Transformer Tap Changer Location - Primary or Secondary 3

[Dobber1978]

Just looking for some information on why tap changers would be spec'd on the primary or secondary of the transformer.

Scenario - 44kV/13.8kV - delta/wye - 15/20MVA - online tap changers

Primary side taps
- This is the less expensive option
- This is the more common option

Secondary side taps
- Was told tap changes when on the secondary are "smoother" but I can't find any literature on this

What other pros/cons of each are people aware of?

Many thanks!!
Cheers,

 

[RRaghunath]

Two things:
1) Current: transformer has higher current on secondary, hence the tap changer if provided on secondary will have to handle higher current magnitudes which will make the tap changer heavier and probably more expensive
2) Finer control: Transformer has more number of turns on primary, hence, it is possible to have finer control - 1% each step etc.
Above are specific to Step-down transformers. In case of Step-up transformers, the tap changer is on Secondary side for the same reasons as above.

 

[bacon4life]

This tutorial has many additional considerations:

http://site.ieee.org/sas-pesias/files/2016/03/Preparation-of-Transformer-Specifications1.pdf

It is also my understanding that USA historically tended towards low side and IEC markets trended towards high side.

 

[dpc]

Can confirm that in over 45 years working in the US, I don't believe I've ever seen an LTC on the primary side of a transformer.

 

[crshears]

Can confirm that in over 45 years working in the US, I don't believe I've ever seen an LTC on the primary side of a transformer.

Can confirm that in over 40 years working for Ontario Hydro and its successor companies in Ontario, Canada I've seen numerous, meaning several dozen, step-down transformers with ratings from 43 MVA up to 750 MVA that have under load tap changers in the primary side windings.

"We" tend to think of trafos as being fully capable of power flow in either direction, and therefore tend to speak more in terms of the tapchangers being in either the HV or LV windings, rather than in the primary or secondary.

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[davidbeach]

I, likewise, much prefer high-side and low-side to primary and secondary. No ambiguity that way. We’ve got two auto transformers on our system with a 13.2kV high-side and a 11.5kV low-side. They each, obviously, have a high-side and a low-side. But I defy you to write a meaningful definition of primary and secondary that could apply equally well to both. Both used radially, one steps up and the other down.

I’ll see your silver lining and raise you two black clouds. - Protection Operations

 

[crshears]

And by convention my utility uses increasing tap numbers varying directly as low voltage increases on all transformers, regardless of whether they're in the HV or LV side; no confusion that way.

Our phase shifters / quad boosters on the other hand are a completely different animal; whether the voltage regulating taps raise or lower our local grid voltage varies by location, same applies to the real power phase shifter taps; some alter the real power flow in the out direction on raise, others do so on lower. This is particularly confusing / challenging on one of our interfaces, where two of them work one way but a third works the other; one has to confirm they are raising and/or lowering the correct taps on the correct quad boosters - and woe betide you if you get it wrong, as very large circulating real power flows can quickly develop and one may get written up for loading equipment above its continuous or even limited time rating.

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[edison123]

Tap changers are provided on the higher voltage winding since they have more turns to give a wider and shorter step range and have lower current whose flow interruption is a pita.

Muthu

http://www.edison.co.in

 

[dpc]

Crshears,

Not sure who "we" is supposed to be but your condescension seems unwarranted. It's just possible that I have some foggy recollection that power can flow in either direction in a transformer. I might actually have designed a few systems where this happened fairly routinely. Just to clarify for the grammarians in the group: I've never seen a transformer in the US with the tap changers on the HIGH VOLTAGE side. It was just an observation based on my limited experience.

 

[prc]

1) In two winding transformers, tap-changers ( DETC or OLTC) can be provided in 4 ways
(a) On the HV side (neutral or line in case of delta connection) for HV variation
(b) on HV side for LV variation
(c) on LV side for LV variation
(d) on LV side for HV variation
In India, we are using all four arrangements but the majority (99 %) are case (a). Advantage simpler tap-changer ( less current) and less maintenance liability ( as tap-changers are at earth potential in service). Disadvantage - when tap-changer is to be operated to take care of voltage drop in transformer and LV line, flux density in the core will go up. To take care of this some utilities ( CEGB in the UK) used to specify a lower max flux density of 1.55 T ( while 1.7 T for GSU) for sub-transmission transformers. In the IEC world, this is the general case, and resistor type OLTC is universally used.
(2) In the US ( to my limited knowledge), case (c) is popular. Disadvantages are higher current tap-changer and difficulty to get fine tap sections ( this is taken care by the use of reactor type OLTC )

 

[davidbeach]

HV side wouldn't be at earth potential. I'm used to the LTC being at the wye-point so that it is close to earth potential and that means that it's on the LV side.

I’ll see your silver lining and raise you two black clouds. - Protection Operations

 

[crshears]

Hello dpc,

"We" is my utility, and I intended no condescension, patronization, head-patting or anything like that; after all, I know that as an electrical operator only and not an engineer I am definitely among my betters here, and participate on sufferance, something I in no way wish to jeopardize . . . but to me your post makes it clear that condescension is what you are choosing to see, and what you choose to see is beyond my control.

Moving on:

Hi David, I could be wrong but I don't believe my utility runs any HV trafos at all [ 115 kV or greater ] that aren't Y-wound on the HV side. This would be distinctly different than much [ most? ] US practice where, so I'm told, delta is almost the standard. Perhaps it is this that is leading to the difference between my and dpc's experience . . . ?

Right now I'm on shift with a manager whose background is Distribution Lines; from his experience, our 27.6, 13.8, 8/8.32, 4.8, and 2.4/4.16 kV are all Y connected, and only our 44 kV is delta.

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[davidbeach]

All of our transmission is wye-wye but the interface to distribution is almost always delta on the transmission side and wye on the distribution side.

I’ll see your silver lining and raise you two black clouds. - Protection Operations

 

[prc]

When I say, tap-changer is on HV side it means on HV winding and normally at neutral end of star winding. Maximum voltage coming on OLTC will be only 10-15 % of line voltage with other end solidly grounded at earth potential. This reduces chances of dielectric failures in service from poor maintenance.Though voltage is low, the current will be that of HV winding.It helps in reducing cost of tap-changer.

 

[davidbeach]

The high-side is delta on the vast majority of transformers I've seen. The LTC is on the low-side so that it can be placed next to the neutral. Auto transformers typically have the LTC at the neutral as well. The only examples of where there'd be a neutral on the high-side that I'm familiar with are GSUs and I don't believe we have an LTC on any of the GSUs.

Obviously there are lots of examples of all sorts, but categorically stating that LTCs are installed at the neutral on the high-side is clearly not applicable to the most common configuration of transformer used at the interface between transmission and distribution.

I’ll see your silver lining and raise you two black clouds. - Protection Operations

 

[crshears]

Different strolks for different fokes . . . and I didn't mean to be categorical in any way; I just didn't want anyone else to be, either.

Truce?

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[davidbeach]

Was more a response to someone else. The point I was try to get to is that there isn’t a single right answer. As with so many things the only general answer is “it depends”..

I’ll see your silver lining and raise you two black clouds. - Protection Operations

 

[prc]

Crshears- nothing categorical. Just explaining the situation as per my experience by supplying transformers around the world.Reality may be different.Learning is when we try to understand the “depends” instead of declaring final word.

1)Early part of transmission history, delta connection on HV side was common for sub transmission transformers. I don’t know why.In India all of them changed to Y-Y long back, except one utility who still insists delta on 110 kV side for 110/33 or 110/22 kV transformers. Still he insists OLTC on 110kV delta and not on LV side. In US, whether star or delta on HV, invariably tap changer on LV side. In IEC world LV side tap changer is ..extremely rare.

2)Certain applications OLTC is a must in GSU eg GSU for pumped storage hydro; units with high percentage impedance ( eg Germany and to some extent UK) ; plants with generator circuit breaker. Due to high current, always on HV side neutral.

3)In auto- transformers, it is always on line end since 1970s ie at MV voltage (even in US) except in high co-ratio units like 400/132 kV auto where neutral end OLTC is economical (UK) ; still in India they insist OLTC at 132 kV in such units. In auto, neutral end tap- changing make units variable flux type, a costly solution.

 

[Dobber1978]

Thanks for all input.

I couple items I was able to sort out in talking with one of the transformer suppliers that may add to the conversation.

If the tap changer goes on the HV (primary in my case) the tap changer typically has to be a high speed resistance based type which using a mechanical contactor under oil but with the resistors being limited in terms of current handling capacity typically can't put on the LV side, of course depends on the size of transformer and voltages in play. For my situation can't use resistance based on the LV side due to amps being too high. The issues with resistance based is they will need a full overhaul about every 7 or so years where the oil and contacts need to get replaced so greater maintenance requirements.

If the tap changer goes on the LV side you can use a vacuum based reactor type which uses a reactor and vacuum contactor. Because the connections are in a vacuum bottle the oil stays in good condition and doesn't need to be changed out. The issue with this type is the vacuum bottles are only good up to about 200BIL so can't be used on the HV side and this solution is more expensive up front.

Resistance based is less expensive up front but needs more maintenance.
Vacuum based is more expensive up front but requires very little maintenance.

And as I was told in the USA they have typically only used tap changers on the LV side. Canada use to be primarily on the HV side but are now switching to more on the LV side to eliminate the maintenance requirements.

Cheers,

 

[prc]

It is true that OLTC with vacuum bottles first came in US with reactor type OLTC. But today resistor type OLTCs are available with vacuum contactors.