TRANSMISSION LEVEL INSTRUMENT TRANSFORMERS AND TRANSIENT EVENT RECORDERS CHARACTERIZATION FOR HARMONIC MEASUREMENTS

This paper presents a technique for laboratory characterization of ins trument transformers designed for transmission level voltage and curre nt measurements. The technique is also extended to Transient Event Rec orders(TERs). The objective of the method is to determine the suitabil ity of existing substation instrument transformers for harmonic measur ements, particularly in the frequency range of 60 to 1500 Hz covering the first 25 harmonics. Specifically, the following characteristics ar e of interest in the frequency range of 60 to 1500 Hz: transfer functi on magnitude and phase, linearity, and sensitivity of the frequency re sponse to burdens. The measurement technique is based on exciting the instrument transformer primary with an impulsive waveform. Both input and output waveforms are recorded using laboratory grade probes and di gitizers. Subsequently, digital signal processing techniques are used to compute the instrument transformer frequency response. Several volt age transformers (both PTs and CCVTs) and current transformers in the 230kV-765kV voltage range were tested. The results of these tests are described in the paper. Conclusions are presented regarding the suitab ility of the instrument transformers and transient event recorders for harmonic measurement and the requirements for such a system. A quanti tative analysis of the measurement accuracy and software based methods to enhance the measurement accuracy is also presented.