Tap changer location in transformers

[prc]

Normally ,in many parts of the world,on-load tap changers are provided on HV side of Transformers for getting voltage variation on HV side or on LV side (variable flux regulation)But in USA most of the cases,tapchangers are provided on LV side for LV variation. This invariably make on- load tapchanger of high current rating with consequent complications in tap changer and tap winding .Historically what is the reason for this?I could not find any advantage for locating tap changer on LV side of the winding. Can any one shed light?

 

[cuky2000]

Large number of transformers used in utility and Industrial substations are HV Delta/Wye-Gnd on LV side.

1- Tap changer (LTC) connected on Delta side will require and Insualtion level (BIL) rated Line-to-Line. For LTC connected to the Wye side, the BIL will be Line-to-Neutral. The cost of LTC appear to favor the LV location for typical transformer size.

2- LTC is the single higher cause of failure in power transformer are caused by moving part mechanism. Operation and maintaintenance cost of LTC on HV is considered more complex and expensive.

3- Reliability index favor LTC on LV side since SLG fault on secondary side including the LTC will not affect the utility side because the Delta winding will not provide zero sequence path to the fault on the HV side.

4- Utility and industrial users have extensive experience on the LTC performance on the LV side. There are not evidence that LTC on HV side will provide better reliability, operation and mainatenability performance.

There are cases of use of LTC on the primary side went the tap changer current on the LV side could be excesive or specific requirement imposed by the system applications.

 

[RAgrawal]

all the answers made by chuky are correct but don;t you think that by providing the LTC on LV side, that is exposed to more current i.e. more sparking i.e more maintenance than the LTC on HV side.

One more thing goes in favour of providing LTC on HT side is that it make the voltage change smooth in because in HT side number of turns are more so corresponding change is LT voltage is smooth but in LT, number of turns are less so for one tap position change, corresponding voltage variation in HT will be more.

 

[jbartos]

Suggestion: Beside historical aspects the following could be considered:
The solid state electronic advances are proliferating into transformer on-load tap changers. The electronic switching on the load side will not have any sparking involved.
Also, the make-before-break versus break-before-make on-load-tap changers shall be scrutinized. Apparently, the make-before-break was more convenient on the low voltage side to avoid high potential differences during switching and resulting consequences.

 

[redtrumpet]

The LTC winding is usually placed in the winding circuit that requires the greatest voltage variation. As cuky2000 pointed out, specific voltage and current capability limits of the load tap changer may require the LTC to be placed in the winding circuit that requires the least voltage variation.

 

[jbartos]

Suggestion: LTC is preferably placed on the interior of the winding to avoid the abnormal voltage stressed to which end coils are usually subjected.

 

[prc]

I am afraid my question was not clear.Iwas referring to a condition where Ltc can be easily put on Hv side but still selected on LV side.As cuky said when Hv side is delta connected probably lv side tap changer will be easier. But in ehv systems Hv side is star connected and always solidly earthed.Of course i remember old industrial customers where going for delta connected Hv systems even when they were drawing power from solidly grounded systems.But modern industial systems seem to have discarded such schemes. But in India, I have seen even in ehv system(110 kv)with delta connection ,utility always go for Hv tap changers.So my question was for star/star trfs only.
Putting Ltc on interior wdg will not reduce voltage stresses.In big trfs for better magnetic symmetry tappings are always provided as a separate winding.In case of HV tappings it will be at neutral end,as outer most wdg and tap achanger will not see much stresses.LV tap changer ask for inner tap winding with consequent difficulties in taking out lead etc,etc.So from mfg angle, design angle,reliability angle Hv tap changers are always better.No doubt manufacturers are capable of providing any requirement.
But from system operation angle any advantage?
HV LTc: HV voltage variation can be done at constant flux in trf. But if LTC is used to get higher lv Terminal voltage it results in variation in flux in core and so change in losses and if excessive, overfluxing in trf.
LV LTC: Opposite condition to above.
I have seen 100 MVA 220/66 Kv trfs star/star with hv or lv tapchanger depending on utility.Some countries go for OCTC on HV side with OLTC on LV side making trf more complicated.

 

[jbartos]

Suggestion: When it comes to high voltages, the electrical field stresses somewhat dictate what is the best, most economical, smallest, etc., e.g. bushings, etc. The other aspects are considered of secondary importance, including on load tap changers.