Sensitivity assessment for HV & EHV field partial discharge measurements via laboratory testing

For the past decade and a half HV and EHV cable systems have been subject to after-laying commissioning withstand and partial discharge commissioning tests. As jointed cable systems constitute a large, distributed capacitance, the apparent charge of a partial discharge pulse is extremely small, and the high-frequency propagating transient is neither related to the apparent charge, nor conserved in propagation. To achieve sufficient sensitivity, a distributed PD measurement, sensitive to the HF (>; 1 MHz) propagating transient, must be performed. The relationship between the actual PD current, and therefore charge, and that of the detected PD current is not trivial and depends highly on the exact location of the discharge. Therefore, two primary problems exist with respect to developing an acceptance level (sensitivity threshold) for field PD measurements: (A) Partial discharge magnitudes are not diagnostic. (B) Injection of known pulses with high-frequency content into HV and EHV accessories is not a trivial matter. A simple calibration constant between apparent charge and the magnitude of the detected transient cannot be established, nor is it necessary. This paper suggests a sensitivity assessment methodology for high-frequency field PD measurements which relies on characterization of the transfer function of individual HV & EHV cable accessories via laboratory tests (following successful type testing) and proposes how the results can be applied to PD field tests on similar accessories.