A control method, device and system for suppressing DC continuous commutation failure

Abstract

The present invention discloses a control method, device, and system for suppressing continuous DC commutation failure in the field of power electronics control, aiming to address the problem of continuous DC commutation failure on the inverter side of a UHV hybrid cascade DC transmission system. The method comprises: obtaining the AC bus voltage at the receiving end of the UHV hybrid cascade DC transmission system, as well as the upper and lower limits of the critical commutation failure voltage; based on the comparison result of the AC bus voltage and the upper and lower limits of the critical commutation failure voltage, coordinating the switching states of the phase regulator excitation control module and the MMC reactive power regulation module to meet preset requirements; wherein the phase regulator excitation control module controls the phase regulator to generate reactive power compensation; and the MMC reactive power regulation module controls each MMC to increase its reactive power output. The present invention can fully utilize the MMC's own reactive power regulation capabilities and the reactive power compensation capabilities under the phase regulator's transient excitation control, suppressing continuous DC commutation failure and improving the transient characteristics of the UHV hybrid cascade DC transmission system after a fault.

Description

Technical Field

[0001] The present invention relates to a control method, device and system for suppressing direct current continuous commutation failure, and belongs to the field of power electronics. Background Art

[0002] Because my country's energy distribution is inversely proportional to its load centers, electricity must be transmitted over long distances. Ultra-high voltage direct current (UHVDC) transmission systems offer advantages for large-scale, long-distance power transmission, including large transmission capacity, rapid regulation and reliable operation, and self-protection capabilities in the event of line faults. Traditional DC transmission systems using line-commuted converters (LCCs) are mature, but they suffer from fixed current direction, high reactive power consumption, and numerous characteristic harmonics. Flexible DC transmission systems utilize fully controlled IGBT devices, overcoming the inherent drawbacks of traditional DC transmission. However, they also suffer from relatively high unit costs and losses.

[0003] The UHV hybrid cascaded DC transmission system combines the advantages of traditional DC transmission and flexible DC transmission. However, when a severe AC fault occurs at the receiving end, the inverter-side LCC still faces the risk of commutation failure. As the fault severity increases, the DC system may even experience continuous commutation failures. Providing reactive power compensation to the system can effectively prevent commutation failures. Traditional reactive power compensation devices include static synchronous compensators (STATCOMs), static VAR compensators (SVCs), and phase regulators. With the successful development of a new generation of phase regulators, the performance of phase regulators has been comprehensively improved. They are now widely used in many national DC projects. However, there are still problems with phase regulators being damaged by prolonged strong excitation, which can affect their service life and normal operation. Summary of the Invention

[0004] The purpose of the present invention is to overcome the deficiencies in the prior art and provide a control method, device and system for suppressing continuous commutation failure of DC power transmission, thereby solving the technical problems of suppressing the occurrence of continuous commutation failure and improving the transient stability of the system after a serious AC fault occurs at the receiving end of a UHV hybrid cascade DC power transmission system.

[0005] To achieve the above object, the present invention is implemented by adopting the following technical solutions:

[0006] In a first aspect, the present invention provides a control method for suppressing DC continuous commutation failure, comprising:

[0007] Obtain the AC bus voltage at the receiving end of the UHV hybrid cascaded DC transmission system, as well as the upper and lower limits of the critical voltage for commutation failure;

[0008] Based on the comparison result of the receiving-end AC bus voltage and the upper and lower limits of the critical voltage for commutation failure, the switching states of the phase regulator excitation control module and the MMC reactive power regulation module are coordinated and controlled so that the relationship between the AC bus voltage and the upper and lower limits of the critical voltage for commutation failure meets the preset requirements; wherein, when the phase regulator excitation control module is put into use, the phase regulator is controlled to emit reactive compensation and increase the excitation current; when the MMC reactive power regulation module is put into use, each MMC is controlled to increase the reactive power output.

[0009] Optionally, the upper and lower limits of the commutation failure critical voltage U cr2 and U cr1 They are defined by the following formulas:

[0010]

[0011]

[0012] Among them, R eq =R cr +R d -R ci , R cr =(3 / π)X cr , R ci =(3 / π)X ci , U LC is the rated effective value of the AC bus voltage at the receiving end; β and γ are the rated trigger advance angle and arc extinction angle of the LCC on the inverter side respectively; γ min is the limit arc extinction angle; X cr 、X ci are the commutation reactances of the rectifier side and the inverter side respectively; R d is the DC resistance; U dr0 is the no-load DC voltage on the rectifier side without considering the trigger delay; α r is the trigger delay angle on the rectifier side; k is the transformation ratio of the converter transformer; b is the number of series bridges.

[0013] Optionally, the coordinated control of the switching states of the phase shifter excitation control module and the MMC reactive power regulation module based on the comparison result of the receiving-end AC bus voltage and the upper and lower limits of the commutation failure critical voltage specifically includes the following steps:

[0014] When an AC fault occurs in the UHV hybrid cascade DC transmission system and the AC bus voltage U L and the upper limit of the critical voltage of commutation failure U cr2 Meet U L cr2 When the phase regulator strong excitation control module is activated, the phase regulator enters the strong excitation state, and causes the AC bus voltage to drop continuously. If the AC bus voltage U L ​' and the lower limit of the critical voltage of commutation failure U cr1 Meet U L ' cr1 When , it is determined that the first commutation failure occurs;

[0015] The MMC reactive power regulation module is put into operation, so that the AC bus voltage rises under the action of each MMC controlled by the MMC reactive power regulation module and the inverter side LCC in the fixed arc extinction angle control mode. If the AC bus voltage U L ″ is greater than the upper limit of the critical voltage of commutation failure U cr2 When the phase shifter excitation control module is cut off, the MMC reactive power regulation module controls each MCC to compensate for the reactive power required by the ultra-high voltage hybrid cascade DC transmission system;

[0016] After the preset delay, if the AC bus voltage can still be kept greater than the upper limit of the critical voltage of the commutation failure U cr2 , it indicates that the AC bus voltage has not experienced a secondary drop;

[0017] When the AC bus voltage is greater than a preset value, the preset value is greater than the upper limit of the critical voltage of the commutation failure U cr2 , indicating that the system has resumed steady-state operation and the MMC reactive power regulation module has been removed.

[0018] Optionally, the inverter-side LCC adopts a constant DC voltage control mode when the ultra-high voltage hybrid cascaded DC transmission system is in steady-state operation; after an AC fault occurs in the ultra-high voltage hybrid cascaded DC transmission system, the inverter-side LCC switches from the constant DC voltage control mode to the fixed arc extinction angle control mode.

[0019] Optionally, the constant DC voltage control mode is the actual value of the DC voltage U dc2 and DC voltage setting value The difference is then adjusted by PI to generate the trigger angle α;

[0020] The fixed extinction angle control method is to take the minimum value of the extinction angle γ within a cycle and then compare it with the extinction angle setting value γ * The trigger angle α is generated after PI adjustment.

[0021] Optionally, when the phase regulator excitation control module is in the on state, the phase regulator enters the excitation state and generates reactive power more than twice the rated capacity.

[0022] Optionally, the receiving end includes MMC1, MMC2 and MMC3 connected in cascade sequence, all of which are connected to the AC bus of the receiving end;

[0023] When the MMC reactive power regulation module is in the on state, MMC1 switches from constant reactive power control to constant AC voltage control to increase reactive power output;​

[0024] MMC2 and MCC3 are in constant reactive power control. The difference between the rated value of the arc extinction angle γN and the minimum value of the arc extinction angle γ in each cycle is calculated. The reference value ΔQ of the required reactive power compensation is obtained through the PI link and limiting. The reference value ΔQ is then evenly distributed to MMC2 and MMC3. In the constant reactive power control of MMC2 and MCC3, ΔQ / 2 is added to increase the reactive power output.

[0025] Optionally, the constant AC voltage control is to set the DC voltage to a value U dc_ref The actual value of DC voltage U dc The difference is then generated through the PI link to generate active current i d_ref Into the inner loop current controller;

[0026] The constant reactive power control is to set the reactive power value Q ref and the actual value of reactive power Q m The difference is then generated through the PI link to generate reactive current i q_ref Enter the inner loop current controller.

[0027] In a second aspect, the present invention provides a control device for suppressing DC continuous commutation failure, comprising:

[0028] An acquisition module is used to obtain the AC bus voltage at the receiving end of the UHV hybrid cascade DC transmission system, as well as the upper and lower limits of the commutation failure critical voltage;

[0029] The coordination control module is used to coordinate the switching status of the phase regulator excitation control module and the MMC reactive power regulation module based on the comparison result of the receiving-end AC bus voltage and the upper and lower limits of the commutation failure critical voltage, so that the relationship between the AC bus voltage and the upper and lower limits of the commutation failure critical voltage meets the preset requirements; wherein, when the phase regulator excitation control module is put into use, the phase regulator is controlled to generate reactive compensation and increase the excitation current; when the MMC reactive power regulation module is put into use, each MMC is controlled to increase the reactive power output.

[0030] In a third aspect, the present invention provides a control system for suppressing DC continuous commutation failure, comprising:

[0031] processors and storage media;

[0032] The storage medium is used to store instructions;

[0033] The processor is configured to operate according to the instructions to execute the steps of the method according to any one of the first aspects.

[0034] Compared with the prior art, the present invention has the following beneficial effects:

[0035] The present invention adopts a UHV hybrid cascade coordinated control method and system based on phase-shifting excitation to suppress continuous DC commutation failure. By combining the phase-shifting excitation characteristics and the MMC reactive power regulation capability, reactive power compensation is provided for the system after a fault occurs, thereby reducing the reactive power absorbed by the DC system from the receiving-end power grid, thereby achieving the purpose of stabilizing the receiving-end AC bus voltage, thereby suppressing continuous DC commutation failure and improving the transient characteristics of the system after the fault. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] Figure 1 This is a topology diagram of a UHV hybrid cascaded DC transmission system in one embodiment of the present invention;

[0037] Figure 2 This is a rectifier-side LCC control diagram in one embodiment of the present invention;

[0038] Figure 3 This is an inverter-side LCC control diagram in one embodiment of the present invention;

[0039] Figure 4 This is a general control diagram of the inner and outer loops of the receiving-end MMC in one embodiment of the present invention;

[0040] Figure 5 A block diagram of a reactive power coordinated control method according to an embodiment of the present invention;

[0041] Figure 6 This is a flow chart of a judgment module in an embodiment of the present invention;

[0042] Figure 7 This is a waveform diagram of electrical quantities in a system with or without a coordinated control method in one embodiment of the present invention;

[0043] Figure 8 This is a reactive power waveform diagram of a phase regulator in an embodiment of the present invention;

[0044] Figure 9 This is a waveform diagram of reactive power output by MMC with and without the coordinated control method in an embodiment of the present invention. DETAILED DESCRIPTION

[0045] The technical solution of the present invention is described in detail below through the accompanying drawings and specific embodiments. It should be understood that the embodiments of the present application and the specific features in the embodiments are detailed descriptions of the technical solution of the present application, rather than limitations on the technical solution of the present application. Unless there is a conflict, the embodiments of the present application and the technical features in the embodiments can be combined with each other.

[0046] The term "and / or" in this article is merely a description of the association relationship between associated objects, indicating that three relationships may exist. For example, A and / or B can mean: A exists alone, A and B exist at the same time, and B exists alone.

[0047] The application principle of the present invention is described in detail below with reference to the accompanying drawings.

[0048] Example 1

[0049] An embodiment of the present invention provides a control method for suppressing DC continuous commutation failure, comprising the following steps:

[0050] Obtain the AC bus voltage at the receiving end of the UHV hybrid cascaded DC transmission system, as well as the upper and lower limits of the critical voltage for commutation failure;

[0051] Based on the comparison result of the receiving-end AC bus voltage and the upper and lower limits of the critical voltage for commutation failure, the switching states of the phase regulator excitation control module and the MMC reactive power regulation module are coordinated and controlled so that the relationship between the AC bus voltage and the upper and lower limits of the critical voltage for commutation failure meets the preset requirements; wherein, when the phase regulator excitation control module is put into use, the phase regulator is controlled to emit reactive compensation and increase the excitation current; when the MMC reactive power regulation module is put into use, each MMC is controlled to increase the reactive power output.

[0052] In a specific implementation of the embodiment of the present invention, the upper and lower limits of the commutation failure critical voltage U cr2 and U cr1 They are defined by the following formulas:

[0053]

[0054]

[0055] Among them, R eq =R cr +R d -R ci , R cr =(3 / π)X cr , R ci =(3 / π)X ci , U LC is the rated effective value of the AC bus voltage at the receiving end; β and γ are the rated trigger advance angle and arc extinction angle of the LCC on the inverter side respectively; γ min is the limit arc extinction angle; X cr 、X ci are the commutation reactances of the rectifier side and the inverter side respectively; R d is the DC resistance; U dr0 is the no-load DC voltage on the rectifier side without considering the trigger delay; α r is the trigger delay angle on the rectifier side; k is the transformation ratio of the converter transformer; b is the number of series bridges.

[0056] In a specific implementation of the embodiment of the present invention, the coordinated control of the switching state of the phase shifter excitation control module and the MMC reactive power regulation module based on the comparison result of the receiving-end AC bus voltage and the upper and lower limits of the commutation failure critical voltage specifically includes the following steps:

[0057] When an AC fault occurs in the UHV hybrid cascade DC transmission system and the AC bus voltage U L and the upper limit of the critical voltage of commutation failure U cr2 Meet U L cr2 When the phase regulator strong excitation control module is activated, the phase regulator enters the strong excitation state, and causes the AC bus voltage to drop continuously. If the AC bus voltage U L ' and the lower limit of the critical voltage of commutation failure U cr1 Meet U L ' cr1 When , it is determined that the first commutation failure occurs;

[0058] The MMC reactive power regulation module is put into operation, so that the AC bus voltage rises under the action of each MMC controlled by the MMC reactive power regulation module and the inverter side LCC in the fixed arc extinction angle control mode. If the AC bus voltage U L ″ is greater than the upper limit of the critical voltage of commutation failure U cr2 When the phase shifter excitation control module is cut off, the MMC reactive power regulation module controls each MCC to compensate for the reactive power required by the ultra-high voltage hybrid cascade DC transmission system;

[0059] After the preset delay, if the AC bus voltage can still be kept greater than the upper limit of the critical voltage of the commutation failure U cr2 , it indicates that the AC bus voltage has not experienced a secondary drop;

[0060] When the AC bus voltage is greater than a preset value, the preset value is greater than the upper limit of the critical voltage of the commutation failure U cr2 , indicating that the system has resumed steady-state operation and the MMC reactive power regulation module has been removed.

[0061] In a specific implementation of the embodiment of the present invention, the inverter-side LCC adopts a constant DC voltage control mode during steady-state operation of the ultra-high voltage hybrid cascaded DC transmission system; after an AC fault occurs in the ultra-high voltage hybrid cascaded DC transmission system, the inverter-side LCC switches from the constant DC voltage control mode to the fixed arc extinction angle control mode.

[0062] In a specific implementation of the embodiment of the present invention, the constant DC voltage control mode is the actual value of the DC voltage U dc2 and DC voltage setting value The difference is then adjusted by PI to generate the trigger angle α;​​

[0063] The fixed extinction angle control method is to take the minimum value of the extinction angle γ within a cycle and then compare it with the extinction angle setting value γ * The trigger angle α is generated after PI adjustment.

[0064] In a specific implementation of the embodiment of the present invention, when the phase modulator excitation control module is in the on state, the phase modulator enters the excitation state and generates reactive power more than twice the rated capacity.

[0065] In a specific implementation of the embodiment of the present invention, the receiving end includes MMC1, MMC2 and MMC3 cascaded in sequence, all of which are connected to the AC bus of the receiving end;

[0066] When the MMC reactive power regulation module is in the on state, MMC1 switches from constant reactive power control to constant AC voltage control to increase reactive power output;

[0067] MMC2 and MCC3 are in constant reactive power control. The difference between the rated value of the arc extinction angle γN and the minimum value of the arc extinction angle γ in each cycle is calculated. The reference value ΔQ of the required reactive power compensation is obtained through the PI link and limiting. The reference value ΔQ is then evenly distributed to MMC2 and MMC3. In the constant reactive power control of MMC2 and MCC3, ΔQ / 2 is added to increase the reactive power output.

[0068] In a specific implementation of the embodiment of the present invention, the constant AC voltage control is to set the DC voltage to a value U dc_ref The actual value of DC voltage U dc The difference is then generated through the PI link to generate active current i d_ref Into the inner loop current controller;

[0069] The constant reactive power control is to set the reactive power value Q ref and the actual value of reactive power Q m The difference is then generated through the PI link to generate reactive current i q_ref Enter the inner loop current controller.

[0070] The method in the embodiment of the present invention is described in detail below with reference to a specific implementation manner.

[0071] First, the topology of the UHV hybrid cascaded DC transmission system and the control methods of each converter on the rectifier and inverter sides are introduced, the important system parameters are given, and the system model is established.

[0072] Figure 1This is the topology of a UHV hybrid cascaded DC transmission system. The model's sending end consists of four sets of 12-pulse converter valves connected in series. The receiving end uses a hybrid cascade of 12-pulse LCCs and modular multilevel converters (MMCs) using half-bridge submodules. The hybrid cascade structure consists of three MMCs connected in parallel and one LCC in series. AC filters are installed on both sides. The main parameters of the UHV hybrid cascaded DC transmission system are shown in Tables 1 and 2.

[0073] Table 1

[0074]

[0075] Table 2

[0076]

[0077] The rectifier side LCC adopts constant DC current control. The rectifier side LCC control diagram is as follows: Figure 2 As shown. Adding low voltage current limiting control (Voltage Dependent Current Order Limiters, VDCOL) prevents the impact on the system caused by the DC voltage being too low and the DC current being too high when a fault occurs. Under normal operating conditions, the rectifier side quickly adjusts the trigger angle α through the current controller to keep the DC current constant. Specifically, the actual value of the DC current I dc1 After being adjusted by the low voltage current limiting link and the DC current setting value I d * c1 Compare and take the smaller value, and then compare it with the actual value of DC current I dc1 The difference is then adjusted by PI to produce the trigger angle α. The inverter side LCC control diagram is as follows Figure 3 As shown. Constant DC voltage control is the control mode of the inverter side LCC. The control part also includes a standby control mode, which is a fixed arc extinction angle control. Constant DC voltage control is the actual value of the DC voltage U dc2 and DC voltage setting value The difference is then adjusted by PI to generate the trigger angle α. The fixed extinction angle control as a backup control method is to take the minimum value of the extinction angle γ within a cycle and then compare it with the extinction angle setting value γ * The difference is then adjusted by PI to generate the trigger angle α.

[0078] MMC total control diagram is as follows Figure 4 As shown. MMC adopts dual-loop control, including an outer loop power / voltage controller and an inner loop current controller. The outer loop power / voltage controller must select an active power physical quantity and a reactive power physical quantity for control. Figure 1The control mode selected by the receiving MMC in the model is shown in Table 3. The receiving MMC adopts an inner loop current controller, which has fast DC response characteristics and good internal current limiting capability. The active power physical quantity control mode MMC1 adopts constant DC voltage control, which is to set the DC voltage setting value U dc_ref The actual value of DC voltage U dc The difference is then generated through the PI link to generate active current i d_ref , MMC2 / 3 adopts fixed active power control, which sets the active power setting value P ref and the actual value of active power P m The difference is then generated through the PI link to generate active current i d_ref The three MMCs all use fixed reactive power control, which is to set the reactive power value Q ref and the actual value of reactive power Q m The difference is then generated through the PI link to generate reactive current i q_ref The MMC inner loop control link receives the reference value i of active and reactive current from the outer loop control. d_ref and i q_ref . And quickly track the reference current to achieve direct control of the current waveform and phase on the AC side of the MMC.

[0079] Table 3

[0080]

[0081] When a serious AC fault occurs at the receiving end of a hybrid cascaded HVDC transmission system, the system response is divided into two stages according to the occurrence of the first commutation failure and the continuous commutation failure.

[0082] Phase 1: From the occurrence of the fault to the first commutation failure. Under normal operation of the DC system, equation (1) holds:

[0083]

[0084] Where: X c is the commutation reactance; I d is the DC current; U LC is the rated effective value of the AC bus voltage at the receiving end; γ and β are the initial arc extinction angle and leading triggering angle of the LCC converter on the inverter side, respectively.

[0085] After the fault occurs, the AC bus voltage at the receiving end continues to drop, causing the rated effective value of the AC bus voltage at the receiving end to decrease to U LC ', the DC current rises to I d', since the first commutation failure occurs shortly after the AC fault, the DC control system does not have enough time to react and take measures. Therefore, when analyzing the first commutation failure, the adjustment action taken by the DC control system on the leading trigger angle β can be ignored. The leading trigger angle β remains unchanged, and γ' is the actual value of the arc extinction angle after the fault. Then, during the fault period, equation (2) holds true:

[0086]

[0087] From formula (2), if both sides of the equation are established, the arc extinction angle γ0 decreases compared with the arc extinction angle setting value γ after the fault occurs. When the arc extinction angle γ' is less than the limit arc extinction angle γ min When , the inverter side LCC converter fails in the first commutation.

[0088] Phase 2: The system returns to steady-state operation after the first commutation failure. After the first commutation failure, the DC control system immediately increases the lead firing angle, and the DC current begins to decrease under the low-voltage current-limiting command. However, the AC bus voltage at the receiving end fluctuates significantly because the DC system absorbs a large amount of reactive power from the AC bus, causing a secondary drop in the AC bus voltage. This puts the inverter-side LCC at risk of continuous commutation failure during system recovery.

[0089] Therefore, in order to improve the transient characteristics of the system during the fault and suppress the continuous commutation failure of the receiving-end LCC converter, the embodiment of the present invention proposes a coordinated control method combining the phase-shifting excitation characteristics and the MMC reactive power regulation to achieve the purpose of coordinated control. cr1 You can take 0.94pu, U cr2 The specific process of the coordinated control method is as follows: Figure 6 As shown. First, measure the receiving end AC bus voltage U L , when U L cr2 When the condenser is in the strong excitation state, it takes a certain amount of time for the condenser to enter the strong excitation state. L It will continue to fall in a short period of time. L cr1 When the MMC reactive power regulation module is activated, the system has reactive power compensation output by the phase regulator. Therefore, the reactive power compensation calculated by the arc extinction angle difference is within the tolerance range of the MMC. Therefore, the required reactive power compensation value will not exceed the reactive power output capacity of the MMC. After the first commutation failure occurs, U L Under the action of reactive power compensation and DC control system, when U L >U cr2 ​​When the phase regulator exits the forced excitation control to protect the equipment itself, the MMC reactive power is adjusted to compensate for the reactive power required by the system to prevent U L A secondary drop occurs, thereby suppressing the occurrence of continuous commutation failure. After a delay, if U L >U cr2 , then U L No secondary fall occurred. L When ≥1, the system returns to steady-state operation and the MMC reactive power regulation module is cut off.

[0090] Phase condensers have a strong excitation characteristic and can generate reactive power at least twice their rated capacity in a short period of time. Preferably, in this embodiment of the present invention, they can generate reactive power at 3.5 times their rated capacity in a short period of time. Phase condensers are installed at the DC receiving end. When a serious system fault causes a significant voltage drop, the phase condensers enter a strong excitation state, providing emergency reactive power and voltage support for the system. This helps quickly restore DC power and system voltage, and stabilizes the AC bus voltage at the receiving end. Phase condensers are connected to the AC bus at the receiving end of the system. The main parameters of the phase condenser system are shown in Table 4.

[0091] Table 4

[0092]

[0093] like Figure 5 As shown in the figure, in the MMC reactive power regulation module, MMC1: after a fault occurs, the DC current of MMC1, which uses a constant DC voltage control method for active power control, will increase. At this time, MMC1 needs to increase its reactive output to compensate for the reactive power required by the system. If the reactive power setting value is directly increased in the constant reactive power control used in the reactive power control method, the reactive current will increase at the same time. In this way, the IGBT used in MMC1 is at risk of being locked due to overcurrent. After a fault occurs, the reactive power control of MMC1 is changed to a constant AC voltage control through the judgment link. Because the system AC voltage U ac After the AC fault occurs, the AC voltage rating U acref With AC voltage U ac The difference between the two increases, and the i generated by the PI link qref After being limited, it is transmitted to the inner loop current controller to increase the reactive power output. Figure 5 As shown, in the MMC reactive power regulation module MMC2 / 3: under the condition of steady-state operation of the system, the inverter side LCC adopts constant DC voltage control. After an AC fault occurs, it switches to the fixed arc extinction angle control mode according to the judgment link instruction. At the same time, the arc extinction angle rated value γ NThe reference value ΔQ for the required reactive power compensation is calculated by subtracting it from the minimum value of the arc extinction angle γ in each cycle, passing through the PI link and limiting. This reference value is then evenly distributed to MMC2 and MMC3. In the MMC2 / 3 constant reactive power control, 1 / 2ΔQ is added to each to increase reactive power output.

[0094] Example 2

[0095] Based on the same inventive concept as that of Example 1, an embodiment of the present invention provides a control device for suppressing DC continuous commutation failure, comprising:

[0096] An acquisition module is used to obtain the AC bus voltage at the receiving end of the UHV hybrid cascade DC transmission system, as well as the upper and lower limits of the commutation failure critical voltage;

[0097] The coordination control module is used to coordinate the switching status of the phase regulator excitation control module and the MMC reactive power regulation module based on the comparison result of the receiving-end AC bus voltage and the upper and lower limits of the commutation failure critical voltage, so that the relationship between the AC bus voltage and the upper and lower limits of the commutation failure critical voltage meets the preset requirements; wherein, when the phase regulator excitation control module is put into use, the phase regulator is controlled to generate reactive compensation and increase the excitation current; when the MMC reactive power regulation module is put into use, each MMC is controlled to increase the reactive power output.

[0098] The rest are the same as in Example 1.

[0099] Example 3

[0100] Based on the same inventive concept as that of Example 1, an embodiment of the present invention provides a control system for suppressing DC continuous commutation failure, comprising:

[0101] processors and storage media;

[0102] The storage medium is used to store instructions;

[0103] The processor is configured to operate according to the instructions to execute the steps of any of the aforementioned methods.

[0104] Example 4

[0105] The UHV hybrid cascaded DC transmission model introduced in this invention was built in PSCAD / EMTDC and the following two simulation cases were compared and analyzed:

[0106] Case 1: UHV hybrid cascaded DC transmission model;

[0107] Case 2: Add reactive power coordinated control method to the UHV hybrid cascaded DC transmission model in Case 1.

[0108] The simulation waveforms of the electrical quantities in the system in different cases are as follows: Figure 7 After the system enters steady-state operation, a three-phase short circuit fault is set at the inverter side commutation bus at 1s through a grounding inductor. The inductance is 0.4H and the fault lasts for 0.1s. Figure 7 (a) The AC bus voltage at the receiving end decreases in the first drop and drops to 0.98 pu for the second time, which is greater than the upper limit of the critical voltage for commutation failure. The system does not have the risk of commutation failure. Figure 7 (c) The arc extinction angle did not drop to 0° during the second drop of the AC bus voltage at the receiving end, and the system did not experience DC continuous commutation failure. Figure 8 This is the reactive power waveform of the phase regulator. Figure 9 The figure below shows the reactive power output waveform of the MMC. It can be seen that during the fault period, the phase regulator and MMC provided corresponding reactive power compensation for the system, improved the waveforms of various electrical quantities in the system, and suppressed the occurrence of continuous commutation failures.

[0109] It can be seen from the simulation results that the control method for suppressing continuous DC commutation failure provided by the present invention reduces the voltage drop of the receiving-end AC bus during the first commutation failure, slows down the DC current fluctuation of the inverter-side LCC, and improves the transient characteristics of the system after the fault. By compensating the reactive power of the DC system, the reactive power absorbed by the DC system from the receiving-end power grid during the fault is reduced, thereby reducing the secondary drop amplitude of the receiving-end AC bus voltage and avoiding the occurrence of continuous commutation failure of the system. It gives full play to the characteristics of the phase regulator's strong excitation operation and the reactive power regulation capability of the MMC itself. Not only does it achieve the purpose of improving the transient characteristics of the system and suppressing continuous commutation failure through the coordination between the two, but it also avoids the problem of the phase regulator's own service life being damaged due to long-term strong excitation operation, and ensures that it maintains its own steady-state operation when the MMC issues additional reactive compensation.

[0110] Those skilled in the art will appreciate that the embodiments of the present application can be provided as methods, systems, or computer program products. Therefore, the present application can adopt the form of a complete hardware embodiment, a complete software embodiment, or an embodiment in combination with software and hardware. Moreover, the present application can adopt the form of a computer program product implemented on one or more computer-usable storage media (including but not limited to magnetic disk storage, CD-ROM, optical storage, etc.) that contain computer-usable program code.

[0111] The present application is described with reference to the flowcharts and / or block diagrams of the methods, devices (systems), and computer program products according to the embodiments of the present application. It should be understood that each process and / or box in the flowchart and / or block diagram, as well as the combination of the processes and / or boxes in the flowchart and / or block diagram, can be implemented by computer program instructions. These computer program instructions can be provided to a processor of a general-purpose computer, a special-purpose computer, an embedded processor, or other programmable data processing device to produce a machine, so that the instructions executed by the processor of the computer or other programmable data processing device generate instructions for implementing the steps in the process. Figure 1 a process or multiple processes and / or boxes Figure 1 A device that provides the functions specified in a block or multiple blocks.

[0112] These computer program instructions may also be stored in a computer readable memory that can direct a computer or other programmable data processing device to work in a specific manner, so that the instructions stored in the computer readable memory produce an article of manufacture comprising an instruction device, which implements the process Figure 1 a process or multiple processes and / or boxes Figure 1 The function specified in one or more boxes.

[0113] These computer program instructions can also be loaded onto a computer or other programmable data processing device so that a series of operational steps are executed on the computer or other programmable device to produce a computer-implemented process, thereby providing the instructions executed on the computer or other programmable device for implementing the process. Figure 1 a process or multiple processes and / or boxes Figure 1 A step that specifies a function in one or more boxes.

[0114] The embodiments of the present invention are described above in conjunction with the accompanying drawings, but the present invention is not limited to the above-mentioned specific implementation methods. The above-mentioned specific implementation methods are merely illustrative and not restrictive. Under the guidance of the present invention, ordinary technicians in this field can also make many forms without departing from the scope of protection of the purpose of the present invention and the claims, which are all protected by the present invention.

[0115] The basic principles, main features, and advantages of the present invention are shown and described above. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The above embodiments and descriptions are merely illustrative of the principles of the present invention. Various changes and modifications may be made to the present invention without departing from the spirit and scope of the present invention. Such changes and modifications are intended to fall within the scope of the present invention. The scope of protection claimed in the present invention is defined by the appended claims and their equivalents.

Claims

1. A control method for suppressing DC continuous commutation failure, characterized in that: include: Obtain the AC bus voltage at the receiving end of the UHV hybrid cascaded DC transmission system, as well as the upper and lower limits of the critical voltage for commutation failure; Based on the comparison result of the receiving-end AC bus voltage and the upper and lower limits of the commutation failure critical voltage, the switching states of the phase-converter excitation control module and the MMC reactive power regulation module are coordinated and controlled so that the relationship between the AC bus voltage and the upper and lower limits of the commutation failure critical voltage meets the preset requirements; wherein, when the phase-converter excitation control module is put into use, the phase-converter is controlled to generate reactive power compensation and increase the excitation current; when the MMC reactive power regulation module is put into use, each MMC is controlled to increase reactive power output; The receiving end includes MMC1, MMC2 and MMC3 which are cascaded in sequence, and all three are connected to the AC busbar of the receiving end; When the MMC reactive power regulation module is in the on state, MMC1 switches from constant reactive power control to constant AC voltage control to increase reactive power output; MMC2 and MMC3 are in constant reactive power control, and the extinction angle rating is γ N The reference value ΔQ of the required reactive power compensation is obtained by subtracting it from the minimum value of the arc extinction angle γ in each cycle through the PI link and after limiting. The reference value ΔQ is then evenly distributed to MMC2 and MMC3. In the constant reactive power control of MMC2 and MMC3, ΔQ / 2 is added to each to increase the reactive power output. The method of coordinating and controlling the switching states of the phase shifter excitation control module and the MMC reactive power regulation module based on the comparison result of the receiving-end AC bus voltage and the upper and lower limits of the commutation failure critical voltage specifically includes the following steps: When an AC fault occurs in the UHV hybrid cascade DC transmission system and the AC bus voltage U L and the upper limit of the critical voltage of commutation failure U cr2 Meet U L <U cr2 When the phase regulator strong excitation control module is activated, the phase regulator enters the strong excitation state, and causes the AC bus voltage to drop continuously. If the AC bus voltage U L ' and the lower limit of the critical voltage of commutation failure U cr1 Meet U L '<U cr1 When , it is determined that the first commutation failure occurs; The MMC reactive power regulation module is put into operation, so that the AC bus voltage rises under the action of each MMC controlled by the MMC reactive power regulation module and the inverter side LCC in the fixed arc extinction angle control mode. If the AC bus voltage U L " is greater than the upper limit of the critical voltage of commutation failure U cr2 When the phase shifter excitation control module is cut off, the MMC reactive power regulation module controls each MMC to compensate for the reactive power required by the ultra-high voltage hybrid cascade DC transmission system; After the preset delay, if the AC bus voltage can still be kept greater than the upper limit of the critical voltage of the commutation failure U cr2 , it indicates that the AC bus voltage has not experienced a secondary drop; When the AC bus voltage is greater than a preset value, the preset value is greater than the upper limit of the critical voltage of the commutation failure U cr2 , indicating that the system has resumed steady-state operation and the MMC reactive power regulation module has been removed.

2. The control method for suppressing DC continuous commutation failure according to claim 1, characterized in that: The upper and lower limits of the commutation failure critical voltage U cr2 and U cr1 They are defined by the following formulas: Among them, R eq =R cr +R d -R ci , R cr =(3 / π)X cr , R ci =(3 / π)X ci , U LC is the rated effective value of the AC bus voltage at the receiving end; β and γ are the rated trigger advance angle and arc extinction angle of the LCC on the inverter side respectively; γ min is the limit arc extinction angle; X cr 、X ci are the commutation reactances of the rectifier side and the inverter side respectively; R d is the DC resistance; U dr0 is the no-load DC voltage on the rectifier side without considering the trigger delay; α r is the trigger delay angle on the rectifier side; k is the transformation ratio of the converter transformer; b is the number of series bridges.

3. The control method for suppressing DC continuous commutation failure according to claim 1, characterized in that: The inverter-side LCC adopts a constant DC voltage control mode when the ultra-high voltage hybrid cascade DC transmission system is in steady-state operation; after an AC fault occurs in the ultra-high voltage hybrid cascade DC transmission system, the inverter-side LCC switches from the constant DC voltage control mode to the fixed arc extinction angle control mode.

4. The control method for suppressing DC continuous commutation failure according to claim 3, characterized in that: The constant DC voltage control mode is the actual value of the DC voltage U dc2 and DC voltage setting value The difference is then adjusted by PI to generate the trigger angle α; The fixed extinction angle control method is to take the minimum value of the extinction angle γ within a cycle and then compare it with the extinction angle setting value γ * The trigger angle α is generated after PI adjustment.

5. The control method for suppressing DC continuous commutation failure according to claim 1, characterized in that: When the phase regulator excitation control module is in the on state, the phase regulator enters the excitation state and generates reactive power more than twice the rated capacity.

6. The control method for suppressing DC continuous commutation failure according to claim 1, characterized in that: The constant AC voltage control is to set the DC voltage to the value U dc_ref The actual value of DC voltage U dc The difference is then generated through the PI link to generate active current i d_ref Into the inner loop current controller; The constant reactive power control is to set the reactive power value Q ref and the actual value of reactive power Q m The difference is then generated through the PI link to generate reactive current i q_ref Enter the inner loop current controller.

7. A control device for suppressing DC continuous commutation failure, characterized in that: include: An acquisition module is used to obtain the AC bus voltage at the receiving end of the UHV hybrid cascade DC transmission system, as well as the upper and lower limits of the commutation failure critical voltage; A coordination control module is configured to coordinately control the switching states of the phase-shifting ... The receiving end includes MMC1, MMC2 and MMC3 which are cascaded in sequence, and all three are connected to the AC busbar of the receiving end; When the MMC reactive power regulation module is in the on state, MMC1 switches from constant reactive power control to constant AC voltage control to increase reactive power output; MMC2 and MMC3 are in constant reactive power control, and the extinction angle rating is γ N The reference value ΔQ of the required reactive power compensation is obtained by subtracting it from the minimum value of the arc extinction angle γ in each cycle through the PI link and after limiting. The reference value ΔQ is then evenly distributed to MMC2 and MMC3. In the constant reactive power control of MMC2 and MMC3, ΔQ / 2 is added to each to increase the reactive power output. The method of coordinating and controlling the switching states of the phase shifter excitation control module and the MMC reactive power regulation module based on the comparison result of the receiving-end AC bus voltage and the upper and lower limits of the commutation failure critical voltage specifically includes the following steps: When an AC fault occurs in the UHV hybrid cascade DC transmission system and the AC bus voltage U L and the upper limit of the critical voltage of commutation failure U cr2 Meet U L <U cr2 When the phase regulator strong excitation control module is activated, the phase regulator enters the strong excitation state, and causes the AC bus voltage to drop continuously. If the AC bus voltage U L ' and the lower limit of the critical voltage of commutation failure U cr1 Meet U L '<U cr1 When , it is determined that the first commutation failure occurs; The MMC reactive power regulation module is put into operation, so that the AC bus voltage rises under the action of each MMC controlled by the MMC reactive power regulation module and the inverter side LCC in the fixed arc extinction angle control mode. If the AC bus voltage U L " is greater than the upper limit of the critical voltage of commutation failure U cr2 When the phase shifter excitation control module is cut off, the MMC reactive power regulation module controls each MCC to compensate for the reactive power required by the ultra-high voltage hybrid cascade DC transmission system; After the preset delay, if the AC bus voltage can still be kept greater than the upper limit of the critical voltage of the commutation failure U cr2 , it indicates that the AC bus voltage has not experienced a secondary drop; When the AC bus voltage is greater than a preset value, the preset value is greater than the upper limit of the critical voltage of the commutation failure U cr2 , indicating that the system has resumed steady-state operation and the MMC reactive power regulation module has been removed.

8. A control system for suppressing DC continuous commutation failure, characterized in that: including processor and storage medium; The storage medium is used to store instructions; The processor is configured to operate according to the instructions to execute the steps of the method according to any one of claims 1 to 6.

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