Lu Wang

A 60-kW 3-kW/kg Five-Level T-Type SiC PV Inverter With 99.2% Peak Efficiency

A silicon carbide (SiC) T-type LCL inverter can achieve smaller device loss than two-level topology, however, its improvement on power density is limited by current ripple loss on magnetic components as switching frequency increases. This paper presents a five-level T-type (5LT2) photovoltaic (PV) inverter that achieves better utilization of SiC devices than the traditional three-level T-type LCL topology at higher switching frequency. The operation principle of the SiC 5LT2 PV inverter has been presented. The key design aspects including magnetic balancing, short-circuit protection, and digital controller computation time have been discussed and methods are developed. A 60-kW PV converter including boost stage and inverter stage has been built in the laboratory, which achieves a power density of 27 W/in3 and 3 kW/kg, and measured peak efficiency of 99.2%.

Ground Leakage Current Analysis a Suppression in a 60-kW 5-Level T-Type Transformerless SiC PV Inverter

In this paper, ground leakage current suppression in a 60-kW 5-level T-type (5LT2) transformerless SiC photovoltaic (PV) inverter has been presented. The common mode (CM) equivalent circuit is analyzed based on a high frequency (HF) CM loop and a low frequency (LF) CM loop, respectively. In the 5LT2 inverter, the derived HF CM voltage (CMV) is found to have 86% reduction compared to that of a 3-level T-type (3LT2) inverter. The simulation and experiment results are provided to demonstrate the advantages of 5LT2 inverter on HF leakage current suppression. In addition, LF CMV caused by neutral point (NP) voltage oscillation has been analyzed in this paper. It is shown that the LF CMV is nearly proportional to the NP voltage oscillation. Furthermore, an LF CMV compensation method is proposed to suppress the LF CMV by 64%, which is verified by simulation and experiment results. Finally, the leakage current shows 79% reduction in the 5LT2 inverter compared to the conventional 3LT2 inverter in the experiment. A further 52% leakage current reduction is achieved by the LF CMV compensation in the 5LT2 inverter.

A 50kW high power density paralleled-five-level PV converter based on SiC T-type MOSFET modules

SiC T-type LCL inverter can achieve smaller device loss than two-level topology, however its improvement on power density is limited by current ripple loss on magnetic components as switching frequency increases. This paper presents a paralleled-five-level (P5L) PV inverter which achieves better utilization of SiC devices than traditional T-type three-level (3L) LCL topology at higher switching frequency. Comparison of the SiC P5L PV inverter with SiC T-type 3L LCL PV inverter has been presented. The design challenges and methods to solve magnetic balancing, short circuit protection, and digital controller computation time issues are analyzed. A 50kW PV converter including boost stage and inverter stage has been built in the laboratory, which achieves a power density of 22.7 W/in3 and 2.5 kW/kg, and measured peak efficiency of 99.2%.

Ground leakage current suppression in a 50 kW 5-level T-type transformerless PV inverter

In this paper ground leakage current suppression in a 50 kW 5-level T-type transformerless PV inverter has been presented. Compared to a 3-level T-type PV inverter, this topology allows a simple modulation method such as carrier-based (CB) PWM can be used to suppress the leakage current without the penalty of the traditional methods. Phase disposition (PD) and phase opposition disposition (POD) based CB PWM method has been applied to this 5-level topology. The spectrum analysis has demonstrated that PD and POD will generate the same phase voltage spectrum. In addition, the common-mode (CM) voltage of a 5-level T-type PV inverter has been derived and the CM choke has been designed. The value of the CM choke is a 73% reduction compared to that of a 3-level T-type PV inverter. The simulation and experimental verification have been provided.

Short-circuit protection of 1200V SiC MOSFET T-type module in PV inverter application

A de-sat based short-circuit protection scheme using commercial driver for SiC MOSFETs is presented and experimentally verified on 1200V 3-level T-type SiC MOSFET module in this paper. Response time is very critical for the short-circuit protection of SiC MOSFETs due to the limited short-circuit withstand time (SCWT). Soft turn-off is required to avoid high voltage spike during the turn-off of the fault current. With the presented circuit, 600 ns response time is realized, and a two-stage soft turn-off circuit with gate voltage clamping is implemented. A gate voltage stabilizing circuit without affecting the switching loss is also proposed to prevent false trigger. Since de-sat protection scheme is not applicable for the neutral branch of T-type module due to polarity changes of branch voltage, the short-circuit protection of neutral branch is realized with the half-bridge device protection. Detail circuit design for the 1200 V T-type SiC module in 1 kV PV application is described, and experimental results demonstrate the effectiveness of the circuit.

Anti-EMI Noise Digital Filter Design for a 60-kW Five-Level SiC Inverter Without Fiber Isolation

Silicon carbide (SiC) power devices are beneficial to the converters in terms of size reduction and efficiency increase. Nevertheless, the fast switching of SiC devices results in more serious electromagnetic interference (EMI) noise issue. Optical fibers based isolation provides a reliable solution to block the EMI noises from the power circuit to the control circuit but with additional cost and size penalty, especially for multilevel converters. This letter aims at the digital filters based solution to suppress the sampling noise caused by EMI without any extra hardware cost. An improved median filter and a novel magnitude bandpass filter are proposed. The design tradeoffs among noises filter capability, delay effect and the computation time have been discussed for proposed filters. The anti-EMI noise function of proposed filters has been experimentally verified in a 60-kW five-level SiC inverter.

Switching Characterization and Short-Circuit Protection of 1200 V SiC MOSFET T-Type Module in PV Inverter Application

In this paper, a 1200 V, 100 A T-type full SiC power module is evaluated in a five-level T-type photovoltaic (PV) inverter. The T-type module is characterized with double pulse test, and based on the results, loss evaluation of the PV inverter is performed. The high power density of 27 W/in3 and 3 kW/kg, and the peak efficiency of 99.2% are achieved for the lab prototype. A de-sat based short-circuit protection scheme using commercial driver chip ACPL339J is presented and experimentally verified on the PV inverter prototype. With the presented circuit, less than 600 ns response time is realized, and a two-stage soft turn-off circuit with gate voltage clamping is implemented. A gate voltage stabilizer circuit without affecting the switching loss is also proposed to prevent false trigger. Experimental results show the superior performance of the T-type module-based PV inverter and demonstrate the effectiveness of the protection scheme.