Rolling Mill Motor fed by transformers of different impedance

[prc]

In a steel rolling mill at finishing line a motor (S= 11.5 MW ,S9= 10 MW, stator= 3x 3.2 kV ,Rotor= 279 V ) is fed by three transformers of rating 4.1 MVA 33/3.3 KV three phase 50Hz Dy5 . 3.3 kV is converted to DC and then back to Ac at 3.2 KV to feed stator of motor. The three supplies after converter-inverter, feed to motor. My query why three supplies to motor ? Are they feed to motor in parallel?

In case one of the transformers impedance in different (say 7.5 % and others 11.5 %) any operational issues?Can the settings in converter/inverter can be adjusted to take care of the regulation differences?

 

[waross]

I don't quite understand the first part of your question. As to the transformer:
Assuming that the transformers have equal KVA ratings;
The transformer with the lower impedance will take a greater share of the current when they are connected in parallel.
If the transformers are not in parallel, then the transformer with the lowest impedance will have a greater available short circuit current.
Under normal operating conditions the transformer with the lowest impedance may have less voltage drop under load.
I say "may" because the PU impedance describes the action under short circuit conditions.
The regulation describes the action under normal loading at a specified power factor.
Under short circuit conditions the transformer impedance is the only impedance in the circuit. Under operating conditions the circuit consists of the transformer resistance, the transformer reactance, the load resistance and the load reactance. Transformer regulation (voltage drop under load) considers these factors at a specified load power factor.
Operating issues? If these transformers are feeding three phases of stator current, the motor will act as an induction generator and try to hold the phase voltages equal. The motor will draw more current from the transformer with the lower impedance. This will tend to create a greater voltage drop in that transformer. The amount of current will depend on the transformer impedance, the transformer X:R ratio, the motor impedance and power factor and the load on the motor.
A seat of the pants estimate:
There is 4% difference in the transformer impedances, The difference in regulation may be 4% or less.
A rule of thumb states that the current unbalance is the square of the voltage unbalance.
With a 4% difference in impedance there may be a 16% greater current in the phase fed from the lower impedance transformer.
Seat of the pants, 10% to 15% greater current in the transformer with the lower impedance.
This is based on the transformers full load current equal to the motor full load current. If the transformers are oversized or the motor is running at less than full load the current estimate will be proportionately less.
If the transformer secondaries are connected in delta a voltage unbalance will create a circulating current which will tend to balance the voltages and reduce the voltage and current differences at the motor.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[prc]

Waross, 4% regulation will come only with a full lagging power factor. We know that actual situation is not like that. Here the transformes are not in parallel Each transformer is connected to a converter-inverter set that feed 3 phase 3.2 kV to stator. I dont know how these 3x3.2 kV is connected to the stator.That is one of my question. Even if slight unbalanced, cant the AC_DC_AC tie be adjusted?

 

[waross]

I was under the impression that the regulation would be equal to the PU impedance when the load X;R ratio is equal to the transformer X:R ratio.
We are considering differences between PU impedances without knowledge of the respective X:R ratios of the transformers.
Hence my use of the somewhat imprecise statement; "The difference in regulation may be 4% or less."
Short answer:
If you feed a motor stator with unequal voltages the motor will try to correct the motor terminal voltages. The motor will transfer energy from the higher voltage phases to the lower voltage phases via the rotor. This will cause increased heating in the rotor and in the higher voltage phase windings. In your case the voltages will be almost equal at no load, and the difference will increase as the load increases.
If the motor is less than fully loaded or if the motor has a cycle of 'rest-full load-rest' there will probably not be an issue.
With some of the possible connections of a delta secondary transformer bank, some of the correction and some of the heating may occur in the transformer bank via circulating currents in the delta.
Again, this effect may not be severe enough to be an issue. If there have been no complaints of the motor running hot, I wouldn't worry.



Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[ScottyUK]

As these transformers are on the output side of the drive, I can see a couple of possibilities:

[li]the drive is a modular type with a number of paralleled modules to reach the required out level. Each transformer forms part of its associated module[/li]
[li]the transformers are necessary to keep energy levels within the capability of semiconductor switches used internally to the drive[/li]

Is the motor a standard type or does it have multiple stator windings, one for each output circuit? The latter is an unusual design but not impossible.

 

[prc]

Scooty, these transformers are on the input side of the drive controls. Motor input is separtaed from the out put of these transformers by individual drive modules. I thought will it be possible to adjust the drive module (AC_DC_AC)to take care of the small difference in regulation?

 

[ScottyUK]

Hi prc, sorry for my mis-understanding.

I wonder if similar logic could apply, i.e. a modular drive with discrete input transformers? The fault level available from a single large transformer may be too large for the equipment design? Yes, I'm sure that the separate modules could be configured to load-share.