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Trfr ended feeder question

Trfr ended feeder question

Trfr ended feeder question

(OP)
Hi

I'm very interested to know if anyone has a solution to the following problem. Consider a transformer ended feeder where the HV is 33kV and the LV is 11kV and the transformer vector group is Dyn. Voltage levels are actually immaterial here.

Suppose you have an EF on the 33kV side (see fault F in attached sketch). Ideally the 33kV feeder breaker A must trip at the source end and the 11kV breaker B at the transformer end due to an intertrip signal received from A. But let's suppose the comms is down such that an intertrip to the 11kV cb from the 33kV side cannot be sent. This leaves the fault being back energised from the 11kV side but there is NO fault current flowing. The 33kV conductor down to earth will still be touching  earth and there is actually no means of detecting it unless voltage displacement is used. But this requires VT's at the trfr end on the 33kV side (point P) which may be very impractical and rather expensive.

I cannot think of any way that any current based protection system will be able to see the fault simply because there is no current flowing. Any 11kV protection looking back through the trfr into the 33kV network cannot see the fault as there is no fault current flowing. Yet it is an extremely dangerous situation to the public as there is an energised conductor down to ground possible within public reach. Of course with a cable feeder the danger is probably not as pronounced and the possibility of a fault is also less.

The only solution I can think of at this stage is to have an alarm issued at the 33kV source end to the control centre, so that as soon as there is an 33kV EF on the feeder and the comms is down, that the operator ensures that the 11kV cb is opened as well.
 
Any other solutions?

Thanks.

RE: Trfr ended feeder question

If you expect current through the transformer under all operating conditions, you could apply an undercurrent relay on the low side to detect loss of supply on the high side.

RE: Trfr ended feeder question

The suggestion is that this is an overhead line feeder in which case it should be fairly simple and relatively cheap to fit 3 x single phase VT's either on the terminal pole or on a structure next to the 33/11kV transformer. You could also look at unit protection of the feeder, which may require additional CT's. You could also duplicate your intertripping channel. Don't forget if your intertripping is down then the transformer protection won't work either.
Regards
Marmite

RE: Trfr ended feeder question

Can you make the transformer non-unit protection go unstable and operate for through faults whenever the communication channel is unavailable? Assuming a non-unit protection exists and it's sensitive enough to pick up the EF current.

I am not a protection specialist and I might be talking nonsense:)
 

May you grow up to be righteous, may you grow up to be true...

RE: Trfr ended feeder question

(OP)
davidbeach
The undercurrent elements is a good suggestion and worthy of further consideration, thanks.smile

Marmite
The trfr has a current diff scheme across it, but of course if the comms is down no CD and no intertripping. Your VT idea would work as well but I would think that my customer (local utility) would consider the undercurrent suggestion as they have many trfr ended feeder sites. I agree that the voltage displacement option will clearly indicate an earthfault better than the undercurrent but it would be more costly.

m3ntosan
As I said above, no comms, no CD, no intertripping. Once the HV breaker trips there is no way of detecting the EF with current based protection.
   

RE: Trfr ended feeder question

If there is a source on the 11 kV, you can use 33 kV VT's with a broken delta and measure zero-sequence voltage with a voltage element across the broken delta.  You can then trip Breaker B on zero-sequence overvoltage 59N.

If there is no source on the 11 kV, then there is no problem.

RE: Trfr ended feeder question

(OP)
Hi jghrist

I guess that's exactly what we're after with voltage displacement. Same principle - needs VT's though.

Regards.

RE: Trfr ended feeder question

Hi.
Undercurrent is possible solution, but you need check it 100 times.
Stnadard undercurrent protection have a setting and this setting iz not zero, used for motors and capacitor banks.
You also need some nominal/work current and some "zero" current,or you can build it with additional logic.
 Lot of chances for unwanted trip in case of low load and CT errors.
Not always VT needed, you can combinated undercurrent functionality with capacity volatge detector on the feeded cable ( we used it for earting switch close enable command).

Good Luck
Slava

RE: Trfr ended feeder question

(OP)
Hi

I am now thinking of using undercurrent combined with voltage depression where the voltage measurement is taken at the LV bus. I have not done my sums yet but if there was an EF there should be an associated voltage depression at the 11kV bus. If the relay can capture that and detect an undercurrent condition within say 3s after that and there is a comms fail then trip. Relay most commonly used is the SEL-351 so I reckon the required functionality is there.

Comments?

Kind regards.

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