Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Power Engineering Society Chicago Chapter Reactive Power: Sources and Solutions 12 February 2003 David E. Mertz, PE Burns & McDonnell Engineers, Inc 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. Reactive Power • Where does it come from? – All conductors are an inductor. – Multiple conductors are inductors with a mutual capacitance. Reactive Power • Where does it come from? – Placing conductors in a magnetic raceway increases their inductance. 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. 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. 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. 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 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. 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: Reactive Power • How do we control it? – The single largest controllable source of lagging (inductive) reactive power is: LAZY MECHANICAL ENGINEERS Reactive Power • Reduction by design: Reactive Power (VAr) – Don’t oversize motors Large Motor, Large Load Total Power (VA) Real Power (Watts) Reactive Power • Reduction by design: Reactive Power (VAr) – Don’t oversize motors Large Motor, Small Load Total Power (VA) Real Power (Watts) Reactive Power • Reduction by design: Reactive Power (VAr) – Don’t oversize motors Small Motor, Small Load Total Power (VA) Real Power (Watts) 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. 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. 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). 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. 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. 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-todiagnose harmonic current flows can result if the resonant frequency or frequencies coincide with the fundament or system frequency or its harmonics. 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. 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. 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. 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. 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. 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 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.