title: Operational and stability analysis of PV inverters with active power filter functionalities
Creators: name: Iqbal, Zafar
Publisher: name: The University of Edinburgh
Year: 2024
Subjects: subject: Operational analysis
subject: stability analysis
subject: photovoltaic (PV)
subject: PV inverters (PVIs)
subject: PVI circuits
subject: PVI controls
subject: active power filter (APF)
subject: PVI instability
subject: over output current (fault current)
subject: under voltage (collapse of DC bus)
subject: PVI topologies
subject: Fast Fourier transform (FFT)
subject: PV generation
subject: maximum power point tracking (MPPT)
subject: PVI output filter

description: Due to the growing concerns over the climate change and continually decreasing costs of photovoltaic (PV) based technologies there was a significant increase of various PV generation systems over the recent years (e.g., there is currently around 14.5 GW of PV generation in the UK). Despite a large difference in their sizes and installed powers, ranging from a few-kilowatt household PV systems to the multi-megawatt PV farms, they all require a power electronics interface—a DC-AC inverter—to connect to the AC grid. The PV inverters (PVIs) employ sophisticated electronic circuits and advanced control systems for converting the variable input DC voltages and currents into the fully regulated AC voltages and currents at the output. Development of accurate models of the PVI circuits and controls is crucial for the correct evaluation of a wide range of interactions between the PVIs and the grid, especially in terms of: analysing the impact of supply voltage distortions on the PVI operation, modifying the PVI controls for emulating active power filter (APF) functionalities, and identifying operating and supply conditions that can potentially cause PVI instability. The unstable operation of a PVI refers to a situation in which the inverter is not performing as expected or desired which could be indicated by over output current (fault current) or under voltage (collapse of DC bus). This PhD thesis focuses on the modelling of common types of PVI topologies and control systems, which are implemented in various reference frames and analysed in the context of both existing and new approaches for identifying, quantifying and reducing harmonic current emissions and resulting voltage harmonic distortion. The full switching models of single-phase and three-phase PVIs with different control schemes are developed and validated using available measurements. A modification of the PVI control system is suggested to allow operation of the PVI with APF functionalities. Deterioration of the overall performance and stability issues of some PVIs are evaluated in terms of the distorted supply voltage conditions and connection to the “weak/strong” grid, as well as the PVI operation at the (very) low output powers. The analysis in the thesis starts with an overview and discussion of various concepts and approaches for analysing non-sinusoidal instantaneous current and voltage waveforms (from which related powers are calculated), including formulation of some novel approaches. The most commonly used approach for analysing harmonic waveform distortions, Fast Fourier transform (FFT), is compared with several other existing approaches (d-q, α-β and instantaneous P-Q theory), as well as with new approaches introduced in the thesis. Using examples of some typical power electronic circuits (e.g., half-bridge and full-bridge diode rectifiers with and without DC link capacitors), the thesis evaluates and compares different options for mitigating harmonic distortions by different methods. Afterwards, the thesis presents details of modelling different components of a typical PV generation system, including: PV panels, DC-DC converter with maximum power point tracking (MPPT) control, DC-AC inverter with associated control system, and output/grid side filter. Different circuit topologies and controls are considered, demonstrating how measured frequency-dependent impedance characteristics of the PVI can be used to identify approximate values of the PVI output filter, as well as the type and parameters of its control. Next, the thesis discusses modifications of the PVI control system for improving its operational performance and for the provision of grid support/ancillary services, such as reactive power compensation and mitigation of harmonic emission. In particular, the operation of the PVI emulating APF functionalities under different supply grid conditions (sinusoidal and distorted supply voltages, as well as different source impedances) and for different PVI operating powers is studied in detail. Particular attention is given to the assessment of the PVI’s operating limits and identification of the most efficient controls for the provision of different compensation/APF functionalities, requiring reduction of the PVI active power output. Finally, after introducing and illustrating stability issues identified in laboratory-based tests with some PVIs, for which accurate models are developed based on the analysis in the previous chapters, the thesis presents a detailed evaluation of the operational performance and stability of the PVIs operating in low power modes and under non-sinusoidal/distorted supply grid conditions. This part of the work aims to fill the present gaps in the existing literature on stability assessment methods, related to a transfer from the stable to the unstable operation regions in the presence of “multitone excitations” (i.e., presence of both fundamental and harmonic components in the supply voltage), under the varying output powers and for the high grid/source impedance values. The analysis of the complex source-device interactions again includes an initial evaluation of some aggregate effects, when two or more PVIs are connected in the local LV distribution network.