Abstract
This paper describes a simplified model and a generic model of high-frequency current transformer (HFCT) sensors. By analyzing the models, a universal charge estimation method based on the double time integral of the measured voltage is inferred. The method is demonstrated to be valid irrespective of HFCT sensor, assuming that its transfer function can be modelled as a combination of real zeros and poles. This paper describes the mathematical foundation of the method and its particularities when applied to measure nanosecond current pulses. In practice, the applicability of the method is subjected to the characteristics and frequency response of the sensor and the current pulse duration. Therefore, a proposal to use the double time integral or the simple time integral of the measured voltage is described depending upon the sensor response. The procedures used to obtain the respective calibration constants based on the frequency response of the HFCT sensors are explained. Two examples, one using a HFCT sensor with a broadband flat frequency response and another using a HFCT sensor with a non-flat frequency response, are presented.
Highlights
Partial discharge (PD) measurements are a fundamental tool for insulation diagnostics.The conventional method, defined in the standard IEC 60270 [1], presents the concept of apparent charge
There is a variety of unconventional partial discharge measurements that rely on high-frequency current transformer (HFCT) sensors, Rogowski coils and magnetic sensors [3,4,5,6,7,8,9]
When the duration of the current pulse is smaller than the time constant determined by 4/p1, the double time integral of the measured voltage converges to the charge value after a certain the integration time can be approximated by 4/p1, p1 being the first pole of the transfer function
Summary
The conventional method, defined in the standard IEC 60270 [1], presents the concept of apparent charge. It sets the procedure for charge calibration and the bandwidths for charge estimation. As presented in [2], the charge estimation method defined in the IEC 60270 is based on the impulse response of bandpass filters—the bandwidth of which is stated in the standard. As described in [2], several techniques are available for charge estimation methods in unconventional partial discharge measurements. One of the proposed charge estimation techniques is based on the frequency domain analysis, while others rely on the evaluation of the time integral of the measured voltage. A charge estimation method is needed for HFCT sensors with a non-flat frequency response
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