Autotransformer teritary 5

[Mbrooke]

Why do utility companies spec tertiary windings on auto transformers? I'm told because of zero sequence current, however a member on here mentioned that a tertiary can be eliminated with no ill effect. How does this work? Is it system/specific dependent?

 

[mgtrp]

No, the opposite--it is very lightly loaded. VArs are being controlled to lower the voltage.

 

[Mbrooke]

I see. Would have reactors have sufficed? In any case thats really cool, I never knew the idea could be put into practice.

 

[FPelec]

How did it work out in Itlay? I take it was a success.

Yes, it was.
Delta connected tertiary windings were at first eliminated in the 70s on 400/135 and 400/155 kV autotransformers (ATRs).
The first short circuit withstand tests carried out in CESI in the late 60s demonstrated that delta connected tertiary and regulating windings were the most common failure locations.
As a consequence, delta connected tertiary was eliminated from 400/135 and 400/155 kV units and the regulating range was significantly reduced.
Since the 70s regulating winding is not present on 400/230 kV autos up to 600 MVA. At the end of the 80s the delta connected tertiary was removed also on 400/230 kV ATRs.
This innovation was introduced in Italy thanks to the precious work of the late prof. Francesco Iliceto, who also operated as a consultant for the Turkish electrical authority, who clearly stated the opportunity of removing the tertiary winding.
These solutions significantly reduced ATRs cost and failure rates. Short circuit withstand tests statistics show that almost 75% of the failures were on ATRs regulating winding (including connection to OLTC) and delta connected tertiary .
In most modern developed systems (i.e. meshed ones), if you use 3-limbs units, there is little need for delta tertiary on ATRs.
As regards voltage regulation on the HV and EHV networks, shunt reactors are normally used to cope with light loading, to effectively deal with reactive power unbalances; in the last 5 years, as many synchronous generators were retired due to renewable distributed energy sources, we installed more than 6 Gvar of shunt reactors on 400 and 230 kV voltage levels.
It is a common misunderstanding that OLTC could cope with reactive power surplus or unbalance on transmission systems; although they can effectively cope with the symptoms of reactive power unbalance on the load side, they cannot solve the cause on the network side.
They can be still very useful in primary distribution centres (230/20 kV or 132/20 kV transformers) but their use on EHV/HV autotransformers is limited: the OLTC already installed in 400/135 kV ATRs typically performs less than 2 manoeuvres per day.

Si duri puer ingeni videtur,
preconem facias vel architectum.

 

[edison123]

Very informative thread. Thanks, prc and FPelec.

Muthu

http://www.edison.co.in

 

[Mbrooke]

FPelec: I want to say thank you! Very informative, and an eye opener.

 

[prc]

Mbrooke, Incase you are interested in official version of the changes in specifications in Italy and Turkey, please refer to CIGRE Paper 12-107 of 2000 Conference titled, "Power Transmission reliability - Technical and Economic issues".It was presented by ABB and utilities of Italy and Turkey. If you are a member of CIGRE, you can down load it from CIGRE web site. In India during the past 10 years we have successfully done short circuit test on more than 10 nos 400 KV auto transformers( of different make) with OLTC and tertiary, at KEMA. Test was done on tertiary also for 3L fault. So technically no manufacturing issue for regulating windings or tertiary, utilities removed as they found not necessary in regular use. In large power transformers of YNyn, tertiary was first removed not by Italy, but by France and US.

 

[Mbrooke]

I would be very interested. Do you have the link by chance, if possible? Around here everything has a tertiary, so this might end up being a beneficial change.

 

[prc]

Mbrooke, CIGRE site is

http://www.cigre.org.

But if you are not a member you will not get access. Some time back, I had prepared a tutorial on the subject for Indian audience. May be of interest to you. Please see attachment.

 

[Mbrooke]

LOL, your good at joking! Of course I am interested- great paper and saved it to the hard drive Again, much thanks.