Bad Power Factor 1

[DonQ]

Hello,

Recently a pumping station was upgraded and the 'part-wind' starters and power factor correction capacitors for three 60hp 480 pumps were replaced with ABB ACQ580 variable frequency drives.

Ever since this upgrade, the power factor has dropped every month, dropping from 90% PF (with the old starters and PFCC) to 19% this month!

Needing some input on what could be causing the problem?

Thanks

 

[IRstuff]

Is 19% even a plausible power factor for machines that do actual work?

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[dpc]

Where is this power factor being measured, and by what? The output of an AFD is highly non-linear.

 

[DonQ]

This installation has been in service for a year now, and every month the 'electric bill'
indicates a 10% loss, now it has leveled out to around 20%. This is a rural electric
utility.

 

[waross]

Typically, all equipment removed during an upgrade becomes the property of the contractor.
If this is a Displacement Power Factor issue, buy the original capacitors back from the contractor and reinstall them.
If this is a Distortion Power Factor issue, I'll let others comment.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[DonQ]

What kind of issues 'could' we have with the VFD's? (Adding the capacitors)

 

[che12345]

Dear Mr DonQ (Electrical)(OP)20 Feb 23 20:09
"...What kind of issues 'could' we have with the VFD's? (Adding the capacitors) "
1. In general, the introduction of the VFD will affect the system.
2. Consider the following cases:
(a) if the pf correction capacitors are connected on the main busbar; your system voltage could be affected by the VFD if you have a weak utility source. The capacitors will fail one-by-one depending on how badly the voltage is distorted. Suggestion: Remove them immediately as the load connected to the VFD do NOT require pf improvement capacitors. However, the other loads may require the caps, in this case check the cap current.
(b) if the pf correction capacitors are connected on the load circuit (i.e. after the VFD), remove them immediately! There shall be NO pf correction cap after the VFD. Attention: these caps will fail very quickly!
3. Check the capacitor current. Some of them may have had failed already.
Che Kuan Yau (Singapore)

 

[EnzoAus]

every month the 'electric bill'
indicates a 10% loss, now it has leveled out to around 20%

Donq, given that the power factor is being measured by the utility, the first thing I would check is the metering data, if it is available. Metering on many commercial installations in Australia provide 30min interval data with volts, current, real power, reactive power, power factor etc. Accessing this information would provide a very good indication as to how/when the poor power factor is being recorded. I suspect it is measured as kwh/kVA over a specific interval defined by your utility or regulator. Perhaps this poor power factor is recorded during the "worst" 30min interval of the month which might point to the cause.

If changes were made to the installation's metering during the pump station upgrade, it is possible that the metering was compromised. There are 40 ways to wire up 3 CTs to a 3 phase meter (3 x 2 x 2 x 2 x 2) and only one configuration is correct. All too often one of the other 47 other options has been inadvertently installed.

Another problem that I have seen is when there are renewables on site, the active load on site could be substantially offset by the behind the meter renewables. As net P reduces, Q remains constant and the PF drops.

I would have thought that the VFDs would be able to display power factor. If available, it would be worth checking such displays before assuming the VFDs are at fault.

 

[DonQ]

Very good point, I'll check the 'power factor' information on the drive(s), I'm sure its there somewhere!

 

[LionelHutz]

I doubt the drive will do any incoming power metering.

You'll need to put some power metering equipment on the system and try to determine what is happening. A drive with a line reactor should have a power factor above 90%. A drive connected directly to line power shouldn't cause the power factor to be anywhere close to 19%.

I was involved with a site that had poor power factor and nothing seemed to help it. I don't know exactly what was wrong because after a bunch of testing to give data to push back to the utility it ended with the customer signing an NDA with the utility.

 

[DonQ]

When the drives were installed, at the same time the electric utility upgraded the service from a 480v corner grounded delta
to a 277/480v wye. Line and load reactors were installed with the drive also. Thanks for all the help!

 

[bacon4life]

How much time the pump run each month?

[anecdote]
I have seen locations with water pumps that only a few minutes in some months and many hours in other months. The pumps themselves have power factor correction applied either via capacitors or by a VFD, so the power factor is good when the pumps run most of the month. In months where the pumps don't run much, the power consumption consists primarily of reactive power for transformer excitation. I have seen power factors lower than 19% for this kind of situation.

Replacing an older transformer that has high real power losses with a modern transformer that has low real power losses could make the power factor even worst in months that the pumps do not run.
[/anecdote]

Another item to verify is that the power factor measured by the utility is lagging rather than leading. Keeping the capacitors and added VFDs with near unity power factor could have pushed the site to delivering reactive power to the grid.

 

[waross]

The worst PF I have seen was under 10% on a large unloaded transformer.
That translated to a 90% penalty, (On a very low bill.)

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[DonQ]

Each pump will run approx. 3 to 4 hrs a day. The pumps are set-up to only run at 55 Hz to keep
the filter tanks from over-flowing. Thanks.

 

[waross]

Seat of the pants engineering.

3. Check the capacitor current. Some of them may have had failed already.

An old and wise maintenance Guru shared the following tip:
"Whenever you walk past a capacitor bank, run your hand across the capacitor cases. If one is cooler than the others, it has probably failed."
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[DonQ]

The power factor correction capacitors were removed when the VFD's were installed.
Thanks..

 

[bacon4life]

The ACQ580 lists the drive with a power factor 0.98. I am stumped how you could be at 0.19 with 3 hours of run time per day.

Does the monthly kWh on your bill still seem reasonable? Did the utility replace the metering when the service was converted from delta to wye-grounded?

 

[DonQ]

I'm still trying to find out if the utility changed-out the metering, I really believe they have
something wrong on their side!

 

[3DDave]

I think the main question remains - how is this being measured and where is it being measured? It is 100% possible they are reporting for an entirely different customer entirely, simple billing error. Or their equipment has failed. Or some other reason.

I feel like those IT support calls where someone says their computer doesn't work, so the IT guy says "check to see if it is plugged in" and the response is that the building power is out and they don't have enough light to see if it is plugged in.

 

[cranky108]

Then again, many utilities have a clause that allows them to measure the highest phase and multiply by three. So if you are very poorly unbalanced, you could be seeing the higher bill.
If the service was changed, the metering likely was changed to accommodate.