Hi All,
Can anyone enlighten me, how to determine how big is the generator size to magnetise 4 x 1600kVa transformers (4% impedance). Is there a calculation on this?

It is a 400V /11000 with 50Hz

Many thanks in advance.

Its not so much the impedance (leakage) which you need to know, its more the magnetisation inductance Xm and (resistive) core losses due to Rc (both at 50Hz) which you need to know, expressed to the side where your generator is.

Most simply, you can translate Rc to a core loss in Watts (at either side of the transformer, it should give the same result). You generator will need to be at least big enough to supply this core loss.

Check http://en.wikipedia.org/wiki/F...quivalent_circuit.svg

Similarly, you should be able to translate Xm to a VAR requirement for excitation. Your generator needs to be able to supply at this many VAR, at the same time as the core loss in Watts.

And if there are 4 transformers, x by 4.

Check http://en.wikipedia.org/wiki/F...quivalent_circuit.svg


My suspicion is that the generator may only need to be a fraction of the size, but if you really only have this tiny generator, it won't be able to do anything else at the same time as energise the Xfmr.

Andrew

-------------------------
Dr. Andrew Roscoe

http://personal.strath.ac.uk/andrew.j.roscoe

Oh, and remember its three-phase, so make sure you take this into account if you have Rc and Xm as single-phase values from a nameplate/spec sheet. If you already have core loss (W) and/or energisation VAR requirement specified, these may already be expressed as 3-phase values.

-------------------------
Dr. Andrew Roscoe

http://personal.strath.ac.uk/andrew.j.roscoe

I would need to know why you wish to just magnetise the transformers or what you are trying to do before I could help.

Words from the ONAN GenSize software:

Looking at the CAT design guide.... it advises 2~4 times if you want to maintain sensitive loads also connected to the generator.
The method of starting/ energisation is the critical aspect.

If we take the scenario of fully excited generator presented with an unmagnetised transformer, the inrush current of the transformers to be 4 x 10 x FLC (1600kVA @ 400V ~2300A) is 4 x 23,000A ~ 92, 400A for a few cycles the sub transient element of the generator decrement curve will need to provide this. Sub transient reactance of the generator which is ~ 12x FLC. This method of starting presents an onerous load on the generator.

Dead bus starting is the preferred method where the AVR (voltage) is increased gradually and typically leads to a transformer/ generator sizing raio of 2~4.

The post says 400/11000 transformers, are we generating at 400V then transforming up to 11kV ?

The principle would be the same, only the current would be reduced. Either way, the method of voltage regulation and excitation is what will govern the maximum no load magnitisation current.

I would like to share our experience in Mumbai , India.
Using a 440 v generator ,We successfully energised entire 145 kv GIS station including 2 nos 90 Mva 110 kv/33 kv transformers and 7 bays of 145 kv GIS . We used 33kv /415 v stations service transformer to back charge 33kv GIS bus.

If we use conventional DG , The MVA requirement goes very high to meet var required to energise Transformers,

Pl furnish following details so that I may suggest the plan to meet your requirement
1) Rating of Transformers , Voltage rating , no load current values , as mesaured at Manufacturing works during factory acceptance test

2) How do you plan charging the transformers ie hopefully one at a time ?

3) Single line diagram fo the installation

I will recommend you the rating of DG set with modified exciter circuit to enable controlled volatge generation unlike convetional one.This technique eliminate inrush totally .
-regards,

Hi,

I would be interested in your modification and the generator/avr details. Please PM me or post here.

Regards

Al