Shunt Capacitor Switching For Power Factor Improvement

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