Phasor measurement unit

A phasor measurement unit (PMU) is a device which measures the electrical waves on an electricity grid using a common time source for synchronization. Time synchronization allows synchronized real-time measurements of multiple remote measurement points on the grid. The resulting measurement is known as a synchrophasor. PMUs are considered to be one of the most important measuring devices in the future of power systems. A PMU can be a dedicated device, or the PMU function can be incorporated into a protective relay or other device.

In 1893, Charles Proteus Steinmetz presented a paper on simplified mathematical description of the waveforms of alternating current electricity. Steinmetz called his representation a phasor. With the invention of phasor measurement units (PMU) in 1988 by Dr. Arun G. Phadke and Dr. James S. Thorp at Virginia Tech, Steinmetz’s technique of phasor calculation evolved into the calculation of real time phasor measurements that are synchronized to an absolute time reference provided by the Global Positioning System. We therefore refer to synchronized phasor measurements as synchrophasors. Early prototypes of the PMU were built at Virginia Tech, and Macrodyne built the first PMU (model 1690) in 1992. With the growth of increasingly more distributed energy resources on the power grid, more observability and controls systems will be needed to monitor power flow. Historically, power has been delivered in a uni-directional fashion through passive components to customers. With an increasingly more complex network of generation and loads, it is imperative that the electrical conditions of transmission and distribution networks are continuously being observed through advanced sensor technology––PMUs and uPMUs.

A PMU can measure 50/60 Hz AC waveforms (voltages and currents) typically at a rate of 48 samples per cycle. The analog AC waveforms are digitized by an analog-to-digital converter for each phase. A phase-locked oscillator along with a Global Positioning System (GPS) reference source provides the needed high-speed synchronized sampling with 1 microsecond accuracy. However, PMUs can take in multiple time sources including non-GPS references as long as they are all calibrated and working synchronously. The resultant time-stamped phasors can be transmitted to a local or remote receiver at rates up to 120 samples per second. Historically, only small numbers of PMUs have been used to monitor transmission lines with acceptable errors of around 1%. These were simply coarser devices were installed to prevent catastrophic blackouts. Now, with the invention of micro-synchronous phasor technology, many more of them are desired to be installed on distribution networks where power can be monitored at a very high degree of precision. This high degree of precision creates the ability to drastically improve system visibility and implement smart and preventative control strategies. No longer are PMUs just required at sub-stations, but are required at several places in the network including tap-changing transformers, complex loads, and PV generation buses.

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