Power Transformer Testing on LTC Taps

[111R]

When testing power transformers with load tap changers, it can be very time consuming to run through all 33 taps on the LTC during each test. The DETC is usually not changed, but that increases the number of tests by a factor of 5 if this needs to be tested on a rebuild or commissioning of a new transformer. I'm trying to make the best use of time for performing useful diagnostics, so I'd like to avoid unnecessary tests.

Winding Resistance:
[ul]
[li]On LTCs that use a series or booster winding (2 winding - not a series auto), does it make any sense to perform static winding resistance measurements on multiple LTC taps? The path for direct current will not run through the actual LTC, but only through the primary of the series transformer. I assume that running a test on a single LTC tap is all that is necessary.[/li]
[li]On LTCs that do not use a series transformer, there is still a reversing switch that changes the direction of the current path through the tap winding. So, aside from the change in position of the reversing switch, 16L-N and N-16R are using the same LTC contacts. Is there any benefit to testing all LTC taps on a transformer with this configuration, or is it sufficient to test 1L-16R which will test all LTC contact and reversing switch positions? There will be no new contact positions touched if tested from 16L-2L, but the LTC would be moving in the opposite direction which may cause contact resistance issues to become visible[/li]
[li]My knowledge of ripple testing on tap changers is limited, but it appears that all 33 taps need to be tested since it's graphing a dynamic resistance during tap change transitions rather than the static, steady-state resistance.
[/ul]

Turns Ratio:
[ul]
[li]Since this test is quick to run, all taps are usually tested. With it using an AC source, it does take into account the series winding on transformers that use this. Is there a benefit to testing all taps versus just testing 1L-16R? If 16L-2L are tested, is there any potential to find something that wouldn't have been seen from 1L-16R? The only difference seems to be that the tap changer is moving in the opposite direction.[/li]
[/ul]

Excitation Tests
[ul]
[li]Excitation tests are typically recommended to be run in one direction from neutral as well as two steps in the opposite direction. For example, 16L and 1L-16R. Why do they recommend the extreme position in the opposite direction? Is it to check for proper seating in the final position when turning in the opposite direction? Should I apply this method to other tests such as winding resistance (ie: 16L, 1L-16R)?[/li]
[/ul]

Thanks!

 

[prc]

Winding Resistance:
1)If tapings are inside the series circuit, you can avoid measurement at all taps
2) You have to measure at all 33 taps as sometimes there can be difference as the movement direction of tap selector contacts are changing
3) same as 2
Turns Ratio:
It has to be done at all taps.1 to 33 and better 33 to 1 also.
Excitation tests:
Extreme positions include the tap winding fully in circuit.
Some tips are available in IEEE Standard C57.152-2013 Diagnostic Field Testing Of Transformers.

 

[prc]

Winding Resistance:
1)If tapings are inside the series circuit, you can avoid measurement at all taps
2) You have to measure at all 33 taps as sometimes there can be difference as the movement direction of tap selector contacts are changing
3) same as 2
Turns Ratio:
It has to be done at all taps.1 to 33 and better 33 to 1 also.
Excitation tests:
Extreme positions include the tap winding fully in circuit.
Some tips are available in IEEE Standard C57.152-2013 Diagnostic Field Testing Of Transformers.

 

[crshears]

Practical point: if you attempt to do too many tests simultaneously in an effort to avoid having to do multiple tap runs, you may end up spending more time swapping test instruments between every tap position than you save in tap runs...

CR

 

[111R]

OK, the direction of tap selector contact movement is a good point. But, I assume you must be sure to start the tests in neutral. For example, a tapchanger can go 16L-16R without changing rotational direction. The current changes direction through the tap winding, but the moving contacts are just making two rotations around the stationary contacts. So, would it essentially be the same to run the tests from 1L-16R and then 16R-1L in the opposite direction?

I'm asking this more so to make sure I understand the principle.

Thanks for the IEEE reference. I wasn't aware of that standard.

 

[prc]

In many of the tap changer models, the tap selector is not taking two rotations, but move back to the original position. In that case, the movement of moving contact in to fixed contact is not the same way and hence the recommendation to measure R at all taps.