Inside the capacitor bank panel: Power factor correction, calculation and schematics

It’s all about loads and angles

Various calculations must be performed during the electrical design stages of any project to deliver a safe electrical system for the end user. One of the essential things that must be taken into account after calculating the total and demand load of the project is to consider the power factor.

But before indulging in the power factor correction, you should be aware of different types of loads. Real and wasted power are the main cause of power factor variation in the system.

Therefore, taking full advantage of increasing real power and minimizing wasted power through improving the power factor is crucial in every system to reduce the monthly electricity bill and avoid penalties imposed by the electrical authorities, as you will know how this could happen in the following paragraphs.

You will learn what it means and how to improve power factor value using capacitor banks and analyze capacitors and reactors control and power circuit diagrams.

Table of contents:

1. Types of Power
2. Types of Loads
3. Lagging and Leading Loads
4. Capacitor Bank Size Calculation
   1. Project Example
5. Automatic Capacitor Bank Power Circuit
   1. Capacitor Bank Switching
   2. Stages in PF Correction
   3. Harmonics and Connection of Reactors
   4. Capacitor Bank Control Circuit
      1. Analyzing Control Circuit of APFC

1. Types of Power

It is very important to understand the different types of loads represented in what is known as the “power triangle” which clearly shows the relationship between different kinds of power. By understanding different types of loads and their output waveforms, it will be easier to analyze how the power factor plays an important role in increasing or decreasing the efficiency of power in the grid.

As you can see in the power triangle, three types of power are influenced when the power factor angle is either increased or decreased.

Figure 1 – Power triangle (apparent, reactive and real power)

Electrical power in the low voltage system is divided into three types:

Active Power (P): The power needed for useful work such as turning a lathe, providing light or pumping water, expressed in Watts or Kilowatts (kW).

Reactive Power (Q): A measure of the stored energy reflected in the source which does not do any useful work, expressed in VAR or Kilovar (kVAR).

Apparent Power (S): The vector sum of active and reactive power, expressed in Volt Amperes (VA) or Kilovolt Amperes (kVA).

These three types are related and have a direct relationship represented in the power triangle. Real power is also called pure and active power, and it is what we need to energize all devices, equipment, and appliances in our homes.

At the same time, the apparent power is the addition of real and wasted reactive power. It is clearly shown in the power triangle that the wider the power factor angle, the more reactive power will result and less real power. In contrast, the less the angle it gets, the more real power and less reactive power will result.

So, how does this relate to the electrical design of a low-voltage system? This question is better answered after you know the types of loads in the coming section.

AC Circuits Analysis: Real, Reactive and Complex Power Calculation

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2. Types of Loads

Most loads in electrical distribution systems fall under one of three categories; resistive, inductive, or capacitive. In most cases, the most common is likely to be inductive.

So, to answer the previous question, “how would this information have its benefit in our electrical low voltage design?” understanding the types of loads would help to answer that question comprehensively. Since we have two major loads in any project – resistive and inductive – the power factor will surely be positively or negatively affected.

Simply put, the more resistive loads present in the system, the more real power in watts will be consumed, keeping the power factor at its high values. In case of more inductive loads, e.g. air conditioners, water pumps, extraction fans etc., are operating in the project, the power factor will decrease.

So let’s define these loads separately:

It may reach a poor value that may put more load on the transformer and heat it due to the higher amount of reactive load (kVAR) in the system. For this reason, improving power factor equipment must be designed to correct and increase it to an efficient value.

In most countries, electrical companies impose on achieving a minimum power factor of 0.9 to avoid penalties.

This means that if the power factor decreases to a lower amount of 0.7, for instance (mainly due to the higher inductive and non-linear loads connected to the system), the main electrical company will penalize the consumers because of the low factor may put the entire system’s equipment (like the transformer) in danger of being damaged or overheated.

Therefore, designing and sizing a capacitor bank in any project is compulsory because it will not only exempt the consumers from the authority’s penalty but it will also increase the amount of real power in the system.

This will allow the addition of more loads to the transformer of the project, reducing voltage drops, reducing electricity bills, and lowering heat generation in cables, switchgear, transformers and alternators of the system.