SINGLE-STAGE THREE-PHASE CURRENT-SOURCE PHOTOVOLTAIC GRID-CONNECTED INVERTER WITH HIGH VOLTAGE TRANSMISSION RATIO
Abstract
This paper proposes a circuit topology of single-stage three-phase current-source photovoltaic (PV) grid-connected inverter with high voltage transmission ratio (VTR). Also, an improved zone SPWM control strategy and an active clamped subcircuit which can suppress the energy storage switch turn-off voltage spike are introduced. The circuit topology, control strategy, steady principle characteristics and high frequency (HF) switching process are analyzed profoundly, as well as the VTR’s expression and design criterion of center-tapped energy storage inductor. The improved zone SPWM control strategy consists of two control loops, namely, the outer-loop of input DC voltage of PV cells with the maximum power point tracking (MPPT) and the inner-loop of the energy storage inductor current. The experimental results of a 3kW 96VDC/380V50Hz3fAC prototype have shown that this kind of three-phase inverter has the excellent performances such as single-stage power conversion, high VTR and power density, high conversion efficiency. Nonetheless, it has small energy storage inductor and output CL filter, low output current THD, and flexible voltage configuration of the PV cells. This work provides an effective design method for single-stage three-phase inverting with high VTR.
EXISTING SYSTEM:
The single-stage three-phase current-source PWM inverter with low voltage transmission ratio (VTR) proposed in the has the advantages of single-stage power conversion, boosting feature and timely over current protection, but there still exists the flaws. The VTR is not high enough, and the output waveform quality and conversion efficiency will be seriously affected when the input voltage is too low or the input voltage variation range is too wide. For example, a 120-200VDC/ 380V50Hz3fAC inverter can be achieved, but when the input voltage is lower than 120V, the duty ratio D is close to the limit value, 1-D is too small, thus the adjusting range of D is limited. This would cause some problems such as poor dynamic characteristics, decrease of the VTR caused by the circuit parasitic parameters, large energy storage inductor current and circuit loss, low conversion efficiency, and worse output waveform. Therefore, it is difficult to invert for low voltage of the PV cells. In order to overcome the limitations of the traditional voltage source PWM inverter and single-stage three-phase current-source PWM inverter with low VTR , this paper proposes a single-stage three-phase CSI with high VTR, as well as the circuit topology and an improved zone SPWM control strategy with two control loops. The loops are consisted of the outer-loop of input DC voltage with MPPT and the inner-loop of the storage inductor current. Besides, an active clamped subcircuit which can suppress the energy storage switch turn-off voltage spike is discussed in this paper, with important conclusions obtained.
PROPOSED SYSTEM:
The circuit topology of single-stage three-phase current-source PV grid-connected inverter with high VTR. The circuit topology is sequentially cascaded by the input filter capacitor C, the center-tapped energy storage inductor L, three-phase inverting bridge with six serial blocking diodes and an output CL filter. An energy storage switch S is connected between the center tap of L and the negative end of the PV cells, the left and right turns number of L are N 1 and N , respectively. Compared to CSI with low VTR, a center tap of the energy storage inductor and an energy storage switch are added to proposed CSI `
CONCLUSION
1) The circuit topology of the proposed inverter is a sequentially cascaded of the input filter capacitor, the center-tapped energy storage inductor, three-phase inverting bridge with six serial blocking diodes and output CL filter, with an energy storage switch connected between the center tap of L and the negative end of the input source. 2) The proposed two-loop improved zone SPWM control strategy can ensure the normal operation of CSI in any high frequency (HF) switching period with the condition that output line voltage are not smaller than input voltage i.e. , and the reactive power adjustment of the system is realized. 2/6 3) There are six operating Intervals in one output line frequency period, each operating Interval has three operating Modes; the VTR expression is derived, which can be adjusted through the coefficient K=kI Lavg and the turns ratio N of L. 4) The active clamped subcircuit can effectively suppress the turn-off voltage spike of S caused by the leakage inductor, and there are 14 different operating Intervals within one HF switching period T s . 5) The design criterion of the center-tapped energy storage inductor is derived. 2 2 /N U /N 1 p 1 =U PV 6) The designed 3kW 96VDC/380V50Hz3fAC PV grid- connected inverter prototype has excellent performances such as higher VTR, much smaller energy storage inductor and higher conversion efficiency, and the experimental results validate theoretical analysis.
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