The photovoltaic power generation system is mainly a DC system, that is, the electric energy generated by the solar cell is charged to the battery, and the battery directly supplies power to the load. For example, the solar user lighting system used more in Northwest my country and the power supply system of the microwave station far away from the power grid are all DC systems.
1. Working principle of pv grid connected inverter
The pv grid connected inverter converts the DC power into AC power. If the DC voltage is low, the voltage is boosted through the AC transformer to obtain the standard AC voltage and frequency. For large-capacity inverters, due to the high DC bus voltage, the AC output can generally reach 220V without a transformer step-up. In medium and small-capacity inverters, due to the low DC voltage, such as 12V, 24V, It is necessary to design a boost circuit.
Medium and small-capacity inverters generally have three types: push-pull inverter circuit, full-bridge inverter circuit and high-frequency boost inverter circuit. The push-pull circuit connects the neutral plug of the step-up transformer to the positive power supply, and two power tubes Alternate work, the output is AC power, because the power transistors are connected to the common ground, the drive and control circuit is simple, and because the transformer has a certain leakage inductance, it can limit the short-circuit current, thus improving the reliability of the circuit. The disadvantage is that the utilization rate of the transformer is low, and the ability to drive the inductive load is poor.
The full-bridge inverter circuit overcomes the shortcomings of the push-pull circuit. The power transistor adjusts the output pulse width, and the effective value of the output AC voltage changes accordingly. Because the circuit 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 bridge arms do not have a common ground, so a special drive circuit or an isolated power supply must be used. In addition, in order to prevent the common conduction of the upper and lower bridge arms, the circuit must be turned off first and then turned on, that is, the dead time must be set, and the circuit structure is more complicated.
2. Definition of pv grid connected inverter
2.1 requires high efficiency. Due to the high price of solar cells, in order to maximize the use of solar cells and improve system efficiency, it is necessary to try to improve the efficiency of the pv grid connected inverter.
2.2 requires high reliability. Photovoltaic power generation systems are mainly used in remote areas, and many power stations are unattended and maintained. This requires the pv grid connected inverter to have a reasonable circuit structure, strict component selection, and requires the pv grid connected inverter to have various protection functions, such as input DC pole Sex reverse connection protection, AC output short circuit protection, overheating, overload protection, etc.
2.3 The DC input voltage is required to have a wide range of adaptation. Since the terminal voltage of the solar cell changes with the load and the intensity of sunlight, although the battery plays an important role in the voltage of the solar cell, the voltage of the battery varies with the remaining capacity and internal resistance of the battery. Changes and fluctuates, especially when the battery is aging, its terminal voltage varies widely, such as 12V battery, its terminal voltage can vary between 10V and 16V, which requires the pv grid connected inverter to operate within a relatively large DC input voltage range To ensure normal operation and ensure the stability of the AC output voltage.
2.4 In medium and large-capacity photovoltaic power generation systems, the output of the pv grid connected inverter power supply should be a sine wave with less distortion. This is because in medium and large-capacity systems, if a square wave is used for power supply, the output will contain more harmonic components, and high-order harmonics will cause additional losses. The loads of many photovoltaic power generation systems are communication or instrumentation equipment. The equipment has high requirements on the quality of the power grid. When the medium and large-capacity photovoltaic power generation systems are connected to the grid, in order to avoid power pollution with the public grid, the pv grid connected inverter is also required to output sine wave current.
3. The function of pv grid connected inverter
The dc/dc conversion boosts or lowers the input voltage and regulates its output to achieve maximum efficiency. After some additional voltage buffering, the dc voltage is converted to ac voltage in the left bridge, usually with a switching frequency of 18-20khz. In general, single-phase H-bridges are a common configuration for dc/ac stages, however, three-phase and other configurations are also possible. Finally, the sinusoidal alternating current output for the grid-connected photovoltaic power generation system is generated through a low-pass filter.
4. Control circuit of pv grid connected inverter
The main circuits of the above-mentioned inverters all need to be realized by a control circuit. Generally, there are two control modes: square wave and sine wave. The pv grid connected inverter power supply circuit with square wave output is simple, low in cost, but low in efficiency and large in harmonic components. . Sine wave output is the development trend of pv grid connected inverters. With the development of microelectronics technology, microprocessors with PWM functions have also come out, so the pv grid connected inverter technology for sine wave output has matured.
4.1 Inverter with square wave output
Inverters with square wave output mostly use pulse width modulation integrated circuits, such as SG3525, TL494, etc. Practice has proved that using SG3525 integrated circuit and using power field effect transistors as switching power components can realize inverters with relatively high performance and price. Because SG3525 has the ability to directly drive power field effect transistors and has internal reference sources and operational amplifiers and Undervoltage protection function, so its peripheral circuit is very simple.
4.2 Inverter with sine wave output
Sine wave output inverter control integrated circuit, the control circuit of the sine wave output inverter can be controlled by microprocessor, such as 80C196MC produced by INTEL, MP16 produced by Motorola and PIC16C73 produced by MI-CROCHIP, etc. , these single-chip microcomputers have multiple PWM generators, and can set the dead time between the upper and lower bridge arms. The 80C196MC of INTEL company is used to realize the sine wave output circuit. The 80C196MC completes the generation of the sine wave signal and detects the AC output. voltage to achieve voltage regulation. The output of the circuit generally uses an LC circuit to filter out high-frequency waves to obtain pure sine waves.