MMC-CLCC novel DC power transmission system coordination control method

Abstract

The invention relates to an MMC-CLCC novel DC power transmission system coordination control method, and belongs to the technical field of hybrid DC power transmission. The method comprises the following steps: establishing an MMC-CLCC hybrid DC power transmission system topology architecture, determining an optimal proportion of full-bridge and half-bridge sub-modules according to requirements of system DC voltage and the like, and determining a control strategy during steady-state operation; analyzing the dynamic process of the system under the alternating current fault of the receiving end, and clarifying the power operation characteristics of the CLCC; designing a calculation process of direct current, active power and reactive power operation limits of the inverter station under multi-condition constraints, and constructing a safe operation domain of the inverter station; and proposing a CLCC turn-off angle adjustment strategy based on reactive sensitivity constraint and an MMC coordination strategy based on maximum power transmission. The method can provide support for the receiving-end AC system during the fault period of the receiving-end power grid, guarantees the DC power transmission capability to the greatest extent, and is suitable for a large-scale new energy cross-regional power transmission scene.

Description

Technical Field

[0001] This invention relates to the field of high-voltage direct current transmission technology, and in particular to a novel coordinated control method for MMC-CLCC direct current transmission systems that takes into account both receiving-end voltage support and DC power transmission. Background Technology

[0002] Currently, my country's new energy sources, primarily wind and solar power, exhibit a "large-scale development and centralized transmission" model, necessitating large-scale, long-distance inter-provincial power transmission for energy dispatch and allocation. High-voltage direct current (HVDC) transmission technology, with its advantages in economy, transmission efficiency, and response speed, has become a crucial support for long-distance power transmission. In recent years, with the emergence of various new converter technologies, hybrid HVDC transmission, combining traditional and new HVDC technologies, represents the future trend for large-capacity power transmission. Currently, hybrid HVDC transmission systems, including LCC and MMC hybrid dual-end and multi-end systems, and hybrid receiving-end interconnected systems, remain dominant.

[0003] The hybrid DC transmission system, which employs a Modular Multilevel Converter (MMC) at the sending end and a Controllable Line Commutated Converter (CLCC) at the receiving end, inherits the advantages of both MMC and CLCC. It features advantages such as no commutation failure and good economic efficiency. Simultaneously, the sending end connects to a weak AC grid containing a high proportion of renewable energy, and the use of MMC can provide some reactive power support to the sending end, making it suitable for large-scale inter-regional transmission of renewable energy. However, research on the MMC-CLCC hybrid DC system is still in its early stages, and existing technologies struggle to simultaneously optimize both AC voltage support at the receiving end and DC power transmission capacity. Summary of the Invention

[0004] The purpose of this invention is to provide a new coordinated control method for MMC-CLCC DC transmission systems, which solves the problem of coordinated optimization of voltage recovery and DC power transmission under AC faults at the receiving end, leverages the advantages of controllable commutation of CLCC, improves the support capability of the MMC-CLCC hybrid DC transmission system for the sending and receiving end AC grids, enhances system stability and transmission efficiency, and dynamically optimizes control parameters under actual operating conditions, making it suitable for different fault severity levels and grid structures.

[0005] The above-mentioned objective of the present invention is achieved through the following technical solution:

[0006] The MMC-CLCC novel coordinated control method for DC transmission systems includes the following steps:

[0007] Step S1: Establish the topology of the MMC-CLCC hybrid DC transmission system, determine the optimal ratio of full and half bridge submodules based on the proportion of full-bridge submodules and the MMC DC voltage regulation range, and determine the control strategy for the steady-state operation of the MMC-CLCC hybrid DC transmission system.

[0008] Step S2: Analyze the dynamic process of the sending and receiving end converters under AC system faults at the receiving end to clarify the power characteristics of the CLCC;

[0009] Step S3: Design the calculation process for the DC current, active power and reactive power operating limits of the inverter station under multiple constraints, and determine the safe operating domain of the inverter station based on this process;

[0010] Step S4: CLCC dynamic reactive power control based on reactive power sensitivity constraints is adopted, and MMC is used to coordinate with the maximum power command to achieve both AC voltage support and maximum DC power transmission during receiving-end faults.

[0011] Preferably, the MMC-CLCC hybrid DC transmission system topology described in step S1 includes a rectifier side and an inverter side. The rectifier side uses two full-and-half-bridge hybrid MMC converters connected in series to form a high- and low-voltage valve group. The inverter side uses dual 12-pulse CLCC converters connected in series. The proportion of the full-and-half-bridge submodules of the MMC is determined according to the MMC DC voltage regulation range, specifically:

[0012] The formula for the modulation ratio m in steady state of MMC is:

[0013] (1)

[0014] In the formula, U dcR The DC output voltage of the rectifier-side MMC, u diffj.peak The peak AC voltage is fitted to the virtual equipotential point of phase j, where a, b, and c represent phases a, b, and c of the three-phase system. During steady-state operation, the system's modulation ratio is less than 1. As the DC voltage decreases, the modulation ratio increases continuously in steady-state. Constrained by the steady-state modulation ratio, the DC component of the half-bridge submodule's arm voltage cannot fall below a lower limit, while the full-bridge submodule can output negative voltage, expanding the adjustable range of the MMC DC voltage.

[0015] Each bridge arm consists of N sub-modules. Ignoring the bridge arm resistance and reactance voltage drop, the number of each type of sub-module has the following relationship:

[0016] (2)

[0017] In the formula, N FB N represents the number of full-bridge submodules. HB U represents the number of half-bridge submodules. dcmin U is the per-unit value of the minimum DC voltage.dcR U is the DC output voltage of the rectifier-side MMC. sm Let U be the capacitor voltage of the submodule. From the above formula, it can be seen that the total number of submodules is only related to the modulation ratio m in steady state, and the number of half-bridge submodules is only related to the per-unit value U of the minimum DC voltage. dcmin The number of full-bridge submodules increases with the increase of the modulation ratio m in steady state or the per-unit value U of the minimum DC voltage. dcmin 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 follow the changes in the inverter-side DC voltage, i.e., the adjustment range is 0~1.0 pu.

[0018] The steady-state control strategy of the MMC-CLCC hybrid DC transmission system includes MMC basic control and CLCC basic control. The d-axis control of the rectifier-side MMC AC side uses stator module capacitor voltage control to prevent submodule capacitor voltage overshoot during transients, while the q-axis control uses constant AC voltage control to provide some reactive power support to the sending end. The DC-side control system achieves constant active power control on the rectifier side by controlling the number of submodules switched on and off. The inverter-side CLCC control is similar to the traditional LCC control strategy, with key control elements including constant DC voltage control, maximum firing angle control, and low-voltage current limiting. The difference is that CLCC does not experience commutation failure, so the inverter side does not need to configure commutation failure prediction or constant turn-off angle control elements to mitigate commutation failure.

[0019] Preferably, step S2 involves analyzing the dynamic process of the system under AC faults at the receiving end to clarify the power operation characteristics of the CLCC. The dynamic process involves the AC voltage dropping after a fault occurs in the receiving-end grid, leading to a decrease in DC voltage. The CLCC switches from constant voltage control mode to maximum firing angle control, increasing the firing angle to boost the DC voltage on the inverter side. When a three-phase short-circuit fault occurs near the AC bus of the inverter station, the DC voltage drops significantly, and even if the CLCC reduces the turn-off angle, it cannot maintain the DC voltage on the inverter side at the set value. If the fault is more severe, the internal potential of the CLCC will drop sharply, and the DC current will rise sharply. However, due to the presence of line inductance and smoothing reactors, the DC current will not rise in a step-like manner but rather undergo a transient process. The DC transmission power of the MMC increases with the increase in DC current, while the active power absorbed by the AC side remains unchanged, resulting in an imbalance of power between the AC and DC sides of the rectifier-side MMC. When the discharge energy of the DC-side submodule capacitor exceeds its charging energy, the submodule voltage decreases. Under the influence of the AC-side d-axis stator voltage, the d-axis current increases, maintaining the submodule capacitor voltage or energy level constant. The DC-side outer-loop stator active power output DC current reference value reaches its upper limit, at which point the MMC operates in constant current mode. The MMC reduces the DC modulation ratio m... dc(Reducing the number of sub-modules) reduces the DC component of the bridge arm voltage, thereby maintaining the stability of the DC current when a fault occurs in the receiving-end grid.

[0020] The CLCC power operation characteristics mentioned in step S2 include the relationship between its active and reactive power and the change of turn-off angle and DC current under different AC voltage levels.

[0021] The expression for the CLCC power characteristic model is:

[0022] (3)

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

[0024] 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:

[0025] (4)

[0026] in:

[0027] (5)

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

[0029] (6)

[0030] In the formula, γ min For the minimum shut-off angle, I γmin U is the DC current value constrained by the minimum turn-off angle. dcI I is the DC voltage on the inverter side. μmax I is the DC current value constrained by the maximum commutation overlap angle. dcmin To avoid discontinuous DC current, the minimum DC current value, I hmax This represents the maximum DC current, constrained by the temperature rise during the period.

[0031] The active and reactive power delivered by the inverter-side CLCC converter station to the AC system are shown in the following formulas:

[0032] (7)

[0033] In the formula, P ac Q represents the active power delivered by the CLCC converter station to the AC system. ac B represents the reactive power transmitted from the CLCC converter station to the AC system. C The equivalent susceptance for reactive power compensation and filtering devices in the inverter station is given by k, which represents the rated turns ratio of the transformer.

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

[0035] First, input the specific parameters of the model, including the number of six-pulse converter bridge groups N and the commutation reactance X. r The equivalent susceptance B of the reactive power compensation and filtering device C Secondly, the initial input value is the AC bus voltage U of 0.1pu. acI and a DC current I with an initial value of 0 dc Then, based on the control mode, determine the inverter DC voltage or turn-off angle, and calculate the inverter-side commutation overlap angle μ according to the formula; when I dc When I <0.1pu dcmin Take 0.1 pu and calculate the minimum active power P transferred to the AC system. acmin and minimum reactive power Q acmin ;if I dc When the value is >0.1 pu, it is determined whether it is within the commutation overlap angle limit. If so, then I dc For I dcmax Otherwise, when the current is less than the maximum limit I of the DC current. dclim Increase the DC current and use it as I for the next cycle. dc The initial value of I is such that if the maximum DC current limit has already been reached, then... dcmax =I dclim And calculate the maximum active power P transmitted to the AC system. acmax and the maximum reactive power Q acmax Finally, increase U. acI Repeat the above process until U acI =1.0 pu;

[0036] The inverter station's safe operation domain includes the DC current operation domain and the active and reactive power operation domains under different AC voltage levels.

[0037] Preferably, the control strategy described in step S4 is characterized as follows: Compared with the original control, after a fault occurs in the receiving-end grid of the MMC-CLCC hybrid DC transmission system, the CLCC turn-off angle on the inverter side is further reduced according to the reactive power sensitivity constraint, so as to reduce the reactive power consumed by the converter station and reduce the impact of DC current on reactive power; at the same time, the inverter side calculates the DC current value under the maximum transmitted active power based on the amplitude after the AC voltage drop, the turn-off angle, and the active power operating range, and transmits it to the sending end through communication as the DC current reference value of the sending end MMC inner loop current controller, thereby calculating the MMC DC modulation ratio m. dc Since the receiving end adopts a reactive power sensitivity control strategy, the impact of DC current changes caused by communication delay on the reactive power of the receiving end will be significantly reduced. Moreover, compared with the sending end relying on the charging and discharging of submodule capacitors to reduce the rectifier-side DC voltage by changing the MMC outer loop DC voltage reference value, the sending end adopts the MMC active current limiting method based on bridge arm voltage control to reduce the rectifier-side DC voltage, which has a faster response speed to faults. Furthermore, the sending end adopts a hybrid bridge topology, which expands the adjustment range of the MMC DC voltage and ensures the DC power transmission capability as much as possible while preventing the MMC AC voltage from overshooting.

[0038] The reactive power sensitivity constraint is as follows:

[0039] (8)

[0040] In the formula, S Q For reactive power sensitivity, I dc1 This represents the current operating value that meets the reactive power sensitivity constraint, γ. Qref To meet the reactive power sensitivity constraint, the reference value for the turn-off angle is S. Qset K is the reactive power sensitivity setpoint. lim This is a constraint on reactive power sensitivity variation. The first term in the formula indicates that the reactive power sensitivity corresponding to the selected turn-off angle reference value should not be less than the set value, that is, the operating point should be above and to the right of the curve corresponding to the set reactive power sensitivity; the second term in the formula indicates that the change of DC current has little effect 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 to the right of the curve inflection point.

[0041] The selection of reactive power sensitivity should consider both the AC voltage and active power at the receiving end, as well as factors such as the strength of the receiving-end power grid. From the above formula, the minimum turn-off angle that meets the reactive power sensitivity constraint can be selected as the reference value for the turn-off angle of the maximum trigger angle control. Furthermore, the final output reference value for the turn-off angle should satisfy the turn-off angle range corresponding to the reactive power operation region on the inverter side, and its determination method is shown in the following formula:

[0042] (9)

[0043] In the formula, Q lim To limit the reactive power transmission from the inverter station to the AC system under the safe operation domain, γ lim K is the shut-off angle limit under the safe operating domain. Q For reactive power limits, k represents the transformer's rated turns ratio.

[0044] Meanwhile, the rectifier-side MMC updates the input signal of the DC-side inner loop control in real time based on the safe operating domain obtained in step S3, so as to provide maximum power support during faults and complete the coordination with the receiving-end converter control strategy.

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

[0046] The data acquisition module is used to collect AC voltage, DC current, DC voltage and power data at the sending and receiving ends in real time.

[0047] The analysis and decision-making module is used to calculate power operating limits and reactive power sensitivity, and to generate turn-off angle adjustment commands and DC current reference values.

[0048] The execution control module, including the CLCC firing angle control unit and the MMC DC modulation ratio adjustment unit, receives commands and performs coordinated control.

[0049] The analysis and decision-making module includes an operating range calculation unit, which uses an operating limit calculation process to determine the safe operating range of DC current and active and reactive power; a sensitivity analysis unit, which generates an initial turn-off angle reference signal through reactive power sensitivity formula and reactive power sensitivity constraints; and a collaborative control unit, which generates CLCC turn-off angle commands and MMC DC control loop inner loop DC current command parameters.

[0050] The beneficial effects of this invention are as follows:

[0051] 1. Improved System Stability: Through coordinated control strategies, it provides effective support for the AC system during receiving-end faults and accelerates AC system voltage recovery after fault clearance. 2. Guaranteed Power Transmission: It balances reactive power support and active power transmission, significantly improving DC power transmission capacity during faults and enabling faster DC power recovery after fault clearance. 3. Strong Adaptability: Applicable to different fault severity levels and grid structures, control parameters can be dynamically optimized according to actual operating conditions. Attached Figure Description

[0052] The accompanying drawings, which are included to provide a further understanding of the invention and form part of this application, illustrate the invention and are used to explain it, but do not constitute an undue limitation of the invention.

[0053] Figure 1 This is a flowchart of the coordinated control method for the MMC-CLCC hybrid DC transmission system of the present invention;

[0054] Figure 2 This is a topology diagram of the +800kV MMC-CLCC hybrid DC transmission system of the present invention;

[0055] Figure 3 This is a basic topology diagram of the MMC converter of the present invention;

[0056] Figure 4 This is a basic topology diagram of the CLCC converter of the present invention;

[0057] Figure 5 for Figure 4 Detailed structural diagram of the intermediate converter valve V1;

[0058] Figure 6 This is a timing diagram of the CLCC switch during commutation from V1 to V3 according to the present invention;

[0059] Figure 7 This is a basic control block diagram of the MMC converter of the present invention;

[0060] Figure 8 This is a basic control block diagram of the CLCC converter of the present invention;

[0061] Figure 9 This is a flowchart illustrating the calculation of the DC current and power operating range of the present invention;

[0062] Figure 10 This is the DC current safe operating range of the present invention;

[0063] Figure 11 This refers to the safe operating range of active power on the inverter side in this invention.

[0064] Figure 12 This is the safe operating range for reactive power on the inverter side according to the present invention;

[0065] Figure 13 , Figure 14 The operating range of the turn-off angle and DC current under different sensitivity conditions of the present invention;

[0066] Figure 15 This is a control block diagram of the MMC-CLCC hybrid DC transmission system of the present invention, which combines AC voltage support at the receiving end and DC power transmission.

[0067] Figure 16 This is a schematic diagram of the simulation results of the firing angle when the AC bus voltage of the receiving-end converter station drops to 0.7 pu according to the present invention;

[0068] Figure 17This is a schematic diagram of the simulation results of the effective value of the AC voltage at the receiving end of the converter station when the AC bus voltage drops to 0.7 pu.

[0069] Figure 18 This is a schematic diagram of the simulation results of DC transmission power when the AC bus voltage of the receiving-end converter station drops to 0.7 pu according to the present invention;

[0070] Figure 19 This is a schematic diagram of the simulation results of the reactive power absorbed by the CLCC when the AC bus voltage of the receiving-end converter station drops to 0.7 pu.

[0071] Figure 20 The present invention relates to a DC power transmission system control system.

[0072] In the diagram: 11. Data acquisition module; 12. Analysis and decision-making module; 121. Operating range calculation unit; 122. Sensitivity analysis unit; 123. Cooperative control unit; 13. Execution control module. Detailed Implementation

[0073] The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention. To make the above-mentioned objects, features and advantages of the present invention more apparent and understandable, the present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0074] Definitions:

[0075] MMC: Modular Multilevel Converter; CLCC: Controlled Phase Commutation Converter; FHMMC: Full-Bridge Hybrid Modular Multilevel Converter; HBSM: Half-Bridge Submodule; FBSM: Full-Bridge Submodule; MMC-CLCC: Modular Multilevel Converter and Controlled Phase Commutation Converter.

[0076] See Figures 1 to 20As shown, the MMC-CLCC hybrid DC transmission system coordinated control method of the present invention, which takes into account both receiving-end voltage support and DC power transmission, can provide support for the receiving-end AC system during receiving-end grid faults, while maximizing DC power transmission capacity, and is applicable to large-scale new energy cross-regional power transmission scenarios. This method establishes the topology of the MMC-CLCC hybrid DC transmission system, determines the optimal proportion of full and half-bridge submodules based on system DC voltage requirements, and determines the control strategy for steady-state operation; analyzes the dynamic process of the system under receiving-end AC faults, and clarifies the power operation characteristics of CLCC; designs the calculation process for the DC current, active power, and reactive power operation limits of the inverter station under multiple constraints, and constructs the safe operation domain of the inverter station; proposes a CLCC turn-off angle adjustment strategy based on reactive power sensitivity constraints and an MMC coordinated control strategy based on maximum power transmission. Furthermore, the present invention also provides a control system for the MMC-CLCC hybrid DC transmission system for improving the support capacity of the sending and receiving ends, including: a data acquisition module, an analysis and decision module, and an execution control module; wherein the analysis and decision module includes an operating range calculation unit, a sensitivity analysis unit, and a cooperative control unit. This invention solves the problem of coordinated optimization of voltage recovery and DC power transmission under AC faults at the receiving end. It has a good effect on improving the stability of AC systems and ensuring power transmission. It is applicable to different fault severity and power grid structure, and the control parameters can be dynamically optimized according to the actual operating conditions.

[0077] See Figure 1 As shown, the coordinated control method for the MMC-CLCC hybrid DC transmission system of the present invention includes the following steps:

[0078] Step S1: Establish the topology of the MMC-CLCC hybrid DC transmission system, determine the proportion of full and half bridge submodules based on the proportion of full bridge submodules and the MMC DC voltage regulation range, and determine the control strategy for the steady-state operation of the MMC-CLCC hybrid DC transmission system.

[0079] S11: MMC-CLCC Hybrid DC Transmission System Topology

[0080] The MMC-CLCC hybrid DC transmission system adopts a true bipolar connection method, and its topology is as follows: Figure 2 As shown. Taking the positive terminal as an example, the rectifier side uses two 400kV MMC converters connected in series to form a +800kV MMC converter station; the inverter side also uses two 12-pulse CLCC converters connected in series. Each 12-pulse unit converter transformer adopts a three-winding structure. One valve-side winding is star-connected and the other is delta-connected. The commutation voltage for the thyristors is provided by the phase difference between the two windings.

[0081] When a half-bridge submodule is used on the rectifier side, a severe fault in the receiving-end grid can cause the DC output voltage of the MMC to fail to follow changes in the inverter-side DC voltage, leading to DC current overcurrent and the risk of overmodulation of the MMC. Therefore, the rectifier-side MMC employs a hybrid structure of full-bridge and half-bridge submodules to ensure that the DC output voltage follows changes in the inverter-side DC voltage during a fault in the receiving-end grid. The MMC topology is as follows: Figure 3 As shown in the figure: L arm For the bridge arm inductance, L g This is the equivalent inductance on the AC side. During normal operation, the MMC adjusts the AC output fitted voltage u... diff The amplitude and phase of the voltage are used to control the active and reactive power on the AC side. The phase of the grid is locked by a phase-locked loop to enable the control of the voltage. s The d-axis coincides with the q-axis, thereby achieving active power loop control by the d-axis and reactive power loop control by the q-axis, i.e., decoupled control of the d-axis and q-axis.

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

[0083] (1)

[0084] In the formula, u diffj.peak To fit the peak AC voltage at the virtual equipotential point of phase j, during steady-state operation, the system's modulation ratio is less than 1. As the DC voltage decreases, the system's modulation ratio will continuously increase. Constrained by the modulation ratio in steady state, the DC component of the half-bridge submodule's arm voltage cannot fall below the lower limit, while the full-bridge submodule can output negative voltage, expanding the adjustable range of the MMC DC voltage.

[0085] Assuming each bridge arm consists of N sub-modules, then the total number of sub-modules in the bridge is N. FB The number of half-bridge submodules is N HB Ignoring bridge arm resistance and reactance voltage drop, the number of each type of submodule should satisfy the following relationship:

[0086] (2)

[0087] In the formula, U dcmin U is the per-unit value of the minimum DC voltage. dcR U is the DC output voltage of the rectifier-side MMC. sm Let U be the capacitor voltage of the submodule. From the above formula, it can be seen that the total number of submodules is only related to the modulation ratio m in steady state, and the number of half-bridge submodules is only related to the per-unit value U of the minimum DC voltage. dcmin The number of full-bridge submodules increases with the increase of the modulation ratio m in steady state or the per-unit value U of the minimum DC voltage. dcminThe 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] In the formula, u diffj U represents the differential mode voltage of the upper and lower arms of the j-phase MMC bridge. comj ωt+φ represents the common-mode voltage of the upper and lower arms of the MMC phase j, and ωt+φ represents the phase angle of the MMC common-mode voltage.

[0095] By introducing the modulation ratios of the MMC AC and DC sides, the AC and DC output voltages can be decoupled, thus enabling independent adjustment of the rectifier-side MMC AC and DC output voltages in the event of a receiving-end fault. This adds an extra control dimension compared to the original control. (The text then abruptly shifts to a description of the voltage values ​​before and after adjustment, seemingly unrelated to the previous sentence.) atop and lower bridge arm voltage u abtm It can be represented as:

[0096] (4)

[0097] The basic control structure of the rectifier-side MMC is as follows: Figure 7 As shown. Under stable operating conditions, the d-axis control of the rectifier-side MMC AC side adopts constant voltage control or stator module capacitor voltage control to avoid the submodule capacitor voltage exceeding the limit during transients. The q-axis control adopts constant AC voltage control to provide a certain reactive power support to the sending end. The DC-side control system achieves constant active power control on the rectifier side by controlling the number of submodules switched on and off.

[0098] The control block diagram of the inverter-side CLCC is as follows: Figure 8 As shown, similar to the traditional LCC control strategy, the main control links include constant DC voltage control, maximum firing angle control, and low voltage current limiting. The difference is that CLCC will not experience commutation failure, and the inverter side does not need to be configured with commutation failure prediction and constant turn-off angle control to resist commutation failure.

[0099] Step S2: Analyze the dynamic process of the sending and receiving end converters under AC system faults at the receiving end to clarify the power characteristics of the CLCC.

[0100] S21: System Dynamics under AC Fault at the Receiving End

[0101] When a fault occurs in the receiving-end power grid, the AC voltage drops, leading to a decrease in DC voltage. The CLCC switches from constant voltage control mode to maximum firing angle control, increasing the firing angle to boost the DC voltage on the inverter side. When a three-phase short-circuit fault occurs near the AC bus of the inverter station, the DC voltage drops significantly, and even if the CLCC lowers the switching angle, it cannot maintain the DC voltage on the inverter side at the set value. If the fault becomes more severe, the internal potential of the CLCC will drop sharply, and the DC current will rise sharply. However, due to the presence of line inductance and smoothing reactors, the DC current will not rise in a step-like manner, but rather undergo a transient process. During this period, the DC current can be expressed as:

[0102] (5)

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

[0104] The voltage at the DC output of the rectifier-side MMC is established through the voltage of the submodule capacitor. The control of its AC / DC side is essentially achieved by controlling the charging and discharging of the submodule capacitor. The relationship between unbalanced power and submodule capacitor energy is as follows:

[0105] (6)

[0106] In the formula, For the unbalanced power of the AC / DC side of the MMC, P ac For the MMC to absorb active power from the AC side, P dc For MMC to transfer active power to the DC side, The difference in energy change of the submodule capacitor can be expressed as follows, based on the MMC submodule capacitor energy expression:

[0107] (7)

[0108] In the formula, C eq C is the equivalent capacitance of the MMC converter. sm For the submodule capacitor. Define the equivalent capacitance time constant τ. c As shown in the following formula. Its physical meaning is that, without considering the AC side injected power, the DC capacitor operates with a constant current I. dcN Discharging the DC system, the DC voltage changes from U dcN The time required to reduce it to 0.

[0109] (8)

[0110] In the formula, U dcN The rated bipolar DC voltage; S N The rated capacity of the converter station, This is the charging and discharging time constant of the MMC submodule capacitor.

[0111] At the moment of a grid fault at the receiving end, the DC current rises rapidly due to the instantaneous drop in DC voltage on the inverter side. The DC transmission power of the MMC increases with the increase in DC current, while the active power absorbed on the AC side remains unchanged. This results in an imbalance of power between the AC and DC sides of the rectifier-side MMC. The discharge energy of the DC-side submodule capacitors exceeds the charging energy, causing the submodule voltage to drop. Under the influence of the AC-side d-axis stator submodule voltage, the d-axis current increases, maintaining a constant voltage / energy ratio for the submodule capacitors. The DC current reference value for the DC-side outer loop stator active power output reaches its upper limit, at which point the MMC operates in a constant current state. The MMC reduces the DC component of the bridge arm voltage by lowering the DC modulation ratio (reducing the number of submodules in operation), thereby maintaining a stable DC current during grid faults at the receiving end.

[0112] S22: Power Operation Characteristics of CLCC

[0113] The relationship between the active power transmitted by CLCC and the reactive power consumed is as follows:

[0114] (9)

[0115] In the formula, P CLCC For active power, Q CLCC For reactive power, U acI The inverter side AC voltage is γ, the turn-off angle is I. dc Where X is the DC current, N is the number of six-pulse commutator bridge groups, and X is the DC current. r For commutation reactance, U dcI0 This is the CLCC no-load DC voltage.

[0116] P in the formula CLCC Q CLCC to I dc Taking the partial derivatives of 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:

[0117] (10)

[0118] in:

[0119] (11)

[0120] From the above, it can be seen that under different AC voltages and DC currents, the partial derivative of active power with respect to DC current is greater than 0, while the partial derivative with respect to the turn-off angle is less than 0. This indicates that the active power of the CLCC is positively correlated with DC current and negatively correlated with the turn-off angle, and that the active power is more sensitive to changes in the turn-off angle when the AC voltage is maintained at a higher level. Similarly, under different AC voltages and DC currents, the partial derivatives of reactive power with respect to DC current and turn-off angle are both greater than 0, indicating that the reactive power consumed by the CLCC is positively correlated with both DC current and turn-off angle. Reducing γ during AC faults at the receiving end is beneficial to the transmission of active power, and the reactive power consumed by the CLCC will also decrease, which is beneficial to U. acI Restore; increase I dc It is beneficial for active power transmission, but it will increase the reactive power consumed by CLCC, which is detrimental to U. acI recover.

[0121] Step S3: Design the calculation process for the DC current, active power and reactive power operating limits of the CLCC converter station under multiple constraints, and construct the safe operating domain of the inverter station based on this.

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

[0123] For the inverter-side CLCC to operate normally, it needs to meet operating conditions such as turn-off angle, commutation angle, maximum DC current, and minimum DC current limits. Based on these operating conditions, IC during normal CLCC operation can be calculated. dc The range of values ​​can be used to determine the operating range of active and reactive power.

[0124] 1) CLCC shut-off angle limitation

[0125] The minimum turn-off angle is the electrical angle corresponding to the minimum time required for the thyristor to regain its forward blocking capability after the current crosses zero. If the turn-off angle is less than the minimum turn-off angle γ, then... min If the signal is not received, the system is considered to have experienced a commutation failure. In traditional LCC converters, the turn-off angle γ is defined as an electrical angle, corresponding to the time when the valve closes, i.e., the positive zero crossing point of the line-to-line voltage on the AC converter bus. The commutation voltage of a CLCC is the sum of the line voltage and the surge arrester operating voltage. Traditional measurement methods do not measure the actual turn-off angle; therefore, the minimum turn-off angle γ of a CLCC is... min It 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 U is the DC current value constrained by the minimum turn-off angle. dcI This is the DC voltage on the inverter side.

[0129] 2) Commutation overlap angle limitation

[0130] During normal operation, the 12-pulse converter operates in condition 4-5 (i.e., 4 valves are conducting during non-commutation periods and 5 valves are conducting during commutation periods), at which point the commutation angle μ < 30°. A decrease in AC bus voltage or an increase in DC current will cause the commutation angle of the 12-pulse converter to increase. When the commutation angle μ = 30°, typically 5 valves in both bridges are conducting simultaneously, meaning the commutation process of the two six-pulse valve groups alternates, and the converter operates in condition 5. When the commutation angle μ > 30°, there will be periods where both valve groups commutate simultaneously; that is, before one pair of valves in one valve group has finished commutating, another pair of valves in the other bridge begins commutating, resulting in 6 valves conducting simultaneously—this is the 5-6 fault condition. When the commutation angle is greater than 30°, if there is coupling inductance between the two bridges of the 12-pulse converter, it will cause the DC voltage to drop, the AC bus voltage to be distorted, and the valve voltage to generate additional commutation teeth. The presence of additional commutation teeth is not conducive to the stable operation of the inverter. Therefore, the commutation angle limit of this invention is μ<30°.

[0131] For a 12-pulse converter, the quasi-steady-state formula can be used to obtain:

[0132] (13)

[0133] The range of DC current limited by the inverter-side commutation angle is:

[0134] (14)

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

[0136] 3) DC current limiting

[0137] Excessive DC current can cause the thyristor junction temperature and converter transformer winding temperature to exceed the specified allowable values. Therefore, the DC current must not exceed the aforementioned allowable limits. In domestic and international high-voltage DC projects, the second-level overload capacity of thyristors is generally set at 1.4 times the rated current, i.e., I0. hmax ≤ 1.4pu, the specific value depends on the design standard of the converter valve.

[0138] On the other hand, when the DC current is low, current fluctuations can cause DC current interruptions. The rate of current change is very high at the moment of interruption, inducing overvoltages in the transformer windings and DC reactors. To prevent current discontinuity, the minimum current limit is generally I. dcmin =0.1pu.

[0139] Therefore, DC current operation should meet the following requirements:

[0140] (15)

[0141] In the formula, I dcmin To avoid discontinuous DC current, the minimum DC current value, I hmax This represents the maximum DC current, constrained by the temperature rise during the period.

[0142] S32: Calculation Flowchart for DC Current and Power Operating Limits of CLCC Converter Stations

[0143] This invention uses AC voltage as the independent variable, based on the DC current I in step S31. dc The operating limits of the converter station's active and reactive power are calculated under the given constraints. The calculation flowchart is as follows: Figure 9 As shown.

[0144] The steps for calculating the limits shown in the diagram are as follows: First, input the specific parameters of the model (Number of six-pulse converter bridge groups N, commutation reactance X). r The equivalent susceptance B of the reactive power compensation and filtering device C ); secondly, the input AC bus voltage U acI (Initial value 0.1pu) and DC current I dc (Initial value is 0); then determine the inverter DC voltage or turn-off angle according to the control mode, and calculate the inverter side commutation overlap angle μ according to formula (13); when I dc When I <0.1pu dcmin Take 0.1 pu and calculate the minimum active power P transferred to the AC system. acmin and minimum reactive power Q acmin ;if I dc When the value is >0.1 pu, it is determined whether it is within the commutation overlap angle limit. If so, then I dc For I dcmax Otherwise, increase the DC current (which should be less than the maximum DC current limit I). dclim ) and as the next loop I dc The initial value of I is such that if the maximum DC current limit has already been reached, then... dcmax =I dclim And calculate the maximum active power P transmitted to the AC system. acmax and the maximum reactive power Q acmax Finally, increase U. acI Repeat the above process until U acI =1.0pu.

[0145] S33: CLCC Converter Station Safe Operation Domain

[0146] By inputting the parameters from Table 1 into the system and following the above process, the safe operation domain of the CLCC converter station can be plotted as follows: Figures 10 to 12 As shown.

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

[0148] like Figure 10 As shown, with U acI The increase of I dc It is successively subject to minimum DC current limit, maximum commutation angle limit, and maximum DC current limit, and different turn-off angles γ will cause the maximum DC current limit to change, with the specific trend being I dc The curve shifts downward as the shut-off angle γ decreases.

[0149] Figure 11 and Figure 12 This describes the variation of active and reactive power on the inverter side with AC voltage, where γ = γ0 = 20°, U acI =0.055 and 0.776pu are the critical points for the inverter-side commutation overlap angle limit, minimum DC current limit, and maximum DC current limit, respectively. When U acI When I > 0.776 pu, dcmin Subject to the maximum DC current value I dclim =1.4 pu limit, with U acI With the increase of P, the upper limit of active power P acmax The increase becomes slow, while the lower limit of reactive power Q acmin It shows a decreasing trend; 0 acI When <0.776pu, I dcmin Subject to maximum commutation angle μ max The 30° limit, with U acI With the increase of P, the upper limit of active power P acmax Lower limit of reactive power Q acmin Both show an increasing trend, and the rate of increase continues; during this process, the trends of the lower limit of active power and the upper limit of reactive power remain basically unchanged.

[0150] Depend on Figures 10 to 12 As can be seen from the different turn-off angle operating ranges, a decrease in the inverter-side turn-off angle shifts the turn-off angle limit and the maximum and minimum DC current limit points to the right, while the DC current and active power operating ranges also become narrower. The maximum and minimum reactive power operating curves both shift downwards. Compared to the change in the active power operating range, the change in the inverter-side reactive power operating range is greater, indicating that the reactive power on the inverter side has a more significant impact on the turn-off angle change.

[0151] ​Step S4: Adopt CLCC dynamic reactive power control based on reactive power sensitivity constraints, and at the same time utilize MMC to coordinate and cooperate through maximum power command to achieve both AC voltage support and maximum DC power transmission during receiving-end faults. This includes proposing a CLCC turn-off angle adjustment strategy based on reactive power sensitivity constraints and an MMC coordination strategy based on maximum power transmission.

[0152] S41: CLCC Reactive Power Control Strategy Based on Reactive Power Sensitivity Constraints

[0153] Traditional LCC control incorporates a commutation failure prediction mechanism, which reduces the firing angle based on the severity of the receiving-end fault, thereby increasing the turn-off angle and mitigating the risk of commutation failure. However, reducing the firing angle inevitably leads to a further drop in DC voltage. While the DC current can be kept constant under the control of the rectifier-side constant DC current, the DC transmission power drops sharply, adversely affecting weak systems with a high proportion connected to the sending-end. CLCC, on the other hand, does not have a commutation failure problem. When the receiving-end AC system fails, there is no need to increase the turn-off angle to prevent commutation failure, thus preventing further DC voltage drops caused by the commutation failure prediction mechanism. This provides some support for weak systems connected to the sending and receiving-end converter stations.

[0154] To further leverage the advantages of CLCC's controllable commutation and enhance its support capability for the sending and receiving end AC systems, the shut-off angle can be further adjusted according to the reactive power exchange between the receiving end converter station and the AC system during receiving end grid faults. This reduces the reactive power consumed by the converter valves during faults and accelerates the recovery of AC system voltage.

[0155] To provide maximum reactive power support to the AC system during AC faults and to ensure the recovery of both reactive and active power during fault recovery, this invention proposes a CLCC reactive power control strategy based on reactive power sensitivity constraints. As shown in equation (16), based on traditional maximum firing angle control, a reactive power sensitivity constraint is used to determine the reference value γ of the turn-off angle. ref .

[0156] (16)

[0157] In the formula, γ ref d is the reference value for the shut-off angle. x For the relative inductive voltage drop of the converter, I dcref I dcN These are the commanded and rated DC current values, U. dcI0N γ represents the rated no-load DC power on the inverter side, and K1 is the current deviation coefficient. The solution for γ follows the principle of minimizing the impact of active power recovery on the converter station's provision of reactive power support to the AC grid during receiving-end faults and recovery periods. α represents the firing angle of the CLCC.

[0158] Define reactive power sensitivity S Q The expression is as follows:

[0159] (17)

[0160] From the above formula, it can be seen that reactive power sensitivity is related to AC voltage, turn-off angle, and DC current. Within a certain range, S Q A larger value indicates a greater coupling between reactive power changes and the turn-off angle, meaning a smaller relationship with DC current. The operating regions of DC current and turn-off angle under specific reactive power sensitivity are shown below. Figure 13 and Figure 14 As shown. During a receiving-end fault, the turn-off angle operating range where the reactive power sensitivity to DC current is low under different operating conditions can be found based on the reactive power sensitivity requirements and the relationship curve. Within this range, the impact of increasing DC current on reactive power consumption can be minimized.

[0161] Based on the above reactive power sensitivity variation curve, constraints are imposed on it, and the constraints are as follows:

[0162] (18)

[0163] In the formula, γ Qref To meet the reactive power sensitivity constraint, the reference value for the turn-off angle is S. Qset K is the reactive power sensitivity setpoint. lim This is a constraint on reactive power sensitivity variation. The first term in the formula indicates that the reactive power sensitivity corresponding to the selected turn-off angle reference value should not be less than the set value, that is, the operating point should be above and to the right of the curve corresponding to the set reactive power sensitivity; the second term in the formula indicates that the change of DC current has little effect 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 to the right of the curve inflection point.

[0164] Regarding the selection of reactive power sensitivity, the requirements of balancing the AC voltage and active power at the receiving end are followed, while also being related to factors such as the strength of the receiving end power grid. This invention approximates the selection by choosing points near the maximum value of the surface, taking S as the value. Qset =5U acI -0.7. The minimum shut-off angle that meets the reactive power sensitivity constraint can be selected as the shut-off angle reference value for maximum trigger angle control. Furthermore, the final output shut-off angle reference value should satisfy the shut-off angle range corresponding to the reactive power operation region on the inverter side, and its determination method is shown in the following formula:

[0165] (19)

[0166] In the formula, Q limTo limit the reactive power transmission from the inverter station to the AC system under the safe operation domain, γ lim This is the shut-off angle limit under the safe operating domain. K Q This is an intermediate variable under the reactive power limit.

[0167] During normal operation, the shutdown angle command value remains unchanged. When a fault is detected at the receiving end, the shutdown angle command is switched to a shutdown angle reference value generated based on reactive power sensitivity constraints. The correction angle is input into the maximum trigger angle generation stage to obtain the trigger angle command value under the reactive power command. Compared with the original control, this control strategy can effectively improve the support for the receiving end power grid during the fault period and is conducive to the recovery of the receiving end power grid.

[0168] S42: MMC Coordination Control Strategy Based on Maximum Active Power Transfer

[0169] As can be seen from step S41, the operating range of DC current is related to the AC voltage and turn-off angle on the inverter side. By reducing the CLCC turn-off angle through reactive power sensitivity constraints, reactive power control is achieved, which ensures the support of AC voltage and reduces the impact of DC current changes on reactive power. However, due to the constraints of the power operating range, the operating range of DC current and active power also changes when the turn-off angle changes. Therefore, it is necessary to calculate the DC current corresponding to the maximum power operating point in real time according to the DC current operating range to ensure the maximum transmission of active power and rapid power recovery when the receiving end is faulty.

[0170] S43: Control System Design

[0171] Based on the analysis in steps S41 and S42, the following was designed: Figure 15 Compared to the original control system, the control system shown in the diagram, after a fault occurs in the receiving-end grid of the MMC-CLCC hybrid DC transmission system, further reduces the CLCC turn-off angle on the inverter side based on reactive power sensitivity constraints. This reduces the reactive power consumed by the converter station while minimizing the impact of DC current on reactive power. Simultaneously, the inverter side calculates the DC current value under maximum transmitted active power based on the amplitude of the AC voltage drop, the turn-off angle, and the active power operating range. This value is then transmitted to the sending end via communication and serves as the DC current reference value for the sending-end MMC inner-loop current controller. Based on this, the MMC DC modulation ratio m is calculated. dc Since the receiving end adopts a reactive power sensitivity control strategy, the impact of DC current changes caused by communication delay on the reactive power of the receiving end will be significantly reduced. Moreover, compared with the sending end relying on the charging and discharging of submodule capacitors to reduce the rectifier-side DC voltage by changing the MMC outer loop DC voltage reference value, the sending end adopts the MMC active current limiting method based on bridge arm voltage control to reduce the rectifier-side DC voltage, which has a faster response speed to faults. Furthermore, the sending end adopts a hybrid bridge topology, which expands the adjustment range of the MMC DC voltage and ensures the DC power transmission capability as much as possible while preventing the MMC AC voltage from overshooting.

[0172] Example:

[0173] The example analysis provided in this embodiment is as follows: Figure 11 and Figure 12 As shown, PSCAD / EMTDC has established the following... Figure 2 The simulation platform for the MMC-CLCC hybrid DC transmission system is shown in Table 1. The relevant parameters of the platform are shown in Table 1.

[0174] The following simulations validate different control strategies of MMC-CLCC during receiving-end grid faults:

[0175] Control Strategy 1: The system adopts the original control strategy;

[0176] Control Strategy 2: The system adopts the coordinated control strategy of the present invention.

[0177] During the simulation, a three-phase short-circuit fault was applied to the receiving-end power grid at 0.8s, lasting for 0.1s. Different fault grounding inductances were used to simulate AC faults of varying severity.

[0178] Figures 16 to 19 The simulation results show the AC bus voltage drop to 0.7 pu at the receiving end converter station. Figure 16 As shown, after adopting the control strategy proposed in this invention, the firing angle command of CLCC is increased by 3°~6°. Figure 17 and Figure 19 The dynamic performance of different control strategies regarding reactive power support and AC voltage recovery during and after a receiving-end fault is demonstrated. In the case of a receiving-end AC fault, reducing the shutdown angle can decrease reactive power consumption. Compared to control strategy 1, control strategy 2 improves reactive power support for the AC system by approximately 489.26 Mvar, increases AC voltage by approximately 3.7%, and shortens the time to recover to near-rated levels after fault clearance. Figure 18 The effectiveness of different control strategies in supporting unipolar DC power during receiving-end faults and fault recovery is illustrated. It can be observed that during a fault, control strategy 2 stabilizes the DC power at approximately 3349.99 MW, while control strategy 1 achieves 2530.75 MW, representing an increase of approximately 20.5%. Therefore, the coordinated control strategy provided by this invention achieves balanced support for both AC voltage and DC power.

[0179] This invention also provides a control system for an MMC-CLCC hybrid DC transmission system to improve the support capability of the sending and receiving ends, such as... Figure 20 As shown, the control system of this DC transmission system includes:

[0180] Data acquisition module 11 is used to acquire AC voltage, DC current, DC voltage and power data of the sending and receiving ends in real time;

[0181] The analysis and decision module 12 is used to calculate the power operating limit and reactive power sensitivity, and generate the turn-off angle adjustment command and DC current reference value. This module mainly includes three units: the operating range calculation unit 121, which determines the DC current and the safe operating range of active and reactive power according to the set operating limit calculation process; the sensitivity analysis unit 122, which generates the initial turn-off angle reference signal according to the reactive power sensitivity formula and reactive power sensitivity constraints; and the collaborative control unit 123, which generates the CLCC turn-off angle command and the DC current command parameters of the inner loop of the MMC DC control loop according to the initial turn-off angle reference signal and the safe operating range.

[0182] The execution control module 13 includes a CLCC firing angle control unit and an MMC DC modulation ratio adjustment unit, which receives instructions and performs coordinated control.

[0183] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the invention by those skilled in the art. Any modifications, equivalent substitutions, or improvements made to the present invention should be included within the scope of protection of the present invention.

Claims

1. A novel coordinated control method for MMC-CLCC DC transmission systems, characterized in that: Includes the following steps: Step S1: Establish the topology of the MMC-CLCC hybrid DC transmission system, determine the proportion of full and half bridge submodules based on the proportion of full bridge submodules and the MMC DC voltage regulation range, and determine the control strategy for the steady-state operation of the MMC-CLCC hybrid DC transmission system. Step S2: Analyze the dynamic process of the sending and receiving end converters under AC system faults at the receiving end to clarify the power characteristics of the CLCC; Step S3: Design the calculation process for the DC current, active power and reactive power operating limits of the inverter station under multiple constraints, and determine the safe operating domain of the inverter station based on this process; Step S4: CLCC dynamic reactive power control based on reactive power sensitivity constraints is adopted, and MMC is used to coordinate with the maximum power command to achieve both AC voltage support and maximum DC power transmission during receiving-end faults.

2. The MMC-CLCC novel DC transmission system coordinated control method according to claim 1, characterized in that: The MMC-CLCC hybrid DC transmission system topology described in step S1 includes a rectifier side and an inverter side. The rectifier side uses two full-and-half-bridge hybrid MMC converters connected in series to form a high- and low-voltage valve group. The inverter side uses dual 12-pulse CLCC converters connected in series. The proportion of the full-and-half-bridge submodules of the MMC is determined according to the MMC DC voltage regulation range, specifically: The formula for the modulation ratio m in steady state of MMC is: ； In the formula, U dcR The DC output voltage of the rectifier-side MMC, u diffj.peak The AC voltage peak is fitted to the virtual equipotential point of phase j. During steady-state operation, the steady-state modulation ratio of the system is less than 1. As the DC voltage decreases, the steady-state modulation ratio will continue to increase. Constrained by the steady-state modulation ratio, the DC component of the half-bridge submodule arm voltage cannot be lower than the lower limit, while the full-bridge submodule can output negative voltage, expanding the adjustable range of the MMC DC voltage. a, b, c represent the three phases a, b, and c of the three-phase system. Each bridge arm consists of N sub-modules. Ignoring the bridge arm resistance and reactance voltage drop, the number of each type of sub-module has the following relationship: ； In the formula, N FB N represents the number of full-bridge submodules. HB U represents the number of half-bridge submodules. dcmin U is the per-unit value of the minimum DC voltage. dcR U is the DC output voltage of the rectifier-side MMC. sm The voltage across the submodule capacitor is given by the above formula. It can be seen from the formula that the total number of submodules depends only on the steady-state modulation ratio m, and the number of half-bridge submodules depends only on the per-unit value U of the minimum DC voltage. dcmin The number of full-bridge submodules increases with the increase of the modulation ratio m in steady state or the per-unit value U of the minimum DC voltage. dcmin The decrease increases; when the configuration ratio of each phase half-bridge submodule to full-bridge submodule of MMC is 1:1, the DC voltage of MMC can meet the requirement of following the change of DC voltage on the inverter side, that is, the adjustment range is 0~1.0pu; The control strategy for steady-state operation of the MMC-CLCC hybrid DC transmission system includes MMC basic control and CLCC basic control. The AC side d-axis control of the rectifier-side MMC adopts constant voltage control or stator module capacitor voltage control to avoid the sub-module capacitor voltage exceeding the limit during transients. The q-axis control adopts constant AC voltage control to provide reactive power support to the sending end. The DC side control system realizes constant active power control on the rectifier side by controlling the number of sub-modules switched on and off. The inverter-side CLCC control includes constant DC voltage control, maximum firing angle control, and low-voltage current limiting. CLCC will not experience commutation failure, and the inverter side does not need to configure commutation failure prediction and constant turn-off angle control to resist commutation failure.

3. The MMC-CLCC novel DC transmission system coordinated control method according to claim 1, characterized in that: The dynamic process described in step S2 is as follows: After a fault occurs in the receiving-end power grid, the AC voltage drops, leading to a decrease in DC voltage. The CLCC switches from constant voltage control mode to maximum firing angle control, increasing the firing angle to boost the DC voltage on the inverter side. When a three-phase short-circuit fault occurs near the AC bus of the inverter station, the DC voltage drops significantly, and even if the CLCC reduces the turn-off angle, it cannot maintain the DC voltage on the inverter side at the set value. If the fault is more severe, the potential inside the CLCC will drop sharply, and the DC current will rise sharply. However, due to the presence of line inductance and smoothing reactor, the DC current will not rise in a step, but rather there will be a transient process. The DC transmission power of the MMC increases with the increase of DC current, while the active power absorbed on the AC side remains unchanged. An imbalance of power is generated between the AC and DC sides of the rectifier-side MMC. The discharge energy of the DC side submodule capacitor is greater than the charging energy of the submodule capacitor, and the submodule voltage decreases. Under the action of the AC side d-axis stator submodule voltage, the d-axis current increases, keeping the submodule capacitor voltage or energy constant. When the DC-side outer loop constant active power output DC current reference value reaches its upper limit, the MMC operates in constant current mode; the MMC reduces the DC modulation ratio m dc This means reducing the number of submodules to lower the DC component of the bridge arm voltage, thereby maintaining the stability of the DC current when a fault occurs in the receiving-end power grid.

4. The MMC-CLCC novel DC transmission system coordinated control method according to claim 1, characterized in that: The power characteristics of the CLCC mentioned in step S2 include the relationship between its active and reactive power and the change of turn-off angle and DC current under different AC voltage levels. The power characteristic model expression for CLCC is: ； In the formula, P CLCC For active power, Q CLCC For reactive power, U acI The inverter side AC voltage is γ, the turn-off angle is I. dc Where X is the DC current, N is the number of six-pulse commutator bridge groups, and X is the DC current. r For commutation reactance, U dcI0 This is the 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: ； in: 。 5. The MMC-CLCC novel DC transmission system coordinated control method according to claim 1, characterized in that: The multiple constraints mentioned in step S3 include: ； In the formula, γ min For the minimum shut-off angle, I γmin U is the DC current value constrained by the minimum turn-off angle. dcI I is the DC voltage on the inverter side. μmax I is the DC current value constrained by the maximum commutation overlap angle. dcmin To avoid discontinuous DC current, the minimum DC current value, I hmax This represents the maximum DC current constrained by the temperature rise during the period. The active and reactive power delivered by the inverter-side CLCC converter station to the AC system are shown in the following formulas: ； In the formula, P ac Q represents the active power delivered by the CLCC converter station to the AC system. ac B represents the reactive power transmitted from the CLCC converter station to the AC system. C This is the equivalent susceptance for reactive power compensation and filtering devices in inverter stations.

6. The MMC-CLCC novel DC transmission system coordinated control method according to claim 1, characterized in that: The calculation process for the DC current, active power, and reactive power operating limits of the inverter station under multiple constraints described in step S3 is as follows: First, input the specific parameters of the model, including the number of six-pulse converter bridge groups N and the commutation reactance X. r The equivalent susceptance B of the reactive power compensation and filtering device C Secondly, the initial input value is the AC bus voltage U of 0.1pu. acI and a DC current I with an initial value of 0 dc Then, based on the control mode, determine the inverter DC voltage or turn-off angle, and calculate the inverter-side commutation overlap angle μ according to the formula; when I dc When I < 0.1 pu dcmin Take 0.1 pu and calculate the minimum active power P transferred to the AC system. acmin and minimum reactive power Q acmin ; when I dc When the value is >0.1 pu, it is determined whether it is within the commutation overlap angle limit. If so, then I dc For I dcmax Otherwise, when the current is less than the maximum limit I of the DC current. dclim Increase the DC current and use it as I for the next cycle. dc The initial value of I is such that if the maximum DC current limit has already been reached, then... dcmax =I dclim And calculate the maximum active power P transmitted to the AC system. acmax and the maximum reactive power Q acmax Finally, increase U. acI Repeat the above process until U acI =1.0 pu; The inverter station's safe operation domain includes the DC current operation domain and the active and reactive power operation domains under different AC voltage levels.

7. The MMC-CLCC novel DC transmission system coordinated control method according to claim 1, characterized in that: Step S4 describes the use of CLCC dynamic reactive power control based on reactive power sensitivity constraints, while simultaneously utilizing MMC through maximum power command coordination, to achieve both AC voltage support and maximum DC power transmission during receiving-end faults. Specifically: After a fault occurs in the receiving-end grid of the MMC-CLCC hybrid DC transmission system, the CLCC turn-off angle on the inverter side is further reduced according to the reactive power sensitivity constraint, in order to reduce the reactive power consumed by the converter station and reduce the impact of DC current on reactive power. At the same time, the inverter side calculates the DC current value under the maximum transmitted active power based on the amplitude after the AC voltage drop, the turn-off angle, and the active power operating range, and transmits it to the sending end via communication. This serves as the DC current reference value for the sending end MMC inner loop current controller, and the MMC DC modulation ratio m is calculated accordingly. dc Since the receiving end adopts a reactive power sensitivity control strategy, the impact of DC current changes caused by communication delay on the reactive power of the receiving end will be significantly reduced. Furthermore, the sending end adopts an MMC active current limiting method based on bridge arm voltage control to reduce the DC voltage on the rectifier side, which can respond to faults more quickly. The hybrid bridge topology expands the adjustment range of the MMC DC voltage, ensuring DC power transmission capability without overshooting the MMC AC voltage.

8. A control system for an MMC-CLCC hybrid DC transmission system using the MMC-CLCC novel DC transmission system coordinated control method according to any one of claims 1 to 7, characterized in that: It can enhance the support capabilities of the sending and receiving ends, including: The data acquisition module is used to collect AC voltage, DC current, DC voltage and power data at the sending and receiving ends in real time. The analysis and decision-making module is used to calculate power operating limits and reactive power sensitivity, and to generate turn-off angle adjustment commands and DC current reference values. The execution control module, including the CLCC firing angle control unit and the MMC DC modulation ratio adjustment unit, receives commands and performs coordinated control.

9. The control system of the MMC-CLCC hybrid DC transmission system according to claim 8, characterized in that: The analysis and decision-making module includes: The operating range calculation unit performs the limit calculation process to determine the safe operating range of DC current and active and reactive power. The sensitivity analysis unit generates an initial turn-off angle reference signal using the reactive power sensitivity formula and reactive power sensitivity constraints. The collaborative control unit generates CLCC turn-off angle commands and MMC DC control loop inner loop DC current command parameters.

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