Generator and VSD motor load 2

[bigamp]

A plant is to have a single high capacity power supply from the grid and this is to run in parallel (normally) with an on-site generator (part of a co-gen plant). The generator is slated to be a 12.5MVA 11kV machine, size determined by waste heat requirements. The electrical load is to be about 8MW, of which approx 4MW will be a single VSD motor. The VSD will most probably be a 12-pulse type. Power to the VSD input transformer will be at 11kV, the VSD and motor will be 3.3kV. The generator will be required to be able to operate in island mode. What thoughts re the generator on its own being able to cope such a relatively large (4MW) VSD. When we issue specifications for the generator, I will certainly identify load types etc. We may need to consider harmonic filtering and it may be an idea to use a 24-pulse VSD. I would like some thoughts please before I approach generator vendors for info. The 4MW VSD sounds a little big for a 12.5MVA machine, are there any rules of thumb.

Regards,

bigamp

 

[electricuwe]

I guess there are at least two things to be considered:

(The following issues apply if you use a voltage source VSD. With a current source VSD or a cycloconverter are more difficult.)

Harmonics: A study should be performed in the same way as for line operation of the equipment. The most relevant value is the available short circuit power at the PCC which will be the genereator in the island operation mode. Compared to operation in parallel mode or to normal operating conditions for such a drive short circuit power will be much lower since the reactance Xd of the generator is usually much higher than the impedance of a transformer of similar rating.

Dynamic issues:
A VSD may be a load changing very rapidly (Think of the drive of a rolling mill, when a large beam enters the rolling mill) It might be necessary to check the dynamic operation of the system if the drive is operated with such a load. It might be necessary to use a special control on the generator or to limit the dynamic performance of the drive to a value the generator can cope with.

 

[bigamp]

Thanks for the reply electricuwe. The application is a centrifugal type compressor on gas service (sorry should have mentioned this). We do not expect rapidly changing load. The VSD may end up being an ABB ACS1000 series, they do make them that big. We will do a harmonics study.

Regards,

bigamp

 

[MqPower]

Bigamp

I have done numerous generator/VSD applications in the past and would just like to point out that be sure the generator manufacturer and alternator manufacturer are in harmony with each other over you application. That all components within the generator package are designed for your specific use and that all warranties apply from those component suppliers to the generator builder. If they are worth a salt they should be able to run a computer model as well as work closely with ABB and/or alternator supplier to insure that this is done.

 

[tomatge]

Bigamp,
When I first read your thread, my immediate response was that the generator would be too small to cope with such a large variable load in island mode, without resultant large changes in frequency and voltage.
I'd be interested to know your final conclusions.

 

[bigamp]

Thanks MgPower, that's good advice. We will certainly provide details of our VSD load to generating plant vendors and ensure that they are aware of our requirements. The bit about warranties is particularly valid.

Have any of your generator/VSD installations included a single VSD load as a similar % of total generator capacity.

Thanks tomatge, those immediate thoughts are the ones to listen to. I also have a feeling that the VSD is uncomfortably big for the generator. Starting should not be a problem as the VSD can do a soft ramp which the AVR & governor ought to be able to keep up with. I think that the harmonics also can be managed, say by going to 24-pulse VSD input and with filtering. The worst situation looks to be when there is a sudden VSD trip. This will subject the generator to a 40% load reject. I am gathering data at present and will do a transient stability study to check out possible responses.

Regards, bigamp

 

[MqPower]

Bigamp-

Yes, most of the applications I am involved with has the generator running one or multiple electric motors with VSD(s) that operate PC oil pumps in the field. Over the last few years there has been a push to use diesel and/or nat gas gensets to pump the oil versus installing utility power. The application is to turn the pump slowly with max torque similar to what has been accomplished with hydraulic packs in the past.
My comments on warranty situations is that some alternator manufacturers have policies in place to address use of their product in conjunction with a VSD, i.e. PMG's, oversizing, etc.
It just takes a little leg work to make sure that all the components are sized properly and will work in harmony with each other over the long haul as well as the short term.
Regards,
MQPOWER

 

[bigamp]

Thanks MQPOWER, you have been very helpful, much appreciated. Sorry about spelling your handle wrong in my previous reply.

Regards

 

[TheDOG]

Have you thought about a PWM front end for the VSD? This will almost eleminate all harmonic distortion. I think Allen Bradley might be able to help you with their new Powerflex 7000 series, which meets IEEE 519.

Good luck and keep us posted!

 

[busbar]

Is it best to write the performance spec tasking the generator design to correctly power the drive, or should the drive be specified to operate properly served from the generator? Ultimately, one vendor has to have the ultimate responsibility for suitable operation. Probably to a lesser degree than in the past, (non-linear) power electronics can cause nonobvious interactions with generator excitation systems.

On a smaller scale, smooth operation of the genset—UPS interconnection is not always automatic or necessarily assured without the two vendors in close communication.

Carefully written specs are needed for source and load, to minimize finger pointing.

 

[NavyPower]

Bigamp,

All above concerns are valid & should be taken into consideration in your design but rest assured it is possible to make your system work. US Navy has successfully tested a 19 MW PWM VSD (with 6-pulse front end) driven by a 21 MW gas turbine generator in island mode as you would expect aboard a ship. 6-pulse rectifier is hard to do as most synchronous generators won't handle the harmonics (we required a 5th harmonic trap filter), common mode currents & grounding are also tricky but most manufacturers can accomodate 12 pulse harmonics. I would recommend contracting with one company to do a turnkey system -- that way you don't end up holding the bag if one of the components is improperly specified. Most of the big-name power system companies (ABB, Siemens, Alstom) are willing to do this and have experience with integrated electric ship propulsion which is not too different from your application.

 

[electricuwe]

To TheDog:

Suggesting a pwm-frontend is easy, but are you aware of the additional cost and reduced efficiency ?

I have seen only very few applications where a pwm-frontend was applied without the possibility of regenerative breaking.

 

[bigamp]

To NavyPower

Many thanks for the reply and the good advice re going for a turn-key type approach. The installation you refer too sounds awesome, it goes to show just what can be done.

By the sound of it, the generator may have been dedicated to the VSD installation, was that the case, or did the generator supply other load as well? (my generator will need to supply other load). Also, how does the system coped with a full load rejection?

Nice to see electric drives being applied as ship propulsion systems. Recently on another website there was a really good thread re the turbo-electric drive propulsion systems that were used on a number of US battleships and the aircraft carriers Lexington and Saratoga in the 1910's and 1920's. Absolutely awesome systems, and not a microprocessor or semiconductor in sight!!

Regards

 

[NavyPower]

Bigamp,

Yes, system is never paralleled with grid. All other loads are rectifier-type (i.e. constant power) and full load rejection has been successfully accomplished but required much tuning of the various controllers (GT engine, AVR, VSD & switchgear setpoints).

I agree with electricuwe that PWM front end can be VERY expensive, especially at these power levels.

Good luck!

 

[TheDOG]

Electricuwe;

Yes the suggestion of an AFE is an easy one, because it will work! An AFE will be more expensive (I think you will be suprised by how little it adds to the total price), but you will not have to oversize the Alternator nor worry about AVR response (harmonics).
The two main reasons an AFE would be used anywhere would be for regenerative braking and Harmonic reduction. At the moment the AFE is not used very widely, but in time it will be - it is the best solution for ANY application. Whether the cost is warranted is another question...

 

[bigamp]

The DOG,

I agree with you about AFE. With harmonics, prevention is better than cure. I have had a good look at AFE's a couple of times in the past and I think they are a really neat concept. Four years ago, we came very close to using a 690V AFE for a 630kW VSD application but ended up going 3.3kV MV 12-pulse instead. The 3.3kV 12-pulse cost more than the 690V AFE (no surprise being MV) but it worked OK.

When comparing AFE's to a 6-pulse in a harmonics sensitive installation, it is also necessary to add the cost of harmonics traps to the cost of the 6-pulse. This narrows the difference in price between 6-pulse and AFE but from recent work, it still looks like the AFE approach would cost more.

The only potential disadvantage with an AFE appears to be reduced reliability due to greater complexity and much higher component count. How less reliable than a 6-pulse I can not say (probably not much at all).

For my present application, the MV VSD I have experience with is not available in AFE (but it is available as a 24-pulse).

I had a look at the AB Powerflex 7000 series you suggested, it looks interesting. Have you had any experience with it?

Regards

 

[electricuwe]

To TheDOG,

some time ago I got quotations for standard inverters and AFE-inverters in the MW-range as well for LV as MV-connection and the AFE-equipment was nearly twice as expensive. Might be the Allen Bradley Aproach using reverse blocking IGCTs is a bit less expensive, but since I got quotes for the IGCTs as well I guess even this type of AFE-inverter will be significantly more expensive than an inverter with 12-pulse diode frontend.

 

[TheDOG]

electricuwe;
The new AB unit actually uses SGCTs with a PWM-CSI topology. The parts count is greatly reduced (compared to multi-level PWM-VSI topology) and the SGCTs have double sided cooling (which IGBTs don't). When using a 6-pulse or 12-pulse MV drive, you typically need a high impedence isolation transformer to reduce the THD (which reduces overall efficiency). Was this added to the price comparison? As bigamp states, you need to compare "apples-to-apples" by including the harmonic filter as well. A lot of MV VSD manufacturers also have to supply a tuned motor filter, which "robs" the motor of torque and reduces efficiency.
bigamp;
I have not actually had any experience in installing MV drives, but have been involved in the design of a large project requiring 2 x 2500hp @ 3.3kV MV drives. That is why I know about the Powerflex.
I'll know more about them soon, if the project ever gets the green light!!

 

Dear all,

I'm new in here. Actually, I'm now working on a project for installing a VSD for a motor (950KW @ 11KV) of a water pump, for energy saving. However, I can only find the supply of the VSD upto 10KV (CDM8000 by Alstom). Could anyone help me to source the right one?

Thanks!

 

[htidke]

What is AFE?
Can U please give a brief Idea