Abstract
In the three-phase photovoltaic (PV) cascaded inverter, the output power of PV arrays is not equal due to the difference of solar radiation, temperature and other factors, which leads to the over modulation of PV cascaded inverter. In order to solve the above problems, the carrier phase-shifted PWM (CPS-PWM) control strategy based on third harmonic injection is proposed in this paper, taking a single-stage high-frequency-link isolated cascaded PV inverter topology with clamping circuits as an example. It can not only improve the utilization of DC voltage, but also enlarge the power balance constraint range between the units in the phase of cascaded PV inverter, so as to ensure that all units can avoid over modulation under some serious power imbalance cases, and will not increase the third harmonic component injection of grid current. Firstly, the topology and working principle of the proposed inverter based on CPS-PWM control strategy are introduced. Then, the improved CPS-PWM control strategy which can improve the DC voltage utilization of the PV cascaded inverter is analyzed, and the control strategy of intra-phase power balance is emphatically analyzed in the overall control strategy of PV cascaded inverter system, and it is discussed that the proposed control strategy can also enlarge the power balance constraint range between the units of the cascaded inverter. Finally, the effectiveness of the proposed PV cascaded inverter based on the improved CPS-PWM control strategy is verified by the simulation and experimental results.
Highlights
Solar energy has made great strides in the past decade
THE THIRD HARMONIC INJECTION MODULATION STRATEGY FOR IMPROVING DC VOLTAGE UTILIZATION Compared with the amplitude of the output voltage of phase A without the improved modulation strategy in Fig. 16(a), that with the third harmonic component injection in Fig. 16(b) has increased by 1.15 times to 9.122kV, so the utilization of DC voltage has increased by 15%
The steady and transient performance of the PV cascaded inverter and the effectiveness of the third harmonic injection method to improve DC voltage utilization are verified by a small prototype built in the laboratory
Summary
Solar energy has made great strides in the past decade. By the end of 2019, the total installed capacity of solar energy in the world has reached 651GW, which has become the fourth largest power source. The single-stage isolated PV inverter based on common DC bus can eliminate the large volume independent capacitor of two-stage converter and avoid the complex control of voltage and current sharing between cascaded units, so as to meet the requirements of grid connected PV system for high reliability, light weight and low cost [12]. The improved control strategy proposed in this paper based on the third harmonic injection can overcome the above shortcomings, and enlarge the power balance constraint range between the units in the phase of cascaded PV inverter to avoid the inherent over modulation and expand the safety margin of grid connected PV system. A. A SINGLE-STAGE HIGH-FREQUENCY-LINK ISOLATED CASCADED PV INVERTER TOPOLOGY the input ports of the power converter units of each phase are connected in parallel with the corresponding capacitors, and the output ports of units are connected in series with the medium voltage power grid. A single-stage high-frequency-link isolated cascaded PV inverter topology with clamping circuits
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