Three-phase string inverter systems convert the DC power generated by the photovoltaic (PV) panel arrays or ‘string’ into AC power fed into a 220V or higher three-phase grid connection. The power rating of these inverters ranges from 8 kW up to more than 300 kW, covering different markets and applications, ranging from residential to commercial or industrial up to utility-scale.
The system’s main components are the PV panels, the DC link capacitors, and the inverter module, which handles the DC/AC conversion. Often DC-DC boost stages are used between the PV strings and the DC link. These systems elevate the output voltage of the PV string to the DC link operating level and also run the MPPT (Maximum Power Point Tracking) function, which maximizes the power generated by the PV strings in different environmental and sun irradiation conditions. When the PV string can reach the DC link operating voltage level, the DC/DC converter is bypassed (via a low VF diode) to maximize efficiency. The design of three-phase systems typically has a DC link operating at 1100 V or 1500 V.
Architecture and technologies
1100 V DC link systems
Power levels range from 8 kW to above 150 kW nowadays, used in large residential, commercial, and decentralized utility-scale applications. The low and mid-power systems of around 100 kW typically implement 3-level T-type topologies utilizing 1200 V and 650 V switches. 3-level I-NPC topologies are employed at the higher power end using only 650 V devices, improving efficiency with lower conduction and switching losses. Solutions for 1100 V systems are implemented both in discrete and modular configurations. Discrete IGBTs or SiC MOSFETs are common, especially at the lower power levels up to approximately 30 kW. Power integrated modules (PIMs) offer a power-dense, robust and easier-to-design solution featuring IGBTs, SiC, or SiC/Hybrid. The DC/DC boost often utilizes 2- and 3-channel topologies at the front-end, either with discrete and power-integrated modules. Several DC/DC stages might be used in parallel as well. Both the high-side and the low-side employ galvanically isolated driver systems, where NCD57XXX and NCP51XXX families offer design flexibility.
1500 V DC link systems
1500 V systems are generally used only in utility-scale inverters, ranging from 100 to above 250 kW. I-NPC-based topology is the typical configuration implemented with PIMs, where SiC/Hybrid designs with 1000 V IGBTs and 1200 V SiC Schottky diodes maximize efficiency and provide an excellent performance to cost ratio. The DC/DC boost is realized with PIMs typically using fully SiC or SiC/Hybrid devices in symmetric or flying capacitor configurations. Both the high-side and the low-side employ galvanically isolated driver systems, where NCD57XXX and NCP51XXX families offer design flexibility.
A three-phase string inverter needs to deliver high efficiencies, very often above 98%, while ensuring high reliability. This accentuates in utility applications, where system-down-time has a direct impact on ROI. The system-level cost is another crucial factor as well to consider.
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