Power factor correction

1. Active and Reactive Power
• When a circuit has resistive and reactive parts, the
resultant power has 2 parts:
– The first is dissipated in the resistive element. This is
the active power, P
– The second is stored and returned by the reactive
element. This is the reactive power, Q , which has
units of volt amperes reactive or var
• While reactive power is not dissipated it does
have an effect on the system
– for example, it increases the current that must be
supplied and increases losses with cables
16.6

2. Reactive power flow in distribution system result in:
• Transformer over loads;
• Voltage drops at the end of the lines;
• Temperature rise supply cables and thus in active
power losses;
• Over sizing of harmonic protection devices.
 Reactive power (KVAR) required by inductive
loads increases the amount of apparent power
(KVA) in distribution system. This increase in
reactive and apparent power results in a larger
angle measured between KW and KVA.
As θ increases, cosine θ (or power factor)
decreases.

3. • Consider an
RL circuit
– the relationship
between the various
forms of power can
be illustrated using
a power triangle

4. • Therefore
Active PowerP = VI cos  watts
Reactive Power Q = VI sin  var
Apparent Power S = VI VA
S2 = P2 + Q2
definition : p.f. = cos  = P/ S
P.F. should be close to unity

5. Power Factor
• Power factor is particularly important in high-power applications
It is the measure of how effectively electrical equipment converts
electric power (supplied by your power utility) into useful power
output.
• In technical terms, it is the ratio of Active Power (kW) to the
Apparent Power (kVA) of an electrical installation.
• The PF play important role in a.c circuits since power consumed
depends on this factor.
• • 𝑷 = 𝑽𝑰 cos ∅ (for SINGLE PHASE)
• 𝑰 = 𝑷/ 𝑽 𝐜𝐨𝐬 ∅ •
• 𝑷 = √𝟑𝑽 𝑰 cos ∅ (for THREE PHASE)
• 𝑰 = 𝑷/ √𝟑𝑽 𝐜𝐨𝐬 ∅
– power companies therefore penalize industrial users who
introduce a poor power factor

6. Causes of low power factor :
Low power factor results when KW small in relation to KVA.
This would occur when KVAR is large.
What causes a large KVAR in a system?
• …………… Inductive loads
• All Ac motors
• Transformers
• Arc & induction heating furnance
• High intensity discharge lighting
• All devices using electromagnetic field.
Disadvantage of low power factor
• large kva rating of equipments.
• greater conductor size.
• large copper losses.
• poor voltage regulation.
• reduced handling capacity of system

7. Benefits for improving power factor
• Lower utility fees by
- Reducing peak KW billing demand
- Eliminating the power factor penalty
• Increased system capacity and reduced system
losses in your electrical system
• Increased voltage level in your electrical
system
• Maintain a better voltage regulation on the
system

8. The problem of poor power factor is tackled by
adding additional components to bring the power
factor back closer to unity
– a capacitor of an appropriate size in parallel with a
lagging load can ‘cancel out’ the inductive element
– this is power factor correction
Consumers of Reactive Power increase power
factor:
• Capacitors
• Synchronous condenser
• Phase advancer

15. • Reactive power (VAR) compensation is defined as the management of
reactive power to improve the performance of ac systems.
• Any device which is connected in series or parallel with load and which is
capable of supplying reactive power demanded by load is called reactive
power compensation device.
There are two aspects:-
• a) Load Compensation – The main objectives are to :-
• i) increase the power factor of the system
• ii) to balance the real power drawn from the system
• iii) compensate voltage regulation
• iv) to eliminate current harmonics.
• b) Voltage Support – The main purpose is to decrease the voltage
fluctuation at a given terminal of transmission line.
• Therefore the VAR compensation improves the stability of ac system by
increasing the maximum active power that can be transmitted.
Methods of Reactive Power Compensation
 Shunt compensation
 Series compensation
 Synchronous condensers
 Static VAR compensators

16. Shunt Compensation
• The device that is connected in parallel with the transmission
line is called the shunt compensator.
• For high voltage transmission line the line capacitance is high and plays a
significant role in voltage conditions of the receiving end.
• When the line is loaded then the reactive power demand of the load is
partially met by the reactive power generated by the line capacitance and
the remaining reactive power demand is met by the reactive power flow
through the line from sending end to the receiving end.
• When load is high (more than SIL) then a large reactive power flows
from sending end to the receiving end resulting in large voltage drop
in the line.
• To improve the voltage at the receiving end shunt capacitors may be
connected at the receiving end to generate and feed the reactive
power to the load so that reactive power flow through the line and
consequently the voltage drop in the line is reduced.

17. • To control the receiving end voltage a bank of capacitors (large
number of capacitors connected in parallel) is installed at the
receiving end and suitable number of capacitors are switched
in during high load condition depending upon the load
demand.
• Thus the capacitors provide leading VAr to partially meet
reactive power demand of the load to control the voltage.
• If XC = 1/ωC be the reactance of the shunt capacitor then the
reactive power generated of leading VAr supplied by the
capacitor:
• where, |V2| is the magnitude of receiving end voltage.
C
V
X
V
Q
C
C 
2
2
2
2



18. • When load is small (less than SIL) then the load reactive
power demand may even be lesser than the reactive power
generated by the line capacitor. Under these conditions the
reactive power flow through the line becomes negative, i.e.,
the reactive power flows from receiving end to sending end,
and the receiving end voltage is higher than sending end
voltage (Ferranti effect).
• To control the voltage at the receiving end it is necessary to
absorb or sink reactive power. This is achieved by
connecting shunt reactors at the receiving end.
• If XL = ωL be the reactance of the shunt reactor (inductor)
then the reactive VAr absorbed by the shunt rector:
• where, |V2| is the magnitude of receiving end voltage.
L
V
X
V
Q
L
L 
/
2
2
2
2



19. • To control the receiving end voltage generally one shunt rector is
installed and switched in during the light load condition.
• To meet the variable reactive power demands requisite number of shunt
capacitors are switched in, in addition to the shunt reactor, which results
in adjustable reactive power absorption by the combination.
• For high voltage transmission line the line capacitance is high and plays
a significant role in voltage conditions of the receiving end.
• When the line is loaded then the reactive power demand of the load is
partially met by the reactive power generated by the line capacitance and
the remaining reactive power demand is met by the reactive power flow
through the line from sending end to the receiving end.
a) uncompensated transmission line b) compensated transmission line

21. Advantages of Series Compensation
1. Increase in transmission capacity
– The power transfer capacity of a line is given by

sin
.
X
V
E
P 
where, E , V are sending and receiving end voltages
X is reactance of line & δ is phase angle between E and V
• Power transfer without and with compensation:
K
X
X
X
X
X
P
P
X
X
V
E
P
X
V
E
P
L
C
C
L
L
C
L
L









1
1
)
/
1
(
1
)
(
sin
)
(
.
sin
.
1
2
2
1



22. 2. Improvement of System Stability
• For same amount of power transfer and same value of E
and V, the δ in the case of series compensated line is less
than that of uncompensated line.
L
C
L
C
L
L
X
X
X
X
X
V
E
P
X
V
E
P
)
(
sin
sin
sin
)
(
.
sin
.
1
2
2
1









• A lower δ means better system stability
• Series compensation offers most economic solution for
system stability as compared to other methods (reducing
generator, x-mer reactance, bundled conductors, increase
no. of parallel circuits

23. Disadvantages
1. Increase in fault current
2. Mal operation of distance relay- if the degree
of compensation and location is not proper.
3. High recovery voltage of lines- across the
circuit breaker contacts and is harmful.

24. Active Compensation
• Synchronous condensers are the active shunt
compensators and have been used to improve the
voltage profile and system stability.
• When machine is overexcited, it acts as shunt
capacitor as it supplies lagging VAr to the system
and when under excited it acts as a shunt coil as it
absorbs reactive power to maintain terminal
voltage.
• The synchronous condenser provides continuous
(step less) adjustment of the reactive power in
both under excited and overexcited mode.

29. Objective Series Capacitors Shunt Capacitors
Improving Power
Factor
Secondary Primary
Improving Voltage
level in overhead line
system with a normal
and low Power
Factor
Primary Secondary
Improving Voltage
level in overhead line
system with a high
Power Factor
Not Used Primary
Reduce Line Losses Secondary Primary
Reduce Voltage
Fluctuations
Primary Not used