Synchronous condensers 2

[wfowfo]

My boss just informed me that I am expected to dine next week with one of the members of our Board of Directors.

In 24 years I've never even observed a Board meeting.

So I ask around and find out the Board member has a buddy who is selling synchronous condensers and is trying to convince the Board to buy them for power factor correction on our Coop distribution lines.

Two questions.
1. Has anyone seen this done for distribution? I know they use them in plants, but is it practical in this application?
2. I understand inductive and capacitive reactance and their relationship to each other and to power factor. What I don't understand is how over-exciting the DC field of a synchronous motor develops a leading power factor in what would normally be an inductive device.

 

[Skogsgurra]

The second question has a very simple answer. As you know, an asynchronous motor consumes reactive power. This power is to produce the magnetic flux in the iron.

A synchronous machine does this another way, by feeding a DC (magnetizing) current to the rotor to produce the magnetic flux. If you feed too little DC, the grid has to supply the rest - and the machine consumes some reactive power. If you feed just the right amount of DC current to the rotor, the machine will have P.F.=1.00, which means that it only consumes power to cover its own losses.

Now, if you feed it a little more DC, it will be more magnetized than it needs for its own flux. The result is that it sends the surplus reactive power to the grid. By taking the magnetizing current high, you can thus generate reactive power. That's how it works.

I could have told you about pole angles and leading/lagging current. But I think that this simplistic approach will give you some sort of start - and that it will make you accept the principle.

The first question? Yes. I have seen them in 110 and 70 kV distribution.

Gunnar Englund

http://www.gke.org

 

[rbulsara]

http://www.pserc.wisc.edu/ecow/get/generalinf/presentati/presentati/heydt_synchronous_mach_sep03.pdf

See if this helps for basics.

 

[Skogsgurra]

That's a very good presentation.

I noticed that I made a gross error in my posting. The lines: "If you feed just the right amount of DC current to the rotor, the machine will have P.F.=1.00, which means that it only consumes power to cover its own losses" shall read "...will have P.F.=0.00, which means that it only consumes power to cover its own losses"


Gunnar Englund

http://www.gke.org

 

[wfowfo]

Quote:
"I could have told you about pole angles and leading/lagging current. But I think that this simplistic approach will give you some sort of start - and that it will make you accept the principle."

This is exactly what I wanted. I've seen several explanations complete with vector analysis and didn't follow a bit of it. Your explanation helped considerably.
One follow up. The distribution system I'm referring to is 12.5 kv and 24.9 Kv and is your typical rural distribution type line. Is a synchronous condenser a practical choice for a system where you don't have a particular load center?

 

[davidbeach]

skogsgurra, no, I believe you had it right the first time. Zero power factor would be all reactive current, which you can't get with a synchronous condenser as there will be some power used in keeping it running. The reactive factor (seldom used, but the other side of the triangle from the power factor) would go to zero if the excitation were such that the field were supplying exactly the excitation required for rotation of the machine.

 

[Skogsgurra]

Did I - did I not???

Anyhow. You got both versions now.

Left as an exercise for the reader.

Tell me what is right. PLZ.

Gunnar Englund

http://www.gke.org

 

[Skogsgurra]

OK. I admit it. I was right the first time. Got confused by that presentation - but it is good.

I am not so sure about those lower voltage levels. If you have a long (low SC capacity) line, then a condenser may help. The possibility to control reactive power continuously is valuable. But there is a problem when you start it. You will probably need a pony motor. Asynch start using the ammortiseur windings (if there are any) is not an option on weak grids.

Gunnar Englund

http://www.gke.org

 

[bacon4life]

I keep hearing about people taking SC out of service rather than putting new ones in service. Maintenance and losses can be much more expensive than capacitors or power electronics. In addition to fixed or mechanically switched capacitor there are also power electronically control ones that provide similar benefits as a SC.

One advantage of a SC over capacitors is that the VAR output of capacitors varies with the voltage squared, so when you need them the most during a voltage sag, they actually are producing the least. Not sure if this would be applicable to a distribution line though.

 

[waross]

With a concentrated load such as a mine or large mill the synchronous condensor can be controlled by varying the field to maintain the power factor at the value you want. Capacitors have to be switched and the switching contactors become expensive. With a distributed load such as a utility, particularly with a lot of residentail and small comercial loads consuming vars, small capacitor banks can be distributed throughout the system and connected with fused cutouts.
12 kv capacitors are fairly inexpensive. As the voltage goes up, the cost per KVAR drops.

A synchronous condensor, physicaly, may be a synchronous motor used for power factor correct. The KVAR capacity will be equal to the maximum allowable KVA of the motor.
It may be a synchronous condensor, which, physically is a synchronous motor without a shaft extension.
It may also be a generator without a prime mover used to correct power factor (it may need a pony motor to get it spinning, but that's not a serious problem).
The output of a synchronous condensor is rated in KVARs and the KVARs are identical to the KVARs supplied by a capacitor bank.

SYNCHRONOUS CONDENSOR
You will have to house a synchronous condensor.
It will require similar control gear and maintainance as a similar sized motor.
The KVARs will be supplied at one point on your system.
If you have a problem with the synchronous condensor, you may not have any correction until it is repaired.
You may want to consider two or more synchronous condensors for redundancy.
CAPACITORS
May be pole mounted.
May be distributed throughout the system to reduce line losses.
Easy and fairly cheap to change out individual failed units.
Capacitors are often used permanently connected.
If control is required, the control gear and contactors will add to the cost.

Your system can probably run all permanently connected or part permanently connected and part controlled.
If your system is fed from one point and you are paying power factor penalties you may consider the advantages of distributed capacitors to improve your voltages and reduce your line losses and a much smaller synchronous condensor at the point of connection to further improve your power factor at the point of connection to your supply.
I'm doing my best to explain the differences without getting into vectors and phase angles.
respectfully

 

[wfowfo]

Quote:
"I'm doing my best to explain the differences without getting into vectors and phase angles.
respectfully"

You're my kinda guy!

Actually, we had the meeting tonight.

We have a 120 Mw (total load) Coop that spreads across 7 counties, has 15 delivery point substations at both 69 and 138 Kv, and operates at an average power factor of around 97%.

According to the rep, we can install a 3.6 Mva, 13.8 Kv synchronous condenser at a single point in the system. We can connect it to a 13.8/138 Kv step up transformer and correct the power factor in our entire system from the one location. Costs about $300,000 plus without the step-up.

Think I'll invest in Enron stock instead.

Thanks for all the help. I always enjoy learning from the pros.

 

[waross]

Thanks for the system information.
A couple of factors:
Are you paying power factor penalties?
If you can save money by correcting to 100%, you may consider adding smaller synchronous capacitors at the feed points where the most penalties are being accrued. The advantage in this case over individual capacitors is that it is quite simple for a controller to maintain 100% power factor by adjusting the field current.
As for correcting the entire system from one point, I pity the engineers who have to try to engineer what the salesman is selling.
Look at the financial benefit to be gained at each tie point and compare that to the cost of a synchronous condensor that can supply the KVARs you need at that point. Don't forget to do a price comparison with switched capacitors.
Another advantage of correcting at the tie points or stations; You can use individual correction, capacitors or synchronous condensors, on the low voltage sides of your station transformers and improve the power factor and the capacity of the transformers slightly. It saves the cost of a transformer dedicated to one synchronous condensor.
BTW, again without vectors, I am sure that you are familiar with plain old induction motors, and the power factor issues.
The power factor issues arise from the need for magnetizing current. The motors draw this current from the power line.
A synchronous motor has a field winding that handles the magnetising. If the field is under excited, the motor will draw extra current from the line to magnetize itself. The power factor will drop.
If the field strength is increased and the synchronous motor is over excited it will produce extra KVARs and these will be put back on the power line. These KVARs are not the lagging KVARs that an induction motor will take to magnetize itself, but are leading KVARs as are supplied by a capacitor. This used to be done in plants to improve the power factor. That is, over excite a synchronous motor.
The power factor of the motor drops again but it is now leading instead of lagging. The current supplying the KVARs back to the line is part of the motor current and tends to add to the heating of the motor. Synchronous motors used this way were derated to allow for the reactive current.
The motor current was the combination of the load current and the magnetizing current.
Sometimes the motor would be derated to zero hoprsepower. All the current would be supplying KVARs to the line. The shaft may be covered with a safety guard or the machine may be built without a shaft extension. It was now called a synchronous condensor.
This was acomplished by increasing the field voltage and as a result the field current. The increased heating of the rotor caused by the higher current limited the minimum power factor that some syncronous motors could be operated at continuously. (Lower power factor means greater magnetizing current, whether it's going into an induction motor or comeing out of an over excited synchronous motor.)
Now the formula is 1.73 times phase voltage times current = VARs
instead of 1.73 times phase voltage times current = watts.

At 50% load, the load current and the magnetizing current must be added with pythagoris' theorem and I'm staying away from vectors and angles today.
respectfully

 

[stevenal]

Does the supplying transmission company totalize the 15 delivery points coming up with a single system pf penalty? If so, the rep is correct. But if each point is metered and billed separately, (more likely) then applying correction at one delivery point does nothing for the others. And injecting vars at one point to consume them somewhere else can increase system losses while decreasing system capacity. For this reason, the transco may penalize leading pf as well as lagging. The best place for correction is near the lagging loads. And are you being penalized for 97%? And just how sophisticated is the transco's metering at each point? In my experience, a transco's rate structure can be well ahead of their metering capability. This sometimes works for you, sometimes against.

 

[wfowfo]

At the moment we are not paying PF penalties, although that should end soon. The PF is monitored at all 15 delivery points, not just the system as a whole.

 

[davidbeach]

In that case you should run away from the idea of a single synchronous condenser. You could "correct" your net pf, so that your whole system together is very close to unity, and make your pf penalties worse. 14 points of delivery would still have exactly the same power factor they do now, but the 15th would have gone way leading. Best to use local cap banks downstream of each metering point.

 

[stevenal]

Okay, to sum up: we have a proposal that increases future pf penalty paid to the transco, increases system losses, decreases system capacity, requires the addition of equipment to maintain, and has absolutely no upside to the members of the coop. But it does put money into the pocket of the board member's friend. A better plan would be to just steal the money from the coop and give it to the friend avoiding all the downsides listed above. Should be a fun diner.

 

[jghrist]

A star for stevenal. I think this sums up the situation perfectly.

 

[jraef]

This is all well and good, but is a .97pf all that bad? Usually penalties kick in when pf is below .95 in my experience. $300,000 to correct a .03 displacement is a lot of money and penalties would need to be excessive. A good lawyer would probably cost less!

...I can't beleive I said that... shame on me.

Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[jraef]

Seriously though, if you are a power distribution co-op and the producers ARE going to ding you for a .03 displacement, then it may be less expensive to find a user on your grid that has a large continuous motor operating and offer them a program to share the cost of installing a synchronous motor on their application to run it in leading pf. Candidates would be large air or refrigeration compressors that don't cycle on and off but rather load and unload; commutators (ball mills, rod mills etc.) at mining operations; large boiler ID fans; the list can go on and on.

I did one like that for an irrigation district that was also a power co-op. They have 3 huge main intake pumps and one of them had to run all the time, the others were cycled on or off by demand. They swapped out the motor on one of the pumps to be synchronous and run that one continuously with a control system that corrects pf on their grid in real time by altering the pf on that motor. That way they get the work out of the motor for pumping, but also take advantage of power factor correction with it as a synchronous condenser.

Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[wfowfo]

Quote:
"Okay, to sum up: we have a proposal that increases future pf penalty paid to the transco, increases system losses, decreases system capacity, requires the addition of equipment to maintain, and has absolutely no upside to the members of the coop. But it does put money into the pocket of the board member's friend. A better plan would be to just steal the money from the coop and give it to the friend avoiding all the downsides listed above. Should be a fun diner."

On the plus side, I got a free Lemon Pepper Fillet out of it.
Thanks for all the help. Ya'lls input got me thru the dinner and I think we have managed to make the issue go away.

Thanks again!