HV intertrip

[matrixabc]

If you have an HV breaker 11kV feeding a 11kV/400VAC transformer, and no breaker on the output from the LV side, the cables go directly to the switchboard, I have normally seen an inter trip to the LV breaker if the HV trips out. I have recommended this to a customer but cant think exactly why you need it, I assume to isolate the tx completely?

 

[lz5pl]

If transformer trips from protection, it must be isolated definitely from all sides.

 

[matrixabc]

Yes, I can see that but if the HV is off then it would be isolated anyway as there would be no other power source?

 

[Marmite]

If there is no other source of infeed to the LV panel then there is no reason to trip the LV breaker if the HV breaker trips. Protection should trip the minimum number of breakers required to isolate a fault from the source of infeed.
Regards
Marmite

 

[lz5pl]

OK, if no other source on the busbars - generator or bus sectionalizer to second source - it could be acceptable. Frankly speaking I don't know how it is according to NEC, but at least in our national regulations it is not explicitly required for LV installations.

 

[matrixabc]

Thanks for the comments, I am going to install the intertrip, it makes sense , there is a generator, so it will prevent the possibility of back feeding in to the faulty transformer.

Regards

Matrixabc

 

[joan271273]

The intertrip also assures that your operational staff has to reset and verify conditions on the LV side prior to energizing the HV side. You will be amazed on the amount of times operators will not verify conditions at both ends. They will check the XFMR conditions, OC relays on the High Side and if everything looks OK. They reset the 86 and energize again.

 

[ruggedscot]

Tranny Protection -

Usually find the tranny input to the first ACB has restricted earth fault REF. This protects the trany secondary and bus to the first oc protection. If a tranny winding goes to earth you may find severe damage to the tranny before the incomer picks it up and trips.

This REF relay provides a back trip signal to trip both the ACB and the HV breaker isolating the transformer.

You mentioned generator back feed - you would need to have some form of interlocking between the tranny and generator to prevent this from happening. If your HV goes down and the genny back feeds the tranny and then the HV is restored then you would have a pretty significant bang.

simple switching schedule HV breaker trips signal trips the LV breaker. LV incommer trips this does not need to back trip the HV - Id be careful with this.

Also bear in mind that some tranys dont like being connected to the HV with a load connected - Inrush can develop some serious spikes on the LV side - hence its best to have the LV open when making up your HV.

Less to worry about.

Rugged