Lan Xiao

Seamless Transfer of Single-Phase Grid-Interactive Inverters Between Grid-Connected and Stand-Alone Modes

This paper presents a novel seamless transfer of single-phase grid-interactive inverters between grid-connected and stand-alone modes. The grid-connected inverter should operate in grid-tied and off-grid modes in order to provide power to the emergency load during system outages. However, the grid current controller and the output voltage controller are switched between the two modes, so the outputs of both controllers may not be equal during the transfer instant, which will cause the current or voltage spikes during the switching process. The transfer between the two controllers does not exist in the proposed method. In grid-tied mode, the voltage controller is used for compensating the filter capacitor current, and the current controller is used to control the grid current. In stand-alone mode, the voltage controller is used to regulate the output voltage, whereas the output of the current controller is zero. With the proposed control method, the seamless transfer can be achieved between both modes, even in polluted grid voltage. The principle and realization conditions of the control methods at both modes are analyzed. The detailed process of the seamless transfer between the two modes is illustrated. Finally, the simulation and experimental results verify the theoretical analysis.

Dual-Buck Full-Bridge Inverter With Hysteresis Current Control

This paper presents a dual-buck full-bridge inverter (DBFBI) to solve the shoot-through problem in conventional bridge-type inverters and reduce the voltage stress of the power device in the dual-buck half-bridge inverter (DBHBI). Hysteresis current control is used in the DBFBI. All switches and diodes operate at each half line cycle, and the freewheeling current flows through the independent freewheeling diodes instead of the body diodes of the switches, so the efficiency can be increased potentially. As the shoot-through problem does not exist in the DBFBI, dead time between the switches need not be set. The input-voltage utilization rate of the DBFBI is twice that of the DBHBI under the same output-voltage condition, i.e., the voltage stress of the power device in DBFBI is half that in the DBHBI. The operating principle, stability and relative stability analyses, and design guidelines and example are provided. Experimental results of a 1-kVA DBFBI verify the theoretical analysis. The comparisons among other inverters and the DBFBI show that the proposed inverter is very promising in 220-240-Vrms output-voltage and high-reliability applications, such as uninterruptible power supplies and grid-connected inverters.

Control Strategy for Series and Parallel Output Dual-Buck Half Bridge Inverters Based on DSP Control

This paper presents a novel control strategy for series and parallel output dual-buck half-bridge inverters (DBHBIs). With the increasing concern of fossil fuel reserves and the environmental aspects, the 2005 International Future Energy Challenge, sponsored by U.S. the Department of Energy and the Institute of Electrical and Electronics Engineers, required that the grid-connected inverter should provide a single-phase utility line with 110-240 V at either 50/60 Hz. The proposed series and parallel output DBHBI can produce those kinds of voltage by series or parallel connection of two inverters and with digital signal processor (DSP) control to meet the aforementioned specifications. The load current of the two inverters at parallel output association and the load voltage of both at series output association can be shared. The waveform quality of the whole output voltage is high at both associations. Stability and relative stability of the inverter are unaffected at the two output associations. Operating principle, control strategy, stability and relative stability, and design guidelines and examples are illustrated. Experimental results of a 1-kVA DSP-based series and parallel output DBHBI at stand-alone mode verify the theoretical analysis. The comparisons between single and double inverter structures show that the proposed inverter is very promising in applications.

Control of Single-Phase Grid-Connected Inverters With Nonlinear Loads

This paper presents a control of single-phase grid-interactive inverters with nonlinear loads. The utility-connected inverter should operate at both stand-alone and grid-connected modes. However, the waveform qualities of the grid current in grid-connected mode and the output voltage in stand-alone mode are poor under the nonlinear critical load with the conventional control. The impact of the nonlinear load on the grid current is analyzed. The proposed control is illustrated in detail. By adding the load current into the filter inductor current loop, the influence of the nonlinear load on the grid current can be eliminated, and the waveform quality of the output voltage in stand-alone mode can be improved. The control method is simple and easy to be achieved. The grid-connected inverter is stable at both modes. The filter inductor is chosen. Simulation and experimental results from a 1-kW dual-buck full-bridge grid-interactive inverter with a diode rectifier load verify the theoretical analysis.

Two-Switch Dual-Buck Grid-Connected Inverter With Hysteresis Current Control

Renewable energy sources, such as solar energy and fuel cells, are desirable due to their pollution-free properties. In order to utilize the present infrastructure of the utility grid for power transmission and distribution, grid-connected inverters are required for distributed generation systems, which should have high reliability. However, a shoot-through problem, which is a major deterrent to the reliability of the inverters, exists in the conventional bridge-type voltage-source inverters. In order to solve the aforementioned problem, this paper proposes a two-switch dual-buck grid-connected inverter. The freewheeling current flows through the independent diodes instead of the body diodes of the switches, so reverse-recovery loss of the diodes can be reduced. Half of the power devices operate in high frequency; the others switch at grid period with zero-current switching. Moreover, unipolar modulation is used. The operating principle, design guidelines and example, and stability analysis are provided. The proposed inverter can be simplified to a current amplifier with hysteresis current control, so it is globally stable. Simulation and experimental results verify the theoretical analysis and satisfy IEEE Std. 929-2000. A comparison of a full-bridge inverter and the proposed inverter shows that the proposed inverter is more attractive in high-reliability applications.

Circulating current's characteristics analysis and the control strategy of parallel system based on double close-loop controlled VSI

Real and reactive power circulation is produced because of the differences of amplitude and phase of each modules output voltage in parallel inverter system. The expression of equivalent output impedance is acquired through the close-loop transfer functions of double close-loop controlled inverter, then the characteristics of circulating current of parallel system are well researched from the point of view of equivalent output impedance. Theoretic analysis and experimental results prove that the magnitude of circulating current is proportional to the voltage difference and to the reciprocal of equivalent output impedances modulus, the ratio of reactive and real power circulation is determined by the tangent of output impedances angle. Finally a new control strategy for this kind of parallel inverter system is proposed and verified by simulation and experimental results.

A novel control strategy for grid-interactive inverter in grid-connected and stand-alone modes

This paper proposed a control strategy for grid-interactive inverter operating in stand-alone and grid-connected modes. This control method has two outer loops and one inner loop. The principle of the control method and the design of the parameters are analyzed. Using this control strategy, seamless transfer can be achieved between the two modes, and the detailed process of the seamless transfer between two modes is illustrated. A 500 W dual buck half-bridge inverter is developed in stand-alone and grid-connected modes based on DSP control. The experimental results verify the analysis

Bi-directional forward-flyback DC-DC converter

Bi-directional DC-DC converter is widely used in more and more areas. The paper proposed a novel type of bi-directional DC-DC converter topologies-forward-flyback bi-directional DC-DC converters. The converter has the following merits: 1) The spike on the switches is much smaller than the current-fed type converter, the energy that cause the spike is much smaller than that in the current-fed converter; 2) The current of one side of the bi-directional DC-DC converter is continuous, the current ripple is small; 3) There is no the start-up problem in the forward-flyback bi-directional DC-DC converter; 4) It is easy to realize soft switching; and 5) The hybrid structure of forward and flyback converter makes it suit for high power situation. The paper analyzed the steady state operation principles in detail. The experimental results verify the analysis. Based on the principle of active clamp forward-flyback bi-directional DC-DC converter, a family of bi-directional DC-DC converter is proposed.

An improved distributed control strategy for parallel inverters

Parallel operation of inverters improves the reliability, maintainability, expandability, and standardization. Many control methods have been proposed. The master- slave method can not realize the redundancy because the slave modules are dependent on the master module, the wireless control method using frequency and voltage droop technique presents too poor output characteristics. Distributed control strategy achieves hot swap and redundancy, the output characteristics is relatively poor due to the inductor current feedback loop. This paper introduces the load current feed-forward control to the distributed control strategy to improve the output characteristics. Meanwhile, the functions of output current limiting and current sharing are remained. The output characteristics and circulating current with and without the load current feed-forward control are compared in this paper. The prototype of parallel inverters is built to verify the effectiveness of the improved method.

A High-Reliability and High-Efficiency Off-Grid Inverter Suitable for Direct-Driven Wind Generator Systems

This paper presents a coupled inductor dual-buck full-bridge inverter (CI DBFBI) to solve the shoot-through problem of the conventional inverter and increase the input-voltage utilization rate of the dual-buck half-bridge inverter (DBHBI). Hysteresis current control is used in the CI DBFBI. All switches and diodes operate at each half line cycle, and the freewheeling current flows through the independent diodes instead of the body diodes of the switches, so the efficiency can be increased. The filter inductors are closely coupled to reduce the weight and volume of the proposed inverter and increase the efficiency. As shoot-through problem does not exist, dead time between the switches need not set, and then waveform quality of the output voltage can be improved. The input-voltage utilization rate of the CI DBFBI is twice that of the DBHBI under the same output-voltage condition. The operating principle is illustrated. A 1-kVA CI DBFBI has been established and tested. Experimental results verify the theoretical analysis.