Power Factor Correction

   The nature of Reactive Energy

     Most of electrical loads in electricity distribution systems like devices and machines convert the consumed electrical power to mechanical power and heat.This consumed electrical power consists of two types of power:

1-     Active power that converted into the machine to useful power, providing the output mechanical power or heat or light. This type of power is measured in kilowatt and generally represented by the letter (P).

2-   Reactive power that used to build up magnetic field into the machine that help in converting the electrical energy to mechanical energy, This type of power generally represented by the letter (Q) measuring in kilovar and takes two forms :

a-    Inductive reactive power consumed by inductive circuits like transformers, motors, etc.

b-    Capacitive reactive power that supplied by capacitive circuits like cable capacitance and power capacitor, etc.

        The apparent power (S) is the combination of (P) and (Q). Consequently, current consumed by any machine is called apparent current which consists of active current (useful current) which converted into the machine to active power and the reactive current (non useful current) which may be inductive or capacitive (but almost this current is inductive because practically most of the loads are inductive) and required to generate magnetic field inside the machine.

       It should be noted that capacitive current is in exact phase opposition to the inductive current and this is the basis in which power factor correction scheme depend on.

       Always in transmission and distribution system , the electrical utility try to reduce the inductive reactive current to reduce power loss and voltage drop as when the inductive current passing throw the inductive reactance,  this will lead to the worst possible condition in voltage drop and this is one of our incentives to improve the power factor. Now let us know the meaning of power factor.

     What is the meaning of power factor (P.F)?

      Power factor is the ratio of P (K.W) to S (K.V.A), its value vary between "0" and "1"  the closer the value to "1" the greater benefit to supplier and customer.

     If voltages and currents are perfectly sinusoidal signals, power factor

will equals cos ϕ, where "ϕ" is the angle between the current and voltage .when P.F close to unity this mean that the reactive energy is small compared with active energy.

      PF = P (kW) / S (kVA)

      Note:

     For single phase load: S = I*V  -  P = V* I* cos ϕ  -  Q = V* I* sin ϕ.

     For three phase load: S =√3*I*U - P =√3*U *I *cos ϕ - Q = √3*U *I *sin ϕ.

    Where:

    V = Voltage between phase and neutral

    U = Voltage between phases

     I = Line current

     ϕ = Phase angle between vectors V and I.

Why you need to improve power factor?

We gain several technical and economical advantages of improving power factor as following:

First (economical): reduction in electricity cost:

In all countries all over the world power supply authorities design a tariff structure that encourage their customers to reduce reactive energy consumption by imposing a penalty on the customers whose P.F don't agree with a standard definite level .The customers bills will be increased incase of their low P.F .In some countries the supply authority impose the penalty on the customers according to tan ϕ value like Europe countries while others like Africa countries use the cos ϕ value.

Second (technical):  

By improving P.F we can gain many technical advantages like:

1- Reduction in power losses (kw) in cables and over head lines , as these losses is proportional to the passed current squared ( power loss = I² *R ) so reduction in the total current by 15 % will result in losses reduction by nearly  30 % .

2- Reduction in voltage drop, as P.F correction capacitors reduce or remove the inductive reactive current consumed by the low P.F loads.

3- Improve voltage profile; also it is a result of the decreased current resulting from connecting P.F correction capacitor panel.

4- Release system power capacity, by increasing the power available in the transformer and this because of the low consumed current from the transformer after improving the P.F of the loads. In some cases it is more economical to improve the P.F of the transformer loads than to replace it.

How can you improve the P.F?

Any an inductive load with low power factor will consume a large amount of reactive inductive current from the power supply with lagging power factor and the result will be associated power loss and voltage drop in the electrical network .

To correct this situation a bank of shunt capacitors will be connected to the low P.F load , as its capacitive consumed current with leading power factor will take the same path of the load inductive current but in direct phase opposition so that ,the two currents will cancel each other at the point of capacitor connection when they are equal in values ( fully compensation) or the inductive reactive current will decreased when the capacitive current value is less than the inductive one (partially compensation ).

So generally to improve the P.F you need to install a bank of capacitors (parallel to the load under consideration) which act as a reactive power generator supplying the load with its need of reactive power instead of the transformer and this type of installation is called reactive energy compensation .

Types of compensations according to capacitors units integration and control:

There are two types of compensation, called fixed and automatic:

1-Fixed compensation: in this type of compensation one or more fixed capacitors units are used for constant level of compensation and they are applied at terminals of motors or transformers or at bus bars supplying reasonably constant loads.

2- Automatic compensation: in this type of compensation we use an automatic controller to control many steps of capacitors units to maintain the P.F around a selected level and they are applied on bus bars where the reactive power variation is relatively high.

Generally it is advisable to use fixed capacitors when the KVAR ratting of the capacitors required are equal to or less than 15% of the transformer rating.

Types of compensations according to capacitors panel point of connection:

There are 3 types as following:

1- Global compensation: the capacitor bank will be connected to the main bus bars of the main distribution board so the tariff penalty will be reduced and the apparent power consumed from the supply bus bars will be reduced. But reactive power still flow in cables downstream the main distribution board consequently power losses and cable sizes are not improved.

2-compensation on sub distribution board: There will be more than one capacitors bank , each one will connect to each local distribution board in the installation, so the tariff penalty will be reduced , the apparent power consumed from the supply bus bars will be reduced and the size of the cable from main distribution board to local distribution boards can be reduced. But reactive power still flow in cables downstream the local distribution board consequently power losses and cable sizes are not improved and when big changes in loads occur, it will be a risk of over compensation leading to overvoltage.

3- Individual compensation: at this type of compensation, capacitors units will be connected directly to the loads terminals so tariff penalty, consumed apparent power from the supply, the sizes of all 
cables and cables losses are reduced, this compensation type is the most effective. 

     Average power factor values for the most commonly used equipments:

Loads	Power factor (cos ϕ)
Induction motor , loading percentage = 0%	0.17
Induction motor , loading percentage = 25%	0.55
Induction motor , loading percentage = 50%	0.73
Induction motor , loading percentage = 75%	0.8
Induction motor , loading percentage = 100%	0.85
Discharge lamp	0.4-0.6
Incandescent lamp	1
Fluorescent lamps (uncompensated)	0.5
Fluorescent lamps (compensated)	0.93
Arc furnace	0.8
Fixed 1-phase arc-welding set	0.5
Arc-welding transformer-rectifier set	0.7 – 0.8

 

   Harmonics and power factor correction capacitor

     Harmonics is defined as undesired signals present in an electrical distribution system that have frequencies multiple of the fundamental frequency that overlap with the fundamental signal and create anew distorted wave form which no longer sinusoidal .Before we apply a power factor correction capacitors in any electrical installation, we should accurately measure and indicate the presence of harmonics currents and voltage to avoid the following:

1-     Capacitors failure.

2-   Fuse of circuit breaker (that feeding the capacitors) nuisance tripping.

3-   Harmonics resonance conditions that may be series or parallel which may damage most of the system installation components.

      We will discuss the relation between harmonics and P.F correction capacitors panel and also the resonance phenomena in details at "Electricity" page at our site: http://electricity2all.blogspot.com .  So please follow us.

     Power factor correction and harmonic filtering study

      Very important issue before installing power factor correction capacitors panel but mainly you need to have a power quality analyzer to do electrical measurements for voltage, current, power, power factor, harmonics and many other parameters before and after installing the P.F correction panel  , let us introduce to you the goals, procedures and the results of the study :

       Goals of electrical measurements by power quality analyzer before buying P.F correction capacitors    panel

(1)   To indicate the panel technical specifications in order to achieve the optimal desired power factor value according to electric utility regulations without exaggeration in capacitor and other panel components ratings. In the other meaning reaching the max economical and technical benefits with a lowest cost of the panel.

(2) To study the presence of harmonics which generated from non linear loads in order to avoid harmonics resonance which may damage the capacitors panel and other installation equipments.

(3) To do an economical study before buying the capacitor panel in order to assure the economical (getting bonus and eliminating penalty) and technical benefits getting from the panel versus the cost of panel, its accessories, life time and installation costs.

Study Procedures:

       (1) Doing electrical measurements by the power quality analyzer with definite time period relevant to the load variation cycle.

       (2)Doing a comprehensive study to indicate  " The optimal sizes and numbers of capacitor steps - The relevant power factor controller program that suit the load KVAR variation - A list of other equipments and accessories required to connect the capacitor panel with the electrical main distribution panel and the contactors, Protection devices (circuit breaker and fuse) and the feeding cables.

 (3) Doing a harmonics filtering study indicating if we need to incorporate harmonic filter (tuned – detuned) equipment with the P.F capacitors panel or if we need to install other types of filters to eliminate harmonics and avoid resonance.     

       Goals of electrical measurements by power quality analyzer after installing P.F correction capacitors panel  

You should do measurements by a power quality analyzer after installing the capacitors panel in order to assure panel efficiency by reaching the desired power factor with a high coordination with the load consumed reactive power variation.

Study results:

At the end of the P.F correction study, the customer should receive the following:

 (1) A comprehensive report containing the capacitor panel components specifications, point of connection, pay back period and the most effective types in the market and equipped with the technical drawings.

(2) Data sheets containing electrical measurements which done by power quality analyzer before and after installing the capacitor panel to realize the success of the study.

      At the end we will be very happy to answer your questions and respond to your inquires at any time for any question related to electricity world " not only power factor correction",........... a brief contact information found in "Contact Us" page at our site: http://electricity2all.blogspot.com .  Also we will write more and more about this issue at "Electricity" page, so please follow us.