A method and system for starting an MMC in a hybrid DC power transmission system

Abstract

The application relates to a starting method and system of an MMC in a hybrid DC power transmission system, which comprises the following steps: uncontrolled charging of the MMC until the capacitor voltage of a half-bridge sub-module and the capacitor voltage of a full-bridge sub-module in the MMC reach first preset voltages and second preset voltages respectively; controlled charging of the MMC until the capacitor voltage of the half-bridge sub-module and the capacitor voltage of the full-bridge sub-module in the MMC reach a starting rated voltage of the MMC; and the technical scheme provided by the application realizes that the capacitor voltages of the full-bridge sub-module and the half-bridge sub-module in the MMC tend to be consistent through the voltage equalization control charging of the MMC after the uncontrolled charging, then the half-bridging of the full-bridge sub-module and the overall rotation conduction charging are sequentially performed on the MMC, so that the capacitor voltages of the sub-modules in the MMC simultaneously reach the rated values, and the failure rate of the MMC starting is reduced.

Description

Technical Field

[0001] The present invention relates to the technical field of flexible direct current (HVDC) transmission, and in particular to a method and system for starting an MMC in a hybrid HVDC transmission system. Background Art

[0002] Modular multilevel converter high voltage direct current (HVDC) has become the development trend of future voltage source converter high voltage direct current (VSC-HVDC) due to its unique technical advantages.

[0003] Currently, most modular multilevel converters (MMCs) in engineering projects employ a half-bridge submodule structure. However, these MMCs cannot handle DC overhead line faults through the converter's own operation. This is because even when the insulated-gate bipolar transistors (IGBTs) in the half-bridge submodule topology are turned off, the AC system still feeds current to the fault point through the IGBT's anti-parallel diodes, resulting in a three-phase short circuit. In the absence of mature high-voltage, high-capacity DC circuit breaker technology, interrupting the DC line fault current relies on the converter locking and simultaneously tripping the AC-side circuit breaker. This results in a long system restart recovery time, typically in seconds, which is detrimental to the transient stability of the AC / DC transmission system.

[0004] To address the shortcomings of the half-bridge MMC, some researchers have proposed a full-bridge MMC with DC fault-clearing capabilities. This MMC-based HVDC system can quickly shut down or output negative voltage to cut off the DC fault current after a DC line fault occurs, without tripping the AC circuit breaker. After the fault is cleared, the system can quickly resume operation. Therefore, the full-bridge MMC is more suitable for hybrid DC transmission systems based on long-distance overhead lines. However, the number of power switching devices used in a full-bridge module is twice that of a half-bridge module, significantly increasing costs and losses.

[0005] For this reason, some scholars have proposed using a mixed MMC of half-bridge sub-modules and full-bridge sub-modules, so that the MMC can reduce costs and losses during operation while also having the ability to clear DC faults.

[0006] However, due to the difference in charging rates between the full-bridge sub-module and the half-bridge sub-module in the MMC during the uncontrolled charging stage, the capacitor voltage of the full-bridge sub-module is inconsistent with the capacitor voltage of the half-bridge sub-module after the uncontrolled charging is completed; at this time, if the MMC is controlled to charge again, the full-bridge sub-module capacitor may reach or even exceed the rated voltage while the half-bridge module capacitor has not reached the rated voltage, and the startup cannot be completed. Summary of the Invention

[0007] In response to the deficiencies of the prior art, the present invention aims to propose a method for starting an MMC in a hybrid direct current transmission system. This method performs voltage-balanced charging on the MMC after uncontrolled charging to achieve consistency in the capacitor voltages of the full-bridge submodule and the half-bridge submodule in the MMC. The full-bridge submodules of the MMC are then converted to half-bridges and the entire MMC is rotated and charged, so that the capacitor voltages of the submodules in the MMC simultaneously reach the rated values.

[0008] The purpose of the present invention is achieved by adopting the following technical solutions:

[0009] The present invention provides a method for starting an MMC in a hybrid direct current (DC) power transmission system. The hybrid DC power transmission system comprises an AC system equivalent power supply, an AC circuit breaker, a starting resistor, and an MMC connected in sequence. A starting resistor bypass switch is connected in parallel across the starting resistor. The MMC comprises a half-bridge submodule and a full-bridge submodule. The improvement is that the method comprises:

[0010] The MMC is charged uncontrolled until the capacitor voltage of the half-bridge submodule and the capacitor voltage of the full-bridge submodule in the MMC reach a first preset voltage and a second preset voltage respectively;

[0011] Controllably charging the MMC until the capacitor voltage of the half-bridge submodule and the capacitor voltage of the full-bridge submodule in the MMC both reach the startup rated voltage of the MMC;

[0012] Among them, the initial state of the AC circuit breaker is disconnected, the initial state of the starting resistor bypass switch is disconnected, the initial state of the half-bridge submodule in the MMC is locked, and the initial state of the full-bridge submodule in the MMC is locked.

[0013] Preferably, the uncontrolled charging of the MMC until the capacitor voltage of the half-bridge submodule and the capacitor voltage of the full-bridge submodule in the MMC reach a first preset voltage and a second preset voltage respectively includes:

[0014] Controlling the working state of the AC circuit breaker to be closed, the working state of the starting resistor bypass switch to be open, the working state of the half-bridge submodule in the MMC to be locked, and the working state of the full-bridge submodule in the MMC to be locked;

[0015] The operation ends when the capacitor voltage of the half-bridge submodule in the MMC is charged to the first preset voltage and the capacitor voltage of the full-bridge submodule in the MMC is charged to the second preset voltage.

[0016] Preferably, the first preset voltage U is determined as follows: F ;

[0017]

[0018] The second preset voltage U is determined as follows: H ;

[0019]

[0020] In the above formula, U M is the peak value of the equivalent power line voltage of the AC system; N F N is the number of full-bridge submodules in the MMC; H is the number of half-bridge submodules in the MMC.

[0021] Preferably, the controllably charging the MMC until the capacitor voltage of the half-bridge submodule and the capacitor voltage of the full-bridge submodule in the MMC reach the startup rated voltage of the MMC includes:

[0022] Performing voltage-balanced charging on the MMC until the capacitor voltage of the half-bridge submodule and the capacitor voltage of the full-bridge submodule in the MMC both reach a third preset voltage;

[0023] Converting the full-bridge submodule of the MMC into a half-bridge, until all power switch tubes T4 of the full-bridge submodule in the MMC are turned on;

[0024] The MMC is turned on and charged as a whole in a rotating manner until the capacitor voltage of the half-bridge submodule and the capacitor voltage of the full-bridge submodule in the MMC both reach the startup rated voltage of the MMC;

[0025] One end of the power switch tube T4 of the full-bridge sub-module in the MMC is connected to the negative voltage electrode of the full-bridge sub-module, and the other end is connected to the negative electrode of the capacitor in the full-bridge sub-module.

[0026] Furthermore, the voltage-balanced charging of the MMC is performed until the capacitor voltage of the half-bridge submodule and the capacitor voltage of the full-bridge submodule in the MMC reach a third preset voltage, including:

[0027] Step 1: Initialize the MMC to perform voltage-balanced control charging at time t=0;

[0028] Step 2: Control the AC circuit breaker to be in a closed state, the starting resistor bypass switch to be in an open state, and the half-bridge submodule in the MMC to be in a locked state;

[0029] Step 3: Select N1 full-bridge sub-modules from all full-bridge sub-modules of the MMC, and control the N1 full-bridge sub-modules to be bypassed, and the remaining full-bridge sub-modules to be locked;

[0030] Step 4: Determine whether the capacitor voltage of the half-bridge submodule and the capacitor voltage of the full-bridge submodule in the MMC have both reached a third preset voltage. If so, end the operation; otherwise, set t=t+1 and return to step 3.

[0031] Furthermore, the step 3 includes:

[0032] Determine the number of bypassed full-bridge submodules N1 according to the following formula:

[0033]

[0034] Where U M is the peak value of the equivalent power line voltage of the AC system; N F N is the number of full-bridge submodules in the MMC; H is the number of half-bridge submodules in the MMC; U is the third preset voltage;

[0035] The full-bridge submodules in the MMC are arranged in descending order according to the capacitance and voltage of the full-bridge submodules in the MMC, and the first N1 full-bridge submodules are selected in the sequence.

[0036] Furthermore, converting the full-bridge submodule of the MMC into a half-bridge, until all power switch tubes T4 of the full-bridge submodule in the MMC are turned on, includes:

[0037] Controlling the working state of the AC circuit breaker to be closed, the working state of the starting resistor bypass switch to be closed, the working state of the half-bridge submodule in the MMC to be locked, and the working state of the full-bridge submodule in the MMC to be locked;

[0038] The operation ends when all the power switch tubes T4 of the full-bridge sub-modules in the MMC are turned on.

[0039] Furthermore, the entire MMC is alternately turned on and charged until the capacitor voltage of the half-bridge submodule and the capacitor voltage of the full-bridge submodule in the MMC reach the startup rated voltage of the MMC, including:

[0040] Step 4: Initialize the MMC to perform overall rotation conduction charging at time m=0;

[0041] Step 5: Control the AC circuit breaker to be in a closed state and the starting resistor bypass switch to be in a closed state;

[0042] Step 6: Select N2 submodules from all submodules of the MMC, and control the N2 submodules to be bypassed, and the remaining submodules to be locked;

[0043] Step 7: Determine whether the capacitor voltage of the half-bridge submodule and the capacitor voltage of the full-bridge submodule in the MMC have both reached the startup rated voltage of the MMC. If so, end the operation; otherwise, set m=m+1 and return to step 6.

[0044] Furthermore, the step 6 includes:

[0045] Determine the number of bypass submodules N2 as follows:

[0046]

[0047] Where U M is the peak value of the equivalent power line voltage of the AC system; N F N is the number of full-bridge submodules in the MMC; H is the number of half-bridge submodules in the MMC, U C is the starting rated voltage value of the modular multilevel converter;

[0048] The MMC neutron modules are arranged in descending order according to the capacitance and voltage of the MMC neutron modules, and the first N2 submodules are selected in the sequence; wherein the MMC neutron modules include the full-bridge submodule and the half-bridge submodule of the MMC.

[0049] The present invention provides a starting system for an MMC in a hybrid direct current (DC) power transmission system. The hybrid DC power transmission system comprises an AC system equivalent power supply, an AC circuit breaker, a starting resistor, and an MMC connected in sequence. A starting resistor bypass switch is connected in parallel across the starting resistor. The MMC comprises a half-bridge submodule and a full-bridge submodule. The improvement is that the system comprises:

[0050] an uncontrolled charging module, configured to perform uncontrolled charging on the MMC until the capacitor voltage of the half-bridge submodule and the capacitor voltage of the full-bridge submodule in the MMC reach a first preset voltage and a second preset voltage respectively;

[0051] A controllable charging module, used to controllably charge the MMC until the capacitor voltage of the half-bridge submodule and the capacitor voltage of the full-bridge submodule in the MMC both reach the startup rated voltage of the MMC;

[0052] Among them, the initial state of the AC circuit breaker is disconnected, the initial state of the starting resistor bypass switch is disconnected, the initial state of the half-bridge submodule in the MMC is locked, and the initial state of the full-bridge submodule in the MMC is locked.

[0053] Preferably, the uncontrolled charging module is used to:

[0054] Controlling the working state of the AC circuit breaker to be closed, the working state of the starting resistor bypass switch to be open, the working state of the half-bridge submodule in the MMC to be locked, and the working state of the full-bridge submodule in the MMC to be locked;

[0055] The operation ends when the capacitor voltage of the half-bridge submodule in the MMC is charged to the first preset voltage and the capacitor voltage of the full-bridge submodule in the MMC is charged to the second preset voltage.

[0056] Preferably, the first preset voltage U is determined as follows: F ;

[0057]

[0058] The second preset voltage U is determined as follows: H ;

[0059]

[0060] In the above formula, U M is the peak value of the equivalent power line voltage of the AC system; N F N is the number of full-bridge submodules in the MMC; H is the number of half-bridge submodules in the MMC.

[0061] Preferably, the controllable charging module includes:

[0062] A voltage-balanced control charging unit, configured to perform voltage-balanced control charging on the MMC until the capacitor voltage of the half-bridge submodule and the capacitor voltage of the full-bridge submodule in the MMC both reach a third preset voltage;

[0063] A full-bridge submodule half-bridge conversion unit, configured to convert the full-bridge submodule into a half-bridge on the MMC until all power switches T4 of the full-bridge submodules in the MMC are turned on;

[0064] An overall rotation conduction charging unit, used to perform overall rotation conduction charging on the MMC until the capacitor voltage of the half-bridge submodule and the capacitor voltage of the full-bridge submodule in the MMC reach the starting rated voltage of the MMC;

[0065] One end of the power switch tube T4 of the full-bridge sub-module in the MMC is connected to the negative voltage electrode of the full-bridge sub-module, and the other end is connected to the negative electrode of the capacitor in the full-bridge sub-module.

[0066] Furthermore, the voltage-balanced charging unit is used to:

[0067] Step 1: Initialize the MMC to perform voltage-balanced control charging at time t=0;

[0068] Step 2: Control the AC circuit breaker to be in a closed state, the starting resistor bypass switch to be in an open state, and the half-bridge submodule in the MMC to be in a locked state;

[0069] Step 3: Select N1 full-bridge sub-modules from all full-bridge sub-modules of the MMC, and control the N1 full-bridge sub-modules to be bypassed, and the remaining full-bridge sub-modules to be locked;

[0070] Step 4: Determine whether the capacitor voltage of the half-bridge submodule and the capacitor voltage of the full-bridge submodule in the MMC have both reached a third preset voltage. If so, end the operation; otherwise, set t=t+1 and return to step 3.

[0071] Furthermore, the step 3 includes:

[0072] Determine the number of bypassed full-bridge submodules N1 according to the following formula:

[0073]

[0074] Where U M is the peak value of the equivalent power line voltage of the AC system; N F N is the number of full-bridge submodules in the MMC; H is the number of half-bridge submodules in the MMC; U is the third preset voltage;

[0075] The full-bridge submodules in the MMC are arranged in descending order according to the capacitance and voltage of the full-bridge submodules in the MMC, and the first N1 full-bridge submodules are selected in the sequence.

[0076] Furthermore, the half-bridge unit of the full-bridge submodule is used to:

[0077] Controlling the working state of the AC circuit breaker to be closed, the working state of the starting resistor bypass switch to be closed, the working state of the half-bridge submodule in the MMC to be locked, and the working state of the full-bridge submodule in the MMC to be locked;

[0078] The operation ends when all the power switch tubes T4 of the full-bridge sub-modules in the MMC are turned on.

[0079] Furthermore, the overall rotation conduction charging unit is used to:

[0080] Step 4: Initialize the MMC to perform overall rotation conduction charging at time m=0;

[0081] Step 5: Control the AC circuit breaker to be in a closed state and the starting resistor bypass switch to be in a closed state;

[0082] Step 6: Select N2 submodules from all submodules of the MMC, and control the N2 submodules to be bypassed, and the remaining submodules to be locked;

[0083] Step 7: Determine whether the capacitor voltage of the half-bridge submodule and the capacitor voltage of the full-bridge submodule in the MMC have both reached the startup rated voltage of the MMC. If so, end the operation; otherwise, set m=m+1 and return to step 6.

[0084] Furthermore, the step 6 includes:

[0085] Determine the number of bypass submodules N2 as follows:

[0086]

[0087] Where U M is the peak value of the equivalent power line voltage of the AC system; N F N is the number of full-bridge submodules in the MMC; H is the number of half-bridge submodules in the MMC, U C is the starting rated voltage value of the modular multilevel converter;

[0088] The MMC neutron modules are arranged in descending order according to the capacitance and voltage of the MMC neutron modules, and the first N2 submodules are selected in the sequence; wherein the MMC neutron modules include the full-bridge submodule and the half-bridge submodule of the MMC.

[0089] Compared with the closest prior art, the present invention has the following beneficial effects:

[0090] The technical solution provided by the present invention performs uncontrolled charging on the MMC until the capacitor voltage of the half-bridge submodule and the capacitor voltage of the full-bridge submodule in the MMC reach a first preset voltage and a second preset voltage respectively; performs controllable charging on the MMC until the capacitor voltage of the half-bridge submodule and the capacitor voltage of the full-bridge submodule in the MMC both reach the startup rated voltage of the MMC; adds a voltage-equalizing control charging stage in the controllable charging stage to sequentially achieve consistency of the capacitor voltages of the full-bridge submodule and the half-bridge submodule in the MMC, and then sequentially performs half-bridge conversion and overall rotation conduction charging on the MMC, so that the capacitor voltages of the submodules in the MMC simultaneously reach the rated values, thereby reducing the error rate of MMC startup. BRIEF DESCRIPTION OF THE DRAWINGS

[0091] Figure 1 It is a flow chart of a method for starting an MMC in a hybrid DC transmission system;

[0092] Figure 2 It is a network topology diagram of the hybrid DC transmission system;

[0093] Figure 3 It is the MMC network topology diagram;

[0094] Figure 4 The figure is a flowchart of the startup system of MMC in a hybrid DC transmission system. DETAILED DESCRIPTION

[0095] The specific embodiments of the present invention will be further described in detail below with reference to the accompanying drawings.

[0096] To make the objectives, technical solutions, and advantages of the embodiments of the present invention more clear, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the accompanying drawings of 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 making creative efforts shall fall within the scope of protection of the present invention.

[0097] The present invention provides a method for starting an MMC in a hybrid DC power transmission system. The hybrid DC power transmission system is composed of an AC system equivalent power supply, an AC circuit breaker, a starting resistor, and an MMC connected in sequence. A starting resistor bypass switch is connected in parallel at both ends of the starting resistor. The MMC is composed of a half-bridge submodule and a full-bridge submodule. Figure 1 As shown, the method includes:

[0098] Step 101: Uncontrolled charging is performed on the MMC until the capacitor voltage of the half-bridge submodule and the capacitor voltage of the full-bridge submodule in the MMC reach a first preset voltage and a second preset voltage respectively;

[0099] In a preferred embodiment of the present invention, the number of half-bridge sub-modules and full-bridge sub-modules on each bridge arm of the MMC is the same.

[0100] Step 102: Controllably charge the MMC until the capacitor voltages of the half-bridge submodule and the full-bridge submodule in the MMC both reach the startup rated voltage of the MMC.

[0101] In the optimal embodiment of the present invention, startup is the basis for the normal operation of the flexible DC transmission system. The essence of startup is that the active AC system performs uncontrolled and controllable charging on the MMC sub-module capacitor through the starting resistor, ultimately making the sub-module capacitor voltage reach the rated value.

[0102] Among them, the initial state of the AC circuit breaker is disconnected, the initial state of the starting resistor bypass switch is disconnected, the initial state of the half-bridge submodule in the MMC is locked, and the initial state of the full-bridge submodule in the MMC is locked.

[0103] In the preferred embodiment of the present invention, the network topology of the hybrid DC transmission system is as follows: Figure 2 As shown, where U S is the AC system equivalent power supply, AC_BRK is the AC circuit breaker, R is the starting resistor, R_BRK is the starting resistor bypass switch, U DC is the DC voltage;

[0104] like Figure 3 As shown, the half-bridge submodule in the MMC is composed of IGBT power device T1, IGBT power device T2 and capacitors;

[0105] The full-bridge sub-module in the MMC consists of IGBT power devices T1, IGBT power devices T2, IGBT power devices T3, IGBT power devices T4 and capacitors. In the figure, point 1 is the positive voltage pole of the full-bridge sub-module; point 2 is the negative voltage pole of the full-bridge sub-module.

[0106] Among them, one end of the power switch tube T1 of the full-bridge sub-module in the MMC is connected to the positive voltage electrode of the full-bridge sub-module, and the other end is connected to the positive electrode of the capacitor in the full-bridge sub-module;

[0107] One end of the power switch tube T2 of the full-bridge sub-module in the MMC is connected to the negative voltage electrode of the full-bridge sub-module, and the other end is connected to the positive electrode of the capacitor in the full-bridge sub-module;

[0108] One end of the power switch tube T3 of the full-bridge sub-module in the MMC is connected to the positive voltage electrode of the full-bridge sub-module, and the other end is connected to the negative electrode of the capacitor in the full-bridge sub-module;

[0109] One end of the power switch tube T4 of the full-bridge sub-module in the MMC is connected to the negative voltage electrode of the full-bridge sub-module, and the other end is connected to the negative electrode of the capacitor in the full-bridge sub-module.

[0110] Specifically, step 101 includes:

[0111] Controlling the working state of the AC circuit breaker to be closed, the working state of the starting resistor bypass switch to be open, the working state of the half-bridge submodule in the MMC to be locked, and the working state of the full-bridge submodule in the MMC to be locked;

[0112] The operation ends when the capacitor voltage of the half-bridge submodule in the MMC is charged to the first preset voltage and the capacitor voltage of the full-bridge submodule in the MMC is charged to the second preset voltage.

[0113] Specifically, the first preset voltage U is determined as follows: F ;

[0114]

[0115] The second preset voltage U is determined as follows: H ;

[0116]

[0117] In the above formula, U M is the peak value of the equivalent power line voltage of the AC system; N F N is the number of full-bridge submodules in the MMC; H is the number of half-bridge submodules in the MMC.

[0118] In the most preferred embodiment of the present invention, Figure 3 As shown in FIG, for the half-bridge submodule in the MMC, charging can be performed only when the charging current direction is positive, and for the full-bridge submodule in the MMC, charging can be performed regardless of whether the charging current direction is positive or negative; therefore, after the uncontrolled charging of the MMC is completed, the capacitor voltage of the full-bridge submodule in the MMC is twice the capacitor voltage of the half-bridge submodule in the MMC, and the sum of the capacitor voltage of the full-bridge submodule in the MMC and the capacitor voltage of the half-bridge submodule in the MMC is the peak value of the equivalent power line voltage of the AC system U M ,Right now:

[0119] U F ×2N F +U H ×N H =U M

[0120] U F =2U H

[0121] The solution is:

[0122]

[0123]

[0124] Furthermore, the step 102 includes:

[0125] Step a. charging the MMC under voltage control until the capacitor voltage of the half-bridge submodule and the capacitor voltage of the full-bridge submodule in the MMC reach a third preset voltage;

[0126] Step b. converting the full-bridge submodule of the MMC into a half-bridge submodule until all power switches T4 of the full-bridge submodule of the MMC are turned on;

[0127] Step c. performing a rotational conduction charging on the MMC as a whole until the capacitor voltage of the half-bridge submodule and the capacitor voltage of the full-bridge submodule in the MMC reach the starting rated voltage of the MMC;

[0128] One end of the power switch tube T4 of the full-bridge sub-module in the MMC is connected to the negative voltage electrode of the full-bridge sub-module, and the other end is connected to the negative electrode of the capacitor in the full-bridge sub-module.

[0129] Specifically, the step a includes:

[0130] Step 1: Initialize the MMC to perform voltage-balanced control charging at time t=0;

[0131] Step 2: Control the AC circuit breaker to be in a closed state, the starting resistor bypass switch to be in an open state, and the half-bridge submodule in the MMC to be in a locked state;

[0132] Step 3: Select N1 full-bridge sub-modules from all full-bridge sub-modules of the MMC, and control the N1 full-bridge sub-modules to be bypassed, and the remaining full-bridge sub-modules to be locked;

[0133] Step 4: Determine whether the capacitor voltage of the half-bridge submodule and the capacitor voltage of the full-bridge submodule in the MMC have both reached a third preset voltage. If so, end the operation; otherwise, set t=t+1 and return to step 3.

[0134] Furthermore, the step 3 includes:

[0135] Determine the number of bypassed full-bridge submodules N1 according to the following formula:

[0136]

[0137] Where U M is the peak value of the equivalent power line voltage of the AC system; N F N is the number of full-bridge submodules in the MMC; H is the number of half-bridge submodules in the MMC; U is the third preset voltage;

[0138] In the preferred embodiment of the present invention, the capacitor voltage of the full-bridge submodule in the MMC is set to be equal to the capacitor voltage of the full-bridge submodule in the MMC. Assuming that the third preset voltage is U, U must satisfy:

[0139] U F <U<U C

[0140] In the above formula, U C is the starting rated voltage of the MMC.

[0141] Then we have:

[0142] U×2(N F -N1)+U×N H =U M

[0143] The solution is:

[0144]

[0145] Arrange the full-bridge submodules in the MMC in descending order according to the capacitance and voltage of the full-bridge submodules in the MMC, and select the first N1 full-bridge submodules in the sequence;

[0146] In the preferred embodiment of the present invention, if there are X bridge arms, then select The full-bridge submodules are bypassed.

[0147] Specifically, the step b includes:

[0148] Controlling the working state of the AC circuit breaker to be closed, the working state of the starting resistor bypass switch to be closed, the working state of the half-bridge submodule in the MMC to be locked, and the working state of the full-bridge submodule in the MMC to be locked;

[0149] The operation ends when all the power switch tubes T4 of the full-bridge sub-modules in the MMC are turned on.

[0150] In the preferred embodiment of the present invention, when the capacitor voltage of the half-bridge submodule in the MMC and the capacitor voltage of the full-bridge submodule in the MMC simultaneously reach U (the third preset voltage), the N1 bypassed full-bridge submodules are first switched back to the locked state, and then the power switch tube T4 of the full-bridge submodule in the MMC is turned on (e.g. Figure 3 As shown, the power switch tube T4 of the full-bridge sub-module in the MMC is the IGBT power switch device with the lower corner in the full-bridge sub-module). At this time, the half-bridge sub-module in the MMC has exactly the same characteristics as the full-bridge sub-module in the MMC, and the half-bridge sub-module in the MMC is realized.

[0151] Specifically, the step c includes:

[0152] Step 4: Initialize the MMC to perform overall rotation conduction charging at time m=0;

[0153] Step 5: Control the AC circuit breaker to be in a closed state and the starting resistor bypass switch to be in a closed state;

[0154] Step 6: Select N2 submodules from all submodules of the MMC, and control the N2 submodules to be bypassed, and the remaining submodules to be locked;

[0155] Step 7: Determine whether the capacitor voltage of the half-bridge submodule and the capacitor voltage of the full-bridge submodule in the MMC have both reached the startup rated voltage of the MMC. If so, end the operation; otherwise, set m=m+1 and return to step 6.

[0156] Furthermore, the step 6 includes:

[0157] Determine the number of bypass submodules N2 as follows:

[0158]

[0159] Where U M is the peak value of the equivalent power line voltage of the AC system; N F N is the number of full-bridge submodules in the MMC; His the number of half-bridge submodules in the MMC, U C is the starting rated voltage value of the modular multilevel converter;

[0160] Arrange the MMC neutron modules in descending order according to the capacitance and voltage of the MMC neutron modules, and select the first N2 submodules in the sequence; wherein the MMC neutron modules include the full-bridge submodule and the half-bridge submodule of the MMC;

[0161] In the preferred embodiment of the present invention, if there are X bridge arms, then select Submodule bypass.

[0162] In the preferred embodiment of the present invention, the startup control operation of the MMC in the hybrid DC transmission system is shown in Table 1:

[0163] Table 1

[0164]

[0165] The present invention provides a starting system for an MMC in a hybrid DC power transmission system. The hybrid DC power transmission system is composed of an AC system equivalent power supply, an AC circuit breaker, a starting resistor, and an MMC connected in sequence. A starting resistor bypass switch is connected in parallel at both ends of the starting resistor. The MMC is composed of a half-bridge submodule and a full-bridge submodule. Figure 4 As shown, the system includes:

[0166] an uncontrolled charging module, configured to perform uncontrolled charging on the MMC until the capacitor voltage of the half-bridge submodule and the capacitor voltage of the full-bridge submodule in the MMC reach a first preset voltage and a second preset voltage respectively;

[0167] A controllable charging module, used to controllably charge the MMC until the capacitor voltage of the half-bridge submodule and the capacitor voltage of the full-bridge submodule in the MMC both reach the startup rated voltage of the MMC;

[0168] Among them, the initial state of the AC circuit breaker is disconnected, the initial state of the starting resistor bypass switch is disconnected, the initial state of the half-bridge submodule in the MMC is locked, and the initial state of the full-bridge submodule in the MMC is locked.

[0169] Specifically, the uncontrolled charging module is used to:

[0170] Controlling the working state of the AC circuit breaker to be closed, the working state of the starting resistor bypass switch to be open, the working state of the half-bridge submodule in the MMC to be locked, and the working state of the full-bridge submodule in the MMC to be locked;

[0171] The operation ends when the capacitor voltage of the half-bridge submodule in the MMC is charged to the first preset voltage and the capacitor voltage of the full-bridge submodule in the MMC is charged to the second preset voltage.

[0172] Specifically, the first preset voltage U is determined as follows: F ;

[0173]

[0174] The second preset voltage U is determined as follows: H ;

[0175]

[0176] In the above formula, U M is the peak value of the equivalent power line voltage of the AC system; N F N is the number of full-bridge submodules in the MMC; H is the number of half-bridge submodules in the MMC.

[0177] Specifically, the controllable charging module includes:

[0178] A voltage-balanced control charging unit, configured to perform voltage-balanced control charging on the MMC until the capacitor voltage of the half-bridge submodule and the capacitor voltage of the full-bridge submodule in the MMC both reach a third preset voltage;

[0179] A full-bridge submodule half-bridge conversion unit, configured to convert the full-bridge submodule into a half-bridge on the MMC until all power switches T4 of the full-bridge submodules in the MMC are turned on;

[0180] An overall rotation conduction charging unit, used to perform overall rotation conduction charging on the MMC until the capacitor voltage of the half-bridge submodule and the capacitor voltage of the full-bridge submodule in the MMC reach the starting rated voltage of the MMC;

[0181] One end of the power switch tube T4 of the full-bridge sub-module in the MMC is connected to the negative voltage electrode of the full-bridge sub-module, and the other end is connected to the negative electrode of the capacitor in the full-bridge sub-module.

[0182] Specifically, the voltage-balanced control charging unit is used to:

[0183] Step 1: Initialize the MMC to perform voltage-balanced control charging at time t=0;

[0184] Step 2: Control the AC circuit breaker to be in a closed state, the starting resistor bypass switch to be in an open state, and the half-bridge submodule in the MMC to be in a locked state;

[0185] Step 3: Select N1 full-bridge sub-modules from all full-bridge sub-modules of the MMC, and control the N1 full-bridge sub-modules to be bypassed, and the remaining full-bridge sub-modules to be locked;

[0186] Step 4: Determine whether the capacitor voltage of the half-bridge submodule and the capacitor voltage of the full-bridge submodule in the MMC have both reached a third preset voltage. If so, end the operation; otherwise, set t=t+1 and return to step 3.

[0187] Specifically, the step 3 includes:

[0188] Determine the number of bypassed full-bridge submodules N1 according to the following formula:

[0189]

[0190] Where U M is the peak value of the equivalent power line voltage of the AC system; N F N is the number of full-bridge submodules in the MMC; H is the number of half-bridge submodules in the MMC; U is the third preset voltage;

[0191] The full-bridge submodules in the MMC are arranged in descending order according to the capacitance and voltage of the full-bridge submodules in the MMC, and the first N1 full-bridge submodules are selected in the sequence.

[0192] Specifically, the half-bridge unit of the full-bridge submodule is used to:

[0193] Controlling the working state of the AC circuit breaker to be closed, the working state of the starting resistor bypass switch to be closed, the working state of the half-bridge submodule in the MMC to be locked, and the working state of the full-bridge submodule in the MMC to be locked;

[0194] The operation ends when all the power switch tubes T of the full-bridge sub-modules in the MMC are turned on.

[0195] Specifically, the overall rotation conduction charging unit is used to:

[0196] Step 4: Initialize the MMC to perform overall rotation conduction charging at time m=0;

[0197] Step 5: Control the AC circuit breaker to be in a closed state and the starting resistor bypass switch to be in a closed state;

[0198] Step 6: Select N2 submodules from all submodules of the MMC, and control the N2 submodules to be bypassed, and the remaining submodules to be locked;

[0199] Step 7: Determine whether the capacitor voltage of the half-bridge submodule and the capacitor voltage of the full-bridge submodule in the MMC have both reached the startup rated voltage of the MMC. If so, end the operation; otherwise, set m=m+1 and return to step 6.

[0200] Specifically, step 6 includes:

[0201] Determine the number of bypass submodules N2 as follows:

[0202]

[0203] Where U M is the peak value of the equivalent power line voltage of the AC system; N F N is the number of full-bridge submodules in the MMC; H is the number of half-bridge submodules in the MMC, U C is the starting rated voltage value of the modular multilevel converter;

[0204] The MMC neutron modules are arranged in descending order according to the capacitance and voltage of the MMC neutron modules, and the first N2 submodules are selected in the sequence; wherein the MMC neutron modules include the full-bridge submodule and the half-bridge submodule of the MMC.

[0205] Those skilled in the art will appreciate that the embodiments of the present application can be provided as methods, systems, or computer program products. Therefore, the present application can adopt the form of a complete hardware embodiment, a complete software embodiment, or an embodiment in combination with software and hardware. Moreover, the present application can adopt the form of a computer program product implemented on one or more computer-usable storage media (including but not limited to magnetic disk storage, CD-ROM, optical storage, etc.) that contain computer-usable program code.

[0206] The present application is described with reference to the flowcharts and / or block diagrams of the methods, devices (systems), and computer program products according to the embodiments of the present application. It should be understood that each process and / or box in the flowchart and / or block diagram, as well as the combination of the processes and / or boxes 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.

[0207] These computer program instructions may also be stored in a computer readable memory that can direct a computer or other programmable data processing device to work 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 1 The function specified in one or more boxes.

[0208] These computer program instructions can also be loaded onto a computer or other programmable data processing device so that a series of operational steps are executed on the computer or other programmable device to produce a computer-implemented process, thereby providing the instructions executed on the computer or other programmable device for implementing the process. Figure 1 a process or multiple processes and / or boxes Figure 1 The steps for the function specified in one or more boxes.

[0209] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit it. Although the present invention has been described in detail with reference to the above embodiments, ordinary technicians in the field should understand that the specific implementation methods of the present invention can still be modified or replaced by equivalents. Any modification or equivalent replacement that does not depart from the spirit and scope of the present invention should be covered by the scope of protection of the claims of the present invention.

Claims

1. A method for starting an MMC in a hybrid DC power transmission system, wherein the hybrid DC power transmission system comprises an AC system equivalent power supply, an AC circuit breaker, a starting resistor, and an MMC connected in sequence, a starting resistor bypass switch being connected in parallel across the starting resistor, and the MMC comprising a half-bridge submodule and a full-bridge submodule, characterized in that: The method comprises: The MMC is charged uncontrolled until the capacitor voltage of the half-bridge submodule and the capacitor voltage of the full-bridge submodule in the MMC reach a first preset voltage and a second preset voltage respectively; Controllably charging the MMC until the capacitor voltage of the half-bridge submodule and the capacitor voltage of the full-bridge submodule in the MMC both reach the startup rated voltage of the MMC; The initial state of the AC circuit breaker is disconnected, the initial state of the starting resistor bypass switch is disconnected, the initial state of the half-bridge submodule in the MMC is locked, and the initial state of the full-bridge submodule in the MMC is locked; The controllably charging the MMC until the capacitor voltage of the half-bridge submodule and the capacitor voltage of the full-bridge submodule in the MMC reach the startup rated voltage of the MMC includes: Performing voltage-balanced charging on the MMC until the capacitor voltage of the half-bridge submodule and the capacitor voltage of the full-bridge submodule in the MMC both reach a third preset voltage; Converting the full-bridge submodule of the MMC into a half-bridge, until all power switch tubes T4 of the full-bridge submodule in the MMC are turned on; The MMC is turned on and charged as a whole in a rotating manner until the capacitor voltage of the half-bridge submodule and the capacitor voltage of the full-bridge submodule in the MMC both reach the startup rated voltage of the MMC; Among them, one end of the power switch tube T4 of the full-bridge sub-module in the MMC is connected to the voltage negative electrode of the full-bridge sub-module, and the other end is connected to the negative electrode of the capacitor in the full-bridge sub-module; The step of performing voltage-balanced control charging on the MMC until the capacitor voltage of the half-bridge submodule and the capacitor voltage of the full-bridge submodule in the MMC both reach a third preset voltage includes: Step 1: Initialize the MMC to perform voltage-balanced control charging at time t=0; Step 2: Control the AC circuit breaker to be in a closed state, the starting resistor bypass switch to be in an open state, and the half-bridge submodule in the MMC to be in a locked state; Step 3: Select N1 full-bridge sub-modules from all full-bridge sub-modules of the MMC, and control the N1 full-bridge sub-modules to be bypassed, and the remaining full-bridge sub-modules to be locked; Step 4: Determine whether the capacitor voltage of the half-bridge submodule and the capacitor voltage of the full-bridge submodule in the MMC have both reached a third preset voltage. If so, end the operation; otherwise, set t=t+1 and return to step 3. The step 3 includes: Determine the number of bypassed full-bridge submodules N1 according to the following formula: Where U M is the peak value of the equivalent power line voltage of the AC system; N F N is the number of full-bridge submodules in the MMC; H is the number of half-bridge submodules in the MMC; U is the third preset voltage; The full-bridge submodules in the MMC are arranged in descending order according to the capacitance and voltage of the full-bridge submodules in the MMC, and the first N1 full-bridge submodules are selected in the sequence.

2. The method according to claim 1, wherein The uncontrolled charging of the MMC until the capacitor voltage of the half-bridge submodule and the capacitor voltage of the full-bridge submodule in the MMC reach a first preset voltage and a second preset voltage respectively includes: Controlling the working state of the AC circuit breaker to be closed, the working state of the starting resistor bypass switch to be open, the working state of the half-bridge submodule in the MMC to be locked, and the working state of the full-bridge submodule in the MMC to be locked; The operation ends when the capacitor voltage of the half-bridge submodule in the MMC is charged to the first preset voltage and the capacitor voltage of the full-bridge submodule in the MMC is charged to the second preset voltage.

3. The method according to claim 1, wherein The first preset voltage U is determined as follows: F ; The second preset voltage U is determined as follows: H ; In the above formula, U M is the peak value of the equivalent power line voltage of the AC system; N F N is the number of full-bridge submodules in the MMC; H is the number of half-bridge submodules in the MMC.

4. The method according to claim 1, wherein Converting the full-bridge submodule of the MMC to a half-bridge until all power switch tubes T4 of the full-bridge submodule in the MMC are turned on includes: Controlling the AC circuit breaker to be in a closed state, the starting resistor bypass switch to be in a closed state, the half-bridge submodule in the MMC to be in a locked state, and the full-bridge submodule in the MMC to be in a locked state; The operation ends when all the power switch tubes T4 of the full-bridge sub-modules in the MMC are turned on.

5. The method according to claim 1, wherein The whole MMC is rotated and charged until the capacitor voltage of the half-bridge submodule and the capacitor voltage of the full-bridge submodule in the MMC reach the startup rated voltage of the MMC, including: Step 4: Initialize the MMC to perform overall rotation conduction charging at time m=0; Step 5: Control the AC circuit breaker to be in a closed state and the starting resistor bypass switch to be in a closed state; Step 6: Select N2 submodules from all submodules of the MMC, and control the N2 submodules to be bypassed, and the remaining submodules to be locked; Step 7: Determine whether the capacitor voltage of the half-bridge submodule and the capacitor voltage of the full-bridge submodule in the MMC have both reached the startup rated voltage of the MMC. If so, end the operation; otherwise, set m=m+1 and return to step 6.

6. The method according to claim 5, wherein The step 6 comprises: Determine the number of bypass submodules N2 as follows: Where U M is the peak value of the equivalent power line voltage of the AC system; N F N is the number of full-bridge submodules in the MMC; H is the number of half-bridge submodules in the MMC, U C is the starting rated voltage value of the modular multilevel converter; The MMC neutron modules are arranged in descending order according to the capacitance and voltage of the MMC neutron modules, and the first N2 submodules are selected in the sequence; wherein the MMC neutron modules include the full-bridge submodule and the half-bridge submodule of the MMC.

7. A starting system for an MMC in a hybrid DC power transmission system, the hybrid DC power transmission system comprising an AC system equivalent power supply, an AC circuit breaker, a starting resistor, and an MMC connected in sequence, a starting resistor bypass switch connected in parallel across the starting resistor, and the MMC comprising a half-bridge submodule and a full-bridge submodule, characterized in that: The system comprises: an uncontrolled charging module, configured to perform uncontrolled charging on the MMC until the capacitor voltage of the half-bridge submodule and the capacitor voltage of the full-bridge submodule in the MMC reach a first preset voltage and a second preset voltage respectively; A controllable charging module, used to controllably charge the MMC until the capacitor voltage of the half-bridge submodule and the capacitor voltage of the full-bridge submodule in the MMC both reach the startup rated voltage of the MMC; The initial state of the AC circuit breaker is disconnected, the initial state of the starting resistor bypass switch is disconnected, the initial state of the half-bridge submodule in the MMC is locked, and the initial state of the full-bridge submodule in the MMC is locked; The controllable charging module includes: A voltage-balanced control charging unit, configured to perform voltage-balanced control charging on the MMC until the capacitor voltage of the half-bridge submodule and the capacitor voltage of the full-bridge submodule in the MMC both reach a third preset voltage; Converting the full-bridge submodule of the MMC into a half-bridge, until all power switch tubes T4 of the full-bridge submodule in the MMC are turned on; The MMC is turned on and charged as a whole in a rotating manner until the capacitor voltage of the half-bridge submodule and the capacitor voltage of the full-bridge submodule in the MMC both reach the startup rated voltage of the MMC; One end of the power switch tube T4 of the full-bridge sub-module in the MMC is connected to the voltage negative electrode of the full-bridge sub-module, and the other end is connected to the negative electrode of the capacitor in the full-bridge sub-module; The voltage-balanced control charging unit is used to: Step 1: Initialize the MMC to perform voltage-balanced control charging at time t=0; Step 2: Control the AC circuit breaker to be in a closed state, the starting resistor bypass switch to be in an open state, and the half-bridge submodule in the MMC to be in a locked state; Step 3: Select N1 full-bridge sub-modules from all full-bridge sub-modules of the MMC, and control the N1 full-bridge sub-modules to be bypassed, and the remaining full-bridge sub-modules to be locked; Step 4: Determine whether the capacitor voltage of the half-bridge submodule and the capacitor voltage of the full-bridge submodule in the MMC have both reached a third preset voltage. If so, end the operation; otherwise, set t=t+1 and return to step 3. The step 3 includes: Determine the number of bypassed full-bridge submodules N1 according to the following formula: Where U M is the peak value of the equivalent power line voltage of the AC system; N F N is the number of full-bridge submodules in the MMC; H is the number of half-bridge submodules in the MMC; U is the third preset voltage; The full-bridge submodules in the MMC are arranged in descending order according to the capacitance and voltage of the full-bridge submodules in the MMC, and the first N1 full-bridge submodules are selected in the sequence.

8. The system according to claim 7, wherein: The uncontrolled charging module is used to: Controlling the working state of the AC circuit breaker to be closed, the working state of the starting resistor bypass switch to be open, the working state of the half-bridge submodule in the MMC to be locked, and the working state of the full-bridge submodule in the MMC to be locked; The operation ends when the capacitor voltage of the half-bridge submodule in the MMC is charged to the first preset voltage and the capacitor voltage of the full-bridge submodule in the MMC is charged to the second preset voltage.

9. The system according to claim 8, wherein The first preset voltage U is determined as follows: F ; The second preset voltage U is determined as follows: H ; In the above formula, U M is the peak value of the equivalent power line voltage of the AC system; N F N is the number of full-bridge submodules in the MMC; H is the number of half-bridge submodules in the MMC.

10. The system according to claim 7, wherein: The half-bridge unit of the full-bridge submodule is used to: Controlling the AC circuit breaker to be in a closed state, the starting resistor bypass switch to be in a closed state, the half-bridge submodule in the MMC to be in a locked state, and the full-bridge submodule in the MMC to be in a locked state; The operation ends when all the power switch tubes T4 of the full-bridge sub-modules in the MMC are turned on.

11. The system according to claim 7, wherein: The overall rotation conduction charging unit is used to: Step 4: Initialize the MMC to perform overall rotation conduction charging at time m=0; Step 5: Control the AC circuit breaker to be in a closed state and the starting resistor bypass switch to be in a closed state; Step 6: Select N2 submodules from all submodules of the MMC, and control the N2 submodules to be bypassed, and the remaining submodules to be locked; Step 7: Determine whether the capacitor voltage of the half-bridge submodule and the capacitor voltage of the full-bridge submodule in the MMC have both reached the startup rated voltage of the MMC. If so, end the operation; otherwise, set m=m+1 and return to step 6.

12. The system according to claim 11, wherein The step 6 comprises: Determine the number of bypass submodules N2 as follows: Where U M is the peak value of the equivalent power line voltage of the AC system; N F N is the number of full-bridge submodules in the MMC; H is the number of half-bridge submodules in the MMC, U C is the starting rated voltage value of the modular multilevel converter; The MMC neutron modules are arranged in descending order according to the capacitance and voltage of the MMC neutron modules, and the first N2 submodules are selected in the sequence; wherein the MMC neutron modules include the full-bridge submodule and the half-bridge submodule of the MMC.

Images