MV Motor Overvoltage Alarm and Trip settings

[adityalagudu3291]

What should be the Overvoltage Alarm and Trip settings on the relays for the 4160V motor according to NEC standards?

 

[Jraef]

There are no “NEC standards” for things like that. Voltage tolerance for a particular motor would have to come from the motor manufacturer. There are NEMA design standards for motors that call for a +-10% voltage tolerance, but NEMA motor design standards don’t apply to MV motors. So again, the only ones who will care are the
Manufacturers.

 

[adityalagudu3291]

There are no “NEC standards” for things like that. Voltage tolerance for a particular motor would have to come from the motor manufacturer. There are NEMA design standards for motors that call for a +-10% voltage tolerance, but NEMA motor design standards don’t apply to MV motors. So again, the only ones who will care are the
Manufacturers.

Thank you Sir for the comments, this is helpful.

 

[paulengr]

There are no “NEC standards” for things like that. Voltage tolerance for a particular motor would have to come from the motor manufacturer. There are NEMA design standards for motors that call for a +-10% voltage tolerance, but NEMA motor design standards don’t apply to MV motors. So again, the only ones who will care are the
Manufacturers.

Yes they do. NEMA goes up to the edge of medium voltage. And IEEE Red book also gives 10% but that’s a nominal/continuous rating and extended ratings for conditions like starting. CBEMA gives time-voltage ratings. Usually if I set it at all I set this to 120%. I’m far less concerned with high voltage that is in general just going to overflux and trip the overloads than I am with undervoltage which causes low torque, stalling out, and consequently overheating the rotor.

One of the fundamental issues with large low voltage and medium voltage motors is protecting the rotor. In a small low voltage motor it is thermally limited by the stator. All we need is an overload relay that uses the proper curve (given by NEMA and IEC). All kinds of faults except short circuits and current unbalance will eventually trigger the overload relay. However medium voltage motors are thermally limited by the stator in high current cases like stalls, current unbalance, and undervoltage. We do not have a way to directly measure the condition (temperature) if the rotor. So we are limited to tripping on these conditions somewhat arbitrarily such as after 2-3 seconds, enough to avoid any startup conditions but quickly enough to protect the rotor. We also use a thermal model or arbitrary rules like 1 or 2 starts per hour to block excessive starts.

 

[adityalagudu3291]

Yes they do. NEMA goes up to the edge of medium voltage. And IEEE Red book also gives 10% but that’s a nominal/continuous rating and extended ratings for conditions like starting. CBEMA gives time-voltage ratings. Usually if I set it at all I set this to 120%. I’m far less concerned with high voltage that is in general just going to overflux and trip the overloads than I am with undervoltage which causes low torque, stalling out, and consequently overheating the rotor.

One of the fundamental issues with large low voltage and medium voltage motors is protecting the rotor. In a small low voltage motor it is thermally limited by the stator. All we need is an overload relay that uses the proper curve (given by NEMA and IEC). All kinds of faults except short circuits and current unbalance will eventually trigger the overload relay. However medium voltage motors are thermally limited by the stator in high current cases like stalls, current unbalance, and undervoltage. We do not have a way to directly measure the condition (temperature) if the rotor. So we are limited to tripping on these conditions somewhat arbitrarily such as after 2-3 seconds, enough to avoid any startup conditions but quickly enough to protect the rotor. We also use a thermal model or arbitrary rules like 1 or 2 starts per hour to block excessive starts.

Thanks for your comments.
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.

 

[paulengr]

Thanks for your comments.
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.

The utility standard voltage is 4160. To allow for voltage drop the utilization standard voltage is 4000 V. However a lot of motors are marked 4160 regardless of this. At 480 we almost instinctively expect transformers marked 480 and we call it “480” but nearly all motors are correctly marked 460. The standards haven’t changed but for some reason a lot more equipment is mid marked at 4160/4000.

As to your issue it is pretty rare to see a soft bus at higher voltage classes. Do you even know that the voltage is drifting and drifting that much? I have one customer where this happens but it’s a unique situation. They are located about 30 miles from a major substation in an extremely rural area. As the natural day/night usage changes the utility switches not just AVRs but ties to keep voltage stable. They are located essentially at one such tie and as such see a lot of transients. The solution was moving them from wye wye to delta wye (they were the utility ground!) and using multiple surge arresters to control it. We used 115% as a trip setting with a fairly long time delay 5-10 seconds to avoid transients.

I’d also point out that switching surges do exist and are significant with most MV motor starters...vacuum starters, especially older models. Newer ones use different metallurgy in the contact tips to combat it. Older ones used snubber circuits and surge arresters. If you are tripping on overvoltage it might be an indication that either you are getting internal or external transients.

Also why this matters is quite simple. A low voltage motor has significantly higher resistance to surges. As voltage goes up this margin disappears because we are working at the limits of insulation materials. This is why for instance shielded cables are mandatory. So you could be looking at a symptom, not an issue.

Install a power meter either temporary or permanent so you can see what is going on.

 

[adityalagudu3291]

The utility standard voltage is 4160. To allow for voltage drop the utilization standard voltage is 4000 V. However a lot of motors are marked 4160 regardless of this. At 480 we almost instinctively expect transformers marked 480 and we call it “480” but nearly all motors are correctly marked 460. The standards haven’t changed but for some reason a lot more equipment is mid marked at 4160/4000.

As to your issue it is pretty rare to see a soft bus at higher voltage classes. Do you even know that the voltage is drifting and drifting that much? I have one customer where this happens but it’s a unique situation. They are located about 30 miles from a major substation in an extremely rural area. As the natural day/night usage changes the utility switches not just AVRs but ties to keep voltage stable. They are located essentially at one such tie and as such see a lot of transients. The solution was moving them from wye wye to delta wye (they were the utility ground!) and using multiple surge arresters to control it. We used 115% as a trip setting with a fairly long time delay 5-10 seconds to avoid transients.

I’d also point out that switching surges do exist and are significant with most MV motor starters...vacuum starters, especially older models. Newer ones use different metallurgy in the contact tips to combat it. Older ones used snubber circuits and surge arresters. If you are tripping on overvoltage it might be an indication that either you are getting internal or external transients.

Also why this matters is quite simple. A low voltage motor has significantly higher resistance to surges. As voltage goes up this margin disappears because we are working at the limits of insulation materials. This is why for instance shielded cables are mandatory. So you could be looking at a symptom, not an issue.

Install a power meter either temporary or permanent so you can see what is going on.

Thanks for your comments.

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.

 

[paulengr]

Thanks for your comments.

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.

Nothing wrong with setting it higher. There is some voltage drop going to the motor. You can calculate (estimate) if you are curious. Typically you won’t see it at the starter because you are seeing voltage directly off the bus.

On MV motors there are few standards but EASA used to recommend hipot at 200% of name plate plus 1 kV, or 9 kV for your motor. Due to damage DC hipot is no longer done but surge tests still use the same voltage. Note that this is peak, not RMS. So either way you should have plenty of margin.

More concerning is this is only a 3% increase. That’s not enough to cause a trip. Better look for the root cause if you had overvoltage trips.

 

[adityalagudu3291]

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.

 

[paulengr]

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.

5% overvoltage is 4200 V. That’s typically the optimal range according to the NEMA charts and at just 60 V above it it’s not worth fixing. If you run right at name plate FLA continuously as some processes do one tap up drops it 2.5% putting you at 4153 V.

If you went to the point of adjusting the relay for voltage, may as well adjust FLA down too (derate). However again you are at the upper end of the optimal range, not outside the acceptable range (4800 V). It gives you slightly more starting torque and slightly less operating torque (thermal limit) but nothing I’d even worry about. Setting up tripping for stall and current or voltage unbalance is far more important than the average voltage. A 2% voltage unbalance will cause a 12-16% current unbalance which definitely should cause a trip because you need to derate by about 30-50% to operate like that.