transformer failing TTR test on 3 phase tester, but passes using phase by phase method 1

[emmjea001]

We have had two (2) new 400/11000 YnD11 step-up transformers, intended to be in parallel operation, that fail primary tests on turns ratio, the test facility are using a Vanguard ATRT-03B tester, and the Results are wildly out.

A - 98, B - 29, C - 18
in addition the instrument says "vector group unknown" - so it cannot establish it

The supplier has produced results which show a consistent 15.88 ratio on all phases but using a single phase ratiometer and testing as follows:

HV side - Neutral and A phase connected. B phase and C phase left open,
LV side - Connect a phase and c phase, b phase leave it open.
They said they Programmed the ratiometer to correct vector group YNd11, and use use formula as HV/(LVxSqrt3) = ratio.,

Questions are:

1. I believe this comes form a field method for testing TX s phase by phase but Anyone got a similar method, or document detailing this procedure - it is not detailed in the BS EN 60079? (sorry, we use EN standards, but happy to have reference to ANSI etc)
2. Why does it seem to pass the above phase by phase method, yet not when all phases are connected and the unit is tested as a three phase transformer using the Vanguard instrument in automatic mode - it is worth noting the unit does not pick up that the TX is a YnD11, and comes up with result unknown?
3. What could this indicate? A mis-configuration on the internal winding connections?
4. What exactly is the prescribed test method over there, ie what do test facilities use - as from what I can see BS EN 60076-1 is pretty vague, section 10.3 does not say much at all?
5. where is it stated you have to use a 3 phase test instrument and not the above / other phase by phase method?

Sorry, a rather long list of questions, but I am a bit lost, and Interested in anyone who has had similar experience, and how the rectification process might go - ultimately we need to be sure these units are right - especially as they will be in parallel operation, and that they are of acceptable quality.

Hopefully someone can answer these and give some technical input as to how this anomaly in the two test can be explained

Many thanks

 

[collies99]

It had to do with the vector option selection on the Vanguatd unit.

Double check the nameplate configuration, x-reference the manual for the config selection all should be good to go if selected properly. Check lead coonections too.

I have found using the Vanguard 3_phase TTR that you can get gross ratio errors if not selected correctly on the winding config selection.

A quick test is to apply low voltage supply power to the HV winding side and read the voltage on the LV with your handheld. If ratio reading comes in the ballpark, you know its your test set; probably working but not configured.

Good luck.

 

[emmjea001]

Thanks Collies,

appreciate that quick feedback - got this from the test facility, as my role is supervisory, but going to the test facility tomorrow, and will make a note of this.

As I said, on the phone I did ask them to repeat using different method, and results seemed ok - I also asked them to connect instrument to another set of TX's, which they did, and with unit set to auto, it detected automatically the right vector grouping - in this other test case though it was a standard DyN11 TX, but to me the question is, since the test unit has been shown to work, and has recent calibration etc certificates, and YnD11 is in its library, then why should it not work on 3 phase auto mode?

Also, is there a specific test sequence prescribed by your regs there? As I said, to my reading the BS EN only says ratios must be tested and vector group established, but it does not prescribe exactly how this should be done?

 

[collies99]

Not sure what you meant on test sequence, but we do standard tests as in:
Ratio/polarity, winding resistance, capacitance/ dissipation factor, polarization index/meggar. On larger unit we do most test with the doble test set including excitation tests. New units are tedted in all tap positions, core ground insulation.

If reqyestef we will include leakage reactance test.

Other tests:
Oil sample/screen tesy, GIO analysis, CT ratio/polarity, circuit checks, bushing/PF tap and lightning/surge arresters, tap changer controls and gauge test, fans/controller test, sudden presdure relay test, funtioning pressure relief micro-switch.

IEEE calls for phasing test but I have yet to perform one. For the most, part if the three-phase TTR comes out OK, is there a need to fo phasing test which dhould have been factory tested anyway.

If i exclude any required test, please let me know.
Chees.

 

[emmjea001]

thanks - sorry, I did not explain that well - and yes we have all the same basic tests, plus additional type tests.

what I meant by test sequence was specifically to the TTR test, and I was looking for a standard that says:

for YnD11 vectro group, you must:

- connect A2 to a2
- then apply 3 phase 400V to A2, B2, C2 with phase rotation as counterclockwise
- then leave all other terminals open
- then test voltage between B2 and b2 it should be ....
- then .... etc etc

in otherwords, if you do not have an automatic 3 phase tester digital tester which you just connect up all the leads and run it - what is the theory and specific tests you undertake to verify the same information

Hope that make better sense?

 

[collies99]

Can you forwatd your nameplate winding diagram and vector notation.

thanks

 

[collies99]

Your above note is a phade rotation check, your original note is a ratio test.

Do you,need instructions for both:
1. Single phase ratio test procedure using the Vanguard on single phase mode ? and
2. Phase rotation check with available 400v three phase supply ?

 

[collies99]

Ynd11 vector group, or high side WYE leads low voltage DELTA by 30 degrees.

Connect H1 to X1

Measure H1-H3, H2-X2, H2-X3, H3-X2, H3-X3

Check:
H3-X2 = H3-X3,
H3-X2 < H1-H3,
H2-X2 < H2-X3,
H2-X2 < H1-H3.

 

[collies99]

Vanguard TTR

This unit is North American and you should be able to test simply by selecting the Wye-delta winding selection without going to the special setting page.

Should pass with flying colors on the three phase test. That is probably why no such vector group was available since it may alreafy be the default setting.

Most my transformers are D-y snd it has been over 20 years that I have done a Y-d.