Control of Three-Phase Inverters in Microgrid Systems

Abstract

In this work, application of two different control strategies to three-phase DC-AC PWM inverter used in smart microgrid system, is analyzed. The objective of control design is to achieve low THD output voltage, fast transient response and asymptotic tracking of the reference output voltage under different loading conditions minimizing the effect of the harmonic frequencies. First, the PID control technique is applied to the three-phase inverter, and it is shown that PID controller works very well for the linear loads and achieves acceptable level of harmonic reduction. However, with nonlinear loads, PID controller cannot achieve satisfactory level of harmonic suppression. The second control technique – robust servomechanism problem (RSP) control design combined with sliding mode control –eliminates this problem, and achieves satisfactory level of harmonic suppression in presence of nonlinear loads. The control design is performed in discrete-time domain. For both control techniques, controller structure is defined, necessary controller parameters are determined and the simulation of the closed-loop system is performed in MATLAB/Simulink.

Typically, smart microgrid systems will have several components. These components include green and renewable energy sources with their associated power converters, efficient transformers, and storage systems. To the end users, residential customers, this power can come from many different energy sources. For example photovoltaic cells, microturbines, fuel cells, etc. The photovoltaic cells (PV) are sources of DC power, whereas, high speed microturbine generators are sources of high frequency AC power. Since these generators are designed for high speed operation, they have a low weight, a low volume, a low foot print. They use natural gas with a lower carbon foot print. The fuel cells are also considered as green energy source since they have close to zero carbon foot prints. The variable wind speed generators have the same operating principle as microturbine generators, except they run at variable speed and generate variable AC power. To utilize the variable frequency AC power sources, the produced power are rectified to DC power using AC/DC rectifier, and then, DC/AC inverters are used to convert the generated DC power output to AC power at the system operating frequency. The wind and PV power sources are depicted in Fig. 5.1 and 5.2, respectively. The renewable green energy sources use three-phase DC/AC inverters, and converts direct current (DC) power to sinusoidal alternating currents (AC).