Partial discharges in high voltage insulations: Mechanism, patterns and diagnosis

In a power system, high voltage equipments play important roles. To support the system operates properly with high reliability; it is necessary to keep all equipments in good conditions. High voltage equipments contain gas, liquid or solid insulation to withstand a high electric field. Under operating condition, excessive electric field may occur and partial discharge (PD) may take place in gas or solid insulations. The appearance of discharges may indicate insulation defects and in long term this may degrade the insulation leading to the failure of the equipments. Recently, Condition Based Maintenance (CBM) of high voltage equipments is widely adopted to replace the conventional Time Based or Breakdown Maintenance. The CBM program is conducted based on the assessment of the equipment conditions. Diagnosis of high voltage equipments is the key for effective CBM implementation. Many researchers and utilities developed diagnosis methods to assess the equipment conditions. Partial discharge measurement is an important method to diagnose high voltage equipments. This paper reports the experimental results and analysis on time-resolved partial discharges in gas, liquid and solid insulation. The partial discharges are presented in various distributions such as φ-q-n, φ-n and three-dimensional distribution. Here φ is phase angle, q is the discharge magnitude and n is the discharge number. Whitehead model for discharges is widely used since long time ago to explain the phenomena. Based on the recent experimental results, new models of partial discharges are also presented. The application of partial discharge data for the diagnosis of insulation in high voltage equipments is discussed.