With more renewable energy integrated into the existing power consumption, power electronics play an important part to convert and control the power. Power inverters employ power semiconductors to converter DC into AC, which is an essential part in the renewable energy utilization. The paralleled transformer-less inverters are well adopted in the industry for large capacity grid-tie application. Compared with the centralized inverter, inverters in parallel can offer higher power rating, higher reliability, and lower grid-side current harmonics. Transformers are commonly used in the grid-tie system to provide galvanic isolation and voltage ratio transformations. Eliminating transformers will be a great benefit to further improve the system efficiency, reduce the size and weight. However, removal of the transformer would result in ground leakage current between the DC input side and the grid ground. The emerging wide band gap (WBG) devices are bringing significant opportunities for inverters towards higher efficiency and higher power density, due to their substantial switching loss reduction over Si devices. Silicon carbide (SiC) adoption also brings new control challenges to the three-phase paralleled transformer-less inverters. The voltage slew rate can be as high as dozens or hundreds of volts per nanosecond and the harmonic frequency related with the turning-on and turning-off of the devices may be up to several hundreds of mega-hertz, these high dv/dt and di/dt can generate high frequency EMI noise that propagates to the whole system including the power stage and control circuits, and raise the issue of increased electromagnetic interference (EMI). With high switching frequency, it is more difficult to control the circulating current among paralleled inverters. The conventional carrier synchronization method cannot be applied due to the impact of communication and sample delay. Limited controller resource also prevents sophisticated control algorithms. In this research, a five-level T-type (5LT2) PV inverter paralleled through inter-cell transformer (ICT) is presented to elaborate the challenges and demonstrate the advantages in three-phase SiC inverter. There are three key current in the 5LT2 PV inverter: circulating current, grid current, and ground leakage current. Circulating current is suppressed by the ICT and further controlled by a current controller. With increased switching frequency and multilevel topology, it is possible for a SiC device based grid connected converter to achieve filter-less function and utilize the grid impedance for its switching harmonic attenuation. Analysis shows that the conventional control method with instantaneous grid voltage feedforward (IGVF) will significantly limit the bandwidth or stability margin of a filter-less grid-connected inverter, thus make the inverter sensitive to grid disturbance. Two proposed grid voltage feedforward control methods, which require little additional computation resources, are presented to suppress the grid voltage disturbance. The increased switching efficiency is beneficial to the high frequency (HF) ground leakage current suppression, since the common mode (CM) choke can be much smaller. The 5LT2 inverter has a significant common mode voltage (CMV) reduction compared to that of a 3-level T-type (3LT2) inverter. However, the low frequency (LF) ground leakage current caused by neutral point (NP) voltage oscillation becomes a new issue in larger power rating multi-level inverters. A LF CMV compensation method is proposed to suppress the LF CMV. In this research, a control system is developed for a 60 kW three-phase paralleled transformer-less filter-less SiC PV inverter, which achieves a power density of 27 W/in3 and 3 kW/kg with nature convection, and measured peak efficiency of 99.2%.
