Method, device, medium and equipment for judging symmetrical fault tripping of alternating-current transmission line of photovoltaic power station system

Abstract

The invention relates to a symmetrical fault tripping judgment method and device for a sending-out alternating-current line of a photovoltaic power station system, a medium and equipment. The method comprises the steps of obtaining a topological structure of the photovoltaic power station system and a preset judgment strategy of a stability control device; respectively analyzing fault characteristics when the following network type new energy and the construction network type new energy are connected to the power grid, and obtaining corresponding fault current changes; analyzing the change rule between the follow-up and network-building type new energy output current and the current measured by the stability control device in the fault state; and symmetrical fault tripping judgment logic of the photovoltaic power station system is constructed and obtained. According to the method, a reason for inapplicability of a traditional symmetric fault tripping criterion of a stability control device is obtained by analyzing fault characteristics of a follow-up network type new energy source and fault current characteristics of an alternating current fault line sent by a photovoltaic power station system, and a symmetric fault tripping criterion suitable for the alternating current line sent by the photovoltaic power station system is reconstructed. Accurate judgment of faults of the follow-up network type converter hybrid parallel system is achieved, and the electric power safety is improved.

Description

Technical Field

[0001] The present application relates to the technical field of power safety, and particularly relates to a method, device, medium and equipment for judging the symmetrical fault tripping of the outgoing AC line of a photovoltaic power station system. Background Art

[0002] With the rapid development of renewable energy represented by wind and light, the existing power system has gradually formed a pattern of centralized development of new energy and long-distance transmission of UHV AC and DC. The operating characteristics of the power system have changed greatly, and the stability problem of the power system is more complex than before. In order to ensure the safe and stable operation of the power grid, a safety and stability control device (referred to as a stability control device for short) is set in the power system. Based on complete power grid fault identification criteria, it correctly judges and identifies various faults or disturbances occurring in the power grid, and takes corresponding control measures. However, the existing traditional power grid fault identification criteria are mainly designed for traditional AC power systems. With the change of the fault characteristics of modern power systems, the existing traditional fault identification criteria based on electrical quantity measurement are no longer fully applicable.

[0003] In the scenario of centralized new energy output, traditional grid-connected converters can no longer provide enough inertia and damping for the system, resulting in a further reduction in the inertia of the system, thus endangering the safe operation of the power system. Grid-forming converters with the external characteristics of synchronous generators are also widely used in power systems. However, the fault characteristics of the hybrid parallel system formed by the combination of grid-connected converters and grid-forming converters are significantly different from those of traditional power systems. Therefore, the symmetrical fault tripping criteria of traditional AC power systems are difficult to apply to hybrid parallel systems. Summary of the Invention

[0004] To overcome the deficiencies of the above-mentioned prior art, the present application provides a method, device, medium and equipment for judging the symmetrical fault tripping of the outgoing AC line of a photovoltaic power station system, and specifically adopts the following technical solutions:

[0005] A method for judging the symmetrical fault tripping of the outgoing AC line of a photovoltaic power station system, the method includes the following steps:

[0006] Obtain the topological structure of the photovoltaic power station system and the preset judgment strategy of the stability control device; the photovoltaic power station system includes a grid-connected converter and a grid-forming converter; the preset judgment strategy includes sudden change start, voltage reduction and increase of the same-phase current, and a fault tripping signal;

[0007] Based on the low-voltage ride-through control strategy adopted by the grid-connected converter during grid connection faults, analyze the fault characteristics when the grid-connected new energy accesses the grid, and obtain the change of the fault current output by the grid-connected new energy;

[0008] Adjust the power loop based on the in-phase operation state of the network-forming converter during grid connection faults, analyze the fault characteristics when the network-forming new energy is connected to the grid, and obtain the change of the fault current output by the network-forming new energy;

[0009] Analyze the change rules among the output current of the grid-following new energy, the output current of the network-forming new energy, and the current measured by the stability control device under fault conditions;

[0010] Based on the change rules obtained from the analysis, construct the symmetric fault tripping judgment logic of the photovoltaic power station system.

[0011] Optional: The grid-following converter adopts a phase-locked loop control and uses a fault ride-through control strategy with reactive power support ability during grid connection faults: When a fault occurs until the voltage recovers to 0.9 pu, the grid-following new energy has the ability to support dynamic reactive current.

[0012] Optional: The steps of analyzing the fault characteristics when the grid-following new energy is connected to the grid and obtaining the change of the fault current output by the grid-following new energy include:

[0013] Based on the fault ride-through control strategy with reactive power support ability adopted when the fault occurs, obtain the reactive current output by the grid-following new energy;

[0014] Limit the positive sequence d-axis component of the short-circuit current based on the current-carrying capacity of the grid-connected inverter to obtain the active current output by the grid-following new energy;

[0015] Based on the reactive current and active current output by the grid-following new energy, obtain the fault current I output by the grid-following new energy GFL.f ;

[0016] Optional: The network-forming converter adopts a virtual synchronous machine control, and adjusts the active power through the phase angle output by the virtual synchronous machine in the active power loop of the power loop, and adjusts the output voltage reference value through the reactive power output by the reactive power loop.

[0017] Optional: The steps of analyzing the fault characteristics when the network-forming new energy is connected to the grid and obtaining the change of the fault current output by the network-forming new energy include:

[0018] Respectively obtain the mathematical models of the active power loop and the reactive power loop of the virtual synchronous machine in the network-forming new energy;

[0019] Based on the mathematical model of the reactive power loop, adjust the reactive power loop of the virtual synchronous machine under fault conditions to make the reactive power output by the virtual synchronous machine equal to the reactive power reference value, and the output voltage setting value is switched to the fault voltage to ensure that the virtual synchronous machine maintains an in-phase operation state;

[0020] Based on the mathematical model of the active loop, adjust the active loop of the virtual synchronous machine in the fault state to set the input power difference of the inertia link to 0, so as to ensure that the virtual synchronous machine maintains the power angle before the voltage dip and continues to operate during the voltage dip;

[0021] On the premise that the virtual synchronous machine maintains the adjusted reactive loop and active loop, obtain the internal potential of the virtual synchronous machine and the impedance value between the internal potential and the grid connection point;

[0022] Based on the internal potential of the virtual synchronous machine and the impedance value between the internal potential and the grid connection point, obtain the fault current I output by the network-forming new energy VSG.f 。

[0023] Optionally: The step of analyzing the variation law among the output current of the grid-following new energy, the output current of the network-forming new energy, and the current measured by the stability control device in the fault state includes:

[0024] When the voltage dip degree at the grid connection point deepens, the amplitude of the fault current output by the grid-following new energy increases until it reaches the maximum current allowed to flow through the inverter, and the angle by which the fault current output by the grid-following new energy lags behind the grid connection point voltage increases; the amplitude of the fault current output by the network-forming new energy decreases, and the angle by which the fault current output by the network-forming new energy leads the grid connection point voltage increases;

[0025] On the premise that the fault current measured by the stability control device is the sum of the fault current output by the grid-following new energy and the fault current output by the network-forming new energy, analyze and obtain that the included angle between the fault current output by the grid-following new energy and the fault current output by the network-forming new energy increases as the voltage dip degree at the grid connection point increases.

[0026] Optionally: The step of constructing the symmetric fault tripping judgment logic of the photovoltaic power station system based on the analyzed variation law includes:

[0027] Based on the fact that the fault current output by the grid-following power source increases as the voltage dip degree at the grid connection point increases, construct a first determination condition for whether the fault current output by the grid-following power source is greater than a preset threshold;

[0028] Take whether the fault current collected by the stability control device is greater than the preset threshold as the second determination condition, and form an OR logic with the first determination condition;

[0029] Form an AND logic with the OR logic formed by the first determination condition and the second determination condition and the preset judgment strategy of the stability control device;

[0030] Based on the combined logic gates, obtain the symmetric fault tripping judgment logic of the photovoltaic power station system.

[0031] Optionally, the stability control device is configured on the grid connection point bus of the photovoltaic power station system, and a stability control execution station is configured on the side of the grid-connected new energy power station. The stability control execution station collects the fault current signal output by the grid-connected new energy and transmits it to the stability control device at the grid connection point bus.

[0032] Optionally, the mathematical model of the active power loop of the virtual synchronous machine is as follows:

[0033]

[0034] where θ vsg is the output phase angle of the virtual synchronous machine; ω vsg is the differential of θ vsg ; P set is the input mechanical power; P is the active power output by the virtual synchronous machine; ω n is the rated angular frequency; J is the moment of inertia; D P is the damping coefficient;

[0035] The mathematical model of the reactive power loop of the virtual synchronous machine is as follows:

[0036] U ref = U0 + k q (Q ref - Q);

[0037] where U ref is the output voltage of the virtual synchronous machine; U0 is the set value of the output voltage; k q is the droop coefficient of the reactive power loop; Q ref is the reference value of the reactive power; Q is the reactive power output by the virtual synchronous machine.

[0038] In addition, the present application also discloses a device for judging the symmetrical fault tripping of the outgoing AC line of the photovoltaic power station system. The device includes:

[0039] A topology structure acquisition module, configured to acquire the topology structure of the photovoltaic power station system and the preset judgment strategy of the stability control device; the photovoltaic power station system includes a grid-connected converter and a grid-forming converter; the preset judgment strategy includes sudden change start, voltage reduction and increase of the same-phase current, and a fault tripping signal;

[0040] A first analysis module, configured to analyze the fault characteristics when the grid-connected new energy is connected to the grid according to the low-voltage ride-through control strategy adopted by the grid-connected converter during the grid connection fault, and acquire the change of the fault current output by the grid-connected new energy;

[0041] A second analysis module, configured to analyze the fault characteristics when the grid-forming new energy is connected to the grid according to the in-phase operation state adjustment power loop of the grid-forming converter during the grid connection fault, and acquire the change of the fault current output by the grid-forming new energy;

[0042] A current analysis module, configured to analyze the variation rules among the output current of grid-connected new energy, the output current of grid-forming new energy, and the current measured by a stability control device under a fault condition;

[0043] A judgment logic construction module, configured to construct a symmetrical fault tripping judgment logic for a photovoltaic power station system based on the analyzed variation rules.

[0044] In addition, this application discloses an electronic device, which includes a memory for storing computer program instructions and a processor for executing the program instructions. Among them, when the computer program instructions are executed by the processor, the device is triggered to execute the method steps described above.

[0045] This application also discloses a computer-readable medium, which stores computer-readable instructions that can be executed by a processor to implement the method steps described above.

[0046] Beneficial effects

[0047] The technical solution of this application has obtained the following beneficial effects:

[0048] The method for judging symmetrical fault tripping of the AC transmission line in the photovoltaic power station system of this application analyzes the fault characteristics of grid-connected new energy and grid-forming new energy, as well as the fault current characteristics of the AC fault transmission line in the photovoltaic power station system, obtains the reason why the traditional symmetrical fault tripping criterion of the stability control device is not applicable to the hybrid parallel system of grid-connected converters and grid-forming converters, and combines the judgment condition that the output fault current of the grid-connected power supply during a fault will be greater than the threshold value to construct a symmetrical fault tripping criterion applicable to the AC transmission line in the photovoltaic power station system, so as to achieve accurate judgment of faults in the hybrid parallel system of grid-connected converters and grid-forming converters and improve power security. Description of the drawings

[0049] Figure 1 It is the topological structure diagram of the photovoltaic power station system including grid-forming converters in the embodiment of this application.

[0050] Figure 2 It is the flowchart of the method for judging symmetrical fault tripping of the AC transmission line in the photovoltaic power station system in the embodiment of this application.

[0051] Figure 3 It is the control schematic diagram of the grid-forming converter adopting a virtual synchronous machine in the embodiment of this application.

[0052] Figure 4 It is the equivalent circuit diagram of the grid-connected system adopting a virtual synchronous machine in the embodiment of this application.

[0053] Figure 5It is the phasor change diagram of the PV power station system after a symmetrical fault occurs in the embodiment of this application.

[0054] Figure 6 It is the phasor change diagram of the network-forming new energy after a symmetrical fault occurs in the embodiment of this application.

[0055] Figure 7 It is the logic block diagram of the symmetrical fault tripping judgment logic of the PV power station system in the embodiment of this application.

[0056] Figure 8 It is the simulation waveform diagram of the output fault current of the grid-following new energy after a symmetrical fault occurs in the embodiment of this application.

[0057] Figure 9 It is the simulation waveform diagram of the current at the grid connection point after a symmetrical fault occurs in the embodiment of this application.

[0058] Figure 10 It is the structure diagram of the symmetrical fault tripping judgment device for the AC transmission line sent out by the PV power station system in the embodiment of this application.

[0059] Figure 11 It is the structure diagram of an electronic device in the embodiment of this application. Specific embodiments

[0060] The following further describes this application with reference to the accompanying drawings. The following embodiments are only used to more clearly illustrate the technical solutions of this application, and cannot be used to limit the protection scope of this application. It should be noted that the following detailed descriptions are all exemplary and are intended to provide further explanations for this application.

[0061] Combined with Figure 1 As shown in the PV power station system, under certain operating conditions, the reactive power in the power grid is excessive, which will cause the voltage of the power grid to rise. Seriously, the voltage will approach or reach the upper limit of the voltage required during system operation, and sometimes even exceed the voltage value specified by the system. At this time, the virtual synchronous machine has the properties of a synchronous machine and can absorb the excess reactive power in the power grid by operating in the leading phase to improve the voltage quality of the power grid.

[0062] Generally, when a symmetrical short - circuit fault occurs in the output line between the grid - connection point and the large power grid, the VSG still maintains the in - phase operation state and appears to absorb reactive power externally, that is, absorb reactive current. The grid - following converter has its own control logic. To avoid the new energy from disconnecting from the grid due to excessive voltage drop, a low - voltage ride - through control strategy (LVRT) is generally adopted, which appears to release reactive power externally, that is, send out reactive current. Since the fault current collected by the stability control device is obtained by adding the fault currents output by the grid - following converter and the grid - forming converter, in this case, the included angle between the two fault currents is greater than ninety degrees, and the fault current collected by the stability control device will be lower than the action threshold of the stability control device under certain circumstances, resulting in the stability control device failing to operate during the fault. Therefore, for a hybrid photovoltaic power station system including a grid - following converter and a grid - forming converter, the traditional fault criterion that only uses the stability control device cannot accurately identify the symmetrical fault of the outgoing AC line.

[0063] Combined with Figure 2 As shown, an embodiment of the present application discloses a method for judging the tripping of a symmetrical fault of an outgoing AC line in a photovoltaic power station system. The method includes the following steps:

[0064] Step 1: Obtain the topological structure of the photovoltaic power station system and the preset judgment strategy of the stability control device. In this embodiment, the photovoltaic power station system includes a grid - following converter and a grid - forming converter, where the grid - following converter uses a phase - locked loop (PLL) control. The grid - forming converter adopts a virtual synchronous generator (VSG) control. And the grid - following converter and the grid - forming converter are connected to the large power grid through a point of common coupling (PCC), as Figure 1 shown.

[0065] It should be noted that the preset judgment strategy of the stability control device (i.e., the traditional fault criterion of the stability control device) in this embodiment includes sudden - change startup, voltage reduction and increase of the same - phase current, and the appearance of a fault - tripping signal. Among them, sudden - change startup means that when the electrical quantities (such as current, voltage, etc.) in the system change suddenly. The reduction of voltage and the increase of the same - phase current are one of the typical characteristics of a symmetrical fault of an AC line. By jointly monitoring these two electrical quantities, the symmetrical fault of the AC line can be accurately identified. And the appearance of a fault - tripping signal indicates that a fault - tripping event has occurred in the current power system. In a traditional AC power system, based on the above signals, the stability control device can timely identify and judge whether a symmetrical fault of an AC line occurs, and take corresponding control measures in time.

[0066] Step 2: Based on the low-voltage ride-through control strategy adopted by the grid-following converter during grid connection faults, analyze the fault characteristics when grid-following new energy is connected to the grid, and obtain the change in the fault current output by the grid-following new energy.

[0067] Specifically, in this Step 2, the grid-following converter adopts a phase-locked loop control and a fault ride-through control strategy with reactive power support ability during grid connection faults: when a fault occurs until the voltage recovers to 0.9 pu, the grid-following new energy has the ability to dynamically support reactive current, and the reactive current output by the grid-following new energy tracks the voltage change at the grid connection point. At this time, based on the fault ride-through control strategy with reactive power support ability adopted when the fault occurs, the reactive current output by the grid-following new energy can be obtained as:

[0068]

[0069] where u pcc.f is the per-unit value of the grid connection point voltage during system faults; I q,f is the reactive current value output by the grid-following new energy during system faults; I N is the rated current value;

[0070] When a grid fault occurs, the grid connection point voltage decreases. According to the LVRT control strategy, the adjustment trends of each reference value are as follows: the decrease in the grid connection point voltage u pcc.f results in a decrease in the output power P out , causing the difference ΔP between the output power P out and the reference power P ref to increase, and finally increasing the d-axis current output by the grid-following new energy. According to the requirements of the grid connection regulations, when outputting reactive current, the maximum active current should be output within the current limit range of the inverter to maintain the active power balance of the grid as much as possible.

[0071] Therefore, based on the current-carrying capacity of the grid-connected inverter, the positive-sequence d-axis component of the short-circuit current is limited, and the active current output by the grid-following new energy is obtained as:

[0072]

[0073] where I d,f is the active current value output by the grid-following new energy during system faults; P m is the active power output by the new energy before the fault during grid connection; I max is the maximum current allowed to flow through the inverter;

[0074] Based on the reactive current and active current output by the grid-following new energy obtained above during system faults, it can be known that the fault current I GFL.f output by the grid-following new energy is determined by the grid connection point voltage u pcc.f during system faults;

[0075] When there is a system fault, the grid-following new energy is equivalent to a current source controlled by the fault voltage at the point of common coupling. The fault current I GFL.f output by the grid-following new energy is as follows:

[0076] I GFL.f = F(u pcc.f );

[0077] Among them, the amplitude of the fault current output by the grid-following new energy is The phase angle difference between the fault current output by the grid-following new energy and the voltage u pcc.f at the point of common coupling is arctan(I q,f / I d,f ).

[0078] Step 3: Adjust the power loop based on the in-phase operation state of the grid-forming converter during grid connection faults, analyze the fault characteristics when the grid-forming new energy is connected to the grid, and obtain the change in the fault current output by the grid-forming new energy.

[0079] Specifically, in this step 3, the grid-forming converter adopts virtual synchronous machine control, and the control part of the virtual synchronous machine includes an active power loop, a reactive power loop, a voltage-current double closed loop, and PWM signal modulation, etc. The control core is the active power loop and the reactive power loop. The virtual synchronous machine realizes synchronization with the grid by imitating the rotor characteristic equation of a synchronous generator. Therefore, the grid-forming converter based on virtual synchronous machine control has the external characteristics of a synchronous generator. As shown in Figure 3 , it adjusts the active power through the phase angle output by the virtual synchronous machine in the active power loop of the power loop, and adjusts the output voltage reference value through the reactive power output by the reactive power loop.

[0080] First, in this step 3, the mathematical models of the active power loop and the reactive power loop of the virtual synchronous machine in the grid-forming new energy can be obtained respectively. Among them, the mathematical model of the active power loop of the virtual synchronous machine is:

[0081]

[0082] where θ vsg is the phase angle output by the virtual synchronous machine; ω vsg is the differential of θ vsg ; P set is the input mechanical power; P is the active power output by the virtual synchronous machine; ω n is the rated angular frequency; J is the moment of inertia; D P is the damping coefficient;

[0083] Based on the above formula, the active power loop can obtain the phase angle θ vsg output by the virtual synchronous machine to adjust the output active power P, and finally make P = P set .

[0084] The mathematical model of the reactive power loop of the virtual synchronous generator is as follows:

[0085] U ref = U0 + k q (Q ref - Q);

[0086] Where U ref is the output voltage of the virtual synchronous generator; U0 is the set value of the output voltage; k q is the droop coefficient of the reactive power loop; Q ref is the reference value of the reactive power; Q is the output reactive power of the virtual synchronous generator.

[0087] Based on the above formula, it can be known that the reactive power loop adjusts the output voltage reference value through the output reactive power.

[0088] Generally, when there is too much reactive power in the system, before a symmetrical short-circuit fault occurs in the external system, the VSG operates in the leading power factor state, which is basically the same as the basic characteristics of the synchronous generator operating in the leading power factor state. At this time, it is necessary to ensure that the voltage amplitude at the connection point is greater than the amplitude of the internal electromotive force. The VSG appears to absorb reactive power externally. After a short-circuit fault occurs in the external system, to avoid the phenomenon of transient overvoltage, the VSG still needs to maintain the leading power factor state. To achieve the above goal, it is necessary to ensure that the voltage amplitude at the connection point is still greater than the amplitude of the internal electromotive force under fault conditions. Therefore, in this embodiment, the reactive power loop of the virtual synchronous generator is adjusted based on the mathematical model of the reactive power loop. When a system fault occurs, the reactive power loop is directly frozen, making Q ref = Q, so that the reference value of the reactive power is equal to the output reactive power of the virtual synchronous generator, and the set value of the output voltage is switched to the fault voltage, thereby ensuring that the amplitude of the internal electromotive force of the virtual synchronous generator and the voltage amplitude at the connection point decrease equally, and enabling the virtual synchronous generator to maintain the leading power factor state.

[0089] Moreover, when the voltage amplitude drops during a system fault, it changes the active power output ability of the inverter. The original VSG strategy still tries to maintain the original active power output by adjusting the phase, which will cause the frequency and phase to be adjusted too much and is not conducive to the stability of the power angle. When using the traditional power loop design, due to the existence of the inertia link, the adjustment of the active power loop takes several seconds. Often, when the voltage drop ends, the active power P has not been adjusted to be equal to the active power command value P ref . Therefore, the method of trying to adjust the active power to be equal to the specified command value through the active power loop will cause the continuous change of the power angle during the voltage drop. Therefore, in this embodiment, the active power loop of the virtual synchronous generator is adjusted based on the mathematical model of the active power loop. When a fault occurs, the active power loop is directly frozen, that is, the input power difference of the inertia link is set to 0, so that the virtual synchronous generator maintains operation at the power angle before the voltage drop during the voltage drop.

[0090] Combined withFigure 4 The equivalent circuit of the virtual synchronous generator grid-connected system shown. At this time, the left side of the grid connection point is the equivalent circuit of the virtual synchronous generator, and \(E = E\angle\delta\). eg is the internal electromotive force of the virtual synchronous generator. In this embodiment, on the premise that the virtual synchronous generator maintains the adjusted reactive power loop and active power loop, the internal electromotive force of the virtual synchronous generator and the impedance value between the internal electromotive force and the grid connection point can be obtained;

[0091] Subsequently, based on the internal electromotive force of the virtual synchronous generator and the impedance value between the internal electromotive force and the grid connection point, the fault current \(I\) output by the network-forming new energy is obtained. VSG.f is:

[0092]

[0093] where \(E\angle\delta\). eg is the internal electromotive force of the virtual synchronous generator; \(u\). pcc.f is the per-unit value of the grid connection point voltage during system fault; \(Z\). f is the impedance value between the internal electromotive force of the virtual synchronous generator and the grid connection point; \(\Delta U\) is the difference between the internal electromotive force of the virtual synchronous generator and the grid connection point voltage.

[0094] Step 4: Analyze the variation law among the output current of the grid-following new energy, the output current of the network-forming new energy, and the current measured by the stability control device under the fault condition.

[0095] Specifically, the following law exists among the fault current output by the grid-following new energy, the fault current output by the network-forming new energy, and the fault current measured by the stability control device:

[0096] As the voltage drop degree at the grid connection point deepens, the amplitude of the fault current output by the grid-following new energy increases until it reaches the maximum current that the inverter allows to flow through, and the angle by which the fault current output by the grid-following new energy lags behind the grid connection point voltage increases; the amplitude of the fault current output by the network-forming new energy decreases, and the angle by which the fault current output by the network-forming new energy leads the grid connection point voltage increases;

[0097] Based on the premise that the fault current measured by the stability control device is the sum of the fault current output by the grid-following new energy and the fault current output by the network-forming new energy, the included angle between the fault current output by the grid-following new energy and the fault current output by the network-forming new energy increases as the voltage drop degree at the grid connection point increases.

[0098] In the photovoltaic power station system of this embodiment, when a symmetrical short-circuit fault occurs in the output line between the grid connection point and the large power grid, the local current value collected by the stability control device installed at the grid connection point bus is the sum of the fault current output by the grid-following new energy and the fault current output by the network-forming new energy, that is:

[0099] I f= I GFL.f + I VSG.f ;

[0100] Since the output fault current of the grid-following new energy and the output fault current of the grid-forming new energy are both related to the voltage drop degree of the grid connection point during the fault. At this time, the variation laws of the two with the grid connection point voltage are specifically analyzed as follows:

[0101] For the grid-following new energy, as the voltage drop degree of the grid connection point deepens, the reactive current output increases continuously to meet the requirement that the reactive current output by the grid-following new energy tracks the voltage change of the grid connection point. When the voltage drop degree of the grid connection point is relatively shallow, the active current output is P m / u pcc.f , and at this time it increases continuously as the voltage drop degree of the grid connection point deepens. When the voltage drop degree of the grid connection point exceeds a certain value, such that the active current output to meet the requirement of the active current value output by the grid-following new energy becomes At this time, it decreases continuously as the voltage drop degree of the grid connection point deepens. Through the above analysis, it can be seen that as the voltage drop degree of the grid connection point deepens, the amplitude of the output fault current of the grid-following new energy increases continuously until it reaches the maximum allowable current value and remains unchanged, and the angle by which the output fault current of the grid-following new energy lags behind the grid connection point voltage u pcc.f increases continuously.

[0102] For the grid-forming new energy, as the voltage drop degree of the grid connection point deepens, combined with Figure 6 the change conditions of each electrical quantity can be obtained. Since the amplitude of the internal electromotive force E decreases equally with the grid connection point voltage u pcc.fn , which leads to the continuous decrease of the amplitude of ΔU and the continuous increase of the phase angle leading the grid connection point voltage. According to the calculation formula of the fault current output by the grid-forming new energy, the change characteristics of the fault current output by the grid-forming new energy are consistent with those of ΔU. That is: as the voltage drop degree of the grid connection point deepens, the amplitude of the fault current output by the grid-forming new energy decreases continuously, and the angle by which the fault current output by the grid-forming new energy leads the grid connection point voltage u pcc.fn increases continuously.

[0103] Based on the above analysis, when the output capacity ratios of the grid-following converter and the grid-forming converter in the photovoltaic power station system are the same, the phasor diagram of the change trends of the output fault current of the grid-following new energy and the output fault current of the grid-forming new energy can be obtained as the voltage drop degree of the grid connection point deepens, as shown in Figure 5 . Among them, the fault causes the grid connection point voltage to drop to u pcc.f , the current output by the grid-forming new energy is I VSG.f , the current output by the grid-following new energy is I GFL.f , and I sum.f is the current measured by the stability control device. Combined withFigure 5 It can be intuitively seen that as the voltage drop degree at the grid connection point deepens, the angle between I GFL.f and I VSG.f keeps increasing. When the voltage drops to a certain extent, the angle between the two is greater than ninety degrees, making the fault current I f measured by the stability control device not necessarily show an obvious increase, and even in extreme cases, the current does not increase. At this time, the traditional fault tripping criterion adopted by the stability control device will no longer be applicable to fault tripping judgment, posing a great safety risk.

[0104] Step Five: Based on the variation law obtained from the analysis, construct the symmetrical fault tripping judgment logic of the photovoltaic power station system.

[0105] Specifically, the symmetrical fault tripping judgment logic in this embodiment is as follows:

[0106] Based on the fact that the fault current output by the grid-connected power source increases as the voltage drop degree at the grid connection point increases, construct the first determination condition of whether the fault current output by the grid-connected power source is greater than the preset threshold;

[0107] Take whether the fault current collected by the stability control device is greater than the preset threshold as the second determination condition, and form an OR logic with the first determination condition and the second determination condition;

[0108] Form an AND logic with the OR logic composed of the first determination condition and the second determination condition and the preset judgment strategy of the stability control device;

[0109] Based on the combined logic gates, obtain the symmetrical fault tripping judgment logic of the photovoltaic power station system.

[0110] Currently, the typical symmetrical fault tripping criterion for AC transmission lines used by traditional stability control devices is: if the three-phase voltage decreases and the three-phase current increases during the fault process, and there is a three-phase tripping signal within the reclosing time, it is judged as a symmetrical short-circuit fault tripping. On this basis, after a symmetrical short-circuit fault occurs in the output line between the grid connection point and the large power grid in the photovoltaic power station system, the fault current collected by the stability control device installed at the grid connection point bus may be less than the stability control action threshold, resulting in the refusal of the stability control device to operate.

[0111] After a symmetrical fault occurs in the transmission line, the fault current output by the grid-connected new energy still satisfies the traditional fault tripping criterion of the fault phase current increasing. Therefore, when the stability control device in this embodiment conducts symmetrical fault judgment, the condition that the fault current output by the grid-connected power source is greater than the threshold value can be added, that is:

[0112] I GFL.f -I N ≥ΔI set ;

[0113] where I Nis the rated current value of the line under normal operating conditions; ΔI set is the threshold value for the stability control device to detect an increase in fault current and operate. In this embodiment, it is generally taken as ΔI set = 10%I N .

[0114] Since the preset judgment strategy of the stability control device includes three aspects: sudden change start, voltage reduction and increase in the same-phase current, and the appearance of a fault trip signal. Among them, the requirement for the fault-phase current is that the effective value of the fault-phase current exceeds the threshold value, that is:

[0115] I f -I N ≥ΔI set ;

[0116] In this embodiment, to ensure the accurate operation of the power system during symmetric fault tripping, the effective value of the fault-phase current exceeding the threshold value and the output fault current of the grid-connected power source being greater than the threshold value are combined to form an "OR" gate logic, and then combined with the three aspects of sudden change start, phase voltage reduction, and the appearance of a fault trip signal to form an "AND" gate logic, forming a new criterion for symmetric fault tripping applicable to a photovoltaic power station system including a grid-connected converter, as Figure 7 shown. By collecting the fault-phase current and the output fault current of the grid-connected power source for judgment, it is possible to avoid the phenomenon that the stability control device refuses to operate due to the insignificant change in the fault current when the grid connection point of the photovoltaic power station system including a grid-connected converter drops to a certain extent, improving the accuracy of fault judgment.

[0117] It should be noted that in this embodiment, the stability control device is configured on the bus at the grid connection point of the photovoltaic power station system, and a stability control execution station is configured on the side of the grid-connected new energy station. The stability control execution station collects the fault current signal output by the grid-connected new energy, while the stability control device collects the fault current. The fault current signal output by the grid-connected new energy collected by the stability control execution station can be transmitted to the stability control device at the bus of the grid connection point, and the stability control device combines the current signal collected by itself for judgment to obtain a fault determination result.

[0118] Furthermore, in the embodiment of the present application, a symmetric short-circuit fault simulation is carried out based on the Figure 1 system structure. Among them, u pcc.f = 0.5 p.u. The simulation results are as Figure 8 and Figure 9 shown, which are the simulation waveform diagrams of the output fault current of the grid-connected new energy and the current at the grid connection point during symmetric faults respectively. From Figure 8 and Figure 9It can be seen that the fault current collected by the stability control device installed at the grid connection point bus is less than the stability control action threshold, while the fault current output by the grid-connected power supply increases normally and meets the condition of being greater than the threshold, enabling the stability control device adopting the new fault criterion of this application to operate correctly, verifying the correctness of the new symmetrical fault tripping criterion proposed in this application.

[0119] In addition, as Figure 10 shown, this application also discloses a device for judging symmetrical fault tripping of an AC transmission line for a photovoltaic power station system. The device includes:

[0120] A topology structure acquisition module for acquiring the topology structure of the photovoltaic power station system and the preset judgment strategy of the stability control device; the photovoltaic power station system includes a grid-connected converter and a grid-forming converter; the preset judgment strategy includes sudden variable start, voltage reduction and increase of the same-phase current, and a fault tripping signal.

[0121] A first analysis module for analyzing the fault characteristics when the grid-connected new energy is connected to the grid according to the low-voltage ride-through control strategy adopted by the grid-connected converter during grid connection faults, and obtaining the change of the fault current output by the grid-connected new energy.

[0122] A second analysis module for analyzing the fault characteristics when the grid-forming new energy is connected to the grid according to the in-phase operation state adjustment of the power loop by the grid-forming converter during grid connection faults, and obtaining the change of the fault current output by the grid-forming new energy.

[0123] A current analysis module for analyzing the change law among the output current of the grid-connected new energy, the output current of the grid-forming new energy, and the current measured by the stability control device under fault conditions.

[0124] A judgment logic construction module for constructing the symmetrical fault tripping judgment logic of the photovoltaic power station system based on the analyzed change law.

[0125] The device provided in the embodiment of this application can achieve Figure 2 each process implemented by the method embodiment. To avoid repetition, it will not be elaborated here.

[0126] As Figure 11 shown, the embodiment of this application also provides an electronic device, including a processor and a memory, a program or instruction stored on the memory and executable on the processor. When the program or instruction is executed by the processor, it realizes each process of the method embodiment as Figure 2 shown, and can achieve the same technical effect. To avoid repetition, it will not be elaborated here.

[0127] The embodiment of this application also provides a readable storage medium, on which a program or instruction is stored. When the program or instruction is executed by the processor, it realizes the above-mentionedFigure 2 Each process of the method embodiment described above can achieve the same technical effect. To avoid repetition, it will not be elaborated here.

[0128] The embodiment of the present application also provides a computer program product, including computer instructions, which when executed by a processor, implement each process of the method embodiment described above Figure 2 Each process of the method embodiment described above can achieve the same technical effect. To avoid repetition, it will not be elaborated here.

[0129] It should be understood that the term "one embodiment" or "an embodiment" mentioned throughout the specification means that a specific feature, structure, or characteristic related to the embodiment is included in at least one embodiment of the present application. Therefore, the appearances of "in one embodiment" or "in an embodiment" throughout the specification do not necessarily refer to the same embodiment. In addition, these specific features, structures, or characteristics can be combined in one or more embodiments in any suitable manner. It should be understood that in various embodiments of the present application, the magnitudes of the sequence numbers of the above processes do not mean the order of execution, and the order of execution of each process should be determined by its function and internal logic, and should not constitute any limitation to the implementation process of the embodiments of the present application. The sequence numbers of the embodiments of the present application are only for description and do not represent the advantages or disadvantages of the embodiments.

[0130] It should be noted that in this article, the term "including", "comprising" or any other variant thereof is intended to cover non-exclusive inclusion, so that a process, method, article or device including a series of elements not only includes those elements, but also includes other elements not explicitly listed, or further includes elements inherent to such process, method, article or device. Without further limitation, an element defined by the statement "including a..." does not exclude the existence of another identical element in the process, method, article or device including the element.

[0131] In several embodiments provided by the present application, it should be understood that the disclosed devices and methods can be implemented in other ways. The device embodiments described above are only illustrative. For example, the division of the units is only a logical function division, and there can be other division methods in actual implementation. For example, multiple units or components can be combined, or can be integrated into another device, or some features can be ignored, or not executed. In addition, the coupling, direct coupling, or communication connection between the components shown or discussed with each other can be through some interfaces, and the indirect coupling or communication connection of the devices or units can be electrical, mechanical, or other forms.

[0132] The units described above as separate components may or may not be physically separated, and the components shown as units may or may not be physical units; they may be located in one place or distributed across multiple network units; some or all of the units can be selected according to actual needs to achieve the purpose of the solution of this embodiment.

[0133] In addition, each functional unit in the embodiments of the present application may be all integrated in one processing unit, or each unit may be separately regarded as one unit, or two or more units may be integrated in one unit; the above-mentioned integrated units may be implemented in the form of hardware, or in the form of a combination of hardware and software functional units.

[0134] Those of ordinary skill in the art can understand that all or part of the steps of implementing the above method embodiments can be completed by hardware related to program instructions. The foregoing program can be stored in a computer-readable storage medium. When the program is executed, it performs the steps including the above method embodiments; and the foregoing storage medium includes: removable storage devices, read-only memory (ROM), magnetic disks or optical discs and other various media that can store program codes.

[0135] Alternatively, if the above-mentioned integrated units of the present application are implemented in the form of software functional modules and sold or used as independent products, they can also be stored in a computer-readable storage medium. Based on such an understanding, the technical solution of the embodiments of the present application essentially or the part that contributes to the prior art can be embodied in the form of a software product. This computer software product is stored in a storage medium and includes several instructions to enable a device (which can be a terminal or a platform, etc.) to execute all or part of the methods described in the embodiments of the present application. And the foregoing storage medium includes: removable storage devices, ROM, magnetic disks or optical discs and other various media that can store program codes.

[0136] The above are only the preferred embodiments of the present application. It should be noted that for those of ordinary skill in the art, without departing from the technical principle of the present application, several improvements and modifications can be made, and these improvements and modifications should also be regarded as the protection scope of the present application.

Claims

1. A method for judging the tripping of a symmetrical fault of an AC outgoing line in a photovoltaic power station system, characterized in that, The method includes the following steps: Obtain the topological structure of the photovoltaic power station system and the preset judgment strategy of the stability control device; the photovoltaic power station system includes a grid-connected converter and a grid-forming converter; the preset judgment strategy includes sudden change start, voltage reduction and increase of the same-phase current, and a fault tripping signal; Based on the low-voltage ride-through control strategy adopted by the grid-connected converter during grid connection faults, analyze the fault characteristics when the grid-connected new energy is connected to the grid, and obtain the change of the fault current output by the grid-connected new energy; Based on the in-phase operation state of the grid-forming converter during grid connection faults, adjust the power loop, analyze the fault characteristics when the grid-forming new energy is connected to the grid, and obtain the change of the fault current output by the grid-forming new energy; Analyze the change rules among the output current of the grid-connected new energy, the output current of the grid-forming new energy, and the current measured by the stability control device in the fault state; Based on the obtained change rules, construct the symmetrical fault tripping judgment logic of the photovoltaic power station system.

2. The method for judging the tripping of the outgoing AC line due to symmetric faults in the PV power station system according to claim 1, characterized in that, The grid-connected converter adopts a phase-locked loop control and adopts a fault ride-through control strategy with reactive power support ability during grid connection faults: when a fault occurs until the voltage recovers to 0.9 pu, the grid-connected new energy has the ability to support dynamic reactive current.

3. The method for judging the tripping of the outgoing AC line due to symmetric faults in the PV power station system according to claim 2, characterized in that, The step of analyzing the fault characteristics when the grid-connected new energy is connected to the grid and obtaining the change of the fault current output by the grid-connected new energy includes: Based on the fault ride-through control strategy with reactive power support ability adopted when a fault occurs, obtain the reactive current output by the grid-connected new energy; Based on the current-carrying capacity of the grid-connected inverter, limit the positive-sequence d-axis component of the short-circuit current to obtain the active current output by the grid-connected new energy; Based on the reactive current and active current output by the grid-following new energy, obtain the fault current I output by the grid-following new energy GFL.f .

4. The method for judging the tripping of the outgoing AC line due to symmetric faults in the PV power station system according to claim 1, wherein, The grid-forming converter adopts a virtual synchronous machine control, and adjusts the active power through the phase angle output by the virtual synchronous machine in the active power loop of the power loop, and adjusts the output voltage reference value through the reactive power output by the reactive power loop.

5. The method for judging the tripping of the outgoing AC line due to a symmetrical fault in the PV power station system according to claim 4, wherein The step of analyzing the fault characteristics when the grid-forming new energy is connected to the grid and obtaining the change of the fault current output by the grid-forming new energy includes: Respectively obtain the mathematical models of the active power loop and the reactive power loop of the virtual synchronous machine in the grid-forming new energy; Based on the mathematical model of the reactive power loop, adjust the reactive power loop of the virtual synchronous machine in the fault state, so that the reactive power output by the virtual synchronous machine is equal to the reactive power reference value, and the output voltage setting value is switched to the fault voltage to satisfy the virtual synchronous machine to maintain the in-phase operation state; Based on the mathematical model of the active power loop, adjust the active power loop of the virtual synchronous machine in the fault state, so that the input power difference of the inertia link is set to 0 to satisfy that the virtual synchronous machine maintains the power angle before the voltage dip and continues to operate during the voltage dip; Based on the premise that the virtual synchronous machine maintains the adjusted reactive power loop and active power loop, obtain the internal potential of the virtual synchronous machine and the impedance value between the internal potential and the grid connection point; Based on the internal potential of the virtual synchronous machine and the impedance value between the internal potential and the grid connection point, obtain the fault current I output by the network-forming new energy VSG.f .

6. The method for judging the tripping of the outgoing AC line due to symmetric faults in the PV power station system according to claim 1, characterized in that, The step of analyzing the change rules among the output current of the grid-connected new energy, the output current of the grid-forming new energy, and the current measured by the stability control device in the fault state includes: When the voltage drop degree at the grid connection point deepens, the amplitude of the fault current output by the grid-following new energy increases until it reaches the maximum current allowed to flow through the inverter, and the angle by which the fault current output by the grid-following new energy lags behind the grid connection point voltage increases; the amplitude of the fault current output by the grid-forming new energy decreases, and the angle by which the fault current output by the grid-forming new energy leads the grid connection point voltage increases; Based on the premise that the fault current measured by the stability control device is the sum of the fault current output by the grid-following new energy and the fault current output by the grid-forming new energy, it is analyzed that the included angle between the fault current output by the grid-following new energy and the fault current output by the grid-forming new energy increases as the voltage drop degree at the grid connection point increases.

7. The method for judging the tripping of the outgoing AC line due to symmetric faults in the PV power station system according to claim 1, characterized in that, The steps of constructing the symmetrical fault tripping judgment logic of the photovoltaic power station system based on the analyzed variation law include: Based on the fact that the fault current output by the grid-following power source increases as the voltage drop degree at the grid connection point increases, a first determination condition for whether the fault current output by the grid-following power source is greater than a preset threshold is constructed; Taking whether the fault current collected by the stability control device is greater than a preset threshold as a second determination condition, and forming an OR logic with the first determination condition and the second determination condition; Forming an AND logic with the OR logic composed of the first determination condition and the second determination condition and the preset judgment strategy of the stability control device; Obtaining the symmetrical fault tripping judgment logic of the photovoltaic power station system based on the combined logic gate.

8. The method for judging the tripping of the symmetrical fault of the AC outgoing line of the photovoltaic power station system according to claim 1, wherein The stability control device is configured on the grid connection point bus of the photovoltaic power station system, and a stability control execution station is configured on the grid-following new energy station side. The stability control execution station collects the fault current signal output by the grid-following new energy and transmits it to the stability control device at the grid connection point bus.

9. The method for judging symmetrical fault tripping of the AC transmission line sent out by the photovoltaic power station system according to claim 4, characterized in that The mathematical model of the active power loop of the virtual synchronous machine is: where θ vsg is the output phase angle of the virtual synchronous machine; ω vsg is the differential of θ vsg ; P set is the input mechanical power; P is the active power output of the virtual synchronous machine; ω n is the rated angular frequency; J is the moment of inertia; D P is the damping coefficient; The mathematical model of the reactive power loop of the virtual synchronous machine is: U ref = U0 + k q (Q ref - Q); Among them, U ref is the output voltage of the virtual synchronous machine; U0 is the set value of the output voltage; k q is the droop coefficient of the reactive power loop; Q ref is the reference value of reactive power; Q is the output reactive power of the virtual synchronous machine.

10. A device for judging the tripping of a symmetrical fault of an AC outgoing line of a photovoltaic power station system, characterized in that, The device includes: A topology structure acquisition module for acquiring the topology structure of the photovoltaic power station system and the preset judgment strategy of the stability control device; the photovoltaic power station system includes a grid-following converter and a grid-forming converter; the preset judgment strategy includes sudden variable startup, voltage reduction and increase in the same-phase current, and the appearance of a fault tripping signal; A first analysis module for analyzing the fault characteristics when the grid-following new energy is connected to the grid according to the low voltage ride-through control strategy adopted by the grid-following converter during the grid connection fault, and obtaining the change of the fault current output by the grid-following new energy; A second analysis module for analyzing the fault characteristics when the grid-forming new energy is connected to the grid according to the in-phase operation state adjustment of the power loop by the grid-forming converter during the grid connection fault, and obtaining the change of the fault current output by the grid-forming new energy; A current analysis module for analyzing the change law among the fault current output by the grid-following new energy, the fault current output by the grid-forming new energy, and the current measured by the stability control device under the fault condition; A judgment logic construction module for constructing the symmetrical fault tripping judgment logic of the photovoltaic power station system based on the analyzed change law.

11. An electronic device, characterized in that, The device includes a memory for storing computer program instructions and a processor for executing the program instructions, wherein when the computer program instructions are executed by the processor, the device is triggered to execute the method steps described in any one of claims 1 to 9.

12. A computer-readable medium, characterized in that, The medium stores computer-readable instructions that can be executed by a processor to implement the method steps described in any one of claims 1 to 9.

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