motor voltage dip problem

[Designer69]

we have an 8000HP 13.8kV motor which is currently under testing. At the test facility they are getting a voltage dip of about 12% so voltage gets down to 88% on start-up, full load connected.

However during our voltage study analysis here at the plant the voltage dip is projected to be about 20% with voltage dropping to 80% on start-up. The torque with 80% voltage available drops and comes real close to the required pump torque taking away any required margin and potentially causing stalling or inability to start.

has anyone here faced a similar problem and how was it resolved?

thanks

-the motor is started across the line btw.

 

[weressl]

we have an 8000HP 13.8kV motor which is currently under testing. At the test facility they are getting a voltage dip of about 12% so voltage gets down to 88% on start-up, full load connected.

However during our voltage study analysis here at the plant the voltage dip is projected to be about 20% with voltage dropping to 80% on start-up. The torque with 80% voltage available drops and comes real close to the required pump torque taking away any required margin and potentially causing stalling or inability to start.

has anyone here faced a similar problem and how was it resolved?

thanks

-the motor is started across the line btw.

Similar problems?

Not really.

We ususally do the loaodflow and transient motor study BEFORE procuring the motor, supply the data to the motor manufacturer, so the motor can be built to meet the system's capability.

There are scenarios where ties are closed to 'stiffen' the system for motor start. I heard of a cement mill in upstate NY, where NiMo needs to be called to do some switching before the mill starts up their 50MW motor.

 

[mull982]

we have an 8000HP 13.8kV motor which is currently under testing. At the test facility they are getting a voltage dip of about 12% so voltage gets down to 88% on start-up, full load connected.

However during our voltage study analysis here at the plant the voltage dip is projected to be about 20% with voltage dropping to 80% on start-up. The torque with 80% voltage available drops and comes real close to the required pump torque taking away any required margin and potentially causing stalling or inability to start.

has anyone here faced a similar problem and how was it resolved?

thanks

-the motor is started across the line btw.

At 80% voltage drop your starting torque will be aproximately 64% of avaliable torque at full voltage. It sounds like you are saying that this 64% torque may not be enough to start the load.

My initial thought is to start the motor with a drive which can provide a constant torque throughout the start but this is probably an expensive solution.

I'm sure others will have better suggestions.

 

[nollij]

At 80% voltage drop your starting torque will be aproximately 64% of avaliable torque at full voltage. It sounds like you are saying that this 64% torque may not be enough to start the load.

My initial thought is to start the motor with a drive which can provide a constant torque throughout the start but this is probably an expensive solution.

I'm sure others will have better suggestions.

Available options:
1. Stiffen the system (as already been mentioned)
2. Use an ASD (again, already mentioned)
3. Lower the process load during startup, ie. only 25% process so that the torque required of the motor is much lower and won't stall out.
4. Talk to the motor manufacturer about the load torque characteristics and your system short circuit ability at the motor (level of stiffness). Motor manufacturers usually require this information for large motors to determine the amount of bracing and material they use. They might be able to modify the motor to provide the necessary torque at the lower voltage.

 

[dkarst]

we have an 8000HP 13.8kV motor which is currently under testing. At the test facility they are getting a voltage dip of about 12% so voltage gets down to 88% on start-up, full load connected.

However during our voltage study analysis here at the plant the voltage dip is projected to be about 20% with voltage dropping to 80% on start-up. The torque with 80% voltage available drops and comes real close to the required pump torque taking away any required margin and potentially causing stalling or inability to start.

has anyone here faced a similar problem and how was it resolved?

thanks

-the motor is started across the line btw.

You already know the starting torque may be an issue and I'm sure you've modeled nearby loads and I hope they don't see the 20% drop or you may have more problems than the motor not starting.

There was an interesting problem with voltage drop caused with the extremely high reactive current draw on a linear motor at an amusement park that was documented here... http://www.powerlogic.com/pdf/cs39.pdf although this wasn't a MV motor.

Please keep posting on this... I'm interested to see how everything turns out.

 

[nollij]

Available options:
1. Stiffen the system (as already been mentioned)
2. Use an ASD (again, already mentioned)
3. Lower the process load during startup, ie. only 25% process so that the torque required of the motor is much lower and won't stall out.
4. Talk to the motor manufacturer about the load torque characteristics and your system short circuit ability at the motor (level of stiffness). Motor manufacturers usually require this information for large motors to determine the amount of bracing and material they use. They might be able to modify the motor to provide the necessary torque at the lower voltage.

5. You might be able to use a large capacitor to provide Reactive Power to reduce the voltage drop on the bus during starting.

 

[conmgt]

I'd like to see a pic of this monster.

 

[SG-1]

I was involved during the start up of a 5000 HP 5KV synchronous motor for a big ball mill used to make cement. There was a clutch between the motor & the mill that was engaged after the motor was synchronized. When the motor was winding up every light in the plant would dim & slowly come back.

 

[zog]

Another advantage to VFD's are the incentives by some utilities. Duke for example will pay you up to $100/HP for every VFD you install.

 

[weressl]

Another advantage to VFD's are the incentives by some utilities. Duke for example will pay you up to $100/HP for every VFD you install.

The cost of the ASD and the power loss associated with it would eat up that 'rebate' so fast that the accountants wouldn't have enough time to change diapers.

Not to mention the reliability degradation we just introduced with the ASD. What kind of cost will THAT incur?!

I am as much in favor of rebates as Government subsidies. You take someone else's money and play favorites. It is the subversion of free market. Supporting ideas and concepts that would NOT survive in a competitive market eventually degrades real value.

 

[zog]

The cost of the ASD and the power loss associated with it would eat up that 'rebate' so fast that the accountants wouldn't have enough time to change diapers.

Not to mention the reliability degradation we just introduced with the ASD. What kind of cost will THAT incur?!

I am as much in favor of rebates as Government subsidies. You take someone else's money and play favorites. It is the subversion of free market. Supporting ideas and concepts that would NOT survive in a competitive market eventually degrades real value.

Yeah, well the Duke chick that gave the presentation was hot.

 

[markstg]

Yeah, well the Duke chick that gave the presentation was hot.

So where do you sign up.

 

[dbuckley]

I'd like to see a pic of this monster.

Yes please. 8,000HP - that must be one mother of a pump!

And if there really is a problem with insufficient supply stiffness; have a chat to Pillar; a flywheel energy storage unit may provide enough of a jump start to get over the hump.

 

[Designer69]

pics as requested

 

[weressl]

Yes please. 8,000HP - that must be one mother of a pump!

And if there really is a problem with insufficient supply stiffness; have a chat to Pillar; a flywheel energy storage unit may provide enough of a jump start to get over the hump.

It is Piller, I believe. I am sure they would LOVE to hear from you, but when you are ready to hear the price you better be sitting down in an armchair. It would be much more cost effective to have a motor built to the existing power system.

 

[kingpb]

The largest motor I've had to deal with is 25,000Hp. We deal with 8000Hp sizes frequently.

You say the driven load is a pump? What is it pumping, i.e. water, oil?

Is the motor starting with valves closed? If it is water, the load torque required during starting is very low. Unlike a fan or belt conveyor which has high starting torque.

If the system voltage is 13.8kV, the motor rated voltage will be 13.2kV, already giving you a 5% margin. So, even though the system voltage drops to 80% of 13.8kV, the motor terminal voltage may only be around 85% at the motor. Assuming large cable between bus and motor.

An issue your going to encounter if the voltage drops too much, is that it will take longer then expected to start, which will cause the windings to heat up too fast, then your going to trip on thermal damage protection on the relay.

A cheaper alternative is to start the big motor with a 480V pony motor. What this does is it gets the big motor spinning at full speed prior to energizing the terminals, so the starting torque of the motor is nil. Don't know of your design could be modified for that, just throwing it out there.

You should get the starting curves for the motor and load, and do a dynamic motor starting analysis on your system. That is the only way to know before the rubber hits the road. If it's a problem, try sequencing your other loads so the system is lightly loaded when you go to start. If it cycles on and off, that may present additional issues.



An ASD for that will cost you dearly, +/- $800k. Not a good option.

 

[Designer69]

I wanted to come back to this post because I have two additional questions...


weressl, You say:

It is Piller, I believe. I am sure they would LOVE to hear from you, but when you are ready to hear the price you better be sitting down in an armchair

I was on their website and they seem to specilize in datacenter UPS systems so I wasn't sure if maybe there are better companies out there.


Also Kingpb ,

A cheaper alternative is to start the big motor with a 480V pony motor. What this does is it gets the big motor spinning at full speed prior to energizing the terminals, so the starting torque of the motor is nil. Don't know of your design could be modified for that, just throwing it out there.

this sounds very interesting, never heard of it before, can you provide some more info on it?

You should get the starting curves for the motor and load, and do a dynamic motor starting analysis on your system.

That is what we did using SKM's DAPPER module and it predicted the motor will not start in time.

Thanks for your help on this!

 

[dbuckley]

I was on their website and they seem to specilize in datacenter UPS systems so I wasn't sure if maybe there are better companies out there.

The reason for this is that datacentres are where most of their market is, as datacentres take power issues seriously. Only a small percentage of industrials need the sort of reliability of power that datacentres do; the sorts of industrials where if the plant stops it takes two months to get the plant running again due to damage caused by the outage.

Their core feature is their PowerBridge flywheel energy storage system, and that technology can be built into into any sort of power system, so providing voltage support to a system during startup load is something the PowerBridge can do. If Piller (and yes, its Piller, not Pillar, slip of the fingers earlier) don't have it off the shelf, then they can point you at someone who can build something using the PowerBridge.

weressl is right; it wont be a low cost option, and you better had be sitting down, but when other options don't work out, you know where to go.

There are other companies that manufacture flywheel energy storage systems, just one example is http://www.power-thru.com, google brings up more.

 

[BJ Conner]

Auto xfmr

Auto xfmr

The largest motor I've had to deal with is 25,000Hp. We deal with 8000Hp sizes frequently.

You say the driven load is a pump? What is it pumping, i.e. water, oil?

Is the motor starting with valves closed? If it is water, the load torque required during starting is very low. Unlike a fan or belt conveyor which has high starting torque.

If the system voltage is 13.8kV, the motor rated voltage will be 13.2kV, already giving you a 5% margin. So, even though the system voltage drops to 80% of 13.8kV, the motor terminal voltage may only be around 85% at the motor. Assuming large cable between bus and motor.

An issue your going to encounter if the voltage drops too much, is that it will take longer then expected to start, which will cause the windings to heat up too fast, then your going to trip on thermal damage protection on the relay.

A cheaper alternative is to start the big motor with a 480V pony motor. What this does is it gets the big motor spinning at full speed prior to energizing the terminals, so the starting torque of the motor is nil. Don't know of your design could be modified for that, just throwing it out there.

You should get the starting curves for the motor and load, and do a dynamic motor starting analysis on your system. That is the only way to know before the rubber hits the road. If it's a problem, try sequencing your other loads so the system is lightly loaded when you go to start. If it cycles on and off, that may present additional issues.

Pony motors can work without getting the motor up to speed. Just getting the rotor moving seem to help. The ones I have seen used rubber pneumatic tires running on a flywheel There was a lever arrangement to lift the tire and electric motor off the fly wheel. I haven't been able to find much information on the theory of pony motors. I think most are just made up by clever people.
An ASD for that will cost you dearly, +/- $800k. Not a good option.

What kind of switchgear ( starter ?) do you have. An autotransformer starter would be cheap, and may fit in the same switch gear you have now.

 

[RoberteFuhr]

Although it will be expensive, installing a Static VAR Compensator will solve your problem. One example is below.

http://www.converteam.com/majic/pag...atic-Var-Compensators--SVC----Converteam.html