What is True Power Factor?
The true power factor is the product of the displacement power factor and the distortion power factor. The displacement power factor depends on the phase angle difference between the voltage and current of a linear load, whereas the distortion power factor depends on the total harmonic distortion caused by the non-linear loads. To understand what is true power factor is, let us first discuss the non-linear and linear loads.
Loads, which have non-linear voltage-current characteristics, are called non-linear loads. When connected to a sinusoidal voltage, these loads draw non-sinusoidal current. Modern power electronic systems result in non-sinusoidal current when connected to the sinusoidal voltage source. Typical non-linear devices are fast static switches like a diode, Silicon Controlled Rectifiers ( SCRs ), Gate Turn Off Transistors ( GTOs ), Insulated Gate Bipolar Transistors ( IGBTs ), Insulated Gate Commutated Thyristors ( IGCTs ).
Almost all semiconductor devices are non-linear or nonohmic. The devices do not follow Ohm’s law, and the current drawn by the devices are not proportional to the applied voltage.
The non-linear devices can be classified under the following three major categories:
Rectifiers, variable speed drives, UPS systems, inverters
Transformer (non-linear magnetizing characteristics)
Arc furnace equipment; generate harmonics due to non-linear characteristics of the arc itself.
Consumer predominantly generates the harmonics and the utility stands to be affected, and so are the other consumers who are connected to the system. The closest example one can cite is that of passive smoking. The harmonic amplitude generated by transformers, motors, and generators is usually insignificant compared to power electronics and arcing equipment unless saturation occurs.
Harmonics voltages affect the performance of other equipment connected to electrical network.
TOTAL HARMONIC DISTORTION (THDv & THDi)
The total harmonic distortion in voltage (THDv) and current (THDi ) is the square root of the sum of squares of the individual harmonic voltage from harmonic order 2 & onwards ( Higher-order may be neglected as their magnitude is low )
THDv= √ (V22+ V32+ V42+V52 +——– )
% THDv = THDv / V1 X 100 Where V1 = Fundamental Voltage
THDi= √ (I22+ I32+ I42+ I52 +——– )
% THDi = THDi / I1 X 100 Where I1 = Fundamental Current.
If the current waveform is symmetrical about the x-axis the even-order harmonics are not present.
What is the difference between True power factor and displacement power factor?
The linear devices draw sinusoidal current from the sinusoidal supply source and these devices do not distort the current waveform. Equipment like an inductor, induction motor, transformer are examples of linear devices. This equipment also consumes magnetizing current for its operation. The current drawn by this equipment is sinusoidal but the current waveform is not in the phase with the voltage. The cosine of the phase angle difference between voltage and current is called the displacement power factor(DPF) or the fundamental power factor.
The semiconductor devices draw the current in phase with the applied voltage, however, the current is not sinusoidal. The non-sinusoidal current generates harmonics in the electrical network. The measurement of the harmonics is checked by calculating total harmonic distortion(THD). The harmonic current is also a reactive current which does not contribute to active power delivery. The power factor due to non-linear current is drawn from a sinusoidal supply source is known as the distortion power factor. The product of the displacement and the distortion power factor is called the True power factor.
Let us understand both terminologies of power factor in detail for linear and non-linear loads.
POWER FACTOR OF LINEAR LOADS:
When the loads connected to the system are linear, and the sinusoidal voltage is applied to the load, the load draws the sinusoidal current. The cosine of the phase angle between the voltage and current is known as the displacement power factor. The displacement power factor formula is as given below.
Pf= CosФ ————————– (i)
Where, pf is power factor and Ф is the phase angle between voltage and current.
The cosine of the angle between voltage and the current is the power factor. Power factor with linear loads is known as displacement power factor. Why is this phase displacement between voltage and current? The phase difference between voltage and current shows that the equipment draws reactive energy for its operation.
Take the case of the resistive load, there is no phase angle difference between voltage and current as the resistance does not consume the reactive energy. The fundamental and the distortion power factor for resistive load is unity. By measuring the active and the apparent power, the displacement power factor can be calculated. The power factor formula is as given below.
Power Factor = Active Power/ Apparent Power
Let’s take the example of induction motor for better understanding the displacement or fundamental power factor. The induction motor takes the magnetizing current for its working. The magnetizing current lags the applied voltage by 90 degrees electrical. That is why the total current drawn by the motor is always more than the active current drawn by the motor.
POWER FACTOR OF NON LINEAR LOADS:
When the loads connected to the system are non-linear, the equation of the power factor of the linear loads is not true, and one more concept of distortion power factor is taken into account for calculating the true power factor.
The semiconductor devices like a diode, Bipolar Junction Transistor, and Insulated Gate Bipolar Transistor(IGBT) are examples of non-linear loads. The non-linear loads generate harmonics, and the harmonic current is reactive in nature. The current drawn by the non-linear devices are not sinusoidal, and it distorts the fundamental current waveform. The power factor gets deteriorated with an increase in the harmonics distortion.
The power factor with non-linear loads is known as the distortion power factor.
Distortion Power Factor:
The non-linear devices( All the semiconductor devices) draw non-sinusoidal current from the sinusoidal source. According to Fourier Series, the non-sinusoidal current can be resolved into the fundamental and integral multiples of the fundamental current. The integral multiples of the fundamental current are the harmonics current that produces the distortion in the electrical network.
Distortion power factor= 1/ (√ 1 +THD2i)
True Power Factor :
The overall power factor of the electrical network not only depends on the displacement power factor but also depends on the harmonic distortion in the electrical network.
The true power factor is calculated using the following formula.
True power factor = Displacement power factor * Distortion Power factor
= CosФ * 1/ (√ 1 +THD2)
The true power factor gets deteriorates with an increase in harmonic distortion. The below-given table shows how the overall power factor(True power factor) is low even if the displacement power factor is 0.9.
In the case of VF drive, the displacement power factor is unity; however, the true power factor may be in the range of 0.996 to 0.998.
How to Improve True Power Factor?
The True Power Factor can be improved by improving the displacement power factor and the distortion power factor.
1. Improvement of Distortion Power Factor:
The harmonics are generated due to non-linear loads like variable frequency drives, soft starters, Slip power recovery systems, etc. Loads, can’t be eradicated but they can be mitigated so that harmonic current does not flow in the electrical network. There are several methods of harmonics mitigation techniques. Some of the methods are
a. Installation of Harmonic Filter:
The tuned or de-tuned filters can be installed to mitigate the harmonics in the electrical network. The Active Filter is also a solution for mitigating harmonics.
b. Installation of 12 Pulse Rectifier:
The harmonics generated in the system depend on the number of pulses. The harmonic order can be expressed by the following formula.
n= Harmonic order
P= Number of pulses
K= integral Value 1,2,3,4………
For 6 pulse rectifier harmonic order(n) is 6*1+/-1=5,7. The order of harmonics for 6 pulse rectifier is 5,7,11,13,17,19….. The order of harmonics for the 12 pulse rectifiers is 11,13,23,25….. The harmonic current gets reduced if the rectifier is selected for a higher number of pulses.
2. Improvement of Displacement Power Factor:
The displacement power factor depends on the phase angle difference between the voltage and current. The inductive loads draw the magnetizing current which lags the voltage. To nullify the effect of lagging current, the capacitor is installed which draws the leading current. Thus the net current reduces as the capacitor current is in the phase opposition with the lagging current.