Commissioning VT 1

[Cerkit]

Hi,

In commissioning a 110V VT with an earthed neutral, if I am I getting 50V across the fuse holders with the fuses out what might be the cause of this? The cables and instruments have already had insulation tests.

Could it be an interconnection via another earthed VT on site?

Thanks

 

[RRaghunath]

Expected voltage across the fuse holder is 63.5V, i.e. L-N voltage on VT secondary.
If you are getting lower than that, first thing to suspect is under voltage in the power system on primary side of VT.

 

[Cerkit]

Does it make a difference that the cables running from the VT through junction boxes is quite long? Could this be a factor?

 

[Distribution73]

Does the VT cable happen to go through several junction boxes (say four) on its route between the supply fuses and the panel instruments (say 300 yards away)?

 

[scottf]

what is the rated secondary voltage of the winding that is fused?

Is the neutral end of the winding grounded and is the fuse clip towards the control house/junction box floating?

I would guess that you measuring voltage between the VT secondary terminal and a floating point/non-grounded point.

 

[RRaghunath]

If other two phase VT secondaries are feeding the load,the neutral point will not be at earth potential due to unbalanced loading. This could cause voltage across the fuse holder to be < 63.5V.
If you read the voltage with all the three phase secondary fuses removed, then the measurement should show ~63.5V.

 

[cunin]

Hi
Check the resistance of the Primary fuses. They all should be about the same. If one is high you will get a low voltage reading on the secondary side side. This will happen if you have a very high impedance load (like a meter). Another way to find a high resistance fuse is to measure the voltage, then with the meter connected put a 40 watt light bulb across the meter leads and see if the voltage goes up. If it does change the fuse.