Educate me

[ptonsparky]

View attachment 10769

Customer complaining about high bills and poor power quality, nothing new, but the POCO set up recording equipment that most of us can only dream of. The graph above shows a section of time that caught the engineers eye as he reviewed the results. The customer is on instrument metering and is being dinged for poor PF.

My understanding is that her billing will be determined by the highest usage during a 15 minute period at the PF during that time. This section seems to fit. Am I wrong? What else do you see?

(The rectangle is mine)

 

[kingpb]

Based on your knowledge of the billing, I can see that the low pf penalty is consistent.

An economic analysis needs to be done to see if the cost of adding pf correction is worth doing in comparison to the pf penalty that is paid.

 

[Iron_Ben]

At the power company I retired from, the demand charge (10$/kW) was the max 15 minute demand (kW) for the month (or billing period, nominally 30 days give or take) A penalty was applied if the pf was below 0.9 *for the month*. Again, the power factor for the 15 minutes of max demand was not used to determine the penalty, it was the month's power factor, calculated as kwh/kvah.

The graph is cool, sort of, but not the best way to view the info. We could provide a pulse data report which showed all the fifteen minute intervals and the real and reactive energy and power in a text format and that was quite helpful, so perhaps the customer could ask for that. Also the kvar scaling implies a negative (leading) power factor. That's possible, but pretty rare.

Lastly, I'd look closely at the customer's bill. Where I worked our power factor penalty was pitifully inadequate. We had a few customers (sugar mills and rice mills) with lots and lots of lightly loaded motors. I've seen plant power factors as low as 0.6, which indicates usage of more reactive power than real power (.707 is the 50-50 point). Even in those cases, our penalty in terms of $$$ never exceeded 2% of the total bill and only seldom got past 1%. It never made economic sense for any of our large customers to install power factor correction capacitors. Now perhaps the serving utility here has a steeper penalty, but I'd look closely at a few monthly bills as soon as possible.

Ben

 

[ptonsparky]

At the power company I retired from, the demand charge (10$/kW) was the max 15 minute demand (kW) for the month (or billing period, nominally 30 days give or take) A penalty was applied if the pf was below 0.9 *for the month*. Again, the power factor for the 15 minutes of max demand was not used to determine the penalty, it was the month's power factor, calculated as kwh/kvah.

The graph is cool, sort of, but not the best way to view the info. We could provide a pulse data report which showed all the fifteen minute intervals and the real and reactive energy and power in a text format and that was quite helpful, so perhaps the customer could ask for that. Also the kvar scaling implies a negative (leading) power factor. That's possible, but pretty rare.

Lastly, I'd look closely at the customer's bill. Where I worked our power factor penalty was pitifully inadequate. We had a few customers (sugar mills and rice mills) with lots and lots of lightly loaded motors. I've seen plant power factors as low as 0.6, which indicates usage of more reactive power than real power (.707 is the 50-50 point). Even in those cases, our penalty in terms of $$$ never exceeded 2% of the total bill and only seldom got past 1%. It never made economic sense for any of our large customers to install power factor correction capacitors. Now perhaps the serving utility here has a steeper penalty, but I'd look closely at a few monthly bills as soon as possible.

Ben

I wondered about the negative kvar but don't have the knowledge needed. I am sure it is lagging PF. They have more graphs than I can look at in an evening and I have requested the spreadsheet for my own curiosity.

Based on your knowledge of the billing, I can see that the low pf penalty is consistent.

An economic analysis needs to be done to see if the cost of adding pf correction is worth doing in comparison to the pf penalty that is paid.

Economic analysis has been done. Recommendation #1 is 300 kvar switched in 50 kvar steps. $25k, plus trying to find a place for a 600 amp disconnect. The next option is having individual PFC at the motor.

 

[kingpb]

I think you will find that adding pf correction at the largest motors contributing to the problem will be cheaper than trying to fix the whole system.

 

[ptonsparky]

I think you will find that adding pf correction at the largest motors contributing to the problem will be cheaper than trying to fix the whole system.

My general thought as well. May not get to the correction possible with the multistage, but close enough where payback is quicker.

 

[Sahib]

0.95pf seems to be economical overall pf to maintain;over voltage transient may also be avoided as there is a lesser chance of pf becoming leading at low load times.