Single-phase Three-level Inverter Research Articles

Full Grid Voltage Feedforward for Critical Mode LCL-Type Single-Phase Grid-Tied Inverters With Physical Interpretations

Full grid voltage feedforward (GVF) is an effective strategy to suppress the current distortion caused by grid voltage harmonics. However, for critical conduction mode (CRM) grid-tied inverters, the time-varying parts of its control scheme make it impossible to derive its full GVF strategy from the point of view of the transfer function. Hence, this study first analyzes the physical interpretation of the full GVF of conventional LCL-type grid-tied inverters, and then reveals the role of each term from the perspective of the circuit. Then, a full GVF strategy for CRM grid-tied inverters is proposed. The design guidelines, parameter sensitivity, and application limitations of the proposed scheme are analyzed in detail. Finally, a 1-kW single-phase three-level grid-tied inverter was built. Experimental results show that the proposed full GVF can significantly suppress the current distortion caused by the grid voltage harmonics and lower the current total harmonic distortion value by 55% within the full power range.

Modeling and Suppression of Circulating Currents Among Parallel Single-Phase Three-Level Grid-Tied Inverters

T-type three-level inverters have attracted extensive attention for their utility in many applications, especially photovoltaic power generation. For these inverters, parallel operation offers superior performance in capacity, reliability, and scalability. However, the introduced circulating current (CC) may cause overcurrent in line, which seriously threatens the safety and stability of the system. The mechanism of CC lacks detailed analysis in single-phase modular inverters. Thus, an accurate mathematical model is established and a CC suppression method is proposed in this article for parallel-operated single-phase T-type three-level inverters. First, the characteristics of port degradation are analyzed, and an improved equivalent circuit is introduced based on the findings. Then, according to the improved equivalent circuit, a detailed mathematical model is established. Using this model, excitation sources of CC are analyzed and divided into three parts. After that, a CC suppression method involving carrier-based pulsewidth modulation with offset voltage injection is proposed, and a deadbeat controller is designed to calculate the offset voltage. Moreover, the proposed method can balance the neutral-point voltage across dc-link capacitors as well. Simulation and experimental results validate the theoretical analysis and effectiveness of the proposed scheme.

Robust DC-Link Voltage-Balancing Strategy Based on Model Predictive Control

For single-phase three-level inverters, the neutral-point (NP) voltage balance problem must be solved to ensure normal operation. This brief proposes a robust voltage balance control method based on the model predictive control. The relative tracking errors of the grid current are used to judge which split capacitor voltage is larger. Then, the larger (smaller) split capacitor voltage will be used as the small vector when releasing power to the grid (absorbing power from the grid). Consequently, there is no need sampling the split capacitor voltages and the voltage sensors can be reduced. Besides, the NP voltage balance is well achieved even when the parameter drift happens to the grid inductor, which enhances the robustness. Finally, a T-type single-phase three-level grid-tied inverter platform is built to verify the effectiveness of the proposed control method.

Third-order current harmonic suppression and neutral-point voltage balance control strategy for single-phase three-level NPC inverter

The imbalance of capacitor voltage on the DC side of the inverter will cause the third-order current harmonics and the device will be damaged greatly with the increase of voltage stress. The mechanism of imbalance capacitors voltage and the third-order current harmonics generated by the double frequency fluctuation are analyzed. This paper presents a feedforward capacitor differential pressure control strategy based on PR controller, which is aimed at the single-phase three-level NPC inverter and considers the cause of double modulation wave. The PR controller is used in the current loop to suppress the third-order harmonic by static-free tracking of the AC signal, and the voltage balance is realized by the feedforward DC-link capacitor voltage difference to offset the DC component of the output side. The simulation model and 1kW prototype driven by the control strategy have been established to demonstrate that the capacitor voltage of the inverter is balanced and the third-order harmonic is suppressed efficiently.

Equal weighted cost function based weighting factor tuning method for model predictive control in power converters

An equal-weighted cost function-based weighting factor tuning method for model predictive control (MPC) in power converters is presented here. In conventional MPC methods developed for power converters, the use of weighting factors is a necessity to determine the importance of sub-terms in multi-objective cost functions. The proposed method is based on equalizing the importance of cost terms by using normalized values rather than the actual values. As a consequence of normalized values, all sub-terms are transformed into a similar range independent from their actual values. Hence, the weights of cost terms are equalized, leading to the elimination of weighting factor tuning necessity. The proposed equal-weighted cost function not only offers simplicity in the MPC design but also guarantees the desired controller performance with the dynamically weighted sub-terms. The proposed control method is validated on a grid-connected single-phase three-level T-type inverter. Experimental results are carried out to demonstrate the effectiveness of the proposed method under steady-state and dynamic conditions. Moreover, the comparative results with the conventional MPC are presented. The results reveal that the proposed approach exhibits excellent performance against the variations in the operating point of the inverter.

Closed-Loop Control System Design for Wireless Charging of Low-Voltage EV Batteries with Time-Delay Constraints

This paper presents an inductive power transfer system on the basis of a double single-phase three-level T-type inverter and two split transmitting coils for constant current and constant voltage wireless charging of low-voltage light electric vehicle batteries with closed-loop control, considering time-delay communication constraints. An optimal control structure and a modified control strategy were chosen and implemented to the wireless power transfer system as a result of a review and analysis of existing solutions. The control system analysis and adjustment of the coefficients of the regulator using Laplace transform were performed. Our study addressed the behavior of the control system with different time delays as well as the dynamic response of the system. The detecting algorithm of a secondary coil was proposed, which ensured efficient system operation and increased the functionality, safety and usability of the device. The efficiency of energy transfer of 90% was reached at the transmitted power of 110 W, which is at the level of existing solutions considered in the article and opens the way to the commercialization of the proposed solution. Therefore, the feasibility of using a nonclassical multilevel inverter, together with split transmitting coils for wireless charging was confirmed.

Novel ripple reduction method using three-level inverters with unipolar PWM

This paper proposes a novel method to reduce voltage and current ripple for the inverters by using three-level inverters with unipolar pulse width modulation (PWM) (3LFB-2U). A simple technique of switching signal generation by using carrier-based dipolar modulation of three-phase three-level inverters is extended to single-phase inverters that can be done by generating all possible switching patterns of the single-phase three-level inverters. Moreover, the concept of carrier-based dipolar modulation and the construction of reference voltages from desired output voltage and added zero voltage to control unipolar switching is also shown. The research results reveal that the proposed method can reduce the voltage and current ripple. Furthermore, the voltage and current harmonics can reduce by 27.80% and 1.79%, respectively less than two-level inverters without a loss of a simple modulation to generate the switching signals.

Model Predictive-Based Voltage Balancing Control for Single-Phase Three-Level Inverters

Neutral-point voltage balance is critical for safe operation of single-phase three-level inverters. This issue can be overcome by selecting proper redundant small vectors. However, two voltage sensors are usually required to sample the split dc-link voltages. This letter proposes a model predictive-based voltage balancing method to reduce the usage of the voltage sensors (saving one voltage sensor). The required voltage information, which is essential for selecting the appropriate small vector, is obtained by comparing the current tracking errors. Consequently, the cost is reduced and the reliability is improved. A prototype is finally built to validate the feasibility and effectiveness of the proposed control method.

A novel PWM technique to eliminate common-mode voltage for single-phase three-level T-NPC inverter

This paper presents a new pulse width modulation (PWM) technique in order to eliminate common mode voltage for a single-phase full H-bridge T-NPC inverter. Three-level NPC inverters are commonly used in grid connected applications or as UPS backup power for industry. In addition to abilities such as to withstand high voltage, and reach output voltage and current with low harmonic distortion, three-level NPC inverter is advantageous for mitigating common mode voltage and leakage current. The new proposed PWM technique is based on the switching state table of the three-level H-bridge T-NPC inverter that containing many switching states with zero common mode voltage. Furthermore, implementation of PWM control for single-phase three-level T-NPC inverter configuration can decrease voltage stress on the output and reduce output voltage distortion compared with bipolar PWM control technique for two-level H-bridge inverter. In this paper, the sine wave carrier PWM technique is analyzed, designed and modelled based on the switching table. The analytical analysis will be evaluated using MATLAB/SIMULINK. Simulation results of the proposed PWM technique for three-level T-NPC inverter are compared with conventional techniques without common mode voltage elimination; the quality of the three-level T-NPC inverter is also compared to that of traditional two-level inverter. The power loss content is evaluated using PLECS software. The obtained results confirm the advantages of the eliminated common mode voltage PWM technique in the three-level T-NPC inverter configuration.

Simultaneous Switching Loss Reduction and Neutral-Point Voltage Balance Scheme for Single-Phase Three-Level T-Type Inverter

Single-phase three-level T-type inverter (3LT <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> I) has revealed superior performances in many applications. However, it inevitably faces the challenge of high switching loss and imbalanced neutral-point voltage with conventional carrier based pulsewidth modulation (CBPWM). To address these issues, an improved CBPWM strategy with maximum offset voltage injection is proposed to reduce the switching loss and balance the neutral voltage simultaneously. Neutral-point voltage balance is maintained by selecting P-type or N-type switching sequence, which depends on the sign of offset voltage. Also, switching loss is mitigated by reducing the number of total switching commutation. First, the restriction of injected offset voltage is investigated and when the offset voltage is maximum, the switching commutation is reduced. Then, to further reduce the number of switching commutation, a new set of carrier signals are proposed to realize zero switching commutation between P-type and N-type switching sequences. After the analysis, the proposed modulation method also has better common-mode voltage restriction and faster response of neutral-point voltage balance. Simulation and experiment have been conducted to verify the theoretical analysis.

Feasibility Study GaN Transistors Application in the Novel Split-Coils Inductive Power Transfer System with T-Type Inverter

A promising solution for inductive power transfer and wireless charging is presented on the basis of a single-phase three-level T-type Neutral Point Clamped GaN-based inverter with two coupled transmitting coils. The article focuses on the feasibility study of GaN transistor application in the wireless power transfer system based on the T-type inverter on the primary side. An analysis of power losses in the main components of the system is performed: semiconductors and magnetic elements. System modeling was performed using Power Electronics Simulation Software (PSIM). It is shown that the main losses of the system are static losses in the filter inductor and rectifier diodes on the secondary side, while GaN transistors can be successfully used for the wireless power transfer system. The main features of the Printed Circuit Board (PCB) design of GaN transistors are considered in advance.

Design and analysis of three‐level hybrid boost converter based on T‐type inverter for solar photovoltaic system

This paper presents the design and analysis of a three-level hybrid boost converter based on a single-phase three-level T-type inverter. The proposed converter can provide high energy conversion efficiency and high voltage gain capability with reduced component count. The pulse-width-modulation signals of the proposed converter are generated by comparing two duty cycles and two triangular carrier signals. The proposed converter consists of four power switches, four diodes, an inductor, and two capacitors. Detailed converter operations at each switching state with the corresponding voltage and current waveforms of each component are discussed. The functionality and performance of the proposed converter are verified through simulation and experimental results.

An optimized harmonic elimination method based on synchronizedmicrocontroller architecture

This paper proposes an optimized synchronous PWM method for harmonic elimination in a quasi square wave inverter. The synchronized PWM method enables online harmonic computation and PWM pulse generation in a multitasking digital controller to eliminate lower order harmonics. The multitasking digital controller reduces the look-up table requirement and helps in realizing efficient implementation to eliminate dominant harmonics. This method offers a simple scalable solution for combined fifth and seventh harmonic elimination using two low-cost eight-bit PIC microcontrollers (PIC18F4550, PIC18F452). Experimental results are demonstrated for a single-phase three-level inverter. The proposed method achieves 90$\% $ reduction of fifth and seventh harmonic components from a quasi square wave inverter output.

A Novel Single-Phase Three-Level Dual-Buck Inverter

This article proposes a new single-phase three-level dual-buck inverter. The topology is derived from a simplified neutral-point clamped inverter that includes fewer active switches. The advantages of the proposed inverter are no shoot-through worries and no reverse recovery issue of the body diode of the power metal-oxide-semiconductor field-effect transistor compared with the conventional inverter; therefore, a high-reliability inverter can be achieved. The operation modes and pulsewidth modulation technique are analyzed in detail. Additionally, a switching scheme is proposed to balance and reduce the direct-current-link capacitance. A 1.4-kW prototype was built and tested to validate the theoretical analysis of the proposed converter.

A Constant Switching Frequency Model Predictive Control Without Weighting Factors for T-Type Single-Phase Three-Level Inverters

This paper presents a novel model predictive control (MPC) for the T-type single-phase three-level inverters. The cumbersome procedure in tuning of weighting factors is eliminated so that the implementation of the proposed MPC becomes easy. The constant switching frequency is achieved during the control implementation in order to facilitate the filter design. In order to optimize the distribution of the output current harmonics, multiple voltage vectors are employed in each control cycle with their application times setting inversely proportion to the respective cost function. The redundant small voltage vectors are utilized to balance the neutral point voltage of the inverter by directly regulating the upper and lower dc-link capacitors voltages. Finally, the proposed MPC algorithm is experimentally evaluated and compared with other two unfixed switching frequency MPC algorithms in terms of the steady state, transient performance, and parameter sensitivity.

Novel Space Vector Pulsewidth Modulation Strategies for Single-Phase Three-Level NPC Impedance-Source Inverters

This paper presents new space vector pulsewidth modulation (PWM) strategies for a single-phase three-level buck-boost neutral point clamped inverter coupled with impedance-source (IS) network. These strategies can be implemented for systems with any IS network with neutral point. The case study system is based on the quasi-Z-source inverter with continuous input current. To demonstrate an improved performance, the strategies are compared with a traditional PWM strategy. The advantages lie in reduced switching number, without output voltage quality distortion. The simulation and experimental results confirm the theoretical predictions.

An optimised harmonic elimination method based on synchronized microcontroller architecture

This paper proposes an optimized synchronous PWM method for harmonic elimination in a quasi square wave inverter. The synchronized PWM method enables online harmonic computation and PWM pulse generation in a multitasking digital controller to eliminate lower order harmonics. The multitasking digital controller reduces the look-up table requirement and helps in realizing efficient implementation to eliminate dominant harmonics. This method offers a simple scalable solution for combined fifth and seventh harmonic elimination using two low-cost eight-bit PIC microcontrollers (PIC18F4550, PIC18F452). Experimental results are demonstrated for a single-phase three-level inverter. The proposed method achieves 90$\% $ reduction of fifth and seventh harmonic components from a quasi square wave inverter output.

Single-phase three-level grid-connected inverter based on direct adaptive fuzzy control

Single-phase three-level grid-connected inverter based on direct adaptive fuzzy control

Design of surface acoustic wave filters for the multiplex transmission system of multilevel inverter circuits

We designed surface acoustic wave (SAW) filters for a multiplex transmission system of multilevel inverter circuits, and applied them to a single-phase three-level inverter. To reduce the transmission delay time of the SAW filters, a four-channel SAW filter array was fabricated and its characteristics were measured. The delay time of the SAW filters was <350 ns, and the delay time difference was reduced to ≤184 ns, less than half that previously reported. The SAW filters withstood up to 990 V, which is sufficient for the inverters used in most domestic appliances. A single-phase three-level inverter with the fabricated SAW filters worked with a total delay time shorter than our target delay time of 2.5 µs. The delay time difference of the proposed system was 0.26 µs, which is sufficient for preventing the inverter circuit from short-circuiting. The SAW filters controlled a multilevel inverter system with simple signal wiring and high dielectric withstanding voltages.

Wavelet PWM Technique for Single-Phase Three-Level Inverters

The wavelet PWM (WPWM) technique has been applied in two-level inverters successfully, but directly applying the WPWM technique to three-level inverters is impossible. This paper proposes a WPWM technique suitable for a single-phase three-level inverter. The work analyzes the control strategy with the WPWM and obtains the design of its parameters. Compared with the SPWM technique for a single-phase three-level inverter under the same conditions, the WPWM can obtain high magnitudes of the output fundamental frequency component, low total harmonic distortion, and simpler digital implementation. The feasibility experiment is given to verify of the proposed WPWM technique.