Photovoltaic Module Integrated dc-dc Converters: Modulation and Advanced Control Techniques.

Abstract

Photovoltaic (PV) energy generation is one of the most prominent technologies for the clean production of electricity. In PV systems of any scale, power electronic converters are the essential interface between the renewable energy generators and the load. Traditionally, multiple PV modules are interfaced to one converter with maximum power point tracking (MPPT) capability. Although this is a well-known and simple architecture, research has highlighted the loss of energy attributed to partial shading and electrical characteristic mismatching among PV modules. During the last two decades, substantial research and industry efforts converged towards distributed maximum power point tracking (DMPPT) architectures adopting interfacing converters, referred to as module integrated converters (MICs), on a per-PV module basis. These architectures maximise energy harvest, regardless of partial shading or electrical characteristic mismatching among PV modules. Other advantages include the possibility of operating PV modules of distinct technology together, or of varying power rating, or installed along different orientations. Moreover, MICs provide opportunities for granular monitoring and diagnostic of PV systems, since the health status of individual PV modules may be observed through the converters. This thesis is focussed on the analysis of a PV interfacing dc-dc converter (dc-MIC), involving modelling, modulation and advanced control techniques. In the studied application, each PV module feeds a dc-MIC, and the dc-MIC outputs are connected in series. The non-inverting buck-boost is the selected converter topology, as it is one of the most promising candidates for DMPPT. Before describing the contributions on dc-MIC control, a chapter of this thesis is dedicated to PV modelling, as the understanding of this topic is necessary to simulate any PV interfacing converter. Firstly, a pragmatic review of PV modelling techniques is undertaken, describing the PV equivalent circuit parameter estimation. Secondly, a contribution to the practical simulation of the PV module electrical characteristic is outlined. The operation of the non-inverting buck-boost dc-MIC is then analysed. One research contribution presented, gives evidence of the improvement of the converter performance during the changeover between buck and boost operating modes. Attention is then devoted to modelling and controller design, according to linear controltechniques. The small-signal model of the non-inverting buck-boost dc-MIC is derived, and a cascaded controller is designed. Its performance is compared against a traditional single-loop voltage controller. It is verified that the cascaded controller yields a considerable improvement in the regulation of the PV module voltage, as it greatly reduces the disturbance caused by the operation of the gate driving circuit. The final contribution regards the utilisation of the feedback linearisation control (FLC) technique to the dc-MIC application. This emerging non-linear control technique provides the benefit of analysing the converter as a linear system, whose dynamic behaviour is independent of the operating point. The dynamic performance of the dc-MIC under FLC and cascaded control is experimentally compared. It is proven that, with FLC, the converter transient response is indeed insensitive to the system operating point, further enhancing the PV module voltageregulation. All converter control techniques are digitally implemented in a floating point microcontroller, driving a 250 W dc-MIC prototype, having a switching frequency of 200 kHz. The outlined converter control techniques can be applied in existing dc-MICs, adding value to state of the art control for PV interfacing dc-dc converters