Slip Ring Motor Rotor Protection

[wrav001]

I am a regular surfer on this website, and found really it very interesting...

 

This is my first post and would appreciate your thoughts.

 

I did come across a 1 MW 3.3kV Slip ring motor with 4 speeds. The resistors are used to control the speed during start up. The cable distance between Rotor and Resistor bank is about 30m. Due to insulation degrading of Rotor to Resistors cable two phases leaked to the ground. Creating a imbalance with other phase. (making the resistors on that phase glowing) Luckily operator did turn it off manually avoiding further destruction.

We had a 30year old motor protection relay upstream (stator). Obviosly this didn't trip. Wasn’t surprised.

 

I belong to VSD generation with very limited knowlege on this sort of a issue. So my questions may be pretty obvious to some of the experts...

 

Question 1 : Do we require any sort of cable protection between Rotor and Resistors. i.e: Fuses,Breakers,advanced motor relay which picks up both rotor and rotor currents? I am bit confused with the NZ/Aus Standard which is valid for this part of the world.

 

Question 2 : My first reaction was if we had a good Motor Protection relay(on Stator side), it should have picked up the imbalace on Rotor Side. Is this correct?

 

Question 3 : I was considering putting a Motor Protection relay with (2 sets of independent CTs) one for stator and rotor. Is this a good idea?

 

Question 4 : This is a dumb one. I want to know how to calculate the current in rotor for a slip ring motor (different speeds) just for personal interest.