The electricity utility industry has experienced many changes in recent years, driven largely by pressures to open the competitive landscape. As a result, new regulatory requirements mandate that utilities should meter previously un-metered locations on the power network with highly accurate revenue metering equipment. The financial burden to upgrade these metering points has forced the industry to explore the possibility of reusing the existing relay-class current transformers (CT) for high accuracy revenue metering applications.
The true accuracy of existing CTs is typically unknown, and utilities and manufacturers are working together to understand the accuracy characteristics and to determine if these CTs can be reused for revenue metering. To address this need, a new CT reclassification system has been developed. The key component is a highly accurate reference current sensor that can easily be deployed using live-line methods in a high voltage environment, which avoids any interruption to service. The system first analyses the accuracy of an existing CT, calculating its characteristic performance. Corresponding error correction parameters are then derived and programmed into an advanced revenue meter, enabling dynamic accuracy reclassification of the existing CT while operational in the field. The new system has been proven in a live, operational high voltage environment at a 138 kV utility substation.
The reference current sensor, known as the primary sensor, can be deployed using live-line methods on voltages up to 765 kV. The sensor is based on a split-core active CT design that provides accurate measurement capabilities. This produces a system traceable to accuracy standards recognised by the National Institute of Standards and Technology (NIST) in the USA, and to related standards recognised by the National Research Council in Canada. In a typical installation, three primary sensors are installed on the high-side bus in an HV substation in series with the circuit breaker containing the existing relay-class CTs under test (see Figure 1).
The primary sensors each wirelessly stream data to a computer workstation in the nearby substation control building. The software simultaneously communicates to an advanced 3-phase meter, which is connected to the secondary wiring of the existing relay-class CT under test. The three primary sensors and the advanced revenue meter are each equipped with a global positioning system (GPS) receiver that supports the accurate time-stamping and synchronisation of all data sent to the computer for analysis. The software compares the data from three primary sensors to the respective 3-phase data collected from the meter, and calculates the appropriate ratio correction factors (RCF) and phase angle correction factors (PACF) for each test point over the dynamic operating range of the relay-class CT under test.
These correction parameters are then programmed into the advanced revenue meter, which in turn provides dynamic correction for the ratio and phase angle errors of the relay-class CT over its operational range. This effectively reclassifies the CT to be of revenue metering accuracy The primary sensors and computer are then removed. The advanced revenue meter is left in place, connected to the secondary wiring of the existing relay-class CTs. The revenue meter, using the correction parameters, measures accurate energy data and reports to the utility data collection and billing system.
PRIMARY SENSOR TECHNOLOGY
The primary sensor acts as a reference standard against which the accuracy of the relay-class CT under test is compared. The innovative design of the primary sensor features a split-core active CT, an on-board GPS receiver, an electronics module for measuring RMS current magnitude and phase angle, wireless communications, and a unique self-powering method (see Figure 2). The GPS units used on the primary sensors and advanced revenue meter produce time-stamping accurate to 100 ns. This enables the software to time-align data points from the sensors and the meter, and also ensures highly accurate phase angle measurements on the sensors and meter. Testing has shown the angular accuracy corresponds to +/- 5 minutes, or +/- 0.083 degrees.
An industrial version of Bluetooth wireless communications featuring a 2.5 GHz spread spectrum radio frequency is used. It was chosen based on its low power consumption, data encryption and highly directional 300 metre range in an HV substation environment. The sensor uses a CT to step down the high primary transmission line current to a level suitable for analog-to-digital conversion and subsequent digital signal processing.
The CT uses a split-core, actively compensated zero flux designi . The split-core enables fast and easy installation by allowing the CT to be clamped over a live transmission line (see Figure 3). However, ‘splitting’ a conventional toroidal CT typically results in degraded accuracy due to reduced effective permeability, increased leakage flux, and the difficulty in designing a mechanism that will properly align the mating core surfaces with repeatability. All these factors will produce ratio and phase errors that prevent use in high accuracy, IEEE Std. C57.13 class 0.3 metering applicationsii . Active compensation effectively removes these sources of error and results in a split-core CT that has a higher ultimate accuracy. In fact, this design results in even higher accuracy over a wider dynamic current range than conventional ‘non-split’ toroidal core CTs.
The CT employs a main and sense set of magnetic cores and windings, mounted within a high-tolerance mechanical structure that mates to form two independent toroidal magnetic circuits when closed. Through special sense amplifier compensation circuitry, a zero flux condition is simulated which produces essentially lossless transformer operation. This in turn provides a near perfect ampereturn balance between the transmission line and secondary winding of the main magnetic core, resulting in minimal ratio and phase error, thus maximising the accuracy of the CT.
Once compensated, the signal is digitally converted at 128 samples-per-cycle, time-stamped with GPS accuracy, and processed by the internal digital signal processor to provide RMS current and phase data. Overall, the primary sensor CT offers a 1000:5 ratio, handling primary transmission line currents of 1000 Amps RMS. The primary sensor employs a unique electric field-based powering system that draws energy from the high voltage transmission line potential. It provides continuous operation at all transmission line current levels, including zero, thus allowing the primary sensor to operate over a very wide dynamic range.
Long term reliability is ensured through simplicity of operation without the use of batteries, solar panels or other energy storage devices. When positioned on a transmission line or substation bus, the sensor’s galvanicallyinsulated tubular aluminium structure is designed to provide corona protection while also developing a finite body capacitance through which a small AC current can flow. On-board circuitry transforms this to a 2.5 W DC power supply that is more than sufficient to operate the measurement and communications components of the primary sensor.
LIVE LINE DEPLOYMENT
Weighing approximately 17 kg, the primary sensor can be installed and removed from an HV transmission line or bus using live-line techniques (see Figure 4). A two-person crew can handle the installation easily, either from a bucket truck or from ground level. The primary sensor is deployed using two shotgun hot sticks. Each hot stick is locked onto the eye ring of one of the two bus clamps on the primary sensor, which is then lifted vertically toward the bus, maintaining proper clearances at all times.
With the split-core CT open, the design allows it to be guided easily into place. Using the hot sticks, the two bus clamps are then screwed closed by turning the eye rings, securing the primary sensor to the bus. Once the primary sensor is secure on the bus, one hot stick holds it steady while the other hot stick is used to screw the split-core CT mechanism closed. The unit can be installed in less than ten minutes. Line crews performing the installation must be trained in live-line work and understand the necessary safety issues when operating equipment in a live, HV substation environment.
LABORATORY AND FIELD TESTS
Laboratory testing of the CT reclassification system was thoroughly performed at the manufacturer’s facility and at the utility laboratory of National Grid. The test qualified the overall end-to-end accuracy performance through in-circuit live current injection operation. The test setup consisted of precision measurement references and a precision current source that duplicated an actual live field scenario. The industry-standard requirements for instrument transformers, under IEEE Std. C57.13-1993 Class 0.3, were used to benchmark system performance.
Tests first confirmed that overall end-to-end performance for ratio correction factor (RCF) and phase angle correction factor (PACF) for the reclassification system fell well within the IEEE Std. C57.13-1993 limits. The next step involved the reclassification of a conventional, large core 1000:5 bushing CT removed from an oil-filled circuit breaker. Performance of the 1000:5 bushing CT was characterised over a 20-150-20 A RMS current range and, as expected, the bushing CT did not comply with ANSI C57.13 -1993 Class 0.3 requirements. The RCF and PACF data was then programmed into the instrument transformer correction (ITC) module of an advanced revenue meter. A precision current source was then applied to the bushing CT with a specific lagging power factor.
The meter was used to measure the current, using the data in its ITC module to compensate for errors in the bushing CT. By comparing to a precision measurement reference, the results showed that overall system accuracy for the CT under test improved dramatically and met the IEEE Std. C57.13 class 0.3 requirements for revenue metering applications. To further ensure success of the CT reclassification system in the field, extensive testing on instrument transformer correction was performed. The research focuses on the variables affecting ratio and phase angle correction factors. Results showed that there are two key operational variables that have the most significant effect on RCF and PACF: percent primary current and the amount of the secondary burden on the CT under test.
The conclusion was that if the ranges of these and other potentially significant operational boundary conditions are well defined, monitored, and maintained, the complexity of the RCF and PACF correction algorithms can be minimised. This effectively maximises the accuracy and reliability of the reclassified CT and metering system. Live-line, high-voltage field tests were performed to further validate the accuracy and performance of the CT reclassification system. The first evaluation site was provided by BC Hydro and BC Transmission Company. Starting in February 2005, tests began in a substation with a 3-phase circuit breaker on a 138 kV line as the evaluation point. The breaker contained relay-class current transformers rated for a nominal current input of 800 A. The secondary wiring of the relay-class CTs is connected to a variety of protective relaying devices located in the substation building. The 138 kV line where the CTs under test are located experiences current flow in both directions; this line is connected to generation stations downstream. The generation operates part time – as a result, the current flow direction changes to accommodate the load change.
As part of the complete CT reclassification system, the advanced revenue meter was located in the substation building and connected to the secondary wiring of the three relay-class CTs under test. Three primary sensors were installed on the 138 kV high voltage line in close proximity to the breaker and in series with the relay-class CTs within the breaker. A data collection computer located in the substation building collected the measurement data from the advanced revenue meter and the three primary sensors.
The test provided proof of end-to-end system operation. This included validating the reliable operation of the sensors’ wireless Bluetooth communications in the HV substation environment, and the successful operation of the primary sensors’ self-powering mechanism. Crews from BC Hydro also confirmed that the three primary sensors were easy to install using live-line methods and could be installed in less than 30 minutes. Further evaluation sites are under way. Thus far the data supports that reclassification of existing relay-class CTs can be performed live, in-situ, during full operation of the CT. The resulting reclassification of the CT ensures improved accuracy to meet the necessary IEEE Std. C57.13 Class 0.3 requirements for revenue metering CTs, allowing the re-use of existing infrastructure and avoiding the high cost of CT upgrade and replacement.
The authors wish to thank the other key people involved in the development of the CT Reclassification System and this article, including Colin Gunn and Marcie Cochrane of Power Measurement, and Clayton
Burns of National Grid, USA Service Company.