Insulation resistance testing
Requirements: to use 2500V megohmmeter. Compared with initial value and the values over the years, the winding insulation resistance should not change significantly and lower than 1000ΜΩ. The insulation resistance of end screen to insulation resistance is not less than 1000ΜΩ. If it is less than 1000ΜΩ, its tan should be measured.
The CT requirements for testing the dielectric loss angle tangent:
1) When it is to test the dielectric loss and capacitance of primary winding, the dielectric loss tester should be measured by positive wiring method; when it is to measure the dielectric loss and capacitance of primary winding of the end screen to the earth, the measuring voltage should be 2000V and the dielectric loss tester should be measured by negative wiring method. Also, the high—pressure end of voltage transformer should connect the shielding wire;
2) Compared with values over the years, the dielectric loss value should not change greatly;
3) Oil immersed type:
500kV: the commission is not more than 0.5%; pre trial is not more than 0.7%; 220kV: the commission is not more than 0.6%; pre trial is not more than 0.8%; 110kV: the commission is not more than 0.8%; pre trial is not more than 1%; 35kV pre test refer to 110kV standard, but the commission is not more than 2.5%; dielectric loss value should be less than 2%.
The test of ac withstand voltage and the requirements for voltage of commissioning test.
1) The primary winding should be underway according to (80%) of factory default;
2) The standard between secondary winding and its withstand test voltage for power frequency of shell should be 2kV;
3) The voltage standard of the power-frequency withstand voltage test for the terminal screen of voltage transformer with voltage grade as 110 KV or more and the earth terminal of potential transformer to the ground should be 3kV.
4) When electromagnetic voltage transformer( it includes electromagnetic units of capacitive voltage transformer ) comes across the core with relatively high magnetic density, the induced over voltage withstand test should be in progress according to the regulations as followed.