The backstage inverter module realizes a stable control of the DC bus voltage and at the same time controls the quality of the connection to the grid, so that the current connected to the grid meets the relevant standards. A complete connection test of both stages is performed and the final result meets the system requirements.
The background of the subject and the purpose and significance of the research
Problem Statement
Aims and Objectives
Structure of the Research Report
Introduction
Research status and analysis of feedback load
- Research status of DC boost circuit topology
- Research status of various load characteristics
- Research status of inverter grid connection
- Project realization and product status
This circuit topology controls the input and output current separately, and without other auxiliary circuits, the soft shutdown of the coupling tube can be realized through proper control, and the coupling loss is reduced. With the complexity of the control algorithm and the requirement of response time, the traditional single-chip microcomputer cannot meet the current demand, so the DSP chip is used to realize the control of the system.
The main design ideas of the system
Topological structure selection of bidirectional DC-DC converter circuit List several typical DC boost circuit topologies: Boost DC boost circuit, push-pull
In addition, the design of capacitance, inductance and other circuit components and other design details are further investigated and researched in the circuit design and simulation process.
Implementation of load working mode
Design of inverter and grid connection
Analysis and comparison of three DC boost circuit topological structures
Actually, the advantage of the buck-boost converter compared to the boost converter is that the capacitor voltage is lower (Vazquez et al., 1999). Similar to a basic:Cuk converter, the two coils can be coupled in the step-up version and the output coil can become ripple. The advantage of this method is that the switching ripple of the output current is small and the response speed is fast.
Topological structure selection of DC boost circuit
- Analysis of Push-Pull Circuit Topology
- Boost DC boost chopper circuit topology analysis
- Topological analysis of DC-DC boost converter with middle tapped inductor
- Sepic chopper circuit
- Zeta chopper circuit
- Topological analysis of capacitive energy storage Cuk circuit
- Comparative analysis and summary
The boost principle is realized by the energy storage of the inductor L and the principle that the capacitor voltage cannot change suddenly. The topological structure of the DC-DC boost converter with a center-tapped inductor is shown in Figure 3.3. The characteristic of the Cuk converter is that the input and output current ripples are small.
Design of control strategy of DC boost circuit
Closed-loop feedback adopts PID link
The main features of PID control are: easy to understand, easy to implement, and a wide range of applications, which can meet more than 95% of objects and control requirements; the controller is suitable for most objects and has strong universality, to be precise, it is not sensitive to parameters, it has a wide range of applications and reliable control. The PID controller can track changes in load voltage, current and power, quickly adjust PWM to adapt to changes in the outside world and achieve better control. The special control algorithm is to collect voltage and current information, then compare with the reference value to generate the difference, modulate through the PID link, and send to the PWM controller to control the on time and power sequence of the device for the difference The absolute value becomes smaller, and the actual value is consistent with the reference value.
The concrete realization plan of the three control strategies
The input current Iin is collected and compared with the reference current Iref to obtain the error value, and then modulated by the PID link to control the duty cycle to make the system output. The input current of corresponds to the reference current, and realizes constant current control. Constant voltage control: By collecting the voltage, compare with the set voltage, get the error by PID controller modulation, control the conduction law of the PWM switching device, realize the minimum error, and then make the actual voltage according to the set voltage effect . Compared to the measured current Iin, the PID controller controls the switching tube to turn on and off.
Overall analysis and simulation verification of capacitive energy storage Cuk circuit
Circuit design parameter analysis
When choosing the parameters of the capacitor C1, the voltage UC1 of the capacitor C1 should be approximately equal to the output voltage Ud. Capacitor C2 is a buffer capacitor, used to realize the soft turn-off of the MOS2 tube. At this time, the main circuit of the intelligent feedback load and the corresponding electrical parameters are determined.
Overall Simulink simulation verification of DC boost circuit
Set the system according to the above parameters, and the simulation result waveform is shown in Figure 3.14-3.16. The waveform of the constant voltage state is shown in the figure, and the control law can be obtained through the waveform. It can be seen from figure 3.21 that the output power is in a stable state with small fluctuations.
Summary of this chapter
A mains converter is a power electronic converter that converts the energy on the DC bus of the previous stage into alternating current and then transmits it to the network. According to the regulations of GB/T 15945, when connecting to the network, the maximum permissible deviation of the converter is ±0.5Hz;. According to the relevant national standards, the total current harmonics do not exceed 5% of the rated power of the inverter, and there is also a limit for individual harmonics.
Circuit topology design of grid-connected inverter
Structure of grid-connected inverter
In addition, for an orderly and controlled entry into the network, it is necessary to observe the appropriate standards when connecting to the network, especially the requirements regarding phase, frequency, quality of electricity and other parameters:. In order to meet the relevant standards and improve the quality of the network connection, it is necessary to design a reasonable inverter topology and control algorithm. The disadvantage of this topology is that it uses multiple devices, but due to its simple and reliable structure and easy realization of various functions, it is used as the main structure of the inverter circuit.
Selection of filter
In an ideal situation, when the values of L and C are large enough, the output of the inverter is a sine wave without any distortion, but in practice, the response speed, the stability of operation and the base voltage of the inverter must be considered. . According to the theory of resonance, it can be concluded that the resonant frequency of the system is 𝑓0 = 1. 2𝛱√𝐿𝐶, and the inverter should avoid the resonant frequency; at the same time, the carrier frequency of the system must be greater than the resonant frequency, so a low-pass filter is designed to eliminate the resonant frequency.
Application of grid-connected transformer
At the same time, LC should be lower than the switching frequency of IGBT. Given the final setting, the capacitance C is 300 μF and the inductance L is 1 mH. When the primary side of the voltage transformer is energized, alternating magnetic flux is generated in the iron core, and the alternating magnetic flux generates alternating voltage on the secondary winding, and then the energy is consumed through the secondary side lobe. . This is the working principle of the transformer and the principle of electrical isolation between the primary and secondary windings.
Control strategy design of inverter circuit
Hysteresis current control mode
The hysteresis current controller plays two roles, as a closed-loop negative feedback controller to adjust the current so that the current is output according to the set value; the other role is to form a PWM wave which modulates the switching command. The hysteresis current control method has a small amount of calculation and no complicated calculation process. Also, since the control accuracy and switching adjustment speed is determined by the hysteresis width and will not change with the current, the stability.
Voltage and current common control scheme
After theoretical analyzes and simulation tests, and at the same time according to the requirements of the current system, when the hysteresis width is chosen to be 0.1A, it is most consistent with the current system, therefore the hysteresis width is chosen as: H =0.1A.
Overall analysis and simulation verification of the inverter circuit
Overall analysis of inverter circuit
Overall Simulink simulation verification of the inverter circuit
Since the grid current waveform is not clear enough, a separate screenshot of the current waveform is as follows. From the waveform analysis, the grid-connected voltage and current have good sine wave characteristics, which can realize the energy transfer from the grid-connected inverter to the grid. Through the harmonic analysis in the powergui tool, 50Hz is selected as the fundamental frequency, and the harmonic analysis of the current waveform is performed.
Summary of this chapter
When the current waveform represents a complete sine wave characteristic, the waveform is the basic waveform with no other frequency waveform components. As shown in Figure 4.9, the obtained data show that the total current distortion level is kept at about 0.01, which is in accordance with the system design. In the previous article, theoretical analysis and simulation experiments were performed on DC boost topology and grid-connected converter and their corresponding topologies.
Overall system analysis
To further explain the mode switching control, a block diagram of the optional control module is shown in Figure 5.3. The connection point of the front-stage amplifier and the back-stage converter is the DC bus, so the DC amplifier has a crucial connection to the next DC bus. The overall architecture of the background converter is shown in the figure above, and its basic control strategy has been described above.
System simulation verification
The figure shows that the voltage of the tested power supply fluctuates widely, but the output current is very stable and meets the design requirements of the system. The current dynamic response time is longer, but can basically meet the design requirements of the system. Based on the above analysis and the conclusion of the third chapter, the design of the system is verified by simulation.
Summary of this chapter
Conclusion
Recommendations
This design is connected to the power grid and is connected to the power grid in single phase; the current and voltage levels of the system design are small, which cannot fully meet the voltage range of the load pile, and the voltage range of the feedback load needs to be expanded later; the system response time is long, can not fully meet the requirements of the load pile response time; at the same time, the control algorithm is not optimized enough, and the output waveform fluctuates greatly, requiring subsequent improvement; the theoretical analysis is not enough, and the simulation depth cannot reach the actual use environment.
