Modification work in GRR (Grid Rotor Resistance) which can reduce spare cost & unnecessary breakdown

Hello Friends / Sirs

Today I am going to present a Case of GRR (Grid Rotor Resistance) which is used in Slip ring induction motor, which is more common in cement plants and few process plants. Hence want to share the experience, so the good modification which was done in one or few plants can be incorporated by other plants.

1st Question What is GRR?

Ans: It is Grid Rotor Resistance, where 3 phase resistances are changed on basis of combination of power contactors.

2nd Question Where it is used?

Ans: It is used in Slip ring induction motor, where motor speed needs to be controlled (Mostly in Fans, Fan speed depends on Process requirement and Air flow required in a system).

In a cement plant, there was a HT Motor of 6.6 kV 750 RPM which was used for Bag House Fan. For controlling the speed of motor there was GRR with its Control panel, control panel has a PLC which control the step of GRR, in turn controlling speed of motor.

PLC normally work on input from control room and feedback from power & auxiliary contactors in GRR. Normally a GRR is placed in field, in the dusty environment. Also, temperature of Rotor resistance is on higher side (around 150-200 Deg), increasing the ambient temperature. Now due to dusty environment and higher ambient temperature, there was malfunctioning happening 2-3 time every year, sometime PLC got hanged, hindering operation of GRR and causing tripping of motor and hence plant, sometime getting not ready and causing tripping. So, a modification was planned to remove the breakdown which has happened due to malfunctioning of PLC.

We decided to remove PLC and incorporate same logic as PLC in DCS. For that just Instrument control cable must be laid from GRR control panel to I/O room and logic must be formed in DCS just like PLC, which we done in shutdown. Also, in last 6 years 3 PLC have failed causing high downtime and breakdown of plants and the cost of single PLC is approx. 1.5 To 2 lacs. Also, in plant there were 2 types of PLC one of 26 steps and other of 29 steps, so high spare cost, much higher breakdown cost and PLC company monopoly. That’s why we planned for such modification, keeping an option open, what if modification failed, but we got success.

Also, during modification, we overlook one condition, which can be happened with anyone, as no one ever thought of such condition. That’s a different case study which “Can Programming in DCS also lead to Tripping of HT Motors” which will be follow immediately after this case study.

The modification was successful and our spare cost for PLC and breakdown cost due to PLC is reduced