DC Fault Ride-Through Method, Device and Medium Based on Energy Dissipation Damping and Circuit Breaker

Abstract

The present invention relates to high-voltage direct current (HVDC) transmission technology, and aims to provide a DC fault ride-through method, device and medium based on energy-consuming damping and a circuit breaker. The method includes: reducing the AC-side voltage of the converter valve, transferring the fault current to the energy-consuming damping module inside the MMC converter valve, and enabling the electromotive force to enter a new steady state stage; gradually reducing the DC fault current, and the DC circuit breaker performs a breaking operation to completely isolate the DC fault point from the high-voltage flexible DC transmission system; restoring the control strategy of the MMC converter valve to the normal operation state, gradually removing the energy-consuming damping module and simultaneously putting into operation the MMC sub-module, so that the high-voltage flexible DC transmission system smoothly transitions to the normal operation state. The present invention can effectively reduce the impact and damage risk of the fault current on the system equipment, ensure the safe and stable operation of the system; a DC circuit breaker with a smaller capacity can be adopted, significantly reducing the procurement cost of the equipment; the system stability can also be maintained under fault conditions, improving the reliability of power supply.

Description

Technical Field

[0001] The present invention belongs to the field of high - voltage direct - current power transmission, and particularly relates to a direct - current fault - ride - through method, a computing device, and a computer - readable storage medium based on energy - dissipating damping and a circuit breaker. Background Art

[0002] In a high - voltage flexible direct - current power transmission system, a direct - current fault is a key factor threatening the stable operation of the system. Direct - current fault protection is crucial for the reliable operation of the direct - current power transmission system, and generally includes the detection, location, interruption, and isolation of pole - to - pole (P - P) and pole - to - ground (P - G) type short - circuit faults, high - impedance grounding faults, and arc faults. Among them, an inter - pole fault occurs between the positive and negative lines, and usually exhibits a relatively low short - circuit impedance. It is the most serious fault type in a direct - current power grid and usually generates a huge fault current in an extremely short time. A pole - to - ground fault occurs when the positive or negative power supply line is short - circuited to the ground, and is usually caused by lightning strikes and component failures. The impact of a pole - to - ground fault depends on the specific grounding scheme used in the direct - current power supply system and may also cause harm. Once a short - circuit fault occurs on the direct - current side, due to the low - damping characteristic of the system itself, the fault current will rapidly climb within an extremely short time, with a very high rising rate, and may reach several times the normal operating current amplitude within a few milliseconds. This will cause a great impact on the devices in the system, such as converter valves, reactors, capacitors, etc., and is very likely to cause device damage, and even trigger the collapse of the entire system, resulting in a large - area power outage and bringing serious losses to the social economy.

[0003] Currently, there are three main technical routes to solve direct - current fault - ride - through:

[0004] The first solution is a direct - current circuit breaker (DCCB). As a key device for solving direct - current faults, due to the need to interrupt a relatively high fault current and withstand the direct - current bus over - voltage, a hybrid structure using multiple groups of IGBTs in series - parallel technology is mostly adopted. This circuit breaker can quickly interrupt the fault current to a certain extent and meet the system's requirement for rapid fault removal. However, in order to achieve reliable interruption under high voltage and large current, a large number of IGBT devices need to be combined in series - parallel, which greatly increases the cost of the fault protection equipment. In addition, in the existing technology during the fault - handling process, the coordination among 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 the direct - current circuit breaker needs to have a large redundancy to avoid the impact of the fault current.

[0005] The second solution is to adopt a hybrid MMC, which uses the additional back electromotive force brought by the reverse input of the full-bridge sub-module to suppress the DC bus voltage and further limit the DC impact current. However, the number of switching devices required for the full-bridge sub-module is twice that of the half-bridge sub-module, and conduction losses will occur under normal operating conditions. For the DC fault protection scheme, the device cost and loss cost are relatively high in this scheme.

[0006] The third solution is to only use an energy-consuming damping module to limit the fault current. This solution can reliably achieve DC fault current limiting, but the DC fault current cannot be directly interrupted after current limiting. The HVDC transmission system is in an uncontrolled rectification state after current limiting. It is necessary to disconnect the AC side breaker of the converter valve and cut off the grid-side AC source before the fault current can be reduced to zero crossing, and then use a disconnector to disconnect the fault circuit. The system restart also requires closing the AC switch. For a single valve body, the overall fault clearing and recovery time is relatively long (more than 500 ms), which poses a hidden danger to maintaining the safety and stability of the power system. Moreover, in the case of a multi-terminal DC power grid, this type of solution cannot achieve selective isolation protection. If there is a faulty line among multiple DC lines connected to the converter valve, the entire converter valve needs to be blocked and the AC breaker needs 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 discloses an MMC converter valve and method for realizing AC-DC fault crossing based on an energy-consuming damping module. In this solution, based on the topology of a conventional MMC converter valve, M energy-consuming damping modules with the same structure and connected in series are respectively arranged between the arm inductor of each arm and the midpoints of the upper and lower arms. The energy-consuming damping module includes a main current-carrying branch, an energy-consuming branch, and a bypass circuit connected in parallel. This solution is a typical case of the third solution and can limit the amplitude of the impact current of the DC short-circuit fault after the DC short-circuit fault occurs. However, in order to reduce the DC fault current to zero crossing, it is necessary to disconnect the AC side breaker of the converter valve. The breaking and reclosing time of the AC breaker is relatively long. At this time, the AC side power grid will lose the support of the converter valve, and the safety and stability of the power system face challenges. Moreover, in the case of a multi-terminal DC power grid, it cannot achieve the function of selective isolation of DC faults.

[0008] Therefore, the present invention intends to propose 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 propose a DC fault crossing method, device, and medium based on energy-consuming damping and a circuit breaker.

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

[0011] Provide a DC fault ride-through method based on energy-consuming damping and DC circuit breaker, including:

[0012] (1) Set up a DC fault ride-through protection system in the high-voltage flexible DC transmission system. The system includes an MMC converter valve, an energy-consuming damping module, a DC circuit breaker, and a detection and control unit. Among them, the energy-consuming damping module is arranged in the upper and lower bridge arms of the MMC converter valve and is connected in series with the arm inductor and the MMC sub-module;

[0013] (2) After detecting a DC fault, adopt a voltage regulation and current limiting strategy through the MMC converter valve to reduce the AC side voltage by a predetermined ratio based on the original voltage value; or adopt a blocking current limiting strategy to limit the AC side voltage to a specified lower limit value;

[0014] (3) Transfer the fault current to the energy-consuming damping module inside the MMC converter valve, so that the energy of the fault current is dissipated in the form of heat; by suppressing the further rise of the DC current, the electromotive force of the fault circuit gradually enters a new steady state stage;

[0015] (4) Gradually reduce the DC fault current through the current limiting operation of the MMC converter valve until the current value is less than the rated operating current of the DC circuit breaker; then the DC circuit breaker performs a breaking operation to completely isolate the DC fault point from the high-voltage flexible DC transmission system;

[0016] (5) Restore the voltage regulation and current limiting strategy or the blocking current limiting strategy executed by the MMC converter valve to the control strategy during normal operation;

[0017] (6) Gradually remove the energy-consuming damping module inside the MMC converter valve, and at the same time correspondingly put into operation the MMC sub-module to make the high-voltage flexible DC transmission system smoothly transition to the normal operation state.

[0018] As a preferred solution of the present invention, the DC circuit breaker is of a hybrid structure and is composed 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.

[0019] As a preferred embodiment of the present invention, in the DC fault ride-through protection system, the topology of the MMC converter valve is composed of six arms in three phases, with each phase including an upper and a lower arm; wherein, the common point of the three-phase arms is used as the DC port for accessing the DC system, and the midpoints of the upper and lower arms are used as the AC ports for accessing the AC system; each arm contains an arm inductor and N MMC sub-modules connected in series in sequence and having the same structure, where N≥1; between the arm inductor of each arm and the midpoints of the upper and lower arms, M energy-consuming damping modules connected in series in sequence and having the same structure are provided, where M≥1;

[0020] The energy-consuming damping module includes a main current-carrying branch, an energy-consuming branch, and a bypass circuit connected in parallel; in the main current-carrying branch, a semi-controlled switching device T1 or a fully-controlled switching device S3 is reversely connected in parallel with its freewheeling diode D1, and then reversely connected in series with a fully-controlled switching device T2 or S4 and its reverse freewheeling diode D2; the energy-consuming branch includes an energy-consuming resistor R d , and the bypass branch includes a bypass switch K;

[0021] Both ends of the MMC converter valve are connected to the bus of the high-voltage flexible DC transmission system through DC circuit breakers.

[0022] As a preferred embodiment of the present invention, the MMC sub-module is a half-bridge power module, including two switching power devices with built-in reverse diodes and a capacitor; in the energy-consuming damping module, the semi-controlled switching device is a thyristor, and the fully-controlled switching device is a three-terminal semiconductor switching device IGBT. A water-cooled heat dissipation device is configured outside the energy-consuming resistor R d .

[0023] As a preferred embodiment 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 be stable according to the monitoring data, and issue control instructions according to a predetermined strategy to coordinate the actions of the energy-consuming damping module, DC circuit breaker, and MMC sub-module; the detection and control unit includes a detection module and a control module, where the detection module is used to monitor the DC-side current and 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 control signals to the MMC converter valve and DC circuit breaker according to a preset control strategy.

[0024] As a preferred embodiment of the present invention, in step (1), when the AC side of the MMC converter valve is connected to a new energy power station, the voltage regulation and current limiting strategy is adopted according to the following formula:

[0025] ;

[0026] where U acfault is the magnitude of the AC-side voltage regulated by the MMC converter valve under DC fault conditions; Uac is the AC side bus voltage under normal operation of the MMC converter valve; k is the voltage regulation coefficient, with a value between 0 and 0.8; the new energy power station refers to an onshore or offshore wind power station, or an onshore photovoltaic power station.

[0027] 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, the following formula is used to adopt a blocking current limiting strategy:

[0028] ;

[0029] where, U acfault is the magnitude of the AC side voltage regulated by the MMC converter valve under DC fault conditions; U acfault_strong represents the lower limit of AC voltage regulation under a strong power grid; the strong power grid specifically refers to a conventional AC large-capacity AC power grid system located on land.

[0030] 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:

[0031] ;

[0032] where, I dc is the DC fault current; T is the power frequency period; t0 is the initial integration time; i u and i d respectively represent the currents of the upper and lower bridge arms of the three phases in the MMC converter valve;

[0033] To effectively ensure that the DC circuit breaker can reliably interrupt the fault current, it is necessary to confirm that the DC fault current I dc is less than the rated operating current I dccb of the DC circuit breaker. This goal is achieved by pre-selecting an energy-consuming damping module with a reasonable damping value and controlling the appropriate amount of energy-consuming damping modules to be put into use during the fault.

[0034] The present invention also provides a computing device, including: 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 to enable the at least one processor to execute the aforementioned DC fault ride-through method based on energy-consuming damping and DC circuit breaker.

[0035] The present invention further provides a computer-readable storage medium, and 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-consuming damping and DC circuit breaker.

[0036] Description of the invention principle:

[0037] Among the mainstream existing technologies for solving DC fault ride-through, although new technologies for improving MMC converter valves using energy-consuming modules have emerged. Constrained by the fault current limiting method and the breaking method, most current research adopts inductive current limiting and DC breaker breaking; research on suppressing current using capacitors in hybrid MMCs; and using energy-consuming damping to achieve fault current limiting. Since the DC fault ride-through mechanisms of different routes are different, current researchers have not overcome the inertial thinking mode under different DC fault ride-through schemes. When solving the DC fault ride-through problem, they are still restricted to the solution ideas of solely adopting the three technical routes. The present invention creatively proposes to organically combine different technical routes to construct a new DC fault ride-through protection scheme integrating a DC breaker, an MMC converter valve, and an energy-consuming module. Specifically, it includes:

[0038] (1) MMC converter valve: The MMC sub-modules inside it play a key role in voltage regulation and current limiting during a fault. When a fault occurs, the MMC sub-modules can select the current limiting control or blocking current limiting mode according to the DC fault state. In the current limiting control mode, the internal control parameters are adjusted, and the MMC sub-modules inside the MMC valve body are reasonably switched to achieve step-down regulation of the AC voltage, thereby effectively limiting the fault current. The control strategy of the MMC converter valve is coordinated with the operation of the energy-consuming damping module and the DC breaker to drive the fault circuit loop into a new steady state, so as to limit the fault current within the breaking capacity of the DC breaker and create favorable conditions for subsequent breaking operations.

[0039] (2) Energy-consuming damping module: It is arranged in the upper and lower arms of the MMC valve body. As the primary link for fault current limiting, this module is immediately activated upon detecting a DC short-circuit fault. The energy-consuming damping module consists of energy-consuming elements and current-carrying power electronic switches. Among them, the energy-consuming elements adopt resistors with reasonable resistance values and corresponding water-cooled heat dissipation devices, and the power electronic switches adopt 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 gate turn-off thyristors (GTOs), etc.). When a fault occurs, the energy-consuming damping module is quickly put into use to suppress the rapid rise of the fault current. The internal energy-consuming resistor efficiently dissipates the energy carried by the fault current in the form of heat, driving the fault circuit loop into a new steady state, so as to limit the fault current within the breaking capacity of the DC breaker and create favorable conditions for subsequent breaking operations.

[0040] (3) DC circuit breaker: Adopting a hybrid structure, it integrates the advantages of fast mechanical switches and power electronic switches. The fast mechanical switch in the DC circuit breaker is a vacuum fast mechanical switch, which has the characteristics of low loss, high reliability, and fast opening and closing. It undertakes the task of conducting current during normal system operation, effectively reducing the on-state loss. The power electronic switch uses fully controlled power electronic devices such as insulated gate bipolar transistors (IGBTs) or integrated gate-commutated thyristors (IGCTs), and is equipped with perfect overcurrent protection and overheat protection functions. Limited by the impact current of the fault current, even under the condition of reasonable reactor input, 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 be paralleled with multiple groups of IGBT valve groups to achieve the goal of fault current conduction and interruption, and this measure greatly increases the cost of the circuit breaker. To solve this problem, the present invention proposes to use the steady-state operation of the energy-consuming damping module to assist the interruption operation of the DC circuit breaker. Through the energy-consuming damping module and the current-limiting measures of the MMC and the commutation valve, the fault current is effectively limited within a certain range, thereby reducing the number of parallel DC circuit breakers, and then greatly reducing the cost of the DC circuit breaker. When the power system protection detection and control unit issues an interruption command, the power electronic switch responds quickly and cuts off the fault current quickly, closely cooperating with the energy-consuming damping module to ensure that the fault current can be reliably interrupted.

[0041] (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 state and whether the current tends to be stable according to the monitoring data, and issuing control commands according to the predetermined strategy to coordinate the actions of the energy-consuming damping module, DC circuit breaker, and MMC sub-module; ensuring precise control and stable operation guarantee of the entire system during the fault ride-through process.

[0042] (5) System regulation and recovery mechanism: When a fault occurs, through current-limiting voltage regulation and switching operations, the rapid limitation of the fault current is quickly restricted, reducing the interruption difficulty of the DC circuit breaker. When the system enters the recovery stage, it orderly controls the energy-consuming damping module to gradually withdraw from operation, avoiding power oscillation of the system; ensuring a smooth transition from the fault state to the normal operation state, reducing the impact on power supply, and improving power supply stability and reliability.

[0043] Through the above-mentioned cooperative working mechanism of the internal components of the DC circuit breaker and the MMC commutation valve, the present invention can quickly and accurately limit the DC fault current and interrupt the DC fault current at a reasonable steady-state value. Ensure 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 handling cost and insufficient power system stability in the existing technology.

[0044] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0045] (1) High-efficiency fault handling ability: The coordinated cooperation of the energy-consuming damping module and the DC circuit breaker realizes the rapid limitation and reliable breaking of the fault current. At the moment of fault occurrence, the energy-consuming damping module quickly limits the rise of the fault current, reducing the breaking difficulty for the DC circuit breaker. The two work closely together, greatly improving the efficiency of fault handling, effectively reducing the impact and damage risk of the fault current on system equipment, and ensuring the safe and stable operation of the system.

[0046] (2) Cost advantage: Based on the cooperation of the MMC converter valve and its internal energy-consuming damping module, a DC circuit breaker with a smaller capacity can be used, which can significantly reduce the procurement cost of equipment. At the same time, by optimizing the fault handling method of the system through the energy-consuming damping module, the dependence on expensive multi-group IGBT series-parallel technology is reduced, further lowering the system cost. This cost optimization strategy makes the high-voltage flexible DC transmission system more economically feasible in large-scale applications, contributing to the development of the industry.

[0047] (3) Enhanced system stability: During a fault, the MMC converter valve works in coordination with the energy-consuming damping module and the DC circuit breaker through a flexible current-limiting control strategy; there is no need to isolate the MMC converter valve from the AC power grid, maintaining the basic operating ability of the system. Even in the event of a fault, a certain degree of stability can be maintained, reducing the possibility of system collapse caused by the fault, improving the reliability of power supply, and ensuring the power consumption needs of users.

[0048] (4) Smooth system recovery: The system recovery mechanism ensures that the system can smoothly transition to the normal operating state 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 oscillations, reduce the impact of the fault on power supply, and improve the stability and power supply continuity of the power system. Description of the Drawings

[0049] Figure 1 It is a schematic diagram of the MMC converter valve, the energy-consuming damping module, and the DC circuit breaker in the DC fault ride-through protection system.

[0050] Figure 2 It is a flow chart of the fault detection and breaking stage in the present invention.

[0051] Figure 3 It is a flow chart of the system recovery stage in the present invention.

[0052] Figure 4 It is a comparison of the DC voltage between the traditional scheme and the present invention under the condition of DC fault clearing.

[0053] Figure 5Comparison between the traditional solution and the DC current of the present invention in the case of DC fault clearing.

[0054] Figure 6 Comparison between the traditional solution and the arm current of the present invention in the case of DC fault clearing. Detailed implementation manners

[0055] The present invention will be further described in detail below with reference to the accompanying drawings.

[0056] I. Structural description of the DC fault ride-through protection system

[0057] Figure 1 It is a schematic diagram of the MMC converter valve, the energy dissipation damping module, and the 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 bus of the high-voltage flexible DC transmission system through DC circuit breakers. The topological structure of the converter valve consists of three-phase six arms, and each phase includes an upper arm and a lower arm; among them, the common point of the three-phase arms is used as the DC port accessing the DC system, and the midpoints of the upper and lower arms are used as the AC ports accessing the AC system; each arm contains an arm inductor and N MMC sub-modules connected in series in sequence and having the same structure, N≥1; between the arm inductor of each arm and the midpoints of the upper and lower arms, M energy dissipation damping modules connected in series in sequence and having the same structure are provided, M≥1; the energy dissipation damping module includes a main current-carrying branch, an energy dissipation branch, and a bypass circuit connected in parallel; in the main current-carrying branch, a semi-controlled switching device T1 or a fully-controlled switching device S3 is reversely connected in parallel with its freewheeling diode D1, and then reversely connected in series with a fully-controlled switching device T2 or S4 and its reverse freewheeling diode D2; the energy dissipation branch includes an energy dissipation resistor R d , and the bypass branch includes a bypass switch K.

[0058] Among them, the MMC sub-module can be a half-bridge power module, including two switching power devices with built-in reverse diodes and a capacitor. The semi-controlled switching device T1 can be a thyristor, and the fully-controlled switching devices T2, S3, and S4 can be three-terminal semiconductor switching devices (IGBTs). A water-cooled heat dissipation device is configured outside the energy dissipation resistor R d . In each arm, the series quantity of the MMC sub-module and the energy dissipation damping module respectively, as well as the selection of the capacitor and the switching device, are all determined by technicians according to the actual situation of the high-voltage flexible DC transmission system.

[0059] The DC circuit breaker has a hybrid structure and is composed of a fast mechanical switch and a power electronic switch; the fast mechanical switch uses 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.

[0060] The detection and control unit includes a detection module and a control module. Among them, the detection module includes a current sensor disposed on the DC bus and a voltage sensor disposed on the AC output side, which are used to monitor the DC side current of the MMC converter valve, the AC side output voltage, and the operating parameters of the DC fault ride-through protection system in real time. The control module is used to receive the monitoring signals and send control signals to the MMC converter valve and the DC circuit breaker according to the preset control strategy.

[0061] II. DC Fault Ride-Through Method Based on Energy Dissipation Damping and DC Circuit Breaker

[0062] Based on the above DC fault ride-through protection system, the present invention proposes a DC fault ride-through method based on energy dissipation damping and DC circuit breaker, including the relevant operation steps in the fault detection and interruption stage and the recovery stage. The actual application scenarios are divided into connecting a new energy power station and a strong power grid on the AC output side. Among them, the new energy power station specifically refers to a wind power station or an onshore photovoltaic power station. The strong power grid specifically refers to an AC power grid system with large capacity and strong inertia. The control strategies for different scenarios in the present invention are slightly different, which will be described separately below:

[0063] 1. The AC side of the MMC converter valve is connected to a new energy power station

[0064] (1) First is the fault diagnosis:

[0065] When the current sensor disposed 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 Equation (1):

[0066] (1)

[0067] Where, U acfault is the magnitude of the AC side phase voltage regulated by the MMC converter valve under DC fault conditions; U ac is the AC side bus voltage of the MMC converter valve under normal operation; k is the voltage regulation coefficient. To meet the fault ride-through requirements of the new energy high-voltage flexible DC transmission system in the national standard, k generally takes values between 0 and 0.8.

[0068] That is, the AC output voltage of the MMC converter valve is adjusted proportionally, and while reducing the voltage, the current in the inner bridge arm of the valve is controlled; by reducing the voltage on the AC side, the impact current is reduced from the source.

[0069] (2) Transfer the fault current to the energy dissipation damping module inside the MMC converter valve, dissipate the energy of the fault current in the form of heat, and suppress the further increase of the DC current, so that the electromotive force of the fault circuit gradually enters a new steady state stage.

[0070] In the new steady state stage, the DC fault current is calculated according to the following formula:

[0071] (2)

[0072] where, I dc is the DC fault current; T is the power frequency period; t0 is the initial integration moment; i u and i d respectively represent the currents of the upper and lower arms of the three phases in the MMC converter valve.

[0073] (3) Gradually reduce the DC fault current I dc through the limited operation of the MMC converter valve. During this process, the current sensor is used to sample the DC side current in real time, and the current change is continuously monitored. When the fault tends to be steady state and the DC fault current I dcfault is less than the rated operating current I dccb of the DC circuit breaker, the control module sends an instruction to the DC circuit breaker, and the latter performs the breaking operation to completely isolate the DC fault point from the high-voltage flexible DC transmission system.

[0074] To effectively ensure that the DC circuit breaker can reliably break the fault current, it is necessary to confirm that the DC fault current I dc is less than the rated operating current I dccb of the DC circuit breaker, that is:

[0075] (3)

[0076] This goal can be achieved by pre-selecting an energy-consuming damping module with a reasonable damping value and controlling the appropriate amount of energy-consuming damping modules to be put into use during the fault.

[0077] (4) Restore the voltage regulation and current limiting strategy or the blocking current limiting strategy executed by the MMC converter valve to the control strategy during normal operation.

[0078] (5) After the problem at the DC fault point is solved, the system can be restored to its original operating state.

[0079] The control module sends an instruction to the MMC converter valve to gradually remove the energy-consuming damping modules inside the MMC converter valve, and at the same time correspondingly put into operation the MMC sub-modules to avoid additional power fluctuations in the DC system.

[0080] At this time, the DC circuit breaker has isolated and removed the fault branch, and then the deionization operation will be carried out to prepare for the reclosing after the subsequent repair of the fault line.

[0081] Through the operations of the above steps, the high-voltage flexible DC transmission system can successfully ride through DC faults, gradually and smoothly transition from the fault state and the cut-off isolation state to the normal operation state, thereby reducing the impact of faults on power supply and improving the stability and reliability of power supply.

[0082] 2. The AC side of the MMC converter valve is connected to a strong power grid

[0083] (1) First is the fault diagnosis:

[0084] When the current sensor installed on the DC bus detects a DC fault, the MMC converter valve adopts a blocking current-limiting strategy:

[0085] (4)

[0086] Among them, U acfault is the magnitude of the AC side phase voltage regulated by the MMC converter valve under DC fault conditions; U acfault_strong represents the lower limit of AC voltage regulation under a strong power grid.

[0087] That is, at this time, the overall fault circuit is that the strong power grid on the AC side conducts current through the diode, which is manifested as a three-phase uncontrolled rectification form; at this time, the current limiting effect of the uncontrolled rectification is better than the original capacitor discharge. Because the MMC AC voltage on the strong power grid side is not adjustable and the lower limit is fixed. If a new energy power station is connected, since the MMC conducts VF control to construct the grid voltage, the AC side voltage is adjustable.

[0088] The subsequent steps (2)-(5) to be executed are the same as the operation content when the AC side of the MMC converter valve is connected to a new energy power station, so they will not be described repeatedly.

[0089] III. Verification experiment

[0090] For easy understanding, the verification experiment is constructed in the PSCAD simulation software Figure 1 the shown topology-based and control model of the MMC converter valve; the simulation parameters of the high-voltage DC transmission system are shown in Table 1, and the simulation results during the control process are as Figures 4 to 6 shown.

[0091] Table 1;

[0092] ;

[0093] As Figure 4 shown, the simulation is set to have a DC bipolar metallic short circuit fault at 0.5 s. 1.315 ms 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 blocking current-limiting scheme to limit the AC side voltage to the lower limit, that is, U acfault= 220 kV. While the MMC converter valve regulates the voltage, an energy-consuming damping module is inserted inside the arm to limit the further rise of the DC current and drive the electromotive force of the fault circuit into a new steady state stage.

[0094] According to Figure 5 it can be seen that at 0.51 s, the fault electromotive force enters the steady state and the fault current gradually tends to 3 kA. When the detection and control unit samples the DC side current in real time, after the fault tends to the steady state and the fault current is less than the rated operating current of the DC breaker (3 kA), the DC breaker is turned on to break the DC fault current and isolate the DC fault point. Figure 5 It further proves that a reasonably designed damping can effectively ensure the reliable breaking of the fault current by the DC breaker.

[0095] 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 the normal operation control strategy from the previous voltage-limiting control strategy, and then gradually removes the energy-consuming damping module to avoid additional power fluctuations in the DC system. Figure 4 、 Figure 5 and Figure 6 respectively show the comparison of DC voltage, current and arm current under the traditional scheme in the case of DC fault clearing. It can be found through comparison that the coordinated cooperation of the energy-consuming damping module and the DC breaker in the present invention realizes the rapid limitation and reliable breaking of the fault current. At the moment of the fault occurrence, the energy-consuming damping module quickly limits the rise of the fault current, reducing the breaking difficulty for the DC breaker. The two cooperate closely, greatly improving the efficiency of fault handling, effectively reducing the impact and damage risk of the fault current on system equipment, and ensuring the safe and stable operation of the system.

[0096] In the present invention, since it is necessary to adopt a voltage regulation and current limiting or a blocking current limiting strategy through the MMC converter valve before turning on the DC breaker to break the DC fault current and isolate the DC fault point, the AC side voltage is reduced on the basis of the original voltage value. Therefore, the required withstand voltage level of the DC breaker will be correspondingly changed to the new withstand voltage U dc : dcfault :

[0097] (5)

[0098] where U dcfault is the withstand voltage of the DC breaker; U acfault is the magnitude of the AC side voltage regulated by the MMC converter valve under DC fault conditions; m is the output voltage modulation ratio of the MMC; the formula is applicable to new energy power stations; the formula is applicable to strong power grids.

[0099] In view of the withstand voltage U required by the DC circuit breaker dcfault being reduced, a DC circuit breaker with fewer internally connected IGBT groups in series can be selected for the component system. Therefore, based on the DC fault ride-through method of the present invention, the usage cost of the DC circuit breaker can actually be significantly reduced. For example, in this experiment, through verification, the traditional scheme requires 4368 IGBTs for the DC circuit breaker, while the present invention requires 2448 IGBTs, significantly reducing the procurement cost of the equipment. At the same time, by reasonably designing the energy-consuming damping module, the fault handling method of the system is optimized, reducing the dependence on the expensive multi-group IGBT series-parallel technology, and further reducing the system cost.

[0100] Based on the above verification, it can be seen that the fault ride-through method proposed by the present invention can achieve cost optimization while ensuring the safe and stable operation of the system based on a brand-new control strategy, making the high-voltage flexible DC transmission system more economically feasible in large-scale applications and contributing to the development of related industries.

Claims

1. A DC fault ride-through method based on energy-consuming damping and DC circuit breaker, characterized in that Including: (1) A DC fault ride-through protection system is set up in a high-voltage flexible DC power transmission system. The system includes an MMC converter valve, a dissipative damping module, a DC circuit breaker, and a detection and control unit. Among them, The topological structure of the MMC converter valve is composed of three-phase six arms, and each phase includes upper and lower arms. Among them, the common point of the three-phase arms is used as the DC port for accessing the DC system, and the midpoints of the upper and lower arms are used as the AC ports for accessing the AC system. Each arm contains an arm inductor and N MMC sub-modules connected in series in sequence and having the same structure, where N≥1. Between the arm inductor of each arm and the midpoints of the upper and lower arms, M dissipative damping modules connected in series in sequence and having the same structure are provided, where M≥1; The energy-consuming damping module includes a main current-carrying branch, an energy-consuming branch, and a bypass circuit connected in parallel; in the main current-carrying branch, a semi-controlled switching device T1 or a fully-controlled switching device S3 is reversely connected in parallel with its freewheeling diode D1, and then reversely connected in series with a fully-controlled switching device T2 or S4 and its reverse freewheeling diode D2; the energy-consuming branch includes an energy-consuming resistor R d , and the bypass branch includes a bypass switch K; Both ends of the MMC converter valve are connected to the bus of the high-voltage flexible DC power transmission system through DC circuit breakers. The dissipative damping module is arranged in the upper and lower arms of the MMC converter valve and is connected in series with the arm inductor and the MMC sub-modules; (2) After detecting a DC fault, adopt a voltage regulation and current limiting strategy through the MMC converter valve to reduce the AC side voltage by a predetermined ratio based on the original voltage value; or adopt a blocking current limiting strategy to limit the AC side voltage to a specified lower limit value; (3) Transfer the fault current to the dissipative damping module inside the MMC converter valve, so that the energy of the fault current is dissipated in the form of heat; by suppressing the further rise of the DC current, the electromotive force of the fault circuit gradually enters a new steady state stage; (4) Gradually reduce the DC fault current through the current limiting operation of the MMC converter valve until the current value is less than the rated operating current of the DC circuit breaker; then the DC circuit breaker performs a breaking operation to completely isolate the DC fault point from the high-voltage flexible DC power transmission system; (5) Restore the voltage regulation and current limiting strategy or the blocking current limiting strategy executed by the MMC converter valve to the control strategy during normal operation; (6) Gradually remove the dissipative damping modules inside the MMC converter valve, and at the same time correspondingly put into operation the MMC sub-modules, so that the high-voltage flexible DC power transmission system smoothly transitions to the normal operation state.

2. The method according to claim 1, characterized in that The DC circuit breaker has a hybrid structure and is composed 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, The MMC sub-module is a half-bridge power module, including two switching power devices with built-in anti-parallel diodes and a capacitor; in the energy-consuming damping module, the semi-controlled switching device is a thyristor, and the fully-controlled switching device is a three-terminal semiconductor switching device IGBT. An external water-cooled heat dissipation device is configured for the energy-consuming resistor R d of.

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

5. 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 new energy power station, the voltage regulation and current limiting strategy is adopted according to the following formula: U acfault = U ac (1 - k) Among them, U acfault is the magnitude of the AC-side voltage regulated by the MMC converter valve under DC fault conditions; U ac is the AC-side bus voltage under normal operation of the MMC converter valve; k is the voltage regulation coefficient, and its value ranges from 0 to 0.8; the new energy power station refers to an onshore or offshore wind power station, or an onshore photovoltaic power station.

6. The method according to claim 1, wherein In step (1), when the AC side of the MMC converter valve is connected to a strong power grid, the following formula is used to adopt a blocking current limiting strategy: U acfault = U acfault_strong Among them, U acfault is the magnitude of the AC-side voltage regulated by the MMC converter valve under DC fault conditions; U acfault_strong represents the lower limit of AC voltage regulation under a strong power grid; the strong power grid refers to the AC power grid system.

7. The method according to claim 1, wherein 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 moment; i u and i d respectively represent the currents of the upper and lower bridge arms of the three phases in the MMC converter valve; To effectively ensure that the DC circuit breaker can reliably interrupt the fault current, it is necessary to confirm that the DC fault current I dc is less than the rated operating current I dccb of the DC circuit breaker. This goal is achieved by pre-selecting an energy-consuming damping module with a reasonable damping value and controlling the use of an appropriate amount of energy-consuming damping modules during faults.

8. A computing device, characterized in that, Including: 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 to cause the at least one processor to execute the DC fault ride-through method based on energy-consuming damping and DC circuit breaker according to any one of claims 1 to 7.

9. A computer-readable storage medium, characterized in that, The computer-readable storage medium stores computer instructions, and the computer instructions are used to cause the computer to execute the DC fault ride-through method based on energy-consuming damping and DC circuit breaker according to any one of claims 1 to 7.

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