What is power factor and why it needs to be improved?

In AC power systems along with active power there is always reactive power. Reactive power is either generated or consumed by system elements that have inductive or capacitive reactance (transformers, generators, motors, capacitors, transmission or distribution lines and others). While reactive power is important for some of these elements to operate, it also causes power losses in transmission, distribution and industrial systems, therefore, usually need to be compensated.

However, the benefits of reactive power compensation are sometimes not sufficiently evaluated. Thus, in this article I will present the importance of reactive power compensation and some useful information about the power factor.

What is the power factor?

vector_diagram

Fig. 1 Power triangles: a – in case of low power factor (big angle), b – in case of high power factor (small angle)

To express the amount of reactive power, power factor (PF) is used.

Basically PF is the ratio of real power P and apparent power S or, on the other hand, it’s a cosine of angle between vectors of P and S as depicted in Fig.1.

The bigger is the angle, the greater is reactive power. As one can see from Fig. 1 in case of low power factor more apparentpower need to flow through power lines in order to transfere the same amount of real power, which is required to perform an efficient work. Therefore, power factor correction is performed by minimizing the angle between P and S (Fig.1.b).

Why it is useful to improve power factor?

Improvement of power factor reduces current flow through the electric lines and other system elements. Because of this power losses are reduced. Compensation of power factor also means that more active power can be transferred without overloading the equipment, thus, system capacity increases. Moreover, capacitive load improves voltage at system bus. And finally, it reduces electric utility bills.

A realistic estimation of power factor correction benefits can be performed with online power system analysis tool ESMOgrid.

Let’s consider a simple one line diagram of distribution system with only one load, which has low power factor PF equal to 0.743, real power P equal to 5 MW and reactive power Q equal to 4.5 MW (Fig. 2).

One-line-diagram-ESMOgrid

Fig. 2 Simple example of distribution system with one load. Capacitor is disconnected.

As can be seen from load flow study results in Fig.2, apparent power flowing through power line is S = 7.092 MVA. It consists of real power P = 5.028 MW and lagging reactive power Q = 5.002 MVAR. Apparent power is in red, because line is overloaded, as nominal power of transformer is only 6 MVA. Another issue to consider is voltage drop at Factory bus that is higher than 5% and may also cause problems for more sensitive equipment.
However, these problems can be solved by connecting optimally sized capacitor to Factory bus (Fig.3). In this case power flow through electric line is around 28% smaller than in system without capacitor.

One-line-diagram-2-ESMOgrid

Fig. 3 Simple example of distribution system with one load. Capacitor is connected.

 

Therefore, power flow does not exceed nominal power of transformer. What is more, voltage drop at Factory bus is only 0.2%. Explicit results are at Table 1.

Table 1 Power flow results in system with capacitor and without it

  Apparent power flow Real power flow Reactive power flow Voltage drop Active power losses Reactive power losses
No capacitor 7.092 MVA 5.028 MW 5.002 MVAR 5.155 % 9315 W 167.4 kVAR
With capacitor 5.014 MVA 5.014 MW 9.752 VAR 0.153 % 4655 W 83.7 kVAR
Benefits 28% smaller power flow P remains nearly unchanged Minimized Q power flow Minimized voltage drop Around 50% smaller active and reactive power losses

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