EARTH FAULT RELAY OPERATION FOR JUMPER OPENING 2

[appunni]

Sir,
We have one substation where 66KV is stepped down to 11KV through two 4 MVA, 66/11KV Ynyn transformers, which are connected in parallel. From 11KV side there are 6 outgoing feeders. In these 11KV feeders there are some 11/0.44KV Dyn transformers for power distribution. The 66KV supply was from another substation through a 66KV line.
Some days ago, the substation was charged and loaded after a maintenance work in the substation yard. The earth fault relay on the 66 KV side of the transformer was acted. After a thorough verification the transformer was charged. While the 11 KV feeders were loading the earth fault relay in 66KV side of the transformer acted again.
Later it was found that one phase of the 66 KV supply line from the remote substation was open due jumper breaking.
How the earthfault relay will act which is connected in the residual circuit of the 66KV side phase CTs for a jumper opening in 66KV line? Is there any possibility or any other reasons? Please note that there is only one circuit breaker and one set (3No.s) of phase CTs in 66 KV side common to both transformers.
I request for your valuable comments.
With regards,
appunni

 

[jghrist]

We had a similar situation recently where an open jumper on a 100 kV line feeding three 100-12 kV distribution transormers caused overvoltage problems on the 12 kV side. When the crews opened the 100 kV breaker feeding one of the transformers, they heard arcing sounds. A power supply board on an RTU fed from a VT connected to the 100 kV bus also burnt up.

I suspect that there was a ferroresonant condition with line capacitance of the open phase being in series with the delta transformer windings. This could cause very high voltages and perhaps in your case caused an arcing ground fault.

 

[appunni]

Sir,
In my case, the primary side of the transformer is connected in star and earthed. Then how would be the effect? The jumper opening is in the 66KV line. The delta star connected transformers are on the 11KV side of the star-star(earthed at both sides) 66/11KV transformers.
I request for your detailed explanation.
with regards,
appunni

 

[appunni]

Sir,

I request for your valuable comments for the explanation I have given last.
With regards,
appunni

 

[jwerthman]

There are two possible reasons that the 66kV earth fault relay operated under the conditions that you have described.

1. With a YnYn connection on the 66-11 kV transformer, the 11kV system would also see only two energized phases when one of the 66kV phases is open. If there are line-to-neutral loads on the 11 kV system, this would result in a neutral unbalance on the 11kV side, which would be reflected back to the 66kV primary neutral, operating the relay when the current reaches the relay threshold.

2. The inrush current to the 66kV primary of the transformers would likely have a significant neutral unbalance with one phase open. This would cause the 66kV earth fault relay to operate during energization of the transformers.

 

[jghrist]

jwerthman,

According to the initial post, all of the 11-0.44 kV transformers are delta-wye, so phase-neutral loads would not produce zero-sequence current on the 11kV or 66kV systems. The trips occurred not while the 66kV transformers were energized, but when the 11kV feeders were being loaded.

 

[jwerthman]

jghrist -

If the 11-0.44kV transformers are the only load on the 11kV system, then I agree with your comment. I assumed that the 11kV system was a typical 4-wire distribution system that would have single-line-to-neutral loads and a few delta-wye transformers that the original post mentioned. My assumption may be totally off-base.

 

[jghrist]

You could be getting a ferroresonance situation. Ferroresonance requires putting an iron core reactance in series with a capacitance. If this series circuit is energized, and the capacitive reactance and the inductive reactance are equal, the voltage between the capacitance and the reactance becomes very high, as well as the current.

The open phase of the 66 kV transformer would be energized through the 11 kV system by the delta winding on the 11-0.44 kV transformers. The delta windings connected to the two intact phases would act as voltage dividers to energize the open phase at 1/2 voltage. The 66 kV transformer winding that is connected to the open phase would be in series with 66 kV line capacitance to ground between the transformer and the open jumper. If the line is long enough, there could be ferroresonance. The current through the transformer winding could get high and would be present only in one winding, so it could cause operation of the earth fault relay.

Energizing the open phase only occurs when the 11 kV line is connected to the delta-wye distribution transformers. This would explain why nothing happens until the 11 kV feeders are loaded.

 

[stevenal]

Is this system radial?

 

[appunni]

sir,
Yes. This system is radial in 11KV side. The same 66KV feeder which supply power to this substation also feeds another similar substation.
With regards,
appunni

 

[sosata]

Dear Appuni,

When one phase in 66kv line is opened, an unballancy occures in your system currents. In this situation you have only 2 phase currents (in your 66kv line) that make a 120 degrees angle. This causes a big neutral current (that is equal to the sum of current vectors) in the neutral circuit of the CT, and so the earth-fault relay operates.

Regards,
SoSaTa

 

[jghrist]

With one phase open and no loads connected to neutral (except on the wye side of delta-wye transformers), the currents will not be 120° apart, they will be 180° apart.

 

[jbartos]

Suggestion: The 66/11kV transformer is star-star (earthed at both sides). Then, currents have 120deg displacement because of the earthed neutral, if one phase is unavailable. Only ungrounded neutrals are sliding.

 

[jghrist]

jbartos,
If there were a phase-to-phase fault on the 11 kV side of the wye-wye 66-11 kV transformer, what would be the phase angle difference of the currents on the 66 kV side?

 

[RAgrawal]

Guys, zero sequence current will flow in the system, in case of open conductor, connecting two grounded systems. so Earth fault relay operated correctly.

 

[sosata]

Jghrist,
The non-broken line currents are 120-degree apart, because both sides of the line are earthed by neutral circuit: one side by the primary of the YY transformer, and on another side, even if there is a Delta winding, the neutral circuit is provided by a high-Z star load or (usually) the primary of the auxiliary transformer which has star or zig-zag winding connection.
Anyway, the current angle follows the line-to-groung voltage angle, and so the difference is 120 degrees. This causes a zero-sequence current in the neutral circuit as i said before.

 

[jghrist]

Let's simplify things with an example. Say you have C phase open on the 66 kV and the only load on the 11 kV is between øA and øB. With no load connected to øC, the open phase makes no difference. The current flowing in 11 kV øA is equal and opposite to the current in øB isn't it? Then the angle between øA and øB current is 180°. There is no phase shift in the 66-11 kV transformer, so the same phase relationship is in the 66 kV currents.

 

[RRaghunath]

jghrist,

I think you are right. The incident is a fit case for ferroresonance. But, I expect the voltage at the time of earth fault relay operation should have been higher than rated.

This is because if the voltage were normal, the inductive reactance of the transformer would be pretty high leaving no scope for the ferroresonance to occur.

May be, Appunni like to say some thing.

Another point that puts doubt in my mind is that what ever I read about ferroresonance incidents, the books talk of severe destruction of the equipment involved such as the OHL insulators, transformer windings / bushings. Have you any case where the earth fault relay has operated leaving the equipment in tact.

Thanks in anticipation.

 

[jghrist]

I've never heard of an earth fault relay operating during ferroresonance. Usually, surge arresters fail. As you say, the inductive reactance of the transformer would be high because with 1/2 voltage, there would be no overexcitation. If there were enough line capacitance (close to the transformer reactance), however, ferroresonance could occur anyway and the transformer voltage would rise.

The transformer reactance at 66 kV, assuming 10% Z would be 54.45 ohm. Shunt capacitive reactance of an OH 66 kV line would be about 0.19 Mohm-mile. For a rule of thumb of keeping XC/XM > 40 to avoid ferroresonance, this would mean keeping the line length less than 87 miles. Is the line this long? I'm assuming OH; for UG it would be much less.

The 40 rule of thumb is for full voltage on a delta-wye transformer; I don't know how this would change in this case.

 

[Bung]

The two transformers are in parallel. Is it possible that there is unequal sharing of the load (on two of three phases, as one line phase is open), leading to a 'false' unbalance in residual connected CTs? In other words, not a true earth fault, merely the result of unbalance in residual connected CTs?

Bung
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