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Diesel generator and transformer inrush

Diesel generator and transformer inrush

Diesel generator and transformer inrush

(OP)
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?

RE: Diesel generator and transformer inrush

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
www.gke.org

RE: Diesel generator and transformer inrush

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...

RE: Diesel generator and transformer inrush

(OP)
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...

RE: Diesel generator and transformer inrush

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.

RE: Diesel generator and transformer inrush

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.

RE: Diesel generator and transformer inrush

(OP)
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

RE: Diesel generator and transformer inrush

I think that you mean "very low"?

Gunnar Englund
www.gke.org

RE: Diesel generator and transformer inrush

(OP)
extremely low

RE: Diesel generator and transformer inrush

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.

RE: Diesel generator and transformer inrush

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

Could you list any requirements, please?

Thank you

RE: Diesel generator and transformer inrush

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.

RE: Diesel generator and transformer inrush

(OP)
Thank you all, friends

RE: Diesel generator and transformer inrush

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?

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

RE: Diesel generator and transformer inrush

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.

RE: Diesel generator and transformer inrush

Interesting - how big is this machine?

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

RE: Diesel generator and transformer inrush

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.

RE: Diesel generator and transformer inrush

(OP)
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.

RE: Diesel generator and transformer inrush

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.

RE: Diesel generator and transformer inrush

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,

RE: Diesel generator and transformer inrush

(OP)
RAVWARD,

Point 1.  I can't be sure that the transformers are de-energised for a long time before a new inrush. I must guarantee reliability in any condition above all in case of emergency.

Point 2.  Connected loads dump inrush currents because they add impedences in parallel to the magnetising impedance of the transformer.

But the operative requirements you suggested can't be respected in case of emergency in a ship.

RE: Diesel generator and transformer inrush

I guess I don't have a full picture of how the system is arranged.  You have four generators and four transformers; are there any connections between generators/transformers other than just one generator to one transformer?  If there is not a means of paralleling generators ahead of the transformers, it would seem that a control scheme could be worked out that would eliminate transformer inrush by energizing the transformer as the generator builds up voltage.

RE: Diesel generator and transformer inrush

(OP)
I tried to depict the scheme of the system:

    |    |    |    |
       ( )     ( )     ( )     ( )
       ( )     ( )     ( )     ( )
    |    |    |    |
    X    X    X    X
    |    |    |    |
--------------------X------------------
    |    |    |    |
    X    X    X    X
    |    |    |    |
       (G)     (G)     (G)     (G)

-Two busbars (one at the forward end of the ship and one at after end) with a coupler,
-two transformers and two generators for each busbar
- each components has a circuit breaker

Any configuration is possible

RE: Diesel generator and transformer inrush

(OP)
Sorry for the previous figure

    |        |       |       |
   ( )      ( )     ( )     ( )
   ( )      ( )     ( )     ( )
    |        |       |       |
    X       X       X       X
    |        |       |       |
--------------X------------------
    |        |        |       |
    X       X        X       X
    |        |        |       |
   (G)     (G)     (G)     (G)

RE: Diesel generator and transformer inrush

Well, in that case you are probably stuck having to deal with transformer inrush currents.

On a side note, when you want to do ASCII graphics, put them in a code box and you will get fixed pitch spacing as:

CODE

    |    |    |    |
   ( )  ( )  ( )  ( )
   ( )  ( )  ( )  ( )
    |    |    |    |
    X    X    X    X
    |    |    |    |
------------X------------------
    |    |    |    |
    X    X    X    X
    |    |    |    |
   (G)  (G)  (G)  (G)

That is done using the [code] and [/code] delimiters.

RE: Diesel generator and transformer inrush

ravward:

I do not think any one of your two points matter. The system shall still be designed for the worst case.

For all practical purposes tranformer inrush is independent of load. (inrush is in the range of 8-12 times the full load and the full load is 1 at the max, plus they are not in phase so it really does not matter).

As for residual flux, unless you are using a static transfer switch to switch sources, 10-15 cycles will be more than enough to dampen most inrush transient. 30 cycles would be the most.


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