Design and Simulation of a Three-Phase Bridge Fully Controlled Rectifier Circuit Based on Simulink

Abstract: Three-phase bridge fully controlled rectifier circuits play a crucial role and have wide applications in modern power electronics technology. This paper, based on the theoretical analysis of three-phase bridge fully controlled rectifier circuits and incorporating the theoretical foundation of fully controlled rectifier circuits, establishes a simulation model of the three-phase bridge fully controlled rectifier circuit using the Simulink simulation tool in Matlab. The circuit's operation under resistive load is then simulated and analyzed. The simulation analysis verifies the correctness of the model designed in this paper.

Keywords : Fully controlled rectifier circuit; Simulink simulation; Modeling; Power electronics

Intermediate Classification Number : TP 9 Document Identification Code: B

0 Introduction

Power electronics technology has a very wide range of applications in power systems. It is estimated that in developed countries, over 60% of the electrical energy ultimately used by users undergoes at least one power electronic converter. Power electronics technology is one of the key technologies in the modernization of power systems. It is no exaggeration to say that without power electronics technology, the modernization of power systems would be unimaginable. Currently, the input rectifier stage of various power electronic converters generally uses uncontrolled or phase-controlled rectifier circuits. These rectifier circuits have simple structures and mature control technologies, but they have low AC input power factors and inject a large amount of harmonic current into the grid. It is estimated that in developed countries, 60% of electrical energy is converted before use, while this figure reached 95% at the beginning of this century.

In electrical power transmission, direct current (DC) transmission has significant advantages for long-distance, high-capacity transmission. Both the rectifier valves at the transmitting end and the inverter valves at the receiving end utilize thyristor converters. Various electronic devices generally require DC power supplies of different voltage levels. The DC power supplies used in program-controlled exchanges in communication equipment, which previously used thyristor rectifiers, have now been replaced by high-frequency switching power supplies with fully controlled devices. The operating power supplies for mainframe computers and the internal power supplies of microcomputers also now use high-frequency switching power supplies. In various electronic devices, linear regulated power supplies were previously widely used, but due to their small size, light weight, and high efficiency, high-frequency switching power supplies have gradually replaced linear power supplies. Because various information technology devices require power from power electronic devices, it can be said that information electronics technology is inseparable from power electronics technology. The flexible alternating current transmission (FACTS) system, which has developed in recent years, is also made possible by power electronic devices.

With the development of social production and science and technology, rectifier circuits are increasingly widely used in automatic control systems, measurement systems, and generator excitation systems. Commonly used three-phase rectifier circuits include three-phase bridge uncontrolled rectifier circuits, three-phase bridge semi-controlled rectifier circuits, and three-phase bridge fully controlled rectifier circuits. Because rectifier circuits involve AC signals, DC signals, and trigger signals, and include various components such as thyristors, capacitors, inductors, and resistors, conventional circuit analysis methods are quite cumbersome, and experiments are difficult to conduct smoothly under high voltage conditions. The visualization simulation tool Simulink provided by Matlab can directly build circuit simulation models, arbitrarily change simulation parameters, and immediately obtain arbitrary simulation results, offering strong intuitiveness and further eliminating the need for programming. This paper uses Simulink to model a three-phase bridge fully controlled rectifier circuit and conducts simulation analysis under different control angles and bridge fault conditions. This not only deepens the theoretical understanding of three-phase bridge fully controlled rectifier circuits but also lays a solid experimental foundation for modern power electronics experimental teaching.

1 Circuit Principle Analysis

The thyristors are turned on in sequence from 1 to 6. Therefore, the thyristors are numbered as shown in Figure 1. Specifically, the three thyristors connected to the three-phase power supply (a, b, c) in the common cathode group are VT1, VT3, and VT5, and the three thyristors connected to the three-phase power supply (a, b, c) in the common anode group are VT4, VT6, and VT2. As shown in Figure 1, the thyristor turn-on sequence is VT1–VT2–VT3–VT4–VT5–VT6.

Figure 1 Main circuit schematic diagram

Its operating characteristic is that at any given time, two thyristors from different groups are simultaneously conducting, forming a current path. Therefore, to ensure normal conduction after circuit startup or current interruption, trigger pulses must be applied simultaneously to the pair of thyristors from different groups that should be conducting. Thus, the width of the trigger pulse should be greater than a π/3 wide pulse. Wide pulse triggering requires high trigger power and can easily saturate the pulse transformer, so a pulse train can be used instead of dual narrow pulses. Commutation occurs every π/3, alternating between the common cathode and common anode groups, but only within the same group. The numbering method of the thyristors in the wiring diagram ensures that the combined conduction sequence of the six thyristors in each cycle is VT1-VT2-VT3-VT4-VT5-VT6; the pulses of the common cathode group T1, T3, and T5 differ by 2π/3 sequentially; the pulses of the upper and lower arms of the same phase, i.e., VT1 and VT4, VT3 and VT6, and VT5 and VT2, differ by π, which facilitates analysis; when α=0, the waveform of the output voltage Ud in one cycle is the envelope of the six line voltages. Therefore, the frequency of the output pulsating DC voltage is 6 times the power supply frequency, which is 1 times higher than that of the three-phase half-wave circuit, reducing pulsation, and the waveform of each pulsation is the same. Therefore, this circuit can also be called a 6-pulse rectifier circuit. Similarly, the three-phase half-wave rectifier circuit is called a 3-pulse rectifier circuit. When α>0, a gap appears in the waveform of Ud. As the angle α increases, the gap increases, and the average value of the output voltage decreases. When α = 2π/3, the output voltage is zero. Therefore, for resistive loads, the phase shift range of α is 0 to 2π/3. When 0 ≤ α ≤ π/3, the current is continuous, and each thyristor conducts for 2π/3 of the current. When π/3 ≤ α ≤ 2π/3, the current is discontinuous, and each thyristor conducts for less than 2π/3 of the current. 23α = π/3 is the dividing point between continuous and discontinuous current in a resistive load.

2 Circuit Design

Based on the principle of a three-phase bridge fully controlled rectifier circuit, a simulation model can be built using modules in Simulink, as shown in Figure 2. Three AC voltage sources, Va, Vb, and Vc, are set with phase angles differing by 120° sequentially, thus obtaining the three-phase power supply for the rectifier bridge. A Universal Bridge is used to construct the rectifier bridge, realizing the conversion from AC voltage to DC voltage. A Synchronized 6-Pulse Generator generates the trigger pulses for the rectifier bridge.

The circuit and device modules are extracted. The main components of the circuit include a three-phase AC power supply, thyristors, and an RLC load. The main components of the three-phase rectifier circuit model are shown in Table 1.

Table 1 Main Components of Three-Phase Rectifier Circuit Model

The three-phase power supplies are configured as follows: Va: 220V, phase angle 0 degrees; Vb: 220V, phase angle -120 degrees; Vc: 220V, phase angle +120 degrees; all frequencies are set to 50Hz. The pulse generator frequency is set to 50Hz, and the pulse width is 10Hz. The Universal Bridge has 3 arms. The load resistor is 10 ohms, and the inductor is 0.01H. Figure 2 shows the simulation model of the three-phase bridge fully controlled rectifier circuit.

Figure 2 Simulation model of three-phase bridge fully controlled rectifier circuit

3. Simulation

1) Power supply parameter settings: The peak voltage of the three-phase power supply is 220V×

, can be represented as “220*sqrt(2)”, with a frequency of 50Hz and phases of 0, -120° and -240° respectively.

2) Three-phase thyristor rectifier parameter settings: Use default values.

3) 6-pulse generator settings: frequency 50Hz, pulse width 1°, double pulse trigger mode.

4) Trigger angle setting: The alph can be set to 30°, 60°, or 90° as needed.

5) The variable step size algorithm ode23tb(stiff/TR.BDF2) is adopted.

6) The load can be set as a pure resistor, a pure inductor, or a resistive-inductor load as needed. In this simulation, the load is a resistive load R=10Ω and a resistive-inductor load R=10Ω, with L=1H.

The simulation time was set to 0.06 seconds, and the numerical algorithm used was ode23tb (stiff/TR.BDF2). The simulation was started, and based on the schematic diagram of the three-phase bridge fully controlled rectifier circuit, the effect of different firing angles α on the output voltage was simulated. The following simulation waveforms show that changing different control angles results in different changes in the output voltage.

Figure 3. Waveform of a three-phase bridge fully controlled rectifier circuit with resistive load a=30°

Figure 4. Waveform of a three-phase bridge fully controlled rectifier circuit with resistive load a=60°.

Figure 5. Waveform of a three-phase bridge fully controlled rectifier circuit with resistive load a=90°

4. Conclusion

For a purely resistive load, when the firing angle is less than or equal to 90°, the Ud waveform is always positive, the DC current Id is proportional to Ud, and the resistance is 10 ohms, so the DC current waveform is the same as the DC voltage waveform. As the firing angle increases, the transistor turns off immediately after the voltage reverses, so the forward conduction time of the thyristor decreases, corresponding to a gradual decrease in the average output voltage. Furthermore, when the firing angle exceeds 60°, the Ud waveform becomes discontinuous. As the firing angle continues to increase, the output voltage decreases sharply, eventually approaching zero at 120°. For a thyristor, in rectification mode, it withstands a reverse blocking voltage. The phase shift range is 0~120°.