Transcript power banks
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