Renewable energy technologies have been growing in their installed capacity
rapidly over the past few years. This growth in solar, wind and other technologies is
fueled by state incentives, renewable energy mandates, increased fossil fuel prices and
environmental consciousness. Utility scale systems form a substantial portion of
electricity capacity addition in modern times. This sets the stage for research activity to
explore new efficient, compact and alternative power electronic topologies to integrate
sources like photovoltaics (PV) to the utility grid, some of which are multilevel
topologies.
Multilevel topologies allow for use of lower voltage semiconductor devices than
two-level converters. They also produce lower distortion output voltage waveforms. This
dissertation proposes a cascaded multilevel converter with medium frequency AC link
which reduces the size of DC bus capacitor and also eliminates power imbalance
between the three phases. A control strategy which modulates the output voltage
magnitude and phase angle of the inverter cells is proposed. This improves differential
power processing amongst cells while keeping the voltage and current ratings of the
devices low.
A battery energy storage system for the multilevel PV converter has also been
proposed. Renewable technologies such as PV and wind suffer from varying degrees of
intermittency, depending on the geographical location. With increased installation of
these sources, management of intermittency is critical to the stability of the grid. The
proposed battery system is rated at 10% of the plant it is designed to support. Energy is stored and extracted by means of a bidirectional DC-DC converter connected to the PV DC bus. Different battery chemistries available for this application are also discussed.
In this dissertation, the analyses of common mode voltages and currents in various PV topologies are detailed. The grid integration of PV power employs a combination of pulse width modulation (PWM) DC-DC converters and inverters. Due to their fast switching nature a common mode voltage is generated with respect to the ground, inducing a circulating current through the ground capacitance. Common mode voltages lead to increased voltage stress, electromagnetic interference and malfunctioning of ground fault protection systems. Common mode voltages and currents present in high and low power PV systems are analyzed and mitigation strategies such as common mode filter and transformer shielding are proposed to minimize them
