Topologies of bridge arm integrated current-limiting energy-consuming devices, commutation valves, and coordinated control methods

Abstract

The present application relates to the field of high-voltage direct current transmission, and aims to provide a topology of bridge arm integrated current-limiting energy-consuming device, a converter valve and a coordinated control method. The topology is a modular MMC architecture of three-phase double bridge arms, which comprises a plurality of MMC sub-modules connected in series in the upper bridge arm, and a current-limiting energy-consuming device composed of a plurality of current-limiting energy-consuming sub-modules connected in series in the lower bridge arm; the current-limiting energy-consuming sub-module comprises a sub-module main power unit and a sub-condition energy-consuming unit, the latter at least contains two energy-consuming units adapted to different fault conditions, and each contains an energy-consuming resistor and an adaptive condition switching device. The present application integrates the functions of direct-current fault current limiting and alternating-current fault energy consumption in the same integrated device, realizes integrated protection of AC and DC faults, does not need to configure multiple sets of separate equipment, and solves the problem of functional redundancy of the traditional scheme. Redundant configuration can be reduced, hardware cost is reduced, system size is reduced, and operation and maintenance complexity is simplified; the risk of device overvoltage breakdown is avoided, and the service life of the equipment is improved.

Description

Technical Field

[0001] This invention belongs to the field of high voltage direct current transmission, specifically a topology, converter valve and coordinated control method for a bridge arm integrated current limiting and energy dissipation device applied to a high voltage flexible direct current transmission system. Background Technology

[0002] In the field of high-voltage direct current (HVDC) transmission and power conversion, the Modular Multilevel Converter (MMC) typically exists in the form of a converter valve (hereinafter referred to as the converter valve) and is a novel topology that is currently widely used and researched. Compared with the traditional two-level or three-level VSC, the converter valve has advantages in terms of higher voltage levels, overall control system flexibility, harmonic suppression effect, and switching loss control, making it one of the core devices in this field. However, its protection system still faces significant technical bottlenecks: traditional solutions treat DC short-circuit fault suppression and AC fault ride-through as independent problems, requiring the configuration of multiple sets of single-function devices, resulting in high system redundancy, high cost, and limited performance.

[0003] Specifically, in DC fault protection, the cost of core equipment, the DC circuit breaker, increases directly with its current breaking capacity and withstand voltage. Although fault current limiters are often used to reduce the breaking requirements of the DC circuit breaker, conventional inductive fault current limiters are limited by grid stability in terms of inductance value, pre-charge inductive fault current limiters require complex auxiliary switching logic, increasing the risk of failure, and superconducting fault current limiters are difficult to popularize due to immature technology and high operating costs. Furthermore, the millisecond-level delay of backup protection when the main DC circuit breaker fails can cause a surge in fault current, forcing the DC circuit breaker to be designed beyond specifications and with increased redundancy, further increasing costs. In terms of AC fault ride-through, AC side faults at the receiving-end converter station can lead to DC side power imbalance and a sudden rise in bus voltage. Although existing chopper circuits can dissipate excess energy through parallel resistors, they cannot participate in DC fault current limiting because they are outside the main current path, and centralized choppers have voltage balance and ripple problems; distributed choppers require high investment and complex water cooling systems, and even if the chopper circuit is integrated into the MMC submodule, its function is still limited to power dissipation. In summary, the core principle of both fault current limiters and chopper circuits is to insert impedance into the system during a fault to achieve current limiting or energy dissipation. However, traditional solutions still rely on separate devices such as DC circuit breakers, fault current limiters, and chopper circuits, resulting in a large system size and high cost.

[0004] While there are technical solutions that integrate AC / DC fault ride-through functionality into the same MMC converter valve, these solutions have not effectively resolved the core technical contradiction of mismatched damping requirements under AC / DC operating conditions: the surplus power dissipation of AC-side faults and the current suppression of DC-side faults have fundamentally different requirements for the damping resistor value and the number of current-limiting and energy-consuming submodules connected in series, and both the "lower limit constraint of resistance value" and the "upper limit constraint of number" must be met simultaneously, ultimately leading to inherent defects in the solutions.

[0005] Specifically, on the one hand, the damping resistor value must be designed to the minimum value: to avoid the voltage across the integrated module exceeding the rated withstand voltage of the thyristor and IGBT under any fault condition (to prevent device overvoltage breakdown), the minimum allowable damping resistor value in AC / DC faults must be used as the benchmark. If the resistance value is greater than this minimum value, it will cause the current to flow through the high-resistance resistor and generate an overrated voltage under a certain fault (such as a DC short circuit), exceeding the safe operating range of the device; however, although this minimum resistance value can meet the breakdown protection requirements, it cannot independently adapt to the protection characteristics of AC and DC respectively. On the other hand, the number of current-limiting and energy-consuming submodules must be configured to the maximum value: because AC dissipation requires medium to high resistance characteristics and DC current limiting requires low resistance characteristics, in order to cover the protection requirements of both faults at the same time, the maximum number of current-limiting and energy-consuming submodules in AC / DC conditions must be used as the unified design standard. The design logic of taking the minimum resistance value and the maximum number directly leads to significant hardware cost redundancy in the existing solution: the current limiting and energy consumption submodules configured with the maximum number additionally increase the hardware cost of the converter valve (including core components such as energy consumption resistors, thyristors, IGBTs and auxiliary control units), while also increasing the size and maintenance complexity of the system.

[0006] It is evident that existing AC / DC fault ride-through integrated solutions require balancing the lower limit of the resistance value for breakdown protection with the upper limit of the number of devices for full coverage, resulting in redundant device numbers and increased protection equipment costs. This makes it impossible to balance functional reliability and system economy. Therefore, it is urgent to optimize the damping adaptation logic at the topology design level to solve the above-mentioned technical bottlenecks. Summary of the Invention

[0007] The technical problem to be solved by this invention is to overcome the shortcomings of the prior art and propose a bridge arm integrated current limiting and energy dissipation device topology and its coordinated control method.

[0008] To solve the technical problem, the solution of the present invention is:

[0009] A topology for an integrated current-limiting and energy-dissipating device in a bridge arm is provided. This topology is a three-phase dual-bridge arm modular multilevel converter (MMC) architecture. In the upper bridge arm of each phase, there is a bridge arm inductor and N MMC sub-modules connected in series with identical structures, where N≥1. In the lower bridge arm of each phase, there is a bridge arm inductor and a current-limiting and energy-dissipating device composed of M current-limiting and energy-dissipating sub-modules connected in series with identical structures, where M≥1.

[0010] The current-limiting energy consumption submodule includes a main power unit for connecting to the bridge arm circuit to achieve the basic function of power conversion;

[0011] The sub-condition energy consumption unit is connected in parallel to both ends of the main power unit of the sub-module to form a fault energy dissipation path. The sub-condition energy consumption unit includes at least two energy consumption units adapted to different fault conditions, namely an AC-adaptive energy consumption unit and a DC-adaptive energy consumption unit. Each energy consumption unit includes an energy consumption resistor and an adaptation switching device.

[0012] As a preferred embodiment of the present invention, the current limiting and energy consumption submodule further includes a bypass branch connected in parallel to both ends of the main power unit of the submodule, and the bypass branch includes a bypass switch.

[0013] As a preferred embodiment of the present invention, the main power unit of the sub-module includes two power electronic switches connected in series, which are semi-controlled and / or fully controlled switching devices; the adapting switching device in the sub-operating condition energy consumption unit is a unidirectional and / or bidirectional semi-controlled and / or fully controlled switching device; the switching device is at least one of IGBT, MOSFET, and thyristor.

[0014] As a preferred embodiment of the present invention, the current-limiting and energy-consuming submodule includes a main current-carrying branch, a current-limiting branch, an energy-consuming branch, and a bypass branch arranged in parallel; wherein, the main power unit of the submodule is arranged in the main current-carrying branch and is composed of two power electronic switches connected in series; the DC-adaptive energy-consuming unit is arranged in the current-limiting branch and is composed of a unidirectional semi-controlled switching device connected in series with a DC current-limiting resistor; the AC-adaptive energy-consuming unit is arranged in the energy-consuming branch and is composed of a bidirectional semi-controlled switching device connected in series with an AC energy-consuming resistor; the bypass branch includes a bypass switch.

[0015] As a preferred embodiment of the present invention, the current-limiting and energy-consuming submodule includes a main current-carrying branch, a current-limiting and energy-consuming branch, and a bypass branch arranged in parallel; wherein, the main power unit of the submodule is arranged in the main current-carrying branch and is composed of two power electronic switches connected in series; the current-limiting and energy-consuming branch includes a bidirectional semi-controlled switching device, a DC current-limiting resistor, and an AC energy-consuming resistor connected in series, and a unidirectional semi-controlled switching device is connected to the midpoint of the two resistors; when a DC fault occurs, the bidirectional semi-controlled switching device, the DC current-limiting resistor, and the unidirectional semi-controlled switching device are turned on to form a DC-adaptive energy-consuming unit; when an AC fault occurs, the bidirectional semi-controlled switching device, the DC current-limiting resistor, and the AC energy-consuming resistor are turned on to form an AC-adaptive energy-consuming unit; the bypass branch includes a bypass switch.

[0016] As a preferred embodiment of the present invention, the upper and lower bridge arms further include a bridge arm current acquisition device.

[0017] As a preferred embodiment of the present invention, the MMC submodule is a half-bridge power module, comprising two switching power devices with built-in reverse diodes and a supporting capacitor, wherein one of the switching power devices is connected in parallel with a bypass thyristor.

[0018] The present invention also provides an MMC converter valve with integrated AC / DC fault ride-through function, wherein the converter valve has the structure of any of the aforementioned topologies; the common point of the three-phase bridge arms is used as the DC port for the converter valve to connect to the DC system, and the midpoint of the upper and lower bridge arms is used as the AC port for the converter valve to connect to the AC system.

[0019] This invention also provides a coordinated control method for the aforementioned MMC converter valve with integrated AC / DC fault ride-through function. This method applies the MMC converter valve to a high-voltage flexible DC transmission system, achieving integrated control of fault current suppression, surplus power dissipation, and DC bus voltage stability through the coordinated operation of the current-limiting energy dissipation submodule and the MMC submodule. Specifically, it includes:

[0020] Under normal operating conditions, only the main power unit of the current-limiting energy-consuming submodule in each bridge arm remains on and is connected in series in the bridge arm circuit of the MMC converter valve; at this time, the bypass switch K is in the open state. Because the DC current-limiting resistor and AC energy-consuming resistor in the sub-condition energy-consuming unit have relatively high resistance values, the bridge arm circuit current only flows through the main current-carrying branch and does not pass through the sub-condition energy-consuming unit or the bypass branch.

[0021] When a DC fault occurs, the DC-adaptive energy dissipation unit is connected in series in the MMC bridge arm circuit, and the DC fault current is limited and suppressed by the DC current limiting resistor in the unit.

[0022] When an AC fault occurs, the AC-adaptive energy dissipation unit is connected in series in the MMC bridge arm circuit, and the AC energy dissipation resistor in the unit dissipates the surplus power on the AC side.

[0023] When a current-limiting and energy-consuming submodule malfunctions, the bypass switch in the bypass branch is turned on to disconnect the current-limiting and energy-consuming submodule from the bridge arm circuit, thus preventing the fault from escalating.

[0024] As a preferred embodiment of the present invention, it further includes:

[0025] (1) When a DC short-circuit fault occurs, the coordinated actions shall be performed in the following sequence:

[0026] Current-limiting energy consumption submodule branch switching: First, the adaptation condition switching device of the DC-adaptive energy consumption unit in the current-limiting energy consumption submodule is triggered to turn on, and then the power electronic switch in the main power unit of the submodule is turned off; since the bypass switch of the bypass branch remains open, the fault current is forcibly transferred to the DC-adaptive energy consumption unit to suppress the current rise and damage the switching device.

[0027] MMC converter valve lockout and bypass protection: Synchronously lockout of all switching devices in the MMC submodule to prevent the supporting capacitor from discharging to the fault point; at the same time, trigger the bypass thyristor in the MMC submodule to conduct, providing a low-impedance path for the fault current and protecting the freewheeling diode from reverse recovery current impact.

[0028] Synergy with DC circuit breakers: After the current limiting and energy consumption submodule completes the current limiting, the DC circuit breaker only needs to interrupt the low peak current suppressed by the DC current limiting resistor; at this time, the MMC converter valve is equivalent to a three-phase uncontrolled rectifier, and the DC side voltage naturally drops to the uncontrolled rectifier output level, reducing the voltage withstand and current breaking requirements of the DC circuit breaker.

[0029] (2) Coordinated control strategy under AC failure

[0030] When an AC-side short-circuit fault occurs, the key is to address power imbalance and DC bus voltage fluctuations through the coordinated operation of the current-limiting energy dissipation submodule and the switching of the MMC submodule.

[0031] Energy-consuming branch switching and surplus power dissipation: The switching device of the AC-adaptive energy-consuming unit is turned on, and the AC energy-consuming resistor is connected to the bridge arm circuit. The surplus power between the receiving and receiving ends is dissipated through Joule heat dissipation to prevent the DC bus voltage from rising continuously.

[0032] Common-mode component adjustment suppresses voltage fluctuations: The DC bias voltage caused by the freewheeling current of the bridge arm inductor when the current-limiting energy-consuming submodule is put into or removed is compensated by adjusting the common-mode component of the MMC converter valve.

[0033] Power flow direction adaptation control: Considering the difference in power flow direction between the sending station and the receiving station, a symmetrical coordination strategy is adopted to increase the number of MMC sub-modules on the sending station side and reduce the number of MMC sub-modules on the receiving station side to ensure synchronous and stable DC bus voltage at both ends.

[0034] The present invention also provides a computer device, comprising: at least one processor, and a memory communicatively connected to the at least one processor, wherein the memory stores instructions that are executed by the at least one processor to cause the at least one processor to perform the aforementioned coordinated control method for an MMC converter valve with integrated AC / DC fault ride-through function.

[0035] The present invention also provides a computer-readable storage medium storing computer instructions for causing the computer to execute the aforementioned coordinated control method for an MMC converter valve with integrated AC / DC fault ride-through function.

[0036] Compared with the prior art, the beneficial effects of the present invention are:

[0037] (1) Compared with the existing technology, AC and DC protection requires different protection design schemes, the present invention integrates the DC fault current limiting and AC fault energy dissipation functions into the same integrated device, realizing integrated protection of AC and DC faults, without the need to configure multiple separate devices, thus solving the problem of functional redundancy in traditional schemes.

[0038] (2) The present invention reduces the redundant configuration of DC circuit breakers, independent chopper circuits and other equipment in the protection system, reduces hardware costs (such as the number of power electronic devices, resistors and auxiliary control units), and at the same time reduces the system size and simplifies the operation and maintenance complexity.

[0039] (3) Compared with the prior art, the present invention uses a differentiated design of the current limiting resistor R dc (Adapted for DC current limiting) and energy-consuming resistor R ac (Adapted to AC power consumption) This avoids the risk of device overvoltage breakdown caused by mismatch between AC and DC damping requirements, and improves the operational reliability and service life of the equipment.

[0040] (4) Compared with the prior art, the present invention designs branch switching logic for different operating conditions such as normal operation, DC fault, AC fault, and module fault, which can intervene in protection in time when a fault occurs, and can not affect the operation of the system under normal operating conditions.

[0041] (5) The topology of the present invention is simple and the function is clear; the operation logic is simple and the internal component parameters are easy to design. Attached Figure Description

[0042] Figure 1 This invention relates to the integrated current-limiting and energy-consuming topology of the bridge arm and the topology of the MMC converter valve.

[0043] Figure 2 This describes the working logic of the current limiting and energy consumption submodule.

[0044] Figure 3 This is the equivalent circuit diagram of the DC side of the power transmission station at both the sending and receiving ends.

[0045] Figure 4 The waveform diagram is a simulation of a DC fault.

[0046] Figure 5 A comparison diagram of the voltage and breaking current of a DC circuit breaker under DC fault conditions.

[0047] Figure 6 The waveform diagram is a simulation of an AC fault.

[0048] Figure 7 This is a comparison chart of DC bus voltage waveforms during AC fault ride-through.

[0049] Figure 8This is the equivalent topology for the current-limiting and energy-consuming submodule. Detailed Implementation

[0050] The present invention will now be described in further detail with reference to the accompanying drawings.

[0051] I. Topology Description of MMC Converter Valve

[0052] Figure 1 This is a bridge arm topology that integrates current-limiting energy dissipation devices and MMC converter valve structure, used in high-voltage flexible DC transmission systems. The AIA in the figure... p / AIA n The bridge arm current acquisition device monitors the current of the upper / lower bridge arm in real time, providing feedback signals for control strategies (such as circulating current suppression and power regulation) and fault protection; the DCCB is a DC circuit breaker that quickly disconnects the DC circuit when there is a fault on the DC side (such as a short circuit), protecting the core components of the MMC, such as sub-modules and bridge arms, from damage.

[0053] This topology is a typical structure of a three-phase modular multilevel converter (MMC), a commonly used power electronic topology in flexible DC transmission (VSC-HVDC) and high-voltage power conversion systems. Its core function is to achieve bidirectional conversion between AC and DC power. Each phase is divided into an upper arm (the upper branch in the diagram) and a lower arm (the lower branch in the diagram). The MMC submodule in each arm is the "basic voltage unit" of the MMC, typically a half-bridge / full-bridge topology (including switching devices and capacitors). By switching different numbers of submodules, a continuously adjustable multilevel voltage can be output, significantly reducing harmonic distortion. By controlling the number of submodules switched in each phase's upper and lower arms of the MMC, the output voltage of the arm is adjusted, ultimately achieving: AC → DC: converting the three-phase AC voltage into a stable DC bus voltage U. dc DC to AC: Converting the DC bus voltage U dc Inverted to three-phase AC voltage U a / U b / U c Bidirectional power transmission: Supports bidirectional adjustment of active and reactive power to adapt to different power grid operation requirements.

[0054] Compared to the traditional MMC topology, this invention proposes an innovative design: in the upper and lower bridge arms of each phase, the circuit structure of the DC current limiting module inserted in series in the bridge arm is improved, so that it becomes a current limiting and energy dissipation sub-module that can realize functionally adjustable current limiting and energy dissipation according to fault conditions, and multiple sub-modules are connected in series to form a current limiting and energy dissipation device.

[0055] This current-limiting energy dissipation submodule includes a main power unit, sub-condition energy dissipation units, and bypass branches. The main power unit comprises two series-connected power electronic switches, which are semi-controlled and / or fully controlled switching devices, used to connect to the MMC bridge arm circuit to achieve the basic function of power conversion. The sub-condition energy dissipation units are connected in parallel across the main power unit, forming a fault energy dissipation path. Each sub-condition energy dissipation unit includes at least two energy dissipation units adapted to different fault conditions: an AC-adaptive energy dissipation unit and a DC-adaptive energy dissipation unit. Each energy dissipation unit includes a dissipation resistor and an adaptation switching device, which is a unidirectional and / or bidirectional semi-controlled and / or fully controlled switching device. The bypass branches are connected in parallel across the main power unit and include bypass switches. Each switching device can be at least one of IGBTs, MOSFETs, and thyristors.

[0056] by Figure 1 Taking this as an example, the topology of the MMC converter valve is a three-phase dual-arm modular multilevel converter (MMC) architecture. The common point of the three-phase arms serves as the DC port for the converter valve to connect to the DC system, and the midpoint between the upper and lower arms serves as the AC port for the converter valve to connect to the AC system. Each phase's upper arm contains one arm inductor and N identically structured MMC sub-modules connected in series, where N ≥ 1. Each MMC sub-module is a half-bridge power module, comprising two switching power devices with built-in reverse diodes and a supporting capacitor, with one of the switching power devices connected in parallel with a bypass thyristor. Each phase's lower arm contains one arm inductor and M identically structured current-limiting and energy-dissipating sub-modules connected in series, where M ≥ 1. Each current-limiting and energy-dissipating sub-module includes a main current-carrying branch, a current-limiting branch, an energy-dissipating branch, and a bypass branch arranged in parallel. The main power unit of the sub-module is located in the main current-carrying branch and consists of two power electronic switches T... AIA1 and T AIA2 Series-connected; DC-adaptive energy-consuming units are arranged in the current-limiting branch, consisting of a unidirectional semi-controlled switching device T. dc With DC current limiting resistor R dc Series-connected; AC-adaptive energy-consuming units are arranged in the energy-consuming branch, consisting of bidirectional semi-controlled switching devices T. ac With AC power dissipation resistor R ac It consists of series connections; the bypass branch includes the bypass switch K.

[0057] In each bridge arm, the number of MMC submodules and current-limiting energy-consuming submodules connected in series, as well as the selection of capacitors and switching devices, are all determined by technical personnel based on the actual conditions of the high-voltage flexible DC transmission system.

[0058] II. Explanation of MMC converter valve control method

[0059] Based on the topology of the aforementioned converter valve, this invention proposes a collaborative control strategy suitable for the MMC converter valve.

[0060] The core requirement for solving DC short-circuit faults is suppressing the surge current amplitude, while the core requirement for solving AC short-circuit faults is rapidly dissipating excess power. Although the fault characteristics and solution objectives differ, both can essentially be achieved through the dual effects of current limiting and energy dissipation via energy-dissipating damping. The former suppresses sudden current surges through damping, while the latter dissipates excess energy through damping, ultimately achieving effective management of both types of faults. Based on the analysis of the above two fault types, this invention proposes the following... Figure 2 The working logic of the current limiting and energy consumption integrated topology is shown.

[0061] (1) Coordination between normal operation and fault detection phases

[0062] Under normal operating conditions, the main current-carrying branch of the current-limiting and energy-consuming submodule remains conducting and is connected in series in the MMC bridge arm circuit. At this time, the (AC) energy-consuming branch, the (DC) current-limiting branch, and the bypass switch K are all in the off state, and the bridge arm current flows through the main current-carrying branch with low impedance, avoiding additional power loss. The DC bus voltage, bridge arm current, and AC side voltage signal are monitored in real time: when a DC short-circuit fault is detected, the coordinated protection action is immediately triggered.

[0063] (2) Coordinated control strategy under DC fault

[0064] When a DC short-circuit fault occurs, the coordinated actions are performed in the following sequence: First, the semi-controlled switching device T of the current-limiting branch in the current-limiting energy consumption submodule is triggered. dc Turn on, then turn off T of the main current branch. AIA1 With T AIA2 Because the bypass switch K remains open, the fault current is forcibly transferred to the DC current-limiting resistor R, which has a lower resistance value. dc The current-limiting branch quickly suppresses the current rise and avoids IGBT overcurrent damage.

[0065] MMC converter valve lockout and bypass protection: Synchronously locks out all IGBTs in the MMC submodule to prevent the supporting capacitors in the MMC submodule from discharging to the fault point; simultaneously triggers the bypass thyristor T in the MMC submodule. m It conducts, providing a low-impedance path for fault current and protecting the freewheeling diode from reverse recovery current surges.

[0066] Coordination with DC circuit breakers: After the current-limiting energy consumption submodule completes current limiting, the DC circuit breaker DCCB only needs to disconnect the circuit through the DC current-limiting resistor R. dcAfter the suppression of the low peak current, the MMC converter valve is equivalent to a three-phase uncontrolled rectifier. The DC side voltage naturally drops to the uncontrolled rectifier output level, which greatly reduces the voltage withstand and current breaking requirements of the DC circuit breaker (DCCB).

[0067] (3) Coordinated control strategy under AC failure

[0068] When an AC-side short-circuit fault occurs, the key is to address power imbalance and DC bus voltage fluctuations through the coordinated action of current-limiting energy dissipation submodules and MMC submodule switching.

[0069] Power consumption branch switching and surplus power dissipation: Switching devices (bidirectional thyristors T) of AC-adaptive power consumption units in the current-limiting power consumption submodule. ac ( ) Turn on, turning on the medium-to-high resistance AC energy-consuming resistor R ac Connect to bridge arm circuit; AC power dissipation resistor R ac By dissipating surplus power between the transmitting and receiving ends through Joule heat dissipation, the DC bus voltage is prevented from continuously rising.

[0070] Common-mode component adjustment suppresses voltage fluctuations: The DC bias voltage caused by the freewheeling current of the bridge arm inductor when the current-limiting energy-consuming submodule is put into or removed is compensated by adjusting the common-mode component of the MMC converter valve.

[0071] When the current-limiting energy-consuming submodule is activated, the bridge arm current flows through the AC energy-consuming resistor R. ac A positive bias voltage is generated, at which point the MMC commutator valve is controlled to reduce the common-mode component and counteract the effect of the bias voltage on the DC bus.

[0072] like Figure 3 As shown, when the current-limiting energy-consuming submodule is activated, the bridge arm current generates a positive bias voltage U through the energy-consuming resistor. Rdiss :

[0073] (1)

[0074] (2)

[0075] Among them, R diss This indicates the total resistance of the energy-consuming resistors connected inside the MMC converter valve bridge arm; N diss Indicates the number of bridge arm integrated sub-modules deployed; R ac This indicates the resistance value of the power-dissipating resistor within a single module; U Rdiss This indicates the common-mode component introduced by the engaged bridge arm submodule, which will cause fluctuations in the DC bus voltage; i cirx This represents the internal circulating current of the three-phase bridge arm of the MMC, where x = a, b, c.

[0076] At this time, control the MMC converter valve to reduce the common mode component u.comx Adjusted common-mode components:

[0077] (3)

[0078] Among them, U fcomx This represents the common-mode component of MMC after incorporating a common-mode suppression strategy; u comx This represents the common-mode component in the initial state of MMC.

[0079] Power flow direction adaptation control: Considering the difference in power flow direction between the sending station (SEC) and the receiving station (REC), the bias voltage generated by the current limiting and energy consumption submodule on the SEC side has the opposite polarity to that on the REC side. Therefore, a symmetrical coordination strategy of "increasing the number of MMC submodules in use" on the SEC side and "reducing the number of MMC submodules in use" on the REC side is required to ensure that the DC bus voltages at both ends are synchronously stable.

[0080] When a current-limiting and energy-consuming submodule in the MMC converter valve needs to be disconnected due to its own fault, it is only necessary to close the bypass switch K in the bypass circuit. Since the resistance is lowest in the bypass circuit, the current will be conducted through it, thus disconnecting the fault-crossing modular equipment from the circuit and preventing it from affecting the normal operation of the high-voltage flexible DC transmission system converter station.

[0081] Based on the knowledge, learning ability, and operational experience of those skilled in the art, AC / DC faults caused by different loads and lines in the power grid may occur during practical applications. Relevant technical personnel can determine which current-limiting and energy-consuming submodule on which bridge arm to activate, and how many integrated current-limiting and energy-consuming submodules to activate, based on the actual fault situation. This part is not part of the core innovation of this invention and will not be elaborated further.

[0082] Third, by applying the MMC converter valve with a modular design of current limiting and energy dissipation sub-module, this invention can achieve smooth control of DC bus voltage fluctuations under AC fault ride-through conditions.

[0083] For ease of understanding, this embodiment is built in PSCAD simulation software. Figure 1 The topology and control model of the MMC converter valve are shown below. Specific parameters are shown in Table 1. Simulation results of the control process are as follows: Figures 4-7 As shown.

[0084] Table 1 Simulation parameters of the HVDC transmission system based on the current-limiting energy consumption submodule

[0085]

[0086] DC fault simulation waveforms are as follows Figure 4As shown. At t=0.5s (T1), an inter-electrode metallic short-circuit fault is applied to the outlet side of the MMC converter valve. After a protection delay of 1.315ms, the system performs the following coordinated actions at t=0.501315s (T2): the MMC converter valve is quickly locked, and the bypass thyristor of the submodule is turned on to shunt the fault current. The IGBT of the main current-carrying branch of the current-limiting energy-consuming submodule is turned off, and the thyristor T of the DC current-limiting circuit is triggered. dc The current-limiting resistor R dc The circuit is engaged to suppress current rise. The DC circuit breaker (DCCB) is triggered and the converter switch (LCS) is locked, preparing for the subsequent transfer and clearing of the fault current. During the T2-T3 phase, the fault current gradually transfers from the current-carrying branch to the converter circuit of the DC circuit breaker. During this period, the MMC topology is equivalent to a three-phase uncontrolled rectifier bridge on the AC side, and the DC voltage drops accordingly from its rated value to the level corresponding to the uncontrolled rectifier. At time T3, the main circuit breaker of the DC circuit breaker operates and begins to cut off the fault current. When the current flows through the metal oxide varistor (MOV) of the DC circuit breaker, it rapidly decays to zero due to its nonlinear volt-ampere characteristic. During this phase, the system experiences the maximum electrical stress: the C-phase upper arm current I... cp Reaching a peak voltage of 3.88kA, the maximum voltage U that a single current-limiting energy-consuming submodule can withstand at this point is... rcp The voltage is 2.5kV. This result shows that both peak voltage and current are within the system's safety margin, verifying the effectiveness of the solution. At time T4, the DC fault current is completely cleared by the DC circuit breaker. To ensure reliable interruption and prevent arc reignition, the system delays for an additional 3ms after detecting the current zero crossing before performing the restart operation. Thanks to the effective maintenance of the submodule capacitor voltage during MMC lockout, the system has rapid recovery capability, restoring normal DC voltage delivery within 10ms of the fault occurrence.

[0087] Figure 5 (a) and (b) in the diagram illustrate the voltage and breaking current of the DC circuit breaker under DC fault conditions, respectively. With the bridge current-limiting energy-consuming submodule, the DC circuit breaker only needs to break at the uncontrolled rectified voltage level. Figure 5 The results show that, compared to the pure DC circuit breaker scheme, the breaking voltage of the DC circuit breaker in the current-limiting energy-consuming submodule scheme is significantly reduced, from 632.15kV to 383.13kV, a reduction of 39.4%. It is worth noting that the DC circuit breaker still needs to withstand the rated DC voltage U during steady-state no-load operation. dc Its parallel metal oxide varistor (MOV) operating voltage must be higher than U. dc To prevent accidental damage, and considering engineering margins, the MOV's motion threshold is set to 1.2U in this example. dcThanks to the current-limiting resistor in the current-limiting energy consumption submodule, the fault current was effectively suppressed. The peak current required to interrupt by the DC circuit breaker was significantly reduced from 7.12kA in the DC circuit breaker scheme to 3.88kA, a reduction of up to 45.5%.

[0088] Figure 6 Simulated waveforms under AC fault conditions are shown. At time t=0.5s (T1), a severe three-phase ground fault is applied to the receiving end of the AC system. From... Figure 6 As can be clearly seen in (a), the AC voltage dropped by 95% instantaneously. This fault completely blocked the power transmission channel at the receiving end. Figure 6 (c) shows the active power P at the receiving end. rec The voltage dropped rapidly from 1.0 pu to 0 p.u. Because the power input at the sending end continued, the power imbalance caused energy to accumulate rapidly on the DC side, leading to a drop in the DC bus voltage U. dc Continue to climb. When U dc When the preset 1.05 pu protection threshold is reached (at time T2), the protection system responds rapidly, simultaneously activating the current-limiting and energy-consuming submodules at both the sending and receiving ends, turning off the IGBTs in the main circuit, and triggering the thyristor T in the AC energy-consuming circuit. ac The current-limiting resistor R ac The circuit is activated to suppress current rise, thus dissipating excess energy caused by power imbalance. At the moment the current-limiting energy-dissipating submodule is activated, it compensates for the introduced equivalent DC voltage U. Rdiss In response to disturbances in the bus voltage, the system synchronously activates the converter valve coordinated control strategy described earlier. This strategy adjusts the common-mode component of the converter valves at both the sending and receiving ends based on the power flow direction:

[0089] At the receiving end station (REC): Since power is transmitted in the forward direction (from DC to AC), the DC current in its bridge arm is positive. Therefore, the integrated current-limiting energy dissipation device in the bridge arm will generate a positive equivalent DC voltage 2U. Rdiss This voltage tends to further increase the DC bus voltage. To achieve accurate compensation, such as... Figure 6 As shown in (e), the control system of the receiving station increases the number of MMC submodules deployed from N. dc / 2=145 was reduced to 133, reducing the investment in 12 sub-modules, thus offsetting U Rdiss The impact.

[0090] Throughout the fault ride-through, the system remained stable; despite experiencing severe AC fault impacts and switching disturbances from the integrated current-limiting and energy-dissipating device in the bridge arm, U remained stable under the precise adjustment of the coordinated control strategy. dc It was successfully clamped around 1.05 pu, effectively preventing system lockout due to overvoltage. Figure 6As shown in (f), the bridge arm integrated current-limiting energy dissipation device is subjected to a peak voltage of 3.28kV after being put into operation, indicating that it is effectively dissipating energy. Meanwhile, Figure 6 As shown in (d), the average voltage of the submodule only rose slightly to 1.04 pu, which is well within the safe range. At t=1.15s (T3), the AC fault was cleared, and U ac Normal operation restored. Upon detecting the restoration, the system immediately disconnected the current-limiting and power-consuming submodule and restored the number of submodules in operation to 145. dc After a brief drop to 0.98 pu, the active power quickly recovered to the rated value at time T4, with P rec It was then restored to 1.0 pu, and the entire system successfully achieved smooth and rapid fault crossing and recovery.

[0091] Figure 7 This image shows a comparison of the DC bus voltage waveforms during AC fault ride-through. Without the coordinated control strategy, the activation and deactivation of the current-limiting and energy-consuming submodules severely impacted the DC bus voltage. Particularly at the moment of activation, the peak DC voltage reached 1.11 pu, potentially causing overvoltage protection malfunction and system lockout. In contrast, the system's dynamic response improved after introducing the converter valve coordinated control strategy. Throughout the AC fault ride-through, the DC bus voltage was controlled within a safe range of 0.98 pu to 1.05 pu. The voltage disturbance caused by the activation and deactivation of the current-limiting and energy-consuming submodules was effectively compensated for through dynamic adjustment of the submodule's common-mode component, verifying the effectiveness and feasibility of the scheme of compensating for the equivalent voltage of the current-limiting and energy-consuming submodules by actively controlling the converter valve's common-mode component. This indicates that the proposed coordinated control strategy plays a crucial role in ensuring the DC voltage stability of the system during AC fault ride-through. Compared to the 43 modules required for traditional AC / DC fault ride-through functions, this invention requires only 31 modules, reducing the number of submodules by 28%.

[0092] Figure 8 The equivalent topology and working principle of the current-limiting energy-consuming submodule are described.

[0093] The current-limiting and energy-consuming submodule in the diagram includes a main current-carrying branch, a current-limiting and energy-consuming branch, and a bypass branch arranged in parallel. The main power unit of the submodule is located in the main current-carrying branch and consists of two power electronic switches connected in series. The current-limiting and energy-consuming branch includes a series-connected bidirectional semi-controlled switching device, a DC current-limiting resistor, and an AC energy-consuming resistor, with a unidirectional semi-controlled switching device connected to the midpoint of the two resistors. In the event of a DC fault, the bidirectional semi-controlled switching device, the DC current-limiting resistor, and the unidirectional semi-controlled switching device are activated to form a DC-adaptive energy-consuming unit. In the event of an AC fault, the bidirectional semi-controlled switching device, the DC current-limiting resistor, and the AC energy-consuming resistor are activated to form an AC-adaptive energy-consuming unit. The bypass branch includes a bypass switch.

[0094] Figure 8 and Figure 1 The current-limiting and energy-consuming submodules in the two modules are mathematically equivalent, but differ slightly in their physical representation. Their topological principles are the same, but the corresponding resistor values ​​can be changed accordingly. Figure 8 The resistance of an intermediate topology compared to Figure 1 The topology can be lower and the cost can be more streamlined, so those skilled in the art can choose the appropriate topology for the protection device system design according to the actual engineering needs.

[0095] In summary, the integrated current-limiting and energy-dissipating topology for bridge arms in high-voltage flexible DC transmission systems provided by this invention can effectively improve the integration of the protection system of high-voltage flexible DC transmission systems. This device integrates DC fault current limiting and AC fault energy dissipation functions into a single device in the MMC bridge arm, replacing the traditional separate DC circuit breaker, fault current limiter, and chopper circuit, avoiding redundant configuration of multiple devices. This device can also significantly reduce the overall cost of the protection device, not only reducing the procurement of core components such as fully controlled IGBTs and resistors, but also reducing the current breaking and voltage withstand requirements of the DC circuit breaker due to its own current-limiting function. Hardware investment can be controlled by reducing the number of series devices in the DC circuit breaker, while simplifying system installation and maintenance processes. Furthermore, this device can effectively smooth DC bus voltage fluctuations during AC fault crossings in high-voltage flexible DC transmission systems. Through its own coordinated control with the MMC converter valve, it suppresses the bias voltage generated by the bridge arm inductor freewheeling when the energy-dissipating branch is switched on, stabilizing the voltage fluctuation range within a safe threshold. This device significantly improves the stability of high-voltage flexible DC transmission systems during AC fault ride-throughs, preventing ride-through failures caused by system blocking triggered by voltage over-limits. This integrated current-limiting and energy-consuming device in the bridge arm is of great significance for promoting the development of flexible DC transmission systems towards high integration, low cost, and high reliability, and for ensuring the grid connection of new energy sources and the safe operation of the power grid.

Claims

1. A topology of a bridge arm integrated current limiting energy dissipating device, characterized in that, This topology is a three-phase dual-bridge-arm modular multilevel converter; in the upper bridge arm of each phase, there is a bridge arm inductor and N MMC sub-modules connected in series with the same structure, N≥1; in the lower bridge arm of each phase, there is a bridge arm inductor and a current limiting and energy dissipation device composed of M current limiting and energy dissipation sub-modules connected in series with the same structure, M≥1. The current-limiting energy-consuming submodule includes a main power unit for connecting to the bridge arm circuit to achieve power conversion; The sub-condition energy consumption unit is connected in parallel to both ends of the main power unit of the sub-module to form a fault energy dissipation path. The sub-condition energy consumption unit includes at least two energy consumption units adapted to different fault conditions, namely an AC-adaptive energy consumption unit and a DC-adaptive energy consumption unit. Each energy consumption unit includes an energy consumption resistor and an adaptation switching device.

2. The topology of claim 1, wherein, The current-limiting energy-consuming submodule also includes a bypass branch connected in parallel to both ends of the main power unit of the submodule, and the bypass branch includes a bypass switch.

3. The topology of claim 1, wherein, The main power unit of the submodule includes two power electronic switches connected in series, which are fully controlled switching devices, specifically selected from either IGBT or MOSFET; the switching devices in the sub-condition energy consumption units are all semi-controlled switching devices, wherein: the AC-adaptive energy consumption unit uses bidirectional thyristors; the DC-adaptive energy consumption unit uses unidirectional thyristors, or uses a method of coordinated control of unidirectional thyristors and bidirectional thyristors.

4. The topology of claim 1, wherein, The current-limiting and energy-consuming submodule includes a main current-carrying branch, a current-limiting branch, an energy-consuming branch, and a bypass branch arranged in parallel. The main power unit of the submodule is located in the main current-carrying branch and consists of two power electronic switches connected in series. The DC-adaptive energy-consuming unit is located in the current-limiting branch and consists of a unidirectional semi-controlled switching device connected in series with a DC current-limiting resistor. The AC-adaptive energy-consuming unit is located in the energy-consuming branch and consists of a bidirectional semi-controlled switching device connected in series with an AC energy-consuming resistor. The bypass branch includes a bypass switch.

5. The topology of claim 1, wherein, The current-limiting and energy-consuming submodule includes a main current-carrying branch, a current-limiting and energy-consuming branch, and a bypass branch arranged in parallel. The main power unit of the submodule is located in the main current-carrying branch and consists of two power electronic switches connected in series. The current-limiting and energy-consuming branch includes a series-connected bidirectional semi-controlled switching device, a DC current-limiting resistor, and an AC energy-consuming resistor. The two ends of the current-limiting and energy-consuming branch are connected in parallel to the two ends of the main power unit of the submodule. One end of the unidirectional semi-controlled switching device is connected to the midpoint of the DC current-limiting resistor and the AC energy-consuming resistor, and the other end is connected to the common connection terminal of the bridge arm of the current-limiting and energy-consuming submodule. In the event of a DC fault, the bidirectional semi-controlled switching device, the DC current-limiting resistor, and the unidirectional semi-controlled switching device are activated to form a DC-adaptive energy-consuming unit. In the event of an AC fault, the bidirectional semi-controlled switching device, the DC current-limiting resistor, and the AC energy-consuming resistor are activated to form an AC-adaptive energy-consuming unit. The bypass branch includes a bypass switch.

6. The topology of claim 1, wherein, The upper and lower bridge arms also include a bridge arm current acquisition device.

7. The topology of claim 1 wherein, The MMC submodule is a half-bridge power module, which includes two switching power devices with built-in reverse diodes and a supporting capacitor, one of which is connected in parallel with a bypass thyristor.

8. An MMC converter valve integrated with an AC-DC fault ride-through function, characterized in that, The converter valve has the topology described in any one of claims 1 to 7; the common point of the three-phase bridge arms is used as the DC port for the converter valve to connect to the DC system, and the midpoint of the upper and lower bridge arms is used as the AC port for the converter valve to connect to the AC system.

9. The method of coordinated control of an MMC converter valve integrated with an AC-DC fault ride-through function according to claim 8, characterized in that, The MMC converter valve is applied to a high-voltage flexible DC transmission system. Through the coordinated operation of the current-limiting energy consumption submodule and the MMC submodule, integrated control of fault current suppression, surplus power dissipation, and DC bus voltage stability is achieved. Specifically, this includes: Under normal operating conditions, only the main power unit of the current-limiting energy-consuming submodule in each bridge arm remains on and is connected in series in the bridge arm circuit of the MMC converter valve; at this time, the bypass switch K is in the open state. Because the DC current-limiting resistor and AC energy-consuming resistor in the sub-condition energy-consuming unit have relatively high resistance values, the bridge arm circuit current only flows through the main current-carrying branch and does not pass through the sub-condition energy-consuming unit or the bypass branch. When a DC fault occurs, the DC-adaptive energy dissipation unit is connected in series in the MMC bridge arm circuit, and the DC fault current is limited and suppressed by the DC current limiting resistor in the unit. When an AC fault occurs, the AC-adaptive energy dissipation unit is connected in series in the MMC bridge arm circuit, and the AC energy dissipation resistor in the unit dissipates the surplus power on the AC side. When a current-limiting and energy-consuming submodule malfunctions, the bypass switch in the bypass branch is turned on to disconnect the current-limiting and energy-consuming submodule from the bridge arm circuit, thus preventing the fault from escalating.

10. The method of claim 9, wherein, Further includes: (1) When a DC short circuit fault occurs: Current-limiting energy consumption submodule branch switching: turn on the DC-adaptive energy consumption unit and turn off the switching devices in the main power unit of the submodule; the fault current is forcibly transferred to the DC-adaptive energy consumption unit; MMC converter valve lockout and bypass protection: synchronously lockout of all switching devices in the MMC submodule, and simultaneously trigger the bypass thyristor in the MMC submodule to conduct; Synergy with DC circuit breakers: After the current limiting and energy consumption submodule completes the current limiting, the DC circuit breaker only needs to interrupt the low peak current suppressed by the DC current limiting resistor; (2) When an AC side short circuit fault occurs: Energy-consuming branch switching and surplus power dissipation: The switching device of the AC-adaptive energy-consuming unit is turned on to connect the AC energy-consuming resistor to the bridge arm circuit. Common-mode component adjustment suppresses voltage fluctuations: by adjusting the common-mode component of the MMC converter valve, the DC bias voltage generated when the current-limiting energy-consuming submodule is put into / out is compensated. Power flow direction adaptation control: A symmetrical coordination strategy is adopted on the sending end station side to increase the number of MMC sub-modules and on the receiving end station side to reduce the number of MMC sub-modules to ensure synchronous and stable DC bus voltage at both ends.

11. A computer device, comprising: include: At least one processor, and a memory communicatively connected to the at least one processor, wherein the memory stores instructions that are executed by the at least one processor to cause the at least one processor to perform the coordinated control method of the MMC converter valve with integrated AC / DC fault ride-through function as described in claim 9.

12. A computer-readable storage medium, characterized in that, The computer readable storage medium stores computer instructions for causing the computer to execute the coordinated control method of the MMC converter valve integrated with the AC / DC fault ride-through function according to claim 9.

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