Multi-objective optimization solution method for short-circuit current control strategy of flexible DC transmission

Abstract

This invention proposes a multi-objective optimization solution method for short-circuit current control strategy in flexible DC transmission, specifically including: Step 1, constructing a two-port model of the receiving-end fault and solving for the parallel fault transition impedance in the receiving-end grid fault model; Step 2, combining the parallel fault transition impedance, deriving the minimum reactive current reference value constraint required to support AC voltage during the fault, and the active current constraint allowed by the DC side energy balance and DC voltage upper limit; Step 3, combining the minimum reactive current required to reduce voltage drop during a short-circuit fault, the active current that the receiving-end converter station MMC can provide, and the short-circuit current amplitude constraint condition output by the receiving-end converter station MMC, obtaining the short-circuit current safety domain under three-phase short-circuit faults. This invention adaptively analyzes and optimizes existing short-circuit current control strategies, ensuring that the optimized control strategy can both reduce unbalanced power accumulation and support AC grid voltage through reserved capacity.

Description

Technical Field

[0001] This invention belongs to the field of protection technology for offshore wind power flexible DC transmission systems, and particularly relates to a multi-objective optimization solution method for short-circuit current control strategy of flexible DC transmission. Background Technology

[0002] The offshore wind power flexible DC transmission project mainly includes offshore wind farms, offshore booster stations, offshore converter stations, submarine cables, onshore cables, and onshore converter stations (receiving-end converter stations), etc., and its system structure is as follows: Figure 1 As shown.

[0003] The electricity generated by offshore wind farms is typically collected at 33kV or 66kV AC voltage levels and fed to offshore substations via submarine cables. The electricity from multiple offshore substations is then connected to the AC side of an offshore converter station via submarine cables. After passing through parallel double-connected transformers, the electricity enters the modular multilevel converter (MMC) at the offshore converter station, completing the AC-to-DC (AC / DC) conversion. Subsequently, the DC power is transmitted via submarine and onshore cables to the onshore converter station's MMC, where it undergoes a DC-to-AC (DC / AC) inversion before being connected to the onshore AC power grid.

[0004] MMC-HVDC (Modular Multilevel High Voltage Direct Current) transmission technology has become the most common topology for flexible DC transmission due to its advantages such as easy expansion, low loss, and low harmonics. Currently, all flexible DC projects in China adopt the MMC-HVDC structure. The MMC topology is as follows: Figure 2 As shown, SM is a submodule, and each SM contains 2 insulated gate transistors (IGBTs) and 2 diodes.

[0005] With the widespread application of flexible DC transmission technology, its impact on the short-circuit current of AC systems has also received increasing attention.

[0006] Offshore wind power systems are more susceptible to short-circuit faults due to their unique operating environment (such as high humidity, salt spray corrosion, and long-distance power transmission). Excessive short-circuit current can burn out generators, transformers, cables, and other equipment. Short-circuit faults can cause a sudden drop in grid voltage, affecting grid stability. They can also trigger fires or other safety accidents. Overly limiting the flexible DC short-circuit current can lead to slow grid fault recovery, while insufficient control can cause the flexible DC system to become unstable due to excessive current.

[0007] Therefore, it is necessary to balance the relationship between short-circuit current suppression and system stability recovery. Exploring and comparing fault ride-through control methods for large-scale deep-sea wind power connected to large receiving-end power grids via flexible DC transmission, and optimizing and forming a composite short-circuit current control strategy for flexible DC transmission adapted to large receiving-end power grids are of great significance for improving the stable operation of the power grid.

[0008] Existing short-circuit current suppression strategies at PCC points include active current suppression control strategies, phase-optimized short-circuit current control strategies, and equipment-added current limiting strategies.

[0009] The active current suppression control strategy dynamically switches the control mode by detecting the voltage drop at the PCC point in real time. It selects active power priority or reactive power priority control according to the system requirements. However, it depends on the voltage detection accuracy. If the voltage detection at the PCC point is delayed or has errors, it may lead to untimely mode switching and reduced current limiting effect.

[0010] Phase-optimized short-circuit current control strategies address the issue of the significant contribution of short-circuit current phase to AC systems. By adjusting the phase angle of the flexible DC transmission output current to be opposite to the phase of the AC fault current, some short-circuit current components are canceled out. However, this approach is highly complex, requiring real-time calculation of the AC fault current phase and placing extremely high demands on the dynamic performance of the phase-locked loop (PLL). Equipment-added current-limiting strategies involve installing superconducting current limiters or DC circuit breakers at key locations in the flexible DC transmission system to limit short-circuit current through physical impedance. This can confine the short-circuit current within the circuit breaker's breaking capacity, preventing converter valve lockout. However, superconducting current limiters are expensive and require a cryogenic system, increasing investment by more than 30%.

[0011] The three-phase short-circuit current control strategies mentioned above at the PCC point each have their advantages, but they also have their limitations. While limiting the short-circuit current, they do not take into account factors such as unbalanced power accumulation safety and reactive voltage support. Summary of the Invention

[0012] The purpose of this invention is to provide a multi-objective optimization solution method for short-circuit current control strategy of flexible DC transmission. This method is particularly applicable to three-phase short-circuit faults occurring at the receiving end PCC point of offshore wind power flexible DC transmission system. The aim is to adaptively analyze and optimize the control strategy that limits the contribution of flexible DC transmission to short-circuit current.

[0013] As the voltage levels of flexible DC transmission projects connected to AC systems continue to increase, the impact of flexible DC transmission on short-circuit currents in AC systems has become increasingly significant and cannot be ignored. Short-circuit current calculation, as a crucial aspect of power system analysis, is the core foundation for power system planning and design, relay protection setting calculations, and the selection and verification of electrical equipment. Therefore, in-depth analysis of the specific impact of modular multilevel converter high-voltage DC transmission (MMC-HVDC) on grid short-circuit currents and the design of optimized short-circuit current protection strategies are particularly important.

[0014] Therefore, this invention designs a fault-safe domain for the receiving-end converter station. By establishing multiple short-circuit current fault-safe domains under multiple electrical quantity constraints, the flexible DC short-circuit current control strategy is adaptively analyzed and optimized, thereby suppressing the contribution of MMC to the short-circuit current.

[0015] This invention first establishes a fault model of the receiving-end power grid, equating the faulted receiving-end power grid with a two-port network, which includes a constant voltage source, series impedance, and parallel equivalent fault transition impedance.

[0016] Furthermore, by establishing a short-circuit current safety domain for the faulted system, adaptive analysis and optimization of the existing short-circuit current control strategy are performed. This ensures that the optimized control strategy can both reduce the accumulation of unbalanced power and support the AC grid voltage through reserved capacity. This plays a significant role in short-circuit current fault ride-through.

[0017] The specific technical solution adopted is as follows:

[0018] A multi-objective optimization solution method for short-circuit current control strategy in flexible DC transmission includes the following steps:

[0019] Step 1: Establish a fault model of the receiving-end power grid, and solve for the equivalent impedance of the receiving-end power grid side before the fault in the fault model. Parallel fault transition impedance ;

[0020] Step 2, combining parallel fault transition impedance Solve for the minimum reactive current required to support the AC voltage. When DC voltage Rise to maximum DC voltage The active current that the MMC of the front and rear receiving-end converter stations can provide;

[0021] Step 3: Minimum reactive current required to support AC voltage The active current that the receiving-end converter station MMC can provide and the short-circuit current amplitude constraint conditions output by the receiving-end converter station MMC are used to obtain the short-circuit current safety domain under three-phase short-circuit fault.

[0022] Preferably, the receiving-end power grid fault model includes: the receiving-end power grid equivalent electromotive force.

[0023] Preferably, the equivalent impedance of the receiving end of the grid before the fault is calculated based on the AC grid equivalent impedance identification principle based on active power injection. Then, the equivalent electromotive force of the AC power grid is calculated by substituting it into the AC power grid equivalent electromotive force model. .

[0024] Preferably, in step 2, the minimum reactive current required to support the AC voltage is the reference value of the reactive current during the fault period. for:

[0025] ;

[0026] - Reference voltage at PCC point;

[0027] - Fault voltage at PCC point;

[0028] -Equivalent impedance at point PCC during the fault;

[0029] - Reactant component of equivalent impedance.

[0030] Preferably, in step 3, the short-circuit current amplitude condition output by the receiving-end converter station MMC is:

[0031] ;

[0032] - shaft current;

[0033] - shaft current;

[0034] - The amplitude of the inner loop current limiter.

[0035] Compared with the prior art, the advantages of the present invention are:

[0036] 1. The fault safety domain of the receiving-end converter station involved in this invention comprehensively considers multiple safety objectives, including short-circuit current, reactive power support, and unbalanced power accumulation. It can adaptively analyze and optimize existing short-circuit current control strategies, enabling the control strategies to go beyond simply suppressing short-circuit current and achieve multi-dimensional fault ride-through.

[0037] The outstanding advantage of this invention is that it links three key variables: short-circuit current (Equation (13)), reactive power support (Equation (14)), and unbalanced power accumulation (Equation (17)), and establishes a safe domain for short-circuit current contribution of receiving-end converter station under multiple objective constraints. This provides a better solution for optimizing short-circuit current control strategy and makes up for the shortcomings of existing short-circuit current control strategies.

[0038] 2. The safety domain theory has the advantage of continuously characterizing the system's operating state, enabling a comprehensive evaluation of the system's safety status. This invention provides an efficient and reliable adaptive analysis and optimization scheme for offshore wind power short-circuit current control strategies, which can significantly improve the operational safety and reliability of offshore wind power systems. Attached Figure Description

[0039] Figure 1 This is a structural diagram of a flexible DC transmission system for offshore wind power.

[0040] Figure 2 This is a topology diagram of MMC;

[0041] Figure 3 For receiving-end power grid fault models;

[0042] Figure 4 This is the safe domain for short-circuit current under three-phase short-circuit faults. Detailed Implementation

[0043] The following will describe in more detail the multi-objective optimization solution method for the short-circuit current control strategy of flexible DC transmission according to the present invention, with reference to the schematic diagrams. Preferred embodiments of the invention are shown. It should be understood that those skilled in the art can modify the invention described herein while still achieving its advantageous effects. Therefore, the following description should be understood as being of general knowledge to those skilled in the art and is not intended to limit the invention.

[0044] A multi-objective optimization solution method for short-circuit current control strategy in flexible DC transmission includes the following steps:

[0045] 1. Establish a fault model for the receiving-end power grid.

[0046] First, a fault model of the receiving-end power grid is established, simplifying the complex receiving-end power grid into a two-port network, which facilitates subsequent analysis and calculation.

[0047] "Receiving-end power grid": Figure 1 The "alternating current power grid" in the context.

[0048] The offshore wind power flexible DC transmission systems studied in this invention all adopt a symmetrical monopole topology, and their typical topologies are shown in [reference needed]. Figure 1 Offshore wind farms typically collect voltages at 33 kV or 66 kV AC to offshore booster stations. Multiple offshore booster stations form an AC collection booster, which is then connected to the AC field of the offshore converter station. After being connected in parallel by two connecting transformers (connecting transformer No. 1 and connecting transformer No. 2), the DC signal is transmitted to the onshore converter station (MMC) via submarine and land cables. After completing the DC / AC conversion, the DC signal is connected to the onshore AC power grid (receiving end grid).

[0049] Figure 1 middle:

[0050] "Sending-end converter station" refers to an offshore converter station; "receiving-end converter station" refers to an onshore converter station.

[0051] "Connecting transformer" refers to the connecting transformer.

[0052] Figure 1 The structure of a DC transmission system including a dual-ended MMC is shown. According to Thevenin's theorem, [the following is a description of the system]. Figure 1 The receiving-end power grid in China ( Figure 1 The AC power grid in the middle is equivalently replaced by an ideal voltage source and an equivalent impedance connected in series.

[0053] When a short-circuit fault occurs in the receiving-end power grid, a new branch will appear in the grid, namely the short-circuit branch of the fault transition resistance. The appearance of this branch changes the original power grid structure, causing a change in the equivalent impedance of the power grid.

[0054] Therefore, according to Thevenin's theorem, when a three-phase short circuit occurs at the receiving-end PCC point, the faulty receiving-end power grid can be equivalent to the equivalent electromotive force of the receiving-end power grid. The equivalent impedance of the receiving end of the power grid during normal operation (before the fault) Parallel equivalent fault transition impedance The constructed two-port network, such as Figure 3 As shown.

[0055] Figure 3 Equivalent Figure 1 The dashed frame containing the receiving-end converter station and Figure 1 The AC power grid in the middle.

[0056] Figure 1 "AC power grid" Figure 3 The same as the "AC power grid" in the text.

[0057] The receiving-end converter station and MMC have the same meaning.

[0058] This invention employs the AC grid equivalent impedance identification principle based on active power injection (Equation 6) to calculate the equivalent impedance of the receiving end grid side during normal operation (before the fault) by calculating the operating parameters before the fault. .

[0059] Figure 3 The equivalent impedance of the connecting transformer in the figure is denoted as In engineering, resistance is often neglected, therefore:

[0060] ;

[0061] -Imaginary unit (known).

[0062] - Reactance of the connecting transformer;

[0063] Relevant values ​​for connecting transformers ( , All values ​​are measured.

[0064] - Equivalent reactance of the receiving-end power grid before the fault.

[0065] -The equivalent resistance of the receiving-end power grid before the fault is often ignored in engineering. Therefore, the equivalent impedance of the receiving-end power grid side before the fault is... for:

[0066] (1)

[0067] ω is the fundamental angular frequency of the receiving-end power grid; ω is a known quantity.

[0068] The actual equivalent impedance of the receiving end of the power grid before the fault (Including the equivalent resistance of the receiving-end power grid before the fault) Equivalent reactance of the receiving-end power grid before the fault ) represents the solution value for the requirement.

[0069] According to Kirchhoff's voltage law for the equivalent circuit of an AC power grid (Thevenin's circuit), we can obtain:

[0070] (2)

[0071] ;

[0072] in, - The vector form of the equivalent electromotive force of the receiving-end power grid; is the quantity of the demand solution;

[0073] - Vector form of the bus voltage at point PCC; is a measured value;

[0074] - Equivalent impedance of the receiving end of the power grid before the fault;

[0075] - Vector form of the current flowing from the receiving-end power grid to the receiving-end converter station;

[0076] - The active power flowing through the PCC point is a measured value;

[0077] - The reactive power flowing through the PCC point is a measured value;

[0078] The power flowing through point PCC is ,but for:

[0079] (3)

[0080] by Using the phase as the reference phase, further eliminating phase angle information and ignoring the resistance in the high-voltage AC grid, the equivalent electromotive force of the receiving-end grid can be derived. Voltage at PCC point Power (active power flowing through the PCC) reactive power and the equivalent impedance of the receiving end of the power grid before the fault. Relationship:

[0081] (4)

[0082] in:

[0083] - Equivalent impedance of the receiving end of the power grid before the fault;

[0084] In equation (4), the equivalent impedance of the receiving end of the power grid before the fault is... Equivalent electromotive force of AC power grid The variable is unknown, and the equivalent electromotive force of the AC power grid before and after active power injection is unknown. They are approximately equal.

[0085] The derivation process of Equation (4) from Equation (2) is the existing technology.

[0086] By injecting two different active power at the PCC , And assume that the equivalent electromotive force of the receiving-end grid is within a short period before and after the active power injection. Without any change, the equivalent impedance of the receiving end of the power grid before the fault can be obtained by combining equation (4) with equation (6). Size and receiving-end grid equivalent electromotive force .

[0087] (6)

[0088] in, and These are the voltages at the grid connection points where two different active power outputs are injected, respectively.

[0089] Based on Kirchhoff's current law and Thevenin's equivalent model, under receiving-end grid faults, the parallel equivalent fault transition impedance is... Connected to the common coupling point between the MMC-HVDC and the receiving-end power grid, it is used to assess the impact of the short-circuit branch on the equivalent impedance of the receiving-end power grid. Under the condition of a short-circuit fault on the AC side of the receiving end, according to Kirchhoff's laws, we can obtain:

[0090] (7)

[0091] in,

[0092] This represents the AC bus voltage vector of the receiving-end converter station during a fault, and is a measured value.

[0093] The active power of the receiving-end converter station during the fault; is the measured value.

[0094] The reactive power of the receiving-end converter station during a fault; is a measured value.

[0095] AC bus voltage at the receiving end converter station The conjugate of; is the measured value;

[0096] - Receiver failure;

[0097] - Equivalent impedance of the connecting transformer; a measured value;

[0098] - Voltage vector at the fault point (PCC point); this is an unknown quantity; It is a modeling quantity, not a physical measurement quantity;

[0099] -PCC point fault status;

[0100] - Equivalent electromotive force of the receiving-end power grid; this is the value that has already been solved;

[0101] -The equivalent impedance of the receiving end of the power grid before the fault is the value that has been solved after combining equation (4) with equation (6);

[0102] - Parallel equivalent fault transition impedance; to be solved.

[0103] Depend on Figure 3 As shown, and Series, then parallel .

[0104] Equivalent electromotive force of receiving-end power grid The equivalent electromotive force of the receiving-end power grid can be approximately constant before and after the fault, and can be solved by equation (6). .

[0105] Eliminate the voltage vector at the fault point by combining equations (7). The parallel equivalent fault transition impedance can be obtained. for:

[0106] (8)

[0107] The fault resistance can be obtained by separating the real and imaginary parts of equation (8). and fault reactance .

[0108] (9)

[0109] (10)

[0110] 2. Fault analysis of the receiving end power grid.

[0111] This section addresses the issue of three-phase short-circuit faults occurring at the PCC point of the receiving-end system in flexible DC transmission. It establishes a fault safety domain for the receiving-end system, thereby providing a reference for the adaptive analysis and optimization of short-circuit current control strategies.

[0112] First, we analyze the contribution of MMC to the short-circuit current, taking the constant DC voltage and constant reactive power control of the receiving-end converter as an example.

[0113] The reactive power (Equation (11)) and the DC voltage mathematical model (Equation (12)) are defined as follows:

[0114] (11)

[0115] (12)

[0116] Equations (11) and (12) represent the active and reactive currents provided by the flexible DC power supply under the control and measurement of "constant reactive power and constant DC voltage".

[0117] in: - Shaft current reference value;

[0118] -Reactive power ratio coefficient;

[0119] -Reactive power reference value;

[0120] - Fixed reactive power integral coefficient;

[0121] - Shaft current reference value;

[0122] - DC voltage proportionality coefficient;

[0123] - DC voltage reference value; - DC voltage;

[0124] - DC voltage integral coefficient;

[0125] -Time variable.

[0126] When a three-phase short-circuit fault occurs at point PCC, the actual reactive power Q output by the receiving-end converter station drops instantaneously, while the reactive power reference value... As can be seen from equation (11), it remains unchanged. The absolute value will keep increasing, but due to the presence of the limiter, eventually... ,in for Axis limiter for amplitude limiting.

[0127] At the same time, the power output of the receiving-end converter station drops instantaneously, causing an imbalance between the input and output power of the receiving-end converter station, resulting in a rapid increase in DC voltage. According to equation (12), the reference current can be obtained. It will continue to rise;

[0128] However, due to the influence of the outer loop current limiter, the reference current will remain at the limit value. ,in for Axis limiter for amplitude limiting.

[0129] To ensure the converter valve does not overload, an inner-loop current limiter is installed in the control system. Therefore, the short-circuit current amplitude output by the MMC is limited. It should meet the following requirements:

[0130] when hour, ;

[0131] when hour, (13)

[0132] ;

[0133] - shaft current;

[0134] - shaft current;

[0135] and They are respectively shaft and The amplitude limit of the axis.

[0136] This refers to the amplitude of the inner loop current limiter. The inner loop current limiter is an internal module of the receiving-end converter station.

[0137] short-circuit current amplitude This refers to the current flowing from the converter station to the PCC power supply.

[0138] , , All are set values.

[0139] When a fault occurs in the receiving-end power grid, the input power of the DC power grid will exceed the output power, resulting in power imbalance. As the fault continues, power will accumulate, causing the DC voltage to rise rapidly. This may lead to DC grid overvoltage, and in severe cases, all converter stations will be shut down due to overvoltage, causing the entire DC power grid to shut down. The capacitor voltage of the converter station submodules will increase. The specific relationship can be expressed as follows:

[0140] (14)

[0141] - Active power input on the DC side; measured value;

[0142] - Active power output from the AC side; this is a measured value;

[0143] Equation (14) lays the foundation for solving equation (15). Equation (14) can be used to solve the minimum active current required to reduce the power imbalance of the converter station.

[0144] The process of deriving equation (15) from equation (14) is as follows:

[0145] (14)

[0146] Multiply both sides of the formula by :

[0147] ;

[0148] Both sides simultaneously adjust the time variable Integration, over a time interval integral:

[0149] ;

[0150] ;

[0151] The default is related to time variables. The function.

[0152] make ;

[0153] Orient the grid voltage vector to Axis, therefore

[0154] ;

[0155] make ;

[0156] ;

[0157] in: The time when the fault occurred; This is the time when the fault ends;

[0158] - Fault termination DC voltage at any given time; this is a measured value.

[0159] - DC voltage at the moment the fault occurred; this is a measured value.

[0160] - Active power input on the DC side; measured value;

[0161] - Active power output from the AC side; this is a measured value;

[0162] -The total equivalent capacitance of all energy storage capacitors inside the MMC is a known quantity.

[0163] - The minimum active current required by the converter station; is the solution value.

[0164] - represents the DC voltage change before and after the fault; is the calculated value.

[0165] - AC voltage of the receiving-end power grid; this is a measured value;

[0166] Duration of the fault; calculated value.

[0167] Equation (14) comprehensively describes the changes in the internal capacitor voltage of the MMC;

[0168] in:

[0169] To minimize the rise in DC voltage caused by unbalanced power, the active power of the receiving-end converter station should be increased. At this time, the active current under constant DC voltage control will continuously increase. However, due to the limitation of the limit value of the inner loop limiter, the active current cannot continue to increase with the continuation of the fault.

[0170] DC voltage after fault Not risen to maximum DC voltage In the second scenario, the minimum active current that the receiving-end converter station can provide is... From equation (14), we can obtain:

[0171] (15)

[0172] In equation (15), the number 2 represents the second case.

[0173] Here The minimum value is set to minimize the short-circuit current while meeting the requirement of reducing DC voltage over-limit.

[0174] The duration of the fault is represented by , and the measured value is .

[0175] This represents the change in DC voltage before and after the fault.

[0176] When the DC voltage rises to the maximum DC voltage In the first case, the active current that the receiving-end converter station can provide is limited by the maximum current. After determining the amount of reactive current required by the receiving-end grid, the maximum active current should be provided as much as possible to limit the rise of DC voltage, provided that the total line current does not exceed the capacity of the IGBT (Insulated Gate Transistor).

[0177] (16)

[0178] - Maximum active current;

[0179] - Limiting value of the inner loop current limiter;

[0180] - Axis current, i.e. ;

[0181] In equation (16), the number 1 represents the first case.

[0182] Although short-circuit current is the root cause of faults, the reactive power it generates plays a crucial role in maintaining the voltage of nearby busbars. Especially when connected to weak AC systems, the system's short-circuit capacity is small, and if the short-circuit current provided by the VSC is also small, the voltage will collapse more rapidly and severely during a fault. Most existing short-circuit current control strategies limit the current during faults, ensuring that the flexible DC transmission only sends active power and not reactive power. Since the receiving-end grid is a weak system with a large reactive power demand from the flexible DC transmission, this strategy will affect the system's voltage stability and post-fault recovery characteristics. Therefore, the short-circuit current control strategy must consider the actual grid structure and analyze the optimal control parameter values ​​for the receiving-end grid's recovery characteristics and adaptability when different offshore wind power flexible DC transmission systems experience faults.

[0183] When the voltage drop at the receiving end of the AC grid is severe, especially during a three-phase short-circuit fault, the voltage at that point (PCC point) will drop significantly. Modern power grid specifications and system stability requirements typically stipulate that MMC-HVDC systems must have low-voltage ride-through capability. This means that when the grid voltage drops to a certain level, the converter station cannot immediately trip but needs to remain connected to the grid and provide support to the grid.

[0184] This invention addresses the situation where a three-phase short-circuit fault occurs in the receiving-end power grid, resulting in a significant voltage drop at the fault point (PPC). When the short-circuit fault causes a substantial voltage drop, approaching a deep short circuit, priority should be given to meeting the reactive current demand to prevent voltage collapse. This is because only after the voltage stabilizes can the transmission of active power be restored. The minimum reactive current during the fault period is [the specific value is missing from the original text]. Determined by voltage deviation and grid short-circuit impedance:

[0185] (17)

[0186] ;

[0187] = ;

[0188] Here The minimum reactive current is set because, while meeting the requirements for reactive power support, the short-circuit current should not be increased as much as possible.

[0189] The reference voltage at the -PCC point is typically 0.9 pu;

[0190] - The voltage at which a fault occurs at the PCC point, i.e., the fault voltage at the PCC point; this is a measured value.

[0191] - After a fault occurs at point Pcc, the equivalent total impedance at point PCC is:

[0192] - Equivalent.

[0193] The derivation process of equation (17):

[0194] To describe the voltage-current relationship at the receiving-end converter station grid connection point (PCC point) during a short-circuit fault, the external AC power grid at the PCC point can be equivalently represented as a Thevenin equivalent power source in series with an equivalent impedance. Let the equivalent power source of the receiving-end power grid be... The equivalent impedance observed at point PCC during the fault is The current injected from the receiving-end converter station to the PCC point is (Right now Figure 3 In Then the voltage at point PCC The current satisfies:

[0195] ;

[0196] To facilitate control and analysis, a voltage-oriented synchronous rotation was established. Coordinate system to align the voltage vector at PCC point. When a short-circuit fault causes the voltage amplitude at the PCC point to drop to [a certain value], The voltage support target is defined as:

[0197] = - ;

[0198] Therefore, the injection current amplitude required to achieve the voltage support target is:

[0199] ;

[0200] = ;

[0201] The shaft current component can be taken as:

[0202] ;

[0203] ;

[0204] in, - Represents the injected current phasor The amplitude;

[0205] - The amplitude of the reactive current reference value during the fault;

[0206] - The minimum reactive current required for a severe voltage drop at the fault point;

[0207] - Target voltage, typically 0.9 pu;

[0208] - The voltage at which a fault occurs at the PCC point, i.e., the fault voltage at the PCC point; this is a measured value.

[0209] - This represents the equivalent total impedance at point PCC during the fault period;

[0210] , is the calculated value, i.e. and After parallel connection, it equals ;

[0211] - is the resistive component of the equivalent impedance;

[0212] - is the reactance component of the equivalent impedance;

[0213] From equations (9) to (10), the parallel equivalent fault transition impedance can be obtained. Substituting into equation (17), we can obtain the minimum value of the reactive current reference value at this time. .

[0214] In summary:

[0215] When a fault occurs at the PCC point, the reactive current support should be rationally allocated according to the voltage sag depth at the fault point. When the voltage sag is deep, the reactive current should be increased first to provide voltage support, followed by short-circuit current suppression strategies. Specifically:

[0216] First, calculate the minimum reactive current required by the power grid at this time according to equation (17). ,

[0217] The magnitude of the active current is then determined based on whether the DC voltage exceeds the limit.

[0218] When the DC voltage does not exceed the limit, the minimum active current required can be solved using equation (15). ;

[0219] When the DC voltage exceeds the limit, the maximum active current that the converter station can provide at this time can be solved using equation (16). .

[0220] By optimizing the limiting strategy described above, DC voltage can be prevented from exceeding the limit and reactive power support can be provided without exceeding the short-circuit current limit.

[0221] The currents obtained in both of the above two cases must satisfy equation (13). < .

[0222] Based on the above analysis, the fault current safety domain can be summarized as follows: (15)~(17)

[0223] (18)

[0224] (19)

[0225] in, Maximum DC voltage; Set value;

[0226] These are measured values;

[0227] 3. Fault current safety domain modeling.

[0228] Based on the aforementioned fault equivalent model (Equations (1)-(12)) and fault analysis conclusions (Equations (13)-(17)), a fault equivalent model can be established. Figure 4 The fault current safety domain shown can be obtained by combining the minimum reactive power region of voltage depth drop and the minimum active power region after voltage recovery.

[0229] "Short-circuit current safety domain under three-phase short-circuit fault", i.e. Figure 4 The region enclosed by points A, B, and C.

[0230] Under three-phase short-circuit faults, the current within the safe domain of the short-circuit current under three-phase short-circuit faults is selected as the current to be output by the MMC. The traditional short-circuit current limiting strategy has been optimized so that the optimized limiting control can not only ensure that the short-circuit current does not exceed the limit, but also reduce the accumulation of unbalanced power and increase reactive power support during the fault period.

[0231] Figure 4 middle:

[0232] Purple curve: Equation (13);

[0233] Red line segment: The minimum reactive current calculated by equation (17) ;

[0234] Green line segment: calculated by equation (15) ;

[0235] Blue dashed line: The range of current values ​​that meet both active and reactive power requirements. This is the AC / DC voltage safety domain.

[0236] Black dashed line: Feasible region for reactive current;

[0237] Green dashed line: Feasible region for active current;

[0238] Point A is the reference point for current before DC voltage exceeds the limit, and point B is the reference point where the current system can provide the maximum active power after DC voltage exceeds the limit. By solving for points A and B, the optimal active and reactive current reference values ​​are obtained, which can provide sufficient reactive power to support the voltage at the fault point when the voltage drops deeply, while avoiding DC voltage exceeding the limit.

[0239] The coordinates of point A and the reference current value at this time can be obtained by using equations (15) and (17).

[0240] Specifically, the x-coordinate of point A is obtained by equation (15), and the y-coordinate of point A is obtained by equation (17).

[0241] The x-coordinate of point B is obtained from equation (16), and the y-coordinate of point B is obtained from equation (17).

[0242] 4. Adaptability Analysis and Comparison of Control Strategies

[0243] Flexible DC transmission systems define safety boundaries by establishing a short-circuit current safety domain and ensure their effectiveness under complex operating conditions by analyzing the adaptability of control strategies. The combination of these two approaches can improve the system's fault ride-through capability, dynamic stability, and compatibility with diverse application scenarios (such as renewable energy grid connection and urban power supply), which is a key guarantee for the large-scale application of flexible DC technology.

[0244] Current research on short-circuit current control strategies for three-phase short-circuit faults on the AC side of flexible DC transmission mainly involves a combination of amplitude limiting control and low-voltage current limiting. Specifically, after a fault is detected, the d-axis and q-axis current reference values ​​of the inner-loop current controller are limited, usually set to fixed thresholds to prevent overcurrent. This is often achieved by reducing the reactive power reference value, thereby decreasing the magnitude of the short-circuit current contributed by the flexible DC transmission. However, simply limiting active and reactive currents cannot balance power surplus and reactive power support, and indiscriminately limiting the short-circuit current can lead to excessively slow grid recovery. Therefore, the parameters of the short-circuit current control strategy should be rationally designed for different fault scenarios when the receiving-end grid experiences a fault.

[0245] According to the short-circuit current safety domain designed in this invention, when a three-phase short-circuit fault occurs in the receiving-end power grid, the active and reactive currents are set to the d-axis and q-axis values ​​of the optimal current reference point (point A) when the DC voltage does not exceed the limit. This can effectively suppress the short-circuit current, reduce the impact of unbalanced power, and provide voltage support at the fault point, preventing grid collapse. If the DC voltage exceeds the limit, the active current reference value can be reasonably increased to suppress the rise in DC voltage, provided that the active power does not exceed the maximum active power that the converter station can provide. That is, selecting point B as the current reference point can effectively support the AC voltage and reduce the accumulation of unbalanced power.

[0246] The above are merely preferred embodiments of the present invention and do not constitute any limitation on the present invention. Any equivalent substitutions or modifications made by those skilled in the art to the technical solutions and content disclosed in the present invention without departing from the scope of the present invention shall be deemed to have remained within the protection scope of the present invention.

Claims

1. A multi-objective optimization solution method for short-circuit current control strategy in flexible DC transmission, characterized in that, Specifically, the following steps are included: Step 1: Establish a fault model of the receiving-end power grid, and solve for the equivalent impedance of the receiving-end power grid side before the fault in the fault model. Parallel fault transition impedance ; Step 2, combining parallel fault transition impedance Solve for the minimum reactive current required to support the AC voltage. When DC voltage Rise to maximum DC voltage The active current that the MMC of the front and rear receiving-end converter stations can provide; Step 3: Combine the minimum reactive current required to support the AC voltage. The active current that the receiving-end converter station MMC can provide. The short-circuit current amplitude constraint condition of the MMC output of the receiving-end converter station is used to obtain the short-circuit current safety domain under three-phase short-circuit fault. The receiving-end power grid fault model also includes: the equivalent electromotive force of the receiving-end power grid. ; The equivalent impedance of the receiving end of the power grid before the fault is calculated based on the principle of AC power grid equivalent impedance identification based on active power injection. ; In step 2, the minimum reactive current required to support the AC voltage is the reference value of the reactive current during the fault. for: ； - Reference voltage at PCC point; - Fault voltage at PCC point; -Equivalent impedance at point PCC during the fault; - Reactant component of equivalent impedance.

2. The multi-objective optimization solution method for the short-circuit current control strategy of flexible DC transmission according to claim 1, characterized in that, In step 3, the constraint condition for the short-circuit current amplitude output by the MMC of the receiving-end converter station is: ； - shaft current; - shaft current; - The amplitude of the inner loop current limiter.

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