A method and system for coordinated control and protection of an ultra-high voltage multi-terminal direct current transmission system

Abstract

The application provides a method and system for coordinated control and protection of an extra-high voltage multi-terminal DC power transmission system, comprising: establishing a DC power transmission model of the DC power transmission system, including a converter subunit for simulating a working state of a converter of the DC power transmission system, a surplus power unit, and a fault protection unit; and establishing a power balance strategy for an AC power grid of a power receiving end of the DC power transmission system according to the DC power transmission model; establishing a fault ride-through strategy and a fault clearing and recovery control strategy of the DC power transmission system and the power grid of the power receiving end according to the DC power transmission model and the power balance strategy; and establishing a safety and stability control unit of the DC power transmission system according to the DC power transmission model, the power balance strategy, and the fault ride-through method, so as to realize control and protection of the DC power transmission system.

Description

Technical Field

[0001] This invention proposes a method and system for coordinated control and protection of ultra-high voltage multi-terminal DC transmission systems, relating to the field of DC transmission technology in power systems. Background Technology

[0002] Existing research on stability control and fault protection technologies for multi-terminal DC transmission systems from renewable energy bases mainly focuses on new energy bases or point-to-point DC transmission scenarios, adhering to the theoretical foundation of power systems dominated by synchronous generators. The research objects are primarily wind and solar new energy bases, and the dynamic impact of ultra-high voltage multi-terminal DC systems has not yet been considered. However, the operating characteristics of multi-terminal DC transmission systems from renewable energy bases are highly dependent on power electronic control. Fault currents are affected by control, exhibiting time-varying characteristics, small short-circuit currents, and high harmonic content. Synchronous operation no longer relies on generators. The system experiences abrupt changes in transient / dynamic characteristics under disturbances, complex coupling mechanisms between various types of equipment, and unclear interaction mechanisms, rendering existing control and protection technologies inapplicable. Therefore, it is urgent to clarify the multi-timescale dynamic characteristics and stability mechanisms of multi-terminal DC transmission systems from various types of renewable energy bases, and to break through the limitations of system stability control and fault protection technologies. Summary of the Invention

[0003] In view of this, in order to fill the gaps and deficiencies in the prior art, the present invention proposes a method and system for collaborative control and protection of ultra-high voltage multi-terminal DC transmission systems, in order to solve the problems in the background art.

[0004] According to a first aspect of the present invention, the present invention provides a method for coordinated control and protection of ultra-high voltage multi-terminal DC transmission systems, characterized in that it includes the following:

[0005] A DC transmission model for the DC transmission system is established, including converter sub-units, surplus power units, and fault protection units for simulating the converter operating state of the DC transmission system; and a power balance strategy for the AC grid at the receiving end of the DC system is established based on the DC transmission model.

[0006] Based on the DC transmission model and power balance strategy, establish fault ride-through strategy and fault clearing and recovery control strategy for DC transmission system and its receiving end grid;

[0007] Based on the DC transmission model, power balance strategy, and fault ride-through method, a safety and stability control unit for the DC transmission system is established to achieve control and protection of the DC transmission system.

[0008] Furthermore, the converter sub-unit of the DC transmission model uses multiple modular multilevel converter units to simulate the converter working state of the DC transmission system; the DC transmission model uses a bipolar symmetrical system to represent the electrical circuit of the DC transmission system; wherein the bipolar symmetrical system is composed of two unipolar asymmetrical systems connected in series.

[0009] Furthermore, the surplus power unit includes the following:

[0010] The relationship between surplus power and voltage can be expressed as:

[0011] ;

[0012] Therefore, the DC voltage after the fault occurred is:

[0013] ;

[0014] The final DC voltage change rate is:

[0015] ;

[0016] Where S' represents surplus power, C eq This represents the equivalent capacitance of a single pole in the converter station, and E represents the rated voltage of a single pole in the converter station.

[0017] Furthermore, the fault protection unit includes the following:

[0018] After a fault occurs in the DC transmission system, the fault protection unit receives the fault signal and then locks all converters through the converter subunit and activates the converter lockout protection. When the converter is locked out and the sum of the capacitor voltages of each arm of the converter is higher than the peak value of the AC line voltage, the capacitor provides back electromotive force to reduce the DC current to zero, thereby realizing the DC fault ride-through function of the fault protection unit.

[0019] Furthermore, the power balancing strategy includes using an AC active power dynamic balancing device to suppress surplus power;

[0020] The total resistance capacity of the AC active power dynamic balancing device is determined by the power of the active power dynamic balancing device and the activation time of the active power dynamic balancing device; in order to meet the fault ride-through requirements under various operating conditions, the total active power of the AC active power dynamic balancing device should meet the following requirements:

[0021] ;

[0022] Where P ACP,min The power of the AC active power dynamic balancing device resistor when the bus voltage is at its minimum; U minis the minimum value of the bus line voltage; m is the number of step-down transformers; N Gi X represents the number of resistor branches in the dynamic power balancing device of the i-th step-down transformer; t,Gi P is the leakage reactance of the i-th step-down transformer; conv This is the rated power of the bipolar converter.

[0023] Furthermore, the AC active power dynamic balancing device includes two operating modes: equal capacity and large / small group. In the equal capacity group operating mode, the power allocation method of the AC active power dynamic balancing device needs to consider factors including: power control accuracy, off-state losses, and power accuracy of new energy switching. In the large / small group operating mode, the power allocation method of the AC active power dynamic balancing device needs to consider factors including: determining the capacity of the large group based on the AC side voltage level and the upper limit of the thyristor current; determining the capacity of the smallest group with the finest switching precision; and rationally allocating the capacity of the intermediate group using a binary method based on the smallest and largest group capacities.

[0024] Furthermore, the fault ride-through strategy includes a dual VF droop control strategy and a VF-PQ control strategy;

[0025] The dual VF droop control strategy mentioned above includes: adjusting the active and reactive power of the two poles in real time to achieve bipolar frequency consistency through the external characteristics of bipolar active power and frequency droop control, reactive power and voltage droop control; and achieving fault ride-through through a dual inner loop current controller.

[0026] The VF-PQ control strategy mentioned above includes: adjusting the active and reactive power of the two poles in real time to achieve bipolar frequency consistency through bipolar frequency PF droop control and QV droop control external characteristics, and achieving fault ride-through through dual inner loop current controllers.

[0027] Furthermore, the fault clearing and recovery control strategy includes controlling the DC voltage to 0 when the DC transmission system fails, and allowing the DC transmission system to pass through the DC short-circuit fault without being shut down throughout the process; at the same time, AC energy-consuming equipment is used to consume the surplus power to prevent converter overcurrent and DC overvoltage.

[0028] The fault clearing and recovery control strategy also includes shutting down AC power-consuming equipment and increasing DC voltage when the fault is cleared, ultimately restoring the active power of the DC system to more than 90% of the level before the fault within 250ms.

[0029] Furthermore, the safety and stability control unit for DC transmission systems includes a future mode data generation unit, a transient calculation basic data conversion unit, a rapid stability quantification and evaluation unit, a control measure sample generation and management unit, and a stability control strategy generation, testing, and update scheduling core unit.

[0030] The future mode data generation unit integrates multiple data sources, including planned maintenance, new energy power forecasting, load forecasting, meteorological system, spot market, and dispatch control, to generate future mode data.

[0031] The transient calculation basic data conversion unit, based on future-mode data samples, completes the data format conversion before calculation, forming a data file that can be recognized by power system simulation calculation software;

[0032] The stability rapid quantitative assessment unit conducts rapid stability quantitative assessment on transient basic data files to screen instability scenarios and initial control measures;

[0033] The control measure sample generation and management unit uses self-developed transient stability simulation calculation software. Based on the above transient calculation data and the current strategy of the stability control system, it realizes the periodic calculation of control measure samples and establishes a dynamically rolling sample center to ensure that the data samples used in the strategy generation process always maintain timeliness and high quality.

[0034] The core unit for the generation, testing, and updating of stability control strategies is designed with a robust scheduler that manages the entire lifecycle of stability control strategy generation, closed-loop testing, and online updates, and has the ability to monitor status and roll back in case of anomalies.

[0035] According to a second aspect of the present invention, the present invention provides a system for coordinated control and protection of ultra-high voltage multi-terminal DC transmission systems, for performing a method for coordinated control and protection of ultra-high voltage multi-terminal DC transmission systems as described in any one of the present invention, characterized in that it includes the following:

[0036] The power transmission system module is used to establish a DC power transmission model for the DC power transmission system, including converter sub-units, surplus power units, and fault protection units for simulating the converter operating state of the DC power transmission system; and to establish a power balance strategy for the AC grid at the receiving end of the DC system based on the DC power transmission model.

[0037] The fault protection, clearing and recovery module is used to establish fault ride-through strategies and fault clearing and recovery control strategies for the DC transmission system and its receiving-end grid based on the DC transmission model and power balance strategy.

[0038] The safety and stability control module is used to establish a safety and stability control unit for the DC transmission system based on the DC transmission model, power balance strategy, and fault ride-through method, so as to realize the control and protection of the DC transmission system.

[0039] The present invention has the following advantages:

[0040] This invention provides theoretical and practical basis for actual engineering; it studies the refined switching control of high-capacity, high-number energy-consuming equipment, the synergistic optimization technology of power recovery speed and voltage stability requirements during fault recovery, and proposes a dynamic switching control strategy with multiple control objectives such as flexible DC and energy consumption to meet the system response constraints at different fault stages; it conducts simulation of DC fault ride-through strategy under metallic transient faults of DC lines, and achieves a power recovery time of ≤250ms after the deionization of metallic transient faults of DC lines.

[0041] This invention conducts research on rapid online stability risk assessment technology for systems, proposing a multi-scheme technical route that takes into account quasi-steady-state equilibrium point information, stability domain, and electromechanical transient simulation. It assesses transient stability under different system operation modes and faults in the sending-end grid scenarios of large-scale renewable energy bases. Under typical fault verification scenarios of multiple operation modes in small-scale node systems, the average verification time for a single scenario is less than 0.4 seconds. Furthermore, it proposes an online generation technology for stability control strategies. To adapt to the automatic design, simulation verification, and system implementation of stability control strategies for sending-end grids of large-scale renewable energy bases, it proposes a standardized modeling method for a stability defense secondary system composed of a strategy simulation model (protection control + stability control model) and a strategy design model (electronic strategy table). It also completes the construction of two model carriers: the electronic strategy table and the simulation software security defense control card, laying the foundation for online generation of stability control strategies. Attached Figure Description

[0042] Figure 1 This is a schematic diagram of the method flow of the present invention.

[0043] Figure 2 This is a schematic diagram of the main wiring of the bipolar structure of the present invention.

[0044] Figure 3 This is a schematic diagram of the MMC two-pole short-circuit fault of the present invention.

[0045] Figure 4 This is a schematic diagram of the equivalent circuit for a short-circuit fault in the MMC poles before the submodule is locked out in this invention.

[0046] Figure 5 This is a schematic diagram of the equivalent circuit in the initial stage after the submodule is locked out according to the present invention.

[0047] Figure 6 This is a schematic diagram of the topology of the half-bridge submodule of the present invention.

[0048] Figure 7 This is a schematic diagram of the topology and output voltage of the full-bridge submodule of the present invention.

[0049] Figure 8 This is a schematic diagram of the full-bridge MMC of the present invention.

[0050] Figure 9 This is a schematic diagram illustrating the equivalent principle of DC short-circuit fault in the MMC full-bridge submodule of the present invention.

[0051] Figure 10 This is a schematic diagram of the topology of a new energy transmission system via flexible direct transmission, which uses a grouped active power dynamic balancing device to smooth out surplus power in one embodiment of the present invention.

[0052] Figure 11 This is a schematic diagram of the equivalent circuit of the resistor in the dynamic balancing device for grouped AC active power under a single step-down transformer according to the present invention.

[0053] Figure 12 This is a schematic diagram comparing the islanding control characteristics and generator characteristics of the present invention.

[0054] Figure 13 This is a schematic diagram of the bipolar VF droop control strategy of the present invention.

[0055] Figure 14 This is a schematic diagram of the asymmetric operation characteristics of the bipolar VF droop control of the present invention.

[0056] Figure 15 This is a schematic diagram of the bipolar VF master-slave control strategy of the present invention.

[0057] Figure 16 This is a schematic diagram of the bipolar VF master-slave control strategy of the present invention.

[0058] Figure 17 This is a schematic diagram of the fixed PQ pole control strategy of the present invention.

[0059] Figure 18 This is a schematic diagram of the system control block of the present invention.

[0060] Figure 19 This is a schematic diagram of the DC line fault clearing and restart process of the present invention.

[0061] Figure 20 This is a schematic diagram of the simulation results of the DC line fault clearing and restart process of the present invention.

[0062] Figure 21 This is a schematic diagram of the program architecture in one embodiment of the system construction of the present invention.

[0063] Figure 22 This is a schematic diagram of the core process state transition in one embodiment of the system construction of the present invention. Detailed Implementation

[0064] The technical solution of the present invention will now be described in detail with reference to the accompanying drawings.

[0065] It should be noted that the following detailed description is illustrative and intended to provide further explanation of the invention. Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains.

[0066] It should be noted that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the exemplary embodiments of the present invention; as used herein, the singular form is intended to include the plural form as well, unless the context clearly indicates otherwise; furthermore, it should be understood that when the terms “comprising” and / or “including” are used in this specification, they indicate the presence of features, steps, operations, devices, components and / or combinations thereof.

[0067] like Figures 1 to 22 As shown, this invention proposes a method and system for coordinated control and protection of ultra-high voltage multi-terminal DC transmission systems, characterized by comprising the following:

[0068] In one embodiment of the present invention, in order to improve the power capacity and voltage level of the flexible DC transmission system and meet the requirements of ultra-high voltage and long-distance high-power transmission, the converter in a single converter station can be composed of two smaller capacity converter units connected in series. For example... Figure 2 As shown, two unipolar asymmetric systems are connected in series to form a bipolar symmetric system similar to traditional high-voltage direct current transmission.

[0069] In bipolar converter stations, the connecting transformer needs to withstand the DC bias voltage caused by DC voltage asymmetry. However, unlike conventional DC transformers, the connecting transformer in an MMC converter does not need to withstand harmonic components generated by the converter station. If a fault occurs at a point in the converter station, only the faulty pole needs to be disconnected. Even if the non-faulty poles are not at full power, the power from the faulty pole can be transferred using a power substitution strategy to ensure maximum power transmission and enhance the reliability of flexible DC power supply.

[0070] High-voltage, high-capacity DC systems require consideration of using overhead lines as transmission lines. Compared to cable-based transmission systems, overhead lines have a much higher probability of experiencing DC line faults.

[0071] This invention is based on the modeling and analysis of a DC transmission system employing a Modular Multilevel Converter (MMC).

[0072] In one embodiment of the present invention, the converter sub-unit for simulating the converter operating state of a DC transmission system in establishing a DC transmission model includes the following:

[0073] In a bipolar topology DC system based on MMC, due to the presence of a DC-side grounding point, a single-pole fault can create a loop through this grounding point, causing a short circuit in the entire converter. Therefore, the fault current rises rapidly after a fault. To protect the converter, all submodules are immediately locked out upon detecting a DC fault. Thus, the fault transient process can be divided into two parts based on whether the submodules are locked out: before and after the lockout.

[0074] In one embodiment of the present invention, the non-locking of the submodule includes the following:

[0075] The equivalent circuit diagram before the submodule is locked is as follows: Figure 3 As shown. During this stage, the fault current mainly consists of submodule capacitor discharge and AC side power supply feed current, as shown in the diagram. Figure 3 (a) and Figure 3 As shown in (b), the former is dominant, so only the discharge current of the submodule capacitors needs to be considered when estimating the fault current. Simultaneously, the submodules are switched on and off according to the normal modulation mode, with a total of N submodules engaged in both upper and lower bridge arms at any given time (assuming the number of levels is N+1). Due to the submodule capacitor voltage balance control principle, all submodules will be engaged or disengaged during this period. All submodules in each phase can be approximately divided into two groups (each group containing N submodules), which discharge alternately. Furthermore, due to the high control frequency of the system, it can be approximated that the two groups of discharging submodules in each phase are in parallel. Therefore, the fault can be equivalent to... Figure 4 The diagram shows a second-order RLC discharge circuit.

[0076] Therefore, the transient process in this stage can also be represented by the following formula.

[0077] ;(1);

[0078] In the formula: , , .

[0079] generally, Therefore, the discharge of the submodule capacitor is a second-order underdamped oscillation process. The characteristic roots of the equation are a pair of conjugate complex numbers.

[0080] (2);

[0081] In the formula , .

[0082] Assuming the instantaneous DC voltage and DC line current are U0 and I0, respectively, they can be approximated as follows: Figure 4 (b) The initial value of the capacitor voltage in the intermediate cascade submodule is also U0. Therefore, the transient solutions for the capacitor voltage and the DC line fault current can be obtained as follows:

[0083] (3);

[0084] In the formula:

[0085] .

[0086] During this stage, the submodule capacitance decreases rapidly, and the DC line fault current increases rapidly. However, the magnitude of the fault current is affected by the submodule capacitance value, the number of bridge arm submodules, and the bridge arm reactance value. When the system capacity and voltage level are fixed, the larger the submodule capacitance, the fewer the number of bridge arm submodules, and the smaller the bridge arm reactance value, the larger the fault current. This characteristic is consistent with physical properties: the larger the submodule capacitance, the greater the energy stored in the capacitor before the fault, and the greater the discharge current after the fault; the fewer the number of submodules, the greater the energy stored in the capacitor, and the greater the discharge current after the fault; the smaller the number of submodules, the greater the energy stored in the capacitor, and the greater the discharge current after the fault. Figure 4 The larger the equivalent capacitance value shown, the more energy is stored, and therefore the larger the discharge current; the smaller the bridge arm reactance value, the less it hinders the discharge of the equivalent capacitance, and therefore the larger the fault current.

[0087] In one embodiment of the present invention, the initial stage after the submodule is locked includes the following:

[0088] When the DC protection detects a DC fault, it will immediately lock out all submodules within the converter to protect the converter. Additionally, the IGBT's self-protection function will shut down the IGBT after the arm current rises. Therefore, the process enters the submodule lockout phase. Before the arm reactance current decays to zero after the submodule lockout, the fault current mainly consists of two parts: the AC power supply feed current and the arm reactance freewheeling current.

[0089] During this stage, because the freewheeling diode is constantly conducting due to the bridge arm reactance, the AC power supply does not exhibit unidirectional conductivity. Figure 5 As shown in (a), the three-phase AC power supply is equivalent to supplying short-circuit current through a symmetrical impedance composed of Rs, Ls, R, and L. At this point, the short-circuit point itself forms a fault current loop; therefore, the fault current supplied by the AC power supply does not feed into the DC line. The equivalent circuit of the AC side feed (single-phase) and the bridge arm reactance freewheeling current can be decomposed into... Figure 5 As shown in (b) and (c).

[0090] Assume that at the initial instant after entering the submodule lockout: the DC line current is I0'; the instantaneous AC power supply voltage is usk=Umsin(ωt+φ0); the attenuated DC component in the AC feed current is very small compared to the bridge arm reactance freewheeling current, and therefore can be ignored. At this point, the following can be calculated:

[0091] (4);

[0092] (5);

[0093] In the formula:

[0094] ; ;

[0095] .

[0096] Therefore, the fault currents of the upper and lower bridge arms are respectively:

[0097] (6);

[0098] The result is shown in equation (6).

[0099] In one embodiment of the present invention, the uncontrolled rectification stage after the submodule is latched includes the following:

[0100] As the reactance current of the bridge arms gradually decreases, each of the six bridge arms will experience a zero-current phenomenon, after which the follower diodes will exhibit unidirectional conduction. Therefore, the MMC will eventually operate as an uncontrolled rectifier bridge. Since the equivalent DC load of the uncontrolled rectifier bridge at this time is the short-circuit impedance, the AC side, DC side, and converter side will still be severely affected by overcurrent during this stage.

[0101] The root cause of short-circuit faults in power systems is the damage to the insulation between conductors in the current-carrying parts of electrical equipment or between conductors and ground. These faults are further classified into transient faults and permanent faults.

[0102] Transient faults mainly refer to short circuits caused by lightning flashover on the insulator surface, discharge caused by dirt on the insulator surface, line discharge to tree branches, short-term contact with the line caused by strong winds, and discharge through the body of birds. After the DC circuit breaker disconnects the faulty circuit, the arc at the fault point will extinguish itself, the insulation strength will be restored, the fault will be eliminated, and closing the DC circuit breaker will restore normal power supply.

[0103] Permanent faults mainly refer to short circuits caused by factors such as the collapse of overhead line towers due to strong winds or ice accumulation on conductors, irreversible insulation breakdown, or operator error. These types of faults cannot be resolved spontaneously and require disconnection of the faulty line for repair.

[0104] Neither transient nor permanent faults will recover on their own without intervention. Both types of faults involve damage to the insulation of electrical equipment (primarily overhead lines). Transient faults are mostly caused by the breakdown of the air insulation medium (conductor to ground, or between conductors) of electrical equipment, resulting in gas discharge phenomena (surface flashover or arcing).

[0105] Once a DC arc is established, it can only be extinguished if the condition u > U - Ri (where U is the power supply voltage, u is the arc voltage, i is the circuit current, and R is the circuit voltage). In other words, the DC voltage of the converter must be reduced below (u - Ri) for the arc to be extinguished. Therefore, measures must be taken (such as disconnecting the DC circuit breaker or using a fault-clearing converter) to extinguish the DC arc.

[0106] In the event of lightning strikes or other incidents, overhead lines may experience overvoltage breakdowns in line towers and other components, leading to transient short-circuit faults. Once the fault is successfully isolated and the arc is extinguished, the insulation of the DC line will be restored, and operation can resume.

[0107] In one embodiment of the present invention, the analysis of steady-state operation and transient fault mechanism of the converter topology includes the following:

[0108] The analysis of the half-bridge submodule (HBSM) includes the following:

[0109] Figure 6 The topology of the half-bridge submodule is shown, consisting of two IGBT devices with anti-parallel diodes and a capacitor.

[0110] The half-bridge MMC submodule has two operating states:

[0111] 1) The submodule alternately outputs positive voltages Uc and 0.

[0112] At this time, when T1 is turned on and T2 is turned off, the submodule outputs a positive voltage Uc; when T2 is turned on and T1 is turned off, the submodule outputs 0.

[0113] 2) Submodule locking

[0114] At this time, both T1 and T2 are turned off, so the submodule is in a locked state.

[0115] Table 1 provides a description of the operating status of the full-bridge submodule.

[0116] Table 1. Working Status of Half-Bridge Submodule

[0117]

[0118] When a bipolar short-circuit fault occurs in DC, the converter performs a lockout protection. Since the short-circuit current flows negatively through the submodule, the submodule output voltage is zero and no reverse electromotive force is provided. Therefore, the MMC converter composed of cascaded half-bridge submodules does not have DC fault clamping capability, but the converter valve should still be locked immediately in the event of a fault to block the discharge circuit of the submodule capacitor.

[0119] The full-bridge submodule (FBSM) includes the following:

[0120] like Figure 7 The diagram shows the topology of a full-bridge submodule, consisting of four IGBT devices with anti-parallel diodes and one capacitor. It can output positive, negative, and zero voltage levels.

[0121] An MMC converter consisting of a single full-bridge submodule has the same topology as a half-bridge MMC converter, the only difference being the submodule.

[0122] Operating mechanism:

[0123] like Figure 7 As shown, the full-bridge MMC submodule has three operating states:

[0124] 1) The submodule alternately outputs positive voltages Uc and 0.

[0125] At this time, when T1 and T4 are turned on and T2 and T3 are turned off, the submodule outputs a positive voltage Uc; when T3 and T4 are turned on and T1 and T2 are turned off, the submodule outputs 0.

[0126] 2) The submodule alternately outputs negative voltages -Uc and 0.

[0127] At this time, when T2 and T3 are turned on and T1 and T4 are turned off, the submodule outputs a negative voltage -Uc; when T1 and T2 are turned on and T3 and T4 are turned off, the submodule outputs 0.

[0128] 3) Submodule locking

[0129] At this time, T1, T2, T3, and T4 are all turned off, and the submodule is in a locked state.

[0130] The table below shows the operating status of the full-bridge submodule.

[0131] Table 2. Working Status Table of Full Bridge Submodule

[0132]

[0133] Because the full-bridge submodule can output a negative level, the AC / DC voltage modulation ratio of an MMC converter composed of full-bridge submodules can be greater than 1, such as... Figure 8 As shown, this can improve DC voltage utilization and indirectly increase system transmission capacity.

[0134] When a bipolar short-circuit fault occurs in the DC power supply, the converter performs a lockout protection. At this time, the equivalent circuit of the full-bridge submodule MMC is as follows: Figure 9 As shown.

[0135] From above Figure 9 It can be seen that when the voltage of the submodule capacitors of the two bridge arms is higher than the peak value of the AC line voltage, the submodule capacitors can provide sufficient back electromotive force to reduce the DC current to zero, thereby realizing the DC fault ride-through function.

[0136] At this point, the DC fault clamping capability of the full-bridge submodule MMC converter is 2NUc=2Udc, which is much greater than the peak value of the AC line voltage.

[0137] In one embodiment of the present invention, the surplus power unit for simulating the converter operating state of a DC transmission system in establishing a DC transmission model includes the following:

[0138] (1) Basic principle and grouping method of AC active power dynamic balancing device in large capacity scenario

[0139] In the scenario of flexible DC transmission of new energy, the sending-end flexible DC converter station often adopts VF control. Currently, the active power control of the sending-end new energy units cannot meet the requirements of millisecond-level response. During a fault, a large amount of surplus power will be stored in the converter station submodule capacitors in the form of electric field energy, causing the DC voltage of the converter station to rise and threatening the safe operation of the equipment.

[0140] The formula for capacitor energy storage is:

[0141] (7);

[0142] Therefore, the relationship between surplus power and voltage can be expressed as:

[0143] (8);

[0144] In the formula, S' represents surplus power, Ceq represents the equivalent capacitance of a single pole of the converter station, and E represents the rated voltage of a single pole of the converter station.

[0145] Therefore, the DC voltage after the fault occurred is:

[0146] ;(9);

[0147] The rate of change of DC voltage can be obtained as follows:

[0148] (10)

[0149] Mode It can quantitatively describe the relationship between unbalanced power and DC voltage rise time.

[0150] As can be seen from the above, the rise time of DC voltage is directly related to the surplus unbalanced power, the equivalent capacitance value, and the duration of capacitor charging. Currently, the main approach to suppressing the rise rate of DC voltage is to reduce the surplus unbalanced power, i.e., to add a dynamic power balancing device.

[0151] For true bipolar systems, since DC active power dynamic balancing devices cannot solve the power surplus problem caused by unipolar blocking at the sending end, true bipolar systems often employ AC active power dynamic balancing devices at the sending end to suppress surplus power, such as... Figure 10 As shown.

[0152] The equivalent circuit diagram of the resistor of the dynamic balancing device for grouped AC active power under a single step-down transformer is as follows: Figure 11 As shown.

[0153] Because the step-down transformer has leakage reactance, the voltage UR across the resistor is:

[0154] (11);

[0155] In the formula, R is the resistance value of a single AC active power dynamic balancing device resistor branch; N is the number of active power dynamic balancing device resistors in operation; Uac is the AC bus voltage; Xt is the leakage reactance of the step-down transformer; and k is the transformation ratio of the step-down transformer.

[0156] Therefore, the active power PR / N consumed by the resistor of the dynamic balancing device for grouped AC active power under a single step-down transformer is:

[0157] (12);

[0158] The total resistance capacity of the AC active power dynamic balancing device is determined by the device's power and activation time. To meet fault ride-through requirements under various operating conditions, the total active power of the AC active power dynamic balancing device should satisfy the following:

[0159] (13)

[0160] In the formula P ACP，min Umin is the power of the AC active power dynamic balancing device resistor when the bus voltage is at its minimum; m is the number of step-down transformers; NGe is the number of active power dynamic balancing device resistor branches under the i-th step-down transformer; Xt,Gi is the leakage reactance of the i-th step-down transformer; Pconv is the rated power of the bipolar converter.

[0161] For ultra-high voltage scenarios, the AC active power dynamic balancing device has two grouping methods: equal capacity and large group.

[0162] (2) Equal capacity grouping method for AC active power balancing devices

[0163] For AC active power dynamic balancing devices of equal capacity, resistors are grouped according to their capacity requirements. The more groups there are, the higher the power control accuracy and the smaller the deviation in surplus power dissipation; however, the cost and footprint also increase accordingly. Furthermore, due to the presence of thyristor off-state resistance, even if the active power dynamic balancing device resistors are not triggered, they still generate some power loss, which increases continuously with the number of groups. In addition, the resistance value of a single AC active power dynamic balancing device should also be matched with the tripping power.

[0164] In particular, the accuracy of power control needs to be considered.

[0165] Because the AC active power dynamic balancing device uses an over-connection method, when the capacity of a single active power dynamic balancing device is large, it will lead to frequent switching of the AC active power dynamic balancing device. For example, if a single AC active power dynamic balancing device has a capacity of 1000MW, and the UHV flexible DC system has a surplus power of 2500MW, then three AC active power dynamic balancing devices with a total capacity of 3000MW will be connected. This will cause a rapid drop in DC voltage, causing the operating logic to disconnect one or all AC active power dynamic balancing devices (the sending-end AC active power dynamic balancing device generally uses an overvoltage criterion). The above process generates a large surplus power, and then the DC voltage continues to rise due to the increase in surplus power, causing one or all AC active power dynamic balancing devices to be connected again. Therefore, the AC active power dynamic balancing device will be switched on and off frequently.

[0166] Therefore, the principle of equal capacity grouping is as follows: 1) Consider power control accuracy; 2) Consider off-state loss; 3) Consider power accuracy of new energy generator switching; 4) Consider cost and land occupation.

[0167] In one embodiment of the present invention, the fault ride-through strategy and fault clearing and recovery control strategy for the DC transmission system and its receiving-end power grid include the following:

[0168] In one embodiment of the present invention, the research on stable control technology for a transmission system considering the coordination of DC and transmitting / receiving power grids includes the following:

[0169] Traditional methods for formulating and converting stable control strategies for power grids rely heavily on manual processes, resulting in low efficiency and long update and testing cycles. This project proposes an online security and stability defense system platform architecture that integrates multiple information sources, connecting planned maintenance, renewable energy power forecasting, load forecasting, meteorological systems, spot markets, and dispatch control. It overcomes the challenges of intelligent generation technology for security defense analysis scenarios, resolving the issues of inefficient and incomplete scenario generation, and providing a data foundation for various security and stability analysis operations within the system. Simultaneously, it proposes the development of strategy generation and automatic conversion technologies centered on electronic strategy tables, establishing a closed-loop testing system composed of virtual devices and virtual testers. This overcomes the technical difficulties of online "analysis and generation - closed-loop testing - automatic update" of power grid security and stability control strategies, addressing the long-standing problems of long offline strategy deployment cycles, high mismatch risks, high control costs, and low reliability and inability to achieve closed-loop control for online strategies. The project also develops online security and stability control equipment to meet the rapid update needs of security and stability control strategies, improving the system's security and stability defense level and ensuring the safe and stable operation of the power system.

[0170] In one embodiment of the present invention, based on a model of a DC transmission system using a Modular Multilevel Converter (MMC), the state of the DC transmission system and the sending-end islanded system based on DC transmission is analyzed when the receiving-end grid is faulty. This includes ensuring that the upper and lower limits of DC voltage fluctuation do not exceed a given value (typically, this value should avoid the normal fluctuation value of the system's DC voltage and the setting value of the protection system, i.e., this value should be greater than the steady-state fluctuation value and less than the protection action value). Finally, the switching principle of the IGBT valve of the series valve centralized resistor energy dissipation device is designed.

[0171] In one embodiment of the present invention, during the switching of the equivalent IGBT switching state, the charging and discharging of the DC capacitor and the fluctuation of the DC voltage will occur.

[0172] When the DC voltage is greater than the upper limit reference value, the series IGBT valve is opened to actively control the power dissipation of the energy-consuming device and dissipate the excess DC power; when the DC voltage is less than the lower limit reference value, the series IGBT valve is closed to restore the DC voltage.

[0173] Based on the above principles, the on-time of the series IGBT valve within one switching cycle is:

[0174] (14);

[0175] In the formula, C eqse d and C eqrec These are the DC equivalent capacitance values ​​at the sending and receiving ends, respectively, P. diss P sed With P recThese represent the power consumption of the power-consuming resistor and the transmission power of the sending and receiving converters, respectively. dcHIGH with U dcLOW These are the upper and lower limit voltage reference values ​​for the defined energy-consuming devices to activate.

[0176] In the formula, C eqsed C eqrec P sed P rec U dcHIGH with U dcLOW When a fault occurs in the AC grid at the receiving end of the isolated DC transmission system at the sending end, the values ​​are constant, while the power dissipation Pdiss of the energy-consuming device varies with the change of the system DC voltage.

[0177] (15);

[0178] In the formula, Rdiss is the dissipation resistance value of the energy-consuming device.

[0179] Typically, since both UdcHIGH and UdcLOW are near the rated voltage UdcN, in engineering applications, the on-time of the series IGBT valve within one switching cycle can be estimated as follows:

[0180] (16)

[0181] Similarly, the turn-off time of the series IGBT valve within one switching cycle can be estimated as follows:

[0182] (17)

[0183] Based on the on-time and off-time of the series IGBT valve, the complete switching cycle of the entire IGBT series valve centralized resistor energy dissipation device can be obtained as follows:

[0184] (18)

[0185] As the above analysis shows, in the IGBT series valve centralized resistor energy dissipation device topology, the switching cycle and duty cycle of the series IGBT valve are closely related to the magnitude of the energy dissipation resistor and the upper and lower limit voltage reference values ​​for the energy dissipation device to operate. For a specific system, once these three values ​​are determined, the switching cycle and duty cycle of the series IGBT valve will only be determined by the fault condition (the difference between Psed and Prec), without the need for active control, making the entire control system simple and reliable.

[0186] In one embodiment of the present invention, the energy coordination control technology for each station under AC receiving end includes the following:

[0187] By optimizing the design through bipolar DC power consumption configuration and detailed control and protection strategies, the entire system achieves full ride-through under AC fault conditions at the receiving end. The specific optimizations to the flexible DC control and protection strategy are as follows:

[0188] a) In the event of an AC fault, the flexible DC converter will be subjected to a large fault current and AC voltage distortion. The overcurrent capacity of the flexible DC converter is limited. In order to ensure that the safety of the valve group equipment is not affected during the fault, the inverter station controls the fault current by setting a current limiting link. In the event of a symmetrical or asymmetrical fault in the AC system, the fault ride-through capability of the flexible DC transmission system can be improved by adopting special ride-through control strategies such as positive and negative sequence current control and utilizing the fast response capability of the converter.

[0189] b) Compared with conventional DC converter stations, flexible DC can provide dynamic reactive power support to the AC grid during faults and during fault recovery, and the magnitude of its reactive power support depends on the reactive current.

[0190] In one embodiment of the present invention, the DC fault ride-through strategy for a renewable energy islanded power transmission system includes the following:

[0191] In one embodiment of the present invention, the islanded system operation control strategy includes the following:

[0192] Maintaining a balance of active power between wind power, solar power, and flexible DC transmission is a prerequisite for the stable operation of islanded systems. The uncertainties inherent in wind and solar power lead to significant fluctuations in the power transmitted from wind farms. Constant active power control is unsuitable for the uncertainties in wind and solar power output, causing system frequency fluctuations and instability. To accurately reflect the impact of frequency fluctuations on power, frequency control is more suitable for islanded wind and solar farm connections. Therefore, frequency control is generally selected for converter stations connecting islanded wind and solar farms, ensuring a real-time balance between the power transmitted by flexible DC transmission and the power sent by wind and solar farms.

[0193] For bipolar flexible DC transmission systems, both poles employ constant-frequency control, resulting in a relatively rigid system. However, the zero-error regulation can lead to repeated power adjustments between the poles, negatively impacting system stability. In contrast, frequency slope control offers greater flexibility, leading to better system stability and avoiding the difficulty in determining the operating point caused by zero-error regulation.

[0194] When the power supply is directly connected to the new energy island system, the converter should supply a stable AC voltage and frequency to the passive network. Therefore, the AC system voltage on the AC grid-connected side of the new energy is directly controlled, that is, passive constant AC voltage and frequency control is adopted.

[0195] When a flexible DC system adopts a true bipolar topology, at least one of the two poles uses VF control to stabilize the voltage and frequency of the AC system at the grid connection point, while the other pole can use either VF control or PQ control, i.e., bipolar VF control or VF-PQ control mode.

[0196] In one embodiment of the present invention, the dual VF droop control strategy includes the following:

[0197] Both poles use constant frequency control, and their zero-error regulation leads to repeated power adjustments between the poles, which is detrimental to system stability. To enable the bipolar converter to simultaneously supply a stable AC voltage to the passive network, a control strategy that does not require switching control modes can be designed, drawing inspiration from the control characteristics of generators. Referring to the frequency differential regulation characteristics of synchronous generators, frequency slope control (frequency droop control) can be used to enable the bipolar converter to simultaneously supply power to the passive network. For example... Figure 12 As shown.

[0198] By controlling the droop of active power and frequency, and the droop of reactive power and voltage in both poles, the active and reactive power are adjusted in real time to achieve bipolar frequency consistency. Fault ride-through is achieved through a dual inner-loop current controller.

[0199] When a converter station is connected to a renewable energy islanded system, the islanded AC voltage controller used provides a stable AC voltage for the islanded system. The bipolar converter needs to coordinate and cooperate to provide voltage support for the islanded system. Currently, there are two typical control strategies: the bipolar VF droop control strategy and the bipolar VF master-slave control strategy.

[0200] like Figure 13 The diagram shows the principle of the bipolar VF droop control strategy. Pf droop control is used to achieve active power coordination of the bipolar converter, and QU droop control is used to achieve reactive power coordination of the bipolar converter.

[0201] For bipolar VF droop control strategies, when bipolar asymmetrical operation is required, it is achieved by setting an overload limiting circuit and adjusting the reference power commands of the two converters, such as... Figure 14 As shown.

[0202] like Figure 15 The diagram shown illustrates the principle of a bipolar VF master-slave control strategy. The bipolar control layer is configured with an AC voltage controller to generate active current command I. dref Reactive current command I qref The reference phase command θ is sent to the pole control layer of the bipolar converter, where the converter current is controlled by the inner loop current control. In bipolar asymmetrical operation, the power limit value P is used... lim The limit value I for modulating the active current generation dmax As a command for inner-loop current control, it enables single-pole constant power control.

[0203] Since the bipolar VF master-slave control strategy is simple to implement in bipolar asymmetrical operation and suitable for engineering applications, it is recommended to adopt this strategy for islanded AC voltage control of DC power grids.

[0204] In one embodiment of the present invention, the VF-PQ control strategy includes the following:

[0205] By utilizing the bipolar frequency PF droop control and QV droop control characteristics, the active and reactive power of the two poles are adjusted in real time to achieve bipolar frequency consistency. Fault ride-through is achieved through a dual inner-loop current controller.

[0206] The VF control electrode provides a constant AC frequency and voltage to the new energy source, and the excess energy generated by the wind turbine is absorbed by this electrode, such as... Figure 16 As shown.

[0207] However, the frequency and voltage phase controlled by PQ-controlled pole lock and VF-controlled power supply are constant, and cannot adapt to changes in renewable energy output in real time. Figure 17 As shown.

[0208] In one embodiment of the present invention, the DC fault clearing and recovery control strategy for a new energy islanded transmission system includes the following:

[0209] For full-bridge submodule MMC and hybrid submodule MMC where FBSM accounts for more than half of the total number of submodules, the DC voltage can be controlled to 0, allowing it to pass through DC short-circuit faults without being blocked.

[0210] The solution adopts the following... Figure 18 The control block diagram shown uses a PI controller in a synchronous rotating coordinate system for the inner current loop. The reactive current setpoint iq_ref is generated by the reactive power controller, and the active current setpoint id_ref is given by one of the three controllers: the submodule voltage controller, the DC bus voltage controller, and the active power controller. The specific control signal to be selected is determined by the fault-crossing sequential control process.

[0211] Upon detecting the activation of the DC line protection, a DC line fault is confirmed in the system. At this point, the DC voltage reference value generated in the modulation scheme is modified to make the DC bus voltage negative, ensuring a rapid decrease in the fault current. After the DC current drops, the MMC simulates the operation of the DC-side resistor (Udc = R * idc) to increase the DC system damping and reduce DC-side current oscillations. The system then enters the DC short-circuit fault ride-through state.

[0212] During DC short-circuit fault ride-through, the MMC cannot perform active power transmission, but can still provide reactive power support to the AC side. Since the DC bus voltage is zero, neither DC bus voltage control nor active power control loops can effectively control the submodule voltage. Therefore, a submodule voltage controller is needed, taking the average voltage of all submodules as the feedback value, and forming a closed loop through a PI controller to stabilize the submodule capacitor voltage.

[0213] It is important to note that the magnitude of the negative voltage Udc- after the DC bus voltage flips is determined by both the AC voltage amplitude Uac and the number n of FBSMs in the bridge arm. As shown in the following formula, the more FBSMs there are, the greater the voltage that can be used to suppress fault current. A full-bridge MMC only contains a full-bridge submodule, therefore it can directly flip the DC bus Udc to -Udc.

[0214] (6);

[0215] The advantage of this approach is that the converter does not need to be locked out throughout the entire process and remains in grid-connected mode. Therefore, the converter can continue to generate reactive current to support the grid during this period. More importantly, maintaining grid connection ensures that the submodule voltage remains at its rated value, guaranteeing that the system can skip the voltage equalization and unlocking processes after the fault is cleared, allowing for a faster recovery.

[0216] DC line fault clearing and restart procedure as follows Figure 19 As shown, in the isolated wind and solar power plant + flexible DC project, the flexible DC system provides a reliable grid connection voltage for wind power and photovoltaic power. Wind power and photovoltaic power are transmitted by adjusting the terminal voltage. The active power flowing into the converter is completely determined by the external system.

[0217] When a DC line fails, the DC power at the faulty pole cannot continue to be transmitted, and the wind turbines and photovoltaic power plants cannot quickly disconnect or reduce the power. The active power flowing into the converter cannot be controlled, which will lead to a serious power surplus in the flexible DC system, causing problems such as converter overcurrent and DC overvoltage. Corresponding power surplus control measures need to be taken.

[0218] For a new energy islanded bipolar flexible DC system, in order to ensure the control of the overall surplus power of the bipolar system when a line fault occurs in one pole, AC energy-consuming equipment needs to be used. The specific strategy is as follows:

[0219] Activation strategy: When a fault signal is received in a single-pole DC line and the bipolar power before the fault is greater than the maximum power of the single pole, activate the AC power dissipation device.

[0220] Exit strategy: When the power of the AC energy-consuming device and the non-faulty pole is not greater than the maximum power of a single pole, it is determined that the wind turbine and photovoltaic power station disconnection operation has been completed, and the AC energy-consuming device is removed.

[0221] In one embodiment of the present invention, the simulation verification of the DC fault ride-through strategy for new energy flexible DC islanding transmission includes the following:

[0222] Under rated operating conditions (DC voltage ±800kV, bipolar DC power 5000MW, dual-valve operation), a 100ms DC line fault was simulated, and the simulation results are as follows: Figure 20 As shown:

[0223] The coordination and control process is as follows:

[0224] 1. Determine if a bipolar operation unipolar DC line is in fault condition and trigger the energy dissipation signal.

[0225] 2. The DC bus voltage is controlled to a negative value to quickly reduce the DC current and extinguish the fault arc;

[0226] 3. Wait for the deionization process to finish before starting to increase the DC voltage.

[0227] 4. Gradually reduce energy consumption and increase active power.

[0228] 5. Active power recovers to more than 90% of the pre-fault level within 250ms.

[0229] In one embodiment of the present invention, the safety and stability control unit for a DC transmission system includes the following:

[0230] In one embodiment of the present invention, establishing a safety and stability control unit for a DC transmission system includes research on transmission system stability control technology that considers the coordination between the DC and the sending and receiving end power grids, wherein the research on transmission system stability control technology includes the following:

[0231] Traditional methods for formulating and converting stable control strategies for power grids rely heavily on manual processes, resulting in low efficiency and long update and testing cycles. This project proposes an online security and stability defense system platform architecture that integrates multiple information sources, connecting planned maintenance, renewable energy power forecasting, load forecasting, meteorological systems, spot markets, and dispatch control. It overcomes the challenges of intelligent generation technology for security defense analysis scenarios, resolving the issues of inefficient and incomplete scenario generation, and providing a data foundation for various security and stability analysis operations within the system. Simultaneously, it proposes the development of strategy generation and automatic conversion technologies centered on electronic strategy tables, establishing a closed-loop testing system composed of virtual devices and virtual testers. This overcomes the technical difficulties of online "analysis and generation - closed-loop testing - automatic update" of power grid security and stability control strategies, addressing the long-standing problems of long offline strategy deployment cycles, high mismatch risks, high control costs, and low reliability and inability to achieve closed-loop control for online strategies. The project also develops online security and stability control equipment to meet the rapid update needs of security and stability control strategies, improving the system's security and stability defense level and ensuring the safe and stable operation of the power system.

[0232] This invention, as a key component of the main station software system for online safe and stable control equipment, aims to solve the following core problems:

[0233] Future mode data generation: Based on the fusion of multiple data sources such as planned maintenance, renewable energy power forecasting, load forecasting, meteorological system, spot market, and dispatch control, future mode data is generated.

[0234] Transient calculation basic data conversion: Based on future data samples, complete the data format conversion before calculation to form a data file that can be recognized by power system simulation calculation software.

[0235] Rapid stability quantitative assessment: For transient basic data files (multiple operating modes), rapid stability quantitative assessment is carried out to screen instability scenarios and initial control measures, which are then provided to the subsequent strategy generation module.

[0236] Control measure sample generation and management: Using self-developed transient stability simulation software, based on the above transient calculation data and the current strategy of the stability control system, the control measure sample periodic calculation is realized and a dynamically rolling sample center is established to ensure that the data samples used in the strategy generation process always maintain timeliness and high quality.

[0237] Robustness of the stability control strategy generation, testing, and adaptive update process: Design a robust scheduler to provide fine-grained management of the entire lifecycle of stability control strategy generation, closed-loop testing, and online updates, with status monitoring and anomaly rollback capabilities.

[0238] Core objective:

[0239] Efficiency: Maximize the automation of data processing and strategy calculation processes to reduce the delay from data to decision.

[0240] Robustness: All modules of the system are highly available and can properly handle various abnormal situations to ensure business continuity.

[0241] Scalability: The system architecture supports horizontal scaling and can cope with future growth in data volume and computational complexity.

[0242] Observability: Provides comprehensive monitoring and logging, facilitating online management and troubleshooting.

[0243] This study covers the following areas:

[0244] Module 1: Future-Oriented Data Generation Module

[0245] Module 2: Transient Computation Basic Data Conversion Module

[0246] Module 3: Rapid Quantitative Stability Assessment Module

[0247] Module 4: Control Measures Sample Generation and Management Module

[0248] Module 5: Core module for generating, testing, and rolling update scheduling of stability control strategies

[0249] Content outside the scope of this study:

[0250] The internal implementation details of the specific future prediction algorithm.

[0251] The specific internal implementation details of the fast stability quantification evaluation algorithm.

[0252] The specific stability control strategy generation algorithm and the internal logic of the testing process.

[0253] The construction and maintenance of underlying infrastructure (such as OSS, RMQ, and databases).

[0254] In one embodiment of the present invention, the overall architecture includes the following:

[0255] The overall architecture of this invention adopts a modular, event-driven, distributed architecture. Each module has a single responsibility, decoupling files through OSS, communicating status and commands through RMQ, and sharing and synchronizing high-frequency data through RTDB.

[0256] like Figure 21 and 22 As shown, the architecture diagram includes the following:

[0257] Computational data generation, transformation, and rapid quantitative evaluation process for stability:

[0258] The Future Mode Data Generation Module executes every 15 minutes, utilizes diverse heterogeneous data, runs prediction algorithms, and uploads the generated Future Mode data files to a designated directory in OSS.

[0259] The transient calculation basic data conversion module detects new future data files by listening to OSS events or performing periodic scans. It immediately downloads and converts the new data files into standard format calculation files, then uploads them to another designated directory in OSS and sends a NewCalcFileReady message via RMQ.

[0260] The stability rapid quantitative assessment module performs rapid quantitative assessment by listening to the NewCalcFileReady message, records the instability results and control measures, generates an interface file which is uploaded to the specified directory in OSS, and writes the corresponding calculation records to the RTDB.

[0261] This process is independent of the main scheduling process and continuously provides the latest computational data sources and stable quantitative evaluation results.

[0262] Core scheduling and decision-making process:

[0263] The scheduling core program is the "brain" of the system, responsible for orchestrating the entire decision-making lifecycle.

[0264] The scheduler triggers the control measure sample generation and management module. Based on the output of the stability rapid quantitative evaluation module, this module obtains the latest calculation file from OSS, completes the measure sample generation, and updates it to the OSS sample center.

[0265] After the sample is updated, the scheduling core program reads the sample from OSS, generates a control strategy, and stores the newly generated stability control strategy in OSS.

[0266] For newly generated control strategies or setpoints, the scheduling core program encapsulates test cases and asynchronously calls the test process via RMQ.

[0267] After the test process is completed, the test report is stored in OSS and the results are returned to the scheduling core via RMQ.

[0268] Based on the test results, the scheduling core issues stability control strategies to the device and performs subsequent operations (such as status release, archiving, etc.).

[0269] The above are preferred embodiments of the present invention. Any changes made to the technical solution of the present invention that do not exceed the scope of the technical solution of the present invention shall fall within the protection scope of the present invention.

Claims

1. A method for coordinated control and protection of ultra-high voltage multi-terminal DC transmission systems, characterized in that, Includes the following: A DC transmission model for the DC transmission system is established, including converter sub-units, surplus power units, and fault protection units for simulating the converter operating state of the DC transmission system; and a power balance strategy for the AC grid at the receiving end of the DC system is established based on the DC transmission model. Based on the DC transmission model and power balance strategy, establish fault ride-through strategy and fault clearing and recovery control strategy for DC transmission system and its receiving end grid; Based on the DC transmission model, power balance strategy, and fault ride-through method, a safety and stability control unit for the DC transmission system is established to achieve control and protection of the DC transmission system.

2. The method for coordinated control and protection of an ultra-high voltage multi-terminal DC transmission system according to claim 1, characterized in that, The converter sub-unit of the DC transmission model uses multiple modular multilevel converter units to simulate the converter working state of the DC transmission system; the DC transmission model uses a bipolar symmetrical system to represent the electrical circuit of the DC transmission system; wherein the bipolar symmetrical system is composed of two unipolar asymmetrical systems connected in series.

3. The method for coordinated control and protection of an ultra-high voltage multi-terminal DC transmission system according to claim 1, characterized in that, The surplus power unit includes the following: The relationship between surplus power and voltage can be expressed as: ； Therefore, the DC voltage after the fault occurred is: ； The final DC voltage change rate is: ； Where S' represents surplus power, C eq This represents the equivalent capacitance of a single pole in the converter station, and E represents the rated voltage of a single pole in the converter station.

4. The method for coordinated control and protection of an ultra-high voltage multi-terminal DC transmission system according to claim 1, characterized in that, The fault protection unit includes the following: After a fault occurs in the DC transmission system, the fault protection unit receives the fault signal and then locks all converters through the converter subunit and activates the converter lockout protection. When the converter is locked out and the sum of the capacitor voltages of each arm of the converter is higher than the peak value of the AC line voltage, the capacitor provides back electromotive force to reduce the DC current to zero, thereby realizing the DC fault ride-through function of the fault protection unit.

5. A method for coordinated control and protection of an ultra-high voltage multi-terminal DC transmission system according to claim 1, characterized in that, The power balancing strategy includes using an AC active power dynamic balancing device to suppress surplus power; The total resistance capacity of the AC active power dynamic balancing device is determined by the power of the active power dynamic balancing device and the activation time of the active power dynamic balancing device; in order to meet the fault ride-through requirements under various operating conditions, the total active power of the AC active power dynamic balancing device should meet the following requirements: ； Where P ACP,min The power of the AC active power dynamic balancing device resistor when the bus voltage is at its minimum; U min is the minimum value of the bus line voltage; m is the number of step-down transformers; N Gi X represents the number of resistor branches in the dynamic power balancing device of the i-th step-down transformer; t,Gi P is the leakage reactance of the i-th step-down transformer; conv This is the rated power of the bipolar converter.

6. A method for coordinated control and protection of an ultra-high voltage multi-terminal DC transmission system according to claim 5, characterized in that, The AC active power dynamic balancing device mentioned above includes two working modes: equal capacity and large / small group. The factors to consider in the power allocation method of the AC active power dynamic balancing device under the equal capacity grouping operation mode include: power control accuracy, off-state loss, and power accuracy of new energy switching. The factors to consider in the power allocation method of the AC active power dynamic balancing device under the large group operation mode include: determining the capacity of the large group based on the AC side voltage level and the upper limit of the thyristor current; determining the capacity of the smallest group with the finest switching precision; and rationally allocating the capacity of the intermediate group using a binary method based on the smallest group capacity and the large group capacity.

7. A method for coordinated control and protection of an ultra-high voltage multi-terminal DC transmission system according to claim 1, characterized in that, The fault ride-through strategies include a dual VF droop control strategy and a VF-PQ control strategy; The dual VF droop control strategy mentioned above includes: adjusting the active and reactive power of the two poles in real time to achieve bipolar frequency consistency through the external characteristics of bipolar active power and frequency droop control, reactive power and voltage droop control; and achieving fault ride-through through a dual inner loop current controller. The VF-PQ control strategy mentioned above includes: adjusting the active and reactive power of the two poles in real time to achieve bipolar frequency consistency through bipolar frequency PF droop control and QV droop control external characteristics, and achieving fault ride-through through dual inner loop current controllers.

8. A method for coordinated control and protection of an ultra-high voltage multi-terminal DC transmission system according to claim 1, characterized in that, The aforementioned fault clearing and recovery control strategy includes controlling the DC voltage to 0 when a DC transmission system fault occurs, and ensuring that the DC transmission system passes through the DC short-circuit fault without being shut down throughout the entire process; at the same time, AC energy-consuming equipment is used to consume the surplus power to prevent converter overcurrent and DC overvoltage. The fault clearing and recovery control strategy also includes shutting down AC power-consuming equipment and increasing DC voltage when the fault is cleared, ultimately restoring the active power of the DC system to more than 90% of the level before the fault within 250ms.

9. A method for coordinated control and protection of an ultra-high voltage multi-terminal DC transmission system according to claim 1, characterized in that, The safety and stability control unit for DC transmission systems includes a future mode data generation unit, a transient calculation basic data conversion unit, a rapid stability quantification and evaluation unit, a control measure sample generation and management unit, and a core unit for stability control strategy generation, testing, and updating scheduling. The future mode data generation unit integrates multiple data sources, including planned maintenance, new energy power forecasting, load forecasting, meteorological system, spot market, and dispatch control, to generate future mode data. The transient calculation basic data conversion unit, based on future-mode data samples, completes the data format conversion before calculation, forming a data file that can be recognized by power system simulation calculation software; The stability rapid quantitative assessment unit conducts rapid stability quantitative assessment on transient basic data files to screen instability scenarios and initial control measures; The control measure sample generation and management unit uses self-developed transient stability simulation calculation software. Based on the above transient calculation data and the current strategy of the stability control system, it realizes the periodic calculation of control measure samples and establishes a dynamically rolling sample center to ensure that the data samples used in the strategy generation process always maintain timeliness and high quality. The core unit for the generation, testing, and updating of stability control strategies is designed with a robust scheduler that manages the entire lifecycle of stability control strategy generation, closed-loop testing, and online updates, and has the ability to monitor status and roll back in case of anomalies.

10. A system for coordinated control and protection of an ultra-high voltage multi-terminal DC transmission system, used to execute a method for coordinated control and protection of an ultra-high voltage multi-terminal DC transmission system as described in any one of claims 1 to 9, characterized in that, Includes the following: The power transmission system module is used to establish a DC power transmission model for the DC power transmission system, including converter sub-units, surplus power units, and fault protection units for simulating the converter operating state of the DC power transmission system; and to establish a power balance strategy for the AC grid at the receiving end of the DC system based on the DC power transmission model. The fault protection, clearing and recovery module is used to establish fault ride-through strategies and fault clearing and recovery control strategies for the DC transmission system and its receiving-end grid based on the DC transmission model and power balance strategy. The safety and stability control module is used to establish a safety and stability control unit for the DC transmission system based on the DC transmission model, power balance strategy, and fault ride-through method, so as to realize the control and protection of the DC transmission system.

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