Electrical power question

[justint]

Hi

Im involved in a project where I am setting up a system to remotely monitor the energy consumed by a industrial plant with a number of motors, pumps etc. Im in Ireland.
I have gotten to the stage where I can send a CSV file each hour that lists the instantaneous KW, KVA, KVAR, power factor and a running cumulative total of the KWhr or KVAhr (depending on the model of power meter I use).
I am also receiving information on the instantaneous current consumed by each individual motor via CTs.

I have a few questions relating to this and how it relates to the bill at the end of the month.

1- The bill for the plant is based on KWhr, if say for example the power factor for the plant got worse and the KVA for the plant increased for the same KW of useful power what then is the point of power factor correction.
I know that the current consumed by the plant will reduce but will this have a bearing on the bill at the end of the month or is it to save on cable sizes etc?

2- Is the KWhr the only parameter that the power company is interested in?
If I have a low power factor and am using large KVA for the KW used should they not be billing me in KVAhr?

3- Does current consumed not come into the power company's equation?

4- From the current consumed by each motor, is there much I can do with this to give an indication of the pumps effeciency over time?

Any help on this appreciated.

Justin

 

[waross]

First stop guessing and ask someone at the utility to explain the tariffs for the plant.
Often one part of the bill is based on the KWHr consumption. A poor power factor may be a penalty. One of the simplest penalty formulas was based on 90 minus the PF in percent equals the percent penalty.
So, if the average power factor for the month based on KWHrs and KVARHrs was 75% a penalty of (90-75=15%) of the consumption charge would be added to the bill. There are much more onerous formulas for PF penalties.
Then for a large plant, there may be demand charges based on the maximum KVA or kW demand for the month. In some instances the demand charges may be the largest component of the power invoice. Some utilities charge the months demand or 90% of the last months demand, whichever is higher. Even if power is disconnected the demand charges may continue for 10 months under some tariffs.
Ask your utility!!!

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[LSpark]

I strongly suspect that the power company will have kW and kVAr tariff meters on the commercial boundary, they aren't interested in the current (as this is instantaneous) but in the half hourly totals of kWhs and KVArhs (I think Ireland is similar to the UK).

As Bill said you will need to ask your supplier what the billing arrangements are, there will be a charge for either average power factor or kVArhs.

Are you connected to the local grid or is it an 'over the wall' supply direct from a generator?

 

[justint]

Thanks

I have tried getting information from the supply company but am still waiting for someone from the industrial pricing section to get back to me.

I have a smart power meter in at the moment (Socomec Diris A40) that Im extracting information on KVArH via modbus. Im presuming this is KVA based and not KVAr based as the notation would suggest(would this be of any use to know i.e. to know the volt amp reactive hours?)

Yes the supply is from the grid.

Justin

 

[rcw retired EE]

Older meters could measure and count watt-hours. By using a phase shift transformer, a similar meter could be wired to measure and count var-hours. The two readings from last month are subtracted from this months readings to get kVArh and kWh consumed this month. If a power factor penalty is to be applied, the average power factor for the month could be calculated from those two values. Note that it is not a true average power factor but a number that is easy to obtain, and close enough to meet the regulatory scrutiny.

With modern meters, we can get more useful data but there still may be a tariff or contract that calculates billing using the monthly kWh and kVArh.

 

[justint]

Ah so the KVAr is measured to be able to work out an average (if not totally accurate) power factor for the month that can be used to check if the power factor is within limits?

From what I can see tho, I have either an option of a unit that provides KWHr information OR KVArHr. Surely this isnt much use just knowing the KVArHr on its own...?

 

[waross]

KWHrs and KVARHrs are used to calculate KVAHrs.
KVAHrs and KWHRs are used to calculate the average power factor.
Historically this was done with two KWHr meters. One meter would have the input voltage phase shifted 90 degrees so as to respond to KVARHrs.
The modern electronic meters are able to calculate and record quite a few parameters.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[LSpark]

Modern meters are usually 4 quadrant meters - they record import & export watts and import & export VArs. So you should be able to get those 4 quantities as instantaneous and half hourly totals from the meter (along with a good few more parameters). The supplier (or settlements) will dial in to get the half hourly totals and probably use those to calculate a rough average power factor.

 

[stevencress]

question 1 The plant's electric bill is based upon a charge for kwhr consumed during the month and a charge for peak monthly kva.

question 2 Dont worry about kvarh or amps, both are proportional to kva. The utility bills for kwhr and peak kva.

question 3 See the answer to question 2.

question 4 Improving the pump efficiencies will reduce the kwhr consumed and will lower utility bills, but I dont know that much about pumps.

 

[justint]

Ok, have a better understanding of whats involved now.

Thanks for the help.

Justin

 

[justint]

Hello again.
Ok, have been monitoring a number of parameters from my smart power meter now for over a week.
The meter applies a CT to each of the incomming 3 phases and a connection to each phases voltage.
While on site the current that we were seeing on the power meter was the same as the reading on our clamp meter for 2 phases, we could not physically get the clamp meter around the 3rd phase.

We also noticed on the meter that 2 of the phases were showing inductive KVAr and 1 was showing capacative.

The past weeks data shows KVAr and KW and PF data that all adds up correctly (will take a longer test to compare with actual bill).

The difference in KVArHr and KWhr also adds up to the same average PF for the week as the instantaneous PF.

1 current consumed is showing way off the other 2, in some cases by up to 80 amps (sometmes smaller, sometimes greater).
Most of the equipment is 3 phase motors, the only single phase equipment woould be lighting and heating.

Q - How is power factor correction capacitance applied?
Could it possibly be across 1 phase and neutral and this is why I am seeing such large differences (including the capacitance) in 1 current?

Justin

 

[DTR2011]

sounds like a CT polarity issue. check the secondary wiring, or simply check plausibility by adding 180 degrees to the phase that is off.

 

[justint]

Just had an after thought there and another look at the results. It seems that it isnt always the same phase current that is different than the other 3.
Could it be that the power meter is taking a very quick sample each minute and is capturing instantaneous current on the wave form as they rise and fall with 120 degrees between them showing greater differences in each phase current?

Maybe I shouldnt look too deep into the current values and look only at the power readings that the unit is calculating from these values.

 

[LSpark]

The smart meters that are used on the systems I am responsible for don't look at the instataneous point on the wave form, I'd be very suprised if yours did. Is the CT wiring sound? I have recently come across inadequate crimps on a system installed in the early 90's, quite scary as there is a strong chance it had been open circuiting intermittently. I'd be very tempted to do an offload tightness check of the wiring.

Is there a vector diagram on the meter? I find that very useful when I'm not getting the results I expect and will immediately show a reversed CT. If you have test blocks you should be able to put a standard to see the circuit condiditons.

Or course it may be that the single phase loads come from different phases and are coming on at different times causing genuine imbalances.

 

[EEIre]

An industrial ESB bill is made up of the following components:

1) Service Capacity Charge: This is related to the size of the contracted MIC (Max Imported Capacity) in kVA

2) Max Demand Change: This is related to the maximum load during the 2 month period in kW

3) kWHR Charge: This is for the kWHrs comsuumed during the billing period

4)Low Power Factor Surcharge in c/kVARh, will apply when the metered kVARh is more than one third of the metered kWh in any two monthly billing period.

Note there are now providers other than the ESB but these principles will apply