Topology of a Neutral Leg - Control of Power Inverters in Renewable Energy and Smart Grid Integration - page 181

CB

Phase lines

CB

CB

Neutral line

Local
Load m

Local
Load 1

Inverter Inverter

Distributed
energy source

Distributed
energy source

DG1 DGn

Figure 10.1 Microgrid equipped with a neutral line

one inverter provides the neutral line is discussed. If multiple inverters are fitted with a neutral
line in a microgrid, then appropriate strategies should be developed to share the neutral current
among them, in addition to the widely-studied load sharing among parallel-operated inverters
(Guerrero et al. 2011; Yao et al. 2011; Zhong 2012c).
Another situation where a balanced neutral point is required is in multi-level pulse width
modulation (PWM) converters, which have the potential to improve the total harmonic dis-
tortion and to reduce the voltage stress across the switches; see (Celanovic and Boroyevich
2000; Nabae et al. 1981; Soto-Sanchez and Green 2001; Wong et al. 2001) and the survey
paper (Rodriguez et al. 2002).

10.2 Split DC Link
A straightforward way to provide a neutral point is to use two capacitors in parallel with
large balancing resistors, as shown in Figure 10.2(a), with the neutral point clamped at half of
the DC link voltage. Such a configuration is called a split DC link. This topology is widely
adopted in active power filters because the neutral current in active power filters does not have
a DC component and the fundamental component is relatively small; see, for example, (Quinn
and Mohan 1992; Verdelho and Marques 1998). It has also been widely adopted in three-level
converters because there is no need to provide a neutral line on the AC side and the aim is to
maintain a balanced mid-point of the DC link. Some special PWM generating schemes are
available to balance the charging of the capacitors; see e.g. (Celanovic and Boroyevich 2000;
Lee et al. 1998; Ratnayake et al. 1999).
However, for applications with a large neutral current, the neutral current flows through the
capacitors and, hence, bulky capacitors are needed to make the voltage ripple on the capacitors
small. Another drawback is that the neutral point usually drifts and becomes unbalanced, in