05-03-2013, 11:15 AM
The Reactive Power, Does It Important For Us
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INTRODUCTION
DEFINITION OF REACTIVE POWER
Almost all power transported or consumed in alternating current (AC) networks. AC systems supply or consume two kind of power: real power and reactive power .Real power accomplishes useful work while reactive power supports the voltage that must be controlled for system reliability.
For AC systems voltage and current pulsate at the system frequency. Although AC voltage and current pulsate at same frequency, they peak at different time power is the algebraic product of voltage and current. Real power is the average of power over cycle and measured by volt-amperes or watt. The portion of power with zero average value called reactive power measured in volt-amperes reactive or vars.
The total power is called the apparent power (symbolized by the capital letter S) and measured by volt-amperes or VA. To describe the reactive power , imagine a person on trampoline , The person real power goes into moving horizontally across trampoline as it bounces , the effort the person expend to keep standing(represent reactive power Q ) during bouncing result no net forward motion(represent real power P) , but it's necessary to walk on trampoline . The motion from trampoline always perpendicular to the direction the person is walking. So that the direction between P and Q 90 degree Out of phase.
Measuring reactive power
The amount and complexity of household electrical equipment has increased tremendously over the last few years. Electronic ballast lighting, computer monitors and air conditioners are welcome additions to our homes but come with additional burdens. One of these is on the electricity grid, as these appliances generate more signal harmonics.
This change in the end-consumer profile is a disadvantage for energy distributors which bill energy based only on active power. With the application of non-linear loads to power lines the active energy no longer represents the total energy delivered. As a response to improve billing, the measurement of reactive energy is gaining interest. For example, Italy’s leading energy distributor has decided to install more than 20 million household energy meters with active and reactive power measurements.
This growing interest in measuring reactive energy leads to the question: What method should an energy meter designer implement to accurately measure the reactive energy?
Although today’s electronic digital signal processing (DSP) enables reactive energy measurements to be closer to the theoretical value, there is no consensus in the field of energy metering on the methods of measurement. This article aims to explain and compare the three main methods in use, namely the Power Triangle, the Time Delay and Low-pass Filter.
System requirements
Electromechanical meters have set a precedent in reactive energy billing. Although they are bandwidth limited and cannot take into account harmonics of the line frequency, they are supported by the international standard for alternating current static var-hour meters for reactive energy (IEC-1268). The standard defines reactive energy measurements at the fundamental line frequency, which implies that it is not mandatory to include harmonics. It also specifies additional testing conditions to check the robustness of the measurements against the third harmonic, the dc offset in the current input, and the line frequency variation. The various reactive power measurement methods presented in this paper are evaluated against these critical tests of the IEC-1268 (Table 1).
Reactive power calculation
As explained above, different methods can be used to calculate the reactive power. The theoretical definition of the reactive power is difficult to implement in an electronic system at a reasonable cost. It requires a dedicated DSP to process the Hilbert transform necessary to get a constant phase shift of 90° at each frequency. Several solutions have been developed to overcome this limitation.
NEEDS OF REACTIVE POWER
Voltage control in an electrical power system is important for proper operation for electrical power equipment to prevent damage such as overheating of generators and motors, to reduce transmission losses and to maintain the ability of the system to withstand and prevent voltage collapse. In general terms, decreasing reactive power causing voltage to fall while increasing it causing voltage to rise. A voltage collapse occurs when the system try to serve much more load than the voltage can support.
When reactive power supply lower voltage, as voltage drops current must increase to maintain power supplied, causing system to consume more reactive power and the voltage drops further . If the current increase too much, transmission lines go off line, overloading other lines and potentially causing cascading failures. If the voltage drops too low, some generators will disconnect automatically to protect themselves. Voltage collapse occurs when an increase in load or less generation or transmission facilities causes dropping voltage, which causes a further reduction in reactive power from capacitor and line charging, and still there further voltage reductions. If voltage reduction continues, these will cause additional elements to trip, leading further reduction in voltage and loss of the load. The result in these entire progressive and uncontrollable declines in voltage is that the system unable to provide the reactive power required supplying the reactive power demands.
Reactive power needs are determined in the planning process, which is a part of engineering, part economics and part judgment. The engineering analysis requires running large, complex mathematical computer models of the electric system. The economical part required putting costs into models to determine how to achieve an efficient, reliable system. The judgment arises due to the large number of modeling choices, expert assumption and approximations that often are necessary.
Reactive power blackouts
Insufficient reactive power leading to voltage collapse has been a causal factor in major blackouts in the worldwide. Voltage collapse occurred in United States in the blackout of July 2, 1996, and August10, 1996 on the West Coast. Voltage collapse also factored in blackouts of December 19, 1978, in France; July 23, 1987, in Tokyo; March 13, 1989, in Québec; August 28, 2003, in London; September 28, 2003, in Sweden and Denmark; and September 28, 2003, in Italy.
While August 14, 2003, blackout in the United States and Canada was not due to a voltage collapse as that term has traditionally used by power system engineers, the task force final report said that" Insufficient reactive power was an issue in the blackout" and the report also "overestimation of dynamics reactive output of system generation " as common factor among major outages in the United States. Due to difficulties modeling dynamic generators output, the amount of dynamic reactive output from generators has been less than expected, worsening voltage problems and resultant power outages
PROBLEMS OF REACTIVE POWER
Though reactive power is needed to run many electrical devices, it can cause harmful effects on your appliances and other motorized loads, as well as your electrical infrastructure. Since the current flowing through your electrical system is higher than that necessary to do the required work, excess power dissipates in the form of heat as the reactive current flows through resistive components like wires, switches and transformers. Keep in mind that whenever energy is expended, you pay. It makes no difference whether the energy is expended in the form of heat or useful work.
We can determine how much reactive power your electrical devices use by measuring their power factor, the ratio between real power and true power. A power factor of 1 (i.e. 100%) ideally means that all electrical power is applied towards real work. Homes typically have overall power factors in the range of 70% to 85%, depending upon which appliances may be running. Newer homes with the latest in energy efficient appliances can have an overall power factor in the nineties.
Conclusion
Efficient completion is a way to achieve efficiency and reduce costs to consumers. Efficient competition is difficult to achieve. Due to innovation and technological progress, the optimal industry structure and mode of regulation may not need to change. As regulated markets move from franchised monopolies toward completion, Regulation needs to move from direct price regulation to market rules. Competitive markets required competitive market design.
Put difficulties, efficient market design does not just happen spontaneously. It is the result of a process that includes full discussion, learning and informed judgment by all affective and responsible parties.