Maloperation of auto-reclosing feeder CB 1

[Bung]

I am having problems with a particular model of auto-recloser failing to operate under fault conditions, and instead the upstream protection operating. The picture is this: Single transformer 33/11kV substation, 5MVA, Dyn. Single incoming 33kV line, 33kV CB with overcurrent protection (IDMT, BS142 std inverse). Transformer has harmonic restrained biassed differential protection. Two 11kV feeders, each with the same type of recloser CB and contoller. The CB is KFME, controller is Form 4C. The reclosers are mounted on spun concrete poles in the substation yard. Earthing appears to be in accordance with the book. Auxiliary volts come from an aux transformer in the substation, connected at 11kV.

The problem: Occasionally (NOT every time) a fault on a feeder(no pattern to the type) will not be tripped by the recloser, but is seen and cleared by the transformer HV overcurrent. The recloser has been found open on at least one occasion after the 33kV protection cleared the fault. There is no transfer tripping or inter-tripping scheme. The recloser behaves perfectly in the lab. There is plenty of grading margin. Records from the Form 4C and the waveforms recorded by the 33kV protection indicate that the F4C should have cleared the fault with a margin of around 600msec.

Anybody got any ideas? Leprechauns sound good at the moment ...

Bung

 

[dpc]

Does the recloser have its own source of control power (internal battery) ?

Is this happening on one particular recloser, or on both?

Is it possible there are external control signals (ground block) that might be blocking operation?

 

[gordonl]

My main thought is co-ordination and grading. I'm not farmiliar with the clearing time of these breakers or even your relays, but I would look carefully at the grading, as it sounds you have already.

One thought, with Dy transformer, during a L-L fault the primary protection can see 115% more current than the secondary protection, so the secondary has to grade with 115% of the primary setting.

 

[stevenal]

Is your high side protection electromechanical, or set to emulate electromechanical relays. If so, consider the ratcheting effect that the recloser can have on it. An induction disk, for example, may not return to its starting point before the recloser closes into the fault again. The time to trip is shortened.

 

[gordonl]

stevenal makes an excellent point, electronic relays often have the option of linear, thermal, or instantaneous reset. If the recloser in inst. but upstream is thermal or linear or mechanical this could cause the unco-ordination.

 

[Bung]

Control power is from station auxiliary supply transformer.

Grading isn't a problem - nor is disk integration on 'pecking' faults - we replaced the old CO rleay on the high side because that was one theory we had. We now have an instantaneous reset electronic relay (Alstom's Micom P123)from which we can get fault waveforms.

Fault types experienced have been both phase and esarth - no pattern discernible. It happens at two different substations, on four different feeders, and it is random. A colleague in a different area 400 miles away has had a similar problem with the same recloser and controller. We do not have any similar problems with other recloser / controller types (so far!).

I suspect some kind of induction or interference in the umbilical between the controller and CB, but can't / don't know how to approach tracking it down.

regards, and thanks for the interest so far...

 

[dpc]

If the controller is seeing the fault, and its data agrees with your primary protection, then either the control is faulty and it is not trying to trip the breaker, or it is trying to trip it but the breaker is not tripping. I assume you do not get a event record indicating that the controller tried to trip the breaker?

If your control power is coming directly from the station service transformer, is is possible that your recloser is losing control voltage during the fault?

Are you using current transformers internal to the breaker/recloser? Did you test operation of controller using primary current injection?

I would talk with the Form 4C supplier (Cooper?) and see if they have had reports of similar problems. Maybe you can talk them into replacing or upgrading to the new version.

 

[peterb]

I think that dpc is definitely on the right track; to expand on a couple of the points -
- Power supply - what backup power option do you have, internal battery or external DC? If battery, is the battery OK?
- Are the tripping capacitors in satisfactory condition? Have you tried a trip test with a simultaneous external power supply failure?
- Do the waveform records indicate that the Form 4C controller issued a trip command to the recloser? If so, then I think you must suspect the trip capacitor arrangement.

 

[jairo]

I live in Colombia and excuse me by my english.
We have the same F4 controls
I´m agree with DPC and we had problems with downstream faults but the force between conductors make a fault upstream; this new fault is only view by the upstream protection.
If you don´t understand me I will try to explain better
Thanks

 

[Bung]

Thanks for all the suggestions. At least it confirms that 1) nobody else has seen this kind of problem to any great extent, so it's probably installation specific; and 2) it apppears the controller is losing its mind during the fault, and I have s few more leads to follow.

Bung

 

[NormGA]

Are your circuit breakers tripped by internal battery, external battery, or capacitor trip device? The problem you describe sounds like what happens when the tripping energy is supplied by capacitor trip device and you have an instant reclose on the breaker. Reclosing must be delayed long enough to allow the capacitor to recharge. I like 3 to 5 seconds just to make sure.

 

[peterb]

jairo, I understand from what you are saying that the upstream bare conductors are contacting each other due the eletromagnetic force present when fault current flows to a downstream fault.
If this is the case, then your conductor spacing is inadequate, or the conductor sag is too great. There are mid-span spacers available that could correct this situation, or you could consider insulated conductor for the problem span(s) - depending on the circuit voltage.