Switched capacitor direct current transformer active damping control method suitable for direct current power distribution network

Abstract

The invention discloses a switched capacitor direct current transformer active damping control method suitable for a direct current power distribution network, and belongs to the field of direct current transformer control. According to the method, discrete capacitor voltage and medium-voltage side bus current are sampled, and zero static error adjustment is carried out through a multi-loop feedback network formed by an outer voltage-sharing controller, an inner current controller, an active damping controller, a voltage stabilization controller and an output voltage controller in sequence, so that duty ratio signals of an SC module and phase shift angle signals of a DAB module are respectively generated; and synthesizing the control instruction to drive the SCDCT-DAB to operate. Virtual impedance is introduced through the active damping controller, the input impedance characteristic of the SCDCT-DAB is remodeled, and the phase margin of equivalent output impedance of the SCDCT-DAB and a medium-voltage direct-current bus at the intersection frequency is remarkably increased, so that the Nyquist stability criterion is met, and voltage oscillation caused by the negative damping characteristic when the multi-machine SCDCT-DAB is connected into a system is effectively restrained.

Description

Technical Field

[0001] This invention belongs to the field of DC transformer control, and more specifically, relates to an active damping control method for switched capacitor DC transformers suitable for DC distribution networks. Background Technology

[0002] The switched-capacitor DC Transformer-Dual Active Bridge (SCDCT-DAB) is a key component in DC distribution networks. This topology functions as a DC circuit breaker to achieve rapid isolation during short circuits on the medium-voltage DC side, while simultaneously improving power transmission capacity, optimizing voltage matching, reducing losses, and increasing efficiency. Its modular structure also provides fault redundancy, enhancing reliability. This topology often uses dual active bridges as its basic module due to their high power density, electrical isolation, soft switching, and modularity, which perfectly match the submodule requirements of the SCDCT-DAB. Therefore, the SCDCT structure composed of dual active bridges has become an important research direction and is widely used in DC distribution networks.

[0003] Current research focuses primarily on input impedance modeling and analysis of traditional ISOP DCTs. For the more widely used SCCT-DAB, linear modeling methods based on active damping are still lacking. Compared to two-stage DC transformers, three-stage SCCT-DABs have higher control complexity and more electrical parameters. The impact mechanism of small state variable disturbances on SCCT-DABs is unclear, thus limiting research on their input impedance. Furthermore, strictly regulated SCCT-DABs in medium-voltage distribution networks often exhibit negative damping characteristics, reducing grid stability. Simultaneously, the integration of multiple Voltage Source Converters (VSCs) and multiple SCCT-DABs introduces significant dynamic interactions, potentially leading to voltage oscillations. In medium-voltage distribution networks with multiple VSCs and multiple SCCT-DABs, the introduction of line impedance exacerbates the complex dynamic interactions between VSC units, SCCT-DAB port impedances, and line impedances. Therefore, considering both the impedance characteristics of the voltage source converter and the line impedance, the connection of SCDCT-DAB may induce bus voltage oscillations, seriously threatening the safe and stable operation of the DC distribution network. Summary of the Invention

[0004] In view of the above-mentioned defects and improvement needs of the existing technology, the present invention provides an active damping control method for switched capacitor DC transformers for DC distribution networks, the purpose of which is to achieve stable optimization of DC transformers.

[0005] The technical solution adopted in this invention is as follows: In a first aspect, the present invention proposes an active damping control method for a switched capacitor DC transformer suitable for DC distribution networks. The switched capacitor DC transformer includes M series-connected switched capacitor (SC) modules and M corresponding dual active bridge (DAB) modules. Each dual active bridge (DAB) module is connected to one SC module via a discrete capacitor, which serves as both the output port of the SC module and the input port of the DAB module. The method includes the following steps: (1) Sample the voltages of M discrete capacitors to obtain the average voltage of the discrete capacitors. ; (2) Sample the input current of the medium voltage side bus. After filtering out the AC component using a low-pass filter module, the DC current component of the medium-voltage side bus is obtained. ; (3) Equalize the voltage of discrete capacitors using an external voltage equalization controller. Reference value for voltage equalization with discrete capacitors Perform zero steady-state error regulation and output the corresponding reference value of the medium-voltage side bus input current. The current is input to the medium-voltage side bus via an internal current controller. Reference value of input current to medium voltage side bus Perform zero steady-state error adjustment and output the first duty cycle signal of the SC module. ; (4) The difference between the DC current component of the medium-voltage side bus and the input current of the medium-voltage side bus is adjusted without steady-state error by the active damping controller, and the second duty cycle signal of the SC module is output. ; (5) The voltage of the first M-1 discrete capacitors is regulated by a voltage regulator. Voltage equalization with discrete capacitors Perform zero steady-state error adjustment and output the corresponding voltage equalization phase shift angle adjustment of the DAB module. , where j = 1 to M-1; (6) The low-voltage DC bus voltage of the output is controlled by the output voltage controller. With output voltage reference value Perform steady-state error-free adjustment, and output the dominant phase shift angle acting on M DAB modules. ; (7) Calculate the final control command using the actual output calculator: ; in, It is the final duty cycle signal of the SC module. It is the phase shift angle signal of the j-th DAB module; (8) Based on the final duty cycle signal of the SC module Phase shift angle signal of DAB module , where j=1 to M, control the operation of the switched capacitor DC transformer (SCDCT-DAB) so that the system impedance satisfies the Nyquist stability criterion.

[0006] Furthermore, the zero steady-state error adjustment is achieved through a PI controller.

[0007] Secondly, the present invention proposes an active damping control device for switched capacitor DC transformers suitable for DC distribution networks, for implementing the above-mentioned active damping control method for switched capacitor DC transformers suitable for DC distribution networks.

[0008] The beneficial effects of this invention are: The control method of this invention maintains stable input and output while reshaping the input impedance of the SCDCT-DAB DC transformer, ensuring that its equivalent impedance with the medium-voltage DC bus satisfies the Nyquist criterion and guaranteeing the stability of the multi-machine SCDCT-DAB connected system. It also characterizes the stability boundary of the multi-terminal medium-voltage DC system and achieves active damping through virtual impedance. The boundary between the source and load converters is defined by the medium-voltage DC equivalent circuit. Specifically, this method samples the discrete capacitor voltage and the medium-voltage side bus current, and sequentially adjusts them without steady-state error through an external voltage equalization controller, an internal current controller, an active damping controller, a voltage regulator, and an output voltage controller to generate the final duty cycle signal of the SC module and the phase shift angle signal of the DAB module. These signals are then synthesized by an actual output calculator to control the operation of the switched-capacitor DC transformer, ensuring that the system impedance satisfies the Nyquist stability criterion. This process directly reshapes the input impedance of the SCDCT-DAB, increasing the phase margin of its equivalent impedance with the medium-voltage DC bus near the intersection frequency. This effectively suppresses voltage oscillations caused by negative damping characteristics when multiple units are connected, widens the stable operating boundary, and ensures the system remains stable under full load and other operating conditions.

[0009] This invention provides an effective analytical method for evaluating the stability of the medium-voltage DC port voltage of SCDCT-DAB, and provides an important theoretical basis for optimizing the design and stable operation of medium-voltage DC applications. Attached Figure Description

[0010] Figure 1 This is a schematic diagram of the circuit structure of a switched capacitor DAB type DC transformer; Figure 2 This is a block diagram of the active damping control structure of SCDCT-DAB; Figure 3It is the SCDCT-DAB that introduces an active damping control method, and its system input and output impedance characteristics are shown in the Bode plot under multi-machine full-load conditions. Figure 4 This is a schematic diagram of the simulated waveform; Figure 5 It is the dynamic waveform when the active damping control is switched on at T=0.225s. Detailed Implementation

[0011] The present invention will be further described and illustrated below with reference to specific embodiments. The embodiments described are merely examples of the content of this disclosure and do not limit the scope of the invention. The technical features of each embodiment in the present invention can be combined accordingly, provided that there is no mutual conflict.

[0012] See Figure 1 The circuit structure of a switched capacitor DC transformer (SCDCT-DAB) mainly consists of three parts: a medium-voltage side DC source. The system consists of M switched-capacitor (SC) modules and M dual active-bridge (DAB) modules. The core function of the M SC modules is to divide the voltage at the medium-voltage DC port. This design creates M independent capacitor DC voltage divider ports for connecting to subsequent DAB modules. The other side (low-voltage side) of each DAB module is connected in parallel to a common low-voltage DC bus, thus completing the medium-to-low voltage conversion and collection of electrical energy. Discrete capacitors serve as both output ports for the SC modules and input ports for the DAB modules. This hybrid architecture, combining switched-capacitor voltage division and DAB isolation conversion, aims to achieve a high voltage conversion ratio and efficient bidirectional power transfer. The low-voltage side of the topology typically also includes support capacitors. .

[0013] The structural block diagram of the active damping control structure of SCDCT-DAB is as follows: Figure 2 As shown. The control structure includes a summation module, a low-pass filter module, an averaging module, an active damping controller, an external voltage equalization controller, an internal current controller, a voltage regulator controller, an output voltage controller, and an actual output calculator.

[0014] The summation module is used to sum the voltages of the M discrete capacitors collected. Summing yields the total voltage across the discrete capacitors. .

[0015] The low-pass filter module is used to filter out the collected medium-voltage side bus input current. The AC component is used to obtain the DC current component of the medium-voltage side bus. This is due to the equivalent reactance of the DC bus on the medium voltage side. and equivalent resistance The existence of It may be a current that is a mixture of DC and AC currents, and the AC component needs to be filtered out.

[0016] The averaging module is used to calculate the total voltage of discrete capacitors. Divide by M to obtain the discrete capacitor voltage equalization. .

[0017] An external voltage equalization controller is used to achieve voltage equalization of discrete capacitors. Reference value for voltage equalization with discrete capacitors Zero steady-state error regulation, outputting the corresponding reference value of the medium-voltage side bus input current. .

[0018] The internal current controller is used to sample the input current of the medium-voltage side bus. Reference value of input current to medium voltage side bus The zero steady-state error adjustment ultimately generates the first duty cycle signal of the SC module. This is to complete closed-loop control.

[0019] An active damping controller is used to implement additional active damping on the medium-voltage side DC bus. It filters the DC current component of the medium-voltage side bus obtained through a low-pass filter. The collected medium-voltage side DC bus input current The difference is calculated, and after adjustment by the controller, a second duty cycle signal is output. In this embodiment, a PI controller is used.

[0020] The voltage regulator controller is used to achieve the voltage of the first M-1 discrete capacitors. Voltage equalization with discrete capacitors Zero steady-state error regulation, output corresponding to the equalization phase shift angle adjustment of the Mth DAB. Specifically, the zero steady-state error regulation process of the voltage regulator controller includes: in the first M-1 DAB modules, each DAB module independently calculates its corresponding discrete capacitor voltage. Voltage equalization with discrete capacitors The difference between them generates the corresponding phase shift angle adjustment amount for the DAB module. The phase shift adjustment is used to correct the reference phase shift, thereby effectively alleviating the voltage imbalance problem between the DAB modules. The phase shift adjustment of the Mth DAB module is determined by the phase shift adjustment of the previous M-1 DAB modules through an algebraic relationship, thereby realizing dynamic decoupling between the main output voltage loop and the capacitor voltage balancing loop in the control structure.

[0021] The output voltage controller is used to realize the low-voltage DC bus voltage output of the SCDCT-DAB. Compared with the set output voltage reference value Zero steady-state error regulation, the dominant phase shift angle of the output system This dominant phase shift angle acts on all modules simultaneously to ensure that the output power is evenly distributed among them.

[0022] The actual output calculator is used to calculate the control command values ​​of each module in the SC module and the DAB module: Specifically, the actual output calculator includes M+1 calculation structures, each used to calculate the duty cycle signal of the SC module. The final phase shift angle of the M DAB modules. Specifically, the SC module obtains the first duty cycle signal through the combined action of the external voltage equalization controller and the internal current controller. The second duty cycle signal obtained by adding the active damping controller The final duty cycle signal is obtained. The dominant phase shift angle obtained by the DAB module through the output voltage controller Subtract the voltage equalization phase shift angle adjustment calculated by the voltage regulator. The final applied phase shift angle signal of the first M-1 DABs of the SCDCT-DAB is obtained. ; By controlling the phase shift angle Add all equalization phase shift angle adjustments The final applied phase shift angle signal of the Mth DAB in SCDCT-DAB is obtained. .

[0023] The above text involves multiple instances of zero steady-state error adjustment. Because the active damping controller, external voltage equalization controller, internal circuit controller, voltage regulator, output voltage controller, and actual output calculator form a feedback adjustment network for the SCDCT-DAB, the voltage equalization of the corresponding discrete capacitors is adjusted by regulating the duty cycle and phase shift angle of each module. DC bus current value on the medium voltage side Discrete capacitor voltage and output low-voltage DC bus voltage All these parameters will change in tandem, and through zero steady-state error adjustment, three points of zero steady-state error tracking will be achieved, thus stabilizing the input and output. The zero steady-state error adjustment structure can refer to conventional designs.

[0024] Furthermore, based on the aforementioned control structure, while maintaining stable input and output, the input impedance of the SCDCT-DAB DC transformer can be reshaped so that its equivalent impedance with the medium-voltage DC bus satisfies the Nyquist criterion, ensuring the stability of the multi-unit SCDCT-DAB access system. To verify the effectiveness of this invention, an equivalent model of a voltage source converter and a switched capacitor DC transformer in a DC distribution network was constructed. In the DC distribution network, there are g VSCs and h SCDCT-DABs simultaneously. The power flow is set to flow from the voltage source converters VSC1 to VSCg to the SCDCT-DABs. The AC side of the VSCs is connected to an ideal large power grid, and the power is supplied to the DC bus through the line impedance. The DC transformer on the right operates under the aforementioned control mode. The line impedance of the output section on the VSCj side is... This refers to the line impedance of the medium-voltage input section on the DCT side.

[0025] The port stability of the bus voltage will be determined by the total output impedance of all voltage source converters and the total input impedance of all DC transformers. This represents the parallel combination of the output impedance and output line impedance of g voltage source converters. This represents the parallel combination of the input impedance and line impedance of h DC transformers. Let j be the output impedance of the j-th VSC. Let be the input impedance of the j-th SCDCT. The parameters of the DC transformer are shown in Table 1: Table 1 DC Transformer Parameters The SCDCT-DAB shown in Table 1 was simulated using MATLAB / Simulink software, where g=1 and h=2. Both SCDCT-DAB units operated at full load. The simulation waveforms are shown below. Figure 4 As shown, (a), (b), and (c) represent the DC bus voltage on the medium-voltage side, the converter input current of the SCDCT-DAB1, and the input current of the SCDCT-DAB1, respectively. and the output low-voltage DC bus voltage of SCDCT-DAB1 The relationship between the current at the input terminal of the SCDCT-DAB1 and time can be observed. Significant oscillations occurred, with an amplitude of 171.6A and a steady-state average of 187A, indicating that the oscillations accounted for more than 50% of the total voltage fluctuation. The voltage oscillation amplitude was 2469V, with a fluctuation rate ranging from -24.69% to +24.69%. This suggests that under the influence of a multi-machine access system, the input port voltage of the SCDCT-DAB will face a serious risk of instability.

[0026] Figure 3 The Bode plots of the input-output impedance characteristics of the SCDCT-DAB system under multi-machine full-load conditions, incorporating the active damping control method of this invention, are shown. Key phenomena can be observed from the plots: the input impedance is reshaped near the intersection frequency. The introduction of virtual impedance alters the amplitude-frequency characteristics of the SCDCT-DAB input impedance near the intersection frequency, increasing the intersection frequency. Since its phase increases with frequency, this expands the number of machines that can be stably connected to the DC transformer. More importantly, within the intersection frequency range, the phase characteristics of the DC transformer input impedance also change, showing a certain degree of increase. These two phase changes directly result in the absolute value of the phase angle of the impedance ratio being less than 180° in this key frequency band. According to the impedance ratio criterion, these two changes effectively increase the phase margin of the system, thereby significantly improving the stability of the cascaded system at this operating point. Table 2 shows the improvement in system stability before and after active damping control when g=1. It can be seen that the system exhibits a steady state after the addition of active damping control.

[0027] Table 2 System stability before and after active damping control (g=1) Figure 5 The dynamic waveform when the active damping control is engaged at T=0.225s is given. The figure shows that only 12.5ms after the control command is issued... It can reach a stable state after 12.5ms. It reaches a steady state and exhibits excellent steady-state control performance.

[0028] In summary, the control strategy proposed in this invention can not only effectively maintain the stability of the input and output voltage and current of the DC transformer, but also reconstruct the input impedance of the SCDCT-DAB to match the output impedance of the VSC medium-voltage DC bus, thus meeting the requirements of the Nyquist stability criterion and further enhancing the overall stability of the system.

[0029] The above-described embodiments are merely illustrative of several implementations of the present invention, and while the descriptions are specific and detailed, they should not be construed as limiting the scope of the invention. Those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these modifications and improvements all fall within the scope of protection of the present invention.

Claims

1. A method for active damping control of a switched-capacitor DC transformer suitable for DC distribution networks, wherein the switched-capacitor DC transformer comprises M series-connected switched-capacitor (SC) modules and M corresponding dual active bridge (DAB) modules, each DAB module being connected to one SC module via discrete capacitors, wherein the discrete capacitors serve as both output ports of the SC modules and input ports of the DAB modules, characterized in that... The method includes the following steps: (1) Sample the voltage of M discrete capacitors to obtain the average voltage of the discrete capacitors; (2) Sample the input current of the medium-voltage side bus, and obtain the DC current component of the medium-voltage side bus after filtering out the AC component through the low-pass filter module; (3) The discrete capacitor equalization voltage and the discrete capacitor equalization voltage reference value are adjusted without steady-state error by the external equalization voltage controller, and the corresponding medium-voltage side bus input current reference value is output. The medium-voltage side bus input current and the medium-voltage side bus input current reference value are adjusted without steady-state error by the internal current controller, and the first duty cycle signal of the SC module is output. (4) The difference between the DC current component of the medium-voltage side bus and the input current of the medium-voltage side bus is adjusted without steady-state error by the active damping controller, and the second duty cycle signal of the SC module is output. (5) The voltage of the first M-1 discrete capacitors and the voltage equalization of the discrete capacitors are adjusted without steady-state error by the voltage regulator controller, and the voltage equalization phase angle adjustment of the first M-1 DAB modules is output. (6) The output low-voltage DC bus voltage and the output voltage reference value are adjusted without steady-state error by the output voltage controller, and the output is applied to the dominant phase shift angle of M DAB modules; (7) Obtain the final duty cycle signal of the SC module based on the first duty cycle signal and the second duty cycle signal of the SC module; combine the dominant phase shift angle and the equalization phase shift angle adjustment of the first M-1 DAB modules to obtain the phase shift angle signals of the M DAB modules; (8) Based on the final duty cycle signal of the SC module and the phase shift angle signals of the M DAB modules, control the operation of the switched capacitor DC transformer so that the system impedance meets the Nyquist stability criterion.

2. The active damping control method for switched capacitor DC transformers applicable to DC distribution networks according to claim 1, characterized in that, The zero steady-state error adjustment is achieved through a PI controller.

3. The active damping control method for switched capacitor DC transformers applicable to DC distribution networks according to claim 1, characterized in that, The final duty cycle signal of the SC module is the sum of the first duty cycle signal and the second duty cycle signal of the SC module.

4. The active damping control method for switched capacitor DC transformers applicable to DC distribution networks according to claim 1, wherein the phase shift angle signal of the first M-1 DAB modules is the difference between the dominant phase shift angle and the voltage equalization phase shift angle adjustment of the corresponding DAB module, and the phase shift angle signal of the Mth DAB module is the sum of the dominant phase shift angle and the voltage equalization phase shift angle adjustment of the first M-1 DAB modules.

5. An active damping control device for switched-capacitor DC transformers suitable for DC distribution networks, characterized in that, include: The summation module is used to sample and sum the voltages of M discrete capacitors; The averaging module is used to average the sum of the voltages of M discrete capacitors to obtain the average voltage of the discrete capacitors. The low-pass filter module is used to filter out the AC component of the sampled medium-voltage side bus input current to obtain the DC current component of the medium-voltage side bus. An external voltage equalization controller is used to adjust the voltage equalization of discrete capacitors and the reference value of voltage equalization of discrete capacitors without steady-state error, and outputs the corresponding reference value of the input current of the medium-voltage side bus. The internal current controller is used to adjust the input current of the medium-voltage side bus and the reference value of the input current of the medium-voltage side bus without steady-state error, and outputs the first duty cycle signal of the SC module. An active damping controller is used to adjust the difference between the DC current component of the medium-voltage side bus and the input current of the medium-voltage side bus without steady-state error, and outputs the second duty cycle signal of the SC module. The voltage regulator controller is used to adjust the voltage of the first M-1 discrete capacitors and the voltage equalization of the discrete capacitors without steady-state error, and outputs the voltage equalization phase angle adjustment of the first M-1 DAB modules. The output voltage controller is used to adjust the output low-voltage DC bus voltage and the output voltage reference value without steady-state error, and the output acts as the dominant phase shift angle for M DAB modules. The actual output calculator is used to obtain the final duty cycle signal of the SC module based on the first and second duty cycle signals of the SC module; combined with the dominant phase shift angle and the equalization phase shift angle adjustment of the first M-1 DAB modules, the phase shift angle signals of the M DAB modules are obtained; the final calculation results are used to control the operation of the SCDCT-DAB so that the system impedance meets the Nyquist stability criterion.

6. The active damping control device for switched capacitor DC transformers suitable for DC distribution networks according to claim 5, characterized in that, The zero steady-state error adjustment is performed using a PI controller.

7. The active damping control device for switched capacitor DC transformers suitable for DC distribution networks according to claim 5, characterized in that, The final duty cycle signal of the SC module is the sum of the first duty cycle signal and the second duty cycle signal of the SC module.

8. The active damping control device for switched capacitor DC transformers suitable for DC distribution networks according to claim 5, characterized in that, In the actual output calculator, the phase shift angle signal of the first M-1 DAB modules is the difference between the dominant phase shift angle and the voltage equalization phase shift angle adjustment of the corresponding DAB module, and the phase shift angle signal of the Mth DAB module is the sum of the dominant phase shift angle and the voltage equalization phase shift angle adjustment of the first M-1 DAB modules.

Images