Research On Solar Ac Module Inverter And Its Control Technique

Along with the quick development of global economic, the world is facing energy crisis because of excessive traditional fossil energy consumption. Countries all over the world are devoting to development and utilization of new energy. Solar energy is resourceful, renewable, non-pollution and has become one of the most potential new energy. Moreover, solar photovoltaic (PV) power generation has got considerable development and become the mainstream of solar energy utilization based on the constant progress of solar cell and power electronic technique. In many kinds of solar PV configuration, solar AC module assembles single PV component and low power inverter and parallels with single-phase low voltage network. This configuration reduces power loss by shade sheltering of PV array and series connection of PV components, and realizes best fit of PV component and inverter easily. AC module is an important development direction of grid-connected PV system now and in the future, but is also facing challenge of optimization of cost, efficiency and life. Concentrating on this object, this paper makes a research on AC module inverter and its control technique deeply according to the characteristic and problem of solar AC module.Grid-connected inverter is the core of whole solar AC module power generation system and improved AC module inverters are proposed in this paper by introducing capacitive idling (CI) technique into traditional flyback inverter. The new inverter uses just four switches, and two of them are working at high frequency to realize maximum power point tracking (MPPT) and grid-connected current waveform adjustment, the other two are working at low frequency to realize positive half wave and negative half wave of the grid-connected current. This simple circuit configuration and operation mode reduces the system cost. The paper analyses the operation principle, gives the PWM control mode, and describes the operation mode of the proposed inverter. By analyzing six working states in one switching period, the energy exchange is also researched.Due to idling operation mode of the middle power decoupling capacitor, the inverter in the paper can be decomposed to relative independent pre-stage DC circuit and last stage inverter circuit and is simplified to a uniform equivalent circuit further which is working like the Buck-Boost circuit. The detailed analysis of inverter operation process indicates that, the discontinuous conductive mode (DCM), CI technique and absorbing capacitor make the high frequency switches working at zero current turn on and zero voltage turn off, which is beneficial to improve system efficiency. The CI technique also makes the power pulsation of single inverter transfer to voltage pulsation on middle power decoupling capacitor and cancels large volume electrolytic capacitor, which is important to system life extending. In addition, the absorbing capacitor provides a channel for energy feedback on leakage inductance of flyback transformer, which greatly reduces the switch turning-off over-voltage and improve system reliability.In order to improve whole system efficiency, besides soft switching of high frequency switches, the PV cell must work around maximum power point (MPP) all the time by real-time adjusting working point according to the working state of the PV module. A MPPT technique is proposed in this paper based on disturbance matching method combining with perturbation observation method (P&O) and increment conductance technique (ICT). This method adjusts duty cycle of main switch to make the inverter input impedance equal to equivalent output impedance of PV cell module, thus locates the MPP. By modulating a perturbation into the duty cycle, and comparing the AC component and the average value of the panel terminal voltage, the MPPT can be realized. Therefore, the method proposed doesn't need to detect output current of PV cell module and complicated computation, which saves the cost of measurement and control, and accuracy of MPPT can be ensured at the same time. Duty cycle or switching frequency perturbation both can adjust input impedance, thus the paper proposes two kinds of disturbance matching tracking methods, duty cycle disturbance matching method and equivalent switching frequency disturbance matching method. And the paper also makes detailed derivation and discussion for the proposed methodsOne hand, AC module inverter needs to capture the maximum power of solar cell, on the other hand, it must provide energy to loads and power network in the form of grid-connected current. According to steady twice frequency fluctuation included in middle decoupling capacitor voltage, the paper proposes feed-forward SPWM correction control using energy equivalence principle to ensure grid-connected current quality based on peak current control concept. And according to the average value variation of middle decoupling capacitor voltage along with the average power variation, the paper proposes close loop control using capacitor voltage average value to realize the control of grid-connected current amplitude. The proposed control scheme combined with amplitude and waveform of grid-connected current just needs to detect middle decoupling capacitor voltage to realize grid-connected current control. It omits primary current measurement of flyback inverter, and saves control system cost.AC module inverter needs perfect islanding detection and protection due to connection with power network, besides power quality control of PV grid-corrected power generation system. The paper proposes an islanding detection method combined with active power disturbance method (APD) and positive feedback active frequency drift method (AFD) based on their advantages and disadvantages analysis. It uses AFD under normal condition, and APD is working when frequency deviation reaches a certain value. The amplitude and frequency of the output voltage are detected at the same time to judge the islanding state. The combination of two methods learns from the strong points to offset some weakness, which avoids deterioration of power quality and periodical power loss, and also speeds up detection time, reduces non-detection zone.Based on the proposed novel AC module inverter topology, MPPT scheme, grid-connected current control method and islanding detection technique, an experimental platform of 100W solar AC module PV grid-connected generation system is established. The main circuit and control system are designed in the paper, which also includes experiment of grid-connected inverter, MPPT, islanding detection, etc. Analysis and discussion of the AC module inverter are given finally. The simulation, experiment and discussion results show that the theory analysis is correct, the practical scheme is feasible, and the operation performance is good, which is laying important theory and practice foundation for further research.