Static Excitation Pt. 1: Compound Boost Current Transformer 1

[n1mr0d]

Hi,

I at my work we got a static exciter for the rotor field
of our 5.5 MW generator. The exciter appears to be of
the 'compound' type e.g. potential + current transformer
shunted. The current transformer below is very unlike the
transformers i have seen before (the coil around the
wire). It seems to consist of a thick wire coil (primary)
sandwiched by 2 other coils.

Does someone know anything about this type of transformer
or excitation system? I need some information for a
report i am working on.

 

[ScottyUK]

A compounded AVR design uses a high power current transformer which is designed to provide additional power to the AVR under heavy fault conditions. It is common for AVRs which are fed from the main output of the generator to use this technique because under fault conditions the machine terminal voltage drops, effectively reducing the gain of the AVR. When a fault occurs the AVR must boost the field in order to maintain generator stability: this is made difficult by the falling terminal voltage, so the compounding CT is used to provide an energy source for the AVR which increases output with fault severity.

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I don't suffer from insanity. I enjoy it...

 

[dpc]

You should try the

http://www.basler.com

website and look for "excitation support" or "current boost". They should have some technical papers that should be of help.

 

[motorspert]

I think it is a form of bar primary CT. For example, for a 500:1 CT there is a single conductor and 500 turns of small wire around the core. At low turns ratios eg 20:1 they struggle and use multiple primary turns and the transformer may have 5 promary turns and 1000 secondary turns. Maybe something to do with sensitivity at low currents or accuracy?

I suspect that a high secondary current is required in your application (ie CT is not driving a relay or ammeter ) and so multiple turns on the primary winding were required.

 

[n1mr0d]

Hi,

This thing actually has some kind of nameplate (on the backside!) which says:

H. 16-1-1
16-1-1
Z. 2-1
2-1 Y3

I assume that the numbers are the no. of windings, which corresponds to the photo. You can clearly see that the primary has 3 windings. I assume the transformer has the following configuration:

S S P S S
|18|3 3 3|18|
H Z Y Z H

This configuration makes things a bit complicated, because of magnetic coupling, which could be mitigated by the metal plate between the Z and H windings.Both the H and Z windings are shunted and connected in series in a way that derives the max amplitude.

Can someone show how to derive the pri/sec amplitude & phase charactaristics of this transformer?