Super-capacitance branch decentration collaborative configuration method and system for network construction BT-EVSC

Abstract

The invention discloses a decentralized collaborative configuration method and system for super-capacitance branches of a network-building BT-EVSC. The method comprises the following steps of: respectively configuring a transmitting-end super-capacitance branch and a receiving-end super-capacitance branch at a transmitting end and a receiving end of a direct-current line; if the decentralized cooperative controller determines that the total support power demand is greater than a starting threshold, setting a power weight coefficient of each super-capacitance branch, and allocating the power weight coefficient to a super-capacitance sub-controller of each super-capacitance branch to determine a power instruction of each super-capacitance branch; if the power instruction of each super-capacitance branch is not greater than the maximum power support level, generating a control pulse of each super-capacitance branch according to the power instruction; if a super-capacitance branch of which the power instruction is greater than the maximum power support level exists, the super-capacitance branch outputs active power according to the maximum power support level, and the decentralized cooperative controller resets power weight coefficients of the rest of the super-capacitance branches according to a difference value between the total support power demand and the maximum power support level of the super-capacitance branch and outputs the active power according to the maximum power support level of the super-capacitance branch. And updating the power instruction of each super-capacitance branch, thereby solving the problem that the transient active power supporting capability of the transmitting and receiving double-end converter station of the flexible direct current system is insufficient.

Description

Technical Field

[0001] The invention belongs to the technical field of UHV DC transmission, and particularly relates to a topology of a bipolar terminal-energy storage voltage source converter (BT-EVSC) with inertia support performance, and a method and system for collaborative configuration of supercapacitor branches with decentralized power. Background Art

[0002] In the context of building a new power system dominated by new energy, the scale of new energy power generation is continuously expanding. The flexible DC transmission technology based on modular multilevel converters (MMC-HVDC) has the characteristics of no commutation failure and flexible control mode, and has been widely applied in engineering. With the accelerated construction of the new power system, traditional synchronous units are largely replaced by power electronic devices, and the system shows characteristics such as low inertia and weak damping. Therefore, a large number of synchronous generators and synchronous condensers still need to be equipped to maintain the stable operation of the system. The cost and flexibility problems brought about by the isolated power grid system will be more prominent.

[0003] In the prior art, the network-forming control builds an internal electromotive force by simulating the inertia, damping, and droop characteristics of a synchronous generator to achieve frequency synchronization between the converter outlet voltage and the grid voltage, and has the ability to actively support the system frequency and voltage under weak grids or frequency disturbances. The network-forming technology has been applied and has good application prospects. However, for the network-forming flexible DC technology, since the sub-module capacitors on the DC side of the flexible DC converter station do not have the ability of large-scale energy storage, when there is a power deficit and frequency drop at the receiving end, it is necessary to achieve it through large-scale DC power support or in-situ active support equipment, etc., which is likely to cause instantaneous overload at the sending end, posing more stringent requirements on the sending-end system. The prior art cannot enable the flexible DC converter station to reduce the DC power transmission demand while having a strong inertia support ability. In addition, with the large-scale replacement of traditional units by the access of "DC + new energy" at the receiving end, the power grid power supply structure has changed greatly, and the system shows high power electronic characteristics, resulting in a reduction in system inertia and voltage support ability, posing challenges to the safe and stable operation of the power grid. Although the prior art alleviates the problem of insufficient system inertia by means of supporting the construction of synchronous units, extending the main power grid, and adopting flexible DC transmission, etc., the flexible DC converter station itself is still difficult to provide inertia support ability under system frequency disturbances, and higher requirements are also put forward for the overload level at the sending end through the method of DC power support. Summary of the Invention

[0004] To address the deficiencies in the existing technologies, the present invention provides a method and system for decentralized collaborative configuration of over-capacity branches in a networked BT-EVSC. The topology of the networked BT-EVSC adopts a two-terminal topology, with over-capacity energy storage device branches directly connected at both the sending end and the receiving end, aiming to solve the problem of insufficient transient active power support capacity of the sending and receiving end converter stations in a flexible DC system, and alleviate the problem of low inertia of the system caused by the reduction of the capacity of synchronous machines such as thermal power in the regional power grid. Through the decentralized collaborative configuration of multiple over-capacity energy storage branches, the collaborative support of energy storage power is realized by comprehensively considering the overload level and response speed of the devices in the energy storage branches, reducing the overload level of the opposite-end system caused by the frequency disturbance of the sending / receiving end power grid of the DC transmission system, and enhancing the inertia support capacity and the safe and stable operation level of the UHV flexible DC system.

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

[0006] The present invention proposes a method for decentralized collaborative configuration of over-capacity branches in a networked BT-EVSC, including:

[0007] Configure a sending-end over-capacity branch on the sending-end side of the DC line and a receiving-end over-capacity branch on the receiving-end side of the DC line; obtain the difference between the output active power signal of the networked converter station and the active power reference value as the total support power demand for all over-capacity branches, and input it to the decentralized collaborative controller;

[0008] During a fault transient period, if the decentralized collaborative controller determines that the total support power demand is greater than the set start threshold, then set the power weight coefficients of each over-capacity branch, and distribute the power weight coefficients to the over-capacity sub-controllers of each over-capacity branch;

[0009] The over-capacity sub-controllers of each over-capacity branch determine the power commands of each over-capacity branch according to the power weight coefficients; if the power commands of each over-capacity branch are not greater than the maximum power support level of each over-capacity branch, then generate control pulses for each over-capacity branch according to the power commands; if there is an over-capacity branch with a power command greater than the maximum power support level, then this over-capacity branch outputs active power according to the maximum power support level, and the decentralized collaborative controller re-sets the power weight coefficients of the remaining over-capacity branches according to the difference between the total support power demand and the maximum power support level of this over-capacity branch, and then the over-capacity sub-controllers of each over-capacity branch update the power commands of each over-capacity branch.

[0010] Preferably, one end of the sending-end over-capacity branch is connected to the positive pole of the DC line and the other end is connected to the grounding pole of the DC line; one end of the receiving-end over-capacity branch is connected to the positive pole of the DC line and the other end is connected to the grounding pole of the DC line;

[0011] Both the sending-end over-capacity branch and the receiving-end over-capacity branch include a plurality of over-capacity sub-modules connected in series and an outlet reactor; wherein, each over-capacity sub-module is controlled to be switched on and off by a corresponding switching device.

[0012] Preferably, based on the active power characteristics of the MMC converter valve adopting the network-forming control strategy, the active power signal output by the network-forming 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-forming converter station and the active power reference value is used as the total support power demand of all over-capacity branches; the total support power demand of all over-capacity branches is input into the decentralized cooperative controller.

[0013] Preferably, the set starting threshold is αP N , where α is the starting threshold coefficient of the over-capacity branch, and P N is the active power reference value.

[0014] Preferably, the power weight coefficient λ i allocated to each over-capacity branch satisfies the following relational expression:

[0015]

[0016] where i = 1, 2, …, m, and m is the total number of over-capacity branches.

[0017] Preferably, the tuning of the power weight coefficient of each over-capacity branch includes:

[0018] 1), taking the over-capacity branch with the closest electrical distance to the converter station as the main branch; among all over-capacity branches, the power weight coefficient of the main branch is the largest, and its value range is [0, 1];

[0019] 2), calculating the maximum over-capacity energy storage capacity according to the overload current level of the switching device in each over-capacity branch as the maximum power support level of the over-capacity branch to establish the power support level constraint of each over-capacity branch, satisfying the following relational expression:

[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 over-capacity branch, I imax is the maximum overload current of the i-th over-capacity branch, and V dc is the DC bus voltage;

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

[0024] When the total support 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 taken as 1; when the total support 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 relational expression:

[0025]

[0026] In the formula, λ j is 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 over-capacity branch, ED j is 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 over-capacity branches, and m is the total number of over-capacity branches;

[0027] 4) Set the power weight coefficients of the remaining over-capacity branches other than the main branch to satisfy the following relational expression:

[0028]

[0029] After providing active power support to the network-forming converter station according to the maximum power support level of the main branch, the power support demand deficit of the network-forming converter station is determined by the remaining over-capacity branches other than the main branch according to the maximum power support level of each branch.

[0030] Preferably, the over-capacity sub-controllers of each over-capacity branch determine the power commands of each over-capacity branch according to the power weight coefficients to satisfy the following relational expression:

[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] In the formula, P ES_j is the power command of the main branch, P ES_iis the power command for the i-th over-capacity branch, ΔP ES is the total support power demand for all over-capacity branches.

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

[0037] The present invention also proposes an over-capacity branch decentralized collaborative configuration system for a grid-forming BT-EVSC, including:

[0038] A decentralized collaborative controller, a total support power demand module, and over-capacitance sub-controllers for each over-capacity branch;

[0039] The total support power demand module is used to obtain the difference between the output active power signal of the grid-forming converter station and the active power reference value as the total support power demand for all over-capacity branches, and input it to the decentralized collaborative controller;

[0040] The decentralized collaborative controller is used to, during a fault transient, if it is determined that the total support power demand is greater than the set start threshold, then set the power weight coefficients of each over-capacity branch, and allocate the power weight coefficients to the over-capacitance sub-controllers of each over-capacity branch;

[0041] The over-capacitance sub-controllers for each over-capacity branch are used to determine the power commands of each over-capacity branch according to the power weight coefficients; if the power commands of each over-capacity branch are not greater than the maximum power support levels of each over-capacity branch, then control pulses for each over-capacity branch are generated according to the power commands; if there is an over-capacity branch with a power command greater than the corresponding maximum power support level, then this over-capacity 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 this over-capacity branch to the decentralized collaborative controller;

[0042] The decentralized collaborative controller is further used to, according to the difference between the total support power demand and the maximum power support level of this over-capacity branch, re-set the power weight coefficients of the remaining over-capacity branches, and send the re-set power weight coefficients of each over-capacity branch to the over-capacitance sub-controllers of each over-capacity branch;

[0043] The over-capacitance sub-controllers for each over-capacity branch are further used to update the power commands of each over-capacity branch according to the re-set power weight coefficients of each over-capacity branch.

[0044] The present invention is also a terminal, including a processor and a storage medium; the storage medium is used for storing instructions; the processor is used for operating according to the instructions to execute the steps of the method.

[0045] 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.

[0046] The beneficial effects of the present invention are at least as follows compared with the prior art. The present invention adopts a multi-drop direct-hanging ultra-capacitor device branch and a network-forming flexible DC technology to enable the flexible DC AC power grid to balance the transmission power at the sending and receiving ends during the fault transient period, reduce the electrical quantity fluctuations of the MMC and the ultra-capacitor device branch at both the sending and receiving ends, and enable the flexible DC converter station to reduce the DC power transmission demand while having the inertia support ability; moreover, the present invention gives full play to the energy storage characteristics of the ultra-capacitor device branch. Through the coordinated configuration of multiple energy storage branches, on the one hand, the capacity required for each energy storage branch is reduced, saving the construction cost of a single energy storage branch; on the other hand, each energy storage branch adopts a decentralized and coordinated configuration, and 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 demand of the converter station, the active power capacity of each energy storage branch is flexibly allocated, improving the active response speed of the ultra-capacitor device and reducing the line loss, realizing the safe and efficient utilization of the transient active support of the energy storage branch. Compared with the existing energy storage control methods, the topology and method proposed by the present invention are more suitable for the short-time inertia continuous support working condition of the converter station, which is beneficial to enhancing the inertia support ability and the safe and stable operation level of the UHV flexible DC system. Description of the Drawings

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

[0048] Figure 2 It is a flowchart of the decentralized and coordinated configuration method of the ultra-capacitor branch of the network-forming BT-EVSC proposed by the present invention;

[0049] Figure 3 It is a control block diagram of the output power of the MMC converter valve of the network-forming control strategy in the embodiment of the present invention;

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

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

[0052] Figure 6 It is a simulation waveform diagram of the DC-side voltage at the sending and receiving ends under a frequency step in the embodiment of the present invention;

[0053] Figure 7 This is the simulation waveform diagram of the active power of the sending and receiving end converters and energy storage devices under frequency step in the embodiments of the present invention. Detailed implementation manners

[0054] 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 with reference to 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 belong to the protection scope of the present invention.

[0055] The present invention proposes a method for decentralized collaborative configuration of the ultra-capacity branch of the network-forming BT-EVSC, which is applicable to the network-forming BT-EVSC topology, such as Figure 1 As shown, the network-forming BT-EVSC includes: a sending-end converter station, a receiving-end converter station, and a UHV DC transmission line; among them, the sending and receiving end converter stations adopt high- and low-voltage converter valve structures, and each converter valve is cascaded to the DC bus. The AC sides of the converter valves are respectively connected to the sending-end and receiving-end power grids through converter transformers;

[0056] In the embodiment, the sending-end converter station is composed of a modular multilevel (MMC) converter valve. The MMC converter valve is connected to the sending-end AC system through a converter transformer; the receiving-end converter station is composed of an MMC converter valve. The MMC converter valve is connected to the receiving-end AC system through a converter transformer; the MMC converter valve is composed of an arm inductor, an upper arm, a lower arm, and an arm resistor. The upper arm and the lower arm contain sub-module strings, and the sub-module strings are composed of multiple full-bridge and half-bridge sub-modules. Each sub-module contains a switching device and a reverse diode. By controlling the number of sub-modules put into operation, the total voltage of the upper and lower arms can be adjusted, and then the AC voltage output value can be adjusted.

[0057] The control strategy of the MMC converter valve usually includes modulation signal generation and sub-module voltage equalization control. The modulation signal generation controls the switching state of the sub-modules according to the modulation voltage command value output by the controller, and outputs the target modulation voltage. The voltage equalization control strategy adjusts the arm voltage by controlling the number of sub-modules put into operation in the upper and lower arms, maintains the balance of the capacitor voltages of each sub-module, and prevents overvoltage or undervoltage of the sub-modules during steady-state operation.

[0058] Figure 1 In, the DC transmission line is a single-pole operation schematic diagram, including a DC line impedance and a smoothing reactor.

[0059] The UHVDC transmission line includes: a DC line, a sending-end over-capacity branch, and a receiving-end over-capacity branch; among them, on the sending-end side of the DC line, one end of the sending-end over-capacity branch is connected to the positive pole of the DC line, and the other end is connected to the grounding electrode of the DC line; on the receiving-end side of the DC line, one end of the receiving-end over-capacity branch is connected to the positive pole of the DC line, and the other end is connected to the grounding electrode of the DC line.

[0060] When the MMC converter valves of the sending-end converter station or / and the receiving-end converter station adopt the network-forming control strategy, the sending-end converter station or / and the receiving-end converter station is a network-forming converter station; the difference between the output power of the MMC converter valve adopting the network-forming control strategy and the AC reference power generates the corresponding endogenous electromotive force amplitude and phase angle through the power outer loop, synthesizes the three-phase AC output reference value, and sends it to the inner loop and the switching modulation wave generation. The upper and lower bridge arms of the MMC converter valve adjust the switching quantity of the sub-modules according to the output signal of the network-forming controller, and adjust the output voltage of the upper and lower bridge arms to adjust the output voltage of the converter valve to the network-forming control output target value.

[0061] As Figure 1 shown, both the sending-end over-capacity branch and the receiving-end over-capacity branch include a plurality of series-connected over-capacity sub-modules SC1, SC2,..., SCn, and an outlet reactor L o , where n is a positive integer; each over-capacity energy storage module generates an over-capacity branch reference current command according to the output command of the decentralized collaborative controller and the DC bus voltage, generates the control signal of each over-capacity energy storage module through the current loop control, and adjusts the switching state of each over-capacity energy storage module to regulate the output current of the over-capacity branch, maintain the stability of the DC bus voltage, and meet the network-forming active power and energy support requirements of the receiving-end converter station.

[0062] To stabilize the DC bus voltage and the sending-receiving end power during the transient period, reduce the overload level of a single over-capacity branch, and give full play to the transient support ability advantages of each directly-connected over-capacity energy storage branch (such as electrical distance or device overload), the present invention also proposes an over-capacity branch decentralized collaborative configuration method applicable to network-forming BT-EVSC. During the transient active response of the network-forming converter station, the decentralized collaborative controller calculates the total support power demand of all over-capacity branches according to the output active power signal of the network-forming converter station, considers the electrical distance between each over-capacity branch and the MMC converter valve and the overload level of the devices in the over-capacity branch, adopts a decentralized collaborative control strategy, dynamically adjusts the output power command value of each over-capacity branch according to the power weight coefficient of each over-capacity branch, and each over-capacity branch adjusts the output active power according to the output power command issued by the decentralized collaborative controller. As Figure 2 shown, the over-capacity branch decentralized collaborative configuration method of network-forming BT-EVSC includes:

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

[0064] In the embodiment, as Figure 3 shown, based on the active power characteristics of the MMC converter valve adopting the network-forming control strategy, according to the reference phase angle θ ref and the reference voltage V mref obtain the active power signal P MMC output by the network-forming converter station. The difference between the active power signal P MMC output by the network-forming converter station and the active power reference value P N is used as the total support power demand ΔP ES for all over-capacity branches; the total support power demand ΔP ES for all over-capacity branches is input to the decentralized collaborative controller.

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

[0066] Step 2, during the fault transient period, if the decentralized collaborative controller determines that the total support power demand of the over-capacity branches is greater than the set start threshold, then set the power weight coefficients of each over-capacity branch and allocate the power weight coefficients to the over-capacity sub-controllers of each over-capacity branch.

[0067] Specifically, during the fault transient period, if the decentralized collaborative controller determines that ΔP ES >αP N , then the decentralized collaborative controller sets the power weight coefficients of each over-capacity branch; where α is the start threshold coefficient of the over-capacity branch, which is related to the steady-state power fluctuation amplitude during the steady-state operation of the HVDC system. The voltage fluctuation of the MMC sub-module usually does not exceed 10%. It is approximately considered that the energy storage is put into operation when the over-capacity branch is close to the threshold with a certain margin. Here, 1.05 is taken; P N is the active power reference value;

[0068] Considering the possible transient active power energy demand of the sending and receiving HVDC converter stations of BT-EVSC, the over-capacity branches are arranged in a double-ended decentralized manner, and the input commands of each over-capacity branch are issued by the decentralized collaborative controller in the sending-end / receiving-end network-forming converter station.

[0069] Specifically, the power weight coefficient λ i, where \(i = 1, 2, \ldots, m\), and \(m\) is the total number of over-capacity branches, satisfying the following relational expression:

[0070]

[0071] The decentralized cooperative controller determines the power weight coefficients of each over-capacity branch according to the electrical distances between each over-capacity branch and the converter station and the overload levels of the devices in each over-capacity branch;

[0072] In the embodiment, the setting of the power weight coefficients of each over-capacity branch includes:

[0073] 1), taking the over-capacity branch with the closest electrical distance to the converter station as the main branch; among all over-capacity branches, the power weight coefficient of the main branch is the largest, and the value range is \([0, 1]\);

[0074] Considering the communication delay caused by the long distance of the UHV flexible DC transmission line, the power weight coefficient is restricted by both physical communication distance and electrical distance. Therefore, among all over-capacity branches, the transient active power support is mainly borne by the over-capacity branch with the closest comprehensive electrical distance and communication distance to the converter station, and this over-capacity branch is defined as the main branch. Since the BT-EVSC is a two-terminal MMC topology and there is no additional branch in the DC line, the weights of the electrical distance and the communication distance are the same. Therefore, the power weight coefficient of the over-capacity main branch with the closest electrical distance to the converter station is the largest and it responds to the active power support demand of the converter station first. In the embodiment, the value range of the power weight coefficient of the main branch is \([0, 1]\);

[0075] The present invention makes full use of the architecture feature that the DC line has no branches and the communication channel construction feature in the DC line, and equivalently replaces the communication distance with the electrical distance between the over-capacity branch and the converter station. The communication distance characterizes the response speed of each over-capacity branch to the power cooperative support demand of the network-forming converter station. Therefore, selecting the main branch based on the closest electrical distance to the converter station improves the response speed of the power cooperative support demand of the network-forming converter station.

[0076] 2), calculating the maximum over-capacity energy storage capacity according to the overload current levels of the switching devices in each over-capacity branch as the maximum power support level of the over-capacity branch to establish the power support level constraint of each over-capacity branch, satisfying the following relational expression:

[0077] P ES_i ≤P ES_imax

[0078] P ES_imax =I imax V dc

[0079] In the formula, P ES_imax is the maximum power support level of the \(i\)-th over-capacity branch, and I imaxis the maximum overload current of the switching device in the i-th over-capacity branch, V dc is the DC bus voltage.

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

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

[0082] When the total support power demand ΔP of all over-capacity branches ES is not greater than the maximum power support level P of the main branch ES_jmax , the power weight coefficient of the main branch takes 1; when the total support power demand ΔP of all over-capacity branches ES is greater than the maximum power support level P of the main branch ES_jmax , the power weight coefficient of the main branch satisfies the following relational expression:

[0083]

[0084] In the formula, λ j is 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 over-capacity branch, ED j is 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 over-capacity branches;

[0085] Since the power weight coefficient of the main branch simultaneously reflects the ability of the main branch to improve the power collaborative support demand response speed of the network-forming converter station and the ability to enhance the inertia support of the UHV flexible DC system.

[0086] 4), set the power weight coefficient of each over-capacity branch other than the main branch

[0087] After providing active power support to the network-forming converter station according to the maximum power support level of the main branch, the power support demand deficit of the network-forming converter station is determined by each over-capacity branch other than the main branch according to the maximum power support level of each branch. The power weight coefficient of each over-capacity branch other than the main branch satisfies the following relational expression:

[0088]

[0089] In the present invention, the power weight coefficients of the other supercapacitor branches except the main branch are used to schedule the output powers of the other supercapacitor branches to meet the inertia support energy requirements. Moreover, on this basis, the failure probability caused by overcurrent of the switching devices in a single supercapacitor energy storage branch is reduced, and the safety and reliability of the DC power transmission system are improved.

[0090] Moreover, the present invention gives full play to the energy storage characteristics of the supercapacitor device branches. Through the collaborative configuration of multiple energy storage branches, on the one hand, the required capacity of each energy storage branch is reduced, and the construction cost of a single energy storage branch is saved; on the other hand, each energy storage branch adopts a decentralized collaborative configuration, and the power weight coefficients of each branch are 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 devices. According to the active power support requirements of the converter station, the active power capacity of each energy storage branch is flexibly allocated, the active power response speed of the supercapacitor device is increased, and the line loss is reduced, so as to realize the safe and efficient utilization of the transient active power support of the energy storage branch.

[0091] Step 3: The supercapacitor sub-controllers of each supercapacitor branch determine the power commands of each supercapacitor branch according to the power weight coefficients. If the power commands of each supercapacitor branch are not greater than the corresponding maximum power support levels, control pulses of each supercapacitor branch are generated according to the power commands. If there is a supercapacitor branch whose power command is greater than the corresponding maximum power support level, then this supercapacitor branch outputs active power according to the maximum power support level, and the decentralized collaborative controller re-tunes the power weight coefficients of the other supercapacitor branches according to the difference between the total support power requirement of all supercapacitor branches and the maximum power support level of this supercapacitor branch, and then updates the power commands of each supercapacitor branch.

[0092] The supercapacitor sub-controllers of each supercapacitor branch determine the power commands of each supercapacitor branch according to the power weight coefficients, 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] In the formula, P ES_j is the power command of the main branch, P ES_iis the power command for the i-th over-capacity branch, P ES_imax is the maximum power support level for the i-th over-capacity branch;

[0098] The output power of each over-capacity branch should be equal to the power command;

[0099] When the total support power demand is greater than the set start threshold, the total support power demand will be jointly responded by the multi-drop direct-connected over-capacity branches. The over-capacity sub-controllers of each over-capacity branch determine the power commands of each over-capacity branch according to the power weight coefficients. The over-capacity sub-controllers first adjust the output current of each over-capacity branch, and then adjust the active power output of each over-capacity branch to respond to the power command, providing active power support for the network-forming converter station;

[0100] When the active power output of each over-capacity 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 over-capacity branch reaches the maximum power support level during the transient output period, then this branch supports with the maximum active power, and the surplus power is distributed by other over-capacity branches according to the power weight coefficient to jointly support the active power demand of the system. By adopting the network-forming type converter valve and the multi-drop direct-connected over-capacity energy storage, the inertia support ability is provided for the converter station, reducing the overloading level at the sending / receiving end caused by the power grid frequency disturbance at the sending / receiving end converter station.

[0101] As Figure 4 shown, the over-capacity sub-controller controls the over-capacity branch according to the power commands of each over-capacity branch, including:

[0102] 1), If the power commands of each over-capacity branch are not greater than the corresponding maximum power support levels, then use the ratio of the power command P ES_i (i = 1, 2,..., m) of the over-capacity branch to the DC bus voltage V dc as the DC current reference value I dcref of the over-capacity branch. The difference between the DC current reference value I dcref and the actual DC current value of the over-capacity branch is summed with the DC bus voltage V dc after passing through a PI controller. The ratio of the obtained sum to the average voltage V cavg of each over-capacity sub-module in the over-capacity branch is used as the number of over-capacity sub-modules to be put into operation in the over-capacity branch. According to the determined number, trigger pulses are sent to the switching devices of the over-capacity sub-modules to be put into operation, and each over-capacity branch jointly supports the active power required during the transient period of the network-forming flexible DC converter station;

[0103] 2) If the power command of a supercapacitor branch is greater than the corresponding maximum power support level, then the supercapacitor branch outputs active power according to the maximum power support level. After the decentralized cooperative controller re-tunes the power weight coefficients of the remaining supercapacitor branches according to the difference between the total support power demand of the supercapacitor branch and the maximum power support level of the supercapacitor branch, the supercapacitor sub-controller updates the power commands of each supercapacitor branch.

[0104] Compared with the existing energy storage control methods, the control strategy for each supercapacitor branch to output active power proposed by the present invention is carried out on the basis of partition cooperative configuration. The topology and power support demand response method of the multi-drop direct-connected supercapacitor branch of the network-forming BT-EVSC proposed by the present invention are more suitable for the short-term inertia continuous support condition of the converter station, which is beneficial to enhancing the inertia support ability and the safe and stable operation level of the UHV flexible DC system.

[0105] Further, after the operation delay, the decentralized cooperative controller updates the total support power demand of all supercapacitor branches. If there is still a shortage in the total support power demand, steps 1 to 3 are repeatedly executed to enable the flexible DC AC grid to balance the transmission power at both the sending and receiving ends during the fault transient period, reduce the electrical quantity fluctuations of the MMCs and the supercapacitor equipment branches at both the sending and receiving ends, and enable the flexible DC converter station to reduce the DC power transmission demand while having the inertia support ability.

[0106] A simulation model of the topology of the network-forming BT-EVSC flexible DC converter and the decentralized cooperative control method was built in PSCAD to verify the effectiveness of the proposed topology and its decentralized cooperative configuration method. The operating parameters of a single MMC converter station are as shown in the following table. The receiving-end converter station in the simulation model adopts network-forming control.

[0107] Table 1 Parameters of the flexible DC system converter

[0108] Converter rated operating power / MW 2000 Valve side rated voltage / kV 196 Rated DC voltage / kV 400 Arm inductance / mH 25 Rated angular frequency / rad / s 314 Number of sub - modules in a single arm of flexible DC MMC / piece 190 Existing capacitance value of sub - module capacitor / mF 24 Converter transformer capacity / MVA 750 Converter transformer rated turns ratio 515 kV / 196 kV Converter transformer rated impedance 20％

[0109] In the embodiment, one supercapacitor branch is adopted, the weight coefficient is considered to be 1.0, and the starting threshold coefficient of the supercapacitor branch is selected to be 5%. The supercapacitor branch configuration and DC voltage that meet the device overcurrent constraint, output power constraint and energy constraint are selected, and a supercapacitor redundant cluster is reserved to improve the reliability of the energy storage device. The DC current of a single branch is not greater than 3 kA. A 144 V / 62.5 F module is selected, the internal resistance of a single module is 96 mΩ, and the total number of supercapacitor valve sub-modules is 325.

[0110] When a frequency disturbance occurs in the receiving-end power grid, due to the grid-forming control of the receiving-end converter station, the converter station will provide transient active power support to the receiving-end power grid. The VSC converter station itself does not have the ability to provide active power support and needs to provide power support through the DC line. At this time, the over-capacity branch will provide the active power and energy required during the converter station fault, meeting the system inertia support capacity requirements. At the same time, since the over-capacity branch can fully cover the power demand during the transient period of the receiving-end converter station fault, it reduces the overload level of the sending-end system while ensuring that the receiving-end converter station provides sufficient inertia response characteristics, which is beneficial to the safe and stable operation of the VSC system.

[0111] Set the system operating condition that the VSC transmission system operates at the rated condition 6 s before, and set the fault condition that the frequency of the receiving-end AC system steps from 50 Hz to 49.5 Hz at 6 s. The system simulation results are as Figure 5 、 Figure 6 、 Figure 7 shown. Figure 5 In the figure, Pm represents the active power output of the AC side of the receiving-end converter, with the unit of 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, with the unit of kV; Figure 7 In the figure, Sdc_SC represents the total active power support of the over-capacity branch, Sdc_R represents the DC power of the receiving-end converter, and Sdc_T represents the DC power of the sending-end converter, with the unit of MW. After the frequency step occurs in the receiving-end system, the receiving-end converter station provides the system inertia active power support, and the directly-connected over-capacity energy storage responds to the change in the active power output of the receiving-end converter station. While maintaining the stability of the DC voltages at the sending and receiving ends, it effectively reduces the overload level of the sending-end system, verifying the effectiveness of the topology of the BT-EVSC grid-forming VSC converter with inertia support ability and the decentralized collaborative control method proposed in the present invention.

[0112] 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.

[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 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 punched 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 being a transient signal per se, 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.

[0114] The computer-readable program instructions described herein can be downloaded from a computer-readable storage medium to respective computing / processing devices, 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 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. A 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 for storage in a 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++, 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., via 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.

[0116] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended 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: it is still possible to modify the specific implementation manners of the present invention or make equivalent substitutions, and any modification or equivalent substitution that does not depart from the spirit and scope of the present invention should be covered by the protection scope of the claims of the present invention.

Claims

1. A method for decentralized collaborative configuration of a supercapacitor branch in a grid-forming BT-EVSC, characterized in that Including: A sending-end over-capacity branch is configured on the sending-end side of the DC line, and a receiving-end over-capacity branch is configured on the receiving-end side of the DC line; Obtain the difference between the active power signal output by the grid-forming converter station and the active power reference value as the total support power demand of all over-capacity branches, and input it to the decentralized collaborative controller; During the fault transient period, if the decentralized collaborative controller determines that the total support power demand is greater than the set startup threshold, then set the power weight coefficients of each over-capacity branch, and distribute the power weight coefficients to the over-capacity sub-controllers of each over-capacity branch; The over-capacity sub-controllers of each over-capacity branch determine the power commands of each over-capacity branch according to the power weight coefficients; if the power commands of each over-capacity branch are not greater than the maximum power support level of each over-capacity branch, then generate the control pulses of each over-capacity branch according to the power commands; if there is an over-capacity branch with a power command greater than the maximum power support level, then this over-capacity branch outputs active power according to the maximum power support level, and the decentralized collaborative controller re-sets the power weight coefficients of the remaining over-capacity branches according to the difference between the total support power demand and the maximum power support level of this over-capacity branch, and then the over-capacity sub-controllers of each over-capacity branch update the power commands of each over-capacity branch.

2. The method for decentralized collaborative configuration of over-capacity branches of the grid-forming BT-EVSC according to claim 1, wherein One end of the sending-end over-capacity branch is connected to the positive pole of the DC line, and the other end is connected to the grounding pole of the DC line; one end of the receiving-end over-capacity branch is connected to the positive pole of the DC line, and the other end is connected to the grounding pole of the DC line; Both the sending-end over-capacity branch and the receiving-end over-capacity branch include a plurality of series-connected over-capacity sub-modules and outlet reactors; wherein, each over-capacity sub-module is controlled to be switched on and off by a corresponding switching device.

3. The method for decentralized collaborative configuration of over-capacity branches of the grid-forming BT-EVSC according to claim 1, wherein Based on the active power characteristics of the MMC converter valve adopting the grid-forming control strategy, obtain the active power signal output by the grid-forming converter station according to the reference phase angle and reference voltage, and the difference between the active power signal output by the grid-forming converter station and the active power reference value is used as the total support power demand of all over-capacity branches; the total support power demand of all over-capacity branches is input into the decentralized collaborative controller.

4. The method for decentralized collaborative configuration of over-capacity branches of the grid-forming BT-EVSC according to claim 1, wherein The set starting threshold is αP N , where α is the starting threshold coefficient of the over-capacity branch, and P N is the reference value of the active power.

5. The method for decentralized collaborative configuration of over-capacity branches of the grid-forming BT-EVSC according to claim 1, wherein The power weight coefficient λ allocated to each over-capacity branch i satisfies the following relational expression: where i = 1, 2, …, m, and m is the total number of over-capacity branches.

6. The method for decentralized collaborative configuration of over-capacity branches of the grid-forming BT-EVSC according to claim 1, wherein The setting of the power weight coefficients of each over-capacity branch includes: 1), Taking the over-capacity branch with the closest electrical distance to the converter station as the main branch; among all over-capacity branches, the power weight coefficient of the main branch is the largest, and the value range is [0, 1]; 2), Calculate the maximum over-capacity energy storage capacity according to the overload current level of the switching device in each over-capacity branch as the maximum power support level of the over-capacity branch, so as to establish the power support level constraint of each over-capacity branch, and satisfy the following relational expression: P ES_i ≤P ES_imax P ES_imax = I imax V dc Wherein, P ES_imax is the maximum power support level of the i-th over-capacity branch, I imax is the maximum overload current of the i-th over-capacity branch, V dc is the DC bus voltage; 3) Set the power weight coefficient of the main branch When the total support 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 taken as 1; when the total support 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 relational expression: where λ j is 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 over-capacity branch, ED j is 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 over-capacity branches, and m is the total number of over-capacity branches; 4) Set the power weight coefficients of the other over-capacity branches except the main branch, which satisfy the following relational expression: After providing active power support to the network-forming converter station according to the maximum power support level of the main branch, the power support demand deficit of the network-forming converter station is determined by the power weight coefficients of the other over-capacity branches except the main branch according to the maximum power support level of each branch.

7. The over-capacity branch decentralized collaborative configuration method of the network-forming BT-EVSC according to claim 6, characterized in that The over-capacity sub-controllers of each over-capacity branch determine the power commands of each over-capacity branch according to the power weight coefficients, which satisfy the following relational expression: P ES_j = λ j ·ΔP ES P ES_j ≤P ES_jmax P ES_i = λ i ·(1 - λ j )ΔP ES i ≠ j, i = 1, 2, …, m P ES_i ≤P ES_imax where i≠j, i = 1, 2, …, m where, P ES_j is the power command of the main branch, P ES_i is the power command of the i-th over-capacity branch, and ΔP ES is the total support power demand of all over-capacity branches.

8. The over-capacity branch decentralized collaborative configuration method of the network-forming BT-EVSC according to claim 7, characterized in that If the power commands of each over-capacity branch are not greater than the corresponding maximum power support levels, then the ratio of the power command of the over-capacity branch to the DC bus voltage is used as the DC current reference value of the over-capacity branch. The value obtained by the difference between the DC current reference value and the actual DC current value of the over-capacity branch after passing through a PI controller is summed with the DC bus voltage, and 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 to be put into operation in 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 operation.

9. A supercapacitor branch decentralized collaborative configuration system for constructing a networked BT-EVSC, characterized in that, Including: Decentralized collaborative controller, total support power demand module, over-capacity sub-controllers of each over-capacity branch; The total support power demand module is used to obtain the difference between the active power signal output by the network-forming converter station and the active power reference value as the total support power demand of all over-capacity branches, and input it to the decentralized collaborative controller; The decentralized collaborative controller is used to, during the fault transient period, if it is determined that the total support power demand is greater than the set start threshold, set the power weight coefficients of each over-capacity branch, and distribute the power weight coefficients to the over-capacity sub-controllers of each over-capacity branch; The over-capacity sub-controllers of each over-capacity branch are used to determine the power commands of each over-capacity branch according to the power weight coefficients; if the power commands of each over-capacity branch are not greater than the maximum power support levels of each over-capacity branch, then control pulses of each over-capacity branch are generated according to the power commands; if there is an over-capacity branch with a power command greater than the corresponding maximum power support level, then the over-capacity 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 over-capacity branch to the decentralized collaborative controller; The decentralized collaborative controller is also used to, according to the difference between the total support power demand and the maximum power support level of the over-capacity branch, re-set the power weight coefficients of the other over-capacity branches, and send the re-set power weight coefficients of each over-capacity branch to the over-capacity sub-controllers of each over-capacity branch; The supercapacitor sub-controller of each supercapacitor branch is further configured to update the power command of each supercapacitor branch according to the re-tuned power weight coefficient of each supercapacitor 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 1-8.

11. 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 1-8 are implemented.

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