Power Engineering Society Chicago Chapter

1. Power Engineering SocietyChicago Chapter Reactive Power: Sources and Solutions 12 February 2003 David E. Mertz, PE Burns & McDonnell Engineers, Inc

2. Reactive Power • What is it? • Current that is 90 degrees out of phase with the voltage in an alternating current system. • Inherent in all alternating current systems • Caused by capacitive (leading) and inductive (lagging) loads. • The complement of “real” power.

3. Reactive Power • Where does it come from? • All conductors are an inductor. • Multiple conductors are inductors with a mutual capacitance.

4. Reactive Power • Where does it come from? • Placing conductors in a magnetic raceway increases their inductance.

5. Reactive Power • Where does it come from? • Magnetic devices are the largest source of lagging (inductive) reactive power • Transformer impedance contributes reactive power, but also limits downstream short-circuit currents.

6. Reactive Power • Where does it come from? • Magnetic devices are the largest source of lagging (inductive) reactive power • Magnetic lamp ballasts also produce lagging reactive power.

7. Reactive Power • Where does it come from? • Magnetic devices are the largest source of lagging (inductive) reactive power • Synchronous electric machines (generators and synchronous motors) can produce either lagging or leading reactive power. • Inductive electric machines (garden-variety motors) produce only lagging reactive power.

8. Reactive Power • What good is it? • In small amounts, it helps transmission line operators control the flow of electric power. • The transmission of high-frequency or step signals in power systems is greatly attenuated by the properties that also give us reactive power. • The same electrical phenomena are used to tune circuits for transmitting and receiving signal broadcast at selected frequencies

9. Reactive Power • Why is it not desirable? • Transmitting reactive together with real power power reduces the conductor ampacity, transformer capacity, and generator capacity available for the real power. • It can lead to the overheating of electrical transmission and distribution equipment. • The same electrical phenomenon attenuates signals on wire-based systems.

10. Reactive Power • How do we control it? • Limit the amount of lagging reactive power required from the electrical power system. • The single largest controllable source of lagging (inductive) reactive power is:

11. Reactive Power • How do we control it? • The single largest controllable source of lagging (inductive) reactive power is: LAZY MECHANICAL ENGINEERS

12. Reactive Power • Reduction by design: • Don’t oversize motors Large Motor, Large Load Reactive Power (VAr) Total Power (VA) Real Power (Watts)

13. Reactive Power • Reduction by design: • Don’t oversize motors Large Motor, Small Load Reactive Power (VAr) Total Power (VA) Real Power (Watts)

14. Reactive Power • Reduction by design: • Don’t oversize motors Small Motor, Small Load Reactive Power (VAr) Total Power (VA) Real Power (Watts)

15. Reactive Power • Reduction by design: • Select high power factor motors • These are often high efficiency motors • Be aware that high power factor motors often have higher starting current requirements. • Ensure that the high efficiency or high power factor motors have the right mechanical characteristics, such as starting torque, for the load.

16. Reactive Power • Reduction by design: • Use variable frequency drives (VFDs) where applicable • Power factor on line side of VFD is usually 0.95 or greater. • Reactive power reduction alone won’t justify cost of the drive, but can be part of the total return on investment. • VFDs have rectifier front ends, which will add harmonic currents to the system.

17. Reactive Power • Reduction by design: • Use synchronous motors for large, constant speed and load applications • Synchronous motors can be run as a source of leading as well as lagging power factor • A large, constant load is necessary to be able to recover the added cost of the synchronous motor and its field controller. • Typically applied at higher voltages (4160, 13 800).

18. Reactive Power • Reduction by design: • Carefully select lighting ballasts • Where possible, use electronic ballasts • Otherwise, select high power factor ballasts. • When using electronic ballasts, be aware of third harmonic consideration.

19. Reactive Power • Lagging power factor countermeasures: • Reduce the demand for reactive power through the measures previously mentioned. • Reducing the amount of lagging reactive power on a system has less potential for creating undesirable conditions than trying to correct it through adding sources of leading reactive power. • Reducing reactive power demand will almost always reduce the real power demand also.

20. Reactive Power • Lagging power factor countermeasures: • Add sources of leading reactive power once opportunities to reduce lagging power demand have been addressed. • Once leading reactive sources have been added to a power system, tuned “LC” circuits have been created. Potentially severe and difficult-to-diagnose harmonic current flows can result if the resonant frequency or frequencies coincide with the fundament or system frequency or its harmonics.

21. Reactive Power • Lagging power factor countermeasures: • Distribution-level Fixed Capacitors: • Most economical on a dollars-per-farad basis. • No control system required • Least flexible in response to changing system conditions. • Load-level Fixed Capacitors • Very economical, easy to install, no control system, switches automatically with the load.

22. Reactive Power • Lagging power factor countermeasures: • Distribution-level Switched Capacitors: • More expensive than fixed banks, but responds to changes in reactive power demand. • Control system required, with added cost, configuration, and maintenance required. • Electromechanical type is less expensive than semiconductor switched banks, but it responds more slowly to load changes, which may be a concern if avoiding utility penalties is a concern.

23. Reactive Power • Lagging power factor countermeasures: • Active Harmonic Compensation Systems: • Most expensive on a dollars-per-farad basis. • Works by “injecting” compensating current into the power system. • Highly responsive control system compensates on a sub-cycle basis for both harmonic and reactive power demands. • Under normal configuration, reactive power takes a back seat to harmonic cancellation.

24. Reactive Power • Benefits of reactive power control: • Better voltage stability • More efficient use of existing power system. • May be able to add load without increasing system ampacities. • Less heating of electrical equipment • Extends useful life of equipment. • Less real power needed to generate that heat. • Potential reduction in utility charges.

25. Reactive Power • Summary: • Reactive power is inherent in AC systems and serves some useful purposes. • Reduce demand for reactive power before adding capacitors to compensate. • Select leading reactive power sources by balancing cost with need for flexibility and responsiveness.

26. Reactive Power • For further reading: • IEEE Std. 141-1993, Electric Power Distribution for Industrial Plants, Chapter 8. • This is a good resource for sizing power factor correction capacitors. • Questions and discussion

27. Reactive Power For further reading: IEEE Std. 141-1993, Electric Power Distribution for Industrial Plants, Chapter 8. This is a good resource for sizing power factor correction capacitors. Questions and discussion.