Control Strategies for Low Voltage DC Microgrids

Abstract

Increasingly, clean renewable energy sources such as wind power and solar power are supplementing and/or replacing, fossil fuels as inputs to electrical grids. Effective and efficient integration of such diverse energy sources, to their full potential, necessitates changes to the architecture and management of those grids. Microgrids provide an effective means for power distribution from renewable energy sources. DC microgrids are well suited to low voltage applications, and have advantages over AC microgrids, including simpler control, higher efficiency and no reactive power. Conventionally, DC microgrids are operated using a hierarchical control scheme. However, complicated control structures add to the cost of microgrids, and limit their application to electrifying remote and developing areas. This thesis focuses on the control of DC microgrids. A set of control strategies based on a modified hierarchical control scheme is proposed for household applications with power coordination, stability enhancement and plug and play performance. First, a dual-window DC bus interacting method is proposed as a replacement for conventional communication infrastructure. It utilises the advantages of both droop control and DC bus signalling methods. Droop control has plug and play performance and DC bus signalling has the function of automatic power regulation. Low bandwidth signalling between distributed sources can be achieved by manipulating relationships between droop coefficients. With the proposed DC bus interacting method, coordination and power management of distributed energy sources in the absence of communication infrastructure in a DC microgrid may be achieved. Second, a passive stabiliser is proposed for the primary controller to compensate non-minimum phase in interface converters. The stability performance of an interface converter in a DC microgrid can be optimised by adjusting the feedforward gain in a passive stabiliser. This affects the quiescent operating point and therefore a modified feedforward controller is used in this system. Consequently, potential instability caused by non-minimum phase can be avoided. As a by-product, the passive stabiliser can reduce the required size of the terminal capacitor. Third, a novel hierarchical control scheme is implemented in a low voltage DC microgrid bench. The passive stabiliser is applied in the primary layer, droop control in the secondary layer, and a six-bit signal series based on the dual-window DC bus voltage interacting method is applied in tertiary layer for power flow control. The impact of bidirectional power flow on battery banks in the system is discussed based on the terminal input admittance analysis. In addition, the small signal model of terminal output admittance under the proposed the passive stabiliser is built. It shows that the proposed system is stable under all possible working modes based on the minor loop gain analysis. The on-grid and off-grid operation modes are conducted, and seamless transfer between them can be achieved. Especially, the surplus power from PV generations is limited, for the power balance consideration, by the proposed modified MPPT algorithm when the system operates under off-grid mode.