Starting method and system for direct-current voltage-reduction hybrid MMC super-capacitance network-building static synchronous phase modifier

Abstract

The invention provides a method and a system for starting a DC step-down hybrid MMC super-capacitance network static synchronous phase modifier, which are applied to the technical field of flexible power transmission of a power system. 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 locked state respectively, so as to enable the MMC to carry out uncontrolled rectification charging; when the direct-current voltage of each full-bridge sub-module of the MMC reaches the working threshold value of the self-energy-taking loop, each full-bridge sub-module is controlled to carry out half-bridge processing; if the DC side voltage of the MMC monitored in real time is greater than a safety threshold, controlling a full-bridge sub-module in each bridge arm of the MMC to carry out half-bridge removal so as to enable the DC side voltage of the MMC to be within the safety threshold; and controlling the working state of each sub-module in the MMC based on the DC voltage balance control strategy, so that each sub-module in the MMC reaches the rated voltage. According to the invention, the problems of current impact generated on the AC side in the starting process of the hybrid MMC super-capacitance networking SC and overvoltage insulation of the DC side equipment caused by the step-down operation of the DC side are solved.

Description

Technical Field

[0001] The present invention relates to the technical field of flexible power transmission in electric power systems, and in particular to a method and system for starting a DC step-down hybrid MMC super-capacity grid-connected static synchronous phase condenser. Background Art

[0002] With the continuous increase in the penetration rate of new energy, 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] In order to improve the reactive voltage control capability of the renewable energy collection system, a static synchronous compensator (SC) is configured in the renewable energy station and the AC collection station. The MMC super-capacitor grid SC has two external ports, AC and DC, and the control of the DC port is more flexible, which has important research and application value. The hybrid MMC converter valve based on the full-bridge and half-bridge submodules has the ability to isolate DC faults while reducing costs and losses. The super-capacitor grid SC formed based on the hybrid MMC has broad application prospects.

[0004] However, when the MMC converter valve with a full-semi hybrid structure is started without controlled charging on the AC side, the external characteristics of the sub-modules with different structures are not completely consistent, resulting in inconsistent charging voltages of the full and half bridges. If the converter valve is unlocked directly, a large impact current will be generated; and when the DC side is step-down operated, it may cause overvoltage insulation problems for the equipment connected to the DC side. Summary of the invention

[0005] In order to overcome the current impact on the AC side caused by the charging startup process of the above-mentioned hybrid MMC super-capacitor grid SC and the overvoltage insulation problem of the DC side equipment caused by the DC side step-down operation, the present invention provides a DC step-down hybrid MMC super-capacitor grid static synchronous phase condenser startup method and system.

[0006] On the one hand, the present invention provides a method for starting a DC step-down hybrid MMC super-capacitor grid-connected static synchronous condenser, comprising:

[0007] Controlling each full-bridge submodule and each half-bridge submodule of the hybrid modular multilevel converter MMC to be in a locked state respectively, so that the hybrid MMC performs uncontrolled rectification charging;

[0008] When the DC voltage of each full-bridge submodule of the hybrid MMC reaches the working threshold of the self-energy extraction loop, controlling each full-bridge submodule to perform half-bridge conversion;

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

[0010] Controlling the working state of each submodule in the hybrid MMC based on a DC voltage balancing control strategy so that each submodule in the hybrid MMC reaches a rated voltage;

[0011] Among them, the safety threshold is determined based on the rated voltage and overload capacity of the overcapacity valve of the super-capacity grid static synchronous condenser; the rated voltage of the overcapacity valve of the super-capacity grid static synchronous condenser is less than the sum of the rated voltages of all sub-modules on a single bridge arm of the hybrid MMC; the working status of each sub-module in the hybrid MMC includes locking and cutting off.

[0012] Optionally, controlling the working state of each submodule in the hybrid MMC based on a DC voltage balancing control strategy so that each submodule in the hybrid MMC reaches a rated voltage includes:

[0013] When the DC side voltage of the hybrid MMC is within a safety threshold, controlling the removal of a target submodule in each bridge arm of the hybrid MMC;

[0014] If the current maximum voltage deviation of the hybrid MMC is greater than the voltage balancing threshold, the working states of the two submodules corresponding to the current maximum voltage deviation are exchanged;

[0015] Repeating the half-bridge removal, the removal of the target submodule, and the switching 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;

[0016] Among them, the target submodule is determined based on the working status and DC voltage of each submodule in each bridge arm; during the repeated execution process, if the cumulative removal number of the target submodule reaches the maximum removal number, the target submodule will no longer be removed; the safety threshold is greater than the voltage balance threshold.

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

[0018] Optionally, the maximum removal quantity of the target submodules is determined based on the peak value of the AC line voltage of the submodules of the hybrid MMC and the rated voltage of the submodules of the hybrid MMC.

[0019] Optionally, the calculation formula for the maximum number of cutouts of the target submodule is:

[0020]

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

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

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

[0024] Controlling 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] The above-mentioned half-bridge removal process is repeatedly performed until the DC side voltage of the hybrid MMC is within the safety threshold.

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

[0027] Optionally, after the controlling of the corresponding full-bridge submodule to convert into 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 a preset current threshold, the slow-start resistor is bypassed.

[0029] On the other hand, the present invention also provides a DC step-down hybrid MMC super-capacitor grid-connected static synchronous phase condenser starting system, comprising:

[0030] An uncontrolled charging unit, used to control each full-bridge submodule and each half-bridge submodule of the hybrid modular multi-level converter MMC to be in a locked state respectively, so that the hybrid MMC performs uncontrolled rectification charging;

[0031] A half-bridge unit, used to control each full-bridge sub-module to perform half-bridge conversion when the DC voltage of each full-bridge sub-module of the hybrid MMC reaches the working threshold of the self-energy extraction loop;

[0032] A half-bridge removal unit, for controlling the full-bridge submodule in each bridge arm of the hybrid MMC to remove the half-bridge if the DC side voltage of the hybrid MMC monitored 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] A voltage-equalizing charging unit, used to control the working state of each submodule in the hybrid MMC based on a DC voltage balancing control strategy, so that each submodule in the hybrid MMC reaches a rated voltage;

[0034] Among them, the safety threshold is determined based on the rated voltage and overload capacity of the overcapacity valve of the super-capacity grid static synchronous condenser; the rated voltage of the overcapacity valve of the super-capacity grid static synchronous condenser is less than the sum of the rated voltages of all sub-modules on a single bridge arm of the hybrid MMC; the working status of each sub-module in the hybrid MMC includes locking and cutting off.

[0035] Optionally, the voltage-balanced charging unit includes:

[0036] A cutting sub-unit, used for controlling the cutting of a target sub-module in each bridge arm of the hybrid MMC when the DC side voltage of the hybrid MMC is within a safety threshold;

[0037] A state exchange subunit, used for exchanging the working states of two submodules corresponding to the current maximum voltage deviation if the current maximum voltage deviation of the hybrid MMC is greater than the voltage balancing threshold;

[0038] A repeating subunit, used for repeatedly performing 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] Among them, the target submodule is determined based on the working status and DC voltage of each submodule in each bridge arm; during the repeated execution process, if the cumulative removal number of the target submodule reaches the maximum removal number, the target submodule will no longer be removed; the safety threshold is greater than the voltage balance threshold.

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

[0041] Optionally, the maximum removal quantity of the target submodules is determined based on the peak value of the AC line voltage of the submodules of the hybrid MMC and the rated voltage of the submodules of the hybrid MMC.

[0042] Optionally, the calculation formula for the maximum number of cutouts of the target submodule is:

[0043]

[0044] Wherein, N1 is the maximum number of target submodules to be removed, N is the number of submodules connected in series in each bridge arm of the hybrid MMC, and U peak is the submodule AC line voltage peak value of the hybrid MMC, Us is the submodule rated voltage of the hybrid MMC, and round is a rounding function.

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

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

[0047] Controlling 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] The above-mentioned half-bridge removal process is repeatedly performed until the DC side voltage of the hybrid MMC is within the safety threshold.

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

[0050] Optionally, the system further comprises:

[0051] A bypass unit is used to bypass the slow-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, any one of the above-mentioned methods for starting a DC step-down hybrid MMC super-capacitor grid-connected static synchronous phase condenser is implemented.

[0055] On the other hand, the present invention also provides a readable storage medium having an execution program stored thereon, and when the execution program is executed, the DC step-down hybrid MMC super-capacitor grid static synchronous phase condenser starting method described in any one of the above items is implemented.

[0056] Compared with the prior art, the present invention has the following beneficial effects:

[0057] The present invention provides a method and system for starting a DC step-down hybrid MMC super-capacity grid-forming static synchronous phase condenser. The method comprises the following steps: firstly, each full-bridge submodule and each half-bridge submodule of a hybrid modular multilevel converter MMC are controlled to be in a locked state respectively, so that the hybrid MMC performs uncontrolled rectification charging; when the DC voltage of each full-bridge submodule of the hybrid MMC reaches the working threshold of a self-energy-taking loop, each full-bridge submodule is controlled to be half-bridged to balance the charging speed of the full and half submodules, and the working state of each submodule in the hybrid MMC is controlled to be adjusted between the locked state and the cut-off state based on a DC voltage balancing control strategy, so that the charging voltages of each submodule of the MMC are basically consistent, thereby avoiding the problem of a large impact current on the AC side when unlocking a converter valve due to inconsistent charging voltages caused by incompletely consistent external characteristics of submodules with different structures, thereby 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 grid static synchronous phase shifter and real-time monitoring of the DC side voltage of the hybrid MMC, and when the DC side voltage is greater than the safety threshold, controlling the full-bridge sub-module in each bridge arm of the hybrid MMC to de-half-bridge, so that the DC side voltage of the hybrid MMC is within the safety threshold, so that in the DC side step-down operation scenario where the rated voltage of the overcapacity valve of the supercapacity grid static synchronous phase shifter is less than the sum of the rated voltages of all sub-modules on a single bridge arm of the hybrid MMC, the overvoltage insulation problem connected to the DC side equipment is avoided, thereby ensuring the insulation safety of the DC side equipment in the DC side step-down operation scenario. BRIEF DESCRIPTION OF THE DRAWINGS

[0058] Figure 1 A schematic flow chart of a method for starting a DC step-down hybrid MMC super-capacitor grid-connected static synchronous condenser according to the present invention;

[0059] Figure 2 A schematic diagram of the topological structure of a DC step-down hybrid MMC super-capacitor network SC according to an example of the present invention;

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

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

[0062] Figure 5 It is a schematic structural diagram of the electronic device of the present invention. DETAILED DESCRIPTION

[0063] The specific implementation modes of the present invention are further described in detail below with reference to the accompanying drawings.

[0064] Example 1

[0065] The present invention provides a DC step-down hybrid MMC super-capacitor grid static synchronous phase condenser starting method, the schematic diagram of which is shown in FIG. Figure 1 As shown, including:

[0066] Step S110, controlling 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 performs uncontrolled rectification charging;

[0067] Step S120, when the DC voltage of each full-bridge sub-module of the hybrid MMC reaches the working threshold of the self-energy extraction loop, controlling each full-bridge sub-module to perform half-bridge conversion;

[0068] Step S130, if the DC side voltage of the hybrid MMC monitored in real time is greater than the safety threshold, control the full-bridge submodule in each bridge arm of the hybrid MMC to de-half-bridge, so that the DC side voltage of the hybrid MMC is within the safety threshold;

[0069] Step S140: Control the working state of each submodule in the hybrid MMC based on the DC voltage balancing control strategy, so that each submodule in the hybrid MMC reaches the rated voltage.

[0070] In this example implementation, the hybrid MMC super-capacitor network static synchronous condenser mainly includes two parts: an MMC valve and a super-capacitor valve. The MMC valve adopts a full-half hybrid structure, that is, the basic topology of the hybrid MMC includes a full-bridge submodule and a half-bridge submodule. The topology of the hybrid MMC super-capacitor network SC is as follows: Figure 2 As shown in the figure, the dotted box is the MMC valve structure. The MMC valve is connected to the AC grid through the AC switch QF and the slow-start resistor loop, and the supercapacitor valve is directly connected to the DC side of the MMC valve. The MMC valve has three phases A / B / C, and each phase is symmetrically provided with an upper bridge arm and a lower bridge arm. The upper / lower bridge arms are respectively composed of multiple submodules (SM) and an inductor in series; for example, for phase A, the upper / lower bridge arms are respectively composed of submodules SM a1 ,SM a2 ,…,SM aN and SM aN+1 ,SM aN+2 ,…,SM a2N and inductor L a The structures of phase B and phase C are similar. The connection point of the upper and lower bridge arms of each phase is the input terminal of the corresponding phase. dc is the DC side voltage of MMC. The half-bridge submodule topology of MMC valve is as follows: Figure 3 As shown, Figure 3 In the figure, T1 / T2 are IGBT switch devices, D1 / D2 are anti-parallel diodes of T1 / T2, and C1 is the DC support capacitor corresponding to the submodule. Full-bridge submodule topology of MMC valve Figure 4 As shown, Figure 4 Among them, 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 supercapacity valve of the supercapacity grid static synchronous phase regulator. For example, the safety threshold can be between 1 and 1.2 times the rated voltage of the supercapacity valve. The rated voltage of the supercapacity valve of the supercapacity grid static synchronous phase regulator is less than the sum of the rated voltages of all submodules on a single bridge arm of the hybrid MMC, that is, the rated operating voltage of the supercapacity valve is less than the algebraic sum of the rated DC voltages of all submodules on a single bridge arm of the MMC valve. The SC that meets this condition is a DC step-down type. Exemplarily, the conditions that the DC step-down type SC needs to meet are U dc <N*U s , U dc is the DC side voltage of the hybrid MMC, i.e. the rated working voltage of the overcapacity valve, U s is the rated voltage of the submodule of the hybrid MMC, and N is the number of submodules connected in series in each bridge arm of the hybrid MMC. For the DC step-down SC, the equipment connected to the DC side may have an overvoltage insulation problem. For this problem, the present invention ensures that the DC side voltage will not be too high by designing real-time monitoring of the DC side voltage, safety thresholds, and de-half-bridge processes, thereby ensuring the safety of the DC side equipment. The working state of each submodule in the hybrid MMC includes locking and cutting off, and the working state of each submodule can be controlled by controlling the switch state in each submodule. For example, the locking state of the half-bridge submodule and the full-bridge submodule respectively locks all power devices in the full-bridge submodule and the half-bridge submodule, that is, closes all IGBT switches. The cut-off state of the full-bridge submodule refers to when both IGBT switches in the full-bridge submodule are in the off state, the submodule is removed from the circuit and no longer participates in the voltage output; the half-bridge of the full-bridge submodule refers to turning on one of the IGBT switches of the full-bridge submodule, such as turning on Figure 4 T3 or T6 in the half-bridge; the de-half-bridge of the full-bridge submodule refers to restoring the full-bridge characteristics of the full-bridge submodule, specifically, restoring the IGBT switch that is turned on during the half-bridge process to the off state. Figure 3 As shown in the figure, the cut-off state of the half-bridge submodule refers to the situation where T1 is turned off and T2 is turned on. At this time, the submodule capacitor is cut out of the circuit. During the uncontrolled rectification charging process, the power devices of all full-bridge submodules and half-bridge submodules in the MMC are locked, the AC side circuit breaker is closed, and the slow-start resistor is turned on. Figure 2The circuit breaker QF is closed in the middle, and the switch KM is disconnected, so that the MMC valve submodule performs uncontrolled rectification charging. During this process, the DC voltage of the full-bridge submodule is usually twice the DC voltage of the half-bridge submodule. By controlling each full-bridge submodule to be half-bridged to balance the charging speed of the full and half submodules, the working state of each submodule in the hybrid MMC is controlled to be adjusted between the locked and cut-off states based on the DC voltage balancing control strategy, so that the charging voltage of each MMC submodule is basically the same, thereby avoiding the problem of large impact current on the AC side when unlocking the converter valve due to inconsistent charging voltage caused by the inconsistency of external characteristics of submodules with different structures. By controlling the DC side voltage of the hybrid MMC to be within the safety threshold, the overvoltage insulation problem of the DC side buck SC connected to the DC side equipment is avoided, and the insulation safety of the DC side equipment in the DC side buck operation scenario is ensured.

[0071] In some example implementations, controlling the working state of each submodule in the hybrid MMC based on the DC voltage balancing control strategy in step S140 so that each submodule in the hybrid MMC reaches a rated voltage includes:

[0072] When the DC side voltage of the hybrid MMC is within a safety threshold, controlling the removal of a target submodule in each bridge arm of the hybrid MMC;

[0073] If the current maximum voltage deviation of the hybrid MMC is greater than the voltage balancing threshold, the working states of the two submodules corresponding to the current maximum voltage deviation are exchanged;

[0074] 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 are repeatedly performed until each submodule in the hybrid MMC reaches the rated voltage.

[0075] In this example implementation, the target submodule is determined based on the working state and DC voltage of each submodule in each bridge arm; illustratively, the target submodule is a submodule in each bridge arm that is in a locked state and has the highest DC voltage. Before each submodule in the hybrid MMC reaches the rated voltage, the charging process can be continued, and during the charging process, the real-time monitoring of the MMC DC side voltage, safety threshold judgment, half-bridge removal, target submodule removal, working state exchange and other processes are repeated, and one target submodule is removed each time the process is repeated. During the repeated execution process, if the cumulative removal number of the target submodule reaches the maximum removal number, the removal of the target submodule is no longer performed; that is, when the cumulative removal number of the target submodule reaches the maximum removal number and the submodules in the hybrid MMC do not reach the rated voltage, the safety threshold judgment, half-bridge removal, and working state exchange are repeated. All submodules in the hybrid MMC reach the rated voltage and the startup is completed. The safety threshold is greater than the voltage balancing threshold. The voltage balancing threshold can be determined based on the rated working voltage of the submodule, such as the voltage balancing threshold is 0.05U s , U s is the rated working voltage of the submodule. Each submodule reaches the rated voltage, which can be the allowable error range of each submodule reaching the rated voltage. For example, each submodule reaches 0.95U s ~1.05U s Exemplarily, the current maximum voltage deviation of the hybrid MMC is the difference between the maximum DC voltage of each submodule in the locked state and the minimum DC voltage of each submodule in the cut-off state in the hybrid MMC. Specifically, the submodule state exchange process is: determine the maximum DC voltage U of all locked submodules. dc_max , record the submodule where it is located as SM j ; Determine the minimum DC voltage U of all removed submodules dc_min , record the submodule where it is located as SM k ; Calculate the current maximum voltage deviation ΔU dc =U dc_max -U dc_min , if ΔU dc > voltage balancing threshold, then the submodule SM is removed j , while locking the submodule SM k .

[0076] Exemplarily, the maximum number of removed target submodules is determined based on the peak value of the AC line voltage of the submodules of the hybrid MMC and the rated voltage of the submodules of the hybrid MMC. Specifically, the maximum number of removed target submodules is calculated as follows:

[0077]

[0078] Wherein, N1 is the maximum number of target submodules to be removed, N is the number of submodules connected in series in each bridge arm of the hybrid MMC, and U peak is the submodule AC line voltage peak value of the hybrid MMC, U s is the submodule rated voltage of the hybrid MMC, and round is a rounding function. Active charging is performed by removing the target submodule to further ensure the safety of the DC side access device.

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

[0080] Controlling 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] The above-mentioned half-bridge removal process is repeatedly performed until the DC side voltage of the hybrid MMC is within the safety 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 is greater than the safety threshold, the full-bridge sub-module in each bridge arm that is in a locked state and has the lowest DC voltage can be de-half-bridged and restored to the full-bridge working mode, that is, one of the IGBT switches of the full-bridge sub-module is turned on, such as turning on Figure 4 At T3 or T6 in the circuit, the magnitude relationship between the MMC DC side voltage and the safety threshold is continued to be determined. If the MMC DC side voltage is greater than the safety threshold, the full-bridge submodule in each bridge arm that is currently in a locked state and has the lowest DC voltage is continued to be de-half-bridged, and the above de-half-bridged process is repeated until the MMC DC side voltage is less than or equal to the safety threshold, and the next step is to remove the target submodule and / or exchange the submodule state. In this example, the DC voltage generated in the upper and lower bridge arms is offset by de-half-bridged the full-bridge submodule in each bridge arm that is in a locked state and has the lowest DC voltage, thereby achieving a voltage reduction effect on the MMC DC side voltage.

[0083] In some example embodiments, the hybrid MMC is connected to the AC side grid via a series slow-start resistor. Figure 2 As shown, the MMC valve is connected to the AC side grid through the AC circuit breaker QF and the 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 avoid overcurrent in the initial stage of AC system charging and damage to system components.

[0084] In some example implementations, after controlling the corresponding full-bridge submodule to perform half-bridge, the method further includes:

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

[0086] In this example implementation, the current threshold can be set to a relatively small value, such as 2-3 amperes. Whether the charging process is finished can be determined based on the preset current threshold. When the effective value of the charging current is less than the current threshold, the switch KM of the slow-start resistor loop can be closed to bypass the slow-start resistor, thereby avoiding a large impact current on the AC side and ensuring the safety of the power equipment.

[0087] A specific embodiment of the present invention is directed to Figure 2 The starting method of the DC step-down hybrid MMC super-capacitor grid SC shown includes the following steps:

[0088] (1) Lock the power devices of all full-bridge submodules and half-bridge submodules, close the AC side circuit breaker QF, put into operation the slow-start resistor R, and the MMC valve submodule is not controlled to rectify and charge. During this process, the DC voltage of the full-bridge submodule is usually twice the DC voltage of the half-bridge submodule;

[0089] (2) When the DC voltage of the full-bridge submodule reaches the working threshold value of the self-energy extraction circuit, 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, which is called half-bridge conversion of the full-bridge submodule;

[0090] (3) When the effective value of the AC charging current is less than the set current threshold, the switch KM is closed, the slow-start resistor is bypassed, and the secondary is uncontrolled for rectification and charging;

[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, de-half-bridge the full-bridge submodule in each bridge arm that is in a locked state and has the lowest DC voltage, that is, lock T3 or T6, and restore to the full-bridge working mode; otherwise, go to step (5);

[0092] (5) Determine whether the number of submodules removed from each bridge arm is less than or equal to the maximum number of removed submodules. If so, control the removal of the submodule with the highest DC voltage in each bridge arm that is in a locked state for active charging. Otherwise, go to step (6);

[0093] (6) Activate the submodule DC voltage balancing control strategy to determine the maximum DC voltage U of all blocking submodules dc_max , record the submodule where it is located as SM j ; Determine the minimum DC voltage U of all removed submodules dc_min , record the submodule where it is located as SM k ; Determine the current maximum voltage deviation ΔU dc Is it greater than the set voltage balance threshold, ΔUdc =U dc_max -U dc_min If yes, then remove the submodule SM j , while locking the submodule SM k Then go to step (7), otherwise, go directly to step (7);

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

[0095] In the full-semi-hybrid structure MMC, due to the inconsistency of charging voltage caused by the inconsistency of external characteristics of submodules of different structures, the present invention will not generate a large impact current when unlocking the converter valve; in the case of DC side step-down operation, the startup algorithm can ensure the insulation safety of the DC side equipment; the startup method is implemented by the control strategy and does not involve the addition or change of hardware, and has strong engineering practicality. In other words, the present invention can ensure the safety of the DC side equipment while ensuring that no current impact is generated on the AC side during the startup of the device through the control algorithm, and has strong engineering practicality.

[0096] Existing SC equipment usually adopts grid-following control, which is highly dependent on the power grid, has low transient overcurrent capacity, and has strong current source characteristics. It cannot meet the needs of new power systems for reactive power compensation equipment. The power system urgently needs a new type of SC device that can provide fast reactive power / voltage support capabilities and has a certain inertia support capability. The MMC super-capacitor SC device has two external ports, AC and DC, and the control of the DC port is more flexible, which has important research and application value. The MMC structure converter valve used in the project mostly adopts half-bridge submodules, which have fewer switches, low losses, and low costs, but cannot isolate DC faults; while the full-bridge submodule has the ability to isolate DC faults, but it has more switches, high losses, and high costs. The hybrid MMC converter valve based on the full-bridge and half-bridge submodules has reduced costs and losses on the basis of having the ability to isolate DC faults, and has broad application prospects. Therefore, the hybrid MMC super-capacitor SC can be configured in the new energy station and AC collection station to improve the functional and non-functional capabilities of the new energy station and AC collection station. The invention patents with application numbers CN202310746136.6, CN201810395466.4, CN201910886504.0 and CN202410549670.2 disclose several conventional full-half hybrid structure MMC converter valve start-up methods, which are basically started through three stages: 1) uncontrolled charging stage with starting resistance, in which all full-bridge modules and half-bridge modules are in a locked state. After charging, the DC voltage of the full-bridge submodule is about twice that of the half-bridge submodule; 2) half-bridge stage of the full-bridge module, continue to charge uncontrolled through the starting resistance; 3) after the submodule voltage is stable, bypass the starting resistance, control the removal of the submodule in the bridge arm, and put the submodule voltage equalization control into operation until the DC voltage of all submodules is charged to the rated value and charging is completed. Among them, the last invention patent adopts the active discharge mode of the full-bridge submodule for active charging in order to speed up the charging speed in the 3rd stage. All these starting methods may cause overvoltage insulation problems of the equipment connected to the DC side.

[0097] In general, if the system can already meet the use requirements of various power equipment under low voltage conditions, there is no need for SC to provide a higher voltage to avoid an increase in system volume and cost, thereby designing a DC step-down SC. The overcapacity of SC is mainly used to provide active power. The configuration of overcapacity can meet the output demand of active power. In other words, the DC step-down SC is a scenario 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 costs and equipment volume. The DC step-down SC has functional capacity and non-functional capacity. The configuration of overcapacity can meet the active power output requirements. The active power can be positive or negative, that is, the equipment can both output active power and absorb active power. For DC step-down application scenarios, if conventional startup methods are used, overvoltage insulation problems may be caused for devices connected to the DC side. In order to ensure that no current shock is generated on the AC side during the startup of the device while ensuring the safety of the DC side equipment, the present invention can ensure that no current shock is generated on the AC side during the startup of the device while ensuring the safety of the DC side equipment by designing the SC startup control logic.

[0098] Example 2

[0099] Based on the same inventive concept, the present invention also provides a DC step-down hybrid MMC super-capacitor grid-connected static synchronous condenser starting system, the system comprising:

[0100] An uncontrolled charging unit, used to control each full-bridge submodule and each half-bridge submodule of the hybrid modular multi-level converter MMC to be in a locked state, so that the hybrid MMC performs uncontrolled rectification charging;

[0101] A half-bridge unit, used to control each full-bridge sub-module to perform half-bridge conversion when the DC voltage of each full-bridge sub-module of the hybrid MMC reaches the working threshold of the self-energy extraction loop;

[0102] A half-bridge removal unit, for controlling the full-bridge submodule in each bridge arm of the hybrid MMC to remove the half-bridge if the DC side voltage of the hybrid MMC monitored 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] A voltage-equalizing charging unit, used to control the working state of each submodule in the hybrid MMC based on a DC voltage balancing control strategy, so that each submodule in the hybrid MMC reaches a rated voltage;

[0104] Among them, the safety threshold is determined based on the rated voltage and overload capacity of the overcapacity valve of the super-capacity grid static synchronous condenser; the rated voltage of the overcapacity valve of the super-capacity grid static synchronous condenser is less than the sum of the rated voltages of all sub-modules on a single bridge arm of the hybrid MMC; the working status of each sub-module in the hybrid MMC includes locking and cutting off.

[0105] In a possible implementation, the voltage-balanced charging unit includes:

[0106] A cutting sub-unit, used for controlling the cutting of a target sub-module in each bridge arm of the hybrid MMC when the DC side voltage of the hybrid MMC is within a safety threshold;

[0107] A state exchange subunit, used for exchanging the working states of two submodules corresponding to the current maximum voltage deviation if the current maximum voltage deviation of the hybrid MMC is greater than the voltage balancing threshold;

[0108] A repeating subunit, used for repeatedly performing 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] Among them, the target submodule is determined based on the working status and DC voltage of each submodule in each bridge arm; during the repeated execution process, if the cumulative removal number of the target submodule reaches the maximum removal number, the target submodule will no longer be removed; the safety threshold is greater than the voltage balance threshold.

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

[0111] In a possible implementation manner, the maximum removal quantity of the target submodules is determined based on the peak value of the AC line voltage of the submodules of the hybrid MMC and the rated voltage of the submodules of the hybrid MMC.

[0112] In a possible implementation manner, the calculation formula for the maximum number of removals of the target submodules is:

[0113]

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

[0115] In a possible implementation manner, the current maximum voltage deviation of the hybrid MMC is a difference between a maximum DC voltage in each submodule in a locked state and a minimum DC voltage in each submodule in a cut-off state in the hybrid MMC.

[0116] In a possible implementation manner, the half-bridge removal unit is specifically used for:

[0117] Controlling 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] The above-mentioned half-bridge removal process is repeatedly performed until the DC side voltage of the hybrid MMC is within the safety threshold.

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

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

[0121] A bypass unit is used to bypass the slow-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 single-chip device, an intelligent mobile device, etc. The electronic device in this embodiment may include a processor, a memory, a transceiver component, etc. The memory, the processor, and the transceiver component are connected via a bus; the memory may be used to store an execution program, and an exemplary execution program may include instructions; the processor is used to execute the instructions stored in the memory. The memory may also be used to store data, which may be called and / or modified when the instructions are executed.

[0124] The processor may be a central processing unit (CPU), or other general-purpose processors, digital signal processors (DSP), application-specific integrated circuits (ASIC), field-programmable gate arrays (FPGA) or other programmable logic devices, discrete gates or transistor logic devices, discrete hardware components, etc. It is the computing core and control core of the terminal, which is suitable for implementing one or more instructions, and is specifically suitable for loading and executing one or more instructions in a storage medium to implement a corresponding method flow or corresponding function, so as to implement the steps of a DC step-down hybrid MMC super-capacitor network static synchronous phase-shifting method in the above-mentioned embodiment.

[0125] Example 4

[0126] Based on the same inventive concept, the present invention also provides a readable storage medium, specifically an electronic device readable storage medium (Memory), which is a memory device in an electronic device for storing programs and data. It can be understood that the storage medium here can include both built-in storage media in electronic devices and, of course, extended storage media supported by electronic devices. The storage medium provides a storage space, which stores the operating system of the terminal. In addition, one or more instructions suitable for being loaded and executed by a processor are also stored in the storage space, and these instructions can be one or more execution programs (including program codes). 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 memory. The processor loads and executes one or more instructions stored in the storage medium, which can implement the steps of a DC step-down hybrid MMC super-capacitor grid static synchronous phase start-up method in the above embodiment.

[0127] Those skilled in the art will appreciate that embodiments of the present invention may be provided as methods, systems, or computer program products. Therefore, the present invention may take the form of a complete hardware embodiment, a complete software embodiment, or an embodiment combining software and hardware. Moreover, the present invention may take the form of a computer program product implemented 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] The present invention is described with reference to flowcharts and / or block diagrams of methods, devices (systems), and computer program products according to embodiments of the present invention. It should be understood that each process and / or block in the flowchart and / or block diagram, as well as the combination of processes and / or blocks in the flowchart and / or block diagram, can be implemented by computer program instructions. These computer program instructions can be provided to a processor of a general-purpose computer, a special-purpose computer, an embedded processor, or other programmable data processing device to produce a machine, so that the instructions executed by the processor of the computer or other programmable data processing device generate instructions for implementing the processes in the flowchart and / or block diagram. Figure 1 A process or multiple processes and / or boxes Figure 1 A device that provides the functions specified in a block or multiple blocks.

[0129] These computer program instructions may also be stored in a computer-readable memory capable of directing a computer or other programmable data processing device to operate in a specific manner, so that the instructions stored in the computer-readable memory produce an article of manufacture comprising an instruction device, which implements the process Figure 1 A process or multiple processes and / or boxes Figure 1A function specified in one or more boxes.

[0130] These computer program instructions can also be loaded onto a computer or other programmable data processing device so that a series of operating steps are executed on the computer or other programmable device to produce a computer-implemented process, thereby providing instructions for implementing the process. Figure 1 A process or multiple processes and / or boxes Figure 1 The steps for the functions 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 rather than to limit its protection scope. Although the present invention has been described in detail with reference to the above embodiments, ordinary technicians in the field should understand that after reading the present invention, those skilled in the art 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 protection scope of the claims to be approved.

Claims

1. A method for starting a DC step-down hybrid MMC super-capacitor grid-connected static synchronous condenser, characterized in that: include: Controlling each full-bridge submodule and each half-bridge submodule of the hybrid modular multilevel converter MMC to be in a locked state respectively, so that the hybrid MMC performs uncontrolled rectification charging; When the DC voltage of each full-bridge submodule of the hybrid MMC reaches the working threshold of the self-energy extraction loop, controlling each full-bridge submodule to perform half-bridge conversion; If the DC side voltage of the hybrid MMC monitored in real time is greater than a safety threshold, control the full-bridge submodule in each bridge arm of the hybrid MMC to de-half-bridge, so that the DC side voltage of the hybrid MMC is within the safety threshold; Controlling the working state of each submodule in the hybrid MMC based on a DC voltage balancing control strategy so that each submodule in the hybrid MMC reaches a rated voltage; Among them, the safety threshold is determined based on the rated voltage and overload capacity of the overcapacity valve of the super-capacity grid static synchronous condenser; the rated voltage of the overcapacity valve of the super-capacity grid static synchronous condenser is less than the sum of the rated voltages of all sub-modules on a single bridge arm of the hybrid MMC; the working status of each sub-module in the hybrid MMC includes locking and cutting off.

2. The method according to claim 1, characterized in that: Controlling the working state of each submodule in the hybrid MMC based on the DC voltage balancing control strategy so that each submodule in the hybrid MMC reaches the rated voltage includes: When the DC side voltage of the hybrid MMC is within a safety threshold, controlling the removal of a target submodule in each bridge arm of the hybrid MMC; If the current maximum voltage deviation of the hybrid MMC is greater than the voltage balancing threshold, the working states of the two submodules corresponding to the current maximum voltage deviation are exchanged; Repeating the half-bridge removal, the removal of the target submodule, and the switching 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; Among them, the target submodule is determined based on the working status and DC voltage of each submodule in each bridge arm; during the repeated execution process, if the cumulative removal number of the target submodule reaches the maximum removal number, the target submodule will no longer be removed; the safety threshold is greater than the voltage balance threshold.

3. The method according to claim 2, characterized in that The target submodule is a submodule in each bridge arm that is in a locked state and has the highest DC voltage.

4. The method according to claim 3, characterized in that The maximum removal quantity of the target submodules is determined based on the peak value of the AC line voltage of the submodules of the hybrid MMC and the rated voltage of the submodules of the hybrid MMC.

5. The method according to claim 4, characterized in that The calculation formula for the maximum number of cutouts of the target submodule is: Wherein, N1 is the maximum number of target submodules to be removed, N is the number of submodules connected in series in each bridge arm of the hybrid MMC, and U peak is the peak value of the submodule AC line voltage of the hybrid MMC, U s is the submodule rated voltage of the hybrid MMC, and round is a rounding function.

6. The method according to claim 2, characterized in that The current maximum voltage deviation of the hybrid MMC is the difference between the maximum value of the DC voltage in each submodule in the locked state and the minimum value of the DC voltage in each submodule in the cut-off state in the hybrid MMC.

7. The method according to claim 2, characterized in that The controlling each full-bridge submodule in each bridge arm of the hybrid MMC to de-half-bridge so that the DC side voltage of the hybrid MMC is within a safety threshold includes: Controlling 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; The above-mentioned half-bridge removal process is repeatedly performed until the DC side voltage of the hybrid MMC is within the safety threshold.

8. The method according to claim 1, characterized in that The hybrid MMC is connected to the AC side grid via a series slow-start resistor.

9. The method according to claim 8, characterized in that After the control corresponding to the full-bridge submodule is converted into a half-bridge, the method further includes: If the effective value of the AC side charging current of the hybrid MMC is less than a preset current threshold, the slow-start resistor is bypassed.

10. A DC step-down hybrid MMC super-capacitor grid-connected static synchronous phase condenser starting system, characterized in that: include: An uncontrolled charging unit, used to control each full-bridge submodule and each half-bridge submodule of the hybrid modular multi-level converter MMC to be in a locked state respectively, so that the hybrid MMC performs uncontrolled rectification charging; A half-bridge unit, used to control each full-bridge sub-module to perform half-bridge conversion when the DC voltage of each full-bridge sub-module of the hybrid MMC reaches the working threshold of the self-energy extraction loop; A half-bridge removal unit, for controlling the full-bridge submodule in each bridge arm of the hybrid MMC to remove the half-bridge if the DC side voltage of the hybrid MMC monitored in real time is greater than a safety threshold, so that the DC side voltage of the hybrid MMC is within the safety threshold; A voltage-equalizing charging unit, used to control the working state of each submodule in the hybrid MMC based on a DC voltage balancing control strategy, so that each submodule in the hybrid MMC reaches a rated voltage; Among them, the safety threshold is determined based on the rated voltage and overload capacity of the overcapacity valve of the super-capacity grid static synchronous condenser; the rated voltage of the overcapacity valve of the super-capacity grid static synchronous condenser is less than the sum of the rated voltages of all sub-modules on a single bridge arm of the hybrid MMC; the working status of each sub-module in the hybrid MMC includes locking and cutting off.

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