Although solar and wind energy have become increasingly important for higher efficiency and cleaner distributed energy worldwide, there are still several challenges in producing energy securely through high-quality connections. Many devices have to be designed and assembled with accurate parameters to ensure greater efficiency and better outputs. In this article, the researchers developed a 50kW PV inverter using SiC-based electronic devices.
Inverters play an important role in supporting equipment that can convert DC to AC in any effective power conversion system. Inverters used in existing solar power plants are typically built using non-silicon components. About 20 per cent of the total cost goes toward silicon devices, even though the losses can be as high as 50 per cent. The 10kW boost converter designed by Cree uses SiC Schottky barrier diodes (SBD) and SiC metal-oxide-semiconductor field effect transistors (MOSFET) wherein the efficiency of inverters using SiC devices increases by more than 3 per cent when the output power is increased from 0 to 1 kW.
Significance of SiC-based devices in high-power applications
In the application of high-power devices, SiC (Silicon Carbide) has superior properties that other semiconductor materials cannot match. At room temperature, 4H-SiC has a significantly higher thermal conductivity than copper. It also has a wider band gap, which is defined as the energy difference between the material’s conduction and valence bands, allowing for a much lower leakage current to Si by several levels of magnitude. Apart from this, the breakdown field strength of SiC material is nearly 9 times that of non-silicon materials. The performance requirements for the entire inverter system are established by SiC-based power devices with diodes and other switching components. To calculate system losses, the following equation can be used–
From the above equation, PD_CON is the diode conduction loss, ED_rr is the diode reverse recovery loss, PS_CON is the switch conduction loss, ES_on is the switch turn-on loss, ES_off is the switch turn-off loss and fSW is the switching frequency.
The proposed three-phase, 50kW PV inverter utilizes SiC MOSFETS and diodes in the power block. It consists of a symmetrical Y-core inductor in the AC filter along with a controller and an enhanced inverter function for grid support functionality. The electrical schematic of the setup of the developed prototype is shown in Figure 1.
1.7-kV 50kW PV Inverter Power Block
During the fabrication process of the PV, the half-bridge SiC devices are connected using 1700V bare die samples of SiC MOSFETs and SiC-based SB diodes. Multiple devices can also be connected in parallel as per the application requirements in achieving the necessary current and voltage ratings. The circuit has several sub-modules, which consist of a direct bonded copper (DBC) substrate material which is attached through the thermal interface material.
As shown in fig 2, the lower side switch of the phase leg module has been depicted. Similar to the low-side switch, the high-side switch of the circuit is designed to be reversely stacked above it. As shown in figure 3, the single-phased assembly along with the fan is depicted wherein the volume of the power block is 20 in3 with a total volume of 60 in3.
Simulations of the prototype inverter
The inverter operation in a voltage control mode and current mode is presented. The quality of the inverter output waveforms is quantified along with efficiency. The electrical schematic of this experimental setup of all the grid-connected tests is shown in figure 1. The output of the inverter is connected to the grid through a Δ-Y transformer. The transformer is used to limit the common mode current as the common-mode choke was not included in the experimental setup. When operating in an open loop, the inverter output waveforms in voltage control mode are connected to a resistive load which is shown in figure 4.
The THD of the load currents for 40kW and 50kW power is supplied by the inverter measured at about 2.35 per cent and 2.14 per cent respectively. Likewise, inverter-output waveforms while operated in a closed-loop, current control mode connected to a grid simulator is shown in the below figure 5.
The peak efficiency of the inverter was computed to be 98.2 ± 0.053% as observed in the simulation. The efficiency calculation included losses in the controller and a thermal management system (fans) was added to provide effective heat sink applications. The measured efficiency was higher than conventional Si-based PV inverters, which were measured with a DC-bus voltage of 900 V. It was noted that the inverter was capable of changing the reactive power injected into the grid through these voltage changes during any conversion process.
Improving the operating frequency of the inverter using SiC MOSFETs
Inverters based on traditional Si devices have an operating frequency of about 20 kHz. During the experiment, the frequency of the inverter was increased to 40 kHz to investigate the changes in its operating efficiency. The boost switching device adopts the N-channel SiC MOSFET and SiC Schottky Barrier Diode (SBD) package integrated chip of model SCH2080KE from ROHM. It could also be seen from the output that although the increase in frequency will intensify the switching loss, the overall efficiency of the inverter will not be reduced. The boost circuit can raise the operating frequency while decreasing the volume and weight of the inductance without affecting the inverter system’s efficiency. Therefore, the total volume and weight as well as the cost of the inverter can be reduced significantly.
A three-phase, two-level, 480 V, 50kW PV inverter designed with SiC power devices has been developed. Its performance has been quantified in terms of the output waveform inverter efficiency at different loads and in voltage and current-control modes of operation. The development of such an inverter and its validation can increasingly encourage the PV industry to transition to wide-bandgap devices in products with higher efficiency and low-cost inverters.
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