Topology Of Photovoltaic Inverters

Centralised Inverters

       The centralised inverter technology is essentially a large number of PV modules connected in series to form a PV string, which is then connected in parallel to form a PV array, where the energy is fed into the DC side of a centralised inverter after convergence in a current sink. The controller is generally composed of a high-performance signal processor such as a DSP, which uses SVPWM modulation algorithms to output the corresponding drive pulses by collecting the signals from the AC output, so that the inverter output is closer to the requirements of the grid.

Among the many centralised inverters, the three-phase two-level inverter topology is the most widely used. This structure consists of a DC-side support capacitor, a three-phase inverter main circuit and an AC measurement filter circuit. The main function of the DC-side support capacitor is to stabilise the DC voltage output from the PV array, and film capacitors are generally used as support capacitors. Compared to the usual electrolytic capacitors, the film capacitors have a higher dielectric constant and higher energy density, which can better stabilise the voltage. The main three-phase inverter circuit is mainly controlled by IGBTs as switching tubes, which reverse the incoming DC power into AC power, and in the process achieve maximum power point tracking (MTTP) and anti-islanding functions to maximise inverter efficiency. However, the inverter circuit does not convert the DC power perfectly into sinusoidal frequency power, which contains various numbers of harmonics. Therefore, an LCL filter circuit is used to filter the AC power in the AC measurement. Compared to LC and L filters, the LCL filter circuit has a better ability to suppress the higher harmonics and at the same time requires a smaller inductance, which facilitates the design of the circuit. Finally, the filtered AC power is connected to the grid via a transformer in order to meet the needs of different voltage levels.

String Inverters

       Several or a dozen PV modules are connected in series to form a PV string, and several PV strings are connected to the DC side of the corresponding inverter, which is the structure of the input side of a string inverter. In contrast to the centralised inverter, the string inverter is a small energy inverter, a decentralised inverter. The string inverters complete the inversion of the respective corresponding input energy and then converge the energy for transmission to the grid.

       String inverters generally use a two-stage single-phase topology. This structure generally consists of a DC/DC boost circuit, a three-phase inverter main circuit, a filter circuit and a support capacitor. The first stage is mainly a DC/DC booster circuit, usually in boost circuit configuration, which controls the output voltage of the PV string so that the voltage level meets the requirements for grid connection. If the output voltage of the PV string already meets the grid requirements, the DC/DC boost circuit can be omitted. At the same time, the MPPT of the PV string can be controlled to improve the efficiency of the inverter. The second stage is mainly the inverter and filter circuit, which completes the DC to AC conversion. Without the DC/DC boost in the first stage, it is necessary to add MPPT to the inverter circuit, as in the case of centralised inverters, to maximise the efficiency of the whole inverting process.

Modular Inverters

       The most typical of the component inverters is the micro-inverter. Only one PV module is connected to the DC side of the microinverter, so that the microinverter operates as a step-up, inverter and filter for each individual PV module.

       There are various microinverter topologies, which can be broadly divided into unipolar and bipolar inverters. At present, the interleaved flyback microinverter in the unipolar type is generally chosen for its simple circuitry, low cost and high efficiency, which is conducive to the promotion of home photovoltaic power generation. The interleaved flyback inverter topology is mainly composed of a dual flyback converter, an I.F. polarity flyback bridge and a filter circuit. The dual flyback converter is connected in parallel to the output side of the PV module to control its output voltage and to implement the MPPT function. The parallel connection of the two flyback converters also increases the power level, reduces the current ripple and improves the power quality. The frequency polarity flyback bridge acts as an inverter and is connected to the grid via a filter circuit.