Calibration of capacitive couplers for online PD detection in HV cables

Possible online quantification and calibration methods for capacitive couplers have been investigated experimentally. The purpose of the calibration is to ensure that if two different systems are used to measure the same sample, they measure the same "apparent PD magnitude," which is the charge transfer at the sample electrodes. Evolved from the conventional electrical method, the terminal injection method for calibration of capacitive couplers has been accepted as a standard approach. However, for online, on-site calibration, alternative methods that approximate the terminal injection method have to be used, because the terminals generally either not accessible or are too distant from the sensor. This paper investigates possible online calibration methods used for capacitive couplers and compares the obtained results. The advantages and limitations as well as the sources of error are discussed in detail. Frequency response test have been undertaken on capacitive couplers to determine their performance over their potential bandwidth. According to linear systems theory, this is an efficient experimental method to determine system behavior when the theoretical model of the system is difficult to obtain. Approximated transfer function model can be obtained from analysis of the frequency response data.