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Shunt Capacitor Switching For Power Factor Improvement Clayton H Reid

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Power Factor Kw is productive power Kvar is non productive

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Industrial Plant Electrical Load Induction Motors Induction Furnaces Fluorescent Lighting

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Advantages of Installing Capacitors Improved Power Factor Released System Capacity Improved Motor and Lighting Performance Reduced Current and Losses Decreased Transformer Losses

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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

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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

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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

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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

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Overvoltage Due To Excessive Capacitance Capacitor connected to the motor and starter de-energized, motor acts as an induction generator with shunt capacitor excitation

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Maximum Voltage Generated Size of capacitor Speed of motor No load characteristics

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Overvoltage Due to Excessive Capacitance

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Magnetizing Current

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Torque Transients

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Location of Capacitors

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Energizing a Single Capacitor Bank

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Capacitor Inrush Current

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Transient Frequency - transient frequency- power frequency

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Recommended Capacitor Rating

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Inrush Currents

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Energizing Additional Banks

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Capacitor Inrush Current Ca La 2 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

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Transient Frequency

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Contactor Switching Capability

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Transient Overvoltage

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Methods Of Limiting Inrush Currents

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Method Of Limiting Inrush Current

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Capacitor Tests

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Air-Core Reactor Design

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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

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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

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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

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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

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