A coordinated start-up control method for a hybrid cascade DC transmission system

Abstract

The present invention discloses a coordinated startup control method for a hybrid cascade type direct current transmission system. In an uncontrollable charging stage, a quantitative setting method is adopted to obtain an optimal current limiting resistor, and the inverter side MMC is charged according to the calculated optimal current limiting resistor until the MMC voltage is stable, and then the converter control unlocking stage is executed; in the converter control unlocking stage, among three MMCs connected in parallel on the inverter side, based on the controller timing coordination between different converters and an adaptive MMC power reference value, the startup unbalanced current on the DC side of the MMC on the inverter side is suppressed, and the DC power of the hybrid cascade type direct current transmission system is increased to stable operation; the beneficial effects of the present invention are to achieve effective suppression of abnormal current at system startup and improve the startup reliability of the hybrid cascade direct current system.

Description

Technical Field

[0001] The present invention relates to the technical field of hybrid direct current power transmission, and in particular to a coordinated startup control method for a hybrid cascade direct current power transmission system. Background Art

[0002] The Baihetan-Jiangsu UHV DC transmission system currently under construction in my country adopts a new topology structure in which a traditional LCC is directly connected in series with three flexible parallel MMC valve groups on the inverter side. Compared with the conventional LCC DC transmission system or the flexible MMC DC transmission system, the hybrid structure of LCC and MMC in series on the inverter side has good AC and DC fault ride-through capability. The flexible power regulation capability of multiple MMCs at the receiving end and the multi-feed point structure can meet the power demand of different load centers, and the parallel relationship of multiple MMCs can solve the disadvantage of the small capacity of the existing flexible DC system, which better combines the economic and technical advantages of both traditional DC transmission and flexible DC transmission.

[0003] When starting the above-mentioned hybrid cascade DC system, it can be divided into two stages according to the time scale of startup: uncontrolled charging of the MMC at the low voltage end and subsequent unlocking of the system controller. However, in the uncontrolled charging stage, the existing setting technology for the starting current limiting resistor lacks consideration of the safety and speed requirements of the converter startup. At the same time, in the unlocking stage of the system controller, since the low-voltage end of the inverter side adopts a structure in which multiple MMCs are connected in parallel, if the coordinated startup control measures between multiple MMCs and between LCC and MMC are improper, an unbalanced current will be generated on the DC side of the parallel MMC when the system starts, which may cause overcurrent in severe cases, posing a great threat to the safe startup of the entire system.

[0004] In view of this, this application is hereby filed. Summary of the invention

[0005] The technical problem to be solved by the present invention is that in the prior art, improper quantitative selection of current limiting resistors in the uncontrolled charging stage of the MMC causes starting overcurrent, and improper coordinated starting control measures between multiple MMCs and between LCC and MMC in the controller unlocking stage cause unbalanced current to be generated on the DC side of the parallel MMCs, which may bring safety hazards to the system in serious cases. The purpose is to provide a coordinated starting control method for a hybrid cascaded DC transmission system, which can effectively suppress abnormal current at system startup.

[0006] The present invention is achieved through the following technical solutions:

[0007] A coordinated startup control method for a hybrid cascade type DC transmission system, wherein the coordinated startup of the hybrid cascade type DC transmission system includes an MMC uncontrollable charging stage and a converter control unlocking stage, and the method steps include:

[0008] In the uncontrollable charging stage, the quantitative setting method is used to obtain the optimal current limiting resistance, and the MMC is charged according to the calculated optimal current limiting resistance until the MMC voltage is stable, and then the converter control unlocking stage is executed;

[0009] In the converter control unlocking stage, in the MMC connected in parallel on the inverter side, based on the controller timing coordination between different converters and the use of an adaptive MMC power reference value, the starting unbalanced current on the DC side of the parallel MMC is suppressed, and the DC power of the hybrid cascaded DC transmission system is increased to stable operation.

[0010] In a traditional hybrid cascade DC transmission system, a hybrid structure of LCC and MMC in series is used on the inverter side, which has good AC and DC fault ride-through capability. The flexible power regulation capability of multiple MMCs at the receiving end and the multi-feed point structure can meet the power demand of different load centers. However, if the coordinated startup control measures between multiple MMCs and between LCC and MMC are improper, an unbalanced current will be generated on the DC side of the parallel MMC when the system is started, which may cause overcurrent in severe cases, posing a great threat to the safe startup of the entire system. The present invention provides a coordinated startup control method for a hybrid cascade DC transmission system, which achieves the suppression of unbalanced starting current on the DC side of the parallel MMC during the controller unlocking stage of the system by timing the MMC outer loop control mode and adopting an adaptive MMC power reference value during the converter control unlocking stage.

[0011] Preferably, the specific method for obtaining the current limiting resistor includes:

[0012] According to the absolute value relationship of the line voltage when the AC side circuit breaker is closed, an uncontrollable charging equivalent circuit model is constructed, combined with the maximum starting impact current I max Less than the system allowable overcurrent I lim , calculate and obtain the lower limit value of the current limiting resistor;

[0013] According to the given precharge time t ref Under this condition, the MMC common DC side voltage can reach a stable charging threshold U dc =0.99U lm , calculate and obtain the upper limit value of the current limiting resistor;

[0014] An intermediate value between the lower limit value of the current limiting resistance and the upper limit value of the current limiting resistance is calculated to obtain an optimal current limiting resistance.

[0015] Preferably, the specific calculation method of the lower limit value of the current limiting resistor includes:

[0016] According to the absolute value relationship of the line voltage when the AC side circuit breaker is closed, an uncontrollable charging equivalent circuit model is constructed, wherein the uncontrollable charging equivalent circuit model is a phase-to-phase circuit model with the largest absolute value of the line voltage;

[0017] According to Kirchhoff's voltage theorem, the electrical characteristics of the MMC half-bridge submodule during uncontrolled charging are solved using the Laplace transform law to obtain the current solution under the initial voltage state.

[0018] Based on the current general solution, combined with the maximum starting impact current I max Less than the system allowable overcurrent I lim , calculate and obtain the lower limit value of the current limiting resistor.

[0019] Preferably, in the inverter control unlocking stage, the specific control method steps include:

[0020] Send a trigger signal to each MMC on the inverter side;

[0021] Under the action of the trigger signal, the control mode of each MMC on the inverter side is converted to the outer loop fixed DC voltage and fixed reactive power control;

[0022] When the voltage of each MMC on the inverter side is stable, the MMC control mode will be converted to fixed active power control;

[0023] The double-side LCC controllers are unlocked, and the power reference value of the slave MMC outer loop active power controller is adaptively adjusted according to the pole bus transmission power until the DC power of the hybrid cascade DC transmission system rises to a stable operation.

[0024] Preferably, the trigger signal is an inner and outer loop PI controller and an IGBT pulse trigger signal corresponding to each MMC on the inverter side.

[0025] Preferably, in the unlocked double-sided LCC controller, the inverter-side LCC adopts a constant DC voltage control mode to continue to raise the system DC voltage, and the rectifier-side LCC adopts a constant DC current control mode, and the current command value rises to the rated value at a fixed slope of 0.9 pu / s.

[0026] Preferably, the electrical characteristic equation is specifically expressed as:

[0027]

[0028] Among them, L is the equivalent inductance in the pre-charging loop, i ph (t) is the loop current, R is the equivalent resistance, u c (t) is the submodule capacitor voltage, U lm is the MMC AC bus voltage amplitude, is the closing phase angle of the AC side circuit breaker, and C is the equivalent capacitance of the charging circuit.

[0029] Preferably, the specific expression of the current general solution is:

[0030] i ph (t) = i p (t)+i α1 (t)+i α2 (t)

[0031] i p (t) is the steady-state component, i α1 (t), i α2 (t) are all free components.

[0032] Preferably, the voltage on the DC side is specifically expressed as:

[0033] U dc (t) = U lm (1-e -t / τ )

[0034] Equivalent time constant: τ=6R ac C SM / N total , N total is the total number of capacitors in the bridge arm submodules.

[0035] Preferably, the specific expression of the power reference value of the slave MMC outer loop active power controller is:

[0036]

[0037] U dc_mmc is the common DC voltage from the MMC, I dc is the DC line current, P mmc Transmit power to the pole bus.

[0038] Compared with the prior art, the present invention has the following advantages and beneficial effects:

[0039] The embodiment of the present invention provides a coordinated startup control method for a hybrid cascade DC transmission system, which quantitatively adjusts the current limiting resistor based on the maximum startup impact current and startup time limit conditions during the MMC uncontrolled charging stage, thereby ensuring the safety and speed of the submodule capacitor when continuously charging during the uncontrolled charging stage. Secondly, by timing the MMC outer loop control method and adopting an adaptive MMC power reference value during the converter control unlocking stage, the system achieves the suppression of the unbalanced startup current on the DC side of the parallel MMC during the controller unlocking stage. BRIEF DESCRIPTION OF THE DRAWINGS

[0040] In order to more clearly illustrate the technical solutions of the exemplary embodiments of the present invention, the drawings required for use in the embodiments will be briefly introduced below. It should be understood that the following drawings only show certain embodiments of the present invention and therefore should not be regarded as limiting the scope. For ordinary technicians in this field, other related drawings can be obtained based on these drawings without paying creative work.

[0041] Figure 1 A schematic diagram of a monopole topology structure of an inverter-side hybrid cascaded DC power transmission system according to an embodiment of the present invention;

[0042] Figure 2 This is an equivalent circuit diagram of MMC uncontrolled charging according to an embodiment of the present invention;

[0043] Figure 3 This is a schematic diagram of the inverter side structure of a hybrid cascaded DC system according to an embodiment of the present invention;

[0044] Figure 4 This is a block diagram of the MMC startup optimization control strategy according to an embodiment of the present invention;

[0045] Figure 5 It is a startup characteristic diagram of the hybrid cascade system when the startup optimization strategy is not put into use in the embodiment of the present invention;

[0046] Figure 6 This is a startup characteristic diagram of the hybrid cascade system after the startup optimization strategy is put into use in an embodiment of the present invention. DETAILED DESCRIPTION

[0047] In order to make the objectives, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with embodiments and drawings. The exemplary embodiments of the present invention and their description are only used to explain the present invention and are not intended to limit the present invention.

[0048] In the following description, a large number of specific details are set forth in order to provide a thorough understanding of the present invention. However, it is apparent to one of ordinary skill in the art that these specific details are not necessarily employed to practice the present invention. In other embodiments, in order to avoid obscuring the present invention, well-known structures, circuits, materials, or methods are not specifically described.

[0049] Throughout the specification, references to "one embodiment," "an embodiment," "an example," or "an example" mean that a particular feature, structure, or characteristic described in conjunction with the embodiment or example is included in at least one embodiment of the present invention. Therefore, the phrases "one embodiment," "an embodiment," "an example," or "an example" appearing in various places throughout the specification do not necessarily all refer to the same embodiment or example. In addition, particular features, structures, or characteristics may be combined in one or more embodiments or examples in any suitable combination and / or sub-combination. In addition, it will be appreciated by those of ordinary skill in the art that the figures provided herein are for illustrative purposes and that the figures are not necessarily drawn to scale. The term "and / or" as used herein includes any and all combinations of one or more of the associated listed items.

[0050] In the description of the present invention, the directions or positional relationships indicated by terms such as “front”, “rear”, “left”, “right”, “up”, “down”, “vertical”, “horizontal”, “high”, “low”, “inside” and “outside” are based on the directions or positional relationships shown in the accompanying drawings and are only for the convenience of describing the present invention and simplifying the description. They do not indicate or imply that the device or element referred to must have a specific direction, be constructed and operated in a specific direction. Therefore, they should not be understood as limiting the scope of protection of the present invention.

[0051] Embodiment 1

[0052] This embodiment discloses a coordinated startup control method for a hybrid cascade DC transmission system, which is used to solve the problem of how to quantitatively select the current limiting resistor in the hybrid cascade DC system during the MMC uncontrolled charging stage and the unbalanced startup current generated by the parallel MMC DC side during the system controller unlocking stage. In this embodiment, the circuit diagram for implementing this method is shown in FIG. Figure 1 shown.

[0053] The coordinated startup of the hybrid cascaded DC transmission system includes an MMC uncontrollable charging stage and a converter control unlocking stage, and the method steps include:

[0054] In the uncontrollable charging stage, the quantitative setting method is used to obtain the optimal current limiting resistance, and the MMC is charged according to the calculated optimal current limiting resistance until the MMC voltage is stable, and then the converter control unlocking stage is executed;

[0055] The specific method for obtaining the current limiting resistor includes:

[0056] According to the absolute value relationship of the line voltage when the AC side circuit breaker is closed, an uncontrollable charging equivalent circuit model is constructed, combined with the maximum starting impact current I max Less than the system allowable overcurrent I lim , calculate and obtain the lower limit value of the current limiting resistor;

[0057] According to the given precharge time t ref Under this condition, the MMC common DC side voltage can reach a stable charging threshold U dc =0.99U lm , calculate and obtain the upper limit value of the current limiting resistor;

[0058] An intermediate value between the lower limit value of the current limiting resistance and the upper limit value of the current limiting resistance is calculated to obtain an optimal current limiting resistance.

[0059] The specific calculation method of the lower limit value of the current limiting resistor includes:

[0060] According to the absolute value relationship of the line voltage when the AC side circuit breaker is closed, an uncontrollable charging equivalent circuit model is constructed, wherein the uncontrollable charging equivalent circuit model is a phase-to-phase circuit model with the largest absolute value of the line voltage;

[0061] According to Kirchhoff's voltage theorem, the electrical characteristics of the MMC half-bridge submodule during uncontrolled charging are solved using the Laplace transform law to obtain the current solution under the initial voltage state.

[0062] Based on the current general solution, combined with the maximum starting impact current I max Less than the system allowable overcurrent I lim , calculate and obtain the lower limit value of the current limiting resistor.

[0063] The specific implementation method process is:

[0064] Get the equivalent impedance Z of the MMC AC side system ac =R ac +jX ac , bridge arm inductance L arm , submodule capacitance C SM , the number of bridge arm submodules N, the MMC AC bus voltage amplitude U lm , AC side circuit breaker closing phase angle And other system parameters.

[0065] During the MMC uncontrolled charging process, according to the absolute value relationship of the line voltage when the AC side circuit breaker is closed, the phase-to-phase circuit with the largest absolute value of the line voltage is selected as the charging equivalent circuit.

[0066] Furthermore, according to Kirchhoff's voltage theorem, the electrical characteristics of MMC during uncontrolled charging are solved as follows:

[0067]

[0068] Among them, the equivalent resistance R = 2R ac , equivalent capacitance C = 2C SM / N, equivalent inductance L = 2L ac+L arm .

[0069] Furthermore, using the Laplace transform law, the MMC voltage U c In the initial state of (0-)=0, the current general solution of the capacitor charging characteristic equation is calculated as:

[0070] i ph (t) = i p (t)+i α1 (t)+i α2 (t)

[0071] Among them, the steady-state component i p (t) and free component i α1 (t), i α2 The expression of (t) is as follows:

[0072]

[0073] The relevant symbolic expressions are:

[0074]

[0075] Furthermore, from the DC side, the three bridge arm capacitors of the MMC in the state of alternate charging maintain a parallel relationship. During this process, the DC side voltage of the inverter side MMC can be expressed as:

[0076] U dc (t) = U lm (1-e -t / τ )

[0077] Among them, the equivalent time constant: τ = 6R ac C SM / N total , N total is the total number of capacitors in the bridge arm submodules.

[0078] Furthermore, according to the relationship between the maximum impact current of the MMC and the current limiting resistor, the current limiting resistor parameter design principle 1: the maximum starting impact current I on the AC side max Less than the system allowable overcurrent I lim , in order to determine the lower limit value of the current limiting resistor parameter R st_min .Right now:

[0079] I ph_max =max(|i ph (t,R st_min )|)≤I lim

[0080] Secondly, according to the relationship between the voltage amplitude and time of the MMC DC side, the current limiting resistor parameters are designed according to the second design principle: at a given pre-charge time tref Under this condition, the MMC common DC side voltage can reach a stable charging threshold U dc =0.99U lm , in order to determine the upper limit value of the current limiting resistor parameter R st_max .Right now:

[0081]

[0082] Finally, in the closed interval (R st_min , R st_max ) range as the parameter setting value of the current limiting resistor in the uncontrolled charging stage of the MMC, ensuring the safety and speed of the flexible MMC during continuous charging in the uncontrolled charging stage.

[0083] In the converter control unlocking stage, in the MMC connected in parallel on the inverter side, based on the controller timing coordination between different converters and the use of an adaptive MMC power reference value, the starting unbalanced current on the DC side of the parallel MMC is suppressed, and the DC power of the hybrid cascaded DC transmission system is increased to stable operation.

[0084] In the inverter control unlocking stage, the specific control method steps include:

[0085] Sending a trigger signal to each of the MMCs;

[0086] Under the action of the trigger signal, the control mode of each MMC on the inverter side is converted to outer loop fixed DC voltage and fixed reactive power control; in this embodiment, the trigger signal is the inner and outer loop PI controller and IGBT pulse trigger signal corresponding to each MMC on the inverter side.

[0087] When the voltage of each MMC on the inverter side is stable, the MMC control mode will be converted to fixed active power control;

[0088] The double-side LCC controllers are unlocked, and the power reference value of the slave MMC outer loop active power controller is adaptively adjusted according to the pole bus transmission power until the DC power of the hybrid cascade DC transmission system rises to a stable operation.

[0089] In the step of unlocking the double-sided LCC controller, the inverter-side LCC adopts a constant DC voltage control mode to continue to raise the system DC voltage, and the rectifier-side LCC adopts a constant DC current control mode, and the current command value increases to the rated value at a fixed slope of 0.9 pu / s.

[0090] The specific implementation process is:

[0091] After determining the current limiting resistor parameters, close the MMC AC side circuit breaker, and the AC power supply uncontrollably charges the three MMCs through the pre-set current limiting resistor. When the submodule capacitor voltage is charged to the first stage stable threshold U c =U lm / N, it is determined that the first stage of uncontrollable charging process of the MMC submodule is completed, the current limiting resistor is bypassed, the internal circuits of the MMC are in a controllable trigger state, and the system startup process enters the second stage.

[0092] In this embodiment, the second stage of the hybrid cascade DC system startup: the converter control unlocking stage is divided into two parts: MMC control unlocking and LCC control unlocking according to the sequence of different converter control unlocking.

[0093] In such Figure 3 In the inverter side topology of the hybrid cascaded DC system shown in the figure, the three MMCs on the low voltage side adopt the master-slave control coordination mode of "constant DC voltage + constant active power" in the stable operation stage, in which the master station MMC1 controls the DC side voltage to a constant value, and the slave stations MMC2 and MMC3 control the AC side active power to a constant value.

[0094] Since the three MMCs are connected in parallel on the DC side, unbalanced starting current will be caused on the MMC DC side when MMC control and LCC control are put into operation in stages. The specific reasons are analyzed as follows:

[0095] The generation mechanism of unbalanced current in the unlocking stage of MMC controller: When MMC is unlocked, when the LCC at the high-voltage end is not unlocked, I d =0, the submodule capacitors in MMC1 that have experienced the uncontrolled charging stage will continue to draw active power from the AC side under the action of the voltage controller, thereby increasing the rated DC voltage. MMC2 and MMC3 use fixed active power control, and the power for the internal submodule capacitors to continue charging and maintain the switching loss comes from MMC1 without power regulation capability, resulting in unbalanced starting currents between the three MMCs at startup. At this time, the DC side current of MMC1 is shown as I1', and MMC1 works in rectification mode.

[0096] The mechanism of unbalanced current generation when the LCC controller is unlocked: After the LCCs on both sides are unlocked, the entire hybrid cascade system enters the DC power rising stage. Due to the time-consuming power transmission of the DC line and the faster power response speed of the MMC at the low-voltage end of the inverter side compared to the LCC at the high-voltage end, in the early stage of LCC unlocking, the line transmission power will not be able to meet the rapidly rising power transmission requirements of MMC2 and MMC3 on the inverter side. The power shortfall of the two will be provided by MMC1 without power control capability, which will again generate unbalanced starting current between the three MMCs.

[0097] In order to suppress the unbalanced current generated on the DC side of the three parallel MMCs during the DC system control unlocking phase, Figure 4 As shown, this embodiment provides a hybrid cascade system startup optimization control strategy based on different converter controller timing coordination and adaptive MMC power reference value. The specific strategy is as follows:

[0098] Get the MMC common DC side voltage U dc_mmc , DC line current I dc ;

[0099] After the MMC uncontrolled charging stage is over, the three MMC inner and outer loop PI controllers and IGBT pulse trigger signals at the low voltage end are unlocked. The three MMCs use the same outer loop fixed DC voltage and fixed reactive power control mode in the 0-t1 stage. The reference value U of the fixed DC voltage controller is dcref =U dcN , set the reference value Q of the reactive power controller ref =0;

[0100] Increase the MMC voltage to reach the stable threshold U c =U DCN / N, the outer loop controllers of MMC2 and MMC3 switch to the fixed active power control mode during stable operation, and set the PI controller input deviation ΔP to zero before the high-voltage end LCC is unlocked;

[0101] Furthermore, at time t2, the LCC controllers on both sides are unlocked, wherein the LCC on the inverter side adopts a constant DC voltage control mode to continue to raise the system DC voltage, and the LCC on the rectifier side adopts a constant DC current control mode, and the current command value increases to the rated value at a fixed slope of 0.9 pu / s;

[0102] Furthermore, in order to prevent the three parallel MMCs from generating unbalanced current again due to the mismatch between the line transmission power and the power reference value of the active power control MMC after the LCC is unlocked, the power reference values ​​P of MMC2 and MMC3 are set. ref2 , P ref3 The transmission power P according to the pole bus mmc Perform adaptive coordinated tuning;

[0103] The power reference values ​​for MMC2 and MMC3 are

[0104] Simulation Implementation:

[0105] In order to verify the coordinated startup method of the hybrid cascade system provided by the present invention, the following Figure 1 The simulation model of the inverter-side hybrid cascaded DC transmission system is shown.

[0106] According to the coordinated startup process of the hybrid cascade system described in the technical solution of the present invention, under the conditions of directly following the steady-state master-slave control strategy and investing in the startup optimization control strategy proposed in this article, the startup characteristics of the hybrid cascade DC system are compared with the simulation results as shown in the following figure. Figure 5 , Figure 6 shown.

[0107] like Figure 5 As shown in the figure, when the MMC directly starts the hybrid cascade DC system according to the steady-state master-slave control strategy, the low-voltage parallel MMC has a starting current imbalance phenomenon in the 0.5s controller unlocking and 1s-1.3s DC power rising stage. The maximum absolute value of the DC side overcurrent reaches 5.92kA, which exceeds the rated operating current by 3.54 times, seriously threatening the safe startup of related converter equipment.

[0108] like Figure 6 As shown, after adopting the startup optimization control strategy of the design scheme of the present invention, the problem of unbalanced starting current on the common DC side of the three MMCs is significantly suppressed, and the stability of the receiving-end power grid of the inverter-side hybrid cascade DC transmission system during the startup process is effectively improved.

[0109] The present embodiment discloses a coordinated startup control method for a hybrid cascade DC transmission system. In the converter control unlocking stage, a plurality of parallel MMC controllers are used to coordinate timings of different control modes and adopt a startup optimization strategy of adaptive power reference values. The method is suitable for suppressing unbalanced currents between a plurality of MMCs during the startup process of an inverter-side LCC-MMC hybrid cascade DC transmission system, thereby improving the startup reliability of the hybrid cascade DC system.

[0110] The specific implementation methods described above further illustrate the objectives, technical solutions and beneficial effects of the present invention in detail. It should be understood that the above description is only a specific implementation method of the present invention and is not intended to limit the scope of protection of the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A coordinated startup control method for a hybrid cascaded DC power transmission system, characterized in that: The coordinated startup of the hybrid cascaded DC transmission system includes an MMC uncontrollable charging stage and a converter control unlocking stage, and the method steps include: In the uncontrollable charging stage, the quantitative setting method is used to obtain the optimal current limiting resistance, and the MMC is charged according to the calculated optimal current limiting resistance until the MMC voltage is stable, and then the converter control unlocking stage is executed; In the converter control unlocking stage, in the MMCs connected in parallel on the inverter side, based on the controller timing coordination between different converters and the use of an adaptive MMC power reference value, the starting unbalanced current on the DC side of the parallel MMC is suppressed, and the DC power of the hybrid cascaded DC transmission system is increased to a stable operation; The specific method for obtaining the current limiting resistor includes: constructing an uncontrollable charging equivalent circuit model based on the absolute value relationship of the line voltage when the AC side circuit breaker is closed, and combining the maximum starting impact current I max Less than the system allowable overcurrent I lim , calculate the lower limit of the current limiting resistor; according to the given pre-charge time t ref Under this condition, the MMC common DC side voltage can reach a stable charging threshold U dc =0.99U lm , calculate and obtain the upper limit value of the current limiting resistor; calculate the intermediate value between the lower limit value of the current limiting resistor and the upper limit value of the current limiting resistor to obtain the optimal current limiting resistor; The specific calculation method of the lower limit value of the current limiting resistor includes: constructing an uncontrolled charging equivalent circuit model according to the absolute value relationship of the line voltage when the AC side circuit breaker is closed, and the uncontrolled charging equivalent circuit model is a phase-to-phase loop model with the largest absolute value of the line voltage; according to Kirchhoff's voltage theorem, and using the Laplace transform law to solve the electrical characteristics of the MMC half-bridge submodule during the uncontrolled charging process, to obtain the current general solution under the initial voltage state; based on the current general solution, combined with the maximum starting impact current I max Less than the system allowable overcurrent I lim , calculate and obtain the lower limit value of the current limiting resistor; In the inverter control unlocking stage, the specific control method steps include: sending a trigger signal to each MMC on the inverter side; under the action of the trigger signal, the control mode of each MMC on the inverter side is converted to outer loop fixed DC voltage and fixed reactive power control; when the voltage of each MMC on the inverter side is stable, the MMC control mode is converted to fixed active power control; unlocking the double-sided LCC controller, according to the pole bus transmission power, the power reference value of the outer loop active power controller of the slave MMC is adaptively adjusted until the DC power of the hybrid cascaded DC transmission system rises to stable operation.

2. The coordinated startup control method of a hybrid cascaded DC power transmission system according to claim 1, characterized in that: The trigger signal is an inner and outer loop PI controller and an IGBT pulse trigger signal corresponding to each MMC on the inverter side.

3. The coordinated startup control method of a hybrid cascaded DC power transmission system according to claim 1, characterized in that: In the unlocked double-sided LCC controller, the inverter-side LCC adopts a constant DC voltage control mode to continue to raise the system DC voltage, and the rectifier-side LCC adopts a constant DC current control mode, and the current command value rises to the rated value at a fixed slope of 0.9 pu / s.

4. The coordinated startup control method of a hybrid cascaded DC power transmission system according to claim 1, characterized in that: The specific expression for solving the electrical characteristics is: Among them, L is the equivalent inductance in the pre-charging loop, i ph (t) is the loop current, R is the equivalent resistance, u c (t) is the submodule capacitor voltage, U lm is the MMC AC bus voltage amplitude, is the closing phase angle of the AC side circuit breaker, and C is the equivalent capacitance of the charging circuit.

5. The coordinated startup control method of a hybrid cascaded DC power transmission system according to claim 4, characterized in that: The specific expression of the current general solution is: i ph (t)=i p (t)+i α1 (t)+i α2 (t) i p (t) is the steady-state component, i α1 (t), i α2 (t) are all free components.

6. The coordinated startup control method of a hybrid cascaded DC power transmission system according to claim 1, characterized in that: The specific expression of the voltage on the DC side is: U dc (t)=U lm (1-e -t / τ ) Equivalent time constant: τ=6R ac C SM / N total , N total is the total number of bridge arm submodule capacitors, C SM is the submodule capacitance, U lm is the MMC AC bus voltage amplitude.

7. The coordinated startup control method of a hybrid cascaded DC power transmission system according to claim 1, characterized in that: The specific expression of the power reference value of the slave MMC outer loop active power controller is: U dc_mmc is the common DC voltage from the MMC, I dc is the DC line current, P mmc Transmit power to the pole bus.

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