Interpretation of Diagnostic Test Results of HV and EHV Equipments in Substation
Current Transformer Capacitance and Tan-delta Test:
Test Frequency: For 400 KV CT the Test is done Bi-Yearly, for 220 KV CT it is done Yearly and for 66 KV to 220 KV CT it is done Bi-yearly.
| Sr. No. | Parameter | Permissible limits |
|---|---|---|
| 1 | Capacitance (pF) | -5 % to +10% of Factory Values |
| 2 | Tan-Delta (%) | 0.7% i.e 0.007 |
| 3 | Rate of Rise of Tan-Delta (%) | 0.1 % |
If Capacitance or Tan-Delta Results exceed above mentioned values, follow-up measurements to be carried out for the next two quaters and the CT can be replaced depending on the Rate of Change and the Oil DGA results.