Topology of a Neutral Leg
For applications in renewable energy, distributed generation and smart grids, there is often a need to have a neutral line to work with inverters so that a current path is provided for unbalanced loads. A neutral line is also needed when the phases of an inverter need to be independently operated so that the coupling effect among the phases is minimised. Indeed, as demonstrated in Part I, the power quality of the phase voltages generated by an inverter is best when the phases are independently controlled. In this chapter, some topologies to provide a neutral line are discussed.
The introduction of microgrids and smart grids improves power quality, reduces transmission line congestion, decreases emission and energy losses, and effectively facilitates the utilisation of renewable energy resources (Chen et al. 2010; Lee and Cheng 2007; Li and Kao 2009; Li et al. 2007; Nikkhajoei and Lasseter 2009; Xiarnay et al. 2008). In some circumstances, the inverters used in microgrids and/or other applications must supply a mixture of single- and three-phase loads via a four-wire three-phase distribution network. A neutral line is often needed to provide a current path for unbalanced loads and the traditional six-switch inverter must be supplemented with a neutral connection (De and Ramanarayanan 2010; Li et al. 2006; Zhong et al. 2006). Figure 10.1 shows a microgrid, where a neutral line is provided for the local loads. If the neutral point of ...