A method and system for hybrid cascaded multi-terminal direct current transmission system (MMC) blocking fault control

Abstract

The application discloses a method and system for MMC blocking fault control of a hybrid cascaded multi-terminal DC power transmission system, and the method comprises the following steps: when MMC of a low-end converter station occurs blocking, the control modes of MMC of three converter stations are acquired; whether surplus power of the MMC of the converter station where blocking occurs can be self-consumed in MMC of the low-end converter station is judged; when it is judged that the MMC of the converter station where blocking occurs cannot self-consume the surplus power, a coordination control strategy is put into operation based on the control mode of the MMC of the converter station where blocking occurs and the control mode of MMC of a sound pole converter station, and consumption of the surplus power is performed.

Description

Technical Field

[0001] This invention relates to the field of control strategy technology for fault ride-through in DC transmission, and more specifically, to a method and system for MMC blocking fault control in hybrid cascaded multi-terminal DC transmission systems. Background Technology

[0002] High-voltage direct current (HVDC) transmission systems with grid-commutated phases offer significant advantages in terms of technology and economics, enabling large-capacity, long-distance power transmission. However, due to the use of thyristors without self-turn-off capability as commutation elements, LCC-HVDC systems suffer from commutation failure during AC voltage dips. Flexible direct current transmission based on MMC (Multi-Mechanical Control Unit) avoids commutation failure, offers flexible control, and can achieve active and reactive power decoupling; however, its higher cost and lower voltage levels require further improvement. Hybrid direct current transmission combines the advantages of both technologies and represents a current trend and research hotspot in HVDC development. The Baihetan-Jiangsu ±800kV hybrid direct current transmission project, currently under construction and operation in my country, employs LCC-HVDC at the sending end and multiple low-end MMC-HVDCs connected in series at the receiving end, with the MMCs using a symmetrical bipolar structure. Compared to all-LCC-HVDC and all-MMC-HVDC systems, hybrid cascaded HVDC systems reduce investment and enhance the reliability and flexibility of the DC system, making them one of the most effective solutions to address the issue of DC feed-in reducing grid stability. However, the low-end MMC on the inverter side of the hybrid cascaded multi-terminal UHVDC transmission system is susceptible to blockage due to faults. After blockage, the surplus power at the low end will cause overcurrent in the DC voltage and arm current of the low-end MMC, and will also affect the high-end DC voltage on the inverter side and the normal operation of the LCC on the rectifier side. Therefore, it is urgent to study the coordinated control strategy after the fault.

[0003] Current coordinated control strategies for hybrid cascaded multi-terminal UHVDC transmission systems mainly focus on coordinated control among multiple low-end MMCs under AC faults at the sending and receiving ends of the system, but cannot solve the overvoltage and overcurrent problems of the system after the low-end MMCs are blocked.

[0004] Therefore, a technology is needed to implement a coordinated control strategy for low-end MMC blocking faults in hybrid cascaded multi-terminal UHVDC transmission systems. Summary of the Invention

[0005] A method and system for controlling MMC blocking faults in hybrid cascaded multi-terminal UHVDC transmission systems are proposed to address the problem of coordinated control of low-end MMC blocking faults in hybrid cascaded multi-terminal UHVDC transmission systems.

[0006] To address the aforementioned problems, this invention provides a method for MMC (Multi-terminal Control) blocking fault control in a hybrid cascaded multi-terminal DC transmission system, the method comprising:

[0007] When the MMC of the lower-end converter station is blocked, the control mode of the MMC of the three converter stations is obtained;

[0008] Determine whether the surplus power of the converter station MMC that experienced a blockage can be self-absorbed by the lower-end converter station MMC;

[0009] When it is determined that the MMC of the converter station that has been blocked cannot absorb the surplus power on its own, a coordinated control strategy is implemented based on the control mode of the MMC of the blocked converter station and the control mode of the MMC of the healthy converter station to absorb the surplus power.

[0010] Preferably, the three converter station MMCs include:

[0011] The first converter station MMC is controlled by constant DC voltage and constant reactive power.

[0012] The control mode of the second converter station MMC is constant active power and constant reactive power control;

[0013] The control mode of the third converter station MMC is constant active power and constant reactive power control.

[0014] Preferably, determining whether the surplus power of the converter station MMC that experienced a power outage can be self-absorbed by the lower-end converter station MMC includes:

[0015] If the reactive power setpoint of the converter station's MMC output is 0 Mvar before the blocking occurs, and the power relationship between the constant DC voltage converter station and the constant power converter station that is blocked is as follows:

[0016]

[0017] Among them, S mmc To determine the capacity of the MMC in a DC-DC converter station, The active power before the blocking occurs at the constant DC voltage converter station. The active power before the blocking of the converter station occurs;

[0018] When the above formula is satisfied, it is determined that the surplus power of the converter station MMC that is blocked cannot be self-absorbed by the lower-end converter station MMC.

[0019] Preferably, when it is determined that the MMC of the converter station experiencing a blockage cannot absorb the surplus power, the coordinated control strategy based on the control mode of the MMC of the converter station experiencing the blockage and the control mode of the MMC of the healthy converter station further includes:

[0020] When the constant DC voltage converter station MMC is blocked, the smaller constant power converter station MMC among the two constant power converter station MMCs is selected to take over the control of the constant DC voltage and the reactive power setpoint is switched to 0p.u.;

[0021] Switch the active power setpoint of the other constant power converter MMC to 1 p.u. and the reactive power setpoint to 0 p.u.

[0022] Preferably, when it is determined that the MMC of the converter station experiencing a blockage cannot absorb the surplus power, the coordinated control strategy based on the control mode of the MMC of the converter station experiencing the blockage and the control mode of the MMC of the healthy converter station further includes:

[0023] When the constant power converter station MMC is blocked, the reactive power setpoint of the constant DC voltage converter station MMC is switched to 0p.u.; the active power setpoint of the constant power converter station MMC of the other healthy pole is switched to 1p.u., and the reactive power setpoint is switched to 0p.u.

[0024] Preferably, after implementing the coordinated control strategy, the method further includes:

[0025] Based on the inverter-side high-side DC voltage to ground as the control target, the reference value of the high-side LCC DC voltage is set to the difference U between 1p.u. and the midpoint-to-ground voltage of the high-side LCC and the converter station MMC. mid .

[0026] Based on another aspect of the present invention, the present invention provides a system for MMC (Multi-terminal Control) interlocking fault control in a hybrid cascaded multi-terminal DC transmission system, the system comprising:

[0027] The acquisition unit is used to acquire the control mode of the three converter station MMCs when the lower-end converter station MMC is blocked.

[0028] The judgment unit is used to determine whether the surplus power of the converter station MMC that has been blocked can be self-absorbed by the lower-end converter station MMC.

[0029] The execution unit is used to implement a coordinated control strategy based on the control mode of the blocked converter station MMC and the control mode of the healthy converter station MMC when it is determined that the blocked converter station MMC cannot absorb the surplus power on its own, so as to absorb the surplus power.

[0030] Preferably, the three converter station MMCs include:

[0031] The first converter station MMC is controlled by constant DC voltage and constant reactive power.

[0032] The control mode of the second converter station MMC is constant active power and constant reactive power control;

[0033] The control mode of the third converter station MMC is constant active power and constant reactive power control.

[0034] Preferably, the judgment unit is used to determine whether the surplus power of the converter station MMC that has experienced a blockage can be self-absorbed by the lower-end converter station MMC, and is also used to:

[0035] If the reactive power setpoint of the converter station's MMC output is 0 Mvar before the blocking occurs, and the power relationship between the constant DC voltage converter station and the constant power converter station that is blocked is as follows:

[0036]

[0037] Among them, S mmc To determine the capacity of the MMC in a DC-DC converter station, The active power before the blocking occurs at the constant DC voltage converter station. The active power before the blocking of the converter station occurs;

[0038] When the above formula is satisfied, it is determined that the surplus power of the converter station MMC that is blocked cannot be self-absorbed by the lower-end converter station MMC.

[0039] Preferably, the execution unit is configured to, when it is determined that the MMC of the converter station experiencing blockage cannot absorb the surplus power, implement a coordinated control strategy based on the control mode of the MMC of the converter station experiencing blockage and the control mode of the MMC of the healthy converter station, and is further configured to:

[0040] When the constant DC voltage converter station MMC is blocked, the smaller constant power converter station MMC among the two constant power converter station MMCs is selected to take over the control of the constant DC voltage and the reactive power setpoint is switched to 0p.u.;

[0041] Switch the active power setpoint of the other constant power converter MMC to 1 p.u. and the reactive power setpoint to 0 p.u.

[0042] Preferably, the execution unit is used to implement a coordinated control strategy based on the control mode of the blocked converter station MMC and the control mode of the healthy converter station MMC when it is determined that the blocked converter station MMC cannot absorb the surplus power. The unit also includes:

[0043] When the constant power converter station MMC is blocked, the reactive power setpoint of the constant DC voltage converter station MMC is switched to 0p.u.; the active power setpoint of the constant power converter station MMC of the other healthy pole is switched to 1p.u., and the reactive power setpoint is switched to 0p.u.

[0044] Preferably, after the coordination control strategy is implemented, the execution unit is further configured to:

[0045] Based on the inverter-side high-side DC voltage to ground as the control target, the reference value of the high-side LCC DC voltage is set to the difference U between 1p.u. and the midpoint-to-ground voltage of the high-side LCC and the converter station MMC. mid .

[0046] This invention provides a method and system for controlling MMC (Multi-terminal Controller) blocking faults in a hybrid cascaded multi-terminal UHVDC transmission system. The method includes: when the lower-end converter station MMC blocks, acquiring the control modes of the three converter station MMCs; determining whether the surplus power of the blocked converter station MMC can be self-absorbed by the lower-end converter station MMC; when it is determined that the blocked converter station MMC cannot self-absorb the surplus power, implementing a coordinated control strategy based on the control modes of the blocked converter station MMC and the healthy converter station MMCs to absorb the surplus power. This invention addresses the problem of DC overvoltage after a lower-end MMC blocking fault in a hybrid cascaded multi-terminal UHVDC transmission system, designing a coordinated control strategy to achieve fault ride-through in the system. Attached Figure Description

[0047] Exemplary embodiments of the present invention can be more fully understood by referring to the following figures:

[0048] Figure 1 This is a flowchart of a method for MMC blocking fault control in a hybrid cascaded multi-terminal DC transmission system according to a preferred embodiment of the present invention.

[0049] Figure 2 This is a schematic diagram of the topology of a hybrid cascaded multi-terminal ultra-high voltage direct current transmission system according to a preferred embodiment of the present invention;

[0050] Figure 3 This is a timing diagram of the interlocking coordination control of a constant voltage converter station according to a preferred embodiment of the present invention;

[0051] Figure 4 This is a timing diagram of the interlocking coordination control of a constant power converter station according to a preferred embodiment of the present invention;

[0052] Figure 5 This is a schematic diagram of the inverter-side high-end LCC coordinated control strategy according to a preferred embodiment of the present invention;

[0053] Figure 6 A schematic diagram of the inverter-side high-end LCC coordinated control strategy according to a preferred embodiment of the present invention; and

[0054] Figure 7 This is a system structure diagram of a preferred embodiment of the present invention for MMC blocking fault control in a hybrid cascaded multi-terminal DC transmission system. Detailed Implementation

[0055] Exemplary embodiments of the invention will now be described with reference to the accompanying drawings. However, the invention may be embodied in many different forms and is not limited to the embodiments described herein. These embodiments are provided to fully and completely disclose the invention and to fully convey its scope to those skilled in the art. The terminology used in the exemplary embodiments illustrated in the drawings is not intended to limit the invention. In the drawings, the same units / elements are referred to by the same reference numerals.

[0056] Unless otherwise stated, the terms used herein (including technical terms) have their common meaning as understood by one of ordinary skill in the art. Furthermore, it is understood that terms defined in commonly used dictionaries should be understood to have a meaning consistent with the context of their relevant field, and not to be interpreted as having an idealized or overly formal meaning.

[0057] Figure 1 This is a flowchart illustrating a method for controlling MMC blockade faults in a hybrid cascaded multi-terminal DC transmission system according to a preferred embodiment of the present invention. For hybrid cascaded multi-terminal UHVDC transmission systems, a coordinated control strategy is designed to address the DC overvoltage problem following a low-end MMC blockade fault, achieving fault ride-through in the system. This invention provides a coordinated control strategy for MMC blockade faults in hybrid cascaded multi-terminal DC transmission systems. This strategy fully utilizes the capacity of the MMC converter stations to absorb the surplus power at the low end through coordinated control between the low-end MMC converter stations. Simultaneously, through coordinated control between the high-end LCC and the low-end MMC on the inverter side, the overall DC voltage stability on the inverter side of the system is maintained after a fault, reducing the impact of low-end MMC faults on high-end LCCs.

[0058] like Figure 1 As shown, the present invention provides a method for MMC (Multi-terminal Control) blocking fault control in a hybrid cascaded multi-terminal DC transmission system, the method comprising:

[0059] Step 101: When the lower-end converter station MMC is blocked, obtain the control mode of the three converter station MMCs; preferably, the three converter station MMCs include:

[0060] The control mode of the first converter station MMC is constant DC voltage and constant reactive power control.

[0061] The control mode of the second converter station MMC is constant active power and constant reactive power control.

[0062] The control mode of the MMC in the third converter station is constant active power and constant reactive power control.

[0063] In this invention, when the low-end MMC is blocked, the current operating power and control mode of the three MMCs are collected to determine the control mode of the blocked converter station and whether the surplus power at the low end can be self-absorbed by the low-end MMC after the blocking.

[0064] Step 102: Determine whether the surplus power of the MMC of the converter station that experienced the blockage can be self-absorbed by the MMC of the lower-end converter station.

[0065] Preferably, determining whether the surplus power of the converter station MMC that experienced a power outage can be self-absorbed by the lower-end converter station MMC includes:

[0066] If the reactive power setpoint of the converter station's MMC output is 0 Mvar before the blocking occurs, and the power relationship between the constant DC voltage converter station and the constant power converter station that is blocked is as follows:

[0067]

[0068] Among them, S mmc To determine the capacity of the MMC in a DC-DC converter station, The active power before the blocking occurs at the constant DC voltage converter station. The active power before the blocking of the converter station occurs;

[0069] If the power surplus of the converter station MMC that is blocked cannot be self-absorbed by the lower-end converter station MMC, then it is determined that the power surplus of the MMC that is blocked cannot be self-absorbed by the lower-end converter station MMC.

[0070] This invention determines whether to activate the coordinated control strategy based on the control mode and operating power of the MMC before the fault. Specifically, if the blocked converter station is a constant power converter station, and the power relationship between the constant DC voltage station and the blocked converter station before the fault is:

[0071]

[0072] Wherein: S mmc The capacity of the MMC in the constant DC voltage converter station; The active power before the fault at the constant voltage converter station; This represents the active power before the converter station fault. At this time, the constant DC voltage station can completely absorb the surplus power of the constant power station, without causing low-end MMC DC voltage overvoltage, and the coordinated control strategy is not activated.

[0073] If the power relationship between the established DC voltage station and the faulty converter station before the fault was:

[0074]

[0075] At this point, whether the constant DC voltage station can absorb the surplus power of the constant power station is questioned. The low-end MMC DC voltage continues to rise, triggering a coordinated control strategy. If the blocked converter station is a constant DC voltage converter station, then the coordinated control strategy is activated.

[0076] If it is determined that the surplus power can be self-absorbed after blocking, then the coordinated control strategy is not activated. Otherwise, the coordinated control strategy is activated.

[0077] Step 103: When it is determined that the MMC of the converter station that has been blocked cannot absorb the surplus power on its own, a coordinated control strategy is implemented based on the control mode of the MMC of the converter station that has been blocked and the control mode of the MMC of the healthy converter station to absorb the surplus power.

[0078] Preferably, when it is determined that the MMC of the converter station experiencing a blockage cannot absorb the surplus power, a coordinated control strategy is implemented based on the control mode of the MMC of the converter station experiencing the blockage and the control mode of the MMC of the healthy converter station, which further includes:

[0079] When the constant DC voltage converter station MMC is blocked, the smaller constant power converter station MMC among the two constant power converter station MMCs is selected to take over the control of the constant DC voltage and the reactive power setpoint is switched to 0p.u.;

[0080] Switch the active power setpoint of the other constant power converter MMC to 1 p.u. and the reactive power setpoint to 0 p.u.

[0081] In this invention, if the blocked converter station is determined to be a constant DC voltage station, the constant power station with the lower operating power before the fault is selected to take over the constant DC voltage control. At the same time, the active power command of the other constant power converter station is set to 1p.u. and the reactive power command is set to zero, so as to make full use of the two healthy converter stations to absorb the surplus power.

[0082] In this invention, when the constant DC voltage station is locked, the constant power station with the lower operating power before the fault takes over the constant DC voltage control, and the reactive power setpoint is switched to 0p.u. to maintain the stability of the low-end DC voltage; the active power setpoint of the other healthy constant power station is switched to 1p.u. and the reactive power setpoint is switched to 0p.u., so that the surplus power of the two healthy converter stations can be fully utilized.

[0083] In this invention, after the constant power station is locked out, the reactive power setpoint of the constant DC voltage station is switched to 0p.u., the active power setpoint of the other healthy constant power station is switched to 1p.u., and the reactive power setpoint is switched to 0p.u.

[0084] Preferably, when it is determined that the MMC of the converter station experiencing a blockage cannot absorb the surplus power, a coordinated control strategy is implemented based on the control mode of the MMC of the converter station experiencing the blockage and the control mode of the MMC of the healthy converter station, which further includes:

[0085] When the constant power converter station MMC is blocked, the reactive power setpoint of the constant DC voltage converter station MMC is switched to 0p.u.; the active power setpoint of the constant power converter station MMC of the other healthy pole is switched to 1p.u., and the reactive power setpoint is switched to 0p.u.

[0086] If the locked converter station is determined to be a constant power station, then the active power setpoint of the other constant power station is set to 1p.u., and the reactive power command is set to zero.

[0087] Preferably, after implementing the coordinated control strategy, it further includes:

[0088] Based on the inverter-side high-side DC voltage to ground as the control target, the reference value of the high-side LCC DC voltage is set to the difference U between 1p.u. and the midpoint-to-ground voltage of the high-side LCC and the converter station MMC. mid .

[0089] In this invention, after the constant power station is locked out, the reactive power setpoint of the constant DC voltage station is switched to 0p.u., the active power setpoint of the other healthy constant power station is switched to 1p.u., and the reactive power setpoint is switched to 0p.u.

[0090] This invention transmits the low-side MMC blocking signal of the inverter side to the high-side LCC of the inverter side, which is controlled by constant DC voltage. The given value of the DC voltage is switched from 400kV to the difference between the high-side voltage to ground of the inverter side (800kV) and the voltage to ground at the midpoint of the LCC and MMC, so as to maintain the stability of the high-side voltage to ground of the DC system and reduce the impact of faults on the rectifier-side LCC.

[0091] This invention utilizes coordinated control between low-end MMC converter stations to switch the control mode and power command of the active MMC converter station according to the fault condition. This effectively avoids low-end DC voltage overvoltage and MMC converter station arm current overcurrent problems after a low-end MMC blockade fault. Through a coordinated control strategy between the inverter-side LCC and MMC, the DC voltage command of the inverter-side high-end LCC is switched to maintain the stability of the inverter-side high-end voltage to ground in the DC system, reducing the impact of MMC blockade faults on the rectifier-side LCC and improving the overall transient characteristics of the DC system during faults.

[0092] This invention provides a coordinated control strategy for MMC (Multi-terminal Control) blocking faults in a hybrid cascaded multi-terminal DC transmission system. The hybrid cascaded multi-terminal DC transmission system provided by this invention is as follows: Figure 2 The rectifier station uses a conventional DC converter valve (LCC), consisting of two cascaded 12-pulse LCCs. The high-side inverter side has one 12-pulse LCC converter station, while the low-side has three parallel MMC converter stations. During steady-state operation, the DC voltage of both the inverter-side LCC and MMCs is 400kV, therefore the high-side inverter voltage to ground is 800kV. The high-side inverter LCC uses constant DC voltage control, with a voltage command of 400kV and the control target being the DC voltage across the LCC. Of the three low-side MMCs on the inverter side, MMC1 uses constant DC voltage and constant reactive power control to control the low-side DC voltage. The remaining MMCs use constant active power and constant reactive power control.

[0093] After a constant-power MMC converter station is locked out, the AC power output from the converter station is blocked, and the steady-state transmission power before the lockout becomes the surplus power of the other two healthy converter stations. At this time, if the constant-DC voltage station cannot absorb the surplus power of the constant-power station, the low-end MMC DC voltage continues to rise. When the DC voltage exceeds 1.5 pu, safety control measures activate, causing overvoltage lockout of the remaining healthy converter stations, resulting in a continuous expansion of the system fault and seriously affecting the stable operation of the system. When a constant-DC voltage MMC converter station is locked out, the low-end DC voltage becomes uncontrolled. At this time, one healthy constant-power MMC converter station needs to switch to constant-DC voltage control to maintain the low-end DC voltage. Subsequently, the low-end surplus power will flood into this converter station; if the surplus power cannot be absorbed, overvoltage problems will also occur.

[0094] Since the inverter-side LCC and MMC use their own 400kV DC voltage as the control target, a blocking fault will cause the low-side DC voltage to rise, leading to an increase in the high-side DC voltage to ground on the inverter side. This reduces the DC current of the hybrid cascaded UHVDC system. When the DC current drops to 10% of its rated value, the inverter side switches from constant voltage control to backup constant current control. The firing angle generated by the rectifier-side controller gradually decreases, and when the firing angle decreases significantly, it will switch to minimum firing angle control. In summary, a blocking fault in the MMC not only causes low-side DC voltage overvoltage, seriously damaging the equipment, but also causes a shift in the system's stable operating point, affecting the normal operation of the high-side LCCs on both the rectifier and inverter sides.

[0095] This invention includes the following steps:

[0096] Step 1: When a lockout fault occurs in the low-side MMC on the inverter side, the lockout signal is transmitted to the remaining healthy MMCs and the high-side LCC. Based on the control mode and operating power of the MMCs before the fault, it is determined whether to activate the coordinated control strategy. The specific method is as follows:

[0097] If the blocked converter station is a constant power converter station, and the power relationship between the constant DC voltage station and the blocked converter station before the fault is as follows:

[0098]

[0099] Where: S mmc For MMC converter station capacity; The active power before the fault at the constant voltage converter station; This represents the active power before the converter station fault. At this time, the constant DC voltage station can completely absorb the surplus power of the constant power station, without causing low-end MMC DC voltage overvoltage, and the coordinated control strategy is not activated.

[0100] If the power relationship between the established DC voltage station and the faulty converter station before the fault was:

[0101]

[0102] At this point, whether the constant DC voltage station can absorb the surplus power of the constant power station is questioned. The low-end MMC DC voltage continues to rise, prompting the implementation of a coordinated control strategy.

[0103] If the blocked converter station is a constant DC voltage converter station, then the coordinated control strategy is activated. The coordinated control strategy is specifically implemented in steps 2 and 3 below.

[0104] Step 2: After the DC voltage station is locked, by Figure 3 The figure shown in the figure t f For the locking time, t c To coordinate the start-up time of the control. When the constant DC voltage station is at t f During a power outage, to improve the ability to absorb surplus power, a station with a lower operating power before the fault is selected at t. c The system switches the DC voltage control to 0p.u., maintaining the stability of the low-end DC voltage. Simultaneously, the active power control of the other constant power station is switched to 1p.u., and the reactive power control to 0p.u., thus fully utilizing the surplus power of the two healthy converter stations.

[0105] After the constant power station is locked, by Figure 4 The constant voltage substation shown can immediately begin absorbing surplus power. At this time, the reactive power setpoint of the constant voltage substation is switched to 0p.u., while the active power setpoint of the other constant power substation is at t. c When switching to 1p.u., the reactive power setpoint is switched to 0p.u.

[0106] Step 3, as follows Figure 5As shown, after implementing the coordinated control strategy, the DC voltage to ground at the high end of the inverter side is taken as the control target. The reference value of the DC voltage at the high end of the LCC is set to the difference U between 1p.u. and the voltage at the midpoint to ground of the LCC and MMC. mid .

[0107] It should be noted that: Figure 6 The diagram shown is a schematic of the inverter-side control of a hybrid cascaded multi-terminal ultra-high voltage direct current system. mid This refers to the DC-to-ground voltage between the LCC and MMC on the inverter side of the system. When both the high-side LCC and the low-side MMC on the inverter side use their respective DC voltages of 0.5 pu (400 kV) as the control target, steady-state operation can ensure that the overall DC voltage on the inverter side is 1 p.u. (800 kV). However, when the DC voltage of the low-side MMC fluctuates, it will cause fluctuations in the overall DC voltage, affecting the normal operation of the LCC.

[0108] Figure 7 This is a system architecture diagram of a hybrid cascaded multi-terminal DC transmission system for MMC (Multi-Connection Control) interlocking fault control according to a preferred embodiment of the present invention. Figure 7 As shown, the present invention provides a system for MMC (Multi-terminal Control) interlocking fault control in a hybrid cascaded multi-terminal DC transmission system, the system comprising:

[0109] The acquisition unit 701 is used to acquire the control modes of the three converter station MMCs when the lower-end converter station MMC is blocked; preferably, the three converter station MMCs include:

[0110] The control mode of the first converter station MMC is constant DC voltage and constant reactive power control.

[0111] The control mode of the second converter station MMC is constant active power and constant reactive power control.

[0112] The control mode of the MMC in the third converter station is constant active power and constant reactive power control.

[0113] The judgment unit 702 is used to determine whether the surplus power of the converter station MMC that has experienced a blockage can be self-absorbed by the lower-end converter station MMC; preferably, the judgment unit is used to determine whether the surplus power of the converter station MMC that has experienced a blockage can be self-absorbed by the lower-end converter station MMC, and is also used to:

[0114] If the reactive power setpoint of the converter station's MMC output is 0 Mvar before the blocking occurs, and the power relationship between the constant DC voltage converter station and the constant power converter station that is blocked is as follows:

[0115]

[0116] Among them, Smmc To determine the capacity of the MMC in a DC-DC converter station, The active power before the blocking occurs at the constant DC voltage converter station. The active power before the blocking of the converter station occurs;

[0117] If the power surplus of the converter station MMC that is blocked cannot be self-absorbed by the lower-end converter station MMC, then it is determined that the power surplus of the MMC that is blocked cannot be self-absorbed by the lower-end converter station MMC.

[0118] The execution unit 703 is used to implement a coordinated control strategy based on the control mode of the blocked converter station MMC and the control mode of the healthy converter station MMC when it is determined that the blocked converter station MMC cannot absorb the surplus power on its own, so as to absorb the surplus power.

[0119] Preferably, the execution unit 703 is used to, when it is determined that the MMC of the converter station that has experienced a blockage cannot absorb the surplus power, implement a coordinated control strategy based on the control mode of the MMC of the converter station that has experienced a blockage and the control mode of the MMC of the healthy converter station, and is also used to:

[0120] When the constant DC voltage converter station MMC is blocked, the smaller constant power converter station MMC among the two constant power converter station MMCs is selected to take over the control of the constant DC voltage and the reactive power setpoint is switched to 0p.u.;

[0121] Switch the active power setpoint of the other constant power converter MMC to 1 p.u. and the reactive power setpoint to 0 p.u.

[0122] Preferably, the execution unit 703 is used to implement a coordinated control strategy based on the control mode of the blocked converter station MMC and the control mode of the healthy converter station MMC when it is determined that the blocked converter station MMC cannot absorb the surplus power. It also includes:

[0123] When the constant power converter station MMC is blocked, the reactive power setpoint of the constant DC voltage converter station MMC is switched to 0p.u.; the active power setpoint of the constant power converter station MMC of the other healthy pole is switched to 1p.u., and the reactive power setpoint is switched to 0p.u.

[0124] Preferably, after the coordination control strategy is implemented, the execution unit 703 is further configured to:

[0125] Based on the inverter-side high-side DC voltage to ground as the control target, the reference value of the high-side LCC DC voltage is set to the difference U between 1p.u. and the midpoint-to-ground voltage of the high-side LCC and the converter station MMC. mid .

[0126] The preferred embodiment of the present invention provides a system 700 for MMC blockade fault control in a hybrid cascaded multi-terminal DC transmission system, which corresponds to the preferred embodiment of the present invention provides a method 100 for MMC blockade fault control in a hybrid cascaded multi-terminal DC transmission system. Further details will not be provided here.

[0127] The invention has been described with reference to a few embodiments. However, as will be known to those skilled in the art, and as defined in the appended claims, other embodiments besides those disclosed above fall equivalently within the scope of the invention.

[0128] Generally, all terms used in the claims are to be interpreted according to their ordinary meaning in the technical field, unless otherwise expressly defined herein. All references to “a / / the [device, component, etc.]” ​​are openly interpreted as at least one instance of the device, component, etc., unless otherwise expressly stated. The steps of any method disclosed herein are not necessarily to be performed in the exact order disclosed, unless explicitly stated otherwise.

Claims

1. A method for hybrid cascaded multi-terminal direct current transmission system (MMC) blocking fault control, the method comprising: obtaining control modes of three converter stations MMC when a blocking occurs in a low-end converter station MMC; the three converter stations MMC comprising: a first converter station MMC, the control mode of the first converter station MMC being constant DC voltage and constant reactive power control; a second converter station MMC, the control mode of the second converter station MMC being constant active power and constant reactive power control; a third converter station MMC, the control mode of the third converter station MMC being constant active power and constant reactive power control; judging whether surplus power of the converter station MMC where the blocking occurs can be self-consumed in the low-end converter station MMC; when it is judged that the converter station MMC where the blocking occurs cannot self-consume the surplus power, inputting a coordinated control strategy based on the control mode of the converter station MMC where the blocking occurs and the control mode of the sound converter station MMC to consume the surplus power; the inputting the coordinated control strategy based on the control mode of the converter station MMC where the blocking occurs and the control mode of the sound converter station MMC when it is judged that the converter station MMC where the blocking occurs cannot self-consume the surplus power further comprising: after the blocking of the constant DC voltage converter station MMC, selecting a constant active power converter station MMC with smaller active power from two constant active power converter stations MMC to take over the control of the constant DC voltage, and switching the given value of the reactive power to 0p.u.; switching the given value of the active power of the other constant active power converter station MMC from the two constant active power converter stations MMC to 1p.u., and switching the given value of the reactive power to 0p.u.; the inputting the coordinated control strategy based on the control mode of the converter station MMC where the blocking occurs and the control mode of the sound converter station MMC when it is judged that the converter station MMC where the blocking occurs cannot self-consume the surplus power further comprising: after the blocking of the constant active power converter station MMC, switching the given value of the reactive power of the constant DC voltage converter station MMC to 0p.u., and switching the given value of the active power of the other constant active power converter station MMC of the sound converter station to 1p.u., and switching the given value of the reactive power to 0p.u. 2.The method of claim 1, the judging whether the surplus power of the converter station MMC where the blocking occurs can be self-consumed in the low-end converter station MMC comprising: if the given values of the output reactive power of the converter station MMC before the blocking are all 0Mvar, and the power relationship between the constant DC voltage converter station and the constant active power converter station where the blocking occurs is: wherein S mmc is the capacity of the MMC of the constant DC voltage converter station, is the active power before the blocking of the constant DC voltage converter station, is the active power before the blocking of the blocking converter station; when the above formula is satisfied, it is judged that the surplus power of the converter station MMC where the blocking occurs cannot be self-consumed in the low-end converter station MMC. 3.The method of claim 1, after the inputting of the coordinated control strategy, further comprising: Based on the high-end DC voltage of the inverter side as the control target, the reference value of the high-end LCC DC voltage is set as the difference U between 1 p.u. and the midpoint voltage of the high-end LCC and the converter station MMC mid . 4.A system for hybrid cascaded multi-terminal direct current transmission system (MMC) blocking fault control, the system comprising: The acquisition unit is used to acquire the control mode of the three converter station MMCs when the lower-end converter station MMC is blocked. The three converter station MMCs include: The first converter station MMC is controlled by constant DC voltage and constant reactive power. The control mode of the second converter station MMC is constant active power and constant reactive power control; The control mode of the third converter station MMC is constant active power and constant reactive power control; The judgment unit is used to determine whether the surplus power of the converter station MMC that has been blocked can be self-absorbed by the lower-end converter station MMC. The execution unit is used to, when it is determined that the MMC of the converter station that has been blocked cannot absorb the surplus power on its own, implement a coordinated control strategy based on the control mode of the MMC of the converter station that has been blocked and the control mode of the MMC of the healthy converter station to absorb the surplus power. The execution unit is used to, when it is determined that the MMC of the converter station that has experienced a blockage cannot absorb the surplus power, implement a coordinated control strategy based on the control mode of the MMC of the converter station that has experienced a blockage and the control mode of the MMC of the healthy converter station, and is also used to: When the constant DC voltage converter station MMC is blocked, the constant active power converter station MMC with the smaller active power among the two constant active power converter station MMCs is selected to take over the control of the constant DC voltage and switch the reactive power setpoint to 0p.u.; Switch the active power setpoint of the other constant active power converter station MMC to 1 p.u. and the reactive power setpoint to 0 p.u.; The execution unit is used to implement a coordinated control strategy based on the control mode of the blocked converter station MMC and the control mode of the healthy converter station MMC when it is determined that the blocked converter station MMC cannot absorb the surplus power. It also includes: When the MMC of a converter station with constant active power is blocked, the reactive power setpoint of the MMC of a converter station with constant DC voltage is switched to 0p.u.; the active power setpoint of the MMC of the other healthy converter station with constant active power is switched to 1p.u., and the reactive power setpoint is switched to 0p.u.

5. The system according to claim 4, wherein the judging unit is used to judge whether the surplus power of the converter station MMC that has experienced a blockage can be self-absorbed by the lower-end converter station MMC, and is further used to: If the reactive power setpoint of the converter station's MMC output is 0 Mvar before the blocking occurs, and the power relationship between the constant DC voltage converter station and the constant active power converter station that is blocked is as follows: wherein S mmc for a constant DC voltage converter station MMC, for a constant DC voltage converter station before the blocking, for a constant DC voltage converter station before the blocking, When the above formula is satisfied, it is determined that the surplus power of the converter station MMC that is blocked cannot be self-absorbed by the lower-end converter station MMC.

6. The system according to claim 4, wherein after the coordination control strategy is implemented, the execution unit is further configured to: Based on the high-end DC voltage of the inverter side as the control target, the reference value of the high-end LCC DC voltage is set as the difference U between 1 p.u. and the midpoint voltage of the high-end LCC and the converter station MMC mid .

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