Direct current fault ride-through method and device based on energy consumption damping and circuit breaker and medium

Abstract

The invention relates to a high-voltage direct-current power transmission technology, and aims to provide a direct-current fault ride-through method and device based on energy-consuming damping and a circuit breaker and a medium. The method comprises the following steps: reducing the AC side voltage of a converter valve, and transferring a fault current to an energy consumption damping module in the MMC converter valve, so that an electromotive force enters a new steady state stage; the direct current fault current is gradually reduced, the direct current circuit breaker executes breaking operation, and a direct current fault point is completely isolated from the high-voltage flexible direct current power transmission system; a control strategy when the MMC converter valve operates normally is recovered, the energy consumption damping module is cut off step by step, and the MMC sub-module is put into operation at the same time, so that the high-voltage flexible direct-current power transmission system is stably transited to the normal operation state. Impact and damage risks of fault current on system equipment can be effectively reduced, and safe and stable operation of the system is guaranteed; a direct-current circuit breaker with a smaller capacity can be adopted, so that the purchase cost of equipment is remarkably reduced; the system stability can be kept under the fault condition, and the power supply reliability is improved.

Description

Technical Field

[0001] The present invention belongs to the field of high voltage direct current transmission, and in particular is a direct current fault riding method based on energy dissipation damping and circuit breaker, a computing device and a computer readable storage medium. Background Art

[0002] In a high-voltage flexible DC transmission system, DC faults are a key factor threatening the stable operation of the system. DC fault protection is essential for the reliable operation of the DC transmission system, and usually includes the detection, location, disconnection and isolation of pole-to-pole (PP) and pole-to-ground (PG) short-circuit faults, high-impedance ground faults and arc faults. Among them, inter-pole faults occur between the positive and negative lines and usually exhibit low short-circuit impedance. It is the most serious type of fault in the DC power grid and usually generates a large fault current in a very short time. Pole-to-ground faults occur when the positive or negative power line is short-circuited to the ground, usually caused by lightning strikes and component failures. The impact of pole-to-ground faults depends on the specific grounding scheme used in the DC power system and may also cause harm. Once a short-circuit fault occurs on the DC side, due to the low damping characteristics of the system itself, the fault current will rise rapidly in a very short time, and the rising rate is extremely fast, which may reach several times the normal operating current amplitude within a few milliseconds. This will have a huge impact on the equipment in the system, such as converter valves, reactors, capacitors, etc., and is very likely to cause equipment damage or even cause the collapse of the entire system, thereby causing large-scale power outages and causing serious losses to the social economy.

[0003] At present, there are three mainstream technical routes to solve DC fault ride-through:

[0004] The first solution is the DC circuit breaker (DCCB). As a key device for solving DC faults, it often adopts a hybrid structure of multiple IGBT series-parallel technology because it needs to break higher fault currents and withstand DC bus overvoltage. This circuit breaker can quickly break fault currents to a certain extent and meet the system's demand for rapid fault removal. However, in order to achieve reliable disconnection under high voltage and high current, a large number of IGBT devices are required to be combined in series and parallel, which greatly increases the cost of fault protection equipment. In addition, in the process of fault handling in the existing technology, the coordination between the various devices is poor, and it is difficult to achieve efficient fault current limiting. Simple fault current limiting measures often cannot accurately control the fault current, and DC circuit breakers need to have a large redundancy to avoid fault current shocks.

[0005] The second solution is to use a hybrid MMC, which uses the additional back electromotive force brought by the reverse input of the full-bridge submodule to suppress the DC bus voltage and further limit the DC inrush current. However, the full-bridge submodule requires twice as many switching devices as the half-bridge submodule, and conduction losses will occur under normal operation. For the DC fault protection solution, the device cost and loss cost under this solution are relatively high.

[0006] The third solution is to use only the energy-consuming damping module to limit the fault current. This solution can reliably achieve DC fault current limiting, but it cannot directly disconnect the DC fault current after current limiting. The high-voltage DC transmission system is in an uncontrolled rectification state after current limiting. It is necessary to disconnect the AC side circuit breaker of the converter valve and remove the AC source on the grid side. Only then can the fault current be reduced to the zero point, and the disconnector is further used to disconnect the fault circuit. The system restart also requires the AC switch to be closed. For a single valve body, the overall fault clearing and recovery time is relatively long (more than 500ms), which poses a hidden danger to maintaining the safety and stability of the power system. In the case of a multi-terminal DC transmission grid, this type of solution cannot achieve selective isolation protection. If there is a fault line in the multiple DC lines connected to the converter valve, it requires the entire converter valve to be locked and the AC circuit breaker to be disconnected. At this time, the energy transmission of all DC lines on the converter valve will be interrupted, which is not conducive to the stability of the entire power grid.

[0007] Chinese patent application CN119315614A records an MMC converter valve and method for achieving AC / DC fault crossing based on an energy-consuming damping module. In this scheme, the topological structure of a conventional MMC converter valve is used as the basis. Between the bridge arm inductance of each bridge arm and the midpoint of the upper and lower bridge arms, M energy-consuming damping modules are respectively provided, which are connected in series and have the same structure. The energy-consuming damping module includes a parallel main flow branch, an energy-consuming branch and a bypass circuit. This scheme is a typical case of the third scheme. After a DC short-circuit fault occurs, the impact current amplitude of the DC short-circuit fault can be limited. However, in order to reduce the DC fault current to the zero-crossing point, it is necessary to disconnect the AC side circuit breaker of the converter valve. The disconnection and reclosing time of the AC circuit breaker is relatively long. At this time, the AC side power grid will lose the supporting role of the converter valve, and the safety and stability of the power system are facing challenges. In addition, in the case of a multi-terminal DC power grid, it is impossible to achieve the selective isolation function of the DC fault.

[0008] Therefore, the present invention proposes a new solution to solve the above technical problems. Summary of the invention

[0009] The technical problem to be solved by the present invention is to overcome the deficiencies in the prior art and to propose a DC fault ride-through method, device and medium based on energy dissipation damping and circuit breaker.

[0010] To solve the technical problem, the solution of the present invention is:

[0011] A DC fault ride-through method based on energy dissipation damping and a DC circuit breaker is provided, comprising: (1) A DC fault ride-through protection system is provided in the high-voltage flexible DC transmission system, the system comprising an MMC converter valve, an energy dissipation damping module, a DC circuit breaker and a detection control unit; wherein the energy dissipation damping module is provided in the upper and lower bridge arms of the MMC converter valve and is connected in series with the bridge arm inductance and the MMC submodule; (2) After a DC fault is detected, the MMC converter valve adopts a voltage regulation and current limiting strategy to reduce the AC side voltage by a predetermined ratio based on the original voltage value; or adopts a blocking current limiting strategy to limit the AC side voltage to a specified lower limit value; (3) Transfer the fault current to the energy dissipation damping module inside the MMC converter valve, so that the energy of the fault current is dissipated in the form of heat energy; by suppressing the further increase of the DC current, the electromotive force of the fault circuit gradually enters a new steady-state stage; (4) The DC fault current is gradually reduced through the current limiting operation of the MMC converter valve until the current value is less than the rated working current of the DC circuit breaker; then the DC circuit breaker performs the disconnection operation to completely isolate the DC fault point from the high-voltage flexible DC transmission system; (5) Restore the pressure regulating and current limiting strategy or the locking and current limiting strategy executed by the MMC converter valve to the control strategy during normal operation; (6) Gradually remove the energy-consuming damping module inside the MMC converter valve and put the MMC sub-module into operation accordingly, so that the high-voltage flexible DC transmission system can smoothly transition to normal operation.

[0012] As a preferred embodiment of the present invention, the DC circuit breaker is a hybrid structure, consisting of a fast mechanical switch and a power electronic switch; the fast mechanical switch adopts a vacuum fast mechanical switch; the power electronic switch is a fully controlled power electronic device, which is an insulated gate bipolar transistor IGBT or an integrated gate commutated thyristor IGCT.

[0013] As a preferred solution of the present invention, in the DC fault ride-through protection system, the topological structure of the MMC converter valve is composed of three-phase six bridge arms, and each phase includes two upper and lower bridge arms; wherein the common point of the three-phase bridge arm is used as the DC port connected to the DC system, and the midpoint of the upper and lower bridge arms is used as the AC port connected to the AC system; each bridge arm includes a bridge arm inductor and N MMC sub-modules connected in series and having the same structure, N≥1; between the bridge arm inductor of each bridge arm and the midpoint of the upper and lower bridge arms, M energy-dissipating damping modules connected in series and having the same structure are provided, M≥1;

[0014] The energy dissipation damping module includes a parallel main current branch, an energy dissipation branch and a bypass loop; in the main current branch, the half-controlled switch device T1 or the fully-controlled switch device S3 is reversely connected in parallel with its freewheeling diode D1, and then reversely connected in series with the fully-controlled switch device T2 or S4 and its reverse freewheeling diode D2; the energy dissipation branch includes an energy dissipation resistor R d , the bypass branch includes a bypass switch K;

[0015] Both ends of the MMC converter valve are connected to the busbar of the high-voltage flexible DC transmission system through DC circuit breakers.

[0016] As a preferred solution of the present invention, the MMC submodule is a half-bridge power module, including two switch power devices with reverse diodes and a capacitor; in the energy dissipation damping module, the half-controlled switch device is a thyristor, and the fully controlled switch device is a three-terminal semiconductor switch device IGBT. d External water cooling device.

[0017] As a preferred solution of the present invention, the detection and control unit is used to monitor the DC side current, AC side voltage and operating parameters of the DC fault ride-through protection system in real time, judge the fault state and whether the current tends to a steady state based on the monitoring data, and issue control instructions according to a predetermined strategy to coordinate the actions of the energy-consuming damping module, the DC circuit breaker and the MMC sub-module; the detection and control unit includes a detection module and a control module, wherein the detection module is used to monitor the DC side current and the AC side output voltage of the MMC converter valve in real time, and the control module is used to receive the monitoring signal and issue a control signal to the MMC converter valve and the DC circuit breaker according to a preset control strategy.

[0018] As a preferred solution of the present invention, in step (1), when the AC side of the MMC converter valve is connected to the new energy station, the voltage regulation and current limiting strategy is adopted according to the following formula:

[0019] ;

[0020] Among them, U acfault is the AC side voltage regulated by the MMC converter valve in the case of a DC fault; U ac is the AC side bus voltage under normal operation of the MMC converter valve; k is the voltage regulation coefficient, which is between 0 and 0.8; the new energy station refers to a wind power station located at sea or on land, or a photovoltaic power station located on land.

[0021] As a preferred solution of the present invention, in step (1), when the AC side of the MMC converter valve is connected to a strong power grid, a locking current limiting strategy is adopted according to the following formula:

[0022] ;

[0023] Among them, U acfault is the AC side voltage regulated by the MMC converter valve in the case of a DC fault; U acfault_strong It indicates the lower limit of AC voltage regulation under a strong power grid; the strong power grid specifically refers to a conventional large-capacity AC power grid system located on land.

[0024] As a preferred solution of the present invention, in step (2), in the new steady-state stage, the DC fault current is calculated according to the following formula:

[0025] ;

[0026] Among them, I dc is the DC fault current; T is the power frequency period; t0 is the initial integration time; i u and i d They respectively represent the currents of the upper and lower bridge arms of the three phases in the MMC converter valve;

[0027] In order to effectively ensure that the DC circuit breaker can reliably disconnect the fault current, it is necessary to confirm the DC fault current I dc Less than the rated operating current I of the DC circuit breaker dccb This goal can be achieved by pre-selecting energy-dissipating damping modules with reasonable damping values ​​and using appropriate amounts of energy-dissipating damping modules when a fault occurs.

[0028] The present invention also provides a computing device, comprising: at least one processor, and a memory communicatively connected to the at least one processor, wherein the memory stores instructions executed by the at least one processor, and the instructions are executed by the at least one processor so that the at least one processor executes the aforementioned DC fault riding method based on energy dissipation damping and DC circuit breaker.

[0029] The present invention further provides a computer-readable storage medium, wherein the computer-readable storage medium stores computer instructions, and the computer instructions are used to enable the computer to execute the aforementioned DC fault ride-through method based on energy dissipation damping and DC circuit breaker.

[0030] Description of the invention principle:

[0031] Among the existing mainstream technologies for solving DC fault crossing, there is a new technology that uses energy-consuming modules to improve MMC converter valves. Due to the fault current limiting method and the disconnection method, most current studies use inductive current limiting and DC circuit breaker disconnection; research on the use of capacitors in hybrid MMCs to suppress current; and the use of energy-consuming damping to achieve fault current limiting. Due to the different DC fault crossing mechanisms of different routes, researchers have not yet overcome the inertial thinking mode under different DC fault crossing schemes. When solving the DC fault crossing problem, we are still confined to the solution of using three technical routes alone. The present invention creatively proposes to organically combine different technical routes to construct a new DC fault crossing protection scheme that integrates DC circuit breakers, MMC converter valves and energy-consuming modules. Specifically including:

[0032] (1) MMC converter valve: The MMC submodule inside it plays a key role in voltage regulation and current limiting during faults. When a fault occurs, the MMC submodule can select current limiting control or locked current limiting mode according to the DC fault status. In the current limiting control mode, the internal control parameters are adjusted and the MMC submodules inside the MMC valve body are switched on and off reasonably to achieve voltage reduction regulation of the AC voltage, thereby effectively limiting the fault current. The control strategy of the MMC converter valve is coordinated with the commissioning operation of the energy dissipation damping module and the DC circuit breaker to push the fault circuit loop into a new steady state, thereby limiting the fault current to the breaking capacity of the DC circuit breaker, creating favorable conditions for subsequent breaking operations.

[0033] (2) Energy dissipation damping module: It is installed in each upper and lower bridge arm of the MMC valve body. As the primary link of fault current limitation, this module starts immediately when a DC short circuit fault is detected. The energy dissipation damping module is composed of energy dissipation elements and current-carrying power electronic switches. The energy dissipation elements use resistors with reasonable resistance values ​​and corresponding water-cooling heat dissipation devices, and the power electronic switches use fully controlled power electronic devices (such as insulated gate bipolar transistors (IGBTs) or integrated gate-commutated thyristors (IGCTs), etc.) or semi-controlled power electronic devices (such as thyristors (GTOs), etc.). When a fault occurs, the energy dissipation damping module is quickly put into use to suppress the rapid rise of the fault current. The internal energy dissipation resistor efficiently dissipates the energy carried by the fault current in the form of heat energy, pushing the fault circuit loop into a new steady state, thereby limiting the fault current within the breaking capacity of the DC circuit breaker, creating favorable conditions for subsequent breaking operations.

[0034] (3) DC circuit breaker: It adopts a hybrid structure that integrates the advantages of fast mechanical switches and power electronic switches. The fast mechanical switch in the DC circuit breaker uses a vacuum fast mechanical switch, which has the characteristics of low loss, high reliability and fast opening and closing. It is responsible for conducting current when the system is operating normally, effectively reducing the on-state loss. The power electronic switch uses fully controlled power electronic devices such as insulated gate bipolar transistors (IGBT) or integrated gate commutated thyristors (IGCT), and is equipped with complete overcurrent protection and overheating protection functions. Limited by the impact current of the fault current, even if a reasonable reactor is invested, the fault current will rise to several times to dozens of times the rated current in a short time. In order to achieve reliable interruption of the fault current, DC circuit breakers usually need to connect multiple groups of IGBT valve groups in parallel to achieve the fault current flow and interruption goals, and this measure greatly increases the cost of the circuit breaker. In order to solve this problem, the present invention proposes to use the steady-state operation of the energy dissipation damping module to assist the disconnection operation of the DC circuit breaker. Through the energy dissipation damping module and the current limiting measures of the MMC and the converter valve, the fault current is effectively limited within a certain range, thereby reducing the number of parallel groups of DC circuit breakers, thereby greatly reducing the cost of the DC circuit breaker. When the power system protection detection and control unit issues a disconnection command, the power electronic switch responds quickly, quickly cuts off the fault current, and closely cooperates with the energy dissipation damping module to ensure that the fault current can be reliably disconnected.

[0035] (4) Detection and control unit: responsible for real-time monitoring of the DC side current and various operating parameters of the DC fault ride-through protection system; judging the fault status and whether the current tends to a steady state based on the monitoring data, and issuing control instructions according to the predetermined strategy to coordinate the actions of the energy-dissipating damping module, DC circuit breaker and MMC sub-module; ensuring that the entire system can achieve precise control and stable operation during the fault ride-through process.

[0036] (5) System control and recovery mechanism: When a fault occurs, the fault current is quickly limited through current limiting, voltage regulation and switching operations, reducing the difficulty of disconnecting the DC circuit breaker. When the system enters the recovery phase, the energy-consuming damping module is orderly controlled to gradually exit work to avoid system power oscillation; ensure a smooth transition from the fault state to the normal operation state, reduce the impact on the power supply, and improve the stability and reliability of power supply.

[0037] Through the collaborative working mechanism of the DC circuit breaker and the internal components of the MMC converter valve, the present invention can quickly and accurately limit the DC fault current and disconnect the DC fault current at a reasonable steady-state value. It ensures that the high-voltage flexible DC transmission system does not disconnect from the large power grid during the fault and can quickly return to normal after the fault, effectively overcoming the problems of high DC fault processing cost and insufficient power system stability in the prior art.

[0038] Compared with the prior art, the present invention has the following beneficial effects:

[0039] (1) Efficient fault handling capability: The energy-dissipating damping module and the DC circuit breaker work together to quickly limit and reliably disconnect the fault current. At the moment of a fault, the energy-dissipating damping module quickly limits the rise of the fault current, reducing the difficulty of disconnecting the DC circuit breaker. The close cooperation between the two greatly improves the efficiency of fault handling, effectively reduces the impact and damage risk of the fault current on the system equipment, and ensures the safe and stable operation of the system.

[0040] (2) Cost advantage: Based on the cooperation of MMC converter valve and its internal energy dissipation damping module, a smaller capacity DC circuit breaker can be used, which can significantly reduce the procurement cost of equipment. At the same time, the energy dissipation damping module optimizes the system's fault handling method, reduces the reliance on expensive multi-group IGBT series-parallel technology, and further reduces the system cost. This cost optimization strategy makes the high-voltage flexible DC transmission system more economically feasible when applied on a large scale, which helps promote the development of the industry.

[0041] (3) Enhanced system stability: During a fault, the MMC converter valve uses a flexible current limiting control strategy to work in conjunction with the energy-consuming damping module and the DC circuit breaker. There is no need to isolate the MMC converter valve from the AC power grid, thus maintaining the basic operating capacity of the system. Even in the event of a fault, it can maintain a certain degree of stability, reducing the possibility of system collapse due to a fault, improving the reliability of power supply, and ensuring the power needs of users.

[0042] (4) Smooth system recovery: The system recovery mechanism ensures that the system can smoothly transition to normal operation after fault handling. The orderly recovery of the MMC converter valve and the gradual withdrawal of the energy-consuming damping module effectively avoid system power oscillation, reduce the impact of the fault on power supply, and improve the stability of the power system and power supply continuity. BRIEF DESCRIPTION OF THE DRAWINGS

[0043] Figure 1 Schematic diagram of the MMC converter valve, energy dissipation damping module and DC circuit breaker in the DC fault ride-through protection system.

[0044] Figure 2 This is a flow chart of the fault detection and disconnection stage in the present invention.

[0045] Figure 3 This is a flow chart of the system recovery phase in the present invention.

[0046] Figure 4 The DC voltage comparison between the conventional solution and the present invention under the condition of DC fault clearing.

[0047] Figure 5Comparison of DC current between the conventional solution and the present invention under DC fault clearing conditions.

[0048] Figure 6 Comparison of bridge arm current between the conventional solution and the present invention under DC fault clearing condition. DETAILED DESCRIPTION

[0049] The present invention is further described in detail below in conjunction with the accompanying drawings.

[0050] 1. Structural description of DC fault ride-through protection system

[0051] Figure 1 It is a schematic diagram of the MMC converter valve, energy dissipation damping module and DC circuit breaker in the DC fault ride-through protection system. In the figure, both ends of the MMC converter valve are connected to the busbar of the high-voltage flexible DC transmission system through the DC circuit breaker. The topological structure of the converter valve is composed of three phases and six bridge arms, and each phase includes two upper and lower bridge arms; wherein the common point of the three-phase bridge arm is used as the DC port connected to the DC system, and the midpoint of the upper and lower bridge arms is used as the AC port connected to the AC system; each bridge arm includes a bridge arm inductor and N MMC sub-modules connected in series in sequence and with the same structure, N≥1; between the bridge arm inductor of each bridge arm and the midpoint of the upper and lower bridge arms, there are M energy-dissipating damping modules connected in series in sequence and with the same structure, M≥1; the energy-dissipating damping module includes a parallel main current branch, an energy-dissipating branch and a bypass loop; in the main current branch, the half-controlled switch device T1 or the fully-controlled switch device S3 is reversely connected in parallel with its freewheeling diode D1, and then reversely connected in series with the fully-controlled switch device T2 or S4 and its reverse freewheeling diode D2; the energy-dissipating branch includes an energy-dissipating resistor R d , the bypass branch includes a bypass switch K.

[0052] Among them, the MMC submodule can be a half-bridge power module, including two switch power devices with built-in reverse diodes and a capacitor. The half-controlled switch device T1 can be a thyristor, and the fully controlled switch devices T2, S3, and S4 can be three-terminal semiconductor switch devices (IGBT). d The number of MMC submodules and energy-dissipating damping modules connected in series in each bridge arm, as well as the selection of capacitors and switching devices, are all determined by technicians based on the actual situation of the high-voltage flexible DC transmission system.

[0053] The DC circuit breaker is a hybrid structure, consisting of a fast mechanical switch and a power electronic switch; the fast mechanical switch adopts a vacuum fast mechanical switch; the power electronic switch is a fully controlled power electronic device, which is an insulated gate bipolar transistor IGBT or an integrated gate commutated thyristor IGCT.

[0054] The detection control unit includes a detection module and a control module. The detection module includes a current sensor arranged on the DC bus and a voltage sensor arranged on the AC output side, which is used to monitor the DC side current, AC side output voltage and operating parameters of the DC fault ride-through protection system of the MMC converter valve in real time. The control module is used to receive monitoring signals and send control signals to the MMC converter valve and DC circuit breaker according to a preset control strategy.

[0055] 2. DC Fault Ride-Through Method Based on Energy Dissipation Damping and DC Circuit Breaker

[0056] Based on the above-mentioned DC fault ride-through protection system, the present invention proposes a DC fault ride-through method based on energy-consuming damping and DC circuit breakers, including a fault detection and disconnection stage, and related operation steps in the recovery stage. The actual application scenarios are divided into the AC output side connecting to a new energy station and connecting to a strong power grid. Among them, the new energy station specifically refers to a wind power station or an onshore photovoltaic power station. A strong power grid specifically refers to a large-capacity, high-inertia AC power grid system. The control strategies for different scenarios in the present invention are slightly different, which are explained below:

[0057] 1. The AC side of the MMC converter valve is connected to the new energy station

[0058] (1) First is fault diagnosis:

[0059] When the current sensor installed on the DC bus detects a DC fault, the MMC converter valve adopts a voltage regulation and current limiting strategy to reduce the AC side voltage by a predetermined ratio based on the original voltage value, as shown in formula (1):

[0060] (1)

[0061] Among them, U acfault is the AC side phase voltage regulated by the MMC converter valve in the case of a DC fault; U ac is the AC side bus voltage under normal operation of the MMC converter valve; k is the voltage regulation coefficient. In order to meet the fault ride-through requirements of the new energy high-voltage flexible DC transmission system in the national standard, k is generally taken between 0 and 0.8.

[0062] That is, the AC output voltage of the MMC converter valve is adjusted proportionally, and the bridge arm current in the valve is controlled while the voltage is lowered; by reducing the voltage on the AC side, the impact current is reduced from the source.

[0063] (2) The fault current is transferred to the energy dissipation damping module inside the MMC converter valve, and the energy of the fault current is dissipated in the form of heat energy. By suppressing the further increase of the DC current, the electromotive force of the fault circuit gradually enters a new steady-state stage.

[0064] In the new steady-state phase, the DC fault current is calculated according to the following formula:

[0065] (2)

[0066] Among them, I dc is the DC fault current; T is the power frequency period; t0 is the initial integration time; i u and i d They respectively represent the currents of the three-phase upper and lower bridge arms in the MMC converter valve.

[0067] (3) Gradually reduce the DC fault current I through the limited operation of the MMC converter valve dc In this process, the current sensor is used to sample the DC side current in real time and continuously monitor the current change. When the fault tends to be steady state and the DC fault current I dcfault Less than the rated operating current I of the DC circuit breaker dccb When the DC fault occurs, the control module sends a command to the DC circuit breaker, which performs the breaking operation to completely isolate the DC fault point from the high-voltage flexible DC transmission system.

[0068] In order to effectively ensure that the DC circuit breaker can reliably disconnect the fault current, it is necessary to confirm the DC fault current I dc Less than the rated operating current I of the DC circuit breaker dccb ,Right now:

[0069] (3)

[0070] This goal can be achieved by preselecting energy dissipation damping modules with reasonable damping values ​​and using an appropriate amount of energy dissipation damping modules when a fault occurs.

[0071] (4) The pressure regulating and current limiting strategy or the locking and current limiting strategy executed by the MMC converter valve is restored to the control strategy during normal operation.

[0072] (5) After the problem at the DC fault point is resolved, the system can be restored to its original operating state.

[0073] The control module sends instructions to the MMC converter valve to gradually remove the energy-consuming damping module inside the MMC converter valve, and at the same time, the MMC sub-module is put into operation accordingly to avoid additional fluctuations in the DC system power.

[0074] At this point, the DC circuit breaker has isolated and cut off the faulty branch, and will then perform a de-ionization operation to prepare for the subsequent reclosing after the faulty line is repaired.

[0075] Through the operation of the above steps, the high-voltage flexible direct current transmission system can smoothly cross the direct current fault and gradually and smoothly transition from the fault state and the cut-off isolation state to the normal operating state, thereby reducing the impact of the fault on the power supply and improving the stability and reliability of the power supply.

[0076] 2. The AC side of the MMC converter valve is connected to a strong power grid

[0077] (1) First is fault diagnosis:

[0078] When the current sensor on the DC bus detects a DC fault, the MMC converter valve adopts a locking current limiting strategy:

[0079] (4)

[0080] Among them, U acfault is the AC side phase voltage regulated by the MMC converter valve in the case of a DC fault; U acfault_strong Indicates the lower limit of AC voltage regulation under strong power grid.

[0081] That is, at this time, the overall fault circuit is the strong power grid on the AC side passing through the diode, which is reflected in the form of three-phase uncontrolled rectification; at this time, the uncontrolled rectification situation has a better current limiting effect than the original capacitor discharge. Because the MMC AC voltage on the strong power grid side is not adjustable, the lower limit is fixed. If connected to a new energy station, the AC side voltage can be adjusted because the MMC performs VF control to build the grid voltage.

[0082] The steps (2) to (5) that need to be performed thereafter are consistent with the operations when the AC side of the MMC converter valve is connected to the new energy station, so they will not be described again.

[0083] 3. Verification Experiment

[0084] For ease of understanding, the verification experiment was constructed in the PSCAD simulation software Figure 1 The simulation parameters of the high-voltage direct current transmission system are shown in Table 1, and the simulation results of the control process are shown in Table 1. Figures 4 to 6 shown.

[0085] Table 1; ;

[0086] like Figure 4 As shown in the figure, the simulation is set to have a DC bipolar metallic short circuit fault at 0.5s. 1.315ms after the fault occurs, the protection system detects the short circuit fault. In this simulation, the MMC AC side is connected to a strong power grid, so the MMC converter valve adopts a closed current limiting scheme to limit the AC side voltage to the lower limit, that is, U acfault=220kV. While the MMC converter valve is regulating the voltage, the energy-dissipating damping module is put into use inside the bridge arm to limit the further rise of the DC current and push the electromotive force of the fault circuit into a new steady-state stage.

[0087] according to Figure 5 It can be seen that at 0.51s, the fault electromotive force enters a steady state, and the fault current gradually approaches 3kA. When the detection and control unit samples the DC side current in real time, when the fault tends to a steady state and the fault current is less than the rated working current of the DC circuit breaker (3kA), the DC circuit breaker is put into operation to interrupt the DC fault current and isolate the DC fault point. Figure 5 It is further proved that the reasonably designed damping can effectively ensure the reliable interruption of fault current by DC circuit breaker.

[0088] After the converter valve protection system ensures that the DC fault point has been completely isolated from the DC system, the MMC converter valve resumes normal operation from the previous voltage limiting control strategy, and then gradually cuts off the energy-consuming damping module to avoid additional fluctuations in the DC system power. Figure 4 , Figure 5 and Figure 6 The comparison of DC voltage, current and bridge arm current under the traditional scheme in the case of DC fault clearing is shown respectively. Through comparison, it can be found that the coordinated cooperation between the energy dissipation damping module of the present invention and the DC circuit breaker realizes the rapid limitation and reliable disconnection of the fault current. At the moment of fault occurrence, the energy dissipation damping module quickly limits the rise of the fault current, which reduces the difficulty of disconnection for the DC circuit breaker. The close cooperation between the two greatly improves the efficiency of fault handling, effectively reduces the impact and damage risk of fault current on system equipment, and ensures the safe and stable operation of the system.

[0089] In the present invention, before the DC circuit breaker is put into operation to interrupt the DC fault current and isolate the DC fault point, it is necessary to adopt a voltage regulation and current limiting strategy or a locking current limiting strategy through the MMC converter valve, thereby lowering the AC side voltage based on the original voltage value. Therefore, the required withstand voltage level of the DC circuit breaker is changed from the original DC voltage U dc Will be changed accordingly to the new withstand voltage U dcfault :

[0090] (5)

[0091] Among them, U dcfault Withstand voltage of DC circuit breaker; U acfault is the AC side voltage regulated by the MMC converter valve in the case of a DC fault; m is the MMC output voltage modulation ratio; formula Applicable to new energy stations; formula Suitable for strong power grid.

[0092] In view of the withstand voltage U dcfault When the DC fault ride-through method of the present invention is used, the cost of using the DC circuit breaker can be greatly reduced. For example, in this experiment, it was verified that the number of IGBTs required for the traditional DC circuit breaker is 4368, while the number of IGBTs required for the present invention is 2448, which significantly reduces the purchase cost of the equipment. At the same time, by rationally designing the energy-consuming damping module, the fault handling method of the system is optimized, the reliance on the expensive technology of connecting multiple IGBTs in series and in parallel is reduced, and the system cost is further reduced.

[0093] Based on the above verification, it can be seen that the fault ride-through method proposed in the present invention is based on a new control strategy and can achieve cost optimization while ensuring safe and stable operation of the system, making the high-voltage flexible direct current transmission system more economically feasible when applied on a large scale, and helping to promote the development of related industries.

Claims

1. A DC fault ride-through method based on energy dissipation damping and DC circuit breaker, characterized in that: include: (1) A DC fault ride-through protection system is provided in the high-voltage flexible DC transmission system, the system comprising an MMC converter valve, an energy dissipation damping module, a DC circuit breaker and a detection control unit; wherein the energy dissipation damping module is provided in the upper and lower bridge arms of the MMC converter valve and is connected in series with the bridge arm inductance and the MMC submodule; (2) After a DC fault is detected, the MMC converter valve adopts a voltage regulation and current limiting strategy to reduce the AC side voltage by a predetermined ratio based on the original voltage value; or adopts a blocking current limiting strategy to limit the AC side voltage to a specified lower limit value; (3) Transfer the fault current to the energy dissipation damping module inside the MMC converter valve, so that the energy of the fault current is dissipated in the form of heat energy; by suppressing the further increase of the DC current, the electromotive force of the fault circuit gradually enters a new steady-state stage; (4) The DC fault current is gradually reduced through the current limiting operation of the MMC converter valve until the current value is less than the rated working current of the DC circuit breaker; then the DC circuit breaker performs the disconnection operation to completely isolate the DC fault point from the high-voltage flexible DC transmission system; (5) Restore the pressure regulating and current limiting strategy or the locking and current limiting strategy executed by the MMC converter valve to the control strategy during normal operation; (6) Gradually remove the energy-consuming damping module inside the MMC converter valve and put the MMC sub-module into operation accordingly, so that the high-voltage flexible DC transmission system can smoothly transition to normal operation.

2. The method according to claim 1, characterized in that The DC circuit breaker is a hybrid structure, consisting of a fast mechanical switch and a power electronic switch; the fast mechanical switch adopts a vacuum fast mechanical switch; the power electronic switch is a fully controlled power electronic device, which is an insulated gate bipolar transistor IGBT or an integrated gate commutated thyristor IGCT.

3. The method according to claim 1, characterized in that In the DC fault ride-through protection system, the topological structure of the MMC converter valve is composed of three phases and six bridge arms, and each phase includes two upper and lower bridge arms; wherein the common point of the three-phase bridge arm is used as the DC port connected to the DC system, and the midpoint of the upper and lower bridge arms is used as the AC port connected to the AC system; each bridge arm includes a bridge arm inductor and N MMC sub-modules connected in series and having the same structure, N≥1; between the bridge arm inductor of each bridge arm and the midpoint of the upper and lower bridge arms, M energy-dissipating damping modules connected in series and having the same structure are provided, M≥1; The energy dissipation damping module includes a parallel main current branch, an energy dissipation branch and a bypass loop; in the main current branch, the half-controlled switch device T1 or the fully-controlled switch device S3 is reversely connected in parallel with its freewheeling diode D1, and then reversely connected in series with the fully-controlled switch device T2 or S4 and its reverse freewheeling diode D2; the energy dissipation branch includes an energy dissipation resistor R d , the bypass branch includes a bypass switch K; Both ends of the MMC converter valve are connected to the busbar of the high-voltage flexible DC transmission system through DC circuit breakers.

4. The method according to claim 3, characterized in that The MMC submodule is a half-bridge power module, including two switch power devices with reverse diodes and a capacitor; in the energy dissipation damping module, the half-controlled switch device is a thyristor, and the fully controlled switch device is a three-terminal semiconductor switch device IGBT. d External water cooling device.

5. The method according to claim 1, characterized in that The detection control unit is used to monitor the DC side current, AC side voltage and operating parameters of the DC fault ride-through protection system in real time, judge the fault state and whether the current tends to a steady state based on the monitoring data, and issue control instructions according to a predetermined strategy to coordinate the actions of the energy-consuming damping module, the DC circuit breaker and the MMC sub-module; the detection control unit includes a detection module and a control module, wherein the detection module is used to monitor the DC side current and the AC side output voltage of the MMC converter valve in real time, and the control module is used to receive the monitoring signal and issue a control signal to the MMC converter valve and the DC circuit breaker according to a preset control strategy.

6. The method according to claim 1, characterized in that In step (1), when the AC side of the MMC converter valve is connected to the new energy station, the voltage regulation and current limiting strategy is adopted according to the following formula: ； Among them, U acfault is the AC side voltage regulated by the MMC converter valve in the case of a DC fault; U ac is the AC side bus voltage under normal operation of the MMC converter valve; k is the voltage regulation coefficient, which is between 0 and 0.8; the new energy station refers to a wind power station located at sea or on land, or a photovoltaic power station located on land.

7. The method according to claim 1, characterized in that In step (1), when the AC side of the MMC converter valve is connected to a strong power grid, a locking current limiting strategy is adopted according to the following formula: ； Among them, U acfault is the AC side voltage regulated by the MMC converter valve in the case of a DC fault; U acfault_strong Indicates the lower limit of AC voltage regulation under a strong power grid; the strong power grid refers to the AC power grid system.

8. The method according to claim 1, characterized in that In step (2), in the new steady-state stage, the DC fault current is calculated according to the following formula: ； Among them, I dc is the DC fault current; T is the power frequency period; t0 is the initial integration time; i u and i d They respectively represent the currents of the upper and lower bridge arms of the three phases in the MMC converter valve; In order to effectively ensure that the DC circuit breaker can reliably disconnect the fault current, it is necessary to confirm the DC fault current I dc Less than the rated operating current I of the DC circuit breaker dccb This goal can be achieved by pre-selecting energy-dissipating damping modules with reasonable damping values ​​and using appropriate amounts of energy-dissipating damping modules when a fault occurs.

9. A computing device, characterized in that include: At least one processor, and a memory communicatively connected to the at least one processor, wherein the memory stores instructions executed by the at least one processor, and the instructions are executed by the at least one processor so that the at least one processor executes the DC fault ride-through method based on energy dissipation damping and DC circuit breaker according to any one of claims 1 to 8.

10. A computer-readable storage medium, characterized in that: The computer-readable storage medium stores computer instructions, and the computer instructions are used to enable the computer to execute the DC fault ride-through method based on energy dissipation damping and DC circuit breaker according to any one of claims 1 to 8.

Images