Improved power electronics topology for premises energy management systems

The numbers of photovoltaic (PV) systems at residential premises are increasing worldwide. This thesis explores new power electronic solutions for premise energy management systems (PEMS) which incorporate a battery energy storage system (BESS) to manage the utilisation of solar and grid power as the load demand varies. A flexible, isolated three port converter (TPC) is proposed that allows unrestricted power flows between the dc connected PV systems and battery storage and the ac connected consumer loads and the external power system. The traditional PEMS solutions will seek to perform the power exchanges at a dc and/or ac bus and use multiple power converters to interface the PV and BESS components and to allow power flows between the dc and ac systems. A number of topologies exist but all require at least two separate power converters for an integrated PV-BESS that results in multistage power conversion steps for some power flows. Several three-port converter (TPC) topologies are reported in the present literature as possible improvements. However, these topologies often have important restrictions on the possible power flows. This thesis develops and demonstrates a high frequency linked three-port converter (HFL-TPC) topology for PEMS employing PV and BESS. A topological and analytical approach was taken and identified that an HFL-TPC topology could be developed by using three coupled active bridges based on dual active bridge (DAB) converters. The converter is symmetric and readily extensible to an arbitrary number of ports. All the ports are bidirectional and have the same power transfer capability. A key challenge for the HFL-TPC is developing appropriate modulation strategies. Several modulation and control strategies are reported for HFL-TPC topologies using a piecewise linear time domain approach. This approach is adequate for the dual active bridge but becomes much more complex if the number of ports increases. The practical implementation of the optimised switching variables on the digital signal processor (DSP) is very difficult to achieve. This thesis adapts the well-known power systems concepts related to real and reactive power flows between generators coupled by inductive lines to establish the key underlying principles that dictate the real and reactive flows in multiport converters. It is shown that the real and reactive flows are strongly dominated by fundamental frequency power flows. For a multiport converter, the fundamental frequency angle differences between the ports determine the real power flows while the magnitude differences largely determine reactive flows. Based on these power system concepts, the thesis develops an improved quintuple phase shift (QPS) modulation for the HFL-TPC. It achieves the required power transfers at minimum current and provides favourable switching operations to improve the efficiency. The proposed algorithm avoids the extensive computational burden associated with traditional optimisation processes which are barely tractable even for converters with a few ports. The thesis also develops the small signal models for the HFL-TPC and control decoupling networks to allow independent closed loop power control strategies. Within the literature, TPCs with a four-quadrant ac port are very rare. It is technically possible to modify one port of the proposed converter by replacing the conventional bridge converter with a matrix converter. The approach was not pursued far as this has significant practical difficulties. It is challenging to maintain soft switching. In a single phase system, the output voltage zero points make reactive power flows difficult to control. In this thesis, the ac port has been developed by adding an inverter stage. Single phase inverters have power fluctuation at the dc bus at twice the output frequency. The thesis presents methods that allow the ripple power to be directed to a nominated port. The operation of the HFL-TPC with the proposed QPS modulation and control strategies has been demonstrated by extensive simulation studies conducted in MATLAB/Simulink. A 60 V, 400 W laboratory scale prototype has been constructed and demonstrated at module level to verify the practical feasibility of the proposed optimised modulation strategies. Simulation and experimental results show that the proposed optimised QPS modulation method controls the reactive power, improves the efficiency, and maintains the zero voltage switching (ZVS) operation over a wide voltage and power range. The proposed converter is a flexible alternative that can facilitate the full range of power flows that might be expected in a PEMS system. It has general applicability across a range of industrial applications where power flows between multiple sources and loads need to be facilitated. The transfer of power systems concepts to the control of the converter modulation is a significant advance in the control of multi-port systems.