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Topic Title: How Big is the generator to magnetise a 4 X 1600KVa Transfomers
Topic Summary: Transformers Calculation sheet
Created On: 12 December 2012 05:33 AM
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 12 December 2012 05:33 AM
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IETUser

Posts: 2
Joined: 12 December 2012

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.
 12 December 2012 09:51 AM
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aroscoe

Posts: 91
Joined: 18 October 2002


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
 12 December 2012 09:54 AM
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aroscoe

Posts: 91
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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
 17 December 2012 09:30 PM
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cookers

Posts: 203
Joined: 10 February 2012

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?


I would need to know why you wish to just magnetise the transformers or what you are trying to do before I could help.
 18 December 2012 09:31 AM
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MickeyB

Posts: 181
Joined: 18 January 2003

Words from the ONAN GenSize software:

FAQ: How do I enter a transformer Load?

GenSize 96 does not include a load type for transformer loads. Although transformers do represent a real load and require a considerable magnetizing current when initially energized, the key thing is to enter the transformer actual secondary load information in determining the required generator set capacity.

Generator sets are capable of supplying magnetizing current for transformers several times the capacity of the generator set with no observable ill effects. Although the potential magnetizing current drawn by transformers can be up to 20 times the transformer rated current when connected to an infinite source, the current drawn from a generator set will be limited to the available current from the generator. This magnetization will last up to several cycles, even at reduced current, but is of such short duration requiring little real power (high kVA but low kW), the generator set hardly reacts before the transformer magnetizes and begins supplying real power to the load.

The real concern is for the actual transformer connected load power requirements. The actual loads must be entered into GenSize 96 to obtain a proper generator set size. You must size for the starting and running requirements of the actual load, not just the transformer kVA capacity. Starting large motor loads connected to the transformer, for instance, will require substantial kVA and kW for significant time durations.
 18 December 2012 10:19 AM
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MickeyB

Posts: 181
Joined: 18 January 2003

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.
 18 December 2012 09:17 PM
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cookers

Posts: 203
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The post says 400/11000 transformers, are we generating at 400V then transforming up to 11kV ?
 18 December 2012 09:41 PM
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MickeyB

Posts: 181
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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.
 23 December 2012 01:40 PM
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ukmaharaja

Posts: 11
Joined: 26 May 2005

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,
 08 January 2013 12:30 AM
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Al

Posts: 60
Joined: 02 February 2009

Originally posted by: ukmaharaja

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
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