Technical Article

Understanding Single- and Three-Phase Power Factor Correction

June 06, 2024 by Munir Ahmad

In today's modern, digitally connected world, reliable, continuous power availability and power quality optimize power consumption, improving distribution system efficiency.

Power factor is the ratio of the power needed to do the useful work (also called true or real power, P) by electric equipment at the consumer end against the power delivered by the power utility facility (called apparent power, S).

Good power factor determines the design quality and effective use of electric power, while poor power factor indicates poor utilization of electric power. So, there is a logical rationale that electric regulatory bodies impel improvement in power factor because bad power factor is a real threat to the modern electric distribution system, causing frequent power failures and losses.

 

Most loads, like a motor, are inductive in nature, meaning that most power factor correction must negate the inductive reactive effects

Figure 1. Most loads, like a motor, are inductive, meaning that most power factor correction must negate the inductive reactive effects. Image used courtesy of Canva

 

Types of Electrical Load

There are three types of electric loads—inductive, resistive, and capacitive—or an amalgam of all three.

Today, the most common loads in the electric distribution system are inductive devices like motors, generators, transformers, etc.; therefore, the reactive load in the power grid is constantly increasing. The increase in the power of transformers, motors, and generators leads to an increase in reactive power, so controlling reactive power load has become imperative.

Power factor depends on the relative phase of the voltage and current, which can be inductive (where current is lagging) or capacitive (where current is leading). It is important to determine what elements cause poor power. There are two main causes: displacement and distortion.

Displacement occurs when voltage and current are out of phase due to the presence of inductive and capacitive components called reactive elements. The linear load causes it. Linear loads draw sinusoidal current, which is always proportional to the voltage.

Distortion is the outcome of harmonic distortion caused by a non-linear circuit such as a rectifier. In non-linear loads, the current is drawn in abrupt, short pulses, distorting the current waveform.

 

What Is Power Factor Correction?

Power Factor Correction (PFC) is a methodology used to improve the device’s power factor or increase the power factor of the load, thereby reducing the current drawn from the mains and improving the efficiency of the distribution system.

PFC deploys different types of devices, such as capacitors, to minimize the reactive power of an AC circuit and improve its efficiency and power factor.

There are two types of power factor corrections: single-phase and three-phase.

In a single-phase system, such as in homes, the capacitor bank is connected in parallel with the load, which helps to reduce reactive power and improve the power factor. It is important to select the suitable type and size of the capacitor.

In a three-phase system, the capacitor bank is connected in parallel with the load in a star or delta scheme for power factor correction. The banks automatically switch on and off to maintain a desired power factor.

 

A three-phase load system with a delta-connected capacitor bank.

Figure 2. A three-phase load system with a delta-connected capacitor bank. Image used courtesy of Control.com

 

Benefits of Improving Power Factor

Multiple benefits can be achieved by applying power factor correction:

  • when a load absorbs a small amount of Q compared with P, the power utility profits from smaller cables, transformer windings, and generator windings
  • reduced utility bills for domestic and industrial users
  • extra KVA is available from the existing supply
  • increased system capacity, power quality, energy saving, and cost-saving
  • increase to the system’s current carrying capacity
  • trimming losses in the distribution equipment
  • reduced voltage drop in long cable
  • improved equipment life by reducing electrical burden
  • fewer failures and downtime

 

Correcting Power Factor 

The two main causes of low power factor are displacement and distortion. To remove the displacement factor, external reactive components are commonly used to compensate for the reactive power in the circuit. Multiple methods for power factor correction exist, and the approach selection depends on the system requirement. Engineers may use combinations of these to achieve power factor correction.

 

Capacitor Banks

In industry, the majority of the inductive loads are motors and transformers. Automatic power factor correction panels or capacitor banks improve the power factor. So why do we use a capacitor bank? The capacitor bank is one of the most common methods to correct and improve the PF by providing the reactive power that needs to be supplied by the source utility.

Inside any industrial system, we have both active and reactive powers. If the percentage of reactive power is high, the system is inefficient, and the power factor will be low. Reactive power has a direct relationship with the power factor; therefore, we should lower the percentage of reactivity to improve the system’s efficiency.

The inductive loads produce lagging reactive power (current lag voltage, lagging PF). To compensate for this, we produce leading reactive power in the system with capacitors. The leading and lagging reactive power both compensate or cancel each other, and this is called reactive power compensation.

 

Capacitor banks may serve facilities or branch circuits, each chosen specifically for the reactance of its own loads

Figure 3. Capacitor banks may serve facilities or branch circuits, each chosen specifically for the reactance of its own loads. Image used courtesy of Canva

 

Power Factor Correction Controller

This controlling device permanently monitors the plant’s reactive power and automatically adjusts it to meet the desired power factor.

The controller activates the control functions by connecting or disconnecting the capacitor banks depending on whether the PF increases or decreases. If the power factor is less than the required values, the controller automatically adds the capacitor bank to improve the PF. The controller also displays multiple real-time parameters, which are easy to monitor, like current, voltage, power, etc.

 

Static Var Compensator

These devices provide reactive power compensation to improve the power factor by injecting or absorbing the reactive power into the electrical system to match the desired voltage level. The main components of Static Var Compensators (SVC) are a thyristor-controlled reactor (TCR), used to control the inductive reactive power, a thyristor-switched capacitor (TSC), used to control the capacitive reactive power, and a harmonic filter. They are typically used for large industrial loads, such as motors and transformers.

 

Poor Power Factor

Poor power factor is generally caused by linear loads like induction motors, transformers, and generators and has multiple disadvantages:

  • excessive energy costs due to low power factor and penalties imposed by regulatory bodies
  • poor power factor means drawing more power from the electricity networks to do the same work, so the cables and equipment need to be larger
  • increased losses (high power means excessively high current) and overheating of transformers, switches, and cables
  • increased voltage drop
  • frequent tripping of breakers and fuses
  • reduction in available power

 

Power Factor Correction Equipment

There are also risks associated with power correction devices if trained or qualified experts do not properly install them:

  • electric shock, short circuit, and fire hazard
  • device damage and malfunction
  • overcorrection, which will increase the voltage level above the acceptable limits and could damage the equipment
  • harmonic currents and distortion can cause electrical equipment failure
  • capacitors have a limited life expectancy, and fire may result without warning

 

The view from an oscilloscope can measure the time delay (leading and lagging) between voltage and current waveforms

Figure 4. The view from an oscilloscope can measure the time delay (leading and lagging) between voltage and current waveforms. Image used courtesy of Canva

 

Power Factor Measurement

Measuring power factor in a facility to record change over time shows how effectively electrical power is being used. This will allow the engineer to take corrective measures, e.g., installing specialized equipment to increase electrical efficiency near the ideal target of unity power factor.

Multiple instruments can be used to monitor PFC in the system, like power factor meters, to measure and monitor the system’s power factor in real-time—likewise, the AC voltmeter and AC ammeter measure system voltage and current. Active and reactive power transducers can be used to integrate the signals into the control system to record values for trends.

Power quality analyzers measure electrical quantities like voltage, current, power, apparent power, power factor, and harmonic distortion.

Finally, oscilloscopes can analyze the voltage and current waveforms to determine the system’s power factor.

Monitoring voltage, current, and power trends at the HMI level can indicate the success of PFC by using proper meters and instruments.