NOTE testing and Calibration of Phasor measurement units

In the evolving Smart Grid, time-variant sources and loads introduce disturbances that can threaten the stability of the grid. Sophisticated protection and control systems are required to preserve reliability. These systems rely on accurate, synchronous measurements of voltage, current and frequency made by Phasor Measurement Units (PMUs). Although PMUs first appeared in 1988, their recent deployment has exposed inconsistent measurements and poor interoperability across brands and models. However, several recent developments promise to enhance the accuracy and consistency of PMU measurements. IEEE C37.118. 1:2011, “Standard for Synchrophasor Measurements for Power Systems”, established new performance limits for PMU test and calibration. The revised standard more clearly defines existing concepts, and enhanced and added material to the sections on steady state and dynamic tests. The emergence of automated systems supports the consistent execution of standard tests during PMU calibrations, and the observance of sound traceability practices for PMU ensures the accuracy of those calibrations. This paper describes the fundamentals of phasor measurements, the recent revisions to the IEEE C37.118. 1 standard, a new automated PMU calibration system developed at Fluke, and the establishment of traceability for PMU calibrations. 1. evolution of the electrical Power Grid In the United States, some 3,000 electrical utility companies operate about 10,000 concentrated generation facilities and 200,000 miles of transmission lines. As one observer noted: “It is often said that electrical grids represent the world’s most complex machines. However, one can argue that this analogy understates the problem. For example, how many airliners or factories are operated by a team whose members are employed by different companies with competing interests or whose members don’t traditionally talk to each other much? While the grid has been run with remarkable reliability in the past, it is likely that business and operating pressures will only increase in the future. ” [1] In 1942, when the power utility station at Grand Coulee Dam came on-line, its highly inertial system was governed by manual and analog controls that could regulate the 60 Hz generators to within a few cycles per day. For reporting and control purposes, time resolution on the order of several seconds was adequate. Electrical power flowed from a few concentrated sources to linear loads that predictably consumed power according to season and time of day. Seventy years later, the environment has radically changed. Today’s “Smart Grid” [2] is a real-time, dynamic network of electrical demand and supply (Fig. 1). There are many distributed, time-variant, non-inertial, renewable sources, such as solar and wind power. Customers can now elect to buy power when it is cheap, and often wish to sell power back to the grid. New electronic power supplies push distortion back into the grid. The demand from electric vehicles is ramping up. With so many low-inertia sources, today’s grid lacks the inherent stability previously enjoyed. Real-time computer protection and control of the grid will be required to preserve the reliability record of the generation, transmission and distribution utilities. Realtime state measurement at widely-spaced nodes, with < 1 μs time accuracy, is the foundation of this control [3]. The Phasor Measurement Unit (PMU) makes these measurements possible. 2. Fundamentals of Phasors, synchroPhasors, and Pmus