The remote calibration of instrument transformers

Abstract

Successful operation and control of a power system is dependent on the accurate measurement of field data. Each measurement received is the result of a chain of instrumentation and data handling processes, and with each process a certain amount of uncertainty is introduced in the measurement result. Instrument transformers, additional transducers, analog-to-digital (A/D) converters, scaling and conversion procedures, synchrophasor recorders and communication equipment all contribute to the uncertainty in measurement. Errors in this measurement chain can either be systematic, random or installation errors. Instrumentation transformers convert (and isolate) primary power system current and voltage waveforms into standardised instrumentation circuit values (i.e. 110 V and 5 A) for more convenient measurement purposes. Nominal conversion ratios, specified on nameplates, may differ from the actual conversion ratios due to manufacturing, drift over time and environmental conditions. To eliminate biased measurements received from instrument transformers, calibration of instrument transformers should be performed periodically. Traditionally this has been done by means field work creating an out-of-service condition. It is time-consuming, expensive and labour intensive. An opportunity exists due to the increased availability of synchronous data for the idea of remote calibration of instrument transformers. This idea estimates a ratio correction factor (RCF) for the instrument transformers using synchrophasor data over a transmission line. It has been researched and verified through various computer-based simulation studies. In this dissertation the opportunity of remote calibration is investigated through the introduction of real-life measurements using synchrophasor recorders over an emulated transmission line. A measurement model is created within a Matlab® Simulink environment to verify to methodology presented in literature and verified by emulating the waveforms using an Omicron™ 256Plus™. It was concluded that measurement uncertainty contributed by using real-life synchrophasor recorders does not defy the original ideas of how synchrophasor data can be used to do much more than small-signal stability analysis such as remotely improve the calibration data of instrument transformers. Other contributions to measurement uncertainty should still be investigated in future research aiming at a pragmatic engineering solution to be used by operators of real power systems.