New energy field station following network construction capacity matching method and system considering transient overvoltage

Abstract

The invention provides a method and a system for matching the following network capacity of a new energy station considering transient overvoltage, and belongs to the technical field of following network capacity calculation, and the method comprises the following steps: simplifying a following network hybrid station to obtain an equivalent model; based on the obtained equivalent model, calculating to obtain a network following and constructing ratio initial value of each station; solving a phase-locked loop stability condition; when a network construction type converter is configured for any station according to the calculated network construction matching initial value, whether a phase-locked loop can be kept stable or not after the station encounters a three-phase short-circuit fault is judged according to a phase-locked loop stability condition, and the transient overvoltage of the station is calculated if the phase-locked loop is unstable; when the transient overvoltage is lower than a safety threshold value, the initial value of the network construction proportion is kept; and when the requirements are not met, deducing to obtain a network construction type capacity ratio expression meeting the transient overvoltage constraint, and correcting the network construction ratio of the station to obtain the network construction type capacity ratio and the following network type capacity ratio in any station.

Description

Technical Field

[0001] The present invention belongs to the technical field of grid-connected capacity calculation of a following structure network, and particularly relates to a method and system for matching the following structure network capacity of a new energy power station considering transient overvoltage. Background Technique

[0002] The statements in this part only provide background technical information related to the present invention and do not necessarily constitute prior art.

[0003] A new energy power station refers to a facility that generates electricity using renewable energy (such as wind energy, solar energy, water energy, biomass energy, etc.), and is an important part of promoting the transformation of the energy structure. Among them, new energy represented by wind power and photovoltaic power has gradually shifted from incremental supplementation to the main power source. In recent years, the combined installed capacity of wind power and solar power generation has exceeded that of thermal power for the first time. This transformation has promoted a fundamental reconstruction of the form of the power system, and a new power system with the "double high" characteristics of a high proportion of renewable energy and a high proportion of power electronic devices is taking shape.

[0004] The proportion of synchronous generators in the new power system is gradually decreasing, resulting in a significant reduction in its voltage support ability. The currently widely used grid-following (GFL) converters rely on a phase-locked loop (PLL) to achieve synchronous operation with the power grid. The PLL has a risk of instability under low voltage conditions and the grid-following type does not have the ability to actively support the grid voltage, which further deteriorates the transient voltage stability of the power system. The lack of voltage support ability of new energy power stations has led to an increasing risk of a sharp increase in transient overvoltage when the system encounters a fault, which has become one of the key obstacles restricting the efficient consumption of new energy.

[0005] The grid-forming (GFM) control has gradually matured. It can independently construct voltage and frequency and has stronger stability under weak grid conditions. Connecting to the grid together with the grid-following type is beneficial to ensuring the voltage stability of the system. However, the introduction of the grid-forming type makes the coupling characteristics of the hybrid power station more complex, the stability of the PLL is not easy to directly judge, and the transient overvoltage is difficult to quantitatively evaluate. Moreover, the cost of the grid-forming converter is relatively high. For economic considerations, the grid-forming capacity ratio should be reduced as much as possible under the premise of meeting the stability requirements. Therefore, it is challenging to solve the matching of the following structure network capacity of each power station in the new energy transmission system considering transient overvoltage constraints.

[0006] The inventor found in the research that in the prior art, a technical solution for quantifying the transient overvoltage of a new energy sending-end system was disclosed. Under commutation failure, an equivalent circuit model of the sending-end system was constructed, including an LCC-HVDC DC system, a new energy unit, and an equivalent AC system at the sending end. Based on the equivalent circuit model of the sending-end system and the point of common coupling, nodal voltage equations were written to derive a voltage calculation formula at the point of common coupling containing DC current and the variable of the leading trigger angle of the rectifier device. The turn-off angle of the inverter was fitted, and the DC current and leading trigger angle of the DC system during commutation failure were analyzed in stages to determine the time-domain expression of the voltage amplitude under commutation failure. By taking the partial derivative of the time-domain expression, the moment of maximum overvoltage and the value of maximum overvoltage were obtained, and the transient overvoltage of the sending-end system under commutation failure was quantified. At this time, the dynamic coupling effect between the new energy and the DC system was accurately characterized, and it was applicable to the transient overvoltage quantification of the current scenario where new energy power stations are sent out through DC.

[0007] The above solution is a fully grid-following type sending system, which does not consider the voltage support effect of the grid-forming converter. Moreover, the method it proposed is only applicable to the commutation failure fault analysis of the DC sending system and cannot quantitatively evaluate the transient overvoltage when a short-circuit fault occurs inside the system.

[0008] In addition, the prior patent also disclosed an optimization scheme for the grid-forming power capacity configuration. Based on the constraint conditions of new energy grid connection stability, the technical indicators corresponding to the grid-forming power were associated, and the initial configuration capacity of the grid-forming power was estimated and given. The grid-forming power access positions were selected as the high-voltage side and low-voltage side of the step-up transformer of the new energy power station. According to the short-circuit ratio constraints, static security constraints, and transient stability constraint conditions at the new energy machine terminal and the connection point of multiple power stations, it was verified whether the grid-forming power met the constraint requirements. If not, the access capacity was adjusted; if it met the requirements, the cost required for the current capacity was calculated according to the comprehensive unit price. The total costs corresponding to the capacities at the two access positions were compared, and the economically optimal one was selected as the final configuration capacity.

[0009] In the above solution, only the constraint conditions for the grid-forming configuration were proposed, and no effective evaluation method for transient overvoltage was proposed. The output characteristics of the grid-following converter and the stability of the PLL could not be taken into account, and the configured grid-forming power capacity might not meet the transient overvoltage constraints under the condition of PLL instability. Summary of the Invention

[0010] To overcome the above deficiencies of the prior art, the present invention provides a method for matching the capacity of the grid-forming converter in a new energy power station considering transient overvoltage, which is used to calculate the minimum capacity configuration of the grid-forming converter in the power station under the condition of transient overvoltage constraints.

[0011] To achieve the above object, one or more embodiments of the present invention provide the following technical solutions: In a first aspect, a method for matching the structure-connected network capacity of a new energy station considering transient overvoltage is disclosed, including: Simplify the structure-connected network hybrid station to obtain an equivalent model; Based on the obtained equivalent model, calculate the initial values of the structure-connected network ratios of each station; Solve the phase-locked loop stability condition; When configuring the structure-connected converter for any station according to the calculated initial value of the structure-connected network ratio, determine whether the phase-locked loop can remain stable after the station encounters a three-phase short-circuit fault through the phase-locked loop stability condition. If it becomes unstable, calculate the transient overvoltage of the station; When the transient overvoltage is lower than the safety threshold U safe maintain the initial value of the structure-connected network ratio; when the requirement is not met, derive the expression of the structure-connected converter capacity ratio under the constraint of transient overvoltage, and correct the structure-connected network ratio of the station to obtain the structure-connected converter capacity ratio and the network-connected converter capacity ratio in any station.

[0012] As a further technical solution, the simplifying the structure-connected network hybrid station to obtain an equivalent model specifically includes: Model the external topology structure of the new energy transmission system station; Model the internal converter of the new energy transmission system station; Model the structure-connected network hybrid station; Make assumptions about the hybrid model; Based on the above process, obtain the equivalent model.

[0013] As a further technical solution, the assumptions about the hybrid model include: Both the network-connected and structure-connected converters switch to the low-voltage ride-through state during the fault and enter the low-voltage ride-through recovery state after the fault is cleared; The control mode delay after the fault occurs and is cleared can be ignored; During the fault, the current loop regulation process can be ignored in the phase-locked loop characteristics, and it is considered that the current loop has reached the command value; The power angle of the structure-connected converter during the fault is equal to the steady-state value before the fault.

[0014] As a further technical solution, calculating the initial values of the structure-connected network ratios of each station based on the obtained equivalent model specifically includes: According to the simplified model of the hybrid station and the circuit superposition theorem, the grid connection point voltage of station i can be obtained; Based on the grid connection point voltage of station i, calculate the structure-connected network capacity ratio of a single station: Based on the structure-connected network capacity ratio of a single station, calculate the initial values of the structure-connected network capacities of each station.

[0015] As a further technical solution, calculate the ratio of the capacity of a single power station to the grid-forming capacity based on the grid connection point voltage of substation i, specifically: Calculate the proportion of the capacity of the grid-forming converter according to the requirement of ensuring that the unit does not trip off the grid ; Calculate the proportion of the capacity of the grid-forming converter according to the transient overvoltage constraint ; When configuring the grid-forming proportion of substation i, It should be ensured that the new energy units do not trip off the grid during the fault and the transient overvoltage is lower than the safety threshold after the fault is cleared.

[0016] As a further technical solution, determine whether the PLL is unstable, and calculate the transient overvoltage of the substation. The calculation process is as follows: Calculate the phase-locked angle error: Substitute the calculated phase angle output by the PLL into the phase angle expression to obtain the phase-locked angle error; Based on the phase-locked angle error, the output current of the current loop before fault clearing, the output current of the grid-following type, and the internal impedance of the grid-forming type, substitute them into the voltage amplitude expression to obtain the transient overvoltage calculation formula of substation i.

[0017] In the second aspect, a new energy power station grid-forming capacity ratio system considering transient overvoltage is disclosed, including: An equivalent model construction module, configured to: simplify the grid-forming and grid-connected hybrid substation to obtain an equivalent model; A grid-forming ratio initial value calculation module, configured to: calculate the initial values of the grid-forming ratios of each substation based on the obtained equivalent model; A phase-locked loop stability condition solving module, configured to: solve the phase-locked loop stability conditions; A transient overvoltage calculation module, configured to: when configuring the grid-forming converter for any substation according to the calculated initial value of the grid-forming ratio, determine whether the phase-locked loop can remain stable after a three-phase short circuit fault through the phase-locked loop stability condition judgment. If it is unstable, calculate the transient overvoltage of the substation; A capacity ratio calculation module, configured to: when the transient overvoltage is lower than the safety threshold U safe Keep the initial value of the grid-forming ratio; when the requirement is not met, derive the expression of the grid-forming capacity ratio under the transient overvoltage constraint, and correct the grid-forming ratio of the substation to obtain the grid-forming capacity ratio and the grid-following capacity ratio in any substation.

[0018] The above one or more technical solutions have the following beneficial effects: Based on Thevenin equivalent and converter characteristics, the new - energy power transmission system of the present invention is reasonably simplified; the initial value of the grid - forming capacity ratio of the substation is calculated by using the circuit superposition theorem; based on the model corrected by the initial value, a PLL stability discrimination method and a phase - locked angle calculation method when PLL is unstable are proposed; considering the phase - locked angle error, a transient over - voltage verification formula is proposed; according to the verification result, the grid - forming capacity ratio of the substation is configured. The present invention has theoretical and practical significance for the grid - forming capacity ratio of new - energy power stations, and can guide the design and operation of new - energy power transmission systems.

[0019] Advantages of additional aspects of the present invention will be given in part in the following description, become apparent in part from the following description, or be learned through the practice of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] The accompanying drawings forming a part of this specification are used to provide a further understanding of the present invention. The schematic embodiments and descriptions thereof of the present invention are used to explain the present invention and do not constitute an improper limitation to the present invention.

[0021] Figure 1 It is a schematic diagram of the topological structure of a new - energy power transmission system in an embodiment of the present invention; Figure 2 It is a control block diagram of a grid - forming converter; Figure 3 It is a control block diagram of a grid - following converter; Figure 4 It is a schematic diagram of the i - model of a grid - forming and grid - following hybrid substation in an embodiment of the present invention; Figure 5 It is a phasor relationship diagram at the moment of fault clearing in an embodiment of the present invention; Figure 6 It is a logic flow chart of an embodiment of the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0022] It should be noted that the following detailed description is exemplary and is intended to provide further explanation of the present invention. Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by those of ordinary skill in the technical field to which the present invention belongs.

[0023] It should be noted that the terms used herein are only for describing specific embodiments and are not intended to limit the exemplary embodiments according to the present invention.

[0024] In the case of no conflict, the embodiments in the present invention and the features in the embodiments can be combined with each other.

[0025] Embodiment 1 This embodiment discloses a method for configuring the grid - forming and grid - following capacity ratio of a new - energy power station considering transient over - voltage, including: Step 1: Based on Thevenin equivalent and converter characteristics, the new energy transmission system is reasonably simplified to obtain an equivalent model; Step 2: Using the circuit superposition theorem, calculate the initial value of the grid-forming capacity ratio of the substation; Step 3: Based on the model after correcting the initial value, a PLL stability discrimination method and a method for calculating the phase-locked angle when PLL is unstable are proposed; Step 4: Considering the phase-locked angle error, a transient overvoltage verification formula is proposed; Step 5: According to the verification results, configure the grid-forming capacity ratio of the substation.

[0026] In Step 1: The simplified model of the grid-forming and grid-connected hybrid substation is equivalent; this equivalent model takes into account the topological structure of the new energy transmission system outside the substation and the characteristics of the internal converter. At the same time, based on the internal and external characteristics of the substation and the research problem requirements, it is reasonably simplified. When comprehensively reflecting the change process of the grid-connected point voltage when a short-circuit fault occurs outside the substation, the complexity and analysis difficulty are reduced.

[0027] Due to the difference in terrain conditions, the topological structure of the new energy transmission system shows significant diversity. At the same time, there are many internal parameters and complex control characteristics in the grid-following type and grid-forming type converters. Therefore, first, make an equivalent simplification and reasonable assumptions for the new energy transmission system. The steps are as follows: Step (1-1) Modeling of the topological structure outside the substation: Different landforms such as mountains, deserts, and coasts directly affect the layout of the transmission corridor, resulting in different forms of topological structure such as long-chain type, radial type, or hybrid mesh type. Figure 1 Represents the structure diagram of a new energy transmission system.

[0028] Assume that the new energy transmission system contains m new energy substations. After the new energy is grid-connected, it is collected and sent to the receiving-end AC grid through the line. The line adopts lumped parameters, ignoring the impedance influence of the substations outside substation i, and only considering the topological part of the system. The resistance value in the new energy transmission system is small and can be ignored. Therefore, through Thevenin equivalent, the corresponding Thevenin reactance outside substation i is X i , and the power supply is U i .

[0029] In this step, using Thevenin's theorem, the topological structure outside the substation is equivalent to X i and U i , and the established model is the Figure 4 on the right side of X i and U i .

[0030] Step (1-2) Modeling of the Converter Inside the Substation: The control modes of the converter can be divided into grid-forming control and grid-following control. The most commonly used control mode in grid-forming control is virtual synchronous generator control, which, as shown by Figure 2 , simulates the operating characteristics of a synchronous generator and can be equivalent to a voltage source with an amplitude of and an output phase angle of . The grid-following converter, as shown by Figure 3 , can be equivalent to a current source, and its output quantity is: (1) In the formula: I T is the output current of the unit; is the current phase angle; is the reactive current; is the active current.

[0031] This formula (1) represents the relationship between I T , and , , and is used in Figure 3 , Figure 4 and step (4-2).

[0032] Step (1-3) Modeling of the Hybrid Substation: Inside the substation, the grid-following and grid-forming converters are jointly connected to the point of common coupling (PCC), and are connected to the power grid through transformers and lines. Without loss of generality, a certain hybrid grid-forming and grid-following substation i can be simplified to the Figure 4 model.

[0033] It should be noted that the modeling step (1-3) of the hybrid substation is based on two parts. One part is the external modeling of the substation, see step (1-1), and the other part is the modeling of the converter inside the substation, see step (1-2). If necessary, the three of them can be combined.

[0034] The base capacity of the system is S base . The total capacity of the converter of substation i is n times the base capacity, that is, S base,i = n S base . The total grid-following capacity of substation i is S GFL , and the total output current is I L = I T ( S GFL / S base);The total capacity of the network-forming type is S GFM , and the output reactance is X GFM . When reduced to the system capacity, it is obtained that X M = X GFM ( S base / S GFM ); The total reactance of the outgoing lines and transformers in the substation is X L .

[0035] Assumptions of the hybrid model in steps (1 - 4): (1) Both the network-following type and network-forming type converters switch to the low-ride-through state during the fault and enter the low-ride-through recovery state after the fault is cleared. Since these control strategies and the transient processes of various quantities are concentrated on the electromechanical transient scale, in order to study the influence of the converter output characteristics on transient overvoltage and improve the voltage evaluation efficiency, the present invention ignores the influence of electromagnetic transient parameters and uses the electromechanical transient model for analysis. (2) Since the switching time between the low-ride-through and low-ride-through recovery modes is generally in the millisecond level, which is much smaller than the fault duration, it can be considered that the control mode delay after the fault occurs and is cleared can be ignored. (3) The current loop bandwidth is several times that of the phase-locked loop bandwidth. Therefore, when studying the characteristics of the phase-locked loop during the fault, the current loop regulation process can be ignored, and it is considered that the current loop has reached the command value, that is I d = I d,ref , I q = I q,ref . (4) It is assumed that the current overload multiple of the network-forming type converter is large enough to ensure that UGFM remains basically unchanged; the inertia coefficient is also large, and the power angle of the network-forming type converter changes little in a short time, and it is considered that the during the fault is equal to the steady-state value before the fault.

[0036] Step two: Method for calculating the initial value of the ratio of network-following capacity to network-forming capacity: During the planning and construction period of the new energy transmission system, the ratio of network-following to network-forming in each substation within the system is unknown. Therefore, ignoring the voltage support of other substations to the connection point of this substation, first calculate the initial value of the ratio of network-following capacity to network-forming capacity of substation i.

[0037] Step (2 - 1) Calculate the connection point voltage: According to the simplified hybrid substation model (the model in the above steps (1 - 3)) and the circuit superposition theorem, the connection point voltage of substation i is: (2) In the formula: U 1 is the amplitude of the connection point voltage; is the phase angle of the grid connection point voltage; is the phase angle of the grid-following current loop; is the phase-locked angle, is the phase angle of the output voltage; U L and U M and U g are the voltage amplitude components of the grid-following type, grid-forming type, and equivalent power source at the grid connection point, respectively, and are respectively: (3) In the formula: X M is the grid-forming type output reactance reduced to the system capacity; X i is the grid-side Thevenin equivalent reactance; X L is the total reactance of the outgoing line and transformer in the substation; I L is the total output current of the grid-following type; U GFM is the internal potential of the grid-forming type; U i is the grid-side Thevenin equivalent power source.

[0038] Substitute formula (3) into formula (2) and decompose according to the phasor relationship to obtain the voltage amplitude and phase angle respectively as (4) (5).

[0039] In this step, the grid connection point voltage is calculated by using the series-parallel simplified model and the circuit superposition theorem, which can take into account the internal characteristics of the converter and fundamentally reveal the causes of transient overvoltage and the mechanism of parameter influence.

[0040] Step (2-2) calculates the capacity ratio of a single substation to the grid-forming capacity: Let the proportion of the grid-forming type capacity = S GFM / n S base , then the proportion of the grid-following type capacity is . At the same time, to ensure reliability, based on the three-phase metallic short circuit near the grid-side substation, calculate the initial value of the capacity ratio of substation i.

[0041] The specific steps are as follows: Calculate the proportion of the grid-forming type converter capacity according to the requirement of ensuring that the unit does not trip off the grid: When the three-phase short circuit occurs instantaneously, U i =0, Xi = 0. At this time, the grid-connected type has no voltage support ability. U L = 0. At the same time, let the minimum grid connection point voltage of the new energy unit without tripping be . Substituting each quantity into equations (3) and (4), it can be calculated that to ensure the unit does not trip, the capacity ratio of the grid-forming converter is The minimum is: (6) In the formula: is the output reactance of the grid-forming type; is the minimum grid connection point voltage of the unit without tripping; is the total reactance of the outgoing lines and transformers in the substation; is the internal electromotive force of the grid-forming type.

[0042] Step (2-2-1) Calculate the capacity ratio of the grid-forming converter according to the transient overvoltage constraint : To simplify the initial value calculation process, it is approximately considered that the grid-connected type PLL can remain stable during the fault. According to the low voltage ride-through characteristic, it can be considered that the active current is 0, and the reactive current reaches the maximum value . Substituting it into equation (5), we get before fault clearing . At the moment of fault clearing and recover, decreases. At this time, the phasor relationship is as Figure 5 shown.

[0043] At this time, the relationship between each quantity is obtained by the cosine theorem: (7).

[0044] Since , , then there is: (8).

[0045] Let the transient overvoltage threshold be U safe , substituting each quantity into the above equation (8), the grid-forming ratio in the new energy substation i can be obtained as The minimum of 2 is: (9) In the formula: X GFM is the output reactance of the grid-forming type; n is the multiple of the converter capacity in substation i to the system capacity; X L is the total reactance of the outgoing lines and transformers in the substation; X iis the Thevenin equivalent reactance on the grid side; I q,max is the maximum value of the grid-connected reactive current; U i is the Thevenin equivalent power supply on the grid side; U GFM is the internal electromotive force of the grid-forming type.

[0046] When the grid-forming type is not overloaded and the voltage safety threshold U safe takes 1.3 p.u., the following can be obtained: (10).

[0047] Step (2-2-2) configures the grid-forming ratio of a single substation: When configuring the grid-forming ratio of substation i, it should be ensured that the new energy units do not trip off the grid during the fault and the transient overvoltage is lower than the safety threshold after the fault is cleared. Therefore, the grid-forming capacity ratio of substation i is: (11).

[0048] The result obtained by this calculation method can enable the substation to meet the constraints of unit non-disconnection from the grid and transient overvoltage after a short-circuit fault. And at this time, the grid-forming capacity ratio is the smallest, and the construction cost of the grid-forming and grid-connected hybrid substation is the lowest.

[0049] Step (2-3) calculates the initial values of the grid-connected and grid-forming capacity ratios of each substation, specifically including: Step (2-3-1) without considering the influence of other substations, calculate the grid-forming capacity ratio of each substation according to the above steps; Step (2-3-2) considering the impedance and power supply of the grid-forming type converters inside the substation, recalculate the Thevenin equivalent impedance and power supply outside substation i to obtain a new Thevenin equivalent model.

[0050] Step (2-3-3) based on the model in Step (2-3-2), use the method in Step (2-2) above to recalculate the grid-forming capacity ratios of individual substations respectively to obtain the initial values of the grid-connected and grid-forming ratios of each substation. At this time, the initial value of the grid-forming capacity ratio inside substation i is , the impedance and power supply in the equivalent new model outside the substation are X i ’ and U i ’ respectively. Substituting them into Equation (3) respectively gives U L ’, U M ’ and U g ’.

[0051] Due to numerous internal parameters and complex structures in new energy power stations, the technical solution of this embodiment can simplify the analysis process and improve the calculation speed. It can quickly calculate the capacity ratio of the structure-connected network that meets the transient overvoltage constraints of most power stations, and then obtain a new model by considering the voltage support of other power stations for this power station. Based on the analysis of the new model, the accuracy of the capacity ratio is continuously improved.

[0052] Step 3: PLL stability discrimination method: Under low voltage conditions during a fault, the PLL cannot always remain stable. As can be seen from equations (4) and (5), the phase-locked angle directly affects the transient overvoltage. Therefore, when configuring the structure-connected network converter according to the initial value in the power station, the transient overvoltage may not meet the requirements in the case of PLL instability. It is necessary to further consider the influence of PLL stability on the ratio and verify whether the initial value of the structure-connected network ratio can meet the transient overvoltage requirements.

[0053] Step (3-1) Solving the PLL stability condition: When during a fault holds, the PLL can be considered stable, otherwise it is unstable. Substituting into equation (5) gives: (12).

[0054] Rearranging the above equation and using the trigonometric sum and difference formula to combine the terms containing U L yields: (13).

[0055] The above equation is in the form of Asin x -Bcos x =C. Introducing the amplitude R and the phase gives: (14).

[0056] Substituting equation (14) into equation (13) simplifies it to: (15).

[0057] For the above equation to have a solution, it is necessary to satisfy: (16).

[0058] Substituting equation (3) into the above equation and simplifying, the condition for equation (16) to have a solution is: (17).

[0059] When there is a three-phase short circuit outside the power station, U i ’=0, the above PLL steady-state condition can be simplified to: (18).

[0060] Therefore, when the substation i encounters a three-phase short-circuit fault and the in-network active output current in the substation does not satisfy the above formula (18), the formula (15) has no solution, and there is no equilibrium point. , and the PLL becomes unstable.

[0061] This formula (18) can determine whether the PLL can remain stable when a three-phase short circuit occurs outside the substation. Its form is simple and intuitively shows that the stability of the PLL is related to the in-network active output current, the internal potential of the network-forming type, and the reactance.

[0062] Step (3-2) Phase angle calculation method when the PLL becomes unstable: From Figure 3 The in-network control characteristics shown, the control equation of the PLL is: (19) In the formula: U 1,q is the q-axis component of the grid-connected point voltage; k p is the proportionality coefficient; k i is the integral coefficient.

[0063] During a three-phase short circuit, according to formula (2) and dq transformation, it can be obtained U 1,q as: (20) In the formula: I d,ref is the in-network output current command value; X M is the network-forming output reactance reduced to the system capacity; X L is the total reactance of the outgoing lines and transformers in the substation; X i ’ is the equivalent reactance of the new model; U M ’ is the network-forming type voltage amplitude component at the grid-connected point under the new model; is the phase-locked angle; is the initial value of the network-forming output voltage phase angle.

[0064] Substituting the above formula (20) into formula (19), it can be obtained: (21).

[0065] Changing the differential equation with variable coefficients into a second-order linear differential equation with constant coefficients. When the equilibrium point does not exist, the PLL phase approximately changes monotonically in a power series and diverges, and within a short time, the PLL output phase angle The change is not significant. Therefore, in Equation (21), the initial value before the fault can be used to replace it. So Equation (21) can be simplified to: (22) (23).

[0066] Therefore, the complete solution of Equation (22) is: (24).

[0067] Where K 1 and K 2 can be obtained from the initial value and the initial value of the derivative respectively: (25).

[0068] This method is based on the dynamic characteristics of the grid-following converter PLL, converts the variable-coefficient differential equation into a constant-coefficient differential equation, greatly reduces the difficulty of solving, and can obtain the analytical solution of the phase-locked angle when the PLL is unstable.

[0069] Step 4: Transient overvoltage calculation method: When the grid-forming converter of substation i is configured according to the initial value , it is judged by Equation (18) whether the PLL can remain stable after encountering a three-phase short-circuit fault. If it is unstable, the transient overvoltage needs to be verified. The following presents the calculation method of the transient overvoltage when the PLL is unstable.

[0070] Step (4-1) Calculate the phase-locked angle error: At the moment of fault clearing, the grid voltage U i ' resumes its supporting effect on the grid-connected point voltage. According to Equation (5), instantly decreases, while cannot change suddenly, and the phase-locked loop generates an error, resulting in the inaccuracy of the grid-following output current, which may worsen or alleviate the transient overvoltage. Therefore, substituting the calculated when the PLL is unstable in the previous section into Equation (5) to calculate .

[0071] Step (4-2) Calculate the transient overvoltage: Before fault clearing, the output current of the current loop reaches the command value, that is, I T = I T,ref , and from Equation (1) it can be seen that . The grid-following output current is proportional to its capacity ratio, . The internal impedance of the grid-forming type is inversely proportional to the grid-forming capacity, .

[0072] Substituting each quantity into Equation (4), the transient overvoltage calculation formula for substation i is obtained as follows: (26) Each quantity in the formula can be obtained from the internal parameters of the substation and the converter control parameters respectively.

[0073] This transient overvoltage calculation formula can reflect the generation mechanism of transient overvoltage and the key influencing factors. It is applicable to the calculation of transient overvoltage under different converter characteristics and different ratios of the capacity of the grid-connected network.

[0074] Step Five: Method for the ratio of the capacity of the grid-connected network: Taking substation i as an example, based on the initial value of the ratio of the grid-connected network , the transient overvoltage is verified based on the PLL stability, and the ratio of the grid-connected network is corrected. The specific steps are as follows: Step (5-1) According to the type and control parameters of the grid-connected converter, calculate the active current output command of the grid-connected network during the fault when substation i encounters a three-phase short-circuit fault externally I d,ref . Substitute I d = I d,ref into Equation (18) to determine whether the PLL can remain stable.

[0075] Step (5-2) If Equation (18) holds, the PLL can be stable. The proportion of the grid-forming capacity of substation i remains unchanged at the initial value, that is .

[0076] Step (5-3) If Equation (18) does not hold, the PLL loses stability. First, calculate the transient overvoltage when the PLL becomes unstable according to Equation (24) in Step Three. Then, use the transient overvoltage calculation method considering the phase-locked loop error and the PLL instability situation in Step Four to verify the transient overvoltage. When the transient overvoltage is lower than the safety threshold U safe , the initial value of the grid-forming ratio remains unchanged; when the requirement is not met, it can be derived from Equation (26) that the grid-forming capacity ratio satisfying the transient overvoltage constraint should be Equation (27), and the grid-forming ratio of substation i is changed from to : (27) In the formula: X GFM is the output reactance of the grid-forming type; n is the multiple of the converter capacity in substation i to the system capacity; U i ’ and X i ’ are the equivalent power supply and reactance of the new model; X Lis the total reactance of the outgoing line and transformer in the substation; I T,ref and are the grid-following output current and phase angle command value; is the phase-locked angle; is the corrected phase angle of the grid connection point voltage; U safe is the voltage safety threshold; is the initial value of the phase angle of the grid-forming output voltage.

[0077] In this embodiment, by using this equation to solve the grid-forming capacity ratio, the characteristics of the grid-following output current and the phase-locked loop error can be taken into account. At the same time, it can effectively ensure that when the PLL is unstable, the transient overvoltage of the substation can still meet the requirement of being less than the safety threshold U safe .

[0078] Similarly for step (5-4), configure the grid-following and grid-forming ratios of the other (m - 1) substations according to the above steps.

[0079] Thus, the grid-following and grid-forming converter capacity ratios considering transient overvoltage constraints in the new energy transmission system are completed. The proportion of the grid-forming capacity in substation i is , and the proportion of the grid-following capacity is . The total grid-forming capacity in the system is , and the total grid-following capacity is .

[0080] In specific implementation, the detailed process is shown in Appendix Figure 6 . Specifically, it includes: simplifying the system based on Thevenin equivalent and converter characteristics, calculating the voltage amplitude and phase angle of the grid connection point of substation i, then configuring the proportion of the grid-forming in a single substation according to the constraints of the unit not disconnecting from the grid and transient overvoltage, and then calculating the initial value of the grid-following and grid-forming capacity ratio outside substation i to obtain a new model. Judge whether the PLL in substation i can be stable. If so, the grid-forming ratio remains unchanged at the initial value, and the grid-forming ratio and grid-following ratio in substation i are obtained. Configure other substations according to the above steps to obtain the grid-following and grid-forming capacity ratios of each substation in the system. When judging whether the PLL in substation i can be stable, if not, calculate the phase-locked angle, solve the transient overvoltage, and correct the grid-forming ratio.

[0081] Embodiment 2 The purpose of this embodiment is to provide a computer device, including a memory, a processor, and a computer program stored on the memory and executable on the processor. When the processor executes the program, the steps of the above method are implemented.

[0082] Embodiment 3 The purpose of this embodiment is to provide a computer-readable storage medium.

[0083] A computer-readable storage medium having a computer program stored thereon, and when the program is executed by a processor, the steps of the above method are executed.

[0084] Embodiment 4 The purpose of this embodiment is to provide a new energy power station structure-network capacity ratio matching system considering transient overvoltage, including: An equivalent model construction module, configured to: simplify the structure-network parallel-connected power station to obtain an equivalent model; A structure-network ratio initial value calculation module, configured to: calculate the initial structure-network ratio values of each power station based on the obtained equivalent model; A phase-locked loop stability condition solving module, configured to: solve the phase-locked loop stability conditions; A transient overvoltage calculation module, configured to: when configuring a grid-forming converter for any power station according to the calculated initial structure-network ratio value, judge whether the phase-locked loop can remain stable after the power station encounters a three-phase short-circuit fault through the phase-locked loop stability condition, and calculate the transient overvoltage of the power station if it becomes unstable; A capacity ratio calculation module, configured to: when the transient overvoltage is lower than the safety threshold U safe keep the initial structure-network ratio value; when the requirement is not met, deduce the expression of the grid-forming capacity ratio under the constraint of transient overvoltage, and correct the structure-network ratio of the power station to obtain the grid-forming capacity ratio and the grid-connected capacity ratio in any power station.

[0085] Embodiment 5 The purpose of this embodiment is to provide a computer program product containing instructions, which when running on a computer, enables the computer to execute the methods and functions involved in any one of the above embodiments.

[0086] The steps involved in the devices of the above embodiments correspond to those of Method Embodiment 1, and the specific implementation manners can be referred to the relevant description part of Embodiment 1. The term "computer-readable storage medium" should be understood to include a single medium or multiple media containing one or more instruction sets; it should also be understood to include any medium that can store, encode, or carry an instruction set for execution by a processor and enable the processor to execute any method in the present invention.

[0087] Those skilled in the art should understand that the above modules or steps of the present invention can be implemented by a general computer device. Optionally, they can be implemented by program codes executable by a computing device, so that they can be stored in a storage device and executed by the computing device, or they can be separately fabricated into individual integrated circuit modules, or multiple modules or steps among them can be fabricated into a single integrated circuit module for implementation. The present invention is not limited to any specific combination of hardware and software.

[0088] Although the specific implementation manners of the present invention have been described above in conjunction with the accompanying drawings, it is not a limitation on the protection scope of the present invention. Those skilled in the art should understand that various modifications or deformations that can be made without creative efforts on the basis of the technical solutions of the present invention are still within the protection scope of the present invention.

Claims

1. A method for matching the capacity ratio of a new energy power station considering transient overvoltage with the grid-forming capacity, characterized in that Including: Simplify the hybrid structure-networked substation to obtain an equivalent model; Based on the obtained equivalent model, calculate the initial values of the structure-network ratio for each substation; Solve the stability conditions of the phase-locked loop; When configuring the structure-network converter for any substation according to the calculated initial value of the structure-network ratio, determine whether the phase-locked loop can remain stable after the substation encounters a three-phase short-circuit fault through the phase-locked loop stability condition judgment. If it becomes unstable, calculate the transient overvoltage of the substation; When the transient overvoltage is lower than the safety threshold U safe Keep the initial value of the grid-forming proportion; when the requirements are not met, the expression of the grid-forming capacity proportion under the transient overvoltage constraint is derived, and the grid-forming proportion of the substation is corrected to obtain the grid-forming capacity proportion and the grid-following capacity proportion in any substation.

2. The method for matching the capacity ratio of a new energy power station considering transient overvoltage to the grid-forming capacity according to claim 1, characterized in that, The process of simplifying the hybrid structure-networked substation to obtain an equivalent model specifically includes: Model the external topology of the new energy transmission system substation; Model the internal converters of the new energy transmission system substation; Model the hybrid structure-networked substation; Make assumptions about the hybrid model; Based on the above process, obtain the equivalent model.

3. The method for matching the structure network capacity of a new energy power station considering transient overvoltage according to claim 1, characterized in that Making assumptions about the hybrid model includes: Both the grid-connected type and structure-network type converters switch to the low-ride-through state during the fault and enter the low-ride-through recovery state after the fault is cleared; The control mode delay during the fault occurrence and clearing can be ignored; During the fault, the current loop regulation process can be ignored for the phase-locked loop characteristics, and it is considered that the current loop has reached the command value; The power angle of the structure-network converter during the fault is equal to the steady-state value before the fault.

4. The method for matching the structure network capacity of a new energy power station considering transient overvoltage as described in claim 1 is characterized in that Based on the obtained equivalent model, calculating the initial values of the structure-network ratio for each substation specifically includes: According to the simplified model of the hybrid substation and the circuit superposition theorem, obtain the grid connection point voltage of substation i; Based on the grid connection point voltage of substation i, calculate the structure-network capacity ratio of a single substation; Based on the structure-network capacity ratio of a single substation, calculate the initial values of the structure-network capacity ratios of each substation.

5. The method for matching the capacity of a new energy power station considering transient overvoltage with the network-forming capacity according to claim 1, wherein Based on the grid connection point voltage of substation i, calculating the structure-network capacity ratio of a single substation is specifically: Calculate the capacity ratio of the structure-network converter according to the requirement of ensuring that the unit does not trip off the grid; Calculate the capacity ratio of the structure-network converter according to the transient overvoltage constraint; When configuring the network connection ratio of substation i, h i it is necessary to ensure that the new energy units do not disconnect from the grid during the fault and the transient overvoltage is lower than the safety threshold after the fault is cleared.

6. The method for matching the structure network capacity of a new energy power station considering transient overvoltage as described in claim 1 is characterized in that instability When calculating the transient overvoltage of the substation, the calculation process is: Calculate the phase-locked angle error: Substitute the calculated PLL output phase angle into the phase angle expression to obtain the phase-locked angle error; Based on the phase-locked angle error, the output current of the current loop before the fault clearance, the output current of the grid-connected type, and the internal impedance of the structure-network type, substitute them into the voltage amplitude expression to obtain the calculation formula for the transient overvoltage of substation i.

7. A new energy power station structure network capacity ratio system considering transient overvoltage, characterized in that, Including: An equivalent model construction module, configured to: simplify the hybrid structure-networked substation to obtain an equivalent model; A structure-network ratio initial value calculation module, configured to: based on the obtained equivalent model, calculate the initial values of the structure-network ratio for each substation; A phase-locked loop stability condition solving module, configured to: solve the stability conditions of the phase-locked loop; A transient overvoltage calculation module, configured to: when configuring the structure-network converter for any substation according to the calculated initial value of the structure-network ratio, determine whether the phase-locked loop can remain stable after the substation encounters a three-phase short-circuit fault through the phase-locked loop stability condition judgment. If it becomes unstable, calculate the transient overvoltage of the substation; The capacity ratio calculation module is configured to: when the transient overvoltage is lower than the safety threshold U safe maintain the initial value of the grid-forming ratio; when the requirements are not met, derive the grid-forming capacity ratio expression under the transient overvoltage constraint, and correct the grid-forming ratio of the substation to obtain the grid-forming capacity ratio and the grid-following capacity ratio in any substation.

8. A computer program product, comprising a computer program, characterized in that, When the computer program is executed by the processor, it implements the method described in any one of claims 1 to 6.

9. A computer device, comprising a memory, a processor, and a computer program stored on the memory and executable on the processor, characterized in that, When the processor executes the program, it implements the steps of the method described in any one of the above claims 1-6.

10. A computer-readable storage medium having a computer program stored thereon, characterized in that, When the program is executed by the processor, it executes the steps of the method described in any one of the above claims 1-6.

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