Environmental Engineering Reference
In-Depth Information
10
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.
10.1 Introduction
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 the inverter that provides the neutral line is not well balanced,
then the neutral-point voltage may deviate severely from the midpoint of the DC source of
the inverter. This may result in an unbalanced or variable output voltage, DC voltage/current
components, large neutral current or even more serious problems. Thus, the generation of a
balanced neutral point in a simple and effective manner has become an important issue (De
and Ramanarayanan 2010; Li et al. 2006; Zhong et al. 2006). As usual, a microgrid equipped
with a neutral line can be operated in the grid-connected mode or the stand-alone mode. If
there are multiple inverters, then one of them can be equipped with the neutral line and the
rest of them can be operated under the balanced three-phase mode. In this part, the case when
Search WWH ::




Custom Search