Automatic power factor correction units use capacitor banks to improve power factor in electrical systems. They monitor power factor and switch capacitors in and out as needed to maintain power factor above a set value. This improves voltage regulation, reduces transmission costs, and allows the same real power to be delivered with less apparent power. The key components are a reactive power control relay, connection points, slow-blow switches, contactors, a capacitor bank, and control transformer. Maintenance involves checking connections, capacitance, current, voltage, and fuses.

1. AUTOMATIC POWER FACTOR
CORRECTION

2. CONTENTS

3. Power factor: An introduction
It is the ratio of real power flowing through
the load to the apparent power in the
circuit
It is also defined as the cosine of the angle
between the voltage and current phases
It is a dimensionless no between 0 and 1
Whether the current is leading or lagging
the power factor is termed as leading or
lagging PF correspondingly

4. Types of Circuit
There are two types of circuits based on the type
of current waveform
• Linear loads
• Non-linear loads
The type of loads which doesn’t change the
current waveform shape are known as linear loads
The loads on the others case which change the
shape (distort) the current waveform are termed
as non-linear loads
Distortion decreases average power transmitted
to the load

5. Variation of PF in transmission
systems
Generally due to inductive loads the power
factor never remains unity in reality
A capacitive network gives reactive power and
an inductive network takes the same
In both the cases the power factor becomes
less than 1 causing more current to be
transferred for the same amount of real
power

6. Disadvantages of low PF
Voltage regulation becomes poor
Greater amount of apparent power needed
to get the same amount of real power
The increase in reactive power increases the
current flowing through the network
Size of the transformer, switch gears etc.
increase thereby increasing the transmission
cost

7. Power Factor Correction for linear
loads
Various methods of power factor correction
(PFC) are adopted to minimize the apparent
power as:
Using capacitor banks in Automatic Power
Factor Correction (APFC) units
For high voltage rating the power electronic
devices such as Static VAR compensator or
STATCOM are used
It’s also a good practice to use an unloaded
synchronous motor

8. APFC unit
It’s an automatic system adjusting itself to
control the power factor above a desired value
by a bank of capacitors switched by means of
contractors
Contractors are controlled by a regulator that
measures PF in the network
Depending upon the load PF the controller will
adjust the PF by switching the necessary no of
capacitors from the bank

9. Parts of the APFC unit
1.Reactive power control
relay
Capable of measuring the
reactive and active power of
the network
Works in conjunction with a
capacitor bank
Controls the programmed
target PF by
activating/deactivating the
capacitors

10. The main functions of the reactive power control relay
Automatic connection detection (Star or Delta)
Automatic detection of capacitor stages
Patented characteristics avoiding compensation of very
low active power
Reactive power requirement-switching delay time
Alarm signals for
• Failure to reach the target PF
• Overcurrent in the capacitor
• Defects at capacitor stages

11. 2.Network connection points
Since the power factor of the loads is to be
compensated hence the connection of the APFC
unit is always done in parallel with the supply
connection. Generally Y- connection of the
capacitor bank is preferred because the
transmission distribution of three phase
distribution system above 132 kv is Y connected
always

12. 3.Slow-blow switches
 Slow blow switches are always used because of
the following requirements
• To sustain the inrush and starting current surges
due to vehicular loads
• It offers a time delay between overloading and
fusing
• They can withstand heavy currents (about ten
times the rated)

13. 4.Inrush current contactors
 Used for damping the inrush current arising due
to
• Remaining capacitor voltage due to fast
switching
• Short circuit power of supply
• Fault level of supply networks
 The contractors also protect the welding of the
main contacts of the contactors
 Prevent the capacitors from high stress

14. 5.Capacitor Bank
The capacitor bank is the main part of
the APFC units. Large capacitor banks
are installed in the APFC’s which switch
in order to meet
the desired power factor requirement.
Each capacitor is individually fused with
appropriately sized current limit fuse.

15. 6.Control Transformer
 Monitors the switchboard load
(or load to be corrected)
 Placed physically above
(towards the supply) the
connections for the PFC system
 In case more than one
transformers are fitted they are
always connected in series as
they work as CT

16. Maintenance of the APFC unit
The major maintenance checks involved for the APFC
unit are
 In “out of maximum demand hours” (eg. Late
nights or weekends) switch off the system after
internal and external checks
 Remove contactor tops and inspect for contact
wear, check capacitance of each capacitor
 Check (and adjust if necessary) all connections to
contactors, capacitors, fuses etc.
 Re-energize system and check current (true RMS)
and harmonics drawn by each capacitor step.
Check voltage level and fuses

17. Conclusion
 Hence the APFC units correct the power
factor above a decided value by switching
the appropriate no of capacitors from the
capacitor bank
 In non-linear loads we use inductors,
filters, valley-fill circuit etc. to suppress
harmonics as they cause the current wave
form to be distorted