Motor Generator

[crystalpalace]

Hello
I have a bit of a strange problem
In work we are running a 1MW MG set which starts OK with no problem, from 1 of 2, 1.5MW Diesel Generators
However a test we have to complete, is to trip the MG and then restart it during its run-down period
on a number of occasions within an hour

This has been dome before on other units with no problem

For some reason we get a number of trips of Phase Inbalance when we try and do this while the MG is running down
The Supply Breaker which feeds the DG with the power opens and says Phase Inbalance on the Reverse Power protection Unit
This happens on either DG

As it runs fine from a stopped position, it is hard to understand why starting with momentum should cause a problem but it does

The insulation may be dropping as it heats up, from over 100MOhm to maybe 2 MOhms
It doesn't trip every time

Any advice welcome, as its all a bit strange

 

[MatthewDB]

Is the motor an induction machine?
Perhaps it is residual flux in the rotor causing back EMF and closing in out of step?

 

[waross]

I agree with MatthewDB. Many engineers will use a timer to prevent a motor from starting when it is coasting. Applying power to a coasting motor may subject the motor to currents and stresses in excess of the design parameters. Applying power to a coasting motor before the back EMF has decayed has been known to cause serious damage including broken shafts.


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

 

[LionelHutz]

I'm not 100% following what you are doing. You're starting a MG (motor-generator) from a DG (diesel generator) and then you say the breaker feeding the DG with power trips. Isn't the DG the source of the power? Is line power also involved?

First thing to do is actually define what that phase imbalance means. Where is the measurement and how is the imbalance calculated. If the protection relay is setup wrong then it may be looking for a voltage or current imbalance when there is either no voltage or no current and then small error reading on each phase are appearing to be a large imbalance.