Case Study - Can Programming in DCS can also lead to Tripping of HT Motors

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Case Study Can Programming in DCS can also lead to Tripping of HT Motors

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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 might be rare in Industries. Hence want to share the experience, so that the problem we faced must not be faced by others or can be avoided In a cement plant, there was a HT Motor of 6.6 kV 750 RPM which was used for Fan. A modification was planned to remove some breakdown which has happened due to malfunctioning of PLC. But during modification, they overlook one condition, which can be happened with anyone, as no one ever thought of such condition.

1st Question What is GRR? Ans: It is Grid Rotor Resistance, where 3 phase resistance is 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) 3rd Question Where it is C1 to C6 ?

Ans: C1, C2, C3, C4 are main power contactors, through combination of which resistance changes, C5 is Star Contactor and C6 is Delta Contactor. If C5 is on it means GRR is in Star and if C6 is on it means GRR is in Delta. Both C5 and C6 will never be on at same time

In GRR there is Local PLC, which controls the step of GRR, which works on the feedback of power and Auxiliary Contactors and Command from DCS and Local to increase and decrease resistance to Rotor, thereby controlling speed of fan.

Team decided to remove PLC and take all the DI, DO and feedback to DCS and make a program just like PLC in their DCS, so as to remove local PLC and reduce breakdown and malfunctioning.

Project was taken and was done during shutdown, every input and output was checked and configured and Just like PLC a programme was made in DCS which removed Local PLC. Now team decided to take trial of fan during shutdown, to ensure that everything is right.

Trial taken in offline mode, GRR was working fine every step as was correct. Now it was decided to take online trial and Motor started successfully. Current was normal, everything was normal. Now it was decided to take motor to full RPM to confirm everything. Step was increased and current was checked, but suddenly after one step Motor was tripped in over current.

What happened? Did motor Gone or did their modification failed. Team was looking at each other. They decided to check megger and healthiness of motor and start again. Motor started again normally but after that same step, Motor was tripped in over current.

Atleast they got that something is wrong after 8th step of GRR, as till 8th step motor runs fine as soon as GRR goes to 9th step, motor gets tripped.

Now the investigation started. GRR Resistance reading of every step and every phase was taken through micro-ohm meter.

But the resistance was balanced for each step and every phase. GRR Step is given below.

Actually this Fan PLC was creating some problem and was malfunctioning, due to which there was problem in increase and decreasing of speed and 2 times caused tripping of plant also, ultimately creating problem on the process side.

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This problem was not solved till 2 days. Both day trial taken 2 times and complete GRR and motor was checked.

Till 8th step of GRR, everything is fine and as soon as it goes ton 9th step Motor trips.

They asked in some other plants, one told them “increase the time delay between change of steps�.

On 3rd Day delay was given between changes of step of GRR. And to everyone surprise, it worked. Now the question was what time delay has done to GRR? Now we knew the problem was in delay. I looked again it into GRR 8th and 9th step and then realize what time delay has done.

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In 8th Step C1, C2, C3 and C5 Contactors were on i.e GRR was in Star. Now as the command comes to GRR to go to 9th Step, Instead of C3 contactor dropping first and then C4 Contactor picking up it was Picking up C4 Contactor first and then it was dropping C3 Contactor, due to which all the resistance were shorting momentarily and GRR was bypassed, which led to increase in Stator current and consequently tripping of Motor.

So the question was during Step change Contactor should drop first or Pick-up first.

It was great learning, a simple PLC logic was tripping our HT motor.

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