Power factor (again)

Hmm possibly. I presume this is a fraction of the total - it sounds like a lot, but I do know you have a big site.

Depends how the sub-metering is working.

If it is measuring KVA, yes,  if KW no.

Most official power meters are more or less power factor agnostic - that is to say the reading does not take RMS amps and multiplicity by RMS volts, (that would be kVA) but also allows for the phase slip type of power factor. (kW)

This is done in electronics these days, where the spot current is multiplied by the spot voltage at perhaps a couple of  hundred sample points  spread more or less uniformly over the main cycle, and then an average deduced. For part of the cycle this has a positive voltage multiplied by a negative current and that is the power returned due to the phase shifts.

In the older units it was a mechanical thing where the forces on the counter were the product of a magnetic field generated proportional to the current and another electromagnet proportional to the voltage. The effect for nice clean sinewaves is the same, some noisy switching waveforms can confuse the sampler kind.

Mike

Hello

It it is a big site could just be the meter errors.  What accuracy standard is the metering and CT set up?

Any on-site generation?

the meters arent MID standard, so it could be metering errors, id have hoped they were a little more accurate than that though.

They do record power factor and you can view KVA on the meter, but we are using kWh because its all to do with our energy monitoring, carbon footprint etc, we have got to keep things fairly simple if we have any chance of consistency.

AJJewsbury: 
 

Any on-site generation?

yes, about 20kW but thats down stream of one of the sub meters, it doesn't take it negative

This is going to get very complex because you need to be sure of several different things. What exactly are your metering devices, and what is the claimed accuracy? You need to know that they really are completely power factor independent, and if so then all the numbers should be exactly equal to overall. However, no meters (even “mechanical" ones) are completely independent unless very sophisticated electronic ones which will undoubtedly give you a PF reading as well. I would not expect an accuracy of better than 0.5-1.0% for most kWh meters. The best way to measure your saving would be a KVAh meter on the PFC capacitor bank connection, or two identical kVAh meters one before and one after the PF corrector. This should give you the best measurement possible, including the metering accuracy. Measuring kWh is never going to give you the number you want with any accuracy.

they are Rayleigh RID240 meters 

They do display PF and log it etc. 

The objective of the system really is to identify areas with the potential to save energy, that's all. I think I need to avoid getting bogged down into the accuracy, but I am enjoying learning about it and how it works on our site. 

it would have been a point of interest if I could say the PFC was saving £x, but no other consequence as I know our PF is .99 on the bill

the other thing, upstream from the meters, the busbar chamber gets pretty toasty, so there will be some losses in that just on its own, and that is between the sub meters and my main income meter

the DNO meter is on the 11kV side of things, so I know that will be different again due to transformer losses too

I may have got the wrong end of the stick but why don't you take a load of readings of all the meters  except the DNO one on the HV side then switch out the PFC bank wait say one hour then re read all your meters  the sub meters should add up to the reading on the DNOs LV meter except for the tiny amount used by the meters themselves. Was this site featured in a youtube video recently? It sounds very similar to an installation featured on there 

I have thought of that, but with one slight concern, depending on how things are running load wise, with the PFC switched off, we can exceed the main ACB rating

we shouldn't have been on youtube at all, all very hush hush

 

Any on-site generation?

yes, about 20kW but thats down stream of one of the sub meters, it doesn't take it negative

 

Most simple meters seem to only record “import” and ignore “export” - rather than clocking backwards for example (there used to be a ratchet-and-pawl symbol on at least the old mechanical ones) - so it seems possible that some of the 20kW is coming out of one sub (not recorded) and then being consumed by a different sub (recorded) without going through the main meter at all.

20kW would be 3,360kWh per week if running 24h/day - so might explain 1,500kWh/week if it wasn't producing 24x7 or half the generation was consumed in the same sub.

Edit - just seen your post about the sub-meters - what do the export registers show?

   - Andy.