I work with testing & commissioning jobs. Recently while charging a 5MVA power transformer DYn11, 33/11KV, I noticed a strange phenomenon. The transformer was drawing about 1.1MW power without any load in secondary. I checked the circuit breaker in primary 33kv side & found that the B-phase pole contact was not proper. I adjusted the penetration of B-phase pole contact & the problem was solved. But my question is how is it possible? How a transformer draws so much current in idle condition & how this circuit breaker problem relates to it?

Thanks and regards
Ashutosh
ashutoshb [at] engineer.com

 

Ashutosh.... please clarify:

A) How was the "power measurement" determined?

B) Does your observation related to the "B" phase pole mean that only "A" and "C" phases were connected to the primary 33kV supply?

Regards, Phil Corso

 

Ashutosh... further to my earlier comments:

C) Where are power measurement PTs and CTs located? Near the source of supply? Or near the Xfmr being energized?

Phil

 

Ashutosh... perhaps my earlier question C) was unclear. Were the PT's and CTs on the supply-side or load-side of the Circuit Breaker?

Phil Corso

 

Dear Mr. Corso,

A) The power was measured rather observed trough an energy meter as well as SIMENS make differential relay panel(7ut6131)

B) No no, apparently while closing the circuit breaker we thought that all three phases were connected....since there was no load in the secondary 11kv side....there was not any question of earth fault pick-up due to B-phase dis connection...I finally adjusted the B-phase penetration of the moving contact and the matter got solved.

C) The cts and pts are connected to 33kv i.e; HV side......

Sorry for the delayed reply .... I could not view the post... control.com doesn't intimate through any e-mails.....

Thanks

 

Ashutosh... my interpretation of the data thus far:

A) During energization of the Delta-winding of the 3-Ph, 5 MVA, 33 kV, 37.5A, transformer, with a single-phase 33kV source, the Siemens Differential Relay, as well as an upstream Energy meter were “observed” to respond to a current flow resulting in a value of 1.1 MW!

B) The inrush-current could have been as high as 20 x its nominal rating, depending on residual magnetic flux, prior to energization. Regardless, the two sound phases should have “seen” exactly the same current magnitude due to the breaker malfunction!

C) Grounded wye-connected PTs have been known to produce overvoltage conditions. Were there any observations of such phenomena?

Questions:

(Q1) Are you sure about it being 1.1 MW and not 1.1 MVA?

(Q2) Was this transformer being paralleled with another?

(Q3) What caused the transformer’s feeder breaker to trip? The Differential Relay or some other protective device?

(Q4) Did you have recording instrumentation, kV, A, and PF, in the 33 kV supply circuitry in addition to the energy meter?

(Q5) Is there any evidence of unbalanced load on the same feeder in parallel with the 5 MVA xfmr?

(Q6) Were any PT fuses blown or Transient Surge Suppressors damaged?

Sincerely, Phil Corso

 

Dear Mr. Corso,

I think I am not able to make you realize the phenomenon! Let’s have another try. Please go through my observations/findings as under. I hope this would also clear your Q1 to Q6 lot put in your last post: -

1.THE SAID TRANSFORMER IS A 33/11KV, 5MVA TRANSFORMER OF VECTOR GROUP DYN11.

2.THE TRANSFORMER WAS RUNNING INDEPENDENTLY WELL UPTILL DATE WE FOUND THE PROBLEM AS FOLLOWED.

3.ONE FINE MORNING THERE WAS A FAULT IN ONE OF THE SUB-STATION’S 11KV OUTGOING FEEDERS;FOR WHICH THERE WAS A TRIPPING OF THE ABOVE MENTIONED 33/11KV 5MVA TRANSFORMER.

4.OUR SUB-STATION’S STAFFS USUALLY DISCONNECT ALL OUTGOING FEEDERS BEFORE ENERGIZING ANY TRANSFORMER. THE CASE REMAINED SAME WITH THE ABOVE MENTIONED TRANSFORMER ALSO. THEY FIRST CHARGED THE TRANSFORMER HV SIDE I.E; 33KV SIDE AND WAITED FOR A WHILE BEFORE CHARGING THE 11KV FEEDERS.

5.THEN SUDDENLY THE SHIFT-IN-CHARGE NOTICED THAT THE SIEMENS DIFF.RELAY (WHICH IS HAVING A NICE DIGITAL DISPLAY OF P/Q/S/AMPS & VOLTS) WAS SHOWING A POWER P=1.1MW NOT MVA.

6.HE DOUBTED THAT THE TRANSFORMER’S LV CIRCUIT BREAKER WAS OPENED AND IT WAS ONLY RUNNING WITHOUT ANY LOAD. THEN WHERE FROM THIS POWER CAME??

7.HE CONTACTED ME AND I CAME AND WITNESSED THE SAME SCENE AS HE DESCRIBED.

8.I GOT PUZZLED AND THOUGHT THE RELAY GOT FAULTY. BUT WHILE ENQUIRING WITH OTHER METERS CONNECTED TO THE SAME PANEL, I FOUND THE CASE IS SAME. THE TRANSFORMER IS ILLOGICALLY DRAWING 1.1MW POWER WITHOUT ANY LOAD.

9.THEN I checked out PTs / CTs INPUT etc. BUT EVERYTHING WAS FINE.

10.FINALLY I FOUND THAT THE B-PHASE OF THE HV (33KV) CIRCUIT BREAKER WAS NOT PROPERLY CONNECTED.

11.I ALSO THOUGHT THAT DUE TO LOW UNBALANCED EARTH FAULT CURRENT GENERATED (IF ANY) DUE TO THIS IMPROPER MALE-FEMALE CONTACT OF THE 33KV CB, THERE WAS NOT ANY TRIPPING OF THIS CB.

12.ACCORDINGLY I ADJUSTED THE BREAKER CONTACTS TO OBTAIN A PROPER CONDUCTION.

13.FINALLY I CHECKED OUT THAT ALL THE THREE POLES OF THE CB WAS WORKING CORRECT AND CONTACTS WERE PERFECT.

14.THEN I CHARGED THE TRANSFORMER AGAIN WITHOUT CHARGING THE 11KV SIDE CB.

15.FOUND THE PROBLEM GOT SOLVED ASTONISHINGLY.

16.NO POWER WAS NOW SHOWN ON EITHER THE RELAY DISPLAY OR ON OTHER METERS.

17.HOW?????

 

Ashutosh... I understand your dilemma so you needn't have shouted!

You may not think so, but, thus far, your answers have proved helpful! However, I need two more pieces to solve the puzzle. So if you're up to the task, please answer the following questions (you have my word, they will be the last two):

(Q7) What is the percent impedance of the 5 MVA transformer?

(Q8) Can you confirm that the Phase A and Phase C line-current magnitudes were equal?

 

Dear Mr.Corso,

first of all I am really delighted to see you taking so much interest on this puzzle. thanks a lot..secondly my laptop's caps lock got unintentionally on..so i never tried shouting I'll be highly grateful if i could share your knowledge in days to come. Now coming to your questions; answers are:-

1.The %impedence is = 6.14%

2.During this incident I fortunately recorded the current in all the phases and they were:-

A-phase= 80Amps
B-phase= 15Amps
C-phase= 65Amps

The above data also specify that the B-phase(where CB contact was found improper) drawn lesser current. But how come at no load or secondary open condition the transformer drawing the above current?

 

Ashutosh... had you provided the info I asked for the answer would have been known a while ago.

However, because of my hang-up with anonymity, please contact me off-forum for the solution!

I want to assure you that no fee is involved. Furthermore, you are free to post any part, or not, of our meeting "face-to-face" (so to speak) to this forum!

Phil

 

Dear Phil,

I don't understand your point....could you please explain a bit more? how can i contact you off forum? and whats the need? why can't you answer my problem here....as you have been doing for last few weeks?

regds
Ashutosh

 

Ashutosh... repeating an earlier Caveat, "I no longer accept anonymous requests for information or help... something to do with the fact that anonymity breeds boorishness"

Therefore, anyone seeking advice or help from me must go the extra mile, i.e., provide me with some personal information. All I ask is that you provide your full name, your affiliation with a company (or school), and your location!

In return, I will send a resume of my qualifications and experience... an abridged or complete version!

Regards, Phil Corso (cepsicon [at] aol [dot] com)

 

Dear Phil!!

firstly i cant' decipher your e-mail!! secondly lemme give my details here....

1 ashutosh bhattacharjee
2. deputy manager(t&c)
3. Assam Elect.Grid Corporation Ltd.
4. Assam/India/Asia

anything more?? now if you cud please mail me your interpretation of the problem......i know you have the answer by now....take care

 

Ashutosh... yes, I have a theory.

My e-mail address is cepsicon [at] aol [dot] com

 

check ur mail please..........

 

Ashutosh... Don't worry, I have not forgotten you, nor your plight!

You have to learn to be patient! Now that you have finally produced the "key" item I had requested early in the investigation, I can proceed with final development of the Mathematical model representing the incident you observed!

BTW, thank you for providing your personal "stats" via Control.com.

Please note I assigned Project File # 1078 to your problem. It is used for tracking only! It is not used (as some think) to bill you for services rendered!

Finally, if you believe the solution I eventually will forward to you can be educational to others then by all means repeat it to Control.com!

Phil Corso

 

A) Conclusion
The electrical parameters observed, namely, high-MW (about 20% of capacity) and unity-PF, following energization of the transformer’s primary-winding [with its secondary-winding unloaded] was erroneous! [Note, paranthesis by author]

B) Incident History
A fault in an outgoing feeder connected to the 11kV bus powered by the transformer’s secondary-winding, resulted in de-energization of the transformer! [There are no details as to what the problem was or which protective device, if any, operated!] Normal procedure to restore power is to first ‘clear’ the Secondary bus by disconnecting [tripping] all secondary feeders, and then energize the transformer’s primary-winding (33kV!)

C) Given or Presumed Parameters
5.0MVA; 3-ph, 33.0kV/6.0kV, 50 Hz, DYn11, %Z = 6.14, X/R = 7.0

D) Observed Electrical Parameters
After some period after energization, operators and supervision "observed" the following electrical values, on two separate measuring instruments [none of which were recorders]:
P = 1.1MW; Q = 0.0MVAr(*); S = 1.1MVA(*); PF = 1.0(*); Line currents were 80A; 15A; 65A, for lines A, B, C, respectively. Note (*) denotes calculated from observed data!

E) Final Action by Plant Personnel
Concluding, somehow, the problem was related to the low B-phase current, the B-phase contact of the transformer’s supply circuit-breaker was realigned or adjusted. Upon energization, everything returned to normal.

F) Analysis Procedure
Mathematical models, representing three scenarios, were investigated:

a) Energization of an un-loaded transformer with only dissipation elements considered, namely, primary-winding resistance (secondary-winding losses ignored), and magnetic core losses. The calculation shows, however, that line-currents are nowhere near the observe magnitudes. Furthermore, the power factors are low, approaching that of typical open-circuit test parameters of power transformers.

b) B-phase line-current precludes single-phase operation. However, contact realignment suggests that the contact could have been arcing. Thus, this model is the same as scenario a) but representing the faulty B-phase contact as a resistive arc. However, even though, this model shows substantial increase in power value, the current-unbalance ratios cannot be corroborated with the observed line-currents. Nor is power-factor elevated anywhere near unity!

c) Energization of a partially-loaded transformer. This model includes secondary-winding impedance and losses! The partial-load represents the unknown problem with an 11kV feeder discussed {but never detailed] in Paragraph B) above!

G) Other Possibilities.
Because the power monitors are digital, and because most use the 2-Wattmeter method for calculation, they could react negatively to arcing-type current.

For additional detail, or if you are interested in a future course-of-action contact me!

Regards, Phil Corso (cepsicon [at] aol [dot] com)

 

Ashutosh... please add the following to Par F) subpar (b):

Furthermore, if the line currents were averaged, i.e., 53.3A, the power reading should have been about 3.0MW!

Phil C.