CABLE DIFFERENTIAL PROTECTION SYSTEMS BASED ON NON-CONVENTIONAL CURRENT SENSORS

A. dos Santos, J. F. Martins, P. Monteiro Rede Eléctrica Nacional Rua Cidade de Goa, 4

2685-038 Sacavém Portugal

Lj. A. Kojovic, M. T. Bishop, T. R. Day Cooper Power Systems

11131 Adams Rd. Franksville, WI 53042

USA

D. Sharma Nucor-Yamato Steel Company

Highway 18 East Armorel, Arkansas 72310

USA

Abstract

This paper presents a novel solution for the protection of high-voltage power cables based on differential protection principles with currents measured by Rogowski Coil current sensors. This solution has been designed for implementation on four in-service 220 kV power cables that interface a gas-insulated substation (GIS) with overhead transmission lines. The protection system has been subjected to extensive testing in a high power laboratory and in a field application that provided extreme power system operating conditions. All tests have verified the high dependability and security of the solution.

1 Introduction

In densely populated areas, substations must be built to distribute electric power to consumers. However, overhead transmission lines are not always an acceptable solution to deliver electric power to these substations because the rights-of-way required for transmission lines cannot be obtained. An accepted solution is to deliver electric power via overhead high-voltage lines close to urban areas and then transition from overhead lines to power cables to deliver electric power to substations inside urban areas. Such network topologies are becoming very common, not only when constructing new circuits but also during the refurbishing of old overhead lines resulting in mixed overhead/cable circuits.

Urban Area

Substation

Power Cables

High-Voltage OverheadLines

Urban Area

Substation

Power Cables

High-Voltage OverheadLines

Figure 1 Line/Cable Transition Solutions in Urban Areas

To ensure reliable power supply, protection systems must differentiate between cable and overhead transmission line faults. For example, after operating for a line fault, the main protection system may initiate auto reclosing only after positive confirmation that there is no fault in the power cable. In this cable/line design, cable differential protection provides reliable discrimination between faults in the cables and on overhead lines. Because of limited space, traditional cable differential protection may not be feasible since the installation of conventional current transformers might be difficult in the cable/line transition stations. The Portuguese Transmission System Operator, REN, desires a new technical solution to overcome these constraints and to be implemented as company practice in such cases. The novel protection solution uses Rogowski Coil current sensors and one multi-function relay at each end of the cable to perform cable differential protection. The Rogowski Coil split-core design allows the installation to be accomplished without disconnecting the power cables. Relays communicate over fiber-optic cables connected to an Ethernet switch. They exchange current phasor information to determine if a fault is on the cable (In-Zone) or on the overhead line (Out-of-Zone). The same fiber-optic cables also serve for remote access to the relays for performing setting changes, event file upload, and other relay observations.

This paper consists of three main sections: project design and implementation, Rogowski Coil current sensor design, and a summary of tests performed on the protection system.

U.S. Government work not protected by U.S. copyright

2 Project Design and Implementation

Novel Rogowski Coil-based line differential protection has been designed for implementation on four in-service 220 kV power cables that interface to gas-insulated switchgear (GIS) in the Sacavem substation with overhead transmission lines in the Prior Velho station as shown in Figure 2. Previously, without cable differential protection between these two substations, fault discrimination was based on the operation of distance protection. However, fault location was not precise enough to discriminate cable faults from faults in the overhead sections of the lines. As a consequence, and for security purposes, some faults in the overhead line sections were cleared without auto reclosing, reducing the reliability of the power supply in the region.

220 kVSacavem Prior Velho

GIS

Cable Differential Protection Zone

Rogowski Coils Remote Substation

Main 1 and Main 2 Cable/Line Protection Zone

Overhead Line

220 kVSacavem Prior Velho

GIS

Cable Differential Protection Zone

Rogowski Coils Remote Substation

Main 1 and Main 2 Cable/Line Protection Zone

Overhead Line

Figure 2 Main 1, Main 2 and Cable Differential Protection Zones

In the new protection system, the overall mix of overhead/cable line is protected by a line differential protection as Main 1 and a distance protection with teleprotection scheme as Main 2. The new cable differential protection system based on Rogowski Coil current sensors creates an internal protection zone as illustrated in Figure 2 and 3. The new protection system will increase the reliability of power supply in the region by providing exact information about the fault location (in the power cables or transmission lines) after a protection trip. The adopted solution minimizes changes in the HV apparatus equipment.

3 x 170 MW

220 kVSacavem

60 kV

Prior Velho

Fanhoes

Carregado

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1 – Rogowski Coils2 – Relays3 – Ethernet Switches4 – Fiber-Optic cables

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System #3

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Carregado

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System #3

Figure 3 Single-Line Diagram of the Substation and Cable/Line

The single-line diagram of the Sacavem GIS substation is shown in Figure 4. The system includes three 170 MVA power transformers serving a 60 kV network and four feeder bays with cable interfaces to remote overhead lines. Split-core Rogowski Coils will be installed around the GIS cable terminals as illustrated in Figure 5. The relays will be installed in an existing equipment cabinet in the Sacavem substation.

TP3220/63/10 kV

170 MVAFanhoes Carregado/

Fanhoes

TP2220/63/10 kV170 MVA

TP1220/63/10 kV170 MVA

Fanhoes 2 Carregado

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170 MVAFanhoes Carregado/

Fanhoes

TP2220/63/10 kV170 MVA

TP1220/63/10 kV170 MVA

Fanhoes 2 Carregado

RogowskiCoils

Figure 4 Single-Line Diagram of Sacavem Substation

Rogowski Coils

Power cable

GIS

Figure 5 Installation of Rogowski Coils in Sacavem Substation

The Prior Velho station layout of all four cables is shown in Figure 6. One bay from Figure 6 is shown in Figure 7. Power cables from the Sacavem to Prior Velho substations reside in a 2-km tunnel and terminated at 220 kV bushings. Split-core Rogowski Coils will be installed just below the bushing base as illustrated in Figure 8. A relay cabinet with the relays will be installed in the cable tunnel near the cable terminations. Single-mode fiber-optic cables will be installed in the same cable tunnel near the power cables for communication between relays.

Cable 1 Cable 2 Cable 3 Cable 4 Future

Cable Terminationsand Rogowski Coil

Locations

High-Voltage Overhead Lines

Cable TunnelCable 1 Cable 2 Cable 3 Cable 4 Future

Cable Terminationsand Rogowski Coil

Locations

High-Voltage Overhead Lines

Cable Tunnel

Figure 6 Prior Velho Substation Layout.

Four Power Cables in Tunnel

Rogowski Coils

Overhead Transmission

Line

Bushing

Surge Arrester

Isolator

Four Power Cables in Tunnel

Rogowski Coils

Overhead Transmission

Line

Bushing

Surge Arrester

Isolator

Figure 7 One Bay at Prior Velho Substation.

Rogowski Coil

Bushing

Power cable

Figure 8 Installation of Rogowski Coils at Prior Velho Substation

The differential protection system uses the GOOSE messaging system over Ethernet for peer-to-peer communication. For reliability, the communication system will be dual-redundant; each relay has two independent, single-mode fiber-optic Ethernet ports (10/100 MBPS) interconnected via two Ethernet switches located in the Sacavem substation. The switches

manage communications between the relays as well as the Ethernet traffic between the substation and the LAN inside the facility. This also enables remote PCs to access various relay event records and to download relay settings.

3 Rogowski Coil Current Sensors

Unlike conventional iron-core current transformers (CT), Rogowski Coils are wound over a non-magnetic core. As a result, Rogowski Coils are linear since the non-magnetic core cannot saturate. Rogowski Coils generate a low voltage output that follows the specific coil scale factor and the signal is presented to low burden (high-impedance input) devices. The output voltage does not change if the wire is disconnected from the relay. This is quite different than a conventional CT, which generates hazardous voltages when the secondary circuit opens. The Rogowski Coil output signal is typically low enough to be considered safer for people and secondary equipment, even when the high currents and voltages exist on the primary side. A broken circuit or short-circuit in the signal cable will cause no hazards or damage. Even under fault conditions such as a primary short-circuit, the transmitted signal is approximately 10 V or less (depending on scale factor in specific application). In addition, Rogowski coils have a large internal resistance ― so they cannot produce significant currents, even if the terminals are shorted. These voltage levels are generally below the values where operating personnel need to apply specific safety precautions and cannot cause hazards to secondary insulation and instruments.

Rogowski Coils are classified as low-power standalone current sensors since their secondary signal is different than the typical CT secondary signal. Unlike CTs that produce secondary current proportional to the primary current, Rogowski Coils produce an output voltage that is a scaled time derivative di(t)/dt of the primary current and require microprocessor-based equipment designed to accept these types of signals. Standards [4], [5], and [6] define the interface between low-power sensors and protective relays or other substation intelligent electronic devices. IEEE Std C37.235™-2007 [7] provides guidelines for the application of Rogowski Coils used for protective relaying purposes.

Rogowski Coils may be designed using printed circuit boards (PCB) with imprinted windings on the boards. Properly designed Rogowski Coils meet these two main criteria:

1. The coil output signal is independent of the primary conductor position inside the coil loop,

2. The impact of nearby conductors that carry high currents on the coil output signal is minimal.

High-precision Rogowski Coils presented in this paper are designed using two PCBs sandwiched together as a multi-layer PCB design. Each PCB has an imprinted coil. These are wound in opposite directions. Rogowski Coils have been designed in a split-core style for installation around primary conductors without the requirement to open primary conductors [8]. Note the compact size in Figure 9. Rogowski Coils also weigh many times less than conventional CTs. The coils as shown in Figure 9 weigh approximately 12 pounds.

15.26’’

19.51’’

1.25’’

0.75’’

10.66’’

15.26’’

19.51’’

1.25’’

0.75’’

10.66’’

15.26’’

19.51’’

1.25’’

0.75’’

10.66’’

Figure 9 Split-Core Style Rogowski Coils

4 Cable Differential Protection System Test Results

The Rogowski Coil-based differential protection systems for power cables and lines has been subjected to extensive testing in a high power laboratory and in a field application that provided extreme power system operating conditions.

4.1 High Power Laboratory Tests

Several test sets were performed in the Cooper Power Systems high power laboratory to evaluate the protection system

performance for different operating and fault conditions. The test setup is shown in Figure 10. Laboratory current transformers (LCT) and one additional set of Rogowski Coils (LRC) were used as reference measuring devices.

R, XRC1 RC2

Load

Ethernet Switch

Fiber-Optic Cables

Relay2Relay1

TransientRecorder

Power CablesLCT LRC

2 km

Relay Trip Signals Figure 10 Test Setup in the High Power Laboratory

Figure 11 shows split-core style Rogowski Coils during testing. The line differential relays were installed in a 19-inch test rack located in the test cell. The Rogowski Coils connected to the relay through twisted pair shielded cables. Communication between relays was through a 2-km single-mode fiber-optic cable and an Ethernet switch.

Relays and Ethernet Switch

2-km Fiber-Optic Cable

Relays and Ethernet Switch

2-km Fiber-Optic Cable

Figure 11 Relays and Rogowski Coils in the High Power Laboratory

The tests included various types of In-Zone and Out-of-Zone faults (single phase-to-ground, phase-to-phase, and three-phase faults). Sensitivity and speed of the protection system operation were tested for low current and high current In-Zone faults. The tests also included fault currents smaller than the load currents. The line/cable charging current compensation logic was tested by connecting different sizes of capacitor banks to the In-Zone line. The effectiveness of the compensation logic was demonstrated by enabling and disabling the compensation algorithm and monitoring the impact on the protection system operation.

Figure 12 and Figure 13 show the results of the protection system dependability tests (In-Zone faults) at low current levels (330 A). The differential element was set with a minimum trip of 300 primary Amperes with a 10% slope characteristic.

Figure 12 shows the protection system operation for a three-phase fault and Figure 13 shows the protection system operation for a phase-to-ground fault. In all of these tests, the protection system operated reliably.

In-Zone Fault

-1000

0

1000

Test #3109Cur

rent

[A]

0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4

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Time [s]

Normal Operation

330 ARMS 3-Phase Fault

Relay Operation

In-Zone Fault

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Test #3109Cur

rent

[A]

0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4

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Time [s]

Normal Operation

330 ARMS 3-Phase Fault

Relay Operation

Figure 12 Protection Operation for a Low Current Three-Phase In-Zone Fault

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500

1000

Test #3116Cur

rent

[A]

0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4

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Normal Operation

330 ARMS Line-to-Ground Fault

Relay Operation

In-Zone Fault

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Normal Operation

330 ARMS Line-to-Ground Fault

Relay Operation

In-Zone Fault

Figure 13 Protection Operation for a Low Current Phase-to-Ground In-Zone Fault

Protection system security for Out-of-Zone faults was tested at high fault currents. Figure 14 shows test results for a 10 kA Out-of-Zone three-phase fault and Figure 15 for a 10 kA Out-of-Zone phase-to-ground fault with reclosing.

Relay Did NOT Operate

Out-of-Zone Fault

-20

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Test #3073Cur

rent

[kA]

0 0.05 0.1 0.15 0.2 0.25 0.3 0.35-1

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Normal Operation

10 kARMS 3-Phase Fault

Relay Did NOT Operate

Out-of-Zone Fault

-20

0

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Test #3073Cur

rent

[kA]

0 0.05 0.1 0.15 0.2 0.25 0.3 0.35-1

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Time [s]

Normal Operation

10 kARMS 3-Phase FaultOut-of-Zone Fault

-20

0

20

Test #3073Cur

rent

[kA]

0 0.05 0.1 0.15 0.2 0.25 0.3 0.35-1

0

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Time [s]

Normal Operation

10 kARMS 3-Phase Fault

Figure 14 Protection Operation for a 10 kA Three-Phase Out-of-Zone Fault

-20

0

20

Test #3094Cur

rent

[kA

]

0 0.1 0.2 0.3 0.4 0.5-1

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Time [s]

10 kARMS Reclosing toLine-to-Ground Fault

Relay Did NOT Operate

10 kARMS Line-to-Ground FaultOut-of-Zone Fault

-20

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20

Test #3094Cur

rent

[kA

]

0 0.1 0.2 0.3 0.4 0.5-1

0

1

Time [s]

10 kARMS Reclosing toLine-to-Ground Fault

Relay Did NOT Operate

10 kARMS Line-to-Ground FaultOut-of-Zone Fault

Figure 15 Protection Operation for a 10 kA Phase-to-Ground Out-of-Zone Fault

Out-of-Zone fault tests at 60 kA were performed in a single-phase configuration because of the high fault current magnitude. Tests were performed by closing the circuit directly onto the fault. The test results are shown in Figure 16. In all tests the protection system preserved high security.

-100

0

100

Fault Current 60 kARMS

Time [s]0 0.1 0.2 0.3

Test #19130

Relay Did NOT Operate

Out-of-Zone Fault

-100

0

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-100

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Fault Current 60 kARMSFault Current 60 kARMS

Time [s]0 0.1 0.2 0.3

Test #19130

Relay Did NOT Operate

Time [s]0 0.1 0.2 0.3

Test #19130

Relay Did NOT Operate

Out-of-Zone Fault

Figure 16 Protection Operation for a 60 kA Phase-to-Ground Out-of-Zone Fault

In addition, to test the impact of the Rogowski Coil types on the protection system performance, different coil types and ratios were used at the line ends. Tests were also performed using non split-core style sensors on one line end and split-core style sensors on the other line end. Testing confirmed that protection system performance was insensitive to different Rogowski Coil types in the same line differential system.

4.2 Field Tests

The line differential protection system was installed in an industrial power system that serves a steel production plant in the autumn shutdown in October 2009. The protection system was applied on a feeder that provides electric power to a 90 MVA electric arc furnace (EAF) transformer. This was the first Rogowski Coil-based line differential protection system implemented in the USA. This system is also unique since it uses one set of Rogowski coils to provide protection for two independent differential protection systems (as described later in the text). To demonstrate the difficult requirements for differential protection of EAF cables, a description of EAF operation is presented next.

In the routine operation of the furnace, a heat cycle starts with charging the furnace with cold scrap. To begin the heat cycle, the electrodes are lowered into the scrap (“bore in” phase) starting the electric arc. This causes momentary short circuits that develop very high currents resulting in excessive forces that blow the scrap away from the electrodes, sometimes interrupting the electric arc. Then the arc quickly re-ignites. (This process can last for several minutes.) During this period, current magnitudes rapidly and chaotically change from low to high values. After 5 to 10 minutes, arc stability improves, but there is still a high degree of current variation as compared to current variation that a utility power cable may experience. To optimize the melting process, the EAF regulator may send a command to change the EAF transformer tap position. In a heat cycle, there is usually more than one scrap charge in order to fill the furnace. Figure 17 shows the RMS values of EAF currents during one heat cycle that includes three heating periods with short breaks for scrap recharging or emptying the furnace. Three periods of current interruptions are intentional and are required for the recharging. EAF transformers typically undergo 70-100

energizations per day. For this type of operation high security of the protection system is essential since even a small number of misoperations would cause unnecessary and costly downtime. The electrical system single-line diagram is shown in Figure 18.

0 10 20 30 40 50 600

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]

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EAF C-Phase CurrentsRMS Values averaged per one second

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EAF C-Phase CurrentsRMS Values averaged per one second

Figure 17 RMS Values of EAF Currents during one Heat Cycle

Power Cables (seven in

parallel/phase)

A-phase B-phase C-phase

Conventional Current Transformers

Circuit Breaker

Rogowski Coils

A-phase

B-phase

C-phase

Rogowski Coils mounted around circuit breaker bushings

Rogowski Coils mounted in the EAF transformer vault

34.5 kV/1000 V90 MVA

161 kV/34.5 kV3 x 50 MVA

EAF

EAFTransformer

Seven Power Cables

in parallel

EthernetSwitch

Relay

50 Ω

- Cable Differential Protection

- EAF Transformer Differential Protection

1

2

1

2

Power Cables (seven in

parallel/phase)

A-phase B-phase C-phase

Conventional Current Transformers

Circuit Breaker

Rogowski Coils

Power Cables (seven in

parallel/phase)

A-phase B-phase C-phase

Conventional Current Transformers

Circuit Breaker

Rogowski Coils

A-phase

B-phase

C-phase

Rogowski Coils mounted around circuit breaker bushings

Rogowski Coils mounted in the EAF transformer vault

34.5 kV/1000 V90 MVA

161 kV/34.5 kV3 x 50 MVA

EAF

EAFTransformer

Seven Power Cables

in parallel

EthernetSwitch

Relay

50 Ω

34.5 kV/1000 V90 MVA

161 kV/34.5 kV3 x 50 MVA

EAFEAF

EAFTransformer

Seven Power Cables

in parallel

EthernetSwitch

Relay

50 Ω50 Ω

- Cable Differential Protection

- EAF Transformer Differential Protection

1

2

1

2 Figure 18 EAF Electric Power System

Two sets of Rogowski Coils are required for the line differential system. One (new) set of Rogowski Coils was installed in the substation on the circuit breaker bushings in an empty source-side CT pocket as shown in Figure 18. In this application split-style Rogowski Coils were used at the circuit breaker location. Figure 19 shows a close-up view of the conventional CT and Rogowski Coil, which demonstrates that Rogowski Coils are much more compact than CTs. The second set of Rogowski Coils required at the other end of the cables, near the EAF transformer at the transition to bus tubes, were solid core-style sensors that were already in place for the vault differential system that has been operational for more than five years. This set of sensors will simultaneously provide current signals for two separate differential protection systems. This is a cost effective solution that is easy to install and maintain, while providing reliable protection operation.

The Rogowski Coil secondary circuit consists of twisted-pair shielded cables that were pulled in conduit to the substation control building. One relay and the Ethernet switch were installed in the substation control room as shown in Figure 20. The second relay was located in the EAF 2 differential relay panel in the EAF control room. Tee style connectors were used to interconnect the twin-ax cable from the same set of Rogowski Coils into both the EAF 2 differential relay and the cable differential relay. Communication between the two line/cable differential relays was provided through an existing fiber-optic cable. One spare fiber-optic cable between the substation control room and the pulpit was used for the protection system.

Rogowski Coil

Conventional Current Transformer

Circuit Breaker’s Bushing

Rogowski Coil

Conventional Current Transformer

Circuit Breaker’s Bushing

Figure 19 Comparison of Current Transformer and Rogowski Coil Sizes

Ethernet SwitchSubstation Relay Ethernet SwitchSubstation Relay

Figure 20 Relay and Ethernet Switch Installed in the Substation Control Room

A typical EAF has extreme operating conditions such as frequent EAF transformer energizing, fast and chaotic current magnitude changes, high EAF currents (can be over 100 kA), significant current distortion and unbalance, and frequent operation of the vacuum circuit breaker bypass for the series reactor. These extreme operating conditions make it difficult to design a protection system to provide both high dependability and high security. In this application, the EAF is energized at least 70 times per day and each energization also involves the operation of a vacuum circuit breaker bypass for the series reactor. Operation of the bypass breaker occurs in the middle of the heat cycle (when the reactors are shorted after the furnace regulator determines that sufficient arc stability is achieved).

The newly implemented Rogowski Coil-based protection system has already experienced several thousand energization inrush events since installation and has performed with high security. Figure 21 shows a manually triggered oscillographic record during normal operation of the EAF at about 2300 primary amps. The differential signals have been on the order of several percent of the total restraint current.

The intention of the differential protection philosophy for the entire EAF electric circuit (from the substation to the secondary bus of the EAF transformer in two separate zones) is fast fault detection and clearing. Differential protection is also desirable because it provides high sensitivity and can detect low fault currents, while it is immune to large load current excursions through the zone. These characteristics make differential protection desirable for EAF circuits. This level of protection cannot be achieved with conventional overcurrent protection since time delayed and instantaneous overcurrent devices must be set in such a way that nuisance operations for normal load current extremes is low[1]. The downside of these high-current settings is that fault detection sensitivity is reduced and tripping times for actual fault events increased. For example, in circuits that use series reactors with a bypass switch, fault currents may drop to less than half the magnitude of the fault current without the series reactor in the circuit. If a fault occurs when the series reactor is in service, overcurrent protection trip times may be on the order of several seconds. The new differential system responds to faults within two cycles, providing both high dependability and security.

Operating Current <2-3% of Restraint Current

Restraint Currents

Operating Currents

Load Currents

Operating Current <2-3% of Restraint Current

Restraint Currents

Operating Currents

Load Currents

Figure 21 Manually Triggered Oscillograph Record during Normal Operation

5 Conclusion

The line differential protection using Rogowski Coil current sensors is the second step in providing differential protection for complete EAF electrical supply circuits. The first step was the application of a differential protection zone enclosing the EAF transformers and other vault-mounted equipment. These vault differential systems have been in operation for more than five years and have successfully detected several fault events, while preserving high security – over 500,000 energizing events. The second step is the protection of the cable between the substation and the EAF vault. At the time of the writing of this paper the cable differential protection has experienced several thousand energizations with no nuisance operations. This promising result indicates that performance similar to the EAF transformer differential protection can be achieved. The line differential protection presented here is the first system based on Rogowski Coil current sensors in the USA. It has provided promising operational data in the months it has been in service.

When comparing Rogowski Coil-based differential protection schemes with current transformer-based schemes, the significant advantage of Rogowski Coil-based schemes is that Rogowski Coils are linear (do not saturate) and preserve scheme security (even at high fault currents such as 60 kA as presented in this paper). There is no big difference between the two technologies at load level and smaller fault currents. However, characteristics such as light weight and compact size may be a determining factor in selecting Rogowski Coil-based schemes. Increased personnel safety is also an important factor in selecting Rogowski Coil-based protection – since opening secondary wiring during operation does not result in hazardous voltages. Split-core style designs that provide easy installation without the need to open primary conductors are an additional advantage of Rogowski Coils.

References

[1] Lj. A. Kojovic, M. T. Bishop, “Comparative Characteristics of Iron-Core Current Transformers and Rogowski Coils for Applications for Protective Relaying Purposes”, 62nd Annual Conference for Protective Relay Engineers, Texas A&M, March 30 – April 2, 2009.

[2] Lj. A. Kojovic, M. T. Bishop, T. R. Day, “Operational Performance of Relay Protection Systems based on Low Power Current Sensors”, Western Power relay Conference, Spokane, WA, October 20-22, 2009.

[3] Lj. A. Kojovic, M. T. Bishop, D. Sharma, “New Fault detection Technology for Electric Arc Furnace Electrical Systems”, Iron & Steel Technology Conference and Exposition, St. Louis, Mo, May 4-7, 2009.

[4] IEEE Standard C37.92™, Analog Inputs to Protective Relays from Electronic Voltage and Current Transducers. [5] IEC Standard 60044-8™, Instrument transformers – Part 8: Electronic current transformers. [6] IEC Standard 61850, Communication networks and systems in substations. [7] IEEE Document C37.235™, Guide for the Application of Rogowski Coils used for Protective Relaying Purposes. [8] Lj. A. Kojovic, “Split Rogowski Coil, Current Measuring Device and Methods”, United States Patent, Patent Number: US

7,538,541 B2; Date of Patent: May 26, 2009. [9] Lj. A. Kojovic, M. Bishop, “Electrical Arc Furnace Protection System”, U.S. Patent: 6,810,069 B2; Date of Patent:

October 26, 2004.