Suitable VFD for MV wound rotor motors 3

[edison123]

For drive gurus

6.6 KV, 3200 KW, 340 Amps 1000 RPM, 50 Hz Motor. Rotor is 2700 V, 720 Amps. It is driving an induced draft (ID) fan.

Motor at present running at variable speeds by rotor resistance manipulation with a PWM slip power recovery system.

If we go for stator MV VFD operation, what would be the required MV drive capacity for this 3200 KW motor, and what would be the efficiency loss in the drive? Would it be more efficient than the present slip power recovery system?

Thanks in advance.

Muthu

http://www.edison.co.in

 

[jraef]

[ul]
[li]VFD would need to be rated for a minimum of 720A @6.6kV. Most MV drives this size are designed as "heavy duty", but an ID fan likely could be used with a Normal Duty rated drive if necessary. I say that because 720A @ 6.6kV is getting into rarified air, you will not have a lot of choices.[/li]
[li]VFD would need to have at least Sensorless Vector Control so that when the slip rings are shorted, you have sufficient control of the current to avoid over stressing the rotor windings.[/li]
[li]VFD will be roughly 96% efficient, but assuming the motor is NOT designed for inverter operation, the addition of an output filter to protect it will add a few more percent losses. Alternatively, you can try to select your MV drive technology to be one of the types that can be safely used with motors not designed to be used with inverters (Current Source-PWM or Cascaded H Bridge topologies). But again, your choices at this size range become limited.[/li]
[/ul]


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[FacEngrPE]

Slip power recovery scheme (SPRS) have a potential for high energy recovery, with an overall system efficiency better than inverter drive and motor. Of course realizing better efficiency is going to be in the details of the two drives applied to your specific motor.

https://www.tmeic.com/sites/default/files/assets/files/library/TMEIC_SEPT_ICR_low_1256736250.pdf

(Attached)

 

[edison123]

Thanks guys.

Agreed MV drive of that rating with filters and bells & whistles is a big cost and time driver. This can be justified if the VFD power saving is much better than SPRS, which obviously would cost less due to lower voltage and KW rating and is already in place and working, and the payback period is less than 3 years.

The head honcho of the unit thinks there will be a big power saving with MV VFD but SPRS guys ABB, TMEIC claim SPRS is better power saving. Hence, my question.

Advantages with VFD are much lower inrush current with unlimited starts, no brush gear to maintain, better speed control and wider speed range.

Muthu

http://www.edison.co.in

 

[LionelHutz]

There is power being lost if there is rotor resistance in the rotor circuit, which isn't clear if it's there or not in your description - "rotor resistance manipulation".

That would be a big drive though, so the payback might not be there.

 

[edison123]

Lionel

They start using the rotor resistors to bring up to rated speed whence the SPRS take over bypassing the resistors.

I agree it might not be cost-effective to change from SPRS to MV VFD.

I have asked them to provide the power saving data with their present SPRS to start with.

Muthu

http://www.edison.co.in

 

[Gr8blu]

edison - make sure you also get the comparable (drive and/or SPRS) efficiencies AT ALL THE EXPECTED STEADY-STATE OPERATING POINTS. A specific drive solution may have an efficiency in high 90s at a point corresponding to full load/speed, but may be considerably less if operating down at a much lower condition (due to the consideration that fixed losses in the system become a much larger chunk of the total).

Converting energy to motion for more than half a century

 

[jraef]

Energy savings on VFDs stem from eliminating losses created by mechanical flow control devices such as dampers. If you are using a WRIM with a slip energy recovery system, it is already a variable speed system, so there will be no energy savings by switching to a VFD. I wouldn’t do it.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[LionelHutz]

It's not worth switching if they are recovering the rotor energy back into the power system.

VFD would need to be rated for a minimum of 720A @6.6kV.

Why? It's a fan load.

 

[edison123]

gr8blu - Yes, the efficiency is not constant for the entire load range for any equipment.

Client is yet to get to back with SPRS data. At this point, MV VFD does not seem to be economically viable.

Lionel - I think Jeff misread rotor current of 720 A as stator current.

Muthu

http://www.edison.co.in

 

[jraef]

I did... sorry.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[FacEngrPE]

The point in time where a VFD conversion decision could be economically viable is the point in time when the SPRS unit must be replaced (technological obsolescence?). However this unit is large enough to be in the size range where some vendors offer technology updates which replace all of the components that are no longer available from the normal supply chain. Technology updates should be considerations when available.
The other reason to change is if operation outside of the speed range available from the SPRS is needed. For a power boiler application it seems unlikely that conditions could change that much.

 

[PersonalProfile]

Perhaps consider the suitability of the motor for use with VSD, despite that you have already etc:

Even though standard motors are commonly used successfully with PWM drives, drive duty places higher levels of dielectric
stress on the motor insulation than are normally present under sine wave duty. If the motor insulation is nearing the end of its useful life, connecting the motor to a PWM drive may lead to more rapid insulation failure.

Ex:

Effects of AC Drives on Motor Insulation
- Knocking Down the Standing Wave

https://library.e.abb.com/public/fec1a7b62d273351c12571b60056a0fd/voltstress.pdf

Regards.