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Shunt Capacitor Switching For Power Factor Improvement

Learn the advantages of installing shunt-capacitor banks for improved power factor, reduced losses, and enhanced electrical system performance in industrial plants. Understand the importance of selecting the correct capacitor size and managing transient inrush currents to prevent overvoltage issues. Discover methods to limit inrush currents and ensure efficient capacitor switching.

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Shunt Capacitor Switching For Power Factor Improvement

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  1. Shunt Capacitor Switching ForPower Factor Improvement Clayton H Reid

  2. Power Factor Kw is productive power Kvar is non productive

  3. Industrial Plant Electrical Load • Induction Motors • Induction Furnaces • Fluorescent Lighting

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

  5. 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

  6. 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

  7. 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

  8. 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

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

  10. Maximum Voltage Generated • Size of capacitor • Speed of motor • No load characteristics

  11. Overvoltage Due to Excessive Capacitance

  12. Magnetizing Current

  13. Torque Transients

  14. Location of Capacitors

  15. Energizing a Single Capacitor Bank

  16. Capacitor Inrush Current

  17. Transient Frequency - transient frequency - power frequency

  18. Recommended Capacitor Rating

  19. Inrush Currents

  20. Energizing Additional Banks

  21. Capacitor Inrush Current Ca Ep 2 Ip = 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

  22. Transient Frequency

  23. Contactor Switching Capability

  24. Transient Overvoltage

  25. Methods Of Limiting Inrush Currents

  26. Method Of Limiting Inrush Current

  27. Capacitor Tests

  28. Air-Core Reactor Design

  29. Air-Core Reactor Design

  30. Air-Core Reactor Design

  31. Capacitor Switching Tests

  32. 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

  33. 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

  34. 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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