Can excessive capacitance at plant #1 impact PF at plant #2?

[11bgrunt]

The POCO has a problem with harmonics caused by many motor loads served by six pole VFDs. A large pumping facility moved to town and saw the high harmonics being supplied. The plant added a static capacitor bank on their 4160 to offset the high harmonics. Other facilities that have nothing in common with the pumping facility except the POCO primary, did run 80-90% PF at their metering points until a few months ago. That seems to have changed and would coincide with the capacitor bank coming on line. For the last four months those accounts are now running 99-100% PF. I have not been involved with anything before with such a large static bank that stayed in the line when the load fell off. When the pumping facility is not running, the capacitance from that plant on the primary line is high enough to affect the POCO primary voltage. I can see that when I measure voltage at those other sites. Could this excess capacitance from the pumping facility impact the PF of those other facilities?

 

[Bugman1400]

Yes, if the other sites share the same primary service as the pumping facility, the caps will affect the PF and the voltage. This effect will be reduced the further away from the primary you go....ie via xfmrs and also the load that is in service. If there is a xfmr between the caps and the other customers, with the large caps, the excess VARs maybe exporting through the lightly load xfmr and onto the primary. If the voltage is excessive, perhaps the caps can be switched out.

 

[Jraef]

The POCO has a problem with harmonics caused by many motor loads served by six pole VFDs. A large pumping facility moved to town and saw the high harmonics being supplied. The plant added a static capacitor bank on their 4160 to offset the high harmonics. Other facilities that have nothing in common with the pumping facility except the POCO primary, did run 80-90% PF at their metering points until a few months ago. That seems to have changed and would coincide with the capacitor bank coming on line. For the last four months those accounts are now running 99-100% PF. I have not been involved with anything before with such a large static bank that stayed in the line when the load fell off. When the pumping facility is not running, the capacitance from that plant on the primary line is high enough to affect the POCO primary voltage. I can see that when I measure voltage at those other sites. Could this excess capacitance from the pumping facility impact the PF of those other facilities?

Whomever made this decision (in bold) should have their HEAD EXAMINED!

I know, not what you asked, but that was answered. I just find this issue incredibly odd / stupid. Adding capacitance to systems with high harmonics is a catastrophe in the making...

Oh wait... Unless maybe you are INTERPRETING what you saw as a static capacitor bank alone and in fact it is a tuned harmonic filter system, which would be capacitors, reactors and RC circuits to trap specific harmonics, usually 5th and 7th. If so, then never mind. But still as Bugman1400 said, if it's on line when it doesn't need to be, the capacitance in the filters will act the same way and cause a leading PF / voltage rise on the entire system. Usually, that sort of system has contactors to take them off-line when the VFDs are not running in order to avoid this. Someone may have been too cheap about it.

 

[steve66]

I was kind of thinking the same thing, that capacitors are added to correct a low power factor, and not for harmonic issues.

Maybe you just meant that they added capacitors to correct the power factor, and didn't really mean harmonics? Harmonics and PF can be related, but they are two separate issues.

At any rate, going back to your question, I think it would depend on where you are measuring the PF. If you are looking at a meter on the customers supply, I don't see how a capacitor at another location could affect the PF. Assume this facility has a single motor. The meter will measure the current to the motor, and the voltage across the motor. It will read the same PF as the motor. If the facility has a bunch of motors, it will read the same voltage, and the total of all currents. It will basically read an average PF of all the motors.

Now add a capacitor at another facility next door. It generally won't make any significant change the voltage across any of the motors, and all the motors will still have to draw the same current. The meter will read exactly the same current and PF.

On the other hand, if you are looking at a meter somewhere upstream that measures the combined current of more than one facility, yes, the caps at one location would affect the PF you see.

But its hard to believe the caps would be sized just right to give a 99-100% PF. More likely, they would be over or under sized and you would get a lower PF, or even a leading PF.

So I have no idea why you are measuring right at 99-100% PF for 3 or 4 months.

 

[Ingenieur]

It will affect the bus pf

assume 1 xfmr feeding a bus with 4 equal loads
10 + 5 j or 11.18/26.57 deg Ohm each
or 2.78/26.57 in parallel equivilent

assume bus voltage is 4160/0 deg
i = 1488/-26.57 deg A total
or 372/-26.57 deg A per load
both bus and load pf = 0.894 lag

now correct 1 load to 10 + 0j by adding some capacitance
equivilent is now 2.77/19.44 deg
i = 1502/-19.44 deg A
bus pf = 0.943 lag
so pf improved

but 3 loads stay the same since bus v is unchanged
4160/0 deg / 11.18/26.57 deg = 372/-26.57 deg A
pf = 0.894 lag

but the load that changed has pf of 1.00
4160/0 deg / 10/0 deg = 416/0 deg A

You help the utility but not your neighbor
check my math

 

[GoldDigger]

Yup.
Even if the capacitance leads to overcorrection and capacitive current flowing on the POCO network that also supplies other customers, the PF of the second plant will be measured by a local meter that sees only that plant's current and the grid voltage.

 

[11bgrunt]

But its hard to believe the caps would be sized just right to give a 99-100% PF. More likely, they would be over or under sized and you would get a lower PF, or even a leading PF.

So I have no idea why you are measuring right at 99-100% PF for 3 or 4 months.[/QUOTE]

The individual POCO metering on these accounts report up to the 100% and do not report as you move into leading PF. The POCO penalty is for being under 95% PF with no penalty for being over or way, way over. I could have a PF of 50% lead and no one would know unless you searched meter data somewhere else. I got excited because the voltage was above standard and found that the circuit metered back at the PWT would be over 4MVAR lead when the pumping facility was off line. Today we can see that static cap bank has been online since August of 2015. It may take a little more snooping but the accounts coming back near 100% may be the 20% part of the motor total of other companies that have had static harmonic filters added. Those filters are always online even when that motor load is off.
Our people state that the lag flows toward the capacitor and the capacitor current back toward the source, not the other way around. I wanted to hear other opinions. A couple years ago the THDv measured in this area was around 15% just about everywhere. Almost all 5th and 7th.

 

[steve66]

The individual POCO metering on these accounts report up to the 100% and do not report as you move into leading PF.

I kind of wondered if something like that was going on. That would explain how they got right to 99% or 100%.

I think the OP must be looking at a meter that is upstream from several different customers, and the overall PF is probably getting pushed into the leading range.

 

[jim dungar]

The POCO penalty is for being under 95% PF with no penalty for being over or way, way over.

Are you sure?
I have had several customers get charged for having a leading power factor. In fact, the 'excess correction' penalty often kicks in at 99%.
A customer needs to involve their utility in their analysis for PF correction. Simply looking at the meter readout or billing statement and then buying capacitors, is all but guaranteed to not lead to a good long term solution.

 

[Jraef]

Are you sure?
I have had several customers get charged for having a leading power factor. In fact, the 'excess correction' penalty often kicks in at 99%.
A customer needs to involve their utility in their analysis for PF correction.

Agreed, most utilities I have dealt with charge for leading PF, not just lagging.

Simply looking at the meter readout or billing statement and then buying capacitors, is all but guaranteed to not lead to a good long term solution.

Scatter guns are not good hunting weapons...