Power factor and grid-connected PV

By Global Sustainable Energy Solutions

As the level of grid-connected photovoltaic penetration continues to rise, the importance of power factor and power factor correction is going to become increasingly relevant from the perspective of the grid and the customer.

This article explains what power factor is, what it is caused by, its impact on the grid, and how grid-connected PV can both degrade and improve power factor in a system.

Power factor is a measure of the phase difference between the voltage and current in an AC power system. In purely resistive loads (such as an incandescent lightbulb or electric kettle) the current is in phase with the voltage and there is ‘unity’ power factor. Capacitive and inductive loads (such as a capacitor banks or inductive motor respectively) will cause the current to ‘lead’ or ‘lag’ the voltage, resulting in a ‘non-unity’ power factor.

A non-unity power factor means a load is consuming both active and reactive power. Active power (also known as real or true power) is the ‘useful’ component of the AC power and is what contributes to the work done in a system (e.g. rotation of a motor shaft or the glowing of a lightbulb). Reactive power oscillates between the generation source and the load, and does no work in the system. Reactive power however is needed to maintain the voltage in the system, provide magnetising power to motors and facilitate the transmission of the active power through the AC circuit.

Power factor is the cosine of the phase angle in a power triangle. It is defined as the ratio between the active power (W) and the apparent power (VA). Power factor will vary between 0 and 1, and be either leading or lagging.

Power factor = cos ø = Active power (W)/ Apparent power (VA)

For purely resistive loads (such as heaters and incandescent lamps) the voltage and current are in synchronisation, therefore they have a power factor of one, or unity power factor. With inductive loads (such as induction motors) the current lags the voltage, therefore they have a lagging power factor. With capacitive loads, (such as capacitor banks), the current leads the voltage, therefore they have a leading power factor.

Power factor and the grid

The supply of reactive power is very important in an AC power grid. The amount of reactive power produced by generators must closely match that which is being consumed. A leading power factor in the system (due to capacitive loads) causes the voltage to rise and a lagging power factor (due to inductive loads) will cause the voltage to fall. If reactive power is either under or over supplied, the voltage on the network may rise or fall to a point where generators must switch off to protect themselves thereby decreasing the generation and cause further problems.

Increasing the reactive power increases the apparent power but has no effect on the active power. This means the generators in the system must supply more apparent power even though there is no additional work being done by the system (as there is no increase in active power). Therefore, power factor is best corrected locally. The most common way for this to be performed is by using banks of capacitors that can be shunted in and out of the system depending on the operation of the load.

Residential customers do not consume enough energy to warrant the additional costs of metering equipment to measure power factor. Large industrial and commercial customers, however, are billed for consuming power at a poor power factor. There is, therefore, an incentive for these customers to improve the power factor of their loads and reduce the amount of reactive power they draw from the grid.

Power factor and grid connected PV systems

Most grid connected PV inverters are only set up to inject power at unity power factor, meaning they only produce active power. In effect this reduces the power factor, as the grid is then supplying less active power, but the same amount of reactive power.

Case 1

Consider the situation; a factory is consuming 100kW of active power (blue line), and 32.9kVAr of reactive power (red) from the grid. The resulting apparent power (black) as drawn from the grid is 105.26kVA. Now using power factor formula, the power factor is 0.95 lagging (phase angle 18.3°).

Case 2

If this factory were to install a 60kW PV system (light green) that exported at a unity power factor, only the active power imported from the grid would be affected. The imported active power from the grid (blue) has been reduced to 40kW, while the reactive power imported from the grid (red) remains constant at 32.9kVAr. The resulting apparent power as drawn from the grid reduces to 51.79kVA. This has the effect of reducing the power factor to 0.77 – lagging (phase angle 39.4°).

This problem of poor power factor however can be addressed through the selection of appropriate inverter products. Inverters with reactive power control can be configured to produce both active and reactive power, i.e. an output at a non-unity power factor. This means the power factor for the load can be kept within reasonable limits.

Case 3

This shows the factory with the inverter set to a power factor of 0.95 – leading. The PV system is now producing 57kW of active power (light green) and 18.7kVAr of reactive power (dark green), reducing the amount of both active and reactive power from the grid. The resultant power factor is therefore maintained at what it was originally at 0.95 – lagging (phase angle 18.3°).

It would be possible to configure this inverter to produce more reactive power and bring the factory to a unity power factor. The optimal power factor that the inverter is programmed to export at will depend on the energy contract of the consumer. Utilities can bill industrial and commercial customers for the energy they consume, their peak demand, and their power factor. Any solar system should therefore be designed to produce the maximum amount of savings across all of these areas. For example, it may be financially beneficial to reduce the amount of active power drawn from the grid at the expense of increased charges due to a poor power factor.