kVA demand costs money
The transmission and distribution networks are under strain due to kVA demand.
Harmonics add to the reactive load, resulting in wider adoption of kVA demand tariffs.
Modern metering measures total demand including harmonics – think of it as kVA caused by the power factor we all know, plus a lot of expensive ‘froth’ on top.
In the old days, kVA was measured by tri-vector meters, and kVArs by a power meter using a 90-degree phase shifter. These were basically electro-dynamic meters – cousins of the Ferranti spinning disk kilowatt/hour meters still seen in older homes.
Old-style kVA demand metering cannot respond to harmonics, with the possible exception of the third. Electronic meters do respond to harmonics. In essence they are sampling meters.
Let’s assume that a 50Hz current is sampled 100 times during the 20 millisecond period. The maximum harmonic to be measured will be the fiftieth.
The lower the power factor, the higher the kVA. With power companies adopting kVA demand tariffs across Australia, there is increasing demand for power factor correction.
But that is not all. Harmonics are lowering the power factor even more. In short, installing a capacitor bank to correct power factor is probably doing half the job at best.
Figure 1 illustrates total kVA demand. The red triangle is one you have seen many times. The kilowatts and the kVAr (reactive) are at right angles. The green triangle shows the kVA due to current harmonics sitting on top of the red one. Total kVA is the thick green line – measured by sampling meters.
Years ago there were harmonics, but the level was much less than today. We had the third due to transformers, welders and arc lamps, but nothing like the situation today with power electronics such as variable-speed drives, IT, electronic ballasts and LEDs.
Back then the power factor was affected by motors drawing mainly lagging current. Capacitors provided a sink for lagging currents and, apart from some switching in and out of more or less capacitance, there was nothing else to do for bringing the power factor towards 1.00 – or as near as dammit.
However, things have changed and new technology is being employed. We’ll look at this in the 2016 autumn issue.
The joined-triangle figure illustrating total demand now becomes very important. In many installations the kVArh (harmonic reactive kVA) is much bigger than the kVAr (reactive kVA).
In most installations, current harmonics are the main feature. The term describing the overall harmonic contribution is THDI (total harmonic distortion for current).
THDI is usually given as a percentage of the 50Hz (fundamental) current. So let’s do some maths. The joined-triangle diagram gives rise to the formula:
PFT = PF1 + TDHI2
PFT is the total power factor, which determines the kVA demand. PF is the displacement power factor, which can be increased by using capacitor power factor correction equipment.
In this installation, with no harmonics present, the demand is kilowatts divided by 0.9. However, with harmonics present, it will be kilowatts divided by 0.805. The demand has gone up 11.8%.
Tariffs around Australia vary a lot, but a realistic example is a kVA charge of about $35 per annum. A consumer using 400 megawatt/hours a year with a power factor of 0.9 could have an increase of about 50kVA, adding $1,800 to the bill.
In case that doesn’t sound frightening, bear this in mind. The above calculation has assumed that the consumer’s demand is 24/7 without bumps – an unusual situation.
Imagine a large building with heating, ventilation and air-conditioning (HVAC), and peak summer demands of perhaps 200kVA. Now the addition to the bill is $7,200. The customer will be charged on that basis for about 12 months, even though demand will drop again. High harmonics (HVAC can inject quite a bit) will only bump up the demand and the kVA charges.
Total power factor made worse by harmonics is a vexing problem. Expect it to become more important to your new – and existing – installation customers.
With proper instrumentation you can assess harmonic levels, displacement power factor, overall power factor, instantaneous kVA demand, etc, and be in a position to suggest solutions. We will go into a bit more detail, but it’s important to summarise:
- kVA demand is straining transmission and distribution networks;
- harmonics add to this, spurring wider adoption of kVA demand tariffs; and,
- modern metering measures the total demand, including harmonics.
We have given kVA demand a bit of exposure, but what is the situation with kilowatts?
There are still distribution folk who base maximum demand on kilowatts. Kilowatts are largely unaffected by harmonics and obviously not by reactive VA. If the point of entry to an installation showed zero source impedance, and the voltage was purely sinusoidal, the power would be as follows:
Rms voltage (50Hz) x rms fundamental current (50Hz) x displacement power factor (determined by the phase angle between voltage and fundamental current).
In that situation, an old-fashioned or modern electronic meter would show the same kilowatts. Harmonic losses in the installation have no effect on the meter reading. However, there is a fundamental (50Hz current) I2R loss in the wiring – but that’s normal.
The sine wave voltage assumption is a good one, but zero source impedance (ie: between the installation and supply network) is not. To understand what’s happening refer to Figure 2.
The load shown relates to a typical variable-speed drive. It pumps out harmonics (illustrated by the little circles representing harmonic current generators) that make for current distortion, or THDI.
The diagram shows that the harmonic current is flowing in the outward direction. At the incomer, where the revenue meter is located, there are harmonic voltages as well as harmonic currents, and they make up the ‘exported’ power to the supply network.
The exported power is subtracted from the power supplied to the load. If an old-fashioned electro-dynamic power meter was employed, there would be no effect on the measured kilowatts. The meter would be ‘blind’ to harmonics.
Electronic meters are sensitive to imported harmonics, and a neighbouring installation could be feeding harmonic power in. Apart from reticulation problems, such as resonance, harmonic power would be consumed and added to the useful kilowatts. Let’s summarise:
- electronic (sampling) meters measure total kVA including harmonics;
- kilowatt measurement is largely unaffected;\
- power factor is the combination of reactive VA and harmonic VA; and,
- power factor correction requires reactive control and harmonic filtering.
It’s a safe assumption that consumers don’t understand many of the aspects controlling demand. There is a good business opportunity for electrical contractors to do some specialised work in energy cost reduction and power quality improvement.