High Inertia Motor Start

[maypot]

We are in the process of installing one 3.8 MW 6.6 kV slip ring motor on our crusher. We have got in our plant one 15.0 MVA 6.6 k V Steam Turbo Generator Set. I know that the starting current of a slip ring motor is around 2 times the FLC and I trust the turbo set should be in a position to start the high inertia load. I am worried that the sudden inrush of current is too fast and the AVR could not respond to this rate of change of current.
Your comments please.

 

[RRaghunath]

Let the AVR be in Voltage regulation mode instead of usual power factor regulation.
Make the dead band in voltage regulator, if any, zero.

With the above in place, the generator should be able to respond fast and maintain the required voltage level even during motor starting.

The AVR response time is typically 0.5sec and not slow, especially the field forcing function of the generator that acts through compound CTs responds instantaneously to boost the field current and the delay is basically due to the time constant of the field windings (inductance doesn't allow instant change in current).

As a thumb rule, it is said a generator rated 3times the motor rating that requires to be started should not pose any problem. There is a FAQ in the forum on DG sizing which may interest you in this context.

 

[PowerfulStuff]

Another option is to control the starting resistances to reduce starting current. Most slipring motors that I have been involved with have had 1 x FLC starting current as this minimises the amount of overrating needed for the supply. This will of course increase start times and the thermal effects on the motor need to be taken into account.

It may even be possible to control the resistance to have a slow increase in start current from nothing - perfect for the generator!

 

[mc5w]

For this big of a motor I agree with Powerfulstuff that you should consider a starting resistance that ramps up motor starting current from say 1/4 of full load. The motor will not turn at all until starting current reaches say 1.5 time full load.

One way to implement this is to use a resistor that consists of three electrodes in a tank of salt water. You also need an auxiliary tank without electrodes. You energize the motor with the tank empty and them fill it up preferably by draining the auxiliary tank by gravity but a sump pump in the auxiliary tank will also work. You can control the rate of fill by attaching a bypass valve to the sump pump that allows some of the water back into the auxiliary tank. You will also need some float switches to make sure that the motor cannot energize with the electrode tank partly full. You will also need a second sump pump to pump back to the auxiliary tank.

Once the electrode tank has been full for specified period of time you would then use a contactor to bypass the electrodes. You can also use your second sump pump for soft stopping as well.