Should I turn off power correction capacitor banks when plant is not running

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The saw mill where I am the only electrician typically only runs 9-10 hours a day. Should I disconnect the power correction capacitor banks when the mill shuts down for the night or weekend? I read something about that, but could not really find a straight answer online. We got a surcharge last month for out power factor being too low. I intend to test all of the capacitors, but am waiting on a grounding stick I ordered to be delivered.
 
Location
NE (9.06 miles @5.9 Degrees from Winged Horses)
Occupation
EC - retired
If you got a surcharge for PF being to low, someone forgot to turn them ON at the start of a day, and it only takes one time. Or they are not functioning.

We had a customer that just left his ON. They knew they should/could turn them off but mostly just forgot or ignored it. Definitely raised the voltage.
 

gar

Senior Member
Location
Ann Arbor, Michigan
Occupation
EE
181114-1521 EST

wyopokesfan:

Suppose your plant ran during the day without the PF capacitors connected, and your power factor was about 0.5 lagging. With the PF capacitors connected it is close to 1.0 . Now turn the plant off, and leave the capacitors connected. What happens to the PF with no load except the capacitors? It will go toward 0.5 leading. Just as bad possibly. So, yes, you would get a PF penalty.

.
 

Besoeker

Senior Member
Location
UK
181114-1521 EST

wyopokesfan:

Suppose your plant ran during the day without the PF capacitors connected, and your power factor was about 0.5 lagging. With the PF capacitors connected it is close to 1.0 . Now turn the plant off, and leave the capacitors connected. What happens to the PF with no load except the capacitors? It will go toward 0.5 leading. Just as bad possibly. So, yes, you would get a PF penalty.

.
So the answer is yes.
 

Jraef

Moderator, OTD
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
Many utilities will penalize you the same for leading or lagging PF, but because a severely leading PF is rare, they may not have a code in their billing computer that differentiates. So their billing computer is just charging you the penalty for POOR power factor, even though the bill says "low".

This is why I always recommend what's called "at load" PF correction; you connect PFC cap bank downstream of the motor contactor of each induction motor so that it is only on-line when the motor is on-line.
 

GoldDigger

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Location
Placerville, CA, USA
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Retired PV System Designer
Many utilities will penalize you the same for leading or lagging PF, but because a severely leading PF is rare, they may not have a code in their billing computer that differentiates. So their billing computer is just charging you the penalty for POOR power factor, even though the bill says "low".

This is why I always recommend what's called "at load" PF correction; you connect PFC cap bank downstream of the motor contactor of each induction motor so that it is only on-line when the motor is on-line.
Actually, both poor leading and poor lagging power factor will actually be "low", since it is always a positive number from 0 to 1.

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drcampbell

Senior Member
Location
The Motor City, Michigan USA
Occupation
Registered Professional Engineer
... I always recommend what's called "at load" PF correction; you connect PFC cap bank downstream of the motor contactor of each induction motor so that it is only on-line when the motor is on-line.
Better yet, connect them via their own independent contactor so they disconnect from both the line and the motor when the motor is turned off.
 

broadgage

Senior Member
Location
London, England
Yes, most power companies will penalise large or industrial consumers for poor power factor including a leading power factor as would be caused by leaving power factor capacitors in circuit when the load is minimal.

Most modern PFC units are switched automatically in stages so as to maintain optimum conditions with a varying load. If working correctly no attention is needed.
Some older units are in circuit continually if not switched manually or automatically.

Here in the UK, and probably elsewhere, the utility will accept a lagging power factor provided that the customer pays any relevant costs or penalties. A leading power factor is specifically prohibited.
A leading power factor can cause the utility voltage to rise, perhaps to undesirable levels.
 

Jraef

Moderator, OTD
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
Actually, both poor leading and poor lagging power factor will actually be "low", since it is always a positive number from 0 to 1.

Sent from my XT1585 using Tapatalk
D’oh! I’ve even had that exact argument with an EE on a project once, because I provided a PF meter that registered the value from 0-1 with no sign, and he wanted one that read 0-1-0 so that he could tell whether it was leading or lagging. This was on a giant Wound Rotor Induction Motor with no PFC correction so it would NEVER be leading, and he was thinking a WRIM was the same as a Synchronous Motor that could adjust the PF. He also didn’t call the meter out that way so I just gave him a simple analog PF meter, adding the argument that from a penalty standpoint it wouldn’t make a difference anyway, because “low is low” to the utility. I totally forgot that entire incident until you said that...
:slaphead:
 
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