Powerfactor Improvement

POWER FACTOR IMPROVEMENT
EETE-327 fabliha027@gmail.com
References:
Principles of Power System
V.K.Mehta
INTRODUCTION
The cosine of angle between voltage and current in an a.c. circuit is known as power factor.
•In an a.c. circuit, there is generally a phase difference φ between voltage and current.
•The term cos φ is called the power factor of the circuit.
•If the circuit is inductive, the current lags behind the voltage and the power factor is referred to as
lagging.
•However, in a capacitive circuit, current leads the voltage and power factor is said to be leading.
INTRODUCTION
Consider an inductive circuit taking a lagging current I from supply voltage V;

the angle of lag being φ. The phasor diagram of the circuit is shown in Fig. 6.1.
The circuit current I can be resolved into two perpendicular components,
namely ;
(a) I cos φ in phase with V
(b) I sin φ 90o out of phase with V
The component I cos φ is known as active or wattful component, whereas
component I sin φ is called the reactive or wattless component. The reactive
component is a measure of the power factor. If the reactive component is small,
the phase angle φ is small and hence power factor cos φ will be high. Therefore,
a circuit having small reactive current (i.e., I sin φ) will have high power factor
and vice-versa.
POWER TRIANGLE
. The analysis of power factor can also be made in terms of power drawn by the
a.c. circuit. If each side of the current triangle OAB of Fig. 6.1 is multiplied by
voltage V, then we get the power triangle OAB shown in Fig. 6.2 where
OA = VI cos φ and represents the active power in watts or kW

AB = VI sin φ and represents the reactive power in VAR or kVAR
OB = VI and represents the apparent power in VA or kVA
DISADVANTAGES OF LOW POWER FACTOR
. •Large KVA rating of equipment

•Greater conductor size
•Large copper losses
•Poor voltage regulation
•Reduced handling capacity of the system
POWER FACTOR IMPROVEMENT EQUIPMENT
1. Static capacitor. The power factor can be improved by connecting capacitors in parallel with the
. equipment operating at lagging power factor. The capacitor (generally known as static capacitor) draws
a leading current and partly or completely neutralizes the lagging reactive component of load current.
This raises the power factor of the load. For three-phase loads, the capacitors can be connected in delta
or star as shown in Fig. 6.4. Static capacitors are invariably used for power factor improvement in
factories.
Advantages
.
(i) They have low losses.
(ii) They require little maintenance as there are no rotating parts.
(iii) They can be easily installed as they are light and require no foundation.
(iv) They can work under ordinary atmospheric conditions.
Disadvantages
(v) They have short service life ranging from 8 to 10 years.
(vi) They are easily damaged if the voltage exceeds the rated value.
(vii) Once the capacitors are damaged, their repair is uneconomical..
2. Synchronous condenser.
.
A synchronous motor takes a leading current when over-excited and, therefore, behaves as a capacitor.
An over-excited synchronous motor running on no load is known as synchronous condenser. When such
a machine is connected in parallel with the supply, it takes a leading current which partly neutralises the
lagging reactive component of the load. Thus the power factor is improved.
Fig 6.5 shows the power factor improvement by synchronous condenser method. The 3φ load takes
current IL at low lagging power factor cos φL. The synchronous condenser takes a current Im which
leads the voltage by an angle φm*. The resultant current I is the phasor sum of Im and IL and lags
behind the voltage by an angle φ. It is clear that φ is less than φL so that cos φ is greater than cos φL.
Thus the power factor is increased from cos φL to cos φ. Synchronous condensers are generally used at
major bulk supply substations for power factor improvement.
.
Advantages
(i)
. By varying the field excitation, the magnitude of current drawn by the motor can be changed by any
amount. This helps in achieving stepless control of power factor.
(ii) The motor windings have high thermal stability to short circuit currents.
(iii) The faults can be removed easily.
Disadvantages
(iv) There are considerable losses in the motor.
(v) The maintenance cost is high.
(vi) It produces noise.
(vii) Except in sizes above 500 kVA, the cost is greater than that of static capacitors of the same rating.
(viii)As a synchronous motor has no self-starting torque, therefore, an auxiliary equipment has to be
provided for this purpose.
CALCULATION OF POWER FACTOR CORRECTION
.
MATH
.
MATH
.
MATH
.
MATH

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POWER FACTOR IMPROVEMENT

Consider an inductive circuit taking a lagging current I from supply voltage V;

OA = VI cos φ and represents the active power in watts or kW

. •Large KVA rating of equipment

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