Direct-current voltage-reduction hybrid MMC super-capacitor grid-connected static synchronous compensator starting method and system

Abstract

The application provides a DC voltage reduction hybrid MMC super-capacitor grid static synchronous compensator starting method and system, and is applied to the technical field of flexible power transmission. The method comprises the following steps: controlling each full-bridge sub-module and each half-bridge sub-module of the MMC to be in a locking state, so that the MMC is charged in a non-controlled rectification mode; when the DC voltage of each full-bridge sub-module of the MMC reaches a self-powered loop working threshold value, controlling each full-bridge sub-module to be half-bridged; if the real-time monitored DC side voltage of the MMC is greater than a safety threshold value, controlling the full-bridge sub-modules in each bridge arm of the MMC to be de-half-bridged, so that the DC side voltage of the MMC is within the safety threshold value; and controlling the working state of each sub-module in the MMC based on a DC voltage equalization control strategy, so that each sub-module in the MMC reaches a rated voltage. The application solves the problems of current impact on the AC side in the hybrid MMC super-capacitor grid SC starting process and overvoltage insulation of the DC side equipment in the DC side voltage reduction operation.

Description

Technical Field

[0001] This invention relates to the field of flexible power transmission technology, specifically to a method and system for starting up a DC step-down hybrid MMC supercapacitive grid static synchronous condenser. Background Technology

[0002] With the increasing penetration of new energy sources, the power grid is gradually showing a trend of high power electronics, and the original power grid operation mechanism and safety and stability characteristics based on synchronous machines are undergoing profound changes.

[0003] To enhance the reactive power and voltage control capabilities of new energy aggregation systems, static synchronous condensers (SCs) are configured in both new energy power plants and AC aggregation stations. MMC (Multi-Capacity Modulated Condenser) grid-based SCs have both AC and DC external ports, with the DC port offering more flexible control, making them valuable for research and application. Furthermore, hybrid MMC converter valves based on full-bridge and half-bridge submodules reduce costs and losses while maintaining DC fault isolation capabilities. Therefore, MMC-based supercapacitive grid-based SCs have broad application prospects.

[0004] However, when the MMC converter valve with a hybrid structure is started without controlled charging on the AC side, the different external characteristics of its sub-modules are not completely consistent, resulting in inconsistent charging voltages between the full and half bridges. If the converter valve is unlocked directly, a large inrush current will be generated. Furthermore, when the DC side is operating at reduced voltage, it may cause overvoltage insulation problems for devices connected to the DC side. Summary of the Invention

[0005] To overcome the current surge on the AC side and the overvoltage insulation problems of DC side equipment caused by the DC side buck operation during the SC charging startup process of the hybrid MMC supercapacitive grid, this invention provides a DC buck hybrid MMC supercapacitive grid static synchronous condenser startup method and system.

[0006] On one hand, the present invention provides a method for starting up a DC-DC buck hybrid MMC supercapacitive grid static synchronous condenser, comprising:

[0007] Each full-bridge submodule and each half-bridge submodule of the hybrid modular multilevel converter (MMC) are kept in a locked state to enable the hybrid MMC to perform uncontrolled rectified charging.

[0008] When the DC voltage of each full-bridge submodule of the hybrid MMC reaches the self-powered circuit operating threshold, each full-bridge submodule is controlled to be half-bridged.

[0009] If the DC-side voltage of the hybrid MMC is monitored in real time and is greater than the safety threshold, the full-bridge submodule in each bridge arm of the hybrid MMC is controlled to be de-half-bridged so that the DC-side voltage of the hybrid MMC is within the safety threshold.

[0010] The operating state of each sub-module in the hybrid MMC is controlled based on a DC voltage equalization control strategy so that each sub-module in the hybrid MMC reaches its rated voltage.

[0011] The safety threshold is determined based on the rated voltage and overload capacity of the overcapacity valve of the supercapacity network static synchronous condenser; the rated voltage of the overcapacity valve of the supercapacity network static synchronous condenser is less than the sum of the rated voltages of all submodules on a single bridge arm of the hybrid MMC; the operating states of each submodule in the hybrid MMC include locked and disconnected.

[0012] Optionally, the operating state of each submodule within the hybrid MMC is controlled based on a DC voltage equalization control strategy to ensure that each submodule within the hybrid MMC reaches its rated voltage, including:

[0013] When the DC-side voltage of the hybrid MMC is within a safe threshold, the target submodule within each bridge arm of the hybrid MMC is controlled to be disconnected.

[0014] If the current maximum voltage deviation of the hybrid MMC is greater than the voltage balancing threshold, the working states of the two sub-modules corresponding to the current maximum voltage deviation are swapped.

[0015] Repeat the process of removing the half-bridge, cutting off the target submodule, and exchanging the working states of the two submodules corresponding to the current maximum voltage deviation until all submodules in the hybrid MMC reach their rated voltage.

[0016] The target submodule is determined based on the operating status and DC voltage of each submodule within each bridge arm; during the repeated execution process, if the cumulative number of target submodules to be removed reaches the maximum number of removals, the removal of the target submodule will no longer be performed; the safety threshold is greater than the voltage balancing threshold.

[0017] Optionally, the target submodule is the submodule in each bridge arm that is in a locked state and has the highest DC voltage.

[0018] Optionally, the maximum number of target submodules to be removed is determined based on the peak AC line voltage of the submodules in the hybrid MMC and the rated voltage of the submodules in the hybrid MMC.

[0019] Optionally, the formula for calculating the maximum number of target submodules to be removed is:

[0020]

[0021] Where N1 is the maximum number of target submodules to be removed, N is the number of submodules connected in series within each bridge arm of the hybrid MMC, and U peak U represents the peak AC line voltage of the submodule of the hybrid MMC. s The rated voltage of the submodule of the hybrid MMC is denoted as , and round is the rounding function.

[0022] Optionally, the current maximum voltage deviation of the hybrid MMC is the difference between the maximum DC voltage in each submodule in the locked state and the minimum DC voltage in each submodule in the disconnected state within the hybrid MMC.

[0023] Optionally, controlling each full-bridge submodule within each arm of the hybrid MMC to de-half-bridge, so that the DC-side voltage of the hybrid MMC is within a safe threshold, includes:

[0024] Control the full-bridge submodule in each bridge arm of the hybrid MMC that is in a locked state and has the lowest DC voltage to de-half-bridge;

[0025] Repeat the above half-bridge removal process until the DC-side voltage of the hybrid MMC is within a safe threshold.

[0026] Optionally, the hybrid MMC is connected to the AC power grid via a series slow-start resistor.

[0027] Optionally, after the control corresponding full-bridge submodule is converted to a half-bridge, the method further includes:

[0028] If the effective value of the AC side charging current of the hybrid MMC is less than the preset current threshold, the soft-start resistor is bypassed.

[0029] On the other hand, the present invention also provides a DC-DC buck hybrid MMC supercapacitive grid static synchronous condenser start-up system, comprising:

[0030] An uncontrolled charging unit is used to control each full-bridge submodule and each half-bridge submodule of the hybrid modular multilevel converter (MMC) to be in a locked state, so that the hybrid MMC can perform uncontrolled rectified charging.

[0031] The half-bridge unit is used to control each full-bridge submodule to be half-bridged when the DC voltage of each full-bridge submodule of the hybrid MMC reaches the self-powered circuit operating threshold.

[0032] The half-bridge de-splitting unit is used to control the full-bridge submodule in each bridge arm of the hybrid MMC to de-splitting if the DC-side voltage of the hybrid MMC detected in real time is greater than a safety threshold, so that the DC-side voltage of the hybrid MMC is within the safety threshold.

[0033] The equalizing charging unit is used to control the working state of each sub-module in the hybrid MMC based on the DC voltage equalization control strategy, so that each sub-module in the hybrid MMC reaches the rated voltage.

[0034] The safety threshold is determined based on the rated voltage and overload capacity of the overcapacity valve of the supercapacity network static synchronous condenser; the rated voltage of the overcapacity valve of the supercapacity network static synchronous condenser is less than the sum of the rated voltages of all submodules on a single bridge arm of the hybrid MMC; the operating states of each submodule in the hybrid MMC include locked and disconnected.

[0035] Optionally, the voltage equalization charging unit includes:

[0036] The cut-off sub-unit is used to control the cut-off of the target sub-module within each bridge arm of the hybrid MMC when the DC-side voltage of the hybrid MMC is within a safe threshold.

[0037] The state exchange subunit is used to exchange the working states of the two sub-modules corresponding to the current maximum voltage deviation if the current maximum voltage deviation of the hybrid MMC is greater than the voltage equalization threshold.

[0038] The repeating subunit is used to repeatedly execute the half-bridge removal, the removal of the target submodule, and the exchange of the working states of the two submodules corresponding to the current maximum voltage deviation, until all submodules in the hybrid MMC reach the rated voltage;

[0039] The target submodule is determined based on the operating status and DC voltage of each submodule within each bridge arm; during the repeated execution process, if the cumulative number of target submodules to be removed reaches the maximum number of removals, the removal of the target submodule will no longer be performed; the safety threshold is greater than the voltage balancing threshold.

[0040] Optionally, the target submodule is the submodule in each bridge arm that is in a locked state and has the highest DC voltage.

[0041] Optionally, the maximum number of target submodules to be removed is determined based on the peak AC line voltage of the submodules in the hybrid MMC and the rated voltage of the submodules in the hybrid MMC.

[0042] Optionally, the formula for calculating the maximum number of target submodules to be removed is:

[0043]

[0044] Where N1 is the maximum number of target submodules to be removed, N is the number of submodules connected in series within each bridge arm of the hybrid MMC, and U peak U represents the peak AC line voltage of the submodule of the hybrid MMC.s The rated voltage of the submodule of the hybrid MMC is denoted as , and round is the rounding function.

[0045] Optionally, the current maximum voltage deviation of the hybrid MMC is the difference between the maximum DC voltage in each submodule in the locked state and the minimum DC voltage in each submodule in the disconnected state within the hybrid MMC.

[0046] Optionally, the de-half-bridge unit is specifically used for:

[0047] Control the full-bridge submodule in each bridge arm of the hybrid MMC that is in a locked state and has the lowest DC voltage to de-half-bridge;

[0048] Repeat the above half-bridge removal process until the DC-side voltage of the hybrid MMC is within a safe threshold.

[0049] Optionally, the hybrid MMC is connected to the AC power grid via a series slow-start resistor.

[0050] Optionally, the system also includes:

[0051] A bypass unit is used to bypass the soft-start resistor if the effective value of the AC side charging current of the hybrid MMC is less than a preset current threshold.

[0052] On the other hand, the present invention also provides an electronic device, comprising: at least one processor and a memory; the memory and the processor are connected via a bus;

[0053] The memory is used to store one or more programs;

[0054] When the one or more programs are executed by the at least one processor, the DC-DC buck hybrid MMC supercapacitive grid static synchronous condenser startup method described in any one of the above-described methods is implemented.

[0055] On the other hand, the present invention also provides a readable storage medium having an executable program stored thereon, wherein when the executable program is executed, it implements the DC-DC buck hybrid MMC supercapacitive grid static synchronous condenser startup method described in any one of the above.

[0056] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0057] This invention provides a method and system for starting a DC-DC step-down hybrid MMC supercapacitive grid static synchronous condenser. First, by controlling each full-bridge submodule and each half-bridge submodule of the hybrid modular multilevel converter (MMC) to be in a locked state, the hybrid MMC undergoes uncontrolled rectified charging. When the DC voltage of each full-bridge submodule of the hybrid MMC reaches the self-powered circuit operating threshold, each full-bridge submodule is controlled to become a half-bridge to balance the charging speed of both full and half submodules. Combined with a DC voltage balancing control strategy, the operating state of each submodule within the hybrid MMC is adjusted between locked and disconnected states, ensuring that the charging voltage of each MMC submodule is essentially consistent. This avoids the problem of large inrush currents on the AC side when unlocking the converter valve due to inconsistent charging voltages caused by submodules with different structures, thus improving the safety of the power system. On the other hand, by determining the safety threshold based on the rated voltage and overload capacity of the overcapacity valve of the supercapacity network static synchronous condenser and by real-time monitoring of the DC-side voltage of the hybrid MMC, and controlling the full-bridge submodules in each bridge arm of the hybrid MMC to de-half-bridge when the DC-side voltage is greater than the safety threshold, so that the DC-side voltage of the hybrid MMC is within the safety threshold, the overvoltage insulation problem connected to the DC-side equipment is avoided in the DC-side step-down operation scenario where the rated voltage of the overcapacity valve of the supercapacity network static synchronous condenser is less than the sum of the rated voltages of all submodules on a single bridge arm of the hybrid MMC, thus ensuring the insulation safety of the DC-side equipment in the DC-side step-down operation scenario. Attached Figure Description

[0058] Figure 1 This is a schematic flowchart of a DC-DC step-down hybrid MMC supercapacitive grid static synchronous condenser startup method according to the present invention;

[0059] Figure 2 This is a schematic diagram of the topology of a DC-DC step-down hybrid MMC supercapacitive network SC according to an example of the present invention;

[0060] Figure 3 This is a schematic diagram of the topology of a half-bridge submodule in a hybrid MMC according to an example of the present invention;

[0061] Figure 4 This is a schematic diagram of the topology of a full-bridge submodule in a hybrid MMC according to an example of the present invention;

[0062] Figure 5 This is a schematic diagram of the electronic device of the present invention. Detailed Implementation

[0063] The specific embodiments of the present invention will be further described in detail below with reference to the accompanying drawings.

[0064] Example 1

[0065] This invention provides a method for starting a DC-DC step-down hybrid MMC supercapacitive grid static synchronous condenser, as shown in the schematic diagram. Figure 1 As shown, it includes:

[0066] Step S110: Control each full-bridge submodule and each half-bridge submodule of the hybrid modular multilevel converter (MMC) to be in a locked state, so that the hybrid MMC can perform uncontrolled rectified charging.

[0067] Step S120: When the DC voltage of each full-bridge submodule of the hybrid MMC reaches the self-powered circuit operating threshold, control each full-bridge submodule to be half-bridged.

[0068] Step S130: If the DC-side voltage of the hybrid MMC detected in real time is greater than the safety threshold, control the full-bridge submodule in each bridge arm of the hybrid MMC to be de-half-bridged so that the DC-side voltage of the hybrid MMC is within the safety threshold.

[0069] Step S140: Control the working state of each sub-module in the hybrid MMC based on the DC voltage equalization control strategy so that each sub-module in the hybrid MMC reaches the rated voltage.

[0070] In this example implementation, the hybrid MMC supercapacitive network static synchronous condenser mainly comprises two parts: an MMC valve and a supercapacitive valve. The MMC valve adopts a full-half hybrid structure, meaning the basic topology of the hybrid MMC includes full-bridge submodules and half-bridge submodules. The topology of the hybrid MMC supercapacitive network SC is as follows: Figure 2 As shown in the diagram, the area within the dashed box represents the MMC valve structure. The MMC valve is connected to the AC power grid via an AC switch QF and a soft-start resistor circuit. The overcapacitance valve is directly connected to the DC side of the MMC valve. The MMC valve has three phases: A, B, and C. Each phase has symmetrically arranged upper and lower bridge arms, each consisting of multiple sub-modules (SMs) connected in series with an inductor. For example, for phase A, the upper and lower bridge arms are each composed of sub-modules SMs. a1 SM a2 ,…,SM aN and SM aN+1 SM aN+2 ,…,SM a2N and inductor L a They are connected in series, with phases B and C having similar structures; the connection points of the upper and lower arms of each phase are the input terminals of the corresponding phases, U dc This is the DC-side voltage of the MMC. The half-bridge submodule topology of the MMC valve is as follows: Figure 3 As shown, Figure 3 In the diagram, T1 / T2 are IGBT switching devices, D1 / D2 are anti-parallel diodes of T1 / T2 respectively, and C1 is the corresponding DC support capacitor for the submodule. This represents the full-bridge submodule topology of the MMC valve. Figure 4 As shown, Figure 4 In this diagram, T3 / T4 / T5 / T6 are IGBT switching devices, D3 / D4 / D5 / D6 are anti-parallel diodes of T3 / T4 / T5 / T6 respectively, and C2 is the DC support capacitor corresponding to the submodule. The safety threshold is determined based on the rated voltage and overload capacity of the overcapacitance valve of the supercapacitance network static synchronous condenser. For example, the safety threshold can be between 1 and 1.2 times the rated voltage of the overcapacitance valve. The rated voltage of the overcapacitance valve of the supercapacitance network static synchronous condenser is less than the sum of the rated voltages of all submodules on a single bridge arm of the hybrid MMC, i.e., the rated operating voltage of the overcapacitance valve is less than the algebraic sum of the rated DC voltages of all submodules on a single bridge arm of the MMC valve. An SC satisfying this condition is a DC buck converter. For example, the condition that a DC buck converter SC needs to satisfy is U... dc <N*U s U dc For the mixed MMC DC side voltage, i.e. the rated operating voltage of the overcapacitance valve, U s N represents the rated voltage of the submodules in the hybrid MMC, and N is the number of submodules connected in series within each arm of the hybrid MMC. For DC-DC step-down SC, devices connected to the DC side may have overvoltage insulation issues. To address this, this invention ensures that the DC side voltage does not become excessively high by designing real-time monitoring of the DC side voltage, a safety threshold, and de-half-bridge conversion processes, thus guaranteeing the safety of DC side devices. The operating states of each submodule within the hybrid MMC include latch-up and disconnection, which can be controlled by controlling the switch states within each submodule. For example, the latch-up states of the half-bridge and full-bridge submodules respectively latch up all power devices within the full-bridge and half-bridge submodules, i.e., close all IGBT switches. The disconnection state of the full-bridge submodule refers to the submodule being removed from the circuit and no longer participating in voltage output when both IGBT switches in the full-bridge submodule are in the off state; the half-bridge conversion of the full-bridge submodule refers to turning on one of the IGBT switches in the full-bridge submodule, such as when it is turned on. Figure 4 T3 or T6 in the text; De-half-bridge conversion of the full-bridge submodule refers to restoring the full-bridge characteristics of the full-bridge submodule, specifically, restoring the IGBT switches that were turned on during the half-bridge conversion to the off state. For example... Figure 3 As shown, the disconnected state of the half-bridge submodule refers to the situation where T1 is off and T2 is on, at which point the submodule capacitor is disconnected from the circuit. During uncontrolled rectifier charging, all power devices in the full-bridge and half-bridge submodules of the MMC are locked out, the AC side circuit breaker is closed, and the slow-start resistor is engaged, which means... Figure 2When the circuit breaker QF is closed and the switch KM is opened, the MMC valve submodule undergoes uncontrolled rectified charging. During this process, the DC voltage of the full-bridge submodule is typically twice that of the half-bridge submodule. By controlling each full-bridge submodule to be half-bridged, the charging speed of both full and half-bridge submodules is balanced. Combined with a DC voltage balancing control strategy, the operating state of each submodule within the hybrid MMC is adjusted between locked and disconnected states, ensuring that the charging voltage of each MMC submodule is basically consistent. This avoids the problem of large inrush current on the AC side when unlocking the converter valve due to inconsistent charging voltage caused by submodules with different structures having completely different external characteristics. By controlling the DC-side voltage of the hybrid MMC within a safe threshold, overvoltage insulation problems are avoided when the DC-side step-down SC is connected to the DC-side equipment, ensuring the insulation safety of the DC-side equipment in the DC-side step-down operation scenario.

[0071] In some example implementations, the DC voltage equalization control strategy in step S140 controls the operating state of each submodule within the hybrid MMC so that each submodule within the hybrid MMC reaches its rated voltage, including:

[0072] When the DC-side voltage of the hybrid MMC is within a safe threshold, the target submodule within each bridge arm of the hybrid MMC is controlled to be disconnected.

[0073] If the current maximum voltage deviation of the hybrid MMC is greater than the voltage balancing threshold, the working states of the two sub-modules corresponding to the current maximum voltage deviation are swapped.

[0074] Repeat the process of removing the half-bridge, cutting off the target submodule, and exchanging the operating states of the two submodules corresponding to the current maximum voltage deviation until all submodules in the hybrid MMC reach their rated voltage.

[0075] In this example implementation, the target submodule is determined based on the operating state and DC voltage of each submodule within each bridge arm; exemplarily, the target submodule is the submodule within each bridge arm that is in a locked state and has the highest DC voltage. Before each submodule within the hybrid MMC reaches its rated voltage, the charging process can continue. During charging, the processes of real-time monitoring of the MMC DC-side voltage, safety threshold judgment, half-bridge removal, target submodule removal, and operating state exchange are repeatedly executed, with one target submodule removed each time. During repeated execution, if the cumulative number of target submodules removed reaches the maximum number of removals, the removal of the target submodules is stopped; that is, if the cumulative number of target submodules removed reaches the maximum number of removals and each submodule within the hybrid MMC has not reached its rated voltage, the safety threshold judgment, half-bridge removal, and operating state exchange are repeated. Once all submodules within the hybrid MMC reach their rated voltage, the startup is complete. The safety threshold is greater than the voltage balancing threshold, which can be determined based on the rated operating voltage of the submodule, such as a voltage balancing threshold of 0.05U. s U s This refers to the rated operating voltage of the submodule. The requirement that each submodule reaches its rated voltage can be defined as the permissible error range within which each submodule reaches its rated voltage; for example, each submodule reaching 0.95U. s ~1.05U s For example, the current maximum voltage deviation of the hybrid MMC is the difference between the maximum DC voltage value in each submodule in the latched state and the minimum DC voltage value in each submodule in the disconnected state. Specifically, the submodule state exchange process is as follows: determine the maximum DC voltage U of all latched submodules. dc_max Record that its submodule is SM j Determine the minimum DC voltage U of all cut-off submodules. dc_min Record that its submodule is SM k ; Calculate the current maximum voltage deviation ΔU dc =U dc_max -U dc_min If ΔU dc >Voltage balancing threshold, then cut off submodule SM j Simultaneously, the SM locking submodule is locked. k .

[0076] For example, the maximum number of target submodules to be removed is determined based on the peak AC line voltage of the submodules in the hybrid MMC and the rated voltage of the submodules in the hybrid MMC. Specifically, the formula for calculating the maximum number of target submodules to be removed is:

[0077]

[0078] Where N1 is the maximum number of target submodules to be removed, N is the number of submodules connected in series within each bridge arm of the hybrid MMC, and U peak U represents the peak AC line voltage of the submodule of the hybrid MMC. s The rated voltage of the submodule of the hybrid MMC is given by 'round', which is a rounding function. Active charging is performed by removing the target submodule, further ensuring the safety of the DC-side connected equipment.

[0079] In some example implementations, the step S130 of controlling each full-bridge submodule within each arm of the hybrid MMC to de-half-bridge, so that the DC-side voltage of the hybrid MMC is within a safe threshold, includes:

[0080] Control the full-bridge submodule in each bridge arm of the hybrid MMC that is in a locked state and has the lowest DC voltage to de-half-bridge;

[0081] Repeat the above half-bridge removal process until the DC-side voltage of the hybrid MMC is within a safe threshold.

[0082] In this example implementation, the DC-side voltage of the MMC is monitored in real time. When the DC-side voltage of the MMC exceeds a safety threshold, the full-bridge submodule with the lowest DC voltage in each bridge arm that is in a locked state can be de-halved and restored to full-bridge operation mode. That is, one IGBT switch of the full-bridge submodule is turned on. Figure 4 In step T3 or T6, the relationship between the MMC DC-side voltage and the safety threshold is further determined. If the MMC DC-side voltage is greater than the safety threshold, the full-bridge submodule with the lowest DC voltage in each bridge arm that is currently in a locked state is de-halved, and the above de-halving process is repeated until the MMC DC-side voltage is less than or equal to the safety threshold, at which point the next step of target submodule removal and / or submodule state exchange is initiated. This example achieves the cancellation of DC voltages generated in the upper and lower bridge arms by de-halving the full-bridge submodule with the lowest DC voltage in each bridge arm that is in a locked state, thereby achieving a voltage reduction effect on the MMC DC-side voltage.

[0083] In some example implementations, the hybrid MMC is connected to the AC-side power grid via a series soft-start resistor. For example, such as... Figure 2 As shown, the MMC valve is connected to the AC power grid via an AC circuit breaker QF and a slow-start resistor circuit. The slow-start resistor circuit includes a switch KM and a slow-start resistor R. The slow-start resistor R can prevent overcurrent from occurring in the initial stage of AC system charging, thus avoiding damage to system components.

[0084] In some example implementations, after the control corresponding full-bridge submodule is half-bridged, the method further includes:

[0085] If the effective value of the AC side charging current of the hybrid MMC is less than the preset current threshold, the soft-start resistor is bypassed.

[0086] In this example implementation, the current threshold can be set to a small value, such as 2-3 amperes. The charging process can be determined based on the preset current threshold. When the effective value of the charging current is less than the current threshold, the slow-start resistor can be bypassed by closing the switch KM of the slow-start resistor circuit. This avoids generating a large inrush current on the AC side and ensures the safety of the power equipment.

[0087] A specific embodiment of the present invention is aimed at Figure 2 The startup method of the DC-DC step-down hybrid MMC supercapacitive network SC shown includes the following steps:

[0088] (1) Block all power devices of the full-bridge submodule and half-bridge submodule, close the AC side circuit breaker QF, put in the slow-start resistor R, and charge the MMC valve submodule uncontrolled rectifier. During this process, the DC voltage of the full-bridge submodule is usually twice that of the half-bridge submodule.

[0089] (2) When the DC voltage of the full-bridge submodule reaches the working threshold value of the self-powered circuit, the T3 or T6 of the full-bridge submodule is triggered to turn on, so that the external characteristics of the full-bridge submodule are consistent with those of the half-bridge submodule. This is called the full-bridge submodule half-bridge conversion.

[0090] (3) When the effective value of the AC charging current is less than the set current threshold, close the switch KM, bypass the soft-start resistor, and charge the secondary uncontrolled rectifier.

[0091] (4) Real-time monitoring of MMC DC side voltage U dc Determine whether the DC side voltage is greater than the set safety threshold. If so, remove the full-bridge submodule with the lowest DC voltage in each bridge arm from the half-bridge state, that is, lock T3 or T6 and restore it to the full-bridge working mode; otherwise, go to step (5).

[0092] (5) Determine whether the number of sub-modules cut off in each bridge arm is less than or equal to the maximum number of cut-offs. If yes, control the cut-off of the sub-module with the highest DC voltage in each bridge arm that is in a locked state for active charging. Otherwise, proceed to step (6).

[0093] (6) Implement the DC voltage equalization control strategy for the sub-modules and determine the maximum DC voltage U of all locked sub-modules. dc_max Record that its submodule is SM j Determine the minimum DC voltage U for all cut-off submodules. dc_min Record that its submodule is SM k Determine the current maximum voltage deviation ΔU dc Is it greater than the set voltage balancing threshold, ΔUdc =U dc_max -U dc_min If so, then remove the submodule SM. j Simultaneously, the SM locking submodule is locked. k Then proceed to step (7); otherwise, proceed directly to step (7).

[0094] (7) Determine whether the DC voltage of all full-bridge and half-bridge submodules of the MMC reaches the rated voltage, i.e., the DC voltage of all submodules is within 0.95U. s ~1.05U s If the range is met, the startup ends; otherwise, proceed to step (4).

[0095] In a hybrid MMC (Multi-Module Control) architecture, where different submodules have inconsistent external characteristics leading to inconsistent charging voltages, this invention avoids generating significant inrush current when unlocking the converter valve. Furthermore, under DC-side step-down operation, the startup algorithm ensures the insulation safety of the DC-side equipment. This startup method is implemented through a control strategy and does not involve any hardware additions or modifications, making it highly practical for engineering applications. In other words, this invention, through its control algorithm, ensures both no current surge to the AC side and the safety of the DC-side equipment during startup, demonstrating strong engineering applicability.

[0096] Existing SC (Supercharged Conversion) equipment typically employs grid-connected control, resulting in strong dependence on the power grid, low transient overcurrent capacity, and strong current source characteristics. This fails to meet the reactive power compensation requirements of modern power systems. Power systems urgently need a new type of grid-connected SC device that provides rapid reactive power / voltage support and a certain degree of inertia support. MMC (Multi-Capacity Modulated Conversion) grid-connected SC devices, with both AC and DC external ports and more flexible DC port control, possess significant research and application value. In engineering applications, MMC-structured converter valves often utilize half-bridge submodules, which have fewer switches, lower losses, and lower costs, but cannot isolate DC faults. Full-bridge submodules, on the other hand, provide DC fault isolation, but have more switches, higher losses, and higher costs. Hybrid MMC converter valves based on full-bridge and half-bridge submodules reduce costs and losses while maintaining DC fault isolation capabilities, showing broad application prospects. Therefore, hybrid MMC supercapacitive grid-connected SC devices can be configured in renewable energy power plants and AC substations to enhance both their reactive and reactive power capabilities. Patent applications CN202310746136.6, CN201810395466.4, CN201910886504.0, and CN202410549670.2 disclose several startup methods for conventional full-semi-hybrid MMC converter valves. These methods generally involve three stages: 1) Uncontrolled charging with a starting resistor, where all full-bridge and half-bridge modules are locked. After charging, the DC voltage of the full-bridge sub-modules is approximately twice that of the half-bridge sub-modules; 2) Half-bridge conversion of the full-bridge modules, continuing uncontrolled charging with the starting resistor; 3) After the sub-module voltage stabilizes, the starting resistor is bypassed, controlling the disconnection of sub-modules in the bridge arms and engaging sub-module voltage equalization control until the DC voltage of all sub-modules reaches its rated value, completing the charging process. The last patent employs an active charging mode with full-bridge sub-module active discharge in stage 3) to accelerate charging. However, these startup methods may cause overvoltage insulation problems when connected to DC-side equipment.

[0097] Generally, if the system can meet the usage requirements of various power equipment under low voltage conditions, there is no need for the SC to provide higher voltage, thus avoiding an increase in system size and cost. Therefore, a DC step-down SC is designed. The overcapacity of the SC is mainly used to provide active power. The overcapacity configuration only needs to meet the active power output demand. In other words, the DC step-down SC is used in scenarios where the active power output demand of the equipment can be met under low voltage conditions. In this scenario, the DC step-down SC can reduce cost and equipment size. The DC step-down SC has both active and non-active capabilities. The overcapacity configuration can meet the active power output requirements, and the active power can be positive or negative; that is, the equipment can both output and absorb active power. For DC step-down applications, if conventional starting methods are used, it may cause overvoltage insulation problems for the equipment connected to the DC side. To ensure that the AC side is not impacted during device startup while ensuring the safety of the DC side equipment, this invention designs the SC startup control logic to ensure that the AC side is not impacted during device startup while ensuring the safety of the DC side equipment.

[0098] Example 2

[0099] Based on the same inventive concept, this invention also provides a DC-DC step-down hybrid MMC supercapacitive grid static synchronous condenser start-up system, the system comprising:

[0100] An uncontrolled charging unit is used to control each full-bridge submodule and each half-bridge submodule of the hybrid modular multilevel converter (MMC) to be in a locked state, so that the hybrid MMC can perform uncontrolled rectified charging.

[0101] The half-bridge unit is used to control each full-bridge submodule to be half-bridged when the DC voltage of each full-bridge submodule of the hybrid MMC reaches the self-powered circuit operating threshold.

[0102] The half-bridge de-splitting unit is used to control the full-bridge submodule in each bridge arm of the hybrid MMC to de-splitting if the DC-side voltage of the hybrid MMC detected in real time is greater than a safety threshold, so that the DC-side voltage of the hybrid MMC is within the safety threshold.

[0103] The equalizing charging unit is used to control the working state of each sub-module in the hybrid MMC based on the DC voltage equalization control strategy, so that each sub-module in the hybrid MMC reaches the rated voltage.

[0104] The safety threshold is determined based on the rated voltage and overload capacity of the overcapacity valve of the supercapacity network static synchronous condenser; the rated voltage of the overcapacity valve of the supercapacity network static synchronous condenser is less than the sum of the rated voltages of all submodules on a single bridge arm of the hybrid MMC; the operating states of each submodule in the hybrid MMC include locked and disconnected.

[0105] In one possible implementation, the voltage equalization charging unit includes:

[0106] The cut-off sub-unit is used to control the cut-off of the target sub-module within each bridge arm of the hybrid MMC when the DC-side voltage of the hybrid MMC is within a safe threshold.

[0107] The state exchange subunit is used to exchange the working states of the two sub-modules corresponding to the current maximum voltage deviation if the current maximum voltage deviation of the hybrid MMC is greater than the voltage equalization threshold.

[0108] The repeating subunit is used to repeatedly execute the half-bridge removal, the removal of the target submodule, and the exchange of the working states of the two submodules corresponding to the current maximum voltage deviation, until all submodules in the hybrid MMC reach the rated voltage;

[0109] The target submodule is determined based on the operating status and DC voltage of each submodule within each bridge arm; during the repeated execution process, if the cumulative number of target submodules to be removed reaches the maximum number of removals, the removal of the target submodule will no longer be performed; the safety threshold is greater than the voltage balancing threshold.

[0110] In one possible implementation, the target submodule is the submodule within each bridge arm that is in a locked state and has the highest DC voltage.

[0111] In one possible implementation, the maximum number of target submodules to be removed is determined based on the peak AC line voltage of the submodules in the hybrid MMC and the rated voltage of the submodules in the hybrid MMC.

[0112] In one possible implementation, the formula for calculating the maximum number of target submodules to be removed is:

[0113]

[0114] Where N1 is the maximum number of target submodules to be removed, N is the number of submodules connected in series within each bridge arm of the hybrid MMC, and U peak U represents the peak AC line voltage of the submodule of the hybrid MMC. s The rated voltage of the submodule of the hybrid MMC is denoted as , and round is the rounding function.

[0115] In one possible implementation, the current maximum voltage deviation of the hybrid MMC is the difference between the maximum DC voltage in each submodule in the locked state and the minimum DC voltage in each submodule in the disconnected state within the hybrid MMC.

[0116] In one possible implementation, the de-half-bridge unit is specifically used for:

[0117] Control the full-bridge submodule in each bridge arm of the hybrid MMC that is in a locked state and has the lowest DC voltage to de-half-bridge;

[0118] Repeat the above half-bridge removal process until the DC-side voltage of the hybrid MMC is within a safe threshold.

[0119] In one possible implementation, the hybrid MMC is connected to the AC power grid via a series slow-start resistor.

[0120] In one possible implementation, the system further includes:

[0121] A bypass unit is used to bypass the soft-start resistor if the effective value of the AC side charging current of the hybrid MMC is less than a preset current threshold.

[0122] Example 3

[0123] like Figure 5 As shown, the present invention also provides an electronic device, which may be a computer device, a microcontroller device, a smart mobile device, etc. The electronic device in this embodiment may include a processor, a memory, a transceiver component, etc. The memory, processor, and transceiver component are connected via a bus; the memory can be used to store executable programs, and an exemplary executable program may include instructions; the processor is used to execute the instructions stored in the memory. The memory can also be used to store data, which can be accessed and / or modified when instructions are executed.

[0124] The processor may be a Central Processing Unit (CPU), or it may be other general-purpose processors, digital signal processors (DSPs), application-specific integrated circuits (ASICs), field-programmable gate arrays (FPGAs), or other programmable logic devices, discrete gate or transistor logic devices, discrete hardware components, etc. It is the computing core and control core of the terminal, and it is suitable for implementing one or more instructions. Specifically, it is suitable for loading and executing one or more instructions in the storage medium to realize the corresponding method flow or corresponding function, so as to realize the steps of the DC buck hybrid MMC supercapacitive network static synchronous condenser startup method in the above embodiments.

[0125] Example 4

[0126] Based on the same inventive concept, this invention also provides a readable storage medium, specifically an electronic device readable storage medium (Memory). An electronic device readable storage medium is a memory device within an electronic device used to store programs and data. It is understood that the storage medium here can include both the built-in storage medium within the electronic device and extended storage media supported by the electronic device. The storage medium provides storage space, which stores the terminal's operating system. Furthermore, this storage space also stores one or more instructions suitable for loading and execution by a processor. These instructions can be one or more executable programs (including program code). It should be noted that the storage medium here can be a high-speed RAM memory or a non-volatile memory, such as at least one disk storage device. Loading and executing one or more instructions stored in the storage medium by the processor can implement the steps of the DC-DC buck hybrid MMC supercapacitive network static synchronous condenser startup method in the above embodiments.

[0127] Those skilled in the art will understand that embodiments of the present invention can be provided as methods, systems, or computer program products. Therefore, the present invention can take the form of a completely hardware embodiment, a completely software embodiment, or an embodiment combining software and hardware aspects. Furthermore, the present invention can take the form of a computer program product embodied on one or more computer-usable storage media (including, but not limited to, disk storage, CD-ROM, optical storage, etc.) containing computer-usable program code.

[0128] This invention is described with reference to flowchart illustrations and / or block diagrams of methods, apparatus (systems), and computer program products according to embodiments of the invention. It will be understood that each block of the flowchart illustrations and / or block diagrams, and combinations of blocks in the flowchart illustrations and / or block diagrams, can be implemented by computer program instructions. These computer program instructions can be provided to a processor of a general-purpose computer, special-purpose computer, embedded processor, or other programmable data processing apparatus to produce a machine, such that the instructions, which execute via the processor of the computer or other programmable data processing apparatus, generate instructions for implementing the flowchart illustrations and / or block diagrams. Figure 1 One or more processes and / or boxes Figure 1 A device that provides the functions specified in one or more boxes.

[0129] These computer program instructions may also be stored in a computer-readable storage medium that can direct a computer or other programmable data processing device to function in a particular manner, such that the instructions stored in the computer-readable storage medium produce an article of manufacture including instruction means, which are implemented in a process Figure 1 One or more processes and / or boxes Figure 1The function specified in one or more boxes.

[0130] These computer program instructions may also be loaded onto a computer or other programmable data processing equipment to cause a series of operational steps to be performed on the computer or other programmable equipment to produce a computer-implemented process, thereby providing instructions that execute on the computer or other programmable equipment for implementing the process. Figure 1 One or more processes and / or boxes Figure 1 The steps of the function specified in one or more boxes.

[0131] 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 its scope of protection. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that after reading the present invention, they can still make various changes, modifications or equivalent substitutions to the specific implementation methods of the application, but these changes, modifications or equivalent substitutions are all within the scope of protection of the claims pending approval.

Claims

1. A starting method of a direct-current voltage-reduction hybrid MMC super-capacitor grid-connected static synchronous compensator, characterized in that, The method comprises the following steps: controlling each full-bridge submodule and each half-bridge submodule of a hybrid modular multilevel converter (MMC) to be in a blocking state, so that the hybrid MMC is charged by uncontrolled rectification; when the direct-current voltage of each full-bridge submodule of the hybrid MMC reaches a self-energizing loop working threshold value, controlling each full-bridge submodule to be half-bridged; if the real-time monitored direct-current side voltage of the hybrid MMC is greater than a safety threshold value, controlling the full-bridge submodules in each bridge arm of the hybrid MMC to be de-half-bridged, so that the direct-current side voltage of the hybrid MMC is within the safety threshold value; under the condition that the direct-current side voltage of the hybrid MMC is within the safety threshold value, controlling a target submodule in each bridge arm of the hybrid MMC to be cut off; if the current maximum voltage deviation of the hybrid MMC is greater than a voltage equalization threshold value, exchanging the working states of two submodules corresponding to the current maximum voltage deviation; repeating the de-half-bridging, the cutting off of the target submodule and the exchanging of the working states of the two submodules corresponding to the current maximum voltage deviation until each submodule in the hybrid MMC reaches a rated voltage; wherein the safety threshold value is determined based on the rated voltage and overload capacity of an overcapacity valve of the overcapacity grid-connected static synchronous compensator; the rated voltage of the overcapacity valve of the overcapacity grid-connected static synchronous compensator is less than the sum of the rated voltages of all submodules on a single bridge arm of the hybrid MMC; the working states of each submodule in the hybrid MMC include blocking and cutting off; the target submodule is determined based on the working state and direct-current voltage of each submodule in each bridge arm; during the repeating, if the cumulative cutting off number of the target submodule reaches a maximum cutting off number, the cutting off of the target submodule is no longer performed; the safety threshold value is greater than the voltage equalization threshold value.

2. The method of claim 1, wherein, The target submodule is a submodule in each bridge arm that is in a blocking state and has the highest direct-current voltage.

3. The method of claim 2, wherein, The maximum cutting off number of the target submodule is determined based on the AC line voltage peak value of the submodules of the hybrid MMC and the rated voltage of the submodules of the hybrid MMC.

4. The method of claim 3, wherein, The calculation formula of the maximum cutting off number of the target submodule is: Wherein, N1 is the maximum number of cut-off of the target submodule, N is the number of submodules in series in each bridge arm of the hybrid MMC, U peak is the peak voltage of the AC line of the submodules of the hybrid MMC, U s is the rated voltage of the submodules of the hybrid MMC, and round is a rounding function.

5. The method of claim 1, wherein, The current maximum voltage deviation of the hybrid MMC is the difference between the maximum direct-current voltage of each submodule in the hybrid MMC that is in a blocking state and the minimum direct-current voltage of each submodule in the hybrid MMC that is in a cutting off state.

6. The method of claim 1, wherein, The control of the full-bridge submodules in each bridge arm of the hybrid MMC to be de-half-bridged so that the direct-current side voltage of the hybrid MMC is within the safety threshold value comprises: controlling the full-bridge submodule in each bridge arm of the hybrid MMC that is in a blocking state and has the lowest direct-current voltage to be de-half-bridged; repeating the above de-half-bridging process until the direct-current side voltage of the hybrid MMC is within the safety threshold value.

7. The method of claim 1, wherein, The hybrid MMC is connected to an AC side grid through a series connection of a slow-start resistor.

8. The method of claim 7, wherein, After the control of the corresponding full-bridge submodule to be half-bridged, the method further comprises: if the effective value of the AC side charging current of the hybrid MMC is less than a preset current threshold value, bypassing the slow-start resistor.

9. A direct current voltage reduction hybrid MMC super-capacitor grid forming static synchronous compensator starting system, characterized in that, The method comprises the following steps: The non-control charging unit is configured to control each full-bridge submodule and each half-bridge submodule of the hybrid MMC to be in a blocking state, so that the hybrid MMC is charged in a non-control rectification mode. The half-bridge unit is configured to control each full-bridge submodule to be half-bridged when a direct-current voltage of each full-bridge submodule of the hybrid MMC reaches a self-powered loop working threshold. The de-half-bridge unit is configured to control the full-bridge submodule in each bridge arm of the hybrid MMC to be de-half-bridged if a real-time monitored direct-current side voltage of the hybrid MMC is greater than a safety threshold, so that the direct-current side voltage of the hybrid MMC is within the safety threshold. The voltage equalization charging unit is configured to control working states of each submodule in the hybrid MMC based on a direct-current voltage equalization control strategy, so that each submodule in the hybrid MMC reaches a rated voltage. The safety threshold is determined based on a rated voltage and an overload capacity of an overcapacity valve of the overcapacity grid-connected static synchronous compensator; the rated voltage of the overcapacity valve of the overcapacity grid-connected static synchronous compensator is less than a sum of rated voltages of all submodules on a single bridge arm of the hybrid MMC; and the working states of each submodule in the hybrid MMC include blocking and removal. The voltage equalization charging unit includes: The removal subunit is configured to control a target submodule in each bridge arm of the hybrid MMC to be removed if the direct-current side voltage of the hybrid MMC is within the safety threshold. The state exchange subunit is configured to exchange working states of two submodules corresponding to a current maximum voltage deviation of the hybrid MMC if the current maximum voltage deviation is greater than a voltage equalization threshold. The repeating subunit is configured to repeatedly execute the de-half-bridging, the removal of the target submodule, and the exchange of the working states of the two submodules corresponding to the current maximum voltage deviation until each submodule in the hybrid MMC reaches the rated voltage. The target submodule is determined based on the working states and the direct-current voltage of each submodule in each bridge arm; during the repeated execution, if a cumulative removal number of the target submodule reaches a maximum removal number, the removal of the target submodule is no longer performed; and the safety threshold is greater than the voltage equalization threshold.

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