21, rue d'artois, F-758 Paris http://www.cigre.org A3-12 Session 24 CIGRÉ PCB ROGOWSKI COILS HIGH PRECISION LOW POWER SENSORS Ljubomir A. Kojovic * Cooper Power Systems USA Summary-This paper presents novel designs of high precision Rogowski coils (RC) for advanced protection, control, and metering systems that use new multifunction relays and fiber optic communication. The coils can be applied at any voltage level (low, medium, and high voltage). One RC consists of two printed circuit boards with imprinted coils, located next to each other. They are highly accurate because manufacturing process is computer controlled, providing accurate geometry of the coil. New protection systems using RCs provide adaptive relaying and fast response times to faults. Since the new RCs are very accurate, protection levels can be set to lower thresholds reducing stress on the protected equipment. Keywords: Relay Protection-Rogowski Coil-Low Power Sensor 1. INTRODUCTION The printed circuit board (PCB) Rogowski coils presented in this paper operate on the same principle as coils that were first time introduced in 1912 for magnetic field measurements. At that time, the coils could not be used for protection because their output power was not sufficient to drive electromechanical relays. However, with today s microprocessor-based equipment, RCs are more suitable for such applications. Current transformers (CTs) have been traditionally used for protection and measurement applications in part because of their ability to produce the high power output needed by electromechanical equipment. Microprocessor-based equipment makes high power output unnecessary and opens the door for other measurement techniques such as RCs, which have many advantages over conventional CTs. Papers [1, 2] explain the theory of RC operation and describe traditional designs and applications. Paper [3] presents an innovative and patented design of RCs that use PCBs. In that design both RC windings are imprinted on the same PCB. This paper presents novel RCs for which an U.S. patent was granted [4]. The RCs have been designed as non-split-core and split-core styles [5-7]. Sections 3, 4, and 5 describe the RC designs, characteristics, and applications for protection, control, and metering. * lkojovic@cooperpower.com
2. PRINCIPLE OF OPERATION Traditional RCs consist of a wire wound on a non-magnetic core. The coil is then placed around conductors whose currents are to be measured. The output voltage is proportional to the rate of change of measured current (Equation 1). vt ( ) = ns d i t M d µ [ j( )] = [ ij( t)] dt dt (1) j µ is permeability of air, n is winding density (turns per unit length), S is core cross-section, and M is mutual coupling. To obtain measured current, coil output voltages must be integrated. For an ideal RC, measurement accuracy is independent of conductor location inside the coil loop. To prevent influence of nearby conductors carrying high currents, RCs must be designed with two wire loops connected in electrically opposite directions. This cancels all electromagnetic fields coming from outside the coil loop. One or both loops can consist of wound wire. If only one loop is constructed as winding on non-magnetic core, then the second wire loop can be constructed by returning the wire through the center of this winding. If both loops are constructed as windings, then they must be wound in opposite directions. In this way, the RC output voltage induced by currents from the inside conductor(s) will be doubled. The traditional method of designing RCs was to use flexible cores such as coaxial cables. The cable shield was removed and the wire wound over the plastic cable core. The existing conductor through the cable core center served as the return (second) loop. To design high performance coils, straight rods have been used. More uniform cross-sectional area can be manufactured with straight rods than with flexible cores. It is also easier to control the winding process using rods. RCs with straight rod design require shielding all ends where rods have to be interconnected. The RC output voltage is proportional to the rate of change of measured current (di/dt) enclosed by the coil. This unique RC feature to measure the speed of current change can be used for special protection algorithms that would make decisions based on the change in the current slope instead on current magnitude. For special applications, it is possible to simultaneously use both the nonintegrated and integrated signals from the same coil. j 3. PCB ROGOWSKI COIL DESIGNS High precision Rogowski coils presented in this paper consist of two printed circuit boards (PCBs) located next to each other (Figure 1). Each PCB contains one imprinted coil wound in opposite directions (clockwise and counter-clockwise). The top and bottom sides of each PCB are imprinted to form a coil around the center of the board. The conductive imprints on the upper and lower sides of the PCB are interconnected by conductive-plated holes. High precision is obtained because the manufacturing process is computer controlled, providing accurate geometry of the coils. New RC designs use multi-layer PCBs, which provides higher accuracy and more proficient manufacturing. PCB Rogowski coils can be designed with different shapes to adjust for the application and be designed in split-core styles for installation without the need to disconnect primary conductors. Figure 2 shows non-encapsulated and encapsulated circular shape RC implemented on multi-layer PCBs. Figure 3 shows an oval shape split-core style RC, designed to embrace all three-phase conductors (for measurement of residual currents) or to embrace parallel conductors that carry heavy currents. The split-core style RC consists of four half loops. The first two half loops are constructed with two PCBs with imprinted windings wound in opposite directions, located next to each other. The remaining two half loops are constructed in the same way, but wound in opposite directions and connected. Figure 3 shows principle of the PCB split-core style RC design and a completed coil.
The PCB Rogowski coil has the following characteristics: measurement accuracy reaching.1 %; measurement range from 1 A to over 1 ka; frequency response linear up to 7 khz; unlimited short-circuit withstand; galvanically isolated from the primary conductors; can be installed around bushings or cables, avoiding the need for high insulation. Rogowski coils can be connected in series to increase output signal. i(t) Figure 1. Principle of the PCB Rogowski Coil Design v(t) Figure 2. PCB Rogowski Coils (Non Split-Core Design) i(t) v(t) Figure 3. PCB Rogowski Coils (Split-Core Design) 4. PERFORMANCE TESTS The RC output voltage is proportional to the rate of change of measured current (di/dt) enclosed by the coil. Therefore, the RC non-integrated signal can be significantly different that the waveform of the measured current since the fault current DC offset will be attenuated and higher frequency components amplified. The waveform will be also shifted by 9. Figure 4 shows non-integrated and integrated RC output signals, which are obviously different. However, the integrated signal is almost identical to the waveform recorded by a precision laboratory current sensor (Figure 5).
8 6 4 Primary current, 24 A RMS (non-integrated and integrated signals) Current [A] 8 6 4 2-2 laboratory CT 2 8 Current [A] -2 Current [A] 6 4 2 integrated RC signal -2.1.2 Time [s] Figure 4. PCB Rogowski Coil Non-Integrated and Integrated Signals.1.2 Time [s] Figure 5. Test Current recorded by a Precision Laboratory CT and by PCB Rogowski Coil (Integrated Signal) Residual current measurements were performed to determine the low range of currents that a residually connected coil can detect. Three split-core style coils were tested in a three-phase arrangement. The test (load) current was maintained at approximately 1 A. Residual current was controlled in the range 1 to 1 A through a current transformer with a variable resistor connected on the secondary side. The results are shown in Figure 6. All three coils have consistent results, demonstrating that can provide sensitive ground fault protection. Differential Current Measurements. The RC performance for capacitor bank differential protection was tested to determine minimum differential currents that can be reliably detected. The RC was connected in a differential mode representing a series capacitor bank carrying 2 A line current. Figure 7 shows test results for differential currents up to 5 A. The RC output was linear to bellow 1 A differential current, demonstrating high performance. When RCs are connected in differential mode shown in Figure 7, the RC insulation class can be much smaller as compared to RCs connected in differential mode in H-connected capacitor bank. RC Output Voltage [mv] 1.4 1.2 1.8.6.4 Three Split-Core Rogowski Coils RC #2 I load =1 A RC #1 RC #3 mv I ground RC Output Voltage [mv].5.4.3.2 1 A 2 A RC 1 A.2.1 2 4 6 8 1 12 Residual Current [A] Figure 6. Residual Current Measurement 1 2 3 4 Differential Current [A] Figure 7. Differential Current Measurement Influence from nearby conductors is one of the most important tests to determine the RC accuracy. The test was performed in a high power laboratory at test current of 6 ka RMS. Two RCs were tested, RC1 was installed to measure the test current and RC2 was located 5 cm next to the primary conductor to test the influence from the primary conductor. Since the induced signal in RC2 was very small, an amplifier with 1 times amplification was used to increase the signal to the level acceptable by the recorder. The results are shown in Figure 9. The influence from the primary conductor was bellow.1 %, verifying very good coil immunity to the external magnetic fields. In actual
applications, RCs will be installed at a distance from nearby conductors where the influence will be near zero. 4 Transient Recorder 3 G Making Switch Circuit Breaker R, X Laboratory CT RC1 RC2 Amplifier O/E Fiber Optic Cables E/O RC Output Voltage [V] 2 1-1 -2-3 Measured current, 6 ka RMS (non-integrated signal) Influence from the Influence from the nearby conductor, 5 cm distance nearby conductor, 5 cm distance (signal magnified 1 times) (signal magnified 1 times) 5 cm Figure 8. High Power Test Setup -4.1.2.3.4 Time[s] Figure 9. Influence from the Primary Conductor 5. Applications 5.1. Low Voltage Applications Spot network systems have been widely applied in the U.S. to supply power to densely loaded urban areas with a high level of reliability and operating flexibility. However, in case of a fault, arcing fault currents in the collector bus are relatively low as compared to maximum available fault currents for bolted faults (in some cases not much higher than the load currents). Since these low currents may not be sufficient to operate the fuses, damage to network equipment can be extensive. PCB Rogowski coils can provide reliable differential protection of spot networks that was not possible by conventional methods [8]. 5.2. Medium Voltage Applications RCs can be designed as combination coils for installation in medium voltage (MV) switchgear to fit around circuit breaker bushings (Figure 1). Combination coils consist of four coils in the same frame, three coils for phase current measurements and one coil for residual current measurements. The coils for phase current measurements are circular non-split-core design, while the coil for residual current measurements is an oval split-core design. Paper [9] presents different designs of low power sensors for applications in medium voltage systems (wire wound RC sensors, resistive dividers, and combi sensor, combining RC and voltage sensors). Split-Core Rogowski coil Secondary Signal A B C Non-Split-Core (Phase) Rogowski Coils (a) (b) Figure 1. Combination PCB Rogowski Coil, Three Phase Coils and One Neutral Coil, (a)-design principle, (b)- prototype coil during high power tests
5.3. High Voltage Applications Transformer Differential Protection. The most common form of protective relaying for large substation transformers is differential relaying. However, CT saturation due to the high current magnitudes is a recognized problem causing protection misoperation. Traditional schemes use restraint currents and multiple slopes to prevent the protection misoperation. Increased setting is also one of the solutions. Differential schemes have not been applied on electric arc furnace (EAF) transformers due to the lack of commercially available current transformers for the secondary leads. Now, PCB RCs and multifunction relays can provide reliable protection. Figure 11 shows RC applications for transformer differential protection. High power tests were performed using test scheme shown in Figure 11a. The results confirmed that the relay properly operates even at low fault currents, 3% load current, confirming that the PCB RCs provide strong differential signal ensuring reliable relaying. Figure 11b demonstrates a RC application for EAF transformer differential scheme employing two sets of RCs and one multifunction relay. Designed as split-core style, coils can be installed around the secondary conductors that normally cannot be opened. An external signal can be supplied to the relay to indicate the operating tap of the transformer. With suitable delays and an algorithm developed for this application, the relay can be programmed to ride through an on-load tap change. The ability of the scheme to adjust to actual transformer operating conditions reduces the main sources of error that force higher percentage differential settings in conventional schemes. The tap position can be supplied to the relay in a variety of formats such as analog or digital. Figure 11c shows installation of RCs around transformer bushings, allowing the use of low voltage insulation class RCs. EAF Transformer G R, X 6 ka Fault current I1 P RC1 RC2 I2 P RC1 RC2 SW Rf I1 S Tap Position I2 S Water Cooled Leads EAF Transformer Differential Protection Algorithm RC Multifunction Relay (a) (b) (c) Figure 11. PCB Rogowski Coils for Transformer Differential Protection, (a)-laboratory test, (b)-eaf transformer application, (c)-rc installation around transformer bushings Gas Insulated Switchgear (GIS). A number of papers have been written on RC applications in GIS because RCs provide superior performance as compared to conventional CTs. RCs have been also effectively combined with capacitive voltage sensors to simultaneously measure voltages and currents in GIS as described in Papers [1-12]. A new digital control panel and electronic instrument based microprocessor techniques developed for GIS is presented in paper [13]. Wire wound type RCs were used for current measurements and capacitive voltage dividers for voltage measurements. Papers [14 and 15] present non-conventional sensors developed and implemented on GIS from 145 kv to 55 kv. Rogowski coils are designed using multi-layer printed circuit boards. Electronic capacitive dividers are used for voltage measurements. Optically Powered Data Link (OPDL) technology has been introduced in high voltage systems to transmit data from the high voltage potential to ground potential, using fiber-optic cables and laser technology [16]. Recent applications include current measurement for protection and metering with metering accuracy. RCs are installed at high-voltage levels, suspended from the primary conductor or a busbar. This eliminates a need for a supporting insulator.
The advantages of these novel methods as compared to the conventional high voltage, free standing iron core CTs are: no oil or SF6 gas, light weight, no seismic or explosion concerns, and use of low voltage insulation class RCs. The system consists of a RC and the OPDL system as shown in Figure 12. The RC output signal is fed into the remote unit of the OPDL, located on the high voltage potential. The remote unit is interfaced to the ground unit over two fiber-optical cables. One cable provides power for the remote unit and the second cable transmits data from the RC. The remote unit is shielded against EMI or RFI noise and converts the RC voltage into digital signals. The electrical power to operate this unit is provided by the photovoltaic power converter that is connected to the laser over one of the fiber optical links with a conversion efficiency of up to 4%. The fiber-optical cables are incorporated into a composite insulator, a lightweight structure similar to suspension insulators made of composite, silicone material. The OPDL ground unit includes the laser with its associated laser driver and the data recovery circuitry. These lasers are reliable with MTBF > 1, h. A self-check function supervises all vital functions of the system. One OPDL unit with two fiber connections (one for power, one for data) can transmit signals out of two RCs to the ground level. i(t) HV Line Fiber-Optical Cables Ground A/D Power Converter Power Supply Laser Diode Data Data Receiver D/A Converter Analog Output Remote DSP Digital Output RC and Remote Ground Ground Fiber-Optical Cables Power Data Remote HV Potential Figure 12. RC Applications fro High Voltage Current Measurements using OPDL Technology RC Junction Box Substation Applications. PCB Rogowski coils can provide advanced protection when applied in substations by integrating primary, fast backup, and conventional backup protection [8]. Protection functions are performed locally and over communication. The RC diagnostic routine, implemented in the relay, continuously monitors the RC condition and performs a contingency operation in case of an RC or secondary lead failure. Providing fast backup protection for another relay is a unique concept. Relays receive direct signals from two RCs, one providing main protection for one feeder and the second providing fast backup protection for another feeder. Relays receive information that other relays are inoperative over peer-to-peer communication. In all cases when the primary relay becomes inoperative, the backup relay will operate fast, without impacting the fault clearing time and protection zone area. Next generation protection systems can use data from low power sensors installed anywhere in the substation over a large communication network. Multifunction relays are capable of executing multiple protection schemes in parallel in the same relay, providing both primary and fast backup protection. Object-oriented programming and code reuse are widely accepted in the industry. Graphical programming languages are sufficiently developed for efficient protection schemes customization. Standardization is becoming internationally unified by developing a global communication standard for substation automation, IEC 6185.
6. CONCLUSIONS Printed circuit board (PCB) Rogowski coils (RCs) can be designed with different shapes to adjust for the application and be designed in split-core styles for installation without the need to disconnect primary conductors. Extensive testing was performed on both types of PCB RCs (non-slit-core and split-core style) to determine their performances. To test the extreme application conditions, no shielding was applied to Rogowski Coils. High power test results confirmed that both the split-core and non-slit-core RC output signals are almost identical to signals obtained by high-precision laboratory current sensors and influence from external electromagnetic fields is minimal. PCB Rogowski coils are accurate and have excellent performance for applications in advanced protection, control, and metering systems with new multifunction relays. 7. REFERENCES [1] K. Heumann, Magnetischer Spannungsmesser hoher Prazision, ETZ-A, 1962. [2] Lj. A. Kojovic, Rogowski Coils Suit Relay Protection and Measurement, IEEE Computer Applications in Power, July 1997. [3] E. Thuries, J. P. Dupraz, C. Baudart, J. P. Gris, Contribution of Digital Signal Processing in the Field of Current Transformers, CIGRE 96, Paris, France, 1996. [4] Lj. A. Kojovic, V. Skendzic, S. E. Williams, High Precision Rogowski Coil, Patent Number: 6,313,623; Date of Patent: November 6, 21. [5] Lj. A. Kojovic, PCB Rogowski Coils Benefit Relay Protection, IEEE Computer Applications in Power, July 22. [6] Lj. A. Kojovic, Split-Core PCB Rogowski Coil Designs and Applications for Protective Relaying, IEEE T&D Conference and Exhibition, Dallas, Texas, September 23. [7] Lj. A. Kojovic, PCB Rogowski Coil Designs and Performances for Novel Protective Relaying, IEEE PES General Meeting, Toronto, Canada, 23. [8] Lj. A. Kojovic, M. T. Bishop, V. Skendzic, Coiled for Protection, IEEE Power & Engineering, May/June, 23. [9] J. M. Sohn, W. J. Choe, B. W. Lee, I. S. Oh, H. S. Kwon, Development of current and voltage sensor for distribution switchgear, T&D Conference and Exhibition 23, Dallas, USA, 23. [1] G. Schet, F. Engler, F. Jaussi, K. Pettersson, A. Kaczkowski, The Intelligent GIS - A Fundamental Change in the Combination of Primary and Secondary Equipment, CIGRE 96, Paris, France, 1996. [11] G. Reiter, T. Looser, D. Fischer, M. Ilar, Digital Protection and Control Systems in Substations using Sensor Technology, CEPSI 2, The 13 th Conference of the Electric Power Supply Industry, Manila, Philippines, October 23 27, 2. [12] H. Aeschbach, J. P. Dupraz, T. Jung, P. Roussel, The integration of Electronic CTs and VTs in Power Switchgear: challenges and choices, CIGRE 2, Paris, 2. [13] J. B. Kim, M. S. Kim, T. W. Kim, W. P. Song, Development of Intelligent Gas Insulated Switchgear using Electronic Technology, T&D Conference and Exhibition 23, Dallas, USA, 23. [14] J. P. Dupraz, G. F. Montillet, A New Method for the Measure of Current: Applications up to 55 kv Gas Insulated Substations, T&D Conference and Exhibition 23, Dallas, USA, 23. [15] J. P. Dupraz, G. F. Montillet, An Innovative Method for Voltage Measurement: Applications up to 55 kv GIS, T&D Conference and Exhibition 23, Dallas, USA, 23. [16] Optically Powered Current Transformers for Metering or Protection, Photonic Power Systems, Inc., Cupertino, CA.