Synchronized Phasor Measurement Unit (PMU)
Electrical Control and Protection II
More recent technological advancements in microprocessor relays, combined with GPS receivers for synchronization and accurate time stamping, is providing users advanced relay systems with synchronized measurements, called synchrophasor measurements Synchronized .Wide Area measurements (WAMs) is a relatively modern practice universal in the operation and control of electric power transmission. It consists of making synchronized measurements of phasor quantities of voltages and currents at several points spaced hundreds or thousands of kilometers points. Phasor Measurement Unit (PMU) technology provides phasor information (both magnitude and phase angle) in real time. The importance of data provided by PMUs has been well recognized after a number of major blackouts in many power systems in different parts of the world. PMU is an important component of WAMs.
Synchronized measurements components:
Synchronized measurements of voltages and currents are done by Phasor measurement Units (PMUs). Each PMU has three essential components.
• GPS signal receiver
• Data acquisition module
• Communication module
The GPS signal receiver consists of an aerial and receiver circuits, which detect the 1PPS (pulse per second) and a time tag consisting of the year, day, hour, minute, and second .
Synchronizing signal from the GPS satellites. This signal is used to correct for drifts in a local clock oscillator, which keeps the basic time in the PMU. The local clock provides the timing signals to the data acquisition module to sample the voltage and current signals with correct time stamps. These samples are digitally processed locally in the PMU, and the processed data are made available to the communication module. The communication module sends out the time-stamped data over a standard communication interface to a wider data concentration and processing system. (Figure 1) shows the block schematic diagram of a PMU .
Figure1: functional block diagram of a typical PMU
Phasor Measurement Unit:
The microprocessor calculates the positive sequence voltage and current phasors, and determines the timing message from the GPS, along with the sample Number at the beginning of a window. If enough PMUs can be installed across a large power transmission network, the real time system operating condition can be directly measured by PMUs.
In addition, as the PMUs have a high data reporting rate, the system dynamics can be captured when the system is subjected to disturbances. Figure 2 compares the voltage angle difference between two substations obtained using PMU measurements and traditional state estimation.
This comparison demonstrates clearly that a real time monitoring system made up of PMUs will provide much more precise and dynamic system operation information than the traditional state estimation. In a real implementation of...