Three Phase String Inverters
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 main components of the system 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, that maximizes the power generated by the PV strings in different environmental and sun irradiation conditions. In situations where the PV string can reach the DC link operating voltage level, the DC/DC converter is bypassed (via a low VF diode) in order to maximize efficiency. Three-phase systems are normally designed with 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 and are used in large residential, commercial and decentralized utility-scale applications. The low and mid powers up to around 100 kW are typically implemented with 3-level T-type topologies utilizing 1200 V and 650 V switches. At the higher power end, 3-level I-NPC topologies are employed with 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. At the front-end, the DC/DC boost often utilizes 2- and 3-channel topologies, either with discrete and power integrated modules. Several DC/DC stages might be used in parallel as well. Galvanically isolated driver systems are employed for both the high-side and the low-side, 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 and implemented with PIMs, where SiC/Hybrid configurations with 1000 V IGBTs and 1200 V SiC Schottky diodes maximize efficiency and provide an achieve an excellent performance to cost ratio. The DC/DC boost is realized with PIMs typically using full SiC or SiC/Hybriddevices in symmetric or flying capacitor configurations. Galvanically isolated driver systems are employed for both the high-side and the low-side, where NCD57XXX and NCP51XXX families offer design flexibility.
Three-phase string inverter need 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 important factor as well to consider.
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