سنجش تخلیه های جزئی در سیستم کابل ولتاژ بالا تحت شرایط آزمایشگاهی
Abstract
1- Introduction
2- Set-up description
3- Verification of the PD measuring system
4- PD measurements
5- Improved PD measurements
6- Conclusions
References
Abstract
A partial discharge (PD) measuring system has been deployed in order to identify and measure PD in a high voltage (HV) cable joint under impulse and superimposed voltages under laboratory conditions. The challenge is to enable the detection of PD during the impulse conditions. The method of measurement has been investigated by introducing an artificial defect in the cable joint in a controlled way to create conditions for partial discharges to occur. Next the HV cable system is subjected to AC, impulse and superimposed voltage. Two high frequency current transformers (HFCT) installed at both ends of the cable joint were used to identify PD from the cable joint and to separate PD from disturbance. Transient voltage suppressors and spark gaps are applied to protect the measuring equipment. Band pass filters with selected characteristics are applied to suppress transient disturbances and increase the chance to detect PD during the impulse. PD signals are separated from transient disturbances during data post processing and by means of pulse polarity analysis. The developed system enables the detection of so-called main and reverse discharges respectively occurring during the rise and tail time of the superimposed impulse. The measurement results obtained show the effectiveness of the presented PD measuring system for investigating the effects of voltage transients on a HV cable system in laboratory conditions.
Introduction
Partial discharge measurements provide a useful tool to obtain information about discharging defects in high-voltage equipment. In power cables, PD occurs at insulation defects in particular in cable joints and terminations, especially at interfaces [1]. Therefore, PD measurement on cable systems can be considered a useful tool to diagnose insulation condition for both laboratory application and on-site application [2–4]. PD in power cables is normally measured under AC voltage by using the conventional technique defined by IEC 60270 [5]. In practice, power cables are not only subjected to AC operating voltage, but also to transient voltages such as lightning and switching impulses, which occasionally will be superimposed on the normal AC voltage. Those transient voltages will have an additional stress on the cable insulation. In that regard it is important to investigate PD under impulse and superimposed voltages. One of the challenges in measuring PD under impulse and superimposed voltages concerns the suppression of the disturbances caused by the transient voltages. In laboratory tests, the applied impulse voltage causes currents in the cable under test that disturb the PD measurement. So the PD measurement system needs to have a strong suppression of the disturbance. In such a case, the conventional PD technique is not suitable anymore. The unconventional method based on the measurements of electrical signals in MHz range is of more interest as a better alternative for these conditions [4,6–10]. Three circuits for PD detection under impulse are provided in [11] with a measurement frequency from hundreds of MHz to GHz, namely: the high frequency current transformer (HFCT) with multipole filter, the coupling capacitor with multipole filter, and the electromagnetic couplers. HFCTs or other sensors are commonly used with wide/ultrawide bandwidth together with filters and a digital oscilloscope to detect PD in insulation specimens or models under impulses [12–17]. A coupling capacitor was used to measure PD in material samples in cases where only impulse [18] and square wave voltage were applied [5]. Those PD measuring systems were able to detect PD during the impulse even during its front time. For superimposed impulses, PD was detected in laminated paper using a current transformer and a high-pass filter by Hayakawa et al. [19]. Nikjoo et al. [20] used a wideband detection system consisting of a coupling capacitor, a detection impedance and a low-pass filter to measure PD in oil-impregnated paper. However, in both works, PD was measured during AC cycles before and after impulses instead of during the impulses. Moreover, PD measurements in the above-mentioned works were performed on material specimens.