
𝐼 ′
𝐴 1
− 𝑎 2
𝐼 ′
𝐴 1
=
(1 − 𝑎 2
)𝑉 𝐴 1
′
(1 − 𝑎 2
)𝐼 𝐴 1
′
=
𝑉 ′
𝐴 1
𝐼 ′
𝐴 1

=
(𝑟 𝑍 𝐿 1
+ 𝑍 𝑆𝐹𝐶𝐿 1
)𝐼 𝐴 1
′
𝐼 ′
𝐴 1

= 𝑟 𝑍 𝐿 1
+ 𝑍 𝑆𝐹𝐶𝐿 1

(3.10)

𝑍 𝑝 ℎ−𝑝 ℎ
′
= 𝑟 𝑍 𝐿 1
+ 𝑍 𝑆𝐹𝐶𝐿 1
= 𝑍 𝑝 ℎ−𝑝 ℎ
+ 𝑍 𝑆𝐹𝐶𝐿 1
(3.11)

Finally, it is possible to obtain the required impedance compensation for the phase-to-phase fault
in (3.12).
𝑍 𝑝 ℎ−𝑝 ℎ
= 𝑍 𝑝 ℎ−𝑝 ℎ
′ − 𝑍 𝑆𝐹𝐶𝐿 1
(3.12)

34
• Analysis of Symmetric Components for Phase-to-Ground Fault
In the event of the phase-ground fault in the power system, the fault current flows both the
negative- and the zero-sequence networks as well as the positive-sequence network as shown in
Figure 3.10. Therefore, the phase-ground fault impedance should take into account the voltage and
current of all sequence networks [15].

Figure 3.10. Sequence Networks for Phase-to-Ground Fault

35
By using the sequence networks, it is possible to find out the symmetrical components from (3.13)
to (3.16).

𝑉 𝐴 0
= 𝑟 𝑍 𝐿 0
𝐼 𝐴 0

𝑉 𝐴 1
= 𝑟 𝑍 𝐿 1
𝐼 𝐴 1

𝑉 𝐴 2
= 𝑟 𝑍 𝐿 2
𝐼 𝐴 2

(3.13)

𝑉 𝐴𝑎
= 𝑉 𝐴 0
+ 𝑉 𝐴 1
+ 𝑉 𝐴 2
= 𝑟 𝑍 𝐿 0
𝐼 𝐴 0
+ 𝑟 𝑍 𝐿 1
𝐼 𝐴 1
+ 𝑟 𝑍 𝐿 2
𝐼 𝐴 2

= (𝑟 𝑍 𝐿 0
+ 𝑟 𝑍 𝐿 1
+ 𝑟 𝑍 𝐿 2
)𝐼 𝐴 1

𝑉 𝐴𝑏
= 𝑉 𝐴 0
+ 𝑎 2
𝑉 𝐴 1
+ 𝑎 𝑉 𝐴 2
= 𝑟 𝑍 𝐿 0
𝐼 𝐴 0
+ 𝑟 𝑍 𝐿 1
∙ 𝑎 2
𝐼 𝐴 1
+ 𝑟 𝑍 𝐿 2
∙ 𝑎 𝐼 𝐴 2

𝑉 𝐴𝑐
= 𝑉 𝐴 0
+ 𝑎 𝑉 𝐴 1
+ 𝑎 2
𝑉 𝐴 2
= 𝑟 𝑍 𝐿 0
𝐼 𝐴 0
+ 𝑟 𝑍 𝐿 1
∙ 𝑎 𝐼 𝐴 1
+ 𝑟 𝑍 𝐿 2
∙ 𝑎 2
𝐼 𝐴 2

(3.14)

𝐼 𝐴 0
= 𝐼 𝐴 1
= 𝐼 𝐴 2
=
𝐸 𝑟 𝑍 𝐿 0
+ 𝑟 𝑍 𝐿 1
+ 𝑟 𝑍 𝐿 2
(3.15)

𝐼 𝐴𝑎
= 𝐼 𝐴 0
+ 𝐼 𝐴 1
+ 𝐼 𝐴 2
= 3𝐼 𝐴 1

𝐼 𝐴𝑏
= 𝐼 𝐴 0
+ 𝑎 2
𝐼 𝐴 1
+ 𝑎 𝐼 𝐴 2
= 0
𝐼 𝐴𝑐
= 𝐼 𝐴 0
+ 𝑎 𝐼 𝐴 1
+ 𝑎 2
𝐼 𝐴 2
= 0
(3.16)

The phase-to-ground fault impedance seen by the distance protection relay can be calculated using
the equation (3.17). The impedance does not include the increased impedance of the SFCL. Since
the phase-to-ground fault impedance includes the zero-sequence impedance, it is necessary to
compensate. Equation (3.18) shows the zero-sequence impedance compensation factor (k).
𝑍 𝑝 ℎ−𝑔 =
𝑉 𝐴𝑎
𝐼 𝐴𝑎
+ 𝑘 𝐼 𝐴 0
=
(𝑟 𝑍 𝐿 0
+ 𝑟 𝑍 𝐿 1
+ 𝑟 𝑍 𝐿 2
)𝐼 𝐴 1
3𝐼 𝐴 1
+ 𝑘 𝐼 𝐴 1
=
𝑟 𝑍 𝐿 0
+ 𝑟 𝑍 𝐿 1
+ 𝑟 𝑍 𝐿 2
3 + 𝑘 (3.17)

𝑘 =
𝑍 𝐿 0
− 𝑍 𝐿 1
𝑍 𝐿 1
(3.18)

36
Next, equation (3.19) through (3.21) represent symmetrical components of the voltage and
current when the impedance of the SFCL is applied. It is assumed that the positive-, negative-,
and zero-sequence impedance of the SFCL have the same amount of the impedance.
(𝑍 𝑆𝐹𝐶𝐿 0
= 𝑍 𝑆𝐹𝐶𝐿 1
= 𝑍 𝑆𝐹𝐶𝐿 2
)

𝑉 ′
𝐴 0
= (𝑟 𝑍 𝐿 0
+ 𝑍 𝑆𝐹𝐶𝐿 0
)𝐼 ′
𝐴 0

𝑉 ′
𝐴 1
= (𝑟 𝑍 𝐿 1
+ 𝑍 𝑆𝐹𝐶𝐿 1
)𝐼 ′
𝐴 1

𝑉 ′
𝐴 2
= (𝑟 𝑍 𝐿 2
+ 𝑍 𝑆𝐹𝐶𝐿 2
)𝐼 ′
𝐴 2

(3.19)

𝑉 𝐴𝑎
′
= 𝑉 𝐴 0
′
+ 𝑉 𝐴 1
′
+ 𝑉 𝐴 2
′

= (𝑟 𝑍 𝐿 0
+ 𝑍 𝑆𝐹𝐶𝐿 0
)𝐼 𝐴 0
′
+ (𝑟 𝑍 𝐿 1
+ 𝑍 𝑆𝐹𝐶𝐿 1
)𝐼 𝐴 1
′
+ (𝑟 𝑍 𝐿 2
+ 𝑍 𝑆𝐹𝐶𝐿 2
)𝐼 𝐴 2
′

= (𝑟 𝑍 𝐿 0
+ 𝑟 𝑍 𝐿 1
+ 𝑟 𝑍 𝐿 2
+ 𝑍 𝑆𝐹𝐶𝐿 0
+ 𝑍 𝑆𝐹𝐶𝐿 1
+ 𝑍 𝑆𝐹𝐶𝐿 2
)𝐼 𝐴 1
′

= (𝑟 𝑍 𝐿 0
+ 𝑟 𝑍 𝐿 1
+ 𝑟 𝑍 𝐿 2
+ 3𝑍 𝑆𝐹𝐶𝐿 1
)𝐼 ′
𝐴 1

𝑉 ′
𝐴𝑏
= 𝑉 ′
𝐴 0
+ 𝑎 2
𝑉 ′
𝐴 1
+ 𝑎 𝑉 ′
𝐴 2

= (𝑟 𝑍 𝐿 0
+ 𝑍 𝑆𝐹𝐶𝐿 0
)𝐼 𝐴 0
′
+ (𝑟 𝑍 𝐿 1
+ 𝑍 𝑆𝐹𝐶𝐿 1
) ∙ 𝑎 2
𝐼 𝐴 1
′

+(𝑟 𝑍 𝐿 2
+ 𝑍 𝑆𝐹𝐶𝐿 2
) ∙ 𝑎 𝐼 𝐴 2
′

𝑉 ′
𝐴𝑐
= 𝑉 ′
𝐴 0
+ 𝑎 𝑉 ′
𝐴 1
+ 𝑎 2
𝑉 ′
𝐴 2

= (𝑟 𝑍 𝐿 0
+ 𝑍 𝑆𝐹𝐶𝐿 0
)𝐼 𝐴 0
′
+ (𝑟 𝑍 𝐿 1
+ 𝑍 𝑆𝐹𝐶𝐿 1
) ∙ 𝑎𝐼
𝐴 1
′

+(𝑟 𝑍 𝐿 2
+ 𝑍 𝑆𝐹𝐶𝐿 2
) ∙ 𝑎 2
𝐼 𝐴 2
′

(3.20)

𝐼 ′
𝐴 0
= 𝐼 ′
𝐴 1
= 𝐼 ′
𝐴 2
=
𝐸 𝑟 𝑍 𝐿 0
+ 𝑟 𝑍 𝐿 1
+ 𝑟 𝑍 𝐿 2
+ 𝑍 𝑆𝐹𝐶 𝐿 0
+ 𝑍 𝑆𝐹𝐶𝐿 1
+ 𝑍 𝑆𝐹𝐶𝐿 2
(3.21)

Equation (3.22) and (3.23) indicate the phase-to-ground fault impedance seen by the distance
protection relay when the SFCL is activated. Numerically, compensating the impedance of the
SFCL in the phase-to-ground fault impedance is similar to the case of the phase-to-phase fault.

37
However, in addition to the compensation above, it is important to consider the zero-impedance
compensation factor (k) as well.

𝑍 ′
𝑝 ℎ−𝑔 =
𝑉 ′
𝐴𝑎
𝐼 ′
𝐴𝑎
+ 𝑘 𝐼 ′
𝐴 0
=
(𝑟 𝑍 𝐿 0
+ 𝑟 𝑍 𝐿 1
+ 𝑟 𝑍 𝐿 2
+ 3𝑍 𝑆𝐹𝐶𝐿 1
)𝐼 𝐴 1
′
3𝐼 ′
𝐴 1
+ 𝑘 𝐼 ′
𝐴 1

=
𝑟 𝑍 𝐿 0
+ 𝑟 𝑍 𝐿 1
+ 𝑟 𝑍 𝐿 2
3 + 𝑘 +
3𝑍 𝑆𝐹𝐶𝐿 1
3 + 𝑘
= 𝑍 𝑝 ℎ−𝑔 +
3𝑍 𝑆𝐹𝐶𝐿 1
3 + 𝑘
(3.22)

Finally, it is possible to obtain the required impedance compensation for the phase-to-ground
fault in (3.23).

𝑍 𝑝 ℎ−𝑔 = 𝑍 ′
𝑝 ℎ−𝑔 −
3𝑍 𝑆𝐹𝐶𝐿 1
3 + 𝑘
(3.23)

38

SYSTEM MODELING
4.1 System Description
4.1.1 Configuration of the Power System
A 154 kV power system modeled in this paper consists of six substations connected by five
transmission lines. A generator is installed at both ends to demonstrate a loop power system with
multiple power sources. A hybrid resistive type SFCL is connected in series with transmission line
1 as shown in Figure 4.1. A distance protection relay is installed in the substation A, and measures
voltages and currents to calculate impedances on a real-time basis.

Figure 4.1. One-Line Diagram of the Loop Power System with SFCL
4.1.2 System Parameters
Table 4.1 shows the parameters for transmission lines and generators. Five transmission
lines have the same impedance. In actual power systems, the length of transmission lines is
different and the cable types vary, so they generally have different impedance. In this paper,
however, it was assumed that all transmission lines have the same impedance so that the effect of
SFCL can be easily identified.

39
Table 4.1. Parameters of Generators and Transmission Lines
Component R1 X1 R0 X0 Unit
Source 0.01921 0.20538 0.09107 0.46388 Ω
Line 0.21800 1.72434 1.19268 5.22573 Ω

4.2 Distance Protection Relay Modeling
4.2.1 Protection Zones and Operating Time
Figure 4.2 shows a simplified quadrilateral distance protection zones. In general, the
distance protection relay has zone 1, 2, and 3 with values of 85%, 125%, and 225% of the
transmission line impedance respectively. Therefore, the distance protection relay in the substation
A can protect the adjacent transmission lines as well as the transmission line where the SFCL is
installed in series. Each zone has a different operating time to remove a fault.

Figure 4.2. Simplified Quadrilateral Distance Protection

40
4.2.2 Setting Parameters
Table 4.2 provides the impedance settings of zone 1, 2, and 3 for the distance protection
relay in the substation A. The setting parameters are determined based on the impedance of the
transmission line. In this simulation, the operating time may increase to 33 – 50 ms from the
predetermined time, including the internal processing time in PSCAD.
Table 4.2. Parameters of Distance Protection Relay
Fault type
Protection Zones
Operating
Time
Remarks
(Length)
Area R X MTA
Phase-to-
phase
Zone 1 3.0 Ω 1.466 Ω 83° Inst. 85%
Zone 2 3.0 Ω 2.155 Ω 83° 333 ms 125%
Zone 3 3.0 Ω 3.880 Ω 83° 1,667 ms 225%
Phase-to-
ground
Zone 1 1.768 Ω 1.466 Ω 83° Inst. 85%
Zone 2 1.768 Ω 2.155 Ω 83° 333 ms 125%
Zone 3 1.768 Ω 3.880 Ω 83° 1,667 ms 225%

The distance protection relay calculates the impedance from the voltage and current. The
fault impedance depends on the distance from the point where the distance protection relay is
installed to the point of failure. The distance protection relay compares the fault impedance to the
predetermined protection zones. When a fault occurs, the fault impedance seen by the protection
relay becomes smaller than the protection zones. This means that it is an internal fault. Therefore,
the distance protection relay operates to remove the fault after the operating time. On the other
hand, the distance protection relay does not operate when the calculated impedance is larger than
the protection zones under the normal condition.

41
4.3 SFCL Modeling
4.3.1 SFCL Module Design
The SFCL is connected in series with the transmission line between the substations A and
B and it consists of the high-temperature superconductor (HTS) and the current limiting reactor
(CLR) as illustrated in Figure 4.3 [13]. Under the normal condition, the SW1 connected to the
CLR opens and the SW2 connected to the HTS is closed, so all current flows through the HTS
without any power losses. If a fault of the power system causes the fault current to exceed the
critical current, the HTS becomes the quenching state and has a large impedance, reducing the
fault current. According to the designed operating sequence, the SW1 is closed and the SW2 is
open. Lastly, all current flows through the CLR.

Figure 4.3. Configuration of SFCL in PSCAD
4.3.2 Sequential Operation of SFCL
Figure 4.4 demonstrates a logic diagram that implements the operating sequence of the
SFCL using PSCAD. The time chart of the logic is provided in Figure 4.5. It is assumed that a
fault occurs at 0.2 seconds. Once a fault current exceeds the quenching current of the HTS, the
logic continuously performs the following actions. The HTS increases the impedance rapidly from
zero to the designed value, reducing the first peak of the fault current. All fault currents flow
through the HTS. After 30 milliseconds from the quenching, the SW1 is closed and the fault current
flows through the HTS and the CLR both. After 30 milliseconds, the SW2 is open and the fault

42
current flows through the CLR only. The impedance of the CLR reduces the continuous fault
current. After 50 milliseconds, the HTS recovers its superconducting state. Even though the fault
is not eliminated, the SW2 is opened and the fault current does not flow to the HTS.

Figure 4.4. Logic Diagram of the Sequential Operation of SFCL in PSCAD

Figure 4.5. Operating Time Chart of the Relay and SFCL

43
4.3.3 Parameters of SFCL
Table 4.3 provides three major parameters to simulate the features of the SFCL in PSCAD.
The quenching current (Iq) is the critical current triggering a quenching state from a
superconducting state. The smaller the value, the more sensitive the SFCL becomes to operate
even further external failures.
Table 4.3. Parameters of SFCL
Components Parameters Values Units
HTS
Quenching current 10 / 20 / 30 /40 kA
Impedance 4 / 6 / 8 / 10 Ω
CLR Impedance 2 / 4 / 6 / 8 mH

The impedance of the HTS is an important parameter that determines how much the first
peak of the fault current will be reduced within one cycle. Figure 4.6 depicts the impedance
characteristics of the HTS in this paper. When a fault occurs, the impedance increases
exponentially to the impedance of the HTS within a cycle [16]. The larger the setting value, the
lower the first peak of the fault current. The impedance of the CLR determines the magnitude of
the continuous fault current. Similarly, the larger the setting value, the lower the fault current.

Figure 4.6. Impedance Characteristics of HTS

44
4.4 Adaptive Distance Protection Scheme Modeling
The following are the parameters and logic diagrams required to apply the adaptive
distance protection scheme in this paper to the distance protection relay.
4.4.1 HTS Zone
The distance protection relay can determine the operation of the SFCL using the HTS Zone.
As shown in Figure 4.7, the blue-lined parallelogram depicts the HTS Zone. The parameters in
Table 4.4 are determined by considering the operating zones of the distance protection relay.

Table 4.4. Additional Parameters of the Adaptive Protection Scheme
Fault Type
HTS Zone
Impedance
Compensation
Factor
Remarks
(Unit)
R1 R2 X1 X2
Phase-to-phase 3 7 - 1 4.88 0.754 Ω
Phase-to-ground 1.179 4.536 - 1 4.88 0.444 Ω

Figure 4.7. Parallelogram Shape of HTS Zone

45
Figure 4.8 shows a logic diagram of the HTS Zone in PSCAD. There are two HTS Zones
for the phase-to-phase fault and phase-to-ground fault. If the fault impedance stays in this area for
more than 10 milliseconds, the distance protection relay detects that the SFCL is activated. On the
other hand, if the SFCL is not activated, the fault impedance passes through this area within a few
seconds. Depending on the result, a signal is sent to the impedance compensation logic.

Figure 4.8. Logic Diagram of the HTS Zone Detection in PSCAD

46
4.4.2 Impedance Compensation Factor
As shown in Figure 4.9, the impedance compensation factor is designed with a module
called the compensator in PSCAD. An internal logic diagram of the module is shown in Figure
4.10. The compensator immediately applies a specified amount of compensation for the fault
impedance after receiving the operational signal from the HTS Zone logic. This makes the distance
protection relay can clear the fault without changing any settings such as protection zones and time.

Figure 4.9. Compensating Module in PSCAD

Figure 4.10. Internal Logic Diagram of the Compensating Module in PSCAD

47
4.4.3 Fast Zone 1 Trip Function
Fast Zone 1 Trip Function is a user's option and the corresponding logic diagram is shown
in Figure 4.11. Ideally, the distance protection relay is supposed to instantaneously eliminate an
internal fault within Zone 1 but it is impossible. In the simulations, it was assumed that the
operating time of Zone 1 is 50 milliseconds due to the internal processing time. This operating
time includes the time the fault impedance stays in zone 1 for at least 16 milliseconds. The fast
zone 1 trip function allows only 10 milliseconds to stay in zone 1, allowing the distance protection
relay to operate more quickly. This function is used only when the adaptive distance protection
scheme determines that the SFCL is activated.

(a) Phase-to-Ground Fault

(b) Phase-to-Phase Fault
Figure 4.11. Logic Diagram of Fast Zone 1 Trip Function

48

Results and Discussions
5.1 Simulations to Analyze the Impact of SFCL
Prior to the simulation of the adaptive distance protection scheme in this paper, it is
necessary to analyze the effect of the operation of the SFCL on the power system. Figure 5.1 shows
the 154 kV loop power system containing an SFCL and a distance protection relay installed in
substation A. For reference, the entire power system is illustrated in Figure 4.1. F1, 2, and 3 refer
to failures that occurred at transmission lines 1, 2, and 3 respectively. Case study A1 simulates
the effect of the SFCL by varying the fault location. Case study A2 simulates the fault occurring
at 10% of the transmission line 1 from the substation A. The effects of various parameters of the
SFCL are analyzed for the same fault.

Figure 5.1. Configuration of Case Study A and B

49
5.1.1 Case Study A1: Faults at the Various Locations
To analyze the fault current, several fault locations, ranging from 10% to 300%, were
simulated for the phase-to-phase fault (3PB) and phase-to-ground fault (SLG) respectively. The
fault current is measured at the substation A and the distance protection relay has three protection
zones as shown in Figure 5.1. Table 5.1 provides the parameters of the SFCL.
Table 5.1. Parameters of SFCL in Case Study A1
Components Parameters Values Remarks
HTS
Quenching current
20 kA Phase-to-phase
10 kA Phase-to-ground
Impedance 6 Ω -
CLR Impedance 2 mH -

Table 5.2 shows the comparisons between the fault currents when the SFCL is not activated
and the fault currents when the SFCL is operated. As well aware, the fault current is divided into
the first peak and the continuous fault current. In general, the first peak is the largest because the
fault current has a large asymmetry at the first peak due to DC Offset [17]. After tens of
milliseconds, the fault current changes to symmetry and maintains the constant fault current. The
initial fault current with the large asymmetry is likely to exceed the rated short-circuit current of
circuit breakers.
According to results, fault no. 1 - 7 showed that the first peak of the fault current decreased
linearly from 53.46 kA to 22.89 kA. Fault no. 8 - 14 also showed that the first peak of the fault
current decreased linearly from 30.37 kA to 14.35 kA. It is obvious that the farther the fault
location away from the power source, the greater the fault impedance. This was the same for

50
continuous fault currents. Fault no. 1 - 7 represented the continuous fault current from 32.58 kA
to 14.19 kA.
The fault currents changed when the SFCL was operated for the same fault scenarios. For
instance, fault no. 1 represented that the first peak was 53.46 kA and the continuous current was
32.58 kA. As the fault current exceeded the quenching current of 20 kA of the HTS, the SFCL was
operated. Accordingly, the first peak of the fault current was reduced to 34.76 kA and the
continuous current was reduced to 27.29 kA. The first peak and continuous current decreased to
35% and 16.2%, respectively. If the rated short-circuit current of the circuit breaker in the
substation A is 40 kA, the SFCL can be an alternative measure for replacing circuit breakers [18].
The magnitude of the fault current is an important factor in determining the range to be
covered by the SFCL. The lower the quenching current of the HTS, the more SFCL operates
against further failures. For example, when the quenching current of the HTS was set to 20 kA for
3PB fault, the SFCL was operated for the range from 0-300%. When the quenching current of the
HTS was set to 30 kA for the same fault, the SFCL only operated for the range of 0-150%.
Therefore, when installing an SFCL, the fault current of the power system should be analyzed
closely to have an accurate range of operation.
Finally, in the power system of this simulation, the fault current of the 3PB fault was larger
than that of SLG fault. This means that as long as the 3PB fault is covered by the SFCL, the SLG
fault is also covered. Therefore, the fault current of the 3PB fault is a more important consideration
in determining the quenching current of the HTS when installing the SFCL.

51

Table 5.2. Comparison between Fault Currents
Location Percentage Type First Peak [kA] Continuous [kA] First Peak [kA] Continuous [kA] First Peak Continuous
1 F1 10% 3PB 20 kA 6 ohm 2 mH 53.46 32.58 34.76 27.29 35.0% 16.2% Zone 1
2 F1 50% 3PB 20 kA 6 ohm 2 mH 45.23 27.61 31.50 23.71 30.4% 14.1% Zone 1
3 F1 100% 3PB 20 kA 6 ohm 2 mH 37.93 23.18 29.42 20.36 22.4% 12.1% Zone 2
4 F2 150% 3PB 20 kA 6 ohm 2 mH 32.67 19.94 27.11 17.81 17.0% 10.7% Zone 3
5 F2 200% 3PB 20 kA 6 ohm 2 mH 28.78 17.56 25.04 15.86 13.0% 9.7% Zone 3
6 F3 250% 3PB 20 kA 6 ohm 2 mH 25.65 15.70 23.29 14.32 9.2% 8.7% -
7 F3 300% 3PB 20 kA 6 ohm 2 mH 22.89 14.19 21.80 13.08 4.8% 7.8% -
8 F1 10% SLG 10 kA 6 ohm 2 mH 30.37 19.81 21.13 17.53 30.4% 11.5% Zone 1
9 F1 50% SLG 10 kA 6 ohm 2 mH 25.79 16.87 18.93 15.17 26.6% 10.1% Zone 1
10 F1 100% SLG 10 kA 6 ohm 2 mH 21.76 14.31 17.14 13.05 21.2% 8.8% Zone 2
11 F2 150% SLG 10 kA 6 ohm 2 mH 18.59 12.16 15.46 11.20 16.9% 7.9% Zone 3
12 F2 200% SLG 10 kA 6 ohm 2 mH 16.28 10.63 14.12 9.86 13.3% 7.2% Zone 3
13 F3 250% SLG 10 kA 6 ohm 2 mH 14.58 9.51 12.91 8.87 11.5% 6.7% -
14 F3 300% SLG 10 kA 6 ohm 2 mH 14.35 9.14 12.76 8.54 11.1% 6.6% -
Fault Current without SFCL Fault Current with SFCL Reduced Rate
Remarks No.
Fault Scenarios Quenching
Current
HTS
Impedance
CLR
Impedance

52
5.1.2 Case Study A2: 3PB Fault at 10% from Substation A
In case study A1, for the faults at various locations, it was possible to compare the fault
current according to the SFCL operation. Through this, it can be seen that the fault current
decreases as it moves farther away from the power source, and the fault current decreases due to
the operation of the SFCL. Case study A2 simulates several faults at one fault location to analyze
the operation of the SFCL in detail.
A. Simulation 1: Analysis of Sequential Operation of the SFCL
A 3PB fault is simulated at a point located at 10% of the transmission line from the
substation A to analyze the fault current according to the sequential operation of the SFCL. The
parameters are provided in Table 5.3.
Table 5.3. Parameters of SFCL in Case Study A2
Components Parameters Values Remarks
HTS
Quenching current 20 kA 3PB
Impedance 6 Ω -
CLR Impedance 6 mH -

The fault current waveforms of case study A2 are illustrated in Figure 5.2. In the period
(a), the HTS has zero impedance in a superconducting state. If a fault occurs at 0.2 ms, a fault
current flows through the SFCL. The dashed blue line shows the original fault current without the
SFCL and the red line describes the reduced fault current due to the operation of the SFCL. As
soon as the fault current exceeds 20kA, the quenching current of the HTS, the fault current
decreases due to the increased impedance of the HTS. The purple line shows the increased HTS
impedance. During the period (b), SFCL has only the HTS impedance.

53

Figure 5.2. Current Waveforms of Case Study A2
In the period (c), the SW1 connected in series with the CLR is closed, causing the fault current to
flow through the HTS and the CLR both. The currents distributed to the HTS and the CLR during
the period (c) are depicted in Figure 5.3. The yellow and green lines (arrows) represent the current
flowing through the HTS and the current flowing through the CLR, respectively.
After tens of milliseconds, the SW2 connected in series with the HTS is open, and all fault currents
flow through the CLR. Therefore, the impedance of the CLR only affects the fault current during
the period (d). If the fault is not removed, the reduced fault current will continue to flow.

Figure 5.3. Current Distributions during the Period (c)
○
a
○
b
○
c
○
d

54
Similarly, Figure 5.4 shows the voltage waveforms measured at substation A according to
the operation of the SFCL. The voltage change of each period is the same as the sequential
operation of the SFCL as previously discussed. The peak voltage without the operation of SFCL
is 5.7kV and it is lower than the voltage after the operation of SFCL. The fault voltage was
recovered to 50kV peak by increasing the impedance of HTS and CLR during the fault. The results
show that the SFCL has the effect of recovering the fault voltage simultaneously while reducing
the fault current. This means that SFCL can help improve the voltage stability of the power system.

Figure 5.4. Voltage Waveforms of Case Study A2
B. Simulation 2: Analysis of the Effects of Each SFCL parameter
In simulation 1, the changes in the current and voltage were analyzed according to the
operation of the SFCL. These changes eventually led to the change in the fault impedance seen by
the distance protection relay. In simulation 2, by varying the detailed parameters of SFCL, the
trajectory of fault impedance seen by the distance protection relay is studied for the same fault
condition (3PB fault at 10% from substation A). Parameters are the quenching current of HTS, the
○
a
○
b
○
c
○
d

55
impedance of HTS and CLR. This allows the analysis of the effect of each parameter on the fault
impedance trajectory.
• The Effect of Quenching Current
There are several trajectories of the fault impedance and three quadrilateral operating zones
in Figure 5.5. To examine the effects of the quenching current only, it was assumed that the
impedance of the HTS is 6 ohms and the CLR impedance is 2mH. As the quenching current
increases from 10 to 40, the impedance trajectories have changed.
A quenching current of 10 kA moves close to the impedance of the HTS along an elliptical
trajectory. The elliptical trajectory of 40 kA is the biggest. This means that the lower the quenching
current, the faster HTS loses superconductivity. It then moves to the same final point having the
CLR impedance. This is because when the SW2 connected in series with the HTS is open, all the
continuous current flows through the CLR.

Figure 5.5. Fault Impedance Trajectories according to the Quenching Current

56
• The Effect of HTS Impedance
In order to analyze the effect of the HTS impedance, the HTS impedance has applied from
4 ohms to 10 ohms. It was assumed that the fault conditions are the same and that the quenching
current is 20 kA and the CLR impedance is fixed at 2 mH. As shown in Figure 5.6, the fault
impedance passes through different paths depending on the impedance of HTS.
As expected, the initial fault impedance increases as similarly as the increased HTS
impedance. This is because all fault currents flow through the HTS until the SW1, which is
connected in series to the CLR, is closed. Therefore, it can be seen that the impedance of the HTS
is a major factor in determining the initial fault impedance as well as the first peak of the fault
current. The final point moves to the point similar to CLR impedance as seen earlier due to the
same reason.

Figure 5.6. Fault Impedance Trajectories according to the HTS impedance

57
• The Effect of CLR Impedance
The following analyses the effect of the CLR impedance on fault impedance. For this
purpose, the CLR impedance has changed from 2 mH to 8 mH and the HTS impedance is fixed at
6 ohms with the quenching current of 20 kA. Various fault impedance trajectories can be seen in
Figure 5.7, depending on the impedance of the HTS.
The initial fault impedance passes near the HTS impedance. As discussed earlier, this is
the most significant effect of the HTS impedance because all the first fault currents flow through
the HTS. The final fault impedance is located similarly to the impedance of the CLR. This is
because all fault currents flow through the CLR after the SW2 connected in series to the HTS is
open. This shows that the impedance of the CLR is the most important factor in determining the
final fault impedance.

Figure 5.7. Fault Impedance Trajectories according to the CLR Impedance

58
5.2 Simulations using the Adaptive Distance Protection Scheme
In the simulation of case study B1, the fault current varies depending on the location of the
failure, and the fault current decreased with the operation of the SFCL. In the simulation of case
study B2, it was found that the fault impedance changed according to the parameters of the SFCL.
It is obvious that the SFCL can affect the distance protection relay because the relay determines
internal and external faults based on the fault impedance. Thus, in this paper, the adaptive distance
protection scheme is proposed for the distance protection relay to determine the operation of the
SFCL and properly respond to the changing impedance. The adaptive distance protection scheme
will be analyzed by simulating phase-to-phase and phase-to-ground faults at various points of the
power system in Figure 5.1.
5.2.1 Case Study B1: Simulations of Phase-to-Phase Faults
In this simulation, three-phase balanced fault (3PB) is used to simulate phase-to-phase fault.
This is because the trajectory of the fault impedance due to the 3PB fault and the phase-to-phase
fault is the same. The SFCL has a quenching current of 20 kA, the HTS impedance of 6 ohms, and
the CLR impedance of 2 mH as parameters. The settings of the HTS Zone and the compensation
factor used in case study B1 are provided in Table 5.4. All parameters were determined in
consideration of the impedance of the transmission line and the SFCL.
Table 5.4. Additional Parameters in Case Study B1

Parameters
HTS Zone Impedance
Compensation
Factor
Remarks
R1 R2 X1 X2
3 7 - 1 4.88 0.754 Unit: Ω

59
Table 5.5 shows the simulation results. In fault no. 1 (10%) and 5 (50%), the fault
impedance is included in Zone 1 (85%), so the relay operated within 50 milliseconds. Fault no. 2
shows that the fault is removed in 56 milliseconds because it remained in Zone 1 even though the
fault impedance increased due to the SFCL. Fault no. 6, on the other hand, took 324 milliseconds
more to remove the fault. This is because the fault impedance was moved to Zone 2 (125%) by the
impedance of the SFCL. In fault no. 7, the distance protection relay operated in 62 milliseconds
when the impedance compensation element was applied. As shown in fault no. 8, after applying
the fast zone 1 trip function, the fault was removed at 56 milliseconds. In general, the actual
distance relay will operate within 50 milliseconds because the internal processing time is faster.

Figure 5.8 Fault Impedance Trajectories of 3PB Fault at 50%

60
Figure 5.8 shows the fault impedance trajectory of fault no. 5-7 as described above. The
green line is the fault impedance trajectory when the SFCL is not activated and the blue line
indicates the fault impedance trajectory when the SFCL is activated. It can be seen that the fault
impedance trajectories of fault no. 5 and 6 are clearly different for the fault that occurred at the
same point (50%). The final fault impedance was also moved from Zone 1 to Zone 2 due to the
operation of the SFCL. Therefore, the impedance compensation factor was applied to adjust these
impedance changes. The red line represents the compensated fault impedance. Using the adaptive
distance protection scheme, the fault impedance became the original fault impedance, and the relay
was able to clear the fault accurately.
Similarly, faults located in Zone 2 and Zone 3 are also moved to Zone 3 and the external
fault area, respectively, due to the operation of the SFCL. For instance, the distance protection
relay eliminated the fault within 367 milliseconds because the failure point was included in Zone
2 (125%) as shown in Fault no. 9 (90%). However, fault no. 10 took 1,340 milliseconds more to
clear the fault. This is because the fault impedance was moved to Zone 3 (225%) because of the
SFCL. With the adaptive distance protection scheme, the relay operated within 380 milliseconds
in fault no. 11.
In fault no. 12, the fault point included in Zone 3 (225%), the distance protection relay
operated in less than 1,700 milliseconds. However, due to the operation of SFCL, it was
determined to be an external fault in fault no. 13. As a result, the relay cannot clear the fault. In
fault no. 14, after applying the adaptive distance protection scheme, the fault impedance entered
Zone 3, and the relay operated within 1,709 milliseconds. The simulation results demonstrated that
the adaptive distance protection scheme is effective and accurate.

61

Table 5.5. Simulation Results of Phase-to-Phase Faults
Location* Percentage Type Protection Zones Operating Time** Fast Zone 1 Trip SFCL Operation Compensation Fault Current Protection Zones Operating Time
1 F1 10% 3PB Zone 1 50 ms - - - 53.463 kA Zone 1 29 ms
2 F1 10% 3PB Zone 1 50 ms - Yes - 41.397 kA Zone 1 56 ms + 6 ms
3 F1 10% 3PB Zone 1 50 ms - Yes Yes 41.397 kA Zone 1 56 ms + 6 ms
4 F1 10% 3PB Zone 1 50 ms Use Yes Yes 41.397 kA Zone 1 50 ms
5 F1 50% 3PB Zone 1 50 ms - - - 45.231 kA Zone 1 30 ms
6 F1 50% 3PB Zone 1 50 ms - Yes - 35.321 kA Zone 2 374 ms + 324 ms
7 F1 50% 3PB Zone 1 50 ms - Yes Yes 35.321 kA Zone 1 62 ms + 12 ms
8 F1 50% 3PB Zone 1 50 ms Use Yes Yes 35.321 kA Zone 1 56 ms + 6 ms
9 F1 90% 3PB Zone 2 367 ms - - - 39.186 kA Zone 2 346 ms
10 F1 90% 3PB Zone 2 367 ms - Yes - 31.089 kA Zone 3 1707 ms + 1340 ms
11 F1 90% 3PB Zone 2 367 ms - Yes Yes 31.089 kA Zone 2 380 ms + 13 ms
12 F2 190% 3PB Zone 3 1700 ms - - - 29.452 kA Zone 3 1681 ms
13 F2 190% 3PB Zone 3 1700 ms - Yes - 25.159 kA × × Not Operated
14 F2 190% 3PB Zone 3 1700 ms - Yes Yes 25.159 kA Zone 3 1709 ms + 9 ms
No.
Fault Conditions Relay Settings Actual Operating Results Remarks
(Increased Time)
* F1, F2, and F3 refer to failures occurring at transmission lines 1, 2, and 3, respectively.
** Operation time includes the internal processing time and does not take into account the operating time of circuit breakers.

62
5.2.2 Case Study B2: Simulations of Phase-to-Ground Faults
Case study B2 simulates the phase-to-ground fault to analyze the changing fault impedance
due to the operation of the SFCL. Same to the previous simulations, the SFCL has a quenching
current of 20 kA, the HTS impedance of 6 ohms, and the CLR impedance of 2 mH. The parameters
added to the case study B2 for applying the adaptive distance protection scheme are shown in
Table 5.6. All values were calculated according to the setting rules in Section 3.3.2, taking into
account the positive- and zero-sequence impedance of the transmission line and the SFCL. The
impedance compensation factor of the phase-to-ground fault is smaller than that of the phase-to-
phase fault due to the zero-sequence impedance compensation factor (k).
Table 5.6. Additional Parameters in Case Study B2
Parameters
HTS Zone
Impedance
Compensation
Factor
Remarks
R1 R2 X1 X2
1.768 4.536 - 1 4.88 0.444 Unit: Ω

The distance protection relay has Zone1, 2, and 3 to protect the range of 85%, 125%, and
225% respectively. The percentage may vary by the setting rules. Considering the protection zones
of the distance protection relay, the phase-to-ground fault in various locations is simulated and the
results are as shown in Table 5.7. Fault no. 1 and 5 refer to the fault at 10% and 60% of the
transmission line, respectively. Both fault impedances are included in Zone 1, so the distance
protection relay operated in less than 50 milliseconds. Fault no. 2, where SFCL was operated for
the same fault, remained in Zone 1 and the fault was removed in 60 milliseconds. However, the
fault impedance of fault no. 6 was moved to Zone 2 by the increased impedance, causing the
distance relay to operate 329 milliseconds later. At this time, the adaptive protection scheme was
applied, the operating time of the distance protection relay reduced to 62 milliseconds, as shown

63
in fault no. 7. By applying the Fast Zone 1 trip function, the distance protection relay operated in
56 milliseconds. In PSCAD, the relay's internal processing time tends to be somewhat longer, but
the actual protection relay is faster, allowing the fault to be eliminated within 100 milliseconds,
the normal instantaneous fault clearing time. Each fault impedance trajectory for a 60% fault
described above is illustrated in Figure 5.9.

Figure 5.9. Fault Impedance Trajectories of SLG Fault at 60%

The green line represents the fault impedance of fault no. 5 with no SFCL operation and
the blue line indicates the fault impedance of fault no. 6 with SFCL operation. Also, the red line
represents the fault impedance of the fault no. 7 applying the adaptive distance protection scheme.
Even though the SFCL is activated, the distance protection relay was able to remove the fault by

64
using the adaptive distance protection scheme. This means that the distance protection relay can
maintain the same performance as removing the fault by applying the same settings such as the
protection zones and operating time. It is useful because existing settings can be used even if the
SFCL is installed to reduce fault current in existing power systems.
Fault no. 9 - 11 showed the operation of the distance protection relay for the phase-to-
ground fault within the range of Zone2. When the SFCL was activated, the distance protection
relay operated 1,346 milliseconds late, and when the adaptive distance protection scheme was
applied, the relay operated at 384 milliseconds. The fault occurring within the range of Zone 3 was
simulated in fault no. 12 - 14. When the SFCL is operated, the distance protection relay could not
recognize the internal fault because of the increased fault impedance. By using the adaptive
distance protection scheme, the relay operated at 1,730 milliseconds.
According to results, the distance protection scheme can be effective for the distance
protection relay to protect the fault at Zone 2 and 3 as well as the fault in Zone 1.

65

Table 5.7. Simulation Results of Phase-to-Ground Faults
* F1, F2, and F3 refer to failures occurring at transmission lines 1, 2, and 3, respectively.
** Operation time includes the internal processing time and does not take into account the operating time of circuit breakers.

Location* Percentage Type Protection Zones Operating Time** Fast Zone 1 Trip SFCL Operation Compensation Fault Current Protection Zones Operating Time
1 F1 10% SLG Zone 1 50 ms - - - 30.380 Zone 1 29 ms
2 F1 10% SLG Zone 1 50 ms - Yes - 23.279 Zone 1 60 ms + 10 ms
3 F1 10% SLG Zone 1 50 ms - Yes Yes 23.279 Zone 1 60 ms + 10 ms
4 F1 10% SLG Zone 1 50 ms Use Yes Yes 23.279 Zone 1 54 ms + 4 ms
5 F1 60% SLG Zone 1 50 ms - - - 24.862 Zone 1 31 ms
6 F1 60% SLG Zone 1 50 ms - Yes - 19.414 Zone 2 379 ms + 329 ms
7 F1 60% SLG Zone 1 50 ms - Yes Yes 19.414 Zone 1 62 ms + 12 ms
8 F1 60% SLG Zone 1 50 ms Use Yes Yes 19.414 Zone 1 56 ms + 6 ms
9 F2 110% SLG Zone 2 367 ms - - - 21.047 Zone 2 348 ms
10 F2 110% SLG Zone 2 367 ms - Yes - 16.648 Zone 3 1713 ms + 1346 ms
11 F2 110% SLG Zone 2 367 ms - Yes Yes 16.648 kA Zone 2 384 ms + 17 ms
12 F3 210% SLG Zone 3 1700 ms - - - 15.898 kA Zone 3 1700 ms
13 F3 210% SLG Zone 3 1700 ms - Yes - 13.818 kA × × Not Operated
14 F3 210% SLG Zone 3 1700 ms - Yes Yes 13.818 kA Zone 3 1730 ms + 30 ms
No.
Fault Conditions Relay Settings Actual Operating Results Remarks
(Increased Time)

66

CONCLUSIONS AND FUTURE WORKS
6.1 Conclusions
This paper proposed an adaptive distance protection scheme after analyzing the effect
of the SFCL on the loop power system. PSCAD was used to simulate the operation of the
SFCL and analyze the trajectory of the fault impedance seen by the distance protection relay.
There are several important findings from the simulations.
• When the SFCL is activated, the voltage increased and the current decreased because of the
additional impedance of the HTS and the CLR. As a result, the trajectory of the fault
impedance seen by the distance protection relay had a different path compared to the fault
without the SFCL.
• The quenching current of the HTS was an important factor to start the operation of the SFCL.
As soon as the fault current exceeded the quenching current, the HTS increased the
impedance. When the quenching current was low, the SFCL was also operated by the through
current caused by the failure of adjacent transmission lines. Therefore, it is important to
choose the appropriate quenching current based on the fault current in the power system.
• The impedance of the HTS is the factor that reduces the first peak of the fault current. Since
all initial fault currents flew through HTS, the HTS impedance greatly affected the trajectory
of the fault impedance. Therefore, the HTS Zone needs to include the impedance of the HTS
when applying the adaptive distance protection scheme.
• The impedance of the CLR is the factor that decreases the continuous fault current. This
parameter had the greatest effect on the operation of the distance protection relay because the

67
final fault impedance seen by the relay was determined. Therefore, it is necessary to decide
the impedance compensation factor by considering the impedance of the CLR.
• The trajectory of the fault impedance was analyzed to design the adaptive distance
protection scheme. The distance protection relay could detect the operation of the SFCL
using the HTS Zone. Then the relay could clear the fault after applying the impedance
compensation factor.
Although the SFCL is a good measure to limit the increasing fault current of the
power system, it was difficult to use the SFCL due to problems in protecting the power
system. The adaptive distance protection scheme proposed in this paper is effective in solving
the changing impedance problem without altering any protection coordination among the
existing protection devices. In particular, all distance protection relays that may be affected
by the operation of the SFCL can perform protection independently. The adaptive distance
protection scheme will contribute to promoting the installation of the SFCL to solve the issue
of fault currents in the power systems.
6.2 Future Works
The adaptive distance protection scheme has been studied on a simplified power
system model with a hybrid resistive SFCL. More research on the adaptive distance
protection scheme is needed to apply SFCLs to actual power systems.
• In this paper, the power system of five transmission lines with the same impedance was used
to calculate the fault impedance. The actual power system has a large number of transmission
lines and substations. Besides, it is common to use a double circuit transmission line to
connect substations. This is because power can be delivered even if a single line fails. This

68
results in more complicated voltage and current changes during a fault. Therefore, it is
necessary to verify the performance of the adaptive protection scheme based on the actual
power systems.
• The adaptive distance protection scheme proposed in this paper is designed based on a
hybrid resistive SFCL. For application to different types of SFCLs, it will be possible to
design the adaptive protection scheme by analyzing the configurations and operating
characteristics of different types of SFCL.
• When applying the adaptive protection scheme to a real distance protection relay, the
performance test is needed using the real-time digital simulator (RTDS). The RTDS
simulator inputs the actual voltage and current to protection devices in an environment
similar to the actual system. The test can verify the performance of the adaptive protection
scheme.
• The distance protection relay has various functions such as load encroachment, power swing
blocking, and auto-reclosing. It is necessary to analyze the impact of the adaptive distance
protection scheme on performing these functions. Accordingly, it is possible to see if the
adaptive distance protection scheme can perform its function while maintaining the existing
function.

69
REFERENCES
[1] The Ministry of Trade, Industry and Energy, South Korea, "The 8th Basic Plan for Long-
term Electricity Supply and Demand," 2017.
[2] S. Lee, K. Lee, Y. Yoon and O. Hyun, "FCL application issues in Korean electric power
grid," pp. 4 pp., 2006.
[3] S. Lee, J. Yoon and B. Yang, "Short Circuit Withstanding Capability of 22.9kV HTS
Cable in Korea," Journal of International Council on Electrical Engineering, vol. 4, pp. 141-
145, Apr 1,. 2014.
[4] H. Kim, S. Yang, S. Yu, H. Kim, B. Park, Y. Han, K. Park and J. Yu, "Development and
Grid Operation of Superconducting Fault Current Limiters in KEPCO," Tasc, vol. 24, pp. 1-
4, Oct. 2014.
[5] W. Choe, LS Industrial Systems, "The Test and Installation of Medium Class (22.9kV)
Hybrid Type Fault Current Limiter in Kepco Grid," Jun. 2011.
[6] IEEE Power and Energy Society, "IEEE Guide for Protective Relay Applications to
Transmission Lines," IEEE Std, pp. 1-113, Feb 29, 2000.
[7] C.M. Rey and A.P. Malozemoff, "2 - Fundamentals of superconductivity," in
Superconductors in the Power Grid, Elsevier Ltd, 2015, pp. 29-73.
[8] M.R. Barzegar-Bafrooei, A. Akbari Foroud, J. Dehghani Ashkezari and M. Niasati, "On
the advance of SFCL: a comprehensive review," IET Generation, Transmission &
Distribution, vol. 13, pp. 3745-3759, Sep 3. 2019.
[9] S. Noh, "Analysis on Malfunction of Distance Relay and Solution in a Power
Transmission System with SFCL," Soong-sil University, Feb. 2014.

70
[10] S. Lee, E. Ko, J. Lee and M. Dinh, "Development and HIL Testing of a Protection
System for the Application of 154 kV SFCL in South Korea," Tasc, vol. 29, pp. 1-4, Aug.
2019.
[11] S. Lee, J. Lee, S. Song, J. Yoon and B. Lee, "Novel adaptive distance relay algorithm
considering the operation of 154 kV SFCL in Korean power transmission system," Physica
C: Superconductivity and its Applications, vol. 518, pp. 134-139, Nov. 2015.
[12] S. Lee, B. Lee and J. Yoon, "Analysis model development and specification proposal of
hybrid Superconducting Fault Current Limiter (SFCL)," Physica C: Superconductivity and
its Applications, vol. 470, pp. 1615-1620, Nov 1, 2010.
[13] O. Hyun, K. Park, J. Sim, H. Kim, S. Yim and I. Oh, "Introduction of a Hybrid SFCL in
KEPCO Grid and Local Points at Issue," Tasc, vol. 19, pp. 1946-1949, Jun. 2009.
[14] W. Romanosky, "Development and Testing of a Transmission Voltage SuperLimiter™
Fault Current Limiter," 2012.
[15] M.R. Barzegar-Bafrooei and A. Akbari Foroud, "Performance evaluation of distance
relay in the presence of hybrid SFCL," IET Science, Measurement & Technology, vol. 12,
pp. 581-593, Aug. 2018.
[16] S. Lee, J. Yoon, B. Yang, Y. Moon and B. Lee, "Analysis model development and
specification proposal of 154 kV SFCL for the application to a live grid in South Korea,"
Physica C: Superconductivity and its Applications, vol. 504, pp. 148-152, Sep. 2014.
[17] IEEE Violet Book "IEEE Recommended Practice for Calculating AC Short-Circuit
Currents in Industrial and Commercial Power Systems," IEEE Std, pp. 1-308, Nov 17, 2006.
[18] IEEE Power and Energy Society, "IEEE Standard for AC High-Voltage Circuit Breakers
Rated on a Symmetrical Current Basis - Preferred Ratings and Related Required Capabilities
for Voltages Above 1000 V," IEEE Std, pp. 1-16, Nov 6, 2009.

Abstract (if available)

Abstract A large fault current is a significant issue to the power system stability from an operational standpoint. If a fault occurs in a power system, fault currents larger than normal load currents will flow from power sources to a fault point. In the meanwhile, large fault impedances decrease fault currents and small fault impedances increase fault currents. Electric utility companies in South Korea have been building more power plants and substations to meet the growing demand for electricity. They have been constructing a double-circuit transmission line to connect electric facilities and making the loop power system. The trends have helped to enhance the reliability of the power system, but have gradually reduced fault impedances. As a result, fault currents in some areas have exceeded the rated short-circuit current of circuit breakers. ❧ This paper will study the characteristics of superconducting fault current limiter (SFCL), a new method of restricting the fault currents, and analyze the impacts of the SFCL on distance protection relays. During a power system failure, the SFCL can reduce the fault current by increasing the impedance of the superconductor immediately. However, the problem is that the increasing impedance also affects the fault impedance seen by distance protection relays. This can cause the distance protection relays to malfunction. Therefore, it is important to consider the changing impedance from a protection perspective. The goal of this paper is to propose an adaptive distance protection scheme that can cope actively with the changing impedance resulting from the operation of the SFCL. Accordingly, distance protection relays can protect electric power systems. This paper will use PSCAD to model a 154kV loop power system containing an SFCL and a distance protection relay to simulate various faults and to make the SFCL operate. In addition to the modeling, symmetrical components and sequence networks will be used to analyze the impact of the SFCL.

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Asset Metadata

Creator Eum, Jaewoo (author)

Core Title An adaptive distance protection scheme for power systems with SFCL

School Viterbi School of Engineering

Degree Master of Science

Degree Program Electrical Engineering (Electric Power)

Publication Date 07/19/2020

Defense Date 05/14/2020

Publisher University of Southern California (original), University of Southern California. Libraries (digital)

Tag fault current,OAI-PMH Harvest,Protection,PSCAD,quadrilateral distance protection,SFCL,short circuit current,superconducting fault current limiter

Language English

Contributor Electronically uploaded by the author (provenance)

Advisor Beshir, Mohammed (committee chair), Jonckheere, Edmond (committee member), Maby, Edward (committee member)

Creator Email jaewooeu@usc.edu,randomy@naver.com

Permanent Link (DOI) https://doi.org/10.25549/usctheses-c89-333100

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