I. Introduction At present, China's photovoltaic power generation system is mainly a direct current system, which means that the energy from the solar battery charges the battery, and the battery directly supplies power to the load. For example, the solar energy household lighting system used in the northwest region of China and the microwave station power supply system far from the power grid are all DC system. Such a system is simple in structure and low in cost. However, due to the difference in the DC voltage of the load (such as 12V, 24V, 48V, etc.), the standardization and compatibility of the system are achieved. Especially for civil power, most of them are AC loads and DC Power-supply photovoltaic power sources are difficult to enter the market as commodities. In addition, photovoltaic power generation will eventually achieve grid-connected operation, which must be adopted maturely. In the future, the exchange of photovoltaic power generation systems will become the mainstream of photovoltaic power generation. Second, photovoltaic power generation system requirements for inverter power Photovoltaic power generation system adopting AC power output consists of four parts: photovoltaic array, charge and discharge controller, battery and inverter power supply (the grid-connected power generation system can generally save the battery), and the inverter power supply is a key component. Photovoltaic power generation system has high requirements for inverter power supply: (1) It requires higher efficiency. Due to the current high price of solar cells, in order to maximize the use of solar cells, improve system efficiency, we must try to improve the efficiency of inverter power supply. (2) It requires high reliability. At present, the photovoltaic power generation system is mainly used in remote areas, and many power stations are unattended and maintained. This requires the inverter power supply to have a reasonable circuit structure, strict component screening, and requires the inverter power supply to have various protection functions, such as input direct current. Reverse polarity protection, AC output short circuit protection, overheating, overload protection, etc. (3) It is required that the DC input voltage has a wide adaptation range. Since the terminal voltage of the solar battery varies with the load and the sunlight intensity, although the battery has a clamping effect on the voltage of the solar battery, the voltage of the storage battery depends on the remaining capacity of the battery and The internal resistance changes and fluctuates, especially when the battery ages when the terminal voltage varies widely, such as a 12V battery, the terminal voltage can vary from 10V to 16V, which requires the inverter power supply must be in a large DC Ensure normal operation within the input voltage range and ensure the stability of the AC output voltage. (4) In medium- and large-capacity photovoltaic power generation systems, the output of the inverter power supply should be a sine wave with small distortion. This is due to the fact that in medium- and large-capacity systems, if square-wave power is used, the output will contain more harmonic components, and higher harmonics will cause additional losses. The load of many photovoltaic power generation systems is communication or instrumentation equipment. When the equipment has a high quality of the power grid, when the medium- and large-capacity photovoltaic power generation system is connected to the grid, it is required that the inverter power supply output a sine wave current in order to avoid the power pollution of the public power grid. Third, the principle of inverter power supply and circuit structure The inverter power supply converts DC power into AC power. The circuit principle shows that the power transistors T1, T3 and T2 and T4 are alternately turned on to obtain AC power. If the DC voltage is low, the AC voltage is boosted through the AC transformer, and the standard AC voltage and frequency are obtained. . For high-capacity inverter power supply, the human DC bus voltage is higher, and the AC output generally does not require transformer boosting to reach 220V. In medium- and small-capacity inverters, the DC voltage is low, such as 12V and 24V. , you must design the boost circuit. Medium and small-capacity inverters generally have three kinds of push-pull circuits: a push-pull inverter circuit, a full-bridge inverter circuit, and a high-frequency step-up inverter circuit. The neutral tap of the step-up transformer is connected to the positive power supply. Only the power tubes work alternately, and the output gets AC power. Since the power transistors are connected in common, the driving and control circuits are simple, and because the transformer has a certain leakage inductance, the short-circuit current can be limited, thus improving the reliability of the circuit. Its disadvantages are low transformer utilization and poor ability to drive inductive loads. The full-bridge inverter circuit shown overcomes the disadvantages of the push-pull circuit. The power transistors T1, T4 and T2, T3 are in opposite phase and the T1 and T2 phases are 180 degrees out of phase. By adjusting the output pulse width of T1 and T2, the rms value of the output AC voltage changes. The control signals and output waveforms of the four power transistors are shown in Fig. 6. Since this circuit has a function that enables T2 and T4 to be turned on together, it has a freewheeling circuit. Even for inductive loads, the output voltage waveform will not be distorted. The disadvantage of this circuit is that the power transistors of the upper and lower arms are not common ground, so special drive circuits or isolated power supplies must be used. In addition, in order to prevent the common conduction of the upper and lower bridges, it is necessary to design the first turn-off and after-conduction circuit between T1, T4 and T2, T3, ie, a dead time must be set, and the circuit structure thereof is relatively complicated. Push-pull circuit and full-bridge circuit output must be added with step-up transformer. Because the power-frequency step-up transformer is large in size, low in efficiency, and relatively expensive, with the development of power electronics and microelectronics technology, high-frequency step-up conversion technology is used. Inverting, high power density inverter can be achieved, this type of inverter circuit booster circuit using a push-pull structure, but the operating frequency is above 20KHZ, step-up transformer uses high frequency magnetic core material, and thus small size / Light weight, high-frequency inverter after high-frequency transformer into a high-frequency alternating current, but also by high-frequency rectifier filter circuit to obtain high-voltage DC (usually above 300V) and then through the power frequency inverter circuit to achieve the inverter. With this circuit structure, the power density of the inverter circuit is greatly increased, the no-load loss of the inverter is reduced accordingly, and the efficiency is improved. The disadvantage of this circuit is that the circuit is complex and the reliability is lower than the above two circuits. Fourth, the control circuit of the inverter circuit The main circuit of the above-mentioned several kinds of inverter power supply needs to have the control circuit to realize, generally have two kinds of control methods of square wave and positive and weak wave, the power circuit of the inverter power of square wave output is simple, the cost is low, but the efficiency is low, the harmonic content is big . Sinusoidal wave output is the development trend of inverter power supply. With the launch of microelectronics technology, microprocessors with PWM functions have also come out. Therefore, the sine wave output inverter technology has matured. 1. The square wave output inverter power supply currently uses pulse width modulation integrated circuits such as SG3525 and TL494. Practice has proved that using the SG3525 integrated circuit and using power MOSFETs as switching power components can achieve a relatively high cost performance inverter power supply, because the SG3525 has the ability to directly drive power MOSFETs (Figure 7) and has an internal reference source And operational amplifier and under voltage protection function, so its peripheral circuit is very simple. 2. Sine wave output inverter power control integrated circuit Sine wave output of the inverter power supply, the control circuit can be controlled by a microprocessor, such as the 80C196MC produced by INTEL, Motorola's MP16 and PIC16C73 produced by MI-CROCHIP, etc. These microcontrollers have multiple PWM generators. The dead time between the upper and upper arms can be set. The circuit using the 80C196MC from INTEL to achieve the sine wave output is shown in Fig. 8. The 80C196MC completes the occurrence of a sine wave signal and detects the AC output voltage to achieve regulation. V. Selection of main circuit power devices for inverter power supply The selection of the main power components of the inverter is of vital importance. Currently, Darlington power transistors (BJTs), power MOSFETs (MOSFETs), insulated gate transistors (IGBTs), and turn-off thyristors (BJTs) are used. GTO), etc., are more commonly used in small-capacity low-voltage systems as MOSFETs because MOSFETs have lower on-state voltage drop and higher switching frequency, and IGBT modules are generally used in high-voltage large-capacity systems. The on-resistance of the MOSFET increases with the increase of the voltage, and the IGBT occupies a large advantage in the medium-capacity system. In the ultra-large-capacity (above 100KVA) system, the GTO is generally used as a power element.
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