Knowledge Centre

The power electronic devices (such as, variable frequency drives-VFDs, phase controlled rectifiers, choppers, and battery chargers) and other major electrical loads in the industries draw currents that have three main components:

• Active current at rated frequency,
• Reactive current at rated frequency, and
• Harmonic currents at higher frequencies (commonly at multiples of rated fundamental frequency).

Among these three parts, the active current is responsible for the actual work done in factory/plant, while the remaining two parts are just circulating between EB & load, and do not contribute to any useful work. These two non-active currents, however, have significant impact on the other connected loads and distribution system. 

• Reactive current is responsible for the poor plant power factors and attracts penalty from the EB.
• Harmonic currents cause adverse effects on other loads in a plant and EB voltage quality..

Power factor can be defined as a quantitative measure to estimate how effectively the active power is being utilized by the loads. Mathematically, it is expressed as:

Once the above parameters are known, the plant total harmonic current (I_H) in amperes can be calculated as: $$Power Factor = PF = {{{Active Power} \over Apparent Power}} = {{{P} \over S}} $$

Where, P is active power and S is apparent power which is product of RMS voltage and net RMS current. When the load is pure resistive, both voltage and current will be in-phase (similar to Fig. 1). In such a case, there will not be any reactive current. That is, both P and S will be equal, and the power factor would be 1 (best possible scenario).

When the voltage and current are sinusoidal, power factor is defined as cosine angle between voltage and current. Due to inductive loads (such as, motors), the current drawn from the system lags the voltage by angle φ The active power and power factor in this case are given as:

$$P = Vrms.Irms.\cos(φ)$$ $$Power Factor = {{{P} \over S}} = {{{Vrms.Irms.\cos(φ)} \over Vrms.Irms}} = {{\cos(φ)}} $$

Note that when there are excessive capacitive banks, the current may lead voltage by angle . This case also results in poor power factor.

When the voltage is pure sinusoidal and current is non-sinusoidal (distorted), power factor is defined by the below expression:

$$Power Factor = {{{I1,rms} \over Irms}} = \cos(θ1-φ1) = distributionfactor \times displacementpowerfactor$$

And, in the worst-case scenario, when both voltage and current are non-sinusoidal (distorted), the power factor is called as true power factor and is defined as:

$$TruePowerFactor = {{{\sum_P} \over Vrms.Irms}} $$

Where the total P is determined as summation of all powers resulted from individual harmonic currents.

Low Power Factor on LT Side

Challenges:

• Dynamic load changes
• Slow response of APFC panels in switching ON/OFF of capacitor banks
• Lack of step-less power factor correction mechanism

Effects:

• Loss of power factor incentives and/or subjected to penalties
• Excess energy bills in kVAH billing systems

Low Power Factor on HT Side

Challenges:

• Transformer has voltage and current dependent reactive power requirements.
• Difficult to meet varying transformer reactive power requirements at all times.
• Fixed capacitor banks cause excess leading KVARH during lite/no-load conditions.
• Lack of fixed capacitor banks cause lagging KVARH in spite of unity PF on LT side.

Effects:

• Loss of power factor incentives and/or subjected to penalties
• Excess energy bills in KVAH billing systems