Transformer %IZ Change 1

[jwilson3]

We have two transformers at a hospital that were recently tested by the manufacturer. Both are 5 years old and are 12.47kv-480v delta/wye. the 5000kva unit's Percent Impedance test revealed an average of 9.5 ohms and a calculated X/R of 1.6. Nameplate is 6.07% and typical X/R is closer to 9. The 3000kva unit also tested high, but not to the same extent. The manufacturer recommended replacing both units.

What could have caused this change? Neither has been subjected to a significant "thru-fault".

 

[jwilson3]

To yenrameshn: Each coil was tested separately and the secondary winding was "shorted" by connecting X1 and X0; no impedance. If the test had been done on all three phases simultaneously, the line current would be used, which would be 5.19a (3a X 1.732). The phase to neutral Z would still have to be calc.d on a phase basis by using L-N voltage.
Z = 27.41/1.732/5.196 = 3.045 ohm
Zbase = 31.1 on a three phase basis.
%Z = 3.045x100/31.1 = 9.79 same as calc'd on single phase basis.
I hope this helps.
jim wilson
To jghrist: I need to review the Standard you refer to. I'm not sure what the 0.866 factor is; probably sqrt3/2.

 

[busbar]

Aside: A poorly connected or undersized jumper on the secondary may have significantly and undesirably jacked up the in-the-field impedance calculation. Also, the tester {nee, manufacturer} may have inadvertently “overlooked” accuracy in the original test-report measurements.

Be careful. Given perceptions of reliability need, the transformer folks may be banking on a hospital being an easily railroaded client.

Table of contents for IEEE C57.12.90-1993 is

http://standards.ieee.org/reading/ieee/std_public/description/dtransformers/C57.12.90-1993_desc.html

§9 and Part II are probably the salient text. It is not exactly “breezy” reading.

 

[BJC]

Jwilson3
is this transfromer a 3 or 5 leg core?
Were all six leads on the high side isolated except for the two being tested. Were there six terminals isolalated on the high side? H1-H2 for A,B & C?

 

[jbartos]

Suggestion: Term "Single phase" can become ambiguous since it may mean the single phase between two phase conductors or single phase between the phase conductor and the neutral.
Notice, that the Zan=Zbn related to Zab is over cos30=0.866, namely, set
Zan=1 p.u.
Zbn=1 p.u.
Zab=Zan x cos30 + Zbn x cos30=(1+1)cos30=1.732 p.u.
and 1.732/2=0.866
Therefore, if one measures Zan and Zbn one gets 2pu, and if one measures Zab, one obtains Zab=0.866(Zan+Zbn)=0.866(1+1)=1.732 pu.

 

[jghrist]

The calculation of %Z is correct if the secondary of the energized winding was shorted to neutral (X0). This is not the procedure for 1ø testing in ANSI/IEEE C57.12.90. In ANSI/IEEE C57.12.90, the 3 secondary terminals are shorted together and neutral is not connected. This eliminates zero-sequence currents. BJC's question on the core construction is very relevant if there are zero-sequence currents. With a 3-legged core, there is no return path for zero-sequence flux except in the insulating the air or metallic connections other than the core. This may have influenced the impedance measurements. You are actually calculating (Z1+Z2+Z0)/3. With a 3-legged core, Z0 does not equal Z1.

I suggest that you get the transformers retested, along with a dc resistance test, according to industry standards before deciding to replace them.

 

[jwilson3]

To busbar: I agree. We're holding off replacement since other test results don't indicate a problem, but I'm still puzzled why the test showed an impedance change and what could have caused this if the test was correct.
To BJC: Three legged core.I think the other leads were open circuit or isolated. If they were not, but were all shorted, I believe this would appear to the leg under test as an increase in secondary load, which would show up as a reduced impedance.
To jbartos: It appears you are referring to a wye-delts transformation, therefore if Zan=Zbn=Zcn=1 pu, then the delta phase impedances Zab=Zbc=Zca=3.
When you measure the Zab of the wye circuit you get 2. When you do the same for the delta circuit you also get 2
(1/Z = 1/3+1/6 and Z = 3x6/3+6 = 2)

 

[jwilson3]

To jghrist: I'm sure the test was done individually, wth X1-X0 shorted. This is a 3 legged core. You've made a good comment. Thanks. I need to review C57 to learn more about the standard test methodology.

 

[jghrist]

yenrameshn,
Do you mean to say that if I connect the X1 terminal to the X0 terminal of an energized Delta-wye transformer, I won't get any zero-sequence current unless I also connect to something that I call "ground"? If I were to also make a connection to a ground rod, no current would flow in the connection to the ground rod (where would it go?). All current would flow from X1 to X0 directly. How would you analyze a fault from X1 to X0 with no "ground" connection using symmetrical components?

 

[jbartos]

Suggestion to jwilson3 (Electrical) May 22, 2003 marked ///\\To jbartos: It appears you are referring to a wye-delts transformation, therefore if Zan=Zbn=Zcn=1 pu, then the delta phase impedances Zab=Zbc=Zca=3.
///Actually, I referred to two wye windings, e.g. Zan+Zbn connected in series and measured from the transformer terminals A and B. Then, considering the one delta connection winding between A and B as Zab only since the delta connections can be opened for convenience of testing.
I agree that the connected delta windings would lead to:
Zabdelta=(1.732)x(1.732+1.732)/(1.732+1.732+1.732)=1.1547pu in terms of Zan=Zbn=Zcn=1pu.\\\

 

[yenrameshn]

Hi jghrist,
Firstly, zero sequence current exist only when there is an unsymmetrical fault and returns through the ground.
In a power system distribution there could be several grounding electrodes which could be interconnected in a earthing ring and may be connected to the star point(neutral point) of the transformer secondary.
If it is connected to the starpoint the zero sequence current will circulate in the transformer winding through the neutral impedance(could also be solidly earthed).
Say when a single line to ground fault occurs at the equipment(say stator ground fault, the current flowing through the fault "I(fault)is=3(Ia0)",here Ia0=Ia1=Ia2. Here the positive,negative,zero sequence flows into the faulted circuit. If the equipment earth is connected to the transformer star point through earthing ring then the zero sequence current returns through neutral impedance and circulates in the transformer winding. For positive and negative sequence currents there is no return current and so they will not pass through neutral reactance.
If the neutral of the transformer is not grounded the zero sequence network is open circuited and Z0 is infinite. under these condition Ia0=Ia1=Ia2=0.

Now coming back to the point, what I meant is there will be no zero sequence current in the secondary of the transformer since there is no condition of a ground fault in the circuit to be short circuit tested. Once the neutral is earthed and by shorting x1 and x0 then atleast we can assume it as a ground fault condition!!! and have the zero sequence component in the circuit.
Ramesh

 

[jghrist]

yenrameshn,
My point is: X1 (Aø) was shorted to X0 during the impedance test. From the standpoint of current flow, it does not matter one bit if X0 was connected to ground. If there are no connections to X2 (Bø) and X3 (Cø), there will be zero current in these phases. There will be current in Aø. Therefore Ia0 = (Ia+Ib+Ic)/3 will not be zero. You can call the wire connecting X1 to X0 "ground" if you want or even connect it to a ground rod. It makes no difference to the flow of current.

 

[advidana]

If the Manufacture's test showed that the transformer is bad I would start making plans to replace these units since the transformers are being used at a hospital and these type of transformers can not be obtained quickly. Remember- transformers do fail, you should be ready. More testing by another test outfit would be good for a second opinion. Don't delay, if those transformers go out people lives may be effected.
A.D.Vidana
5-28-03

 

[Kiribanda]

Hello Gentlemen,

I am a new comer for this forum. I saw this thread on transformer impedance change, but decided to not to comment, as I also cannot agree in anyway with the so called “impedance test” and also due to the fact that JWILSON3 has not witnessed such an important test.

Since YENRAMESHN had pushed this posting to a different way and made some comments on the zero sequence current components, which has no relevance to this thread as he himself says, I too got some interest to get involved.

YENRAMESHN
I cannot agree with your comment on May 23 rd which says “ For a zero sequence current to flow in the secondary winding, the star point (neutral point) has to be grounded”. IT IS NOT NECESSARILY TO BE. If there is a 4 th conductor on a three phase system then zero sequence components will exit. In that way JGHRIST is fully correct and I agree with his immediate reply on May 23 rd. Load side of a Delta/Star transformer is not grounded. Then how do you analyze a Phase to neutral fault ( X1 to X0)on the load side using sym. components where zero seq. components are involved in the calculation? In a 3-phase 4-wire distribution system if a fault occurred between a phase and the neutral, will the residual earth fault protection (51N) or zero sequence protection (51GS) scheme respond? The answer is No. The transformer will trip from overcurrent (50/51) and not from any of the ground fault schemes (51N or 50GS). The reason is that the zero sequence currents DO exist but they are balanced by the 3I0 flowing in the neutral which is the 4 th conductor. Similarly, take a 30 A, 3 phase, 4-wire household ELCB (RCCB) and if we bridge one phase and the neutral terminals using a test lamp, it will not trip on earth leakage but depending on the fault the main or branch MCCB/MCB will trip on overcurrent. Same thing will happen for a single phase RCCB too. But if we bridge a phase terminal and “ground”, then definitely it will trip. Reason is even though the zero sequence currents DO exist in the first case they are all balanced in all 4 conductors. Nothing exist in the earth path. In the second case, yes they DO exist and flow in the earth return path, but this time nothing in the earth path. I hope this will help you to think about it again.

JWILSON3
With regard to impedance change, I can reasonably claim that the manufacturer had done a test which is not correct neither as per ANSI/IEEE C57.12.90 nor per IEC 76. Normally the impedance test is done along with the test for load losses so that we can estimate transformer load losses too. If we are doing the impedance test

1)Using a three phase supply then a VARIABLE 3-phase supply has to be connected to H1, H2 & H3 while short circuiting X1, X2 & X3. (no connection with X0). Supply is raised till we get the primary current equal to 138 Amps corresponds to 3 MVA (AA rating) and not to 5 MVA(AFA) and then to be corrected to 75 deg C for accurate results to compare with original test results.

2)Using a single phase supply then it has to be connected to H1 (H2 or H3) while H2(H3 or H1) is earthed while short circuiting X1, X2 & X3. (no relevance with X0). Supply is raised till we get the primary current equal to 138 Amps correspond to 3 MVA (AA rating) and not to 5 MVA(AFA) and then to be corrected to 75 deg C for accurate results to compare with original test results.

As far as I know this is the standard routine test suitable for measuring the impedance voltage irrespective of whether the secondary is delta or star. That means, for your
3/ 5 MVA(AA/AFA) dry type transformer having a primary rated voltage of 12470 Volts with a name plate impedance of 6.07%, we should apply at least 756 Volts to the primary so that 138 Amps is flowing in the primary while X1,X2,X3 terminals are short circuited. Therefore the results of 27.41 V. 3.0 Amps, 135.4 Amps does not sound good for me.
Since the manufacturer has not followed the standard method, I think before any replacement you should ask them to repeat the test so that you also can witness. If all the other routine tests ( specified by NETA-MTS-2002) are OK and if the impedance test results is then become “SENSIBLE”, my advice is to energize the transformer without a second thought.

Regards!

 

[jbartos]

Suggestion to jwilson3 (Electrical) May 22, 2003 marked ///\\To jbartos: It appears you are referring to a wye-delts transformation, therefore if Zan=Zbn=Zcn=1 pu, then the delta phase impedances Zab=Zbc=Zca=3.
When you measure the Zab of the wye circuit you get 2. When you do the same for the delta circuit you also get 2
(1/Z = 1/3+1/6 and Z = 3x6/3+6 = 2)
///Yes, I agree with the delta-wye transformation (transfiguration). I was addressing the test of two windings connected in wye with Ean=1pu, Zan=1pu, Ia=Ib=1pu, Ebn=1pu, Zbn=1pu
versus
Eab=1.732pu, Ia=Ib=1pu and Zab=1.732pu.
To comply with the transfiguration relationships between Zan=Zbn=Zcn=1pu and Zab=(Zan x Zbn + Zbn x Zcn + Zcn x Zan)/Zcn=(1 x 1 + 1 x 1 + 1 x 1)/1 = 3pu
current would have to be 1/1.732pu to obtain:
Eab=Zab x Ia = 3 x 1/1.732 = 1.732 pu = 1.732 x Ean, in pu
For line-to-line fault:
Eab=Isc x (Zan + Zbn)
or
Zan + Zbn = Eab/Isc = 1.732pu/0.866pu = 2pu since Zan + Zbn = 1pu + 1pu
Therefore, the line-to-line short circuit current Isc is 0.866 x Iline-to-ground since
Iline-to-ground=Ia=Ib=Ic=1pu for Ean/Zan=Ebn/Zbn=Ecn/Zcn=1pu/1pu\\\

 

[jwilson3]

I've been in the Carribean on a 60 ft schooner for the last two weeks, so I've not been able to participate in the "thread" nor give it much thought.
A generous amount of interesting discussion has been posted and it has forced me to think thru the testing procedure in relation to ANSI/IEEE C57.12.91 for dry transformers.
The field test was done on each HV/LV coil pair separately: that is with H1-H2 energized and X1-X0 shorted for the first test with all other leads opened. Then the other coils were similarly tested. They tested at lower amperage than the normal full load values and got an impedance value of 9.14 ohms for one phase, which when divided by a Zbase of .933 gives a % Impedance of 9.79%. If they had raised the primary voltage from 27.41v to 1221v, the pri current would have been at full load of 133a(phase current). The 1221 impedance volts is 9.79% of full voltage.
The single phase test procedure in ANSI/IEEE C57.12.91 is with all primary connections between the phases intact, and X1-X2-X3 connected together on the secondary. If 27.41v is applied to H1-H2, 3a flows in the H1-H2 leg and 1.5a in the parallel legs. The line current is 3a+1.5a = 4.5a. If the voltage was raised to 1410v, the pri. line current would be at full load of 231a(line current). Each leg is similarly tested, averaged, and multiplied by 0.866 per ANSI/IEEE. Assuming each phase is at 1410v, the % impedance volts is 1410 X .866 = 1221v or % IZ of 9.79.
So, I feel the field test procedures, done on a single phase/single coil basis gives the same result as the ANSI/IEEE procedure, which is done without disconnecting internal connections.
The big question is, assuming their readings were accurate, is why the results varied from the factory results?