Chapter 5 - Dynamics of power flow in a stand-alone microgrid using four-leg inverters for nonlinear and unbalanced loads

Abstract

The four-leg inverter has some proven benefits over three-leg split direct current (DC) link inverters or three-leg inverters followed by a Δ-Y transformer when used for three-phase, four-wire low voltage systems. When a three-leg split DC link inverter is used in the presence of nonlinear and unbalanced loads, zero-sequence currents find a path through the DC link capacitors. In the presence of such loads, this topology fails to maintain balanced and sinusoidal terminal voltages. Another conventional way is to use a Δ-Y transformer followed by a three-leg inverter for three-phase, four-wire loads. A Δ-Y transformer will not allow zero-sequence currents to flow to the input side but the whole system becomes bulky. Also, the three-phase, four-wire loads will be fed without affecting the DC link voltages but terminal voltages cannot be maintained in a balanced way. Four-leg inverters on the other hand are able to handle zero-sequence currents and can maintain balanced sinusoidal voltages at their terminals. The load neutral is be connected to the center of the fourth leg. Switching of the fourth leg is taken care of by a special modulation technique and thus avoids ripples in DC link voltage. It eliminates the need to use bulky transformers and the need to use large-sized DC link capacitors. This motivates the use of four-leg inverters as a power interface in a stand-alone microgrid (MG). The concept is applicable in remote areas where the grid supply is not available and renewable energy sources are distributed with a random nature of loads on three phases.