1 1 Applications of High-Voltage Fiber Optic Current Sensors Farnoosh Rahmatian, Member, IEEE-PES, and James N. Blake Abstract Various applications of a fiber optic current sensor are explored. Flexibility and inherent beneficial features of the technology make it appropriate for many applications, including some novel protection applications. These applications include regular high-voltage AC metering and protection, accurate wide dynamic range metering, high voltage DC, high current DC, portable calibration reference, generator monitoring, generator protection, and high-current AC applications. Some novel uses of the technology, including optical summation of currents where conductors are quite large or far from one another, are also discussed. Index Terms-- current measurement, high-voltage techniques, optics, optical current sensor, transducers, optical fiber devices, power measurement. O I. INTRODUCTION PTICAL voltage and current sensors used for highvoltage (HV) and/or high-current (HC) measurements can offer several attractive features. These benefits include High accuracy over wide dynamic range, Wide bandwidth from dc to > 100th harmonic, Light-weight and small size: Excellent seismic performance, Safe, easy, flexible, and cost-effective installation, User-adjustable turn-ratio, No CT saturation, Excellent phase accuracy, Voltage and current sensing in one device, and Safety & environmental benefits: No oil or SF6, No open secondaries, No ferro-resonance, and Galvanic isolation from HV line. Fiber-optic current sensors offer additional advantages of flexible form factor, window-ct design, and ability to measure very high currents. The combination of all these features creates a great deal of flexibility in the use of optical voltage and current sensors. In other words, the same optical products or technology can be used for several applications where traditionally different types of products or even F. Rahmatian is with NxtPhase T&D Corp., Vancouver, BC V6M 1Z4 Canada ( J. N. Blake is with NxtPhase T&D Inc., Phoenix, AZ USA ( different technologies were used. In this paper we provide a review of this application flexibility for a fiber optic current sensor (NXCT) technology. II. APPLICATIONS The NXCT uses an in-line fiber optic interferometric design described in detail in  and . The sensing head is in the form of an optical fiber encircling the current carrying conductor in a full turn or several turns. It effectively and accurately integrates the magnetic field around the current carrying conductors that it encircles, via the Faraday Effect, measuring the net current through its aperture. The sensing fiber can be packaged in fixed size windows or in a flexible cable. Figure 1 shows a 362 kv class NXCT with several primary conductor connection options for used in air-insulted substations (AIS). The sensing fiber packaging is the same for all these configurations, and the window aperture is 110 mm in diameter, rated for 4000 A continuous operation. The options shown use different clamps for holding the primary conductor. Figure 1.b shows a flat conductor bar, supplied for connection to standard 4-hole or 6-hole NEMA connections. Figure 1.c shows the dual cable clamp option, allowing the user to run cables through the CT and reduce the number of HV current carrying connections, which are a source of heat and reliability issues for HC CTs. Figure 1.d, shows yet another clamp option, for mounting the NXCT on a 4 solid bus in the substation. In addition to saving on electrical interfaces at high voltage and high current, this option (suspension) can allow elimination of civil work and costs associated with pedestal-mount CTs and can be very attractive in seismically active regions. Figure 2 shows 145 kv class optical CTs, mounted conventionally, using the 6-hole NEMA conductor bar. In this application, high accuracy (0.15S class) over a wide dynamic range was the main motivation for using the NXCT. The optical current sensor can offer better than 0.15% class accuracy from 0.1% to 200% of rated current . Energy measurements where wide dynamic range is required, e.g., with Independent Power Produces (IPPs) and wind farms, are ideal applications for the NXCT due to its linearity. Figure 3 shows 72 kv class NXCTs, mounted horizontally from other substation structures. In this case, the light weight (<65 kg), solid insulation (no oil to leak), and high accuracy features combined to provide a very cost-effective solution for
2 2 (b) (c) Figure 3. Zero foot-print installations: two 69 kv NXCT systems installed horizontally off existing civil structures. (a) Figure 1. A 362 kv class NXCT (a) with several primary conductor connection options: (b) flat conductor bar with standard 6-hole NEMA pads, (c) dual cable clamp, and (d) 4 solid bus suspension clamp. the user, providing significant savings in the civil work for the (d) installation. Figure 4 shows accuracy measurement data for an NXCT for primary currents from 2 A to 3600 A. The optical sensor shows 0.1% accuracy over this entire range. The measurements are taken using three different metrological circuits over this current range, such as those given in  - . The error bars around data points in Figure 4 show the uncertainties of the measurements. Figure 5 shows an NXCT attached to a 420 kv class live tank circuit breaker. Here, the solid bus clamp is used, and the NXCT is suspended from the breaker. As compared to using a conventional free-standing CT for this application, the 0.2 S Accuracy Class Error (%) Current (A) Figure 2. A 0.15S class 138 kv NXCT system installed for an IPP revenue metering application. Figure 4. Accuracy measurement results from 2 A to 3600 A of primary current for an NXCT optical current sensor. The red line shows boundaries of 0.2S accuracy class requirements per IEC
3 3 NXCT offers significant cost savings by eliminating the footing and civil work, reducing the need for substation realestate, excellent seismic performance, and no CT saturation. In summary, this solution provides a very attractive alternative to using conventional HV CTs. The dynamic range of measurement provided by NXCT can also be exploited for protection applications. For example, a key application where use of conventional magnetic wire-wound CTs presents significant safety challenges is on low ratio CTs for protection of series or shunt capacitor banks. In these applications, usually a very sensitive CT (low ratio) is used to detect a small in-balance current which may appear when a small subset of capacitors has failed. The challenge is, however, that in case of a major capacitance unbalance (for example if a significant portion of capacitors are externally shorted for a short period of time), significant voltages and energy can appear on the secondary side of a magnetic-core wire-wound CT. Use of an optical CT completely eliminates this safety concern. However, to address this application effectively, the optical CT has to be designed and fabricated to yield acceptable signal-to-noise ratio. The primary current rating of the CTs used for these applications is usually less than 25 A. Protection settings of the relays used in this application are usually set at <10% of the rated current, 2.5 A in this case. Therefore, the noise on the output of the CT should be significantly less than 2.5 A (primary equivalent) to avoid any false trips. Figure 6 shows the measurement result made on a 40 fiberturn NXCT manufactured for this application. The ratio setting of this NXCT is 25A:1A, and the bandwidth is limited to 300 Hz for this application. The NXCT generally shows 200 ma-turn/ Hz of equivalent-to-primary-current noise. Accordingly, the expected noise for a 40 fiber-turn CT is 5 ma/ Hz, or about 100 ma (rms) for a 300 Hz bandwidth sensor. Many relays for this application filter the power frequency signal with a filter time constant of about one second, effectively further eliminating the noise to less than 5 ma. Both of these levels (5 ma and 100 ma) are significantly less than 2.5 A, satisfying the application requirement. Figure 6 shows compliance of this NXCT with the requirements of 0.5% and 1% accuracy classes specified in IEC , for a rated current of 25 A. The uncertainty of the measurements given in Figure 6 is less than 0.07%. The same NXCT represented in Figure 6 is also configured to provide a separate analog output rated at 25A:200mV, suitable for use at up to 40 times rated current (1000 A), with an output bandwidth of 6 khz. The rated time delay of this NXCT is 50 µs. This output can be used for usual instantaneous over current protection application. Wider bandwidth of this output (6 khz, as compared to 300 Hz) translates into an effective noise about 5 times larger Figure 5. An NXCT-420 mounted on a 420 kv class live tank circuit breaker Primary Current (A) Figure 6. Accuracy results for an NXCT-245 rated at 25A:1A, IEC class 1, used for unbalance shunt capacitor bank protection Phase Error (deg.) Phase Error (deg.) Figure 7. Accuracy measurements on a second output of the same NXCT-245 of Figure 6, rated at 25A:200 mv, with 40 times over current capability Primary Current (A) Phase Error (degrees) Phase Error (degrees)
4 4 (equivalent to 0.5 A rms of primary current). Figure 7 shows results of accuracy measurements on the 200 mv rated output of this device at up to 800 A rms. The test equipment used was an Arbiter 931A Power System Analyzer together with a traceable reference magnetic CT, giving <0.07% uncertainty in the measurements reported here. The NXCT can also be used in high-voltage DC (HVDC) applications. The HVDC sensor is practically the same as the HV AC product, as the real bandwidth of the CT starts at DC. Both protection and high-accuracy metering applications (better than 0.2% class) can be served with the NXCT. Figure 8 shows the sensing head of a flexible form factor (F3) fiber optic CT, NXCT-F3. The flexible sensing head is an all-dielectric fiber cable which can be wrapped around the current carrying conductor to measure current. NXCT-F3 provides a whole new level of flexibility in application because of its form factor and all-dielectric structure. Some of its key features are: Ability to measure very high currents, in excess of 500 ka, Very easy installation around large conductors, No need to break the current carrying path for installing the head, Capability to be installed on live lines (depending on user practices), Insensitive to conductor positioning through the current sensing loop, Ability to measure AC, DC, and high frequency currents These features make the NXCT-F3 ideal for application such as DC current measurement at very high currents (e.g., aluminum smelters and electro-chemical processes), electric power generation applications (measurement, protection, and monitoring), retrofit applications, live-line measurement, portable calibration systems, and HVDC testing. Figure 9 shows 0.1% class NXCT-F3 s installed at a chemical plant, operating at nominal DC currents of 25 ka to 50 ka. The flexible and light sensor-head cable is routed around the large DC conductor, with the open end of the loop plugging back into the box to make one complete loop. Ability to measure DC, as well as AC, compact size, ability to Figure 8. Sensing head of an NXCT-F3. The flexible sensing head is an all-dielectric fiber cable permanently attached to the grey enclosure at one end. The other end of the sensing cable will be secured in the same enclosure after wrapping the cable around the current carrying conductor(s). Figure % class NXCT-F3 current sensor systems used in highcurrent DC applications. The NXCT-F3 is used for metering, protection, and process control at 25 to 50 ka DC. The sensing head cable is routed through an insulating conduit in this installation. be installed without breaking the large rigid bus, and insensitivity to positioning have made the NXCT-F3 an ideal solution for high current DC applications. A novel application for the NXCT-F3 in AC power systems is protection based on net zero current measurements (requiring very large low ratio CTs). In many applications, the net multi-conductor vector sum current (or zero sequence current in case of a 3-phase system) is zero under normal operating conditions. In the case of a high-impedance fault, for example, the net current may not be zero (due to the addition of the fault current), and this information can be used for fault protection applications. Using conventional magnetic core CT technologies, it can be very difficult to measure several currents with enough accuracy to detect small differences between them, especially when each conductor is carrying large currents. The NXCT-F3 can be installed such that all the conductors of interest are inside one sensing loop, and the net sum current is measured. The sensing loop can be several meters in diameter, or have irregular shapes to include conductors from various phases. For example, a 200 MW generator may be producing 10 ka to 20 ka at 10 kv. As this large current flows through the generator, the two conductors connected to the generator should have exactly the same current (sum of currents into a node should be zero). However, if a high-impedance stator fault occurs in the generator, and some current leaks through it, the input and output currents will not be the same; they may be different by 10 A for example. The NXCT-F3 with its thin and long sensing loop can be easily wrapped around both conductors such that it effectively measures the vector sum (difference in this case) of the currents. It can easily detect 10 A net difference, even when each individual conductor is carrying 20 ka. One NXCT-F3 can actually be wrapped around all three phases (in typical three-phase generation plants) and be used for high-impedance fault protection on the entire system. To do the same with six (one per conductor) separate 20 ka conventional magnetic CTs is most likely not possible due to the accuracy requirements. Even if the technical issues are
5 5 worked out, the cost and size of a hypothetical 10 kv, 20 ka conventional CT core that can encompass all 6 conductors is prohibitively high and impractical. Similar solutions maybe used at higher voltages too. For example, a three-phase 69 kv system may be operating at 2000 A/phase nominal current, with phase-to-phase fault current level of 20 ka, while a transformer ground fault current is limited to 100 A. The solution for transformer ground fault protection with conventional CTs would require installing three perfectly matching 69 kv CTs, and rating the CTs such that while they can continuously operate at 2000A, they would produce accurate and well detectable signals at 100A (both individually and superimposed on 2000A). Further more, the CT s need to be linear to much better than 0.5% (100A in 20,000 A) at up to 20,000 to avoid false operation for the transformer ground fault protection scheme under phase-to-phase faults. The optical solution can use one (as opposed to three) NXCT-F3 that wraps around all three conductors, without touching the HV elements. Simply, a loop of sensing cable, 6 meters wide and 2 meters high, can be placed around all three HV buses in the substation (phase-tophase spacing at 72 kv can be ~ 1 meter). The installation would be much safer, faster, and less expensive than that of a three-phase conventional solution, and the solution will not be affected by phase-to-phase faults. Additionally, the output of the NXCT can be scaled for the 100A net primary current so that relay input dynamic range doesn t become an issue. Similarly, an NXCT-F3 is an easy-to-retrofit, accurate, and safe solution when high accuracy metering or power quality measurements are needed for medium voltage high current generators or other high-current applications such as AC/DC converter systems. Figure 10 show the frequency response of an NXCT-F3 with 20 khz bandwidth, used for testing thyristor valves of HVDC Converters. The data is normalized relative to sensors DC sensitivity. The system has about 33 µs time delay, accuracy of 0.2% at 80 ka, and maximum current measurement capability of 100 ka. The metrology used for measurements at frequencies above 60 Hz had <1% uncertainty. Another application for NXCT or NXCT-F3 is power cable Normalized Amplitude Frequency (Hz) Figure 10. Amplitude sensitivity of an NXCT-F3 rated at 100 ka peak as a function of frequency. monitoring. For example, NXCTs can be mounted at the two ends of a HV underground cable and measure the difference between the current entering and the current exiting the cable. The relay connected to the NXCTs can be located at one end of the cable, and fiber optics can be run along the power cable without any concerns about CT signal degradation and electromagnetic interference. Similar solutions with conventional CTs would create significant challenges with respect to electrical wiring, burden issues related to long wire runs, and electromagnetic interference on CT secondary wires from the high-current power cable running in parallel. Owing to the characteristics of optical fibers (thin, flexible, insulating) the NXCT sensing head can also be easily integrated into other substation devices. For example, the same NXCT technology is already integrated into some GIS switchgear. At 420 kv, for instance, the GIS-NXCT gives significant reduction in the size of the CT compartment, from ~ 150 cm to 6 cm, and overall results in a 10% to 15% smaller GIS Bay. GIS systems are usually used where real estate is a significant limitation (either cost or other practical limitations) and size reduction can have very significant value. Fiber optic CTs can be integrated into various live-tank and dead-tank equipment and completely eliminate the footprint of the CTs in HV substations. Flexibility also extends beyond the initial installation for all CTs of this type. The turns ratio of the fiber optic CT is a parameter set in software. The device is inherently linear, so an NXCT set with one ratio at installation can be changed to a new ratio at a later point in time as required with a simple lap top connection and no change to hardware. This feature is a benefit in any application where flexibility is desired such as a wind farm where turbines are added and generation capacity is growing resulting in the need for a change in CT ratio. III. SUMMARY AND CONCLUSION In this paper, several applications of the NXCT optical fiber sensor are explored, and data related to its accuracy, linearity, dynamic range, bandwidth, and noise considerations are provided. The inherent features and attractive characteristics of the technology allow the same sensor to be used effectively for many different applications. Several of these applications, both in AC and in DC systems, have been presented. Further, some of the sensor s unique features, particularly its simple and safe dielectric design and its flexible form factor, enable some novel applications, which otherwise could not be practically addressed. An outstanding example of these new applications is protective relaying based on optical summation of currents where conductors are quite large or far from one another. Finally, as power system technologists gain more familiarity with optical sensors and their characteristics, it is expected that further new concepts and applications will be uncovered. IV. REFERENCES
6 6  G. A. Sanders, J. N. Blake, A. H. Rose, F. Rahmatian, and C. Herdman, Commercialization of Fiber-Optic Current and Voltage Sensors at NxtPhase, 15 th Optical Fiber Sensors Conference, Portland, OR, May 2002, pp  J. Blake, P. Tantaswadi, R. T. de Carvalho, In-line Sagnac interferometer current sensor, IEEE Trans. Power Delivery, vol. 11, Jan. 1996, pp  J. N. Blake and A. H. Rose, Fiber-Optic Current Transducer Optimized for Power Metering Applications, Proceedings of the IEEE T&D meeting, Dallas, TX, Sept. 2003, pp  IEEE Standard Requirements for Instrument Transformers, IEEE Standard C57.13,  E. So, R. Arseneau, and D. Bennett, A current-comparator-based system for calibration of optical instrument transformers with analog and digital outputs, in Dig. CPEM 2002, Ottawa, ON, Canada, Jun. 2002, Dig. No. 02CH37279, p  E. So, The application of the current comparator technique in instrumentation and measurement equipment for the calibration of nonconventional instrument transformers with non standard rated outputs, IEEE Trans. Power Del., vol. 7, no. 1, pp , Jan V. BIOGRAPHY Farnoosh Rahmatian (S 89, M 91) was born in He received the B.A.Sc. (Hon.), M.A.Sc., and Ph.D. degrees from the University of British Columbia, Vancouver, BC, Canada, in 1991, 1993, and 1997, respectively, all in electrical engineering. From 1997 to 2004, he was a Director of Research & Development at NxtPhase Corporation, also in Vancouver, working on precision high-voltage optical instrument transformers for use in high-voltage electric power transmission systems. Since 2004, he has been the Director of Optical Systems at NxtPhase T&D Corporation, focusing on application and commercial use of optical voltage and current sensors. Dr Rahmatian has also been an adjunct professor at the Department of Electrical and Computer Engineering at the University of British Columbia, a member of IEC TC38 Working Group on instrument transformers, Standards Council of Canada, Canadian Standards Association, IEEE Power Engineering Society, IEEE Lasers and Electro-Optics Society, and an active member of IEEE/PES working group working on optical instrument transformer systems. Dr. Rahmatian has received an R&D 100 award for the development of the optical fiber current and voltage sensor in James Blake was born in Oakland, CA in He received his B.S.E.E. from U.C. Berkeley in 1981, and his Ph.D. in Electrical Engineering from Stanford University in He worked as a microwave antenna engineer at Ford Aerospace in Palo Alto, CA from 1981 to From 1988 to 1991 he was a Research Scientist at Honeywell in Phoenix, AZ working on fiber optic gyroscopes. From 1991 to 1999 Dr. Blake was a Professor of Electrical Engineering at Texas A&M University in College Station, TX. His research at Texas A&M concentrated on fiber optic gyros, flow sensors and current sensors. Since 1999, Dr. Blake has been Director of Research and Development at NxtPhase in Phoenix, AZ where he has concentrated on commercializing fiber optic current sensors for high-voltage applications. Also, Dr. Blake formed Precision Lightwave Instruments in 1998 to work on fiber-optic current standards. Dr. Blake has received an R&D 100 award for the development of the optical fiber current and voltage sensor in 2002.