MV Motor Overvoltage Alarm and Trip settings

[adityalagudu]

One of our essential old motor is running on 4160V, but the Electromechanical relay is set at 4000V. The current overvoltage trip for the motor is at 1.1 X rated (i.e 4400V) and the alarm is 1.09 X rated (i.e 4360V). Recently the motor tripped due to Overvoltage. We are looking for options to keep this motor running safely. As this is an essential motor we want keep the Alarm settings and disable the trip setting. Or we would like to change the voltage set value to 4160V instead of 4000V, as it doesn't make any sense to keep the voltage as 4000V when the motor is running at 4160V. Would appreciate it if you could provide your comments

 

[waross]

First I would not accept the settings on the old relays as accurate.
Verify the calibration of the relays.
What you see may not be what you get.
How old is the motor?
What HP?
Induction motor?
What is the rated voltage of the motor?
How long has it been in service?
Does it have a history of overvoltage trips?
Has something changed in the plant or on the grid to raise the incidence of overvoltage events?
Have you considered dropping the taps on your supply transformer one step?
Has the integrity of the motor insulation been verified lately?

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

 

[Kiribanda]

Before any comments could you please confirm that,
1) Your motor rated voltage on its nameplate is 4000V?
2) Your system nominal voltage is 4160Volts of which the motor is connected?

 

[RRaghunath]

Overvoltage protection on motor suggests that the motor is synchronous type, connected through a dedicated transformer. The protection relay is elctro-mechanical type. This suggests that the motor is not new and probably in service for a couple of decades at least. If true, why is it tripping only now. In this I tend to agree with Mr. Bill's suggestion that the relay probably lost its calibration. Point to check.
Other points to check are
1) Is the motor being left unloaded for too long
2) Problem could be with Excitation current control - Under unloaded condition, PF mode of AVR doesn't work well. It shall be MVAR mode of automatic control or FCR mode.

 

[adityalagudu]

It is a 3 phase, 2300/4000V, Induction motor, 1250HP, FLA 279A. Please see the attached for motor nameplate details.

Our system's nominal voltage is 4160V. It has been in service for more than 15 years. The trip occurred due to overcurrent, not overvoltage, due to blockage in some valve, it draws more current.

We don't want to drop the taps on the supply transformer right now, one it is a manual operation and second, it will impact other loads. I don't have any info on the motor insulation testing recently.

The question is, could we still able to change the voltage settings on the relay to 4160V as the motor is rated for 4000V? Please help to provide your comments.

 

[waross]

In North America:
Motor voltages are multiples of 115 Volts,
System voltages are multiples of 120 Volts.
In the dim past, the multiple of 115 Volts was 2300 Volts.
The corresponding multiple of 120 Volts was 2400 Volts.
In the 40s and 50s, many 2400 Volt delta systems were upgraded to star systems.
The system voltages became 4156.9 Volts, rounded to 4160 Volts.
The 2300 Volts delta motors became 3983.7 Volts, rounded to 4000 Volts.
It is common and accepted practice to connect 4000 Volt rated motors to 4160 Volt systems in North America.
By the way, your full load Amps is 160 Amps.
I hope that your O/C settings reflect that current.
Nothing new here!
Have you checked your incoming voltage?
A possible cause may have been an upstream voltage regulator.
Depending on the stiffness of your supply system, the utility may have voltage regulators upstream, or On-Load-Tap-Changers on the supply transformer at the substation, possibly both.
If the 1250 HP motor is causing line voltage drops, the voltage regulators and tap changers will be ramping up the supply voltage to compensate for the line drops.
If the nature of the overload was such that it took some seconds to trip, then the voltage regulators and tap changers may have time to go up one more tap, possibly making the situation worse.
When the motor did trip, the reduced line current would result in less voltage drop in the distribution circuit.
This could easily cause an over-voltage trip, after the motor tripped off-line.
An over-voltage relay will often be energized when the motor is not, so as to prevent start attempts in the event that the voltage is too high.
You may consider inhibiting an over-voltage trip in the event of an over-load trip.
But I could be wrong.


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

 

[RRaghunath]

There is no way Induction motor can be responsible for over voltage! I don't understand why the poor fellow should be blamed / tripped??
Over voltage is due to upstream disturbance and will affect all the loads connected to the bus, not the motor alone. Why trip the motor? In fact, tripping the motor is going to worsen the overvoltage situation. Instead, focus should be to address whatever is responsible for overvoltage?
It is not that the motor insulation is weak and needs to be protected against overvoltages. The motor insulation is weak when it comes to steep-fronted voltage surges. But when it comes to temporary overvoltages, motor insulation is as good as any other in its withstand capability.
A few thoughts for you to review the overvoltage protection location / application.

 

[LionelHutz]

Why change the over-voltage trip or the transformer taps if the motor is not tripping on over-voltage?

 

[waross]

Hi Lionel. I understood from the first and subsequent post that both the overload and the over-voltage relays activated.

adityalagudu Can you clarify this for us?


RRaghunath Magnetic saturation due to overvoltage is not related to insulation quality.

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

 

[RRaghunath]

Saturation is an issue with transformers with iron core but not so much with motors as the flux linkage happens through air between stator & rotor. Correct me please if I am wrong.
I haven't come across over voltage protection exclusively for an induction motor.
Overfluxing protection is standard with all grid connected as well as generator connected transformers.

 

[waross]

I see that you are familiar with saturation, but not this aspect of saturation.
The same issues arise with the iron core of motors.
When changing frequencies of motors we must be aware of the V/Hz ratio of the motor.
Going over the V/Hz ratio may lead to rapid overheating and burn out.
You are familiar with the X/R ratio of transformers.
There is a similar ratio in motors; The ratio may change as the load increases but when a motor goes into saturation, further increases in the current will be limited by the resistance alone. Whatever the effective X/R ratio is, you may expect an increase in current of several hundred percent with a small rise in voltage.
Over-voltage protection on a large motor will often be in circuit at all times, regardless of whether the motor is running.
The purpose is to avoid connecting the motor to too high a line voltage.
Most industrial motors are designed to accept 10% over-voltage.
Much more over-voltage and things can get very hot, very fast.


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

 

[adityalagudu]

waross, RRaghunath

The trip is not due to overvoltage, it is due to overload only. Sorry for the confusion, I got myself clarified recently.

Currently, the transformers are at Tap-03, 12.5KV/4160V. According to motor data, it is rated for 2300V/4000V.

Currently, the voltage on GE multiline is set for 4000V and the current running voltage is 4260V.

Should I still go ahead and change the Overvoltage set limits on the GE relay based on 4160V as a base or should I keep the existing 4000V as a base?

See attached for your reference, help to provide your comments.

 

[waross]

Time for tough love.
Please forgive my lack of tack, BUT:
Your relay is doing exactly as it is intended to do.
The trip was not related to voltage.
Changing the voltage setting will not prevent future similar trips.
The solution is not with relays but with operating issues.
AVOID SUBJECTING THE MOTOR TO 498% OVERLOADS.
The relay saved your motor from a possible burn-out.

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

 

[Kiribanda]

1) From the last trip on August 28, 2020 it can be seen that the motor input voltage
had gone down to 3887V which is still with in the opearting range of +/-10% from
its rated name plate voltage of 4000V.
2) The reason for trip is an overload of nearly 5xFLA. So in my opinion the motor is
healthy and SR469 had done his job.
3) From the 469 setting file it is seen that the mechanical jam function of the relay
had not been enabled. It may be the reason that it had tripped by overload at 5xFLA
because max mechanical jam pick-up is 3xFLA.
4) From the motor data sheet the motor is having a NEMA LR code=H means it takes
8.8MVA at start @ the rated voltage of 4000V. Therefore, it is better to confirm
starting conditions too.