Electrical power network system is considered as a valuable asset. So it is important to manage such an asset with criticality criteria by adopting different predictive maintenance techniques to increase reliability (availability) and reduce direct & indirect costs. Such techniques constitute the basis for an effective asset management system, as they can ultimately decrease the electrical failure rate. HV transformers & switchgears are considered to be expensive and delicate components of the electrical energy network. Increasing load on transformers & switchgears causes the more stress on it. Continuous stress weakens the insulation and producing arcing called partial discharge (PD). Partial discharges seriously affect the reliability of transformers & switchgears. Electrical energy networks may experience transformers & switchgears failures because of insulation degradation. Failure of high voltage insulation is the No-1 cause of high voltage system breakdown. IEEE statistics indicates that electrical insulation deterioration causes upto 90% of electrical failures of high voltage equipment. The major and most effective tool to detect insulation deterioration, as is well known, the measurement and analysis of partial discharges. However, this technique is not as well accepted as one might expect based on its great benefit. Reasons behind it are (1) PD testing is a fairly new technology compared to usual Hi-pot test. PD testing has matured over the years and today it has already been proven to be much more powerful indicator for insulation defects compared to offline test like Hi-pot & Megger etc. (2) PD testing may be more powerful indicator for detecting insulation defect but it is also the most sophisticated test to be carried out. The interpretations of the result are extremely challenging and require vast experience to do further analysis. Once results are correctly interpreted, PD testing will give the best detailed diagnosis of the insulation condition of equipment compared to other conventional test. (3) PD testing is sophisticated and the cost of instrument is comparatively higher. Based on many advantages, this PdM technique is gaining popularity with time.

This paper describes PD theory, PD detection and analysis approach to the diagnosis of HV Transformers & Switchgears insulation system along with few case studies. The results of PD measurements performed on-line on HV transformers & switchgears insulation system, just after commissioning or after sometime in operation helps in identifying PD-generating defect. Decision is made by comparing reading with baseline and periodically recorded data.

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