Transformer Winding Resistance

[janmeinard]

I have a transformers winding resistance:
T1(LV SIDE), 3MVA, 34.5/.480 KV
X1-X0= 0.130 mΩ
x2-x0= 0.408 mΩ
x3-x0= 0.320 mΩ
T2(LV SIDE), 3MVA, 34.5/.480 KV
X1-X0= 0.462 mΩ
x2-x0= 0.442 mΩ
x3-x0= 0.241 mΩ
T3(LV SIDE), 3MVA, 34.5/.480 KV
X1-X0= 0.75 mΩ
x2-x0= 1.200 mΩ
x3-x0= 0.480 mΩ
Are they considered balanced as per NETA standard?
I had been finding and searching for the standards on these transformers.

[Kalbi_Rob]

I have a transformers winding resistance:
T1(LV SIDE), 3MVA, 34.5/.480 KV
X1-X0= 0.130 mΩ
x2-x0= 0.408 mΩ
x3-x0= 0.320 mΩ
T2(LV SIDE), 3MVA, 34.5/.480 KV
X1-X0= 0.462 mΩ
x2-x0= 0.442 mΩ
x3-x0= 0.241 mΩ
T3(LV SIDE), 3MVA, 34.5/.480 KV
X1-X0= 0.75 mΩ
x2-x0= 1.200 mΩ
x3-x0= 0.480 mΩ
Are they considered balanced as per NETA standard?
I had been finding and searching for the standards on these transformers.

First, I would say NO! How far off depends on a few factors. First, is this an Oil Filled Padmount, GSU, Large Dry-Type, or Small Dry-Type transformer. If dry type, what is the insulation? Each type of transformer has a different requirement for winding resistance due to configurations. I assume you do not have the original factory winding resistance measurements?
Lastly, what winding resistance equipment are you using, and are you allowing it to stabilize before taking readings? I also have seen erratic measurements as seen above when leads are hooked up incorrectly, are you sure you are connecting per manufacturer recommendations?

[rofo42]

I have a transformers winding resistance:
T1(LV SIDE), 3MVA, 34.5/.480 KV
X1-X0= 0.130 mΩ
x2-x0= 0.408 mΩ
x3-x0= 0.320 mΩ
T2(LV SIDE), 3MVA, 34.5/.480 KV
X1-X0= 0.462 mΩ
x2-x0= 0.442 mΩ
x3-x0= 0.241 mΩ
T3(LV SIDE), 3MVA, 34.5/.480 KV
X1-X0= 0.75 mΩ
x2-x0= 1.200 mΩ
x3-x0= 0.480 mΩ
Are they considered balanced as per NETA standard?
I had been finding and searching for the standards on these transformers.

T3 for sure is odd. However, winding resistance can be influenced by some factors from what I have noticed.

1. What test equipment? What amperage used for testing? Same across all 3 transformers? 10-15% of rated current is maximum. Heating of the windings can cause erroneous values.
2. Are you using connections based off of the nameplate vectors or a pre determined sheet? I have seen our procedures not line up with certain vector diagrams. Any additional bus/cable included will affect results. Testing needs to be exactly the same across all three transformers if comparing to each other with no previous data.
3. Equally long wait times for stabilizing? It can take a good amount of time to stabilize.
4. All three transformers are the same rating it appears, are test connections exactly the same? Are the transformers the same temperature during testing?
5. Off chance that these are dual bushing low sides? If so, you need to short the respective low side bushings of the same phase together but also test from the same bushing. This has actually shown a major change is test values in my experience.
6. Have you tried demagging before testing? Try a wait period between tests?
7. Any past results from these transformers? Sister units?
8. A small detail but worth noting, make certain that the potential leads are on the inside of the current leads.

Post more information/nameplates/test equipment. More than likely it is test connections/procedures that are influencing results.

[Kalbi_Rob]

1. What test equipment? What amperage used for testing? Same across all 3 transformers? 10-15% of rated current is maximum. Heating of the windings can cause erroneous values.

Never exceed 10%

Per Megger "A Guide to Transformer Winding Resistance Measurements"

6) Selecting the Proper Current Range
• Always try to saturate the core. This happens typically when the test current is about 1% of
rated current
• Never exceed 10% of rated current. This could cause erroneous readings due to heating of the
winding
• Typical test currents are from about 0.1% to a few percent of rated current
• If test current is too low, measured resistance is not consistent (pending magnetic status before
the test is started) and a specific winding will have different readings when measured from
time to time
Note: If MTO indicates input voltage overload or does not charge the transformer with the selected
test current, you have selected a test current > 10%. Select next lower current range and restart.

[Kalbi_Rob]

I have a transformers winding resistance:
T1(LV SIDE), 3MVA, 34.5/.480 KV
X1-X0= 0.130 mΩ
x2-x0= 0.408 mΩ
x3-x0= 0.320 mΩ
T2(LV SIDE), 3MVA, 34.5/.480 KV
X1-X0= 0.462 mΩ
x2-x0= 0.442 mΩ
x3-x0= 0.241 mΩ
T3(LV SIDE), 3MVA, 34.5/.480 KV
X1-X0= 0.75 mΩ
x2-x0= 1.200 mΩ
x3-x0= 0.480 mΩ
Are they considered balanced as per NETA standard?
I had been finding and searching for the standards on these transformers.

Small Dry-Type Transformer (<500 KVA 3 phase, or <167 KVA 1 phase)

-Not a recommended test and no recommended test results.

Large Dry-Type Transformer (>/=500 KVA 3 phase, or >/=167 KVA 1 phase)

-Optional Test

-Temperature-corrected winding-resistance values shall compare within one percent of
previously obtained results.

Liquid Filled Transformer:

-Required Test

-Consult the manufacturer if winding-resistance test values vary by more than two percent
from factory test values or between adjacent phases.

[rofo42]

Never exceed 10%

Per Megger "A Guide to Transformer Winding Resistance Measurements"

6) Selecting the Proper Current Range
• Always try to saturate the core. This happens typically when the test current is about 1% of
rated current
• Never exceed 10% of rated current. This could cause erroneous readings due to heating of the
winding
• Typical test currents are from about 0.1% to a few percent of rated current
• If test current is too low, measured resistance is not consistent (pending magnetic status before
the test is started) and a specific winding will have different readings when measured from
time to time
Note: If MTO indicates input voltage overload or does not charge the transformer with the selected
test current, you have selected a test current > 10%. Select next lower current range and restart.

You are correct, 10% is the top. I had some cold beverages that night 🙂.
Good info also. We have had issues with the transformer charging and taking an extremely long time to stabilize. These were large GSU and tertiary transformers(22KV to 6.9/6.9KV), however. No where near going over 10% at 50 amp max output of test set. The 1% of rated current is good information.

I have thought before that long extension cables to the test set (Vanguard) could be dropping the input voltage enough to cause issues but not low enough for the test set to fail to operate. What do you think?

[Kalbi_Rob]

You are correct, 10% is the top. I had some cold beverages that night 🙂.
Good info also. We have had issues with the transformer charging and taking an extremely long time to stabilize. These were large GSU and tertiary transformers(22KV to 6.9/6.9KV), however. No where near going over 10% at 50 amp max output of test set. The 1% of rated current is good information.

I have thought before that long extension cables to the test set (Vanguard) could be dropping the input voltage enough to cause issues but not low enough for the test set to fail to operate. What do you think?

Winding resistance test set, similar to DLRO (some DLRO units can be used as winding resistance test sets), use a Kelvin bridge. We are talking about charging a winding, thus we are more concerned with the amount of current, which is indirectly proportional to the voltage, so a lowering of the voltage would increase the current. The current is regulated by the machine, if properly calibrated, thus no issue should occur.

I have used a 10A DLRO on a 22 story building bus duct with one lead running the length of the building and no issues, I think using the winding resistance test set on a GSU unit isn't gonna be a problem, even the new 1100kV HVDC 587 MVA transformer Siemens installed in China.

[rofo42]

Never exceed 10%

Per Megger "A Guide to Transformer Winding Resistance Measurements"

6) Selecting the Proper Current Range
• Always try to saturate the core. This happens typically when the test current is about 1% of
rated current
• Never exceed 10% of rated current. This could cause erroneous readings due to heating of the
winding
• Typical test currents are from about 0.1% to a few percent of rated current
• If test current is too low, measured resistance is not consistent (pending magnetic status before
the test is started) and a specific winding will have different readings when measured from
time to time
Note: If MTO indicates input voltage overload or does not charge the transformer with the selected
test current, you have selected a test current > 10%. Select next lower current range and restart.

Just an FYI that Megger has some possible conflicting info on the maximum rated current. This is from a Megger webinar. I have best results staying at 5% or below.