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CIGRE 2012
This paper presents novel solutions for integration of protection, control, and metering functions based on Rogowski coil current sensors, implemented for two 90 MVA electric-arc furnace (EAF) power systems in two stages. These are probably the first Rogowski coil-based protection solutions of this kind in the world. The first phase of the project was the EAF transformer differential protection systems installed in 2004. Since installation, protection systems performed reliably, achieving both high dependability and security. Several In-Zone fault events have occurred and in all events protection operated fast, resulting in minimal damage to equipment despite high-fault current magnitudes (250 kA). During the same period, EAF transformers experienced more than one million energizing and heat cycle operations, preserving protection scheme security (no relay mis-operation).
Because the EAF transformer differential protection systems proved reliable, these systems were expanded in 2010 to integrate protection of power cables as a supplement to the differential protection systems. An additional and unique solution was integration of the electric-arc regulation and control functions by using (sharing) secondary signals from the existing Rogowski coils. These solutions simplify both design and installation, and reduce the project cost.
Rogowski coils have linear volt-current characteristics and provide a wide current range of application. They generate a low-voltage output signal during operation. This signal does not increase if the wire is disconnected from the relay. This is quite different from a conventional CT, which may generate 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. High-precision Rogowski coils presented in this paper are designed using printed-circuit boards. The split-core style design allows for installation around primary conductors without the requirement to open conductors. Rogowski coils are compact and robust providing long-lasting reliable operation. They weigh multiple times less than conventional
CTs.
The intention of the differential protection philosophy for the entire EAF electric circuit is fast fault detection and fault clearing. In addition, low-fault currents can be detected, and the protection security is immune to large load current excursions through the protection zones. Integration of the electric-arc regulation and control functions improves EAF operating efficiency.
KEYWORDS
Rogowski Coil, Relay Protection, Control, Metering, Power Transformer, Power Cable ljubomir.kojovic@cooperindustries.com

Rogowski coils have linear volt-current characteristics because the wire is wound over a non-magnetic core. When design criteria are met, Rogowski coils achieve high accuracy and the same sensor can be used for both protection and metering [1]. They are low-power current sensors and generate low- voltage output signals during operation. Standard IEC 61869-10 (under development) defines requirements for Rogowski coil applications for metering and protection. 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 IEEE C37.92™-2005, IEC 60044-8, and IEC
61850 define the interface between low-power sensors and protective relays or other substation intelligent electronic devices [2]-[4]. IEEE Std C37.235™-2007 provides guidelines for the application of Rogowski coils used for protective relaying purposes [5].
High-precision Rogowski coils presented here are designed using printed circuit boards (PCB) with imprinted windings on the boards (non-magnetic material), sandwiched together as a multi-layer PCB design. Each PCB has an imprinted coil and is wound in opposite directions. Rogowski coils have been designed in a non-split-core style and in a split-core style for installation around primary conductors without the requirement to open primary conductors. Figure 1 shows application of split- core style coils around multiple parallel power cables carrying high currents [6]. Such applications are possible because the coils are immune to external electromagnetic fields resulting from nearby conductors. Despite the close proximity of power cables and high primary currents, the coils provide accurate metering. Also, because of the precise design of the coil, conductors inside the coil opening do not need to be centered to preserve metering accuracy. Additional benefits when using Rogowski coils are that they can be connected in a non-standard method. Such an application is shown in Figure
2. Because of high currents, two power cables are used per each phase. The cables are terminated by porcelain insulators separated by approximately one meter. Since one non-split-core style Rogowski coil could not be installed around two cables, one coil was used for each cable (installed around the insulator) and then two coils connected in-series to sum their signals. Rogowski coils are compact in size and weigh many times less than conventional CTs.
A-Phase
A-Phase
B-Phase
C-Phase
A-Phase
Figure 1 Rogowski Coil around Power Cables
(Split-Core Style)
Figure 2 Series-connected Two Rogowski Coils per
Phase on an EAF Application (Non-Split-
Core Style)
The routine operation of an electric-arc furnace (EAF) creates a challenging electrical environment for any protection system. The heat cycle begins with the “bore in” phase when the electrodes are lowered into the cold scrap 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. During this time period, phase current magnitudes change rapidly and chaotically 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 the current variation that a utility power transformer or 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. EAF transformers typically undergo about100 energizations per day. For this type of operation high security of the protection system is essential since even a small number of mis-operations would cause unnecessary and costly downtime.
The expanded (integrated) protection system of the entire EAF electric circuit from the substation to the secondary bus of the EAF transformer includes two separate protection zones. The intention of the differential protection philosophy is fast fault detection and fault clearing. Differential protection is also desirable because it provides high sensitivity, can detect low-fault currents, and is immune to large load current excursions through the protection zones. 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 minimizes nuisance operations for normal load current extremes. The downside of these high-current settings is that fault detection sensitivity is reduced and tripping times for actual fault events is 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.
2.1 Differential Protection of EAF Transformer
The Rogowski coil protection system was implemented for the first time on two 90 MVA, 34.5/1 kV
EAF transformers equipped with a load-tap changer. Primary Rogowski coils were designed as non- split-core style and installed in protective aluminum shrouds as shown in Figure 3. Because of high secondary currents exceeding 50 kA, the EAF transformer secondary has a delta closure consisting of two water-cooled tubes per phase (23 cm diameter each). Since the secondary tubes cannot be opened, the Rogowski coils were designed in split-core styles and installed as shown in Figure 4. One set of coils provide signals for relay protection and the second set of coils provide signals for electric-arc regulation system (explained later in the text).
Figure 3 Rogowski Coil on the EAF Transformer
Primary Side (Non-Split-Core Style)
Figure 4 Rogowski Coils for Protection and Arc
Regulation on the EAF Transformer
Secondary Side (Split-Core Style)
Rogowski coils can accurately sense low In-Zone fault currents and are linear (cannot saturate) at high
Out-of-Zone currents. In addition, Rogowski coil secondary signals, which are a scaled time derivative di(t)/dt of the primary current, were used for improved relay protection security during through-fault events and transformer energizing. High security is preserved even for high-current Out-of-Zone faults well exceeding 60 kA. The protection algorithms are simple since Rogowski coils do not saturate. In addition, multiple slopes are not required. Protection is set at a lower current threshold as compared to

conventional solutions based on CTs. The load-tap changer position has also been used by the relay to adaptively adjust the transformer ratio allowing the set threshold to be further reduced. Power transformer inrush currents can be reliably detected by performing waveform recognition of the
Rogowski coil di(t)/dt output signal. Actual current waveform and the Rogowski coil original (non- integrated) output signal waveform during a power transformer energizing are shown in Figure 5. A relay utilizing 16 samples per cycle will derive this signal as shown in Figure 6. Instead of estimating the second harmonic component, this method determines segments and duration of inrush current low- rate changes (low di(t)/dt values). A number of Rogowski coil-based protection solutions with inrush- current detectors based on this principle have been installed in actual projects. Superior performance of this method was confirmed in EAF transformer protection systems, where requirements for differential protection are the most difficult of all power transformers. Traditional methods cannot meet these requirements. A serious challenge when protecting EAF transformers is to achieve high security of the protection scheme since these types of transformers are subject to frequent energizing.
As of now, over a million power transformer energizations have been performed. In all these cases the scheme security was preserved.
Actual Inrush Current Waveform
Inrush Current di(t)/dt
0
Rogowski Coil Output Signal (di(t)/dt)
0
0
Low di(t)/dt slopes indicate
Inrush Current
Time
Time
Figure 1 Power Transformer Inrush Current and
Rogowski Coil Secondary Signals
Figure 2 Rogowski Coils Secondary Signal for an
Inrush Current (16 samples/cycle)
Backup Ground Fault Element - Many EAF facilities apply ground overcurrent elements on conventional CT-based relays at the vault switching device and/or at the substation circuit breaker. In one application, the plant was experiencing nuisance operations of the instantaneous ground element
(50N) that was using three iron-core CTs in the three phases. The EAF transformer was connected in
Delta on the primary side, so there should not be residual current flowing on the primary circuit from the transformer. The 50N device was set at 2000 primary amps and there were a number of nuisance operations during transformer energizing, so the operator disabled this protection function. The cause of the relay nuisance operation was the CT saturation during transformer energization, producing false secondary residual currents.
The Rogowski coil-based primary-side backup ground element operates on the residual current derived by the relay based on phase currents. This protection was set to operate at low-fault currents to achieve high sensitivity without concerns of nuisance operation. In this project, the ground element was set to 500 primary amps, which is 25% of the CT-based ground relay setting that was causing nuisance operations. Rogowski coil-based ground fault protection did not experience nuisance operation during transformer energizing or during any period of the EAF heat cycle since the protection implementation. This protection function provides back-up for the differential (87-1) element in events such as a line-to-ground fault.
An additional backup protection was implemented by adding one compact-size Rogowski coil in the ground connection of the EAF transformer tank as shown in Figure 7. This function provides a sensitive overcurrent protection.

2.2 Differential Protection of EAF Power Cables
Figure 7 shows the EAF 1 power system layout upgraded (integrated) with the power cable differential protection zone. Design for EAF 2 is identical to EAF 1. Power cable differential systems were implemented by adding a set of sensors in the substation around the circuit breaker bushings, mounted in similar fashion to conventional CTs. The split-core style coils were installed in an empty source- side CT pocket as shown in Figure 8. This project employed pre-connectorized and tested Rogowski coil-to-relay interface cables; these resulted in an easy installation and prevented wiring mistakes. A summing block electrically adds signals from the two half-sections of the split-core style Rogowski coils and presents this combined signal to the relay. At the other end of the cables, the EAF transformer primary-side sensors were utilized to share signals for both the cable and EAF transformer differential protection systems. One relay was added on both ends of the cable differential protection zones and interconnected with a fiber-optic communication link. Note that Figure 8 shows a view of conventional CTs below Rogowski coils, demonstrating how much more compact Rogowski coils are as compared to CTs.
161 kV/34.5 kV
3 x 50 MVA
Twisted-Pair Wire
Shielded Cables
EAF 1
Rogowski Coils RC 1
EAF 2 to other Load
Relay 1
Ethernet Switch
Series Reactors and
By-pass Switch
Identical to
EAF 1
EAF Vault to LAN
Fiber-Optic Cables
Relay 2
Seven Parallel
Power Cables
RC 2 Tap
Position
RC 4
EAF Transformer
34.5 kV/1 kV
90 MVA
RC 3
Relay 3
Twisted-Pair Wire
Shielded Cables
Water-Cooled
Conductors
Electric Arc
Regulation
Integrated Protection, Metering, and Control Functions:
1. Differential and Ground Fault Protection of EAF
Transformer
2. Differential Protection of Power Cables
3. Metering of Power, Currents, Harmonics, THD
4. Electric Arc Control and Regulation
Molten Steel
Electric Arc Furnace (EAF)
Figure 7 Integrated Protection, Control, and Metering Functions for EAF Electric Systems
Protected Power
Cables (seven in parallel)
Rogowski Coils
Conventional CTs
Figure 8 Rogowski Coils installed in Substation around Circuit Breaker Bushings

The twisted-pair signal cables connecting the Rogowski coils to the relay cabinet are heavy-duty for mechanical strength and double-shielded to minimize the impact of switching transients. The signal cables are approximately 50 meters long. The Ethernet switches used in this project are installed in the same cabinet next to the relays. There is enough room to install three more relays in the same cabinet, planned for 2012. Three future systems will provide differential protection for power cables supplying power to three other EAFs. Relay cabinets in the pulpit control room were redesigned to accommodate both the relay for EAF transformer differential protection and the relay for power cable differential protection. In this way it was easy to implement interconnections between relays to share signals from the Rogowski coils that previously provided signals only for differential protection systems (Figure
10).
The relays communicate over fiber-optic cables connected to Ethernet switches exchanging current phasor information. The same fiber-optic cables also serve for remote access to the relays for performing setting changes, event file upload, and other relay observations. The differential protection system uses the GOOSE messaging system over Ethernet for peer-to-peer communication. For reliability, the communication system is dual-redundant; each relay has two independent, single-mode fiber-optic Ethernet ports (100 MBPS) interconnected via two Ethernet switches. The switches manage communications between the relays as well as the Ethernet traffic between the substation and the local area network inside the facility
The cable differential protection system settings are similar to most differential types of systems. The settings include a minimum trip level in differential amperes and a percentage differential slope characteristic. With the Rogowski coil current sensor, a lower slope is normally selected (as compared to CT-based systems). The protection scheme has logic that can be enabled to compensate for the charging current of the cable or line (when charging currents are high they would be seen as In-Zone faults by the protection system). This feature can be disabled if charging currents are small.
Figure 9 Relays and Ethernet Switch installed in
Substation Control House for EAF 1 and EAF 2 (Relay 1 from Figure 7)
Figure 10 Relays installed in Pulpit Control Room for
EAF1 and EAF 2 (Relays 2 and Relay 3 from Figure 7)
2.3 Metering and Control
Figure 4 shows two sets of Rogowski coils, one set used for protection and the second set used for electric arc regulation and control. It is also feasible to use the same set of secondary coils that provide signals for relay protection to share signals with the electric arc regulation system. In this case, the second set of sensors is not required. Rogowski coils provide improved electric arc regulation because they are linear and accurately measure EAF currents during all periods of a heat-cycle. This results in more efficient EAF operation.
The same sets of Rogowski coils used in these projects also perform all metering functions such as measurement of currents, electric power, harmonics, and THD factor.

The newly expanded (integrated) Rogowski coil-based power cable protection systems have already experienced tens of thousands of EAF energization (inrush current events) and heat-cycle operations.
Since installation the system performance is reliable.
A serious challenge when protecting EAF power cables is to achieve high security of the protection scheme. First, during EAF startup, currents rapidly change from low to high, close to full fault currents
(Figure 11). Furthermore, currents are very unbalanced and harmonic-rich. Second, the EAF transformers are subject to frequent energizing. Even a small percentage of reduced scheme security can result in many nuisance operations that would be unacceptable. The scheme security was verified by monitoring the level of differential current during the furnace operation. Figure 12 shows a manually triggered oscillographic record during normal operation of EAF 1 at about 2100 A primary current (73 kA secondary current). The differential current signals are typically a few percent of the total restraint current. Dependability was verified by simulating In-Zone faults. In all tests both systems responded as designed.
Two Phases Involved All Three Phase Involved
5000
]
0
-5000
0 0.2
Time [s]
0.4 0.6 0.8
Figure 11 EAF Currents Rapidly Change during the Initial Stage of Heat Cycle
Primary Currents (non-integrated)
Primary Currents (integrated)
2100 A RMS
Residual Currents
12 A
RMS
96 A RMS
Differential Operate Currents
Figure 12 Manually Triggered Oscillographic Record during Normal EAF 1 Operation
Figure 13 and Figure 14 show comparative performance of conventional 3000/5 A current transformers (shown in Figure 8) and Rogowski coils during EAF 1 transformer energizing. Figure 13 indicates that current transformers saturated, resulting in spurious residual current. This may cause unnecessary ground relay operation. Rogowski coils accurately reproduce primary currents. Residual currents are approximately 1% of primary currents.
As described in the previous text, flat spots of inrush currents of non-integrated Rogowski coil signals are used to identify transformer energizing conditions and block the scheme operation. Figure 14 shows non-integrated Rogowski coil signals with distinct flat spots of inrush currents. However, if a fault occurs during energizing, the blocking algorithm is overruled and the relay may operate to speed up the response during actual fault conditions.

] 5000
CT Currents Recorded by a Transient Recorder
0
]
-5000
1000
Spurious Residual CT Current
500
0
0 0.2 0.4 0.6 0.8 1
Time [s]
Figure 13 EAF 1 Transformer Inrush Currents (CT Currents Recorded by a Transient Recorder)
] 5000 RC Primary Currents Recorded by Relays
0
]
]
-5000
5000
0
-5000
100
Integrated Primary Currents, 60 Hz Component
Residual Current I
RMS
= 14 A
Ib
RMS
= 1550 A
0
-100
0.02 0.06
Time [s]
0.1 0.14
Figure 14 EAF 1 Transformer Inrush Currents (Rogowski Coil Currents Recorded and Derived by Relays)
Traditionally, EAF circuits included only overcurrent protection with phase and ground elements connected to operate the substation circuit breaker. Setting and coordination of overcurrent protection is difficult due to the balance required between fault sensitivity, speed, and avoiding nuisance-trip operations on normal overcurrent events. In the past, EAF transformers were not protected individually by differential protection since CTs were not available for such application due to the high currents and large physical size required.
The integrated protection systems presented here are fault-sensitive (can detect low-fault currents without jeopardizing the scheme security) and operate fast (no intentional time delays). In addition, the protection schemes are immune to high-current load swings that are common to EAF circuits. The protection systems using the new technology allow the conventional overcurrent devices to be set as a true backup protection device. Problems with protection coordination are eliminated.
BIBLIOGRAPHY
[1] Lj. A. Kojovic, “Innovative Non-conventional Current Transformers for Advanced Substation Designs and Improved
Power System Performance”, 42 nd CIGRE Session 2008, Paris, France, 2008.
[2] IEEE Standard C37.92™-2005, Analog Inputs to Protective Relays from Electronic Voltage and Current Transducers.
[3] IEC Standard 60044-8™-2002, Instrument transformers – Part 8: Electronic current transformers.
[4] IEC Standard 61850-2004, Communication networks and systems in substations.
[5] IEEE Document C37.235™-2007, Guide for the Application of Rogowski Coils used for Protective Relaying Purposes.
[6] Lj. A. Kojovic, 2004, “Measuring Current Through An Electrical Conductor”, U.S. Patent: 6,680,608 B2; Date of
Patent: January 20, 2004.
[7] Lj. A. Kojovic, M. Bishop, “Electrical Arc Furnace Protection System”, U.S. Patent: 6,810,069 B2; Date of Patent:
October 26, 2004.