A control method of a hybrid dc power electronic transformer

Abstract

The application discloses a control method of a hybrid DC power electronic transformer, which is composed of a dual active full-bridge DC converter and a resonant DC converter, and the control method comprises voltage normal state control and abnormal state control. When the system voltage is normal, the dual active full-bridge DC converter adopts phase-shift control for realizing active adjustment of output power and voltage; the resonant DC converter adopts open-loop control and directly works in a resonant state; when the system voltage rises or falls, the resonant DC converter adopts active frequency modulation control, the switching frequency of the resonant converter is adjusted, the input voltage of the resonant converter is changed, and the purpose of input voltage balance of each DC converter is achieved. The proposed control method can not affect the system problem operation, and when the input voltage fluctuates, the input voltage balance of each converter is realized, and the stability of the system is ensured.

Description

TECHNICAL FIELD

[0001] The application relates to a control method of a hybrid direct-current power electronic transformer. BACKGROUND

[0002] In recent years, flexible direct-current transmission technology and its application in a direct-current power grid have gradually become a research hotspot and received extensive attention. A power electronic transformer (PET) or also called a solid state transformer (SST) is a key link in a flexible direct-current distribution network, can realize flexible control and rapid management of voltage and power between a high-voltage direct-current distribution network and a low-voltage direct-current distribution network or a microgrid, and has important theoretical value and engineering significance.

[0003] A hybrid direct-current power electronic transformer composed of a dual active bridge (DAB) type direct-current converter and a series resonant converter (SRC) type converter can realize active regulation of voltage or power and has high operation efficiency. However, due to different circuit structures of the sub-modules of the hybrid direct-current power electronic transformer, there is an input voltage deviation between the sub-modules of different types, and a large voltage deviation not only affects the operation efficiency, but also causes device damage due to overvoltage and affects the service life of the converter.

[0004] Simply making the series resonant converter always work in a resonant state can ensure the operation efficiency of the system, but when the input side voltage greatly deviates from the rated value, a large input voltage deviation of each module occurs, and the use reliability is reduced. Therefore, the application provides a control strategy to solve the problem of the large input voltage deviation of the modules of the power electronic transformer. SUMMARY

[0005] The application solves the problem of how to provide a control strategy to solve the problem of the large input voltage deviation between the modules while ensuring active regulation of output voltage and power and having certain operation efficiency.

[0006] To solve the above problems, the technical scheme adopted by the application is: a control method of a hybrid direct-current power electronic transformer, characterized in that: the control method comprises voltage normal state control and voltage abnormal state control; when the system voltage is normal, the dual active full-bridge type direct-current converter adopts phase-shift control for active regulation of output power and voltage; the resonant type direct-current converter adopts phase-shift control with zero phase-shift ratio and directly works in a resonant state; when the system voltage is abnormal, the resonant type direct-current converter adopts active frequency modulation control, the switching frequency of the resonant converter is actively adjusted, the input voltage of the resonant converter is changed, and the purpose of balancing the input voltages of the direct-current converters is achieved.

[0007] The further technical scheme is that the state monitoring is that the main controller first performs voltage and current sampling and detects the system voltage state to determine the normal state or abnormal state control instruction.

[0008] The further technical scheme is that, in the voltage normal state control instruction, the dual active full-bridge type direct-current converter adopts phase-shift control, the phase-shift control adopts single phase-shift control or other multiple phase-shift control, and the phase-shift control adopts a PI controller, the input of the controller being a voltage or power difference value and the output of the controller being a phase-shift ratio in the phase-shift control.

[0009] The further technical scheme is that, in the voltage normal state control instruction, the resonant type direct-current converter adopts phase-shift control with zero phase-shift ratio and directly works in a resonant state.

[0010] The further technical scheme is that, in the voltage abnormal state control instruction, the dual active full-bridge type direct-current converter still adopts phase-shift control to implement constant output voltage or constant output power control; the resonant type direct-current converter uses active frequency modulation control to change its input voltage and keep the input voltages of the direct-current converters balanced.

[0011] The further technical scheme is that the active frequency modulation control is characterized in that the main controller determines the number of resonant type direct-current converters participating in the active frequency modulation control according to the degree of deviation of the system input voltage from the rated value, and the resonant type direct-current converters not participating in the active frequency modulation are still in a zero phase-shift state and work in a resonant state.

[0012] The further technical scheme is that the main controller numbers and sorts the resonant type direct-current converters, and each resonant type direct-current converter determines whether to perform active frequency modulation control according to its number order.

[0013] The further technical scheme is that the active frequency modulation control adopts a PI controller, the input of the controller being a voltage difference value, and the output of the controller being a change amount of the switching frequency of the switching tube.

[0014] The beneficial effects of the above technical solution are that the hybrid direct current power electronic transformer utilizes the control method of the application, can realize active adjustment of voltage and power when the system voltage is normal, ensures high operation efficiency, and can keep the input voltage of each direct current converter balanced when the system voltage is temporarily lowered or temporarily raised. BRIEF DESCRIPTION OF DRAWINGS

[0015] The drawings of the application are as follows:

[0016] Figure 1 is a control method block diagram in the embodiment of the application;

[0017] Figure 2 is a hybrid direct current power electronic transformer topology in the embodiment of the application;

[0018] Fig. 3 is a voltage and power control principle diagram in the embodiment of the application;

[0019] Figure 4 is an active frequency modulation control schematic diagram in the embodiment of the application;

[0020] Figure 5 is a simulation topology in the embodiment of the application;

[0021] Figure 6 is a voltage control waveform of the hybrid direct current power electronic transformer when the conventional control is adopted in the embodiment of the application;

[0022] Fig. 7 is the input voltage waveform of each converter, the output voltage and current waveform of the hybrid direct current power electronic transformer when the control proposed in the application is adopted in the embodiment of the application;

[0023] Figure 8 is a frequency change amount diagram of active frequency modulation when the control proposed in the application is adopted in the embodiment of the application. DETAILED DESCRIPTION

[0024] The drawings of the application are as follows: Figure 1 -7, a control method of a hybrid direct current power electronic transformer proposed in the application is described in detail.

[0025] Figure 1 is a control method block diagram in the embodiment of the application, in which, V o is an output side voltage; V in is an input side voltage; I o is an output side voltage, V in(i) (i=1, 2,..., n) is an input voltage of the i-th sub-module.

[0026] Figure 2The hybrid DC power electronic transformer topology is connected with high and low DC bus on input and output sides respectively, basic DC / DC converter unit is double active full bridge type DC converter and resonance type DC converter, C in 、C o are series (high voltage) side and parallel (low voltage) side filter capacitors respectively.

[0027] Fig. 3(a) is a fixed voltage control schematic diagram of the hybrid DC power electronic transformer in the embodiment of the present application, the double active full bridge type DC converter adopts single phase shift control, the single phase shift control adopts PI controller, the input of the controller is output voltage difference, and the output of the controller is phase shift ratio in phase shift control.

[0028] Fig. 3(b) is a fixed power control schematic diagram of the hybrid DC power electronic transformer in the embodiment of the present application, the double active full bridge type DC converter adopts single phase shift control, the single phase shift control adopts PI controller, the input of the controller is output power difference, and the output of the controller is phase shift ratio in phase shift control.

[0029] Figure 4 is an active frequency modulation control schematic diagram in the embodiment of the present application, the input of the control is input voltage difference of the converter, and the output of the controller is switching pulse signal of each switch tube.

[0030] Figure 5 is a simulation topology in the embodiment of the present application, the hybrid DC power electronic transformer is composed of one double active full bridge type DC converter and two resonance type DC converters, and simulation example parameters are set as shown in Table 1.

[0031] Figure 6 is a voltage control waveform of the hybrid DC power electronic transformer when the conventional control is adopted in the embodiment of the present application, the initial input voltage is 3200V, the total voltage rises to 3500V at 0.20s, and the input voltage returns to 3000V at 0.30s. As can be seen from the result, the double active full bridge type DC converter adopts fixed voltage phase shift control, the resonance type DC converter works in open loop resonance state, and the increased voltage on the system input side is all added to the double active full bridge type DC converter, and the actual input voltage exceeds the rated value by 45%.

[0032] Figure 7(a) is the input voltage waveform of the hybrid DC power electronic transformer in the embodiment of the present application using the control proposed in the present application, the initial input voltage is 3200V, the control strategy is started at 0.10s, the resonant DC converter 1 participates in the active frequency modulation, and the resonant DC converter 2 works in the resonant state. At 0.20s, the total voltage rises to 3500V, at 0.30s, the input voltage returns to 3000V. As can be seen from the results, after the hybrid DC power electronic transformer uses the control proposed in the present application, the phenomenon of the input voltage of the dual active full-bridge DC converter being too high is alleviated, the actual input voltage exceeds the rated value by 20%, which is greatly reduced compared with the traditional control strategy.

[0033] Table 1 Simulation parameters

[0034]

[0035] Figure 7(b) is the output voltage and current waveform of the hybrid DC power electronic transformer in the embodiment of the present application using the control proposed in the present application, the output voltage is 1000V, and the output current is 80A. As can be seen from the figure, the control strategy proposed in the present application does not affect the output side when adjusting the input voltage.

[0036] Figure 8 is the frequency variation amount diagram of the active frequency modulation when the control proposed in the present application is used in the embodiment of the present application, as can be seen from the figure, the frequency during the active frequency modulation period is relatively stable, and there is no large frequency fluctuation phenomenon.

Claims

1. A control method of a hybrid DC power electronic transformer, characterized by: The control method comprises voltage normal state control and voltage abnormal state control, the voltage normal state control is used for realizing active regulation of output power and voltage when the system voltage is normal, and the double active full-bridge type direct current converter adopts phase shift control; The resonant type direct current converter directly works in the resonant state by adopting phase shift control with zero phase shift ratio when the system voltage is abnormal, and the resonant type direct current converter adopts active frequency modulation control, the switching frequency of the resonant converter is actively regulated, the input voltage of the resonant converter is changed, and the purpose of equalizing the input voltages of all direct current converters is achieved.

2. The control method of a hybrid DC power electronic transformer according to claim 1, characterized in that, The main controller firstly carries out voltage and current sampling, detects the system voltage state, and determines to adopt normal or abnormal control instructions.

3. The control method of a hybrid DC power electronic transformer according to claim 1, characterized in that, When the system is in the normal working state, the control method executes voltage normal state control instructions, the double active full-bridge type direct current converter adopts phase shift control, the phase shift control adopts single phase shift control or multiple phase shift control, the phase shift control adopts a PI controller, the input of the controller is a voltage or power difference value, and the output of the controller is a phase shift ratio in the phase shift control.

4. The method of claim 1, wherein the control method of the hybrid DC power electronic transformer is characterized by, When the system voltage is in the normal state, the control method executes voltage normal state control instructions, and the resonant type direct current converter adopts phase shift control with zero phase shift ratio and directly works in the resonant state.

5. The method of claim 1, wherein the control method of the hybrid DC power electronic transformer is characterized by, When the system voltage is in the abnormal state, the control method executes voltage abnormal state control instructions, the double active full-bridge type direct current converter still adopts phase shift control, the control target of fixed output voltage or fixed output power is realized, the resonant type direct current converter adopts active frequency modulation control, the input voltage of the resonant type direct current converter is changed, and the input voltages of all direct current converters are equalized.

6. A control method of a hybrid DC power electronic transformer as claimed in claim 5, characterized in that, The main controller determines the number of resonant type direct current converters participating in active frequency modulation control according to the degree of deviation of the system input voltage from the rated value, and the resonant type direct current converters not participating in active frequency modulation are still in the zero phase shift state and work in the resonant state.

7. The method of claim 5, wherein the control method of the hybrid DC power electronic transformer is characterized by, The main controller numbers and sorts all resonant type direct current converters, and the resonant type direct current converters determine whether to execute active frequency modulation control according to the number order.

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