General info on On Load Tap Changer

[JLuc]

Hi,

when designing a new substation (industrial plant), what criterion do you base your decision on whether or not to include an On Load Tap Changer on the main power transformers.

What kind of load will require an OLTC?

OLTC are quite expensive and I wonder how often do you have to use it.

Thanks

JL

 

[davidbeach]

Constant loads probably don't need an OLTC, highly variable loads probably do. What does your daily and yearly voltage profile look like? Is it within acceptable limits under all conditions (both internal and external)? If so, you probably don't need an OLTC, if it gets out of range, then an OLTC would probably be a good investment.

 

[dpc]

The incoming distribution or transmission line is supposed to be maintained within a certain voltage profile based on utility standards. If you can live with fluctuations in that range, factoring in the voltage drop across your own transformer based on your system's load profile, you probably don't need an LTC.

It's important to keep in mind that an LTC does not provide constant voltage. Temporary voltage sags and swell will still occur because it takes time for the LTC to tap up and down.

And what David Beach said.

 

[electricpete]

Yes, the need for an LTC comes when a fixed tap transformer would cause unacceptable swings in voltage.

In addition to the initial cost, LTC's can be a maintenance bear.

Three single-phase voltage regulators mounted external to the transformer are sometimes selected as an alternative to the LTC. A lot easier to service.

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[slavag]

Dear JLuc.
Could you please answer on one, not so correct Q.
Who are you, end-user, main contractor, subcontractor or designer in this project?
I put attention on your several Q's on the last time, as, OLTC use or not, CB's use or not, etc.
Please don't take this , as some sort of negative Q's, I'm only try understand, you are technical guy or project manager or salesman.
If you only technical, my answer for you:
Is not "good" design for the new industrial substation
1. Trafo paralleled w/o separated CB's
2. Plant with full load about 40MW on the two xfr's w/o OLTC. I've never see industrial plant w/o variable load.
3. More from this : you have two parallel xfr's, that mean you also must parallel operation of OLTC control.
4. Series connection of several diff protection, today???, additional cores or CT's is not issue

If you project manager with money problems, becouse your bosses wined this project with dumping price, will try make min of min, but not base it on some right technical solution.

If you end-user, EE of this plant, sorry, I'm don't understand you. You need work with this in future.

I'm also project manager and usually work as subcontractor of big companies, have same problems, but cost is cost,
technical responsible is responsible.
Regards.
Slava

 

[rohra]

Dear JLuc,
we are using OLTc for trnsformers that are 75 MVa & 220KV/33 KV,star-star.There are two transformers in parallel.
we have Remote tap changing Cubicle (RTCC) seperate for both Transformers.Auto voltage relay senses & gives command to cahnge the taps in Auto.its setting has to be done.
PT of 220 KV/110 v used there
One is kept in master other in Follower.master senses & gives command to follower.The Transformers are for state electricity board supply to company.normally no load.in case of tripping of generator they take Load
tap changing takes place in uto control,though u have manual control also

thanks

 

[ScottyUK]

Be careful when designing master-follower tap changer control solutions. Particularly plan for contingencies such as failure of the master controller - some implementations of this scheme result in the follower units tapping to one or other extremity of the tapping range. Not good, but I could name at least one installation where this is precisely what happens and it is very inconvenient!


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If we learn from our mistakes I'm getting a great education!

 

[slavag]

Hi Scotty.
What do you prefer?
M/F logic
Negative reactance principle
Minim. circulating current.
Some external PLC.
Regards.
Slava

 

[isquaredr]

Hi Slavag
I prefer to use the negative reactance principle as it allows for temporary paralleling to other networks without load grabbing and tap runaway without the need to fix taps non auto. The relays I use are manufactured by Siemens (ex Reyrolle) Supertapp & Microtapp.

http://www.reyrolle-protection.com/product_list.php

 

[lz5pl]

By the way, how often AVR's are used in your practice? I found that in almost all my projects AVR's are set by the end user on "Hand mode" immediately after hand-over because "they reduce the life of OLTC". Probably they are right, but I always wonder why they invest in AVR? I hope the reason is not in my work
I usually use REGsys from A.Eberle.

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It may be like this in theory and practice, but in real life it is completely different.
The favourite sentence of my army sergeant

 

[jghrist]

Iz5pl,
What do you mean by AVR? Do you mean a voltage regulator separate from the OLTC? If so, why spend the money for OLTC and then have a separate voltage regulator?

If you mean the voltage regulator control for the OLTC, why would you spend the money for OLTC and then not use it for automatic voltage regulation? Sure you can reduce maintenance on the OLTC by never using it, but you would have even less maintenance by not buying OLTC and use the off-load tap changers for infrequent tap changes.

 

[slavag]

Hi.
Isquaredr, thanks for information, as I understand you work in utilities.
Iz5pl,
Transmission and Main Distribution transformers we use only
with automatic voltage control. Industrial plant main transformer (connected to grid)or main(first) transformer of separate voltage level we used only with automatic voltage control. Others xfr's, is depend.
But if you don't need automatic voltage control, don't bay xfr with OLTC, as Jghrist saied, use off-load tap changer.
BTW, I heard, that REGsys is excelent product. I think in our area 99% of voltage contollers is MR ( MK20,MK30, Tapcon)
Jghrist, AVR that mean, OLTC voltage control, many companies and technical guys use this term AVR. It's not so right term: AVR ( I'm also start think about generator AVR).
Paralleling work of xfr's is our weak place, up today is not used as need. Or M/F logic or some external PLC's.
I would like try used Min circulating current instead M/F
logic .
Regards
Good day and good week to all.
Slava

 

[lz5pl]

jghrist, under AVR I mean Automatic Voltage Regulator - in fact voltage controller, as slavaq say. Actually I work in engineering side, not in utility, and I follow customer requirements what to install and what not. So I do my part and just wonder why controllers are not used after that. Normally my projects are manned substations, where operators change taps of transformers one or two times per shift, typically according to day load and evening load. If they operate them in automatic mode transformers will tapchange may be 10 times per day. Not a big deal according to me, but this is not my decision. I was just curious what is the usual practice in other areas.

slavaq, REGsys are really good product, and even better is customer support - it is may be the best from all manufacturers I have used, even Big Three (ABB, Siemens, AREVA) are more difficult in spite of their bigger recourses. I usually use M/F program because it is most logical for me. Usually my projects are new stations with all transformers from same type, so circulating currents or other programs are generally not necessary. You can also program a topological view of your station for automatic recognizing of parallel conditions and entering of "M/F" or "Independent" program. I have used before MVGC from GEC Alstom, but REGsys is another generation, of course. Tapcon's I have seen in a project where only commissioning of relay protection was in my scope, so I just from curiosity passed trough menus of already commissioned by others Tapcon. It seems it is not very complicated too, but I cannot say something more.


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It may be like this in theory and practice, but in real life it is completely different.
The favourite sentence of my army sergeant

 

[slavag]

Hi Iz5pl.
Are you finished commissioning of substation with REF54_ and RET521?
What is your opinion about those relays?
Regards.
Slava

 

[lz5pl]

Hi slavaq,

Yes, it is in operation from 11 september (easy to remember !)
RET 521 doesn't surprise me. I have worked with REL521 on another project in Egypt in 2000 year, nothing special here too.
About REF541/3 I have mixed feelings. Relays themselves are looking well, less technically than Siemens and more serious than AREVA - this is if a blonde girl should choose the supplier.
The main idea is like high end REL's, and for me it is a matter of taste to like it or not. They are VERY flexible, what according to me is not so necessary. I prefer mostly Siemens and AREVA concept to configure I/O's in conventional way, like a matrix, and to use graphical editor only for some more specific tasks. It is easier according to me, but again it is a matter of taste.
CAP505 software is not very convenient comparing with CAP531. It is pity that they just didn't implement these relays into CAP540/CAP531 package.

But all these things are secondary, important is to have new projects ;-)
BTW, after this station we completed one much bigger station (actually started from April) 220/110kV for Siemens. Well, now we are looking for next one !

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It may be like this in theory and practice, but in real life it is completely different.
The favourite sentence of my army sergeant