New energy networking flexible direct current system fault current sequence component characteristic characterization method

Abstract

The invention provides a new energy networking flexible direct current system fault current sequence component characteristic characterization method, which comprises the following steps of: considering a fault ride-through control strategy of alternating current transmission line side equipment in a new energy networking flexible direct current system, and establishing an equivalent sequence network diagram of the system; solving analytic expressions of fault current sequence components on two sides of the line under different fault types based on the equivalent sequence network diagram; quantizing a fault current sequence component corresponding to each fault type through an analytical expression; and extracting influence parameters of the fault current sequence component, and representing the characteristics of the fault current sequence component in combination with the analytical expression. According to the method, the fault current sequence components under different fault types are systematically quantified, the association rule of the influence parameters and the fault current sequence components is represented, a theoretical basis is provided for formulation, optimization and parameter adjustment of a conventional control strategy, the adaptability and the operation reliability of the control strategy to different fault scenes are remarkably improved, and the method is suitable for large-scale popularization and application. And technical support can be directly provided for safe and stable operation of the new energy network construction flexible direct current system.

Description

Technical Field

[0001] This invention relates to the field of power technology, specifically to a method for characterizing the fault current sequence component characteristics of a new energy grid-connected flexible DC system. Background Technology

[0002] Driven by carbon reduction goals, the flexible direct transmission system for new energy power grids has become an important application technology. Both sides of its AC transmission line are equipped with power electronic equipment. After a fault, it is affected by fault ride-through control, and the fault current exhibits significant weak-feed controlled characteristics, which differs markedly from the fault characteristics of traditional synchronous machine systems. However, existing protection strategies are still designed based on the power frequency fault characteristics of traditional synchronous machine systems, making it difficult to adapt to the changes in fault current under new scenarios.

[0003] Existing technologies focus on system stability analysis and low-voltage control strategy optimization, but lack sufficient research on the specific characteristics of fault current sequence components on both sides of AC lines. This results in a lack of targeted adjustment basis for conventional control strategies in practical applications, which in turn leads to the inability of conventional control strategies to accurately adapt to system states under different fault types and parameter conditions, affecting the reliability and adaptability of control effects.

[0004] In summary, the ability to characterize the sequence component of AC line fault current in new energy grid-connected flexible DC systems is insufficient, resulting in a lack of basis for conventional control strategies and technical problems with poor adaptability and reliability. Summary of the Invention

[0005] This invention addresses the problems existing in the prior art by providing a method that can clearly characterize the fault current sequence component characteristics of a new energy grid-connected flexible DC system, thereby solving the aforementioned problems.

[0006] To achieve the above objectives, the technical solution adopted by the present invention is as follows: This invention provides a method for characterizing the fault current sequence component of a new energy grid-connected flexible DC system, which includes the following steps: The fault ride-through control strategy for AC transmission line side equipment in the new energy grid flexible DC system is considered, and the equivalent sequence network diagram of the system is established. Based on the fault ride-through control strategy and the equivalent sequence network diagram, the analytical expressions of the fault current sequence components on both sides of the line under different fault types are solved. The fault current sequence components corresponding to each fault type are quantified using the analytical expression. Based on the fault control characteristics of the new energy grid-connected flexible DC system, the influence parameters of the fault current sequence component are extracted. The fault current sequence component characteristics are characterized by the analytical expression and the correlation of the influencing parameters.

[0007] Optionally, the fault ride-through control strategy includes low-voltage ride-through control for new energy converters and virtual synchronous machine fault ride-through control for flexible DC converter stations. The low-voltage ride-through control of the new energy converter includes reactive current support control and negative sequence current suppression control. The virtual synchronous machine fault ride-through control of the flexible DC converter station includes power reference value switching, internal potential adjustment, and virtual impedance input control.

[0008] Optionally, the fault types include single-phase ground fault, two-phase-to-phase fault, and two-phase-to-ground fault.

[0009] Optionally, in the equivalent sequence network diagram, the positive sequence network, negative sequence network, and zero sequence network of the system corresponding to the single-phase ground fault are connected in series at the fault point; The positive-sequence network and negative-sequence network of the system corresponding to the two-phase interphase fault are connected in parallel at the fault point; The positive-sequence network, negative-sequence network, and zero-sequence network of the system corresponding to the two-phase ground fault are connected in parallel at the fault point.

[0010] Optionally, extracting the influencing parameters includes: The adjustment parameters of the fault ride-through control and / or the inherent physical parameters of the fault scenario are selected as the influencing parameters.

[0011] Optionally, the influencing parameters include the reference value of the virtual synchronous machine potential in the flexible DC converter station and the transition resistance at the fault point.

[0012] Optional, also includes: A simulation model needs to be built based on the simulation platform to match the topology of the new energy grid flexible DC system and the fault ride-through control characteristics of the equipment; Based on the characterization results of the fault current sequence component, the verification is performed using the controlled variable method, and the verification results are output. The method is evaluated and / or modified based on the verification results.

[0013] Optionally, in the controlled variable method, the range of the reference value of the internal potential of the virtual synchronous machine is determined based on the rated voltage of the flexible DC converter station and the fault ride-through control strategy.

[0014] Optionally, in the controlled variable method, the range of values ​​for the transition resistance shall at least include the minimum equivalent grounding resistance to the maximum equivalent grounding resistance under fault scenarios; The range of values ​​for the transition resistor also includes typical fault grounding resistance values.

[0015] Optionally, based on the electrical characteristics of the system equipment, analytical expressions can be obtained by solving the node current equations and loop voltage equations of the equivalent sequence network diagram.

[0016] Compared with the prior art, the present invention has the following beneficial effects: This invention quantifies the fault current sequence components under different fault types, characterizes the correlation between influencing parameters and fault current sequence components, provides a basis for the formulation, optimization and parameter adjustment of conventional control strategies, significantly improves the adaptability and operational reliability of control strategies to different fault scenarios, and can directly provide technical support for the safe and stable operation of new energy grid-connected flexible DC systems. Attached Figure Description

[0017] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0018] Figure 1 This is a flowchart illustrating a specific embodiment of the method of the present invention; Figure 2 This is a system topology diagram of a specific embodiment of the present invention; Figure 3 This is a trend diagram of the zero-sequence component variation of the new energy side current in a specific embodiment of the present invention; Figure 4a This is a graph showing the variation trend of the positive sequence component of the flexible DC side current under a ground fault in a specific embodiment of the present invention. Figure 4b This is a graph showing the variation trend of the negative sequence component of the flexible DC side current under a ground fault in a specific embodiment of the present invention. Figure 4c This is a graph showing the variation trend of the zero-sequence component of the flexible DC side current under a ground fault, according to a specific embodiment of the present invention. Figure 5a This is a graph showing the variation trend of the positive sequence component of the flexible DC side current under phase-to-phase faults in a specific embodiment of the present invention. Figure 5b This is a graph showing the variation trend of the negative sequence component of the flexible DC side current under phase-to-phase faults in a specific embodiment of the present invention. Figure 6 This is a schematic diagram illustrating the setting of fault points in the outgoing line according to a specific embodiment of the present invention; Figure 7a This is a sequence component diagram of renewable energy sources during a single-phase grounding fault, as shown in a specific embodiment of the present invention. g =0 ohms); Figure 7b This is a sequence component diagram of renewable energy sources during a single-phase grounding fault, as shown in a specific embodiment of the present invention. g =10 ohms); Figure 8a This is a sequence component diagram of renewable energy sources during a two-phase ground fault, as shown in a specific embodiment of the present invention (R).g =0 ohms); Figure 8b This is a sequence component diagram of renewable energy sources during a two-phase ground fault, as shown in a specific embodiment of the present invention (R). g =10 ohms); Figure 9a This is a single-phase ground fault flexible DC sequence component diagram (R) according to a specific embodiment of the present invention. g =0 ohms); Figure 9b This is a single-phase ground fault flexible DC sequence component diagram (R) according to a specific embodiment of the present invention. g =10 ohms); Figure 10a This is a two-phase interphase fault flexible direct sequence component diagram (R) of a specific embodiment of the present invention. g =0 ohms); Figure 10b This is a two-phase interphase fault flexible direct sequence component diagram (R) of a specific embodiment of the present invention. g =10 ohms); Figure 11 This is a trend diagram of the zero-sequence component variation on the renewable energy side during a single-phase grounding fault, according to a specific embodiment of the present invention. Figure 12 This is a trend diagram of the variation of the flexible DC side sequence component of a single-phase grounding fault according to a specific embodiment of the present invention; Figure 13 This is a trend diagram of the change of the flexible direct sequence component of the two-phase interphase fault in a specific embodiment of the present invention. Detailed Implementation

[0019] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0020] It should be noted that similar labels and letters in the following figures indicate similar items. Therefore, once an item is defined in one figure, it does not need to be further defined and explained in subsequent figures.

[0021] It should be noted that, for ease of understanding, the method steps in the specific embodiments of the present invention are described in a certain order, but those skilled in the art can change the order of the steps according to actual needs, so this should not be used as a limiting condition; further, in the description of the following specific embodiments, the superscripts and subscripts of each parameter should be understood as distinguishing marks of similar identifiers in accordance with common interpretations, representing parameters of the related or corresponding devices, and should not be understood as specific models or special marks.

[0022] like Figure 1 As shown in the figure, this embodiment provides a method for characterizing the fault current sequence component characteristics of a new energy grid-connected flexible DC system.

[0023] First, to facilitate the explanation of the method of this application, this embodiment takes a specific scenario and the corresponding new energy transmission system via flexible direct transmission as an example. Specifically, the new energy scenario is offshore wind power generation, and the topology of its flexible direct transmission system is as follows: Figure 2 As shown, the system mainly consists of a direct-drive wind farm, transformers, a flexible DC converter station, submarine cables, a DC system, and an onshore AC power grid. The offshore wind farm is connected to the flexible DC transmitting converter station via a 230kV AC submarine cable, and then connected to the flexible DC receiving converter station via a DC cable before being integrated into the AC power grid, enabling large-scale transmission of wind power from isolated offshore islands.

[0024] Among them, the new energy converter, namely the wind turbine converter, is a back-to-back voltage source converter. The turbine-side rectifier often adopts maximum power point tracking (MPPT) control to ensure that the wind turbine operates at optimal efficiency. It is connected to the grid-side inverter through the DC bus, without an intermediate gearbox, and the generator supplies full power to the grid. The grid-side inverter adopts direct current control, that is, power outer loop-current inner loop control, and achieves synchronization with the grid voltage through a phase-locked loop (PLL). The power outer loop controls the DC bus voltage U. dc The reactive power Q can stabilize the DC bus voltage and achieve unity power factor grid connection. The inner current loop uses a PI controller to achieve rapid tracking of the dq-axis current, and feedforward decoupling control is used to achieve independent regulation of active and reactive currents. Therefore, when the grid experiences a voltage drop due to a fault, the power imbalance at both ends of the wind farm's DC line causes a rise in DC voltage. To address this, a DC unloading circuit is introduced to absorb the surplus power generated during the fault, maintaining the system's instantaneous power balance and voltage stability.

[0025] The method includes the following steps: The fault ride-through control strategy for AC transmission line side equipment in the new energy grid flexible DC system is considered, and the equivalent sequence network diagram of the system is established.

[0026] First, regarding the fault ride-through control strategy, this embodiment includes low-voltage ride-through control for new energy converters and virtual synchronous machine fault ride-through control for flexible DC converter stations.

[0027] Specifically, in this embodiment, the new energy transmission system via flexible direct transmission takes a full-power inverter power supply, represented by a direct-drive wind turbine, as an example, and the converter station at the flexible direct transmission end adopts a virtual synchronous machine (VSG) control strategy.

[0028] Low-voltage ride-through control of renewable energy converters includes reactive current support control and negative sequence current suppression control. During fault periods, permanent magnet direct-drive wind turbines need to output reactive current to support the grid voltage. Under asymmetrical faults, the reactive current value is set according to the degree of voltage drop at the grid connection point to increase reactive power generation and achieve low-voltage ride-through. Simultaneously, to reduce the power second harmonic component, a negative sequence current suppression strategy is adopted. Therefore, when the grid connection point voltage drops to 0.9 pu, it switches to low-voltage ride-through control, the power outer loop saturates, and the positive and negative sequence current command values ​​are: ; ; In the formula, , These are the reference values ​​for the dq-axis components of the positive sequence current, respectively. , These are the reference values ​​for the dq-axis components of the negative sequence current, respectively. This represents the positive sequence voltage amplitude at the grid connection point of the wind turbine grid-side converter. The dynamic reactive current proportionality coefficient of the wind farm (with a value range of 1.5-3). This is the rated current value on the fan side; This represents the maximum allowable output current value of the converter.

[0029] The virtual synchronous generator (VSG) fault ride-through control in a flexible DC-DC converter station includes power reference value switching, internal potential adjustment, and virtual impedance switching control. At the flexible DC-DC converter station, the VSG control, by simulating the characteristics of a synchronous generator, provides inertia, damping, and voltage support for the power system, mainly consisting of a power control loop and a virtual impedance loop.

[0030] The active power-frequency control loop incorporates the generator rotor motion equations to simulate the rotational inertia and damping characteristics of a synchronous generator, thus mitigating grid frequency variations and improving system frequency stability. A second-order model of the synchronous generator is used to simulate the rotor motion equations, namely: ; In the formula, For virtual rotational inertia, This is the virtual damping coefficient; , These are the commanded and actual values ​​of the active power output by the VSG, respectively. , These are the actual and rated values ​​of the electrical angular velocity of the power grid, respectively. The phase angle of the internal potential of the virtual synchronous machine.

[0031] The reactive power-voltage control loop regulates the reactive power output of the converter, maintains grid voltage stability, and optimizes the power factor. The control equation is as follows: ; In the formula, This is the integral gain coefficient of the reactive power controller; The magnitude of the VSG virtual internal potential. This is the reference value for the VSG's no-load internal potential; under normal operating conditions, it is the rated value. ; , These are the commanded and actual values ​​of the reactive power output by the VSG, respectively.

[0032] The virtual impedance loop simulates the stator impedance, providing system inertia and thus regulating system stability. The modulation wave dq-axis voltage of the inverter is obtained by subtracting the voltage drop across the virtual impedance from the internal potential. , ,Right now: ; In the formula, For virtual inductance, For virtual resistance; , These are the dq-axis components of the output current, respectively.

[0033] After a fault, when the grid connection point voltage drops below 0.9 pu, the flexible DC converter switches to low-voltage ride-through control. During this fault period, the actual transmitted power of the VSG changes abruptly, resulting in a significant power difference between the actual and reference power. This causes the VSG output phase angle to swing during the system fault, potentially even causing the system to lose synchronization. Therefore, to maintain a constant VSG output phase angle during the fault, a low-voltage ride-through control strategy that switches the power reference value is adopted.

[0034] After the fault, the active power reference value will be... Switch to actual active power ,Right now To keep the phase angle of the converter's internal potential constant before and after the fault, i.e.: ; In the formula, the active power difference ; The internal potential of VSG is given.

[0035] To suppress short-circuit current and prevent damage to power electronic devices, the reference value of the VSG internal potential is typically reduced, and the virtual impedance is increased, indirectly limiting the voltage vector difference between the converter grid connection point and the fault point. Furthermore, the reactive power loop is frozen. This makes the VSG output voltage equal to the reference voltage. No deviation.

[0036] Therefore, the internal voltage reference value during the fault for: ; In the formula, The voltage sag ratio at the converter grid connection point; , These are the VSG output voltages before and after the fault. , The VSG output currents before and after the fault are shown respectively. , These are the virtual impedance values ​​before and after the fault, respectively; , These are the reference values ​​of the internal potential of the VSG before and after the fault.

[0037] During the fault phase, the VSG indirectly limits the fault current by reducing its internal potential and applying a virtual impedance, while also employing voltage vector control. , Therefore, it exhibits the characteristics of a positive-sequence voltage source, that is, the positive-sequence network is equivalent to a controlled voltage source with internal impedance, and the negative-sequence network is equivalent to a constant internal impedance, which is the virtual impedance value, and the positive-sequence voltage amplitude is the voltage reference value after the fault.

[0038] Based on the equivalent sequence network diagram, the analytical expressions of the fault current sequence components on both sides of the line under different fault types are solved. By simplifying the circuit and solving the circuit equations using Kirchhoff's voltage and current laws, an analytical expression for the sequence component of the fault current on the flexible DC side is obtained. At the same time, the influencing factors and changing trends of the sequence component of the fault current on the flexible DC side are analyzed to obtain the fault current characteristics, providing a basis for subsequent steps.

[0039] Furthermore, by analyzing the expressions, the fault current sequence components corresponding to each fault type are quantified. Among them, the positive sequence component of the fault current on the new energy side is the positive sequence current value of the converter output, with no negative sequence component; at the same time, the fault point is the only zero-sequence voltage source, and the zero-sequence component is related to the fault point voltage and the zero-sequence impedance values ​​of electrical components such as lines and transformers.

[0040] Therefore, the fault current on the new energy side is: ; ; In the formula, This refers to the positive-sequence fault current phasor output from the new energy converter side. The zero-sequence fault current phasor output from the new energy converter side; This represents the zero-sequence current allocation coefficient on the new energy side of the system. , For the system's flexible DC side zero-sequence impedance, For the zero-sequence impedance of the new energy side; This is the synchronous phase angle output by the phase-locked loop (PLL), which is the grid voltage phase angle reference value obtained by the converter when tracking the grid voltage through the PLL. For the zero-sequence component of the fault point current, we have: ; In the formula, , , These represent single-phase ground fault, two-phase phase-to-phase fault, and two-phase ground fault, respectively, with a transition resistance of... ; This represents the positive sequence component of the flexible DC side current. This represents the distribution coefficient of the zero-sequence current at the fault point under a two-phase-to-ground fault. ;in, The negative sequence impedance of the flexible direct side of the system. This is the system's overall zero-sequence impedance.

[0041] Based on the fault control characteristics of the flexible DC-DC converter system connected to the new energy grid, the influencing parameters of the fault current sequence component are extracted. Specifically, the fault current on the new energy side consists of a positive-sequence component and a zero-sequence component. Among them, the amplitude of the positive-sequence component is related to the maximum current limiting value of the new energy converter, and the phase angle is related to the voltage at the converter's grid connection point; the zero-sequence component is related to factors such as line parameters, grounding transformer impedance, reference value of VSG internal potential in the flexible DC-DC converter station, and the magnitude of transition resistance.

[0042] In the flexible DC-DC converter station, the equivalent sequence network diagrams of the system under different fault types have different connection forms at the fault point. Specifically, for a single-phase-to-ground fault system, the positive, negative, and zero-sequence networks are connected in series at the fault point; for a two-phase-to-phase fault system, the positive and negative sequence networks are connected in parallel at the fault point, with no zero-sequence network; and for a two-phase-to-ground fault system, the positive, negative, and zero-sequence networks are connected in parallel at the fault point. Using Kirchhoff's voltage and current laws in the equivalent sequence network diagram, by writing the equations for the node currents and loop voltages, the positive-sequence component of the fault current on the flexible DC side can be obtained as follows: ; In the formula, This is the reference value for the internal potential of the VSG in the flexible DC converter; This represents the positive-sequence impedance of the flexible DC side of the system. Simultaneously, the negative-sequence component of the current on the flexible DC side can be obtained. and zero-sequence components for: ; ; ; In the formula, This represents the distribution coefficient of the negative sequence and zero sequence currents at the fault point under a two-phase ground fault. ; This represents the zero-sequence current distribution coefficient on the flexible DC side of the system. .

[0043] Furthermore, the fault current on the renewable energy side consists of positive-sequence, negative-sequence, and zero-sequence components. Each component is related to factors such as line parameters, grounding transformer impedance, reference value of VSG potential in the flexible DC converter station, magnitude of transition resistance, and positive-sequence current component of the renewable energy converter output.

[0044] The adjustment parameters of fault ride-through control and / or the inherent physical parameters of the fault scenario are selected as influencing parameters. From the analytical expression of the fault current sequence component on both sides of the line and the influencing factors, it can be seen that when the electrical components of the power system and their related impedance parameters are constant, the sequence component mainly depends on the reference value of the VSG internal potential of the flexible DC converter station and the magnitude of the transition resistance. Therefore, the influencing parameters in this embodiment include the reference value of the VSG internal potential of the flexible DC converter station and the transition resistance at the fault point.

[0045] The characteristics of the fault current sequence component are characterized based on the analytical expression and the correlation of influencing parameters. Based on the analytical expressions for the current sequence components on both sides of the line (new energy side and flexible DC side), under a ground fault, the current on the new energy side consists of positive-sequence and zero-sequence components. The former is the equivalent current source of the converter, and its amplitude remains constant after the fault. The latter is related to the zero-sequence component of the fault point voltage and the zero-sequence impedance values ​​of the electrical components on both sides. The changes in its sequence components are as follows: Figure 3 As shown, the current decreases with increasing transition resistance and increases with increasing VSG internal potential. Under phase-to-phase fault conditions, the current on the new energy side only has a positive sequence component, and the three-phase waveforms are symmetrical.

[0046] The fault current on the flexible DC side consists of positive-sequence, negative-sequence, and zero-sequence components. The magnitude of each sequence component is mainly affected by the reference value of the VSG internal potential and the transition resistance. The changing trends of the sequence components of the flexible DC side current under ground faults and phase-to-phase faults are as follows: Figures 4a-4c and Figure 5a , Figure 5b As shown.

[0047] After a system fault occurs, when the internal potential is constant, as the transition resistance increases, the system approaches normal operating conditions, and the three-phase imbalance decreases. Therefore, the positive-sequence component of the fault current on the flexible DC side increases with the increase of the transition resistance, while the negative-sequence and zero-sequence components show a decreasing trend. When the transition resistance is small, the current on both sides flows to the fault point. The fault current on the flexible DC side increases with the increase of the VSG internal potential amplitude, so the positive-sequence, negative-sequence, and zero-sequence components all show an increasing trend. When the transition resistance is large, the current on the fan side is split into two loops: the flexible DC system and the fault point. The current on the flexible DC side exhibits a through current. The larger the VSG internal potential, the greater the voltage difference between the equivalent voltage source of the flexible DC converter station and the voltage at the fault point, and the greater the system imbalance. Therefore, the positive-sequence component of the fault current on the flexible DC side shows a decreasing trend, while the negative-sequence and zero-sequence components show an increasing trend.

[0048] Furthermore, the method in this embodiment also includes: To verify the magnitude and impact of fault current components on the flexible DC side of renewable energy transmission lines, a simulation model was built based on a simulation platform to match the topology of the renewable energy grid-connected flexible DC system and the fault ride-through control characteristics of the equipment. A fault point was then set at the midpoint of the AC transmission line. Figure 6 As shown.

[0049] Based on the characterization results of the fault current sequence component, the verification is performed using the controlled variable method, and the verification results are output.

[0050] 1. Control the reference value of the internal potential of the flexible DC side VSG; Different fault types are set at the fault point, the reference value of the internal potential of the VSG on the flexible DC side is fixed, and the value of the transition resistance is adjusted. Its range generally includes the minimum to maximum equivalent grounding resistance under the fault scenario. In this embodiment, for ease of explanation, the value of the transition resistance is a typical fault grounding resistance value, that is, the transition resistance is set to 0Ω and 10Ω respectively, and the fault current sequence components on both sides of the line are obtained as follows: Figures 7a-10b As shown.

[0051] In order to reflect the trend of the fault current sequence component with the transition resistance on both sides, the zero sequence component exists only under ground fault on the new energy side, so single-phase ground fault and two-phase ground fault are taken as examples; the flexible DC side is taken as symmetrical fault, single-phase ground fault and two-phase fault.

[0052] Under ground faults with different transition resistance values, the amplitude of the positive-sequence component of the fault current on the new energy converter side remains unchanged, while the zero-sequence component decreases as the transition resistance increases. Figure 3 The pattern shown is the same.

[0053] Depend on Figures 9a-10b As shown, under ground faults with different transition resistance values, the positive sequence component of the fault current on the flexible DC side increases with increasing transition resistance, while the negative sequence and zero sequence components decrease with increasing transition resistance. Figures 4a-4c and Figure 5a , Figure 5b The pattern shown is the same.

[0054] 2. Control the transition resistance; Different fault types are set at the fault point, the transition resistance is fixed, and the reference value E of the VSG internal potential of the flexible DC converter is changed. mmc The range of its reference value is determined based on the rated voltage of the flexible DC converter station and the fault ride-through control strategy. This yields the fault current sequence components on both sides of the line as follows: Figures 11-13 As shown. To illustrate the trend of the fault current sequence component on both sides with the reference value of the potential inside the VSG of the flexible DC converter station, the zero-sequence component exists only under ground fault on the new energy side, so a single-phase ground fault is taken as an example; on the flexible DC side, single-phase ground fault and two-phase-to-phase fault are taken as examples.

[0055] Depend on Figure 11 It can be seen that under ground faults with different VSG internal potential reference values ​​in flexible DC converter stations, the zero-sequence component of the fault current on the new energy converter side increases with the increase of the internal potential reference value, and is related to... Figure 3 The pattern shown is the same.

[0056] Depend on Figures 12-13 As shown, under ground faults and phase-to-phase faults with different VSG internal potential reference values ​​in flexible DC converter stations, the positive sequence component of the fault current on the flexible DC side decreases with increasing VSG internal potential reference value, while the negative sequence and zero sequence components increase with increasing VSG internal potential reference value. Figures 4a-4c and Figure 5a , Figure 5b The pattern shown is the same.

[0057] Based on the above verification results, the method of this specific embodiment can be evaluated and modified.

[0058] The above analysis and verification show that, on the power frequency timescale, the fault current mainly consists of a periodic component. The positive-sequence component on the flexible DC side decreases with increasing VSG internal potential reference value and increases with increasing transition resistance. The negative-sequence and zero-sequence components are affected by the degree of system imbalance and show the opposite trend to the positive-sequence component, i.e., they increase with increasing VSG internal potential reference value and decrease with increasing transition resistance. Simultaneously, the method of this application provides corresponding characterization formulas to determine the changes in the current sequence components, enabling more accurate characterization.

[0059] Furthermore, to illustrate the beneficial technical effects of this application, an exemplary application of the characterization results is as follows: based on the fact that existing engineering still uses current differential protection based on power frequency as the main line protection, its sensitivity is significantly reduced under high-resistance grounding faults. Considering the main influencing factors and changing trends of the flexible DC side sequence component in this application, after a fault, the magnitude of the potential inside the VSG of the flexible DC converter station can be quantitatively increased to increase the proportion of negative sequence and zero sequence components, thereby improving the adaptability of differential protection and improving the phenomenon of protection failure caused by weak double-end feeder characteristics after AC line faults in the scenario of new energy network flexible DC system transmission.

[0060] It should also be noted that the above exemplary content is only used to illustrate one application of the characterization results of this application. Similarly, the characterization results can also be used to update the judgment criteria of power grid protection devices. For example, if the original protection device has an ambiguous judgment on the current situation, the preset criteria based on the characterization results can clearly determine which fault corresponds to which current, thereby redesigning the protection logic (such as adding judgment of negative sequence and zero sequence current) to avoid misjudgment. The characterization results can also guide the precise adjustment of equipment parameters. For example, if the impact of increasing or decreasing the VSG internal potential on the current is known, an optimal value can be set to avoid the fault current being too large and burning out the equipment, while also allowing the protection device to accurately detect the fault. The characterization results can also assist in the rapid troubleshooting of faults: based on the ratio of the three sequence components of the current, maintenance personnel can quickly determine which type of fault it is and the severity of the fault (for example, a large zero sequence current may indicate a ground fault; a large negative sequence current may indicate an asymmetrical fault).

[0061] Finally, it should be noted that the above content is only used to illustrate the technical solution of the present invention, and is not intended to limit the scope of protection of the present invention. Simple modifications or equivalent substitutions made by those skilled in the art to the technical solution of the present invention do not depart from the essence and scope of the technical solution of the present invention.

Claims

1. A method for characterizing the fault current sequence component of a new energy grid-connected flexible DC system, characterized in that, Includes the following steps: The fault ride-through control strategy for AC transmission line side equipment in the new energy grid flexible DC system is considered, and the equivalent sequence network diagram of the system is established. Based on the fault ride-through control strategy and the equivalent sequence network diagram, the analytical expressions of the fault current sequence components on both sides of the line under different fault types are solved. The fault current sequence components corresponding to each fault type are quantified using the analytical expression. Based on the fault control characteristics of the new energy grid-connected flexible DC system, the influence parameters of the fault current sequence component are extracted. The fault current sequence component characteristics are characterized by the analytical expression and the correlation of the influencing parameters.

2. The method for characterizing the fault current sequence component of a new energy grid-connected flexible DC system according to claim 1, characterized in that, The fault ride-through control strategy includes low-voltage ride-through control for new energy converters and virtual synchronous machine fault ride-through control for flexible DC converter stations. The low-voltage ride-through control of the new energy converter includes reactive current support control and negative sequence current suppression control. The virtual synchronous machine fault ride-through control of the flexible DC converter station includes power reference value switching, internal potential adjustment, and virtual impedance input control.

3. The method for characterizing the fault current sequence component of a flexible DC power grid system according to claim 2, characterized in that, The fault types include single-phase ground fault, two-phase-to-phase fault, and two-phase-to-ground fault.

4. The method for characterizing the fault current sequence component of a new energy grid-connected flexible DC system according to claim 3, characterized in that, In the equivalent sequence network diagram, the positive sequence network, negative sequence network, and zero sequence network of the system corresponding to the single-phase ground fault are connected in series at the fault point; The positive-sequence network and negative-sequence network of the system corresponding to the two-phase interphase fault are connected in parallel at the fault point; The positive-sequence network, negative-sequence network, and zero-sequence network of the system corresponding to the two-phase ground fault are connected in parallel at the fault point.

5. The method for characterizing the fault current sequence component of a new energy grid-connected flexible DC system according to claim 3 or 4, characterized in that, Extracting the influencing parameters includes: The adjustment parameters of the fault ride-through control and / or the inherent physical parameters of the fault scenario are selected as the influencing parameters.

6. The method for characterizing the fault current sequence component of a new energy grid-connected flexible DC system according to claim 5, characterized in that, The influencing parameters include the reference value of the internal potential of the virtual synchronous machine in the flexible DC converter station and the transition resistance of the fault point.

7. The method for characterizing the fault current sequence component of a new energy grid-connected flexible DC system according to claim 6, characterized in that, Also includes: A simulation model needs to be built based on the simulation platform to match the topology of the new energy grid flexible DC system and the fault ride-through control characteristics of the equipment; Based on the characterization results of the fault current sequence component, the verification is performed using the controlled variable method, and the verification results are output. The method is evaluated and / or modified based on the verification results.

8. The method for characterizing the fault current sequence component of a new energy grid-connected flexible DC system according to claim 7, characterized in that, In the controlled variable method, the range of the reference value of the internal potential of the virtual synchronous machine is determined based on the rated voltage of the flexible DC converter station and the fault ride-through control strategy.

9. The method for characterizing the fault current sequence component of a new energy grid-connected flexible DC system according to claim 8, characterized in that, In the controlled variable method, the range of values ​​for the transition resistance should at least include the minimum equivalent grounding resistance to the maximum equivalent grounding resistance under fault scenarios. The range of values ​​for the transition resistor also includes typical fault grounding resistance values.

10. The method for characterizing the fault current sequence component of a new energy grid-connected flexible DC system according to claim 1, characterized in that, Based on the electrical characteristics of the system equipment, analytical expressions are obtained by solving the node current equations and loop voltage equations of the equivalent sequence network diagram.

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