High Voltage Substation Surveillance using Radio Frequency Interference Measurement

Partial discharge RFI Thermal Light Chemical Pressure/ Acoustic Electric Detecting partial discharge (indirect measurement) Acoustic

Partial discharge pulseRFI emission FFT frequency spectrum of PD pulse RF pulse from a PD related source

CASE Inkoo 110 kV CT Selective testing targeting mainly old 110 kV CTs discovered a unit with a high C2 power factor ~0,95 %. (Strömberg KOTU F41 type, manuf. 1972) A DGA follow up showed considerable gassing TCG ppm (H ppm, CH ppm, C2H ppm) diagnosed as D1 low energy discharge Suitable test case...? Doble were asked for a PDS100 blind test. 5

PvmEsityksen nimi / Tekijä 6

Baseline measurement 7 GSM Typical frequency zone for air discharges <150 MHz Discharges in oil will have higher frequency content Baseline measured outside the fences Low backround noise levels

8 Measuring points on the service road between CBs and CTs Suspected item No signals Baseline

Measurements The faulty unit was succesfully located –A small PD signal was registered ~15dB above baseline –Attennuation of high frequencies is useful in the location process –The 110 kV CT was replaced - no signal after replacement (one month later) 9

HV Lab testing of the CT PD Ignites at 74 kV phase-to-earth voltage (128 kV main voltage), apparent charge was pC Extinguishing voltage is considerably lower around 45 kV (77 kV main voltage) At 1,2 x Um/ (85 kV) discharge level was 150 pC Internal pressure 0,75 bar (0,6 considered normal) Tandelta slightly increasing as a function of voltage up to 70 kV, 0,43...0,59 % Tandelta increases rapidly when PD ignites at 74 kV being 1,76 % at 123 kV CT passed the 0,75 x 230 kV, 50 Hz 1 min overvoltage test 10

Non-invasive, no need for physical or electrical connection. Does not require equipment under test to be taken out of service. May be adopted for first- line substation surveillance. Rate and severity of discharge can be monitored and trended. Advantages of RFI

Thanks for your kind attention! Questions? Contact: Alan Nesbitt Brian Stewart Glasgow Caledonian University Scotland, UK Contact: Micael Hellquist Hans Ove Kristiansen Doble TransiNor AS Norway