Further development of a PMU calibration system

Motivation and relevance for the National Metrology Institute and for Sweden

There is a need for calibrattion of PMUs in the Swedish electricity grid and Swedish industry. PMUs are used to control the electricity grid both for transmission reliability and to be able to maximize resources for the transmission of electrical energy. PMUs can measure quantities such as voltage, current, harmonics, frequency, power and other mathematical quantities. All these measurements are based on amplitude and phase, and it is foremost the amplitude and above all the phase that need to be calibrated.

Those responsible for energy transfer in Sweden face major challenges. An increasing proportion of renewable energy sources are being introduced, which are more or less uncontrollable at the same time as nuclear power plants are being closed down and charging of electric vehicles is being added. At the same time, the demands on energy efficiency increase. The energy flow is largely controlled by phase angle. Therefore, correct measurement of phase becomes very important when you want to control the energy flow for best safety and economy. The availability of a more complete calibration of PMUs gives the electric power industry a better control of the measuring capacity of their PMUs.

Background and overview of the area

Absolute phase ("Phasor") refers to the phase of an AC signal measured in relation to Coordinated Universal Time (UTC) instead of in relation to any other (AC) signal. This makes it possible to compare phase positions measured far apart physically. Absolute phase has gained a large and important use in the context of electric power, especially in the transmission of electricity. Absolute phase measuring devices are usually called Phasor Measurement Units, PMU, but the functionality can also be built into more comprehensive measuring devices.

A few international metrology laboratories, such as METAS (Switzerland), VSL (Netherlands) and NPL (UK), have the ability to measure and calibrate absolute phase. RISE also now has this capacity after the implementation of a previous RMP R&D project, see article in CPEM 2018 digest [3].

The IEEE standard C37.118.1 [1, 2] describes the functionality of a PMU and also an extended functionality to measure frequency, which is calculated from the phase change, and the rate of change of frequency (ROCOF) which is calculated from the frequency. For a complete calibration of PMUs, it is therefore important to be able to calibrate frequency and ROCOF.

som beskriver funktionaliteten för en PMU, beskrivs också en utökad funktionalitet som är att mäta frekvens, som beräknas ur fasändringen, samt frekvensändringstakten (Rate Of Change Of Frequency, ROCOF) som beräknas ur frekvensen. För en fullständig kalibrering av PMUer är det därför angeläget att kunna kalibrera frekvens och ROCOF.

The project is a collaboration between the National Laboratory for Electricity DC/LF and the National Laboratory for Time and Frequency.

Referenser

[1] IEEE standard, C37.118.1-2011 - IEEE Standard for Synchrophasor Measurements for Power Systems

[2] IEEE standard, C37.118.1a-2014 - IEEE Standard for Synchrophasor Measurements for Power Systems -- Amendment 1: Modification of Selected Performance Requirements

[3] S. Svensson, C Rieck, G Bideberg, B Larsson, “A PMU Calibration System, Conference of Precision Electromagnetic Measurements”, CPEM 2018 Digest.