Transfomer tap changer vs voltage regulator 2

[Mbrooke]

When utility engineers have the option, do they choose between an on board tap changer or individual voltage regulators in the feeder bays? What are the pros and cons to either?

 

[magoo2]

I think more utilities in North America tend to employ LTC in their distribution substation transformers and regulate the bus voltage for a few feeders. I know a couple of utilities that use a separate stand-alone three phase regulator to do something similar; this way you can get a simpler transformer design and add on a regulator. You can also do maintenance on the regulator without taking the transformer out of service by simply bypassing the regulator. Since the LTC or voltage regulator requires periodic maintenance, this may influence your decision too.

A key advantage to using separate sets of voltage regulators is that you can regulate the voltage for each feeder rather than making a compromise setting for the 2 or 3 feeders if you had bus regulation.

In addition, voltage regulators are more limited in their short circuit handling capability compared to LTC. For a large MVA transformer, you might be able to use only LTC. For smaller ratings, LTC and regulators can be used. You can check the standards on this.

 

[Mbrooke]

Good point! The idea of individualized voltage regulation is very attractive, especially with lines having varying load patterns.

Would you happen to know to what degree individual feeder regulators have fault current limitations?

 

[racobb]

Another advantage to individual circuit regulators is the ability to keep a spare unit or units on hand for quick replacement and.....only one circuit affected at a time.

I think the standards require a minimum fault rating of 25 times the base load current raring of the regulator. Don't have the standard in front of me, so you probably need to check.

Alan

 

[Mbrooke]

Good info. With higher impedance substation transformers and smaller intermediate substations fault current will easily be under 10ka.


The only disadvantage that comes to mind is each feeder will have reduced contingency capacity, but that may not be an issue on high performing circuits.


In regards to the regulator configuration, if a closed delta system is employed the regulators must be slaved, buck/boosting in synchronism where as wye connected regulators may be advance at any position regardless of the other two?

 

[bacon4life]

LTCs take up a lot less room in the substation and work well with enclosed bus. Regulator are typically used with open air bus.

 

[MahdiHooshyar]

On board tap changer is suitable for large power transmission. If you use individual voltage regulators in such large scales you must expend a lot of money for that applications. so if your application are very sensitive or smaller consumption voltage regulators are better option.
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[krisys]

At a substation level hardly anyone use the voltage regulator per feeder. All the electrical equipments are designed to operate for a certain voltage tolerance (say +/-5% or +/-10%) and frequency tolerance (say, +/-1%). Thus voltage regulation at a bus level shall be normally adequate.

If you are talking about the power generator voltage regulator and the unit transformer with OLTC, probably, AVR shall be used for the fast response and for the finer voltage regulation. For the coarse regulation, OLTC may be used. But the priority is for the generator AVR to operate.

 

[jghrist]

At a substation level hardly anyone use the voltage regulator per feeder.

This must be very location dependent. In the Southeast USA where I am, practically all outdoor distribution substations have feeder regulators. This would not be the case where metal-clad switchgear is used.

 

[Mbrooke]

"This must be very location dependent. In the Southeast USA where I am, practically all outdoor distribution substations have feeder regulators. This would not be the case where metal-clad switchgear is used."


In cases like this what is the typical loading per feeder bay?

 

[Mbrooke]

Normally regulators per bay are limited to 10MVA is size? Where as on board tap changers are unlimited?

 

[krisys]

When the voltage is regulated at bus level, then the voltage of the radial feeders will be regulated by using the capacitor banks switched in steps.

 

[Mbrooke]

It depends though, in some cases yes but in longer lines there usually another regulator toward the end to buck voltage back up.

 

[jghrist]

Most feeder regulators that I've dealt with at 12.5 kV are 250 kVA (regulated rating) single phase. This limits feeder loading to 7.5 MVA if full 10% regulation is allowed.

 

[cuky2000]

The voltage regulation at the feeder or bus changes on the utility preference for reliability, economics, O&M practice among other factors mentioned in the above posts.

LTCs in main power transformers are notorious to be prone to fail do to the moving part technology used in most units.

Although more expensive and larger space taking, some utilities prefer regulate the individual feeder with 3-single phase voltage regulator transformers(VRT). In case one VRT fail, it is easy to replace it and even operate the feeder by-passing the VRT in many instances.

Many distribution substations use automatic mechanical switching cap banks for additional voltage regulation during heavy load conditions.

In high density populated area in the East Coast, one of the oldest utility use feeder voltage regulators in a 4 kV switchgear with dedicated bay in the SWGR line up for 3-single phase VRT bank.