MMC pre-charging method based on bridge arm module alternate bypass

Abstract

The invention discloses an MMC pre-charging method based on bridge arm module alternate bypass. The method comprises the following steps: detecting the voltage of a grid-connected point at the AC side of an MMC; detecting a medium-voltage energy storage unit and a low-voltage energy storage unit, and selecting an energy storage unit participating in MMC pre-charging and a corresponding control mode according to a detection result; controlling a DC bus voltage in the DC power distribution system to be raised to a rated value of a capacitor voltage of an MMC bridge arm sub-module, and executing an MMC pre-charging mode; on the premise that the voltage fluctuation amplitude of the direct-current bus does not exceed the allowable range, different sub-modules on the same bridge arm of the MMC are bypassed alternately in sequence until the difference value between the capacitor voltage of the bridge arm sub-module and the rated working voltage of the bridge arm sub-module is within the allowable range; and closing a circuit breaker at the AC side of the MMC, unlocking the MMC, switching the pre-charging mode of the MMC to a normal operation mode, executing constant DC voltage control by the MMC, and recovering the determined control mode of the energy storage unit to a constant power control mode.

Description

Technical Field

[0001] The invention relates to the technical field of direct current power distribution of electric power systems, and in particular to an MMC precharging method based on alternate bypass of bridge arm modules. Background Art

[0002] In the DC power distribution system, new energy sources, AC and DC loads, and energy storage are collected through DC lines and then connected to the AC grid after being inverted by modular multilevel converters (MMCs). This has the advantages of reducing the number of cables used and improving the quality of power. It is one of the important development directions for large-scale new energy consumption. After the DC collection system stops operating due to an external AC fault, it needs to be restarted and restored to the operating state before the fault. Among them, the pre-charging of the MMC is one of the key links in the system restart.

[0003] There are two methods for precharging the capacitor of the MMC bridge arm submodule in the DC power distribution system:

[0004] (1) Self-excited pre-charging: The MMC bridge arm submodule capacitor is pre-charged through the AC power grid. The specific implementation method is: lock the MMC, and put a current limiting resistor on the AC side of the MMC, and then close the MMC AC side circuit breaker to make the MMC enter the uncontrolled rectification mode, and pre-charge the three-phase bridge arm submodule capacitor until the series voltage of the single-phase bridge arm submodule capacitor is close to the AC voltage peak.

[0005] (2) Externally excited pre-charging: The MMC bridge arm sub-module capacitor is pre-charged through an external DC voltage source. The specific implementation method is: a DC voltage source is connected in parallel at both ends of a certain MMC bridge arm sub-module capacitor to pre-charge the MMC sub-module capacitor to its rated working voltage, and then the next bridge arm sub-module capacitor is charged; or a DC voltage source is connected at both ends of each bridge arm sub-module capacitor to pre-charge the three-phase bridge arm sub-module capacitor synchronously.

[0006] In the existing MMC pre-charging method, the self-excitation method performs uncontrolled rectification pre-charging of the MMC through the AC power grid. After the pre-charging is completed, the series voltage of the MMC bridge arm submodule capacitor reaches the peak voltage of the AC power grid, but still does not reach the rated working voltage of the MMC bridge arm submodule capacitor. At this time, if the MMC is unlocked, a large impact current may still appear, causing the MM to lock again, and then the system restart fails. The external excitation method can raise the MM bridge arm submodule capacitor voltage to its rated working voltage due to the external DC voltage source, but it increases the investment cost and maintenance cost. Summary of the invention

[0007] In view of the deficiencies in the prior art, the purpose of the present invention is to provide an MMC pre-charging method based on alternating bypass of the bridge arm module, so as to solve the problem that in the existing DC power distribution system, during the pre-charging process of MMC using a self-excitation method, the MMC is subjected to a large impact current, causing the MMC to lock again and thus causing the system to fail to restart.

[0008] A MMC pre-charging method based on alternate bypass of a bridge arm module, the MMC pre-charging method comprising:

[0009] S1, detect the voltage of the MMC AC side grid connection point. If the detected voltage is greater than the set threshold, the DC power distribution system MMC pre-charging mode is started, otherwise, the voltage of the MMC AC side grid connection point is continuously detected;

[0010] S2, detecting the remaining power of the medium voltage energy storage unit and the low voltage energy storage unit of the DC power distribution system, and selecting the energy storage unit and the corresponding control mode to participate in the MMC pre-charging according to the detection results;

[0011] S3, after determining the MMC pre-charged energy storage unit and control mode, control the DC bus voltage in the DC power distribution system to rise to the rated value of the capacitor voltage of the MMC bridge arm submodule, and then execute the MMC pre-charging mode;

[0012] S4. Under the premise of ensuring that the fluctuation amplitude of the DC bus voltage does not exceed the allowable range, different sub-modules on the same bridge arm of the MMC are bypassed alternately in sequence until the difference between the capacitor voltage of the bridge arm sub-module and its rated working voltage is within the allowable range.

[0013] S5. Close the circuit breaker on the AC side of the MMC, unlock the MMC, switch the DC power distribution system MMC pre-charging mode to the normal operating mode, and let the MMC perform constant DC voltage control. At the same time, restore the control mode of the determined energy storage unit to the constant power control mode.

[0014] Preferably, in step S1, the voltage u of the MMC AC side port is detected. mmc Has it recovered to the set threshold value U? set , if the port voltage u mmc Restored to U set , then execute the pre-charge mode, otherwise continue to detect the port voltage;

[0015] Preferably, the energy storage unit participating in the MMC pre-charging and the corresponding control mode are selected in step S2 in the following manner: the remaining power (SOC mid ) and the remaining capacity of the low-voltage energy storage unit (SOC low ), if SOC mid and SOC low Satisfies the following formula:

[0016]

[0017] In the formula, min and max represent the minimum and maximum values ​​respectively, [SOC mid_min ,SOC mid_max ] represents the power source section of the medium voltage energy storage unit, [SOC low_min ,SOC low_max ] represents the power source section of the low-voltage energy storage unit;

[0018] At this time, the medium voltage energy storage unit performs constant DC voltage control, and takes the rated DC bus voltage as the control target. At the same time, the low voltage energy storage unit is set as the power source. At this time, the MMC pre-charging mode can be executed;

[0019] In the formula, if SOC mid If the value is below the minimum, the MMC precharge mode cannot be executed.

[0020] Preferably, in step S3, the DC bus voltage is raised to the rated value U of the capacitor voltage of the MMC bridge arm submodule by the selected medium voltage energy storage unit. dc_N , at this time, the U dc_N for:

[0021]

[0022] Where N represents the number of upper (lower) bridge arm submodules, u sm_i represents the capacitor voltage of the i-th bridge arm submodule, and the superscript tp and superscript bm represent the upper bridge arm and the lower bridge arm respectively;

[0023] At this time, the medium voltage energy storage unit simultaneously charges the capacitors of the upper and lower bridge arm submodules of the three-phase MMC. After this stage, the capacitor voltages of the bridge arm submodules of the MMC are equal, specifically:

[0024]

[0025] Where U sm Indicates the average voltage of the bridge arm submodule capacitor.

[0026] Preferably, in step S4, the fluctuation range of the DC bus voltage is [0.95pu, 1.05pu].

[0027] Preferably, the MMC bridge arm submodule alternate bypass method comprises the following steps:

[0028] S10, sorting the bridge arm submodules on each phase of the MMC, where the upper bridge arm and the lower bridge arm on each phase each have N submodules, the first submodule of the upper bridge arm is sorted as 1, the last submodule of the upper bridge arm is sorted as N, the first submodule of the lower bridge arm is sorted as N+1, and the last submodule of the lower bridge arm is sorted as 2N;

[0029] S20, selecting the bridge arm submodules on each phase of the MMC, the number of the selected bridge arm submodules is 2m, and the number of the bridge arm submodules m is calculated by rounding up function, and the specific calculation method is:

[0030]

[0031] Where U sm (k-1) is the average voltage of the bridge arm submodule capacitor during the last bridge arm alternate bypass (after the first m and the last m bridge arm submodules are bypassed, which is counted as one time), k is the number of alternate bypasses, U sm (0) = U dc_N / 2N; k = 1, 2, 3, ..., n;

[0032] S30, alternately bypass the bridge arm submodules selected for each phase of the MMC, wherein when alternately bypassing the bridge arm submodules selected for phase A of the MMC, first bypass the first m submodules, and the number of bridge arm submodules turned on in the current bridge arm is 2N-m, and after the DC bus voltage is stable, the capacitor voltage of the A-phase bridge arm submodule is:

[0033]

[0034] In the formula, u sm_1(k) to u sm_m(k) represents the capacitor voltage of the first m bridge arm submodules among the selected 2m bridge arm submodules, u sm_m+1(k) to u sm_2N(k) Represents the capacitor voltage of the remaining 2N-m bridge arm submodules in phase A;

[0035] Subsequently, the first m bridge arm submodules are turned on, and the bridge arm submodules from m+1 to 2m are bypassed. The bridge arm submodules from m+1 to 2m are the remaining m bridge arm submodules that have not been bypassed among the selected 2m, i.e., the last m bridge arm submodules. Therefore, the number of bridge arm submodules turned on in phase A is still 2N-m. After the DC bus voltage stabilizes again, the capacitor voltage of the bridge arm submodule in phase A is:

[0036]

[0037] Repeat the above alternate bypass steps until the capacitor voltage of the A-phase bridge arm submodule meets the following conditions:

[0038] |min[u sm_i (k)]-u sm_N |≤ε

[0039] In the formula, min[u sm_i (k)] is the minimum value of the capacitor voltage of the A-phase bridge arm submodule; u sm_Nis the rated working voltage of the bridge arm submodule capacitor; ε is a positive number greater than 0;

[0040] Wherein, when the bridge arm submodules of the B phase and the C phase of the MMC are alternately bypassed, the alternate bypass method of the bridge arm submodule of the A phase of the MMC described above is referred to.

[0041] The present invention provides an MMC pre-charging method based on bridge arm module alternate bypass, which has the following beneficial effects:

[0042] 1. The MMC pre-charging method for the DC power distribution system based on the bridge arm module alternate bypass provided by the present invention can maximize the use of the power distribution system's own energy storage to pre-charge the MMC bridge arm sub-module, thereby reducing the investment cost and maintenance cost of purchasing additional DC voltage sources.

[0043] 2. Compared with other pre-charging methods, the pre-charging method using alternate bypass of the bridge arm sub-modules can quickly raise the capacitor voltage of all bridge arm sub-modules to near the rated value, which can reduce the impact current caused by MMC unlocking and AC side circuit breaker closing, thereby ensuring that the DC distribution system quickly enters normal operation. BRIEF DESCRIPTION OF THE DRAWINGS

[0044] Figure 1 A structural diagram of a DC power distribution system provided by an embodiment of the present invention;

[0045] Figure 2 A flow chart of an MMC pre-charging method based on alternate bypass of bridge arm modules provided by an embodiment of the present invention;

[0046] Figure 3 A comparison diagram of the capacitor voltage of the bridge arm submodule under different pre-charging methods for phase A of the MMC provided by an embodiment of the present invention;

[0047] Figure 4 A comparison diagram of the voltage of the MMC AC port under different MMC pre-charging methods provided by an embodiment of the present invention;

[0048] Figure 5 A comparison diagram of DC bus voltage under different pre-charging methods of MMC provided in an embodiment of the present invention. DETAILED DESCRIPTION

[0049] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0050] In the description of the present invention, it should be noted that the terms "center", "longitudinal", "lateral", "up", "down", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "top", "bottom", "top surface", "bottom surface", "inside", "outside", "inside", "outside" and the like indicate orientations or positional relationships based on the orientations or positional relationships shown in the accompanying drawings, and are only for the convenience of describing the present invention and simplifying the description, and do not indicate or imply that the referred device or element must have a specific orientation, be constructed and operated in a specific orientation, and therefore cannot be understood as limiting the present invention.

[0051] In the description of the present invention, "several" means one or more, "more" means more than two, "greater than", "less than", "exceed" etc. are understood to exclude the number itself, and "above", "below", "within" etc. are understood to include the number itself. If the terms "first", "second", "third" are described, they are only used for the purpose of description and distinguishing technical features, and cannot be understood as indicating or implying relative importance or implicitly indicating the number of the indicated technical features or implicitly indicating the order of the indicated technical features.

[0052] In the description of the present invention, it should be noted that, unless otherwise clearly specified and limited, the terms "installed", "connected", "connected", and "set" should be understood in a broad sense. For example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be a direct connection, or it can be indirectly connected through an intermediate medium, or it can be the internal connection of two components. For ordinary technicians in this field, the specific meanings of the above terms in the present invention can be understood according to specific circumstances. The following is an explanation of the embodiments of the present invention based on its overall structure.

[0053] See also Figure 1 and Figure 2 , an MMC pre-charging method based on bridge arm module alternate bypass includes:

[0054] S1. Detect the grid-connected point voltage on the MMC AC side. If the detected voltage is greater than the set threshold, start the MMC pre-charging mode of the DC power distribution system. Otherwise, continue to detect the grid-connected point voltage on the MMC AC side. In a specific implementation scheme, the MMC has three phases, each phase has an upper bridge arm and a lower bridge arm. When pre-charging the MMC, first detect the grid-connected point voltage on the MMC AC side, according to the following formula:

[0055]

[0056] In the formula, u φ is the effective value of the phase voltage, where φ is phase A, B or C; U set To set the threshold.

[0057] The voltage of the grid connection point on the AC side of the MMC is tested. The test result is based on the above formula. φ Value greater than or equal to U set threshold, the MMC in the DC system is pre-charged. If the detection result is u φ Value is less than U set Threshold, the MMC in the DC system is not pre-charged, and the voltage of the MMC AC side grid connection point is continued to be detected. As long as u φ Value greater than or equal to U set Threshold, MMC pre-charge can be performed.

[0058] Wherein, the voltage u of the MMC AC side port is detected in step S1. mmc Has it recovered to the set threshold value U? set , if the port voltage u mmc Restored to U set , the pre-charge mode is executed, otherwise the port voltage continues to be detected. The MMC AC side port voltage u mmc Restored to the set threshold U set u represents MMC φ The value has reached U set Threshold.

[0059] S2. Detect the remaining power of the medium voltage energy storage unit and the low voltage energy storage unit of the DC power distribution system, and select the energy storage unit participating in the MMC pre-charging and the corresponding control mode according to the detection result.

[0060] Specifically, the energy storage unit participating in the MMC pre-charging and the corresponding control mode are selected in step S2 in the following manner: the remaining power (SOC mid ) and the remaining capacity of the low-voltage energy storage unit (SOC low ), if SOC mid and SOC low Satisfies the following formula:

[0061]

[0062] In the formula, min and max represent the minimum and maximum values ​​respectively, [SOC mid_min ,SOC mid_max ] represents the power source section of the medium voltage energy storage unit, [SOC low_min ,SOC low_max ] represents the power source section of the low-voltage energy storage unit;

[0063] At this time, the medium voltage energy storage unit performs constant DC voltage control, and takes the rated DC bus voltage as the control target. At the same time, the low voltage energy storage unit is set as the power source. At this time, the MMC pre-charging mode can be executed.

[0064] In the formula, if SOC mid If the value is below the minimum, the MMC precharge mode cannot be executed.

[0065] Among them, SOC mid and SOC low The lower limit SOC mid_min , SOC low_min and upper limit SOC mid_max , SOC low_max It needs to be determined based on the actual situation of the energy storage unit, and is usually taken as 10% and 90% respectively.

[0066] S3, after determining the MMC pre-charged energy storage unit and control mode, control the DC bus voltage in the DC power distribution system to rise to the rated value of the capacitor voltage of the MMC bridge arm submodule, and then execute the MMC pre-charging mode.

[0067] In a specific implementation plan, after the control mode of the energy storage unit is determined, the medium voltage energy storage unit performs constant DC voltage control to raise the DC bus voltage to the rated value ±10kV, and then the MMC switches from the standby mode to the pre-charging mode.

[0068] In a specific implementation scheme, in step S3, the DC bus voltage is raised to the rated value U of the capacitor voltage of the MMC bridge arm submodule by the selected medium voltage energy storage unit. dc_N , at this time, the U dc_N for:

[0069]

[0070] Where N represents the number of upper (lower) bridge arm submodules, u sm_i represents the capacitor voltage of the i-th bridge arm submodule, and the superscript tp and superscript bm represent the upper bridge arm and the lower bridge arm, respectively.

[0071] At this time, the medium voltage energy storage unit charges the capacitors of the upper and lower bridge arm submodules of the three-phase MMC at the same time. Therefore, after this stage, the capacitor voltages of the bridge arm submodules of the MMC are equal, specifically:

[0072]

[0073] Where U sm Indicates the average voltage of the bridge arm submodule capacitor.

[0074] S4. Under the premise of ensuring that the fluctuation amplitude of the DC bus voltage does not exceed the allowable range, different sub-modules on the same bridge arm of the MMC are bypassed alternately in sequence until the difference between the capacitor voltage of the bridge arm sub-module and its rated working voltage is within the allowable range.

[0075] In a specific implementation scheme, the fluctuation range of the DC bus voltage is [0.95pu, 1.05pu], and alternating bypass is performed under the premise of ensuring that the fluctuation amplitude of the DC bus voltage does not exceed the allowable range (±5% of the rated DC bus voltage).

[0076] The MMC bridge arm submodule alternate bypass method comprises the following steps:

[0077] S10. Sort the bridge arm submodules on each phase of the MMC. The upper bridge arm and the lower bridge arm on each phase have N submodules respectively. The first submodule of the upper bridge arm is sorted as 1, the last submodule of the upper bridge arm is sorted as N, the first submodule of the lower bridge arm is sorted as N+1, and the last submodule of the lower bridge arm is sorted as 2N.

[0078] S20, selecting the bridge arm submodules on each phase of the MMC, the number of the selected bridge arm submodules is 2m, and the number of the bridge arm submodules m is calculated by rounding up function, and the specific calculation method is:

[0079]

[0080] Where U sm (k-1) is the average voltage of the bridge arm submodule capacitor during the last bridge arm alternate bypass (after the first m and the last m bridge arm submodules are bypassed, which is counted as one time), k is the number of alternate bypasses, U sm (0) = U dc_N / 2N; k=1,2,3,…,n.

[0081] S30, alternately bypass the bridge arm submodules selected for each phase of the MMC. Taking phase A as an example, when alternately bypassing the bridge arm submodules selected for phase A of the MMC, first bypass the first m submodules. The number of bridge arm submodules that are turned on in the current bridge arm is 2N-m. After the DC bus voltage is stable, the capacitor voltage of the bridge arm submodule of phase A is:

[0082]

[0083] In the formula, u sm_1(k) to u sm_m(k) represents the capacitor voltage of the first m bridge arm submodules among the selected 2m bridge arm submodules, u sm_m+1(k) to U sm_2N(k) It represents the remaining 2N-m bridge arm sub-modules in phase A, which include the remaining m bridge arm sub-modules in the selected 2m.

[0084] Subsequently, the first m bridge arm submodules are turned on, and the bridge arm submodules from m+1 to 2m are bypassed. The bridge arm submodules from m+1 to 2m are the remaining m bridge arm submodules that have not been bypassed among the selected 2m, i.e., the last m bridge arm submodules. Therefore, the number of bridge arm submodules turned on in phase A is still 2N-m. After the DC bus voltage stabilizes again, the capacitor voltage of the bridge arm submodule in phase A is:

[0085]

[0086] Repeat the above alternate bypass steps until the capacitor voltage of the A-phase bridge arm submodule meets the following conditions:

[0087] |min[u sm_i (k)]-u sm_N |≤ε

[0088] In the formula, min[u sm_i (k)] is the minimum value of the capacitor voltage of the A-phase bridge arm submodule; u sm_N is the rated working voltage of the bridge arm submodule capacitor; ε is a positive number greater than 0.

[0089] That is, the difference between the capacitor voltage of the A-phase bridge arm submodule and its rated operating voltage satisfies the above formula.

[0090] Referring to the above-mentioned alternating bypass method, when the bridge arm submodules of the B phase and the C phase of the MMC are alternately bypassed, the determination of m and the difference between the capacitor voltage of the bridge arm submodules of the B phase and the C phase and their rated working voltage also refer to the above formula.

[0091] S5. Close the circuit breaker on the AC side of the MMC, unlock the MMC, switch the DC power distribution system MMC pre-charging mode to the normal operating mode, and let the MMC perform constant DC voltage control. At the same time, restore the control mode of the determined energy storage unit to the constant power control mode.

[0092] In a preferred implementation scheme, the MMC AC side circuit breaker is first closed, and then the MMC is unlocked (i.e., the conduction mode of the bridge arm submodule in the alternating bypass mode is switched to the nearest level approximation modulation mode), and the MMC executes the constant DC voltage control mode; at the same time, the medium voltage energy storage unit is switched to the constant power control mode.

[0093] According to the method provided by the present invention, by utilizing the energy storage unit of the DC power distribution system, the investment cost and maintenance cost are reduced; at the same time, by utilizing the flexible control capability of the MMC bridge arm submodule, the bridge arm submodule capacitor voltage is raised to its rated working voltage, the impact current of the MMC unlocking process is reduced, and the DC power distribution system is guaranteed to start safely and reliably. Among them, the order and number of alternate bypasses of the bridge arm submodules can be adjusted according to actual conditions. Therefore, different bridge arm submodules have different alternate bypass orders and different numbers of alternate bypass bridge arm submodules each time, which can be carried out according to actual conditions.

[0094] Figure 3 A comparison diagram of the capacitance voltage of the bridge arm submodule under different pre-charging methods for phase A of the MMC provided by an embodiment of the present invention, Figure 3 (a) shows the capacitor voltage of the bridge arm submodule of the A phase of the MMC for the step-by-step bypass pre-charging. Figure 3 (b) in the figure shows the capacitor voltage of the bridge arm submodule of the A phase alternate bypass pre-charging of the MMC.

[0095] pass Figure 3 It can be seen that the alternating bypass pre-charging is faster than the gradual bypass pre-charging to a certain extent. For large-scale, multi-module MMC systems, it can effectively shorten the system startup time, while the gradual bypass pre-charging speed is relatively slow. Alternating bypass pre-charging can better achieve balanced charging between capacitors. Different bypass branches can charge different capacitors or capacitor groups, and can flexibly adjust the charging path and charging current, so that the voltage rise of each capacitor is more balanced.

[0096] Figure 4 A comparison diagram of the voltage of the MMC AC port under different MMC pre-charging methods provided by the embodiment of the present invention, Figure 4 (a) shows the MMC AC port voltage under MMC step-by-step bypass pre-charging. Figure 4 (b) in the middle shows the MMC AC port voltage under MMC alternate bypass pre-charging. Figure 5 A comparison diagram of DC bus voltage under different pre-charging methods of MMC provided in an embodiment of the present invention.

[0097] pass Figure 4 and Figure 5, since alternating bypass pre-charging has advantages in balanced charging effect, it can better handle the voltage balance problem between multiple capacitors. During the process of alternating operation of multiple bypass branches, certain voltage fluctuations will be generated at the switching moment. Because the input and removal of different bypass branches will cause the current path in the circuit to change, which may affect the voltage stability of the DC bus. By adopting the control strategy provided by the present invention, this voltage fluctuation is controlled within a certain range, so that the voltage stability of the DC bus is more stable and less volatile than that of step-by-step bypass pre-charging. Alternating bypass pre-charging can reduce the impact current caused by the unlocking of the MMC and the closing of the AC side circuit breaker, thereby ensuring that the DC power distribution system quickly enters a normal operating state.

[0098] In summary, the present invention provides an MMC pre-charging method based on alternate bypass of bridge arm modules, which utilizes the cooperation between medium voltage and low voltage energy storage of the DC power distribution system, and provides a solution for the cooperation between medium voltage energy storage and low voltage energy storage to participate in MMC pre-charging. By controlling the bypass quantity and sequence of the bridge arm sub-modules, the bridge arm sub-modules are alternately bypassed, and finally the capacitor voltage of all bridge arm sub-modules reaches its rated working voltage. This can solve the problem that in the existing DC power distribution system, during the process of MMC pre-charging using a self-excitation method, the MMC is subjected to a large impact current, causing the MMC to be locked again and thus causing the system to fail to restart.

[0099] The above is only a specific implementation of the present application. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principles of the present application. These improvements and modifications should also be regarded as the scope of protection of the present application.

Claims

1. A MMC pre-charging method based on alternate bypass of bridge arm modules, characterized in that: The MMC pre-charging method comprises: S1, detect the voltage of the MMC AC side grid connection point. If the detected voltage is greater than the set threshold, the DC power distribution system MMC pre-charging mode is started, otherwise, the voltage of the MMC AC side grid connection point is continuously detected; S2, detecting the remaining power of the medium voltage energy storage unit and the low voltage energy storage unit of the DC power distribution system, and selecting the energy storage unit and the corresponding control mode to participate in the MMC pre-charging according to the detection results; S3, after determining the MMC pre-charged energy storage unit and control mode, control the DC bus voltage in the DC power distribution system to rise to the rated value of the capacitor voltage of the MMC bridge arm submodule, and then execute the MMC pre-charging mode; S4. Under the premise of ensuring that the fluctuation amplitude of the DC bus voltage does not exceed the allowable range, the different submodules on the same bridge arm of the MMC are bypassed alternately in sequence until the difference between the capacitor voltage of the bridge arm submodule and its rated working voltage is within the allowable range; S5, close the circuit breaker on the AC side of the MMC, unlock the MMC, switch the DC distribution system MMC pre-charging mode to the normal operating mode, and let the MMC perform constant DC voltage control. At the same time, restore the control mode of the determined energy storage unit to the constant power control mode.

2. A MMC pre-charging method based on alternate bypass of bridge arm modules according to claim 1, characterized in that: In step S1, the voltage u of the MMC AC side port is detected mmc Has it recovered to the set threshold value U? set , if the port voltage u mmc Restored to U set , the pre-charge mode is executed, otherwise the port voltage continues to be detected.

3. A MMC pre-charging method based on alternate bypass of bridge arm modules according to claim 1, characterized in that: The energy storage unit and the corresponding control mode for participating in the MMC pre-charging in step S2 are selected in the following manner: detecting the remaining power SOC of the medium voltage energy storage unit mid And the remaining power SOC of the low-voltage energy storage unit low : In the formula, min and max represent the minimum and maximum values ​​respectively, [SOC mid_min ,SOC mid_max ] represents the power source section of the medium voltage energy storage unit, [SOC low_min ,SOC low_max ] represents the power source section of the low-voltage energy storage unit; If SOC mid and SOC low If the above formula is satisfied, the medium voltage energy storage unit will perform constant DC voltage control, and the rated DC bus voltage will be used as the control target. At the same time, the low voltage energy storage unit will be set as the power source. At this time, the MMC pre-charging mode can be executed. In the formula, if SOC mid If the value is below the minimum, the MMC precharge mode cannot be executed.

4. A MMC pre-charging method based on alternate bypass of bridge arm modules according to claim 1, characterized in that: In step S3, the DC bus voltage is raised to the rated value U of the capacitor voltage of the MMC bridge arm submodule by the selected medium voltage energy storage unit. dc_N , the U dc_N for: Where N represents the number of upper (lower) bridge arm submodules, u sm_i represents the capacitor voltage of the i-th bridge arm submodule, and the superscript tp and superscript bm represent the upper bridge arm and the lower bridge arm respectively; The medium voltage energy storage unit charges the capacitors of the upper and lower bridge arm submodules of the three-phase MMC at the same time. After this stage, the capacitor voltages of the bridge arm submodules of the MMC are equal, specifically: Where U sm Indicates the average voltage of the bridge arm submodule capacitor.

5. A MMC pre-charging method based on alternate bypass of bridge arm modules according to claim 1, characterized in that: The fluctuation range of the DC bus voltage in step S4 is [0.95pu, 1.05pu].

6. A MMC pre-charging method based on alternate bypass of bridge arm modules according to claim 5, characterized in that: The MMC bridge arm submodule alternate bypass method comprises the following steps: S10, sorting the bridge arm submodules on each phase of the MMC, the upper bridge arm and the lower bridge arm on each phase of the MMC each have N submodules, the first submodule of the upper bridge arm is sorted as 1, the last submodule of the upper bridge arm is sorted as N, the first submodule of the lower bridge arm is sorted as N+1, and the last submodule of the lower bridge arm is sorted as 2N; S20, selecting the bridge arm submodules on each phase of the MMC, the number of the selected bridge arm submodules is 2m, and the number of the bridge arm submodules m is calculated by rounding up function, and the specific calculation method is: Where U sm (k-1) is the average voltage of the bridge arm submodule capacitor during the last bridge arm alternate bypass (after the first m and the last m bridge arm submodules are bypassed, which is counted as one time), k is the number of alternate bypasses, U sm (0) = U dc_N / 2N; k = 1, 2, 3, ..., n; S30, alternately bypass the bridge arm submodules selected for each phase of the MMC, wherein when alternately bypassing the bridge arm submodules selected for phase A of the MMC, first bypass the first m submodules, and the number of bridge arm submodules turned on in the current bridge arm is 2N-m, and after the DC bus voltage is stable, the capacitor voltage of the A-phase bridge arm submodule is: In the formula, u sm_1(k) to u sm_m(k) represents the capacitor voltage of the first m bridge arm submodules among the selected 2m bridge arm submodules, u sm_m+1(k) to u sm_2N(k) Represents the capacitor voltage of the remaining 2N-m bridge arm submodules in phase A; Subsequently, the first m bridge arm submodules are turned on, and the bridge arm submodules from m+1 to 2m are bypassed. The bridge arm submodules from m+1 to 2m are the remaining m bridge arm submodules that have not been bypassed among the selected 2m. The number of bridge arm submodules turned on in phase A is 2N-m. After the DC bus voltage stabilizes again, the capacitor voltage of the bridge arm submodule in phase A is: Repeat the above alternate bypass steps until the capacitor voltage of the A-phase bridge arm submodule meets the following conditions: |min[u sm_i (k)]-in sm_N |≤ε In the formula, min[u sm_i (k)] is the minimum value of the capacitor voltage of the A-phase bridge arm submodule; u sm_N is the rated working voltage of the bridge arm submodule capacitor; ε is a positive number greater than 0.

Images