Virtual capacitance-based v2g power distribution network voltage sag support method and system

Abstract

The application discloses a kind of V2G distribution network voltage sag support method and system based on virtual capacitor, and the method is: based on the voltage current characteristics in the process of capacitor support direct current voltage, by being equivalent to virtual capacitor in two-phase rotating coordinate system before AC / DC converter of charging pile front stage;When V2G charging pile grid-connected point voltage sag, the differential of the equivalent virtual capacitor of AC / DC converter is used to generate virtual capacitor active current instruction, then with basic active current instruction superposition obtains d-axis active current instruction, drives AC / DC converter to extract energy from the battery of V2G charging pile and injects power grid in the form of active power.The application makes V2G AC / DC converter equivalent to shunt capacitor on port external characteristic by control algorithm, to provide more rapid, direct virtual inertia for distribution network voltage dynamic support required.

Description

Technical Field

[0001] This invention belongs to the field of power distribution network technology, specifically relating to a method and system for supporting voltage sag in V2G power distribution networks based on virtual capacitance. Background Technology

[0002] With the continuous emergence of new technology industries, power quality has become an increasingly important concern for power companies. Currently, voltage sags are a relatively frequent power quality disturbance in power systems, typically referring to a sudden drop in the effective value of the power frequency voltage to 10%–90% of its rated value, followed by a return to normal power quality after 10ms–1 minute. Frequent voltage sag events can cause significant economic losses to sensitive load users such as chip manufacturers and precision machining processes.

[0003] Currently, voltage sag mitigation equipment, such as CN114825351B, is typically installed in distribution networks to suppress or compensate for voltage sags. However, adding dedicated mitigation equipment at the distribution network level faces the dual challenges of high investment costs and low equipment utilization, making it difficult to achieve cost-effective and large-scale deployment. With the large-scale promotion of electric vehicles, V2G technology has become an important means to achieve friendly interaction between vehicles and the network. V2G charging piles can not only manage battery charging and discharging but also have the potential to participate in grid voltage regulation. When grid voltage sags or fluctuations occur, it is feasible for V2G equipment to quickly provide dynamic support and significantly improve the voltage stability of the local grid.

[0004] Currently, current-compensated voltage sag mitigation is mainly achieved through QU droop control or virtual synchronous machine control. However, droop control has limited response speed, and its reactive power output during voltage drops is positively correlated with the voltage amplitude, resulting in insufficient support capacity at the most critical moments. Virtual synchronous machine control provides inertia and frequency / voltage regulation capabilities by simulating the electromechanical transients of a synchronous generator, but its control structure is complex, parameter tuning is difficult, and it faces transient power angle instability issues during grid voltage sags and other faults. V2G charging piles are directly connected to the end of the distribution network, and their onboard power batteries have short-term high-power discharge capabilities, making them naturally suitable as distributed energy sources for transient voltage support. Therefore, to achieve low-cost, high-reliability distribution network voltage sag mitigation, it is necessary to combine the characteristics of V2G's local deployment and ability to instantly provide active power surges, and research a voltage support method that balances speed and stability. Summary of the Invention

[0005] This invention provides a method and system for supporting voltage sags in V2G distribution networks based on virtual capacitors. By using a control algorithm, the AC / DC converter of the V2G is made equivalent to a parallel capacitor in terms of its external port characteristics, thereby providing the virtual inertia required for dynamic voltage support of the distribution network.

[0006] To achieve the above technical objectives, the present invention adopts the following technical solution: A method for supporting voltage sags in V2G distribution networks based on virtual capacitors is proposed. Based on the voltage and current characteristics during the process of capacitor-supported DC voltage, the AC / DC converter before the charging pile is equivalent to a virtual capacitor in a two-phase rotating coordinate system. When the voltage at the grid connection point of the V2G charging pile sags, the active current command of the virtual capacitor is generated by the derivative of the virtual capacitor equivalent to the AC / DC converter. This command is then superimposed with the basic active current command to obtain the d-axis active current command, which drives the AC / DC converter to extract energy from the battery of the V2G charging pile and inject it into the grid in the form of active power.

[0007] Furthermore, the d-axis basic active current command The voltage is generated by a PI controller after comparing the real-time voltage of the DC bus of the V2G charging pile with the reference value.

[0008] Furthermore, the virtual capacitor active current command With basic active current command Before overlaying, amplitude limiting is required. The upper limit of amplitude is based on Confirmed, among which This refers to the peak rated current of the AC / DC converter.

[0009] Furthermore, a virtual active current command is generated using the derivative of the virtual capacitor, expressed as: ; In the formula, This is a virtual active current command; This is the preset value for the virtual capacitor; The d-axis component of the grid connection point voltage is obtained by performing Park transformation on the real-time three-phase AC voltage at the grid connection point of the V2G charging pile. The d-axis component of the grid connection point voltage The time derivative.

[0010] Furthermore, the preset value of the virtual capacitor The setting is based on the rated capacity of the V2G charging pile, and the calculation formula is as follows: ; in, For voltage virtual inertia, The three-phase apparent power of the AC / DC converter. This refers to the peak value of the rated phase voltage of the AC / DC converter. This is the angular frequency of the power grid.

[0011] Furthermore, the d-axis active current command drives the AC / DC converter to extract energy from the battery of the V2G charging station and inject it into the grid as active power, specifically: Step a1: Send the d-axis active current command Substituting into the AC / DC decoupling mathematical model, the feedforward voltage command for voltage outer loop control based on virtual capacitor is obtained. : ; In the formula, , These are the filter inductance and equivalent resistance of the line between the AC / DC converter and the grid connection point, respectively. This is a virtual active current command; This is the d-axis basic active current command; This is the preset value for the virtual capacitor; The d-axis component of the grid connection point voltage; Step a2, send the d-axis active current command and q-axis active current command The current is fed into two inner-loop controllers respectively, and each controller will... With real-time active current After comparison, the initial d-axis voltage command is obtained through the PI controller. ,Will With real-time reactive current After comparison, the initial q-axis voltage command is obtained through the PI controller. Then, based on the initial voltage command, the dq cross-decoupling term, the grid voltage feedforward, and the feedforward voltage command of the virtual capacitor are superimposed to finally obtain the d-axis modulated voltage. and q-axis modulation voltage ; Step a3: Modulate the voltage along the dq axis using the phase angle of the mains voltage. , The inverse Park transformation is performed, and a PWM signal is generated through sinusoidal pulse width modulation to drive the AC / DC converter of the V2G charging pile.

[0012] Furthermore, step a2 is expressed as the following expression: ; In the formula, , These are the d-axis modulation voltage and the q-axis modulation voltage, respectively. , These are the proportional and integral coefficients of the d-axis current inner loop controller, respectively. , These are the proportional and integral coefficients of the q-axis current inner loop controller, respectively. The q-axis component of the grid connection point voltage is obtained by performing Park transformation on the real-time three-phase AC voltage at the grid connection point of the V2G charging pile. Let Laplace be the complex frequency variable. This is the angular frequency of the power grid.

[0013] Furthermore, the q-axis active current command It is generated based on voltage support requirements and by adopting a constant power control mode.

[0014] Furthermore, the differentiation operation is implemented using a bandwidth-limited differentiator, and the transfer function is... ,in The filtering time constant is Let be the Laplace complex frequency variable.

[0015] A V2G distribution network voltage sag support system based on virtual capacitors is proposed. The V2G charging pile includes a front-end AC / DC converter, a DC-side capacitor, and a rear-end DC / DC converter. The AC / DC converter consists of three half-bridges, with the DC side of each half-bridge connected in parallel across the DC-side capacitor, and the AC side connected to the grid's point of common coupling via filter impedances R and L. Under normal grid voltage conditions, the V2G charging pile absorbs charging power from the grid through the AC / DC converter. During grid voltage dips, V2G charging stations utilize the aforementioned virtual capacitor-based V2G distribution network voltage dip support method to deliver active power to the grid. .

[0016] Compared with the prior art, the beneficial effects of the present invention are:

[0017] 1. This invention directly utilizes the derivative of the d-axis component of the grid voltage to generate an active current command. When the voltage dips, the derivative term instantaneously outputs a positive command, driving the converter to draw energy from the battery and inject it into the grid as active power. The change in voltage drop across the line resistance or inductive reactance supports the grid voltage. This active power-supporting voltage mechanism is faster and more direct than traditional reactive power compensation.

[0018] 2. Ideal grid voltage in the dq coordinate system As a direct current, its time derivative is zero in steady state, and the virtual capacitor current command automatically returns to zero. Therefore, this invention does not generate any additional active circulating current during normal grid operation, is completely decoupled from the V2G charging plan, and only plays a role during grid fluctuations.

[0019] 3. This invention only requires adding a differential calculation unit, a current command superposition channel, and a voltage command superposition channel to the existing V2G charging pile system's dq control software. It does not increase any hardware costs and is a pure software upgrade that can be quickly deployed to existing charging pile controllers. Attached Figure Description

[0020] Figure 1 The V2G electrical structure diagram is shown; Figure 2 This invention presents the V2G equivalent electrical model based on virtual capacitance in the dq coordinate system. Figure 3 This diagram illustrates the control strategy block diagram of the distribution network voltage sag support method based on virtual capacitors according to the present invention, and its sub-... Figure 3 (a) Figure 3 (b) Figure 3 (c) Figure 3 (d) correspond to Park transformation, outer loop voltage control based on virtual capacitor, inner loop decoupling control of current, and modulation, respectively; Figure 4 The simulation comparison before and after the application of the method of the present invention is shown, and its sub- Figure 4 (a) Before application, sub Figure 4 (b) is after application. Detailed Implementation

[0021] The embodiments of the present invention will be described in detail below. These embodiments are based on the technical solutions of the present invention and provide detailed implementation methods and specific operation processes to further explain the technical solutions of the present invention.

[0022] This embodiment provides a V2G distribution network voltage sag support method based on virtual capacitance, applied to a V2G charging pile system. When the grid voltage sags, the AC / DC converter in the V2G charging pile is driven to extract energy from the battery and inject it into the grid as active power, utilizing the voltage drop across the line resistance or inductive reactance to support the grid voltage. The electrical connection of the V2G charging pile is also described.

[0023] like Figure 1 The electrical connection of the V2G charging pile shown is as follows: the V2G charging pile includes a front-end AC / DC converter, a DC-side capacitor, a rear-end DC / DC converter, and an electric vehicle. The AC / DC converter consists of three half-bridges, with the DC side of each half-bridge connected in parallel across the DC-side capacitor, and the AC side connected to the grid's common coupling point via filter impedances R and L. Under normal grid voltage conditions, the V2G charging pile absorbs charging power from the grid through the AC / DC converter. .

[0024] In a rotating coordinate system Figure 1 The electrical diagram is subjected to Parker transformation, and data models in a rotating coordinate system are established for the grid side, the V2G upstream AC / DC converter, and the load side, respectively: Web-side dq mathematical model: (1); in, These are the dq components of the grid voltage, These are the dq components of the grid current, respectively; For the inductive reactance of the grid-side line, The angular frequency of the power grid; The dq-axis component of the grid connection point voltage is obtained by performing Park transformation on the real-time three-phase AC voltage at the grid connection point of the V2G charging pile.

[0025] V2G front-end AC / DC converter dq mathematical model: (2); in, These are the dq-axis components of the V2G AC port voltage, respectively. These are the dq-axis components of the V2G AC port current, respectively. These are the filter inductance and equivalent resistance on the line between the AC / DC converter and the grid connection point, respectively.

[0026] The load-side dq mathematical model is: (3); in, This represents the dq-axis component of the AC port current of the AC / DC converter.

[0027] Since the essence of grid voltage sag is a drop in AC voltage amplitude, it is directly reflected as the d-axis component in the dq coordinate system. The reduction in voltage and current characteristics is achieved by treating the AC / DC converter of the V2G charging pile as a virtual capacitor in the d-axis equivalent circuit. This makes its voltage and current characteristics consistent with those of a DC capacitor. Through the differential characteristic of "voltage change rate - current", it automatically outputs active current at the moment of voltage sag, providing rapid inertia support for the power grid. The V2G equivalent electrical model based on the virtual capacitor in the dq coordinate system is as follows: Figure 2 As shown.

[0028] When the voltage at the V2G charging pile's grid connection point drops, a virtual capacitor active current command is generated by differentiating the equivalent virtual capacitor of the AC / DC converter. This command is then superimposed with the basic active current command to obtain the d-axis active current command, driving the AC / DC converter to extract energy from the V2G charging pile's battery and inject it into the grid as active power. Specifically, refer to... Figure 3 As shown, the voltage sag support method for distribution networks based on virtual capacitors in this embodiment includes the following steps:

[0029] Step 1: Real-time acquisition of three-phase AC voltage at the V2G charging pile grid connection point , , and three-phase alternating current , , The real-time phase angle θ of the grid voltage is obtained through a phase-locked loop.

[0030] Step 2, as follows Figure 3 As shown in (a), the Park transformation is performed on the collected three-phase voltage and current using the phase angle θ to obtain the dq-axis component of the grid-connected point voltage in the dq synchronous rotating coordinate system. , and the dq-axis components of the alternating current , .

[0031] Step 3: Based on the voltage support requirements, generate the basic reactive current command using constant power control mode. And as a q-axis reactive current command .

[0032] Step 4: Based on the DC bus voltage adjustment requirements, the V2G DC voltage reference value U dc,ref With sampled value U dc After comparison, the PI controller generates a basic active current command. ,like Figure 3 As shown in (b).

[0033] Step 5, as follows Figure 3 As shown in (b), a virtual capacitor control loop is constructed in the dq coordinate system to generate a virtual capacitor active current command. : (4); in, The preset virtual capacitance value, Let be the time derivative of the d-axis component of the grid connection point voltage. In equation (4), the differentiation operation is implemented through a bandwidth-limited differentiator, and the transfer function is... ,in This is the filtering time constant.

[0034] Among them, virtual capacitance value The setting is based on the rated capacity of the V2G charging pile, and the calculation formula is as follows: (5); in, For voltage virtual inertia, The three-phase apparent power of the AC / DC converter. This refers to the peak value of the rated phase voltage of the AC / DC converter. This is the angular frequency of the power grid.

[0035] Step 6, send the basic active current command With virtual capacitor active current command Superimposed, such as Figure 3 As shown in (b), the synthesized d-axis current command is obtained. In this embodiment, the virtual capacitor current command... With basic active current command Before superposition, the virtual capacitor current command is executed. Amplitude limiting is applied, with the upper limit of the amplitude based on... Confirmed, among which This refers to the peak rated current of the AC / DC converter.

[0036] Step 7, as follows Figure 3 As shown in (b), the d-axis current command is... Substituting the V2G front-end AC / DC decoupling mathematical model, the feedforward voltage command under virtual capacitor control is obtained. : (6); In equation (6), the differentiation operation is implemented using a bandwidth-limited differentiator, and the transfer function is: ,in This is the filtering time constant.

[0037] Step 8, as follows Figure 3 As shown in (c), and The current is fed into two separate inner-loop controllers. With real-time active current After comparison, the initial voltage command for the d-axis is obtained via a PI controller. , With real-time reactive current After comparison, the PI controller obtains the initial voltage command for the q-axis. Finally, the initial voltage command for the d-axis. Superimposed feedforward cross decoupling Grid voltage feedforward and virtual capacitor feedforward voltage command Then, generate the d-axis modulation voltage command. This allows the actual current to quickly track the command value; the initial voltage command for the q-axis. Superimposed grid voltage feedforward and cross-decoupling terms Generate q-axis modulation voltage command .

[0038] Specifically, such as Figure 3 As shown in (c), the current inner loop controller in this embodiment adopts a composite control structure including RL model feedforward, and its dq axis modulation voltage is: (7); Where L and R are the filter inductance and equivalent resistance on the AC side of the AC / DC converter. , These are the proportional and integral coefficients of the d-axis current inner loop controller, respectively. , These are the proportional and integral coefficients of the q-axis current inner loop controller, respectively. The feedforward term of the RL model is used... It can achieve fast and lag-free tracking of the output current to the virtual capacitor command.

[0039] Step 9, as follows Figure 3 As shown in (d), the voltage along the dq axis is modulated using the phase angle θ. , Perform inverse Park transform to obtain three-phase modulated voltage command , , Then divide by the DC voltage respectively. Normalization is performed, and a PWM signal is generated through sinusoidal pulse width modulation to drive the V2G charging pile's front-end AC / DC converter.

[0040] Figure 4 This is a simulation comparison before and after applying the method of the present invention. To verify the supporting effect of the present invention on the voltage sag of the distribution network, the following simulation conditions were set: a grid voltage sag with an amplitude of 50% and a duration of 10ms was applied at 5s, and the d-axis components of the V2G grid connection point voltage were recorded before and after applying the method of the present invention. The simulation results show that before applying the method of this invention, the V2G network connection port... The voltage drop reached 122V; after applying the method of this invention, the drop was reduced to 13V, and the voltage deviation met the requirement of GB / T 12325-2008 that the voltage deviation of three-phase power supply of 20kV and below should not exceed ±7% of the nominal voltage.

[0041] The above embodiments are preferred embodiments of this application. Those skilled in the art can make various changes or improvements based on them. Without departing from the overall concept of this application, these changes or improvements should fall within the scope of protection claimed in this application.

Claims

1. A method for supporting voltage sags in V2G distribution networks based on virtual capacitance, characterized in that, Based on the voltage and current characteristics during the process of capacitor-supported DC voltage, the AC / DC converter of the charging pile is equivalent to a virtual capacitor in a two-phase rotating coordinate system. When the voltage at the grid connection point of the V2G charging pile drops, the derivative of the virtual capacitor equivalent to the AC / DC converter is used to generate a virtual capacitor active current command, which is then superimposed with the d-axis basic active current command to obtain the d-axis active current command. This drives the AC / DC converter to extract energy from the battery of the V2G charging pile and inject it into the grid in the form of active power.

2. The method for supporting voltage sag in a V2G distribution network using virtual capacitors according to claim 1, characterized in that, The d-axis foundation active current command The voltage is generated by a PI controller after comparing the real-time voltage of the DC bus of the V2G charging pile with the reference value.

3. The method for supporting voltage sags in a V2G distribution network using virtual capacitors according to claim 1, characterized in that, Virtual capacitor active current command With basic active current command Before overlaying, amplitude limiting is required, and the upper limit of the amplitude limiting value is based on... Confirmed, among which This refers to the peak rated current of the AC / DC converter.

4. The method for supporting voltage sag in a V2G distribution network using virtual capacitors according to claim 1, characterized in that, The virtual active current command is generated by the derivative of the virtual capacitor, and is expressed as: ； In the formula, This is a virtual active current command; This is the preset value for the virtual capacitor; The d-axis component of the grid connection point voltage is obtained by performing Park transformation on the real-time three-phase AC voltage at the grid connection point of the V2G charging pile. The d-axis component of the grid connection point voltage The time derivative.

5. The method for supporting V2G distribution network voltage sag using virtual capacitors according to claim 4, characterized in that, Preset value of virtual capacitor The setting is based on the rated capacity of the V2G charging pile, and the calculation formula is as follows: ； in, For voltage virtual inertia, The three-phase apparent power of the AC / DC converter. This refers to the peak value of the rated phase voltage of the AC / DC converter. This is the angular frequency of the power grid.

6. The method for supporting voltage sag in a V2G distribution network using virtual capacitors according to claim 4, characterized in that, The d-axis active current command drives the AC / DC converter to extract energy from the battery of the V2G charging station and inject it into the grid in the form of active power. Specifically: Step a1: Send the d-axis active current command Substituting into the AC / DC decoupling mathematical model, the feedforward voltage command for voltage outer loop control based on virtual capacitors is obtained. : ； In the formula, , These are the filter inductance and equivalent resistance of the line between the AC / DC converter and the grid connection point, respectively. This is a virtual active current command; This is the d-axis basic active current command; This is the preset value for the virtual capacitor; The d-axis component of the grid connection point voltage; Step a2, send the d-axis active current command and q-axis active current command The current is fed into two inner-loop controllers respectively, and each controller will... With real-time active current After comparison, the initial d-axis voltage command is obtained through the PI controller. ,Will With real-time reactive current After comparison, the initial q-axis voltage command is obtained through the PI controller. Then, based on the initial voltage command, a cross-decoupling term, a grid voltage feedforward, and a virtual capacitor feedforward voltage command are superimposed to finally obtain the d-axis modulated voltage. and q-axis modulation voltage ; Step a3: Modulate the voltage along the dq axis using the phase angle of the mains voltage. , The inverse Park transformation is performed, and a PWM signal is generated through sinusoidal pulse width modulation to drive the AC / DC converter of the V2G charging pile.

7. The method for supporting voltage sag in a V2G distribution network using virtual capacitors according to claim 6, characterized in that, Step a2 is represented by the following expression: ； In the formula, , These are the d-axis modulation voltage and the q-axis modulation voltage, respectively. , These are the proportional and integral coefficients of the d-axis current inner loop controller, respectively. , These are the proportional and integral coefficients of the q-axis current inner loop controller, respectively. The q-axis component of the grid connection point voltage is obtained by performing Park transformation on the real-time three-phase AC voltage at the grid connection point of the V2G charging pile. Let Laplace be the complex frequency variable. This is the angular frequency of the power grid.

8. The method for supporting voltage sag in a V2G distribution network using virtual capacitors according to claim 6, characterized in that, The q-axis active current command It is generated based on voltage support requirements and by adopting a constant power control mode.

9. The method for supporting V2G distribution network voltage sag using virtual capacitors according to any one of claims 4-7, characterized in that, Differentiation is implemented using a bandwidth-limited differentiator, and the transfer function is: ,in The filtering time constant is Let be the Laplace complex frequency variable.

10. A V2G distribution network voltage sag support system based on virtual capacitance, characterized in that, V2G charging piles consist of a front-end AC / DC converter, a DC-side capacitor, and a rear-end DC / DC converter. The AC / DC converter comprises three half-bridges, with the DC side of each half-bridge connected in parallel across the DC-side capacitor, and the AC side connected to the grid's point of common coupling via filter impedances R and L. Under normal grid voltage conditions, the V2G charging pile absorbs charging power from the grid through the AC / DC converter. During grid voltage dips, V2G charging stations utilize the V2G distribution network voltage dip support method with virtual capacitors as described in any one of claims 1-9 to supply active power to the grid. .

Images