Test Transformer The formula for determining capacity Pn: Pn=kVn 2ωCt×10-9
Where: Pn——Test Transformer capacity (kVA)
Vn——Effective value of the rated output high voltage of the test transformer (kV)
k ——Safety factor. k≥1, when the nominal voltage Vn≥1MV, k=2; when the nominal voltage is lower, the k value can be taken higher.
Ct——Capacitance of the test object (pF)
ω——Angular frequency, ω=2πf, f——frequency of the test power supply
The capacitance Ct of the test equipment can be measured by an AC bridge. Ct varies greatly and can be determined by the type of equipment. Typical data are as follows:
Simple rod or suspension insulators: tens of microfarads
Simple graded bushings: 100~1000pF
Voltage transformers: 200 ~ 500pF
Power transformers: < 1000kVA ~1000pF
>1000kVA 1000~10000pF
High-voltage power cables and oil-impregnated paper insulation: 250~300pF/m
Gas insulation: ~60pF/m
Enclosed substations,SF6 gas insulation: 100~10000pF
For different test voltages Vn, different safety factors k are selected. The following lists the k values selected for different Vn for reference.
Vn =50~100kV k = 4
Vn =150~300kV k = 3
Vn >300kV k = 2
