Measurement, control and protection of microgrids at low frame rates supporting security of supply

Roscoe, Andrew and Burt, Graeme and McDonald, James (2009) Measurement, control and protection of microgrids at low frame rates supporting security of supply. PhD thesis, Electronic And Electrical Engineering.

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Abstract

Increasing penetrations of distributed generation at low power levels within electricity networks leads to the requirement for cheap, integrated, protection and control systems. To minimise unit cost, algorithms for the measurement of AC voltage and current waveforms should be implemented on a single microcontroller, which also carries out all other protection and control tasks, including communication and data logging. This limits the frame rate of the major algorithms, although ADCs can be over-sampled using peripheral control processors on suitable microcontrollers. Measurement algorithms also have to be tolerant of poor power quality which may arise, even transiently, within a microgrid, battlefield, or disaster-relief scenario. This thesis analyses the potential magnitude of these interfering signals, and presents suitably tolerant architectures and algorithms for measurements of AC waveforms (amplitude, phase and frequency). These algorithms are shown to be robust and accurate, with harmonic content up to the level of 53% THD, and with the major algorithms executing at only 500 samples per second. This is achieved by the careful optimisation and cascaded use of exact-time averaging techniques, which prove to be useful at all stages of the measurements: from DC bias removal to low-sample-rate Fourier analysis to sub-harmonic ripple removal. Algorithms for three-phase nodal power flow analysis are benchmarked on the Infineon TC1796 microcontroller and require less than 8% of the 2000μs frame time, leaving the remainder free for other algorithms. Furthermore, to optimise security of supply in a microgrid scenario, loss-of-mains must be detected quickly even when there is an accidental or deliberate balance between local active power generation and demand. The measurement techniques are extended to the detection of loss-of-mains using a new Phase Offset relay, in combination with a novel reactive power control technique to avoid the non-detection-zone. These techniques are tested using simulation, captured network transient events, and a real hardware microgrid including a synchronous generator and inverter.

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