Module 7. Electric power economics
Lesson 32
LOAD FACTOR AND POWER FACTOR CORRECTION
32.1 Power Factor Basics
32.1.1 Active & reactive power
Active power, measured in kilowatt (kW), is the real power (shaft power, true power) used by a load to perform a certain task. However, there are certain loads like motors, which require another form of power called reactive power (kVAR) to establish the magnetic field. Although reactive power is virtual, it actually determines the load (demand) on an electrical system. The utility has to pay for total power (or demand) as depicted in Figure 32.1.
Fig. 32.1 Active, reactive and total power
The vector sum of the active power and reactive power is the total (or apparent) power, measured in kVA (kilo Volts-Amperes). This is the power sent by the power company to customers. Mathematically it may be represented as:
The power factor is the ratio between active power (kW) and total power (kVA), or the cosine of the angle between active and total power. A high reactive power will increase this angle and as a result the power factor will be lower (See Figure 32. 2).
Fig. 32.2 Relationship between active, reactive and total power
The power factor is always less than or equal to one. Theoretically, if all loads of the power supplied by electricity companies have a power factor of one, the maximum power transferred equals the distribution system capacity. However, as the loads are inductive and if power factors range from 0.2 to 0.3, the electrical distribution network’s capacity is stressed. Hence, the reactive power (kVAR) should be as low as possible for the same kW output in order to minimize the total power (kVA) demand.
32.2 Disadvantages of Low Power Factor
Considering fixed power and voltage, the load current is inversely proportional to the power factor. Smaller the power factor, higher is the load current and vice-versa.
Large current due to poor power factor results in the following disadvantages:
32.2.1 Large kVA rating of equipment
The electrical machinery such as motors, A.C. generators, transformers, distribution and control system etc. are rated in kVA. Because the power factor is not known when the machine is manufactured in the factory.
kVA rating of the equipment is inversely proportional to power factor which implies smaller the power factor, the larger is the kVA rating. At low power factor kVA rating of the equipment required is more which makes the equipment larger and costly.
1. Large copper losses: The large current drawn by the machine because of poor power factor causes more I2R losses.
2. Poor voltage management: The large current at low power factor causes greater voltage drops in the electrical distribution network and system. This causes improper functioning of the electrical machines.
32.3 Improving Power Factor (PF)
The solution to improve the power factor is to add power factor correction capacitors to the plant power distribution system. They act as reactive power generators, and provide the needed reactive power to accomplish kW of work. This reduces the amount of reactive power, and thus total power, generated by the utilities.
32.4 The advantages of PF improvement by Capacitor Addition
a) Reactive component of the network is reduced and so also the total current in the system from the source end.
b) I2R power losses are reduced in the system because of reduction in current.
c) Voltage level at the load end is increased.
d) kVA loading on the source generators as also on the transformers and lines upto the capacitors reduces giving capacity relief. A high power factor can help in utilising the full capacity of your electrical system.
32.5 Cost Benefits of PF Improvement
Costs of PF improvement are in terms of investment need for capacitor addition. The benefits to be quantified for feasibility analysis are:
a) Reduced kVA (Maximum demand) charges in utility bill
b) Reduced distribution losses (kWh) within the plant network
c) Better voltage at motor terminals and improved performance of motors
d) A high power factor eliminates penalty charges imposed when operating with a low power factor
e) Investment on system facilities such as transformers, cables, switchgears etc for delivering load is reduced.