Abstract
Frequency measurement is an important issue in electrical engineering. Electric power systems have become increasing complex over the last decade. The use of distributed generation, the connection of non-linear loads and the presence of unexpected system faults are the main causes of frequency variations. In addition, power quality includes frequency as an important index. From a hardware instrumentation point of view, frequency measurement has different requirements: i.) Large power systems have slow frequency variation due to the high inertia of the overall network. This kind of problem requires frequency measuring methods capable of detecting small and also slow frequency variations. ii.) Small power systems can have frequency variation due to their reduced short-circuit power capacity. This type of problem requires fast methods with the capacity to detect large frequency variations. There are different groups of methods intended for frequency measurement. The methods can be compared in terms of computation and dynamic response, especially when the main voltage is disturbed. This research work focuses on frequency measurement under non-sinusoidal conditions. The paper studies the behaviour of a modified version of Sezi’s method and its hardware implementation using a microcontroller. This system can be used for frequency measurement or as a synchronized sampling source in harmonic measurement (e.g. EN 61000-4-7) The fast time response of the system enables it to be used in almost all kinds of application: small and slow frequency variations; frequency triggering in power system protection and power quality index characterisation.
Highlights
Most power system experts do not consider frequency variation as an important topic because it is rather difficult to find large frequency variation in continental power networks
From the point of view of the instrumentation design, some instrumentation devices used to register power quality disturbances exhibit a great sensitivity to small frequency variations
The European standard EN 61000-4-7 [1] defines the characteristics of an instrumentation device used for harmonic distortion measurement based on analog to digital conversion
Summary
Most power system experts do not consider frequency variation as an important topic because it is rather difficult to find large frequency variation in continental power networks. The European standard EN 61000-4-7 [1] defines the characteristics of an instrumentation device used for harmonic distortion measurement based on analog to digital conversion. This device includes a phase locked loop in order to adjust the sampling frequency dynamically but does not include any comment about the behaviour of the method used to compute the fundamental frequency. An iterative algorithm based on FIR filter has showed a good behaviour under non-sinusoidal conditions [4]. The method is not dependent upon zero-crossing of the measured voltage or current This frequency meter is basically insensitive to steady state disturbances like harmonics or low frequency amplitude variations so it can be used as a reference method in order to obtain an accurate sampling frequency in harmonic meters
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