Motors of around 330kW in 11kV system

[RRaghunath]

330kW Motors rated at 6.6kV or 3.3kV probably cheaper than those rated 11kV. But in many cases clients may have only 11kV as next higher voltage rating after 415V in the plant and for obvious reasons would prefer to go for 11kV rated motor though the kW rating is low.

In case the switchgear is vacuum type, I think one of the problems could be the switching surges and probable motor failures caused by that.

Any experiences!

 

[jraef]

Switching surges will be no different, they are always a percentage (usually 600%) of FLA. In 11kV, your FLA will be roughly 1/2 that of 6.6kV, so electrically it is technically a little better from a current standpoint. Starting kVA will of course be the same no matter what your voltage is. The main cost factor will be in the higher cost of 11kV switchgear opposed to 6.6kV. They are a different class of components.

"Venditori de oleum-vipera non vigere excordis populi"

 

[aolalde]

jraef:

I think that the surges refered are voltage spikes developed when the contactor opens.
These voltage spikes kill the motor winding turn to turn insulation and are of a higher intensity as the voltage supply increases.

 

[aolalde]

If the conductor of a 4 kV winding has 0.006” of insulation, when the contactors are closed the full incoming voltage falls across the first turns of the winding coils. The peak voltage stress is 4000*SQRT (2)/SQRT (3)/.012” = 272,166 Volts/inch = 272.2 Volts/mil.
The above regular stress is increased several times (3 to 6) when the vacuum contactors are open (voltage spike). This could become 19.6 kV peak, increasing the conductor stress on the first turns to 1,633.2 Volts per mil.
For 11 kV machines the peak voltages increase at least proportional to the supply voltage, 11/4=2.75 times. More insulation is provided in order to handle those figures, but if air voids are present, Partial Discharge (PD) is developed, with the resulting erosion of the insulation and reduced insulation life.
Normally turn to turn insulation failures lead ground insulation failures of motor windings.

 

[RRaghunath]

I think the inherently low capacitance of low kW, high voltage rated motors is inadequate to reduce the steepness of the voltage surge and thus contributes to the terminal end insulation.

Anyone can please elaborate!

 

[hsj]

Now a days switchboards are built in three ranges:
-Upto 12 kV
-24 kV
-36 kV
Therefore price wise not much difference between 11 kV switchboard and 6.6 or 3.3 kV switchboard.

The difference is in motor price. 3.3 kV cheapest. If the cost of 3.3kV motor. But for one motor 3.3 kV option may be expensive considering cost of transformer plus 3.3 kV switchgear.

The other factor is not all manufacturers make 11 kV motors. Therefore pricing may not be competetive.

Facility to repair/service 11 kV motors may not be available in all places.

The switching surges are overvoltage due to chopping of inductive current of low magnitude such as motor no load current. A voltage of 2.25 or 2.5 times reated voltage can be generated by vacuum breakers. Surge arrestors are provided in the breaker panel(at cable terminations ) to overcome this problem.