Power factor (again)

these meters do go backwards, i discussed it with them, its moot anyway, as the sub always consumes well more than what we generate. 

an interesting point, none the less 

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

That could amount to an error between 2% and 50%+

Regards

BOD

Ok Johno, can you supply the reactive power readings on the TX feed (overall) and after the PF corrector. I would expect the Tx to be very low (comparatively) as the PF reads 0.99. The readings on the outgoing feeders will show the reactive power and the total will be about how much you are saving. It may be about the 1500 kVAh you mention, presumably over a period, but a significant amount of money. The TX primary meter will be reading KVAh, and should approximately agree with the TX secondary meter, less 2-3% for TX loss etc. The PF correction could well be saving several hundred pounds over your metering period, and choosing a day for this would certainly help others understand the savings. Another saving may well come from the replacement of the main transformer, modern high-efficiency types may well save their cost reasonably quickly. Taking daily and hourly readings may also isolate more savings from the use of particular plant, or lighting loads that would benefit from changes or replacement.

Regards

David

Johno12345: 
 

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.

There may be CT errors too.  I doubt that these will have been tested together or calibrated.  These errors can soon add up and when you are talking about the difference between two large numbers they can appear considerable.

If you have power factor correction at the incomer it wont affect current and losses down stream of it.  It should reduce losses upstream.

davezawadi (David Stone): 
 

Ok Johno, can you supply the reactive power readings on the TX feed (overall) and after the PF corrector. I would expect the Tx to be very low (comparatively) as the PF reads 0.99. The readings on the outgoing feeders will show the reactive power and the total will be about how much you are saving. It may be about the 1500 kVAh you mention, presumably over a period, but a significant amount of money. The TX primary meter will be reading KVAh, and should approximately agree with the TX secondary meter, less 2-3% for TX loss etc. The PF correction could well be saving several hundred pounds over your metering period, and choosing a day for this would certainly help others understand the savings. Another saving may well come from the replacement of the main transformer, modern high-efficiency types may well save their cost reasonably quickly. Taking daily and hourly readings may also isolate more savings from the use of particular plant, or lighting loads that would benefit from changes or replacement.

Regards

David

I cant monitor the PF on the TX feed, I don't think its feasible to get it really. All I can really say is that the bill, which is obviously from the TX feed shows a PF of, usually, .99. 

What I might do is take the bill reading, take the incomer reading which will give me something like the TX losses, then take the sum of the sub-meters, which seems to be higher, and that should be what the PFC is saving, with the uncertainty of measurement. Its going to take about six weeks to get a full months of data to do this, its all an interesting learning curve.

the main focus is on the sub mains, as we can see loads switching in and out on timers. Armed with some good site knowledge and the size and duration of the load, you can make a good guess of what equipment is turning on and whether a more effective system is needed to shut it down

The transformer is from about 2005, a Schneider ONAN air cooled one, I'd like to think because of its age, its reasonably efficient. 

There may be CT errors too.  I doubt that these will have been tested together or calibrated.  These errors can soon add up and when you are talking about the difference between two large numbers they can appear considerable.

If you have power factor correction at the incomer it wont affect current and losses down stream of it.  It should reduce losses upstream.

Im sure there are going to be CT errors, none of it is calibrated, just bought as matching kits if you like. 

yep, I get that the PFC being all together isn't the best for that reason but its probably a good compromise in my situation

have worked out the TX losses, its about 0.8% based on the half hour data from our supplier vs the meter on the incomer

Good, that is quite low. Now your savings are the difference between the now zero reactive power measured by the supply meter and your measured reactive power, correcting the phase angle uses no “extra power”, it just changes kVA into kW in a more or less lossless way. The PF correction is simply moving the current phase to match the voltage phase, rather than asking the supply to provide current when the voltage is not at its matching value. Reactive power (that is power demanded when the voltage doesn't match) looks to the source generator as extra torque, so costs in terms of excess driving power and therefore fuel.

I think it would be reasonable to say in answer to the OP, that the PF correction is saving 1500 kWh per week as you said. I wonder what that makes the payback period for its purchase? The next problem is to work out why your loads have poor PFs. If the cables are long you may get some significant savings by correcting the load PF at the load, cable losses are increased by poor PF.