A method and system for reactive coupling control to inhibit commutation failure

Abstract

The application discloses a reactive coupling control method and system for inhibiting commutation failure, and the method comprises the following steps: after fault removal, the equivalent voltage and current of the LCC loop on the inverter side of the hybrid cascaded DC power transmission system are measured, the equivalent impedance of the LCC loop is calculated, and the risk of commutation failure on the inverter side is evaluated; when it is evaluated that there is a risk of commutation failure, the increasing speed of the trigger angle under the constant DC current control of the LCC on the rectifier side is improved through the trigger angle control of the rectifier side; and the reactive power response speed of the MMC on the inverter side is accelerated through reactive power compensation, the reactive power of the LCC on the inverter side is supplemented, and whether the risk of commutation failure is eliminated or not. The application can effectively avoid the influence of communication delay, improve the response speed of the coupled reactive power of the commutation failure receiving system, and inhibit transient low voltage and transient high voltage.

Description

Technical Field

[0001] This invention belongs to the field of high voltage direct current transmission technology, and relates to a reactive power coupling control method and system for suppressing commutation failure in a hybrid cascaded DC transmission system. Background Technology

[0002] Currently, the technology of LCC-based high voltage direct current (HVDC) transmission based on line-commutated converters is very mature, possessing many advantages such as adjustable transmission power and direction, low transmission line cost, and the ability to achieve DC fault ride-through. It is widely used in asynchronous interconnection of power grids and large-capacity long-distance power transmission. However, because line-commutated converters (LCCs) use semi-controlled switching elements and require AC grid access for commutation, LCC-HVDC suffers from insurmountable defects such as commutation failure.

[0003] High-voltage direct current (VSC) transmission based on voltage source converters (VSC-HVDC) employs fully controlled switching devices, fundamentally avoiding commutation failure. However, VSC has two degrees of control freedom, allowing independent control of active and reactive power, but it faces challenges such as high cost, low transmission capacity, and inability to overcome DC faults. LCC-VSC hybrid DC transmission, on the other hand, integrates the advantages of both systems and has significant application value in long-distance, high-capacity power transmission applications.

[0004] The novel hybrid cascaded DC transmission system employs an LCC (Liquid Crystal Capacitor) at the sending end and multiple Modular Multilevel Converters (MMCs) connected in series at the receiving end, with the MMCs belonging to the VSC (Variable Reactive Power Controller). This system can select different connection methods based on the strength of the receiving-end grid, offering advantages such as high reliability, large capacity, and multiple connection points. During AC faults at the receiving end, the MMCs can provide dynamic reactive power compensation, improving the voltage support capability of the receiving-end grid and thus mitigating LCC commutation failure. However, under severe fault conditions, the MMCs suffer from insufficient and inflexible reactive power support, leading to LCC commutation failure. Furthermore, existing research primarily focuses on the active power response under typical transient conditions such as system startup, power step shifts, and AC / DC faults, and designs control and protection strategies accordingly. For this novel hybrid cascaded DC transmission system, which is being applied for the first time, research on reactive power and voltage coordination control strategies is limited. Summary of the Invention

[0005] To address the shortcomings of existing technologies, this invention provides a reactive power coupling control method and system for suppressing commutation failure in a hybrid cascaded DC transmission system. By assessing the risk of commutation failure, controlling the firing angle on the rectifier side, and using reactive power compensation by the MMC on the inverter side, the characteristics of the receiving-end system are improved. The reactive power control capability of the MMC and the overall coordination of the system are utilized to enhance the ability of the receiving-end LCC to resist commutation failure, thereby improving the power supply capacity and stability level of the hybrid cascaded DC transmission system.

[0006] The present invention adopts the following technical solution.

[0007] A reactive power coupling control method for suppressing commutation failure includes the following steps:

[0008] S1: After the fault is cleared, measure the equivalent voltage and current of the LCC circuit on the inverter side of the hybrid cascaded DC transmission system and calculate the equivalent impedance of the LCC circuit. Based on the equivalent impedance of the LCC circuit, conduct a risk assessment of the LCC commutation failure on the inverter side.

[0009] S2: When a risk of commutation failure is assessed, the firing angle of the rectifier-side LCC under constant DC current control is increased by controlling the firing angle on the rectifier side to suppress overcurrent; and the reactive power response speed of the inverter-side MMC is accelerated by reactive power compensation to supplement the reactive power deficit of the inverter-side LCC and realize reactive power coupling at the receiving end.

[0010] S3: Determine whether the risk of commutation failure has been eliminated based on the current setting value of the LCC circuit on the inverter side. If so, end the rectifier side trigger angle control and the inverter side MMC reactive power compensation; otherwise, return to S2.

[0011] Preferably, the hybrid cascaded DC transmission system includes a series LCC on the sending end rectifier side, a high-voltage valve group on the receiving end inverter side consisting of one LCC, and a low-voltage valve group consisting of several identical MMCs connected in parallel.

[0012] Preferably, the sending-end LCC uses constant DC current control, the receiving-end LCC uses constant DC voltage control, and the MMC uses constant reactive power control.

[0013] Preferably, in S1, the commutation failure risk criterion is:

[0014]

[0015] Among them, R d The equivalent impedance of the LCC circuit;

[0016] β is the lead-out firing angle;

[0017] θ is the critical angle for commutation failure;

[0018] r0 is the equivalent resistance of the transformer leakage inductance.

[0019] Preferably, in S2, the strategy for controlling the rectifier-side firing angle is as follows:

[0020] Based on the DC current I on the rectifier side dc With the DC current setting value I on the rectifier side dcref The deviation value ΔI between the two outputs a firing angle change Δα, which is then added to the constant DC current control on the rectifier side to change the firing angle α of the rectifier side LCC. R This is to increase the rate of increase of the firing angle under the constant DC current control of the rectifier-side LCC, and to better suppress overcurrent.

[0021] Preferably, Δα = 10ΔI.

[0022] Preferably, in S2, the reactive current setting value i of any MMC on the inverter side after reactive power compensation is the constant reactive power control value. qref for:

[0023]

[0024] In the formula: V MMC V LCCI The DC voltages measured at the inverter-side MMC and inverter-side LCC are respectively.

[0025] K p K I Here, s is the PI control parameter, and s is a Laplace complex variable;

[0026] K ΔP and K ΔI The proportional gain and integral constant of the PI controller for reactive power compensation;

[0027] X ΔQ The compensation component output by the PI controller for constant reactive power control is the reactive power deficit of the inverter-side LCC compensated by MMC.

[0028] Preferably, in S3, every 50ms, it is determined whether the current setting value of the inverter-side LCC circuit has recovered to the value before the risk of commutation failure. If so, it means that the risk of commutation failure has been eliminated.

[0029] A reactive power coupling control system for suppressing commutation failure includes:

[0030] The commutation failure risk prediction module is used to measure the equivalent voltage and current of the LCC circuit on the inverter side of the hybrid cascaded DC transmission system and calculate the equivalent impedance of the LCC circuit after the fault is cleared. Based on the equivalent impedance of the LCC circuit, the module assesses the risk of commutation failure of the LCC on the inverter side.

[0031] The rectifier-side firing angle control module is used to increase the rate of increase of the firing angle under the constant DC current control of the rectifier-side LCC by controlling the firing angle of the rectifier side when a risk of commutation failure is assessed, so as to suppress overcurrent.

[0032] The reactive power compensation module is used to accelerate the reactive power response speed of the inverter-side MMC when a commutation failure risk is assessed, supplement the reactive power deficit of the inverter-side LCC, and realize reactive power coupling at the receiving end.

[0033] The commutation failure risk elimination detection module is used to determine whether the commutation failure risk has been eliminated based on the inverter-side LCC circuit current setting value. If it is eliminated, the rectifier-side trigger angle control and inverter-side MMC reactive power compensation will end; otherwise, it will return to the rectifier-side trigger angle control module and reactive power compensation module.

[0034] A terminal includes a processor and a storage medium; the storage medium is used to store instructions; the processor is used to perform operations according to the instructions to execute the steps of the method.

[0035] A computer-readable storage medium having a computer program stored thereon that, when executed by a processor, implements the steps of the method.

[0036] The beneficial effects of this invention are compared with those of the prior art:

[0037] (1) This invention addresses the problem of overcurrent and transient voltage instability on the rectifier side caused by LCC commutation failure on the inverter side of a hybrid cascaded DC transmission system. Based on the characteristics of DC voltage and current changes, the invention utilizes the characteristics of electrical quantity changes on the rectifier side as the basis for judging commutation failure, and obtains a commutation failure risk criterion based on the equivalent impedance of the LCC circuit. This can effectively avoid the impact of communication delay and improve the transient characteristics of the receiving end of the system.

[0038] (2) When the risk of commutation failure is assessed, the present invention increases the rate of increase of the firing angle under the constant DC current control of the LCC on the rectifier side by controlling the firing angle on the rectifier side, so as to suppress overcurrent and increase the stability of the DC system after a fault.

[0039] (3) Based on the reactive power consumed by the inverter-side LCC and the active and reactive power decoupling characteristics of the MMC, the reactive power compensation module of the present invention couples the inverter-side MMC with the inverter-side LCC when a commutation failure risk is assessed. The reactive power generated by the MMC is used to compensate the inverter-side LCC to supplement the reactive power deficiency of the inverter-side LCC. That is, the deviation value between the compensated reactive power is calculated by the PI controller and added to the MMC constant reactive power control to realize the reactive power coupling at the receiving end. This improves the response speed of the receiving end system coupled reactive power when there is a risk of commutation failure and can effectively suppress transient low voltage and transient high voltage. Attached Figure Description

[0040] Figure 1 This is a topology diagram of a hybrid cascaded DC transmission system with a single-pole structure;

[0041] Figure 2 This is the equivalent circuit diagram for impedance measurement of the LCC loop on the inverter side of a hybrid cascaded DC transmission system.

[0042] Figure 3 This is a flowchart for assessing the risk of commutation failure;

[0043] Figure 4 This is a block diagram illustrating the principle of the reactive power coupling control method of the present invention;

[0044] Figure 5 This is a control structure diagram for the normal operation of an ultra-high voltage direct current transmission system. Detailed Implementation

[0045] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions of this invention will be clearly and completely described below with reference to the accompanying drawings of the embodiments of this invention. The embodiments described in this application are merely some embodiments of this invention, and not all embodiments. Based on the spirit of this invention, other embodiments obtained by those skilled in the art without creative effort are all within the protection scope of this invention.

[0046] Embodiment 1 of this invention provides a reactive power coupling control method to suppress commutation failure. Generally, if no measures are taken after a serious fault is cleared, it will lead to the risk of commutation failure of the inverter-side LCC. Therefore, the method of this invention plays a role in preventing the risk of commutation failure during the stage when the system resumes normal operation after the fault is cleared. It is based on the characteristics of the constant DC current of the sending-end LCC, the constant DC voltage of the receiving-end high-end LCC, and the active and reactive power control of the receiving-end low-end MMC in a hybrid cascaded DC transmission system, to achieve reactive power coupling control to suppress commutation failure in the hybrid cascaded DC transmission system. Figure 1 This is a topology diagram of a hybrid cascaded DC transmission system with a single-pole structure. Figure 1In this hybrid cascaded DC transmission system, the rectifier station consists of two 12-pulse LCCs connected in series; the inverter station's high-voltage valve group consists of one 12-pulse LCC, and the low-voltage valve group consists of three identical half-bridge modular MMCs connected in parallel to match the LCC's transmission capacity. The sending-end LCC of this hybrid cascaded DC transmission system uses Constant DC Current Control (CCC) and employs Minimum Firing Angle Control (MFAC), typically 5°, to ensure reliable thyristor triggering. The inverter station LCC uses Constant DC Voltage Control (CVC) and works in parallel with the low-voltage MMCs to maintain stable DC voltage. Backup constant current control is also configured to improve AC fault ride-through capability. Simultaneously, a low-voltage dependent current order limiter (VDCOL) is typically added to the control strategies of the sending-end LCC and receiving-end LCC. This allows the system to automatically limit the current flowing through the DC line based on the degree of DC voltage drop upon detecting it, thus improving the system's transient characteristics. When the MMCs are put into operation, they can achieve independent decoupled control of active and reactive power. For active power control, this can be divided into constant active power control and constant DC voltage control, with only one option selected in actual operation. For reactive power control, this can be divided into constant reactive power control and constant AC voltage control, with one option selected in actual operation. In this invention, the three MMCs at the receiving end employ master-slave control. One MMC acts as the master station for the three stations, typically using a constant DC voltage mode for the active power outer loop; the other two MMCs act as slave stations, with their active power outer loops operating in a constant active power mode; the reactive power control target for all three MMCs is set to constant reactive power. The normal operation control structure of the hybrid cascaded DC transmission system is as follows: Figure 5 As shown. Since DC transmission often requires the delivery of a certain DC power P, the power calculation formula is P = UI, where U is the DC voltage and I is the DC current. The LCC on the sending-end rectifier side can control the DC current to reach the set value under constant DC current control. As a hybrid system, the LCC on the inverter side can control the DC voltage of the high-end valve group under constant voltage control, while the DC voltage of the low-end valve group is controlled by any MMC. The high and low-end DC voltages are added together to obtain the required DC voltage for the system. Since the above control parameters are not in the same converter station, inter-station communication is used to integrate the parameters to achieve the control objective. Therefore, this control mode is only used as the basic architecture for the scenario construction of this invention; this control exists in any DC transmission system containing an LCC.

[0047] In a preferred but non-limiting embodiment of the present invention, the reactive power coupling control method for suppressing commutation failure of the present invention includes the following steps:

[0048] S1: After the fault is cleared, measure the equivalent voltage and current of the LCC circuit on the inverter side of the hybrid cascaded DC transmission system and calculate the equivalent impedance of the LCC circuit. Based on the equivalent impedance of the LCC circuit, conduct a risk assessment of the LCC commutation failure on the inverter side.

[0049] More preferably, the commutation failure prediction module is based on the value of the extinction angle γ used to determine commutation failure. This invention converts the microscopic extinction angle into a easily measurable loop impedance, thus simplifying and facilitating the assessment of commutation failure risk and subsequent reactive power coordination control strategies. Specifically, the extinction angle formula based on a three-phase bridge rectifier circuit is expressed as:

[0050]

[0051] Among them, R d R is the equivalent impedance of the LCC circuit; r0 is the equivalent resistance of the transformer leakage inductance; β is the lead firing angle; R d Both β and β are known quantities, provided by the power grid system operator.

[0052] Suppose that a commutation failure occurs when γ < θ, that is:

[0053]

[0054] Right now:

[0055]

[0056] Further analysis revealed:

[0057]

[0058] in The condition that is naturally satisfied can be derived from γ > 0. Therefore:

[0059] When the commutation failure risk criterion shown in formula (5) is met, it can be determined that the system has a commutation failure risk.

[0060]

[0061] Preferably, θ is generally set to 15° to 18°.

[0062] Figure 2 This is an equivalent circuit diagram for impedance measurement of the LCC loop on the inverter side of a hybrid cascaded DC transmission system. Figure 2 The marked measurement point locations measure the circuit impedance. When an AC fault occurs in the system, the voltage U... d Decrease, current i d Increase, loop impedance R d Decrease, when R dWhen the commutation failure risk criterion is reduced, it can be determined that the system has a commutation failure risk.

[0063] Therefore, such as Figure 3 As shown, S1 specifically includes the following sub-steps:

[0064] S11: Measure the equivalent voltage U of the LCC circuit on the inverter side. d Current i d The parameters are: lead trigger angle β, transformer leakage inductance equivalent resistance r0, and commutation failure critical angle θ.

[0065] S12: According to U d i d Calculate the equivalent impedance R of the LCC circuit. d ;

[0066] S13: According to R d Commutation failure risk is assessed using β, r0, and θ. When the following commutation failure risk criterion is met, it indicates that the inverter-side LCC will face commutation failure risk:

[0067]

[0068] S2: When a risk of commutation failure is assessed, the firing angle of the rectifier-side LCC under constant DC current control is increased by controlling the firing angle on the rectifier side to suppress overcurrent; and the reactive power response speed of the inverter-side MMC is accelerated by reactive power compensation to supplement the reactive power deficit of the inverter-side LCC and realize reactive power coupling at the receiving end.

[0069] The aforementioned fast firing angle on the rectifier side is solely for suppressing overcurrent in the DC system, and this operation can increase the stability of the DC system after a fault. The reactive power coupling at the receiving end refers to the coupling between the inverter-side MMC and the inverter-side LCC. Originally, on the inverter side, the MMC and the inverter-side LCC were not interconnected. This invention compensates the reactive power generated by the MMC to the inverter-side LCC, hence the term "coupling."

[0070] More preferably, the rectifier-side firing angle control and reactive power compensation are respectively controlled by... Figure 4 The rectifier-side firing angle control module and reactive power compensation module are implemented in the middle. Figure 4 The rectifier-side firing angle control module and the reactive power compensation module in the system perform rectifier-side firing angle control and reactive power compensation after receiving the start signal "on".

[0071] 1) The rectifier-side firing angle control module controls the rectifier-side firing angle:

[0072] After S3 assessment identified a commutation failure risk in the LCC, the DC voltage on the inverter side of the hybrid cascaded transmission system plummeted. The reactive power consumed by both the rectifier-side converter LCC and the inverter-side converter LCC increased rapidly, leading to a surge in the rectifier-side DC current I. dcThe current increases rapidly, and the actual value of the rectifier-side DC current exceeds the rectifier-side DC current setting value I. dcref Based on the deviation ΔI between the two, a trigger angle change Δα is output, where Δα = 10ΔI and ΔI = I. dc -I dcref This change is then added to the constant DC current control on the rectifier side, that is, Δα is added to the firing angle α of the LCC on the rectifier side. R Above, change the LCC firing angle α R This increases the rate of increase in the firing angle under constant DC current control of the rectifier-side LCC, thus better suppressing overcurrent. The principle is that without rectifier-side firing angle control, the actual output rectifier-side firing angle is only α. R After adding Δα, it becomes α. R +Δα. Normally, the firing angle would increase after a fault to suppress overcurrent. This addition of firing angle control on the rectifier side will trigger this process earlier, further suppressing overcurrent and increasing the stability of the DC system after a fault.

[0073] 2) The reactive power compensation module performs reactive power compensation:

[0074] After the fault is cleared, and the risk of LCC commutation failure is assessed by S3, the reactive power compensation module is activated in this invention.

[0075] Before reactive power compensation is implemented, the reactive current setting value i of any MMC's constant reactive power control is... qref It can be represented as:

[0076] i qref =(Q MMCref -Q MMC (K) p +K I / s) (6)

[0077] In the formula K p K I These are the two control parameters of the PI controller.

[0078] i qref It is the setting value of the MMC reactive current;

[0079] Q MMCref This is the reactive power setting value for any of the three MMCs; since the reactive power control mode of all three MMCs is constant reactive power, the reactive current setting value Q is... MMCref The value is 0, so Q in formula (6) is 0. MMCref =0;

[0080] Q MMC The actual value of reactive power output to MMC;

[0081] s is a Laplace complex variable;

[0082] In this invention scenario, the three MMCs are identical in specifications. As mentioned earlier, each MMC has a decoupling function for active and reactive power control, allowing for independent control of active and reactive power. The MMC controlling the master station uses constant DC voltage control for its active power, working in conjunction with the inverter-side LCC to achieve the required DC voltage setting. The two slave MMCs use constant active power control to meet the DC power transmission requirements. Furthermore, all three MMCs use constant reactive power control, with a reactive current setting value Q. MMCref It is 0.

[0083] Q MMCref -Q MMC Let it be ΔQ, then i qref This can be further expressed as:

[0084]

[0085] K P ΔQ is denoted as X ΔQP K I / s·ΔQ is denoted as X ΔQI .

[0086] Formula (7) yields i when the strategy of this invention is not adopted. qref value.

[0087] The reactive current i controlled after the reactive power compensation module is put into operation qref The setting value is expressed as:

[0088]

[0089] In the formula: V MMC V LCCI The DC voltages measured at the inverter-side MMC and inverter-side LCC are respectively.

[0090] K ΔP and K ΔI This represents the proportional coefficient and integral constant of the PI controller in the reactive power compensation module, which is used to calculate the LCC compensation amount on the inverter side.

[0091] X ΔQ The compensation component is the output of the PI controller for constant reactive power control.

[0092] The · / 3 is because three MMCs are needed to simultaneously compensate for the reactive power demand of the inverter-side LCC. X ΔQ / 3 is the value after reactive power compensation. qref The parameters that are changed in the expression are represented by the formula, which is a concrete representation of the compensation model.

[0093] Formula (8) yields i after adopting the strategy proposed in this invention. qref value.

[0094] From formula (8), we can see that X ΔQ The larger the absolute value, the better for i qref The greater the impact of the value, the more V will increase as the circuit gradually returns to normal. MMC With V LCCI The smaller the difference, the better X will be. ΔQ The smaller the absolute value, the more it affects the speed at which MMC reactive power is generated / absorbed.

[0095] After the fault is cleared, the reactive power consumed by the high-side LCC on the inverter side increases rapidly, and the DC voltage drops sharply. Because the reactive power generated by the series-connected MMC constant reactive power control cannot immediately follow the reactive power support demand of the LCC, at this time V MMC -V LCCI If X > 0, then X ΔQ >0, and combining with formula (8), it can be seen that MMC and LCC were originally unrelated, i qref The change is only slow due to the actual geographical distance and electrical transmission distance. Compared with the expression (7) without reactive power compensation, the formula (8) after reactive power compensation is implemented, i qref It's even bigger now, meaning that the reactive power compensation module has increased i after its implementation. qref Increase the rate, while the reactive power generated by the MMC is related to i qref The magnitude is directly related, thus accelerating the response speed of the MMC in generating reactive power and playing a role in reactive power coupling at the receiving end. The increased reactive power can support the LCC on the inverter side, thereby suppressing the occurrence of commutation failure (the risk of commutation failure is caused by the low DC voltage of the LCC on the inverter side).

[0096] Because the voltage drop of the inverter-side LCC is large, the reactive power output of the MMC is relatively high, which may cause the inverter-side LCC voltage to be higher than normal, resulting in a transient high voltage. At this time, the reactive power output of the MMC decreases to allow the voltage to stabilize as quickly as possible. That is, for a period of time after the converter bus voltage of the inverter-side LCC changes from a transient low voltage to a transient high voltage, the MMC will continue to output reactive power due to the influence of the integral component output of the PI controller. At this time, V... MMC -V LCCI <0, then X ΔQ When the value is less than 0, the MMC changes from generating reactive power to absorbing reactive power, which further stabilizes the LCC voltage curve on the inverter side.

[0097] S3: Determine whether the risk of commutation failure has been eliminated based on the current setting value of the LCC circuit on the inverter side. If yes, end the current rectifier side trigger angle control and reactive power compensation; otherwise, return to S2.

[0098] Understandably, S1 detects the risk of subsequent commutation failure in the system; S3 detects the recovery of the current and can quickly determine that the system has stabilized, and the DC system switches from standby control triggered by the fault to normal operation control. That is, S1 addresses the transient process after the fault, while S3 addresses the process of the system approaching steady state. During the transient process, the current setting value alone cannot assess the risk of commutation failure, so the equivalent impedance of the LCC circuit in S1 is introduced as an evaluation index.

[0099] Figure 4 After receiving the start signal "on", the rectifier-side firing angle control module and the reactive power compensation module perform reactive power coupling control at the receiving end based on rectifier-side firing angle control and reactive power compensation. After the fault is cleared, under the action of VDCOL, the current setting value gradually recovers to the setting value I before the risk of commutation failure. d * (0) means that the commutation failure prediction module outputs a stop signal off. In addition, considering that the control system has a certain delay, in order to avoid erroneous output, it is generally believed that the duration of the DC low current stage after the risk of commutation failure is 50 to 100 ms. After setting a delay of 50 ms, it is judged whether the current setting value has recovered to the value before the risk of commutation failure.

[0100] That is, when S2 assesses the risk of commutation failure of the LCC on the inverter side, it simultaneously informs... Figure 4 The rectifier-side firing angle control module and reactive power compensation module send an on start signal to perform receiver-side reactive power coupling control based on rectifier-side firing angle control and reactive power coupling compensation, and check the inverter-side LCC circuit current setting value I every 50ms. d * Has it been restored to the previous value I with the risk of commutation failure? d * (0), if so, it means the risk of commutation failure has been eliminated, and at this time... Figure 4 The rectifier-side trigger angle control module and reactive power compensation module send a stop signal "off" to end the current inverter-side LCC commutation failure risk adjustment.

[0101] Embodiment 2 of the present invention provides a reactive power coupling control system for suppressing commutation failure, comprising:

[0102] The commutation failure risk prediction module is used to measure the equivalent voltage and current of the LCC circuit on the inverter side of the hybrid cascaded DC transmission system and calculate the equivalent impedance of the LCC circuit after the fault is cleared. Based on the equivalent impedance of the LCC circuit, the module assesses the risk of commutation failure of the LCC on the inverter side.

[0103] The rectifier-side firing angle control module is used to increase the rate of increase of the firing angle under the constant DC current control of the rectifier-side LCC by controlling the firing angle of the rectifier side when a risk of commutation failure is assessed, so as to suppress overcurrent.

[0104] The reactive power compensation module is used to accelerate the reactive power response speed of the inverter-side MMC when a commutation failure risk is assessed, supplement the reactive power deficit of the inverter-side LCC, and realize reactive power coupling at the receiving end.

[0105] The commutation failure risk elimination detection module is used to determine whether the commutation failure risk has been eliminated based on the inverter-side LCC circuit current setting value. If it is eliminated, the current rectifier-side trigger angle control and inverter-side MMC reactive power compensation will end; otherwise, it will return to the rectifier-side trigger angle control module and reactive power compensation module.

[0106] A terminal includes a processor and a storage medium; the storage medium is used to store instructions.

[0107] The processor is configured to operate according to the instructions to execute the steps of the method.

[0108] A computer-readable storage medium having a computer program stored thereon that, when executed by a processor, implements the steps of the method.

[0109] The beneficial effects of this invention are compared with those of the prior art:

[0110] (1) This invention addresses the problem of overcurrent and transient voltage instability on the rectifier side caused by LCC commutation failure on the inverter side of a hybrid cascaded DC transmission system. Based on the characteristics of DC voltage and current changes, the invention utilizes the characteristics of electrical quantity changes on the rectifier side as the basis for judging commutation failure, and obtains a commutation failure risk criterion based on the equivalent impedance of the LCC circuit. This can effectively avoid the impact of communication delay and improve the transient characteristics of the receiving end of the system.

[0111] (2) When the risk of commutation failure is assessed, the present invention increases the rate of increase of the firing angle under the constant DC current control of the LCC on the rectifier side by controlling the firing angle on the rectifier side, so as to suppress overcurrent and increase the stability of the DC system after a fault.

[0112] (3) Based on the reactive power consumed by the inverter-side LCC and the active and reactive power decoupling characteristics of the MMC, the reactive power compensation module of the present invention couples the inverter-side MMC with the inverter-side LCC when a commutation failure risk is assessed. The reactive power generated by the MMC is used to compensate the inverter-side LCC to supplement the reactive power deficiency of the inverter-side LCC. That is, the deviation value between the compensated reactive power is calculated by the PI controller and added to the MMC constant reactive power control to realize the reactive power coupling at the receiving end. This improves the response speed of the receiving end system coupled reactive power when there is a risk of commutation failure and can effectively suppress transient low voltage and transient high voltage.

[0113] This disclosure can be a system, method, and / or computer program product. A computer program product may include a computer-readable storage medium having computer-readable program instructions loaded thereon for causing a processor to implement various aspects of this disclosure.

[0114] Computer-readable storage media can be tangible devices capable of holding and storing instructions for use by an instruction execution device. Computer-readable storage media can be, for example—but not limited to—electrical storage devices, magnetic storage devices, optical storage devices, electromagnetic storage devices, semiconductor storage devices, or any suitable combination of the foregoing. More specific examples (a non-exhaustive list) of computer-readable storage media include: portable computer disks, hard disks, random access memory (RAM), read-only memory (ROM), erasable programmable read-only memory (EPROM or flash memory), static random access memory (SRAM), portable compact disc read-only memory (CD-ROM), digital multifunction disc (DVD), memory sticks, floppy disks, mechanical encoding devices, such as punch cards or recessed protrusions storing instructions thereon, and any suitable combination of the foregoing. The computer-readable storage media used herein are not to be construed as transient signals themselves, such as radio waves or other freely propagating electromagnetic waves, electromagnetic waves propagating through waveguides or other transmission media (e.g., light pulses through fiber optic cables), or electrical signals transmitted through wires.

[0115] The computer-readable program instructions described herein can be downloaded from computer-readable storage media to various computing / processing devices, or downloaded via a network, such as the Internet, local area network, wide area network, and / or wireless network, to an external computer or external storage device. The network may include copper transmission cables, fiber optic transmission, wireless transmission, routers, firewalls, switches, gateway computers, and / or edge servers. A network adapter card or network interface in each computing / processing device receives the computer-readable program instructions from the network and forwards them to the computer-readable storage media in the respective computing / processing device.

[0116] Computer program instructions used to perform the operations of this disclosure may be assembly instructions, instruction set architecture (ISA) instructions, machine instructions, machine-dependent instructions, microcode, firmware instructions, status setting data, or source code or object code written in any combination of one or more programming languages, including object-oriented programming languages ​​such as Smalltalk, C++, etc., and conventional procedural programming languages ​​such as the "C" language or similar programming languages. The computer-readable program instructions may execute entirely on the user's computer, partially on the user's computer, as a standalone software package, partially on the user's computer and partially on a remote computer, or entirely on a remote computer or server. In cases involving a remote computer, the remote computer may be connected to the user's computer via any type of network—including a local area network (LAN) or a wide area network (WAN)—or may be connected to an external computer (e.g., via the Internet using an Internet service provider). In some embodiments, electronic circuitry, such as programmable logic circuitry, field-programmable gate arrays (FPGAs), or programmable logic arrays (PLAs), is personalized by utilizing the status information of the computer-readable program instructions to implement various aspects of this disclosure.

[0117] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit it. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the specific implementation of the present invention. Any modifications or equivalent substitutions that do not depart from the spirit and scope of the present invention should be covered within the protection scope of the claims of the present invention.

Claims

1. A reactive power coupling control method for suppressing commutation failure, characterized in that: The method includes the following steps: S1: After fault clearance, measure the equivalent voltage and current of the LCC circuit on the inverter side of the hybrid cascaded DC transmission system and calculate the equivalent impedance of the LCC circuit. Based on the equivalent impedance of the LCC circuit, assess the risk of commutation failure on the inverter side LCC. The commutation failure risk criterion is as follows: （5） in, The equivalent impedance of the LCC circuit; It is a leading trigger angle; This is the critical angle for commutation failure. The equivalent resistance of the transformer leakage inductance; S2: When a commutation failure risk is assessed, the rate of increase of the firing angle under constant DC current control of the rectifier-side LCC is increased through rectifier-side firing angle control to suppress overcurrent; and the reactive power response speed of the inverter-side MMC is accelerated through reactive power compensation to supplement the reactive power deficit of the inverter-side LCC and achieve reactive power coupling at the receiving end; wherein, the reactive current setting value of the constant reactive power control of any MMC on the inverter side after reactive power compensation is... for: （8） In the formula: The DC voltages measured at the inverter-side MMC and inverter-side LCC are respectively. K p , K I Here, s is the PI control parameter, and s is a Laplace complex variable; and The proportional gain and integral constant of the PI controller for reactive power compensation; To compensate for the reactive power deficit of the inverter-side LCC using MMC; S3: Determine whether the risk of commutation failure has been eliminated based on the inverter-side LCC circuit current setting value. If yes, end the rectifier-side trigger angle control and inverter-side MMC reactive power compensation; otherwise, return to S2. Every 50ms, check whether the inverter-side LCC circuit current setting value has been restored to the value before the risk of commutation failure. If yes, it means that the risk of commutation failure has been eliminated.

2. The reactive power coupling control method for suppressing commutation failure according to claim 1, characterized in that: The hybrid cascaded DC transmission system includes a series LCC on the sending end rectifier side, a high-voltage valve group on the receiving end inverter side consisting of one LCC, and a low-voltage valve group consisting of several identical MMCs connected in parallel.

3. The reactive power coupling control method for suppressing commutation failure according to claim 2, characterized in that: The sending-end LCC uses constant DC current control, the receiving-end LCC uses constant DC voltage control, and the MMC uses constant reactive power control.

4. The reactive power coupling control method for suppressing commutation failure according to claim 1, characterized in that: In S2, the strategy for controlling the rectifier-side firing angle is as follows: Based on the DC current on the rectifier side I dc With the DC current setting value on the rectifier side I dcref Deviation value between Output a change in trigger angle This change is then added to the constant DC current control on the rectifier side, altering the firing angle of the rectifier-side LCC. α R This is to increase the rate of increase of the firing angle under the constant DC current control of the rectifier-side LCC, and to better suppress overcurrent.

5. The reactive power coupling control method for suppressing commutation failure according to claim 4, characterized in that: =10D I 。 6. A reactive power coupling control system for suppressing commutation failure, comprising the method described in any one of claims 1-5, characterized in that: The system includes: The commutation failure risk prediction module is used to measure the equivalent voltage and current of the LCC circuit on the inverter side of the hybrid cascaded DC transmission system and calculate the equivalent impedance of the LCC circuit after the fault is cleared. Based on the equivalent impedance of the LCC circuit, the module assesses the risk of commutation failure of the LCC on the inverter side. The rectifier-side firing angle control module is used to increase the rate of increase of the firing angle under the constant DC current control of the rectifier-side LCC by controlling the firing angle of the rectifier side when a risk of commutation failure is assessed, so as to suppress overcurrent. The reactive power compensation module is used to accelerate the reactive power response speed of the inverter-side MMC when a commutation failure risk is assessed, supplement the reactive power deficit of the inverter-side LCC, and realize reactive power coupling at the receiving end. The commutation failure risk elimination detection module is used to determine whether the commutation failure risk has been eliminated based on the inverter-side LCC circuit current setting value. If it is eliminated, the rectifier-side trigger angle control and inverter-side MMC reactive power compensation will end; otherwise, it will return to the rectifier-side trigger angle control module and reactive power compensation module.

7. A terminal, comprising a processor and a storage medium; characterized in that: The storage medium is used to store instructions; The processor is configured to operate according to the instructions to perform the steps of the method according to any one of claims 1-5.

8. A computer-readable storage medium having a computer program stored thereon, characterized in that, When executed by a processor, the program implements the steps of the method according to any one of claims 1-5.

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