Network construction HC-EVSC device and super-capacity energy dispersion balance configuration method thereof

Abstract

The invention discloses a networking HC-EVSC device and a super-capacitance energy dispersion balance configuration method thereof. N parallel super-capacitance energy storage branches are configured on an inversion side; according to the active power variation and the power recovery time of the MMC adopting the network construction control strategy under the power grid frequency step, determining the number of input super-capacitance energy storage branches; determining the total transmission power of all the super-capacitance energy storage branches and the energy coefficient of each super-capacitance energy storage branch according to the active power vacancy of the MMC adopting the network construction control strategy, and determining the output power instruction of the super-capacitance energy storage branch according to the total transmission power and the energy coefficient; and when the output power instruction of the super-capacitance energy storage branch meets the maximum power support horizontal constraint, the active power output by each super-capacitance energy storage branch is dynamically adjusted according to the output power instruction, so that the super-capacitance energy dispersion balance configuration is realized, the problems of rotational inertia and insufficient reactive power support capability of the hybrid cascade power transmission system are effectively solved, and the power transmission efficiency of the hybrid cascade power transmission system is improved. And the operation reliability and stability of the hybrid cascade power transmission system are improved.

Description

Technical Field

[0001] The present invention belongs to the technical field of UHV DC power transmission, and particularly relates to a grid-forming hybrid cascade energy storage type voltage source converter (Hybrid cascade-Energy Storage Voltage Source Converter, HC-EVSC) topology with inertia support performance and a supercapacitor energy dispersion equalization configuration method therefor. Background Art

[0002] LCC DC power transmission technology has been widely used in large-capacity long-distance power transmission scenarios due to its many advantages such as large transmission capacity, strong economy, and mature technology. However, the commutation failure problem of the inverter-side LCC restricts its application to a certain extent. Compared with LCC technology, the flexible DC power transmission technology based on modular multilevel converter (MMC) has characteristics such as no commutation failure and flexible control mode. However, compared with LCC technology, the investment is relatively large. Combining the technical advantages of LCC and MMC, the Baihetan-Jiangsu UHV project adopts a structure in which the sending-end LCC, the receiving-end LCC are connected in series with 3 parallel MMCs to achieve a power transmission of 800 kV / 8000 MW. In this hybrid cascade DC power transmission system, the 3 parallel MMCs can provide reactive power support for the LCC of the inverter station, reducing the risk of LCC commutation failure to a certain extent. At the same time, the LCC converter can block the fault current of the MMC when a DC fault occurs. Therefore, it has good technical application prospects.

[0003] In the prior art, the traditional MMC converter adopts a grid-following control strategy, showing a current source characteristic. It cannot track the load power fluctuation and needs to rely on a strong power grid for operation, which is suitable for a power grid environment dominated by synchronous generators. While the grid-forming control shows a voltage source external characteristic similar to that of a synchronous generator, which can autonomously form a network and actively support the grid frequency / voltage, and is suitable for a new power system with a high degree of power electronics. The MMC part adopts grid-forming control to maintain the stability of the DC bus voltage, which is beneficial to the stable and reliable operation of the LCC. However, in order to provide sufficient inertia and reactive power support, the power of the converter changes greatly during the transient period, and the required transient power will cause a sudden change in the power of the opposite station, and the opposite station shows a high overload state, seriously threatening the safe and stable operation of the HVDC equipment. Therefore, it is necessary to propose a hybrid cascade topology and its control method suitable for the new power system. Summary of the Invention

[0004] To address the deficiencies in the existing technologies, the present invention provides a grid-forming HC-EVSC device with inertia support performance and a method for ultra-capacitor energy dispersion and equalization configuration. Based on the hybrid cascaded DC transmission topology, an ultra-capacitor branch is installed on the DC side of the MMC, and the energy dispersion and equalization configuration under decentralized cooperation of the ultra-capacitor branch is realized, effectively solving the problems of insufficient rotational inertia and reactive power support capacity in the hybrid cascaded transmission system, and improving the operation reliability and stability of the hybrid cascaded transmission system.

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

[0006] The present invention proposes a grid-forming HC-EVSC device. The inverter side adopts a structure in which multiple parallel modular multilevel converters are cascaded with one phase-commutated converter. The device includes: n parallel ultra-capacitor energy storage branches configured on the inverter side, where n is a positive integer; one end of each ultra-capacitor energy storage 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;

[0007] The device further includes: an ultra-capacitor energy equalization controller and n ultra-capacitor energy storage branch controllers; the ultra-capacitor energy equalization controller is used to determine the number of ultra-capacitor energy storage branches to be put into operation according to the active power change amount and power recovery time of the modular multilevel converter adopting the grid-forming control strategy under the grid frequency step; according to the active power deficit of the modular multilevel converter adopting the grid-forming control strategy, determine the total transmission power of all ultra-capacitor energy storage branches and the energy coefficients of each ultra-capacitor energy storage branch, and send the total transmission power and the energy coefficients of each ultra-capacitor energy storage branch to the corresponding ultra-capacitor energy storage branch controller;

[0008] The ultra-capacitor energy storage branch controller is used to determine the output power command of the ultra-capacitor energy storage branch according to the received total transmission power and energy coefficient; when the output power command of the ultra-capacitor energy storage branch satisfies the maximum power support level constraint, the ultra-capacitor energy storage branch controller adjusts the active power output by each ultra-capacitor energy storage branch according to the output power command.

[0009] Each ultra-capacitor energy storage branch includes multiple series-connected ultra-capacitor sub-modules; each ultra-capacitor sub-module includes a power device and an ultra-capacitor cluster.

[0010] The ultra-capacitor energy equalization controller includes: an ultra-capacitor energy storage branch parameter configuration module;

[0011] The ultra-capacitor energy storage branch parameter configuration module is used to determine the maximum current value of all ultra-capacitor energy storage branches according to the active power change amount and power recovery time of the modular multilevel converter adopting the grid-forming control strategy under the grid frequency step; use the maximum current value and the rated current of each ultra-capacitor energy storage branch to determine the number of parallel ultra-capacitor energy storage branches.

[0012] The parameter configuration module of the supercapacitor energy storage branch includes: a frequency step acquisition unit, a simulation unit, a branch current calculation unit, and a branch number calculation unit;

[0013] The frequency step acquisition unit is used to obtain the maximum frequency step value of the AC system connected to the modular multilevel converter adopting the grid-forming control strategy;

[0014] The simulation unit is used to determine the maximum value of the active power change amount of the modular multilevel converter adopting the grid-forming control strategy under the maximum frequency step value, and the power recovery time required for the active power to recover to the rated active power by means of simulation in the scenario where all modular multilevel converters output the rated active power and no supercapacitor energy storage branch is connected;

[0015] The branch current calculation unit takes the maximum value of the active power change amount output by the simulation unit as the maximum active power supported by all supercapacitor energy storage branches, and the power recovery time output by the simulation unit as the support time of all supercapacitor energy storage branches; and calculates the maximum current of all supercapacitor energy storage branches by using the maximum active power supported by all supercapacitor energy storage branches, the support time, and the rated voltage of each supercapacitor energy storage branch; where the DC bus voltage is the rated voltage of each supercapacitor energy storage branch;

[0016] The branch number calculation unit is used to set the number of supercapacitor energy storage branches to 1 if the maximum current of all supercapacitor energy storage branches is not greater than the maximum current-carrying capacity of the power device; if the maximum current of all supercapacitor energy storage branches is greater than the maximum current-carrying capacity of the power device, determine the number of parallel supercapacitor energy storage branches according to the maximum current of all supercapacitor energy storage branches and the rated current of each supercapacitor energy storage branch; where the rated current of each supercapacitor energy storage branch is determined according to the current-carrying capacity and overload capacity of the power device in each supercapacitor energy storage branch.

[0017] The maximum current of all supercapacitor energy storage branches satisfies the following relational expression:

[0018]

[0019] In the formula, I SCmax is the maximum current of all supercapacitor energy storage branches, ΔP max is the maximum active power supported by all supercapacitor energy storage branches, ΔT max is the support time of all supercapacitor energy storage branches, V SCN is the rated voltage of each supercapacitor energy storage branch.

[0020] The supercapacitor energy balance controller further includes: an energy balance control module;

[0021] The energy balance control module is used to determine the total transmission power of all over-capacity energy storage branches and the energy coefficients of each over-capacity energy storage branch according to the active power deficit of the modular multilevel converter adopting the network-forming control strategy; and send the total transmission power and the energy coefficients of each over-capacity energy storage branch to the corresponding over-capacity energy storage branch controller respectively.

[0022] The energy balance control module includes: a total transmission power calculation unit;

[0023] The total transmission power calculation unit is used to take the difference between the active power output by the modular multilevel converter adopting the network-forming control strategy and the active power reference value as the active power deficit, and use the active power deficit as the total transmission power of all over-capacity energy storage branches.

[0024] The total transmission power of all over-capacity energy storage branches satisfies the following relational expression:

[0025] ΔP SC =P mmc -P mmc_ref

[0026] In the formula, ΔP SC is the total transmission power of all over-capacity energy storage branches, P mmc is the active power output by the MMC adopting the network-forming control strategy, and P mmc_ref is the active power reference value output by the MMC adopting the network-forming control strategy.

[0027] The energy balance control module further includes: an energy coefficient equalization configuration unit;

[0028] The energy coefficient equalization configuration unit is used to assign the largest energy coefficient, which is α′, to the over-capacity energy storage branch with the shortest electrical distance from the receiving-end converter station among all over-capacity energy storage branches. The value range of α′ is [0.8, 1.0]; the energy coefficients of the remaining over-capacity energy storage branches are assigned equally, all of which are α″, and satisfy α′+(n - 1)α″ = 1;

[0029] Calculate the output power commands of each over-capacity energy storage branch by using the assigned values of the energy coefficients of each over-capacity energy storage branch; based on the output power commands of each over-capacity energy storage branch, determine the regulation cost and regulation duration of each over-capacity energy storage branch;

[0030] Taking the comprehensive optimization of the regulation cost and regulation duration of each over-capacity energy storage branch and the comprehensive optimization of the regulation cost and regulation duration of all over-capacity energy storage branches as the objective function, and taking the sum of the output power commands of all over-capacity energy storage branches satisfying the total transmission power of all over-capacity energy storage branches as the constraint condition, optimize the energy coefficients of each over-capacity energy storage branch through multiple iterations; the optimized energy coefficients of each over-capacity energy storage branch are α i 、i = 1, 2, …, n, and satisfy

[0031] The objective function satisfies the following relational expression:

[0032]

[0033] Wherein, F is the objective function, λ is the weight, and C i , τ i are respectively the adjustment cost and adjustment duration of the response output power command of the i-th over-capacity energy storage branch, and C i = f(ΔP i ), τ i = f(ΔP i ), ΔP i is the difference between the actual output power value and the output power command of the i-th over-capacity energy storage branch, and f() represents a functional relationship;

[0034] The sum of the output power commands of all over-capacity energy storage branches satisfies the total transmission power of all over-capacity energy storage branches as a constraint condition, and satisfies the following relational expression:

[0035]

[0036] Wherein, is the output power command of the i-th over-capacity energy storage branch, and ΔP SC is the total transmission power of all over-capacity energy storage branches.

[0037] The over-capacity energy storage branch controller includes: an output power command generation module;

[0038] The output power command generation module is used to determine the output power command of the over-capacity energy storage branch according to the received total transmission power and energy coefficient; when the output power command of the over-capacity energy storage branch satisfies the maximum power support level constraint, the over-capacity energy storage branch controller adjusts the active power output by each over-capacity energy storage branch according to the output power command;

[0039] When the output power command of the over-capacity energy storage branch does not satisfy the maximum power support level constraint, the active power output by the over-capacity energy storage branch remains at the maximum power support level, and a signal for re-optimizing the energy coefficient of each over-capacity energy storage branch is sent to the over-capacity energy balance controller.

[0040] The output power command of the over-capacity energy storage branch determined by the output power command generation module should satisfy the maximum power support level constraint shown in the following relational expression:

[0041]

[0042] Wherein, is the output power command of the i-th over-capacity energy storage branch, and PSC_imax is the maximum power support level of the i-th over-capacity energy storage branch at the rated DC voltage of the receiving-end converter station. The maximum power support levels of each over-capacity energy storage branch are determined according to the current-carrying capacity and overload capacity of the power devices in different over-capacity energy storage branches under the rated voltage.

[0043] The over-capacity energy storage branch controller further includes: an over-capacity sub-module parameter configuration module;

[0044] The over-capacity sub-module parameter configuration module is used to establish an equivalent circuit of the over-capacity energy storage branch before adjusting the active power output of each over-capacity energy storage branch according to the output power instruction. The equivalent circuit includes an equivalent capacitor and an equivalent internal resistance. Based on the equivalent circuit, when the power released by the equivalent capacitor is not less than the sum of the power consumed by the equivalent internal resistance and the active power demand of the over-capacity energy storage branch, determine the number of over-capacity sub-modules connected in series in the over-capacity energy storage branch according to the output power instruction, the open-circuit voltage, and the equivalent capacitor value.

[0045] The over-capacity sub-module parameter configuration module includes: an equivalent circuit establishment unit;

[0046] The equivalent circuit establishment unit is used to establish an equivalent circuit of each over-capacity energy storage branch. The equivalent circuit is a series connection of an ideal capacitor and an ideal resistor. And the capacitance value of the ideal capacitor is the equivalent capacitance value C in the state where all over-capacity sub-modules in all over-capacity energy storage branches are put into operation SC_i and the resistance value of the ideal resistor is the equivalent internal resistance value R in the state where all over-capacity sub-modules in all over-capacity energy storage branches are put into operation SC_i ;

[0047] The power consumed by the equivalent internal resistance value of the i-th over-capacity energy storage branch satisfies the following relational expression:

[0048]

[0049] In the formula, P SCR_i is the power consumed by the equivalent internal resistance value of the i-th over-capacity energy storage branch, I SCN_i is the rated current of the i-th over-capacity energy storage branch, R SC_i is the equivalent internal resistance value of the i-th over-capacity energy storage branch, ΔT max is the support time of all over-capacity energy storage branches;

[0050] The power released by the equivalent capacitance value of the i-th over-capacity energy storage branch satisfies the following relational expression:

[0051] P SCC_i ≥P SCN_i +P SCR_i

[0052] In the formula, P SCC_iThe power released for the equivalent capacitance value of the i-th over-capacity energy storage branch, P SCN_i The active power demand of the i-th over-capacity energy storage branch based on the equivalent circuit, satisfying ΔP max Is the maximum value of the active power supported by all over-capacity energy storage branches.

[0053] The over-capacity sub-module parameter configuration module further includes: an over-capacity sub-module quantity calculation unit;

[0054] In the over-capacity sub-module quantity calculation unit, it is determined that the equivalent capacitance value in the over-capacity energy storage branch satisfies the following relational expression:

[0055]

[0056] In the formula, V SC_i Is the open-circuit voltage of the i-th over-capacity energy storage branch, V SCN Is the rated voltage of each over-capacity energy storage branch, Is the output power command of the i-th over-capacity energy storage branch;

[0057] Combined with the withstand voltage capacity of the over-capacity cluster in the over-capacity sub-module and the equipment insulation level, the open-circuit voltage V of the i-th over-capacity energy storage branch is selected SC_i And the equivalent capacitance value C SC_i , reserve redundant over-capacity clusters, and determine the number of over-capacity sub-modules connected in series in each over-capacity energy storage branch.

[0058] The over-capacity energy storage branch controller further includes: an active power balance control module;

[0059] In the active power balance control module, the active power P output by the i-th over-capacity energy storage branch SC_i And the ratio of the DC bus voltage V of the receiving-end converter station dc Is used as the DC current reference value I of the i-th over-capacity energy storage branch dc_refi ; The difference between the current I of the i-th over-capacity energy storage branch and the DC current reference value I SC_i After passing through the PI controller, the obtained value is summed with the DC bus voltage V of the receiving-end converter station dc_refi To obtain the voltage reference value of the i-th over-capacity energy storage branch, and the ratio of the voltage reference value of the i-th over-capacity energy storage branch to the average voltage V of each over-capacity sub-module in the i-th over-capacity energy storage branch dc Is used to determine the number of over-capacity sub-modules to be put into the i-th over-capacity energy storage branch. According to the number of over-capacity sub-modules put in, the modulation link is used to generate switching pulses to trigger the corresponding over-capacity sub-modules in the i-th over-capacity energy storage branch. SC_sm SC_sm

[0060] The present invention also provides a method for ultra-capacitor energy dispersion and equalization configuration of a grid-forming HC-EVSC device, including:

[0061] Collect the DC bus voltage as the rated voltage of each ultra-capacitor energy storage branch;

[0062] The ultra-capacitor energy equalization controller determines the maximum current of all ultra-capacitor energy storage branches according to the active power change amount and power recovery time of the modular multi-level converter adopting the grid-forming control strategy under the grid frequency step; and determines the number of ultra-capacitor energy storage branches to be put into use by using the maximum current and the rated current of each ultra-capacitor energy storage branch;

[0063] The ultra-capacitor energy equalization controller determines the total transmission power of all ultra-capacitor energy storage branches and the energy coefficient of each ultra-capacitor energy storage branch according to the active power deficit of the modular multi-level converter adopting the grid-forming control strategy; and sends the total transmission power and the energy coefficient of each ultra-capacitor energy storage branch to the corresponding ultra-capacitor energy storage branch controller respectively

[0064] The ultra-capacitor energy storage branch controller determines the output power command of the ultra-capacitor energy storage branch according to the received total transmission power and energy coefficient; when the output power command of the ultra-capacitor energy storage branch meets the maximum power support level constraint, the ultra-capacitor energy storage branch controller dynamically adjusts the active power output by each ultra-capacitor energy storage branch according to the output power command.

[0065] Establish an equivalent circuit for each ultra-capacitor energy storage branch, and the equivalent circuit includes an equivalent capacitor and an equivalent internal resistance; based on the equivalent circuit, when the power released by the equivalent capacitor is not less than the sum of the power consumed by the equivalent internal resistance and the active power demand of the ultra-capacitor energy storage branch, determine the number of ultra-capacitor sub-modules connected in series in the ultra-capacitor energy storage branch according to the output power command, the open-circuit voltage, and the equivalent capacitor value.

[0066] The present invention is also a terminal, including a processor and a storage medium; the storage medium is used to store instructions; the processor is used to operate according to the instructions to execute the steps of the method.

[0067] 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.

[0068] The beneficial effects of the present invention are at least as follows compared with the prior art. The grid-forming HC-EVSC device proposed by the present invention improves the inertia support and reactive power compensation capabilities of the converter under the fault of the inverter-side AC system by configuring ultra-capacitor energy storage branches on the inverter side and performing ultra-capacitor energy dispersion and equalization control. At least one MMC converter on the inverter side adopts a grid-forming control strategy, and adjusts the converter transmission power according to the system frequency and voltage changes to achieve active support for the active and reactive power of the AC grid.

[0069] Through the balanced and optimized allocation of the energy coefficients of the branch circuits, maximizing the energy coefficient of the super-capacity energy storage branch circuit with the shortest electrical distance from the receiving-end converter station is the basis for improving the dynamic response speed, while the balance of the remaining energy coefficients is the basis for achieving stable active power support. Through the feedback optimization of the energy coefficients, the problem of insufficient active power support caused by the active power deficit due to the failure of a super-capacity energy storage branch circuit with a continuous output at the maximum power support level is avoided. Moreover, through the feedback optimization of the energy coefficients, the goal of balanced energy allocation for each super-capacity energy storage branch circuit is ultimately achieved, improving the service life of each super-capacity energy storage branch circuit, which is conducive to the standardized construction, operation, and maintenance of each super-capacity energy storage branch circuit. On the basis of achieving the balanced and dispersed allocation of super-capacity energy, the number of super-capacity sub-modules connected in series in each super-capacity energy storage branch circuit is further determined, improving the utilization rate of super-capacity sub-modules in each super-capacity energy storage branch circuit, reducing the redundant number of super-capacity sub-modules, and effectively controlling the construction cost. Description of the Drawings

[0070] Figure 1 Topological diagram of the grid-forming HC-EVSC device with inertia support performance proposed in the embodiment of the present invention;

[0071] Figure 2 Topological diagram of the MMC in the embodiment of the present invention;

[0072] Figure 3 Schematic block diagram of the converter grid-forming control strategy based on a virtual synchronous machine in the embodiment of the present invention;

[0073] Figure 4 Balanced control block diagram of the active power output by each super-capacity energy storage branch circuit proposed in the embodiment of the present invention;

[0074] Figure 5 Active power simulation waveform diagram of the MMC converter under a frequency step in the embodiment of the present invention. Detailed Embodiment

[0075] To make the objectives, technical solutions, and advantages of the present invention clearer, the technical solutions of the present invention will be clearly and completely described below in conjunction with the accompanying drawings in the embodiments of the present invention. The embodiments described in this application are only a part of the embodiments of the present invention, rather than all embodiments. Based on the spirit of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the protection scope of the present invention.

[0076] A grid-forming HC-EVSC device with inertia support performance proposed by the present invention adopts a hybrid cascaded DC transmission topology. The rectifier side uses a line-commuted converter (LCC), and the inverter side uses a structure in which multiple parallel modular multilevel converters (MMCs) are cascaded with one LCC; n parallel supercapacitor energy storage branches are configured on the inverter side, where n is a positive integer; one end of each supercapacitor energy storage 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.

[0077] Considering the overload capacity of the rectifier side equipment and the DC bus voltage fluctuation during the fault transient, at least one supercapacitor energy storage branch is directly connected to the MMC side. By calculating the fault transient active power / energy constraint equation of the flexible DC converter station and combining the device overload level, the parameter configuration of the supercapacitor branch is determined; the equipment of the supercapacitor energy storage branch tracks the change of the active power on the inverter side in real time, and adjusts the power compensation amount in real time according to the change of the transmitted power, supports the DC side voltage of the MMC, reduces the risk of LCC commutation failure, effectively improves the fault recovery characteristics of the system, and ensures the safe and reliable operation of the system.

[0078] In the embodiment, as Figure 1 shown, the rectifier side uses a dual 12-pulse LCC, and the inverter side uses a structure in which 3 parallel MMCs are cascaded with one LCC. The rectifier side and the receiving-end converter station are connected by a DC bus, and the LCC and the MMC are respectively connected to the AC grid through corresponding transformers.

[0079] Among them, the structures of the 3 parallel MMCs are the same. Each phase arm includes 1 arm inductor, 1 arm resistor and the same number of sub-modules connected in series. The sub-module consists of multiple full-bridge and half-bridge sub-modules. In the embodiment, the topology of each MMC is as Figure 2 shown. The MMC is connected to the point of common coupling (PCC) of the AC grid through a transformer. The MMC adopts a three-phase six-arm structure. The number of sub-modules SM connected in series in each arm is N. L arm and R arm are the arm inductor and the arm resistor respectively. u va 、u vb 、u vc are the three-phase voltages on the AC side, i sa 、i sb 、i sc are the three-phase currents on the AC side, i pa 、i pb 、i pc are the three-phase currents of the upper arm, i na 、i nb 、i ncis the three-phase current of the lower bridge arm, and P and N are the positive and negative poles.

[0080] The inverter side of the MMC is configured with n supercapacitor energy storage branches, where n is a positive integer. As Figure 1 shown. Each supercapacitor energy storage branch includes a plurality of series-connected supercapacitor sub-modules; each supercapacitor sub-module includes a power device and a supercapacitor cluster; in the embodiment, the supercapacitor energy storage branch includes m supercapacitor sub-modules CM1, CM2, ……, CMm, where m is a positive integer; one end of the supercapacitor energy storage branch is connected to the positive pole of the DC line, and the other end of the supercapacitor energy storage branch is connected to the grounding pole of the DC line.

[0081] At least one of the three MMCs adopts a network-forming control strategy; in the embodiment, any one of the MMCs adopts a constant voltage control strategy, and the remaining two MMCs adopt a constant power control strategy. In the device proposed by the present invention, at least one MMC adopts a network-forming control strategy. The MMC adopting the network-forming control strategy does not rely on the external AC system voltage to achieve synchronization, and externally shows the characteristics of a synchronous machine. It adjusts the converter transmission power value according to the changes in the inverter side frequency and voltage, and realizes the active support for the active and reactive power characteristics of the AC power grid; moreover, the MMC adopting the network-forming control strategy outputs the power change value in real time during the transient period. The supercapacitor energy storage branch receives the power change command from the MMC adopting the network-forming control strategy and tracks the change in the converter transmission power in real time.

[0082] Figure 3 is a schematic block diagram of the network-forming control strategy of the converter based on the virtual synchronous machine. As Figure 3 (a) shown, P ref is the given active power reference value, P m is the actually measured active power of the MMC, ω g is the actually measured angular frequency, ω0 is the system rated angular frequency, D is the damping coefficient, H is the inertia time constant, and θ is the phase angle reference value. As Figure 3 (b) shown, Q ref is the given reactive power reference value, Q m is the actually measured reactive power of the MMC, Dq is the reactive power droop coefficient, E0 is the internal potential amplitude, and E is the internal potential amplitude reference value; the three-phase modulation voltage of the MMC is determined according to the phase angle reference value and the internal potential amplitude reference value.

[0083] The grid-forming HC-EVSC device further includes: a supercapacitor energy balancing controller and n supercapacitor energy storage branch controllers; the supercapacitor energy balancing controller is used to determine the number of supercapacitor energy storage branches to be put into operation according to the active power change amount and the power recovery time of the modular multilevel converter adopting the grid-forming control strategy under the grid frequency step; according to the active power deficit of the modular multilevel converter adopting the grid-forming control strategy, determine the total transmission power of all supercapacitor energy storage branches and the energy coefficients of each supercapacitor energy storage branch, and send the total transmission power and the energy coefficients of each supercapacitor energy storage branch to the corresponding supercapacitor energy storage branch controllers respectively;

[0084] Specifically, the supercapacitor energy balancing controller includes: a supercapacitor energy storage branch parameter configuration module and an energy balancing control module;

[0085] A1), the supercapacitor energy storage branch parameter configuration module is used to determine the maximum current of all supercapacitor energy storage branches according to the active power change amount and the power recovery time of the modular multilevel converter adopting the grid-forming control strategy under the grid frequency step; use the maximum current and the rated current of each supercapacitor energy storage branch to determine the number of parallel supercapacitor energy storage branches.

[0086] The supercapacitor energy storage branch parameter configuration module includes: a frequency step acquisition unit, a simulation unit, a branch current calculation unit, and a branch number calculation unit; among them,

[0087] The frequency step acquisition unit is used to obtain the maximum frequency step value of the AC system connected to the modular multilevel converter adopting the grid-forming control strategy;

[0088] The simulation unit is used to determine the maximum value of the active power change amount of the modular multilevel converter adopting the grid-forming control strategy under the maximum frequency step value and the power recovery time required for the active power to recover to the rated active power by using the simulation method in the scenario where all modular multilevel converters output the rated active power and no supercapacitor energy storage branches are connected;

[0089] The branch current calculation unit is used to take the maximum value of the active power change amount output by the simulation unit as the maximum value of the active power supported by all supercapacitor energy storage branches, and the power recovery time output by the simulation unit as the support time of all supercapacitor energy storage branches; use the maximum value of the active power supported by all supercapacitor energy storage branches, the support time, and the rated voltage of each supercapacitor energy storage branch to calculate the maximum current of all supercapacitor energy storage branches; among them, the DC bus voltage is the rated voltage of each supercapacitor energy storage branch;

[0090] The maximum current of all supercapacitor energy storage branches satisfies the following relational expression:

[0091]

[0092] Wherein, I SCmax is the maximum current of all supercapacitor energy storage branches, and ΔP max is the maximum active power supported by all supercapacitor energy storage branches, and ΔT max is the support time of all supercapacitor energy storage branches, and V SCN is the rated voltage of each supercapacitor energy storage branch;

[0093] A branch number calculation unit, configured to, if the maximum current of all supercapacitor energy storage branches is not greater than the maximum current-carrying capacity of the power device, the number of supercapacitor energy storage branches is 1; if the maximum current of all supercapacitor energy storage branches is greater than the maximum current-carrying capacity of the power device, determine the number of parallel supercapacitor energy storage branches according to the maximum current of all supercapacitor energy storage branches and the rated current of each supercapacitor energy storage branch; wherein, the rated current of each supercapacitor energy storage branch is determined according to the current-carrying capacity and overload capacity of the power device in each supercapacitor energy storage branch.

[0094] In the grid-forming HC-EVSC device proposed by the present invention, through the supercapacitor energy balance controller, according to the power change of the modular multilevel converter adopting the grid-forming control strategy, the number of supercapacitor energy storage branches put into operation is adjusted in real time, realizing the decentralized configuration of supercapacitor energy in a flexible and variable manner, providing active power support for the receiving-end converter station, and ensuring the system stability.

[0095] A2), an energy balance control module, configured to determine the total transmission power of all supercapacitor energy storage branches and the energy coefficient of each supercapacitor energy storage branch according to the active power deficit of the modular multilevel converter adopting the grid-forming control strategy; and send the total transmission power and the energy coefficient of each supercapacitor energy storage branch to the corresponding supercapacitor energy storage branch controller respectively.

[0096] The energy balance control module includes: a total transmission power calculation unit and an energy coefficient balance configuration unit;

[0097] The total transmission power calculation unit is configured to use the difference between the active power output by the modular multilevel converter adopting the grid-forming control strategy and the active power reference value as the active power deficit, and use the active power deficit as the total transmission power of all supercapacitor energy storage branches;

[0098] The total transmission power of all supercapacitor energy storage branches satisfies the following relationship:

[0099] ΔP SC = P mmc - P mmc_ref

[0100] Wherein, ΔP SC is the total transmission power of all supercapacitor energy storage branches, P mmc is the active power output by the MMC adopting the grid-forming control strategy, Pmmc_ref is the reference value of the active power output by the MMC adopting the network-forming control strategy;

[0101] The energy coefficient equalization configuration unit is used to assign the largest energy coefficient of α′, where the value range of α′ is [0.8, 1.0], to the ultra-capacitive energy storage branch with the shortest electrical distance from the receiving-end converter station among all ultra-capacitive energy storage branches; the energy coefficients of the remaining ultra-capacitive energy storage branches are assigned equally, all being α″, and satisfying α′+(n - 1)α″ = 1;

[0102] Calculate the output power commands of each ultra-capacitive energy storage branch by using the assigned energy coefficients of each ultra-capacitive energy storage branch; determine the regulation cost and regulation duration of each ultra-capacitive energy storage branch based on the output power commands of each ultra-capacitive energy storage branch;

[0103] Taking the comprehensive optimization of the regulation cost and regulation duration of each ultra-capacitive energy storage branch and the comprehensive optimization of the regulation cost and regulation duration of all ultra-capacitive energy storage branches as the objective function, and taking the sum of the output power commands of all ultra-capacitive energy storage branches satisfying the total transmission power of all ultra-capacitive energy storage branches as the constraint condition, optimize the energy coefficients of each ultra-capacitive energy storage branch through multiple iterations; the optimized energy coefficients of each ultra-capacitive energy storage branch are α i , i = 1, 2, …, n, satisfying

[0104] Among them, the objective function satisfies the following relational expression:

[0105]

[0106] In the formula, F is the objective function, λ is the weight, C i , τ i are respectively the regulation cost and regulation duration for the i-th ultra-capacitive energy storage branch to respond to the output power command, and, C i = f(ΔP i ), τ i = f(ΔP i ), ΔP i is the difference between the actual output power value and the output power command of the i-th ultra-capacitive energy storage branch currently, and f() represents the functional relationship;

[0107] Taking the sum of the output power commands of all ultra-capacitive energy storage branches satisfying the total transmission power of all ultra-capacitive energy storage branches as the constraint condition, it satisfies the following relational expression:

[0108]

[0109] In the formula, is the output power command of the i-th ultra-capacitive energy storage branch, and ΔP SC is the total transmission power of all ultra-capacitive energy storage branches.

[0110] On the basis of realizing the decentralized configuration of supercapacitor energy by using the supercapacitor energy storage branch parameter configuration module, the energy balance control module determines and optimizes the energy coefficients of each branch, assigns the largest energy coefficient to the supercapacitor energy storage branch with the shortest electrical distance from the receiving-end converter station, so as to improve the response speed of power support, and makes the adjustment cost and adjustment duration of each supercapacitor energy storage branch optimal comprehensively, and the adjustment cost and adjustment duration of all supercapacitor energy storage branches optimal comprehensively, so as to realize the energy balance configuration of each supercapacitor energy storage branch with the optimal economic benefit. Moreover, the process of the energy balance control module determining and optimizing the energy coefficients of each branch is carried out before sending a signal to the supercapacitor energy storage branch controller, and takes the sum of the output power commands of all supercapacitor energy storage branches satisfying the total transmission power of all supercapacitor energy storage branches as a constraint condition, which becomes a prerequisite for avoiding the output power oscillation of each supercapacitor energy storage branch.

[0111] The supercapacitor energy storage branch controller is used to determine the output power command of the supercapacitor energy storage branch according to the received total transmission power and energy coefficient; when the output power command of the supercapacitor energy storage branch satisfies the maximum power support level constraint, the supercapacitor energy storage branch controller adjusts the active power output by each supercapacitor energy storage branch according to the output power command.

[0112] Specifically, the supercapacitor energy storage branch controller includes: an output power command generation module, a supercapacitor sub-module parameter configuration module, and an active power balance control module;

[0113] B1), the output power command generation module is used to determine the output power command of the supercapacitor energy storage branch according to the received total transmission power and energy coefficient; when the output power command of the supercapacitor energy storage branch satisfies the maximum power support level constraint, the supercapacitor energy storage branch controller adjusts the active power output by each supercapacitor energy storage branch according to the output power command;

[0114] When the output power command of the supercapacitor energy storage branch does not satisfy the maximum power support level constraint, the active power output by the supercapacitor energy storage branch remains at the maximum power support level, and a signal for re-optimizing the energy coefficients of each supercapacitor energy storage branch is sent to the supercapacitor energy balance controller;

[0115] The output power command of the supercapacitor energy storage branch determined by the output power command generation module should satisfy the maximum power support level constraint shown by the following relational expression:

[0116]

[0117] In the formula, is the output power command of the i-th supercapacitor energy storage branch, P SC_imaxis the maximum power support level of the i-th over-capacity energy storage branch at the rated DC voltage of the receiving-end converter station. The maximum power support levels of each over-capacity energy storage branch are determined according to the current-carrying capacity and overload capacity of the power devices in different over-capacity energy storage branches under the rated voltage.

[0118] B2) The over-capacity sub-module parameter configuration module is used to establish an equivalent circuit of the over-capacity energy storage branch before adjusting the active power output of each over-capacity energy storage branch according to the output power command. The equivalent circuit includes an equivalent capacitor and an equivalent internal resistance; based on the equivalent circuit, when the power released by the equivalent capacitor is not less than the sum of the power consumed by the equivalent internal resistance and the active power demand of the over-capacity energy storage branch, determine the number of over-capacity sub-modules connected in series in the over-capacity energy storage branch according to the output power command, the open-circuit voltage, and the equivalent capacitance value.

[0119] The over-capacity sub-module parameter configuration module includes: an equivalent circuit establishment unit and an over-capacity sub-module number calculation unit;

[0120] The equivalent circuit establishment unit is used to establish an equivalent circuit of each over-capacity energy storage branch. The equivalent circuit is a series connection of an ideal capacitor and an ideal resistor. And, the capacitance value of the ideal capacitor is the equivalent capacitance value C in the state where all over-capacity sub-modules in all over-capacity energy storage branches are put into operation SC_i , and the resistance value of the ideal resistor is the equivalent internal resistance value R in the state where all over-capacity sub-modules in all over-capacity energy storage branches are put into operation SC_i ;

[0121] The power consumed by the equivalent internal resistance value of the i-th over-capacity energy storage branch satisfies the following relational expression:

[0122]

[0123] In the formula, P SCR_i is the power consumed by the equivalent internal resistance value of the i-th over-capacity energy storage branch, I SCN_i is the rated current of the i-th over-capacity energy storage branch, R SC_i is the equivalent internal resistance value of the i-th over-capacity energy storage branch, ΔT max is the support time of all over-capacity energy storage branches;

[0124] The power released by the equivalent capacitance value of the i-th over-capacity energy storage branch satisfies the following relational expression:

[0125] P SCC_i ≥P SCN_i +P SCR_i

[0126] In the formula, P SCC_i is the power released by the equivalent capacitance value of the i-th over-capacity energy storage branch, P SCN_iThe active power demand of the i-th supercapacitor energy storage branch based on the equivalent circuit satisfies ΔP max is the maximum value of the active power supported by all supercapacitor energy storage branches.

[0127] In the supercapacitor sub-module quantity calculation unit, it is determined that the equivalent capacitance value in the supercapacitor energy storage branch satisfies the following relational expression:

[0128]

[0129] In the formula, V SC_i is the open-circuit voltage of the i-th supercapacitor energy storage branch, V SCN is the rated voltage of each supercapacitor energy storage branch, is the output power command of the i-th supercapacitor energy storage branch;

[0130] Combined with the voltage withstand capacity of the supercapacitor cluster in the supercapacitor sub-module and the equipment insulation level, the open-circuit voltage V SC_i and the equivalent capacitance value C SC_i of the i-th supercapacitor energy storage branch are selected, and redundant supercapacitor clusters are reserved to determine the number of series-connected supercapacitor sub-modules in each supercapacitor energy storage branch.

[0131] B3) In the active power balance control module, the ratio of the active power P SC_i output by the i-th supercapacitor energy storage branch to the DC bus voltage V dc of the receiving-end converter station is used as the DC current reference value I dc_refi of the i-th supercapacitor energy storage branch; the difference between the current I SC_i of the i-th supercapacitor energy storage branch and the DC current reference value I dc_refi is obtained through a PI controller, and then summed with the DC bus voltage V dc of the receiving-end converter station to obtain the voltage reference value of the i-th supercapacitor energy storage branch. The ratio of the voltage reference value of the i-th supercapacitor energy storage branch to the average voltage V SC_sm of each supercapacitor sub-module in the i-th supercapacitor energy storage branch is used to determine the number of supercapacitor sub-modules to be put into the i-th supercapacitor energy storage branch. According to the number of supercapacitor sub-modules put in, a switching pulse is generated through the modulation link to trigger the corresponding supercapacitor sub-module in the i-th supercapacitor energy storage branch.

[0132] To improve the energy supply performance of the supercapacitor energy storage branch and reduce the DC side voltage fluctuation level of the MMC, the present invention also proposes a supercapacitor energy dispersion and balance configuration method for a grid-connected HC-EVSC device, including:

[0133] Step 1, collect the DC bus voltage as the rated voltage of each supercapacitor energy storage branch;

[0134] In the embodiment, taking the DC bus voltage V of the receiving-end converter stationdc As the rated voltage V of each ultra-capacitor energy storage branch SCN ;

[0135] Step 2: The ultra-capacitor energy balance controller determines the maximum current value of all ultra-capacitor energy storage branches according to the active power change amount and power recovery time of the MMC adopting the grid-forming control strategy under the grid frequency step; The number n of ultra-capacitor energy storage branches to be put into operation is determined by using the maximum current value and the rated current of each ultra-capacitor energy storage branch;

[0136] In the embodiment, the maximum frequency step value f of the AC system connected to the MMC adopting the grid-forming control strategy is obtained max , in the scenario where the three parallel MMCs all output the rated active power and no ultra-capacitor energy storage branch is connected, the simulation method is adopted to determine the maximum value of the active power change amount of the MMC adopting the grid-forming control strategy under the maximum frequency step value f max and the recovery time required for the active power to recover to the rated active power are respectively used as the maximum value of the active power ΔP supported by all ultra-capacitor energy storage branches and the support time ΔT max ; max ;

[0137] In the embodiment, the maximum value of the active power support ΔP max and the support time ΔT max are the upper limit of the energy demand in the most extreme case; The maximum value of the active power supported by all ultra-capacitor energy storage branches, the support time and the rated voltage are used to calculate the maximum current value of all ultra-capacitor energy storage branches, satisfying the following relationship:

[0138]

[0139] In the formula, I SCmax is the maximum current value of all ultra-capacitor energy storage branches;

[0140] Each ultra-capacitor sub-module includes power devices and an ultra-capacitor cluster; If I SCmax is not greater than the maximum current-carrying capacity of the power device, the number of ultra-capacitor energy storage branches to be put into operation is 1; If I SCmax is greater than the maximum current-carrying capacity of the power device, multiple parallel ultra-capacitor energy storage branches are put into operation, and the number of ultra-capacitor energy storage branches to be put into operation is determined according to the maximum current value of all ultra-capacitor energy storage branches and the rated current of each ultra-capacitor energy storage branch;

[0141] If the installation method of multiple parallel ultra-capacitor energy storage branches is adopted, further considering that there are certain differences in the current-carrying and overload capabilities of power devices and ultra-capacitor clusters from different manufacturers, when the rated currents of each ultra-capacitor energy storage branch satisfy the following relationship, the number of ultra-capacitor energy storage branches to be put into operation is determined:

[0142]

[0143] In the formula, I SCN_i is the rated current of the i-th over-capacity energy storage branch, which is determined dispersedly according to the current-carrying capacity and overload capacity of the power devices in different over-capacity energy storage branches, and n' is the number of over-capacity energy storage branches put into operation.

[0144] Step 3: The over-capacity energy balance controller determines the total transmission power of all over-capacity energy storage branches and the energy coefficients of each over-capacity energy storage branch according to the active power deficit of the MMC adopting the grid-forming control strategy; and sends the total transmission power and the energy coefficients of each over-capacity energy storage branch to the corresponding over-capacity energy storage branch controller respectively;

[0145] Specifically, the difference between the active power output by the MMC adopting the grid-forming control strategy and the active power reference value is the active power deficit; the over-capacity energy balance controller takes the active power deficit as the total transmission power of all over-capacity energy storage branches, and satisfies the following relationship:

[0146] ΔP SC =P mmc -P mmc_ref

[0147] In the formula, ΔP SC is the total transmission power of all over-capacity energy storage branches, P mmc is the active power output by the MMC adopting the grid-forming control strategy, and P mmc_ref is the active power reference value output by the MMC adopting the grid-forming control strategy;

[0148] The over-capacity energy balance controller determines the energy coefficients of each over-capacity energy storage branch, including:

[0149] 1). Among all over-capacity energy storage branches, the energy coefficient of the over-capacity energy storage branch with the shortest electrical distance to the receiving-end converter station is assigned the largest value, which is α', and in the embodiment, the value range of α' is [0.8, 1.0]; the energy coefficients of the remaining over-capacity energy storage branches are assigned equally, all of which are α'', and satisfy α'+(n - 1)α'' = 1;

[0150] 2). Using the assigned values of the energy coefficients of each over-capacity energy storage branch, calculate the output power commands of each over-capacity energy storage branch; based on the output power commands of each over-capacity energy storage branch, determine the regulation cost and regulation duration of each over-capacity energy storage branch;

[0151] 3) When each supercapacitor energy storage branch responds to the output power command, there is a corresponding adjustment cost and adjustment duration. The adjustment cost characterizes the economy of the difference between the actual value of the current output power of the branch and the output power command, and the adjustment duration is the time required for the branch to adjust from the actual value of the current output power to the output power command, which characterizes the technical performance of each supercapacitor sub-module in the branch. Therefore, taking the comprehensive optimization of the adjustment cost and adjustment duration of each supercapacitor energy storage branch and the comprehensive optimization of the adjustment cost and adjustment duration of all supercapacitor energy storage branches as the objective function, and taking the sum of the output power commands of all supercapacitor energy storage branches satisfying the total transmission power of all supercapacitor energy storage branches as the constraint condition, the energy coefficient of each supercapacitor energy storage branch is optimized through multiple iterations;

[0152] Among them, the objective function satisfies the following relationship:

[0153]

[0154] In the formula, F is the objective function, λ is the weight, and its value is 0.5 in the embodiment. Those skilled in the art can set different weights according to the actual engineering requirements according to different requirements of economy or technology, C i 、τ i are respectively the adjustment cost and adjustment duration of the i-th supercapacitor energy storage branch responding to the output power command, and, C i =f(ΔP i ), τ i =f(ΔP i ), ΔP i is the difference between the actual value of the current output power of the i-th supercapacitor energy storage branch and the output power command, and f() represents a functional relationship;

[0155] Taking the sum of the output power commands of all supercapacitor energy storage branches satisfying the total transmission power of all supercapacitor energy storage branches as the constraint condition, it satisfies the following relationship:

[0156]

[0157] In the formula, is the output power command of the i-th supercapacitor energy storage branch, and ΔP SC is the total transmission power of all supercapacitor energy storage branches;

[0158] The optimized energy coefficient of each supercapacitor energy storage branch is α i 、i = 1, 2,..., n, satisfying

[0159] In the supercapacitor energy balance controller, the process of iteratively optimizing the energy coefficient according to the objective function is actually a process of realizing the energy coefficient balance design among the branches other than the supercapacitor energy storage branch with the shortest electrical distance from the receiving-end converter station. The maximum energy coefficient of the supercapacitor energy storage branch with the shortest electrical distance from the receiving-end converter station is the basis for improving the dynamic response speed, while the balance of the remaining energy coefficients is the basis for realizing active power stable support.

[0160] Step 4: The supercapacitor energy storage branch controller determines the output power command of the supercapacitor energy storage branch according to the received total transmission power and energy coefficient; when the output power command of the supercapacitor energy storage branch satisfies the maximum power support level constraint, the supercapacitor energy storage branch controller dynamically adjusts the active power output by each supercapacitor energy storage branch according to the output power command.

[0161] In the embodiment, during the transient period, the output power command of the supercapacitor energy storage branch determined by the supercapacitor energy storage branch controller using the total transmission power and energy coefficient should satisfy the maximum power support level constraint shown by the following relational expression:

[0162]

[0163] In the formula, is the output power command of the i-th supercapacitor energy storage branch, and P SC_imax is the maximum power support level of the i-th supercapacitor energy storage branch under the rated DC voltage of the receiving-end converter station. The maximum power support levels of each supercapacitor energy storage branch are determined dispersedly according to the current-carrying capacity and overload capacity of the power devices in different supercapacitor energy storage branches under the rated voltage.

[0164] When the output power command of the supercapacitor energy storage branch does not satisfy the maximum power support level constraint, the active power output by the supercapacitor energy storage branch remains at the maximum power support level, and step 3 is returned. The supercapacitor energy balance controller re-optimizes the energy coefficients of each supercapacitor energy storage branch; through the feedback optimization of the energy coefficients, the problem of insufficient active power support caused by the active power shortage due to the failure of a certain supercapacitor energy storage branch with the maximum power support level as the continuous output is avoided, and the goal of balanced energy configuration of each supercapacitor energy storage branch is finally realized through the feedback optimization of the energy coefficients, improving the service life of each supercapacitor energy storage branch, which is beneficial to the standardized construction and operation and maintenance of each supercapacitor energy storage branch; through steps 1 to 4, on the basis of realizing the decentralized and balanced configuration of supercapacitor energy, the number of series-connected supercapacitor sub-modules in each supercapacitor energy storage branch is further determined, improving the utilization rate of supercapacitor sub-modules in each supercapacitor energy storage branch, reducing the redundant number of supercapacitor sub-modules, and effectively controlling the construction cost.

[0165] Step 5: Establish the equivalent circuit of each supercapacitor energy storage branch. The equivalent circuit includes an equivalent capacitor and an equivalent internal resistance. Based on the equivalent circuit, when the power released by the equivalent capacitor is not less than the sum of the power consumed by the equivalent internal resistance and the active power demand of the supercapacitor energy storage branch, determine the number of supercapacitor sub-modules connected in series in the supercapacitor energy storage branch according to the output power instruction, the open-circuit voltage, and the equivalent capacitance value.

[0166] Specifically, establish the equivalent circuit of each supercapacitor energy storage branch. The equivalent circuit is a series connection of an ideal capacitor and an ideal resistor. And the capacitance value of the ideal capacitor is the equivalent capacitance value C in the state where all supercapacitor sub-modules in all supercapacitor energy storage branches are put into operation. SC_i The resistance value of the ideal resistor is the equivalent internal resistance value R in the state where all supercapacitor sub-modules in all supercapacitor energy storage branches are put into operation. SC_i ;

[0167] The power consumed by the equivalent internal resistance value of the i-th supercapacitor energy storage branch satisfies the following relational expression:

[0168]

[0169] In the formula, P SCR_i is the power consumed by the equivalent internal resistance value of the i-th supercapacitor energy storage branch, I SCN_i is the rated current of the i-th supercapacitor energy storage branch, R SC_i is the equivalent internal resistance value of the i-th supercapacitor energy storage branch, ΔT max is the support time of all supercapacitor energy storage branches.

[0170] The power released by the equivalent capacitance value of the i-th supercapacitor energy storage branch satisfies the following relational expression:

[0171] P SCC_i ≥P SCN_i +P SCR_i

[0172] In the formula, P SCC_i is the power released by the equivalent capacitance value of the i-th supercapacitor energy storage branch, P SCN_i is the active power demand of the i-th supercapacitor energy storage branch based on the equivalent circuit, and it satisfies

[0173] On the basis that all supercapacitor energy storage branches are exactly the same, the established equivalent circuits and their parameters are the same. Therefore, the active power demand of each supercapacitor energy storage branch is the evenly distributed value of the product of the maximum active power supported in the extreme case and the support time, which well matches the result of the balanced configuration of the energy coefficients of each branch, and further promotes the realization of the goal of balanced energy configuration of each supercapacitor energy storage branch.

[0174] According to Step 4, ideally, the output power of the i-th supercapacitor energy storage branch is equal to the output power command of the i-th supercapacitor energy storage branch, satisfying the following relational expression:

[0175]

[0176] In the formula, P SC_i is the active power output by the i-th supercapacitor energy storage branch, α i is the energy coefficient of the i-th supercapacitor energy storage branch, ΔP SC is the total transmission power of all supercapacitor energy storage branches, V SC_i is the open-circuit voltage of the i-th supercapacitor energy storage branch, V dc is the DC bus voltage of the receiving-end converter station, R SC_i is the equivalent internal resistance of the i-th supercapacitor energy storage branch;

[0177] Therefore, when the overcurrent constraint, output power constraint, and energy constraint of the power device are simultaneously satisfied, the equivalent capacitance value in the supercapacitor energy storage branch satisfies the following relational expression:

[0178]

[0179] In the formula, V SC_i is the open-circuit voltage of the i-th supercapacitor energy storage branch, V SCN is the rated voltage of each supercapacitor energy storage branch, is the output power command of the i-th supercapacitor energy storage branch;

[0180] Combined with the withstand voltage capacity of the supercapacitor cluster in the supercapacitor sub-module and the equipment insulation level, the open-circuit voltage V SC_i of the i-th supercapacitor energy storage branch and the equivalent capacitance value C SC_i are selected, and redundant supercapacitor clusters are reserved to determine the number of supercapacitor sub-modules connected in series in each supercapacitor energy storage branch.

[0181] The balanced control of the active power output by each supercapacitor energy storage branch is as Figure 4 shown. In the controller of the i-th supercapacitor energy storage branch, the ratio of the active power P SC_i output by the i-th supercapacitor energy storage branch to the DC bus voltage V dc of the receiving-end converter station is used as the DC current reference value I dc_refi of the i-th supercapacitor energy storage branch; the difference between the current I SC_i of the i-th supercapacitor energy storage branch and the DC current reference value I dc_refi is obtained through a PI controller, and then added to the DC bus voltage V dc to obtain the voltage reference value of the i-th supercapacitor energy storage branch. The voltage reference value of the i-th supercapacitor energy storage branch and the average voltage V SC_smDetermine the number of supercapacitor sub-modules to be put into the \(i\)-th over-capacity energy storage branch according to the ratio, and use the modulation link to generate switching pulses to trigger the corresponding supercapacitor sub-modules in the \(i\)-th over-capacity energy storage branch according to the number of supercapacitor sub-modules to be put into.

[0182] Specifically, it includes two parts: the decentralized selection of supercapacitor parameters and the balanced control of the active power output of each supercapacitor energy storage branch through the design of the supercapacitor energy balance controller. There are differences in the withstand voltage and over-current capacity of the supercapacitor clusters and semiconductor devices produced by different manufacturers. Therefore, in the determination of supercapacitor parameters, a decentralized selection method is adopted to ensure the safe and stable operation of branch equipment. At the same time, in order to give full play to the energy support capabilities of different supercapacitor branches, a supercapacitor energy balance controller is further used to realize the reliable supply of energy for the super-capacity branches.

[0183] A simulation model of the HC-EVSC grid-forming flexible DC converter topology and its control method was built in PSCAD to verify the effectiveness of the proposed topology and its control method. In the simulation model, one of the two MMC converters uses the grid-forming control method. The operating parameters of a single MMC converter station are as follows in the table.

[0184] Table 1 Parameters of the receiving-end converter of the flexible DC system

[0185] Converter rated operating power / MW 667 Valve side rated voltage / kV 182.6 Rated DC voltage / kV 400 Arm inductance / mH 50 Rated angular frequency / rad / s 314 Number of sub-modules in a single arm of flexible DC MMC / piece 174 Existing capacitance value of sub-module capacitor / mF 18 Converter transformer capacity / MVA 375 Rated transformation ratio of converter transformer 525 / 182.6 Rated impedance of converter transformer 20％

[0186] For the design of the supercapacitor energy balance controller in the first part of the supercapacitor configuration, the number of supercapacitor branches is considered as 1 branch in the simulation, and then according to the above-mentioned supercapacitor energy balance controller, the branch energy coefficient \(\alpha_1 = 1\). For the second part of the supercapacitor configuration, the detailed selection process is as follows:

[0187] 1). Select the rated voltage \(V\) of the supercapacitor energy storage branch SCN : \(V\) SCN to be consistent with the MMC DC bus voltage \(V\) dc In this example, \(V\) SCN = 400 kV;

[0188] 2). Select the active power support \(\Delta P\) max and the support time \(\Delta T\) max of the supercapacitor branch: The maximum frequency step value \(f\) max of the AC system connected to the grid-forming control MMC converter station is selected as 0.5 Hz. Through the established system simulation model, when testing the rated power output of the MMC system and in the case of no additional supercapacitor branches, the maximum value of the change in active power \(\Delta P\) fmax of the grid-forming MMC converter station under the maximum frequency step value of 0.5 Hz of the AC system is 1200 MW, and the time \(\Delta T\) max required for the active power to recover to the rated power is 0.5 s; among them, the active power simulation waveform of the MMC converter under the frequency step is asFigure 5 as shown;

[0189] 3) Select the number of supercapacitor branches \(n\): According to the active power and duration requirements of the system, determine the number and maximum capacity of the directly-connected supercapacitor energy storage branches that meet the overcurrent constraints of the energy storage branch devices. In this example, the number of branches is taken as 1, and the total released energy is 600 MJ;

[0190] 4) Select the detailed configuration of the selected number of supercapacitor branches. Select the supercapacitor branch configuration that meets the overcurrent constraints, output power constraints, and energy constraints of the devices, and reserve supercapacitor redundant clusters to improve the reliability of the energy storage device. Considering that the DC current of a single branch is not greater than 3 kA, select a 144 V / 62.5 F module, the internal resistance of a single module is 96 mΩ, and the total number of supercapacitor valve sub-modules is 325.

[0191] The specific simulation process is as follows: The receiving-end single valve group configuration network control converter operates at the rated condition before 8 s. In the simulation, at 8 s, the AC system frequency is stepped from 50 Hz to 49.5 Hz. The configuration network control converter's active power overload provides inertia support for the system. The supercapacitor branch identifies the power change at the receiving end and then outputs power, and the DC side voltage and power at the sending and receiving ends are maintained stable.

[0192] The present disclosure may be a system, method, and / or computer program product. The computer program product may include a computer-readable storage medium having thereon computer-readable program instructions for causing a processor to implement various aspects of the present disclosure.

[0193] The computer-readable storage medium may be a tangible device that can retain and store instructions for use by an instruction execution device. The computer-readable storage medium can be, for example, but is 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 (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 a groove having instructions stored thereon, and any suitable combination of the foregoing. The computer-readable storage medium used herein is not construed as an instantaneous signal itself, such as a radio wave or other freely propagating electromagnetic wave, an electromagnetic wave propagated through a waveguide or other transmission medium (e.g., an optical pulse through an optical fiber cable), or an electrical signal transmitted through a wire.

[0194] The computer-readable program instructions described herein can be downloaded to various computing / processing devices from a computer-readable storage medium or downloaded to an external computer or external storage device through a network, such as the Internet, a local area network, a wide area network, and / or a wireless network. The network may include copper transmission cables, optical fiber transmission, wireless transmission, routers, firewalls, switches, gateway computers, and / or edge servers. 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.

[0195] The computer program instructions for performing the operations of the present disclosure may be assembly instructions, instruction set architecture (ISA) instructions, machine instructions, machine-related instructions, microcode, firmware instructions, state-setting data, or source code or object code written in any combination of one or more programming languages, including object-oriented programming languages such as Smalltalk, C++, etc., and conventional procedural programming languages such as the "C" language or similar programming languages. The computer-readable program instructions may be executed entirely on the user's computer, partially on the user's computer, executed as a stand-alone software package, partially on the user's computer and partially on a remote computer, or entirely on a remote computer or server. In the case of a remote computer, the remote computer may be connected to the user's computer through any type of network, including a local area network (LAN) or a wide area network (WAN), or may be connected to an external computer (e.g., via an Internet service provider through the Internet). 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.

[0196] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit them. Although the present invention has been described in detail with reference to the above embodiments, those of ordinary skill in the art should understand that: modifications or equivalent replacements can still be made to the specific embodiments of the present invention, and any modifications or equivalent replacements that do not depart from the spirit and scope of the present invention shall be covered by the protection scope of the claims of the present invention.

Claims

1. A grid-forming HC-EVSC device, in which the inverter side adopts a structure of cascading multiple parallel modular multilevel converters and 1 phase-commutated converter, characterized in that the device includes: n parallel supercapacitor energy storage branches configured on the inverter side, where n is a positive integer; one end of each supercapacitor energy storage 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; the device further includes: a supercapacitor energy balance controller and n supercapacitor energy storage branch controllers; the supercapacitor energy balance controller is used to determine the number of supercapacitor energy storage branches to be put into operation according to the active power change amount and power recovery time of the modular multilevel converter adopting the grid-forming control strategy under the grid frequency step; according to the active power deficit of the modular multilevel converter adopting the grid-forming control strategy, determine the total transmission power of all supercapacitor energy storage branches and the energy coefficients of each supercapacitor energy storage branch, and send the total transmission power and the energy coefficients of each supercapacitor energy storage branch to the corresponding supercapacitor energy storage branch controller respectively; the supercapacitor energy storage branch controller is used to determine the output power command of the supercapacitor energy storage branch according to the received total transmission power and energy coefficient; when the output power command of the supercapacitor energy storage branch meets the maximum power support level constraint, the supercapacitor energy storage branch controller adjusts the active power output by each supercapacitor energy storage branch according to the output power command.

2. The grid-forming HC-EVSC device according to claim 1, characterized in that each supercapacitor energy storage branch includes multiple series-connected supercapacitor sub-modules; each supercapacitor sub-module includes a power device and a supercapacitor cluster.

3. The grid-forming HC-EVSC device according to claim 2, characterized in that the supercapacitor energy balance controller includes: a supercapacitor energy storage branch parameter configuration module; the supercapacitor energy storage branch parameter configuration module is used to determine the maximum current value of all supercapacitor energy storage branches according to the active power change amount and power recovery time of the modular multilevel converter adopting the grid-forming control strategy under the grid frequency step; use the maximum current value and the rated current of each supercapacitor energy storage branch to determine the number of parallel supercapacitor energy storage branches.

4. The grid-forming HC-EVSC device according to claim 3, characterized in that the supercapacitor energy storage branch parameter configuration module includes: a frequency step acquisition unit, a simulation unit, a branch current calculation unit, and a branch number calculation unit; the frequency step acquisition unit is used to obtain the maximum frequency step value of the AC system connected to the modular multilevel converter adopting the grid-forming control strategy; the simulation unit is used to determine the maximum value of the active power change amount of the modular multilevel converter adopting the grid-forming control strategy under the maximum frequency step value and the power recovery time required for the active power to recover to the rated active power by using the simulation method in the scenario where all modular multilevel converters output the rated active power and no supercapacitor energy storage branch is connected; The branch current calculation unit is used to take the maximum value of the active power change amount output by the simulation unit as the maximum active power supported by all supercapacitor energy storage branches, and take the power recovery time output by the simulation unit as the support time of all supercapacitor energy storage branches; calculate the maximum current of all supercapacitor energy storage branches by using the maximum active power supported by all supercapacitor energy storage branches, the support time, and the rated voltage of each supercapacitor energy storage branch; wherein, the DC bus voltage is the rated voltage of each supercapacitor energy storage branch. The branch number calculation unit is used to determine that the number of supercapacitor energy storage branches is 1 if the maximum current of all supercapacitor energy storage branches is not greater than the maximum current-carrying capacity of the power device; if the maximum current of all supercapacitor energy storage branches is greater than the maximum current-carrying capacity of the power device, determine the number of parallel supercapacitor energy storage branches according to the maximum current of all supercapacitor energy storage branches and the rated current of each supercapacitor energy storage branch; wherein, the rated current of each supercapacitor energy storage branch is determined according to the current-carrying capacity and overload capacity of the power device in each supercapacitor energy storage branch.

5. The grid-forming HC-EVSC device according to claim 4, wherein The maximum current of all supercapacitor energy storage branches satisfies the following relational expression: Where, I SCmax is the maximum current of all supercapacitor energy storage branches, ΔP max is the maximum active power supported by all supercapacitor energy storage branches, ΔT max is the support time of all supercapacitor energy storage branches, V SCN is the rated voltage of each supercapacitor energy storage branch.

6. The grid-forming HC-EVSC device according to claim 3, wherein The supercapacitor energy balance controller further includes: an energy balance control module; The energy balance control module is used to determine the total transmission power of all supercapacitor energy storage branches and the energy coefficient of each supercapacitor energy storage branch according to the active power deficit of the modular multilevel converter adopting the grid-forming control strategy; and send the total transmission power and the energy coefficient of each supercapacitor energy storage branch to the corresponding supercapacitor energy storage branch controller respectively.

7. The grid-forming HC-EVSC device according to claim 6, wherein The energy balance control module includes: a total transmission power calculation unit; The total transmission power calculation unit is used to take the difference between the active power output by the modular multilevel converter adopting the grid-forming control strategy and the active power reference value as the active power deficit, and take the active power deficit as the total transmission power of all supercapacitor energy storage branches.

8. The grid-forming HC-EVSC device according to claim 7, wherein The total transmission power of all supercapacitor energy storage branches satisfies the following relational expression: ΔP SC = P mmc - P mmc_ref where, ΔP SC is the total transmission power of all supercapacitor energy storage branches, P mmc is the active power output by the MMC adopting the network-forming control strategy, P mmc_ref is the reference value of the active power output by the MMC adopting the network-forming control strategy.

9. The grid-forming HC-EVSC device according to claim 7, wherein The energy balance control module further includes: an energy coefficient equalization configuration unit; The energy coefficient equalization configuration unit is used to assign the largest energy coefficient to the supercapacitor energy storage branch with the shortest electrical distance from the receiving-end converter station among all supercapacitor energy storage branches, which is α′, and the value range of α′ is [0.8, 1.0]; the energy coefficients of the remaining supercapacitor energy storage branches are assigned equally, all of which are α″, and satisfy α′+(n - 1)α″ = 1; Calculate the output power command of each supercapacitor energy storage branch by using the assigned value of the energy coefficient of each supercapacitor energy storage branch; based on the output power command of each supercapacitor energy storage branch, determine the adjustment cost and adjustment duration of each supercapacitor energy storage branch. Taking the comprehensive optimization of the adjustment cost and adjustment duration of each over-capacity energy storage branch and the comprehensive optimization of the adjustment cost and adjustment duration of all over-capacity energy storage branches as the objective function, and taking the sum of the output power commands of all over-capacity energy storage branches to satisfy the total transmission power of all over-capacity energy storage branches as the constraint condition, the energy coefficients of each over-capacity energy storage branch are optimized through multiple iterations; the optimized energy coefficients of each over-capacity energy storage branch are α i , i = 1, 2, …, n, satisfying 10. The network-forming HC-EVSC device according to claim 9, characterized in that The objective function satisfies the following relational expression: where F is the objective function, λ is the weight, and C i , τ i are the regulation cost and regulation duration of the response output power command of the i-th over-capacity energy storage branch, respectively, and C i = f(ΔP i ), τ i = f(ΔP i ), ΔP i is the difference between the actual output power value and the output power command of the i-th over-capacity energy storage branch, and f() represents a functional relationship; The sum of the output power commands of all over-capacity energy storage branches satisfies the total transmission power of all over-capacity energy storage branches as a constraint condition, and satisfies the following relational expression: Wherein, is the output power command of the i-th super-capacitor energy storage branch, and ΔP SC is the total transmission power of all super-capacitor energy storage branches.

11. The network-forming HC-EVSC device according to claim 9, characterized in that The over-capacity energy storage branch controller includes: an output power command generation module; The output power command generation module is used to determine the output power command of the over-capacity energy storage branch according to the received total transmission power and energy coefficient; when the output power command of the over-capacity energy storage branch satisfies the maximum power support level constraint, the over-capacity energy storage branch controller adjusts the active power output by each over-capacity energy storage branch according to the output power command. When the output power command of the over-capacity energy storage branch does not satisfy the maximum power support level constraint, the active power output by the over-capacity energy storage branch remains at the maximum power support level, and a signal for re-optimizing the energy coefficient of each over-capacity energy storage branch is sent to the over-capacity energy balance controller.

12. The network-forming HC-EVSC device according to claim 11, characterized in that The output power command of the over-capacity energy storage branch determined by the output power command generation module should satisfy the maximum power support level constraint shown in the following relational expression: In the formula, is the output power command of the i-th over-capacity energy storage branch, and P SC_imax is the maximum power support level of the i-th over-capacity energy storage branch under the rated DC voltage of the receiving-end converter station. The maximum power support levels of each over-capacity energy storage branch are determined according to the current-carrying capacity and overload capacity of the power devices in different over-capacity energy storage branches under the rated voltage.

13. The network-forming HC-EVSC device according to claim 11, characterized in that The over-capacity energy storage branch controller further includes: an over-capacity sub-module parameter configuration module; The over-capacity sub-module parameter configuration module is used to establish an equivalent circuit of the over-capacity energy storage branch before adjusting the active power output by each over-capacity energy storage branch according to the output power command. The equivalent circuit includes an equivalent capacitor and an equivalent internal resistance; based on the equivalent circuit, when the power released by the equivalent capacitor is not less than the sum of the power consumed by the equivalent internal resistance and the active power demand of the over-capacity energy storage branch, the number of over-capacity sub-modules connected in series in the over-capacity energy storage branch is determined according to the output power command, the open-circuit voltage, and the equivalent capacitance value.

14. The network-forming HC-EVSC device according to claim 13, characterized in that The over-capacity sub-module parameter configuration module includes: an equivalent circuit establishment unit; An equivalent circuit establishing unit is configured to establish an equivalent circuit for each supercapacitor energy storage branch. The equivalent circuit is a series connection of an ideal capacitor and an ideal resistor. Moreover, the capacitance value of the ideal capacitor is the equivalent capacitance value C in the state where all supercapacitor sub-modules in all supercapacitor energy storage branches are put into operation. SC_i The resistance value of the ideal resistor is the equivalent internal resistance value R in the state where all supercapacitor sub-modules in all supercapacitor energy storage branches are put into operation. SC_i ; The power consumed by the equivalent internal resistance value of the i-th over-capacity energy storage branch satisfies the following relational expression: Where, P SCR_i is the power consumed by the equivalent internal resistance value of the i-th over-capacity energy storage branch, I SCN_i is the rated current of the i-th over-capacity energy storage branch, R SC_i is the equivalent internal resistance value of the i-th over-capacity energy storage branch, ΔT max is the support time of all over-capacity energy storage branches; The power released by the equivalent capacitance value of the i-th over-capacity energy storage branch satisfies the following relational expression: P SCC_i ≥P SCN_i +P SCR_i Where, P SCC_i is the power released by the equivalent capacitance value of the i-th over-capacity energy storage branch, and P SCN_i is the active power demand of the i-th over-capacity energy storage branch based on the equivalent circuit, satisfying ΔP max is the maximum active power supported by all over-capacity energy storage branches.

15. The network-forming HC-EVSC device according to claim 14, characterized in that The over-capacity sub-module parameter configuration module further includes: an over-capacity sub-module number calculation unit; In the over-capacity sub-module number calculation unit, it is determined that the equivalent capacitance value in the over-capacity energy storage branch satisfies the following relational expression: where, V SC_i is the open-circuit voltage of the i-th supercapacitor energy storage branch, V SCN is the rated voltage of each supercapacitor energy storage branch, is the output power command of the i-th supercapacitor energy storage branch; Combining the withstand voltage capacity of the supercapacitor clusters in the supercapacitor sub-module with the equipment insulation level, select the open-circuit voltage V of the i-th supercapacitor energy storage branch SC_i and the equivalent capacitance value C SC_i , reserve redundant supercapacitor clusters, and determine the number of supercapacitor sub-modules connected in series in each supercapacitor energy storage branch.

16. The network-forming HC-EVSC device according to claim 11, characterized in that The over-capacity energy storage branch controller further includes: an active power balance control module; In the active power balance control module, the active power P output by the i-th supercapacitor energy storage branch SC_i and the DC bus voltage V of the receiving-end converter station dc The ratio is used as the DC current reference value I of the i-th supercapacitor energy storage branch dc_refi ; the current I of the i-th supercapacitor energy storage branch SC_i The difference from the DC current reference value I dc_refi After passing through the PI controller, the obtained value is summed with the DC bus voltage V of the receiving-end converter station dc To obtain the voltage reference value of the i-th supercapacitor energy storage branch, use the voltage reference value of the i-th supercapacitor energy storage branch and the average voltage V of each supercapacitor sub-module in the i-th supercapacitor energy storage branch SC_sm The ratio determines the number of supercapacitor sub-modules to be put into the i-th supercapacitor energy storage branch. According to the number of supercapacitor sub-modules put in, the modulation link is used to generate switching pulses to trigger the corresponding supercapacitor sub-modules in the i-th supercapacitor energy storage branch.

17. A method for ultra-capacity energy dispersion and equalization configuration of a grid-forming HC-EVSC device, which is applicable to the device described in any one of claims 1 to 16, characterized in that, including: Collect the DC bus voltage as the rated voltage of each over-capacity energy storage branch; The supercapacitor energy balancing controller determines the maximum current value of all supercapacitor energy storage branches according to the active power change amount and power recovery time of the modular multilevel converter adopting the grid-forming control strategy under the grid frequency step; and determines the number of supercapacitor energy storage branches to be put into use by using the maximum current value and the rated current of each supercapacitor energy storage branch. The supercapacitor energy balancing controller determines the total transmission power of all supercapacitor energy storage branches and the energy coefficient of each supercapacitor energy storage branch according to the active power deficit of the modular multilevel converter adopting the grid-forming control strategy; and sends the total transmission power and the energy coefficient of each supercapacitor energy storage branch to the corresponding supercapacitor energy storage branch controller respectively. The supercapacitor energy storage branch controller determines the output power command of the supercapacitor energy storage branch according to the received total transmission power and energy coefficient; when the output power command of the supercapacitor energy storage branch meets the maximum power support level constraint, the supercapacitor energy storage branch controller dynamically adjusts the active power output by each supercapacitor energy storage branch according to the output power command. An equivalent circuit of each supercapacitor energy storage branch is established, and the equivalent circuit includes an equivalent capacitor and an equivalent internal resistance; based on the equivalent circuit, when the power released by the equivalent capacitor is not less than the sum of the power consumed by the equivalent internal resistance and the active power demand of the supercapacitor energy storage branch, the number of supercapacitor sub-modules connected in series in the supercapacitor energy storage branch is determined according to the output power command, the open-circuit voltage and the equivalent capacitor value.

18. A terminal, comprising a processor and a storage medium; characterized in that: 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 described in claim 17.

19. 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 described in claim 17 are implemented.

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