Power electronic transformers, also known as solid-state transformers, are new types of power grid distribution transformers that have attracted people's attention in recent years with the development of power electronics technology. The advantages of the power electronic transformer are high stability of the power supply voltage, guaranteed power supply quality, low work loss, small size, and light weight. Although the current power electronic transformer is lower in efficiency than the traditional power frequency transformer and the cost is higher, it can be expected that the power electronic transformer will become a key equipment for the transmission and distribution network in the next 35 years, and will be widely used in AC flexible transmission and high voltage DC. In power transmission, it has become a new symbol of power electronics technology to update traditional power system equipment.
The original intention of studying power electronic transformers was to reduce the size and weight of conventional transformers. The volume and weight of the transformer are inversely proportional to its operating frequency, and by increasing its switching frequency by means of power electronics, volume and weight can be reduced. The US Navy conducted research work in the late 1970s and early 1980s and achieved certain results. The American Academy of Electric Power conducted research on related PET. In the above two projects, the test prototype does not suppress the input harmonic current well, and the transformer input and output are not isolated. At the end of the 1990s, the University of Missouri completed PET 10kVA, 7200V/240V).
The primary side high-voltage voltage control scheme The control method of the three-phase PWM rectifier circuit is as follows (1), -, the requirements of the PWM rectifier are: the AC of the power grid is changed to DC, the DC output voltage is controllable, and the network side unit power factor operation is performed. For the three-phase PWM rectifier circuit, a voltage outer loop and a double closed-loop control scheme of the current inner loop can be used. The voltage outer loop is for realizing the control of the output voltage, and the current inner loop is for realizing unit power factor control. The input current satisfies the following formula (1): the d and q axis currents are affected by the controlled voltages sdUdc and sqUdc, and are also affected by the coupled voltages Liq, -Liq and the grid voltage ed, 6. Therefore, the dq axis is simply The negative feedback of the current does not cancel the current coupling between the dq axes. The effect is not very satisfactory. To design a decoupling controller, the d, q component Md of the AC side voltage fMc of the rectifier circuit is the output of the controller. And the current and voltage regulators are all PI regulators, and satisfy the relationship of: analysis, such as >0, then 4=1, otherwise A=0; if; -%>0, then 5 is as shown, set d The 9-axis voltage component is selected from the interval/internal test vector. Substituting equation (8) into equation (6) can be derived as follows: It can be summarized as the following expression: Calculate the switching point of the space voltage comparator, and define the following expression Knowing the sector where the voltage is located, you can find the corresponding adjacent vector vector duty cycle according to Table 1.
Table 1 The switching point of the sector N space vector of the assignment table is shown in Table 2. The electrical system IT voltage maintains good stability.
Table 2 Simulation of the switching point of the space vector and its result analysis According to the control scheme proposed in this paper, the main simulation parameters in the Matlab6.5/Simulmk environment are as follows: the power supply side three-phase input voltage is 1V, the input terminal inductance is 1.3e-3mH, primary high-voltage DC capacitor is 2000e-6, high-frequency transformer is 30:9, secondary low-voltage DC capacitor is 5000e-6, secondary filter capacitor is 4700e-6, secondary The side filter inductor is 2.15e-3, the primary side high voltage stage switching frequency is 9000, the secondary side low voltage stage switching frequency is 10000, and the output voltage operating frequency is 50Hz. It is the output voltage and current of the primary side power supply side with a power factor of 1. The waveform diagram shows that the phase difference between the voltage and current waveforms is close to zero, indicating that the power supply is outputting active power to the load side. According to the method described herein, the sector that can be determined is as follows, and the interval sequence is in the order of 4-6-2-3-1-5-4, which conforms to the vector synthesis direction of the control algorithm of the principle analysis: 4 (100), is the method of SPWM control rectifier side on the primary side, which is the method of SVPWM control inverter on the secondary side, where (b) is the enlarged view at 0.1s switch, the output voltage and current waveform are at 0.1 There is no change before and after dynamic switching. It can be clearly seen from the locally amplified waveform diagram that the current at 0.1s has a small jitter, but after this transient occurs, the secondary waveform of the output is simulated by the SVPWM control method. In the inverter method, when the load is switched from inductive to capacitive at 0.1 s, the output voltage and current waveforms do not change significantly before and after this dynamic switching. It can be clearly seen from the partially amplified waveform (b) that the voltage rises slightly before and after 0.1 s, and the voltage amplitude rises by about 40 V transient unstable voltage after the transient of 0.1 s. However, it has good stability after switching; it can be seen through comparative analysis. After 1 s, the output voltage still maintains a stable output voltage.
The simulation waveform diagram of the secondary side inverter adopting the SPWM control method is analyzed and compared: in the AC-DC-AC type power electronic transformer, the power factor of the control grid is adopted on the primary side, and the control method of the double closed loop is used to load the load on the secondary side. The output of the side adopts two different control schemes, both of which have relatively stable output characteristics, wherein the output controlled by the SVPWM algorithm has voltage stability on the switching side. The SPWM and SVPWM control algorithms on the load side have achieved good results in the application of power electronic transformers. In the primary power factor control, the power factor of the grid side is better. Two different control strategies achieve a stable constant frequency and constant voltage output.
5 Conclusions This paper proposes a power electronic transformer algorithm control scheme with easy to implement digitalization, which is used in systems with different load characteristics on the load side that require stable and easy to implement output. In order to control the power factor of the primary grid side, the power factor is close to 1, so the primary side rectifier circuit is controlled by the decoupled voltage and current double closed loop control method; when the load is switched from the capacitive load to the inductive load, The secondary side secondary inverter is controlled by the SVPWM algorithm. The change is small before and after the 0.1s transient, and the output after switching is stable, the amplitude change is small, and the voltage recovery is fast. In this paper, the control scheme of the power electronic transformer based on double PWM transform is simulated. The results show that the power electronic transformer of this structure can not only realize the unit power factor operation of the power grid, but also output the AC voltage with constant voltage and constant frequency. With anti-load disturbance characteristics.
The original intention of studying power electronic transformers was to reduce the size and weight of conventional transformers. The volume and weight of the transformer are inversely proportional to its operating frequency, and by increasing its switching frequency by means of power electronics, volume and weight can be reduced. The US Navy conducted research work in the late 1970s and early 1980s and achieved certain results. The American Academy of Electric Power conducted research on related PET. In the above two projects, the test prototype does not suppress the input harmonic current well, and the transformer input and output are not isolated. At the end of the 1990s, the University of Missouri completed PET 10kVA, 7200V/240V).
The primary side high-voltage voltage control scheme The control method of the three-phase PWM rectifier circuit is as follows (1), -, the requirements of the PWM rectifier are: the AC of the power grid is changed to DC, the DC output voltage is controllable, and the network side unit power factor operation is performed. For the three-phase PWM rectifier circuit, a voltage outer loop and a double closed-loop control scheme of the current inner loop can be used. The voltage outer loop is for realizing the control of the output voltage, and the current inner loop is for realizing unit power factor control. The input current satisfies the following formula (1): the d and q axis currents are affected by the controlled voltages sdUdc and sqUdc, and are also affected by the coupled voltages Liq, -Liq and the grid voltage ed, 6. Therefore, the dq axis is simply The negative feedback of the current does not cancel the current coupling between the dq axes. The effect is not very satisfactory. To design a decoupling controller, the d, q component Md of the AC side voltage fMc of the rectifier circuit is the output of the controller. And the current and voltage regulators are all PI regulators, and satisfy the relationship of: analysis, such as >0, then 4=1, otherwise A=0; if; -%>0, then 5 is as shown, set d The 9-axis voltage component is selected from the interval/internal test vector. Substituting equation (8) into equation (6) can be derived as follows: It can be summarized as the following expression: Calculate the switching point of the space voltage comparator, and define the following expression Knowing the sector where the voltage is located, you can find the corresponding adjacent vector vector duty cycle according to Table 1.
Table 1 The switching point of the sector N space vector of the assignment table is shown in Table 2. The electrical system IT voltage maintains good stability.
Table 2 Simulation of the switching point of the space vector and its result analysis According to the control scheme proposed in this paper, the main simulation parameters in the Matlab6.5/Simulmk environment are as follows: the power supply side three-phase input voltage is 1V, the input terminal inductance is 1.3e-3mH, primary high-voltage DC capacitor is 2000e-6, high-frequency transformer is 30:9, secondary low-voltage DC capacitor is 5000e-6, secondary filter capacitor is 4700e-6, secondary The side filter inductor is 2.15e-3, the primary side high voltage stage switching frequency is 9000, the secondary side low voltage stage switching frequency is 10000, and the output voltage operating frequency is 50Hz. It is the output voltage and current of the primary side power supply side with a power factor of 1. The waveform diagram shows that the phase difference between the voltage and current waveforms is close to zero, indicating that the power supply is outputting active power to the load side. According to the method described herein, the sector that can be determined is as follows, and the interval sequence is in the order of 4-6-2-3-1-5-4, which conforms to the vector synthesis direction of the control algorithm of the principle analysis: 4 (100), is the method of SPWM control rectifier side on the primary side, which is the method of SVPWM control inverter on the secondary side, where (b) is the enlarged view at 0.1s switch, the output voltage and current waveform are at 0.1 There is no change before and after dynamic switching. It can be clearly seen from the locally amplified waveform diagram that the current at 0.1s has a small jitter, but after this transient occurs, the secondary waveform of the output is simulated by the SVPWM control method. In the inverter method, when the load is switched from inductive to capacitive at 0.1 s, the output voltage and current waveforms do not change significantly before and after this dynamic switching. It can be clearly seen from the partially amplified waveform (b) that the voltage rises slightly before and after 0.1 s, and the voltage amplitude rises by about 40 V transient unstable voltage after the transient of 0.1 s. However, it has good stability after switching; it can be seen through comparative analysis. After 1 s, the output voltage still maintains a stable output voltage.
The simulation waveform diagram of the secondary side inverter adopting the SPWM control method is analyzed and compared: in the AC-DC-AC type power electronic transformer, the power factor of the control grid is adopted on the primary side, and the control method of the double closed loop is used to load the load on the secondary side. The output of the side adopts two different control schemes, both of which have relatively stable output characteristics, wherein the output controlled by the SVPWM algorithm has voltage stability on the switching side. The SPWM and SVPWM control algorithms on the load side have achieved good results in the application of power electronic transformers. In the primary power factor control, the power factor of the grid side is better. Two different control strategies achieve a stable constant frequency and constant voltage output.
5 Conclusions This paper proposes a power electronic transformer algorithm control scheme with easy to implement digitalization, which is used in systems with different load characteristics on the load side that require stable and easy to implement output. In order to control the power factor of the primary grid side, the power factor is close to 1, so the primary side rectifier circuit is controlled by the decoupled voltage and current double closed loop control method; when the load is switched from the capacitive load to the inductive load, The secondary side secondary inverter is controlled by the SVPWM algorithm. The change is small before and after the 0.1s transient, and the output after switching is stable, the amplitude change is small, and the voltage recovery is fast. In this paper, the control scheme of the power electronic transformer based on double PWM transform is simulated. The results show that the power electronic transformer of this structure can not only realize the unit power factor operation of the power grid, but also output the AC voltage with constant voltage and constant frequency. With anti-load disturbance characteristics.
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