Coordinated control method for MMC-CLCC novel direct current transmission system

Abstract

The present application relates to a kind of MMC-CLCC novel direct current transmission system coordinated control method, belong to hybrid direct current transmission technical field.The method includes: establishing the topological architecture of MMC-CLCC hybrid direct current transmission system, determines the best proportion of full-bridge sub-module according to system dc voltage and so on requirement, determines the control strategy when steady state runs;Analysis system dynamic process under the receiving end ac fault, illustrate the power operating characteristics of CLCC;Design the calculation process of inverter station dc current, active and reactive power operating limit under multi-condition constraint, build inverter station safe operation area;Propose the CLCC off-angle regulation strategy based on reactive power sensitivity constraint and the MMC coordinated cooperation strategy based on maximum power transmission.The method can provide support for receiving end ac system during receiving end grid fault, while maximumly guaranteeing direct current power transmission capacity, applicable to large-scale new energy cross-regional power transmission scene.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of high-voltage direct current transmission, in particular to a MMC-CLCC novel direct current transmission system coordinated control method considering voltage support and direct current power transmission of the receiving end. BACKGROUND

[0002] At present, the new energy mainly including wind power and photovoltaic in China presents a mode of "large-scale development and centralized delivery", which must rely on large-scale and long-distance cross-provincial power transmission to realize energy scheduling and configuration. High-voltage direct current transmission technology has become an important support for long-distance power transmission due to its advantages in economy, transmission efficiency and response speed. In recent years, with the proposal of various new types of converters, the hybrid direct current transmission formed by the combination of traditional direct current transmission technology and new direct current technology has become the development trend of future large-capacity power transmission technology, and at present, the hybrid direct current transmission system mainly includes LCC, MMC hybrid double-end, multi-end system and receiving-end hybrid system.

[0003] The hybrid direct current transmission system using modular multilevel converter (MMC) at the sending end and controllable line commutated converter (CLCC) at the receiving end inherits the advantages of MMC and CLCC, has the advantages of no commutation failure and good economy, and the sending end is connected to a weak AC power grid containing a high proportion of new energy, and the use of MMC can provide certain reactive power support for the sending end, which is suitable for large-scale new energy cross-regional power transmission. However, the research on MMC-CLCC hybrid direct current system is still in its infancy, and the existing technology cannot consider the optimization of the receiving end AC voltage support and the direct current power transmission capacity. SUMMARY

[0004] The present application relates to the technical field of high-voltage direct current transmission, in particular to a MMC-CLCC novel direct current transmission system coordinated control method considering voltage support and direct current power transmission of the receiving end.

[0005] The above-mentioned purpose of the present application is achieved by the following technical scheme:

[0006] The MMC-CLCC novel direct current transmission system coordinated control method comprises the following steps:

[0007] Step S1: establishing a topology architecture of the MMC-CLCC hybrid DC power transmission system, determining an optimal proportion of full-bridge sub-modules and half-bridge sub-modules according to the proportion of the full-bridge sub-modules and a DC voltage regulation range of the MMC, and determining a control strategy of the MMC-CLCC hybrid DC power transmission system in a steady state operation;

[0008] Step S2: analyzing a dynamic process of the sending and receiving converters under a fault of the receiving end AC system, and clarifying a power characteristic of the CLCC;

[0009] Step S3: designing a calculation process of DC current, active power and reactive power operation limits of the inverter station under multiple condition constraints, and determining a safe operation region of the inverter station according to the process;

[0010] Step S4: realizing a consideration of the AC voltage support of the receiving end and the maximum transmission of the DC power during the fault of the receiving end by using a dynamic reactive power control of the CLCC based on a reactive power sensitivity constraint and a coordination of the MMC through a maximum power instruction.

[0011] Preferably, the topology architecture of the MMC-CLCC hybrid DC power transmission system in step S1 comprises a rectifying side and an inverting side, wherein the rectifying side adopts two full- and half-bridge hybrid MMC converters in series to form a high- and low-voltage valve group, and the inverting side adopts a form of double 12-pulse CLCC in series; the proportion of the full- and half-bridge sub-modules of the MMC is determined according to a DC voltage regulation range of the MMC, and specifically:

[0012] A definition formula of a modulation ratio m of the MMC in a steady state is:

[0013] (1)

[0014] In the formula, U dcR is a DC outlet voltage of the rectifying side MMC, u diffj.peak is a fitting AC voltage peak value of a j-phase virtual equipotential point, and a, b and c represent three phases a, b and c of a three-phase system. In a steady state operation, the modulation ratio of the system is less than 1, and with a decrease of the DC voltage, the modulation ratio of the system in the steady state will always increase. Under the constraint of the modulation ratio in the steady state, the DC component of the half-bridge sub-module bridge arm voltage cannot be lower than a lower limit value, while the full-bridge sub-module can output negative voltage, thereby expanding the adjustable range of the DC voltage of the MMC.

[0015] Each bridge arm is composed of N sub-modules, and the number of various sub-modules has the following relationship by ignoring the bridge arm resistance and reactance voltage drop:

[0016] (2)

[0017] In the formula, N FB is the number of the full-bridge sub-modules, N HB is the number of the half-bridge sub-modules, U dcmin is a per-unit value of the minimum DC voltage, and UdcR U is the rectifier-side MMC DC output voltage, sm U is the submodule capacitor voltage. From the above equation, it can be seen that the total number of submodules is only related to the modulation ratio m at steady state, and the number of half-bridge submodules is only related to the per-unit value of the minimum DC voltage U dcmin . The number of full-bridge submodules increases with the increase of the modulation ratio m at steady state or the decrease of the per-unit value of the minimum DC voltage U dcmin . When the configuration ratio of the half-bridge submodules to the full-bridge submodules of each phase of the MMC is 1:1, the DC voltage of the MMC can meet the change of the DC voltage of the inverter side, that is, the regulation range is 0~1.0p.u.

[0018] The steady-state control strategy of the MMC-CLCC hybrid DC power transmission system includes MMC basic control and CLCC basic control. The d-axis control of the rectifier-side MMC AC side adopts stator submodule capacitor voltage control to avoid submodule capacitor voltage over-limiting in transient state, and the q-axis control adopts fixed AC voltage control to provide certain reactive power support to the sending end. The rectifier-side control system realizes fixed active power control of the rectifier side through the control mode of the number of submodules switched; the inverter-side CLCC control is similar to the traditional LCC control strategy, and the main control links include fixed DC voltage control link, maximum trigger angle control link and low-voltage current limiting link, and the difference lies in that the CLCC will not have commutation failure, and the inverter side does not need to configure a commutation failure prediction link and a fixed turn-off angle control link to resist commutation failure.

[0019] Preferably, the analysis of the dynamic process of the system under the AC fault of the receiving end in step S2 is used to determine the power operating characteristics of the CLCC, wherein the dynamic process is that after the fault of the receiving end power grid, the AC voltage drop leads to the decrease of the DC voltage, the CLCC switches from the fixed voltage control mode to the maximum trigger angle control, and the trigger angle is increased to increase the DC voltage of the inverter side. When a three-phase short-circuit fault occurs in the near area of the AC bus of the inverter station, the DC voltage decreases greatly, and the CLCC cannot maintain the DC voltage of the inverter side at the set value even if the turn-off angle is reduced. If the fault is more serious, the potential in the CLCC will drop suddenly, and the DC current will rise suddenly, but due to the existence of the line inductance and the smoothing reactor, the DC current will not rise step by step, but there will be a transient process. The DC transmission power of the MMC increases with the increase of the DC current, while the active power absorbed by the AC side does not change, and the AC side and the DC side of the rectifier-side MMC produce unbalanced power. The discharge energy of the DC side submodule capacitor is greater than the charging energy of the submodule capacitor, the submodule voltage decreases, and under the action of the d-axis stator submodule voltage on the AC side, the d-axis current increases, so as to keep the submodule capacitor voltage or energy unchanged. The DC side outer ring fixed active power output DC current reference value reaches the upper limit, at which time the MMC works in the constant current state. The MMC reduces the DC modulation ratio m dcThe DC component of the bridge arm voltage is reduced by reducing the number of sub-modules, so that the stability of the DC current is maintained when the receiving end power grid fails.

[0020] The CLCC power operating characteristics in step S2 include the active and reactive power variation with the turn-off angle and DC current at different AC voltage levels.

[0021] The CLCC power characteristic model expression is:

[0022] (3)

[0023] In the formula, P CLCC is the active power, Q CLCC is the reactive power, U acI is the AC voltage at the inverter side, γ is the turn-off angle, I dc is the DC current, N is the number of six-pulse converter bridges, X r is the commutation reactance, and U dcI0 is the CLCC no-load DC voltage.

[0024] The partial derivatives of the active power P CLCC and the reactive power Q CLCC with respect to the DC current I dc and the turn-off angle γ are taken, and the active power and the reactive power of the CLCC converter at different AC voltages are obtained as functions of the DC current and the turn-off angle:

[0025] (4)

[0026] Wherein:

[0027] (5)

[0028] Preferably, in step S3, the DC current operating constraints subject to multiple conditions include:

[0029] (6)

[0030] In the formula, γ min is the minimum turn-off angle, I γmin is the DC current value subject to the minimum turn-off angle, U dcI is the DC voltage at the inverter side, I μmax is the DC current value subject to the maximum commutation overlap angle, I dcmin is the minimum DC current to avoid DC current discontinuity, and I hmax is the maximum DC current subject to the temperature rise constraint.

[0031] The active and reactive power delivered by the CLCC converter station at the inverter side to the AC system is as follows:

[0032] (7)

[0033] where P ac is the active power delivered to the AC system by the CLCC converter station, Q ac is the reactive power delivered to the AC system by the CLCC converter station, B C is the equivalent susceptance of the reactive power compensation and filter devices of the inverter station, and k represents the rated transformer ratio.

[0034] The calculation process of the DC current, active power and reactive power operating limits in step S3 is as follows:

[0035] First, the specific parameters of the model are input, including the number of six-pulse converter bridge groups N, commutation reactance X r , equivalent susceptance B C of the reactive power compensation and filter devices; second, the initial value of the AC bus voltage U acI is 0.1 p.u. and the initial value of the DC current I dc is 0; then, the inverter DC voltage or turn-off angle is determined according to the control mode, and the inverter-side commutation overlap angle μ is calculated according to the formula; when I dc < 0.1 p.u., I dcmin is taken as 0.1 p.u. and the minimum active power P acmin and the minimum reactive power Q acmin delivered to the AC system are calculated; if I dc > 0.1 p.u., it is determined whether it is within the limit of the commutation overlap angle; if yes, I dc is I dcmax at this time, otherwise, the DC current is increased to less than the maximum limit I dclim and taken as the initial value of I dc for the next cycle; if the maximum limit of the DC current has been reached at this time, I dcmax = I dclim , and the maximum active power P acmax and the maximum reactive power Q acmax delivered to the AC system are calculated; finally, U acI is increased and the above process is repeated until U acI = 1.0 p.u.

[0036] The safe operating domain of the inverter station includes the DC current operating domain, the active power and reactive power operating domain under different AC voltage levels.

[0037] Preferably, the control strategy in step S4 is characterized in that, compared with the original control, after the receiving-end power grid of the MMC-CLCC hybrid DC power transmission system fails, the CLCC turn-off angle of the inverter side is further reduced according to the reactive power sensitivity constraint condition, so as to reduce the reactive power consumed by the converter station and reduce the influence of the DC current on the reactive power; meanwhile, the inverter side calculates the DC current value under the maximum transmission active power according to the amplitude of the AC voltage drop, the turn-off angle and the active power operation interval, and transmits the DC current value to the sending end through communication as the DC current reference value of the inner ring current controller of the sending-end MMC, so as to calculate the DC modulation ratio m of the MMC dc of the receiving end will be significantly reduced, and the sending end relies on the charging and discharging of the sub-module capacitor to reduce the DC voltage of the rectifier side by changing the DC voltage reference value of the outer ring of the MMC, which is faster than the active current limiting method of the MMC based on the bridge arm voltage control in response to the fault, and the sending end uses the hybrid bridge topology to expand the adjustment range of the DC voltage of the MMC, which ensures the DC power transmission capacity as much as possible under the premise of preventing the AC voltage of the MMC from overshooting.

[0038] The reactive power sensitivity constraint condition is:

[0039] (8)

[0040] In the formula, S Q is the reactive power sensitivity, I dc1 represents the current operating value meeting the reactive power sensitivity constraint condition, γ Qref is the turn-off angle reference value meeting the reactive power sensitivity constraint condition, S Qset is the set value of the reactive power sensitivity, and K lim is the reactive power sensitivity change constraint. In the first term, it is indicated that the reactive power sensitivity corresponding to the selected turn-off angle reference value should be not less than the set value, that is, the operating point should be in the upper right of the set reactive power sensitivity corresponding curve; in the second term, it is indicated that the change of the DC current has little influence on the reactive power sensitivity corresponding to the selected turn-off angle reference value, that is, the operating point is located near the reactive power sensitivity curve and on the right side of the inflection point of the curve.

[0041] Regarding the selection of the reactive power sensitivity, the requirements of the receiving-end AC voltage and active power are taken into account, and the strength of the receiving-end power grid and other factors are also related. The minimum value of the turn-off angle meeting the reactive power sensitivity constraint condition can be selected as the turn-off angle reference value of the maximum trigger angle control according to the above formula. In addition, the finally output turn-off angle reference value should meet the turn-off angle range corresponding to the reactive power operation region of the inverter side, and the determination method is shown in the following formula:

[0042] (9)

[0043] In the formula, Q lim γ is the reactive power limit value of the inverter station under the safe operation domain to the alternating current system, lim K is the off-angle limit value under the safe operation domain, Q K is the intermediate variable under the reactive power limit value, and k represents the rated ratio of the transformer.

[0044] At the same time, the rectifier side MMC updates the input signal of the inner loop control of the DC side in real time according to the safe operation domain obtained in step S3, so as to maximize the power support during the fault and complete the coordination with the control strategy of the receiving end converter.

[0045] Another object of the application is to provide an MMC-CLCC hybrid DC power transmission system control system for improving the sending and receiving end support capability, comprising:

[0046] The data acquisition module is used for collecting the sending and receiving end alternating current voltage, DC current, DC voltage and power data in real time.

[0047] The analysis and decision module is used for calculating the power operation limit and reactive power sensitivity, generating the off-angle adjustment instruction and the DC current reference value.

[0048] The execution control module includes a CLCC trigger angle control unit and an MMC DC modulation ratio adjustment unit, which receives the instruction and performs coordinated control.

[0049] The analysis and decision module includes an operation range calculation unit, which determines the DC current and active and reactive power safe operation domain through the operation limit calculation process; a sensitivity analysis unit, which generates the initial off-angle reference signal through the reactive power sensitivity formula and the reactive power sensitivity constraint condition; and a coordinated control unit, which generates the CLCC off-angle instruction and the MMC DC control loop inner loop DC current instruction parameter.

[0050] The application has the following beneficial effects:

[0051] 1. Improve system stability: through the coordinated control strategy, effective support is provided for the alternating current system during the fault at the receiving end, and the alternating current system voltage recovery is accelerated after the fault is removed. 2. Ensure power transmission: considering the reactive power support and active power transmission, the DC power transmission capacity is greatly improved during the fault, and the DC power recovery is more rapid after the fault is removed. 3. Strong adaptability: applicable to different fault severity and power grid structure, and the control parameters can be dynamically optimized according to the actual working conditions. BRIEF DESCRIPTION OF DRAWINGS

[0052] The drawings described herein are used to provide further understanding of the application, constitute a part of the application, and the illustrative examples and their descriptions of the application are used to explain the application, and do not constitute an improper limitation on the application.

[0053] Figure 1 Flow chart of the MMC-CLCC hybrid DC power transmission system coordination control method of the present application;

[0054] Figure 2 Topological structure diagram of the ±800 kV MMC-CLCC hybrid DC power transmission system of the present application;

[0055] Figure 3 Basic topological structure diagram of the MMC converter of the present application;

[0056] Figure 4 Basic topological structure diagram of the CLCC converter of the present application;

[0057] Figure 5 Detailed structure diagram of the converter valve V1 of the present application; Figure 4 Detailed structure diagram of the converter valve V1 of the present application;

[0058] Figure 6 CLCC switch timing diagram when V1 commutates to V3 of the present application;

[0059] Figure 7 Basic control block diagram of the MMC converter of the present application;

[0060] Figure 8 Basic control block diagram of the CLCC converter of the present application;

[0061] Figure 9 Flow chart of the DC current and power operating range calculation of the present application;

[0062] Figure 10 DC current safe operating domain of the present application;

[0063] Figure 11 Inverter-side active power safe operating region of the present application;

[0064] Figure 12 Inverter-side reactive power safe operating region of the present application;

[0065] Figure 13 , Figure 14 Working region of the turn-off angle and DC current under different sensitivity selection conditions of the present application;

[0066] Figure 15 Control block diagram of the MMC-CLCC hybrid DC power transmission system of the present application, which takes into account the AC voltage support at the receiving end and DC power transmission;

[0067] Figure 16 Simulation result diagram of the trigger angle when the AC bus voltage at the receiving end converter station of the present application drops to 0.7 p.u.

[0068] Figure 17A simulation result diagram of the effective value of the receiving end AC voltage when the AC bus voltage of the receiving end converter station of the application drops to 0.7 p.u.

[0069] Figure 18 A simulation result diagram of the DC transmission power when the AC bus voltage of the receiving end converter station of the application drops to 0.7 p.u.

[0070] Figure 19 A simulation result diagram of the CLCC absorbed reactive power when the AC bus voltage of the receiving end converter station of the application drops to 0.7 p.u.

[0071] Figure 20 A DC power transmission system control system of the application.

[0072] In the figure: 11, data acquisition module; 12, analysis and decision module; 121, operating range calculation unit; 122, sensitivity analysis unit; 123, cooperative control unit; 13, execution control module. DETAILED DESCRIPTION

[0073] The technical solutions in the embodiments of the application will be described clearly and completely below with reference to the drawings. Obviously, the described embodiments are only part of the embodiments of the application, rather than all the embodiments. Based on the embodiments in the application, all other embodiments obtained by those skilled in the art without creative work fall within the protection scope of the application. In order to make the above-mentioned purposes, features and advantages of the application more obvious and easy to understand, the application will be further described in detail below with reference to the drawings and specific embodiments.

[0074] Noun explanation:

[0075] MMC: modular multilevel converter; CLCC: controllable commutation converter; FHMMC: full half-bridge hybrid modular multilevel converter; HBSM: half-bridge sub-module; FBSM: full-bridge sub-module; MMC-CLCC: modular multilevel converter and controllable commutation converter.

[0076] Reference Figures 1 to 20As shown, the MMC-CLCC hybrid DC power transmission system coordinated control method of the application considering voltage support and DC power transmission at the receiving end can provide support for the receiving end AC system during grid failure, while maximizing the DC power transmission capacity, and is suitable for large-scale new energy cross-regional transmission scenarios. The method establishes the MMC-CLCC hybrid DC power transmission system topology architecture, determines the optimal proportion of full-bridge and half-bridge sub-modules according to the system DC voltage requirements, and determines the control strategy during steady-state operation; analyzes the dynamic process of the system under receiving end AC fault, and clarifies the power operating characteristics of the CLCC; designs a calculation process of the DC current, active and reactive power operating limits of the inverter station under multiple condition constraints, and constructs the safe operating region of the inverter station; proposes a CLCC off-angle regulation strategy based on reactive power sensitivity constraints and an MMC coordinated control strategy based on maximum power transmission. In addition, the application also provides an MMC-CLCC hybrid DC power transmission system control system for improving the sending and receiving end support capability, which comprises a data acquisition module, an analysis and decision module and an execution control module; wherein the analysis and decision module comprises an operating range calculation unit, a sensitivity analysis unit and a collaborative control unit. The application solves the problem of coordinated optimization of voltage recovery and DC power transmission under receiving end AC fault, has good effect on improving the stability of the AC system and ensuring power transmission, and is suitable for different fault severity and power grid structure, and can dynamically optimize the control parameters according to the actual working conditions.

[0077] Reference Figure 1 As shown, the MMC-CLCC hybrid DC power transmission system coordinated control method of the application comprises the following steps:

[0078] Step S1: Establishing the MMC-CLCC hybrid DC power transmission system topology architecture, determining the proportion of full-bridge and half-bridge sub-modules according to the proportion of full-bridge sub-modules and the MMC DC voltage regulation interval, and determining the control strategy of the MMC-CLCC hybrid DC power transmission system during steady-state operation.

[0079] S11: MMC-CLCC hybrid DC power transmission system topology

[0080] The MMC-CLCC hybrid DC power transmission system adopts true bipolar wiring mode, and its topology is as shown in Figure 2 Taking the positive electrode as an example, the rectifier side adopts the form of double 400kV MMC converters in series, forming a +800kV MMC converter station; the inverter side also adopts the form of double 12-pulse CLCC in series, and each 12-pulse unit converter adopts a three-winding structure, one valve-side winding is star-connected and the other is angle-connected, relying on the phase difference between the two to provide a commutation voltage for the thyristor.

[0081] When the rectifier side adopts half-bridge sub-modules, serious faults of the receiving end grid can cause the DC output voltage of the MMC to fail to follow the change of the inverter side DC voltage, thereby causing the DC current to overcurrent, and the MMC also faces the risk of over-modulation. Therefore, the rectifier side MMC adopts a hybrid structure of full-bridge sub-modules and half-bridge sub-modules, the purpose of which is to ensure that the DC output voltage can follow the change of the inverter side DC voltage when the receiving end grid fails. The topology structure of the MMC is shown in Figure 3 , in which: L arm is the bridge arm inductance, L g is the equivalent inductance on the AC side. In normal operation, the MMC realizes the control of the active power and the reactive power on the AC side by changing the amplitude and phase of the fitting voltage u diff . By locking the grid phase through a phase-locked loop, u s and the d-axis are made to coincide, thereby realizing the d-axis control of the active power ring and the q-axis control of the reactive power ring, i.e., the decoupling control of the d-axis and the q-axis.

[0082] The modulation ratio of the MMC in the steady state can be defined as:

[0083] (1)

[0084] In the formula, u diffj.peak is the peak value of the fitting AC voltage at the j-phase virtual equipotential point. In the steady state operation, the modulation ratio of the system is less than 1, and with the decrease of the DC voltage, the modulation ratio of the system will always increase. Under the constraint of the modulation ratio in the steady state, the DC component of the bridge arm voltage of the half-bridge sub-module cannot be lower than the lower limit value, while the full-bridge sub-module can output negative voltage, thereby expanding the adjustable range of the DC voltage of the MMC.

[0085] It is assumed that each bridge arm is composed of N sub-modules, the number of full-bridge sub-modules is N FB , and the number of half-bridge sub-modules is N HB . Ignoring the bridge arm resistance and reactance voltage drop, the number of various sub-modules should satisfy the following relationship:

[0086] (2)

[0087] In the formula, U dcmin is the per-unit value of the minimum DC voltage, U dcR is the DC output voltage of the rectifier side MMC, and U sm is the capacitor voltage of the sub-module. It can be seen from the above formula that the total number of sub-modules is only related to the modulation ratio m in the steady state, the number of half-bridge sub-modules is only related to the per-unit value U dcmin of the minimum DC voltage, and the number of full-bridge sub-modules increases with the increase of the modulation ratio m in the steady state or the per-unit value U dcmin of the minimum DC voltage.The voltage increases as the inverter voltage decreases. When the configuration ratio of each phase half-bridge submodule to the full-bridge submodule of the MMC is 1:1, the MMC DC voltage can meet the requirement of following the change of the inverter side DC voltage, that is, the adjustment range is 0~1.0pu.

[0088] The controllable phase-commutation converter is based on the working principle of DC circuit breakers and borrows from LCC. Each bridge arm is composed of a main branch and an auxiliary branch connected in parallel. Figure 4 and Figure 5 The topology is a 6-pulse unit CLCC, with the main branch being a traditional thyristor converter valve V. 11 Series low-voltage high-current IGBT valve V 12 Composition, auxiliary branch is high-pressure IGBT valve V 13 Series high-voltage thyristor valve V 14 In addition, thyristors and IGBTs are connected in parallel with RC damping and RCD damping circuits respectively to achieve dynamic voltage equalization. Surge arresters of different specifications are connected in parallel with power electronic devices and bridge arms to prevent overvoltage damage to equipment.

[0089] The CLCC switching timing sequence during V1 to V3 commutation is as follows: Figure 6 As shown. Under normal commutation conditions, when the main branch current decreases to a certain set value, V 12 Off, V 13 and V 14 When the circuit is turned on, the current is transferred from the main branch to the auxiliary branch. After the current in the main branch crosses zero, the thyristor V in the main branch... 11 Under pressure, it enters the recovery period. Within the specified delay... Afterwards, V 13 After completing zero-current turn-off, the bridge arm regains its blocking capability, and natural commutation is completed.

[0090] After a fault occurs in the receiving-end power grid, the CLCC operates in forced commutation mode. The commutation current cannot be reduced to the set value. When the main branch current-carrying time exceeds the set value, V... 12 Forced shutdown, V 13 and V 14 When the circuit is turned on, the main branch current is forced to transfer to the auxiliary branch, waiting for V... 11 V after restoring blocking ability 13 When the circuit is turned off, the current is transferred to the parallel surge arrester. The operating voltage established by the surge arrester can increase the commutation voltage, complete the forced commutation between the bridge arms, and avoid commutation failure.

[0091] S12: Basic Control Strategy for MMC-CLCC Hybrid DC Transmission System

[0092] For a single-phase MMC system at the sending end, define m dc m ac Let the modulation ratios on the DC and AC sides of the MMC be respectively the modulation ratios on the rectifier side and the AC side. Then, the AC and DC side voltages of the MMC can be expressed as:

[0093] (3)

[0094] where u diffj represents the differential-mode voltage of the upper and lower bridge arms of MMC phase j, u comj represents the common-mode voltage of the upper and lower bridge arms of MMC phase j, and ωt+φ represents the phase angle of the common-mode voltage of MMC.

[0095] By introducing the modulation ratios of the AC side and the DC side of MMC, the AC outlet voltage and the DC outlet voltage can be decoupled, and then the rectifier-side MMC AC outlet voltage and the DC voltage during the fault of the receiving end can be adjusted separately, which adds one more control dimension compared to the original control. The upper bridge arm voltage u atop and the lower bridge arm voltage u abtm of phase a before and after adjustment can be represented as:

[0096] (4)

[0097] The basic control structure of the rectifier-side MMC is shown in Figure 7 . In the stable operation state, the d-axis control of the AC side of the rectifier-side MMC adopts the constant voltage control or the constant capacitor voltage control of the sub-module to avoid the over-limit of the capacitor voltage of the sub-module in the transient state, and the q-axis control adopts the constant AC voltage control to provide a certain reactive power support for the sending end. The control system of the DC side realizes the constant active power control of the rectifier side by the control mode of the number of sub-modules.

[0098] The control block diagram of the inverter-side CLCC is shown in Figure 8 . Similar to the traditional LCC control strategy, the main control links include the constant DC voltage control link, the maximum trigger angle control link and the low-voltage current limiting link, and the difference lies in that the CLCC will not occur commutation failure, and the inverter side does not need to configure the commutation failure prediction link and the constant turn-off angle control link to resist the commutation failure.

[0099] Step S2: Analyzing the dynamic process of the sending and receiving converter under the fault of the receiving end AC system, and clarifying the power characteristics of the CLCC.

[0100] S21: System dynamic process under the fault of the receiving end AC system

[0101] After the failure of the receiving end power grid, the AC voltage drop leads to the decrease of DC voltage, and the CLCC switches from constant voltage control mode to maximum trigger angle control, increasing the trigger angle to improve the DC voltage at the inverter side. When a three-phase short-circuit fault occurs near the AC bus of the inverter station, the DC voltage decreases greatly, and the CLCC cannot maintain the DC voltage at the inverter side at the set value even if the turn-off angle is reduced. If the fault is more serious, it will lead to a sudden drop of the potential in the CLCC and a sudden rise of the DC current. However, due to the existence of line inductance and smoothing reactor, the DC current will not rise step by step, but there will be a transient process, during which the DC current can be expressed as:

[0102] (5)

[0103] In the formula, E dcI represents the potential in the CLCC at the inverter side, R l is the DC line resistance value, L l is the DC circuit inductance value, L ’ r is the equivalent commutation inductance of the CLCC. e represents the natural constant, which is about 2.71828. t represents time.

[0104] The voltage at the DC outlet of the rectifier side MMC is established by the sub-module capacitor voltage, and the control of its AC and DC sides is essentially realized by controlling the charging and discharging of the sub-module capacitor. The relationship between unbalanced power and sub-module capacitor energy is as follows:

[0105] (6)

[0106] In the formula, is the unbalanced power of the AC and DC sides of the MMC, P ac is the active power absorbed by the MMC from the AC side, P dc is the active power transmitted by the MMC to the DC side, is the difference of the change of the sub-module capacitor energy, which can be expressed as follows according to the expression of the MMC sub-module capacitor energy:

[0107] (7)

[0108] In the formula, C eq is the equivalent capacitance of the MMC converter, C sm is the sub-module capacitor. The equivalent capacitance time constant τ c is defined as follows. Its physical meaning is that when the AC side injected power is not considered, the DC capacitor is charged with a constant current I dcN , and the time required for the DC voltage to decrease from U dcN to 0.

[0109] (8)

[0110] wherein U dcN is the rated DC voltage; S N is the rated capacity of the converter station, is the charging and discharging time constant of the MMC sub-module capacitor.

[0111] At the moment of the fault of the receiving-end power grid, due to the instantaneous drop of the DC voltage at the inverter side, the DC current rapidly increases, the DC transmission power of the MMC increases with the increase of the DC current, while the active power absorbed at the AC side remains unchanged, and the AC side and the DC side of the rectifier-side MMC generate unbalanced power. The discharging energy of the DC-side sub-module capacitor is greater than the charging energy of the sub-module capacitor, the sub-module voltage decreases, under the action of the d-axis stator module voltage at the AC side, the d-axis current increases, and the sub-module capacitor voltage / energy remains unchanged. The DC current reference value of the outer ring active power output at the DC side reaches the upper limit, at this time, the MMC works in the constant current state. The MMC reduces the DC component of the bridge arm voltage by reducing the DC modulation ratio (reducing the number of sub-modules put into operation), thereby maintaining the stability of the DC current when the receiving-end power grid fails.

[0112] S22: Power operating characteristics of the CLCC

[0113] The active power transmitted by the CLCC and the reactive power consumed are related as follows:

[0114] (9)

[0115] wherein P CLCC is the active power, Q CLCC is the reactive power, U acI is the AC voltage at the inverter side, γ is the turn-off angle, I dc is the DC current, N is the number of six-pulse converter bridge groups, X r is the commutation reactance, U dcI0 is the no-load DC voltage of the CLCC.

[0116] The partial derivatives of P CLCC and Q CLCC with respect to I dc and γ are obtained as follows:

[0117] (10)

[0118] wherein:

[0119] (11)

[0120] From the above, the partial derivative of active power with respect to DC current is greater than 0 and the partial derivative of active power with respect to turn-off angle is less than 0 under different AC voltages and DC currents, which indicates that the active power of the CLCC is positively correlated with the DC current and negatively correlated with the turn-off angle, and the active power is more sensitive to the change of the turn-off angle when the AC voltage is maintained at a high level. Similarly, the partial derivatives of reactive power with respect to DC current and turn-off angle are both greater than 0 under different AC voltages and DC currents, which indicates that the reactive power consumed by the CLCC is positively correlated with the DC current and the turn-off angle. Reducing γ during the AC fault at the receiving end is beneficial to the transmission of active power, and the reactive power consumed by the CLCC will also be reduced, which is beneficial to the recovery of the DC voltage at the inverter side. acI ; increasing I dc is beneficial to the transmission of active power, but will increase the reactive power consumed by the CLCC, which is not conducive to the recovery of the DC voltage at the inverter side. acI .

[0121] Step S3: design a calculation process of the DC current, active power and reactive power operating limits of the CLCC converter station under multi-condition constraints, and construct the safe operating region of the inverter station according to the calculation process.

[0122] S31: multi-condition constraints of the CLCC converter station

[0123] The CLCC needs to meet the operating conditions of the turn-off angle, commutation angle, maximum DC current and minimum DC current during normal operation, and the value range of I dc during normal operation of the CLCC can be calculated through these operating conditions, and then the operating range of active power and reactive power can be obtained.

[0124] 1) turn-off angle limit of the CLCC

[0125] The minimum turn-off angle corresponds to the minimum time required for the thyristor to restore the positive blocking capability from the current zero crossing time, and if the turn-off angle γ min is less than the minimum turn-off angle γ min , it is judged that the system has failed to commutate. In the traditional LCC converter, the turn-off angle γ is defined as an electrical angle, which corresponds to the time when the valve is closed, i.e. the positive zero crossing point of the line-to-line voltage on the AC converter bus. The commutation voltage of the CLCC is the sum of the line voltage and the arrester operating voltage, and the actual turn-off angle is not measured by the traditional measurement method, so the minimum turn-off angle γ min of the CLCC can be reduced to 0 degrees.

[0126] Therefore, the DC current operating range limited by the turn-off angle is:

[0127] (12)

[0128] In the formula, I γmin is the DC current value constrained by the minimum turn-off angle, and U dcI is the DC voltage at the inverter side.

[0129] 2) Commutation overlap angle limitation

[0130] The 12-pulse converter operates in 4-5 mode (i.e. 4 valves conducting in each bridge during non-commutation period and 5 valves conducting during commutation period) in normal operation, and the commutation angle μ < 30°. When the AC bus voltage decreases or the DC current increases, the commutation angle of the 12-pulse converter increases. When the commutation angle μ = 30°, 5 valves in each bridge conduct simultaneously, i.e. the commutation of two six-pulse valve groups is alternately performed, and the converter operates in 5 mode; when the commutation angle μ > 30°, the commutation of two valve groups occurs simultaneously, i.e. when the commutation of a pair of valves in one valve group has not ended, a pair of valves in the other bridge starts commutation, and 6 valves conduct simultaneously, which is a 5-6 fault mode. When the commutation angle is greater than 30°, if there is a coupling inductance between the two bridges of the 12-pulse converter, the DC voltage will decrease, the AC bus voltage will be distorted, and additional commutation teeth will be generated in the valve voltage, which is not conducive to the stable operation of the inverter. Therefore, the commutation angle of the present application is limited to μ < 30°.

[0131] For the 12-pulse converter, the quasi-steady state formula is as follows:

[0132] (13)

[0133] Therefore, the range of DC current limited by the commutation angle of the inverter is:

[0134] (14)

[0135] In the formula, I μmax is the DC current value limited by the maximum commutation overlap angle.

[0136] 3) DC current limitation

[0137] Excessive DC current will cause the junction temperature of the thyristor and the temperature of the winding of the converter transformer to exceed the specified allowable value. Therefore, the DC current cannot exceed the above-mentioned allowable range. The second-level overload capacity of the thyristor of the high-voltage DC project at home and abroad is generally set to 1.4 times the rated current, i.e. I hmax ≤ 1.4pu, and the specific value depends on the design standard of the converter valve.

[0138] On the other hand, when the DC current is low, the fluctuation of the current will cause the DC current to be interrupted, and the current change rate is very high at the moment of current interruption, which will induce overvoltage on the transformer winding and the DC reactor. In order to prevent current interruption, the minimum current limit value is generally I dcmin = 0.1p.u..

[0139] Therefore, the DC current operation should satisfy:

[0140] (15)

[0141] I dcmin =0.1p.u. hmax =0.1p.u.

[0142] S32: Calculation process of DC current and power operation limits of CLCC converter station

[0143] The present application takes AC voltage as independent variable, and calculates operation limits of active power and reactive power of the converter station according to the limiting condition of DC current I dc of step S31, and the calculation process chart is shown in Figure 9 .

[0144] In the operation limit calculation step, first, the specific parameters of the model (the number of six-pulse converter bridge groups N, commutation reactance X r , equivalent susceptance B C of reactive power compensation and filtering device, etc.) are inputted; second, AC bus voltage U acI (initial value is 0.1 p.u.) and DC current I dc (initial value is 0) are inputted; then, the inverter DC voltage or turn-off angle is determined according to the control mode, and the commutation overlap angle μ of the inverter side is calculated according to formula (13); when I dc <0.1 p.u., I dcmin takes 0.1 p.u., and the minimum active power P acmin and the minimum reactive power Q acmin transmitted to the AC system are calculated; if I dc >0.1 p.u., it is judged whether it is within the limit of commutation overlap angle, if yes, I dc is I dcmax at this time, otherwise, the DC current is increased (which is less than the maximum limit of DC current I dclim ) and taken as the initial value of I dc of next cycle, if the maximum limit of DC current is reached at this time, I dcmax =I dclim , and the maximum active power P acmax and the maximum reactive power Q acmax transmitted to the AC system are calculated; finally, U acI is increased, and the above process is repeated until U acI =1.0 p.u.

[0145] S33: Safe operation domain of CLCC converter station

[0146] The parameters in table 1 are brought into the system, and the safe operation domain of the CLCC converter station can be plotted according to the above process, as shown in Figures 10 to 12 .

[0147] Table 1 Simulation parameters of ±800 kV MMC-CLCC hybrid DC transmission system

[0148]

[0149] As shown in Figure 10 , with the increase of U acI , I dc is first limited by the minimum DC current, then by the maximum commutation angle, and finally by the maximum DC current, and different off angles γ will cause the maximum limit of DC current to change, and the specific trend is that the I dc curve moves downward with the decrease of off angle γ.

[0150] Figure 11 and Figure 12 are the operating ranges of active power and reactive power at the inverter side with the change of AC voltage, U acI = 0.055, 0.776 p.u. are the critical points of the inverter side commutation overlap angle limit and minimum DC current limit, maximum DC current limit respectively, when U acI > 0.776 p.u., I dcmin is limited by the maximum DC current I dclim = 1.4 p.u., with the increase of U acI , the increase of the upper limit of active power P acmax becomes slow, while the lower limit of reactive power Q acmin presents a decreasing trend; when 0 < U acI < 0.776 p.u., I dcmin is limited by the maximum commutation angle μ max = 30°, with the increase of U acI , the upper limit of active power P acmax and the lower limit of reactive power Q acmin both present an increasing trend, and the rate is always increasing; in this process, the lower limit of active power and the upper limit of reactive power are basically unchanged.

[0151] As can be seen from the operating ranges of different off angles in Figures 10 to 12 , the decrease of off angle at the inverter side will make the off angle limit and the maximum and minimum limit points of DC current move to the right, and at the same time, the operating range of DC current and active power also becomes narrower. While the maximum and minimum reactive power operating curves move downward, compared with the change of active power operating range, the change of reactive power operating range at the inverter side is greater, which shows that the reactive power at the inverter side is more sensitive to the change of off angle.

[0152] Step S4: dynamic reactive power control of CLCC based on reactive power sensitivity constraint, and coordination of MMC through maximum power instruction to realize both support of AC voltage at receiving end during fault and maximum transmission of DC power, including a CLCC turn-off angle adjustment strategy based on reactive power sensitivity constraint and a MMC coordination strategy based on maximum power transmission.

[0153] S41: CLCC reactive power control strategy based on reactive power sensitivity constraint

[0154] The traditional LCC control is provided with a commutation failure prediction link, which can reduce the trigger angle according to the fault degree of the receiving end, thereby increasing the turn-off angle and reducing the risk of commutation failure. However, the reduction of the trigger angle inevitably leads to further reduction of the DC voltage. Although the DC current can be kept constant under the control of the rectifier side DC current, the DC transmission power drops sharply, which has an adverse effect on the weak system connected to the sending end. The CLCC does not have the problem of commutation failure, and the turn-off angle does not need to be increased to avoid commutation failure during the fault of the receiving end AC system, which prevents the further drop of the DC voltage caused by the commutation failure prediction link and supports the weak system connected to the sending and receiving stations.

[0155] In order to further exert the advantage of controllable commutation of CLCC and improve the support ability of CLCC to the sending and receiving end AC system, the turn-off angle can be further adjusted according to the reactive power exchange between the receiving end converter station and the AC system during the fault of the receiving end grid, so as to reduce the reactive power consumed by the converter valve during the fault and speed up the recovery of the AC system voltage.

[0156] In order to provide as much reactive power support as possible for the AC system during the AC fault, and to consider the recovery of reactive power and active power during the fault recovery, the present application proposes a CLCC reactive power control strategy based on reactive power sensitivity constraint. As shown in formula (16), on the basis of the traditional maximum trigger angle control, the reactive power sensitivity constraint is used to determine the turn-off angle reference value γ ref .

[0157] (16)

[0158] In the formula, γ ref is the turn-off angle reference value, d x is the relative inductive voltage drop of the converter, I dcref and I dcN are the DC current instruction value and the rated value respectively, U dcI0N is the inverter side no-load DC power rating, and K1 is the current deviation coefficient. For the solution of γ value, the influence of the active power recovery on the provision of reactive power support for the AC grid by the converter station during the fault and recovery of the receiving end is minimized, and α represents the trigger angle of the CLCC.

[0159] Definition of reactive power sensitivity S Q The expression is as follows:

[0160] (17)

[0161] From the above formula, the reactive power sensitivity is related to the AC voltage, the extinction angle and the DC current. Within a certain range, the greater the value of S Q indicates that the change of the reactive power is more coupled with the extinction angle, i.e. less related to the DC current. The operating region of the DC current and the extinction angle under a certain reactive power sensitivity is shown in FIG. 5 and FIG. 6. During the fault at the receiving end, the operating region of the extinction angle with low sensitivity of the reactive power to the DC current under different operating conditions can be found according to the requirement of the reactive power sensitivity and the relationship curve, and the increase of the DC current has the least impact on the consumed reactive power in this region. Figure 13 Figure 14 Based on the above-mentioned reactive power sensitivity variation curve, the constraint condition is as follows:

[0162]

[0163] (18)

[0164] In the formula, γ Qref is the reference value of the extinction angle that meets the constraint condition of the reactive power sensitivity, S Qset is the set value of the reactive power sensitivity, and K lim is the variation constraint of the reactive power sensitivity. The first term in the formula indicates that the reactive power sensitivity corresponding to the selected reference value of the extinction angle should not be less than the set value, i.e. the operating point should be on the right upper side of the corresponding curve of the set reactive power sensitivity; the second term in the formula indicates that the change of the DC current has little impact on the reactive power sensitivity corresponding to the selected reference value of the extinction angle, i.e. the operating point is near the curve of the reactive power sensitivity and on the right side of the inflection point of the curve.

[0165] Regarding the selection of the reactive power sensitivity, the requirements of the AC voltage and the active power at the receiving end should be considered, and the strength of the receiving end power grid and other factors are also related. In the present application, the maximum value near the curve is selected and approximately processed, and S Qset = 5U acI - 0.7. The minimum value of the extinction angle that meets the constraint condition of the reactive power sensitivity can be selected as the reference value of the extinction angle for the maximum trigger angle control according to the constraint formula of the reactive power sensitivity. In addition, the output reference value of the extinction angle should meet the range of the extinction angle corresponding to the reactive power operating region at the inverter side, and the determination method is shown in the following formula:

[0166] (19)

[0167] In the formula, Q lim ​​For the safe operation of the domain under the inverter station to the AC system transmission reactive power limit, γ lim For the safe operation of the domain under the off angle limit. K Q For the intermediate variable under the reactive power limit.

[0168] Normal operation, off angle instruction value is unchanged, when the detection of the receiving end fault, will off angle instruction switching for the off angle reference value based on the reactive power sensitivity constraints generated, will be corrected angle input to the maximum trigger angle generation link can get the trigger angle instruction value under the reactive instruction, compared to the original control, this control strategy can effectively improve the support of the receiving end power grid during the fault, and is conducive to the recovery of the receiving end power grid.

[0169] S42: MMC cooperative control strategy based on maximum active power transmission

[0170] From the analysis of step S41, the operating range of the DC current is related to the AC voltage on the inverter side and the off angle. By reducing the off angle of the CLCC based on the reactive power sensitivity constraint, the reactive power control is realized, the support of the AC voltage is ensured, and the influence of the change of the DC current on the reactive power is reduced. However, due to the constraint of the power operating interval, the DC current and the active power operating range change when the off angle changes. Therefore, the DC current corresponding to the maximum power operating point needs to be calculated in real time according to the DC current operating range to ensure the maximum transmission of the active power and the rapid recovery of the power during the receiving end fault.

[0171] S43: Control system design

[0172] Based on the analysis of steps S41 and S42, the control system as shown in Figure 15 Compared with the original control system, after the receiving end power grid of the MMC-CLCC hybrid DC power transmission system fails, the off angle of the CLCC on the inverter side is further reduced based on the reactive power sensitivity constraint condition, so as to reduce the reactive power consumed by the converter station and reduce the influence of the DC current on the reactive power. At the same time, the DC current value under the maximum transmission of the active power is calculated according to the amplitude of the AC voltage after the drop, the off angle and the active power operating interval, and is transmitted to the sending end through communication as the DC current reference value of the inner loop current controller of the sending end MMC, so as to calculate the DC modulation ratio m dc of the MMC. Since the receiving end adopts the reactive power sensitivity control strategy at this time, the influence of the change of the DC current on the reactive power of the receiving end will be significantly reduced due to the communication delay. Compared with changing the DC voltage reference value of the MMC outer loop by relying on the charging and discharging of the sub-module capacitor to reduce the DC voltage on the rectifier side, the MMC active current limiting method based on the bridge arm voltage control reduces the response speed of the rectifier side DC voltage to the fault, and the sending end adopts the hybrid bridge topology to expand the adjustment range of the DC voltage of the MMC. Under the premise of preventing the overmodulation of the AC voltage of the MMC, the DC power transmission capacity is ensured as much as possible.

[0173] Embodiment:

[0174] The example analysis provided by the embodiment is shown in Figure 11 and Figure 12 A MMC-CLCC hybrid DC power transmission system simulation platform shown in Figure 2 is established in PSCAD / EMTDC, and the platform related parameters are shown in Table 1.

[0175] The different control strategies of MMC-CLCC when the receiving end power grid fails are simulated and verified as follows:

[0176] Control strategy 1: the system adopts the original control strategy;

[0177] Control strategy 2: the system adopts the coordinated control strategy of the application.

[0178] In the simulation process, a three-phase short-circuit fault is applied to the receiving end power grid at 0.8s, and the duration is 0.1s. Different fault grounding inductances are used to simulate different severity of AC faults.

[0179] Figures 16 to 19 The simulation results when the receiving end converter station AC bus voltage drops to 0.7p.u. are shown in Figure 16 After the control strategy proposed in the application is adopted, the trigger angle instruction of the CLCC is increased by 3°~6°. Figure 17 and Figure 19 The dynamic performance of different control strategies about reactive power support and AC voltage recovery during the receiving end fault and the fault recovery is shown. In the case of receiving end AC fault, reducing the turn-off angle can reduce the reactive power consumption. Compared with control strategy 1, the reactive power support of control strategy 2 to the AC system is increased by about 489.26Mvar, the AC voltage is increased by about 3.7%, and the time to recover to the near rated level after the fault is cleared is shortened. Figure 18 The support effect of different control on monopolar DC power during the receiving end fault and the fault recovery is shown. It can be observed that during the fault, the DC power of control strategy 2 is stabilized at about 3349.99MW, and the value of control strategy 1 is 2530.75MW, which is increased by about 20.5%. Therefore, the coordinated control strategy provided by the application realizes the balanced support of AC voltage and DC power.

[0180] The application also provides an MMC-CLCC hybrid DC power transmission system control system for improving the sending and receiving end support capability, as shown in Figure 20 The DC power transmission system control system comprises:

[0181] A data acquisition module 11 is used to acquire the sending and receiving end AC voltage, DC current, DC voltage and power data in real time.

[0182] The analysis decision module 12 is used for calculating power operation limits, reactive power sensitivity, generating a shutdown angle regulation instruction and a direct current reference value; the module mainly includes three units: an operation range calculation unit 121, which determines a direct current and active and reactive power safe operation domain according to a set operation limit calculation process; a sensitivity analysis unit 122, which generates an initial shutdown angle reference signal according to a reactive power sensitivity formula and a reactive power sensitivity constraint condition; and a coordinated control unit 123, which generates a CLCC shutdown angle instruction and an MMC direct current control loop inner ring direct current instruction parameter according to the initial shutdown angle reference signal and the safe operation domain.

[0183] The execution control module 13 includes a CLCC trigger angle control unit and an MMC direct current modulation ratio regulation unit, receives instructions and performs coordinated control.

[0184] The above only describes the preferred examples of the present application and is not used to limit the present application. For those skilled in the art, the present application can have various modifications and changes. Any modification, equivalent replacement, improvement, etc. made to the present application shall be included in the protection scope of the present application.

Claims

1. A novel MMC-CLCC HVDC power transmission system coordinated control method, characterized in that: Comprising the following steps: Step S1: establishing a MMC-CLCC hybrid DC power transmission system topology architecture, determining the full half bridge sub-module ratio according to the full bridge sub-module proportion and the MMC DC voltage regulation interval, determining the control strategy of the MMC-CLCC hybrid DC power transmission system in steady state operation; Step S2: analyzing the dynamic process of the sending and receiving converters under the fault of the receiving end AC system, and clarifying the power characteristics of the CLCC; Step S3: designing a calculation process of the DC current, active and reactive power operating limits of the inverter station under multiple condition constraints, and determining the safe operation domain of the inverter station according to the process; Step S4: using the CLCC dynamic reactive power control based on the reactive power sensitivity constraint and the MMC coordinated cooperation through the maximum power instruction, realizing the maximum transmission of the DC power and the support of the receiving end AC voltage during the fault of the receiving end, specifically: After the receiving end power grid of MMC-CLCC hybrid HVDC system fails, the turn-off angle of the inverter side CLCC is further reduced according to the reactive power sensitivity constraint condition, so as to reduce the reactive power consumed by the converter station and the influence of the DC current on the reactive power; at the same time, the inverter side calculates the DC current value under the maximum transmission active power according to the amplitude of the AC voltage drop, the turn-off angle and the active power operation interval, and transmits it to the sending end through communication as the DC current reference value of the sending end MMC inner loop current controller, so as to calculate the MMC DC modulation ratio m dc ; since the receiving end adopts the reactive power sensitivity control strategy at this time, the influence of the change of the DC current on the reactive power of the receiving end caused by the communication delay will be significantly reduced, the response speed of the rectifier side DC voltage to the fault is faster by using the MMC active current limiting method based on the bridge arm voltage control of the sending end, the adjustment range of the MMC DC voltage is expanded by using the hybrid bridge topology, and the DC power transmission capacity is ensured under the premise that the MMC AC voltage is not overmodulated.

2. The MMC-CLCC novel HVDC power transmission system coordinated control method according to claim 1, characterized in that: The MMC-CLCC hybrid DC power transmission system topology architecture of step S1 comprises a rectifier side and an inverter side, wherein the rectifier side adopts two full half bridge hybrid MMC converters in series to form a high low voltage valve group, and the inverter side adopts a double 12 pulsation CLCC series form; the full half bridge sub-module ratio of the MMC is determined according to the MMC DC voltage regulation range, specifically: The modulation ratio m of the MMC in steady state is defined as: ； In the formula, U dcR is the rectifier side MMC DC outlet voltage, u diffj.peak is the j-phase virtual equipotential point fitting AC voltage peak value, in the steady state operation, the steady-state modulation ratio of the system is less than 1, and as the DC voltage decreases, the steady-state modulation ratio will always increase; constrained by the steady-state modulation ratio, the DC component of the half-bridge submodule bridge arm voltage cannot be lower than the lower limit value, while the full-bridge submodule can output negative voltage, expanding the adjustable range of the MMC DC voltage; a, b, and c represent the three-phase system a, b, and c three-phase. Each bridge arm is composed of N sub-modules, ignoring the bridge arm resistance and reactance voltage drop, and the number of various sub-modules has the following relationship: ； where N FB is the number of full-bridge submodules, N HB is the number of half-bridge submodules, U dcmin is the per-unit value of the minimum DC voltage, U dcR is the rectifier-side MMC DC outlet voltage, U sm is the submodule capacitor voltage; as can be seen from the above equation, the total number of submodules is only related to the modulation ratio m at steady state, and the number of half-bridge submodules is only related to the per-unit value of the minimum DC voltage U dcmin , and the number of full-bridge submodules increases with an increase in the modulation ratio m at steady state or a decrease in the per-unit value of the minimum DC voltage U dcmin ; when the configuration ratio of the half-bridge submodules to the full-bridge submodules of each phase of the MMC is 1:1, the MMC DC voltage can follow the change in the inverter-side DC voltage, i.e., the regulation range is 0~1.0 p.u. The control strategy of the MMC-CLCC hybrid DC power transmission system in steady state operation includes MMC basic control and CLCC basic control, the d-axis control of the AC side of the rectifier side MMC adopts constant voltage control or constant sub-module capacitor voltage control to avoid sub-module capacitor voltage over-limit in transient state, and the q-axis control adopts constant AC voltage control to provide reactive power support for the sending end; the DC side control system realizes the constant active power control of the rectifier side through the control mode of the number of sub-modules; the CLCC control link of the inverter side includes constant DC voltage control link, maximum trigger angle control link and low voltage current limiting link, the CLCC will not occur commutation failure, and the inverter side does not need to configure commutation failure prediction link and constant extinction angle control link to resist commutation failure.

3. The MMC-CLCC novel HVDC power transmission system coordinated control method of claim 1, characterized in that: The dynamic process of step S2 is that after the receiving end power grid fails, the AC voltage drops, the DC voltage decreases, the CLCC switches from the constant voltage control mode to the maximum trigger angle control, and the trigger angle is increased to increase the DC voltage at the inverter side; when a three-phase short-circuit fault occurs in the vicinity of the AC bus of the inverter station, the DC voltage decreases greatly, and the CLCC cannot maintain the DC voltage at the inverter side to be a set value even if the turn-off angle is reduced; if the fault is more serious, the potential in the CLCC will drop suddenly, and the DC current will rise suddenly, but due to the existence of the line inductance and the smoothing reactor, the DC current will not rise step by step, but will have a transient process; the DC transmission power of the MMC increases with the increase of the DC current, while the active power absorbed by the AC side does not change, and the AC side and the DC side of the MMC at the rectifier side produce unbalanced power; the discharge energy of the DC side sub-module capacitor is greater than the charging energy of the sub-module capacitor, the sub-module voltage decreases, and under the action of the d-axis stator module voltage at the AC side, the d-axis current increases, so as to keep the sub-module capacitor voltage or energy unchanged; When the active power output DC current reference value reaches the upper limit, the MMC works in the constant current state; the MMC reduces the DC modulation ratio m dc , i.e. reduces the number of sub-modules, to reduce the DC component of the bridge arm voltage, thereby maintaining the stability of the DC current when the receiving end power grid fails.

4. The MMC-CLCC novel HVDC power transmission system coordinated control method of claim 1, characterized in that: The power characteristics of the CLCC of step S2 include the relationship between the active power and the reactive power of the CLCC and the turn-off angle and the DC current at different AC voltage levels; The expression of the power characteristic model of the CLCC is: ； In the formula, P CLCC is active power, Q CLCC is reactive power, U acI is AC voltage on the inverter side, γ is turn-off angle, I dc is DC current, N is the number of six-pulse converter bridge groups, X r is commutation reactance, U dcI0 is CLCC no-load DC voltage; The active power P in the formula CLCC Reactive power Q CLCC For DC current I dc Taking the partial derivatives of the turn-off angle γ and γ respectively, we can obtain the changes in active and reactive power of the CLCC converter with respect to DC current and turn-off angle under different AC voltages: ； Wherein: 。 5. The MMC-CLCC novel HVDC power transmission system coordinated control method of claim 4, characterized in that: The multi-condition constraint of step S3 includes: ； In the formula, The active power and the reactive power transmitted by the CLCC inverter station at the inverter side to the AC system are as follows: min is the minimum off angle, I γmin is the direct current value constrained by the minimum off angle, U dcI is the inverter-side direct current voltage, I μmax is the direct current value constrained by the maximum commutation overlap angle, I dcmin is the minimum direct current value to avoid direct current discontinuity, I hmax is the maximum direct current value constrained by the period temperature rise; The calculation process of the DC current, the active power and the reactive power operating limits of the inverter station under the multi-condition constraint of step S3 is as follows: ； In the formula, P ac is the active power transmitted by the CLCC converter station to the AC system, Q ac is the reactive power transmitted by the CLCC converter station to the AC system, B C is the equivalent susceptance of the reactive power compensation and filtering device of the inverter station.

6. The MMC-CLCC novel HVDC power transmission system coordinated control method of claim 1, wherein: The safe operation domain of the inverter station includes the DC current operating domain, the active power and the reactive power operating domain at different AC voltage levels. Firstly, the specific parameters of the model are inputted, including six-pulse converter bridge group number N, commutation reactance X r , reactive power compensation and filter device equivalent susceptance B C ; secondly, the initial value of 0.1 p.u. of AC bus voltage U acI and the initial value of 0 of DC current I dc are inputted; then, according to the control mode, the inverter DC voltage or the turn-off angle is determined, and the commutation overlap angle μ of the inverter side is calculated according to the formula; when I dc <0.1 p.u., I dcmin takes 0.1 p.u. and the minimum active power P acmin and the minimum reactive power Q acmin transmitted to the AC system are calculated; when I dc >0.1 p.u., it is judged whether it is within the limit of the commutation overlap angle, if yes, I dc is I dcmax , otherwise, the DC current is increased under the maximum limit of the DC current I dclim and taken as the initial value of I dc of the next cycle, if the maximum limit of the DC current is reached at this time, I dcmax =I dclim , and the maximum active power P acmax and the maximum reactive power Q acmax transmitted to the AC system are calculated; finally, U acI is increased; the above process is repeated until U acI =1.0 p.u. The sending and receiving end support capability can be improved, including:

7. A control system of the MMC-CLCC hybrid DC power transmission system for the coordinated control method of the MMC-CLCC novel DC power transmission system according to any one of claims 1 to 6, characterized in that: A data acquisition module is configured to acquire real-time data of AC voltage, DC current, DC voltage and power at the sending and receiving end; An analysis and decision module is configured to calculate power operating limits and reactive power sensitivity, and generate turn-off angle adjustment instructions and DC current reference values; An execution control module includes a CLCC trigger angle control unit and an MMC DC modulation ratio adjustment unit, which receives instructions and performs coordinated control. The analysis and decision module includes:

8. The control system of the MMC-CLCC hybrid DC power transmission system according to claim 7, characterized in that: An operating range calculation unit, an operating limit calculation process, and a DC current and active power and reactive power safe operating domain; A sensitivity analysis unit generates an initial turn-off angle reference signal through a reactive power sensitivity formula and a reactive power sensitivity constraint condition; A cooperative control unit generates a CLCC turn-off angle instruction and an MMC DC control loop inner loop DC current instruction parameter. ​

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