Jump to content

Use Of Regen Drives For Var Correction


Matt303

Recommended Posts

 

I work as an elecrical engineer in the water and sewage treatment industry. In recent years many of the owners have intallated combined heat and power plants to take advantage of the bio-gas which is a by product of sewage digesters. These machines are mainly spark ignition gas engines and are often run by a third party company under an energy purchasing agreemnt. In the UK we have a scheme called ROC (renewable obligation certificates) which is essentially a carbon trading market. The power generated from these sets can be sold and the offset purchased as if imported onto the site. As the value of the ROC is higher than the imported power these sets are generally run in this offset mode.

Forgive this drawn out description but now I will get to the point. These sets run at a power factor of near unity. They therefore change the balance between kW and kVAr on the site. The tariff does not charge for VAr for a power factor above 0.95 inductive but as these sets effectively reduce the kW import but leave the reactive unaffected they bring down the power factor. Although the VAr imports are about the same the proportion is lower as a proportion of MVA and the site starts paying for them as the power factor <0.95. There is no option to export VAr from the sets as they are part of a third party agreement.

It has been suggested that as a large number of partly loaded regernerative variable speed drives are installed these could be used to export VAr (essentially run them with leading power factors) and this could correct the site power factor the right side of the threshold.

I don't really understand how this works it seems as if the drives operate a bit like 4 quadrant DC systems and I suppose could be used as an alternative to a DC system on a crane i.e. to provide regenerative braking. I have no idea if they could be used for PF correction or if this is worth looking at.

Is this a sensible proposition?

 

Thanks in advance for your views.

Link to comment
Share on other sites

Hello Matt303

 

I am not sure that I understand the concept of using regen drives to reduce the VARs. This would assume that you have motors that are regenerating?? not common in your type of environment.

 

If the power plants are using proper alternators, the power factor can be adjusted by controlling the excitation.

If the power plants use induction generators, then there is little that can be done other than adding power factor correction.

 

The other issue to be aware of, is the distortion power factor. If you have a number of drives, and I would be very surprised if you do not, then you can have a high harmonic current and this will increase the KVA and that can not be easily reduced.

 

There are active correction units that you can use to reduce the harmonics and VARs. These are like an inverter that feed back into the supply, but are expensive.

Good luck

Link to comment
Share on other sites

Hello Matt303

 

I am not sure that I understand the concept of using regen drives to reduce the VARs. This would assume that you have motors that are regenerating?? not common in your type of environment.

 

If the power plants are using proper alternators, the power factor can be adjusted by controlling the excitation.

If the power plants use induction generators, then there is little that can be done other than adding power factor correction.

 

The other issue to be aware of, is the distortion power factor. If you have a number of drives, and I would be very surprised if you do not, then you can have a high harmonic current and this will increase the KVA and that can not be easily reduced.

 

There are active correction units that you can use to reduce the harmonics and VARs. These are like an inverter that feed back into the supply, but are expensive.

Good luck

Marke,

 

Thanks for your comment.

I've done a bit more investigation. The regenerative drives are variable speed units that we generally use instead of a standard 6 pulse unit and active filter. The units have the capability to correct their own harmonic distortion. If the existing system is not dirty already they work out cheaper than adding the filter seperately. Apparently this was not their original design purpose and is more an advantageous side effect of their design that we are currently exploiting. Te supplier twlls me that the units have a fully rated first stage and they were intended to be used as a replacement for DC systems. Because of this feature the the drive is capable of acting like an active filter and can generate back into the system up to the MVA rating of the nameplate. On this basis if the unit is not running at its full output the balance is available (or so it is claimed). In principle and assuming the description is accurate it seems like a good idea but I have the sense that the implementation and control of a system (particularly voltage regulation) might be much more complex. If we throw VAr onto the system at 400V other than increasing the voltage at the imediate bus I'm not clear on how much of this will be translated through the step up transformer. The offending drives reponsible for lowering the power factor are based elsewhere on the distribution system. The reason they have not employed bulk power factor correction capacitors is due to voltage regulation problems and I don't see why this method shoud be more effective.

Do you think an active filter correct the power factor and provide fast enough transient response to prevent overvoltage conditions?

 

 

Link to comment
Share on other sites

Hello Matt303

 

Yes, an active front end drive (regenerative drive) can draw a sinusoidal current from the supply. This will improve the true power factor of the input current by the reduction in the level of harmonic currents produced by the drive. In both cases, the reactive current is minimal and so VARs do not enter the equation. The use of the active front end drive, according to my understanding, will not improve the power factor or reduce the VARs produced by other equipment, so I am not sure where this comment about VARs fits at all.

 

drive is capable of acting like an active filter and can generate back into the system up to the MVA rating of the nameplate
Yes, this is true if the motor is regenerating. In the case of a pump load, this will never happen, however a downhill conveyor will regenerate and feed power back into the grid. The regeneration will only occur where energy can be pulled out of the load, it will not happen where the load is requiring energy to run.

 

If you add capacitors to compensate for the VARs of other equipment, it will improve the power factor on the input of the transformer provided that the capacitors are connected on the same secondary as the poor power factor load.

It is possible to over correct one transformer in order to provide a leading power factor in the primary of that transformer and thereby improve the power factor on the HT supply, but you do need to be careful if the loading on the transformer is low as you can have resonance problems between the transformer reactance and the power factor correction.

 

Best regards,

Link to comment
Share on other sites

Matt303,

there is a big difference between an active filter and an AFE converter: the switching frequency.

To realize a good performance, active filters should run with a switching frequency 4.... 10 times higher than AFE converters.

 

Regards

Mario

Mario Maggi - Italy - http://www.evlist.it - https://www.axu.it

Link to comment
Share on other sites

Hello Matt303,

 

On some VSDs with an active front end, the power factor that the AFE operates at can be set in the parameters to be leading or lagging but is by default set to be unity. To dynamically change the vars that he drive is absorbing or generating will require you talk to the manufacturer. The actual amount of reactive power that the drive can absorb/genreate will be limited by the motor load at that instant.

 

Cheers

Niallnz

Link to comment
Share on other sites

Thanks for all your comments. It seems that the main point here is that the available power at the VSD terminals is limited to the drive load characteristics. This is fundementally different from the description the manufacturer has indicated. They intimated that the rating of the fist stage was equivalent to the final drive stage and that the drive could act as a VAr generator, the only reference to the attached load is the MVA rating of the unit i.e. that the available current to correct the powe factor is the rated current minus current due to the connected motor. I am now even less convinced that this is viable but will investigate further.

 

Your comments are much appreciated, I'm way past the limits of my knowledge on this issue

Link to comment
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
×
×
  • Create New...