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Interpretation of the principle and application of inverter in photovoltaic power generation system
- Apr 16, 2018 -

At present, China's photovoltaic power generation system is mainly a direct current system, which means that the energy from the solar cells will be used to charge the batteries, and the batteries will directly supply power to the loads. For example, solar energy household lighting systems used in the northwest region of China and power supply systems for microwave stations far from the power grid are DC system. Such a system is simple in structure and low in cost. However, due to different DC voltages of the load (such as 12V, 24V, 48V, etc.), it is difficult to achieve standardization and compatibility of the system, especially for civil power, since most of the AC loads use DC power. The supply of photovoltaic power is difficult to enter the market as a commodity. In addition, photovoltaic power generation will eventually achieve grid-connected operation. This requires the adoption of a mature market model. In the future, the exchange of photovoltaic power generation systems will become the mainstream of photovoltaic power generation.

Inverter requirements in the application:

1. Requirements have a higher efficiency. Due to the current high price of solar cells, in order to maximize the use of solar cells and improve system efficiency, we must try to improve the efficiency of the inverter.

2. Requirements have a higher reliability. At present, photovoltaic power generation systems are mainly used in remote areas, and many power stations are unattended and maintained. This requires inverters to have reasonable circuit structures, strict component screening, and require inverters to have various protection functions, such as input direct current. Reverse polarity protection, AC output short circuit protection, overheating, overload protection, etc.

3. Requires a wide range of DC input voltage, because the terminal voltage of the solar battery varies with the load and the sunlight intensity. Although the battery plays an important role in the voltage of the solar battery, the battery voltage depends on the remaining battery capacity and internal resistance. The fluctuation of the change, especially when the battery is aging, its terminal voltage has a wide range of changes, such as 12V battery, the terminal voltage can be changed between 10V~16V, which requires the inverter must have a large DC input voltage. Ensure normal operation within the 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. The equipment has higher requirements on the quality of the power grid. When the large-capacity photovoltaic power generation system is connected to the grid, it is required that the inverter output a sine wave current in order to avoid power pollution with the public power grid.

The inverter converts the DC power into 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 inverters, because the DC bus voltage is relatively high, 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. It is necessary to design a boost circuit.

Medium and small-capacity inverters generally have three kinds of push-pull inverter circuits, full-bridge inverter circuits and high-frequency boost inverter circuits. The push-pull circuit connects the neutral plug of the step-up transformer to the positive power supply, and the two power transistors Alternating the work, 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 overcomes the disadvantages of the push-pull circuit. The power transistor regulates the output pulse width and the rms value of the output AC voltage changes accordingly. Since this circuit has a freewheeling circuit, the output voltage waveform will not be distorted even for inductive loads. 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 arms, it is necessary to design the first turn-off and turn-on circuit, that is, the dead time must be set, and the circuit structure thereof is relatively complicated.

Inverter control circuit

The main circuit of the above-mentioned several kinds of inverters needs a control circuit to realize, generally there are two kinds of control methods of square wave and positive weak wave, the inverter power supply circuit of square wave output is simple, the cost is low, but the efficiency is low, and the harmonic components are large . Sinusoidal wave output is the development trend of the inverter. With the development of microelectronics technology, microprocessors with PWM functions have also come out. Therefore, the sine wave output inverter technology has matured.

1. Inverters with square-wave output currently use pulse width modulation integrated circuits such as SG3525 and TL494. Practice has proved that using SG3525 integrated circuit and using power FET as switching power element can realize inverter with high performance and price, because SG3525 has the ability to directly drive power MOSFET and has internal reference source and operational amplifier and Undervoltage protection, so its peripheral circuitry is simple.

2. Sine wave output inverter control integrated circuit, inverter with sine wave output, its control circuit can be controlled by microprocessor, such as 80C196MC produced by INTEL company, MP16 produced by Motorola company and produced by MI-CROCHIP company. PIC16C73, etc., these microcontrollers have multiple PWM generators, and can set the dead time between the upper and upper arm, using the 80C196MC INTEL company to achieve a sine wave output circuit, 80C196MC complete the occurrence of sine wave signals, and detection AC output voltage to achieve voltage regulation.

Inverter main circuit power device selection

The selection of the main power components of the inverter is critical. Currently used power components include Darlington power transistors (BJT), power MOSFETs (MOS-FET), insulated gate transistors (IGBT) and shutdown Thyristors (GTO), etc., which are used more often in low-volume and low-voltage systems, are 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. This is because the on-resistance of the MOSFET increases with the increase of the voltage, while the IGBT occupies a large advantage in the medium-capacity system. In the ultra-large-capacity (above 100 kVA) system, the GTO is generally used as the power element.