Voltage Transformer Isolation

[Arevaengineer]

We have 132kV GIS switchgear with outgoing cables connnected. The cable needs to be tested at HV AC at 75kV. The voltage transformer is a fixed type above the cable box. Disconnecting mean are provided inside the cable box, but degassing would be required to remove that link.
Should the GIS be disconnected from the cable?

 

[rasevskii]

Yes you have to disconnect the VT, obviously, even when it means degassing that section. Furthermore, the VT should be earthed on the HV side in case there is a flashover from the cable under test. Is there really enough clearance inside that section to withstand 75KV? You might have to disconnect the link after degassing, then earth the VT (is there a switch provided for this?) then gas up again.

Get the OEM procedures for that particular switchgear design and follow them. Is this an older existing installation or is new being just now commissioned?

rasevskii

 

[rasevskii]

Rereading that again, 75kV Ac is about 132kV divided by 1.732, therefore the test voltage phase to earth is about the normal operating voltage anyway, so theoretically the VT could be left in circuit. BUT that would ruin the test due the magnetizing current of the VT. It is the cable you are testing only is it not? The section concerned has to stay gassed up in any case to withstand the test voltage.

What is the point of testing at 75KV AC as that is the normal operating voltage anyway? Usually a higher test voltage would be used:

Cable experts offered to comment here....

rasevskii