Real, Apparent & Reactive Power

Wednesday, September 16, 2020
Training

Power, is it real?

This week we will be looking at power; real, apparent and reactive and how power factor fits into the context of a commercial solar system

What is Power Factor?

Power factor is the relationship between voltage and current in an AC system and with purely resistive loads, current and voltage is together resulting in a power factor of unity.

In reality this is never the case as sites usually have a mix of both capacitive and inductive loads so the power factor is always less than one.

On sites with capacitive and inductive loads the current is not in phase with voltage. 


Non unity power factor

On-site there is a consumption of both real and reactive power. The real power is what counts, it does all the work but the customer actually pays for the apparent power which is equal to  real power/power factor.

What is reactive power?

Reactive power is a necessary part of the whole equation and is something required to maintain voltage and it helps the flow of real power in the AC circuit. The more reactive power there is, the lower the power factor, and if there was zero reactive power then power factor would be unity.


Importance of reactive power

It is required to maintain the voltage to deliver active power (watts) through transmission lines and all motor loads require reactive power to convert the flow of electrons into useful work. If there is not enough reactive power, the voltage sags down and it’s not possible to push the power demanded by loads through the lines.

What is apparent power?

Real power and reactive power vectors are added together to give apparent power and the site pays for the apparent power. There is a vector relationship between real, reactive and apparent


Site: inductive/capacitive loads

Motors etc are inductive loads and on these sites, current lags the voltage which results in a lagging power factor. So the current lags the voltage. This is the most common situation encountered on commercial sites.

If the majority of loads are capacitive, current leads the voltage, resulting in a leading power factor, a less common situation.

The grid is affected by all this

With a site experiencing a leading power factor this can  lead to a rise in voltage whereas a lagging power factor leads to a fall in voltage

Because of these extreme rises and falls in voltage, the grid tries to protect itself.

Now if reactive power increases, apparent power increases also, but there is no change to real power at all because of their relationship. The generators are doing more real work but they still have to provide more apparent power.


So what do you do

For sites experiencing low power factor there are techniques and products that can be employed to correct this power factor situation. A site with lagging power factor can use a large amount of capacitors to offset and reduce the amount of reactive power that comes from the grid.

Power Factor and inverters

There is some semblance of control with the use of inverters with reactive power control.

Usually the default setting is PF of 1 and this means they contribute only active power.


Conclusion

No site will have a perfect PF of 1 as there is invariably inductive and capacitive loads

All power is made up of real, apparent and reactive power but it is the apparent power that you pay for on your bill. The commercial solar designer must take into consideration the site’s specific power factor and, if fairly low, certain strategies may have to be adopted to correct in addition to planning for a commercial solar system.


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