Transformer inrush current while switching off

[powercontroller]

Many discussions are found on inrush current when energising a power transformer. Has anyone got any experience of a current unbalance on the star-point to ground connection during the instant that a transformer was switched off ? Has any analysis been done on the currents that exist as the different circuit breaker poles sequentially interrupt the transformer current ?

 

[rbulsara]

Replace the circuit breaker. Ferroresonance may be a possible cause.

Rafiq Bulsara

http://www.srengineersct.com

 

[waross]

I'll go with Rafiq. If the breaker is interupting the poles sequentially, replace the breaker.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[davidbeach]

Of course it interrupts sequentially. Each pole interrupts on a current zero, and these occur sequentially. Certainly one pole will open first. Whether the other two open sequentially or together depends on the nature of the load. All sorts of interesting things can happen between the time the first pole interrupts and the final pole interrupts. All depends on the specific interruption.

 

[Bronzeado]

Hi powercontroller,

We have an experience with a current unbalance on the star-point to ground connection during the interruption of inrush currents, which I think may be used in your case.

In our case, the instantaneous unit of the neutral overcurrent relay operated due to the high curent in the neutral. Of course, that relay operated during the circuit breaker openning so it had no real function at that moment. However, we waste some time to find the real reason of relay operation.

Regrads,

H. Bronzeado

 

[powercontroller]

Thank you for all your posts.

The actual problem that we experienced occurred in a power station when one of the generator transformers was disconnected from the 33 kV busbars.

The other generator transformers, that are also star connected on the 33 kV side, detected a 20 ms current flow between the star point and ground at the moment that the particular transformer was disconnected.

Working along the lines of sequential interruption of the poles of the circuit breaker, the opening of the circuit breaker will appear first as an open circuit on one phase. Analysing this first step using symmetrical components, with sequence networks connected in parallel, I found that a zero sequence current does in fact flow in the star winding of all generator transformers !

The current that was recorded as flowing between the star point and ground is three times this zero sequence current.

The problem can therefore be explained without the need to refer to the magnetic circuit of the transformer.

Thanks again for your help, which pointed me in the right direction to solve this problem !

 

[Bronzeado]

powercontroller,

Could you, please, post the diagrams and the waveforms you have recorded?


Best regards,

Herivelto S. Bronzeado
Chesf, Recife, Brazil

 

[powercontroller]

Hi Bronzeado,

The event that occurred was a bit more complex than what I described in my previous email.

The setup basically consists of a number of generators connected to a common 33 kV busbar by means of star delta transformers (Star winding is on the 33 kV side, with a solid connection to ground).

For an unknown reason, Generator no 3 issued a trip signal, and its transformer was disconnected from the 33 kV busbar by its 33 kV ciruit breaker.

The protection on Generator transformer 5 detected a current flowing to ground from its star point, and due to a fault in the connection of the restricted earth fault relay, issued a trip command to the 33 kV circuit breaker of Generator transformer no 5. (We now know that an inverted CT caused the maloperation of the restricted earth fault relay of Generator transformer no 5).

From the waveforms attached, you can see that two current pulses were recorded on the connection of the star point of generator transformer no 5 to ground (yellow curve). These correspond to the tripping of first Generator 3, and then Generator 5. My original question was about why there should have been a current to ground, when all that happened was an opening of a circuit breaker.

I have also attached a sketch I made of the flow of current in the sequence networks (connected in parallel, assuming that one pole of the circuit breaker opened while the other two were still closed). This explains the presence of a zero sequence current in the star point connection of both the disconnecting transformer and the remaining transformers in the system (only one shown in my sketch).

It also shows that the zero sequence current in the disconnecting transformer and that in the remaining transformers should be 180 degrees out of phase.

I expected to see this phase shift of 180 degrees between the two phasor diagrams, as for the LHS diagram, Generator transformer 5 was one of the remaining transformers, while for the RHS diagram, Generator 5 was the disconnecting transformer.

However, the diagram shows approximately 60 degrees of phase shift. This can be explained if a different pole first interrupted the current in each of the two cases, as this phenomenon would introduce a 120 degree phase shift (180 - 120 = 60).

Unfortunately I do not have the software to model a single phase open circuit ffault, so I cannot confirm my interpretation of the results.

Best regards,

Alan

PS Waveform file is in the next post

 

[powercontroller]

Here is the waveform file that I was not able to attach to the previous post.

 

[karalahana]

if not so much relevant excuse me but is there any possibility that for a smooth three phase opening or closing there might be a current imbalance in other transformers' neutral wire?? ?mbalance current must occur if there is an unsymmetrical situatin true?

 

[waross]

Your initial post was a little misleading. No inrush.
BUT
The current from each phase returns to the wye point and back to the source through the other two phases when the load and voltages are balanced.
When the breaker opens, it may not interupt the current until the next zero crossing. So when "A" phase interupts, the current returning to the wye point from "B" phase and "C" phase will return partly on the neutral and partly on each other. 1/6 of a cycle later "B" phase will interupt and the "C" phase current will return on the neutral. It should all be over in 1/2 cycle.
But if you have violated the "One connection between neutral and ground" rule, the current may share between the neutral conductor and the ground path and cause ground fault tripping.
And you stated it correctly when you stated that the breaker poles sequentially interupted theCURRENT. I am afraid that I and possibly others read that as the breaker poles Open sequentially and visualized three single pole breakers with timing issues when they opened.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[powercontroller]

My initial thoughts were that the phenomenon was due to inrush currents.

However, the theory of sequential interruption of current of differnt poles explains the observed currents.

The system is a three wire system, with no neutral. All transformer star points are, however, connected to ground. No tripping usually occurs, as the duration of the current flowing to ground is too short to cause overcurrent relays to operate.