Transformerless grid-connected inverters (TLI) feature high efficiency, low cost, low volume, and weight due to using neither line-frequency transformers nor high-frequency transformers Therefore, TLIs have been extensively investigated in the academic community and popularly installed in distributed photovoltaic grid-connected systems during the past decade This article analyzes and summarizes the state of the art of TLI techniques, three rules of maintaining constant common-mode voltage (CMV) of TLIs at switching frequency have been concluded from a generic CMV analysis model at the beginning Second, suppression methods of leakage current (LC) and dc current injection (DCCI), so-called two key challenges for first-generation TLIs, have been classified and discussed in detail, respectively Finally, future trends and some challenges of TLIs based on wide bandgap devices named second-generation TLIs have been presented in this article

Overview of Transformerless Photovoltaic Grid-Connected Inverters

The hybrid switch concept of paralleling a higher-current main Si IGBT and a lower-current auxiliary SiC mosfet offers an improved cost/performance tradeoff in power converters. Currently, the gate control strategy of these two internal devices emphasizes on minimizing the total power loss, and is referred to as the efficiency control mode in this paper. However, there is a serious risk of overheating and reliability degradation of the SiC mosfet if solely relying on this control strategy. In this paper, we propose a new method of gate control optimization, referred to as the thermal balance control mode, to keep the junction temperature of both devices within the specified temperature range, and to minimize the total power loss simultaneously. We first investigate the dependency of the hybrid switch switching losses on the gate control pattern both theoretically and experimentally. We then extensively study control optimization in these two distinct control modes in a dc–dc boost converter. It is found that the thermal balance control mode can achieve almost the same total power loss as the efficiency control mode, but much lower and more balanced junction temperatures of the two internal devices. Experimental results demonstrate that the Si/SiC hybrid switch in an optimal thermal balance control mode can achieve a 163% higher power handling capability in the 20-kHz boost converter or four times higher switching frequency in the 4-kW boost converter than a single IGBT solution with hard switching condition, and yet a considerably lower component cost than a single SiC mosfet solution in the boost converter.

Gate Control Optimization of Si/SiC Hybrid Switch for Junction Temperature Balance and Power Loss Reduction

For high-power drives, switching frequency is usually restricted to several hundred hertz to minimize the switching losses. To maintain the current distortions and torque ripples at a reasonable level, synchronized pulse patterns with half-wave and quarter-wave symmetries are employed. The analytic compensation is derived by Fourier analysis to ensure the proportionality between the voltage reference and the output voltage of an inverter for pulse width modulation (PWM) with low pulse ratio. A simple yet very effective method with varying sampling rate is proposed to maintain synchronization even for fast dynamic processes. The fast and smooth transition between different PWM patterns is achieved by compensating phase angle of the voltage reference through the analysis of stator flux trajectories. The effectiveness of the proposed method is validated on a down-scaled 2.2-kW induction motor drives.

/pdf/hybrid-synchronized-pwm-schemes-for-closed-loop-current-xnyvmzal7f.pdf

Hybrid Synchronized PWM Schemes for Closed-Loop Current Control of High-Power Motor Drives

Analysis of electrical and thermal characteristics of PV array under mismatching conditions caused by partial shading and short circuit failure of bypass diodes

A dual-dc-port dc–ac converter with virtual space vector pulsewidth modulation (SVPWM) based flexible power control is proposed for photovoltaic (PV)-battery hybrid systems. Two dc ports, in which one dc port is interfaced with PV and the other dc port is interfaced with the battery, are provided simultaneously by the dual-dc-port dc–ac converter. Hence, all the active power is transferred within single power conversion stage, and the dc–dc converter between PV and battery is removed. To realize flexible power control between PV, battery, and ac grid, a new control scheme consisting of an outer power loop and an inner virtual SVPWM is proposed, with fully considering the voltage variation of both PV and battery. Operation principle and power control capability of the dual-dc-port dc–ac converter are analyzed in detail. The effectiveness of the proposed solution is validated by experimental results.

Virtual SVPWM-Based Flexible Power Control for Dual-DC-Port DC–AC Converters in PV–Battery Hybrid Systems

This paper proposes a general control scheme for the dual-input three-level inverter, which comes from a neutral point clamped three-level inverter. It can be used for photovoltaic (PV) systems or battery storage systems with two dc sources. The dual-input three-level inverter can be connected with two PV arrays, which are able to realize maximum power point tracking independently. First, the dynamic model of the dual-input three-level inverter is established, which describes the relationship between the dual-input voltages and the duty cycle. After discussing the control theory of the dual-input voltages, a typical control scheme is proposed. The design of a control loop is introduced in details. Afterward, modulation and restriction of the proposed method are discussed. Finally, experiment on a 125 kW dual-input three-level PV inverter is implemented to verify the validity and feasibility of the proposed method.

/pdf/general-control-scheme-for-a-dual-input-three-level-inverter-2i01y05pe2.pdf

General Control Scheme for a Dual-Input Three-Level Inverter

In this paper, a hybrid switch with the combination of large-current silicon (Si) insulated gate bipolar transistor (IGBT) and small-current silicon carbide (SiC) metal-oxide semiconductor field-effect transistor ( mosfet ) is utilized in a three-phase T-type three-level grid inverter. The hybrid switch can optimize the cost of the system while achieving high efficiency. The relationship between efficiency improvement and cost of the hybrid switch is investigated. Several switching patterns, which can decrease the loss of the small-current SiC mosfet for the hybrid switch, are studied. Different switching patterns for the hybrid switch of the grid inverter are compared based on the inverter efficiency. Finally, a 20 kW three-phase T-type three-level grid inverter prototype with the hybrid switch of large-current Si IGBT and small-current SiC mosfet (1:2.4 SiC/Si current ratio) is built to verify the main results.

Efficiency Improvement of Grid Inverters With Hybrid Devices

A hybrid pulsewidth modulation (PWM) scheme for the grid inverter is investigated, which combines the merits of space-vector PWM (SVM) and selected harmonic elimination PWM (SHEPWM). SHEPWM is adopted at steady state, while SVM is adopted to achieve better dynamic performance at grid fault. A transition strategy between these two PWM schemes is proposed, which achieves smooth and quick transition by optimizing the switching state during transition. Finally, the proposed transition strategy for the hybrid PWM scheme is validated by the simulation and the experiment.

Hybrid PWM Scheme for the Grid Inverter

In this paper, efficiency comparison of 2- and 3-level inverters based on a 50 kW grid-connected solar inverter is presented. A 3-level Inverter with combination of SiC MOSFET and Si IGBT is proposed. An improved switching pattern is employed to realize the zero-voltage switching (ZVS) operation for Si IGBTs. Therefore, optimal efficiency can be achieved by taking advantage of both Si-IGBT and SiC MOSFET. It shows that utilizing hybrid switches in T-type 3-level grid-connected inverters can achieve the highest efficiency. A 4 kW 3-level single phase inverter is built to verify the analysis.

Efficiency improvement of 2- and 3-level inverters for distributed photovoltaic application using hybrid devices

Flying capacitor three-level (FCTL) inverter is one of the most important multi-level inverters, which may be used in renewable energy, motor drives, uninterrupted power supplies, etc. Fault-tolerant strategy is important for reliable operation of the inverters in critical applications. In this paper, a short-circuit fault-tolerant strategy for FCTL inverter is studied. First, short-circuit fault of power devices for FCTL inverters are analyzed. Then, operating modes transfer of inverter and rebalance of flying capacitor voltage after short-circuit fault are discussed. Implementation of the fault-tolerant strategy is presented. With the proposed strategy, the rated output voltage of FCTL inverter is guaranteed without adding any extra hardware after the short-circuit fault. Finally, prototype is built to verify the proposed strategy.

A Short-Circuit Fault-Tolerant Strategy for Three-Phase Four-Wire Flying Capacitor Three-Level Inverters

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