A method for selecting the line with small current grounding fault in a distribution network containing parallel lines on the same pole

Abstract

The present invention discloses a method for selecting a small current grounding fault line in a distribution network with double-circuit lines on the same pole, which relates to the field of relay protection and specifically includes the following steps: Step S1: Group the double-circuit lines on the same pole in the distribution network, and obtain the in-phase component and the anti-phase component of the zero-sequence current of each group; Step S2: Perform in-group in-bound line selection based on the anti-phase component of the zero-sequence current, which specifically includes two parts: amplitude determination for screening the fault group and reactive power calculation for line selection of the fault group; Step S3: Perform out-of-group out-bound line selection based on the in-phase component of the zero-sequence current, which specifically includes two parts: amplitude screening for selecting possible fault lines and phase comparison for determining the fault line. By grouping the double-circuit lines on the same pole, the present invention divides the line selection process of the distribution network into an in-bound line selection process and an out-bound line selection process, improves the sensitivity of line selection, and avoids the problem of misjudgment caused by the small fault characteristics of short feeders through two amplitude determinations, which is beneficial to the fault handling of relevant distribution systems.

Description

Technical Field

[0001] The present invention relates to the field of fiber lasers for outputting high-repetition-rate mode-locked pulses, and more particularly to a method for selecting small-current grounding faults in a distribution network with a same-pole and parallel-line configuration. Background Art

[0002] In recent years, with the increase in people's electricity demand, in order to meet economic benefits and maintain the safe and reliable operation of the power system, the distribution network often modifies existing lines, converting single-circuit lines into same-pole and parallel-line configurations. This not only increases the capacity of the distribution line but also saves floor space and reduces the cost of pole construction. In addition to phase coupling, there is also line-to-line coupling in the same-pole and parallel-line configuration, which may change the fault characteristics when a small-current grounding fault occurs in a system with a same-pole and parallel-line configuration. Currently, research on fault line selection for distribution networks with a same-pole and parallel-line configuration is extremely rare. There is an urgent need to study relevant theories, analyze the applicability of existing fault line selection methods, and further propose new fault line selection methods for distribution networks with a same-pole and parallel-line configuration. Summary of the Invention

[0003] To solve the above problems, the present invention provides a method for selecting small-current grounding faults in a distribution network with a same-pole and parallel-line configuration. By exploring the influence of the same-pole installation on the single-phase grounding fault characteristics of the distribution network based on a simulation model, a steady-state fault line selection method for single-phase grounding faults in a non-grounded system with a same-pole and parallel double-circuit line is provided. By grouping the same-pole and parallel double-circuit lines, the distribution network fault line selection process is divided into an in-group fault line selection within the same-pole and parallel-line group and an out-group fault line selection for other single-circuit lines, improving the sensitivity of fault line selection. By performing two amplitude determinations, the problem of misjudgment caused by small fault characteristics of short feeders is avoided, which is beneficial to the fault handling of relevant distribution systems.

[0004] To achieve the above object, the present invention provides a method for selecting small-current grounding faults in a distribution network with a same-pole and parallel-line configuration. Step S1: Group the same-pole and parallel double-circuit lines in the distribution network, and obtain the in-phase and anti-phase components of the zero-sequence current of each group;

[0005] Step S2: Perform in-group in-network fault line selection based on the anti-phase component of the zero-sequence current, which specifically includes two parts: amplitude determination for screening the fault group and reactive power calculation for fault line selection in the fault group;

[0006] Step S3: Perform out-group out-of-network fault line selection based on the in-phase component of the zero-sequence current, which specifically includes two parts: amplitude screening for selecting possible fault lines and phase comparison for determining the fault line.

[0007] Preferably, in step S1, when a fault occurs, it is determined whether a single-phase grounding fault occurs in each group of same-pole and parallel double-circuit lines;

[0008] In case of a fault, line selection within the fault group is performed;

[0009] If the common feeling lines of each group are all non-faulty lines, line selection outside the group is performed for the single-circuit lines outside the group;

[0010] Based on the idea of the six-sequence component method, the zero-sequence current of the double-circuit lines on the same tower is transformed into the zero-sequence in-phase component and the zero-sequence out-of-phase component, as shown in the following formula:

[0011] ;

[0012] Where i T refers to the in-phase component of the instantaneous zero-sequence current and is the sum of the zero-sequence currents of the double-circuit lines on the same tower. i F refers to the out-of-phase component of the instantaneous zero-sequence current and is the difference between the zero-sequence currents of the double-circuit lines on the same tower. i 01 and i 02 are the instantaneous zero-sequence currents flowing through the bus outlet of the lines L1 and L2 on the same tower respectively. Line selection within the group is performed using i F , and line selection outside the group is performed using i T .

[0013] Preferably, in step S2, the process of line selection within the group based on the zero-sequence current out-of-phase component is as follows:

[0014] After a single-phase ground fault occurs, the line selection process is started through the zero-sequence voltage for line selection within the group. The effective value of the zero-sequence current out-of-phase component I F is judged by amplitude. When performing the steady-state analysis of the single-phase ground fault according to the fault equivalent circuit, the series impedances of the zero-mode and line-mode in the equivalent circuit are ignored, and only the transition resistance is retained. The zero-sequence current phases of the fault feeder and the non-fault feeder are completely opposite. The zero-sequence current phase of the non-fault feeder leads the bus zero-sequence voltage by 90°, and the zero-sequence current phase of the fault feeder lags the bus zero-sequence voltage by 90°;

[0015] When the lines L1 and L2 are non-faulty lines, the phasor values of the zero-sequence currents flowing through the bus outlet of the lines L1 and L2 are denoted as , , and the calculation process is as follows:

[0016] ;

[0017] Where C 0_1s , C 0_2s are the equivalent non-parallel line-to-ground capacitances of the parallel lines L1 and L2 respectively, is the phasor value of the system zero-sequence voltage,j is the rotation factor, is the system operating frequency, , The instantaneous values of and are respectively denoted as 、 ;

[0018] In the case where the double-circuit line on the same pole is a non-faulty line, when the double-circuit line is completely on the same pole, the phasor values of the zero-sequence currents of the two parallel feeders should be the same: , and at this time, the effective value of the reverse component of the zero-sequence current ;

[0019] When the double-circuit line is not completely on the same pole, there is a difference in the lengths of the two feeders. At this time , but the zero-sequence currents on the two feeders are in the same phase. The effective value of the reverse component of the zero-sequence current is expressed by the difference between the effective value of the zero-sequence current of line L1 and the effective value of the zero-sequence current of line L2 as:

[0020] ;

[0021] For the case where there is a faulty line in the double-circuit line on the same pole, the phases of the zero-sequence currents of the two parallel feeders are opposite, I F is approximately the sum of the effective values of the zero-sequence currents of the two feeders. Since the zero-sequence currents of other feeders outside the boundary flow to the faulty line, at this time I F is larger, ;

[0022] When there is no in-boundary fault, I F is smaller. After an in-boundary fault occurs, I F is larger. By setting a threshold α , for I F amplitude determination is performed to determine whether an in-boundary fault has occurred:

[0023] ;

[0024] If the amplitude determination determines that an in-boundary fault has occurred, line selection is performed on the two lines on the same pole. In the equivalent circuit, the non-faulty line is represented by an equivalent zero-sequence capacitor, and the reactive power flows from the line to the bus, and the measured zero-sequence reactive power at the bus outlet is negative;

[0025] In the faulty line, the reactive power flows from the bus to the line, and the measured zero-sequence reactive power at the bus outlet is positive. Throughi F The calculated reactive power is exactly the reactive power of line L1 and the reactive power of line L2 The difference is used for fault line selection of the two lines within the boundary. For lines L1 and L2 that are completely on the same pole and in parallel, there is , and correspondingly there is ; if the reactive power is positive, then line L1 has a fault;

[0026] if the reactive power is negative, then line L2 has a fault;

[0027] For lines that are not completely on the same pole and in parallel, correct i F , and use the corrected to calculate the reactive power. The criterion and the line selection result are the same as when they are completely on the same pole and in parallel; If the amplitude determination determines that the double-circuit line on the same pole is a non-fault line, then perform out-of-boundary line selection.

[0028] Preferably, in step S3, the out-of-boundary line selection process outside the group is based on the zero-sequence current in-phase component as follows: The condition for starting the out-of-boundary line selection is that the double-circuit line on the same pole in the in-boundary line selection is determined to be a non-fault line,

[0029] is in the same phase as i 01 and i 02 , then the effective value of the zero-sequence current in-phase component is expressed as . For an ungrounded system, the zero-sequence current of the fault feeder is the sum of the zero-sequence currents of the non-fault feeders, and the zero-sequence current on the line with a single-phase ground fault is not less than the sum of the effective values of the zero-sequence current in-phase components of each group of double-circuit lines on the same pole . Based on set a threshold β to screen the faulty lines and filter out the shorter lines that are prone to misjudgment in the zero-sequence current phase comparison. The threshold β is determined according to the actual distribution network structure and is set in the example as:

[0030] ;

[0031] If the zero-sequence currents of all single-circuit lines outside the boundary are less than β , it indicates a bus fault; if there is a line with a zero-sequence current greater than β , compare the zero-sequence current with the sum of the instantaneous zero-sequence current in-phase component . The line with the opposite phase is the faulty line. If they are all in the same phase, then it is determined as a bus fault.

[0032] Preferably, after the example analysis of the line selection method under the typical distribution network structure is completed, simulation verification is carried out: build a typical 10 kV distribution network Matlab / Simulink simulation model, and set the faults to occur on the short parallel feeders on the same pole, the incomplete parallel feeders with different length differences on the same pole, the short non-parallel feeders on different poles, the long non-parallel feeders on different poles, and the busbar respectively.

[0033] Preferably, the results of the simulation feature verification are as follows:

[0034] The faulty line has a changing pattern of fault characteristics under different transition resistances, fault point positions, and fault initial phase angles for the parallel feeders on the same pole:

[0035] The larger the transition resistance, the smaller the first half-wave peak value of the zero-sequence current of the faulty line, the faster the transient attenuation speed, and the steady-state current is determined by the ground currents of other feeders, with little overall change and slight distortion at higher transition resistances.

[0036] As the fault point is farther from the busbar, the amplitude of the first half-wave of the zero-sequence current becomes smaller, the transient main resonance frequency becomes lower, the zero-sequence steady-state current of the faulty line is determined by the zero-sequence currents of the non-faulty feeders, and is less affected by the change of the fault point position. The fault initial phase angle only has a great influence on the transient amplitude of the zero-sequence current.

[0037] The coupling relationship of the parallel feeders on the same pole has no influence on the steady-state zero-sequence current of the faulty line, but when the faulty line is a parallel feeder on the same pole, the transient zero-sequence current changes, manifested as an increase in the transient resonance main frequency.

[0038] Therefore, the present invention adopts the above-mentioned small current grounding fault line selection method for a distribution network with parallel feeders on the same pole, and has the following beneficial effects:

[0039] (1) Based on the simulation model, the present invention explores the influence of the same-pole erection on the single-phase grounding fault characteristics of the distribution network, and further provides a steady-state line selection method for single-phase grounding faults in a neutral-ungrounded system with double parallel feeders on the same pole. By grouping the double parallel feeders on the same pole, the distribution network line selection process is divided into an in-bound line selection within the group of parallel feeders on the same pole and an out-bound line selection for other single feeders, improving the sensitivity of line selection. By making two amplitude judgments, the problem of misjudgment caused by the small fault characteristics of short feeders is avoided, which is beneficial to the fault handling of related distribution systems.

[0040] Next, through the attached drawings and embodiments, the technical solutions of the present invention will be further described in detail. Description of the Drawings

[0041] Figure 1 It is the distribution network model with parallel feeders on the same pole built in the first embodiment of the present invention;

[0042] Figure 2It is the waveform diagram of the zero-sequence current of the faulty feeder under different transition resistances measured by the same-pole erection system in the present invention;

[0043] Figure 3 It is the transient waveform diagram of the zero-sequence current of the faulty feeder under different transition resistances in the same-pole erection system of the present invention;

[0044] Figure 4 It is the steady-state waveform diagram of the zero-sequence current of the faulty feeder under different transition resistances in the same-pole erection system of the present invention;

[0045] Figure 5 It is the steady-state waveform diagram of the zero-sequence current of the faulty feeder at different fault point positions in the same-pole erection system of the present invention;

[0046] Figure 6 It is the transient waveform diagram of the zero-sequence current of the faulty feeder at different fault point positions in the same-pole erection system of the present invention;

[0047] Figure 7 It is the transient waveform diagram of the zero-sequence current of the faulty feeder under different fault initial phase angles in the same-pole erection system of the present invention;

[0048] Figure 8 It is the steady-state waveform diagram of the zero-sequence current of the faulty feeder with a metallic ground fault under different parallel pole arrangements in the present invention;

[0049] Figure 9 It is the transient waveform diagram of the zero-sequence current of the faulty feeder with a metallic ground fault under different parallel pole arrangements in the present invention;

[0050] Figure 10 It is the flow chart for line selection of the present invention;

[0051] Figure 11 It is the power distribution network model for example analysis built in the third embodiment of the present invention;

[0052] Figure 12 It is the phase comparison image in the third embodiment of the present invention. Detailed implementation manners

[0053] The technical solutions of the present invention will be further described below with reference to the accompanying drawings and embodiments.

[0054] Unless otherwise defined, the technical terms or scientific terms used in the present invention shall have the ordinary meanings understood by those of ordinary skill in the field to which the present invention belongs.

[0055] The words "include" or "comprises" and the like used in the present invention mean that the elements before the word include the elements listed after the word, and do not exclude the possibility of also including other elements. The orientation or position relationship indicated by the terms "inside", "outside", "upper", "lower", etc. is based on the orientation or position relationship shown in the drawings, which is only for the convenience of describing the present invention and simplifying the description, and does not indicate or imply that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation. Therefore, it cannot be understood as a limitation of the present invention. When the absolute position of the described object changes, the relative position relationship may also change accordingly. In the present invention, unless otherwise clearly specified and limited, the terms "attachment" and the like should be understood in a broad sense, for example, it can be a fixed connection, a detachable connection, or an integral body; it can be directly connected, or indirectly connected through an intermediate medium, and it can be the internal connection of two elements or the interaction relationship between two elements. For those of ordinary skill in the art, the specific meanings of the above terms in the present invention can be understood according to the specific circumstances.

[0056] Embodiment 1

[0057] This embodiment discloses the influence of the same-pole installation on the single-phase grounding fault characteristics of the distribution network, and specifically includes the following steps:

[0058] Step (1) build a simulation model of a distribution network with two parallel lines on the same pole and an equivalent distribution network simulation model without a parallel system under the same operating conditions; Step (1) build a simulation model of a distribution network with two parallel lines on the same pole and an equivalent distribution network simulation model without a parallel system under the same operating conditions:

[0059] like Figure 1 As shown, there are 6 overhead lines in total, of which L1 and L2 are installed on the same pole, and L4 and L5 are installed on the same pole.

[0060] For a single loop, set the line mode resistance to 0.12Ω / km, the zero mode resistance to 0.29Ω / km, and the line mode inductance to 1.08×10 - 3 H / km, zero mode inductance 4.49×10 -3 H / km, line mode capacitance 11.49×10 -9 F / km, zero mode capacitance 5.475×10 -9F / km.

[0061] For the lines on the same pole, a six-phase distributed parameter line model is used for simulation. The corresponding single-loop line parameters are used, and the corresponding phase model parameters are calculated by Karenbauer transformation. After taking into account the zero-sequence coupling, the resistance matrix, inductance matrix, and capacitance matrix are formed respectively. The parameters are self-resistance 0.177Ω / km, phase-to-phase mutual resistance 0.057Ω / km, line-to-line mutual resistance 0.08Ω / km, self-inductance 2.215×10 -3H / km, the mutual inductance between phases is 1.133×10 -3 H / km, the mutual inductance between lines is 0.913×10 -3 H / km, the self-capacitance is 12.09×10 -9 F / km, the mutual capacitance between phases is -2×10 -9 F / km, the mutual capacitance between lines is -0.87×10 -9 F / km.

[0062] The transformer is connected in YgD1, with a rated power of 20 MVA. The loads are all set as 1 MW constant impedance loads, and the power factor is set as 0.9.

[0063] The fault is set on L1, and parameters such as the fault point location, transition resistance, and fault initial phase angle are adjusted to conduct multiple simulations. To explore the influence of the same-pole erection on the fault characteristics, in addition to the above simulation settings, the cases of non-coupling between L1 and L2 and non-coupling of all lines are additionally considered during the simulation.

[0064] Step (2) Analyze the fault waveforms after single-phase grounding faults in the parallel and non-parallel systems;

[0065] Taking the transition resistance as a variable, set the fault to occur at 10 km from the bus on L1, with a fault initial phase angle of 90°, to explore the influence of the transition resistance on the zero-sequence current. Figure 2 、 Figure 3 、 Figure 4 are the zero-sequence currents of the fault feeder under different transition resistances, and their amplified waveform images in the transient and steady states. The larger the transition resistance, the smaller the peak value of the first half-wave of the zero-sequence current of the fault line, and the faster the transient attenuation speed. The zero-sequence steady-state current of the fault line is determined by the ground currents of other feeders, and the overall change is not significant. Due to the influence of the transition resistance on the zero-sequence voltage of the bus, it indirectly causes a slight lag in the phase and a decrease in the amplitude when the transition resistance is 1000 Ω.

[0066] Taking the fault point location as a variable, set the fault to occur on L1, with a transition resistance of 10 Ω and a fault initial phase angle of 90°, to explore the influence of the fault point location on the zero-sequence current. Figure 5 、 Figure 6 are the transient and steady-state zero-sequence current waveform images of the fault feeder under different fault point locations. As the fault point is farther from the bus, the amplitude of the first half-wave of the zero-sequence current becomes smaller, and the transient main resonance frequency becomes lower. The zero-sequence steady-state current of the fault line is determined by the ground currents of other feeders and is less affected by the change in the fault point location.

[0067] Taking the fault initial phase angle as a variable, set the fault to occur at 10 km from the bus on L1, with a transition resistance of 10 Ω, to explore the influence of the fault initial phase angle on the zero-sequence current. Figure 7It is the waveform image of the transient zero-sequence current of the faulty feeder under different fault initial phase angles. The fault initial phase angle only has a greater impact on the transient amplitude of the zero-sequence current. The larger the fault initial phase angle, the larger the transient amplitude, and it has no influence on the transient main resonance frequency.

[0068] To explore the influence of parallel lines on fault characteristics, in addition to the above simulation settings, the cases of non-coupling between L1 and L2 and non-coupling of all lines were additionally considered during the simulation. The fault was set to occur at 10 km from the bus on L1, with a transition resistance of 0.001 Ω and a fault initial phase angle of 90°.

[0069] Figure 8 and Figure 9 are the steady-state and transient zero-sequence currents measured at the outlet of the bus for the faulty feeder (L1). In the steady-state waveform of the zero-sequence current of the faulty feeder, the simulation waveforms in the three cases of the faulty feeder having parallel lines, the non-faulty feeder having parallel lines, and no parallel lines completely coincide. The coupling relationship of the parallel lines has no influence on the steady-state zero-sequence current of the faulty feeder. In the transient waveform, there are certain differences in the simulation waveforms of the three cases. The transient zero-sequence current decays more slowly under a metallic ground connection. It can be clearly seen from the figure that only the waveforms of L4 and L5 being parallel and not being parallel are approximately the same, and the transient resonance main frequency of the zero-sequence current when the faulty feeder L1 has parallel lines is higher than the previous two.

[0070] The parameters such as the fault point location, transition resistance, and fault initial phase angle were adjusted for multiple simulations, and the waveform characteristics and their variation laws observed based on the simulation data under multiple working conditions were all consistent with the above typical cases.

[0071] Step (3) presents the variation law of fault characteristics.

[0072] Parallel lines have a fault characteristic variation law similar to that of a single-circuit line under different transition resistances, fault point locations, and fault initial phase angle parameters. The larger the transition resistance, the smaller the peak value of the first half-wave of the zero-sequence current of the faulty line, the faster the transient decay speed, and the steady-state current is determined by the ground currents of other feeders, with little overall change and slight distortion at a higher transition resistance. As the fault point is farther from the bus, the amplitude of the first half-wave of the zero-sequence current is smaller, the transient main resonance frequency is lower, and the zero-sequence steady-state current of the faulty line is determined by the zero-sequence current of the non-faulty feeder, with little influence from the change in the fault point location. The fault initial phase angle only has a greater impact on the transient amplitude of the zero-sequence current.

[0073] The coupling relationship of parallel lines has no influence on the steady-state zero-sequence current of the faulty feeder, but when the faulty line is a parallel line, its transient zero-sequence current changes, mainly manifested as an increase in the transient resonance main frequency.

[0074] This embodiment provides the single-phase grounding fault characteristics of a distribution network with double circuits on the same pole and the influence of the same-pole erection on the single-phase grounding fault characteristics of the distribution network compared with a non-parallel system. The double circuits on the same pole have a similar law of change in fault characteristics as single circuits under different transition resistances, fault point positions, and fault initial phase angle parameters. The coupling relationship of the double circuits on the same pole has no influence on the steady-state zero-sequence currents of the fault feeder and non-fault feeders. The line selection of the same-pole parallel system can be studied based on the steady-state characteristics of the zero-sequence current after the fault of the equivalent non-parallel line.

[0075] Embodiment 2

[0076] This embodiment provides a small current grounding fault line selection method for a distribution network with double circuits on the same pole, reveals the influence of the same-pole erection on the single-phase grounding fault characteristics of the distribution network; proposes a steady-state line selection method for single-phase grounding faults in a non-grounded system with double circuits on the same pole; completes the example analysis of the line selection method under a typical distribution network structure, and verifies the effectiveness of the line selection method proposed by the present invention through a typical distribution network model. The specific line selection process is as Figure 10 shown, and the specific steps are as follows:

[0077] Step S1: Group the double circuits on the same pole in the distribution network, and obtain the in-phase component and anti-phase component of the zero-sequence current of each group;

[0078] In step S1, when a fault occurs, determine whether a single-phase grounding fault occurs in each group of double circuits on the same pole;

[0079] If a fault occurs, perform in-bound line selection for the fault group;

[0080] If all the in-phase lines in each group are non-fault lines, perform out-of-bound line selection for the single circuits outside the group;

[0081] Based on the idea of the six-sequence component method, transform the zero-sequence current of the double circuits on the same pole into the zero-sequence in-phase component and zero-sequence anti-phase component, as shown in the following formula:

[0082] ;

[0083] Where i T refers to the in-phase component of the instantaneous zero-sequence current, which is the sum of the zero-sequence currents of the double circuits on the same pole. i F refers to the anti-phase component of the instantaneous zero-sequence current, which is the difference between the zero-sequence currents of the double circuits on the same pole, i 01 and i 02 are the instantaneous zero-sequence currents flowing through the bus outlet of the lines L1 and L2 on the same pole respectively, and the instantaneous zero-sequence current flowing through the bus outlet of the single circuit L3 is denoted as i 03。Use i F to select the in - zone line, and use i T to select the out - of - zone line.

[0084] Step S2: Select the in - zone line within the group according to the reverse component of zero - sequence current, which specifically includes two parts: amplitude determination for screening fault groups and reactive power calculation for selecting the line of the fault group;

[0085] In step S2, the process of selecting the in - zone line within the group according to the reverse component of zero - sequence current is as follows:

[0086] After a single - phase ground fault occurs, start the line - selection process through zero - sequence voltage to select the in - zone line, and perform amplitude determination on the effective value of the reverse component of zero - sequence current I F When performing steady - state analysis of a single - phase ground fault according to the fault equivalent circuit, ignore the series impedance of the zero - mode and line - mode in the equivalent circuit, and only retain the transition resistance. The zero - sequence current phases of the fault feeder and the non - fault feeder are completely opposite. The zero - sequence current phase of the non - fault feeder leads the zero - sequence voltage of the bus by 90°, and the zero - sequence current phase of the fault feeder lags the zero - sequence voltage of the bus by 90°;

[0087] When lines L1 and L2 are non - fault lines, record the zero - sequence current phasor values flowing through the bus outlet of lines L1 and L2 as i 01n 、 i 02n , and the calculation process is as follows:

[0088] ;

[0089] where C 0_1s 、C 0_2s are respectively the equivalent non - parallel - rack line - to - ground capacitances of the parallel - rack lines L1 and L2, is the system zero - sequence voltage phasor value, j is the rotation factor, is the system operating frequency, 、 The instantaneous values of are respectively recorded as 、 , and the effective values are respectively recorded as 、 ;

[0090] In the case where the double - circuit line on the same pole is a non - fault line, when the double - circuit line is completely on the same pole, the zero - sequence current phasor values of the two parallel - rack feeders should be the same: , and at this time, the effective value of the reverse component of zero - sequence current ;

[0091] When the double-circuit lines are not completely mounted on the same pole, there are certain differences in the lengths of the two feeders. At this time , but the zero-sequence currents on the two feeders are in the same phase, and the effective value of the reverse component of the zero-sequence current is represented by the difference between the effective value of the zero-sequence current of line L1 and the effective value of the zero-sequence current of line L2 as:

[0092] ;

[0093] For the case where there is a faulty line in the double-circuit lines mounted on the same pole, the phases of the zero-sequence currents of the two parallel feeders are opposite, I F which is approximately the sum of the effective values of the zero-sequence currents of the two feeders. Since the zero-sequence currents of other feeders outside the boundary flow towards the faulty line, at this time I F is larger, ;

[0094] When there is no in-boundary fault, I F is smaller. After an in-boundary fault occurs, I F is larger. By setting a threshold value α , for I F the amplitude is judged to determine whether an in-boundary fault has occurred:

[0095] ;

[0096] If the amplitude judgment determines that an in-boundary fault has occurred, the faulty line selection is performed on the two lines mounted on the same pole. In the equivalent circuit, the non-faulty line is equivalently represented by a zero-sequence capacitor, and the reactive power flows from the line to the bus, and the measured zero-sequence reactive power at the bus outlet is negative;

[0097] In the faulty line, the reactive power flows from the bus to the line, and the measured zero-sequence reactive power at the bus outlet is positive. The reactive power calculated through i F is exactly the difference between the reactive power of line L1 and the reactive power of line L2 . The positive and negative of the difference are used for the faulty line selection of the two in-boundary lines. For the completely same-pole-mounted lines L1 and L2, there is , and correspondingly there is ;

[0098] If the reactive power is positive, then line L1 is faulty;

[0099] If the reactive power is negative, then line L2 is faulty;

[0100] For non-identical double-circuit lines on the same pole, i F perform correction, , and use the corrected to calculate the reactive power. The criterion and the line selection result are the same as those in the case of identical double-circuit lines on the same pole;

[0101] If the amplitude determination determines that the double-circuit line on the same pole is a non-faulty line, then perform out-of-zone line selection.

[0102] Step S3: Perform out-of-zone line selection outside the group based on the zero-sequence current in-phase component, which specifically includes two parts: amplitude screening for possible faulty lines and phase comparison for determining the faulty line. In step S3, the process of performing out-of-zone line selection outside the group based on the zero-sequence current in-phase component is as follows:

[0103] The condition for starting out-of-zone line selection is that the in-zone line selection determines that both double-circuit lines on the same pole are non-faulty lines, i 01 and i 02 are in the same phase. Then, the effective value of the zero-sequence current in-phase component is expressed as . For an ungrounded system, the zero-sequence current of the faulty feeder is the sum of the zero-sequence currents of the non-faulty feeders, and the zero-sequence current on the line with a single-phase ground fault is not less than the sum of the effective values of the zero-sequence current in-phase components of each group of double-circuit lines on the same pole , based on set a threshold β to screen the faulty lines and filter out the shorter lines that are prone to misjudgment in the zero-sequence current phase comparison. The threshold β is determined according to the actual distribution network structure and is set in the numerical example as:

[0104] ;

[0105] If the zero-sequence current of all single-circuit lines outside the zone is less than β , it indicates a bus fault; if there is a line with a zero-sequence current greater than β , compare the zero-sequence current with the sum of the instantaneous zero-sequence current in-phase component . The line with an opposite phase is the faulty line. If they are all in the same phase, then determine a bus fault.

[0106] Embodiment III

[0107] After completing the numerical example analysis of the line selection method under the typical distribution network structure, perform simulation verification: Build a typical 10 kV distribution network matlab / simulink simulation model, and set the faults to occur on the short double-circuit line on the same pole, the non-identical double-circuit lines with different length differences, the short non-identical double-circuit line, the long non-identical double-circuit line, and the bus respectively. As Figure 11As shown, a total of 8 lines L1 - L8 are set. Lines L1 - L6 are parallel lines on the same pole, and lines L7 and L8 are single - circuit lines. Among them, lines L1 and L2 are completely parallel lines on the same pole with a length of 5 km, simulating the shorter parallel double - circuit lines on the same pole in the system; lines L3 and L4, and lines L5 and L6 are not completely parallel lines on the same pole, and their parallel lengths are both 10 km, simulating the incompletely parallel lines on the same pole with length differences in the system, where the length difference between lines L5 and L6 is relatively large; the single - circuit line L7 has a length of 5 km, simulating the short line in the system where phase misjudgment is likely to occur; the single - circuit line L8 has a length of 30 km. In this embodiment, the system parameters, the parameters of the parallel lines on the same pole, and the parameters of the single - circuit lines are the same as those in Embodiment 1, and will not be elaborated here. Single - phase ground faults are respectively set at the ends of lines L2 - L8 and single - line ground faults are set at the bus. Simulations are carried out under various transition resistances to verify the proposed line - selection method.

[0108] First, group the parallel lines on the same pole. Lines L1 and L2 are in Group 1, lines L3 and L4 are in Group 2, and lines L5 and L6 are in Group 3. Taking the single - phase ground fault with a transition resistance of 10 Ω occurring at the end of line L3 as an example, the in - zone line - selection process is introduced.

[0109] After the fault occurs, first judge the amplitude of the reverse component of the zero - sequence current of each group of parallel lines on the same pole in the zone. The relevant data is shown in Table 1:

[0110] Table 1

[0111] ;

[0112] After determining that the fault occurs in Group 2, calculate the reactive power generated by the reverse component of the zero - sequence current in Group 2. After correcting the reverse component of the zero - sequence current, the measured reactive power Q F2 = 4617 var > 0, and the first line in the group, that is, line L3, is the fault line, and the line - selection ends.

[0113] Taking the single - phase ground fault with a transition resistance of 1000 Ω occurring at the end of line L8 as an example, the out - of - zone line - selection process is introduced.

[0114] After the fault occurs, first judge the amplitude of the reverse component of the zero - sequence current of each group of parallel lines on the same pole in the zone. The relevant data is shown in Table 2:

[0115] Table 2

[0116] ;

[0117] Judge that an out - of - zone fault has occurred. The threshold β = 0.497, I 07 = 0.041 A, I08 = 0.662 A. The single - circuit line L8 may be the faulty line. To determine whether it is a bus - bar fault, it is also necessary to i 08 compare the phases with . As shown in Figure 12 , the two are out of phase, and it is determined that the line L8 is the faulty line.

[0118] All the simulation data of single - line - to - ground faults occurring at the ends of lines L2 to L8 and on the bus - bar are set as shown in Table 3:

[0119] Table 3

[0120] ;

[0121] The steps and methods involved in the second and third embodiments are based on the first embodiment.

[0122] The verification results of the simulation characteristics are as follows:

[0123] The faulty line has a changing rule of fault characteristics under different transition resistances, fault - point positions, and fault initial phase angles for the double - circuit lines on the same pole:

[0124] The larger the transition resistance, the smaller the peak value of the first half - wave of the zero - sequence current of the faulty line, the faster the transient attenuation speed, the steady - state current is determined by the ground currents of other feeders, and the overall change is small, with a slight distortion at a relatively high transition resistance;

[0125] As the fault - point position is farther from the bus - bar, the amplitude of the first half - wave of the zero - sequence current is smaller, the transient main resonance frequency is lower, the zero - sequence steady - state current of the faulty line is determined by the zero - sequence currents of the non - faulty feeders, and the influence of the change in the fault - point position is small. The fault initial phase angle only has a great influence on the transient amplitude of the zero - sequence current;

[0126] The coupling relationship of the double - circuit lines on the same pole has no influence on the steady - state zero - sequence current of the faulty feeder, but when the faulty line is a double - circuit line on the same pole, the transient zero - sequence current changes, manifested as an increase in the transient resonance main frequency.

[0127] Therefore, the present invention adopts the above - mentioned method for selecting the faulty line of a small - current - grounded distribution network with double - circuit lines on the same pole, using negative - dispersion ytterbium - doped fiber as the gain medium of a high - repetition - rate fiber laser to simplify dispersion management, reduce the threshold, optimize the control of nonlinear effects, and improve the repetition - rate performance, and finally obtains a low - threshold mode - locked pulse output with a repetition rate of hundreds of GHz.

[0128] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit them. Although the present invention has been described in detail with reference to the preferred embodiments, those of ordinary skill in the art should understand that they can still modify the technical solutions of the present invention or make equivalent replacements, and these modifications or equivalent replacements cannot make the modified technical solutions deviate from the spirit and scope of the technical solutions of the present invention.

Claims

1. A method for selecting a line for a small current ground fault in a distribution network containing lines on the same pole, characterized in that: The specific steps include: Step S1: grouping the double-circuit lines on the same pole in the distribution network, and obtaining the same component and opposite component of the zero-sequence current of each group; Step S2: selecting the line within the group according to the zero-sequence current reverse component, specifically including two parts: amplitude determination for screening the fault group and reactive power calculation for the line selected for the fault group; In step S2, the process of selecting the line within the group based on the zero-sequence current reverse component is as follows: After a single-phase grounding fault occurs, the zero-sequence voltage is used to start the line selection process to select the line within the boundary, and the effective value of the zero-sequence current reverse component is I F When making amplitude determination and performing steady-state analysis of single-phase grounding fault according to the fault equivalent circuit, the series impedance of the zero mode and line mode of the equivalent circuit is ignored, and only the transition resistance is retained. The zero-sequence current phases of the fault feeder and the non-fault feeder are completely opposite. The zero-sequence current phase of the non-fault feeder leads the zero-sequence voltage of the bus by 90°, and the zero-sequence current phase of the fault feeder lags the zero-sequence voltage of the bus by 90°. When lines L1 and L2 are non-fault lines, the zero-sequence current phasor value flowing through lines L1 and L2 at the bus outlet is , , the calculation process is as follows: ； Among them C 0_1s , C 0_2s They are the equivalent non-parallel line capacitance to ground of the parallel line L1 and line L2, is the system zero-sequence voltage phasor value, j is the rotation factor, is the system operating frequency, , The instantaneous values ​​of , , and the effective values ​​are recorded as 、 ; When the double-circuit lines on the same pole are non-fault lines, the zero-sequence current phasor values ​​of the two feeders on the same pole should be consistent: , at this time the effective value of the zero-sequence current reverse component ; When the double-circuit lines are not completely installed on the same pole, the lengths of the two feeder lines are different. , but the zero-sequence currents on the two feeders are in phase, and the effective value of the zero-sequence current reverse component is the effective value of the zero-sequence current of line L1 And the effective value of zero-sequence current of line L2 The difference is expressed as: ； In the case of a faulty line in a double-circuit line on the same pole, the zero-sequence currents of the two feeders are in opposite phases. I F It is approximately the sum of the effective values ​​of the zero-sequence currents of the two feeders. Since the zero-sequence currents of the other feeders outside the boundary all flow to the fault line, at this time I F Larger, ; When no internal fault occurs, I F Small, after an internal fault occurs, I F Larger, by setting the threshold α ,right I F The amplitude is used to determine whether an in-bounds fault has occurred: ； If the amplitude judgment determines that an in-boundary fault has occurred, the two lines on the same pole are selected. In the equivalent circuit, the non-fault line is equivalently represented by zero-sequence capacitance. Reactive power flows from the line to the busbar, and the zero-sequence reactive power measured at the busbar outlet is negative. In the fault line, reactive power flows from the busbar to the line, and the zero-sequence reactive power measured at the busbar outlet is positive. The calculated reactive power is exactly the reactive power of line L1 Reactive power of line L2 The positive and negative values ​​of the difference are used to select the fault line of the two lines within the boundary. For the lines L1 and L2 that are completely installed on the same pole, there are , correspondingly ; If no effect If it is positive, line L1 is faulty; If no effect If it is negative, line L2 is faulty; For lines that are not completely installed on the same pole, Make corrections, , using the corrected The reactive power calculation criteria and line selection results are consistent with those when all the cables are installed on the same pole. If the amplitude determines that the double-circuit line on the same pole is a non-fault line, then the out-of-bounds line selection is performed; Step S3: selecting the out-of-bounds line outside the group according to the zero-sequence current component, specifically including two parts: amplitude screening of possible fault lines and phase comparison of the fault line; In step S3, the process of selecting the outer line outside the group according to the zero-sequence current component is as follows: The condition for starting the out-of-bounds line selection is that the in-bounds line selection determines that the double-circuit lines on the same pole are all non-fault lines. and The same-phase, zero-sequence current component effective value is expressed as For an ungrounded system, the zero-sequence current of the faulty feeder is the sum of the zero-sequence currents of the non-faulty feeders. The zero-sequence current on the line with a single-phase grounding fault is not less than the sum of the effective values ​​of the zero-sequence currents of the same components of the lines on the same pole. ,based on Setting Thresholds β To screen the faulty lines and filter out the shorter lines that are easily misjudged in the zero-sequence current phase comparison, the threshold β According to the actual distribution network structure, the following settings are made in the example: ； If the zero-sequence current of a single-circuit line outside the boundary is less than β , indicating a busbar fault; if there is a zero-sequence current greater than β The sum of the zero-sequence current and the instantaneous zero-sequence current is Compare the phases, the one with opposite phase is the faulty line, if they are all in phase, it is determined to be a bus fault.

2. According to the method for selecting a line for a small current ground fault in a distribution network containing lines in parallel on the same pole as described in claim 1, it is characterized in that: In step S1, when a fault occurs, it is determined whether a single-phase grounding fault occurs in each group of double-circuit lines on the same pole; If a fault occurs, the fault group is selected within the boundary; If all the groups of common-sensing lines are non-fault lines, the single-circuit lines outside the group are selected outside the boundary; Based on the idea of ​​the six-sequence component method, the zero-sequence current of the double-circuit line on the same pole is transformed into zero-sequence same component and zero-sequence opposite component, as shown in the following formula: ； in It refers to the same component of instantaneous zero-sequence current, which is the sum of zero-sequence currents of double-circuit lines on the same pole. It refers to the reverse component of the instantaneous zero-sequence current, which is the difference between the zero-sequence currents of the double-circuit lines on the same pole. and are the instantaneous zero-sequence currents flowing through the bus outlet of the lines L1 and L2 on the same pole, respectively. To select a line in the boundary, use Make an out-of-bounds line selection.

3. According to the method for selecting a line for a small current ground fault in a distribution network containing lines in parallel on the same pole as described in claim 2, it is characterized in that: After completing the case analysis of the line selection method under the typical distribution network structure, simulation verification is carried out: a typical 10kV distribution network matlab / simulink simulation model is built, and the faults are set to occur in the short feeders on the same pole, incomplete feeders on the same pole with different length differences, short feeders on different poles, long feeders on different poles and busbars.

4. A method for selecting a line for a small current ground fault in a distribution network containing lines in parallel on the same pole as claimed in claim 3, characterized in that: The simulation feature verification results are: The fault characteristics of the lines installed on the same pole vary under different transition resistances, fault point locations, and fault initial phase angle parameters: The larger the transition resistance, the smaller the first half-wave peak value of the zero-sequence current of the fault line, the faster the transient decay speed, and the steady-state current is determined by the current of other feeders to the ground, with little overall change, and slightly distorted when the transition resistance is high; As the fault point is farther from the busbar, the amplitude of the first half-wave of the zero-sequence current is smaller, the transient main resonant frequency is lower, and the zero-sequence steady-state current of the fault line is determined by the zero-sequence current of the non-fault feeder, and is less affected by the change of the fault point location. The initial phase angle of the fault only has a great influence on the transient amplitude of the zero-sequence current. The coupling relationship of the lines installed on the same pole has no effect on the steady-state zero-sequence current of the faulty feeder. However, when the faulty line is a line installed on the same pole, the transient zero-sequence current changes, which is manifested as an increase in the transient resonant main frequency.

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