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Test transformer capacity Pn determination formula: Pn=kVn 2ωCt×10-9 

Where: Pn——test transformer capacity (kVA) 

Vn——rated output high voltage effective value of the test transformer (kV) 

k ——safety factor. k≥1, when nominal voltage Vn≥1MV, k=2; when nominal voltage is lower, 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 for different Vn for reference. 

Vn =50~100kV k = 4 

Vn =150~300kV k = 3 

Vn > 300kV k = 2