Ratio test for transformer bushing CT

[Powerykh]

Hello all, I am from IEC world. We have a 275/13.8 kV 3 phase transformer. Vector group is YNd1. There is a 4700/5A bushing CT installed at low voltage b phase. Attached is the detail of this Tx.
As the bushing CT primary side is not accessible, I can think of two option (other than the automatic test set like omicron ct analyser) to carry out the ratio check for this bushing CT:
1. Ratio to be carried our during transformer is on load
2. Injecting 3 phase low voltage (415V) to Tx HV side phase terminal (A6, B6, C6), and shorted at Tx LV side (a2, b2 , c2)
Calculation for short circuit current found at TX LV side:
Assuming tap 5 in use: 275/13.8kV, and % impedance is 11%.
HV side full load current: 235A
HV side impedance= HV side voltage/HV side Full load current=275000/235=1170 ohms
HV side impedance/phase = HV side impedance*% impedance=1170*11%=128.7ohms
HV side current/phase=Injecting voltage/HV side impedance/phase = 415/128.7=3.224A
Transformer ratio = 275000/13800=19.92
LV side current = 19.92*3.224A=64.2A
CT ratio = 4700/5=940
CT secondary current measured when 64.2A flowing in CT primary side= 64.2/940=0.068A

The secondary current measured is only around 68mA. Would like to know whether got other method to carry out this ratio test?

 

[prc]

Please see attached paper.

 

[ScottyUK]

Have done a similar test to prove CT polarities for unit protection by applying a short circuit using the HV side earth switches and applying power to the LV winding using a large rental generator. You can calculate the currents which will circulate from the transfofrmer impedance and rating, and then choose the size of your test generator accordingly.

You can 'steer' current through windings by selective operation of the earth switches, and if you temporarily replace the generator's AVR with a laboratory-type variable DC power supply then you have the basis of a very high power primary injection test set. Take appropriate precautions as you may be dealing with very large test currents.

 

[unclebob]

I would try using a standard TTR tester. A CT ratio can be found by inverting H's and X's leads.

Put H1 on S1b and H2 on S2b

Put X1 on a2 and X2 on b2.

 

[stevenal]

Inject on the low voltage side rather than the high voltage. Short A to N, with the other terminals open. With A shorted, the impedance of b2 to a2 becomes negligible allowing you to inject up to rated current (4690 A).

 

[Powerykh]

Thank you all for spending time to reply this thread. I have a question regarding this bushing CT as following:

When we carry out CT excitation test (Finding the magnetizing curve for CT), we inject secondary of CT, and keep CT primary side open.
For the Bushing CT in this case, I able to do the excitation test as usual. I have a question here, the transformer primary side all three bushing are grounded in this case, and when we in inject the CT secondary side, the primary side of CT will induce a short circuit current flow and make the excitation test failed to be carried out right? But i can carried out the test easily, any idea?(please see the attached sketch drawing if my expression not clear)

 

[stevenal]

Your sketch and text do not indicate the primary windings are short circuited. Extend your ground to YN and your test will fail.

 

[pwrengrds]

Response to:
I have a question here, the transformer primary side all three bushing are grounded in this case, and when we in inject the CT secondary side, the primary side of CT will induce a short circuit current flow and make the excitation test failed to be carried out right? But i can carried out the test easily, any idea?

I expect that the CT would likely saturate at 3-400 volts, the current ratio is 4700/5 or 1140/1, assume it saturates at 400 volts, then the voltage on the primary is 400/1140 or 0.35 volts, the amount of current pushed through the TX at that voltage is very low, so the results are not affected.

Putting voltage on the secondary and measuring the primary (which is what Uncle Bob suggested) is a typical way to measure ratio (if you want better results on the ratio test the TX grounds should be lifted).

David

 

[Powerykh]

Hi Stevenal, my mistake on the sketch drawing. Actually the YN terminal was connected to ground. I redraw as per attachment.

 

[Powerykh]

Hi David,
The said CT saturated around 53V when i carried out the excitation test. Like that, smaller voltage existed at CT primary side, hardly pushed any current through the TX.
Thank you so much, you help clear my doubt.

 

[stevenal]

Powerykh and unclebob,

If grounds are lifted and TTR is connected as suggested, how can you be sure the voltage measured at the b2 to a2 bushings is representative of the voltage from bushing B2 to a1 (the primary)? Any voltage drop across winding a will affect your result.

 

[Powerykh]

Hi Stevenal, We not carried out ratio test for this CT this time, only Ct excitation test only, so not able to see the actual result based on the suggestion above. Maybe next time i will come across again this type situation, so i still figuring/studying all the suggestion from yours all above.

 

[Powerykh]

Hi Stevenal,
I rethink your question. Ya, You are right, in order to use the method suggested by unclebob, we need to at least short out the HV terminal of A6 and YN, otherwise voltage drop across winding a2 to a1 will affect the ratio result.
Am i thinking on the right way?