1、 Preface

At present, the photovoltaic power generation system in China is mainly DC system, that is, the power from solar cells is charged to the battery, and the battery directly supplies power to the load. For example, the solar household lighting system in Northwest China and the power supply system of microwave station far away from the grid are DC systems. This kind of system has simple structure and low cost, but due to the different DC voltage of load (such as 12V, 24V, 48V, etc.), it is very difficult to realize the standardization and compatibility of the system, especially for civil power, because most of the AC load, the photovoltaic power supply powered by DC power is difficult to enter the market as a commodity. In addition, photovoltaic power generation will eventually achieve grid connected operation, which must be mature. In the future, AC photovoltaic power generation system will become the mainstream of photovoltaic power generation.

2、 Requirements of photovoltaic power generation system for inverter

Photovoltaic power generation system with AC power output is composed of photovoltaic array, charge and discharge controller, battery and inverter (grid connected power generation system can generally save battery), and inverter is the key component. Photovoltaic power generation system has high requirements for inverter power supply

(1) High efficiency is required. Due to the high price of solar cells, in order to maximize the use of solar cells and improve the efficiency of the system, we must try to improve the efficiency of the inverter.

(2) High reliability is required. At present, photovoltaic power generation system is mainly used in remote areas, and many power stations are unattended and maintained, which requires inverter power supply with reasonable circuit structure, strict component selection, and various protection functions, such as input DC polarity reverse protection, AC output short circuit protection, overheating, overload protection, etc.

(3) The DC input voltage is required to have a wide range of adaptability. Because the terminal voltage of the solar cell varies with the load and sunshine intensity, although the battery has a clamping effect on the voltage of the solar cell, the voltage of the battery fluctuates with the change of the residual capacity and internal resistance of the battery, especially when the battery is aging, the terminal voltage varies widely, such as 12V storage battery The terminal voltage of the battery can vary from 10V to 16V, which requires the inverter to work normally in a large range of DC input voltage and ensure the stability of AC output voltage.

(4) In medium and large capacity photovoltaic power generation system, the output of inverter should be sine wave with less distortion. This is because in medium and large capacity systems, if square wave power supply is used, the output will contain more harmonic components, and higher harmonics will produce additional losses. Many photovoltaic power generation systems are loaded with communication or instrument equipment, which has a high impact on the quality of power grid. In addition, when medium and large capacity photovoltaic power generation systems are connected to the grid, in order to avoid power pollution of Duo public power grid, The inverter is also required to output sine wave current.

3、 Principle and circuit structure of inverter

The circuit principle of the inverter is shown in Figure 3. The power transistors T1, T3 and T2, T4 are turned on alternately to get AC power. If the DC voltage is low, the standard AC voltage and frequency can be obtained by boosting the voltage through the AC transformer. For large capacity inverter power supply, the DC bus voltage is high, and the AC output can reach 220 V without transformer boost. In medium and small capacity inverter power supply, because the DC voltage is low, such as 12 V, 24 V, the boost circuit must be designed.

There are three kinds of medium and small capacity inverter: push pull inverter, full bridge inverter and high frequency boost inverter, The neutral tap of the step-up transformer is connected to the positive power supply, and the two power transistors work alternately to output AC power. Because the power transistors are connected to the same ground, the driving and control circuit is simple. In addition, because the transformer has a certain leakage inductance, the short-circuit current can be limited, so the reliability of the circuit is improved. Its disadvantages are low utilization rate of transformer and poor ability of dynamic load.

The full bridge inverter circuit overcomes the shortcomings of push-pull circuit. The power transistors T1, T4 and T2, T3 are inverted, and the phase difference between T1 and T2 is 180 degrees. Adjust the output pulse width of T1 and T2, the effective value of output AC voltage will change accordingly. The control signals and output waveforms of the four power transistors have a freewheeling circuit because the circuit can make T2 and T4 conduct together. Even for inductive load, 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 are not in common ground, so special driving circuit or isolated power supply must be used. In addition, in order to prevent the upper and lower bridge arms from conducting together, the first off and then on circuit must be designed between T1, T4 and T2, T3, that is, the dead time must be set, and the circuit structure is complex.

The output of push-pull circuit and full bridge circuit must be added with step-up transformer. Because the power frequency step-up transformer is large, low efficiency and expensive, with the development of power electronics and microelectronics technology, high-frequency step-up conversion technology is used to realize inverter, which can realize high-power density inverter. The front step-up circuit of this kind of inverter circuit adopts push-pull structure, but the working frequency is higher Above 20kHz, the step-up transformer is made of high-frequency magnetic core material, so it is small in volume / light in weight. After high-frequency inverter, it turns into high-frequency AC through high-frequency transformer, and then gets high-voltage DC (generally above 300V) through high-frequency rectifier filter circuit, and then realizes inverter through power frequency inverter circuit.

Using this circuit structure, the power density of the inverter circuit is greatly improved, the no-load loss of the inverter power supply is also reduced correspondingly, 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.

4、 Control circuit of inverter circuit

The main circuit of the above inverter need to have a control circuit to achieve, generally there are two control modes of square wave and positive weak wave. The square wave output inverter has simple circuit, low cost, low efficiency and large harmonic component. Sine wave output is the development trend of inverter power supply. With the development of microelectronics technology, microprocessor with PWM function has come out, so the inverter technology of sine wave output has been mature.

1. At present, PWM integrated circuits are widely used in square wave inverter, such as SG3525, TL494 and so on. It has been proved that using SG3525 integrated circuit and power field effect transistor as switching power element can realize high performance and high price inverter power supply. Because SG3525 has the ability of directly driving power field effect transistor and has internal reference source, operational amplifier and undervoltage protection function, its peripheral circuit is very simple.

2. Sine wave output inverter control IC

The control circuit of inverter power supply with sine wave output can be controlled by microprocessor, such as 80C196MC produced by Intel, MP16 produced by Motorola and PIC16C73 produced by mi-chip.these microcontrollers all have multi-channel PWM generator, and can set the dead time between upper and upper bridge arms. 80C196MC produced by Intel can realize the circuit of sine wave output As shown in Figure 8, 80C196MC completes the generation of sine wave signal, and detects the AC output voltage to realize voltage stabilization.

5、 Selection of power devices in main circuit of inverter

It is very important to select the main power components of inverter. At present, the most commonly used power components are Darlington power transistor (BJT), power field effect transistor (MOSFET), insulated gate transistor (IGBT) and turn off thyristor (GTO), etc. in small capacity and low voltage system, MOSFET is the most commonly used device, because mosfe is the most important IGBT is widely used in high voltage and large capacity systems because of its low on state voltage drop and high switching frequency. This is because the on state resistance of MOSFET increases with the increase of voltage. IGBT has a great advantage in medium capacity systems, while GTO is generally used in super large capacity (above 100kVA) systems As a power element.