Diesel generator and transformer inrush

[Alex68]

A client of mine asked me to make a list of the requirements of a diesel generator which must be able to withstand the inrush of one transformer.

Generator data:
Rated voltage: 6.6 kV
Rated Freq: 60Hz
Rated power: 2.1 MW
power factor: 0.8

Transformer:
Vltg ratio: 6.6 / 0.45 kV
Rated power: 1.6 MVA
Inrush current: < 8*Irated

Could you suggest any requirements of the generator or the transfomer?

 

[Skogsgurra]

Since the inrush current does not produce a lot of active power and certainly not for any long time, I think that you will be fine with this combination.

Your generator voltage will look very nasty for a split second and there is a risk that your AVR will overcompensate. The frequency will probably also do excursions, so the speed governor will work hard.

But these things are normal and there are no problems as long as you do not have any loads connected when you switch in the transformer.

Having load connected and then switching the transformer in is something quite different. Then you need to be very careful. I would not do it.



Gunnar Englund

http://www.gke.org

 

[ScottyUK]

Can you leave the transformer close-coupled to the generator - i.e. no MV breaker, or no online switching of the MV breaker - and avoid the inrush completely? Large generators frequently use this technique and have a solid connection from the generator terminals to the GSU tranformer.



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Sometimes I only open my mouth to swap feet...

 

[Alex68]

As ScottyUK suggestes, I proposed to avoid the transformer inrush performing a slow energisation.

I would like to avoid this condition, Skogsgurra.
But I can't because this is only one possibility.

The gen and trafo will be installed on a ship. The gen set will be composed by 2 MV switchboards, 4 generators and 4 transformers.
The energisation of the first gen and the first trafo will be as indicated by ScottyUK but during a cruise any other condition will be possible.

Skogsgurra, I agree with you that the combination will be fine but I must produce a report in which I describe the technical motivations. And this is not so easy. I must convert my sensation in something technical...

 

[mc5w]

In such a system you would need circuit breakers between the generators and the transformers to effect load sharing and synchronization but I think that once you start up the system you can leave all 4 transformers energized. Just do not close the secondary circuit breakers until after the transformers are energized.

i take it that generators will be switched on and off as the load varies. What you need for startup is to start 1 generator, energize 1 transformer, connect some load to that transformer's secondary such a lighting or refrigeration that will not be bothered by frequency and voltage excursions too much, then synchronize in the other generators, and then energize each of the 3 other transformers. The reason why you need to connect some load during startup is so that the load acts as a damper that absorbs the bumps that occur during synchronization.

For utilty supplied power it is not a problem to have some load connected to the secondary of a tranaformer when the primary is energized.

 

[hvcad]

As the transformer inrush current is almost wholly reactive, there will be minimal transient real power demand on the system, and consequently there will be very little generator speed/frequency variation, the governor will not have to work very hard.

The key factor is voltage dip, which will depend on four factors:

Generator source impedance
Point on wave switching
The magnetic characteristics of the transformer core (esp in the saturated region)
Magnetic residual flux in the core before the energisation.

The best way to quantify the impact of these is to perform a time domain inrush calculation using a program such as PSCAD-EMTDC. See

http://www.ipst.org/TechPapers/2005/IPST05_Paper026.pdf

for details of the type of modelling required.

In this particular case it would be relatively straight forward for an experienced consultant to model the transformer, the generator, and the AVRs.


Dr K S Smith
Mott-MacDonald, Power Systems Division
Glasgow, Scotland.

 

[Alex68]

mc5w,
thank you for suggesting some operative procedures.
The biggest problem is related to guarantee reliability also in case of emergency, that nobody can define previously (Tytanic experience teaches).
So the electrical components must be able to withstand any stress.

hvcad,
you are right when you list the basic factors.
Even if nothing I can say regarding wave switching and residual flux, maybe I could impose that:

1- Generator sub-transient reactance shall be very high
2- Transformer core shall have very high saturation point and low hysteresis

Right?

Thank you all for your help

 

[Skogsgurra]

I think that you mean "very low"?

Gunnar Englund

http://www.gke.org

 

[Alex68]

extremely low

 

[TheBlacksmith]

This situation is not dissimiliar to a bolted fault; you should make sure the end basket of the stator can withstand the repetitive high mechanical forces caused by the brief high current's magnetic field. I have worked a project where this was a design consideration, but also the transformer was shared with a larger genrator and therefore was larger relative to the generator rating.

 

[Alex68]

TheBlacksmith
Do you suggest to include also some requirements concerning mechanical efforts?

Could you list any requirements, please?

Thank you

 

[TheBlacksmith]

You should determine maximum inrush current and have the generator designer ensure through analysis that the resultant magnetic forces in the end basket do not fatigue the mechanical supports and cause a failure. My application included a lot of cycling and many inrush events, the supplier beefed up his standard end turn support system after said analysis.

 

[Alex68]

Thank you all, friends

 

[ScottyUK]

Hi Blacksmith,

Any generator designed to withstand a fault shouldn't require additional bracing for load inrush - what did OEM think would happen to their un-modified design during fault conditions? I'm really surprised by this - what size generator was it, and what brand?


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Sometimes I only open my mouth to swap feet...

 

[TheBlacksmith]

I decline to mention the supplier or application as it is an ongoing contract, but part of the problem was repetitive inrushes, numbering in the hundreds, not a handful of faults and the supplier did uncover a marginal design and has incorporated some of the changes into all machines. There were actually two end turn failures of similar machines during testing due to inadequate bracing that occurred during the design phase of my machine.

 

[ScottyUK]

Interesting - how big is this machine?


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Sometimes I only open my mouth to swap feet...

 

[rbulsara]

Alex68:

Firstly, what gunnar and ScottyUK advised is very appropriate.

If you want a practical case in point, I just installed a 2000kVA transformer, which is energized by a 2000kW, 13.8kV generator via a vacuum circuit breaker. This configuration while is now working fine, had one issue. The transformer had high inrush current for a long time. It came to light because the generator differential protection set at 10% and 15 cycles occasionally tripped. The immediate remedy was to open up the settings of the 87 function. In fact I captured several waveforms, which indicated the inrush is just above 11% at 15 cycles. The long-term remedy to be implemented is to temporarily defeat the 87 function (or have expanded settings) during the initial energization, say for 0.5 seconds, and then revert back to tighter settings. This is easily accomplished by modern (for example, GE SR-489) digital multifunction relays. There was no other issue with generator power quality.

The moral of the story is that the generator itself will not have any problems like voltage dip or anything, but you may have to set your differential protection correctly, if it is a problem. I strongly recommend capturing waveforms of the inrush transients during initial test (at least a dozen) to be sure what is happening.

 

[Alex68]

Hi rbulsara,

thank you for your very interesting contribution.

A little suggestion: instead of modifying the 87G settings, you can block it with the 2nd and 5th harmonic like for transformers, if the relay can do that.

 

[rbulsara]

alex:

I beleive this particular relay does not have harmonic restraint. (May be because it is a generator relay, the tranformer protection relay may have it) The solutions I mentioned are published in GE's tech bulletin. But thank you for the suggestion.

 

[RAVWARD]

Hi
This is an interesting post. I did some emerg. gen. installations as a jr. engg. consultant but that confined to 600 V only. But some time, if I come across a transformer connected to a MV generator: I can use the Moral of this story. Please advise me, if I am right.

As per Rubulsara, change settings of protection

From rest:

Point # 1 : Make sure the transformers was deenerized for long time so that there is no residual flux as that may lead to increase inrush.

Point # 2 : Connect some lighting load to transformer for damping("I had an impression than an unloaded transformer has less inrush than a loaded one").

I beleive that is all that matters.
Note: What is Cold inrush and Hot inrush ?

Regards,