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Topic Title: Synchronous Condenser
Topic Summary: Tap changer for transformer
Created On: 24 September 2013 12:39 PM
Status: Read Only
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 24 September 2013 12:39 PM
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iochoa

Posts: 18
Joined: 19 November 2002

I have recently seen a design of tap-changer for a transformer which will link a synchronous condenser with the network system. In my opinion this is not correct since the purpose of a tap-changer is to regulate the ratio of the transformer in order to control the voltage at the secondary side, so if the transformer is connected to a VAR regulator, this regulator will play the role as voltage regulator, thus the tap-changer is useless. I have seen SVC system and none of them use a tap-changer at the transformer. Have anybody seen this type of configuration tap-changer+SC?
 24 September 2013 10:28 PM
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statter

Posts: 124
Joined: 06 February 2013

Is the SC driving a load as well as providing reactive power?
 25 September 2013 01:14 AM
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iochoa

Posts: 18
Joined: 19 November 2002

No the SC is not driving a load but the transformer has a tertiary to feed auxiliary services.
 27 September 2013 01:55 PM
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statter

Posts: 124
Joined: 06 February 2013

Sounds unusual, but the tap change might be necessary if the line voltage is very variable and independent control of reactive power production was required.

The SC can only control the voltage by the effects of the reactive power it produces on the rest of the network it cant compensate for e.g. the full range of active power voltage variation.

Hope this helps
 27 September 2013 09:27 PM
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Avatar for JonathanHill.
JonathanHill

Posts: 225
Joined: 09 September 2002

Iochoa, thanks for this nice challenging post. To give us a fighting chance to make appropriate guesses, can you please give some further details of this design, eg:

- application - what is feeding the trafo, and what is the load
- trafo voltages Primary, Secondary, Tertiary
- Trafo rating and impedance
- Source Fault Level
- Winding where the tap changer is fitted
- Confirmation that the tap changer is On Load type
- Winding where the AVC senses the voltage
- Winding that the SC is connected
- SC rating
- Anything known about the SC control

Thoughts I have so far:
- Apart from the capital cost, SCs are typically quite lossy (windage & friction) with maintenance demands. There therefore must be a good reason for this combination
- SC provides dynamic response (order of tens of milliseconds), whereas an OLTC responds to trends (usually first tap operation is delayed by 90 - 120s). SC could compensate for fast V fluctuations that would otherwise cause flicker. The SC may also be providing tight V control if the load is very sensitive to voltage changes, allowing the tap changer to operate in coarse mode tracking trends in the source voltage.
- SC could provide a short-term current boost to allow protection to operate to clear a short circuit fault where otherwise the voltage might collapse (especially if the network has a very low fault level).

Looking foward .....

-------------------------
Jonno
IET » Energy » Synchronous Condenser

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