A fault processing method for a hybrid DC transmission system with parallel connection of a generator and a network

Abstract

The application discloses a fault processing method for a hybrid DC transmission system of a net following and net constructing, and relates to the technical field of power system dynamic stability. The method of the application proposes an improved current limiting fault ride through control which sacrifices a part of transient reactive power support capability of GFM to increase a power factor angle on the basis of maintaining system operation on a current limiting boundary, and gives a boundary condition of corresponding control switching. The control strategy proposed by the embodiment of the method of the application is verified on a PSCAD simulation platform, and has good application necessity and reliability.

Description

Technical Field

[0001] This invention discloses a fault handling method for a hybrid grid-connected and DC-transmitted power system, relating to the field of power system dynamic stability technology. Background Technology

[0002] The "Shagohuang" region is rich in wind energy resources, but it is far from load centers. LCC-HVDC plays a significant role in long-distance inter-regional power transmission due to its characteristics such as large transmission capacity, strong power regulation capability, and relatively low cost.

[0003] Traditional grid-connected renewable energy base DC transmission systems are typically equipped with conventional power sources at the sending end to provide reactive power support for wind power transmission and commutation voltage for LCC-HVDC systems. However, due to their unique geographical location, the "desert desert" region cannot provide thermal power units as conventional power sources. Therefore, many scholars have proposed applying grid-based control strategies to address this issue. Compared to conventional GFL (Gas-Powered Fluid Transmission) grid-connected systems using phase-locked loops (PLLs), voltage-source grid-connected converters are not affected by PLLs and can actively provide voltage support. Currently, mainstream grid-connection methods include power synchronization control, power droop control, and virtual synchronization control. Among these, grid-connected energy storage based on virtual synchronization control shows promising prospects in solving the problem of high-proportion renewable energy grid connection and has become a research hotspot.

[0004] In existing technologies, although grid-based control utilizes the flexibility and controllability of power electronic devices to simulate the rotor motion equation of a synchronous generator and achieve reactive power and inertial support for large-scale grid connection of new energy sources, it is limited by the weak short-circuit current tolerance of power electronic converter equipment.

[0005] To address the aforementioned short-circuit current problem, many scholars have proposed current limiting mechanisms. However, these mechanisms cause significant differences in the transient characteristics of networked devices compared to synchronous machines. Some scholars have pointed out that directly limiting the inner current loop reference value would prevent the GFM from maintaining the networked characteristics of the voltage source, thereby worsening the system's transient stability margin.

[0006] Currently, how to effectively limit transient overcurrent problems while maximizing the potential of grid-based power regulation and maintaining its voltage source characteristics has become a research hotspot for grid-based control and grid connection, and has not yet been well solved.

[0007] While some scholars have proposed adaptive virtual impedance control strategies that can calculate the virtual impedance value of the sequence component based on the positive and negative sequence voltage drops to generate a virtual impedance coefficient to effectively limit the maximum short-circuit current, their ability to provide transient support to the power grid during faults has not been clearly defined.

[0008] Most studies focus on the scenario of a single GFM being connected to an infinite power grid, while the application of current limiting control in the scenario of GFM being transmitted via LCC-HVDC is rarely mentioned. Summary of the Invention

[0009] This invention addresses the problems of existing technologies by providing a fault handling method, electronic equipment, and computer storage medium for a hybrid GFL and GFM system connected via LCC-HVDC. The technical solution adopted is as follows:

[0010] A first aspect includes a fault handling method for a DC transmission system that is connected to and interoperates with a grid, the method comprising:

[0011] S1. Establish a hybrid power supply system with GFL and GFM converters connected in series via LCC – HVDC, and adopt the equivalent Thevenin model of voltage source series impedance at the receiving end.

[0012] S2, obtain the three active and reactive current distribution limit points of the grid converter during the commutation failure process;

[0013] S3, based on the allocation limit point, obtain the corresponding transient characteristics through the grid-type converter after DC commutation failure;

[0014] S4, Based on the hybrid output system, analyze the transient characteristics through the commutation failure stage;

[0015] S5. Substitute the current distribution angle into the GFM current command and boundary conditions to obtain an improved current limiting control diagram.

[0016] In some implementations, S2 specifically includes, based on the steady state of the hybrid transmission system, splitting the output currents of the sending-end GFL and GFM at the converter station, analyzing the relationship between the current splitting, power and power distribution coefficient of the output line, and obtaining the current distribution coefficient angle of the GFM output current.

[0017] In some implementations, S3 specifically includes:

[0018] S31, based on the current distribution coefficient angle, obtain the phasor relationship of the GFM converter under the critical state of the power distribution coefficient;

[0019] S32, calculate the angle of the GFM current distribution coefficient using the law of cosines, and obtain the GFM current command value at this time.

[0020] In some implementations, S32 specifically includes:

[0021] S321, when the current distribution coefficient angle is 90°, reactive power regulation is performed to the limit state through the GFM converter;

[0022] S322, when the current distribution coefficient angle is greater than 90°, the current limit control state is entered through the GFM converter;

[0023] S323, when the current distribution coefficient angle is less than 90°, the power distribution coefficient is overshooted through the GFM converter.

[0024] In some implementations, S4 specifically includes:

[0025] S41, Based on the transient characteristics, the distribution angle of the active and reactive power distribution limit point current during the commutation failure stage is obtained.

[0026] S42, based on the transient characteristics, the boundary condition formula for the active and reactive power distribution limit point current in the commutation failure stage is obtained.

[0027] In some implementations, S41 specifically includes, when the GFM enters current limiting control, increasing the current distribution coefficient angle while the operating point is at the current limiting boundary, to obtain a new current distribution coefficient angle.

[0028] In some implementations, S42 specifically includes,

[0029] S421, based on the new current distribution coefficient angle, the current command value is obtained through the output current of the GFM;

[0030] S423, Based on the current command value, obtain the rectifier-side converter bus voltage value of the GFM output current at the critical moment through boundary conditions.

[0031] Thirdly, embodiments of the present invention provide an electronic device, including a memory and a processor, wherein the memory is used to store one or more computer instructions, wherein when the one or more computer instructions are executed by the processor, they implement the method described in the first aspect above.

[0032] Fourthly, embodiments of the present invention provide a computer storage medium, wherein a computer program is stored in the computer-readable storage medium, and when the computer program is executed by a processor, it implements the method described in the first aspect.

[0033] One or more embodiments of the present invention can bring at least the following beneficial effects:

[0034] Traditional control of grid-connected converters is prone to power distribution coefficient overshoot during DC commutation failure, leading to delayed exit of current limiting control. The fundamental reason is that conventional GFM current limiting control can only passively meet the switching conditions by relying on changes in external grid electrical quantities to restore constant voltage / power control. However, the commutation failure timescale is relatively short, and delayed exit of current limiting can result in poor transient overvoltage suppression or even subsequent commutation failures. Therefore, this invention proposes an improved current limiting fault ride-through control that sacrifices a portion of the transient reactive power support capability of the GFM to increase the power factor angle while maintaining system operation within the current limiting boundary. The invention also provides corresponding boundary conditions for control switching. The implementation of this method on the PSCAD simulation platform demonstrates the proposed control strategy's strong applicability and reliability. Attached Figure Description

[0035] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0036] Figure 1 This is a structural diagram of a hybrid GFL and GFM system via LCC-HVDC provided in an embodiment of the present invention, wherein (a) is a system structural diagram and (b) is a simplified system topology diagram;

[0037] Figure 2 This is the system phasor relationship diagram provided in the embodiment of the present invention, wherein (a) is the system vector diagram and (b) is the vector diagram of the power reference decomposition result of the transmitting line;

[0038] Figure 3 This is provided by the embodiments of the present invention. =90° Power coefficient critical state GFM phasor relation schematic diagram;

[0039] Figure 4 This is provided by the embodiments of the present invention. Schematic diagram of the GFM phasor relationship at the critical state of power coefficient >90°;

[0040] Figure 5 This is provided by the embodiments of the present invention. Schematic diagram of the GFM phasor relationship at the critical state of power coefficient <90°;

[0041] Figure 6 This is provided by the embodiments of the present invention. CD Phase diagram of the system;

[0042] Figure 7This is provided by the embodiments of the present invention. DF Phase diagram of the system;

[0043] Figure 8 This is a system assembly diagram of the DC recovery process provided in the embodiment of the present invention; wherein (a) is a schematic diagram of the recovery progress not exceeding the current limit, and (b) is a schematic diagram of the recovery progress exceeding the current limit;

[0044] Figure 9 This is provided by the embodiments of the present invention. CD Phase-based improved control strategy GFM phasor diagram;

[0045] Figure 10 This is provided by the embodiments of the present invention. DF Phase-based improved control strategy GFM phasor diagram;

[0046] Figure 11 This is a schematic diagram of the improved GFM fault ride-through control provided in an embodiment of the present invention;

[0047] Figure 12 This is a simulation diagram of traditional GFM control provided in an embodiment of the present invention; wherein, (a) is a schematic diagram of transient overvoltage at the new energy generator end, (b) is a schematic diagram of DC current on the rectifier side, (c) is a schematic diagram of DC system turn-off angle, and (d) is a schematic diagram of GFM current limiting on / off status;

[0048] Figure 13 This is a schematic diagram for verifying the phasor relationship of electrical quantities under current limiting provided in an embodiment of the present invention; wherein, (a) is a schematic diagram of the active power of the GFM branch, (b) is a virtual internal potential, (c) is a schematic diagram of the voltage at the converter bus terminal, and (d) is a schematic diagram of the voltage at the GFM terminal.

[0049] Figure 14 This is a schematic diagram of the improved control simulation verification provided in an embodiment of the present invention;

[0050] Figure 15 This is a schematic diagram illustrating the improved GFM fault-crossing performance verification provided in this embodiment;

[0051] Figure 16 These are schematic diagrams of GFM fault-crossing simulation results under different fault types provided in this embodiment; where (a) is a schematic diagram of GFM deployment in the fault-crossing strategy proposed in this embodiment, and (b) is a schematic diagram of not deploying GFM equipment. Detailed Implementation

[0052] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. The components of the embodiments of the present invention described and shown in the accompanying drawings can generally be arranged and designed in various different configurations. Therefore, the following detailed description of the embodiments of the present invention provided in the accompanying drawings is not intended to limit the scope of the claimed invention, but merely to illustrate selected embodiments of the invention. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort are within the scope of protection of the present invention.

[0053] Example 1:

[0054] Figure 1 A structural diagram of a hybrid GFL and GFM system via LCC-HVDC is shown, as follows: Figure 1 As shown, the fault handling method for a DC transmission system that is hybrid with and connected to the grid, provided in this embodiment, includes:

[0055] S1. Establish a hybrid power supply system with GFL and GFM converters connected in series via LCC – HVDC, and adopt the equivalent Thevenin model of voltage source series impedance at the receiving end.

[0056] S2, obtain the three active and reactive current distribution limit points of the grid converter during the commutation failure process;

[0057] S3, based on the allocation limit point, obtain the corresponding transient characteristics through the grid-type converter after DC commutation failure;

[0058] S4, Based on the hybrid output system, analyze the transient characteristics through the commutation failure stage;

[0059] S5. Substitute the current distribution angle into the GFM current command and boundary conditions to obtain an improved current limiting control diagram.

[0060] Furthermore, S2 specifically includes, according to the steady state of the hybrid transmission system, splitting the output current of the sending end GFL and GFM at the converter station, analyzing the relationship between the current split, power and power distribution coefficient of the output line, and obtaining the current distribution coefficient angle of the GFM output current.

[0061] Furthermore, S3 specifically includes:

[0062] S31, based on the current distribution coefficient angle, obtain the phasor relationship of the GFM converter under the critical state of the power distribution coefficient;

[0063] S32, calculate the angle of the GFM current distribution coefficient using the law of cosines, and obtain the GFM current command value at this time.

[0064] Furthermore, S32 specifically includes:

[0065] S321, when the current distribution coefficient angle is 90°, reactive power regulation is performed to the limit state through the GFM converter;

[0066] S322, when the current distribution coefficient angle is greater than 90°, the current limit control state is entered through the GFM converter;

[0067] S323, when the current distribution coefficient angle is less than 90°, the power distribution coefficient is overshooted through the GFM converter.

[0068] Furthermore, S4 specifically includes:

[0069] S41, Based on the transient characteristics, the distribution angle of the active and reactive power distribution limit point current during the commutation failure stage is obtained.

[0070] S42, based on the transient characteristics, the boundary condition formula for the active and reactive power distribution limit point current in the commutation failure stage is obtained.

[0071] Furthermore, S41 specifically includes, when the GFM enters current limiting control, increasing the current distribution coefficient angle while the operating point is at the current limiting boundary, to obtain a new current distribution coefficient angle;

[0072] Furthermore, S42 specifically includes,

[0073] S421, based on the new current distribution coefficient angle, the current command value is obtained through the output current of the GFM;

[0074] S423, Based on the current command value, obtain the rectifier-side converter bus voltage value of the GFM output current at the critical moment through boundary conditions.

[0075] Example 2:

[0076] Based on Example 1, this example establishes a system where GFL and GFM converters are connected in parallel and then transmitted via LCC-HVDC, as shown below. Figure 1As shown in (a), the system topology adopts the Ximeng AC / DC hybrid topology of the North China Power Grid. The GFL / GFM converter is connected in parallel to the 220kV grid, stepped up to the 500kV grid, and then connected to the LCC-HVDC rectifier bus. There is no conventional power source at the sending end; it is supported nearby. The LCC-HVDC control system adopts the CIGRE-HVDC standard control mode, using a ±800kV bipolar DC transmission model with two sets of 12-pulse converter valves per pole. The receiving end uses the equivalent Thevenin model with voltage source series impedance.

[0077] Simplified system topology as follows Figure 1 As shown in (b), where, X M It is the sum of the reactance of the GFM transformer and the line reactance. X L For new energy step-up transformers and line reactors, X s For the equivalent reactance of transmission lines and transformers, E s The voltage amplitude within the equivalent electromotive force (EMF) of the receiving-end power grid is given, with its rotor angle δs as a reference value. The voltage at the GFM grid connection point is related to... δ s The relative angle is δ g The amplitude of the DC sending-end converter bus voltage is U Lr ,and δ s The relative angle is δ Lr .

[0078] When the system is in steady state, the output currents of the sending-end GFL and GFM are shunt at the converter station, and the following relationship applies:

[0079] (1)

[0080] In the formula The current flowing into the converter station, For AC line current, For GFM output current, This is the output current of the GFL. Considering that the renewable energy source only outputs active power during steady-state operation, i.e. and In phase, current limiting is equivalent to an adaptive increase in virtual impedance. Neglecting line resistance, in a synchronous rotating coordinate system, according to equation (1), the relationships between the phasors in the system are as follows: Figure 2 As shown in (a). Let the phasors be... Starting point O, ending point S, phasor The endpoint is H. At this point, the active power Pg transmitted via GFM is:

[0081] (2)

[0082] In the formula U g sin( δ gl ) Direction is Figure 2 (a) The pink dashed arrow indicates that the active power transmitted by the AC system can be multiplied by α. U g sin( δ gl This indicates that its trend is consistent with the trend of power transmission in a DC system. Shifting this phasor to the origin, as shown... Figure 2 As shown in (b), using this phasor as a reference, the phasor of the sending system can be... Figure 2 (b) Combined with a power-characterizing DC system. In this case, the sending-end system exchanges reactive power. Q g for:

[0083] (3)

[0084] In the diagram U gN The dashed arc represents the GFM reactive power-voltage droop control reference, SH and U gN The arcs intersect at a point T (N) Intersects the current limiting circle at point T (k) Let K be the power distribution coefficient of the GFM output line. Then, according to equations (2) and (3), in steady state... K N for:

[0085] (4)

[0086] It can be seen K N for OS and SH The absolute value of the tangent of the included angle, according to Figure 2 (b) This angle is simultaneously dq Shaft current distribution angle, in order to φ GFM This indicates that during the transient period, GFM adjusts... E ref , θ ref Adjust the output P g and Q g This will affect K The size, further discussion under existing control K The regulatory boundaries.

[0087] a) φ GFM =90°

[0088] when φ GFM When the angle is 90°, the denominator of K equals 0, reaching its maximum value. The corresponding electrical quantity topology is as follows: Figure 3 As shown, this is equivalent to the external circuit power deficiency causing GFM reactive power overshoot, the q-axis current reaching the output limit, and the current distribution coefficient angle reaching 90 degrees. When GFM enters current limiting control, according to equation (1), the q-axis output is limited to... I lim The reactive power regulation capability of GFM reaches the limit under current limiting control.

[0089] b) φ GFM >90°

[0090] when φ GFM When the angle is greater than 90°, the maximum value of K varies with... φ GFM As the value decreases, the value increases, therefore its maximum value is infinitely close to φGFM=90°, while its minimum value appears at... U g sin δ gl When =0, the corresponding electrical quantity topology is as follows: Figure 4 As shown, at this point, the value of K is infinitely close to 0, and GFM will inevitably enter current limiting control. T (k) When the circuit moves to the horizontal axis, the d-axis current output reaches its limit.

[0091] c) φ GFM <90°

[0092] when φ GFM When the angle is less than 90°, according to the Law of Cosines, we have:

[0093] (5)

[0094] When GFM enters current limiting control ST (k) The minimum value is I lim X M Equation (5) can be rearranged to obtain:

[0095] (6)

[0096] Then when U Lr cos δ lg - U g When = 0, equation (4-123) reaches its minimum value. I lim X M ) 2 . U Lr cos δ lg - U g =0 can be rewritten in the following form:

[0097] (7)

[0098] when U g Values U gN ,Right now T (N) and T (k) During reconciliation, reactive power-voltage arc U gN At T (N)(k) The tangent at a point can be represented by the following formula:

[0099] (8)

[0100] Substitute equation (8) into S Point coordinates ( U Lr ,0) is exactly the same as the form of equation (4-124), therefore when U Lr cos δ gl - U gN When =0, the corresponding electrical quantity topology is as follows: Figure 5 As shown, its physical meaning is equivalent to SH With arc U gN Tangent to T (N)(k) At the same time K molecules U g sin δ lg The value is the maximum value, therefore K Reaching the control limit is equivalent to the external circuit having a power surplus, causing the GFM power factor to overshoot. qThe shaft current reaches the output limit. Based on Example 1, the entire commutation failure process is divided into three stages: AC, CD, and DF. According to the above conclusions, the interaction between the rectifier-side AC system and DC system is most intense in the DF stage during the entire commutation failure process. The AC stage has a small time scale, usually in the millisecond range, and the rectifier-side control action is relatively simple. Therefore, we will only focus on analyzing the system interaction process in the CD and DF stages.

[0101] The DC system current increases at point T, leading to a significant increase in reactive power consumption by the rectifier and causing voltage drops on the rectifier-side converter bus. U Lr A sudden drop, at which point:

[0102] (9)

[0103] According to equation (9), the reactive current of the GFM branch increases rapidly. U g The decrease, according to equation (9), E GFM This reduces the reactive current to suppress its rapid rise, thus providing voltage support and suppressing its rapid increase. U g Reduced. However, as the fault persisted, I M As the current gradually increases, the GFM enters current limiting mode and loses its ability to regulate power and voltage.

[0104] During the CD phase, the DC current decreases, and the current rises at the sending-end converter bus. U Lr Gradually rise, in the arc U gN During the process of separation from the current-limiting circle, T (k) and T (N) Two points at Figure 5 At this point, the GFM power factor reaches its maximum value, as analyzed earlier. This will continue in the subsequent... U Lr As it continues to rise, such as Figure 6 As shown, T (k) Gradually increase, T (k) S The dashed line represents the maximum power factor regulation value at this point. Its intersection with the current limiting point is located outside the reactive power-voltage loop, therefore, in subsequent... U Lr During the continuous increase, regardless of how the GFM power factor is adjusted, it cannot return to the reactive power-voltage circle. E GFM As it continues to decrease, its terminal voltage Ug Only with U Lr A rapid increase in elevation leads to U g and E GFM The difference increases again, at which point the GFM current limiting cannot be deactivated, significantly limiting the GFM's ability to suppress transient overvoltages at the sending end. Until... U g Value close to U Lr When the current limiting control reaches the activation / deactivation condition, the GFM returns to constant voltage / power control. At this point, the GFM's ability to regulate transient overvoltages is extremely limited.

[0105] In the DE stage, U g and U Lr Approaching the GFM line I Md , I Mq Lower, according to equation (4-115), E GFM Increase to improve output reactive current and suppress voltage. U Lr Increase, at the same time δ GFM Increasing the swing angle increases the active current of the line. As analyzed above, the high active current deviation during the commutation failure (CD) phase of the GFL causes power oscillations. Therefore, the GFM can alleviate the power fluctuations caused by the GFL during the CD phase and suppress subsequent commutation failures induced by inverter-side control switching anomalies due to rectifier-side power fluctuations. However, at this time, the external circuit has not yet returned to its steady-state value, and with... E GFM Increase U g The reduction is due to the fact that the external circuit DC system has not yet completed the constant off-angle control switching, and the DC system has reached the fifth reactive power change stagnation point. E dr(di) forward, E GFM It can only continue to increase, and U g As the current decrease continues, GFM once again enters current limiting control. During the EF phase, U Lr Reduce, corresponding to OS reduction, in arc U gN During the process of intersecting with the current limiting circle, the point where its output power coefficient is at its maximum is Figure 3 Obtained from there, after T (N) rise and T(k) reduce, T (k) It will be located within the reactive power-voltage regulation loop, such as Figure 7 As shown, GFM consistently experiences a reactive power deficit. E GFM Increase I Mq Increase ST (k) Towards ST (N) Close, however, due to ST (N) Located inside the reactive power-voltage circle and subject to current limiting, GFM cannot recover to the reactive power-voltage circle, therefore E GFM As the current continues to increase, the current limiting cannot be deactivated.

[0106] During the EF phase, the DC system enters current deviation control. When the DC system power recovers to point N, it is assumed that the active power of the GFM line and the current limiting point intersect at this time. N 1. However, according to the above analysis, the DC system reaches the reactive power change stagnation point at this time, the reactive power consumption decreases, and the point on the current limiting circle that satisfies its reactive power change is... N Position 2, therefore T (k) The point must be on the arc N 1 N 2. Sliding back and forth causes power oscillation, such as Figure 8 As shown in (a), when the DC power further recovers to position M, there is no intersection between the active power and current limiting of the GFM line. T (k) Once an equilibrium point is no longer available, the system can only perform circular motion, leading to even greater power oscillations, such as... Figure 8 As shown in (b), based on the analysis above, continuous power oscillations cause abnormal switching of inverter-side control, resulting in a continuous decrease in the lead-out trigger angle, which in turn leads to subsequent commutation failure. At this time, the power loop control of the GFM actually limits the DC system fault recovery process.

[0107] In summary, during the entire commutation failure process of a conventional GFM / GFL hybrid LCC-HVDC system, GFM can effectively provide low-voltage support, alleviate the power difference at the sending end, and suppress subsequent commutation failures induced by inverter-side control switching anomalies caused by electrical quantity fluctuations. However, it has the following two limitations: 1) The current limiting is not promptly deactivated after providing low-voltage support, resulting in poor transient overvoltage suppression; 2) The current limiting is not promptly deactivated during DC system fault recovery, which not only fails to suppress subsequent commutation failures induced by power fluctuations but also triggers subsequent commutation failures. The root cause of these two limitations is that GFM current limiting has clear activation conditions. However, after the current limiting is activated, it can only passively meet the activation / deactivation conditions by relying on changes in the electrical quantity of the external power grid to restore constant voltage / power control. The time scale of commutation failure is relatively small, and the activation time of the current limiting is very likely to coincide with the DC system fault recovery CC control and the CC-CEA control switching process. Oscillating components will be transmitted to the inverter-side control system during this stage, ultimately causing more serious subsequent commutation failures.

[0108] Based on the transient characteristics mentioned above, the GFM fault ride-through control strategy applicable to the DC commutation failure timescale is as follows:

[0109] 1) During the CD stage, the equilibrium point of the control system is at Figure 9 middle T (k) The system's operating point exceeds the reactive power-voltage control boundary, causing the internal potential to continuously decrease. Simultaneously, the terminal voltage continuously increases with the external circuit's electrical quantities, making it difficult to exit the current limiting mechanism. Therefore, the system equilibrium point is changed to point T(s). First, we discuss changing the system operating point to... T (s) The feasibility of this. T (s) It is an arc U gN The intersection with the current limiting condition satisfies the requirement for stable reactive-voltage operation of GFM, while... H (s) Running in O With the center of the circle, OH (s) On an arc of radius , therefore OH (s) S This satisfies system stability requirements. At this point, it's equivalent to appropriately increasing the reactive current of the GFM line and decreasing its active current, based on the original electrical quantities of the system. While maintaining system operation within the current limiting boundary, it sacrifices some of its reactive power support capacity to increase the current distribution coefficient angle and suppress... E GFM The continuous reduction and U gThe current continues to increase. At this point, the GFM current distribution coefficient angle can be obtained using the cosine theorem:

[0110] (10)

[0111] At this time, the GFM current command value can be expressed by the following formula:

[0112] (11)

[0113] Next, we will discuss the boundary conditions under this control mode. Based on the above analysis,

[0114] when T (k) Point and T (k) S When the reactive power-voltage arc tangent point coincides with the critical point, the GFM reaches its critical value, and therefore the value at this point can be calculated. U Lr(1) The possible values ​​are:

[0115] (12)

[0116] Then at this time U Lr(1) The boundary is:

[0117] (13)

[0118] 2) During the DF phase, the equilibrium point of the control system is at Figure 10 middle T (k) The system's internal potential continues to increase after its operating point lags behind the reactive power-voltage control boundary. U g As the current continues to decrease, it cannot be released from the current limiting state until the external circuit returns to steady state. Therefore, the system operating point is changed to... T (s) The feasibility analysis will not be elaborated further, but the goal is to suppress the continuous increase of internal potential. At this point, the GFM current distribution coefficient angle is:

[0119] (14)

[0120] Based on the above analysis, when T (k) Point and T (k) S Perpendicular to OS At this point, GFM reaches a critical value, therefore the value at this time can be calculated. U Lr(2) The possible values ​​are:

[0121] (15)

[0122] Then at this time U Lr(2) The boundary is:

[0123] (16)

[0124] Therefore, the overall improvement strategy is implemented as follows: Figure 11 As shown;

[0125] according to Figure 11 As can be seen, the control method proposed in this invention adaptively adjusts the power distribution coefficient by extracting the terminal voltage value. In the overall control loop, only during the conversion of the transient voltage signal into a current signal does the presence of the PI link generate a small system response time constant, and the system current limiting response time can be controlled in the millisecond range. The control timing diagrams for each stage of GFL, GFM, and LCC-HVDC are shown in Appendix C. It can be seen that using the conventional virtual impedance method, due to the simultaneous PI delay in both the current and voltage loops, and even the inability of the feedback signal to change abruptly in time when extracting the port fault current signal due to the presence of inductive components in the line, the response time of this method is typically in the tens to hundreds of milliseconds range. However, commutation failure is a hundreds of milliseconds-level electromagnetic transient process, in which the CC and CEA switching process during transient recovery typically lasts only tens of milliseconds. Applying the conventional virtual impedance method would result in the DC system switching to the next state before the limiting link has responded, and the GFM cannot quickly track changes in the system's electrical quantities, causing the current to fail to be successfully limited or even affecting the system recovery process. Therefore, for the commutation failure time scale, the improved current limiting control response speed proposed in this invention is more suitable.

[0126] Example 3:

[0127] This embodiment also provides an electronic device, including a memory and a processor, wherein the memory is used to store one or more computer instructions, wherein the one or more computer instructions are executed by the processor to implement the method of Embodiment 1;

[0128] In practical applications, the processor can be implemented as an Application Specific Integrated Circuit (ASIC), Digital Signal Processor (DSP), Digital Signal Processing Device (DSPD), Programmable Logic Device (PLD), Field Programmable Gate Array (FPGA), controller, microcontroller unit (MCU), microprocessor, or other electronic components to execute the methods described in the above embodiments.

[0129] The method implemented in this embodiment is as described in Embodiment 1.

[0130] Example 4:

[0131] This embodiment also provides a computer storage medium, in which a computer program is stored, and when the computer program is executed by one or more processors, it implements the method of embodiment one.

[0132] The computer-readable storage medium can be implemented by any type of volatile or non-volatile storage device or a combination thereof, such as Static Random Access Memory (SRAM), Electrically Erasable Programmable Read-Only Memory (EEPROM), Erasable Programmable Read-Only Memory (EPROM), Programmable Read-Only Memory (PROM), Read-Only Memory (ROM), magnetic storage, flash memory, magnetic disk, or optical disk.

[0133] The method implemented in this embodiment is as described in Embodiment 1.

[0134] Based on the characteristics revealed above and the proposed improved fault ride-through control strategy, a full electromagnetic simulation was performed using PSCAD to verify the results. The test system topology is shown below. Figure 12As shown in (a). The AC system SCR=1.5, a three-phase short-circuit fault is set on the inverter side at 2.5s, the duration is 0.06s, the transition resistance is 100Ω, and five operating conditions are set to verify the correctness of the theory proposed in this invention and the effectiveness of the proposed strategy.

[0135] Operating Condition 1: The GFL's operational power (Pw) is 200MW. Record the relevant electrical quantities with and without a 50MW GFM (Gross Flow Meter) in operation, where the GFM uses a conventional current limiting strategy. With the GFM in operation, the total power at the sending end is 250MW, and the GFM:GFL ratio is 1:4.

[0136] The simulation results for working condition 1 are as follows: Figure 12 , Figure 13 As shown, according to Figure 12 The simulation system shows that... t At moment 1, a high-low voltage ride-through occurs, and the GFM fails to disengage from current limiting control in time. This results in a significant effect on transient low voltage support, but a weak effect on transient overvoltage suppression. Figure 12 As shown in (a). The simulation system is in t At time 2, the circuit enters the CEC phase, switching between CC and CEA. At this point, electrical quantities have largely recovered, and the current limiting is delayed again before exiting, ultimately... t At time 3, a subsequent commutation failure occurred. After the subsequent commutation failure, the power grid electrical quantity changed, and the GFM was released from current limiting control, proving the correctness of the transient characteristic analysis of the system and the characterization of the dynamic process of commutation failure described above in this invention.

[0137] according to Figure 13 It can be observed that even after the GFM enters current limiting mode, it still retains a certain degree of regulatory capability over the electrical quantities of the external circuit, such as... Figure 13 As shown in (a) and (d), the active power of the GFM branch continues to increase slowly, and the terminal voltage drop is not significant. However, during the high and low voltage fault crossing of the external circuit, the GFM terminal voltage increases instantaneously with the electrical quantity of the external circuit, and the active power loses its regulation capability, proving that the power distribution coefficient is overshooting. Meanwhile, the EGFM continues to decrease, so the current limiting cannot be released, which is consistent with the above analysis.

[0138] Quantitative calculation of the phasor relationship of electrical quantities at this instant. actual U Lr =1.1263. After the second overshoot of the power distribution factor, the terminal voltage dropped. E GFM However, the current continued to rise, the current limit could not be released, the power oscillated, the oscillation amplitude gradually increased, and finally the second commutation failure occurred, which is consistent with the previous analysis.

[0139] The phasor relationship of an electrical quantity is calculated by quantizing the instant at which the inflection point occurs. ,actual U Lr =1.0377. The above quantitative calculation ignores the influence of reactive power, so there is a certain error, but it proves the correctness of the electrical quantity relationship analysis of this invention. Operating condition 2 of this invention: Based on operating condition 1, the GFM is equipped with the control strategy proposed in this invention, and the relevant electrical quantities are recorded. The total power at the sending end remains unchanged at 250MW, and the ratio of GFM to GFL is 1:4.

[0140] The simulation results for working condition 2 are as follows: Figure 12 The control strategy proposed in this invention allows the GFM to quickly break free from current limiting control during the CD stage, achieving a better transient overvoltage suppression effect. During the DF stage, before the DC system enters the current deviation stage, it breaks free from current limiting control again, avoiding subsequent commutation failure.

[0141] Operating Condition 3: Simulations were performed on the electrical quantities of the sending-end system under three conditions: when the control strategy proposed in this invention is applied, when the conventional current limiting control strategy is adopted, and when the GFM is not applied. The GFL applied power Pw is 100MW, 300MW, 500MW, 750MW and 1000MW, the GFM applied power is 100MW, and the GFM:GFL ratios are 1 / 1, 1 / 3, 1 / 5, 1 / 7.5 and 1 / 10 respectively.

[0142] The simulation results for working condition 3 are as follows: Figure 13 .according to Figure 13 (a) As can be seen, after implementing the control strategy proposed in this invention, the effect of GFM in suppressing transient overvoltage is fully utilized, greatly suppressing the power oscillation of conventional converters and subsequent commutation failures caused by commutation failures. Although GFM under conventional current limiting also has a certain ability to suppress subsequent commutation failures caused by power oscillations at the sending end, its suppression effect is poor, and its effect on suppressing transient overvoltages is also poor. At the same time, when the grid-connected power of GFL is small, it is easy to cause subsequent commutation failures, which is consistent with the above analysis. Under the condition of not implementing GFM at all, as the grid-connected power of GFL gradually increases, its power oscillation becomes more and more violent. After one commutation failure occurs, it causes multiple commutation failures again, resulting in repeated high and low voltage ride-throughs at the sending end, which greatly limits the ability of LCC-HVDC to transmit new energy. Therefore, the control strategy proposed in this invention has good application necessity and reliability.

[0143] Operating Condition 4: The inverter side fault scenarios are set as three-phase short circuit, single-phase grounding, two-phase phase-to-phase, and two-phase grounding, respectively. The GFL input power Pw is 700MW. Simulations are performed on the 100MW GFM with the control strategy proposed in this invention and the scenario without GFM.

[0144] The simulation results for working condition 4 are as follows: Figure 16 .according to Figure 16 It can be seen that regardless of the fault type on the inverter side, when the grid-connected power of the rectifier-side GFL is high, power oscillation will be triggered, leading to subsequent commutation failure. Since the nature of this problem is the same as the research scenario of this invention, the control strategy proposed in this invention can still effectively suppress the subsequent commutation failure caused by transient overvoltage and power oscillation.

[0145] In the several embodiments provided in this invention, it should be understood that the disclosed systems and methods can also be implemented in other ways. The system and method embodiments described above are merely illustrative.

[0146] It should be noted that, in this invention, the terms "first," "second," etc., in the description, claims, and accompanying drawings of the method of this invention are used to distinguish similar objects and are not necessarily used to describe a specific order or sequence. The terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or apparatus. Without further limitations, an element defined by the phrase "comprising one..." does not exclude the presence of other identical elements in the process, method, article, or apparatus that includes said element.

[0147] While the embodiments disclosed in this invention are as described above, the content is merely for the purpose of facilitating understanding of the invention and is not intended to limit the invention. Any person skilled in the art to which this invention pertains may make any modifications and variations in form and detail of the implementation without departing from the spirit and scope disclosed herein; however, the scope of patent protection for this invention shall still be determined by the scope defined in the appended claims.

Claims

1. A fault handling method for a DC transmission system that is hybrid with and connected to the grid, characterized in that, The method includes: S1. Establish a hybrid power supply system with GFL and GFM converters connected in series via LCC – HVDC, and adopt the equivalent Thevenin model of voltage source series impedance at the receiving end. S2, obtain the three active and reactive current distribution limit points of the grid converter during the commutation failure process; S3, based on the allocation limit point, obtain the corresponding transient characteristics through the grid-type converter after DC commutation failure; S4. Based on the hybrid power transmission system established in S1, analyze the changing characteristics of the power distribution coefficient angle of the grid converter and the coupling relationship of transient electrical quantities during the voltage recovery stage and DC power recovery stage of the sending-end converter bus after DC commutation failure, and obtain the current distribution coefficient angle, GFM current command and control switching boundary conditions for the corresponding stage. S5, Substitute the current distribution coefficient angle into the GFM current command formula and the boundary conditions to obtain the improved current limiting control diagram; S3 includes: S31, based on the current distribution coefficient angle, obtain the phasor relationship of the GFM converter under the critical state of the power distribution coefficient; S32, calculate the GFM current distribution coefficient angle using the law of cosines, and obtain the GFM current command value at this time; S32 includes: S321, when the current distribution coefficient angle is 90°, reactive power regulation is performed to the limit state through the GFM converter; S322, when the current distribution coefficient angle is greater than 90°, the current limit control state is entered through the GFM converter; S323, when the current distribution coefficient angle is less than 90°, the power distribution coefficient is overshooted through the GFM converter; S4 includes: S41, based on the transient characteristics, obtain the current distribution angle of the active and reactive power distribution limit point during the commutation failure stage; S41 includes, when the GFM enters the current limiting control, while the operating point is at the current limiting boundary, the current distribution coefficient angle is increased to obtain a new current distribution coefficient angle. S42, based on the transient characteristics, obtain the boundary condition formula for the active and reactive power distribution limit point current during the commutation failure stage; S42 includes, S421, based on the new current distribution coefficient angle, obtain the current command value through the GFM output current; S423, Based on the current command value, obtain the rectifier-side converter bus voltage value of the GFM output current at the critical moment through boundary conditions.

2. The method according to claim 1, characterized in that, S2 specifically includes, according to the steady state of the hybrid transmission system, splitting the output current of the sending end GFL and GFM at the converter station, analyzing the relationship between the current split, power and power distribution coefficient of the output line, and obtaining the current distribution coefficient angle of the GFM output current.

3. An electronic device, characterized in that, The system includes a memory and a processor, the memory being used to store one or more computer instructions, wherein the one or more computer instructions, when executed by the processor, implement the method as described in any one of claims 1-2 above.

4. A computer-readable storage medium, characterized in that, The computer-readable storage medium contains a computer program that, when executed by a processor, is used to implement the method as described in any one of claims 1-2.

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