Fault processing method for following network and constructing network series-parallel direct current sending-out system

Abstract

The invention discloses a fault processing method for a network following and network construction hybrid direct current transmission system, and relates to the technical field of power system dynamic stability. According to the method disclosed by the invention, on the basis of maintaining the operation of the system at a current amplitude limiting boundary, a part of transient reactive support capability of the GFM is sacrificed to improve current amplitude limiting fault ride-through control of a power coefficient angle, and corresponding boundary conditions for controlling switching are given; a PSCAD simulation platform verifies that the control strategy provided by the embodiment of the method has good application necessity and reliability.

Description

Technical Field

[0001] The present invention discloses a fault handling method for a hybrid series and parallel DC transmission system for grid-following and grid-forming, which relates to the technical field of power system dynamic stability. Background Art

[0002] The commissioning of large-scale new energy wind power bases in deserts, gobi, and wastelands has become a key point in the country's new energy development. The "desert-gobi-wasteland" region is rich in wind energy resources but far from load centers. LCC-HVDC plays a significant role in long-distance cross-regional power transmission due to its large transmission capacity, strong power regulation ability, and relatively low cost.

[0003] In the traditional DC transmission system at the sending end of a grid-following new energy base, conventional power sources are often equipped to provide reactive power support for wind power transmission and commutation voltage for the LCC-HVDC system. However, due to the special geographical location of the "desert-gobi-wasteland" region, thermal power units cannot be provided as conventional power source support. For this reason, a large number of scholars have proposed applying grid-forming control strategies to solve this problem. Compared with the conventional GFL application of a phase-locked loop grid-connected system, the voltage-source grid-forming converter is not affected by the phase-locked loop and can actively provide voltage support. Currently, the mainstream grid-forming methods include power synchronization control, power droop control, and virtual synchronization control. Among them, the grid-forming energy storage based on virtual synchronization control has good prospects in solving the problem of high-proportion renewable energy grid connection and has become a research hotspot.

[0004] In the prior art, although the grid-forming control uses the flexibility and controllability of power electronic devices to simulate the rotor motion equation of a synchronous generator to achieve reactive power and inertia support for large-scale new energy grid connection, it is limited by the problem of weak short-circuit current tolerance of power electronic converters.

[0005] To solve the above short-circuit current problem, many scholars have proposed a current limiting link. However, this link causes a large difference in the transient characteristics between the grid-forming device and the synchronous machine. Some scholars have pointed out that directly restricting the reference value of the current inner loop will cause the GFM to be unable to maintain the grid-forming characteristics of the voltage source, thereby deteriorating the system transient stability margin.

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

[0007] The adaptive virtual impedance control strategy proposed by some scholars can effectively limit the maximum short-circuit current by calculating the sequence component virtual impedance value based on the positive and negative sequence voltage dips to generate a virtual impedance coefficient, but it does not clarify its transient support ability for the power grid during a fault.

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

[0009] In view of the problems of the prior art, the present invention provides a fault handling method, an electronic device, and a computer storage medium for a hybrid connection of GFL and GFM via an LCC-HVDC system. The technical solution adopted is as follows:

[0010] In a first aspect, a fault handling method for a hybrid connection of grid-connected and network-forming via a DC transmission system, the method comprising:

[0011] S1, establish a hybrid connection of GFL and GFM converters via an LCC–HVDC as a hybrid transmission system, and adopt an equivalent Thevenin model with a voltage source in series with an impedance at the receiving end;

[0012] S2, obtain the distribution limit points of the three active currents and reactive currents of the network-forming converter during the commutation failure process;

[0013] S3, according to the distribution limit points, obtain the corresponding transient characteristics through the network-forming converter after DC commutation failure;

[0014] S4, according to the hybrid transmission system, analyze the transient characteristics according to the stages of commutation failure;

[0015] S5, substitute the current distribution angle into the GFM current command formula and boundary conditions to obtain an improved current limit control diagram.

[0016] In some implementation manners, S2 specifically includes, when the hybrid transmission system is in a steady state, shunt the output currents of the sending-end GFL and GFM at the converter station, analyze the relationship between the current shunt, power, and power distribution coefficient of the output line, and obtain the current distribution coefficient angle of the GFM output current.

[0017] In some implementation manners, S3 specifically includes:

[0018] S31, according to the current distribution coefficient angle, obtain the phasor relationship of the GFM converter in the critical state of the power distribution coefficient;

[0019] S32, use the cosine theorem to calculate the GFM current distribution coefficient angle and obtain the GFM current command value at this time.

[0020] In some implementation manners, S32 specifically includes:

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

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

[0023] S323: When the current distribution coefficient angle is less than 90°, cause the power distribution coefficient to overshoot through the GFM converter.

[0024] In some implementation manners, S4 specifically includes:

[0025] S41: Obtain the distribution angle of the active and reactive power distribution limit point currents in the commutation failure stage according to the transient characteristics;

[0026] S42: Obtain the boundary condition formula of the active and reactive power distribution limit point currents in the commutation failure stage according to the transient characteristics.

[0027] In some implementation manners, S41 specifically includes that when the GFM enters the current limit control, increase the current distribution coefficient angle while the operating point is at the current limit boundary to obtain a new current distribution coefficient angle.

[0028] In some implementation manners, S42 specifically includes:

[0029] S421: Obtain a current command value through the output current of the GFM according to the new current distribution coefficient angle;

[0030] S423: Obtain the commutation bus voltage value on the rectifier side when the output current of the GFM is critical according to the current command value through the boundary conditions.

[0031] In a third aspect, an embodiment of the present invention provides an electronic device, including a memory and a processor, where the memory is used to store one or more computer instructions, and when the one or more computer instructions are executed by the processor, the method described in the first aspect above is implemented.

[0032] In a fourth aspect, an embodiment of the present invention provides a computer storage medium, where a computer program is stored in the computer-readable storage medium, and when the computer program is executed by a processor, the method described in the first aspect is implemented.

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

[0034] In the traditional control of the network-forming converter, during the process of DC commutation failure, the power distribution coefficient is prone to overshoot, resulting in a delay in the exit of the current limiting control. The essential reason is that the conventional GFM current limiting control can only passively meet the switching conditions by relying on the change of the external power grid electrical quantities to restore the constant voltage / power control. Moreover, the time scale of commutation failure is relatively small, and the delay in the exit of the current limiting is likely to cause poor transient overvoltage suppression effect or even lead to subsequent commutation failures. Therefore, the method of the present invention proposes an improved current limiting fault ride-through control that sacrifices a part of the transient reactive power support ability of the GFM to increase the power coefficient angle on the basis of maintaining the system operation at the current limiting boundary, and gives the boundary conditions for the corresponding control switching. The control strategy proposed in the embodiments of the method of the present invention is verified on the PSCAD simulation platform to have good application necessity and reliability. Description of the Drawings

[0035] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required for the description of the embodiments or the prior art. Obviously, the drawings in the following description are some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can also be obtained based on these drawings.

[0036] Figure 1 It is a structural diagram of a system for the parallel connection of GFL and GFM via the LCC-HVDC system provided by an embodiment of the present invention, where (a) is the system structural diagram and (b) is the simplified system topology diagram; Figure 2 It is the phasor relationship diagram of the system provided by an embodiment of the present invention, where (a) is the system vector diagram and (b) is the vector diagram of the decomposition result of the sending line power reference;

[0037] Figure 3 It is provided by an embodiment of the present invention Schematic diagram of the GFM phasor relationship in the critical state of the power coefficient;

[0038] Figure 4 It is provided by an embodiment of the present invention Schematic diagram of the GFM phasor relationship in the critical state of the power coefficient;

[0039] Figure 5 It is provided by an embodiment of the present invention Schematic diagram of the GFM phasor relationship in the critical state of the power coefficient;

[0040] Figure 6 It is the system phasor relationship diagram in the CD stage provided by an embodiment of the present invention;

[0041] Figure 7 It is the system phasor relationship diagram in the DF stage provided by an embodiment of the present invention;

[0042] Figure 8 It is a combined diagram of the DC recovery process system provided by an embodiment of the present invention; among them, (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;

[0043] Figure 9 It is a phasor diagram of the improved control strategy in the CD stage provided by an embodiment of the present invention;

[0044] Figure 10 It is a phasor diagram of the improved control strategy in the DF stage provided by an embodiment of the present invention;

[0045] Figure 11 It is a schematic diagram of the improved GFM fault ride-through control provided by an embodiment of the present invention;

[0046] Figure 12 It is a simulation diagram of the input of the traditional GFM control provided by an embodiment of the present invention; among them, (a) is a schematic diagram of the transient overvoltage at the new energy machine terminal, (b) is a schematic diagram of the DC current on the rectifier side; (c) is a schematic diagram of the DC system turn-off angle; (d) is a schematic diagram of the on / off situation of the GFM current limit;

[0047] Figure 13 It is a schematic diagram for verifying the phasor relationship of electrical quantities under current limit provided by an embodiment of the present invention; among them, (a) is a schematic diagram of the active power of the GFM branch, (b) is the virtual internal potential, (c) is a schematic diagram of the converter bus terminal voltage, (d) is a schematic diagram of the GFM terminal voltage;

[0048] Figure 14 It is a schematic diagram for verifying the improved control simulation provided by an embodiment of the present invention;

[0049] Figure 15 It is a schematic diagram for verifying the effect of the improved GFM fault ride-through provided by this embodiment;

[0050] Figure 16 It is a schematic diagram of the GFM fault ride-through simulation results under different fault types provided by this embodiment; among them, (a) is a schematic diagram of the GFM inputting the fault ride-through strategy proposed in this embodiment, and (b) is a schematic diagram of not inputting the GFM device. Detailed implementation manners

[0051] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. The components of the embodiments of the present invention described and illustrated herein can 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 represents the selected embodiments of the present invention. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative efforts belong to the scope of protection of the present invention.

[0052] Embodiment 1:

[0053] Figure 1 A structural diagram of a GFL and GFM hybrid connection via an LCC-HVDC system is shown. As Figure 1 shown, the fault handling method for the hybrid connection of grid-following and grid-forming via a DC transmission system provided in this embodiment includes:

[0054] S1. Establish a hybrid connection of GFL and GFM converters via an LCC–HVDC as a hybrid transmission system, and adopt an equivalent Thevenin model with a voltage source in series with an impedance at the receiving end;

[0055] S2. Obtain the distribution limit points of the three active currents and reactive currents of the grid-forming converter during the commutation failure process;

[0056] S3. According to the distribution limit points, obtain the corresponding transient characteristics through the grid-forming converter after DC commutation failure;

[0057] S4. According to the hybrid transmission system, analyze the transient characteristics through the stages of commutation failure;

[0058] S5. Substitute the current distribution angle into the GFM current command formula and boundary conditions to obtain an improved current limit control diagram.

[0059] Further, S2 specifically includes, when the hybrid transmission system is in a steady state, shunting the output currents of the sending-end GFL and GFM at the converter station, analyzing the relationship between the current shunting, power, and power distribution coefficient of the output line, and obtaining the current distribution coefficient angle of the GFM output current.

[0060] Further, S3 specifically includes:

[0061] S31. According to the current distribution coefficient angle, obtain the phasor relationship of the GFM converter under the critical state of the power distribution coefficient;

[0062] S32. Calculate the angle of the GFM current distribution coefficient using the cosine theorem, and obtain the GFM current command value at this time.

[0063] Further, S32 specifically includes:

[0064] S321. When the current distribution coefficient angle is 90°, perform reactive power regulation to the limit state through the GFM converter.

[0065] S322. When the current distribution coefficient angle is greater than 90°, enter the current limit control state through the GFM converter.

[0066] S323. When the current distribution coefficient angle is less than 90°, cause the power distribution coefficient to overshoot through the GFM converter.

[0067] Further, S4 specifically includes:

[0068] S41. According to the transient characteristics, obtain the distribution angles of the active and reactive power distribution limit point currents in the commutation failure stage.

[0069] S42. According to the transient characteristics, obtain the boundary condition equations of the active and reactive power distribution limit point currents in the commutation failure stage.

[0070] Further, S41 specifically includes that when the GFM enters the current limit control, while the operating point is at the current limit boundary, increase the current distribution coefficient angle to obtain a new current distribution coefficient angle.

[0071] Further, S42 specifically includes:

[0072] S421. According to the new current distribution coefficient angle, obtain the current command value through the GFM output current.

[0073] S423. According to the current command value, obtain the rectifier side commutation bus voltage value at the critical time of the GFM output current through the boundary conditions.

[0074] Embodiment 2:

[0075] Based on Embodiment 1, in this embodiment, a series - parallel connection of GFL and GFM converters is established and sent out through the LCC - HVDC system as Figure 1(a) As shown. The system topology adopts the Ximeng AC / DC hybrid topology of the North China Power Grid. The GFL / GFM converters are connected in parallel to the 220 kV power grid, stepped up to the 500 kV power grid and then connected to the LCC-HVDC rectifier busbar, and there is no conventional power source nearby at the sending end for support. The LCC-HVDC control system adopts the CIGRE-HVDC standard control mode, and uses a ±800 kV bipolar DC transmission model, with two groups of 12-pulse converter valves for each pole. The receiving end adopts an equivalent Thevenin model of a voltage source in series with an impedance.

[0076] The simplified system topology is as Figure 1 (b) As shown, where X M is the sum of the reactance of the GFM transformer and the line reactance, X L is the reactance of the new energy step-up transformer and the line, X s is the equivalent reactance of the transmission line and the transformer, E s is the amplitude of the internal electromotive force of the equivalent machine at the receiving end power grid, with its rotor angle δs as the reference value. The relative angle between the voltage at the GFM connection point and δ s is δ g , the amplitude of the DC sending-end converter busbar voltage is U Lr , and the relative angle with δ s is δ Lr .

[0077] When the system is operating in a steady state, the output currents of the sending-end GFL and GFM are shunted at the converter station, and there is a relational expression:

[0078]

[0079] In the formula is the current flowing into the converter station, is the AC line current, is the GFM output current, is the GFL output current. Considering that the new energy only outputs active power during the steady state period, that is is in phase with , the current limit is equivalent to an adaptive increase in the virtual impedance. Ignoring the line resistance, in the synchronous rotating coordinate system, according to Equation (1), the phasor relationships in the system can be obtained as Figure 2 (a) As shown. Let the starting point of the phasor be O and the end point be S, and the end point of the phasor be H. At this time, the active power Pg transmitted by the GFM is:

[0080]

[0081] In the formula U g sin(δ gl ) is in the direction of Figure 2As shown in the pink dashed arrow in (a), the AC system can transmit active power by α times U g sin(δ gl ) indicates that its changing trend is consistent with the changing trend of the DC system transmission power. Shift the phasor to the origin, as Figure 2 As shown in (b), taking this phase as the reference, the phase of the sending end system can be Figure 2 (b) Combined with the power characterization DC system. At this time, the sending end system exchanges reactive power Q g for:

[0082]

[0083] Take U in the figure gN The dashed arc represents the GFM reactive power-voltage droop control benchmark, SH and U gN The arc intersects at point T (N) Intersects with the current limit circle at point T (k) Assume K is the GFM output line power allocation coefficient, then according to equations (2) and (3), K is N for:

[0084]

[0085] It can be seen that K N is the absolute value of the tangent of the angle between OS and SH, according to Figure 2 (b) This angle is also the dq axis current distribution angle. During the transient period, GFM adjusts E ref ,θ ref Adjust the output P g and Q g This in turn affects the size of K, and further discusses the regulatory boundaries of K under existing control.

[0086] a)

[0087] when When the denominator of K is equal to 0, it takes the maximum value, that is, min[U Lr -U g cos(δ gl )]=0, the corresponding electrical quantity topological relationship is as follows Figure 3 As shown, it is equivalent to the power shortage of the external circuit causing the GFM reactive overshoot, the q-axis current reaches the output limit, and the current distribution coefficient angle reaches 90 degrees. When the GFM enters the current limiting control, according to formula (1), the q-axis output is limited to I lim , the reactive power regulation capability of GFM reaches the limit under current limiting control.

[0088] b)

[0089] When happens, the maximum value of K increases with the decrease of , so its maximum value is infinitely close to and its minimum value appears when U g sinδ gl = 0. The corresponding topological relationship of electrical quantities is as Figure 4 shown. At this time, the value of K is infinitely close to 0, and GFM will surely enter the current limit control. T (k) runs to the horizontal axis, and the d-axis current output reaches the limit.

[0090] c)

[0091] When happens, according to the cosine theorem, there is:

[0092]

[0093] After GFM enters the current limit control, the minimum value of ST (k) is I lim X M . After formula (5) is sorted out, there is:

[0094]

[0095] Then when U Lr cosδ lg - U g = 0, formula (4 - 123) gets the minimum value (I lim X M ). U 2 . U Lr cosδ lg - U g = 0 can be rewritten in the following form:

[0096]

[0097] When the value of U g is U gN , that is, when T (N) coincides with T (k) , the tangent of the reactive power - voltage arc U gN at the T (N)(k) point can be expressed by the following formula:

[0098]

[0099] Substituting formula (8) into the coordinates of point S (U Lr , 0) is exactly the same as the form of formula (4 - 124). Therefore, when U Lr cosδ gl - U gNWhen it is equal to 0, the topological relationship of the electrical quantity is as follows Figure 5 shown, and its physical meaning is equivalent to SH being tangent to arc U gN at T (N)(k) . At the same time, the numerator U of K at this time g sinδ lg takes the maximum value. Therefore, K reaches the regulation limit, which is equivalent to the overshoot of the GFM power coefficient caused by the power surplus in the external circuit, and the q-axis current reaches the output limit. Based on Example 1, the whole process of commutation failure is divided into three stages: AC, CD, and DF. According to the above conclusion, the interaction between the rectifier-side AC system and the DC system is the most intense in the DF stage during the whole process of commutation failure, and the time scale of the AC stage is small, usually in milliseconds, and the control actions on the rectifier side are relatively single. Therefore, only the interaction process of the system in the CD stage and the DF stage is analyzed in detail.

[0100] The DC system current at point T increases, and the rectifier consumes a large amount of reactive power, resulting in a sudden drop in the commutation bus voltage U on the rectifier side Lr . At this time, there is:

[0101]

[0102] According to Equation (9), the reactive current of the GFM branch increases rapidly, and U g decreases. According to Equation (9), E GFM decreases to suppress the rapid increase of the reactive current, playing a role in voltage support, and further suppressing the decrease of U g . However, as the fault persists, I M gradually increases, and the GFM enters current limiting, losing its regulation effect on power and voltage.

[0103] In the CD stage, the DC current decreases, the sending-end commutation bus rebounds, and U Lr gradually increases. During the process of the arc U gN intersecting and separating outside the current limiting circle, points T (k) and T (N) coincide at Figure 5 . According to the previous analysis, the GFM power coefficient reaches the maximum value at this time. During the subsequent continuous increase of U Lr , as Figure 6 shown, T (k) gradually increases, and T (k) The S dotted line represents the maximum value of the power coefficient regulation at this time, and its intersection with the current limiting is located outside the reactive power-voltage loop. Therefore, during the subsequent continuous increase of U Lr , no matter how the GFM power coefficient is adjusted, it cannot return to the reactive power-voltage circle, and E GFM continues to decrease, and its terminal voltage U g can only increase rapidly with the increase of U Lr , resulting in Ug With E GFM The difference increases again. At this time, the GFM current limiting cannot be exited, which greatly limits the ability of GFM to suppress the transient overvoltage at the sending end. Until U g value approaches U Lr , the current limiting control reaches the switching condition, and GFM returns to the constant voltage / power control again. At this time, the ability of GFM to adjust the transient overvoltage is extremely limited.

[0104] In the DE stage, U g and U Lr being close leads to relatively low GFM line I Md , I Mq . According to Equation (4-115), E GFM increases to increase the output reactive current and suppress the increase of voltage U Lr . At the same time, the swing angle of δ GFM increases to increase the active current of the line. According to the analysis described above, it is the relatively high active power deviation in the commutation failure CD stage of GFL that causes power oscillation. Therefore, GFM can alleviate the power fluctuation caused by GFL in the CD stage and suppress the subsequent commutation failure induced by the abnormal control switching on the inverter side caused by the power fluctuation on the rectifier side. However, at this time, the external circuit has not yet returned to the steady-state value. As E GFM increases, U g decreases. The DC system of the external circuit has not completed the switching of the constant extinction angle control. Before the DC system reaches the fifth reactive power change stationary point E dr(di) , E GFM can only continue to increase, while U g continues to decrease, and GFM enters the current limiting control again. In the EF stage, U Lr decreases, corresponding to the decrease of OS. During the process of the intersection of the arc U gN and the current limiting circle, the maximum point of its output power coefficient is obtained at Figure 3 . After that, T (N) increases while T (k) decreases. T (k) will be located within the reactive power-voltage regulation loop. As shown in Figure 7 , GFM always senses a reactive power deficit. E GFM increases, I Mq increases, and ST (k) moves closer to ST (N) . However, since ST (N) is located inside the reactive power-voltage circle and there is a current limiting restriction, GFM cannot return to the reactive power-voltage circle. Therefore, E GFM continues to increase and the current limiting cannot be exited.

[0105] In the EF stage, the DC system enters the current deviation control. When the power of the DC system recovers to point N, assuming that there is an intersection point N1 between the active power of the GFM line and the current limit at this time, however, according to the above analysis, the DC system reaches the reactive power change stationary point at this time, the reactive power consumption decreases, and the corresponding point on the current limit circle that satisfies its reactive power change is at the position of N2. Therefore, point T (k) has to slide back and forth on the arc N1N2, triggering power oscillation, as Figure 8 (a) shows. When the DC power further recovers to position M, there is no intersection point between the active power of the GFM line and the current limit at this time. After point T (k) has no equilibrium point, it can only perform circular motion on the circle, triggering a larger power oscillation, as Figure 8 (b) shows. According to the analysis described above, continuous power oscillation causes abnormal switching of the inverter-side control, the leading trigger angle continuously decreases, and then subsequent commutation failures occur. At this time, the power loop control of GFM instead restricts the fault recovery process of the DC system.

[0106] In summary, during the entire process of commutation failure in the conventional GFM / GFL hybrid-connected 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 abnormal switching of the inverter-side control caused by fluctuations in electrical quantities. However, it has the following two limitations: 1) The current limit is not withdrawn in time after providing low-voltage support, resulting in poor suppression effect of transient overvoltage; 2) The current limit is not withdrawn in time during the fault recovery process of the DC system, which not only cannot suppress subsequent commutation failures induced by power fluctuations, but instead triggers subsequent commutation failures. The essential reason for the above two limitations is that the GFM current limit has clear input conditions. However, after the current limit is input, it can only rely on changes in the electrical quantities of the external power grid to passively meet the input and withdrawal conditions to restore constant voltage / power control. Moreover, the time scale of commutation failure is relatively small, and the input time of the current limit is very likely to coincide with the CC control of the DC system fault recovery and the switching process between CC and CEA control. The oscillation component will be transmitted to the inverter-side control system at this stage, ultimately causing more serious subsequent commutation failures.

[0107] According to the transient characteristics proposed above, the GFM fault ride-through control strategy applicable to the time scale of DC commutation failure is as follows:

[0108] 1) In the CD stage, the control system equilibrium point is at Figure 9 the position of middle T (k) . After the system operating point exceeds the reactive power-voltage control boundary, the internal potential continuously decreases. At the same time, the terminal voltage continuously increases with the electrical quantities of the external circuit, making it difficult to withdraw the current limit. Therefore, change the system equilibrium point to point T(s). First, discuss the feasibility of changing the system operating point to T (s) . T (s) is the circular arc U gNThe intersection point with current limiting satisfies the stable operation of GFM reactive power - voltage, and at the same time H (s) operates on the arc with O as the center and OH (s) as the radius. Therefore, ΔOH (s) S meets the system stability requirements. At this time, it is equivalent to appropriately increasing the reactive current of the GFM line and decreasing its active current on the basis of the original electrical quantities of the system, sacrificing a part of its reactive power support ability while maintaining the system operation at the current limiting boundary to increase the current distribution coefficient angle and suppress the continuous decrease of E GFM and the continuous increase of U g . At this time, the GFM current distribution coefficient angle can be obtained by the cosine theorem:

[0109]

[0110] At this time, the GFM current command value can be expressed by the following formula:

[0111]

[0112] Next, discuss the boundary conditions under this control mode. According to the above analysis,

[0113] When the T (k) point coincides with the tangent point of T (k) S and the reactive - voltage arc, GFM reaches the critical state. Therefore, the value of U Lr(1) at this time can be obtained as:

[0114]

[0115] Then the U Lr(1) boundary is:

[0116]

[0117] 2) In the DF stage, the control system equilibrium point is at Figure 10 at the T (k) position. The system operating point lags behind the reactive - voltage control boundary and the internal potential continues to increase, and U g continues to decrease. Before the external circuit returns to steady state, the current limiting cannot be exited. Therefore, change the system operating point to the T (s) point. The feasibility analysis will not be elaborated here, and the continuous increase of the internal potential is suppressed. At this time, the GFM current distribution coefficient angle is:

[0118]

[0119] According to the above analysis, when the T (k) point is perpendicular to OS at the T (k) point, GFM reaches the critical state. Therefore, the value of U Lr(2) at this time can be obtained as:

[0120]

[0121] Then at this time, U Lr(2) The boundary is:

[0122]

[0123] Therefore, the implementation principle of the overall improvement strategy is as Figure 11 shown;

[0124] According to Figure 11 It can be seen that the control method proposed in the present invention adaptively adjusts the power distribution coefficient by extracting the terminal voltage value. In the overall control loop, only a small system response time constant is generated due to the existence of the PI link during the process of converting the transient voltage signal into a current signal. The system current limiting response time can be controlled within the millisecond level. The control timing diagrams of each stage of GFL, GFM, and LCC-HVDC are shown in Appendix C. It can be seen that when using the conventional virtual impedance method, due to the PI delay of both the current loop and the voltage loop, and even when extracting the port fault current signal, the feedback signal cannot change suddenly in time due to the existence of inductive elements in the line. The response time of this method is usually in the range of dozens of milliseconds to hundreds of milliseconds. And commutation failure is an electromagnetic transient process at the level of hundreds of milliseconds. The switching process between CC and CEA during the transient recovery process usually only lasts for more than a dozen milliseconds. Applying the conventional virtual impedance method will result in the situation that the limiting link has not responded yet, while the DC system has already switched the control and entered the next state, and GFM cannot quickly track the changes in the electrical quantities of the system, resulting in the current not being successfully limited and even affecting the system recovery process. Therefore, for the time scale of commutation failure, the response speed of the improved current limiting control proposed in the present invention is relatively well-matched.

[0125] Embodiment 3:

[0126] This embodiment also provides an electronic device, including a memory and a processor. The memory is used to store one or more computer instructions. Among them, when the one or more computer instructions are executed by the processor, the method of Embodiment 1 is implemented;

[0127] In practical applications, the processor can be implemented by an application specific integrated circuit (ASIC), a digital signal processor (DSP), a digital signal processing device (DSPD), a programmable logic device (PLD), a field programmable gate array (FPGA), a controller, a microcontroller unit (MCU), a microprocessor or other electronic components, and is used to execute the method in the above embodiments.

[0128] The method implemented in this embodiment is as described in the content of Embodiment 1.

[0129] Embodiment 4:

[0130] This embodiment also provides a computer storage medium. A computer program is stored in the computer-readable storage medium. When the computer program is executed by one or more processors, the method of Embodiment 1 is implemented;

[0131] Among them, the computer-readable storage medium can be implemented by any type of volatile or non-volatile storage device or a combination thereof, such as a static random access memory (SRAM), an electrically erasable programmable read-only memory (EEPROM), an erasable programmable read-only memory (EPROM), a programmable read-only memory (PROM), a read-only memory (ROM), a magnetic memory, a flash memory, a magnetic disk or an optical disc.

[0132] The method implemented in this embodiment is as described in the content of Embodiment 1.

[0133] For the above-disclosed characteristics and the proposed improved fault ride-through control strategy, PSCAD is used for full electromagnetic simulation verification. The test system topology diagram is as Figure 12(a). The SCR of the AC system is 1.5. A three-phase short-circuit fault is set on the inverter side at 2.5 s, with a duration of 0.06 s and a transition resistance of 100 Ω. Five working conditions are set to verify the correctness of the theory proposed in the present invention and the effectiveness of the proposed strategy.

[0134] Condition 1: The input power Pw of the GFL is 200 MW. With and without the input of 50 MW GFM, relevant electrical quantities are recorded. The GFM adopts the conventional current limiting strategy. The total sending-end power is 250 MW when the GFM is input. The ratio of GFM to GFL is 1:4.

[0135] The simulation results of Condition 1 are as shown in Figure 12 、 Figure 13 shown. According to Figure 12 the display, the simulation system experiences high and low voltage ride-through at time t1, but the GFM does not promptly disengage from the current limiting control, resulting in an obvious transient low voltage support effect, while the transient overvoltage suppression effect is not obvious, as shown in Figure 12 (a). The simulation system enters the CEC link at time t2, and the CC and CEA are switched. At this time, the electrical quantities have basically recovered, and the current limiting delays to exit again, and subsequent commutation failures finally occur at time t3. After the subsequent commutation failure occurs, the grid electrical quantities change and the GFM then disengages from the current limiting control, proving the correctness of the above analysis of the transient characteristics of the system and the description of the dynamic process of commutation failure in the present invention.

[0136] According to Figure 13 it can be found that after the GFM just enters the current limiting, it still has a certain ability to adjust the external circuit electrical quantities. As shown in Figure 13 (a) and (d), the active power of the GFM branch still slowly increases, and the terminal voltage drops slightly. However, when the external circuit experiences high and low voltage faults and rides through, the terminal voltage of the GFM instantaneously increases with the external circuit electrical quantities, and the active power loses the adjustment ability, proving that the power distribution coefficient overshoots, while the EGFM continues to decrease, so the current limiting cannot exit, which is consistent with the above analysis.

[0137] Quantitatively calculate the phasor relationship of the electrical quantities at this moment, the actual U Lr = 1.1263. After the power distribution coefficient overshoots for the second time, the terminal voltage drops, while the E GFM continues to rise, the current limiting cannot exit, the power oscillates, and the oscillation amplitude gradually increases, and finally a second commutation failure occurs, which is consistent with the previous analysis.

[0138] Quantitatively calculate the phasor relationship of the electrical quantities at the moment when the inflection point of the electrical quantities appears, the 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 analysis of the relationship between electrical quantities in the present invention. Working condition 2 of the present invention: Based on working condition 1, GFM is put into the control strategy proposed by the present invention to record the relevant electrical quantities. The total power at the sending end remains unchanged at 250MW, and the ratio of GFM:GFL is 1:4.

[0139] The simulation results of working condition 2 are as follows Figure 12 The control strategy proposed in the present invention can allow the GFM to quickly break away from the current limiting control in the CD stage, achieving a better transient overvoltage suppression effect. In the DF stage, before the DC system enters the current deviation link, it breaks away from the current limiting control again to avoid the subsequent commutation failure.

[0140] Working condition 3: The electrical quantities of the sending-end system are simulated under three conditions: the control strategy proposed by the present invention, the conventional current limiting control strategy and without GFM. The GFL input power Pw is 100MW, 300MW, 500MW, 750MW and 1000MW, the GFM input power is 100MW, and the GFM:GFL ratios are 1 / 1, 1 / 3, 1 / 5, 1 / 7.5 and 1 / 10 respectively.

[0141] The simulation results of working condition 3 are as follows Figure 13 .according to Figure 13 (a) It can be seen that after the control strategy proposed in the present invention is put into use, the effect of GFM in suppressing transient overvoltage is fully exerted, and the power oscillation of the conventional converter and the subsequent commutation failure caused by the commutation failure process are greatly suppressed. Although the GFM under conventional current limiting also has the ability to suppress the subsequent commutation failure caused by the power oscillation at the sending end to a certain extent, its suppression effect is not good, and the suppression effect on transient overvoltage is relatively poor. At the same time, when the GFL grid-connected power is small, it is easy to cause subsequent commutation failure, which is consistent with the above analysis. Under the condition where GFM is not put into use at all, as the GFL grid-connected power gradually increases, its power oscillation becomes more and more violent. After a commutation failure occurs, it causes multiple commutation failures again, resulting in repeated high and low voltage crossing of the sending end voltage, which greatly limits the LCC-HVDC's ability to send new energy. Therefore, the control strategy proposed in the present invention has good application necessity and reliability.

[0142] Working condition 4: The inverter side fault scenarios are set to three-phase short circuit, single-phase grounding, two-phase phase-to-phase, and two-phase grounding. The GFL input power Pw is 700MW. The 100MWGFM with the control strategy proposed in the present invention and the GFM without GFM are simulated respectively.

[0143] The simulation results of working condition 4 are as follows Figure 16 .according to Figure 16It can be seen that regardless of the fault type on the inverter side, when the grid-connected power of the rectifier side GFL is relatively high, power oscillation will be triggered, which will in turn cause subsequent commutation failures. Since the essence of its problem is the same as the research scenario of the present invention, the control strategy proposed in the present invention can still effectively suppress the subsequent commutation failures caused by transient overvoltage and power oscillation when it is put into use.

[0144] In several embodiments provided by the embodiments of the present invention, it should be understood that the disclosed systems and methods can also be implemented in other ways. The above-described system and method embodiments are merely illustrative.

[0145] It should be noted that in the present invention, the terms "first", "second", etc. in the description and claims of the method of the present invention and the above drawings are used to distinguish similar objects, and do not necessarily have to be used to describe a specific order or sequence. The term "comprising", "including" or any other variant thereof is intended to cover a non-exclusive inclusion, so that a process, method, article or device comprising a series of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such process, method, article or device. Without further limitation, an element defined by the statement "comprising an..." does not exclude the presence of additional identical elements in the process, method, article or device comprising the element.

[0146] Although the disclosed embodiments of the present invention are as above, the content described above is only an embodiment adopted for the convenience of understanding the present invention, and is not intended to limit the present invention. Any person skilled in the art within the technical field to which the present invention pertains may make any modifications and changes in the form of implementation and details without departing from the spirit and scope disclosed by the present invention. However, the scope of patent protection of the present invention shall still be subject to the scope defined by the appended claims.

Claims

1. A fault handling method for a DC transmission system for a grid-connected and grid-building hybrid system, characterized in that: The method comprises: S1, establish a hybrid transmission system of GFL and GFM converters via LCC-HVDC, and use the equivalent Thevenin model of voltage source series impedance at the receiving end; S2, obtaining the three distribution limit points of active current and reactive current of the grid-connected converter during the commutation failure process; S3, according to the allocation limit point, obtaining corresponding transient characteristics through the grid-type converter after the DC commutation failure; S4, analyzing transient characteristics through a commutation failure stage according to the hybrid transmission system; S5, bringing the current distribution angle into the GFM current command formula and boundary conditions to obtain an improved current limit control diagram.

2. The method according to claim 1, characterized in that S2 specifically includes: according to the steady state of the hybrid transmission system, the output current of the GFL at the sending end and the GFM are shunted at the converter station, the relationship between the current shunting, power and power distribution coefficient of the output line is analyzed, and the current distribution coefficient angle of the GFM output current is obtained.

3. The method according to claim 2, characterized in that S3 specifically includes: S31, obtaining a phasor relationship of the GFM converter in a critical state of the power distribution coefficient according to the current distribution coefficient angle; S32, using the cosine theorem to calculate the GFM current distribution coefficient angle, and obtain the GFM current command value at this time.

4. The method according to claim 3, characterized in that S32 specifically includes: S321, when the current distribution coefficient angle is 90°, the GFM converter is used to perform reactive power regulation to a limit state; S322, when the current distribution coefficient angle is greater than 90°, entering a current limit control state through the GFM converter; S323, when the current distribution coefficient angle is less than 90°, the power distribution coefficient is overregulated by the GFM converter.

5. The method according to claim 4, characterized in that S4 specifically includes: S41, obtaining the distribution angle of the active and reactive power distribution limit point currents in the commutation failure stage according to the transient characteristics; S42, according to the transient characteristics, obtaining the boundary condition formula of the active and reactive power distribution limit point current in the commutation failure stage.

6. The method according to claim 5, characterized in that S41 specifically includes, when the GFM enters the current limiting control, increasing the current distribution coefficient angle while setting the operating point at the current limiting boundary to obtain a new current distribution coefficient angle.

7. The method according to claim 6, characterized in that S42 specifically includes: S421, obtaining a current command value by outputting the current through the GFM according to the new current distribution coefficient angle; S423, according to the current command value, obtaining the rectifier-side commutation bus voltage value when the GFM output current is critical through boundary conditions.

8. An electronic device, characterized in that: The invention comprises a memory and a processor, wherein the memory is 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 to 7.

9. A computer-readable storage medium, characterized in that: The computer-readable storage medium stores a computer program, and when the computer program is executed by a processor, it is used to implement the method as described in any one of claims 1 to 7.

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