Solar photovoltaic power generation is the most promising new energy technology in the world. Solar photovoltaic power generation systems can be divided into off-grid photovoltaic power generation systems, grid-connected photovoltaic power generation systems and hybrid photovoltaic power generation systems according to different system operation modes. With the rapid development of China's photovoltaic power generation system, especially the implementation of the photovoltaic roof plan, the demand for off-grid PV inverters in China will become larger and larger. The off-grid photovoltaic power generation system is mainly composed of photovoltaic cell arrays, controllers, inverters, energy storage devices, etc., as shown in Figure 1, where the inverter is one of the important devices in the photovoltaic system, and its reliability. And conversion efficiency is critical to the implementation of photovoltaic systems and the reduction of system cost. At present, the domestic similar products mainly have the following shortcomings: a. Most of them use single-chip control, poor real-time performance, limited data processing and communication capabilities; b. use transformers, large and bulky; c. output voltage accuracy is not high, can not meet social development need. This paper proposes the design scheme of 5kW photovoltaic controller, which can be widely used in off-grid photovoltaic power generation system and wind-solar hybrid power generation system. It has the characteristics of small size, light weight, high output voltage precision, good waveform and intelligent monitoring of field bus. 1, 5kW off-grid photovoltaic inverter basic structure The structure of the photovoltaic inverter is as follows, including the primary circuit and the secondary circuit. The primary circuit is composed of input filter circuit, boost voltage boost circuit, full bridge inverter circuit and output filter circuit. The secondary circuit is controlled by TMS320Fz812. The circuit, the signal detection circuit, the human-machine interaction circuit and the communication circuit are composed. The hardware main circuit and control strategy of the 5kW off-grid PV inverter are designed below. Figure 2 Photovoltaic controller structure 2, 5kW off-grid photovoltaic inverter hardware design At present, the commonly used off-grid inverter circuits mainly have three kinds of topologies: a power frequency isolated single-stage inverter, a high-frequency isolated two-stage inverter, and a non-isolated two-stage inverter. Through theoretical calculations and practical verification, a circuit topology that is more suitable for use in photovoltaic power generation systems is used: no isolation two-stage inverter, also called Boost inverter, as shown in Figure 3. The input filter circuit is used to filter the 48V DC input of the photovoltaic solar energy, and then boosted by the Boost boost circuit, the inverter is processed by the full bridge inverter, the SPWM wave is output, and finally filtered by the LC low-pass filter, and the output is output. Sine wave at 50 Hz. 2.1 Input Filter Circuit Design The input filter circuit is composed of a filter capacitor, which is used to reduce the ripple of the input voltage. It is assumed that the maximum power of the converter is Pmax, and the input and output powers are equal to obtain the energy provided by the input filter capacitor in one cycle. 2.2 Boost circuit The Boost circuit is shown in Figure 4, where Q is a fully-controlled power device IGBT. The Boost circuit is a non-isolated DC conversion circuit with an output voltage equal to or higher than the input voltage. When the input voltage of the PV controller fluctuates within the allowable range. At the time, the output voltage is kept stable by controlling the conduction ratio D of the power switching device Q. According to whether the inductor current in the Boost circuit is continuous, it can be divided into three working modes: continuous inductor current, intermittent inductor current and critical inductor current. When operating in the critical mode of operation, the value of the inductor satisfies equation (3). 2.3 single-phase full-bridge inverter circuit In this paper, the driving waveform of the single-phase full-bridge inverter circuit is obtained by the modulation method. The generation and modulation of the signal wave and carrier are realized by the DSP2812. SPWM has three modulation modes: synchronous modulation, asynchronous modulation and piecewise synchronous modulation. The output frequency of this design is 50Hz, and the frequency is not too low, so synchronous modulation is adopted. 2.4 LC low pass filter The SPWM wave contains an integer multiple of the carrier frequency and harmonic components in the vicinity thereof. In order to obtain a good output voltage waveform, the LC low-pass filter must be used to eliminate higher harmonics. As the carrier ratio increases, the farther the lowest harmonic is from the fundamental, the easier it is to filter. The higher the carrier ratio will effectively improve the output voltage quality, but the increase of the carrier ratio is limited by the switching speed and switching of the power switching device. For factors such as loss, the selection of the LC low-pass filter mainly considers several factors, noise, suppression capability, output impedance, and inverter current stress. In the design, the volume, weight and manufacturing cost of the filter circuit should be considered comprehensively. Generally, the cutoff frequency is selected from 1/10~1/20 of the switching frequency. In this design, the system switching frequency is selected as 18kHz, and the inverter output AC power frequency is 50Hz, the initial determination of the cutoff frequency is 1kHz, there are two parameters to be determined in the filter, namely the filter inductor and the filter capacitor. The structure of LC low-pass filter is shown in Figure 5. The control strategy of 3,5kW off-grid PV inverter is widely used in inverter circuit. This paper combines PID control with closed-loop negative feedback control. Digital control strategy.
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