What is reactive power?
This document covers; The Concepts of Reactive Power, Low Power Factor and Methods of Power Factor Improvement.
Power factor is defined as the ratio of real power to apparent power. This definition is often mathematically represented as kW/kVA, where the numerator is the active (real) power and the denominator is the (active+ reactive) or apparent power. Though the definition is very simple, the concept of reactive power is vague or confusing even to many of those who are technically knowledgeable.
Explanation for reactive power says that in an alternating current system, when the voltage and current go up and down at the same time, only real power is transmitted and when there is a time shift between voltage and current both active and reactive power are transmitted. But, when the average in time is calculated, the average active power exists causing a net flow of energy from one point to another, whereas average reactive power is zero, irrespective of the network or state of the system. In the case of reactive power, the amount of energy flowing in one direction is equal to the amount of energy flowing in the opposite direction (or different parts -capacitors, inductors, etc- of a network, exchange the reactive power). That means reactive power is neither produced nor consumed.
But, in reality we can measure reactive power losses, many different types of devices, equipment and systems can be introduced to manage or mitigate reactive power. These types of compensation are to reduce electricity consumption and cost.
Confusions
The indisputable law of conservation of energy states, “energy can neither be created nor be destroyed”; yet we talk about Conservation of Energy!! The confusions erupt when we yell out the theory of conservation ignoring other theories of thermodynamics – like one, which states that entropy (low quality energy) is ever increasing. Mathematical sum of total energy has no meaning to an energy user, and hence he must be concerned about the efficiency of conversion and conservation of energy. Similarly, though we can mathematically prove that loss in reactive power is no real loss and no reactive energy is lost, we have several other reasons to be concerned about reactive power improvement. This can be better explained by physical analogies.
Physical Analogies
Suppose I want to fill a water tank with water, one bucket at a time. Only way is to climb a ladder, carrying a bucket of water and pouring the water into the tank. Once I fill up the tank, then I have to go down the ladder to get more water. In this one cycle of going up the ladder and coming down I have done some “work” or “the energy required to go up is more than the energy required for coming down.”
If I had climbed the ladder with an empty bucket, and I had come down with the same bucket I am not doing any work. The energy for upward and downward motion is the same. Though I have not done any work – worth paying for- I require some energy.
That is, the energy that it takes to go up and down a ladder carrying nothing either way requires reactive power, but no real power. The energy that it takes to go up a ladder carrying something and come down without carrying anything requires both real power and reactive power.
The analogy can be extended for explaining 3 phase system; it is like putting 3 ladders going up to the tank and having 3 people climb up in sequence and pouring their water into the tank such that there is always a steady flow.
Here is a simplistic analogy called, the “Beer Mug analogy”
Power Factor = Active power/Apparent power = kW/kVA
= Active power/ (Active Power+Reactive Power)
= kW/(kW+kVAr)
= Beer/(Beer+Foam)
The more foam (higher kVAr) indicates low power factor and vice versa.
(In Electrical terms kW, kVA, and kVAr are vectors and we have to take the vector sum).
What causes low power factor in Electrical System
Various causes, which can be attributed for low PF, may be listed as follows.
- Inductive loads. Especially lightly loaded induction motors, and transformers.
- Induction Furnaces
- Arc Lamps and arc furnaces with reactors.
- Fault limiting reactors
- High Voltage.
- Harmonic distortion up to 63rd harmonic
The reactive power required by these loads increases the amount of apparent power in the distribution system and this increase in reactive power and apparent power results in a lower power factor.
How to improve Power Factor
Power factor can be improved by adding consumers of reactive power in the system like Capacitors or Synchronous Motors. It can also be improved by fully loading induction motors and transformers and also by using higher rpm machines. Removing or reducing the harmonic distortion. Improving and regulating of the voltage sine wave. Usage of automatic tap changing system in transformers can also help to maintain better power factor.
Question: Under which circumstances may power factor corrections..
A) reduce electricity consumption in a plant
Answer: Power factor improvement in plant, by adopting any one of the aforementioned options, will generally compensate for the losses and reduce current loadings on supply equipment, i.e.; cables, switchgear, transformers, generating plant, etc. That means power factor corrections – whenever there is scope for correction- will reduce electricity consumption in the plant and in turn the electricity cost. Many of these losses are not properly monitored in many industries and hence the savings are not quantified. This may be one of the reasons for the argument that PF improvement reduces only electricity costs; in case the power utility is offering a tariff where a reactive power demand charge are part of the monthly electricity bill.
Power factor improvement will lead to reduction in electricity consumption, when it is done at the equipment level or at the Control Center level (case studies have shown the savings in both these instances)
B) reduce electricity costs only
Answer: Power factor correction will reduce electricity cost only, when the plant receiving power from a common grid carries out the correction at the supply voltage/incoming voltage level, just to compensate for the reactive power drawn from the grid. But, even this improvement in PF may not always reduce the electricity cost as the contract demand in a plant is very often fixed on a fictitious consumption in the plant. On many occasions contract demand is fixed based on the future expansion plans, and based on the high diversity factor taken during design stages. In most of the cases the Utilities charge for a minimum contract demand irrespective of the consumption and a reduction in kVA may not produce any benefit as long as the contract demand is re-fixed to actual value.
Generally PF is improved to 0.95-0.98, as improving PF further to unity (1.0) may lead to higher payback periods.
C) reduce both electricity costs and electricity consumption
Answer: In all other cases, other than the above mentioned exception, whenever improvement of power factor is carried out, it will eventually lead to reduction in electricity consumption and hence electricity cost.
However, payback on investment due to power factor correction depends on the type of installation and various other factors like power tariff, loading pattern of equipment, method of power generation/utilization, operating philosophy of the plant etc.
