Evaluation of the Effect of the Location of the Corona Measurement Devices on the Corona Inception Voltage Under HVDC Application

摘要

Instruments for corona measurements form an integral part in the detection, measurement and isolation of corona discharges. IEC 60270 address the electrical method using a coupling capacitor and acquisition unit for the detection and measurement of partial discharge However, there are also non-conventional methods available namely: the use of high frequency current transformers (HFCT), acoustic methods and optical methods for detection of corona discharges (corona camera). The different measurement methods have various advantages and disadvantages with the main difference in the location of the sensor. The coupling capacitor is connected directly to the main circuitry of the power system, the HFCT is connected in the earth connection and the acoustic sensor as well as the corona camera being placed at a safe distance away from the system. The electrical method using a coupling capacitor is the only method that can be calibrated to measure current by means of electromagnetic coupling. The corona camera visualizes, locates the corona discharge point and displays the partial discharge intensity by detecting the ultraviolet radiation photon count rate. In this paper, the aim is to elaborate how the location of the corona measurement sensor influences the measured corona inception voltage under HVDC application. The tests were conducted in a high voltage laboratory where the ambient temperature was 20 ℃ using an indoor laboratory corona cage. The test methodology was based on gradually increasing the voltage until the corona inception voltage is reached for both positive and negative polarities. The results show a difference in the measured inception voltage levels for the different methods and that the location of the corona measurement devices has an impact on the measured corona inception voltage. The results indicate that connecting the corona measurement device directly to the main circuitry of the system as is the case with the electrical measurement method using a coupling capacitor will be the most sensitive method and result in the lowest measured corona inception voltage. The results are analysed and the reasons for the differences explained.