Open CT secondary causing high impedance in primary? 1

[BrkfldEE]

In a high voltage substation there is an incoming source transmission line that connects to 2 circuit breakers, so the current from the incoming source splits between these 2 circuit breakers. One of these circuit breakers tripped due to a "pole discrepency", which means that one phase current was lost. We can see from our oscillograph records that indeed one phase current was lost. The CT that had the current "lost" goes through the "pole discrepency" relay and then sums with another CT circuit from another breaker and then goes onto a DFR. We can see from the DFR that the current remained unchanged. This brings me to my question, if one phase CT has an open secondary, could that open secondary cause a high impedance on the primary which would then cause all the primary phase current to go through the other circuit breaker? And if that is in fact true, are we lucky that the current had a second path to go, because if it did not, then the CT would have catastrophically failed? I hope I have explained this clearly.

Thanks

 

[thermionic1]

Do you have a schematic you can post? Most PD schemes I've encountered are CB aux contacts and a timer. The timer trips the IPO breaker and also initiates BF. Is this PD scheme implemented via a protection relay or out in the CB control cabinet?

 

[BrkfldEE]

It is implemented by a protection relay. Sorry I can't post the schematic. This particular protection looks at the loss of current for pole discrepancy. My main question though, is if there was an open CT secondary, does it cause a high impedance on the primary of the CT, which would make the load current look for a different direction to go (if it had any)?

 

[Fischstabchen]

No, a CT can't do this because it would saturate almost instantly. It would take hardly any current to magnetize the open CTs core. Once it is saturated, it is just a series inductance. Current splits on ring busses and whatever take very little extra impedance to path weirdly.

 

[che12345]

Dear Mr. BrkfldEE (Electrical)(OP)21 Dec 21 19:25
"... This particular protection looks at the loss of current for pole discrepancy. My main question though, is if there was an open CT secondary, does it cause a high impedance on the primary of the CT, which would make the load current look for a different direction to go (if it had any)?..."
1. If there was (an open CT secondary), [it does NOT cause a high impedance on the primary of the CT].
2. For your case, the event could be i) say one phase "A" CT secondary open circuit, >> ii) protective relay senses phase "A" current = 0 A. >>> iii) protection relay operates.
3. Attention: during the phase "A" CT secondary open circuit, the phase "A" primary current remains at what is was, (NOT 0 A).
3.1 Do NOT be mistaken thinking that (the primary current is 0A) [due to high impedance].
4. It is only the secondary is 0A, NOT the primary.
Che Kuan Yau Singapore)

 

[bacon4life]

Luck might explain the lack of catastrophic CT failure.

1)Under steady state conditions, the open circuit CT will produce voltage proportional to it's relaying class. This steady state voltage may not be high enough to actually damage any components. Perhaps there was little change is current flow while the open circuit existed?

2)Transients can cause very high voltages on the open secondary. However, often instead of the CT insulation failing, the wiring at a terminal block flashes over instead. I have seen a terminal block only be slightly damaged after many minutes of an open circuit CT intermittently causing flashovers. Before putting the equipment back in service, I would inspect for evidence of flashovers and megger anything that was possibly exposed to high voltage.

 

[prc]

1) Opening out of CT secondary will not cause any catastrophic failure. The voltage and energy involved in the secondary arcs are comparatively low. There are cases where HV CT remained in the circuit with secondary opened out for months.

2) Open secondary will not increase primary impedance or current sharing pattern on the primary side.

 

[stevenal]

I think folks are looking at this too much like a typical open CT. The parallel path makes the difference. Yes, an open CT with a strong primary source will saturate completely, but small changes in parallel path impedance cause huge differences in current division. I would postulate the open circuit voltage will be transformed back to the primary by the inverse of the turns ratio. This back voltage on the primary will force most of the primary current to take the lower impedance path through the healthy CT primary. Saturation will probably occur to some degree, but not the same as if there was no other low impedance paths.

 

[Fischstabchen]

Stevenal,

I don't think the open circuit voltage can be transferred back the primary just due to the fact that there is no current flowing on the secondary side. If the impedance was reflected, there would have to be a voltage drop due to the secondary on the primary side. The only thing that the primary side will see is self-impedance of the primary winding of the CT. I don't think a CT winding has much inductance but there are a number of other things that could have existed that were imbalanced prior to the event. I have looked at the currents that flow around a ring bus and it is not unusually for them to flow in weird manners just do to slightly different impedance paths due to strength of connections or geometry. I think it is a mistake to assume that the currents were ever split like you would expect prior to the event.

 

[sushilksk]

IMO there will be minor increase in impedance on primary side for open circuit CT, but not enough to make significant change in current distribution in primary currents and cause operation of any relay. I think there can be measurable difference in current for open circuit / short circuit condition.

Reason for no current in one branch may be due to loose connection / switchgear's high resistance etc.

 

[thermionic1]

Did the pole discrepancy relay trip the offending breaker open on all poles? Can it be confirmed via a breaker status contact that the phase that showed 0A indeed opened followed by the other phases?

 

[BrkfldEE]

the pole discrepancy relay did trip the breaker on all poles. No we don't have any records of breaker status contacts to show that the one phase opened before the other 2. One more piece of evidence that was found was there was a second relay connected to a different CT winding that also showed that the phase current went away. So 2 relays connected to 2 different CT's both showed the loss of one phase current.

 

[stevenal]

If you look at an equivalent circuit, you will see sufficient current flowing on the secondary side of the ideal transformer. Since there is an open circuit further down, it flows in the non-linear magnetizing branch. The voltage drop in this branch is equal to the voltage across the open point. Adjust this voltage downward by the turns ratio to see the voltage across the single turn primary.

I have looked at the currents that flow around a ring bus and it is not unusually for them to flow in weird manners just do to slightly different impedance paths due to strength of connections or geometry.

Exactly my point: Small differences in impedance cause big changes in current when parallel paths are involved.

Consider current division through two parallel paths of a micro-ohm each. then add a half a milliohm to one side (a small amount in the absolute sense). If I did my math right, current shifted by 99.6 % from the balanced state. Sure it wasn't perfectly balanced to begin with (although the pole discrepancy did not pick up). Did the the added unbalance subtract from or add to the existing unbalance? How lucky are you?

 

[RRaghunath]

"So 2 relays connected to 2 different CT's both showed the loss of one phase current."
This definitely can't be due to CT secondary open circuit. Have you checked primary circuit??