I can't answer directly, but having an integral LTC can increase the risk of losing the entire unit in the event of a LTC failure. LTC failures are fairly high on the components of a transformer that can fail. There are some designs that have a Non LTC transformer connected to external voltage regulators. These regulators can be bypassed for maintenance and typically 3 single phase units are utilized. In the event of regulator maintenance or failure, the xfmr can still operate with the units bypassed. The regulators can be removed, repaired or replaced with much less effort than trying to work on an in situ LTC.

If you completely eliminate OLTC and opt for a simple tap less transformer, approximate cost saving is 10-15%. Weight and space savings are not appreciable.

There is a weight and space saving for the whole transformer tank if you eliminate the OLTC.



Space its mostly saved for the total lenght of the tank (could go down to -20%), as you no longer need the preventive autotransformer if your OLCT is reactive type (like Reinhausen RMV), or the in-tank diverter switch - tap selector if its a resistive type OLTC.

And you also save the space and weight of the OLTC body itself if its external or "on-tank" (like RMV).

Thats what I was thinking. I've seen 50MVA trafos one with an LTC and one without an LTC and the one without an LTC seems smaller by about maybe 5-10%. I've also read on here that the majority of trafo failures come from the LTC itself.

Let me stick to my earlier statement. Transformer weight is mainly in the core &coil. Contribution from tank and oil is rather small.Tank length will come down. But overall size is more decided by radiators mounted on tank.

The type of transformers that you mentioned-two winding stepdown units do require OLTC and not easy to eliminate from power quality angle. Since these units are normally YNyn, the OLTC will be at neutral end. OLTC failures are comparatively rare in such application and OLTC failure will not lead to disastrous consequences like fire as fault current will be limited by winding impedance.

We were discussing earlier in this forum about eliminating OLTC in EHV auto transformers like 400/220 kV where tap-changer is at line end and at 220 kV potential. Any deterioration in tap changer chamber oil will lead to a creep failure to ground with 220 kV system fault current discharging inside tank. This will result in explosion and fire from the huge volume of hot gases generated from oil.

In strong interconnected grids, EHV voltage will not change much and the tap changer will not be effective. This is the reason many utilities (eg Italy &Turkey) eliminated this type of line end OLTC in such auto-transformers with cost savings of nearly 20 % + reduced losses + slight reduction in size and weight. Any voltage variation will be taken care by the OLTC in down side two winding transformers to keep input voltage to distribution transformers under control.

I beg to disagree PRC ;) They do not need or require on OLTC. Some install separate voltage regulators on each feeder which has several advantages over an OLTC.

Agree. I am not familiar with such voltage regulators!

Allow me to introduce these gems:


https://www.eaton.com/content/dam/eaton/products/m...

MBrooke, Those aren’t for EHV.

That is the same regulators found in about every utility.

Personally I wouldn’t have an OLTC on our system. 40% of XF failures are with the OLTC. We have ONE that is due to come out this year. It’s a very old (40-50s era) transformer we have serving one mill.

Actually this unit still has two of the ORIGINAL U type bushings. First year productions..

A 20-80MVA trafo outputting 13.8-34.5kv is (typically) not fed with EHV. Voltage regulation is done either on the secondary winding of the power transformer or through 32 step regulators on the outgoing feeder circuit.

True, but the post you were introducing those regulators to was talking about EHV.
I also feel quite sure prc knows about those regulators you “introduced” him to. ;)


I know all about regulators and their placement, as I actually work in the utility industry.

I should have quoted this, my mistake:

Quote:

The type of transformers that you mentioned-two winding stepdown units do require OLTC and not easy to eliminate from power quality angle.

Two winding 115-13.8kv trafos do not require an OLTC. A feeder regulator is a legit substitute for an OLTC.

And in some cases (United Illuminating in Connecticut as one example) was able to get away without any voltage regulation for decades when the transmission system was lightly loaded and local generation was ample. In fact the West Haven Substation still doesn't have LTCs or any voltage regulation as I understand it (except a 115kv cap bank, if that counts :P).

Anyway, back on topic. @PalletJack- This: "40% of XF failures are with the OLTC" Most compelling reason to go for feeder regulation IMHO. Not to mention maintenance of the LTC. In addition line compensation can be set to tailor each individual feeder greatly improving the overall system voltage profile.

I don’t dispute that. We are doing that now.
Our remaining bus regulated stations are put into work plans to change to feeder regulation, but this takes time, and we aren’t rich. We have to phase in gradually like so many others.

No disagreement here :)

I'm with prc on this one.
It could be an ANSI trend?, I've never seen voltage regulators used like that in my country (IEC based). Something similar is used in particular cases to boost overloaded distribution lines.

Just to understand it better: these voltage regulators are installed in every feeder right in the same substation the step-down transformer is located?

While could be the best technical solution for voltage control, it doesnt seem to be the most cost-effective for 20-80 MVA.

Also: I agree OLTC are one of the most critical component, but is the first time I hear the need to avoid them in this kind of application.

Very interesting thread.

Several utilities in the opted to use three single-phase transformer voltage regulator per feeder. The original driver back in the 1950s & 1960s was associated with the concern of poor reliability of the Conventional On-load-tap-changer (OLTC) have moving parts and historically is one of the weakest points in the transformer.

Although there is more flexibility to repair and replace the voltage regulator transformer in each distribution feeder, this option is not cheap since requires additional space, foundation, installation, and O&M capital budget. Recently one of the utilities that we work for is concerned with the potential leak of oil and associated liability and environmental compliance on a large amount of single-phase regulating transformers.

The good news today is that there are promising options commercially available with a new vacuum OLTC design in the market that show promising expectancy to improve the transformer reliability.

_{NOTE: The enclosed thread and the video could supplement some of the issues in this discussion}

Once you start getting above 20-25 MVA I can see the full range regulators getting extremely heavy, expensive, and not really an option. Even limiting them to 5% your still not going to get over the 35MVA 100% loading.
Once you get that big maintenance and machinery to change them out becomes a problem.

Especially with some of the older station designs where the bus is built over the regulators rather than the bus coming into and leaving the regulators from behind them.

But remember most feeders don't carry more than 5-12MVA. 3 400-440 amper regs per feeder ain't bad.

1) True, when I said I am not familiar with voltage regulators, I only meant I have not designed or made them. My company is making such regulators in Europe and US. I had heard that in US these are popular instead of providing OLTC in step down transformers. But it is a costly proposition and in IEC world we never used them. In IEC world OLTC is provided on HV neutral of step down transformers while US prefer OLTC on MV side. But in IEC world we had a need for such regulators recently.( these are small auto-transformers with a simple OLTC for controlling input voltage by +- 10 %) When small distributed power generators (roof top solar and wind turbines) started feeding power to grid through distribution transformers, it became necessary to automatically control the input or output voltage of DTs ( these are usually provided with DETC for +-5 %) to feed power to grid. So in Europe a need arose to provide voltage regulators on MV or LV side of distribution transformers. From 2015 onwards ABB is marketing such voltage regulators- LV up to 250KVA and MV up to 8 MVA to fit near to existing DTs. In new DTS, OLTC is provided in transformer itself and MR has developed a new simple low cost model, vacuum type, for use in DTS.

2) In 1970s&80s,when we were facing acute power shortage in India, voltage dip at consumer end was a real problem. We used to make DTs with OLTC during those days for +_15 % voltage variation. Now no more we require them except in special situations as above.

3) There were separate IEC and IEEE standards for voltage regulators.In 2018 these were combined and issued as a common standard viz. IEC 60076-21/IEEE C57.15.But Maximum rating is limited to 1000 KVA single phase or 3 MVA three phase; max 33 kV. So I am not sure whether 20-30 MVA voltage regulators are normal as Mbrooke mentions.

4) Of course nothing is impossible. In India, some years back we used to make 100 MVA 100/110 kV auto-transformers with OLTC. In India, 100 kV transmission voltage came in 1910 at Mumbai.At that time it was the highest transmission voltage in the world. But later 110 or 132 KV became standard, so a need came to interconnect this 110 KV grid with old 100 KV lines and the need for such special auto-transformers.

5) Vacuum type OLTC came in to use more than 20 years back, first in US with reactor type OLTC. But it is replacing only the arcing contacts and all other parts remain the same. So it may reduce maintenance liability ( a great advantage where tap changing is frequent eg HV DC transformers, furnace transformers etc) but I am not sure of reliability improvement.

@PRC- no hard feelings :) I hold profound respect for your knowledge. And the fact you are from India? is a ++. I think they have some of the most beautiful power systems on earth.


FWIW I've seen some US utilities put on giant 3 phase regulator in front of the power trafo and regulate a whole bus:


https://new.siemens.com/us/en/products/energy/medi...

One a side not since we are comparing oddities. In the IEC world, have you ever seen a power transformer with a fused primary?

https://www.sandc.com/globalassets/sac-electric/do...

We have a couple of 115/15 kV stations that do not have regulation. These transformers are basically useless when attempting a to provide backup service to adjacent stations. Just the voltage drop change when going from lightly loaded to the ONAF2 rating uses up almost the entire allowable voltage range. This leave no room for the transmission voltage to vary or for voltage drop along the feeder. As a rule of thumb, we start run into voltage issues if the unregulated banks are loaded to more than the ONAN rating. The lack of regulation on the medium voltage side means the transmission voltage has to be very closely regulated.

The cost, weight and size goes up dramatically when using external regulator. Compared to the photos argotier posted, these external regulators are huge.

@Bacon4life: What is the power factor when the trafo is loaded to the ONAF2 rating? What is the voltage drop across the trafo? See slides near the bottom:


http://voltage-disturbance.com/power-engineering/t...

Mbrooke-We are learning from each other. Each country has its own unique technical solutions and practices,many times dictated by tradition rather than engineering judgments.
1) You are right; in US a fuse will be used for 10 MVA transformer primary protection(to save money), but in this part of the world a breaker will be used even for a 5 MVA unit.

2) Till the middle of last century, US was preferring three phase banks (using three single phase units)from 1MVA to 1000 MVA banks. US was looking for flexibility, thought a costly proposition. Europe switched over to three phase units even in 19 th century.

3) US opted for separate voltage regulators while in other parts of the world OLTC is used on HV neutral end to handle voltage variations in incoming voltages and load terminal voltages.

Yes and yes. Fascinating to compare the two. As I understand it the UK played a huge role in current foreign standards.

Mbrooke, I had assumed 0.95% power factor. The sliders on the page you linked to shows that a IEEE 12/16/20 MVA 8% transformer at the ONAF2 (13.3%) rating at 95% power factor has voltage drop of 5.19%.

What X/R did you use? At 100% power factor 5.19 drops down to 2.5- a notable difference.

Mbrooke-Even with a perfect power factor, our transformers without LTCs cannot be fully loaded (much less overloaded). Our allowable voltage variation for any specific customer is 8%. Allowing a 3% swing in the transmission system and a 2.5% drop through the transformer only leaves 2.5% for voltage drop through the feeder, MV/LV service transformer and LV service wiring to the meter.

Although I am hoping to add a few more switched capacitors to these LTC-less transformers as marginal improvement, switched capacitors seem to have even more reliability issues than LTCs.

Good explanation which has given me something to think about it.

A side question: are those sliders accurate? I've been using them lol.

I remember an old guy telling me that tap changers were 50% for compensating the large impedance of power transformers 50% for T line voltage variants.

IMHO, OLTC should banally be adopted only then really necessary.
By removing a +/- 10% OLTC (and, of course, the regulating winding) on a 250 MVA 400/155 kV ATR you could reduce the weight of about 10 to 15% and the cost of about 15 to 20%, keeping the same losses figures.
In the EU practice, the neutral-connected regulating winding is almost never the outermost winding, so that if you eliminate it, you will also reduce the diameter (and the copper weight!) of other windings.
A significant part of the failures during the short circuit withstand test are related to the regulation winding and to the connections between the regulation winding and the OLTC, so that removing the latter you can push a bit more on the design margins.

Si duri puer ingeni videtur,
preconem facias vel architectum.