Reactive Power Compensation using Capacitor banks

Many industries are not effectively using their electrical power fully, and as a result incur higher energy bills and inefficient power usage. We can correct this by using a technique known as reactive power compensation. This is done by increasing the power factor of the system. Read on to find out more about what the power factor is and why it is necessary to improve it.

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Figure 1: 3-D representation of PF Capacitor panel.

1. What is power factor?

It expresses the ratio between the active power that can really be used for applications (mechanical, thermal) and the apparent power (VA) supplied by the network—that is, it measures the electrical efficiency of the installation. The best solution is to keep it close to unity, resulting in as much useful power consumed as possible.

Active power: is also called the real power which is responsible for the work done in an electrical system. Our purpose in using the power factor capacitor panel is to increase the real power.

Reactive Power: can best be described as the quantity of “unused” power that is developed by reactive components in an AC circuit or system. Reactive power can also be described as the resultant power in watts of an AC circuit when the current waveform is out of phase with the waveform of the voltage, usually by 90 degrees if the load is purely reactive, and is the result of either capacitive or inductive loads.

Apparent Power (S): It is the vector sum of active and reactive powers.

Power Factor can be expressed as,

It is best for electrical consumers to have power factor as close to unity as possible.

When power frequency current absent of harmonics flows through a network, this ratio equals cos(𝜑) where 𝜑 is the phase shift between current and voltage of the installation:

PF=P∕S= cos𝜑.

Note: This equation, however, only applies for the fundamental component.

Power factor measurements can also be monitored using a PF relay. Power Factor can be measured over a period, KW, V and A and finding the average PF over a time (day/week etc.).

2. Why is it required to improve Power Factor?

The following technical and economic advantages are associated with power factor improvement:

Economic Advantages:

These advantages are linked to the impact of compensation on the installation and allow evaluation of compensation payback time.

  • • Reduction in bills which may be a result of excess consumption of reactive power
  • • Reduction in subscribed power in kVA

  • Technical Advantages:

  • • Line voltage drop can be reduced as the reactive current drawn is reduced
  • • An increase of the active power available at the transformer secondary
  • • A reduction of line losses at constant active power.
  • • Easy installation, low maintenance as moving parts are absent and product design is straightforward
  • • High Life expectancy and simple protection devices.

3. Power factor correction using Capacitor Banks

Power factor correction capacitors are an efficient and simple solution to low power factor issues. A capacitor or bank of capacitors installed parallel to the load provides this reactive power. Reactive power is supplied by these banks and apparent power drawn from line is reduced. This process is known as reactive power compensation.

How do we determine Reactive Power Compensation?

The first step is to calculate the reactive power required for an installation. The aim is to determine the reactive power Qc (kvar) to be installed to increase the power factor (cos𝜑, where 𝜑 is the phase angle) of the installation to reach a given target power factor, cos𝜑2.

Take a look at figure 1 and note the following steps:

  • • Calculate the phase angles 𝜑1 and 𝜑2 based on the current (cos𝜑1) and target (cos𝜑2) power factors
  • • Calculate the tangent of the phase angles tan𝜑1 and tan𝜑2, where tan𝜑2 < tan𝜑1
  • • Remember, Q = QL–Qc is the target reactive power of the system, where QL is load reactive power and QC is the reactive power of the Capacitor bank. Thus, apparent power
    goes from S1 to S2, where S2 < S1.

  • Figure 1: Representation of Reactive Power Compensation

    Pa - Active Power (horizontal axis)
    S1 - Initial Apparent Power
    S2 - Final apparent Power
    QL - Line Reactive Power
    QC - Capacitor bank reactive Power

    Qc=Pa(tan𝜑1−tan𝜑2) which results from the figure.

    Thus, Qc is the reactive power compensation for your system for which you need to build “reactive” capacitor panels to improve your system power factor.

    4. What are the different power compensation methods and where in the system should you compensate power?

    Reactive power compensation schemes can be :

    • • Global
    • • By sector
    • • By individual load.

    Global :  This is when the capacitor panel is connected at the supply end of the system. It provides compensation for the entire system. The advantage is that it is economical and the banks are small. On the contrary, the disadvantage stems from the possibility of the system’s no-load voltage is increasing due to the capacitor bank.

    Sector :  In this method, the compensation is provided in specific sectors with bad power factors. The advantage is that the capacitors are localized to the area with a low power factor. The system may be exposed to a risk of overcompensation following large load variations within the sector. This risk can be avoided by means of automatic compensation with an automatic capacitor bank or by dividing banks into smaller sections and switching them accordingly.

    Individual Compensation :  In this method the reactive loads are compensated individually. The advantage is that reactive power is produced at the point of consumption, and is a technically ideal scheme. The disadvantage of this scheme is that it is not economical, and is only ideal for consumers with large motors.

    Other methods of Reactive power compensation:

    1. Synchronous Condensers

    Synchronous condensers are overexcited synchronous motors.

    The synchronous motor has the characteristics of operating under any power factor leading, lagging, or unity depending upon the excitation. For inductive loads, a synchronous condenser is connected towards the load side and is overexcited. Over excitation of the synchronous condenser results in providing reactive power to the connected system or drawing the lagging current. Although the Power factor control is smooth, the synchronous condenser is an expensive and complex solution as compared to Static Capacitor Banks.

    2. Phase Advancers

    This is an AC exciter mainly used to improve the PF of an induction motor

    Phase advancers are usually mounted to the shaft of the motor load and connected to the rotor circuit. It then provides the exciting amperes required to produce the required flux at the given slip frequency. Further, if ampere-turns increase, it can be made to operate at the leading power factor.