Unbalance load condition effect on Transformer

[LalitKumar]

In our operation at factory, we have total six rings of load(heating elements) fed by a 5MVA transformer, DY5 33kv/500-2500V (5tapes). Each secondary phase is connected to equal load of heatingelement rings through thyristor and we recently carried out a thyristor firing program change that allows even one ring in operation and supporting the production. Earlier one ring stops due to any genuine interlock/reason caused all the rings down - loosing production.

Now after program change, sometimes one ring trips and loses power and then we have to re-start it within15-20minute.

Recently we have undergone one phase failure of one such transformer under operation. I believe this type of switching program change which allows other phase ON, may have caused neutral shift and increasing high current in remaining phases for say 10-15 minutes and thus this transformer burnt. Even after re-startup we graduallyincrease the current at faster rate to reach up to the last ampere value beforetripping. So unbalance situation lies roughly 3-4 hours.


How much unbalance typically a transformer is allowed which supposed to run at equal loading ? our relay at upstream 33kV side is set at 2times of FLC for 51N.

 

[mivey]

A transformer can be 100% unbalanced if sized correctly.

Do you have a nameplate diagram and something showing the connections/grounding as well as load sizes and connections?

 

[LalitKumar]

Unbalance load condition effect on Transformer

A transformer can be 100% unbalanced if sized correctly.

Do you have a nameplate diagram and something showing the connections/grounding as well as load sizes and connections?

attached here pics

 

[Sahib]

Lalitkumar:
You said transformer burnt due to high current. High current happened on primary side or secondary side? Any damage to heater banks? Transformer neutral grounded or not?

 

[Sahib]

One possibility for transformer burnout is harmonics from thyristor switching. Check whether THD on transformer is within limit or whether the transformer is K-rated.

 

[junkhound]

Yep, probably triplets.

OP did say DY (delta -WYE)

Thyristor angle changes likely producing much higher triplett harmonics (3rd, 9th) which, due to being IN PHASE on the delta side, are a short circuit.
Thus, all the triplett harmonic currents are running short circuited in the primary = overheat and burnout if high enough harmonics.

 

[mivey]

LalitKumar,

Please interpret the nameplate and what it means about how the loads are connected. Are they mutiple windings in parallel or separate or what?

For phase R (and the same question for Y and B phase), if MR1 and OR1 are together why are we seeing two different voltage readings (559V and 624V)?

How are we getting 650V if we are at the 585V tap?

What are the load readings about? Take IR for example: 470 A at 555 V is 452 kVA but the reading is 68.2 kW. That is a 15.1% power factor. If these are heating elements I would expect near unity. Why such large distortion? If IR is the first stage why is it not almost continuously on? What did I miss?

Maybe I'm being slow today but I'm not picturing the wiring diagram.

Is this all saying we have about 746A, 901A, and 631A on the different phases with a 1600A capacity?

Is it the IR load by itself at 470A the case that is causing issues?

 

[LalitKumar]

Unbalance load condition effect on Transformer

LalitKumar,

Please interpret the nameplate and what it means about how the loads are connected. Are they mutiple windings in parallel or separate or what?

For phase R (and the same question for Y and B phase), if MR1 and OR1 are together why are we seeing two different voltage readings (559V and 624V)?

How are we getting 650V if we are at the 585V tap?

What are the load readings about? Take IR for example: 470 A at 555 V is 452 kVA but the reading is 68.2 kW. That is a 15.1% power factor. If these are heating elements I would expect near unity. Why such large distortion? If IR is the first stage why is it not almost continuously on? What did I miss?

Maybe I'm being slow today but I'm not picturing the wiring diagram.

Is this all saying we have about 746A, 901A, and 631A on the different phases with a 1600A capacity?

Is it the IR load by itself at 470A the case that is causing issues?

Please interpret the nameplate and what it means about how the loads are connected. Are they mutiple windings in parallel or separate or what?
Each phase feeding two different load banks (Rings) by having neutral link in between.

For phase R (and the same question for Y and B phase), if MR1 and OR1 are together why are we seeing two different voltage readings (559V and 624V)?
Each load bank current is controlled by a multistep controller, can feed different current and can accept different set point.

How are we getting 650V if we are at the 585V tap
We can go as high as 2500V by MSC setpoint

What are the load readings about? Take IR for example: 470 A at 555 V is 452 kVA but the reading is 68.2 kW. That is a 15.1% power factor. If these are heating elements I would expect near unity. Why such large distortion? If IR is the first stage why is it not almost continuously on? What did I miss?
Yes , you are right. nothing missed. its a wrong kW reading, needs correction. PF is unity.

Maybe I'm being slow today but I'm not picturing the wiring diagram.


Is this all saying we have about 746A, 901A, and 631A on the different phases with a 1600A capacity?
Current can be fed as high as 3000A at low voltage , different rings and different set point. Thats' what my query is how much max %unbalance we can go in this case.

Is it the IR load by itself at 470A the case that is causing issues?
Current can be fed as high as 3000A at low voltage , different rings and different set point. Thats' what my query is how much max %unbalance we can go in this case.

 

[LalitKumar]

Unbalance load condition effect on Transformer

Lalitkumar:
You said transformer burnt due to high current. High current happened on primary side or secondary side? Any damage to heater banks? Transformer neutral grounded or not?

I suspect the high current caused by load unbalance and neutral shift , neutral is grounded with NGR. NGR OK. heater bank ok.

 

[mivey]

I'm still not clear on your setup but you can be 100% unbalanced as long as you do not overload the single winding.

Some transformers will handle single-winding load beyond their rated capacity. Some do not like unbalanced loading below winding capacity (delta-delta?).

I do not remember a restriction on up to rated winding load on a delta-wye but would have to look to be positive when I get back to my office. Not something I think about every day.

 

[mivey]

I suspect the high current caused by load unbalance and neutral shift , neutral is grounded with NGR. NGR OK. heater bank ok.

High or low impedance NGR?

 

[LalitKumar]

Unbalance load condition effect on Transformer

One possibility for transformer burnout is harmonics from thyristor switching. Check whether THD on transformer is within limit or whether the transformer is K-rated.

THD is 3.15% and below 5%, transformer is not K rated. However Delta side mitigate triplet. Still cannt deny hand of harmonics as it makes one of the probable cause. More or less transformer is experiencing load unbalance as normal operational process.

Do you have any reference to let me find more info in this regard. Our transformer is Vacuum-cast dry-type resin-encapsulated transformers manufactured in accordance with VDE 0532, Part 76-11, IEC 60076-11, and DIN 42523.

 

[LalitKumar]

High or low impedance NGR?

High

 

[LalitKumar]

Unbalance load condition effect on Transformer

I'm still not clear on your setup but you can be 100% unbalanced as long as you do not overload the single winding.

Some transformers will handle single-winding load beyond their rated capacity. Some do not like unbalanced loading below winding capacity (delta-delta?).

I do not remember a restriction on up to rated winding load on a delta-wye but would have to look to be positive when I get back to my office. Not something I think about every day.

Thats' great of you. Just because of loading values v/s full load rating, I allowed individual control and unbalance of load. well I gave few probable causes for this incident:1) manufacturing defect - dry soldering of neutral tap taken right from the middle where exact V phase gave me sign of overheating. local flashover caused nearby primary winding shorted and caused Ig tripping which is set at 2times of FLC (150/1)=300A. too high but it tripped on Ig. The intensity of flashover is severe. Whole room has carbon black smoke, still sometimes carbon remains there on wall. 2) load unbalance 3) Harmonic current but then it should affect other transformer running last ten days with no any issue, it tells me point No.1 most probable cause 4) foreign particle - cannt go it is such a rugged cast resin insulated, even a lizard would go nothing will happen.

 

[mivey]

You should be able to fully load a single winding on a delta-wye unit. You could actually overload it since the other two windings will share about a third of the load but there is no guarantee the leads will like it.

If you are not overloading the windings, something else is wrong. I don't see unbalance causing a problem. Do you know the core type?

Adequate ventilation? Ambient temperature okay?

 

[Sahib]

THD is 3.15% and below 5%, transformer is not K rated. However Delta side mitigate triplet. Still cannt deny hand of harmonics as it makes one of the probable cause. More or less transformer is experiencing load unbalance as normal operational process.

Do you have any reference to let me find more info in this regard. Our transformer is Vacuum-cast dry-type resin-encapsulated transformers manufactured in accordance with VDE 0532, Part 76-11, IEC 60076-11, and DIN 42523.

When harmonics is present, the transformer neutral is likely to be overloaded. With your transformer loaded to a level which caused earlier burnout, check with a true RMS measuring meter whether neutral current is more than phase current or not.