Back charging of Power transformer 2

[sakaran51]

Hi,everybody,
I am new entrant to this Eng-tips forum and hope to have an enjoyable company of friends, sharing and learning.
Can anyone clarify whether a 2500 KVA Dyn11 6.3/0.433 KV,Z=6.25% transformer be back charged safely from LV side by a 1250 KVA Diesel generator? The x"d of generator is around 17%.There are two nos 1250 KVA DGs connected to the transformer LV bus,which can run in parallel, if needed.There is always a reservation to charge the transformer with DG. Any protection issues to be taken care due to inrush current?

 

[ScottyUK]

If possible, pre-close the LV breaker and allow the machine to run up with the transformer coupled to the alternator. No inrush problems. I doubt you will be able to energise a 2.5MVA transformer from less than 5MVA - 6MVA without causing a massive voltage excursion.

 

[prc]

Back charging of large Generator Transformers can be a tricky issue.Please see attached

 

[sakaran51]

Hi,Scotty,
Thanks.However I have following queries:
1.Will paralleling of other generator help equaling the transformer capacity?
2.Will the generator sees the magnetizing inrush as a fault like event and trip?
3.Is not the magnitude of magnetizing inrush current depends on the source(weak or strong)to some extent?-some elaboration is required.
This kind of back charging situation arise sometimes,for trial run of HT motors during commissioning when one is struck up with LV generators at site, when the HT Supply is not yet available.
4.What could be the solution without compromising the safety of the equipments?

 

[ScottyUK]

[li]It will help, but probably not enough to make it successful. You need a lot more reactive capability than you can get from those 1250kVA sets.[/li]
[li]Probably. It depends how your protection is set up, but it is a likely outcome. Damaging the AVR is another possibility, because the AVRs fitted to small sets have nowhere near the level of protection of those on large sets.[/li]
[li]Yes, that is true to some extent. The interaction between the load and the source which needs to be studied as a system rather than either part individually. The instantaneous conditions at breaker closure also has an influence, as does the magnetic state of the transformer core.[/li]
[li]Increase your generator capacity, or do as I initially suggested and configure the generator / transformer as a unit and allow the generator to build up voltage with the transformer already connected via the LV breaker. This will cause no inrush at all.[/li]

 

[waross]

When you try to energize that transformer with that generator one or more of the following trips may occur;
1> Under voltage trip.
2> Over current trip.
3> Under frequency trip.
However if those protections are disabled or are not present (Small sets below about 500 KVA, with inverse time main breakers.) The load will pull down the speed of the prime mover. As the speed/frequency drops, the Under Frequency Roll Off function of the will reduce the voltage. As the voltage drops, the amperage, KW and KVA will drop. A point will be reached at some value of frequency and voltage below normal levels where the prime mover power output matches the reduced KW demand of the transformer inrush. As the inrush passes the set will accelerate back up to normal speed and the AVR will restore the voltage to the set value.
Whether connecting at the same voltage or connecting through a transformer, you may expect problems if the set rating is less than 300% of the motor rating. For prime power sets you may use the 110% short time rating when comparing capacities.

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

 

[itsmoked]

Bill, you've confused things a bit by mentioning a "motor" while this is a transformer discussion. Want to clarify that?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[sakaran51]

Hi,Scotty,
I fully agree with you.With the present set up, the only way to avoid the inrush current problem is to start the system as "DG-Trafnsformer" single Unit.
By the way,can we assume the short time reactive capability of an alternator as 140% of alternator reactive rating?
Hi,Bill,
QUOTE:
Whether connecting at the same voltage or connecting through a transformer, you may expect problems if the set rating is less than 300% of the motor rating.

Is the motor starting current is 6 times the FLC?

 

[waross]

This kind of back charging situation arise sometimes,for trial run of HT motors during commissioning when one is struck up with LV generators at site, when the HT Supply is not yet available.

For smaller sets and motors, I have found that the minimum capability to start motors, based on current ratings of the motor and generator is 250%. This will result in severe voltage drops and frequency dips. This is not recommended with any other equipment connected to the gen-set.
If there is other equipment on-line the practical minimum rating is 300%. This keeps the voltage and frequency dips within limits that are generally acceptable.
Gen set manufacturers usually have sizing software available which will call for a larger generator capacity then 300%.
If the set is standby rated, use the nameplate current for sizing.
If the set is prime rated use 110% of the nameplate rating for sizing.
If a motor is too large for direct starting it may often be started by connecting to the set and then starting the set. Note; a separate source may be required to energize the controls and some trip functions may have to be disabled.



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

 

[sakaran51]

Hi,Keith,
The link you have sent is not opening.

 

[5000Dollars]

Wouldn't it be overfrequency and not underfrequency if you suddenly loaded down a generator with transformer inrush? The stator would heat up and the voltage would drop. If the voltage lowered enough the generator would stability-wise be disconnected from the system and start accelerating due to being unable to deliver real power. I thought generator frequency only dropped due to not meeting real power, not reactive power demands.

 

[5000Dollars]

Nevermind. I see it now.

 

[itsmoked]

Hi,Keith,
The link you have sent is not opening.

I sent you a link? Oh, if you meant my signature link, it works fine as far as I can tell.

Keith Cress
kcress -

http://www.flaminsystems.com