Power Factor Improvement , Lecture notes of Electrical Engineering

Download Power Factor Improvement and more Lecture notes Electrical Engineering in PDF only on Docsity!

Power Factor Improvement

Lectures

ELECTRICAL POWER ENGINEERING

Prepared by: S M Ismail

Chapter Outline

• Power Factor
• Power Triangle
• Disadvantages of low power factor
• Causes of low power factor
• Improvement of low power factor

Power Factor (P.F=cosФ)

“ The cosine of angle between voltage and current

in an A.C. circuit is known as power factor”.

• In an A.C circuit generally there is an angle

between voltage and current

• If circuit is inductive, current lags behind voltage

(lagging P.F)

• If circuit is capacitive, current leads the voltage

(leading P.F)

PF of different Load types

 When current passes through inductive loads, it lags voltage, because inductance do not allow current to change in 0+^ time and hence voltage gets lead and current gets lagged (for pure inductance lag is 90o).

 When current passes through Capacitive loads, it leads the applied voltage because capacitors do not allow voltage to change in 0+ time and hence voltage gets behind the current and current advanced. (For pure capacitive load current will lead voltage by 90 o).

 When Current passes through Resistance it does not react against voltage or current and hence both remain in phase (i.e., neither lag nor lead).

 At any point in a power circuit, the power flow carries current which is composed of all three characteristic properties of conductor R, XL and XC.

Introduction

• From engineering and economic point of view,

it is desirable to have power factor to be close

to unity.

• This will lead to more efficient and low cost

electricity generation and transmission system

Power Factor (P.F=cosФ)

“ The cosine of angle between voltage and current

in an A.C. circuit is known as power factor”.

• In an A.C circuit generally there is an angle

between voltage and current

• If circuit is inductive, I lags behind voltage

(lagging P.F)

• If circuit is capacitive, I leads the voltage

(leading P.F)

Expressing P.F

• Usually we attach lagging or leading with the

numerical value of P.F to show current is

leading or lagging the voltage

• P.F can be expressed as a percentage, e.g. 0.

lagging P.F may be expressed as 80% lagging

• P.F cannot be more than unity or more than

100% (because CosФ ranges between - 1 to zero to 1)

Power Triangle

• P.F can be found in terms of power drawn as well
• Multiply V on each side of current triangle we get power triangle OA= 𝑉𝐼 cos Ф (active power kW) AB= 𝑉𝐼 sin Ф (reactive power kVAR) OB= 𝑉𝐼(apparent power kVA) 𝑂𝐵^2 = 𝑂𝐴^2 + 𝐴𝐵^2 𝑎𝑝𝑝𝑎𝑟𝑒𝑛𝑡 2 = 𝑎𝑐𝑡𝑖𝑣𝑒 𝑝𝑜𝑤𝑒𝑟 2 + 𝑟𝑒𝑎𝑐𝑡𝑖𝑣𝑒 𝑝𝑜𝑤𝑒𝑟 2

𝑐𝑜𝑠Ф =

𝑂𝐴 𝑂𝐵

=

𝐴𝑐𝑡𝑖𝑣𝑒 𝑃𝑜𝑤𝑒𝑟 𝐴𝑝𝑝𝑎𝑟𝑒𝑛𝑡 𝑃𝑜𝑤𝑒𝑟

=

𝑘𝑊 𝑘𝑉𝐴 The Power Factor is the percentage of Apparent Power that does real work

Power Triangle

• Power Factor =

𝑅 𝑍

𝑅𝑒𝑠𝑖𝑠𝑡𝑎𝑛𝑐𝑒 𝐼𝑚𝑝𝑒𝑑𝑎𝑛𝑐𝑒

• Power factor= cos Ф = cosine angle

between V and I

• Power Factor =

𝑉𝐼 cos Ф

𝑉𝐼

𝐴𝑐𝑡𝑖𝑣𝑒 𝑃𝑜𝑤𝑒𝑟 𝐴𝑝𝑝𝑎𝑟𝑒𝑛𝑡 𝑃𝑜𝑤𝑒𝑟

Power Factor Correction

• P. F issue arises whenever reactive power is not balanced.
• Normally the low P. F is due to increase in inductive nature of load, which causes drop in system voltage and low voltages issues are more serious in power grid (related to high currents and high losses and low efficiency)
• Further low voltages may at any time lead to collapse of the system i.e. de-synchronization.
• Hence, P.F correction required at all points of power grid

Means of P. F. Correction

• Generally, P.F range is 0.8-0.9 on system
• If P.F is lower, it’s desirable to take steps to

improve P.F

• Device taking leading power should be connected

in parallel with the load

P.F improving equipment

• Static Capacitors
• Synchronous condensers
• Phase Advancers

Illustration of PF Improvement

• Capacitor draws current 𝐼𝐶 which leads the

supply voltage by 90 degrees

• Resulting line current I ′ is the phasor sum of I and

𝐼𝐶 and its angle of lag is φ 2

• φ 2 is less than φ 1 ,
• Hence, the power factor of the load is improved

Disadvantages of Low P.F

• Large kVA rating of equipment.

𝑘𝑉𝐴 =

𝐾𝑊 𝑐𝑜𝑠Ф 𝑘𝑉𝐴 ∝

1 𝑃.𝐹 , larger and expensive equipment E.g. P=10kW on full load and terminal V= 250V What happens at unity and 0.8 lagging PF?

• Greater conductor size.

Have to carry more current

• Large copper losses

More 𝐼^2 𝑅 losses Results in poor efficiency

Disadvantages of Low P.F

• Poor Voltage Regulation
  • Large current at low PF causes more voltage drops in every equipment on grid (alternators, transformers, transmission lines, distributors, etc.)
  • Decreased voltage is available at the supply end thus impairing the performance of utilization devices
  • Extra equipment e.g. voltage regulator are required
• Reduced handling capacity of system

Reactive component of current prevents the full utilization of installed capacity