What Are Power System VARs?

Most people involved in building automation are familiar with kW, which is the rate at which a building is consuming energy from the power company. But there is another power system parameter known as VAR/kVAR which is less well understood. In this article we will attempt to dispel some of the VAR mystery.

The term “VAR” stands for “Volt-Amperes Reactive.” Guess we’re done here, right? What’s that? You were hoping for a little more explanation. OK, let’s dig a little deeper.

First, for any readers that are complete newbies to power monitoring, let’s explain the ‘k’ prefix frequently found on power system readings. “k” simply means “times 1000.” So three kV is three thousand volts, two kW is two thousand watts, one kVAR is one thousand VARs, etc…

What is a VAR?

Let’s begin our VAR discussion by saying that some of the electrical loads in a building (motors, transformers, classic style fluorescent lighting ballasts) use rising and falling magnetic fields to perform their intended functions. We call these “inductive” loads. When an inductive load is drawing power from the power company, some of this power is used to build up the load’s magnetic field during one part of the power cycle. The magnetic field stores part of the energy being delivered to the load.

Here is the interesting part – at a later point in the power cycle, the magnetic field which was built up earlier collapses. When that happens, the energy that was stored in the magnetic field is converted back into power which is returned to the power company! So with inductive loads, extra power is “borrowed” from the power company temporarily but is later “returned” to the power company. The extra power needed by inductive loads essentially bounces back and forth between the power company generator and the loads. This power is called reactive power and given the name VAR (Volt-Amperes Reactive) power.

VAR power does not show up on a conventional kW-only power meter. The kW meter only shows power that is actually consumed by the load. However, many modern electronic power meters such as Kele’s endicator will display both kW and kVAR power being drawn by a load.

Are VARs a problem?

So… if VAR power is not actually consumed by the load, but is returned to the power company, then there is no problem, right? Wrong. The extra VAR power, even though it’s not consumed by the load, causes larger currents to flow through the power company’s generators and power distribution system. So the power company has to install beefier generators and distribution equipment to handle that extra current flowing. Therefore they are not happy when a customer’s load is drawing high VARs.

To discourage customers from presenting high-VAR loads to the power system, the power company will sometimes install a VAR meter on a commercial or industrial building and tack a penalty on to the power bill if the VAR reading goes over a certain limit. This is normally not done for residential customers (good news for your home power bill).

How can I compensate for a high-VAR inductive load?

The good news is that there is a way to compensate for a high-VAR inductive load. There is an electrical component called a “capacitor” which also draws reactive power but stores the energy in an electric field instead of a magnetic field. Now, here’s the cool part – the capacitor stores and releases its reactive energy at the opposite times of an inductive load. That is, just as the inductive load needs extra energy to build up its magnetic field the capacitor is ready to give up the extra energy stored in its electric field, and vice-versa.

So by installing the correct amount of capacitance in parallel with an inductive load, the extra reactive power needed just cycles back and forth between the capacitor and the inductive loads, and the power company does not ever see the reactive power on their system.

Note we said the “correct amount” of capacitance. The value of the capacitance must match the value of the load inductance for complete cancellation of the reactive power seen by the power company. Of course, in the real world they won’t be perfectly matched; but still, the reactive power seen by the power company can be reduced to a low level.

If your building has a relatively constant inductive VAR load, then a fixed bank of capacitors can be installed for “nominal” reactive power cancellation. If your building has inductive loads that are dynamically cycled so that the inductive VARs fluctuate a lot, there are “smart” controllers that can measure the instantaneous inductive VARs and switch different values of capacitance in or out of the system to maintain on-the-fly dynamic cancellation of the inductive VARs.

Conclusion

The inductive VAR load presented by a building to the power company is always undesirable. You may or may not be penalized by the power company for a high-VAR load, depending on your situation. Capacitor banks can be added to a load to cancel the inductive VARs seen by the power company. Capacitor banks can be provided as fixed-value or dynamically-adjusted devices depending on whether your inductive VARs are steady or fluctuate widely. Kele’s endicator power monitor will give your building automation system the information it needs – both kW and kVAR, to control capacitor banks and minimize those utility penalties if they are causing you a pain in the wallet. Call Kele today to find out how!