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A half-bridge photovoltaic (PV) system is proposed, which can not only deal with bidirectional power flowing but also improve power quality. According to varying insolation, the system conditions real power for dc and ac loads to accommodate different amounts of PV power. Furthermore, the system eliminates current harmonics and improves power factor simultaneously. As compared with conventional PV inverter, the total number of active switches and current sensors can be reduced so that its cost is lower significantly. For current command determination, a linear-approximation method (LAM) is applied to avoid the complicated calculation and achieve the maximum power point tracking (MPPT) feature. For current controlling, a direct-source-current-shaping (DSCS) algorithm is presented to shape the waveform of line current. Simulation results and practical measurements also demonstrate the feasibility of the proposed half-bridge PV system.

Solar energy is clean, pollution-free, and inexhaustible, so developing solar energy power system can solve the energy crisis of exhausting in fossil fuel. Recently, photovoltaic arrays are widely used for power supply [

A block diagram of a conventional grid-connection PV system.

Illustration for a two-stage grid-connection PV system.

A block diagram to represent a single-stage grid-connection PV system.

In this paper a half-bridge single-stage PV system is proposed to reduce the total number of active switches and current sensors. As a result, the proposed PV inverter system is compact and cost can be reduced significantly. Furthermore, the proposed system can not only process real power bidirectionally but also improve power factor and eliminate harmonic currents. To draw maximum power from PV arrays, a linear-approximation method (LAM) is developed to complete the maximum power point tracking (MPPT). Based on the LAM, a reference dc-link voltage is chosen. With an outer-voltage controller, source current commands are determined, which avoids optimal current determination from complicated calculations. A direct-source-current-shaping (DSCS) algorithm is applied to perform wave shaping for bidirection power flow controlling and power factor improvement. A prototype is established, simulated, tested, and measured. The simulation results and experimental measurements have verified the feasibility of the proposed PV system.

Figure

Configuration of the PV inverter system.

A block diagram of the system controller.

To understand the operation principle of the PV system, power flow controlling is discussed. The PV system processes real power, reactive power, and distortion power simultaneously. Figure

Power tetrahedron diagram.

A trajectory to indicate operation points varying with insolation.

Illustration of power flow during the interval of high insolation.

Illustration of power flow during the interval of medium insolation.

Illustration of power flow during the interval of low insolation.

Illustration of power flow during the interval of no insolation.

In the PV system, once a current command is determined, the output current of the half-bridge inverter will trace the waveform of the reference current to perform power flow controlling and power quality improvement. In the following, an optimal current command is derived.

According to the current and voltage definitions shown in Figure

Notation

In (

Illustration of the trajectory of maximum power point varying with insolation while PV temperature is invariant.

Illustration of the trajectory of maximum power point varying with temperature while insolation is constant.

Illustration of the relationship between reference voltage and temperature.

An example of 110 V 60 Hz half-bridge PV system is designed, simulated, and implemented, of which operation range of input voltage is from 395 to 420 V. Component values and important parameters are determined as

power switches: IGBT, TOSHIBA GT25Q101, 1200 V/25 A,

PV arrays: SHARP NT-KR5EX (12 pieces in series),

PV power: from 200 W to 1.8 kW.

Nonlinear loads are connected to utility, of which power dissipation is 650 W. Figure

Load current while nonlinear loads are connected to utility.

Filtered line current and line voltage during the interval of high insolation.

The corresponding inverter current during the interval of high insolation.

Filtered line current and line voltage during the interval of medium insolation.

The corresponding inverter current during the interval of medium insolation.

Filtered line current and line voltage during the interval of low insolation.

The corresponding inverter current during the interval of low insolation.

Filtered line current and line voltage during the interval of no insolation.

The corresponding inverter current during the interval of no insolation.

Experimental result: the filtered line current and line voltage during the interval of high insolation.

Experimental result: the filtered line current and line voltage during the interval of medium insolation.

Experimental result: the line voltage and the filtered line current during the interval of no insolation.

A half-bridge PV inverter capable of dealing with power flow bidirectionally is presented in this paper. As compared with full-bridge one, the total number of active switches is reduced by half so that the system configuration is simplified and its cost is lowered significantly. The LAM is applied to obtain an optimal reference voltage for the determination of a current command and to achieve MPPT feature, which avoids sophisticated calculation. The DSCS algorithm is embedded to perform wave shaping for line current so as to achieve power quality improvement directly. Simulation results and practical measurements have demonstrated the feasibility of the PV inverter system.