【作者】 刘鸿鹏；

【导师】 王卫；

【作者基本信息】 哈尔滨工业大学 ， 电力电子与电力传动， 2011， 博士

【摘要】 光伏并网发电是太阳能最重要的应用形式,具有广阔的发展前景。并网逆变器是连接光伏阵列模块和电网的关键部件,其拓扑结构和控制策略是当前研究的热点。本文所研究的Z源光伏并网逆变器属于单级系统,具有结构简单、允许逆变桥同一桥臂上下功率器件直通、输出波形畸变小等优点。该拓扑结构特别适用于光伏发电等输入电压变换范围比较大的场合。本文对单相Z源光伏并网逆变器的拓扑结构和并网控制策略进行了深入的研究。具体内容包括以下几个方面:深入分析了单相Z源光伏并网逆变器的电路拓扑、升压原理、换流过程和开关管电流电压应力,并详细分析了单相Z源光伏并网逆变器的损耗,从而为系统的散热设计、器件和开关频率的选取提供了重要的依据。建立了Z源网络电路的小信号模型,并分析了Z源网络各参数变化对其输出特性的影响。Z源光伏并网逆变器采用了单级系统的双级控制策略,通过调节直通占空比和调制因子分别实现最大功率跟踪控制和逆变器并网电流控制。在光伏并网系统中,并网电流的频繁变化将会对电网的电能质量带来负面影响。为此,本文在分析已有并网电流幅值控制方法的基础上,提出了单周期电容电压调节法来控制并网电流幅值。该方法能够在一个工频周期内完成对光伏并网电流的调整,从而改善了并网电流的波形质量,减小了并网电流的波动。无变压器型光伏并网逆变器体积小、效率高、造价低,被广泛地应用于小功率光伏并网系统中。本文分析了Z源光伏并网逆变器和全桥逆变器共模电流的产生机理。其中,Z源逆变器和采用单极性调制方式的全桥逆变器的共模电压是高频脉冲电压,会产生共模电流。因此需要在电网侧加入工频变压器来消除共模电流。而在双极性PWM调制方式下,全桥逆变器的共模电压恒定,不产生共模电流。但是,此调制方式不能消除输出电流的直流分量。过大的直流分量会使变电所变压器工作点偏移,增加电网电缆的腐蚀,烧毁输入保险,甚至增加谐波含量。为此本文提出一种新型控制算法来抑制直流分量的输出。该算法使用两个补偿环节分别抑制由于调制脉宽不对称和并网电流检测误差引起的直流分量。它无需增加外围硬件电路,并且所增加的环节只占用很少的控制芯片资源。定电压跟踪法控制精度差,但在低光照强度下具有更高的效率,而扰动观察法在高光照强度下具有较高的效率。为此,本文将定电压跟踪法、扰动观察法与Z源逆变器的特点有机的结合,提出一种适用于Z源光伏并网逆变器的变直通占空比扰动观察方法。采用定电压法启动Z源光伏并网逆变器,从而减小了系统在启动过程中频繁的扰动,提高逆变器稳定性。在低光照强度下应用定电压跟踪法,而在高光照强度下使用扰动观察法,从而提高了整个系统在不同的光照强度下最大功率跟踪算法的效率。当电网因为故障或者停电维修而跳脱时,若系统不能及时检测出断电状态且迅速将自身切离电网,将会发生孤岛效应。孤岛情况一旦发生,常会导致一系列严重的问题。为此,光伏并网系统应具有防止孤岛现象的功能,从而保证系统的正常工作。本文详细分析过/欠压、过/欠频被动式孤岛检测方案和滑模频移主动式检测方案的工作原理以及在不同坐标系下的非检测区域。由于滑模频移法未设初始相位,理论上不能保证电网瞬间触发频率偏移。为此,本文提出一种新型的自适应滑模频移方法。该方法设置了初始相位角,并且在实测频率偏差超出误差范围时,以正反馈方式增加最大相位角和初始相位角,从而加速了公共节点电压频率的偏移,缩短了检测孤岛的时间。在上述工作的基础上,研制开发了一套1.5kW单相Z源光伏并网逆变器装置。本装置以TMS320F2812为主控制芯片,整机的最大效率在94%以上,功率因数超过了0.99。实验结果验证了所提出的新型控制算法的正确性和可行性。更多还原

【Abstract】 The grid-connected photovoltaic (PV) system power generation is the most important way in the solar energy application area, and has broad prospects. In PV system, the grid-connected inverter is a critical component responsible for the control of electricity flow between the PV module and power grid. Nowadays, the topology and control strategy of PV inverter has become the research hotpot. Z-source inverter is a single-stage system. It has some advantages such as simple structure, high reliability of the inverter for avoiding influence of the shoot-through by EMI, and small output waveform distortion. Especially, this topology applies to the occasions of large rang input voltage such as PV power generation, etc.In this paper, the topology and control strategy of the single-phase Z-source grid-connected PV inverter was analyzed in detailed. The whole paper is organized as followed:The topology, operating principle, commutation process and current and voltage stresses of the power switches was analyzed in this paper. And the power losses of the single-phase Z-source grid-connected PV inverter were studied, which can provide important guidance of the thermal design, devices and switching frequency selection.This paper presented small-signal modeling of the Z-source network. The output response characteristics had been analyzed with change of Z-source network parameters. The double-loop control strategy was adopted in this system. By regulating shoot-through duty cycle and modulation index, maximum power point tracking (MPPT) control and grid-connected current control was achieved, respectively. This paper proposed the one-cycle capacitor voltage-change adjustment method to control ac line current amplitude, combined with the characteristics of the single-phase Z-source PV grid-connected inverter. The proposed method can achieve Z-source capacitor voltage regulation within one ac mains cycle, and thus improve the power quality of the grid and minimize the injected ac line current variation.Due to the small size, high efficiency and low cost, the transformerless PV grid-connected inverter was widely used in low-power PV grid-connected systems. The paper analyzed the generating mechanism of common-mode currents in the Z-source inverter and full-bridge inverter. The shortcoming of common-mode voltage of Z-source inverter and full-bridge inverter with unipolar modulation is the high-frequency PWM pulse voltage. In order to overcome the generated common-mode current, the common method is to add transformer. While bipolar PWM modulation, no changes appear in the common-mode voltage and no common-mode current is generated. However, this modulation mode could not prevent the injection of DC current into the grid. A dc current fed from the customer’s side into the grid can shift the transformers operating point and might cause saturation. This would result in high primary current peaks, which might trip the input fuse and thus cause a power outage to that specific section of the grid. Furtherly, it would cause increased harmonics. Therefore, a new control algorithm was proposed in this paper. The algorithm uses two compensation links to suppress the DC components, which were caused by pulse width imbalance in PWM process and the error in the actual current measurement. Without adding auxiliary circuit, the algorithm occupied a few control chip resources.In the case of low irradiation, the Constant Voltage (CV) method has a high efficiency, and the Perturb and Observe (PO) method has a high efficiency under high irradiation. Therefore, in order to improve the efficiency of solar panel effectively, a new MPPT method is proposed combined with the characteristics of Z-source inverter. In order to reduce frequency disturbance and improve the stability of the inverter, CV method is adopted during the startup process. The proposed scheme can track the maximum power point with high efficiency at both low and high irradiation, which combines the CV method with variable shoot-through duty cycle PO method.Islanding operations occurs when power supply from the main grid is interrupted due to fault and intentional disconnection of the line for servicing but the Distributed Generation (DG) keeps supplying power into distribution networks. Inadvertent islanding causes serious problems. For these reasons islanding detection is an indispensable feature for DG-systems. The over/under voltage, over/under frequency method and slip-mode frequency shift (SMS) method was discussed. And the non-detection zones of above two methods were analyzed. In order to overcome the disadvantage of SMS, an adaptive sliding-mode frequency shift method is proposed. This method sets the starting angle. When the measured frequency deviation is beyond the error range, the maximum phase angle and starting angle will be increased in the positive feedback manner. Thus, the frequency deviation of point of common coupling is accelerated, and the time of islanding detection is shortened.A 1.5kW experimental device of Z-source PV grid-connected inverter was built after the above research. By using TMS320F2812, the proposed methods were realized on this platform. The experimental tests showed the maximum efficiency and power factor reaches above 94% and 0.99, respectively. The experimental results verified the proposed control algorithms.更多还原