Bad Power Factor? – A reason to oversize your inverter

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In a previous blog, we discussed some good reasons to oversize your PV array. In this blog we will discuss how, by oversizing your inverter, you can correct a site’s poor power factor.

Electricity used in our homes and businesses is (almost always) alternating current. Put simply, voltage and current that are transmitted throughout the electric power grid in a sinusoidal waveform averaging 0. When these current and voltage waveforms are perfectly synchronised in time, they have a power factor of 1 or pure active power.

Example of pure active power (left) with current and voltage perfectly in-phase, and of pure reactive power (right) with current and voltage perfectly out-of-phase.

Example of pure active power (left) with current and voltage perfectly in-phase, and of pure reactive power (right) with current and voltage perfectly out-of-phase.

When we consume electricity (in pumps, fridges, lights, etc) current and voltage waveforms can  go out of alignment. This will lead to a power factor ≠ 1. As a site’s power factor moves further away from 1, they will typically incur increased grid quality supply charges from their electricity provider. This is where your SMA inverter can begin to help save you even more money. By utilising SMA inverter’s built in grid support functionality, you can correct a bad power factor by feeding reactive power as well as active power and hence reduce the grid quality charge component of your electricity bill. This can often be cheaper than using additional power factor correction equipment such a capacitor banks.  Often active power  is just as valuable to a site as reactive power for correcting power factor. This creates a financial driver to oversize your inverter.

How much should I oversize my inverter?

Since this is an abstract concept for a lot of system designers and installers, let’s work through an example.

First, we need to understand the relationship between ACTIVE, REACTIVE and APPARENT power. Apparent power consist of active and reactive power, two different types of power existing only in its pure nature. Because active and reactive power don’t have a relationship, it is impossible to convert one power type to the other. A graphical model of such relationship is a cartesian coordinate system with active power for the x-axis and reactive power in the y-axis. These 3 power types are related together according to Pythagoras’ theorem as shown in the following diagram.

Relationship between Apparent, Active and Reactive power.

Relationship between Apparent, Active and Reactive power.

Now let’s assume we have a site which is consuming 80kW of active power with a site power factor of 0.85 due to some inductive loads such as pumps and motors. This would result in the following relationship:

Power consumption without power factor correction or generation

Power consumption without power factor correction or generation

Now let’s assume the site needs to correct its power factor back to 0.90, and they also want to reduce their active power consumption by ~60%. If we begin with a 60kW solar system (60kW PV array, 60kW inverter), and this system generated power with a cos(φ) of 1.0, we would have the following power consumption. We can see that if we did nothing to the way the solar system operated, it could actually make the site’s power factor (and hence power quality charges) significantly worse from the utility’s point of view.

Power consumption with Generation at cos(φ) 1.0

Power consumption with Generation at cosphi 1.0

Now let’s operate the solar system with a cos (φ) of 0.82 to try and correct the site’s power factor. We would have the following power consumption and generation relationships:

Power consumption with generation at cosphi 0.82

Power consumption with generation at cosphi 0.82

This would then have the resultant power consumption for the site according to the following:

Resultant power consumption with inverters correcting power factor

Resultant power consumption with inverters correcting power factor

We could then consider to implement a cos(φ) function similar to the following could help to ensure that as the solar system increases its output power, it will change its cos(φ) to compensate for site power factor.

Dynamic cosphi function to assist correcting a poor site power factor

Dynamic cosphi function to assist correcting a poor site power factor

In this example, we require 60kVA of inverter capacity, but only 49kW of active power generation, meaning we can oversize our inverters by about 20% compared to the size of our PV array. SMA inverters can generate reactive power without using any active power. Within SMA, were refer to this capability as Q @ Night ( read more about Q @ Night here ).

 

Conclusion

By oversizing inverters, you have reserve reactive power capacity which can be utilised without sacrificing active power generation. Utilising the built-in grid support functionality in SMA inverters, such as a dynamic cos(φ) function, can help to improve a site’s power factor and in turn help to reduce grid quality supply charges a customer might incur from their electricity provider.

You can read more about this topic in detail by reading SMA’s PV Grid Integration technical compendium.

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