A Multilevel Energy Buffer and Voltage Modulator for Grid-Interfaced Microinverters

Abstract

Abstract-Micro-inverters operating into the single-phase grid from solar photovoltaic (PV) panels or other low-voltage sources must buffer the twice-line-frequency variations between the energy sourced by the PV panel and that required for the grid. Moreover, in addition to operating over wide average power ranges, they inherently operate over a wide range of voltage conversion ratios as the line voltage traverses a cycle. These factors make the design of micro-inverters challenging. This paper presents a multilevel energy buffer and voltage modulator (MEB) that significantly reduces the range of voltage conversion ratios that the dc-ac converter portion of the micro-inverter must operate over by stepping its effective input voltage in pace with the line voltage. The MEB also functions as an active energy buffer to reduce the twice-line-frequency voltage ripple at the output of the solar panel. The small additional loss of the MEB can be compensated by the improved efficiency of the dc-ac converter stage, leading to a higher overall system efficiency. A prototype micro-inverter incorporating a MEB, designed for 27 V to 38 V dc input voltage, 230 V rms ac output voltage, and rated for line cycle average power of 70 W, has been built and tested in grid-connected mode. It is shown that the MEB can successfully enhance the performance of a single-phase grid-interfaced microinverter by increasing its efficiency and reducing the total size of the twice-line-frequency energy buffering capacitance.

A Two-Stage Universal Input Charger With Wide Output Voltage Range

“…Transitioning from the preliminary line current draw scheme of [18] to that of Fig. 6a reduced worst case loss by 0.25W (a 0.5% efficiency improvement) 9 .…”

“…The back-and front-end stages of the protoype converter are shown in Figs. 7 and 8, respectively, and the component 9 Introducing higher order harmonics into this current draw worsened overall efficiency. Note that we omit the volume of logic and control circuitry in this metric as these represent circuit components that can be miniaturized by other established means, such as implementation in an integrated circuit.…”

“…The key constraint at the heart of sizing the energy buffer is that the capacitance is selected to satisfy ripple requirements when the input voltage is lowest, but it must be physically sized for the voltage rating when the input voltage is highest. This motivates the concept of an "active buffer" in which an additional conversion stage is interfaced to the capacitor in order to better utilize its energy storage capability [7]- [9]. This idea has primarily been investigated in designs which must satisfy power factor correction (PFC) regulations (e.g.…”

Direct storage hybrid (DSH) inverter: A new concept of intelligent hybrid inverter

“…A transformer-less topology such as HERIC [23] with 6 active switches 3 -8 is used to connect the dc link capacitor 1 to the grid, but only operated at line frequency to generate a rectangular output voltage. A series active filter with 4 high-frequency switches 9 -12 and an LC filter ( f , f ) are employed to compensate the high harmonic voltages from the rectangular one [24]- [26]. The output is connected to a load and/or to an ac grid and operated in either off-grid mode or grid-connected mode.…”

Design of a highly efficient microinverter

“…The main advantage of using a microinverter system to a string inverter system is that microinverters isolate light obstructions and faults from solar panels in the entire system [4]. A microinverter is constituted of two main conversion units: 1) a DC-DC boost converter to step up the DC voltage generated as well as guarantee its operation at maximum power, and 2) a DC-AC converter to generate a desired AC current to ensure a unity power factor with the connected grid.…”