Design of Thyristor Inverter in Photovoltaic Power Generation System
Article reproduced from: China Power Electronics Industry Network

Abstract: With the continuous expansion of the scale and scope of photovoltaic power generation in China, the demand for inverters in the photovoltaic market is increasing rapidly. At the same time, high-quality, low-cost inverter products have gradually become the focus of optoelectronic system developers and users. Inverter is an important application of power electronics technology.

1. Foreword

With the continuous expansion of the scale and scope of photovoltaic power generation applications in China, the demand for inverters in the photovoltaic market is increasing rapidly. At the same time, high-quality, low-cost inverter products have gradually become the focus of optoelectronic system developers and users. Thyristor inverter is an important application of power electronics technology. Power electronics technology is a comprehensive technology of mutual penetration and organic combination of power, electronics, automatic control and semiconductor technology. Therefore, the inverter involves a wide range of knowledge and technical content, this paper only from the perspective of the application of photovoltaic power generation system, the basic working principle of the inverter, the composition of the circuit system for a brief introduction. In recent years, with the increasing number of "small household appliances" products, more and more people are using rechargeable batteries, and many families are equipped with small battery chargers. The core component of the charger is the rectifier, which functions to rectify 50 cycles of alternating current into direct current. The inverter is the opposite of the rectifier. Its function is to convert direct current to alternating current. This reverse process, which corresponds to rectification, is called "inversion". Solar cells produce direct current under sunlight, but systems powered by direct current have significant limitations. For example: fluorescent lamps, televisions, refrigerators, electric fans, etc. can not be directly powered by DC power supply, the vast majority of power machinery is the same. In addition, when the power supply system needs to increase or decrease the voltage, the AC system only needs to add a transformer, and the step-up and step-down technology and device in the DC system are much more complicated. Therefore, in addition to special use, in the photovoltaic power generation system need to be equipped with inverter. The inverter also has the function of automatic voltage regulation or manual voltage regulation, which can improve the power supply quality of the photovoltaic power generation system. In summary, the inverter has become an indispensable and important supporting equipment in the photovoltaic power generation system.

Basic working principle of 2. inverter

There are many types of inverters, and their specific working principles and working processes are different, but the most basic inverter process is the same. The following is the most basic inverter circuit-single-phase bridge inverter circuit as an example

The "inversion" process of the inverter is specified. The single-phase bridge inverter circuit is shown in Figure 1 (a). The input DC voltage is E, and R represents the pure resistive load of the inverter. When switches K1 and K3 are turned on, current flows through K1, R and K3, and the voltage polarity on the load is positive left and negative right. When switches K1 and K3 are turned off and K2 and K4 are turned on, current flows through K2, R and K4, and the voltage polarity on the load is reversed. If the two groups of switches K123 and K224 are alternately switched at a frequency f, an alternating voltage Ur with a frequency f can be obtained on the load R, and its waveform is shown in FIG. 1 (B). The waveform is a square wave with a period T = 1/f. The switches K1, K2, K3 and K4 in the circuit of FIG. 1 (a) are actually ideal models of various semiconductor switching devices. Power switching devices commonly used in inverter circuits include power transistors (GTR), power field effect transistors (POWER MOSF E T), turn-off thyristors (GTO) and fast thyristors (SCR). In recent years, the insulated gate bipolar transistor (IGB T) with lower power consumption and faster switching speed has been developed.

3. inverter circuit system composition

The circuit shown in FIG. 1 ( a) is a schematic diagram of the inverting process of the inverter. In fact, to form a practical inverter, many important functional circuits and auxiliary circuits need to be added. The output is a sine wave voltage,

The working process is briefly as follows: the direct current sent by the solar cell array (or battery) enters the main circuit of the inverter, is converted into an AC square wave by inversion, and then filtered by a filter to become a sine wave voltage, and finally is boosted by the transformer and sent to the electric load. The switching process of the power switch tube in the main circuit of the inverter is controlled by the system control unit through the driving circuit. The working state and working parameters of each part of the inverter circuit, after being transformed into identifiable electrical signals by sensors with different functions, the information is sent to the system control unit through the detection circuit for comparison, analysis and processing. According to the judgment results, the system control unit regulates the working conditions of each circuit of the inverter. For example, the output voltage value of the inverter can be adjusted through the voltage adjustment circuit. When the detection circuit sends short circuit information, the system control unit passes through the protection circuit, immediately turn off the power switch tube of the main circuit of the inverter to protect the inverter. The main status information and fault conditions of the inverter can be sent to the display and alarm circuit through the system control unit. According to the inverter power size and function, the system control unit in Figure 2 can simply be a logic circuit composed of components or a dedicated chip; the complex can be a single-chip microprocessor or a 16-bit microprocessor, etc. In addition, Figure 2 shows the typical circuit system principle of the inverter. The actual inverter circuit system can be much simpler than Figure 2. It can also be more complicated. Finally, it should be noted that an inverter with perfect functions and good performance has all the functional circuits shown in Figure 2, there is also a secondary power supply. The power supply is responsible for providing all electrical components, components, instruments, etc. of the inverter with different levels of low-voltage working power.

Classification and Characteristics of Inverter in 4. Photovoltaic System

There are many principles for the classification of inverters. For example, according to the number of phases of the inverter output AC voltage, it can be divided into single-phase inverters and three-phase inverters. According to the different types of semiconductor devices used in the inverter, it can be divided into transistor inverter, thyristor inverter and turn-off thyristor inverter. According to the different circuit principles of the inverter, it can also be divided into self-excited oscillation inverter, step-wave superposition inverter and pulse width modulation inverter. In order to facilitate the photoelectric users to choose the inverter, here only the inverter output AC voltage waveform of different classification, and the characteristics of different output waveform inverter to do a brief description.

1. Square wave inverter

The AC voltage waveform output by the square wave inverter is a square wave, as shown in Figure 3(a). The inverter lines used by such inverters are not exactly the same, but the common feature is that the lines are relatively simple and the number of power switches used is small. The design power is generally between tens of watts and hundreds of watts. The advantages of square wave inverter are: cheap price and simple maintenance. The disadvantage is: because the square wave voltage contains a large number of high harmonics, in the transformer as the load of the electrical appliances will produce additional losses, the radio and some communication equipment also have interference. In addition, some of this type of inverter voltage regulation range is not wide enough, some protection function is not perfect, the noise is relatively large.

2. Step wave inverter

The AC voltage waveform output by this type of inverter is a step wave, as shown in Figure 3 (B). There are many different lines for the inverter to realize the step wave output, and the number of steps of the output waveform is also different. The advantage of the step wave inverter is that the output waveform is significantly improved than the square wave, and the high harmonic content is reduced. When the step reaches more than 17, the output waveform can be quasi-sine wave. When the transformer-free output is used, the efficiency of the whole machine is very high. The disadvantage is that the power switch tube used in the step wave superposition line is more, and some of the line forms also require multiple sets of DC power input. This brings trouble to the grouping and wiring of the solar cell array and the balanced charging of the battery. In addition, the step wave voltage still has some high-frequency interference to the radio and some communication equipment.

3. Sine wave inverter

The AC voltage waveform output by this type of inverter is a sine wave, as shown in FIG. 3 (c). The characteristics of sine wave inverter are: good comprehensive technical performance, perfect function, but complex circuit. The advantages of the sine wave inverter are: good output waveform, low distortion, no interference to the radio and communication equipment, and low noise. In addition, the protection function is complete, and the efficiency of the whole machine is high. Disadvantages are: the line is relatively complex, the maintenance technology requirements are high, the price is more expensive. The classification of the above three types of inverters is conducive to the identification and selection of inverters by optoelectronic system developers and users. In fact, the inverter with the same waveform is still very different in terms of line principle, use of devices and control methods.