Abnormal sound from no load transformer

[asgkrishnan]

Hi All,
I face a strange problem in the converter transformers connected to 11 KV sub station. There is abnormal variation in humming sound of the Transformer 11KV/520 V, 2.25 KVA Dd0 feeding to converter/ inverter drive for rolling mill stand motors. Another transformer 11KV/520V, 4.5 KVA Dd0y11 in the same station behaves similar and the humming sound varies at different times. These transformers are in no-load condition only as the mill is not started yet. There is no variation in either the voltages or currents at 11 KV sub-station (Source to these transformers)

On detailed observation it is found that there is fluctuation in leakage current between 0.25 to 1.0 mA (through ammeter provided with surge monitor ) and when leakage current increase, the sound also increases. Other wise at normal sound, ammeter indicates 0.25 mA continuously. These transformers are of cast resin dry type.

Please suggest the reason for this strange behavior in humming sound of these Converter Transformers. What could be the probable cause for it?
(Kindly note that transformers are not loaded; only outgoing cables are in charged up to converter panel)
Also please tell me why these surge monitor ammeter indicate 0.25 mA at normal sound. I feel that ammeter should read zero .

Regards

 

[Knobhead]

You need to explain more fully what these transformers are doing. IE one per stand / motor? Stand motors are beasts of things and the control are as equally beefy.

Harmonics going through the idle drive are the most likely cause of the noise.

The transformers and drives should have been matched at the design stage. Yes I’ve come across situations like this before but there’s very little you can do about it.

 

[Engineer54]

Hi All,
I face a strange problem in the converter transformers connected to 11 KV sub station. There is abnormal variation in humming sound of the Transformer 11KV/520 V, 2.25 KVA Dd0 feeding to converter/ inverter drive for rolling mill stand motors. Another transformer 11KV/520V, 4.5 KVA Dd0y11 in the same station behaves similar and the humming sound varies at different times. These transformers are in no-load condition only as the mill is not started yet. There is no variation in either the voltages or currents at 11 KV sub-station (Source to these transformers)

On detailed observation it is found that there is fluctuation in leakage current between 0.25 to 1.0 mA (through ammeter provided with surge monitor ) and when leakage current increase, the sound also increases. Other wise at normal sound, ammeter indicates 0.25 mA continuously. These transformers are of cast resin dry type.

Please suggest the reason for this strange behavior in humming sound of these Converter Transformers. What could be the probable cause for it?
(Kindly note that transformers are not loaded; only outgoing cables are in charged up to converter panel)
Also please tell me why these surge monitor ammeter indicate 0.25 mA at normal sound. I feel that ammeter should read zero .

Regards

Cast Resin type transformers are i'm afraid inherently noisy beasts at the best of times, far noisier than oil filled, which muffles the effect of winding hum and even core lamination hum to a good degree.

I don't have much experience with these smaller TX's, so Tony is your best man to follow on this one...

 

[Knobhead]

Why do transformers hum….
Because they’re happy!

 

[Knobhead]

I’ve never dealt with dry resin cast transformers so E54 has a lead on me there. I just noticed that you say the drives haven’t run at load yet. You also don’t say if they are DC drives, which I would expect for a stand motor.

I also assume you meant 2.25MVA Not KVA

For those that don’t understand what a stand motor is look up “cogging mill” on Google. They are absolute beasts of things.

OK this is a very old picture of a mill motor (and before any of you bar stewards say anything it’s not me).
View attachment 9983

 

[johnboy6083]

I dont think many people will have much experience with cast resin transformers, other trhan maybe to install them, because as far as im aware they are maintenance free. A company i used to work for was primarily a panel building copmpany, and used to specuialise in LUL work. most of there big DB's would have an 11Kv cast resin tranny feeding a .44Kv ACB, and then onto the smaller switches. Im not even sure if they have thermistors in them.

 

[Engineer54]

I dont think many people will have much experience with cast resin transformers, other trhan maybe to install them, because as far as im aware they are maintenance free. A company i used to work for was primarily a panel building copmpany, and used to specuialise in LUL work. most of there big DB's would have an 11Kv cast resin tranny feeding a .44Kv ACB, and then onto the smaller switches. Im not even sure if they have thermistors in them.

You can specify all sorts of monitoring to be included in these Cast Resin TX's , especially on the 1 MVA and above distribution TX's, not that thay go too much further up than that.
Depending on where and how they are positioned, many need forced air cooling when ambient and therefore internal temperatures rise above certain levels...

I personally prefer the tried, tested and proved traditional oil filled TX's for main distribution work...

 

[Knobhead]

Right on with you there E54, tried and tested. I’ve only ever had one oil filled MV Tx fail in 30+ years and that was a brand new unit not fitted correctly.

 

[asgkrishnan]

These transformers are feeding to ABB converters/inverter PWM drives to 690VAC squirrel cage motors of 700/900/1000 KW capacity. One transformer feed to one line up drive (feeding to 4 stand motors).The problem posed is not due to load as these transformers are not loaded yet. So no question of harmonics..

 

[asgkrishnan]

Sorry, transformer capacity is in MVA only..and motors are 690VAC motors driven by PWM drives. These drives are still to be made ready, presently only transformer with cable up to converter/rectifier section is charged.

 

[Engineer54]

Where are you in the world Asgkrishnan??

I can only tell you that Cast Resin TXs are very much noisier than oil TXs as any humming, or core lamination vibrations are amplified by the solid Cast Resin they are sealed in. We have in the past had to replace a 500KVA Dry Cast Resin TX because of the high noise was just not acceptable being close to a hospitals out patients clinic... And they get noisier the larger the load is being drawn, so it's not going to get any better than it is now, without any loading!! lol!!!

 

[Knobhead]

Asgkrishnan these have got to be beasts of things, I’ve not been near one since I was an apprentice 30+ years ago. One thing that sticks in my mind is when a fresh “bloom” came along the motor would stall, then get it’s shoulder in to the job and maul the bloom through the stand. This is why I’m surprised they are squirel cage and not DC.
Even if I can’t help you would you be good enough to send me more infofmation about them? They sound fantastic bits of kit.