What Is a Phasor Measurement Unit?

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 track the quality of alternating current (AC) power being provided by electric power utility companies on the electricity grid. The phasor in a phasor measurement unit refers to the mathematical model of a vector with both direction and amplitude of electrical waves. When referring to the periodic nature of AC, the phasor is a specific type of vector that only has a phase relationship to its reference phase. If there is only one generator in electric power transmission, there is little need for a phasor measuring tool. Grids, however, have several generators that feed the same power distribution system.

One generator unit in a power generation system needs to be able to synchronize its phase and amplitude with other generating units. For instance, a utility frequency of 60 cycles per second (cps) or 60 hertz (Hz) completes one cycle every about 0.0166667 second. The phasor difference between the outputs of individual generators in the system must be very close to zero. This phasor difference increases when one generator runs just a bit faster or just a bit slower than the other generators, or has an output that was eventually higher or lower than the net level required.

Two 5-kilovolt-ampere (kVA) generators at 110 volts (V) and 60 Hz demonstrate the significance of a phasor measurement unit. Without synchronization, these two generators will be running and each delivering an output of 110 volts alternating current (VAC) at 60 Hz. These two generators at half capacity may be delivering emergency power to two sets of lights and appliances about 2.5 kilowatts (kW) each, which corresponds to about 22.8 A. This system of dual generator does not require any PMU because the two generators are not in the same circuit.

Connecting two generators, which need to be synchronized in frequency and in phase, to a grid requires a phasor measurement unit. Two generators may be generating 110 VAC at exactly 60 Hz. If these generator outputs are exactly zero degrees phase difference, the outputs of these generators may be wired together to form a grid. Special hardware and software are used to make sure generators are synchronized at all times. Any exception may result in partial to total grid blackout.

The phasor measurement unit is required for power system automation to form a smart grid. With the help of a global positioning system (GPS), accurate frequency and phase outputs of several power plant generators may be synchronized by the PMU and control equipment. Both accurate and precise, the control of these power plants grid-wide, together with calculated compensations for the electrical characteristics of the transmission system and demand statistics, makes continuous electric utility power a reality.