Power Factor Correction Guidelines

Introduction

We have learned the fundamentals of Power Quality in our earlier article “Power Quality Basics”. We have also learned the importance of good Power Quality on Low Voltage distribution networks along with some basic calculations. Now we will go a little deeper and understand the implementation of power quality improvement methods. Power factor is a very important component of good power quality. We will also know about the various components and discuss their functions, needed to design power factor correction solutions.

In commercial establishments be it Commercial Buildings, Large Residential complexes, or Institutional Buildings, Hospitals, Hotels and Industries. They all get supply at 11kV or 33kV from the concerned utility at a certain power factor. We need transformers to step down voltage at the level of 440V so that it can directly be fed to various equipments.

Starting from Transformers to motors and other utilities, most of the devices are inductive in nature and need a certain amount of reactive power along with active power. This reactive power will be drawn from the source but if it can be locally fed to the equipment then it will not draw this power from the source thereby reducing the overall power consumption.

We will take a few equipments as an example to understand this.

Power Factor Correction for Transformers

We all know that Power Transformers works on the principle of mutual induction. To generate a voltage on secondary winding there is a no-load current in primary winding to generate flux to induce secondary voltage. This current is known as magnetizing current. This no-load primary current is purely inductive and the transformer draws the associated reactive power from the grid. But if we compensate it locally at the transformer end then there is no additional power requirement from the grid as this reactive power does not do any active work but is still needed.

This will be consumed by the transformer throughout the year even if it is not feeding the load, but it is charged. Let us understand this by an example. Refer to the single-phase equivalent diagram of a transformer below.

You can see two reactances on the secondary side. One refers to the impedance responsible for generating magnetic flux in the transformer called shunt magnetizing reactance whereas the other one is series leakage reactance refers to the amount of magnetizing current that does not link between the primary and secondary winding. This series impedance is a very important parameter and decides the transformer’s contribution to the short circuit. So we need to do the power factor correction for transformers as well. See below for how.

Let us assume:

Magnetizing reactance as XM,

Leakage reactance as  XL and

Secondary voltage as VS

Then magnetizing current IM = VS/(XM+XL)

Since  XM ≫ XL

IM =  VS/(XM)      →     XM =  VS/(IM)

Reactive power consumed per phase = (VS²)/XM

Three phase reactive power consumed by transformer Q0 = 3*(VS²)/XM

Here Q0 is the no-load reactive power requirement of the Transformer. This requirement comes to about 2% of the transformer kVA rating.

Now let us discuss the situation when the transformer secondary is closed, and it is feeding connected loads. In closed circuit conditions other than shunt reactance, series leakage reactance will also consume reactive power. To know how much reactive power will be consumed by series leakage reactance, see the below calculation.

Let us assume:

Series leakage reactance is XL, also known as % impedance %Z

Then,

Reactive power consumed by series leakage reactance QL = (%Z/100)*(kVAL/kVAT)²*kVAT

Total reactive power consumed by Transformer at full load QT=Q0+QL

Where,

kVAT is Transformer kVA and

This is the story on the source side. A similar calculation needs to be done for load-side equipments. Let’s consider 3-phase induction motors as one of the load-side equipments.

Power Factor Correction for Induction Motors

Consider a 100kW motor as a load in a distribution network in an installation. The designed power factor of the motor is 0.8 and the desired power factor is unity.

Reactive power compensation required Q = P * (tan∅1-tan∅2)

= 100*0.75

= 75kVAr

The value of (tan∅1-tan∅2) can be directly taken from Table 1 in another article “Power Quality Fundamentals”.

Similarly, the reactive power compensation requirements of other connected loads to achieve the desired power factor can be calculated. To arrive at the total reactive power requirement of the complete installation, individual requirements of the connected equipments to be summed up and an equivalent capacitor bank can be connected in the network.

Power Factor Correction Methods

So, the next question. Will these capacitor banks be connected directly to the network or through switching devices controlled by Automatic Power Factor Correction Relay (APFC)? We all know that at any of the installations, whether it is industry, building or any other application, all the loads are not run simultaneously so the requirement of reactive power to achieve the desired power factor varies along with the connected loads at the moment. So using fixed capacitor banks will not work as it will feed fixed reactive power all the time.

It is recommended to use discrete capacitor banks controlled through the Automatic Power Factor Correction relay. APFC relay will switch “ON” only the number of capacitor banks required at the moment equivalent reactive power need of active loads. To make an automatic solution for power factor correction we need many components. All the components are assembled in the enclosure and wired together. This kind of assembly is called the APFC Panel.

Components of APFC Panel

The APFC Panel consists of the following components:

• Main Incomer
• APFC Relay
• Switching Contactors
• Protective Fuses/MCCB’s
• Capacitor Banks
• Reactors

Refer below pictures to have some idea of the above components:

The function of Various Components

Main Incomer

It is used as a gateway for the main supply to energise the APFC Panel. It is used for switching “ON” main supply and protection of the APFC Panel. This can be an Air Circuit Breaker (ACB) or a Moulded Case Circuit Breaker (MCCB).

APFC Relay

This is a device that continuously monitors the power factor of the installation and calculates the required number of capacitor banks by comparing it with the desired power factor set in the relay. Then it switches ON the desired number of capacitor banks to compensate for the reactive power requirement. It comes out with various numbers of relay outputs like 8, 12 or 16. It can control as many capacitor banks as it has the number of relay outputs.

Contactor

It is a switching device used for switching “ON” of capacitor banks. They are being controlled through the APFC relay.

Protective Fuses/MCCB

The capacitor feeder is protected by either a fuse or a MCCB. Nowadays it is prevalent to use MCCB instead of Fuse as MCCB offers protection against overload, short circuit and earth leakage whereas Fuse offers protection only against short circuit.

Capacitor Bank

This device generates reactor power for consumption of load or source thereby reducing the total load on the source.

Reactor

It is also called a detuned reactor and is used for the mitigation of harmonics in the system.

Now what next? How to integrate these components to make a solution for power factor correction? How to design a power factor correction panel or APFC Panel? Let us hold for a while. We will take up the design of the APFC Panel in the next article.