TERU-based direct-current power transmission system and fault suppression and energy adjustment method thereof

Abstract

The invention provides a direct-current power transmission system based on a TERU and a fault suppression and energy adjustment method thereof, and belongs to the technical field of light flexible direct-current power transmission systems. Aiming at the problems that a receiving-end power grid in the offshore wind power flexible direct current power transmission system is easy to be fluctuated by new energy, direct current faults are rapidly suppressed and traversed, fault points are accurately positioned, and energy is dynamically adjusted, on the basis of comprehensive analysis of a transient energy transmission mechanism and alternating current and direct current coupling characteristics in the light-weight flexible direct current power transmission system of the DRU-MMC architecture, the power transmission efficiency is improved. The invention provides a highly integrated TERU (Transient Energy Regulation Unit) to integrate the functions of DC voltage stability control, fault energy absorption, fault point detection and positioning, secondary reclosing and the like, which is helpful for reducing the power coupling degree of the transmitting / receiving side of the power transmission system, effectively reducing the influence of the power or load fluctuation of the transmitting / receiving end on the voltage stability of the DC power transmission line, and improving the reliability of the system. And the direct-current fault ride-through performance of the system is improved.

Description

Technical Field

[0001] The present invention belongs to the technical field of power electronics, and in particular relates to a modular TERU (Transient Energy Regulating Unit) applied to an offshore wind power grid-connected system. Background Art

[0002] With the continuous development of offshore wind power resources, VSC-HVDC (High Voltage Direct Current based on Voltage Source Converter) has become the representative of the next generation of DC transmission technology. Its core uses fully controlled power electronic devices such as IGBTs (Insulated-Gate Bipolar Transistors) composed of VSC (Voltage Source Converter). It has advantages such as independent control of active and reactive power, no risk of commutation failure, and adaptability to weak grid access. It is particularly suitable for scenarios such as offshore wind power grid connection, island power supply, and asynchronous grid interconnection. Compared with traditional thyristor-based LCC-HVDC (Line Commutated Converter Based High Voltage Direct Current, grid-commutated converter type high voltage direct current), VSC-HVDC can flexibly adjust voltage and frequency, support multi-terminal DC networking, and does not rely on AC grid to provide commutation voltage. It has become the mainstream technology direction for long-distance, large-capacity renewable energy transmission.

[0003] In VSC-HVDC technology, the Modular Multilevel Converter (MMC) has become the mainstream converter topology due to its advantages such as low harmonics, low losses, and high scalability. To further optimize system cost and complexity, the asymmetric DRU-MMC (Diode Rectifier Unit-Modular Multilevel Converter) architecture has been derived. In this architecture, the receiving end uses a traditional MMC converter, while the sending end is simplified to a diode rectifier unit, which only configures a diode rectifier bridge, DC bus capacitors, current-limiting reactors, and some control modules, eliminating the need for a complete MMC sub-module cascade structure.

[0004] The advantages of the DRU-MMC architecture include: (1) Lightweight: The sending-end DRU eliminates a large number of MMC sub-modules, reducing equipment size and cost; (2) Simple control: The sending end is uncontrolled rectifier and does not require complex control strategies; (3) Adaptability to new energy scenarios: Suitable for scenarios where the sending end is a weak power grid or fluctuating load (such as offshore wind power grid connection).

[0005] However, the lightweight flexible direct current transmission system based on the DRU-MMC architecture has significant drawbacks:

[0006] (1) Insufficient transient energy support: The receiving-end power grid lacks active support measures and relies on the reverse-end power grid, which is easily affected by renewable energy fluctuations and fault response delays, resulting in an increased risk of voltage instability;

[0007] (2) Cost and feasibility issues: Existing solutions improve support capabilities by integrating energy storage units into converter submodules, but this has the problems of high cost, redundant configuration, and high engineering difficulty;

[0008] (3) Weak fault handling capability: The simplified structure of the DRU-MMC leads to the complication of the fault characteristics of the AC and DC systems. Existing technologies make it difficult to achieve rapid fault current suppression, accurate fault point location, and secondary reclosing control, resulting in limited fault ride-through capability.

[0009] (4) Power coupling impact: Power fluctuations at the transmitting and receiving ends are directly coupled through the DC line. There is no fast power balancing mechanism on the DRU side, which exacerbates the risk of voltage fluctuations.

[0010] (5) Secondary reclosing problem: After the fault is isolated, the DRU side lacks active detection and energy buffering, making it difficult to support rapid reclosing of the circuit breaker, which can easily cause secondary shock.

[0011] To address these issues, existing technologies often use DC circuit breakers or AC-side energy dissipation devices. However, these require high interrupting capacity, long de-energizing times, and cannot balance power support and dynamic energy regulation during faults. Therefore, a highly integrated, low-cost, and multifunctional transient energy regulation solution is urgently needed to improve the system's dynamic response capabilities and fault ride-through performance. Summary of the Invention

[0012] The technical problem to be solved by the present invention is to address the problems of the reverse-end power grid being susceptible to fluctuations in renewable energy, rapid suppression and ride-through of DC faults, precise fault point positioning, and dynamic energy regulation. Based on a comprehensive analysis of the transient energy transmission mechanism and AC / DC coupling characteristics in the lightweight flexible DC transmission system with a DRU-MMC architecture, a highly integrated transient energy regulation unit (TERU) is proposed to integrate functions such as DC voltage stability control, fault energy absorption, fault point detection and positioning, and secondary reclosing. This aims to solve the problems of insufficient transient energy support, strong power coupling between the sending and receiving ends, weak fault ride-through capability, and lack of effective fault location and ranging means under the DRU-MMC architecture.

[0013] The present invention adopts the following technical solutions to solve the above technical problems:

[0014] The present invention first proposes a transient energy regulation unit for a direct current transmission system, comprising:

[0015] The TERU series branch consists of multiple TERU submodules connected in series with a current-limiting reactor and is connected between the positive and negative poles of the DC transmission line;

[0016] A DC switch group includes a first DC switch and a second DC switch installed on the positive DC transmission line and the negative DC transmission line respectively, and the DC switch group is arranged on the line between the series branch and the converter station;

[0017] A circuit breaker current limiting assembly, comprising a first DC circuit breaker and a second DC circuit breaker, respectively provided on the positive DC transmission line and the negative DC transmission line, and a first DC line current limiting inductor and a second DC line current limiting inductor provided correspondingly, wherein the circuit breaker current limiting assembly is located between the series branch and the DC transmission line;

[0018] A first diode branch, composed of a plurality of diodes connected in series in a forward direction, with a negative terminal of the first diode being provided at a connection node between a first DC line current limiting inductor of the positive DC transmission line and the transmission line, and a positive terminal of the first diode being provided at a connection node between a second DC switch of the negative DC transmission line and a second DC circuit breaker;

[0019] The second diode branch is composed of multiple diodes connected in series in the forward direction, and its negative connection end is set at the connection node between the first DC knife switch and the first DC circuit breaker of the positive DC transmission line, and the positive connection end is set at the connection node between the second DC line current limiting inductor of the negative DC transmission line and the transmission line.

[0020] Furthermore, in the transient energy regulation unit proposed in the present invention, the TERU submodule includes:

[0021] The full-bridge circuit consists of four insulated gate bipolar transistors (IGBTs) forming an H-bridge topology, with each IGBT connected in reverse parallel to a diode to provide a freewheeling path. The external interface of the TERU submodule is the midpoint of the first bridge arm and the midpoint of the second bridge arm of the full-bridge circuit, and its output voltage polarity is controlled by the conduction state of the IGBTs in the full-bridge circuit.

[0022] A bidirectional DC-DC converter, wherein the input end is connected to the DC bus capacitor of the full-bridge circuit, and the output end is connected to the energy storage device; the bidirectional DC-DC converter comprises: two complementary conductive IGBT switching tubes, whose emitters and collectors are connected in reverse parallel with freewheeling diodes; a filter inductor connected between the midpoints of the bridge arms of the two IGBT switching tubes; and a filter capacitor connected in parallel to the output end of the energy storage device.

[0023] An energy storage device is coupled to the DC bus capacitance of the full-bridge circuit through the bidirectional DC-DC converter; the energy storage device is composed of a plurality of lithium-ion battery cells connected in series and / or in parallel.

[0024] Furthermore, the number of TERU submodules in the TERU series branch is determined according to the following parameters:

[0025] the rated voltage of the DC transmission line, and

[0026] The voltage tolerance of the IGBTs, diodes, and capacitors in the TERU submodule;

[0027] The number of series connections satisfies the requirement that the voltage stress borne by each submodule does not exceed a preset threshold value of its component tolerance.

[0028] The number of diodes in the first diode branch and the second diode branch is determined according to the rated voltage of the DC transmission line, and each diode branch is formed by connecting a plurality of diodes in series in the forward direction.

[0029] The first DC circuit breaker and the first DC line current limiting inductor are connected in series to form a positive pole protection branch, and the second DC circuit breaker and the second DC line current limiting inductor are connected in series to form a negative pole protection branch; the first and second DC circuit breakers are both arranged on the side close to the converter station.

[0030] At the same time, the present invention also proposes a direct current transmission system based on a transient energy regulation unit, comprising:

[0031] The sending-end AC grid is connected to the offshore wind farm;

[0032] A sending-end diode rectifier unit DRU, whose AC side is connected to the sending-end AC power grid;

[0033] A DC transmission line connecting the DC side output terminal of the sending-end diode rectifier unit and the DC side input terminal of the receiving-end modular multilevel converter MMC;

[0034] a transient energy regulation unit connected in parallel between the positive and negative poles of the DC transmission line, for suppressing transient overvoltage and fault energy absorption of the DC line;

[0035] The receiving-end modular multilevel converter MMC has its AC side connected to the receiving-end AC grid.

[0036] Furthermore, the present invention also provides a method for fault suppression and energy regulation of a DC power transmission system, comprising the steps of:

[0037] S1. Build a voltage and current dual closed-loop control system, control the energy storage unit through a bidirectional DC-DC converter to participate in transient energy regulation, and indirectly control energy absorption and output with the goal of stabilizing the DC bus capacitor voltage;

[0038] S2, real-time monitoring of the DC transmission line current, the DC side voltage of the receiving converter station, the actual value and command value of the capacitor voltage, and the actual value and reference value of the energy storage device input current;

[0039] S3, let L l is the current limiting inductance of the DC transmission line, i Line is the DC line current, U dc is the DC side voltage of the receiving converter station, is the capacitor voltage command, U c is the actual value of the voltage across the capacitor, Input current reference value for energy storage device, i B Input the actual current value of the energy storage device, and use 0 or 1 to represent the off or on state of the switch tubes S1 to S4 in the TERU submodule, SOC, SOC min Respectively represent the voltage of the energy storage device and the preset minimum threshold, and switch the following control modes according to the line operation status:

[0040] when i Line >0 when it enters normal power transmission mode:

[0041] a), when U dc dcmin 、SOC>SOC min When , the TERU submodule switch state is controlled to 1001, so that each module is connected in series to form a weak voltage source group to provide power support for the receiving end;

[0042] b), when U dc dcmin 、SOC≤SOC min When the TERU submodule switch state is controlled to 0000, it enters the locked state; when U dcmin dc dcmax ​​​​When , it indicates that the power fluctuation of the sending / receiving end is within the normal range. At this time, the switch status of S1~S4 is 0000, and each module is locked and does not act externally;

[0043] c) When i Line >0、U dc >U dcmax When , the switch state is kept at 0000 and the excess energy is absorbed through the diode branch;

[0044] when i Line Entering fault ride-through mode when ≤0:

[0045] d) When the DC current zero crossing is detected, the DC circuit breaker is disconnected and the receiving end power is completely provided by the TERU;

[0046] e) Compare SOC with the preset threshold SOC min :

[0047] If SOC>SOC min , control the TERU switch status to 1001 to provide emergency power support;

[0048] If SOC≤SOC min , control the TERU switch status to 0000 locked;

[0049] S4. Perform fault location during a bipolar short circuit fault:

[0050] f) Control the TERU switch state to switch to 0110 and keep it for 50ms before returning to 0000, injecting a voltage pulse into the line;

[0051] g) Calculate the fault location based on the time difference of traveling wave reflection.

[0052] Compared with the prior art, the present invention adopts the above technical solution and has the following technical effects:

[0053] The topological structure proposed in this invention provides a basis for providing relatively continuous grid sequential support for lightweight flexible direct current transmission systems, helps to reduce the degree of power coupling between the sending and receiving sides of the transmission system, and effectively reduces the impact of power or load fluctuations at the sending and receiving ends on the voltage stability of the direct current transmission line. In addition, it can cooperate with the DC circuit breaker and the isolation switch to improve the DC fault ride-through performance of the system, especially providing a feasible solution for providing continuous power support during DC faults and active detection and positioning functions in the later stage of faults. BRIEF DESCRIPTION OF THE DRAWINGS

[0054] Figure 1 It is the topology of the DRU-MMC system based on TERU and its steady-state control strategy.

[0055] Figure 2It is the basic working mode of TERU.

[0056] Figure 3 Figure 1 is a schematic diagram of the simulation results: (a) shows the waveforms of the receiving-end power P2 and the DC bus voltage when the sending-end power fluctuates by 100 MW; (b) shows the waveforms of the receiving-end power P2 and the DC bus voltage when the receiving-end grid frequency fluctuates by 0.5 Hz; and (c) shows the waveforms of the receiving-end power P2 and the line current when a bipolar short-circuit fault occurs in the DC line. DETAILED DESCRIPTION

[0057] The technical solution of the present invention is further described in detail below with reference to the accompanying drawings:

[0058] It will be understood by those skilled in the art that, unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by those skilled in the art in the art to which the present invention belongs. It should also be understood that terms such as those defined in common dictionaries should be understood to have meanings consistent with their meanings in the context of the prior art and, unless defined as such, will not be interpreted in an idealized or overly formal sense.

[0059] First, as Figure 1 , is the DRU-MMC DC transmission system topology applicable to the present invention, which consists of a sending-end AC grid, a sending-end diode rectifier unit (DRU), a DC transmission line, a receiving-end modular multilevel converter station (MMC), and a receiving-end AC grid. The sending end is equipped with an AC chopper device (AC chopper) to achieve overvoltage protection and fault response for offshore wind farms. Among them, L l is the current limiting inductance of the DC transmission line, i Line is the DC line current, U dc is the DC side voltage of the receiving converter station, i Line 、U dc The positive direction is shown in the figure.

[0060] The TERU submodule consists of a full-bridge circuit and an energy storage unit. The energy storage unit is connected to the full-bridge circuit capacitor via a bidirectional DC-DC converter. The connection and orientation of the electrical components are shown in the diagram. The full-bridge circuit uses four bipolar insulated gate bipolar transistors (IGBTs) S1-S4 as switches to improve the circuit's withstand voltage rating. Each switch is connected in antiparallel with a diode to provide a freewheeling path for current flow. The pair of switches S1 and S4 are turned on and off simultaneously, complementing the on and off states of S2 and S3. The switching states of S1-S4 are denoted by XXXX (e.g., 1001 indicates S1 and S4 are on, S2 and S3 are off). The bidirectional DC-DC converter consists of two complementary IGBTs, a filter inductor, and a filter capacitor. The IGBTs are also connected in antiparallel with a diode to provide a commutation path for the current flowing through the inductor.

[0061] Typical control strategies of TERU are as follows: Figure 1 As shown in the figure, the bidirectional DCDC converter control strategy adopts voltage and current dual closed-loop control. is the capacitor voltage command, U c is the actual value of the voltage across the capacitor, Input current reference value for energy storage device, i B Input the actual current value for the energy storage device. In essence, the way the energy storage unit participates in transient energy regulation is to indirectly absorb and output energy by stabilizing the voltage across the capacitor. Figure 1 The flowchart in the lower right corner and Figure 2 For detailed explanation:

[0062] (1) When i Line >0、U dc dcmin 、SOC>SOC min When , it indicates that the DC transmission line is in normal transmission state, the transmission power at the sending end drops or the power demand of the receiving end grid increases to beyond the acceptable range, and the energy storage unit has sufficient energy to provide support. At this time, adjust the switch state of S1~S4 to 1001, and the current path is as follows Figure 2 As shown in (b), the TERU modules are connected in series, appearing as a weak voltage source group to the outside world, providing power support for the receiving end.

[0063] (2) When i Line >0、U dc dcmin 、SOC≤SOC min When the DC transmission line is in normal transmission mode, the transmission power at the sending end has dropped, or the power demand of the receiving grid has increased beyond an acceptable range, but the energy storage unit does not have sufficient energy to support it. At this point, adjust the S1-S4 switches to 0000, locking all modules and preventing external operation. ​​

[0064] (3) When i Line >0、U dcmin dc dcmax When , it indicates that the DC transmission line is in normal transmission state, and the power fluctuation of the sending / receiving end is within the normal range. At this time, the switch status of S1~S4 is 0000, and each module is locked and does not act externally; In addition, when i Line >0、U dc >U dcmax When the transmission power at the sending end increases or the power demanded by the receiving end grid decreases to outside the acceptable range. At this time, the switch states of S1 to S4 are also 0000, and the current path is as follows: Figure 2 As shown in (a), TERU absorbs surplus power spontaneously through the diode, without additional control. Since the switch state in the above two cases is 0000, the flow chart shows that U dc >U dcmax No additional explanation is given for the situation.

[0065] (4) When i Line When ≤0, it indicates that a large-scale fault in the wind farm or a bipolar short circuit in the DC transmission line causes the DC bus transmission power to drop rapidly, and the DC transmission line crosses zero. At this time, it enters the fault ride-through mode, such as Figure 2 As shown in (d), at the instant of zero crossing, DC circuit breakers DCCB1 and DCCB2 are disconnected, and the receiving end power is completely provided by TERU. min , then adjust the switch state of S1~S4 to 1001 to provide short-term power support for the receiving end. If SOC≤SOC min , then adjust the switch states of S1~S4 to 0000 and TERU is locked. During a bipolar short circuit fault, the fault can be detected and located by transmitting voltage pulses through TERU. First, disconnect the DC knife switch, then adjust the switch states of S1~S4 to 0110, and then adjust them to 0000 after 50ms. The current path is as follows Figure 2 As shown in (e), a voltage pulse is output to the DC line, and the fault point location is calculated based on the traveling wave reflection principle.

[0066] To clearly describe the functions and effects of the present invention on a DC transmission system, the TERU can be divided into the following five modes according to the switch state and operating mode:

[0067] (1) Energy absorption: Figure 2 In (a), this mode occurs when the transmission system is operating normally and there is a sudden increase in power at the sending end or a rise in grid frequency at the receiving end. In this mode, all switches are off, and excess power naturally flows through the diodes into the energy storage device.

[0068] (2) Energy support: Figure 2 ​​In (b), this mode operates when the transmission system is operating normally and there is a sudden power drop at the sending end or a drop in the grid frequency at the receiving end. In this mode, the TERU outputs active power. When the energy storage SOC drops to a threshold, the switch turns off, ceasing power output.

[0069] (3) DC fault suppression and ride-through: Figure 2 In (c), this mode operates before a bipolar short circuit occurs and the zero crossing occurs. The operating mechanism of this mode is similar to that of Mode 2, except that after a bipolar short circuit occurs, the current in the current-limiting inductor rapidly drops until it reverses phase, and the DC circuit breaker in the TERU opens when the current crosses zero.

[0070] (4) Power support and residual energy transfer during DC faults: Figure 2 In (d), this mode operates during a bipolar short circuit in the transmission line after the DC circuit breaker is activated. In this mode, by changing the voltage command in the MMC control loop at the receiving end, This controls the power supported by the TERU. When the energy storage SOC drops to a threshold, the switch is turned off, stopping power output.

[0071] (5) Fault point detection and location: Figure 2 In (e), this mode operates during a fault to locate the fault point. Before operating this mode, all switches must be turned off and the DC circuit breaker must be opened to prevent reverse voltage from causing current surges at the receiving converter station.

[0072] To further verify the theoretical effect, a DC transmission model based on the DRU-MMC architecture was built in Matlab / Simulink, and a TERU model was configured at the corresponding location. The simulation results are shown in the figure below. Figure 3 The following is an analysis of the simulation results:

[0073] Figure 2 (a) shows the receiving end power P2 and receiving end DC bus voltage U when the sending end power fluctuation is 100MW. dc Compared with the non-energy unloading device and the traditional energy unloading device, TERU can significantly reduce the voltage fluctuation at the moment of power imbalance and reduce the power impact of the power drop at the sending end on the receiving end. Figure 2 In (b), when the receiving-end grid frequency fluctuates by 0.5Hz, the receiving-end power P2 and the receiving-end DC bus voltage U dc The receiving-end converter station adopts voltage closed-loop control, U / f matching control strategy, and TERU-based control strategy respectively. Comparison shows that the U / f matching control strategy has primary frequency modulation characteristics and can provide certain active power support when the receiving-end frequency fluctuates. The TERU also has primary frequency modulation function and can provide more active power support and better transient characteristics.

[0074] In terms of fault ride-through, Figure 2 (b) shows the receiving end power P2, TERU absorbed power ΔP, and receiving end transmission line current I during bipolar short circuit fault ride-through of the DC line. Line , receiving end converter station entrance current I MMC , TERU branch current I TERU Waveform (positive direction of current reference Figure 1 ). When a bipolar short circuit fault occurs, the TERU enters mode 3, where the TERU absorbs line energy, effectively reducing the power impact of the fault on the receiving end. Line When the current drops to 0, a zero-crossing occurs, or the current is low, the TERU switches from Mode 3 to Mode 4, providing short-term power support to the receiving grid, thereby allowing time for power scheduling. When the energy storage is depleted, the TERU ceases support. During this period, the line completes fault isolation and line de-isolation. Then, at t = 2.5 seconds, the fault is detected and located, and the TERU sends a voltage / current pulse to the line. Finally, the fault is eliminated and the system recloses. Simulation results show that the TERU effectively implements energy support, fault ride-through, and fault detection and location during the fault.

[0075] After Matlab / Simulink simulation verification, the envisioned performance can be preliminarily verified, such as Figure 3 In (a), when the power fluctuation at the sending end is 100MW, the TERU can fill part of the fluctuating power, achieve the effect of transient energy support / absorption, and effectively reduce the DC bus voltage fluctuation caused by power fluctuation. In addition, Figure 3 In (b), when the U / f matching control strategy is adopted, when facing the frequency fluctuation of the receiving main network, the TERU-based DRU-MMC system can provide more active power support for the main network and have better primary frequency regulation performance. Figure 3 Figure (c) shows the fault ride-through waveform during a bipolar short-circuit fault. During the fault, the TERU can still provide power support to the receiving main grid for a certain period of time, thus providing a buffer for power dispatch. Simulations also verified the active post-fault detection and location capabilities, as well as the fault clearance and reclosing process, further validating the feasibility of the invention.

[0076] The above descriptions are only partial embodiments of the present invention. It should be pointed out that ordinary technicians in this technical field can make several improvements and modifications without departing from the principles of the present invention. These improvements and modifications should also be regarded as within the scope of protection of the present invention.

Claims

1. A transient energy regulation unit applied to a direct current transmission system, characterized in that: include: The TERU series branch consists of multiple TERU submodules connected in series with a current-limiting reactor and is connected between the positive and negative poles of the DC transmission line; A DC switch group includes a first DC switch and a second DC switch installed on the positive DC transmission line and the negative DC transmission line respectively, and the DC switch group is arranged on the line between the series branch and the converter station; A circuit breaker current limiting assembly, comprising a first DC circuit breaker and a second DC circuit breaker, respectively provided on the positive DC transmission line and the negative DC transmission line, and a first DC line current limiting inductor and a second DC line current limiting inductor provided correspondingly, wherein the circuit breaker current limiting assembly is located between the series branch and the DC transmission line; A first diode branch, composed of a plurality of diodes connected in series in a forward direction, with a negative terminal of the first diode being provided at a connection node between a first DC line current limiting inductor of the positive DC transmission line and the transmission line, and a positive terminal of the first diode being provided at a connection node between a second DC switch of the negative DC transmission line and a second DC circuit breaker; The second diode branch is composed of multiple diodes connected in series in the forward direction, and its negative connection end is set at the connection node between the first DC knife switch and the first DC circuit breaker of the positive DC transmission line, and the positive connection end is set at the connection node between the second DC line current limiting inductor of the negative DC transmission line and the transmission line.

2. The transient energy regulation unit according to claim 1, wherein: The TERU submodule includes: The full-bridge circuit consists of four insulated gate bipolar transistors (IGBTs) forming an H-bridge topology, with each IGBT connected in reverse parallel with a diode to provide a freewheeling path; a bidirectional DC-DC converter, wherein the input end is connected to the DC bus capacitor of the full-bridge circuit and the output end is connected to the energy storage device; an energy storage device coupled to the DC bus capacitance of the full-bridge circuit via the bidirectional DC-DC converter; The external interface of the TERU submodule is the midpoint of the first bridge arm and the midpoint of the second bridge arm of the full-bridge circuit, and the output voltage polarity is controlled by the conduction state of the IGBT in the full-bridge circuit.

3. The transient energy regulation unit according to claim 2, wherein: The bidirectional DC-DC converter comprises: Two complementary conducting IGBT switch tubes, whose emitters and collectors are connected in reverse parallel with freewheeling diodes; The filter inductor is connected between the midpoints of the bridge arms of the two IGBT switch tubes; A filter capacitor is connected in parallel to the output end of the energy storage device.

4. The transient energy regulation unit according to claim 2, wherein: The energy storage device is composed of a plurality of lithium-ion battery cells connected in series and / or in parallel.

5. The transient energy regulation unit for a DC power transmission system according to claim 1, characterized in that: The number of TERU submodules in a TERU series branch is determined by the following parameters: the rated voltage of the DC transmission line, and The voltage tolerance of the IGBTs, diodes, and capacitors in the TERU submodule; The number of series connections satisfies the requirement that the voltage stress borne by each submodule does not exceed a preset threshold value of its component tolerance.

6. The transient energy regulation unit according to claim 1, wherein: The number of diodes in the first diode branch and the second diode branch is determined according to the rated voltage of the DC transmission line, and each diode branch is composed of a plurality of diodes connected in series in the forward direction.

7. The transient energy regulation unit according to claim 1, wherein: The first DC circuit breaker and the first DC line current limiting inductor are connected in series to form a positive pole protection branch, and the second DC circuit breaker and the second DC line current limiting inductor are connected in series to form a negative pole protection branch; wherein the first and second DC circuit breakers are both arranged on the side close to the converter station.

8. A direct current transmission system based on the transient energy regulation unit according to any one of claims 1 to 7, characterized in that: include: The sending-end AC grid is connected to the offshore wind farm; A sending-end diode rectifier unit DRU, whose AC side is connected to the sending-end AC power grid; A DC transmission line connecting the DC side output terminal of the sending-end diode rectifier unit and the DC side input terminal of the receiving-end modular multilevel converter MMC; a transient energy regulation unit connected in parallel between the positive and negative poles of the DC transmission line, for suppressing transient overvoltage and fault energy absorption of the DC line; The receiving-end modular multilevel converter MMC has its AC side connected to the receiving-end AC grid.

9. The direct current transmission system according to claim 8, wherein: Also includes: An AC chopper device is connected in parallel between the sending-end AC grid and the sending-end diode rectifier unit, and is used to conduct the energy-consuming branch when a fault occurs in the offshore wind farm, thereby achieving overvoltage protection and active fault current suppression.

10. A fault suppression and energy regulation method for a DC power transmission system according to any one of claims 1 to 9, characterized in that: Including steps: S1. Build a voltage and current dual closed-loop control system, control the energy storage unit through a bidirectional DC-DC converter to participate in transient energy regulation, and indirectly control energy absorption and output with the goal of stabilizing the DC bus capacitor voltage; S2, real-time monitoring of the DC transmission line current, the DC side voltage of the receiving converter station, the actual value and command value of the capacitor voltage, and the actual value and reference value of the energy storage device input current; S3, let L l is the current limiting inductance of the DC transmission line, i Line is the DC line current, U dc is the DC side voltage of the receiving converter station, is the capacitor voltage command, U c is the actual value of the voltage across the capacitor, Input current reference value for energy storage device, i B Input the actual current value of the energy storage device, and use 0 or 1 to represent the off or on state of the switch tubes S1 to S4 in the TERU submodule, SOC, SOC min Respectively represent the voltage of the energy storage device and the preset minimum threshold, and switch the following control modes according to the line operation status: when i Line >0 when it enters normal power transmission mode: a), when U dc dcmin 、SOC>SOC min When , the TERU submodule switch state is controlled to 1001, so that each module is connected in series to form a weak voltage source group to provide power support for the receiving end;​ b), when U dc dcmin 、SOC≤SOC min When the TERU submodule switch state is controlled to 0000, it enters the locked state; when U dcmin dc dcmax When , it indicates that the power fluctuation of the sending / receiving end is within the normal range. At this time, the switch status of S1~S4 is 0000, and each module is locked and does not act externally;​​​ c) When i Line >0、U dc >U dcmax When , the switch state is kept at 0000 and the excess energy is absorbed through the diode branch; when i Line Entering fault ride-through mode when ≤0: d) When the DC current zero crossing is detected, the DC circuit breaker is disconnected and the receiving end power is completely provided by the TERU; e) Compare SOC with the preset threshold SOC min : If SOC>SOC min , control the TERU switch status to 1001 to provide emergency power support; If SOC≤SOC min , control the TERU switch status to 0000 locked; S4. Perform fault location during a bipolar short circuit fault: f) Control the TERU switch state to switch to 0110 and keep it for 50ms before returning to 0000, injecting a voltage pulse into the line; g) Calculate the fault location based on the time difference of traveling wave reflection.

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