Larnacman : You are correct, DNO meters register kW (well, kWh but that is being picky). The reason that I worked my answer in kVA is that, from KME's orginal example, we don't have enough information to calculate the kW - we would need to know either the reactive power or the power factor before we could do that.I'll try and explain ....... there are three kinds of power in an AC system: Real, Reactive & Apparent powers.
Real power is the thing that we are really interested in, it is measured in Watts, equates to 'work' and is the energy which is consumed by a simple resistive load like a heater or filament lamp load.
Reactive power is the exchange of magnetic energy, it is measured in VAr, is sometimes called 'wattless power' and is the power used to magnetise transformers, motors & other wound items. Just to complicate matters further, reactive power comes in two flavours - lagging (motors, inductors, transformers) and leading (Capactive loads)
Apparent power is the thing that we can easily measure - it is the product of the applied voltage & the current flowing and is measured in VA.
These three powers are related by the power triangle - real power (watts) is horizontal, reactive power is vertical & apparent power is the hypotenuse. Also, from this triangle comes the powert factor - it is the cosine of the angle between the real power & the apparent power. When we apply 'power factor correction' we are attempting to reduce the vertical 'reactive power' part to zero so that the apparent power is equal to the real power.
So what does all this have to do with metering, well, as stated above & also correctly stated by Larnacaman, we buy electricity from the DNO in kwh - ie we buy real power not apparent power. So how does the meter work this out?
An old analogue (single phase) meter consisted of two coils - a potential coil which was connected between phase & neutral and a current coil which was connected in series with the entire load current. These coils were arranged one above the other with the aluminium disc in between them. When a load was applied, the interaction between these two coils induced an eddy current in the aluminium disc, thus creating a magnetic field which repelled the field from the two coils hence creating a torque on the disc causing it to rotate. The higher the load current, the higher the torque & the faster the disc spun.
Now for part two of the power engineering lesson ........ when we measure the current in a circuit we are actually measuring the 'apparent' current - we can't easily measure the part of it which is responsible for the real power (Watts) or that which is responsible for the reactive power (VAr) however there is one way to separate them. If we could look at the voltage waveform & the current waveforms on an oscilloscope we would discover something interesting - the 'real' current is in phase with the voltage (ie. the peaks line up) while the 'reactive' current would be 90 degs out of phase. (ie. 1/4 cycle behind or ahead depending on whether it is inductive or capacitive).
So, back to metering ....... as stated above, the component of the load current which is responsible for 'Watts' is in-phase with the voltage - it is exactly this principle that the kwh meter uses to separate the real & reactive powers - only the magnetic field produced by the 'real' current in the winding will be in phase with the field produced by the voltage coil and hence will act upon the disc. Clever really.
A three phase meter works on exactly the same principles except that it contains three voltage coils (one per phase, connected phase to neutral) and three current coils - these are arranged such that the torque imposed on the disc is the sum of the torques created by each coil pair.
Modern digital meters use DSP (digital signal processing) to separate the currents into their components but the principles remain the same - it doesnt need to have three displays, the math is done internally and only the answer is displayed.
So, back to the original question ..... why did balancing the load currents reduce the bill. Without seeing the actual bills it is hard to say but I have a good idea:
If the customer is industrial he may well be on a traffic which penalises him for drawing more current than agreed in his contract (remember that the current flowing dictates the sizes of cables, cutouts etc all of which cost money to install & maintain). The metering equipment may well be arranged to detect this and penalise by charging a higher rate per kWh during each month the agreed capacity is exceeded. By balancing the phases, you will have reduced the biggest phase current seen by the metering apparatus and hence may no longer exceeding the maximum agreed figure - this would have a substaintial cost benefit while not actually changing the number of units recorded by the meter.
For those still not sure, consider this - If the meter did simply find this bigest & triple it, that would result in gross over charging for energy not supplied .... wouldn't that be fraud?
I hope I havent put you all to sleep...... :yawn
Adrian
