Yanjun Shi

Abnormal Operation State Analysis and Control of Asymmetric Impedance Network-Based Quasi-Z-Source PV Inverter (AIN-qZSI)

Since its conception, the quasi-Z-source inverter (qZSI) has been extensively studied, especially in renewable energy applications fields due to its ability to withstand shoot-through and provide continuous current. However, research thus far has only explored symmetric qZSI/ZSI impedance network with large inductances and capacitances. By utilizing next-gen wideband-gap devices at high-switching frequency, the impedance network can be significantly reduced. However, the reduced qZS impedance network will result in abnormal operation modes, in addition to frequency resonances, in several kilohertz range. Thus, this paper first thoroughly explains the occurrence of the abnormal states, analyzes the effects of the abnormal states on the frequency resonance, and then proposes a control method to suppress the resonance that result from the reduced impedance network. A 1-kW prototype is built and experimental results verify the effectiveness of the analysis and proposed control method.

A unified Impedance-based Stability Criterion (UIBSC) for paralleled grid-tied inverters using global minor loop gain (GMLG)

In this paper a unified Impedance-based Stability Criterion (UIBSC) has been proposed for paralleled grid-tied inverters. Compared to the traditional Impedance-based Stability Criterion (IBSC) which needs to evaluate all minor loop gains (MLGs) of each individual inverter, the UIBSC only checks the derived global minor loop gain (GMLG) once to determine system stability. As a result, the computation efforts can be significantly reduced when system contains a large number of inverters. In addition, a stability-oriented design guideline based on the GMLG has been developed for the paralleled grid-tied inverters. Therefore, the grid impedance, inverter filter parameters, time delays of digital control and control parameters can be analyzed or designed to meet the system stability requirement. Theoretical analysis and simulation results have been presented to verify the proposed methods.

Individual Phase Current Control Based on Optimal Zero-Sequence Current Separation for a Star-Connected Cascade STATCOM Under Unbalanced Conditions

This paper proposes an individual phase current control (IPCC) method based on optimal zero-sequence current separation for star-connected cascade STATCOMs under unbalanced grid voltage. The IPCC is adopted to get better adaptation to unbalanced conditions. Each cluster of the cascade STATCOM has its own phase current control loop and dc-voltage feedback loop, and it can generate the necessary negative-sequence current automatically to rebalance power. However, the zero-sequence current which is unnecessary and harmful in star configuration is introduced in the process. An optimal zero-sequence current separation is presented to remove the zero-sequence current by reconstructing the reactive current command. This improved IPCC enables star-connected STATCOMs adapt to unbalanced conditions without the need for any power balancing algorithms. Also, the operation range of the proposed control method is analyzed. Simulation research studies are performed to verify the operation range analysis, and the experimental operation is tested on a 25-level ±10 Mvar/10-kV STATCOM product installed at a wind farm.

A resonance suppression method for GaN-based single-phase quasi-Z-source PV inverter with high switching frequency

By utilizing next-gen wide band-gap (WBG) devices, such as GaN and SiC, at high switching frequency in a single-phase Quasi-Z-Source PV inverter (qZSI), the impedance network can be significantly reduced. Although, the reduced qZS impedance network will decrease cost and improved power density, it will also result in frequency resonance that will introduce oscillations in the capacitor voltage and increase system losses. Thus, these resonances must be properly mitigated. Traditional methods only apply to two-stage power conditioning systems and/or require a low voltage control bandwidth, which is unsuitable for the GaN-based qZSI. Therefore, this paper proposes an active damping control method to suppress the high frequency resonance that results from the reduced impedance network in a single phase qZSI. A 1-kW prototype is built and experimental results verify the effectiveness of the proposed control method.

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.

Modular multilevel DAB (M 2 DAB) converter for shipboard MVDC system with fault protection and ride-through capability

Shipboard Medium Voltage Direct Current (MVDC) system has gained increasing popularity. However, fault management of MVDC system is one critical challenge. In this paper, a modular multilevel dual active bridge (M2DAB) converter is proposed to be applied in shipboard MVDC system. Under the presented unified control strategy, the bi-directional isolated dc/dc converter with fault protection and ride-through capability can limit the fault current and maintain operation with very wide input voltage range. Detailed system analysis and control strategy are given. The simulation results are presented to verify the operation and functionality of the proposed method.

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.