Maintaining Voltage-Current Phase Relationships in Power Quality Monitoring Systems

Transcription

1 Maintaining Voltage-Current Phase Relationships in Power Quality Monitoring Systems Brian Kingham, Utility Market Manager, Schneider Electric, PMC Division Abstract: Historical power quality measurement devices focused on voltage-only measurements for analysis, survey and troubleshooting purposes. In order to increase their return on capital investment, utilities are looking for measurement devices capable of meeting more than one business need: for example, a device capable of simultaneously monitoring power quality and providing revenue-accurate data. This desire drives a new requirement to maintain phaseangle relationships between previously independent voltage and current circuits so that energy accuracy is preserved. This paper describes the system components (such as transformers and PQ monitor components) that can affect V-I phase relationships and methods to eliminate or compensate for these errors. In conclusion, it is shown how requirements for leading revenue accuracy standards and IEC Class A Power Quality compliance can not only be met but exceeded. T I. Introduction HE increasing cost of energy has intensified the need for more accurate end-to-end metering systems. At the same time, utilities are looking to leverage new technologies to reduce capital expenditure costs, leading them to install multifunction devices within their measurement and monitoring systems. This paper describes how total system measurement accuracy can be affected by components both internal and external to metering devices. The paper then introduces available technologies that, with proper application, are capable of mitigating the error introduced to provide system measurement accuracy that is orders of magnitude greater than was historically possible. II. Sources of Phase Error There are many potential sources of phase and magnitude errors within an electrical network. For the purposes of this paper, we are focused on those components near the metering point that can affect the accuracy of the metered parameters. As shown in Figure 2, these sources are identified as the power transformer (A), the instrument transformers (B), the current transformers inside the meter (C), and the analog front-end of the meter (D). B A C Front-End D CPU Figure 2. Sources of phase and magnitude error in metering systems Figure 1. Rise in US energy price, Brian Kingham is with Schneider Electric in Victoria, BC ( brian.kingham@ca.schneider-electric.com)

2 A. Power Transformer Error Substation power transformers provide highly efficient voltage and current magnitude transformation but still introduce both ratio error (the difference, expressed in RMS volts or amps, between the primary and secondary magnitudes after the turns ratio has been accounted for) and phase error (the difference in phase, expressed in degrees, between the primary and secondary sinusoidal waveforms). Both these sources of errors are due to the magnetizing inductance required by the core and the impedance of the windings (see Figure 3). The resistive losses will cause ratio error and the inductance of the windings and core will cause both ratio and phase error from the primary to the secondary side of the transformer. of the core, phase shift through a CT will be nonlinear. C. Metrology CT Error An often overlooked component of the metering system is the current transformer used inside the meter itself. Like any other transformer, this internal CT introduces another source of ratio and phase error. Passive CTs are inexpensive magnetic transformers which produce output current proportional to input current but which incorporate both a magnitude loss and phase error. Figure 4. Equivalent circuit of a passive CT Figure 3. Equivalent circuit of a power transformer B. Instrument Transformer Error Instrument transformers, including current transformers (CTs) and potential transformers (PTs), are used to provide low-voltage and lowcurrent inputs to meters. Like power transformers, instrument transformers are a source of both ratio and phase error. In order to maximize the accuracy of voltage transformation, PTs are designed to minimize voltage drop in the transformer windings and magnetization current. The equivalent circuit for a PT is essentially the same as shown in Figure 3 for a power transformer. Phase error introduced by passive CTs varies with the impedance of the core, and is therefore susceptible to changes in load or temperature. Because of this multi-variable dependency, it is difficult to fully compensate for passive CT phase error through calibration. Active CTs are also a source of magnitude error, but use a feedback loop to replace lost current and maintain a zero-flux core. Because of this feedback loop, there is no phase error though an active CT. CTs, however, are more sensitive to the magnetization current flowing through the transformer core. Due to the complex impedance

3 Figure 5. Equivalent circuit of an active CT D. Analog Front-End Error Finally, a source of error particular to multifunction power quality and energy meters is the introduction of low-pass RLC filters in the analog front-end. These filters are required by international power quality standards such as IEC and IEC to ensure that high frequency signals are not aliased into the reported values for individual harmonic magnitudes or total harmonic distortion. The insertion of any RLC filter will produce ratio and phase shift error between measurement channels which must also be corrected if power and energy accuracy is to be maintained. III. Phase Error Correction Methods A. Power Transformer Correction Method 2 uses empirical data from the in-situ transformer to determine the amount of compensation necessary. The measured Watt and VAR losses are entered into the loss compensation calculations along with system resistance and reactance information. Like Method 1, these coefficients can either be programmed into the meter for real-time correction or used in postprocessing applications. Rather than attempting to compensate for power transformer ratio and phase errors measured at a meter s three-phase voltage and current input channels, the industry-standard approach is to correct for errors in the resulting Watt and VAR totalized values. One of two sets of calculations is used to achieve this, dependent on the input coefficients available. Method 1 uses manufacturer-supplied test sheet data to calculate the Watt and VAR losses caused by the transformer. The test sheet data (VA TXtest, LWFe TXtest and LWCu TXtest ) are entered into the loss compensation calculations along with actual voltage and current levels, %Excitation and %Impedance. The results of these calculations can then be either programmed into a meter for realtime compensation or used for post-processing of energy data.

4 B. Instrument Transformer Correction Unlike power transformer correction, instrument transformer correction (ITC) corrects for error in the individual voltage and current circuits in real time. This produces the same benefit to power and energy calculations as the power transformer correction method but with the advantage of also correcting the individual voltage and current readings. Additionally, since each phase of a three-phase PT or CT can have a unique error correction curve, per-phase rather than total error correction is preferred as it will be more accurate in unbalanced systems than total error correction. To implement ITC, a series of ratio correction and phase angle correction data points are provided from transformer manufacturers specifications or from test results (see Figure 6). These coefficients are programmed into an algorithm that interpolates between data points to best fit the error curve of the transformer. curves can be calculated for the entire current range and for multiple power factors. Because the error curves vary by power factor, an approximation is always employed to compensate for points not covered by the calibration process. Because of the active compensation circuit, active CTs have no phase error and are thereby immune to changes in power factor. This allows manufacturers to use standard calibration techniques to compensate for the magnitude error of the CT in real time. An additional benefit of an active CT s dynamic compensation is correction for core magnetization. Half-wave signals (such as transients) can shift the magnetization of a CT core away from its ideal state. For passive core CTs, this will result in additional phase error. The feedback circuit in an active CT will compensate for the magnetization effect by injecting the necessary current to maintain zero magnetic flux. D. Analog Front-End Correction Figure 6. Phase angle error curve C. Metrology CT Correction To correct for errors introduced by the meter s metrology CT, passive correction is dependent on the ability to correctly model the phase and magnitude error. Given a highly accurate reference, sufficient memory and processing power to handle high-order polynomials, and enough time during the calibration process, error correction The amount of correction needed to compensate for ratio and phase errors introduced in the analog front-end depends greatly on the architecture being used. At a minimum, to comply with international harmonics measurement standards such as IEC , sufficient filtering must exist in the signal chain to prevent aliasing of higher frequency components into lower harmonic measurements. Low-pass filters, introduced to prevent aliasing, will cause undesirable phase shift errors between voltage and current channels which then affect power and energy calculations. These phase shift errors can be corrected through calibration in a similar manner to instrument transformer correction. By modeling the error curves using laboratory-precision devices, compensation curves can be programmed into the meter to correct for errors in real-time. If matched filter packs are used to keep ratio and phase error

5 consistent between voltage and current channels, the correction can completely mitigate the error. IV. Results of Phase and Magnitude Error Correction A. Power Transformer Correction Results By its nature and the formulas used, power transformer correction applies correction factors directly to the Watt and VAR values of interest. This means that the effect of correction is dependent on the accuracy of the input variables used rather than an empirical measurement as in other correction methods. Using data supplied by the transformer manufacturer, a typical 7500 kva transformer operating at 3780 kva and unity power factor will introduce an error of 37.2 kw or 0.98%. This includes a phase error of 2.9 degrees. Power transformer correction will completely eliminate this error. B. Instrument Transformer Correction Results Because the voltage input to a PT tends to be fairly stable, PT accuracy limits are typically defined between 80 and 120% of nominal voltage. Within this range, PT phase error for a Class 0.5 PT is limited to about 0.35 degrees. The allowable ratio error is included in the accuracy class number (0.5% at nominal voltage for a Class 0.5 PT). These errors are fairly small, and unless in-situ data is provided the effect of instrument transformer correction on a PT will be limited. CT error, particularly phase error, can be much higher. CTs are expected to be subjected to most of their dynamic range as loading levels change. Because of the relationship between current, magnetic flux and inductance within a CT, phase error is non-linear and larger than within a PT. The same accuracy Class 0.5 CT can introduce phase errors between 0.5 degrees and 1.5 degrees and a Class 1 CT can be up to 3 degrees. The ability to use high-voltage, highly-accurate current sensors has allowed empirical observations of ITC results. Table 1 shows the results with and without ITC, where it can be seen that an improvement of up to 1.5% has been realized through error compensation of an instrument CT. Voltage V RMS Current A RMS Power Factor Error (%) Without ITC Error (% ) With ITC Unity lag lag lag lag Table 1 Effect of instrument transformer correction C. Metrology CT Correction Results With passive CT error approximation and correction, power and energy accuracy can be brought to within industry standard requirements of 0.2%. Due to the non-linear phase error, it is difficult to improve error correction beyond this level except at discrete calibration points (e.g. unity power factor and full load). Active CT correction results are much better and more consistent over the entire operating range of the meter due to the zero phase error. Using active CT correction, the error of the meter can be brought to within the bounds of calibration reference standards; typically four to ten times more accurate than the most stringent 0.2% energy error tolerance. Empirical results of active CT correction show an improvement in metering error (and thereby system error) of 0.15%. D. Analog Front-End Correction Results The total benefit of analog front-end correction depends greatly on the architecture chosen for the meter. If all filter components are perfectly matched on all input channels, phase error is negated. Using a combination of matched component filter packs and calibration

6 compensation, improvements in power and energy accuracy of up to 0.2% have been achieved. V. Conclusion With system measurement error becoming increasingly important as energy prices rise, power transformer correction, instrument transformer correction, active CTs and analog front-end calibration can all contribute to increased accuracy in power and energy measurements. Proper selection and application of these methods has shown total system error improvements of over 3%, and should be considered integral to the implementation of a Class 0.2 metering system. VI. Author Biography Brian Kingham received his Bachelor of Electrical Engineering from the University of Victoria in In 1995 he joined Power Measurement (now Schneider Electric) and is currently the Utility Market Manager for the company s Power Monitoring and Control (PMC) division. VII. References [1] P. Doig, C. Gunn, L. Durante, C. Burns & M. Cochrane, "Reclassification of Relay-Class Current Transformers for Revenue Metering Applications", IEEE 2005