power factor correction on synchronous machines 1

[stevenal]

It was real easy in machines lab. Adjusting the field current could make the load current go from lagging to leading. I can see where adding capacitors could be a pain, but why would an industrial customer with big synchronous machines not use them to reduce their bill?

 

[jbartos]

Suggestion: The big industrial consumer would reduce bill in case that it is penalized for the low power factor. Actually, by adding the power factor compensating capacitors, the voltage drop on the incoming lines decreases, the incoming utility voltage increases, which in turn causes customer's higher energy consumption and the higher bills because of higher utility voltage due to lower voltage drop. Therefore, the consumers are not much interested in a compensation of the power factors.

 

[stevenal]

I thought I was being clear, guess I'll try again. Why not use synchronous motors that are already in place and operating to avoid power factor penalties? My only direct experience comes from college, where we could simply increase the field current to improve pf. Is there a real world down side?

Jbartos,

The customer is penalized for poor pf, and I don't buy the would if they could idea. Even if true, such a dismissal does little to help me understand the situation.

Motors take enough power to move the mechanical load and cover the losses. Increasing the voltage has little effect on this equation. On the other hand, this particular customer finds voltage sag induced interruptions to their process to be very costly. Keeping the voltage high helps them to ride through more of these events.

 

[dpc]

I agree with stevenal, and in fact we have clients that do use their synchronous motors to regulate overall power factor.

Higher incoming voltage can cause slightly higher power consumption by resistive loads, but these loads are generally temperature-controlled so any increase in power consumption averaged over time is probably negligible.

But many industrial plants don't really understand how their reactive power charges are computed, or even if they are being penalized. And in many cases, utilities do not meter for reactive power or bill for it when it is metered.

But if you have synchronous motors and want to reduce reactive power, it makes sense to use the synch motors to the extent that is possible.

 

[jraef]

stevenal,
The point is, just because they have synchronous machines and are not using them to correct power factor dosen't necessarily mean that their bill is higher as a result. I have a client in Southern California with 5 synchronous machines in his plant. I just built new solid state starters for them with new solid state exciter packs, and offered them automatic PF correction capability for about a $250.00 adder. They declined. Why? Because the utility, Southern California Edison, does not penalize them for poor power factor! I tried to make the argument that correcting their PF was going to increase the efficiency of the transmission equipment, but when I investigated it I found that 90% of their load was resistive (electric melting furnaces) so I couldn't prove that either. It turns out that the only reason these motors were synchronous was that they were installed years ago as part of a larger facility who DID need PF carrection, but it was split up when the paernt company went under. These folks bought the partial plant that included these compressors, but did not want to relace the motors since they worked.
Subvert the dominant paradigm... Think first, then act!

 

[stevenal]

jraef,

This is frustrating. I am giving X, and asking answer Y of the Eng-tips folk. Instead of getting Y, I am getting arguments about whether X is really so. X is so. I am in a position to know.

Given: Industrial synchronous motor load customer who is billed for and pays for poor pf.

The rest of your answer is interesting regarding automatic pf control. I'm not sure in this case that automatic control would be needed. The load is fairly steady, and I think (this part I don't know for sure) they are penalized for the average pf over the billing period. Can't a higher field current simply be dialed in to see what the next bill looks like?

 

[bigamp]

stevenal,

If a plant has big synchronous motors available then they most certainly should be used to correct overall plant power factor. Make sure that they are adequately rated to be operated "overexcited". It is fairly normal for such motors to be rated at "x" kW's at a power factor of 0.8 leading so that they can be operated overexcited.

In days gone by when synchronous motors were more common it was usual to operate them in the way you propose. Often, a number of drive motors in a plant would be synchronous for this very purpose. Companies such as Basler can provide suitable excitation systems.

You tend not to see so many synchronous motors these days because they cost so much more than cage induction type motors. Synchronous type motors seem to have very limited applications such as for large low speed drives where they are clearly superior to cage induction motors.

Synchronous motors connected to a plant MV system will correct the overall plant power factor as seen by the utility. This will minimise/eliminate any penalty imposed by the utility for poor power factor. The synchronous motors will not however correct LV system power factor at the LV system. It is still desirable to have static PF correction on LV systems to maximise MV/LV transformer utilisation and to minimise volt drop in feeder cables to the LV MCC's etc.

Regards

 

[stevenal]

bigamp,

Thanks for suggesting Basler. Found a good reference at

http://www.basler.com/downloads/synchmtr.pdf

If automatic controls are needed, I see a possible down side due to the added complexity. But I fail to see why automatic controls are necessary. The reference above says "Using a fixed source of excitation also eliminates the possibility of using motors to provide
local var support or to assist in controlling power factor on the supply bus" with no further explanation. What would be the risk of running a fixed source on the over excited side?

 

[bigamp]

stevenal,

If the plant has a reasonably steady load you should be able to operate as you suggest; no automatic control, just set up the excitation on the synchronous motors so that there is zero import/export of VAr's to/from the grid. Operations people would need to be able to manually adjust the excitation if there was any change in plant operating conditions or demand.

If a synchronous motor tripped or was shut down, there would be a need to crank up the excitation on other motors to compensate. If a large block of induction motor load tripped or was shut down, there would be a need to back off the excitation to stop export of reactive power. No prizes for being a nett VAr exporter. Automatic control would potentially avoid the need for manual intervention for these sorts of ecvents.

Automatic control systems can work well, but as you suggest they can get tricky. In addition to controlling the overall plant reactive power import/export, you probably also need to be able to ensure equal reactive load sharing with your in-service synchronous motors. For sure Basler should be able to suggest a suitable control scheme and provide suitable control equipment. It would probably be worth talking to them just to get an idea of costs involved if nothing else. Operations people tend to have different skills these days and there do not seem to be as many on plants as there used to be. They may not take too kindly to having an additional task added to their work load, such as periodically checking plant power factor and adjusting synchronous motor excitation as required.

Regards,

 

[jraef]

stevenal,

Sorry fopr missing your initial point. As the others have said, this is done all the time. What I meant by automatic PF control was not automatic with relation to the motor but rather the pf of the entire plant. It would do exactly as you are stating is needed. We used a company named Kinetics out of New Jersey. This is all they do and they are very good at it. I suggest contacting them as well.

http://www.kinetics-industries.com/prodb1.htm

Subvert the dominant paradigm... Think first, then act!

 

[jbartos]

Comments: With increasing amount of nonlinear load, the harmonic compensation is needed. There is a way to improve both, namely, to minimize the harmonic content and improve power factor. The synchronous machines do not have capability of harmonics compensation (this is why they are much harder sold). Solid state power electronics and filtering do. When it comes to a consumer, the potential penalties are for both low power factor and high voltage harmonic THD. I still think that the consumer is not very happy about the low power factor and harmonics. If there happen to be DC power distribution, the consumer would appreciate it. In the beginning, there was the DC power distribution.

 

[stevenal]

The pf in this case is mainly determined by displacement rather than distortion.