Understanding the relationship between these three quantities lies at the heart of understanding power engineering. It does, however, serve an important function in electrical grids and its lack has been cited as a significant factor in the Northeast Blackout of 2003. To the load, it is sometimes called "wattless" power. Since reactive power transfers no net energy The unit for reactive power is expressed as var, which stands for volt-amperes reactive. Apparent power is conventionally expressed in volt-amperes (VA) since it is the product of rms voltage and rms current.
The unit for all forms of power is the watt (symbol: W), but this unit is generally reserved for real power. Real power moves energy, so it is the real axis. Reactive power does not transfer energy, so it is represented as the imaginary axis of the vector diagram. In the diagram, P is the real power, Q is the reactive power (in this case positive), S is the complex power and the length of S is the apparent power. P h ase of Current ( φ), the angle of difference (in degrees) between voltage and current Current lagging Voltage (Quadrant I Vector), Current leading voltage.Apparent Power (| S|), that is, the absolute value of complex power S: volt- ampere.Reactive power ( Q): volt-amperes reactive.Real power ( P) or active power : watt.The apparent power is the vector sum of real and reactive power.Įngineers use the following terms to describe energy flow in a system (and assign each of them a different unit to differentiate between them): This is the fundamental mechanism for controlling the power factor in electric power transmission capacitors (or inductors) are inserted in a circuit to partially cancel reactive power 'consumed' by the load. Conventionally, capacitors are considered to generate reactive power and inductors to consume it. If a capacitor and an inductor are placed in parallel, then the currents flowing through the inductor and the capacitor tend to cancel out rather than adding. Conductors, transformers and generators must be sized to carry the total current, not just the current that does useful work.Īnother consequence is that adding the apparent power for two loads will not accurately give the total apparent power unless they have the same displacement between current and voltage (the same power factor). Apparent power is the product of the root-mean-square (rms) voltage and current.Įngineers care about apparent power, because even though the current associated with reactive power does no work at the load, it heats the wires, wasting energy. Power engineers measure apparent power as the vector sum of real and reactive power.
Practical loads have resistance, inductance, and capacitance, so both real and reactive power will flow to real loads. In this case, only reactive energy flows-there is no net transfer of energy to the load. There is no net energy flow over one cycle. For half of each cycle, the product of voltage and current is positive, but on the other half of the cycle, the product is negative, indicating that on average, exactly as much energy flows toward the load as flows back. If the load is purely reactive, then the voltage and current are 90 degrees out of phase. In this case, only real power is transferred. At every instant the product of voltage and current is positive, indicating that the direction of energy flow does not reverse. If the load is purely resistive, the two quantities reverse their polarity at the same time. In a simple alternating current (AC) circuit consisting of a source and a linear load, both the current and voltage are sinusoidal.
The following ―in quote is taken from under the Creative Commons Attribution-ShareAlike License The relevant theory related to real, reactive, and apparent power should be reviewed in your ECE 231 book before doing the experiment. Submit your one-line diagram file via Moodle. Values you’ll need to enter for the generator, transmission line and loads are given in the appendix on Power world. Place a variable capacitor across the load.
Create an oneline Diagram with a transmission line and a series R-L Load connected to a generator.
PreLab:ĭownload and Install PowerWorld on your computer. To understand the power diagram, active, reactive, apparent power, power factor correction and the effect on transmission line losses. Experiment 2: Power Factor Correction Objectives