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Shunt Capacitor Switching
For
Power Factor Improvement
Clayton H Reid
Power Factor
Kw is productive power
Kvar is non productive
Industrial Plant Electrical Load
• Induction Motors
• Induction Furnaces
• Fluorescent Lighting
Advantages of Installing Capacitors
•
•
•
•
•
Improved Power Factor
Released System Capacity
Improved Motor and Lighting Performance
Reduced Current and Losses
Decreased Transformer Losses
Shunt-capacitor Banks
• Automatic switching of capacitor banks
•
Voltage Control-a voltage sensitive relay is
used which responds to changes in line voltage
• Current Control-a current sensitive relay is used
which responds to changes in line current
• Kilovar Control-a kilovar relay is used which
responds to changes in reactive loads
Capacitors Switched with Motor
• Another means of obtaining automatic
switching is to connect the capacitor to the
motor and switch the motor and capacitor
as a single unit
Capacitors Switched with Motor
• The importance of selecting the correct
size of capacitor to be switched with a
given motor load
• Location of capacitor connected points
• Capacitor switching for special motors and
for special motor-starting applications
Capacitors Switched with Motor
• Transient inrush current and frequency for
the following cases:
• When a single capacitor is energized on
a system
• When a capacitor is energized in parallel
with capacitor banks already connected
• Effect of transient currents on contactors
• Use of air-core reactors to limit transient
current in parallel switching of capacitors
Overvoltage Due To Excessive
Capacitance
• Capacitor connected to the motor and
starter de-energized, motor acts as an
induction generator with shunt capacitor
excitation
Maximum Voltage Generated
• Size of capacitor
• Speed of motor
• No load characteristics
Overvoltage Due to Excessive
Capacitance
Magnetizing Current
Torque Transients
Location of Capacitors
Energizing a Single Capacitor Bank
Capacitor Inrush Current
Transient Frequency
- transient frequency
- power frequency
Recommended Capacitor
Rating
Inrush Currents
Energizing Additional Banks
Capacitor Inrush Current
Ip
=
2
Ep
Ca
La
Ip= peak in rush current in amps
Ep= r.m.s phase voltages in volts
Ca= total circuit capacitance in farads
La= total circuit inductance in henries
Between C1 and C2
Transient Frequency
Contactor Switching Capability
Transient Overvoltage
Methods Of Limiting Inrush
Currents
Method Of Limiting Inrush Current
Capacitor Tests
Air-Core Reactor Design
Air-Core Reactor Design
Air-Core Reactor Design
Capacitor Switching Tests
Single 10 kvar
capacitor
Inrush Current
725 A
Parallel
Parallel
switching of 10 switching of 10
kvar capacitors kvar capacitor
with reactors
Inrush Current Inrush Current
1153 A
595 A
Transient
frequency
1057 Hz
Transient
frequency
3340 Hz
Transient
frequency
1750 Hz
Summary
• Capacitor selection can be made from
manufactures literature. Will provide correction
to approx. 95% lagging, voltage will be limited to
110% when motor disconnected.
• Capacitors should be connected ahead of
overload relays. If connected after the relays
Overload section should be selected based on
reduced current through the relays.
• Do not connect capacitors to the winding of a
motor driving a high inertia load.as torque
transients up to 20 times can occur resulting in
mechanical damage to motor shaft and driven
machinery
Summary
• To avoid torque transient problems for motor
and driven machinery,capacitors should not be
connected directly to the motor in the following :
• a) any open transition reduced voltage starter
• b) reversing starters, or starters which are used
for for jogging the motor
• c) two speed motors
• d) wye-delta motors
• Use a separate contactor to switch the capacitor
Summary
• When capacitors are installed in motor control
centers additional inductance should be installed
in series with the capacitors to limit transient
charging current.This will reduce contact erosion
in the contactor