Does PF affect kWh for POCO billing?

[11bgrunt]

We are involved with a company that has five 300 horsepower motors and the average recorded power factor for the plant runs 80%. They do receive a PF penalty. Not much, maybe $500 a month. Years ago we talked about adding a switched cap bank that would get them above 95% and there would be no penalty. Payback was over ten years so nothing ever happened. We are looking at a borderline transformer loading issue now and believe if their power factor was better it could reduce the load to the transformer as much as 1000kVA in the worst of times that we can document.
It was suggested this week that power factor affects POCO billing demand charge and can affect kWh. If that is correct, the additional charge for power regardless of PF penalty would have changed the payback period for the proposed cap bank considerably. Because the metering CTs are on the secondary of the POCO transformer, the capacitors would have to be on the 277/480 side.
This billing idea seems odd to me and I will be visiting with metering folks next week. i am interested in what this forum has to say about this.

 

[Fulthrotl]

We are involved with a company that has five 300 horsepower motors and the average recorded power factor for the plant runs 80%. They do receive a PF penalty. Not much, maybe $500 a month. Years ago we talked about adding a switched cap bank that would get them above 95% and there would be no penalty. Payback was over ten years so nothing ever happened. We are looking at a borderline transformer loading issue now and believe if their power factor was better it could reduce the load to the transformer as much as 1000kVA in the worst of times that we can document.
It was suggested this week that power factor affects POCO billing demand charge and can affect kWh. If that is correct, the additional charge for power regardless of PF penalty would have changed the payback period for the proposed cap bank considerably. Because the metering CTs are on the secondary of the POCO transformer, the capacitors would have to be on the 277/480 side.
This billing idea seems odd to me and I will be visiting with metering folks next week. i am interested in what this forum has to say about this.

i posed a question here on PF correction a while ago, and learned the caps need to be near the actual motors
to make maximum benefit.... so if you size caps for those motors specifically, and put them physically near
the motors, that may change your strategy a bit.

 

[Besoeker]

i posed a question here on PF correction a while ago, and learned the caps need to be near the actual motors
to make maximum benefit.... so if you size caps for those motors specifically, and put them physically near
the motors, that may change your strategy a bit.

Yes, that would reduce conductor losses.

 

[winnie]

Power factor will increase losses, which will probably cause an increase in power consumption and thus demand at any point in time.

But I think the OP's question is: does power factor _directly_ increase the recorded demand value and thus demand charges.

I would think that if the demand charges are based on _power_ consumption then power factor will only have an indirect effect.

If the demand charges are based on _current_ consumption, then power factor will directly change the measured 'demand'.

I don't know if any utility measures 'demand' by current level, but I think the answer has to be 'it depends on what the utility is actually metering'.

-Jon

 

[ptonsparky]

It gets more complicated then just throwing a few PFCs on. VFDs don't mix in well and a proper study should be done to prevent other problems.

 

[zach_zachary1@yahoo.com]

PF has a direct impact on KWH in the form of reactive power. One of the PEs could probably way in on this better then I can, but I was taught to calculate real power ( the power that actually does real work):

P_(kW) = √3 × PF × I_(A) × V_L-L(V) / 1000


"For instance, a power factor of 68 percent (0.68) means that only 68 percent of the total current supplied is actually doing work; the remaining 32 percent is reactive and has to be made up by the utility. Usually, utilities do not charge consumers for the reactive power losses as they do no real work for the consumer. However, if there are inefficiencies at the customer’s load source that causes the power factor to fall below a certain level, utilities may charge customers in order to cover an increase in their power plant fuel use and their worse line and plant capacity."
Source Wikipedia

At 0.80 PF the motor is only using 80% of the KWH it is demanding for the utility.

I have only seen PF correcting Capacitor Banks on reduced voltage starters. I was told most VFDs have PF correction built into them.

Interesting topic. Looking forward to additional comments.

 

[kwired]

More current in conductors means more conductor losses as heat.

That is why placing correction close to motors results in most efficiency, there is less loss in conductors because of less current in conductors.

How effective and cost worthy depends on details of each installation. The shorter the conductor run, the less loss. Oversizing conductors for voltage drop does have an impact as well.

 

[GoldDigger]

PF has a direct impact on KWH in the form of reactive power. One of the PEs could probably way in on this better then I can, but I was taught to calculate real power ( the power that actually does real work):




At 0.80 PF the motor is only using 80% of the KWH it is demanding for the utility.


Interesting topic. Looking forward to additional comments.

At .80 PF the motor is using 80% of the KVA ....(not kWh)

 

[kwired]

PF has a direct impact on KWH in the form of reactive power. One of the PEs could probably way in on this better then I can, but I was taught to calculate real power ( the power that actually does real work):

P_(kW) = √3 × PF × I_(A) × V_L-L(V) / 1000


"For instance, a power factor of 68 percent (0.68) means that only 68 percent of the total current supplied is actually doing work; the remaining 32 percent is reactive and has to be made up by the utility. Usually, utilities do not charge consumers for the reactive power losses as they do no real work for the consumer. However, if there are inefficiencies at the customer’s load source that causes the power factor to fall below a certain level, utilities may charge customers in order to cover an increase in their power plant fuel use and their worse line and plant capacity."
Source Wikipedia

At 0.80 PF the motor is only using 80% of the KWH it is demanding for the utility.

I have only seen PF correcting Capacitor Banks on reduced voltage starters. I was told most VFDs have PF correction built into them.

Interesting topic. Looking forward to additional comments.

VFD's don't specifically have PFC built into them - it is just an inherent property. VFD's rectify incoming AC power to DC, then convert that DC voltage back to a simulated controlled sine wave via the electronics. The reactive power does not pass through the rectifier and into the supply circuit.

Reactive power is between the source and the motor if there is no capacitor in between to exchange power with. With a VFD you basically have a new source on the load side of the rectifier and there is reactive power between this source and the motor, the motor needs reactive power to work properly.

 

[texie]

We are involved with a company that has five 300 horsepower motors and the average recorded power factor for the plant runs 80%. They do receive a PF penalty. Not much, maybe $500 a month. Years ago we talked about adding a switched cap bank that would get them above 95% and there would be no penalty. Payback was over ten years so nothing ever happened. We are looking at a borderline transformer loading issue now and believe if their power factor was better it could reduce the load to the transformer as much as 1000kVA in the worst of times that we can document.
It was suggested this week that power factor affects POCO billing demand charge and can affect kWh. If that is correct, the additional charge for power regardless of PF penalty would have changed the payback period for the proposed cap bank considerably. Because the metering CTs are on the secondary of the POCO transformer, the capacitors would have to be on the 277/480 side.
This billing idea seems odd to me and I will be visiting with metering folks next week. i am interested in what this forum has to say about this.

If you have a POCO that bills demand in KVA instead of KW that could give you real incentive to correct the PF. Some do and this really changes the calculations for pay back as compared to a POCO that bills demand in KW with a separate PF penalty.

 

[Jraef]

If you have a POCO that bills demand in KVA instead of KW that could give you real incentive to correct the PF. Some do and this really changes the calculations for pay back as compared to a POCO that bills demand in KW with a separate PF penalty.

That's the key issue right there. The place to start is with the PoCo, find out exactly how they are billing for demand. If it is kW demand, you will not have a noticeable demand reduction, only a small reduction in losses as kWH. If they bill on kVA demand, you will likely see a reduction in the demand charges too.

I'm never a fan of switching PFC cap banks, my experience is that they do more harm than good in modern power systems that are full of power supplies. As mentioned, VFDs are not at all friendly with them, but also anything with an SMPS (Switch Mode Power Supply), which is now virtually everything. PC power supplies, monitors, energy saving ballasts, HID lighting, servos, DC drives, etc.

To me, the better approach is to place the caps nearest the motor down stream of the contactor, not only for the best results, but also because that way they ONLY come on when the motor comes on. You can't possibly over correct that way. But at 300HP, if you have soft starters, be aware that you must use a separate PFC Cap Contactor that pulls in only AFTER the soft starter has finished ramping.

 

[mgookin]

That's the key issue right there. The place to start is with the PoCo, find out exactly how they are billing for demand. If it is kW demand, you will not have a noticeable demand reduction, only a small reduction in losses as kWH. If they bill on kVA demand, you will likely see a reduction in the demand charges too.

I'm never a fan of switching PFC cap banks, my experience is that they do more harm than good in modern power systems that are full of power supplies. As mentioned, VFDs are not at all friendly with them, but also anything with an SMPS (Switch Mode Power Supply), which is now virtually everything. PC power supplies, monitors, energy saving ballasts, HID lighting, servos, DC drives, etc.

To me, the better approach is to place the caps nearest the motor down stream of the contactor, not only for the best results, but also because that way they ONLY come on when the motor comes on. You can't possibly over correct that way. But at 300HP, if you have soft starters, be aware that you must use a separate PFC Cap Contactor that pulls in only AFTER the soft starter has finished ramping.

One problem I see is figuring out who at POCO to ask if you want reliable information. It's not going to be the billing center. And then figuring out how to get to that person.

Can they figure out what make & model the meter is and research that?

 

[Besoeker]

To me, the better approach is to place the caps nearest the motor down stream of the contactor, not only for the best results, but also because that way they ONLY come on when the motor comes on. You can't possibly over correct that way. But at 300HP, if you have soft starters, be aware that you must use a separate PFC Cap Contactor that pulls in only AFTER the soft starter has finished ramping.

I would always use a separate PFC contactor. We've been down this road before.

 

[Ingenieur]

You need details on the metering and billing scheme

as far as central vs local/remote/distributed pf caps not enough information
% motor/total load
single line
physical layout/geometry
etc

if you have a mech room with all the motors in it and a central located mcc in that room central is the way to go

it you have numerous motors scattered about, but 2 large ones that account for 50% of the total motor load on opposite ends of the building local would seem a better choice