We have a medium voltage induction motor of following specifications installed at our Plantsite.

1400 kW, 6300 V, 3-Phase, 50 Hz, rated current 150.8 amperes, Insulation Class: F
Noraml Running current of the motor ranges form 145 to 154A.
and winding Temperature ranges from 100 to 120  degree Celsius.

01. The motor is being protected by a Muti-Function Motor protecrtion relay IMM7990 Make: CEE.
Settings of Protection Relay are:

49: 157.5A, 04 min
46: 78.75A
50: 1433.25A,0.1 sec
51LR: 1023.5A, 60 sec
51N: 6A, 0.1 sec

02. We have an additional secuirty on DCS which is:
Over Current Secuirty: 158A for 05 sec.
Under Current Secuirty: 92A for 03 sec

We are facing nuisance tripping of the motor on this DCS Over Current secuirty during power system transient conditions i.e. due to cuttin of some other MV motor or voltage dips in the system.

Please share your opinion that while having a Muti-Function Motor protecrtion relay IMM7990 installed on this motor(including thermal overload 49), Is this additional Overcurrent Secuirty on DCS is required or not?
Note: No other MV motor at our plant have this secuirty.

I think DCS over current security are for remote monitoring probably sending signals in your control room etc. You need to check the current variations in scenarios where motor gets tripped. Further you can change the settings in DCS over current relay to see the behavior.

Thanks for your comments.When motor trips we have seen current reaching 165A. But my opinion is that even at that value of current motor thermal relay should be sufficient to follow its curve and trip the breaker instead of tripping it from DCS at 158A for 05 sec OC setting.We can change the time delay setting for 05 sec to say 10 sec to accommodate system transient conditions which normally lasts for less than 10 sec to save tripping. But first I want to be sure that this will not affect our motor.Moreover, I have another opinion about this OC security that it is the security of pump rather than the motor, to protect it from overloading in normal conditions. But unfortunately no data is available about its significance in vendors manuals of both motor and pump.

maybe you can bypass or modify the settings of your DCS, make your DCS as monitoring of voltage and current ratings for your motor (get alarm and trip signal from the protection relays instead, so that your DCS can safely trip the unit like close/open valves, back-up oil motor running, etc.), but still can halt the operations by other parameters (pressure, flow, temperature & vibration)... most 51 relays are set to extremely inverse time curve settings, and the rest if the protection relay settings comply with the drive motor OEM requirements, then leave it as it is...

if you're worrying about the no-dwell zone/ condition of your pump-side then you can set your 50 relay to 500-700% of full load current (since your 50 settings at 1400A is quite far from the full-load current)... i guess other parameters like pressure, vibration, temperature, etc., once there is an electrical related variations/ transients, will safeguard your pump assembly... try to consult with your machine/ equipment manufacturers first before doing anything instead...

Motor protection should be from Motor protection relay only and not from DCS. DCS should monitor the motor current and temperature only.
In your case the DCS is acting as a definite time overcurrent relay and is causing nuisance tripping.A motor having a full load current of 150 amps can go upto 157.5 amps for 5 seconds without any concern.The DCS protection is not required unless your motor protection relay fails to perform.Even if you want to put additional backup protection from DCS then the setting of 157.5 for 5 sec is not correct.The definite time protection should be of high set nature with minimum time delay.There is nothing called overload of pumps but yes the undeloading of pumps is of concern.

I have seen the concept of a DCS trip set finer than the Motor Protection Relay before. Usually for these systems you only allow a Max of 3 resets per hour before allowing the MPR to trip the drive. The intent is to provide a system that enables a quick reset after overload conditions. It depends on how critical the drive is and the response time of getting a qualified person to push the rest button on the MPR. The other consideration is that if the MPR trips on I*I*T then you may have to wait up to 20 minutes for everything to cool down before a reset.

Have you had a look at the root cause of your problem and if there is any other way to manage the situation? For example is it possible to reduce the load on this drive whilst the other are being started. It could be built into the DCS code when a start request for the other drives is initiated.

In another factory where i worked we had to shut some sensitive drives down before energising a transformer with high inrush current or we would blow $12000 worth of fuses. The transformer inrush current caused a voltage sag similar to your situation.

Looking at your indicated Overcurrent Protection Settings, I noticed that there is some thing wrong with the OC Security Setting. see the comparison below:

Protection #49 setting: 157.5 amps at 04 minutes
Over Current Security at DCS; 158 amps at 05 seconds

The 157.5 amps setting at 04 minutes appears a valid / correct setting while OCS at DCS setting of 158 amps at 05 seconds is incorrect, because it is irrational to set the next OC setiing at a difference of 0.5 amps (158 - 157.5 = 0.5) to trip at a very fast time setting of 05 seconds. Momentary voltage dip in the system can create nuisance tripping on the motor overcurrent protection set at 05 seconds. Overload protection are normally set at 60 seconds or more.

we have 1800kW induction motor in our plant, maybe i will check the settings of our protection relays and DCS interlocks... since, electrical-related abnormalities will be triggered by protection relays (DCS overcurrent/ undercurrent tripping will act only as support, if in case), in which the trip signal will be sent to the DCS, together with the AI signal for monitoring... (but by-passed for 30sec during motor energization)...

May be there are some special protective requirements in the driven equipment that a conventional protection relay is not able to provide. It could be the driven equipment (pump or compressor) which has a very narrow window of safe operation.

I had experienced a similar case in liquid oxygen and nitrogen pumps, which must operate only at temperature around -180deg.C.

If you have a similar case like what I had experience, your only solution is to go back to the pump vendor and discuss the alternatives, eg, whether the definite time delay can be made longer, or using multi-stage time delays at different loading levels.

Motor protection Relay is fully capable to protect the motor it self so there is no need of DCS tripping. DCS don't understand block rotor rotor current(starting Current), instantaneous over current and IDMT.

You said you faced with nuisance tripping. At what stage of the operation does this happen?
Starting, normal running, picking up a load sudden or gradual? pls state exact before tripping.

I agree with Dinesh. Your MPR, if properly configured, will be fully capable of protecting the motor without any trips coming from the DCS. I have seen some plants have DCS trips based on motor winding temperature, but your MPR should be capable of that too, provided you have RTDs installed in the stator.
Also, it appears your existing protection settings in both the relay and the DCS are too conservative. Assuming this motor has a Service Factor of 1.0, your overload settings should be 15% higher than rated current, which in this case would be 173 amps.

your thermal protection Relay (49) setting is 157.5A for 04 min and DCS setting is 158A for 05 sec you can see easily the time setting is not matching with each other.please confirm the time setting of relay for motor from manufacturer and same setting you will use in DCS. Both should be operate in same time.otherwise you can remove the DCS setting

amen to Mr. Dinesh... We only get AI signal (motor current and voltage) and DI signal (alarm and trip signals) from MPRs for our induction and synchronous motors ranging from 1800kW and beyond... also thermal overload relay settings should be 115-125% of nameplate current, depending on the safety factor... DCS will monitor and control for other process parameters (pressure, temperature, level, vibration, flow. etc.)...