Capacity Selection



The formula for determining the rated capacity Pn of the test transformer: Pn=KVn2ωCt×10-9.



Where: Pn----Rated capacity of the test transformer (KVA).



Vn-----Rated output high voltage RMS value of the test transformer (KV).



K-------Safety factor. K≥1, when the rated voltage Vn≥1MV, K=2; when the rated voltage is lower, a higher K value can be taken.



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 depends on the type of equipment. Typical data are as follows:



Simple bridge 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, select different (appropriate) safety factors K. The K values selected for different Vn are listed below for reference:



Vn = 50 – 100KV K=4 Vn = 150 – 300KV K=3 Vn > 300KV K=2.