A phasor measurement unit (PMU) measures the phase and amplitude of sine waves to monitor the quality of AC supplied by power companies. PMUs are necessary for synchronizing generators in a power grid and forming an intelligent grid with accurate control of power plants.
A phasor measurement unit (PMU) is a means of providing a measurement of the phase difference and amplitudes of periodic sine waves and is intended to monitor the quality of alternating current (AC) supplied by power utility companies on the electricity grid. Phasor in a unit of phasor measurement refers to the mathematical model of a vector with direction and amplitude of electric waves. When referring to the periodic nature of AC, the phasor is a specific type of vector that has only a phase relationship with its reference phase. If there is only one generator in the transmission of electrical energy, there is little need for a phasor measuring instrument. The grids, however, have several generators feeding the same energy distribution system.
A generating unit in a power generation system must be able to synchronize its phase and amplitude with other generating units. For example, a utility frequency of 60 cycles per second (cps) or 60 hertz (Hz) completes one cycle approximately every 0.0166667 seconds. The phase difference between the outputs of the individual generators in the system must be very close to zero. This phase difference increases when one generator runs just a little faster or just a little slower than other generators, or has an output that was ultimately above or below the required net level.
Two 5 kilovolt-ampere (kVA) generators at 110 volts (V) and 60 Hz demonstrate the significance of a unit of phasor measurement. Without synchronization, these two generators will operate and each provide an output of 110 volts alternating current (VAC) at 60 Hz. These two half-capacity generators can provide emergency power for two sets of lights and appliances of approximately 2.5 kilowatts ( kW) each, which corresponds to approximately 22.8 A. This dual generator system requires no PMU because the two generators are not in the same circuit.
Connecting two generators, which must be synchronized in frequency and phase, to a network requires a phasor unit of measurement. Two generators can generate 110 VAC at exactly 60 Hz. If these generator outputs have exactly zero degrees of phase difference, the outputs of these generators can be wired together to form a grid. Special hardware and software are used to ensure that the generators are always in sync. Any exception can result in a partial or total blackout of the network.
The phasor measurement unit is required for power system automation to form an intelligent grid. With the help of a global positioning system (GPS), the precise frequency and phase outputs of several power plant generators can be synchronized by the PMU and control equipment. Accurate and precise, grid-wide control of these power plants, together with calculated offsets for the electrical characteristics of the transmission system and demand statistics, make continuous electric power a reality.
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