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Figure 1Relationship between Real and Reactive Power

WHITE PAPER - SOLAR PV SYSTEMS AND POWER FACTOR

INTRODUCTION

With the rise in electricity tarrifs, many companies and commercial users have switched to renewable sources of energy especially solar energy. The solar PV plants reduces the utility bills and has a fast payback compared to other renewable power systems, but they also do something surprising which could reduce the overall savings.

The grid tied solar inverters produce only real power (pre-set function) and many industrial and commercial consumers consume large amount of reactive power and also do not have capacitor banks to offset this. This leads to the deterioration of the power factor and hence will lead to penalties for maintaining low power factor.

1. UTILITY POWER Before we discuss how a solar PV plant affects the power factor, let us first see the different types of commercial power. The real power as the name suggests is the power that is used to do real work, example lighting and heating. The resistive loads are the users of real power. In resistive loads, the supply voltage and load current are in phase and constructively work together. The reactive power can be classified into two types, namely lagging reactive power and leading reactive power. Reactive power

is lagging if the loads are inductive in nature and is leading if the loads are capacitive in nature. Reactive power do not do any work as the supply voltage and load current are 90 out of phase (lag or lead).

The apparent power is the vector sum of the real power and reactive power. The real, reactive and apparent power can be represented as the 3 sides of a right angled triangle as shown. The cosine of the angle between the real power and apparent power is called power factor (PF) and is the direct measure of the quality of power.

2. GRID TIED SOLAR PV SYSTEMS Let us now understand how a solar PV plant will affect the power factor. Consider a commercial client consumes 100 kW real power and 30 kVAr reactive power which will be monitored by the utility meter. The power factor is 0.958.

= ( )

()

=

! + !

= 100

100! + 30!= 0.958

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Now consider that a part of the load is supported by a solar PV plant (70 kW) that produces power at unity power factor. Now the utility meter will read 30 kW real power and 30 kVAr reactive power and hence the new power factor is 0.707.

= ( )

()

=

! + !

= 30

30! + 30!= 0.707

The reason for the 26 % drop in power factor is that the solar grid tied inverters are only producing real power.

For instance, the cost incurred due to low power factor maintenance in Tamil Nadu when the PF falls below 0.85 and above 0.75 is 1 % of units consumed for every 0.01 drop in power factor and 1.5 % of units consumed for every drop in power factor below 0.75

Let us assume the energy consumption for a given month is 12,000 kWh and the average power factor is 0.707 then the client will be fined INR 20,592 (Cost = INR 8/kWh).

!"#$%&' = 0.85 0.707 1.512000

!"#$%&' = 20,592

As it can be seen that the penalty increases as the power factor keeps falling. Therefore, the savings brought in by the PV system will reduce significantly. 3. POSSIBLE SOLUTION The power factor can be mainted in two different ways:

a) Configuring the inverter to produce reactive power.

b) Using an automatic capacitor bank.

The grid-tied solar inverter in its pre-set state will only produce real or active power and this can be configured to produce reactive power. However, this configuration reduces the real power production and it also requires an expensive dynamic control system. An automatic capacitor bank provides necessary power factor correction based on the load by providing reactive power. This will be the recommended solution as the PV system is left at the pre set function to produce maximum possible active power, which is the ideal case. 4. CONCLUSION The power factor is an element that has been overlooked by many PV system integrators in India and the main reason is ignorance and inexperience. It is necessary to include PF as an important parameter when integrating a PV system to the energy mix, inorder to avoid unnecessary penalities and correction complications.