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

CaLa2 Ep

Ip =

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

Parallel switching of 10 kvar capacitors

Parallel switching of 10 kvar capacitor with reactors

Inrush Current

725 A

Inrush Current

1153 A

Inrush Current

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