Multi-terminal flexible interconnection system for large-scale charging pile access and control method thereof

Abstract

According to the large-scale charging pile access-oriented multi-terminal flexible interconnection system and the control method thereof, when a direct current bus voltage is greater than a direct current voltage threshold value controlled by a virtual synchronous machine connected with a slave line and is smaller than a direct current bus voltage reference value, the virtual synchronous machine connected with the slave line adopts an active and reactive cooperative voltage regulation control strategy; otherwise, the virtual synchronous machine connected with the slave line adopts a direct-current voltage reactive power control strategy; the voltage at the tail end of the slave line is mapped into a direct-current voltage threshold value controlled by the virtual synchronous machine connected with the slave line; when the direct-current bus voltage is greater than the lower limit of the direct-current bus voltage reference value and less than the direct-current bus voltage reference value, the virtual synchronous machine connected with the main line adopts a direct-current voltage reactive power control strategy; on the contrary, the virtual synchronous machine connected with the main line adopts an alternating current feeder tail end voltage control strategy; and the main line tail end voltage is mapped to a direct current bus voltage reference value controlled by the virtual synchronous machine connected with the main line. And low and out-of-limit voltage at the tail end of the feeder line can be quickly avoided when the power distribution network has no communication.

Description

Technical Field

[0001] The present invention belongs to the technical field of flexible interconnection, and specifically relates to a multi-terminal flexible interconnection system for large-scale charging pile access and a control method thereof. Background Art

[0002] In recent years, for the radial residential distribution network, the access of electric vehicles (EVs) will affect the voltage distribution. Especially during the peak electricity consumption period, the distribution network voltage may be significantly reduced or fluctuate beyond the allowable range.

[0003] Regarding the power quality problems caused by the large-scale access of electric vehicles to the grid during peak hours, the existing technologies mainly include optimizing the distribution network architecture, installing reactive power compensation devices, implementing demand-side management, accessing distributed power sources, applying energy storage systems, etc. Although optimizing the distribution network architecture can improve the overall power supply capacity, it often requires large-scale infrastructure investment and a long construction period. Installing reactive power compensation devices can improve the local voltage, but the adjustment range and effect are limited. Demand-side management disperses the load by guiding the charging behavior of users, but its effect depends on the reaction of users and the implementation of the strategy. The access of distributed power sources can provide additional power support, but there are intermittency and uncertainty. The application of an energy storage system (ESS) can stabilize the voltage to a certain extent, but the cost is high and the capacity is limited.

[0004] To solve the low voltage problem of the distribution feeder, a low-voltage flexible interconnected distribution network with a DC link is formed by a voltage source converter (VSC) and a DC line to achieve power and voltage regulation in a wider area, and it also has the advantages of large transmission capacity and low line loss. One solution is to use a flexible interconnection device to optimize the distribution of the power flow of the distribution network according to the node voltage information of the entire low-voltage distribution system, so as to improve the voltage curve of the distribution feeder under the wide access of charging piles. However, with the existing communication conditions of the low-voltage distribution network, in the face of the randomness and rapidity of the end voltage exceeding the limit, the distribution network cannot regulate the local voltage in real time and safely. Another solution is to use the DC bus voltage of the flexible interconnection device to transmit the AC voltage information of each feeder, and then realize the unified coordinated control between multiple regional feeders in the distribution network. However, this solution needs to keep the end voltages of each feeder consistent. When the voltage does not exceed the limit, the feeders will also continuously exchange power through the VSC and the DC link, resulting in high operating losses and lack of economy. Summary of the Invention

[0005] To solve the deficiencies existing in the prior art, the present invention provides a multi-terminal flexible interconnection system for large-scale charging pile access and its control method. A flexible interconnection device is used to realize the interconnection of the ends of different distribution network feeders, and the active power and reactive power between the interconnected feeders can be flexibly adjusted quickly when there is no communication in the distribution network, which not only solves the technical problems of low voltage and over-limit at the end of the distribution network feeder, but also reduces the operation loss.

[0006] The present invention adopts the following technical solutions.

[0007] The present invention proposes a multi-terminal flexible interconnection system for large-scale charging pile access, with the AC feeder with the largest transmission power in the distribution network as the main line, and the other AC feeders as the slave lines. The voltage of the grid-connected bus connected by the AC feeder is used as the end voltage of the AC feeder. All grid-connected buses are connected to intelligent soft-switching devices. The intelligent soft-switching device includes a plurality of virtual synchronous machines. The number of virtual synchronous machines is the same as the number of AC feeders. The AC side of the virtual synchronous machine is connected to the grid-connected bus, and the DC sides of all virtual synchronous machines are connected to a common DC bus; When the DC bus voltage is greater than the DC voltage threshold controlled by the virtual synchronous machine connected to the slave line and less than the DC bus voltage reference value, the virtual synchronous machine connected to the slave line adopts a control strategy of coordinated active and reactive power voltage regulation; when the DC bus voltage is not less than the DC bus voltage reference value or the DC bus voltage is not greater than the DC voltage threshold controlled by the virtual synchronous machine connected to the slave line, the virtual synchronous machine connected to the slave line adopts a DC voltage reactive power control strategy; wherein, based on the relationship between the end voltage of the slave line and the DC bus voltage, the end voltage of the slave line is mapped to the DC voltage threshold controlled by the virtual synchronous machine connected to the slave line; When the DC bus voltage is greater than the lower limit of the DC bus voltage reference value and less than the DC bus voltage reference value, the virtual synchronous machine connected to the main line adopts a DC voltage reactive power control strategy; when the DC bus voltage is not less than the DC bus voltage reference value or the DC bus voltage is not greater than the lower limit of the DC bus voltage reference value, the virtual synchronous machine connected to the main line adopts an AC feeder end voltage control strategy; wherein, based on the relationship between the end voltage of the main line and the DC bus voltage, the end voltage of the main line is mapped to the DC bus voltage reference value controlled by the virtual synchronous machine connected to the main line.

[0008] The control system structures of the virtual synchronous machines connected to each slave line are the same, including: a first voltage outer loop module, a first current reference value module, a first current inner loop module, and a first coordinate conversion module; Among them, the first voltage outer loop module uses the difference between the line end voltage and the line end voltage threshold, and the difference between the DC bus voltage and the DC voltage threshold controlled by the virtual synchronous machine connected to the line as input signals; when adopting the active and reactive coordinated voltage regulation control strategy, the first voltage outer loop module sends a first signal to the first current reference value module, and when adopting the DC voltage reactive power control strategy, the first voltage outer loop module sends a second signal to the first current reference value module; The first current reference value module is used to generate the reference value of the d-axis component of the current according to the signal sent by the first voltage outer loop module; The first current inner loop module uses the difference between the reference value of the d-axis component of the current and the d-axis component of the current, and the difference between the reference value of the q-axis component of the current and the q-axis component of the current as input signals to generate the reference value of the d-axis component of the voltage and the reference value of the q-axis component of the voltage of the virtual synchronous machine; moreover, the d-axis component of the voltage and the q-axis component of the voltage of the virtual synchronous machine are introduced to correct the reference value of the d-axis component of the voltage and the reference value of the q-axis component respectively; Among them, the reference value of the q-axis component of the current satisfies the following relational expression:

[0009] In the formula, is the reference value of the q-axis component of the current, , are the reactance and resistance of the line, is the reference value of the d-axis component of the current; The corrected reference values of the d-axis component of the voltage and the q-axis component of the voltage pass through the first coordinate conversion module to obtain the three-phase voltage reference value of the virtual synchronous machine.

[0010] When the line end voltage does not exceed the limit, the line end voltage threshold is the line end voltage reference value; when the line end voltage exceeds the limit, the line end voltage threshold is the lower limit of the line end voltage.

[0011] The first voltage outer loop module includes: 6 switches, the first voltage loop, the second voltage loop, the first feedback unit, and the second feedback unit; Among them, one end of the first switch inputs the line end voltage, the other end of the first switch is connected to the first input end of the first voltage loop, the other end of the second switch is connected to the second input end of the first voltage loop, the output end of the first voltage loop is connected to the positive input end of the first feedback unit, the output end of the first feedback unit is connected to one end of the second switch, the output end of the first voltage loop is also connected to one end of the third switch, and the other end of the third switch is connected to the first input end of the first current reference value generation module and the negative input end of the second feedback unit; One end of the fifth switch inputs the DC bus voltage. The other end of the fifth switch is connected to the first input end of the second voltage loop. The other end of the fourth switch is connected to the second input end of the second voltage loop. The output end of the second voltage loop is connected to the positive input end of the second feedback unit. The output end of the second feedback unit is connected to one end of the fourth switch. The output end of the second voltage loop is also connected to one end of the sixth switch. The other end of the sixth switch is connected to the second input end of the first current reference value generation module and the negative input end of the first feedback unit; When adopting the control strategy of coordinated active and reactive voltage regulation, perform the operation of closing the first switch, the third switch, and the fourth switch and opening the second switch, the fifth switch, and the sixth switch; when adopting the DC voltage reactive power control strategy, perform the operation of closing the second switch, the fifth switch, and the sixth switch and opening the first switch, the third switch, and the fourth switch; Both the first voltage loop and the second voltage loop include PI adjustment units; the difference between the line end voltage and the line end voltage threshold value is input to the first voltage loop, and the first voltage loop outputs a first signal; the difference between the DC bus voltage and the DC voltage threshold value controlled by the virtual synchronous machine connected to the line is input to the second voltage loop, and the second voltage loop outputs a second signal; The first feedback unit feeds back the difference between the first signal and the second signal to the first voltage loop; the second feedback unit feeds back the difference between the second signal and the first signal to the second voltage loop.

[0012] The control system of the virtual synchronous machine connected to the main line includes: a second voltage outer loop module, a second current reference value module, a second current inner loop module, and a second coordinate transformation module; Among them, the second voltage outer loop module is used to take the difference between the main line end voltage and the main line end voltage reference value, and the difference between the DC bus voltage and the DC bus voltage reference value controlled by the virtual synchronous machine connected to the main line as input signals; when adopting the DC voltage reactive power control strategy, the second voltage outer loop module sends a third signal to the second current reference value module, and when adopting the AC feeder end voltage control strategy, the second voltage outer loop module sends a fourth signal to the second current reference value module; The second current reference value module is used to generate the current d-axis component reference value according to the signal sent by the second voltage outer loop module; The second current inner loop module is used to take the difference between the current d-axis component reference value and the current d-axis component, and the difference between the current q-axis component reference value and the current q-axis component as input signals to generate the voltage d-axis component reference value and the voltage q-axis component reference value of the virtual synchronous machine, where the current q-axis component reference value is 0; and, introduce the voltage d-axis component and the voltage q-axis component of the virtual synchronous machine to correct the voltage d-axis component reference value and the q-axis component reference value respectively; The corrected d-axis component reference value and q-axis component reference value of the voltage are input into a second coordinate conversion module to obtain the three-phase voltage reference values of the virtual synchronous machine.

[0013] The voltage threshold at the end of the main line is the voltage reference value at the end of the main line; when the voltage at the end of the main line reaches the minimum value, the DC bus voltage reference value of the virtual synchronous machine connected to the main line is the third DC voltage threshold, and when the voltage at the end of the main line reaches the reference value, the DC bus voltage reference value of the virtual synchronous machine connected to the main line is the DC bus voltage reference value.

[0014] The second voltage outer loop module includes: six switches, a third voltage loop, a fourth voltage loop, a third feedback unit, and a fourth feedback unit; Among them, one end of the seventh switch inputs the DC bus voltage, the other end of the seventh switch is connected to the first input end of the third voltage loop, the other end of the eighth switch is connected to the second input end of the third voltage loop, the output end of the third voltage loop is connected to the positive input end of the third feedback unit, the output end of the third feedback unit is connected to one end of the eighth switch, the output end of the third voltage loop is also connected to one end of the ninth switch, and the other end of the ninth switch is connected to the first input end of the second current reference value generation module and the negative input end of the fourth feedback unit; One end of the eleventh switch inputs the voltage at the end of the main line, the other end of the eleventh switch is connected to the first input end of the fourth voltage loop, the other end of the tenth switch is connected to the second input end of the fourth voltage loop, the output end of the fourth voltage loop is connected to the positive input end of the fourth feedback unit, the output end of the fourth feedback unit is connected to one end of the tenth switch, the output end of the fourth voltage loop is also connected to one end of the twelfth switch, and the other end of the twelfth switch is connected to the second input end of the second current reference value generation module and the negative input end of the third feedback unit; When the DC voltage reactive power control strategy is adopted, the seventh switch, the ninth switch, and the tenth switch are closed and the eighth switch, the eleventh switch, and the twelfth switch are opened; when the AC feeder end voltage control strategy is adopted, the eighth switch, the eleventh switch, and the twelfth switch are closed and the seventh switch, the ninth switch, and the tenth switch are opened; Both the third voltage loop and the fourth voltage loop include PI adjustment units; the difference between the voltage at the end of the main line and the voltage reference value at the end of the main line is input into the third voltage loop, and the third voltage loop outputs a third signal; the difference between the DC bus voltage and the DC bus voltage reference value controlled by the virtual synchronous machine connected to the main line is input into the fourth voltage loop, and the fourth voltage loop outputs a fourth signal; The third feedback unit feeds back the difference between the third signal and the fourth signal to the third voltage loop; the fourth feedback unit feeds back the difference between the fourth signal and the third signal to the fourth voltage loop.

[0015] The voltage at the end of the main line is mapped to the DC bus voltage reference value controlled by the virtual synchronous machine connected to the main line, satisfying the following relationship:

[0016] In the formula, is the DC bus voltage reference value controlled by the virtual synchronous machine connected to the main line, and are the mapping coefficient and mapping error of the main line, is the voltage at the end of the main line; Among them, the lower limit of the DC bus voltage reference value controlled by the virtual synchronous machine connected to the main line corresponds to the minimum value of the voltage at the end of the main line; The voltage at the end of the slave line is mapped to the DC voltage threshold controlled by the virtual synchronous machine connected to the slave line, and the following relational expression is satisfied:

[0017] In the formula, is the DC voltage threshold controlled by the virtual synchronous machine connected to the slave line, is the voltage at the end of the slave line, and are the mapping coefficient and mapping error of the slave line.

[0018] The present invention also proposes a control method for a multi-terminal flexible interconnected system for large-scale charging pile access, including: Collect the voltages of each grid-connected bus, the transmission power of each AC feeder, and the charging pile loads connected to the grid-connected bus; When the voltages of each grid-connected bus are all not less than 0.9 p.u., and the transmission power of each AC feeder is all not less than the corresponding charging pile load, the virtual synchronous machine connected to the slave line does not work, and the virtual synchronous machine connected to the main line adopts a DC voltage reactive power control strategy; When the voltage of at least one grid-connected bus connected to the slave line is less than 0.9 p.u. and the voltage of the grid-connected bus connected to the main line is not less than 0.9 p.u., the transmission power of at least one slave line is less than the corresponding charging pile load and the transmission power of the main line is not less than the corresponding charging pile load, the virtual synchronous machine connected to the slave line adopts an active and reactive power coordinated voltage regulation control strategy, and the virtual synchronous machine connected to the main line adopts a DC voltage reactive power control strategy; when the active power regulated by the virtual synchronous machine connected to the slave line reaches the set power threshold, the virtual synchronous machine connected to the slave line does not work, and the virtual synchronous machine connected to the main line adopts a DC voltage reactive power control strategy; When the voltage of the grid-connected bus connected to at least one slave line is less than 0.9pu and the voltage of the grid-connected bus connected to the main line is less than 0.9pu, the transmission power of at least one slave line is less than the corresponding charging pile load and the transmission power of the main line is less than the corresponding charging pile load, the virtual synchronous machine connected to the main line switches from the DC voltage and reactive power control strategy to the AC feeder end voltage control strategy, and the virtual synchronous machine connected to the main line controls the DC bus voltage to be the lower limit of the DC bus voltage reference value; for the slave line whose DC voltage threshold obtained by the end voltage mapping is less than the lower limit of the DC bus voltage reference value, the connected virtual synchronous machine is switched from the control strategy of active and reactive coordinated voltage regulation to the DC voltage and reactive power control strategy, and the virtual synchronous machines connected to the remaining slave lines adopt the control strategy of active and reactive coordinated voltage regulation.

[0019] For multiple slave lines whose connected virtual synchronous machines all adopt the DC voltage reactive power control strategy, if the DC voltage thresholds obtained by the terminal voltage mapping are all less than the lower limit of the DC bus voltage reference value, then the slave line corresponding to the maximum value among the DC voltage thresholds is The corresponding charging pile load switches from disordered charging mode to orderly charging mode.

[0020] When the line After the corresponding charging pile load switches from the disordered charging mode to the ordered charging mode, the control method further includes: When the line When the terminal voltage of the slave line is not less than 0.9pu, the DC voltage reactive power control strategy is adopted; when the terminal voltage of the other slave lines is not less than 0.9pu, the corresponding virtual synchronous machine adopts the active and reactive coordinated voltage regulation control strategy; the virtual synchronous machine connected to the main line adopts the AC feeder terminal voltage control strategy; When the line The terminal voltage reaches 0.9pu, from the line The connected virtual synchronous machine switches from the DC voltage reactive power control strategy to the active and reactive coordinated voltage regulation control strategy; the DC voltage threshold obtained by the terminal voltage mapping is greater than the DC bus voltage of the slave line The connected virtual synchronous machine adopts DC voltage reactive power control strategy; at this time, if the line The terminal voltage reaches 0.9pu, from the line The corresponding charging pile load switches from disordered charging mode to ordered charging mode; if the line The terminal voltage is greater than 0.9pu, from the line The connected virtual synchronous machine adopts a control strategy of coordinated active and reactive voltage regulation; when the DC bus voltage is not less than the lower limit of the DC bus voltage reference value, the virtual synchronous machine connected to the main line switches from the AC feeder end voltage control strategy to the DC voltage reactive power control strategy; when the active power regulated by the virtual synchronous machine connected to the slave line reaches the set power threshold, the virtual synchronous machine connected to the slave line stops working, and the virtual synchronous machine connected to the main line adopts the DC voltage reactive power control strategy.

[0021] The present invention is also a terminal, including a processor and a storage medium; the storage medium is used to store instructions; the processor is used to operate according to the instructions to execute the steps of the method.

[0022] The present invention is also a computer-readable storage medium, on which a computer program is stored, and when the program is executed by a processor, the steps of the method are implemented.

[0023] The beneficial effects of the present invention are at least as follows compared with the prior art. The present invention uses a flexible interconnection device to realize the interconnection of the ends of different distribution network feeders. Based on the DC voltage mapping method, the low-voltage distribution network can still accurately obtain the end voltages of each AC feeder without communication, so that the low-voltage distribution network can realize "AC-DC" collaborative voltage over-limit governance in a communication-free scenario. Since the virtual synchronizers installed at the ends of each feeder can sense the DC bus voltage, by setting reasonable action thresholds for the DC voltage, unnecessary power interaction between multiple distribution lines is avoided, the operation loss is significantly reduced, and the system's acceptance capacity for electric vehicle charging piles is improved. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 It is a schematic diagram of a typical radial low-voltage distribution system with EVS in the embodiment of the present invention; Figure 2 It is a schematic diagram of the control system of the virtual synchronous machine connected to the slave line in the embodiment of the present invention; Figure 3 It is a schematic diagram of the control system of the virtual synchronous machine connected to the main line in the embodiment of the present invention; Figure 4 It is a relationship diagram between the end voltage and the DC bus voltage in the embodiment of the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0025] To make the objectives, technical solutions, and advantages of the present invention clearer, the technical solutions of the present invention will be clearly and completely described below in conjunction with the accompanying drawings in the embodiments of the present invention. The embodiments described in this application are only a part of the embodiments of the present invention, rather than all the embodiments. Based on the spirit of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts fall within the protection scope of the present invention.

[0026] Currently, due to the problem of low voltage at the end of the distribution network feeder or even voltage violation caused by the large-scale access of charging piles, the existing technology uses the DC bus voltage in the flexible interconnection device to transmit the AC voltage information of each feeder, and then realizes the unified coordinated control among the multi-region feeders in the distribution network. However, the existing technology needs to keep the voltage at the end of each feeder consistent. When the voltage does not exceed the limit, the feeders will also continuously exchange power through the virtual synchronous machine and the DC link, resulting in high operating losses.

[0027] In view of the above problems, the present invention proposes a multi-terminal flexible interconnection system for large-scale charging pile access. As Figure 1 shown, the distribution network includes: multiple AC feeders, each AC feeder is connected to a grid-connected bus, and each grid-connected bus is connected to an AC load and a charging pile; among them, the AC feeder with the largest transmitted power among all AC feeders is the main line, and the remaining AC feeders are the slave lines; Figure 1 In , , ……, , after being stepped down by a 10kV / 380V distribution transformer, they are respectively connected to the AC grid-connected buses , through the AC feeder 1, AC feeder 2, ……, AC feeder , , ……, , , where , , ……, , are the impedances of each AC feeder, and the transmitted powers of each AC feeder are respectively , , ……, , , and the voltages of each grid-connected bus are respectively , , ……, , , and the grid-connected bus voltage is used as the AC feeder end voltage. Each grid-connected bus is respectively connected to the AC load , , ……, , , and each grid-connected bus is respectively connected to the charging pile load , , ……, , ; the AC feeder with the largest transmitted power among all AC feeders is the main line, and the AC feeder 1, AC feeder 2, ……, AC feeder For the slave line; the virtual synchronous machine connected to the main line (AC feeder ) The output power is ; for the slave lines (AC feeder 1, AC feeder 2,..., AC feeder ) connecting the virtual synchronous machines , ,..., The output powers are respectively , ,..., .

[0028] All grid-connected busbars are connected to the intelligent soft-switching device; among them, the intelligent soft-switching device includes multiple virtual synchronous machines, the number of virtual synchronous machines is the same as the number of AC feeders, the AC side of each virtual synchronous machine is connected to a grid-connected busbar, and the DC sides of all virtual synchronous machines are connected to the common DC busbar; Figure 1 Among them, the common DC busbar includes a DC positive busbar and a DC negative busbar, and the DC positive busbar and the DC negative busbar are connected by a DC capacitor; the DC busbar voltage is ; Among them, the virtual synchronous machine connected to the main line adopts a DC voltage reactive power control strategy or an AC feeder end voltage control strategy, and the virtual synchronous machine connected to the slave line adopts an active and reactive power coordinated voltage regulation control strategy or a DC voltage reactive power control strategy.

[0029] The multi-terminal flexible interconnection system proposed by the present invention realizes the interconnection of multiple AC feeders through an intelligent soft-switching device (soft open point, SOP). The SOP is formed by connecting multiple virtual synchronous machines through an internal common DC busbar (DC capacitor), so as to form a single multi-port back-to-back structure SOP to increase the power flow regulation channel. The SOP controls the active power transmitted between different AC feeders and provides a certain amount of reactive power support to realize the spatial transfer of active power flow between different feeders, achieving the effect of flexible regulation of the power flow in the distribution system. In addition, based on the SOP, reactive power compensation is further realized on the basis of active power regulation, thereby improving the accommodation capacity of the distribution network for EVS.

[0030] The control system structures of the virtual synchronous machines connected to each slave line , ,..., are the same. Taking the AC feeder ( = 1, 2) as an example, as Figure 2 shown, the control system of the virtual synchronous machine connected to the AC feeder includes: a first voltage outer loop module, a first current reference value module, a first current inner loop module, and a first coordinate transformation module; The first voltage outer loop module is used to use the AC feeder terminal voltage and the AC feeder terminal voltage threshold The difference between , the DC bus voltage and the DC voltage threshold controlled by the virtual synchronous machine connected to the line from The difference between is used as the input signal; when the DC bus voltage is less than the DC bus voltage reference value and greater than the DC voltage threshold controlled by the virtual synchronous machine connected to the line from , the virtual synchronous machine adopts a control strategy of coordinated active and reactive voltage regulation. At this time, the first voltage outer loop module sends a first signal to the first current reference value module ; when the DC bus voltage is not less than the DC bus voltage reference value or the DC bus voltage is not greater than the DC voltage threshold controlled by the virtual synchronous machine connected to the line from , the virtual synchronous machine adopts a DC voltage reactive power control strategy. At this time, the first voltage outer loop module sends a second signal to the first current reference value module ; Figure 2 Among them, the AC feeder terminal voltage threshold indicates that the virtual synchronous machine selects a value from the AC feeder terminal voltage reference value and the AC feeder terminal voltage lower limit according to whether the terminal voltage of the AC feeder is out of limit; when the terminal voltage of the AC feeder is not out of limit, the AC feeder terminal voltage threshold is the AC feeder terminal voltage reference value ; when the terminal voltage of the AC feeder is out of limit, then the AC feeder terminal voltage threshold is the AC feeder terminal voltage lower limit ; each virtual synchronous machine connected to the line from selects different terminal voltage thresholds according to whether the terminal voltage of the AC feeder is out of limit to meet different control strategies under non-out-of-limit and out-of-limit conditions; Based on the relationship between the terminal voltage of the line from and the DC bus voltage, the DC voltage threshold controlled by the virtual synchronous machine connected to the line from Obtained by mapping the voltage at the end of the line, Figure 2 In, when The DC voltage threshold Is the DC voltage threshold obtained by mapping the voltage at the end of AC feeder 1. When The DC voltage threshold Is the DC voltage threshold obtained by mapping the voltage at the end of AC feeder 2. Since there are differences in the over-limit conditions of the voltages at the ends of each slave line, the DC voltage thresholds controlled by the virtual synchronous machines connected to different slave lines are related to the voltages at the ends of the slave lines. The voltage regulation control strategy is more in line with the actual situation of the voltages at the ends of the slave lines, and the voltage regulation control strategy is adjusted in a timely manner for the working conditions where the voltages at the ends of the slave lines change from not over-limited to over-limited, forming a flexible and variable voltage regulation mode.

[0031] Specifically, the first voltage outer loop module includes: 6 switches , , , , , , the first voltage loop, the second voltage loop, the first feedback unit, and the second feedback unit; Among them, one end of the first switch Inputs the voltage at the end of the AC feeder , and the other end of the first switch Is connected to the first input end of the first voltage loop. The other end of the second switch Is connected to the second input end of the first voltage loop. The output end of the first voltage loop is connected to the positive input end of the first feedback unit. The output end of the first feedback unit is connected to one end of the second switch . The output end of the first voltage loop is also connected to one end of the third switch . The other end of the third switch Is connected to the first input end of the first current reference value generation module and the negative input end of the second feedback unit; One end of the fifth switch Inputs the DC bus voltage , and the other end of the fifth switch Is connected to the first input end of the second voltage loop. The other end of the fourth switch Is connected to the second input end of the second voltage loop. The output end of the second voltage loop is connected to the positive input end of the second feedback unit. The output end of the second feedback unit is connected to one end of the fourth switch . The output end of the second voltage loop is also connected to one end of the sixth switch . The other end of the sixth switch Is connected to the second input end of the first current reference value generation module and the negative input end of the first feedback unit; The first voltage loop includes a PI adjustment unit, and the AC feeder Terminal voltage The difference between the terminal voltage and the AC feeder Terminal voltage threshold Is the input signal of the first voltage loop, and the first signal is the output signal of the first voltage loop; the second voltage loop includes a PI adjustment unit, and the difference between the DC bus voltage and the DC voltage threshold controlled by the virtual synchronous machine connected to the line is the input signal of the second voltage loop, and the second signal is the output signal of the second voltage loop; The first feedback unit feeds back the difference between the first signal and the second signal to the first voltage loop; the second feedback unit feeds back the difference between the second signal and the first signal to the second voltage loop; When the virtual synchronous machine adopts the active and reactive coordinated voltage regulation control strategy, execute Virtual synchronous machine The operations of closing and , , Closing and , , Disconnecting operations, at this time the first voltage outer loop module outputs the first signal ; Virtual synchronous machine When adopting the DC voltage reactive power control strategy, execute , , Closing and , , Disconnecting operations; at this time the first voltage outer loop module outputs the second signal ; The first current reference value generation module is used to generate the reference value of the d-axis component of the current of the virtual synchronous machine according to the signal sent by the first voltage outer loop module; Figure 2 Among them, when satisfying , use the first signal As the reference value of the d-axis component of the current of the virtual synchronous machine connected to the AC feeder , when satisfying Or Or , use the second signal As the reference value of the d-axis component of the current of the virtual synchronous machine connected to the AC feeder ; The current reference value module proposed in the present invention sets an algorithm for judging and selecting the reference value of the d-axis component of the current according to the control strategy of the virtual synchronous machine, which can control the terminal voltage of the AC feeder not to exceed the limit while meeting the requirements of DC voltage stability under different control strategies; ; The first current inner loop module is used to use the reference value of the d-axis component of the current and the d-axis component of the current The difference between , Current q-axis component reference value and the q-axis component of the current The difference between As an input signal, a voltage d-axis component reference value and a voltage q-axis component reference value of the virtual synchronous machine are generated; and the voltage d-axis component and the voltage q-axis component of the virtual synchronous machine are introduced to respectively correct the voltage d-axis component reference value and the voltage q-axis component reference value of the virtual synchronous machine; The current d-axis component reference value is processed by phase difference angle offset to obtain the current q-axis component reference value ; The reference value of the current q-axis component satisfies the following relationship:

[0032] Where, is the reference value of the current q-axis component, 、 is the reactance and resistance of the slave line, is the reference value of the current d-axis component; The corrected voltage d-axis component reference value and q-axis component reference value are converted by the first coordinate conversion module Get the three-phase voltage reference value of the virtual synchronous machine 、 、 .

[0033] AC feeder Connected virtual sync machine The control system structure, such as Figure 3 As shown, it includes: a second voltage outer loop module, a second current reference value module, a second current inner loop module, and a second coordinate conversion module; The second voltage outer loop module is used to use the voltage at the end of the main line and the main line end voltage reference value The difference between the DC bus voltage DC bus voltage reference value for controlling the virtual synchronous machine connected to the main line The difference between them is the input signal. When the DC bus voltage Less than the DC bus voltage reference value And greater than the lower limit of the DC bus voltage reference value When the virtual synchronous machine connected to the main line adopts the DC voltage reactive control strategy, the second voltage outer loop module sends a third signal to the second current reference value module. Not greater than the DC bus voltage reference value Or DC bus voltage Not less than the lower limit of the DC bus voltage reference value When this is the case, the virtual synchronous machine connected to the main line adopts the AC feeder end voltage control strategy, and at this time, the second voltage outer loop module sends the fourth signal to the second current reference value module; The relationship between the voltage at the end of the main line and the DC bus voltage is as Figure 4 shown Figure 4 In it, the lower limit of the DC bus voltage reference value corresponds to the minimum value of the voltage at the end of the main line ; the DC bus voltage reference value corresponds to the reference value of the voltage at the end of the main line ; Therefore, based on the relationship between the voltage at the end of the main line and the DC bus voltage, the voltage at the end of the main line is mapped to the DC bus voltage reference value controlled by the virtual synchronous machine connected to the main line, satisfying the following relationship:

[0034] In the formula, is the DC bus voltage reference value controlled by the virtual synchronous machine connected to the main line, is the voltage at the end of the main line, and are the mapping coefficient and mapping error of the main line; among them, the lower limit of the DC bus voltage reference value controlled by the virtual synchronous machine connected to the main line corresponds to the minimum value of the voltage at the end of the main line; According to this article = 1.00 (p.u.), when = 1.00 (p.u.); when the feeder y is below the lower limit = 0.90 (p.u.), the DC bus voltage reference value = 0.98 (p.u.); according to the calculated proportional relationship = 0.2, = 0.8.

[0035] The second voltage outer loop module proposed by the present invention not only realizes the switching of the control strategy, but also based on the relationship between the voltage at the end of the main line and the DC bus voltage, maps the voltage at the end of the main line to the DC bus voltage reference value controlled by the virtual synchronous machine connected to the main line, and the virtual synchronous machine connected to the main line adjusts the DC bus voltage so that is equal to .

[0036] The relationship between the voltage at the end of the slave line and the DC bus voltage also satisfies the curve shown in Figure 4 , maps the voltage at the end of the slave line to the DC voltage threshold controlled by the virtual synchronous machine connected to the slave line, and satisfies the following relationship:

[0037] In the formula, is the DC voltage threshold controlled by the virtual synchronous machine connected to the slave line, is the voltage at the end of the slave line, and are the mapping coefficient and mapping error of the slave line. In the embodiment, = 0.2, = 0.8 The first voltage outer loop module proposed by the present invention not only realizes the switching of the control strategy, but also, based on the relationship between the voltage at the end of the slave line and the DC bus voltage, maps the voltage at the end of the slave line to the DC voltage threshold controlled by the virtual synchronous machine connected to the slave line, and the virtual synchronous machine connected to the slave line adjusts the DC bus voltage so that is equal to .

[0038] Therefore, in the scenario of no communication in the low-voltage distribution network, when the virtual synchronous machine connected to the main line controls the DC bus voltage to meet the reference value, the virtual synchronous machine connected to the slave line only needs to sense the DC bus voltage to determine the voltage at the end of the main line, and the voltage at the end of the main line represents the remaining capacity of the main line to absorb the excess power of the slave line, which helps the virtual synchronous machines connected to each slave line to adopt reasonable control strategies and use the main line to absorb the excess power of the slave line to control the over-limit of the voltage at the end of the slave line. On the contrary, when the virtual synchronous machine connected to the slave line controls the DC bus voltage to meet the threshold, the virtual synchronous machine connected to the main line senses whether the voltage at the end of the slave line is over-limit according to the DC bus voltage, which helps the virtual synchronous machine connected to the main line to adopt reasonable control strategies and realize the optimal distribution of transmission power.

[0039] The second voltage outer loop module includes: six switches , , , , , , the third voltage loop, the fourth voltage loop, the third feedback unit, and the fourth feedback unit; Among them, one end of the seventh switch inputs the DC bus voltage , the other end of the seventh switch is connected to the first input end of the third voltage loop, the other end of the eighth switch is connected to the second input end of the third voltage loop, the output end of the third voltage loop is connected to the positive input end of the third feedback unit, the output end of the third feedback unit is connected to one end of the eighth switch , the output end of the third voltage loop is also connected to one end of the ninth switch , and one end of the ninth switch The other end is connected to the first input terminal of the second current reference value generation module and the negative input terminal of the fourth feedback unit; The eleventh switch One end of which inputs an AC feeder Terminal voltage , the eleventh switch The other end is connected to the first input terminal of the fourth voltage loop, and the tenth switch The other end is connected to the second input terminal of the fourth voltage loop, the output terminal of the fourth voltage loop is connected to the positive input terminal of the fourth feedback unit, and the output terminal of the fourth feedback unit is connected to the tenth switch One end, the output terminal of the fourth voltage loop is also connected to the twelfth switch One end, the twelfth switch The other end is connected to the second input terminal of the second current reference value generation module and the negative input terminal of the third feedback unit; Both the third voltage loop and the fourth voltage loop include PI adjustment units; the difference between the main line terminal voltage and the main line terminal voltage reference value is input to the third voltage loop, and the third voltage loop outputs a third signal; the difference between the DC bus voltage and the DC bus voltage reference value controlled by the virtual synchronous machine connected to the main line is input to the fourth voltage loop, and the fourth voltage loop outputs a fourth signal; The third feedback unit feeds back the difference between the third signal and the fourth signal to the third voltage loop; the fourth feedback unit feeds back the difference between the fourth signal and the third signal to the fourth voltage loop; Virtual synchronous machine When adopting the DC voltage reactive power control strategy, execute , , Closed and , , Open the operation, and at this time the voltage outer loop module outputs the third signal ; Virtual synchronous machine When adopting the AC feeder terminal voltage control strategy, execute , , Closed and , , Open the operation; at this time the voltage outer loop module outputs the fourth signal ; The second current reference value module is used to generate the reference value of the d-axis component of the current of the virtual synchronous machine according to the signal sent by the second voltage outer loop module; Figure 3 Among them, when satisfying , using the third signal As the AC feeder Reference value of the d-axis component of the current of the connected virtual synchronous machine , satisfying or When, with the fourth signal as the AC feeder Reference value of the d-axis component of the current of the connected virtual synchronous machine ; In the current reference value module proposed by the present invention, an algorithm for judging and selecting the reference value of the d-axis component of the current according to the control strategy of the virtual synchronous machine is set, which controls the voltage at the end of the AC feeder not to exceed the limit while meeting the requirements of DC voltage stability under different control strategies; The second current inner loop module uses the difference between the reference value of the d-axis component of the current and the d-axis component of the current, and the difference between the reference value of the q-axis component of the current and the q-axis component of the current as input signals, where the reference value of the q-axis component of the current is 0; And, the voltage d-axis component and voltage q-axis component of the virtual synchronous machine are introduced to correct the reference value of the voltage d-axis component and q-axis component of the virtual synchronous machine respectively; The output signal of the second current inner loop module passes through the second coordinate conversion module to obtain the three-phase voltage reference values of the virtual synchronous machine , , .

[0040] The system proposed by the present invention uses a common DC bus to realize the interconnection of the ends of multiple low-voltage distribution network feeders. The SOP device replaces the traditional tie switch to realize flexible power regulation of each distribution feeder, avoid line congestion, reduce system power loss, improve the utilization rate of distribution transformers and feeders, and optimize the overall operation level of the distribution network; With the wide access of electric vehicles, the risk of voltage over-limit at the end of the low-voltage distribution feeder increases. By optimizing the topological structure and switching control strategies, the power flow between the low-voltage distribution feeder and the industrial distribution feeder is adjusted to realize the treatment of voltage over-limit of the low-voltage distribution feeder. On the basis of combined active and reactive voltage regulation, it is more suitable for multi-terminal flexible interconnection control under different operation modes of the distribution system. Without sacrificing the voltage safety of other feeders, the voltage over-limit treatment of the distribution system is realized, and the system's acceptance capacity for electric vehicles is effectively improved. In addition, since there are still a large number of weak communication scenarios in the low-voltage distribution network, the challenges brought by the imperfect communication network in the low-voltage distribution network are effectively overcome without relying on communication conditions.

[0041] Aiming at managing voltage over-limit in a flexible interconnected distribution network without communication, with the goal of economy and low loss, and taking margin into consideration, the present invention proposes a control method for a multi-terminal flexible interconnected system for large-scale charging pile access. The distribution network includes: multiple AC feeders, each AC feeder is connected to a grid-connected bus, and each grid-connected bus is connected to an AC load and a charging pile; the AC feeder with the highest transmission power among all AC feeders is the main line, and the remaining AC feeders are slave lines. The grid-connected bus voltage is used as the AC feeder terminal voltage; the method includes: Step 1: Collect the voltage of each grid-connected bus, the transmission power of each AC feeder, and the load of the charging pile connected to the grid-connected bus.

[0042] Specifically, the voltage of each grid-connected busbar is the voltage at the end of each AC feeder line.

[0043] Step 2: When the voltage of each grid-connected busbar is not less than 0.9 pu and the transmission power of each AC feeder is not less than the corresponding charging pile load, the virtual synchronous machine connected to the slave line does not work, and the virtual synchronous machine connected to the main line adopts the DC voltage reactive power control strategy; Specifically, when the voltage of each grid-connected bus 、 、……、 、 are not less than 0.9 pu, and the transmission power of each AC feeder 、 、……、 、 No less than the corresponding charging pile load 、 、……、 、 When , it is determined to be the first operating condition; at this time, the terminal voltages of all AC feeders are within the limit and the regulation capabilities are sufficient, and the multi-terminal flexible interconnection system operates normally; The control strategy for the first working condition includes: AC feeder 1, AC feeder 2, ..., AC feeder The connected virtual synchronous machine is not working, the AC feeder The connected virtual synchronous machine adopts DC voltage reactive power control strategy, and the AC feeder The reference value of the current d-axis component of the connected virtual synchronous machine satisfies , and the third signal With AC feeder Connected virtual synchronization machine The regulated active power is positively correlated to maintain DC voltage stability.

[0044] Step 3, when the voltage of at least one grid-connected bus connected by a secondary line is less than 0.9 p.u. and the voltage of the grid-connected bus connected by the main line is not less than 0.9 p.u., and the transmission power of at least one secondary line is less than the corresponding charging pile load and the transmission power of the main line is not less than the corresponding charging pile load, the virtual synchronous machine connected to the secondary line adopts an active and reactive coordinated voltage regulation control strategy, and the virtual synchronous machine connected to the main line adopts a DC voltage reactive power control strategy; when the active power regulated by the virtual synchronous machine connected to the secondary line reaches the set power threshold, the virtual synchronous machine connected to the secondary line stops working, and the virtual synchronous machine connected to the main line adopts a DC voltage reactive power control strategy; Specifically, when the voltage of each grid-connected bus 、 、……、 is at least one less than 0.9 p.u., and is not less than 0.9 p.u., and the transmission power of each AC feeder 、 、……、 is at least one less than the corresponding charging pile load 、 、……、 、and is not less than , it is determined as the second working condition; at this time, the terminal voltage of at least one AC feeder among AC feeder 1, AC feeder 2, ……, AC feeder is out of limit and the regulation ability is insufficient, and the terminal voltage of AC feeder is not out of limit and the regulation ability is sufficient; The control strategy for the second working condition includes: the virtual synchronous machines connected to AC feeder 1, AC feeder ②, ……, AC feeder adopt an active and reactive coordinated voltage regulation control strategy, and at this time the reference value of the d-axis component of the current of each virtual synchronous machine is the first signal ; the virtual synchronous machine connected to AC feeder adopts a DC voltage reactive power control strategy, and at this time the reference value of the d-axis component of the current of the virtual synchronous machine is the third signal ; and, the terminal voltage of AC feeder reflects the remaining capacity of the active power transmitted by AC feeder after meeting the corresponding charging pile load. By adjusting the DC bus voltage, the DC bus voltage is made equal to the reference value of the DC voltage controlled by the virtual synchronous machine connected to AC feeder . Therefore, when the reference value of the DC voltage controlled by the virtual synchronous machine connected to AC feeder is less than the DC bus voltage reference value and greater than the lower limit of the DC bus voltage reference value, AC feeder Transmitting active power to an AC feeder with a power deficit or the main line having sufficient remaining capacity to bear the excess of the electric vehicle charging power on the secondary line; wherein, the lower limit of the DC bus voltage reference value is taken as 0.98 p.u., and the DC bus voltage reference value is taken as 1.0 p.u.; In the embodiment, due to the randomness of the electric vehicle charging power, when the electric vehicle charging power on the secondary line decreases significantly, the active power transmitted from the main line to the secondary line decreases. Based on the voltage at the end of the main line being mapped to the DC bus voltage reference value controlled by the virtual synchronous machine connected to the main line, the DC bus voltage reference value controlled by the virtual synchronous machine connected to the main line increases as the voltage at the end of the main line increases; at a certain moment, the virtual synchronous machine connected to the main line will transmit active power reversely to maintain the voltage at the end of the secondary line not less than the minimum value; when the active power regulated by the virtual synchronous machine connected to the secondary line is transmitted reversely, it indicates that the secondary line can fully bear the charging pile load. To prevent the virtual synchronous machine from frequently switching control modes, when the active power regulated by the virtual synchronous machine connected to the secondary line reaches the set power threshold of 0.02 p.u. (1 kW), the control strategy under the second working condition is exited, and the control strategy under the first working condition is returned; Step 4, when the voltage of the grid-connected bus connected to at least one secondary line is less than 0.9 p.u. and the voltage of the grid-connected bus connected to the main line is less than 0.9 p.u., the transmission power of at least one secondary line is less than the corresponding charging pile load, and the transmission power of the main line is less than the corresponding charging pile load, the virtual synchronous machine connected to the main line switches from the DC voltage and reactive power control strategy to the AC feeder end voltage control strategy, and the virtual synchronous machine connected to the main line controls the DC bus voltage to the lower limit of the DC bus voltage reference value; for the secondary line where the DC voltage threshold obtained by mapping the end voltage is less than the lower limit of the DC bus voltage reference value, the virtual synchronous machine connected thereto switches from the active and reactive power coordinated voltage regulation control strategy to the DC voltage and reactive power control strategy, and the virtual synchronous machines connected to the remaining secondary lines adopt the active and reactive power coordinated voltage regulation control strategy; Specifically, when the voltages of each grid-connected bus , , ……, at least one of which is less than 0.9 p.u., and is less than 0.9 p.u., the transmission powers of each AC feeder , , ……, at least one of which is less than the corresponding charging pile load , , ……, , and is less than When , it is determined to be the third working condition; at this time, AC feeder 1, AC feeder 2, ..., AC feeder At least one of the AC feeder terminals has an over-limit voltage and insufficient regulation capability. The terminal voltage exceeds the limit and reaches the carrying capacity limit; In the third working condition, if the AC feeder Virtual synchronous machine obtained by terminal voltage mapping Controlled DC bus voltage reference value is the lower limit of DC bus voltage When the AC feeder The terminal voltage reaches the minimum value. Switch from DC voltage reactive power control strategy to AC feeder end voltage control strategy; if AC feeder 1, AC feeder 2, ..., AC feeder The DC voltage threshold value obtained by mapping the terminal voltage of any AC feeder is less than the lower limit of the DC bus voltage reference value. When , the virtual synchronous machine connected to the slave line switches from the active and reactive coordinated voltage regulation control strategy to the DC voltage reactive control strategy, and the virtual synchronous machines connected to the remaining slave lines adopt the active and reactive coordinated voltage regulation control strategy; The present invention mainly aims to solve the voltage limit of feeder end node caused by large-scale charging piles by using flexible interconnection device in non-communication scenario. The priority of voltage regulation is set as follows: AC feeder The voltage regulation of the connected virtual synchronous machine is greater than that of the electric vehicle charging. In order to ensure the maximum power operation of the charging pile, the AC feeder with larger capacity is used first. To AC feeder 1, AC feeder 2, ..., AC feeder Increase the missing active power. Connected virtual synchronization machine Under the control of When the AC feeder The lack of residual capacity causes the terminal voltage to exceed the lower limit, and an AC feeder terminal voltage control strategy needs to be adopted.

[0045] When the DC bus voltage controlled by the virtual synchronous machine connected to the slave line using the DC voltage reactive power control strategy is lower than the DC voltage threshold obtained by terminal voltage mapping, and if the voltage of the grid-connected bus connected to the slave line is lower than 0.9pu, the charging pile load corresponding to the slave line switches from the disordered charging mode to the ordered charging mode; The DC voltage threshold for controlling the virtual synchronous machine connected to the secondary line is obtained by mapping the voltage at the end of the secondary line. In the intelligent soft-switching device, each virtual synchronous machine is affected by the transmission power and the terminal voltage of the connected AC feeder, and the same DC voltage threshold cannot be adopted. Therefore, the DC voltage threshold for controlling the corresponding virtual synchronous machine is obtained based on the voltage at the end of each secondary line. When the DC voltage reaches the DC voltage threshold for controlling the virtual synchronous machine connected to the secondary line, the voltage at the end of the secondary line is monitored, and further consideration is given to whether to cut off the charging pile load connected to the secondary line, so as to achieve the purpose of controlling the voltage over-limit in the large-scale charging pile access system.

[0046] In the embodiment, is 1.00 p.u., is 0.98 p.u., is 0.96 p.u., is 0.94 p.u.; In the embodiment, as the AC feeder continuously transmits active power to AC feeder 1, AC feeder 2,..., AC feeder , the terminal voltage of the AC feeder is lower than the minimum value . Through mapping, the obtained is less than , indicating that the capacity of the AC feeder is not enough to bear the deficit power of AC feeder 1, AC feeder 2,..., AC feeder . Therefore, in an environment without communication, after the virtual synchronous machine connected to the main line switches from the DC voltage reactive power control strategy to the AC feeder terminal voltage control strategy, if the grid-connected bus voltage connected to the main line is less than 0.9 p.u., set equal to , and the voltage source converter switches from the DC voltage reactive power control strategy to the terminal voltage control strategy of the AC feeder . , the terminal voltage of the AC feeder [[ID=4)]]is controlled to the minimum value to ensure that the terminal voltage of the AC feeder does not exceed the limit; For multiple secondary lines where the virtual synchronous machines connected to them all adopt the DC voltage reactive power control strategy, if the DC voltage thresholds obtained by mapping the terminal voltage are all less than the lower limit of the DC bus voltage reference value, the charging pile load corresponding to the secondary line with the maximum value among the DC voltage thresholds is switched from the disordered charging mode to the ordered charging mode.

[0047] However, as the charging power of electric vehicles , With the increase of less than ; In the embodiment, when the DC voltage threshold value mapped from the terminal voltage is less than the lower limit of the DC bus voltage reference value , switch from the active and reactive coordinated voltage regulation control strategy to the DC voltage reactive power control strategy. , the DC voltage threshold value mapped from the terminal voltage is less than the lower limit of the DC bus voltage reference value , switch from the active and reactive coordinated voltage regulation control strategy to the DC voltage reactive power control strategy. ; At this time, if , it means that due to the continuous increase of the charging power of the electric vehicle on AC feeder 1 causing to drop, then the electric vehicle on AC feeder 1 switches from the unordered charging mode to the ordered charging mode, and some charging piles stop operating to avoid from exceeding the limit.

[0048] In the embodiment, after the electric vehicle on AC feeder 1 switches from the unordered charging mode to the ordered charging mode, the control method further includes: 1). Case 1: The terminal voltage of AC feeder 1 is not less than 0.9 p.u., and the whole system is in normal operation; when the electric vehicle on AC feeder 1 switches from the unordered charging mode to the ordered charging mode, it means that the charging power is no longer increasing, and the terminal voltage of AC feeder 1 no longer decreases and does not exceed the lower limit, then maintain the current control strategy (DC voltage reactive power control strategy); When the terminal voltage of AC feeder 2 is not less than 0.9 p.u., the virtual synchronous machine of AC feeder 2 switches to the active and reactive coordinated voltage regulation control strategy to ensure does not exceed the limit. The terminal of feeder y adopts the terminal voltage control strategy of AC feeder to ensure does not exceed the limit. The node voltages of all feeders do not exceed the limit, and the system is in normal operation.

[0049] 2). Case 2: The terminal voltage of AC feeder 1 continues to drop, and the charging piles on AC feeder 2 continue to charge the electric vehicle. When the terminal voltage of AC feeder 1 continues to drop to the minimum value , switch from the DC voltage reactive power control strategy to the active and reactive coordinated voltage regulation control strategy, and control the terminal voltage of AC feeder 1 to the minimum value , at this time, the DC bus provides the deficit power to the AC feeder 2, resulting in the DC bus voltage being less than the DC voltage threshold mapped from the voltage at the end of the AC feeder 2 , Adopt the DC voltage and reactive power control strategy to control the DC bus voltage to . Due to the continuous increase in the electric vehicle charging power of the AC feeder 2 , the voltage at the end of the AC feeder 2 drops to the minimum value , and the electric vehicles on the AC feeder 2 are switched from the disordered charging mode to the ordered charging mode, and some charging piles stop operating to avoid over-limit; At a certain moment, with the reduction of the electric vehicle charging power 、 , the voltage at the end of the AC feeder 2 rises to the reference value (greater than 0.9 p.u.,), Switch to the control strategy of coordinated active and reactive power voltage regulation; with the further reduction of the electric vehicle charging power 、 , the DC bus voltage is not less than , Switch from the AC feeder end voltage control strategy to the DC voltage and reactive power control strategy, and transmit active power to 、 ; control the voltage at the end of the AC feeder 1 to the minimum value 、control the voltage at the end of the AC feeder 2 to , when 、 the regulated active power 、 is transmitted in the reverse direction, indicating that the AC feeder 1 can fully bear the electric vehicle charging power 、the AC feeder 2 can fully bear the electric vehicle charging power , to prevent the VSC from frequently switching control modes, when 、 the regulated active power 、 reaches the set power threshold of 0.02 p.u. (1 kW), exit the control strategy under condition ⅡI, and return to the control strategy under condition I.

[0050] With the above control method, when the electric vehicle charging power of AC feeder 1 and / or 2 is large enough to cause the voltage at the end of AC feeder 1 and / or 2 to exceed the limit, the SOP device uses the remaining capacity of AC feeder y to adjust the voltage at the end of AC feeder 1 and / or 2, so as to ensure the normal charging of electric vehicles on AC feeder 1 and / or 2 and the treatment of voltage over-limit. Since the voltage at the end of AC feeder y is also affected when using AC feeder y for voltage regulation, when the voltage at the end of AC feeder y also reaches the lower limit, it indicates that the system's acceptance capacity for electric vehicles reaches the maximum. Then, the electric vehicles on AC feeder 1 and / or 2 are switched from the disordered charging mode to the ordered charging mode according to the threshold of the DC bus voltage, so as to avoid the voltage at the end of AC feeder 1 and / or 2 falling below the lower limit.

[0051] The present disclosure may be a system, a method, and / or a computer program product. The computer program product may include a computer-readable storage medium having computer-readable program instructions thereon for causing a processor to implement various aspects of the present disclosure.

[0052] The computer-readable storage medium may be a tangible device that can retain and store instructions for use by an instruction execution device. The computer-readable storage medium may be, for example, but not limited to, an electrical storage device, a magnetic storage device, an optical storage device, an electromagnetic storage device, a semiconductor storage device, or any suitable combination of the foregoing. More specific examples (a non-exhaustive list) of the computer-readable storage medium include: a portable computer disk, a hard disk, a random access memory (RAM), a read-only memory (ROM), an erasable programmable read-only memory (EPROM or Flash memory), a static random access memory (SRAM), a portable compact disc read-only memory (CD-ROM), a digital versatile disc (DVD), a memory stick, a floppy disk, a mechanically encoded device such as a punch card or raised structures in grooves having instructions stored thereon, and any suitable combination of the foregoing. The computer-readable storage medium as used herein is not construed as an instantaneous signal itself, such as a radio wave or other freely propagating electromagnetic wave, an electromagnetic wave propagated through a waveguide or other transmission medium (e.g., an optical pulse through an optical fiber cable), or an electrical signal transmitted through a wire.

[0053] The computer-readable program instructions described herein may be downloaded from a computer-readable storage medium to various computing / processing devices, or downloaded to an external computer or external storage device through a network, such as the Internet, a local area network, a wide area network, and / or a wireless network. The network may include a copper transmission cable, an optical fiber transmission, a wireless transmission, a router, a firewall, a switch, a gateway computer, and / or an edge server. The network adapter or network interface in each computing / processing device receives the computer-readable program instructions from the network and forwards the computer-readable program instructions for storage in the computer-readable storage medium in each computing / processing device.

[0054] The computer program instructions for performing the operations of the present disclosure may be assembly instructions, instruction set architecture (ISA) instructions, machine instructions, machine - related instructions, microcode, firmware instructions, state - setting data, or source code or object code written in any combination of one or more programming languages, including object - oriented programming languages such as Smalltalk, C++, etc., and conventional procedural programming languages such as the "C" language or similar programming languages. The computer - readable program instructions may be executed entirely on the user's computer, partially on the user's computer, executed as a stand - alone software package, partially on the user's computer and partially on a remote computer, or entirely on a remote computer or server. In the case of a remote computer, the remote computer may be connected to the user's computer through any type of network, including a local area network (LAN) or a wide area network (WAN), or may be connected to an external computer (e.g., through the Internet using an Internet service provider). In some embodiments, by using the state information of the computer - readable program instructions to customize an electronic circuit, such as a programmable logic circuit, a field - programmable gate array (FPGA), or a programmable logic array (PLA), the electronic circuit can execute the computer - readable program instructions to implement various aspects of the present disclosure.

[0055] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit them. Although the present invention has been described in detail with reference to the above embodiments, those of ordinary skill in the art should understand that: modifications or equivalent replacements can still be made to the specific embodiments of the present invention, and any modification or equivalent replacement that does not depart from the spirit and scope of the present invention shall be covered by the protection scope of the claims of the present invention.​

Claims

1. A multi-terminal flexible interconnection system for large-scale charging pile access, with the AC feeder with the largest transmission power in the distribution network as the main line, and the other AC feeders as the secondary lines. The voltage of the grid-connected bus connected to the AC feeder is used as the terminal voltage of the AC feeder. All grid-connected buses are connected to intelligent soft-switching devices. It is characterized in that The intelligent soft-switching device includes a plurality of virtual synchronous machines. The number of virtual synchronous machines is the same as the number of AC feeders. The AC sides of the virtual synchronous machines are connected to the grid-connected buses, and the DC sides of all virtual synchronous machines are connected to a common DC bus; When the DC bus voltage is greater than the DC voltage threshold controlled by the virtual synchronous machine connected to the secondary line and less than the DC bus voltage reference value, the virtual synchronous machine connected to the secondary line adopts a control strategy of coordinated active and reactive voltage regulation; when the DC bus voltage is not less than the DC bus voltage reference value or the DC bus voltage is not greater than the DC voltage threshold controlled by the virtual synchronous machine connected to the secondary line, the virtual synchronous machine connected to the secondary line adopts a DC voltage reactive power control strategy; among them, based on the relationship between the terminal voltage of the secondary line and the DC bus voltage, the terminal voltage of the secondary line is mapped to the DC voltage threshold controlled by the virtual synchronous machine connected to the secondary line; When the DC bus voltage is greater than the lower limit of the DC bus voltage reference value and less than the DC bus voltage reference value, the virtual synchronous machine connected to the main line adopts a DC voltage reactive power control strategy; when the DC bus voltage is not less than the DC bus voltage reference value or the DC bus voltage is not greater than the lower limit of the DC bus voltage reference value, the virtual synchronous machine connected to the main line adopts an AC feeder terminal voltage control strategy; among them, based on the relationship between the terminal voltage of the main line and the DC bus voltage, the terminal voltage of the main line is mapped to the DC bus voltage reference value controlled by the virtual synchronous machine connected to the main line.

2. The multi-terminal flexible interconnection system for large-scale charging pile access according to claim 1, characterized in that The control system structures of the virtual synchronous machines connected to each secondary line are the same, including: a first voltage outer loop module, a first current reference value module, a first current inner loop module, and a first coordinate transformation module; Among them, the first voltage outer loop module is used to take the difference between the terminal voltage of the secondary line and the terminal voltage threshold of the secondary line, and the difference between the DC bus voltage and the DC voltage threshold controlled by the virtual synchronous machine connected to the secondary line as input signals; when adopting the control strategy of coordinated active and reactive voltage regulation, the first voltage outer loop module sends a first signal to the first current reference value module, and when adopting the DC voltage reactive power control strategy, the first voltage outer loop module sends a second signal to the first current reference value module; The first current reference value module is used to generate a reference value for the d-axis component of the current according to the signal sent by the first voltage outer loop module; The first current inner loop module is used to take the difference between the reference value of the d-axis component of the current and the d-axis component of the current, and the difference between the reference value of the q-axis component of the current and the q-axis component of the current as input signals, and generate reference values for the d-axis component of the voltage and the q-axis component of the voltage of the virtual synchronous machine; and, the d-axis component of the voltage and the q-axis component of the voltage of the virtual synchronous machine are introduced to correct the reference values of the d-axis component of the voltage and the q-axis component of the voltage respectively; Among them, the reference value of the q-axis component of the current satisfies the following relational expression: Wherein, is the reference value of the q-axis component of the current, , are the reactance and resistance of the line, is the reference value of the d-axis component of the current; The reference values of the corrected d-axis component and q-axis component of the voltage are input to the first coordinate conversion module to obtain the three-phase voltage reference values of the virtual synchronous machine.

3. The multi-terminal flexible interconnected system for large-scale charging pile access according to claim 2, wherein When the voltage at the end of the slave line is not out of limit, the voltage threshold at the end of the slave line is the reference value of the voltage at the end of the slave line; When the voltage at the end of the slave line is out of limit, the voltage threshold at the end of the slave line is the lower limit of the voltage at the end of the slave line.

4. The multi-terminal flexible interconnected system for large-scale charging pile access according to claim 2, wherein The first voltage outer loop module includes: 6 switches, a first voltage loop, a second voltage loop, a first feedback unit, and a second feedback unit; Among them, one end of the first switch inputs the voltage at the end of the slave line, the other end of the first switch is connected to the first input end of the first voltage loop, the other end of the second switch is connected to the second input end of the first voltage loop, the output end of the first voltage loop is connected to the positive input end of the first feedback unit, the output end of the first feedback unit is connected to one end of the second switch, the output end of the first voltage loop is also connected to one end of the third switch, and the other end of the third switch is connected to the first input end of the first current reference value generation module and the negative input end of the second feedback unit; One end of the fifth switch inputs the DC bus voltage, the other end of the fifth switch is connected to the first input end of the second voltage loop, the other end of the fourth switch is connected to the second input end of the second voltage loop, the output end of the second voltage loop is connected to the positive input end of the second feedback unit, the output end of the second feedback unit is connected to one end of the fourth switch, the output end of the second voltage loop is also connected to one end of the sixth switch, and the other end of the sixth switch is connected to the second input end of the first current reference value generation module and the negative input end of the first feedback unit; When the active and reactive power coordinated voltage regulation control strategy is adopted, the operations of closing the first switch, the third switch, and the fourth switch and opening the second switch, the fifth switch, and the sixth switch are performed; when the DC voltage reactive power control strategy is adopted, the operations of closing the second switch, the fifth switch, and the sixth switch and opening the first switch, the third switch, and the fourth switch are performed; Both the first voltage loop and the second voltage loop include PI adjustment units; the difference between the voltage at the end of the slave line and the voltage threshold at the end of the slave line is input to the first voltage loop, and the first voltage loop outputs a first signal; the difference between the DC bus voltage and the DC voltage threshold controlled by the virtual synchronous machine connected to the slave line is input to the second voltage loop, and the second voltage loop outputs a second signal; The first feedback unit feeds back the difference between the first signal and the second signal to the first voltage loop; the second feedback unit feeds back the difference between the second signal and the first signal to the second voltage loop.

5. The multi-terminal flexible interconnected system for large-scale charging pile access according to claim 1, wherein The control system of the virtual synchronous machine connected to the main line includes: a second voltage outer loop module, a second current reference value module, a second current inner loop module, and a second coordinate conversion module; Among them, the second voltage outer loop module is used to take the difference between the voltage at the end of the main line and the reference value of the voltage at the end of the main line, and the difference between the DC bus voltage and the reference value of the DC bus voltage controlled by the virtual synchronous machine connected to the main line as input signals; when the DC voltage reactive power control strategy is adopted, the second voltage outer loop module sends a third signal to the second current reference value module, and when the AC feeder end voltage control strategy is adopted, the second voltage outer loop module sends a fourth signal to the second current reference value module; The second current reference value module is used to generate the reference value of the d-axis component of the current according to the signal sent by the second voltage outer loop module; The second current inner loop module is used to take the difference between the reference value of the d-axis component of the current and the d-axis component of the current, and the difference between the reference value of the q-axis component of the current and the q-axis component of the current as input signals, and generate the reference value of the d-axis component of the voltage and the reference value of the q-axis component of the voltage of the virtual synchronous machine, where the reference value of the q-axis component of the current is 0; and, the d-axis component of the voltage and the q-axis component of the voltage of the virtual synchronous machine are introduced to correct the reference value of the d-axis component of the voltage and the reference value of the q-axis component of the voltage respectively; The corrected reference value of the d-axis component of the voltage and the reference value of the q-axis component of the voltage are converted by the second coordinate conversion module to obtain the three-phase voltage reference value of the virtual synchronous machine.

6. The multi-terminal flexible interconnected system for large-scale charging pile access according to claim 5, characterized in that The threshold value of the voltage at the end of the main line is the reference value of the voltage at the end of the main line; when the voltage at the end of the main line reaches the minimum value, the reference value of the DC bus voltage of the virtual synchronous machine connected to the main line is the third threshold value of the DC voltage, and when the voltage at the end of the main line reaches the reference value, the reference value of the DC bus voltage of the virtual synchronous machine connected to the main line is the reference value of the DC bus voltage.

7. The multi-terminal flexible interconnected system for large-scale charging pile access according to claim 5, characterized in that The second voltage outer loop module includes: 6 switches, a third voltage loop, a fourth voltage loop, a third feedback unit, and a fourth feedback unit; Among them, one end of the seventh switch inputs the DC bus voltage, the other end of the seventh switch is connected to the first input end of the third voltage loop, the other end of the eighth switch is connected to the second input end of the third voltage loop, the output end of the third voltage loop is connected to the positive input end of the third feedback unit, the output end of the third feedback unit is connected to one end of the eighth switch, the output end of the third voltage loop is also connected to one end of the ninth switch, and the other end of the ninth switch is connected to the first input end of the second current reference value generation module and the negative input end of the fourth feedback unit; One end of the eleventh switch inputs the voltage at the end of the main line, the other end of the eleventh switch is connected to the first input end of the fourth voltage loop, the other end of the tenth switch is connected to the second input end of the fourth voltage loop, the output end of the fourth voltage loop is connected to the positive input end of the fourth feedback unit, the output end of the fourth feedback unit is connected to one end of the tenth switch, the output end of the fourth voltage loop is also connected to one end of the twelfth switch, and the other end of the twelfth switch is connected to the second input end of the second current reference value generation module and the negative input end of the third feedback unit; When the DC voltage and reactive power control strategy is adopted, the seventh switch, the ninth switch, and the tenth switch are closed, and the eighth switch, the eleventh switch, and the twelfth switch are opened; when the AC feeder end voltage control strategy is adopted, the eighth switch, the eleventh switch, and the twelfth switch are closed, and the seventh switch, the ninth switch, and the tenth switch are opened; Both the third voltage loop and the fourth voltage loop include PI adjustment units; the difference between the main line end voltage and the main line end voltage reference value is input to the third voltage loop, and the third voltage loop outputs a third signal; the difference between the DC bus voltage and the DC bus voltage reference value controlled by the virtual synchronous machine connected to the main line is input to the fourth voltage loop, and the fourth voltage loop outputs a fourth signal; The third feedback unit feeds back the difference between the third signal and the fourth signal to the third voltage loop; the fourth feedback unit feeds back the difference between the fourth signal and the third signal to the fourth voltage loop.

8. The multi-terminal flexible interconnected system for large-scale charging pile access according to claim 1, characterized in that The main line end voltage is mapped to the DC bus voltage reference value controlled by the virtual synchronous machine connected to the main line, satisfying the following relationship: In the formula, is the DC bus voltage reference value controlled by the virtual synchronous machine connected to the main line, is the voltage at the end of the main line, and are the mapping coefficient and mapping error of the main line; Among them, the lower limit of the DC bus voltage reference value controlled by the virtual synchronous machine connected to the main line corresponds to the minimum value of the main line end voltage; The slave line end voltage is mapped to the DC voltage threshold value controlled by the virtual synchronous machine connected to the slave line, satisfying the following relationship: In the formula, is the DC voltage threshold controlled by the virtual synchronous machine connected to the slave line, is the voltage at the end of the slave line, and are the mapping coefficient and mapping error of the slave line.

9. A control method for a multi-terminal flexible interconnection system for large-scale charging pile access, implemented by using the multi-terminal flexible interconnection system for large-scale charging pile access according to any one of claims 1 to 8, characterized in that, The method includes: Collect the voltages of each grid-connected bus, the transmission powers of each AC feeder, and the charging pile loads connected to the grid-connected buses; When the voltages of each grid-connected bus are not less than 0.9 p.u., and the transmission powers of each AC feeder are not less than the corresponding charging pile loads, the virtual synchronous machine connected to the slave line does not work, and the virtual synchronous machine connected to the main line adopts the DC voltage and reactive power control strategy; When the voltage of at least one grid-connected bus connected to the slave line is less than 0.9 p.u. and the voltage of the grid-connected bus connected to the main line is not less than 0.9 p.u., the transmission power of at least one slave line is less than the corresponding charging pile load and the transmission power of the main line is not less than the corresponding charging pile load, the virtual synchronous machine connected to the slave line adopts the active and reactive power coordinated voltage regulation control strategy, and the virtual synchronous machine connected to the main line adopts the DC voltage and reactive power control strategy; when the active power adjusted by the virtual synchronous machine connected to the slave line reaches the set power threshold, the virtual synchronous machine connected to the slave line does not work, and the virtual synchronous machine connected to the main line adopts the DC voltage and reactive power control strategy; When the voltage of at least one grid-connected bus connected by a secondary line is less than 0.9 p.u. and the voltage of the grid-connected bus connected by the main line is less than 0.9 p.u., and the transmission power of at least one secondary line is less than the corresponding charging pile load and the transmission power of the main line is less than the corresponding charging pile load, the virtual synchronous machine connected to the main line switches from the DC voltage reactive power control strategy to the AC feeder end voltage control strategy, and the virtual synchronous machine connected to the main line controls the DC bus voltage to the lower limit of the DC bus voltage reference value; for the secondary line where the DC voltage threshold obtained by mapping the end voltage is less than the lower limit of the DC bus voltage reference value, the virtual synchronous machine connected thereto switches from the active and reactive power coordinated voltage regulation control strategy to the DC voltage reactive power control strategy, and the virtual synchronous machines connected to the remaining secondary lines adopt the active and reactive power coordinated voltage regulation control strategy.

10. The control method of a multi-terminal flexible interconnected system for large-scale charging pile access according to claim 9, characterized in that For multiple slave lines where the connected virtual synchronous machines all adopt the DC voltage reactive power control strategy, if the DC voltage thresholds obtained by mapping the terminal voltage are all less than the lower limit of the DC bus voltage reference value, then the slave line corresponding to the maximum value among the DC voltage thresholds The corresponding charging pile load is switched from the unordered charging mode to the ordered charging mode.

11. The control method of a multi-terminal flexible interconnected system for large-scale charging pile access according to claim 10, characterized in that When from the line After the corresponding charging pile load is switched from the disordered charging mode to the ordered charging mode, the control method further includes: When the terminal voltage of the outgoing line is not less than 0.9 p.u., a DC voltage and reactive power control strategy is adopted; when the terminal voltages of the remaining outgoing lines are not less than 0.9 p.u., the corresponding virtual synchronous machines adopt an active and reactive power coordinated voltage regulation control strategy; the virtual synchronous machine connected to the main line adopts an AC feeder terminal voltage control strategy; When the terminal voltage of line reaches 0.9 p.u., the virtual synchronous machine connected to line switches from the DC voltage reactive power control strategy to the active and reactive power coordinated voltage regulation control strategy; for the line where the DC voltage threshold obtained by mapping the terminal voltage is greater than the DC bus voltage, the connected virtual synchronous machine adopts the DC voltage reactive power control strategy; at this time, if the terminal voltage of line reaches 0.9 p.u., the corresponding charging pile load of line switches from the unordered charging mode to the ordered charging mode; if the terminal voltage of line is greater than 0.9 p.u., the virtual synchronous machine connected to line adopts the active and reactive power coordinated voltage regulation control strategy; when the DC bus voltage is not less than the lower limit of the DC bus voltage reference value, the virtual synchronous machine connected to the main line switches from the AC feeder terminal voltage control strategy to the DC voltage reactive power control strategy; when the active power regulated by the virtual synchronous machine connected to the branch line reaches the set power threshold, the virtual synchronous machine connected to the branch line stops working, and the virtual synchronous machine connected to the main line adopts the DC voltage reactive power control strategy.

12. A terminal, comprising a processor and a storage medium; characterized in that The storage medium is used to store instructions; The processor is used to operate according to the instructions to execute the steps of the method according to any one of claims 9-11.

13. A computer-readable storage medium having a computer program stored thereon, characterized in that, When the program is executed by the processor, the steps of the method according to any one of claims 9-11 are implemented.

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