Power Factor Correction 1

[bass1mj]

I am doing the design of a new 6.6kV Substation consisting of two Incomers, a Bussection,two motor feeders and two spare panels in an industrial plant. The supply for the substation come from a main substation which is fed from the supply authority.

The above is just background info and my question is on the PFCorrection on the motors.

The following options exist
1. Use Static power factor correction i.e. connect capacitors in parallel with the motor in which case you have to consider taking it from either after or before the protection CT's. Before and you need to install protection for the caps or after and you need to compensate for it in the motor protection.
All other factors like resonance will be taken into consideration.
2. Increase the size of the existing capacitance at the feeder sub to cater for the additional inductive load on the new substation downstream. Upgrade control and protection on old system.

I would like to hear what option other people would prefer and why?

More info: New Sub rating = 6MVA
Motor1 data : 620kW :Ifl = 100A: pf=0.57 @FL: Inl = 70A
Motor2 data : 500kW :ifl = 74A : pf=0.63@FL: Inl = 49A

Thank you

Thank you

 

[aolalde]

The closest to the load, the more effective PF corrective capacitors are. This reduces the kVA requirements of immediate lines and transformers.

Cost analysis could result in lower cost for primary PF capacitors, but the benefits are not for the plant installations, just for the electricity bill.

 

[hsj]

1. For the kW rating, power factor of the motors appear to be extremely low. Should be around 0.9. You have mentioned pf of about 0.6 which is unsual. Please check. Why do you have to by such a motor?

2. If both the motors are operating simultaneously then the pf compensation could be at the motors or at switchboard.

3. If only one motor is going to operate at a time(which seems unlikely because the ratings are different) then the pf compensation should be at the switchboard.

4.The main objective of pf compensation in your case is to avoid penanlty due to bad power factor and therefore the location of pf compensation is immaterial.

5. Problem number 1 with pf compensation at motors is if motors are running at light load then this will result in overcompensation and leading power factor which the power company don't allow. Problem no.2 is protection of the bank.
6. Better option is to provide automatic PF compensation at switchboard. Proper protection (including from harmonics) can then be provided.

 

[busbar]

Although good engineering practice involves economic reduction of power-system losses, the prime payback is most often through reduction in electric billing by the electricity suppler through careful review of their rate-schedule's reduced-PF penalties. As implied by others, setting up a spreadsheet to view changes in capacitor size and switching, and their direct effect on monthly billing is often at the top of the priority list at the design stage.

 

[Marke]

Hello bass1mj

Static power factor correction is an easy way to control the power factor of the load, but there are some constraints.
1) With static correction, it is very important that you do not correct to more than 80% of the reactive current in the load. If you apply more correction, you can end up in resonance problems when the correction and motor are disconnected from the supply.
2) With static correction, it is preferable to use a separate contactor for the motor and for the capacitors. This is commonly not done, but will reduce the potential of harmonic resonance and interaction between the motor and the capacitors.
3) With static correction, you can not achieve as high a level of correction as you can with correctly controlled bulk correction.
4) Bulk correction at the supply can cause switching transients and resonances on the supply, particularly with weak and/or inductive supplies.
5) I would recommend for both types of correction, detuning reactors are used to reduce the harmonic current loading of the capacitors, and to reduce resonance effects.

Best regards,

Mark Empson

http://www.lmphotonics.com

 

[Rodmcm]

The point of harmonics is noted but not stressed here, and it should be. If the pF are on the motor side then it can set up ringing in the motor when turned off, also the combination of motors, chokes (transformer supply) and capacitors can resnonate through the protection relay on the motor. When on the busbar the same resonance can occur, however you can correct for the other loads as well as the motors.

 

[TomG33]

Bass1Mj
Just to step back from the problem to widen the field of view a little, Have the motors been purchased or is this a new supply to an existing installation ?

I ask this because if the motors have not been purchased, You could consider using at least one Synchronous motor. I know this is kind of on the small side for synchronous motors however if one of the loads is reasonably constant and is on most of the time, Buy making this one synchronous, you could adjust the Pf at which the motor operates, by changing the excitation, to achieve whatever power factor you need from the total installation free of harmonics and contactors etc.

For a motor of such size, you would not need complicated power factor regulators, a simple resistor setpoint would do if the load was reasonably constant.

There are many things to consider before going down this route, however, you were asking for options and opinions.

Tom