Phasor Measurement Units (PMU) Information
Phasor measurement units (PMUs) are devices that record and transmit synchronized phasors, frequency, and rate of change of frequency (ROCOF) estimates. The observed phasors represent both the magnitude and phase angle of electrical waves flowing through the electricity grid. A common time source is used to synchronize several phasors and the resulting measurement is referred to as a synchrophasor.
Synchrophasors are recorded at finite time intervals from which frequency and the ROCOF are calculated. A PMU can be a standalone device or may be integrated into another piece of equipment such as a protective relay. They are deployed at local substations and monitor power distribution. They provide input to a situational awareness tool known as a wide area monitoring system (WAMS) or are otherwise integrated into a SCADA system for monitoring, measurement, and control applications.
The term synchrophasor refers to the synchronized phasor measurements taken by the PMU, although it may also be used to describe the device itself. The phasor is an AC waveform that can be represented by a sinusoidal function whose amplitude (Xm), angular frequency (ω), and initial phase (θ) are time-invariant. The amplitude describes the rms voltage or current. A common time source such as a Global Positioning System (GPS) is used to synchronize phasor vectors with respect to a Coordinated Universal Time (UTC).
The importance of synchrophasor is twofold. Integration of the PMU into a WAMS allows for visualization of power distribution across multiple nodes in a power grid in real-time. Time accuracy issues stemming from network latency, time-zone differentials, and data processing time can be corrected as each measurement is related to UTC.
Secondly, power systems seldom transmit power at a nominal frequency. Synchrophasors take into account the actual frequency of the system at the time of measurement from which phase angle and ROCOF are inferred. These measurements are important in AC power systems as power flows from a higher to a lower voltage phase angle and the difference between the two relates to power flow. Large phase angle differences are indicative of increased static stress and may be used to identify power grid instability, which can then be corrected.
IEEE developed standards IEEE Std C37.118.1-2011 and its amendment IEEE Std C37.118.1a-2014 to define measurement requirements and compliance tests for PMUs operated in a steady state as well as in dynamic conditions.
IEEE Std C37.118.1-2011: This standard defines synchrophasor, frequency, and ROCOF measurements under both steady-state and dynamic conditions as well as compliance requirements and evaluation methods.
IEEE Std C37.118.1a-2014: This standard is an amendment to IEEE Std C37.118.1-2011 that removes limitations to frequency and ROCOF measurements, corrects for discrepancies between latency tests and measurements, and refines ramp tests to ensure repeatable results when evaluating PMU’s with anti-aliasing filters.
Performance classes: IEEE Std C37.118.1-2011 defines two performance classes:
- P class is defined by faster response times and mandates no explicit filtering. They are designed for protection application where faster response times are critical.
- M class is defined by the inclusion of anti-aliasing filters that are desired when the measurement could be adversely affected by aliased signals. Response time is compromised in exchange for greater precision as may be required for analytical measurements.
Compliance to either performance class simply denotes that the device meets the minimum requirements and does not guarantee that the device is adequate for a specific application. PMUs can be filtered by their respective performance class, but should be selected based upon the requirements of the intended application.
The following parameters represent the accuracy of the device.
ROCOF error (RFE): ROCOF is the first derivative of the frequency function. It describes the magnitude to which frequency changes in respect to time and can be expressed by the function df/dt. The importance of ROCOF is that it gives keen insight into load shedding, power disturbances, and loss of grid in real time. ROCOF may be expressed by the units Hz/s and may not exceed an error of ±0.44 Hz/s for a P class PMU operating in steady-state conditions.
Total vector error (TVE): TVE is an expression that quantifies the difference between theoretical values and estimated values of amplitude, phase, and frequency by the device under test. It is defined as the square root of the difference squared between the theoretical and the estimated synchrophasors in ratio to the magnitude of the theoretical synchrophasor and is expressed as a percentage.
Accuracy of the PMU is largely dependent on the time source integrated into the device and the nominal frequency of the waveform being measured. A timer error of 1 µs corresponds to a phase error of 0.022 degrees for a 60Hz system and 0.018 degrees for a 50Hz system. The time source and time tagging event must provide time traceable to UTC with sufficient accuracy in order to satisfy compliance requirements and maintain TVE, frequency error (FE), and RFE within the required limits.