Fault location method using negative sequence suppression strategy of opposite end grid-connected converter

Abstract

The application discloses a fault positioning method using a negative sequence suppression strategy of a grid-connected converter at an opposite end, and particularly relates to the following steps: 1, a sending-out line of an AC system is connected with a new energy system, when a single-phase grounding fault occurs in the sending-out line, starting judgment is performed, and a starting element is actuated when a starting criterion is met; 2, a power frequency fault sequence component phase difference is used on the fault line to realize fault phase selection, three-phase voltage and current phasors are extracted and decomposed into symmetrical sequence components; 3, a new energy system switching control strategy is used to obtain positive and negative sequence components, an open circuit condition is constructed using a negative sequence current strategy and is used as a control constraint; and 4, a composite network graph is constructed to calculate a fault distance. The application solves the problem that the existing fault positioning method is not accurate in line fault positioning.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of power transmission line fault location, and particularly relates to a fault location method using a negative sequence suppression strategy of a grid-connected converter at the opposite end. BACKGROUND

[0002] New energy power generation technology is developing rapidly, and the installed capacity of new energy power sources represented by photovoltaic and wind power is rapidly increasing, which has become the main development direction of global energy transformation. New energy units represented by direct-drive wind turbines and photovoltaic devices are increasingly applied in power grids, and their impact on power grid protection is becoming more and more significant.

[0003] New energy units are connected to the grid through power electronic devices, and the power electronic devices have flexible regulation and control capabilities. The inherent characteristics of power electronic devices result in the short-circuit current during power system faults, which is significantly different from that of synchronous generators in terms of amplitude limitation, frequency non-power frequency, and phase control. The characteristics of line faults change, and the traditional relay protection principle based on power frequency steady-state quantities faces severe challenges. The traditional fault location method is based on the assumption that the phase of the fault point current is consistent with that of the measuring point current, and is only applicable when the phase of the power source and the line impedance is similar. When the line is connected to a power electronic device at one end, due to the high controllability of the power electronic device, the phase of the fault current at the new energy side is controlled, and the amplitude is controlled. At this time, the assumption that the phase of the fault point current is consistent with that of the measuring point current no longer holds, and accurate fault location of the line cannot be achieved. Moreover, the traditional fault location method focuses on solving the problem of the opposite side system being a traditional synchronous generator system. When the opposite side system is a power electronic system, the parameters to be identified in the opposite side system are affected by the control strategy of the power electronic device before and after the fault, and have time-varying characteristics, which cannot be accurately identified. For the scenario where the opposite side of the line is connected to a new energy source through a power electronic device, when a single-phase ground fault occurs, the existing fault location method has poor transition resistance resistance, the opposite side system parameters cannot be accurately identified, and the fault location is not accurate. SUMMARY

[0004] The purpose of the application is to provide a line fault location method using a negative sequence suppression strategy of a grid-connected converter at the opposite end, which solves the problem of inaccurate line fault location caused by the inability to identify the parameters of the opposite side system and poor transition resistance resistance in the existing fault location method.

[0005] The technical solution adopted by the application is a fault location method using a negative sequence suppression strategy of a grid-connected converter at the opposite end, which is implemented according to the following steps:

[0006] Step 1, when a new energy system is connected to an AC system through an AC transmission line, and a single-phase ground fault occurs in the transmission line, a start-up judgment is performed on the fault line, and when the start-up criterion is met, a start-up element is actuated.

[0007] Step 2, the phase selection of fault is realized by using the phase difference of power frequency fault sequence component on the fault line, and the three-phase voltage and current phasors under power frequency are extracted and decomposed into symmetric sequence components;

[0008] Step 3, the switching control strategy of the new energy system is obtained, the open circuit condition is constructed by using the negative sequence current strategy as a control constraint;

[0009] Step 4, the composite network graph is constructed by using the control constraint and the sequence component, and the fault distance is calculated.

[0010] The characteristics of the application also lie in,

[0011] Step 1, the new energy system is used as a counter element, specifically for the centralized photovoltaic sending line scene, including the photovoltaic grid-connected topological structure, the photovoltaic grid-connected topological structure includes a photovoltaic panel, the photovoltaic panel is connected through a direct current boost circuit Boost module to improve the voltage level of the direct current side, and then connected to a converter, the converter is connected to a controller, the converter is inverted to an alternating current system, the alternating current system includes a transformer T1 and a transformer T2, the voltage level of the alternating current side is improved through the transformer T1 and the transformer T2, the transformer T2 is finally connected to a 110kV alternating current sending line, and the fault line to be measured in the 110kV alternating current sending line is L1, located at the position of the measuring point on one side of the fault line L1, denoted as K 1m , the other side of the fault line L1 is the position of the measuring point, denoted as K 1n , the fault point between the measuring points K 1m and K 1n is denoted as f 1, the back side of the measuring point K 1m is a large grid Grid of the alternating current system, and the counter side of the line where the measuring point K 1n is located is the photovoltaic grid-connected topological structure.

[0012] When a single-phase ground fault occurs in the alternating current sending line of the alternating current system, the starting judgment is performed on the measuring point K 1m of the fault line L1, and when the following starting criterion is met, the starting element acts;

[0013] (1)

[0014] Among them, the voltage amplitude in the time domain collected by the measuring point K1m, is the rated voltage of the alternating current system in normal operation, t is the time after the fault occurs.

[0015] Step 2, on the measuring point K 1m of the fault line L1, the fault selection element realizes the fault selection by using the phase difference of the power frequency fault sequence component, and then extracts the three-phase voltage and current phasors under power frequency; the three-phase voltage phasors are respectively , the three-phase current phasors are respectively ; using the symmetrical component method, the extracted phase voltage and phase current are respectively decomposed into three sets of symmetrical sequence components with the fault phase as the reference, namely positive sequence voltage, negative sequence voltage, zero sequence voltage, positive sequence current, negative sequence current, and zero sequence current, to realize the decoupling of mutual inductance elements in the line parameters;

[0016] (2)

[0017] wherein, The formula represents the electrical quantity under three-phase components, represents the electrical quantity under positive sequence, negative sequence, and zero sequence components, and α is the complex representation of 120° phase angle difference.

[0018] Step 3 is specifically,

[0019] Step 3.1, after starting step 1, the converter on the AC side of the photovoltaic grid-connected access as the grid-side converter, detects the voltage drop at the grid connection point K 1n on the fault line L1, the controller of the grid-side converter is switched from normal operation mode to low voltage ride-through mode, and the controller of the grid-side converter collects the voltage and current at the grid connection point;

[0020] Step 3.2, the voltage and current collected in step 3.1 are separated into positive sequence components and negative sequence components;

[0021] Step 3.3, the negative sequence current suppression strategy is adopted for the negative sequence components separated in step 3.2, the negative sequence current at the outlet of the grid-side converter is approximately equal to zero, and an open circuit condition is constructed in the negative sequence network, and the photovoltaic side is approximately equivalent to an open circuit.

[0022] When the negative sequence current suppression strategy is adopted, PI control is designed in the negative sequence coordinate system, the output voltage of the converter is adjusted to offset the negative sequence component in the voltage of the fault line L1; in the rotating coordinate system, the d-axis negative sequence current and the q-axis negative sequence current are suppressed respectively, the reference value is set to 0, and negative feedback control is performed through the PI control link to realize zero-error adjustment, and the output result is the negative sequence voltage, which is then used as a modulation wave to enter the modulation link.

[0023] The negative sequence voltage is calculated as:

[0024] (3)

[0025] wherein, , are the negative sequence inner loop currents on the d-axis and q-axis respectively, is the angular velocity under power frequency, is the equivalent inductance of the converter, , are the negative sequence voltages output on the d-axis and q-axis respectively, is a complex frequency domain variable, is a proportional parameter of PI control, is an integral parameter of PI control, , are the negative sequence components of the grid-side voltage on the d-axis and q-axis respectively;

[0026] is the control result under PI control, after the fault occurs, K 1n is the negative sequence current at the measuring point, which is attenuated to 0 on the d-axis and q-axis respectively through the negative sequence current suppression strategy, so as to achieve the control effect.

[0027] Step 4 is specifically,

[0028] Step 4.1, according to the boundary condition of single-phase grounding fault, a composite sequence network graph of series connection of positive sequence, negative sequence and zero sequence networks is constructed on the fault line L1, the positive sequence, negative sequence and zero sequence networks are connected in series as series branches, each series branch is a fault branch, and three transition resistors are connected in series between the three fault branches. 、 and respectively represent the positive sequence, negative sequence and zero sequence components of the voltage at the fault branch, is the transition resistance of the fault branch;

[0029] 、 and respectively represent the positive sequence, negative sequence and zero sequence components of the voltage at the measuring point K 1m , 、 and respectively represent the positive sequence, negative sequence and zero sequence components of the current at the measuring point K 1m ;

[0030] 、 and respectively represent the positive sequence, negative sequence and zero sequence components of the current at the measuring point K 1n ;

[0031] The unit line impedance of the positive sequence, negative sequence and zero sequence of the fault line L1 is respectively 、 and , the equivalent positive sequence, negative sequence and zero sequence impedance of the grid side in the alternating current system is respectively 、 and , the equivalent source potential of the grid side is ; the equivalent positive sequence and zero sequence impedance of the topology structure of photovoltaic grid connection is respectively and , the equivalent source potential of the photovoltaic grid-connected topology is ; L is the full length of the fault line L1, d is the fault distance, 、 and respectively represent the positive sequence, negative sequence, zero sequence total impedance from the measurement point K 1m to the fault point, 、 and respectively represent the positive sequence, negative sequence, zero sequence total impedance from the measurement point K 1n to the fault point.

[0032] Step 4.2, according to the circuit relationship of the composite sequence network diagram in step 4.1, the negative sequence current measured at the measurement point K 1m is equal to the negative sequence current at the fault branch, that is:

[0033] (4);

[0034] The total current at the fault branch is:

[0035] (5);

[0036] A single-phase grounding fault is taken as an example of A-phase grounding. From the measurement point K 1m , the equivalent impedance of the fault line L1 to be measured is calculated ;

[0037] (6);

[0038] (7)

[0039] wherein, is the zero sequence compensation coefficient, 、 are the zero sequence and positive sequence unit line impedance of the fault line L1, are the positive, negative and zero sequence fault currents measured at the K 1m measurement point; is the negative sequence current of the fault branch; are the voltage and current of A-phase grounding fault;

[0040] When the transition resistance is short-circuited, the equivalent impedance of the fault line L1 to be measured is ;

[0041] (8)

[0042] wherein, K 1m measured current at the measuring point, K 1m line impedance from the measuring point to the fault point, so as to obtain a complex equation about the fault distance d , transition resistance .

[0043] (9)

[0044] The complex equation is split into real and imaginary equations, and the fault distance d is;

[0045] (10)

[0046] wherein Re represents the real equation, and Im represents the imaginary equation.

[0047] The converter adopts a two-level voltage source type converter.

[0048] The method has the advantages that: the method for fault positioning of the opposite end grid-connected converter negative sequence suppression strategy is used, the high controllability of the new energy power electronic equipment is used, when a single-phase ground fault occurs in the new energy sending line, the control strategy constraint condition when the power electronic equipment fails is introduced, so that the composite sequence network graph during the fault is constructed, the negative sequence network of the new energy side is opened, the purpose of fault positioning only by using single-end electrical quantity is achieved, communication is not needed, and the accuracy is greatly improved. BRIEF DESCRIPTION OF DRAWINGS

[0049] Figure 1 is a flow chart of the method for fault positioning of the opposite end grid-connected converter negative sequence suppression strategy of the application;

[0050] Figure 2 is a 110kV centralized photovoltaic sending line connection model graph in embodiment 2 of the application;

[0051] Figure 3 is a negative sequence current suppression module graph of the controller of the grid-connected inverter of the application;

[0052] Figure 4 is a d-axis negative sequence current dynamic response curve graph of the dynamic tracking result under the negative sequence current suppression strategy of the application;

[0053] Figure 5 is a q-axis negative sequence current dynamic response curve graph of the dynamic tracking result under the negative sequence current suppression strategy of the application;

[0054] Figure 6 is a composite sequence network graph constructed in embodiment 2 of the application;

[0055] Figure 7is a comparison chart of actual error and standard error in the simulation test of the application. DETAILED DESCRIPTION

[0056] The application will be described in detail below in combination with the drawings and specific embodiments.

[0057] Embodiment 1

[0058] The application utilizes the fault positioning method of the negative sequence suppression strategy of the opposite end grid-connected converter, and the flow is as shown in Figure 1 The following steps are implemented in detail:

[0059] Step 1, when the single-phase grounding fault occurs in the AC transmission line of the new energy system connected to the AC system, the starting element acts when the starting criterion is met by starting judgment on the fault line.

[0060] Step 2, the phase difference of the power frequency fault sequence component is used on the fault line to realize fault phase selection, and the three-phase voltage and current phasors under power frequency are extracted and decomposed into symmetric sequence components.

[0061] Step 3, the new energy system switching control strategy is obtained, the open circuit condition is constructed by using the negative sequence current strategy as a control constraint.

[0062] Step 4, the composite network graph is constructed by using the control constraint and the sequence component, and the fault distance is calculated.

[0063] Embodiment 2

[0064] The application utilizes the fault positioning method of the negative sequence suppression strategy of the opposite end grid-connected converter, and the following steps are implemented in detail:

[0065] Step 1, when the single-phase grounding fault occurs in the new energy transmission line, the new energy system as the opposite end element accesses the line and connects to the AC system, and the starting element acts when the starting criterion is met by starting judgment on the AC transmission fault line of the AC system.

[0066] The new energy system as the opposite end element is specifically the centralized photovoltaic transmission line scene, as shown in Figure 2 The photovoltaic grid-connected topology includes a photovoltaic panel, the photovoltaic panel passes through a Boost module of a direct current boost circuit to improve the voltage level of the direct current side, and then connects a converter, the converter connects a controller, the converter inverts to an AC system, the AC system includes a transformer T1 and a transformer T2, the voltage level of the AC side is improved through the transformer T1 and the transformer T2, the transformer T2 is finally connected to a 110kV AC transmission line, and the fault line L1 to be measured in the 110kV AC transmission line is located at the position of the measuring point on one side of the fault line L1, and is recorded as K 1m, the fault point between the measuring points K 1n , K 1m and K 1n is recorded as f 1, the back side of the measuring point K 1m is the large grid Grid of the alternating current system, and the opposite end of the line where the measuring point K 1n is located is the photovoltaic grid-connected topology.

[0067] Step 2, the phase difference of the power frequency fault sequence component is used on the fault line to realize fault phase selection, the three-phase voltage and current phasors under power frequency are extracted and decomposed into symmetric sequence components;

[0068] Step 3, the new energy system switching control strategy is obtained, the open circuit condition is constructed by using the negative sequence current strategy as a control constraint;

[0069] Step 4, the composite network graph is constructed by using the control constraint and the sequence component, and the fault distance is calculated.

[0070] Embodiment 3

[0071] The fault positioning method of the application utilizes the negative sequence suppression strategy of the opposite end grid-connected converter, and is specifically implemented according to the following steps:

[0072] Step 1, when a single-phase ground fault occurs in the new energy sending line, the new energy system is connected to the line as an opposite end element and connected to the alternating current system, the starting element is actuated when the starting criterion is met;

[0073] The new energy system is an opposite end element, specifically a centralized photovoltaic sending line scene, including a photovoltaic grid-connected topology, the photovoltaic grid-connected topology includes a photovoltaic panel, the photovoltaic panel is connected to a direct current boost circuit Boost module to improve the voltage level of the direct current side, and then connected to a converter, the converter is connected to a controller, the converter is inverted into an alternating current system, the alternating current system includes a transformer T1 and a transformer T2, the voltage level of the alternating current side is improved through the transformer T1 and the transformer T2, the transformer T2 is finally connected to a 110kV alternating current sending line, and the fault line L1 to be measured in the 110kV alternating current sending line is located at the position of the measuring point K 1m , the position of the measuring point K 1n on the other side of the fault line L1 is recorded as K 1m , the fault point between the measuring points K 1n is recorded as f 1, the back side of the measuring point K 1m is the large grid Grid of the alternating current system, and the opposite end of the line where the measuring point K 1n is located is the photovoltaic grid-connected topology.

[0074] Step 2, the phase selection of the fault is realized by using the phase difference of the power frequency fault sequence component on the fault line, and the three-phase voltage and current phasors under power frequency are extracted and decomposed into symmetric sequence components; at the measuring point K 1m of the fault line L1, the phase selection of the fault is realized by using the phase difference of the power frequency fault sequence component, and then the three-phase voltage and current phasors under power frequency are extracted; the three-phase voltage phasors are respectively , and the three-phase current phasors are respectively ; by using the symmetric component method, the extracted phase voltage and phase current are decomposed into three groups of symmetric sequence components with the fault phase as the reference, which are positive sequence voltage, negative sequence voltage, zero sequence voltage, positive sequence current, negative sequence current and zero sequence current, and the decoupling of the mutual inductance element in the line parameter is realized.

[0075] Step 3, the new energy system switching control strategy is obtained, the open circuit condition is constructed by using the negative sequence current strategy as a control constraint;

[0076] Step 4, the composite network graph is constructed by using the control constraint and the sequence component, and the fault distance is calculated.

[0077] Embodiment 4

[0078] The fault positioning method of the application utilizes the negative sequence suppression strategy of the grid-connected converter at the opposite end, and is specifically implemented according to the following steps:

[0079] Step 1, when a single-phase ground fault occurs in the new energy sending line, the new energy system is connected to the line as the opposite end element, connects to the alternating current system, and judges the starting of the alternating current sending fault line of the alternating current system; when the starting criterion is met, the starting element acts;

[0080] The new energy system is connected to the line as the opposite end element, specifically for the centralized photovoltaic sending line scene, including the topology structure of photovoltaic grid connection, and the topology structure of photovoltaic grid connection includes a photovoltaic panel, the photovoltaic panel is connected to a direct current boost circuit Boost module to improve the voltage level of the direct current side, and then connected to a converter, the converter is connected to a controller, the converter is inverted to an alternating current system, the alternating current system includes a transformer T1 and a transformer T2, the voltage level of the alternating current side is improved through the transformer T1 and the transformer T2, the transformer T2 is finally connected to a 110kV alternating current sending line, and the fault line L1 to be measured in the 110kV alternating current sending line is located at the measuring point on one side of the fault line L1, denoted as K 1m , the other side of the fault line L1 is denoted as K 1n , and the fault point between the measuring points K 1m and K 1n is denoted as f 1, the back side of the measuring point K 1m is a large power grid Grid of the alternating current system, and the measuring point K1n The opposite end of the line is a photovoltaic grid-connected topology.

[0081] Step 2, on the fault line, the phase difference of the power frequency fault sequence component is used to realize fault phase selection, and the three-phase voltage and current phasors under power frequency are extracted and decomposed into symmetric sequence components; on the measuring point K 1m of the fault line L1, the fault phase selection element realizes fault phase selection by using the phase difference of the power frequency fault sequence component, and then extracts the three-phase voltage and current phasors under power frequency; the three-phase voltage phasors are respectively , and the three-phase current phasors are respectively ; the extracted phase voltage and phase current are decomposed into three groups of symmetric sequence components, i.e. positive sequence voltage, negative sequence voltage, zero sequence voltage, positive sequence current, negative sequence current and zero sequence current, by using the symmetric component method, and the mutual inductance element in the line parameter is decoupled.

[0082] Step 3, new energy system switching control strategy, get positive and negative sequence components, use negative sequence current strategy to build open circuit condition as control constraint;

[0083] Step 3.1, after starting in step 1, the photovoltaic grid-connected access to the AC side of the converter as the grid-side converter, detects the voltage drop at the measuring point K 1n of the fault line L1, the controller of the grid-side converter is switched from normal operation mode to low voltage ride through mode, and the controller of the grid-side converter collects the voltage and current of the grid-connected point respectively;

[0084] Step 3.2, the voltage and current collected in step 3.1 are separated into positive sequence components and negative sequence components respectively;

[0085] Step 3.3, the negative sequence components separated in step 3.2 are suppressed by using the negative sequence current suppression strategy, and the negative sequence current at the outlet of the grid-side converter is approximately equal to zero, which constructs an open circuit condition in the negative sequence network, and the photovoltaic side is approximately equivalent to an open circuit.

[0086] As Figure 3 shown, when the negative sequence current suppression strategy is used, PI control is designed in the negative sequence coordinate system, and the output voltage of the converter is adjusted to offset the negative sequence component in the voltage of the fault line L1;

[0087] In the rotating coordinate system, the d-axis negative sequence current and q-axis negative sequence current are suppressed respectively, the reference value is set to 0, and negative feedback control is performed through the PI control link to realize zero error adjustment, and the output result is negative sequence voltage, which is then used as a modulation wave to enter the modulation link.

[0088] The negative sequence voltage is calculated as:

[0089] (3)

[0090] where, , are the negative sequence inner loop currents on d-axis and q-axis respectively, is the angular speed at power frequency, is the equivalent inductance of the converter, , are the negative sequence voltages output on d-axis and q-axis respectively, is the complex frequency domain variable, is the proportional parameter of PI control, is the integral parameter of PI control, , are the negative sequence components of grid side voltages on d-axis and q-axis respectively;

[0091] The control result under PI control, as shown in Figure 4 and Figure 5 , after the fault occurs, the negative sequence current of the measuring point is attenuated to 0 on the d-axis and the q-axis respectively through the negative sequence current suppression strategy, so that the control effect is achieved. 1n

[0092] Step 4, using the control constraints and sequence components, a composite network graph is constructed to calculate the fault distance.

[0093] Embodiment 5

[0094] The fault positioning method of the application utilizes the negative sequence suppression strategy of the grid-connected converter at the opposite end, and is specifically implemented according to the following steps:

[0095] Step 1, when a single-phase ground fault occurs in a new energy sending line, a new energy system as an opposite end element is connected to the line, and the new energy system is specifically a centralized photovoltaic sending line scene, including a photovoltaic grid-connected topology, the photovoltaic grid-connected topology includes a photovoltaic panel (PV1), the photovoltaic panel is connected through a direct current boost circuit Boost module to improve the voltage level on the direct current side, and then connected to a converter (VSC), the converter is connected to a controller, the converter adopts a two-level voltage source converter, the two-level voltage source converter is inverted into an alternating current system, the alternating current system includes a transformer T1 and a transformer T2, the voltage level on the alternating current side is improved through the transformer T1 and the transformer T2, the transformer T2 is finally connected to a 110kV alternating current sending line, the fault line to be measured in the 110kV alternating current sending line is L1, the measuring point on one side of the fault line L1 is located, denoted as K 1m , the measuring point on the other side of the fault line L1 is denoted as K 1n , the fault point between the measuring points K 1m and K 1n is denoted as f 1, the back side of the measuring point K 1m is a large power grid Grid of the alternating current system, and the measuring point K 1n ​The opposite end of the line is a photovoltaic grid-connected topology.

[0096] When the single-phase grounding fault of the AC transmission line of the AC system occurs, the starting judgment is performed at the measuring point K 1m of the fault line L1, and when the following starting criterion is met, the starting element acts;

[0097] (1)

[0098] wherein, the voltage amplitude in the time domain collected by the measuring point K1m, is the rated voltage of the normal operation of the AC system, t is the time after the fault occurs.

[0099] Step 2, after starting in step 1, the phase selection of the fault is realized by using the phase difference of the power frequency fault sequence component on the fault line, and the three-phase voltage and current phasors under power frequency are extracted and decomposed into symmetric sequence components;

[0100] At the measuring point K 1m of the fault line L1, the fault selection element realizes the fault selection by using the phase difference of the power frequency fault sequence component, and then extracts the three-phase voltage and current phasors under power frequency; the three-phase voltage phasors are respectively , and the three-phase current phasors are respectively ; the extracted phase voltage and phase current are decomposed into three groups of symmetric sequence components by using the symmetric component method, respectively, as the positive sequence voltage, the negative sequence voltage, the zero sequence voltage, the positive sequence current, the negative sequence current, and the zero sequence current, to realize the decoupling of the mutual inductance element in the line parameter;

[0101] (2)

[0102] wherein, the formula represents the electrical quantity under the three-phase component, represents the electrical quantity under the positive sequence, the negative sequence, and the zero sequence component, and α is the complex representation form of the 120° phase angle difference.

[0103] Step 3, the new energy system switching control strategy is obtained, the negative sequence current strategy is used to construct the open circuit condition as a control constraint;

[0104] Step 3.1, after starting in step 1, the photovoltaic grid-connected converter connected to the AC side is used as the grid-side converter, and the voltage drop of the grid-connected point is detected at the measuring point K 1n of the fault line L1, the controller of the grid-side converter is switched from the normal operation mode to the low voltage ride-through mode, and the controller of the grid-side converter collects the voltage and current of the grid-connected point respectively;

[0105] Step 3.2, the voltage and current collected in step 3.1 are respectively separated into positive sequence components and negative sequence components;

[0106] Step 3.3, the negative sequence current suppression strategy is adopted for the negative sequence components separated in step 3.2, the negative sequence current at the outlet of the grid-side converter is approximately equal to zero, and an open circuit condition is constructed in the negative sequence network, and the photovoltaic side is approximately equivalent to an open circuit.

[0107] When the negative sequence current suppression strategy is adopted, PI control is designed in the negative sequence coordinate system, and the output voltage of the converter is adjusted to offset the negative sequence component in the fault line L1 voltage;

[0108] In the rotating coordinate system, the d-axis negative sequence current and the q-axis negative sequence current are suppressed respectively, the reference value is set to 0, and negative feedback control is performed through the PI control link to realize zero-error regulation, and the output result is the negative sequence voltage, which is then used as the modulation wave to enter the modulation link.

[0109] The negative sequence voltage is calculated as:

[0110] (3)

[0111] wherein, 、 are the negative sequence inner loop currents on the d-axis and the q-axis respectively, is the angular velocity under power frequency, is the equivalent inductance of the converter, 、 are the negative sequence voltages output on the d-axis and the q-axis respectively, is a complex frequency domain variable, is the proportional parameter of PI control, is the integral parameter of PI control, 、 are the negative sequence components of the grid-side voltage on the d-axis and the q-axis respectively;

[0112] The control result under PI control, after the fault occurs, K 1n The negative sequence current at the measurement point is attenuated to 0 on the d-axis and the q-axis respectively through the negative sequence current suppression strategy, and the control effect is achieved.

[0113] Step 4, using the control constraints obtained in step 3 and the sequence components obtained in step 2, a composite network graph is constructed;

[0114] Step 4.1, according to the boundary condition of single-phase ground fault, a composite sequence network graph of series connection of positive sequence, negative sequence and zero sequence networks is constructed on the fault line L1, as shown in Figure 6 The positive sequence, negative sequence and zero sequence networks are connected in series as series branches, and each series branch is a fault branch, and three transition resistors are connected in series between the three fault branches. 、 and denote the positive, negative and zero sequence components of the voltage at the fault branch, is the transition resistance of the fault branch.

[0115] the boundary conditions of the equivalent single-phase-to-ground fault, where K 1m is located at one side of the positive, negative and zero sequence networks, K 1m is located at the other side of the positive, negative and zero sequence networks, K 、 and denote the positive, negative and zero sequence components of the voltage at the fault branch, 1m is located at one side of the positive, negative and zero sequence networks, K 1m is located at the other side of the positive, negative and zero sequence networks, K 、 and denote the positive, negative and zero sequence components of the current at the fault branch, 1n is located at one side of the positive, negative and zero sequence networks, K 1n is located at the other side of the positive, negative and zero sequence networks, K

[0116] is located at one side of the positive, negative and zero sequence networks, K 1n is located at the other side of the positive, negative and zero sequence networks, K 、 and denote the positive, negative and zero sequence components of the current at the fault branch, 1n is located at one side of the positive, negative and zero sequence networks, K 1n is located at the other side of the positive, negative and zero sequence networks, K

[0117] the unit line impedance of the positive, negative and zero sequence of the fault line L1 are 、 and , the equivalent positive, negative and zero sequence impedance of the grid side in the AC system are 、 and , the equivalent source potential of the grid side is , the equivalent positive and zero sequence impedance of the PV grid-connected topology are and , the equivalent source potential of the PV grid-connected topology is .

[0118] L is the full length of the fault line L1, d is the fault distance, 、 and denote the positive, negative and zero sequence total impedance from the measuring point K 1m to the fault point, 、 and denote the positive, negative and zero sequence total impedance from the measuring point K 1n to the fault point.

[0119] Step 4.2, according to the circuit relationship of the composite sequence network diagram in step 4.1, the measuring point K 1m The measured negative sequence current is equal to the negative sequence current at the fault branch, that is:

[0120] (4);

[0121] The total current at the fault point is:

[0122] (5);

[0123] Taking the A-phase grounding as an example, the equivalent impedance of the fault line L1 to be measured is calculated from the measuring point K 1m to the measuring point ;

[0124] (6);

[0125] (7)

[0126] wherein, is the zero sequence compensation coefficient, , are the zero sequence and positive sequence unit line impedances of the fault line L1 respectively, are the positive, negative and zero sequence fault currents measured at the K 1m measuring point; is the negative sequence current of the fault branch; are the voltage and current of the A-phase grounding respectively;

[0127] After the transition resistance short-circuit, the equivalent impedance of the fault line L1 to be measured is measured as ;

[0128] (8)

[0129] wherein, is the measured current at the K 1m measuring point, is the line impedance from the K 1m measuring point to the fault point, so as to obtain the complex equation about the fault distance d , the transition resistance :

[0130] (9)

[0131] The complex equation is split into real and imaginary part equations, and the fault distance d is:

[0132] (10)

[0133] wherein Re represents a real part, and Im represents a virtual part equation.

[0134] The application utilizes the fault positioning method of the negative sequence suppression strategy of the opposite end grid-connected converter, controls the grid-connected converter of the outgoing line photovoltaic system after the fault, three sets of symmetrical sequence components, realizes the decoupling of the mutual inductance element in the line parameter, provides the network topology convenient for analyzing the fault distance, and the controller of the grid-side converter controls the positive sequence component and the negative sequence component separation of the voltage and current collected at the grid connection point respectively for the asymmetric fault of the single-phase ground fault; reliable mathematical constraints are provided for the time-varying power electronic equipment, and the solution of the fault distance is assisted; the voltage and current measured by the measuring point and the fault distance are used to realize the single-end positioning of the single-phase ground fault.

[0135] Embodiment 6

[0136] In order to verify the accuracy of the fault positioning result of the fault positioning method of the negative sequence suppression strategy of the opposite end grid-connected converter, this embodiment takes the 110kV transmission line as an example, adopts the Bergeron model of the electromagnetic transient simulation software PSCAD, establishes a simulation photovoltaic grid-connected system model, and the parameters of the model are shown in Table 1.

[0137] Table 1 is the main parameters of the photovoltaic grid-connected system model

[0138]

[0139] wherein the fault line L1 section is the transmission line for fault distance measurement, the single-end electrical quantity is collected through the opposite end measuring point, the length of the fault line L1 section is L L1=100km; the fault with different fault distances and transition resistances is set in the middle section of the line L1, and the accuracy of the single-end fault positioning in the application is verified.

[0140] The controller is switched to control, and the distance measurement is solved under different fault distances and transition resistances, and the results are shown in Table 2.

[0141] Table 2 is the distance measurement result under different fault distances and transition resistances

[0142]

[0143] In the test of simulation, the actual fault distance is set to 10, 30, 50, 70 and 90 km respectively, the actual transition resistance is set to 0, 10, 50, 100 and 300 Ω respectively, the fault distance result is respectively corresponding in the table, the data in the same row is the ranging result of different transition resistances under the same fault distance, and the ranging result is always approximately equal to the actual simulated fault distance with the change of the transition resistance, therefore, the fault positioning method has good transition resistance tolerance.

[0144] The ranging error is shown in Table 3;

[0145] Table 3 is the ranging error

[0146]

[0147] The ranging error is shown in Table 3, and the error curve is shown in Figure 7 , x The axis coordinate represents the fault distance, y The axis coordinate represents the transition resistance, z The axis coordinate variable is the ranging error, the black plane part represents the maximum allowable ranging error under different transition resistances and fault distances, and the gray gradient surface part represents the actual ranging error under different transition resistances and fault distances. By observing and comparing the two kinds of surfaces, it is found that with the increase of the fault distance and the transition resistance, the actual ranging error also increases, but the ranging error is within the allowable error range, the ranging result of the fault positioning method has feasibility, and the problems of the existing fault positioning method, such as the inability to identify the end system parameters, poor transition resistance tolerance and inaccurate line fault positioning, are solved.

Claims

1. A fault location method using a counter-terminating grid-connected converter negative sequence suppression strategy, characterized in that, The implementation is specifically performed according to the following steps: Step 1, the new energy system is connected to the AC sending line of the AC system, when the sending line is single-phase grounded fault, the starting judgment is performed on the fault line, when the starting criterion is met, the starting element acts; Step 2, the phase difference of the power frequency fault sequence component is used on the fault line to realize fault phase selection, the three-phase voltage and current phasors under power frequency are extracted and decomposed into symmetric sequence components; Step 3, the new energy system switching control strategy is obtained, the positive and negative sequence components are obtained, the open circuit condition is constructed by using the negative sequence current strategy as a control constraint; Step 4, the composite network graph is constructed by using the control constraint and the sequence component, and the fault distance is calculated; Step 1 The new energy system is a peer element, specifically a centralized photovoltaic sending line scene, including a photovoltaic grid-connected topology, the photovoltaic grid-connected topology including a photovoltaic panel, the photovoltaic panel being connected through a direct current boost circuit Boost module to improve the voltage level of the direct current side, then being connected to a converter, the converter being connected to a controller, the converter being inverted to an alternating current system, the alternating current system including a transformer T1 and a transformer T2, the voltage level of the alternating current side being improved through the transformer T1 and the transformer T2, the transformer T2 being finally connected to a 110kV alternating current sending line, a fault line to be measured in the 110kV alternating current sending line being L1, a measuring point being located at one side of the fault line L1, denoted as K 1m , the other side of the fault line L1 being a measuring point, denoted as K 1n , a fault point between the measuring points K 1m and K 1n , denoted as f 1, a back side of the measuring point K 1m being a large grid Grid of the alternating current system, a peer of the line where the measuring point K 1n is located being the photovoltaic grid-connected topology; When a single-phase ground fault occurs in the AC transmission line of the AC system, the starting element is started at the measuring point K 1m of the fault line L1, and when the following starting criterion is satisfied, the starting element operates. （1） wherein, the voltage amplitude in time domain collected by the measuring point K1m, a rated voltage for normal operation of the alternating current system, t a time after the fault occurs; Step 2 at the measuring point K of the faulty line L1 1m The fault phase selection element adopts the phase difference of the power frequency fault sequence component to realize fault phase selection, and then extracts three-phase voltage and current phasors under power frequency; the three-phase voltage phasors are respectively , and the three-phase current phasors are respectively ; the extracted phase voltage and phase current are respectively decomposed into three groups of symmetrical sequence components, i.e. positive sequence voltage, negative sequence voltage, zero sequence voltage, positive sequence current, negative sequence current and zero sequence current, by taking the fault phase as a reference, to realize decoupling of mutual inductance elements in line parameters; （2） wherein The formula represents the electrical quantity under three-phase components, represents the electrical quantity under positive, negative, and zero sequence components, and α is the complex representation of 120° phase angle difference. Step 3 is specifically, Step 3.1, after starting step 1, the inverter on the AC side of the photovoltaic grid-connected access as the grid-side inverter, at the measuring point K of the fault line L1 1n Above, detecting the voltage drop of the grid-connected point, the controller of the grid-side inverter is switched from the normal operation mode to the low-voltage ride-through mode, and the controller of the grid-side inverter collects the voltage and current of the grid-connected point respectively; Step 3.2, the voltage and current collected in step 3.1 are separated into positive sequence components and negative sequence components respectively; Step 3.3, the negative sequence current suppression strategy is used for the negative sequence component separated in step 3.2, the negative sequence current at the outlet of the grid-side converter is approximately equal to zero, and the open circuit condition is constructed in the negative sequence network, and the photovoltaic side is approximately equivalent to an open circuit.

2. The fault location method using the opposite end grid-connected converter negative sequence suppression strategy according to claim 1, characterized in that, When the negative sequence current suppression strategy is used, the PI control is designed in the negative sequence coordinate system, the output voltage of the converter is adjusted to offset the negative sequence component in the fault line L1 voltage; in the rotating coordinate system, the d-axis negative sequence current and the q-axis negative sequence current are suppressed respectively, the reference value is set to 0, and the negative feedback control is performed through the PI control link, the differenceless regulation is realized, and the output result is the negative sequence voltage, which is then used as a modulation wave to enter the modulation link.

3. The fault location method using the opposite end grid-connected converter negative sequence suppression strategy according to claim 2, characterized in that, The calculation of the negative sequence voltage is: （3） wherein, , are negative sequence inner currents on d-axis and q-axis, respectively, is angular speed under power frequency, is equivalent inductance of the converter, , are negative sequence voltages output on d-axis and q-axis, respectively, is complex frequency domain variable, is proportional parameter of PI control, is integral parameter of PI control, , are negative sequence components of grid side voltages on d-axis and q-axis, respectively; The control result under PI control, after the fault occurs, K 1n The negative sequence current of the measuring point is attenuated to 0 on the d-axis and the q-axis respectively through the negative sequence current suppression strategy, and the control effect is achieved.

4. The fault location method using the opposite end grid-connected converter negative sequence suppression strategy according to claim 3, characterized in that, Step 4 is specifically, Step 4.1, according to the boundary condition of single-phase ground fault, on the fault line L1, the composite sequence network graph of the series of positive sequence, negative sequence, zero sequence network is constructed, the positive sequence, negative sequence, zero sequence network is taken as a series branch, each series branch is a fault branch, three transition resistances are connected in series between three fault branches, 、 and respectively represent the positive sequence, negative sequence and zero sequence components of the voltage at the fault branch, is the transition resistance of the fault branch; , and K 1m Positive-sequence, negative-sequence, and zero-sequence components of the voltage at the measuring point. , and K 1m Positive sequence, negative sequence, and zero sequence components of the current at the measuring point; , and K 1n The positive sequence, negative sequence, and zero sequence components of the current at the measuring point; The unit line impedance of the positive sequence, the negative sequence and the zero sequence of the fault line L1 are , and respectively, the equivalent positive sequence, the negative sequence and the zero sequence impedance of the grid side in the AC system are , and respectively, the equivalent power supply potential of the grid side is ; the equivalent positive sequence and the zero sequence impedance of the topology structure of the photovoltaic grid-connected are and respectively, the equivalent power supply potential of the topology structure of the photovoltaic grid-connected is ; L is the full length of the fault line L1, d is the fault distance, , and respectively represent the positive sequence, the negative sequence and the zero sequence total impedance from the measuring point K 1m to the fault point, , and respectively represent the positive sequence, the negative sequence and the zero sequence total impedance from the measuring point K 1n to the fault point; Step 4.2, according to the circuit relationship of the composite sequence network diagram in step 4.1, the measuring point K is obtained 1m The measured negative sequence current is equal to the negative sequence current at the fault branch, that is: （4）； total current at the fault branch is: （5）； Single-phase ground fault, taking A-phase ground fault as an example, from measuring point K 1m At the measuring point, the equivalent impedance of the fault line L1 to be measured is calculated ; （6）； （7） wherein, is a zero sequence compensation factor, , are zero sequence and positive sequence unit line impedances of the fault line L1, respectively, are the positive and negative sequence fault currents measured at the measurement point K 1m are the positive and negative sequence fault currents measured at the measurement point K is the negative sequence current of the fault branch; are the voltage and current of the A-phase ground fault, respectively. through the transition resistor the equivalent impedance of the fault line L1 to be measured at the time of short circuit is; （8） wherein is K 1m the measured current at the measuring point, is K 1m the line impedance from the measuring point to the fault point, so that the complex equation for the fault distance d , the transition resistance is obtained. （9） Split complex equation into real and imaginary parts, to fault distance d is; （10） Wherein, Re represents the real part, and Im represents the imaginary part equation.

5. The fault location method using the opposite end grid-connected converter negative sequence suppression strategy according to claim 1, characterized in that, The converter uses a two-level voltage source converter.

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