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Principle and Application of Inverter in Photovoltaic Power Generation System
- Apr 11, 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 system standardization and compatibility, especially for civil power, since most of the load is AC, DC power is used. Powered photovoltaic power sources are difficult to enter the market as commodities. In addition, photovoltaic power generation will eventually achieve grid-connected operation, which must adopt a mature market model. In the future, AC photovoltaic power generation systems will become the mainstream of photovoltaic power generation.


Photovoltaic Power Generation System Requirements for Inverter Power Supply


Photovoltaic power generation system adopting AC power output is composed of four parts: photovoltaic array, charge and discharge controller, battery and inverter (the grid-connected power generation system can generally save the battery), and the inverter The device is a key component. Photovoltaic power generation system requires higher inverter:


1. Requires 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. Requires high 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 to adapt to, because the terminal voltage of the solar cell varies with the load and the intensity of sunlight, although the battery has an important role in the voltage of the solar battery, but because the battery voltage varies with the remaining battery capacity and internal resistance The fluctuation, especially when the battery is aging, its terminal voltage has a wide range of changes, such as 12V battery, the terminal voltage can be varied from 10V to 16V, which requires the inverter must be within a large DC input voltage range. Ensure normal operation and ensure the stability of the AC output voltage.


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 step-up inverter circuits. Push-pull circuits connect the neutral plug of the step-up transformer to the positive power supply and 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 shortcomings of the push-pull circuit. The power transistor regulates the output pulse width and the rms value of the output AC voltage changes. 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 after-conduction circuit, that is, the 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 transformers. Because step-up transformers are bulky, inefficient, and expensive, with the development of power electronics and microelectronics technology, high-frequency boost converter technology is used to implement inverse Change, can achieve high power density inverter, the inverter circuit of the pre-stage boost circuit using push-pull structure, but the operating frequency is above 20KHz, step-up transformer uses high-frequency magnetic core material, thus small size, light weight After the high-frequency inverter is transformed into a high-frequency alternating current through a high-frequency transformer, and then high-voltage direct current (usually more than 300V) is obtained through the high-frequency rectifier filter circuit, and then the inverter is again realized through the power frequency inverter circuit.

With this circuit structure, the power of the inverter is greatly increased, the no-load loss of the inverter is correspondingly reduced, 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.


Inverter Circuit 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. The square wave output inverter currently uses 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, sine wave output inverter, the control circuit can be controlled by a microprocessor, such as 80C196MC produced by INTEL company, MP16 produced by Motorola company and PIC16C73 produced by MI-CROCHIP company, etc. These single-chip microcomputers have multi-channel PWM generators and can set the dead time between the upper and upper arms. The INTEL 80C196MC realizes a sine wave output circuit. The 80C196MC completes the occurrence of a sine wave signal and detects the AC output. Voltage to achieve voltage regulation.


Inverter main circuit power device selection


The choice of the main power components of the inverter is of utmost importance. Currently used Darlington Power Transistors (BJT), Power MOSFETs (MOS-FET), and Insulation Gate-transistor (IGBT) and turn-off thyristor (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 in high-voltage large-capacity The IGBT module is generally used in the system 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, and in the mega-capacity (above 100 kVA) system. In general, GTOs are used as power components.