Tap Changer vs No Tap Changer 4

[Mbrooke]

How much cost, weight, and size is saved by not having an automatic on load tap changer? 20-80MVA size, 115-230kv/13.8-34.5kv.

 

[prc]

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.

 

[Mbrooke]

@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/produ...door-distribution/sfr-voltage-regulators.html

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/...on-bulletin-210-110.pdf?dt=637205789821007210

 

[bacon4life]

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.

 

[Mbrooke]

@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/transformer-voltage-regulation/

 

[prc]

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.

 

[Mbrooke]

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

 

[bacon4life]

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%.

 

[Mbrooke]

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

 

[bacon4life]

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.

 

[Mbrooke]

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.

 

[FPelec]

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.


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