Capacity Selection



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



Where: Pn----nominal test transformer capacity (KVA).



Vn-----the 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 equipment under test 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 bridge or suspension insulators: tens of microfarads.



Simple grading 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 (appropriate) safety factors K are selected. The K values selected for different Vn are listed above for reference:



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