Resumen de Single-stage single-phase inverter with multimode modulation and zvs for pv applications
Photovoltaic energy has grown rapidly over the last few decades driven by the decrease in cost and volume of the photovoltaic systems. These systems are composed of solar panels, batteries, and power converters. This tendency has driven the research for more compact and efficient solar inverters for residential applications. To push the research in this field, Google and the IEEE cosponsored the Little Box Challenge in 2014-2015, a contest with a million-dollar prize for the designers of the smallest 2 kVA single-phase inverter in the world. The goal was to obtain a ten times reduction in size compared to commercially available inverters. To obtain this dramatic size reduction, most teams participating in the competition used wide-bandgap devices in their designs and proposed different architectures. The CEI@UPM team proposed an innovative but complex solution. Unfortunately, the concept was not ready for the date of the competition. Nevertheless, the research group has continued to develop the architecture to prove that high efficiency and power density are achievable.
A key component in the design of single-phase inverters is the storage element needed to decouple the input and output power. This storage element is typically a bulky capacitor that comprises a large part of the total volume of the inverter. Recently, converters to interface the input port and the capacitor are emerging, allowing a higher voltage swing in the capacitor to lower its volume. The novelty of the studied topology is that it integrates the inverter and the converter for the storage capacitor in a single power stage. Therefore, instead of interfacing just the input and output ports, as conventional inverters do, this inverter has an additional port for the storage capacitor. As a result, the topology can be considered a three-port inverter.
The power stage is simple since it requires only one inductor and six switches. Besides the reduced component count, the topology and modulation are designed aiming to reduce the size and losses of the inductor through the minimization of the indirect power. The indirect power is a measure of the energy stored or delivered by reactive components in a switching cycle. Therefore, it should be minimized to optimize the inductor design. The modulation of the inverter is divided into several operation modes used in different sectors of the line cycle to allow the inverter to interface the three ports with minimum indirect power. More than one operation mode can be used at each point of the line cycle, and, for the modulation, those with minimum indirect power in the inductor are selected.
Regarding the control, three-port inverters require two control variables. As a result, the current in the inductor in the different operation modes is divided into three or four different states, as opposed to common architectures where the waveform is divided into just two states. The inverter can work under resistive, inductive, and capacitive loads, with different sets of operation modes in each case. As a result, this inverter has a multi-mode modulation. All operation modes work ideally in boundary conduction mode and with variable switching frequency.
The modulation is designed to achieve Zero Voltage Switching (ZVS) in all transitions to improve the efficiency of the inverter. ZVS not only helps to reduce the losses in switching devices, but it reduces the electromagnetic noise introduced in the grid. For ZVS purposes, the current in the inductor is used to discharge the drain-source capacitance of MOSFETs before the turn-on event. ZVS transitions in this inverter are challenging to analyze due to the different voltages withstood by switching devices, and because more than a device can be turned on at once. In this dissertation, the modulation is modified to achieve ZVS in the transitions where the inductor current is zero. This modification includes additional states when the inductor current is between a positive and negative threshold value. As a result, this modified modulation is based on triangular conduction mode. The proposed modulation also solves some issues associated with the wide variations in switching frequency along the line cycle and for different loads.
As the topology integrates both the DC-AC conversion and the power decoupling functions and the modulation is divided into several operation modes, the control of the inverter is sophisticated. A decoupling network is used to enable the use of conventional control loops and make the control independent of the operation mode. This decoupling network solves a set of equations that relate the time each state lasts in the different modes with the desired currents in the three ports. To improve the previous solution, a numerical method is proposed to solve these equations. This control is implemented in an FPGA.
One of the challenges of nonisolated photovoltaic inverters is the leakage current generated in the parasitic capacitance of the solar panels to ground due to the common-mode voltage between the input and output ports. This leakage current is limited by legislation. In this topology, a filter is required to limit the leakage current.
This dissertation details technical advancements since the end of the competition in the research of this inverter. The multi-mode modulation is explored in detail to obtain all the operation modes required to operate the inverter under any load condition. Also, the ZVS transitions are analyzed, aiming to obtain the current in the inductor needed for ZVS purposes in all possible scenarios. With this information, a complex modulation based on triangular conduction mode is proposed. The modulation is seamless, obtains ZVS in all transitions, and helps limiting the switching frequency variations during the operation. This work also describes how the modulation is implemented.
Finally, the design of the leakage current filter is also covered. The concepts discussed in the dissertation are validated through simulations and a 1 kVA proof-of-concept prototype.
