Decentralized collaborative configuration method and system for overcapacity branches in BT-EVSC network

Abstract

A decentralized collaborative configuration method and system for overcapacity branches in a BT-EVSC network configures overcapacity branches at the sending and receiving ends of a DC line, respectively. If the decentralized collaborative controller determines that the total support power demand is greater than the starting threshold, it adjusts the power weight coefficient of each overcapacity branch and distributes it to the overcapacity sub-controller of each overcapacity branch to determine the power instruction of each overcapacity branch. If the power instruction of each overcapacity branch is not greater than the maximum power support level, a control pulse is generated for each overcapacity branch according to the power instruction. If there is an overcapacity branch with a power instruction greater than the maximum power support level, the overcapacity branch outputs active power according to the maximum power support level. The decentralized collaborative controller re-adjusts the power weight coefficients of the remaining overcapacity branches based on the difference between the total support power demand and the maximum power support level of the overcapacity branch, and then updates the power instruction of each overcapacity branch, thereby solving the problem of insufficient transient active power support capacity of the converter stations at both the sending and receiving ends of the flexible DC system.

Description

Technical Field

[0001] The present invention belongs to the field of ultra-high voltage direct current (UHVDC) transmission technology, and specifically relates to a bipolar terminal-energy storage voltage source converter (BT-EVSC) topology with inertia support performance and a method and system for decentralized collaborative configuration of its super-capacity branches. Background Art

[0002] As new power systems dominated by renewable energy sources are being constructed, the scale of renewable energy generation continues to expand. High voltage direct current (HVDC) transmission technology based on modular multilevel converters (MMC-HVDC), with its advantages of no commutation failures and flexible control methods, has been widely adopted in engineering projects. With the accelerated construction of new power systems, traditional synchronous generators are being largely replaced by power electronic devices, resulting in systems with low inertia and weak damping. Consequently, a large number of synchronous generators and phase regulators are still required to maintain stable system operation, exacerbating the cost and flexibility issues associated with isolated grid systems.

[0003] In existing technologies, grid-based control simulates the inertia, damping, and droop characteristics of synchronous generators to generate an endogenous electromotive force, achieving frequency synchronization between the converter output voltage and the grid voltage. This provides the ability to actively support system frequency and voltage in the event of a weak grid or frequency disturbance. Grid-based technology has been widely adopted and holds promising prospects. However, because the DC-side submodule capacitors of flexible DC converter stations lack the capacity for large-scale energy storage, large-scale DC power support or local active power support equipment are required when power shortages and frequency drops occur at the receiving end. This can easily lead to transient overloads at the sending end, placing more stringent demands on the sending-end system. Existing technologies are unable to achieve a flexible DC converter station with strong inertia support capabilities while simultaneously reducing DC power transmission demand. Furthermore, with the widespread replacement of traditional generators with "DC + renewable energy" at the receiving end, the grid power structure has undergone a significant transformation, with systems characterized by a high degree of power electronics. This has reduced system inertia and voltage support capabilities, posing challenges to the safe and stable operation of the grid. Although existing technologies have alleviated the problem of insufficient system inertia through the construction of supporting synchronous units, extension of the main power grid, and the use of flexible direct current transmission, the flexible direct current converter station itself still cannot provide the inertia support capability under system frequency disturbances, and the overload level at the sending end through direct current power support has also put forward higher requirements. Summary of the Invention

[0004] In order to address the deficiencies in the prior art, the present invention provides a method and system for decentralized collaborative configuration of overcapacity branches for BT-EVSC networking. The BT-EVSC networking topology adopts a dual-end topology, with direct-hung overcapacity energy storage device branches at both the sending and receiving ends. This aims to address the problem of insufficient transient active power support capacity of the flexible direct current system's sending and receiving dual-end converter stations, and alleviate the low inertia problem of the system caused by the reduction in capacity of synchronous machines such as thermal power in the regional power grid. Through decentralized collaborative configuration of multiple overcapacity energy storage branches, collaborative support of energy storage power is achieved that comprehensively considers the overload level and response speed of the energy storage branch devices, reduces the overload level of the opposite-end system caused by frequency disturbances in the sending / receiving end grids of the direct current transmission system, and enhances the inertia support capability and safe and stable operation level of the ultra-high voltage flexible direct current system.

[0005] The present invention adopts the following technical solutions.

[0006] The present invention proposes a decentralized collaborative configuration method for super-capacity branches in a BT-EVSC network, comprising:

[0007] Configure a sending-end overcapacity branch on the sending end of the DC line, and configure a receiving-end overcapacity branch on the receiving end of the DC line. Obtain the difference between the active power signal output by the network converter station and the active power reference value as the total support power demand of all overcapacity branches and input it into the decentralized collaborative controller.

[0008] During a fault transient, if the decentralized cooperative controller determines that the total supported power demand is greater than the set start-up threshold, it adjusts the power weight coefficient of each over-capacity branch and distributes the power weight coefficient to the over-capacity sub-controller of each over-capacity branch;

[0009] The super-capacity sub-controller of each super-capacity branch determines the power instruction of each super-capacity branch according to the power weight coefficient; if the power instruction of each super-capacity branch is not greater than the maximum power support level of each super-capacity branch, a control pulse of each super-capacity branch is generated according to the power instruction; if there is an super-capacity branch whose power instruction is greater than the maximum power support level, the super-capacity branch outputs active power according to the maximum power support level, and the decentralized collaborative controller re-adjusts the power weight coefficients of the remaining super-capacity branches according to the difference between the total support power demand and the maximum power support level of the super-capacity branch, and then the super-capacity sub-controller of each super-capacity branch updates the power instruction of each super-capacity branch.

[0010] Preferably, one end of the sending-end overcapacity branch is connected to the DC line positive electrode, and the other end is connected to the DC line ground electrode; one end of the receiving-end overcapacity branch is connected to the DC line positive electrode, and the other end is connected to the DC line ground electrode;

[0011] The sending-end overcapacity branch and the receiving-end overcapacity branch both include a plurality of overcapacity submodules connected in series and an export reactor; wherein each overcapacity submodule is switched on and off by a corresponding switch device.

[0012] Preferably, based on the active characteristics of the MMC converter valve adopting the network control strategy, the active power signal output by the network converter station is obtained according to the reference phase angle and the reference voltage, and the difference between the active power signal output by the network converter station and the active power reference value is used as the total support power demand of all excess capacity branches; the total support power demand of all excess capacity branches is input into the decentralized collaborative controller.

[0013] Preferably, the start threshold is set to αP N , where α is the startup threshold coefficient of the overcapacity branch, P N is the active power reference value.

[0014] Preferably, the power weight coefficient λ allocated to each excess capacity branch is i , satisfying the following relationship:

[0015]

[0016] Where i = 1, 2, ..., m, and m is the total number of excess capacity branches.

[0017] Preferably, the setting of the power weight coefficient of each excess capacity branch includes:

[0018] 1) The over-capacity branch closest to the converter station is the main branch; among all over-capacity branches, the main branch has the largest power weight coefficient, and its value range is [0,1];

[0019] 2) Calculate the maximum capacity of the overcapacity energy storage based on the overload current level of the switching devices in each overcapacity branch, and use it as the maximum power support level of the overcapacity branch to establish the power support level constraints of each overcapacity branch, satisfying the following relationship:

[0020] P ES_i ≤P ES_imax

[0021] P ES_imax =I imax V dc

[0022] Where, P ES_imax is the maximum power support level of the i-th overcapacity branch, I imax is the maximum overload current of the i-th overcapacity branch, V dc is the DC bus voltage;

[0023] 3) Adjust the power weight coefficient of the main branch

[0024] When the total supported power demand of all over-capacity branches is not greater than the maximum power support level of the main branch, the power weight coefficient of the main branch is 1; when the total supported power demand of all over-capacity branches is greater than the maximum power support level of the main branch, the power weight coefficient of the main branch satisfies the following relationship:

[0025]

[0026] Where λ j The power weight coefficient of the main branch, P ES_jmax is the maximum power support level of the main branch, P ES_imax is the maximum power support level of the i-th overcapacity branch, ED j The electrical distance between the main branch and the converter station, ED i is the electrical distance between the ith over-capacity branch and the converter station; is the sum of the maximum power support levels of all overcapacity branches, and m is the total number of overcapacity branches;

[0027] 4) Adjust the power weight coefficients of all excess capacity branches other than the main branch to satisfy the following relationship:

[0028]

[0029] After active power support is provided to the network converter station according to the maximum power support level of the main branch, the power support demand shortfall of the network converter station is met by the power weight coefficients of the remaining over-capacity branches other than the main branch, each of which is determined according to the maximum power support level of each branch.

[0030] Preferably, the overcapacity sub-controller of each overcapacity branch determines the power instruction of each overcapacity branch according to the power weight coefficient to satisfy the following relationship:

[0031] P ES_j =λ j ΔP ES

[0032] P ES_j ≤P ES_jmax

[0033] P ES_i =λ i ·(1-λ j )ΔP ES i≠j,i=1,2,…,m

[0034] P ES_i ≤P ES_imax i≠j,i=1,2,…,m

[0035] Where, P ES_j The power command of the main branch, P ES_iis the power command of the i-th overcapacity branch, ΔP ES is the total supported power requirement of all overcapacity branches.

[0036] Preferably, if the power instructions of each over-capacity branch are not greater than the corresponding maximum power support level, the ratio of the power instruction of the over-capacity branch to the DC bus voltage is used as the DC current reference value of the over-capacity branch, and the difference between the DC current reference value and the actual DC current value of the over-capacity branch after passing through the PI controller is then summed with the DC bus voltage. The ratio of the obtained sum to the average voltage of each over-capacity sub-module in the over-capacity branch is used as the number of over-capacity sub-modules put into the over-capacity branch, and according to the determined number, a trigger pulse is sent to the switching device corresponding to the over-capacity sub-module to be put into use.

[0037] The present invention also proposes a decentralized collaborative configuration system for super-capacity branches in a BT-EVSC network, comprising:

[0038] Decentralized collaborative controller, total support power demand module, and overcapacity sub-controllers for each overcapacity branch;

[0039] The total support power demand module is used to obtain the difference between the active power signal output by the network converter station and the active power reference value as the total support power demand of all overcapacity branches, and input it into the decentralized collaborative controller;

[0040] A decentralized cooperative controller is used to adjust the power weight coefficient of each over-capacity branch and distribute the power weight coefficient to the over-capacity sub-controller of each over-capacity branch if the total support power demand is determined to be greater than the set start threshold during the fault transient period;

[0041] The overcapacity sub-controller of each overcapacity branch is used to determine the power instruction of each overcapacity branch according to the power weight coefficient; if the power instruction of each overcapacity branch is not greater than the maximum power support level of each overcapacity branch, a control pulse for each overcapacity branch is generated according to the power instruction; if there is an overcapacity branch whose power instruction is greater than the corresponding maximum power support level, the overcapacity branch outputs active power according to the maximum power support level, and sends the difference between the total support power demand and the maximum power support level of the overcapacity branch to the decentralized collaborative controller;

[0042] The decentralized cooperative controller is further configured to readjust the power weight coefficients of the remaining over-capacity branches based on the difference between the total supported power demand and the maximum power support level of the over-capacity branch, and send the readjusted power weight coefficients of the over-capacity branches to the over-capacity sub-controllers of the over-capacity branches;

[0043] The over-capacity sub-controller of each over-capacity branch is further used to update the power instruction of each over-capacity branch according to the re-adjusted power weight coefficient of each over-capacity branch.

[0044] The present invention also provides a terminal, comprising a processor and a storage medium; the storage medium is used to store instructions; and the processor is used to operate according to the instructions to execute steps of the method.

[0045] The present invention also relates to a computer-readable storage medium having a computer program stored thereon, which implements the steps of the method when the program is executed by a processor.

[0046] The beneficial effects of the present invention are as follows: compared with the prior art, the present invention adopts multi-point direct hanging of super-capacity equipment branches and grid-type flexible direct current technology to achieve balanced transmission power at the sending and receiving ends of the flexible direct current AC power grid during transient fault conditions, reduce fluctuations in electrical quantities between the MMCs at the sending and receiving ends and the super-capacity equipment branches, and enable the flexible direct current converter station to have inertia support capabilities while reducing DC power transmission requirements; moreover, the present invention gives full play to the energy storage characteristics of the super-capacity equipment branches, and through the coordinated configuration of multiple energy storage branches, on the one hand reduces the capacity required for each energy storage branch, saving the construction cost of a single energy storage branch; on the other hand Each energy storage branch adopts a decentralized and coordinated configuration. The power weight coefficient of each branch is determined by the electrical distance between the energy storage branch and the flexible DC converter valve and the overload level of the energy storage branch device. According to the active power support requirements of the converter station, the active capacity of each energy storage branch is flexibly allocated, the active response speed of the overcapacity equipment is improved, and the line loss is reduced, so as to achieve safe and efficient utilization of the transient active power support of the energy storage branch. Compared with existing energy storage control methods, the topology and method proposed in the present invention are more suitable for the short-term inertia continuous support working condition of the converter station, which is conducive to enhancing the inertia support capability and safe and stable operation level of the UHV flexible DC system. BRIEF DESCRIPTION OF THE DRAWINGS

[0047] Figure 1 This is the topology diagram of the BT-EVSC network with inertia support performance proposed by the present invention;

[0048] Figure 2 Flowchart of the decentralized collaborative configuration method for super-capacity branches in BT-EVSC network proposed by the present invention;

[0049] Figure 3 This is a block diagram of the control of output power by the MMC converter valve in the network control strategy according to an embodiment of the present invention;

[0050] Figure 4 This is a block diagram of the decentralized configuration logic of the decentralized collaborative controller and the control logic of each sub-controller proposed by the present invention;

[0051] Figure 5 This is a simulated waveform diagram of the active power of the receiving-end converter under frequency step in an embodiment of the present invention;

[0052] Figure 6 This is a simulated waveform diagram of the DC side voltage at the transmitting and receiving ends under frequency step in an embodiment of the present invention;

[0053] Figure 7 This is a simulation waveform of the active power of the transmitting and receiving end converters and energy storage equipment under frequency steps in an embodiment of the present invention. DETAILED DESCRIPTION

[0054] To make the objectives, technical solutions, and advantages of the present invention more clear, 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 part of the embodiments of the present invention, not all of them. Based on the spirit of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0055] The present invention proposes a method for decentralized collaborative configuration of super-capacity branches in BT-EVSC network, which is applicable to BT-EVSC network topology, such as Figure 1 As shown in the figure, the BT-EVSC network includes: a sending-end converter station, a receiving-end converter station, and an ultra-high voltage direct current transmission line. The sending and receiving-end converter stations adopt a high- and low-voltage converter valve structure. Each converter valve is cascaded to the DC bus. The AC side of the converter valve is connected to the sending-end and receiving-end power grids respectively through converter transformers.

[0056] In the embodiment, the sending-end converter station is composed of a modular multi-level (MMC) converter valve, which is connected to the sending-end AC system via a converter transformer; the receiving-end converter station is composed of an MMC converter valve, which is connected to the receiving-end AC system via a converter transformer; the MMC converter valve is composed of a bridge arm inductor, an upper bridge arm, a lower bridge arm, and a bridge arm resistor, and the upper bridge arm and the lower bridge arm contain a sub-module string, which is composed of multiple full-bridge and half-bridge sub-modules, each sub-module includes a switching device and a reverse diode, and by controlling the number of sub-modules put into operation, the total voltage of the upper and lower bridge arms can be adjusted, thereby adjusting the AC voltage output value.

[0057] The control strategy for an MMC converter valve typically involves modulation signal generation and submodule voltage balancing control. Modulation signal generation controls the switching states of the submodules based on the modulation voltage command output by the controller, resulting in the target modulation voltage. The voltage balancing control strategy adjusts the bridge arm voltage by controlling the number of upper and lower bridge arm submodules in operation, maintaining capacitor voltage balance across the submodules and preventing overvoltage or undervoltage in the submodules during steady-state operation.

[0058] Figure 1 In the figure, the DC transmission line is shown as a unipolar operation diagram, including the DC line impedance and smoothing reactor.

[0059] An ultra-high voltage direct current (UHVDC) transmission line includes a direct current (DC) line, a sending-end overcapacity branch, and a receiving-end overcapacity branch. On the sending-end side of the DC line, one end of the sending-end overcapacity branch is connected to the DC line positive electrode, and the other end is connected to the DC line ground electrode. On the receiving-end side of the DC line, one end of the receiving-end overcapacity branch is connected to the DC line positive electrode, and the other end is connected to the DC line ground electrode.

[0060] When the MMC converter valves at the sending-end converter station and / or the MMC converter valves at the receiving-end converter station adopt a grid-type control strategy, the sending-end converter station and / or the receiving-end converter station serve as grid-type converter stations. The difference between the output power of the MMC converter valves adopting the grid-type control strategy and the AC reference power is generated through the power outer loop to generate the corresponding endogenous electromotive force amplitude and phase angle, which are synthesized into a three-phase AC output reference value and sent to the inner loop and the switch modulation wave generation. The upper and lower bridge arms in the MMC converter valves adjust the number of submodules switched in accordance with the output signal of the grid-type controller, and by changing the output voltage of the upper and lower bridge arms, the converter valve output voltage is adjusted to the grid-type control output target value.

[0061] like Figure 1 As shown, both the sending-end overcapacity branch and the receiving-end overcapacity branch include multiple overcapacity submodules SC1, SC2, ..., SCn connected in series, and an export reactor L o , n is a positive integer; each super-capacity energy storage module generates a super-capacity branch reference current command based on the output command of the decentralized cooperative controller and the DC bus voltage, and generates a control signal for each super-capacity energy storage module through current loop control. By adjusting the switching status of each super-capacity energy storage module, the output current of the super-capacity branch is adjusted to maintain the stability of the DC bus voltage and meet the active power and energy support requirements of the receiving-end converter station network.

[0062] In order to stabilize the DC bus voltage and the power at the transmitting and receiving ends during the transient period, reduce the overload level of a single overcapacity branch, and give full play to the transient support capability advantages of each directly-hung overcapacity energy storage branch (such as electrical distance or device overload), the present invention also proposes a decentralized collaborative configuration method for overcapacity branches suitable for BT-EVSC networking. During the transient active power response period of the networking converter station, the decentralized collaborative controller calculates the total support power demand of all overcapacity branches based on the active power signal output by the networking converter station, considers the electrical distance between each overcapacity branch and the MMC converter valve and the overload level of the devices in the overcapacity branch, adopts a decentralized collaborative control strategy, and dynamically adjusts the output power instruction value of each overcapacity branch according to the power weight coefficient of each overcapacity branch. Each overcapacity branch adjusts the output active power according to the output power instruction issued by the decentralized collaborative controller. Figure 2 As shown in FIG, the decentralized collaborative configuration method for super-capacity branches in the BT-EVSC network includes:

[0063] Step 1: Configure a sending-end overcapacity branch on the sending-end side of the DC line and a receiving-end overcapacity branch on the receiving-end side of the DC line; obtain the difference between the active power signal output by the network converter station and the active power reference value as the total support power demand of all overcapacity branches and input it into the decentralized collaborative controller.

[0064] In the embodiment, Figure 3 As shown in the figure, based on the active characteristics of the MMC converter valve using the grid control strategy, according to the reference phase angle θ ref and reference voltage V mref Get the active power signal P output by the network converter station MMC , the network converter station outputs active power signal P MMC and active power reference value P N The difference between the two is taken as the total supported power demand ΔP of all over-capacity branches. ES ; The total supported power demand of all overcapacity branches ΔP ES Input into the decentralized collaborative controller.

[0065] Specifically, all over-capacity branches include sending-end over-capacity branches and receiving-end over-capacity branches; on the sending-end side of the DC line, one end of the sending-end over-capacity branch is connected to the DC line positive pole and the other end is connected to the DC line grounding electrode; on the receiving-end side of the DC line, one end of the receiving-end over-capacity branch is connected to the DC line positive pole and the other end is connected to the DC line grounding electrode.

[0066] Step 2: During the fault transient, if the decentralized collaborative controller determines that the total supported power demand of the over-capacity branch is greater than the set start-up threshold, the power weight coefficient of each over-capacity branch is adjusted, and the power weight coefficient is allocated to the over-capacity sub-controller of each over-capacity branch.

[0067] Specifically, during the fault transient, if the decentralized cooperative controller determines that ΔP ES >αP N , the decentralized cooperative controller adjusts the power weight coefficient of each overcapacity branch; where α is the startup threshold coefficient of the overcapacity branch, which is related to the steady-state power fluctuation amplitude during the steady-state operation of the flexible DC system. The voltage fluctuation of the MMC submodule usually does not exceed 10%. It is approximately assumed that the energy storage is put into use when the overcapacity branch approaches a certain margin of the threshold, which is 1.05 here; P N is the active power reference value;

[0068] Considering the possible transient active power energy demand of the BT-EVSC sending and receiving end flexible DC converter stations, the excess capacity branches adopt a double-end decentralized layout, and the input instructions of each excess capacity branch are issued by the decentralized cooperative controller in the sending / receiving end network converter station.

[0069] Specifically, the power weight coefficient λ allocated to each excess capacity branch is i, i=1,2,…,m, m is the total number of excess capacity branches, satisfying the following relationship:

[0070]

[0071] The decentralized cooperative controller determines the power weight coefficient of each overcapacity branch based on the electrical distance between each overcapacity branch and the converter station and the overload level of the components in each overcapacity branch;

[0072] In the embodiment, the power weight coefficient of each excess capacity branch is set as follows:

[0073] 1) The over-capacity branch closest to the converter station is the main branch; among all over-capacity branches, the main branch has the largest power weight coefficient, and its value range is [0,1];

[0074] Considering the communication delay caused by the long distance of the ultra-high voltage flexible direct current transmission line, the power weight coefficient is constrained by both the physical communication distance and the electrical distance. Therefore, among all the overcapacity branches, the transient active power support is mainly undertaken by the overcapacity branch with the closest electrical distance and communication distance to the converter station, and this overcapacity branch is defined as the main branch. Since BT-EVSC is a two-terminal MMC topology and the DC line has no additional branches, the electrical distance and the communication distance have the same weight. Therefore, the power weight coefficient of the overcapacity main branch with the closest electrical distance to the converter station is the largest, and it responds first to the active power support demand of the converter station. In the embodiment, the power weight coefficient of the main branch ranges from [0,1];

[0075] The present invention fully utilizes the branchless architecture of DC lines and the characteristics of communication channel construction in DC lines, and replaces the communication distance with the electrical distance between the overcapacity branch and the converter station. The communication distance represents the response speed of each overcapacity branch to the power collaborative support demand of the networked converter station. Therefore, the main branch is selected based on the closest electrical distance to the converter station, thereby improving the response speed of the networked converter station to the power collaborative support demand.

[0076] 2) Calculate the maximum capacity of the overcapacity energy storage based on the overload current level of the switching devices in each overcapacity branch, and use it as the maximum power support level of the overcapacity branch to establish the power support level constraints of each overcapacity branch, satisfying the following relationship:

[0077] P ES_i ≤P ES_imax

[0078] P ES_imax =I imax V dc

[0079] Where, P ES_imax is the maximum power support level of the i-th overcapacity branch, I imaxis the maximum overload current of the switch device in the i-th overcapacity branch, V dc is the DC bus voltage.

[0080] In the embodiment, each super-capacity sub-module in each super-capacity branch is switched on and off by a corresponding switching device, which includes but is not limited to: IGBT; the DC bus voltage is the rated DC bus voltage;

[0081] 3) Adjust the power weight coefficient of the main branch

[0082] When the total supported power demand of all overcapacity branches ΔP ES Not greater than the maximum power support level P of the main branch ES_jmax , then the power weight coefficient of the main branch is 1; when the total supported power demand of all over-capacity branches ΔP ES Greater than the maximum power support level P of the main branch ES_jmax , then the power weight coefficient of the main branch satisfies the following relationship:

[0083]

[0084] Where λ j The power weight coefficient of the main branch, P ES_jmax is the maximum power support level of the main branch, P ES_imax is the maximum power support level of the i-th overcapacity branch, ED j The electrical distance between the main branch and the converter station, ED i is the electrical distance between the ith over-capacity branch and the converter station; is the sum of the maximum power support levels of all overcapacity branches;

[0085] Due to the power weight coefficient of the main branch, it also reflects the main branch's ability to improve the power collaborative support demand response speed of the network converter station and enhance the inertia support capability of the UHV flexible DC system.

[0086] 4) Adjust the power weight coefficients of all excess capacity branches other than the main branch.

[0087] After active power support is provided to the network converter station according to the maximum power support level of the main branch, the power support demand shortfall of the network converter station is met by the power weight coefficients of the remaining excess branches other than the main branch, which are determined according to the maximum power support level of each branch. The power weight coefficients of the remaining excess branches other than the main branch satisfy the following relationship:

[0088]

[0089] In the present invention, the power weight coefficients of the remaining over-capacity branches other than the main branch are used to dispatch the output power of the remaining over-capacity branches to meet the inertia support energy demand. On this basis, the failure probability caused by overcurrent of the switching device in a single over-capacity energy storage branch is reduced, thereby improving the safety and reliability of the direct current transmission system.

[0090] Moreover, the present invention fully utilizes the energy storage characteristics of the super-capacity equipment branch. Through the coordinated configuration of multiple energy storage branches, on the one hand, it reduces the capacity required for each energy storage branch and saves the construction cost of a single energy storage branch. On the other hand, each energy storage branch adopts a decentralized and coordinated configuration. The power weight coefficient of each branch is jointly determined by the electrical distance between the energy storage branch and the flexible DC converter valve and the overload level of the energy storage branch device. According to the active power support requirements of the converter station, the active capacity of each energy storage branch is flexibly allocated, the active response speed of the super-capacity equipment is improved, and the line loss is reduced, thereby realizing the safe and efficient use of the transient active support of the energy storage branch.

[0091] Step 3: The over-capacity sub-controller of each over-capacity branch determines the power instruction of each over-capacity branch according to the power weight coefficient; if the power instruction of each over-capacity branch is not greater than the corresponding maximum power support level, a control pulse of each over-capacity branch is generated according to the power instruction; if there is an over-capacity branch whose power instruction is greater than the corresponding maximum power support level, the over-capacity branch outputs active power according to the maximum power support level, and the decentralized collaborative controller re-adjusts the power weight coefficients of the remaining over-capacity branches according to the difference between the total support power requirements of all over-capacity branches and the maximum power support level of the over-capacity branch, and then updates the power instructions of each over-capacity branch.

[0092] The supercapacitor controller of each supercapacitor branch determines the power instruction of each supercapacitor branch according to the power weight coefficient, satisfying the following relationship:

[0093] P ES_j =λ j ΔP ES

[0094] P ES_j ≤P ES_jmax

[0095] P ES_i =λ i ·(1-λ j )ΔP ES i≠j,i=1,2,…,m

[0096] P ES_i ≤P ES_imax i≠j,i=1,2,…,m

[0097] Where, P ES_j The power command of the main branch, P ES_iis the power instruction of the i-th overcapacity branch, P ES_imax is the maximum power support level of the i-th overcapacity branch;

[0098] The output power of each overcapacity branch should be equal to the power instruction;

[0099] When the total supported power demand exceeds the set start threshold, the total supported power demand will be jointly responded to by the multiple drop-point direct-connected overcapacity branches. The overcapacity sub-controllers of each overcapacity branch determine the power instructions of each overcapacity branch according to the power weight coefficient. The overcapacity sub-controllers first adjust the output current of each overcapacity branch and then adjust the active power output of each overcapacity branch to respond to the power instruction, providing active power support for the network converter station.

[0100] When the active power output of each overcapacity branch meets the maximum power support level, each branch outputs the corresponding transient active power support according to the given power weight coefficient. If the i-th overcapacity branch reaches the maximum power support level during the transient output period, it will provide support at its maximum active power, and the surplus power will be distributed among the other overcapacity branches according to the power weight coefficients to collaboratively support the system's active power demand. The use of grid-type converter valves and multi-point direct-mounted overcapacity energy storage provides inertia support for the converter station, reducing the overload level on the sending and receiving ends caused by grid frequency disturbances at the receiving and sending converter stations.

[0101] like Figure 4 As shown, the super-capacitor sub-controller controls the super-capacitance branches according to the power instructions of each super-capacitance branch, including:

[0102] 1) If the power instructions of each excess capacity branch are not greater than the corresponding maximum power support level, the power instruction P of the excess capacity branch is used. ES_i (i=1,2,…,m) and DC bus voltage V dc The ratio of is used as the DC current reference value I of the overcapacity branch dcref , DC current reference value I dcref The difference between the actual value of the DC current of the over-capacity branch and the value after the PI controller is compared with the DC bus voltage V dc Sum the obtained sum and the average voltage V of each super-capacitor module in the super-capacitor branch cavg The ratio of is used as the number of super-capacitor modules put into the super-capacitor branch. According to the determined number, trigger pulses are sent to the switching devices of the super-capacitor modules to be put into use. Each super-capacitor branch jointly supports the active power required by the flexible DC converter station during the transient period of the network.

[0103] 2) If the power instruction of an over-capacity branch is greater than the corresponding maximum power support level, the over-capacity branch outputs active power according to the maximum power support level. The decentralized collaborative controller readjusts the power weight coefficients of the remaining over-capacity branches based on the difference between the total support power demand of the over-capacity branch and the maximum power support level of the over-capacity branch, and the over-capacity sub-controller updates the power instructions of each over-capacity branch.

[0104] Compared with existing energy storage control methods, the control strategy for the active power output of each over-capacity branch proposed in the present invention is based on partitioned collaborative configuration. The topology and power support demand response method of the multi-drop point direct-hanging over-capacity branch for BT-EVSC networking proposed in the present invention are more suitable for the short-term inertia continuous support conditions of the converter station, which is conducive to enhancing the inertia support capability and safe and stable operation level of the UHV flexible direct current system.

[0105] Furthermore, after the operation delay, the decentralized cooperative controller updates the total support power demand of all overcapacity branches. If there is still a shortfall in the total support power demand, steps 1 to 3 are repeated to achieve balanced transmission power at the sending and receiving ends of the flexible DC AC grid during the fault transient period, reduce the electrical quantity fluctuations of the MMCs at the sending and receiving ends and the overcapacity equipment branches, and enable the flexible DC converter station to have inertia support capabilities while reducing the DC power transmission demand.

[0106] A simulation model of the BT-EVSC grid-connected flexible DC converter topology and decentralized collaborative control method was built in PSCAD to verify the effectiveness of the topology and decentralized collaborative configuration method proposed in this invention. The operating parameters of a single MMC converter station are shown in the following table. In the simulation model, the receiving-end converter station adopts grid-connected control.

[0107] Table 1 Parameters of converters for flexible DC systems

[0108] Converter rated operating power / MW 2000 Valve side rated voltage / kV 196 Rated DC voltage / kV 400 Bridge arm inductance / mH 25 Rated angular frequency / rad / s 314 Number of flexible direct current MMC single bridge arm submodules 190 Existing submodule capacitance value / mF 24 Converter capacity / MVA 750 Rated ratio of commutation transformer 515kV / 196kV Rated impedance of commutation transformer 20％

[0109] In this embodiment, a single overcapacity branch is used, with a weight coefficient of 1.0 and a 5% threshold coefficient for the overcapacity branch startup. An overcapacity branch configuration and DC voltage are selected to meet device overcurrent, output power, and energy constraints. Overcapacity redundant clusters are reserved to improve energy storage device reliability. The DC current of a single branch is no greater than 3kA. A 144V / 62.5F module is selected, with a single module internal resistance of 96mΩ. The total number of overcapacity valve submodules is 325.

[0110] When a frequency disturbance occurs in the receiving-end power grid, due to the network control function of the receiving-end converter station, the converter station will provide transient active power support to the receiving-end power grid. The flexible DC converter station itself does not have the active power support capability and needs to provide power support through the DC line. At this time, the overcapacity branch will provide the active power and energy required during the converter station fault, meeting the system inertia support capability requirements. At the same time, because the overcapacity branch can completely cover the power demand during the transient period of the receiving-end converter station fault, it ensures that the receiving-end converter station provides sufficient inertia response characteristics while reducing the overload level of the sending-end system, which is conducive to the safe and stable operation of the flexible DC system.

[0111] The system operating condition is that the flexible DC transmission system is operating at rated conditions 6s ago, and the fault condition is that the AC system frequency at the receiving end is stepped from 50Hz to 49.5Hz at 6s. The system simulation results are as follows: Figure 5 、 Figure 6 、 Figure 7 shown. Figure 5 In the formula, Pm represents the active power output on the AC side of the receiving converter, in MW. Figure 6 In the figure, Edc_T represents the DC side voltage of the sending-end converter, and Edc_R represents the DC side voltage of the receiving-end converter, both in kV. Figure 7 In the figure, Sdc_SC represents the total active power support of the overcapacity branch, Sdc_R represents the DC power of the receiving-end converter, and Sdc_T represents the DC power of the sending-end converter, both in MW. After a frequency step occurs in the receiving-end system, the receiving-end converter station provides active support for the system inertia, and the directly connected overcapacity energy storage responds to the change in active output of the receiving-end converter station. While maintaining the stability of the DC voltage at the sending and receiving ends, it effectively reduces the overload level of the sending-end system, verifying the effectiveness of the BT-EVSC network flexible DC converter topology with inertia support capability and the decentralized collaborative control method proposed in this invention.

[0112] The present disclosure may be a system, method and / or computer program product. The computer program product may include a computer-readable storage medium carrying computer-readable program instructions for causing a processor to implement various aspects of the present disclosure.

[0113] A computer-readable storage medium can be a tangible device that can hold and store instructions for use by an instruction execution device. A computer-readable storage medium can 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 thereof. More specific examples (a non-exhaustive list) of computer-readable storage media 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 disk (DVD), a memory stick, a floppy disk, a mechanical encoding device, such as a punch card or a raised structure in a groove on which instructions are stored, and any suitable combination thereof. As used herein, a computer-readable storage medium is not to be construed as a transient signal per se, such as a radio wave or other freely propagating electromagnetic wave, an electromagnetic wave propagating through a waveguide or other transmission medium (e.g., a light pulse through a fiber optic cable), or an electrical signal transmitted through an electrical wire.

[0114] The computer-readable program instructions described herein can be downloaded from a computer-readable storage medium to each computing / processing device, or downloaded to an external computer or external storage device via a network, such as the Internet, a local area network, a wide area network, and / or a wireless network. The network can include copper transmission cables, fiber optic transmission, wireless transmission, routers, firewalls, switches, gateway computers, and / or edge servers. The network adapter card or network interface in each computing / processing device receives the computer-readable program instructions from the network and forwards the computer-readable program instructions to be stored in the computer-readable storage medium in each computing / processing device.

[0115] The computer program instructions for performing the operations of the present disclosure may be assembly instructions, instruction set architecture (ISA) instructions, machine instructions, machine-dependent 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++, and conventional procedural programming languages ​​such as "C" language or similar programming languages. Computer-readable program instructions may be executed entirely on a user's computer, partially on a user's computer, as an independent software package, partially on a user's computer, 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 via 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., utilizing an Internet service provider to connect via the Internet). In some embodiments, an electronic circuit, such as a programmable logic circuit, a field programmable gate array (FPGA), or a programmable logic array (PLA), may be personalized by utilizing the state information of the computer-readable program instructions. The electronic circuit may execute the computer-readable program instructions, thereby realizing various aspects of the present disclosure.

[0116] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention rather than to limit it. Although the present invention has been described in detail with reference to the above embodiments, ordinary technicians in the field should understand that the specific implementation methods of the present invention can still be modified or replaced by equivalents. Any modification or equivalent replacement that does not depart from the spirit and scope of the present invention should be covered by the scope of protection of the claims of the present invention.

Claims

1. A method for decentralized collaborative configuration of super-capacity branches in BT-EVSC networks, characterized in that: include: A sending-end overcapacity branch is configured on the sending-end side of the DC line, and a receiving-end overcapacity branch is configured on the receiving-end side of the DC line; The difference between the active power signal output by the network converter station and the active power reference value is obtained as the total support power demand of all overcapacity branches and input into the decentralized collaborative controller; During a fault transient, if the decentralized cooperative controller determines that the total supported power demand is greater than the set start-up threshold, it adjusts the power weight coefficient of each over-capacity branch and distributes the power weight coefficient to the over-capacity sub-controller of each over-capacity branch; The super-capacity sub-controller of each super-capacity branch determines the power instruction of each super-capacity branch according to the power weight coefficient; if the power instruction of each super-capacity branch is not greater than the maximum power support level of each super-capacity branch, a control pulse of each super-capacity branch is generated according to the power instruction; if there is an super-capacity branch whose power instruction is greater than the maximum power support level, the super-capacity branch outputs active power according to the maximum power support level, and the decentralized collaborative controller re-adjusts the power weight coefficients of the remaining super-capacity branches according to the difference between the total support power demand and the maximum power support level of the super-capacity branch, and then the super-capacity sub-controller of each super-capacity branch updates the power instruction of each super-capacity branch.

2. The method for decentralized collaborative configuration of super-capacity branches for building a BT-EVSC network according to claim 1 is characterized in that: One end of the over-capacity branch at the sending end is connected to the positive pole of the DC line, and the other end is connected to the grounding electrode of the DC line; one end of the over-capacity branch at the receiving end is connected to the positive pole of the DC line, and the other end is connected to the grounding electrode of the DC line; The sending-end overcapacity branch and the receiving-end overcapacity branch both include a plurality of overcapacity submodules connected in series and an export reactor; wherein each overcapacity submodule is switched on and off by a corresponding switch device.

3. The method for decentralized collaborative configuration of super-capacity branches in BT-EVSC networking according to claim 1 is characterized in that: Based on the active power characteristics of the MMC converter valves adopting the networking control strategy, the output active power signal of the networking converter station is obtained according to the reference phase angle and reference voltage. The difference between the output active power signal of the networking converter station and the active power reference value is used as the total supporting power demand of all over-capacity branches; the total supporting power demand of all over-capacity branches is input into the decentralized collaborative controller.

4. The method for decentralized collaborative configuration of super-capacity branches for building a BT-EVSC network according to claim 1, characterized in that: The starting threshold is set to αP N , where α is the startup threshold coefficient of the overcapacity branch, P N is the active power reference value.

5. The method for decentralized collaborative configuration of super-capacity branches in BT-EVSC networking according to claim 1 is characterized in that: Power weight coefficient λ allocated to each overcapacity branch i , satisfying the following relationship: Where i = 1, 2, ..., m, and m is the total number of excess capacity branches.

6. The method for decentralized collaborative configuration of super-capacity branches for building a BT-EVSC network according to claim 1, characterized in that: The setting of the power weight coefficient of each overcapacity branch includes: 1) The over-capacity branch closest to the converter station is the main branch; among all over-capacity branches, the main branch has the largest power weight coefficient, and its value range is [0,1]; 2) Calculate the maximum capacity of the overcapacity energy storage based on the overload current level of the switching devices in each overcapacity branch, and use it as the maximum power support level of the overcapacity branch to establish the power support level constraints of each overcapacity branch, satisfying the following relationship: P ES_i ≤P ES_imax P ES_imax =I imax V dc Where, P ES_imax is the maximum power support level of the i-th overcapacity branch, I imax is the maximum overload current of the i-th overcapacity branch, V dc is the DC bus voltage; 3) Adjust the power weight coefficient of the main branch When the total supported power demand of all over-capacity branches is not greater than the maximum power support level of the main branch, the power weight coefficient of the main branch is 1; when the total supported power demand of all over-capacity branches is greater than the maximum power support level of the main branch, the power weight coefficient of the main branch satisfies the following relationship: Where λ j The power weight coefficient of the main branch, P ES_jmax is the maximum power support level of the main branch, P ES_imax is the maximum power support level of the i-th overcapacity branch, ED j The electrical distance between the main branch and the converter station, ED i is the electrical distance between the i-th over-capacity branch and the converter station; is the sum of the maximum power support levels of all overcapacity branches, and m is the total number of overcapacity branches; 4) Adjust the power weight coefficients of all excess capacity branches other than the main branch to satisfy the following relationship: After active power support is provided to the network converter station according to the maximum power support level of the main branch, the power support demand shortfall of the network converter station is met by the power weight coefficients of the remaining over-capacity branches other than the main branch, each of which is determined according to the maximum power support level of each branch.

7. The method for decentralized collaborative configuration of super-capacity branches in BT-EVSC networking according to claim 6 is characterized in that: The overcapacity sub-controller of each overcapacity branch determines the power instruction of each overcapacity branch according to the power weight coefficient to satisfy the following relationship: P ES_j =λ j ·ΔP ES P ES_j ≤P ES_jmax P ES_i =λ i ·(1-l j )ΔP ES i≠j,i=1,2,…,m P ES_i ≤P ES_imax i≠j,i=1,2,…,m Where, P ES_j The power command of the main branch, P ES_i is the power command of the i-th overcapacity branch, ΔP ES is the total supported power requirement of all overcapacity branches.

8. The method for decentralized collaborative configuration of super-capacity branches for building a BT-EVSC network according to claim 7 is characterized in that: If the power instructions of each over-capacity branch are not greater than the corresponding maximum power support level, the ratio of the power instruction of the over-capacity branch to the DC bus voltage is used as the DC current reference value of the over-capacity branch. The difference between the DC current reference value and the actual DC current value of the over-capacity branch after passing through the PI controller is then summed with the DC bus voltage. The ratio of the obtained sum to the average voltage of each over-capacity sub-module in the over-capacity branch is used as the number of over-capacity sub-modules put into the over-capacity branch. According to the determined number, trigger pulses are sent to the switching devices corresponding to the over-capacity sub-modules to be put into use.

9. A decentralized collaborative configuration system for super-capacity branches in BT-EVSC network, characterized by: include: Decentralized collaborative controller, total support power demand module, and overcapacity sub-controllers for each overcapacity branch; The total support power demand module is used to obtain the difference between the active power signal output by the network converter station and the active power reference value as the total support power demand of all overcapacity branches, and input it into the decentralized collaborative controller; A decentralized cooperative controller is used to adjust the power weight coefficient of each over-capacity branch and distribute the power weight coefficient to the over-capacity sub-controller of each over-capacity branch if the total support power demand is determined to be greater than the set start threshold during the fault transient period; The overcapacity sub-controller of each overcapacity branch is used to determine the power instruction of each overcapacity branch according to the power weight coefficient; if the power instruction of each overcapacity branch is not greater than the maximum power support level of each overcapacity branch, a control pulse for each overcapacity branch is generated according to the power instruction; if there is an overcapacity branch whose power instruction is greater than the corresponding maximum power support level, the overcapacity branch outputs active power according to the maximum power support level, and sends the difference between the total support power demand and the maximum power support level of the overcapacity branch to the decentralized collaborative controller; The decentralized cooperative controller is further configured to readjust the power weight coefficients of the remaining over-capacity branches based on the difference between the total supported power demand and the maximum power support level of the over-capacity branch, and send the readjusted power weight coefficients of the over-capacity branches to the over-capacity sub-controllers of the over-capacity branches; The over-capacity sub-controller of each over-capacity branch is further used to update the power instruction of each over-capacity branch according to the re-adjusted power weight coefficient of each over-capacity branch.

10. A terminal comprising a processor and a storage medium; characterized in that: The storage medium is used to store instructions; The processor is configured to operate according to the instructions to execute the steps of the method according to any one of claims 1 to 8.

11. A computer-readable storage medium having a computer program stored thereon, characterized in that: When the program is executed by a processor, the steps of the method according to any one of claims 1 to 8 are implemented.

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