Power quality in microgrids

Rapidly increasing energy demand from the industrial and commercial sector, especially in the current climate of high oil prices, steadily reducing energy sources and at the same time increased concerns about environmental changes, have caused fast development of Distributed Power Generation Systems (DPGS) based on renewable energy. A recent concept is to group DPGS and the associated loads to a common local area forming a small power system called a microgrid. This small autonomous system formed by DPGS can offer increased reliability and effciency of future power system networks. Furthermore, the improvement of the control capabilities and operational features of microgrids brings environmental and economic benefits. The introduction of microgrids improves power quality, reduces transmission line congestion, decreases emission and energy losses, and effectively facilitates the utilisation of renewable energy resources. As a consequence of the fast expanding DPGS based on renewable energy sources, Transmission System Operators (TSO) have issued strict interconnection requirements (grid code compliance), e.g., on power quality control, reactive power control, fault ride-through etc. Among these different requirements issued by the grid operators, power quality have recently gained a lot of attention due to excessive non-linear and unbalanced loads over-stressing the power systems and causing system failure. As nonlinear and/or unbalanced loads can represent a high proportion of the total load in small-scale systems, the problem with power quality is a particular concern in microgrids. In this work, different control strategies are proposed and implemented for the grid and microgrid connected voltage-source inverters (VSI), based on H^inf and repetitive control techniques. The repetitive control, which is regarded as a simple learning control method, offers very good performance for voltage and current tracking as it can deal with a very large number of harmonics simultaneously. This leads to a very low Total Harmonic Distortion (THD) of the output voltage and/or the current even in the presence of nonlinear loads and/or grid distortions. Initially, a voltage controller proposed in the literature for microgrid applications is further developed and experimentally tested. The aim is to improve power quality and tracking performance, while considerably reducing the complexity of the controller design. The model of the plant is reduced for single-input-single-output (SISO) repetitive control design. As a consequence, the design becomes much simpler and the stability evaluation easier. Moreover, a frequency adaptive mechanism is proposed so that the controller can cope with grid frequency variations in the grid-connected mode. This mechanism allows the controller to maintain very good tracking performance over a wide range of grid frequencies. Then, a H^inf repetitive control strategy for the inverter current is proposed and validated with experiments. As a result, the power quality and tracking performance are considerably improved. In order to demonstrate the improvements, the proposed controller is compared with the traditional proportional-resonant (PR), proportional- integral (PI) and predictive deadbeat (DB) controllers. Finally, the advantages of the proposed voltage and current controllers based on H^inf and repetitive control techniques are put together for consideration in microgrid applications and experimentally tested. The proposed cascaded current-voltage control strategy is not a simple combination of the two control strategies, but a complete re-design after realising that the inverter LCL filter can be split into two separate partsfor the design of the controllers. As a consequence, the cascaded controller is able to maintain low THD in both the microgrid voltage and the current following into/from the grid at the same time. It also enables seamless transfer of the operation mode from standalone to grid-connected or vice versa. It turns out that the voltage controller can be reduced to a proportional gain cascaded with the internal model (in a re-arranged form), which can be easily implemented in real applications. Experiments under different scenarios (e.g. in the standalone mode or in the grid-connected mode, with linear, nonlinear or unbalanced loads etc.) are presented to demonstrate the excellent performance of the controllers