A method and device for detecting high-resistance grounding faults in a distribution network

Abstract

The present invention provides a fault detection method and device for high-resistance grounding faults in a distribution network. The method comprises: injecting a compensation current into the distribution network through a controllable voltage source; reducing the output voltage amplitude of the controllable voltage source to obtain the zero-sequence current value of each distribution line after the reduction; and determining the fault line based on the zero-sequence current value of each distribution line after the reduction. The present invention can achieve full current compensation of the fault current, quickly extinguish the fault arc, and prevent the fault from expanding. At the same time, in the case of a high-resistance grounding fault, the fault current can also be appropriately increased to achieve accurate positioning of the fault point.

Description

Technical Field

[0001] The present invention relates to the field of distribution network measurement, and in particular to a fault detection method and device for a high-resistance grounding fault in a distribution network. Background Art

[0002] When a single-phase ground fault occurs in a distribution network with a low-current grounding system, the magnitude of the fault current depends primarily on the system capacitive current and the fault resistance. When the fault current is high, arcing can easily cause the fault to expand or even catch fire in cables and switchgear. However, when the ground fault current is low, ground fault line selection and location cannot be achieved based on fault characteristics. Existing capacitive current compensation devices for distribution networks primarily compensate for system capacitive current but cannot precisely control ground current. Consequently, existing fault line selection methods have numerous shortcomings. Summary of the Invention

[0003] In order to solve the above problems, the present invention provides the following technical solutions:

[0004] A first embodiment of the present invention provides a method for detecting a high-resistance grounding fault in a distribution network, wherein the distribution network includes a plurality of distribution lines, including:

[0005] If a high-resistance ground fault occurs in the distribution network and the fault current is greater than a set threshold, a compensation current is injected into the distribution network through a controllable voltage source until the fault current drops to a near-zero range;

[0006] After a set interval, the injected compensation current is gradually reduced, and if the system damping rate changes and the fault grounding current is less than a set threshold, the output voltage amplitude of the controllable voltage source is gradually reduced;

[0007] The fault line is determined or the fault point is located according to the current characteristics of the distribution network during the curtailment operation; wherein the topological structure combination of the controllable voltage source is determined based on the characteristics of the distribution network.

[0008] In a preferred embodiment, before injecting the compensation current into the distribution network through the controllable voltage source, the fault detection method further includes:

[0009] The compensation current is generated according to a system damping rate during normal operation of the power distribution network.

[0010] In a preferred embodiment, the current characteristics of the distribution network include current values ​​of each line; and determining the fault line according to the current characteristics of the distribution network during the curtailment operation includes:

[0011] It is determined whether the difference between the current values ​​of any two distribution lines after each power reduction is greater than a set threshold. If so, the distribution line with the larger current among the two distribution lines is the fault line.

[0012] In a preferred embodiment, the current characteristics of the distribution network include current values ​​before and after the compensation current is injected into each current detection point on each line; locating the fault point according to the current characteristics of the distribution network during the curtailment operation includes:

[0013] Calculate the current difference before and after the compensation current is injected into each current detection point on each line;

[0014] Searching for a current detection point where the current difference is greater than a set threshold, and searching for the next current detection point adjacent to the current detection point where the current difference is greater than the set threshold along the current direction on the power distribution line;

[0015] If the current difference corresponding to the next current detection point is smaller than the set threshold, the fault point of the high-resistance grounding fault is located between the current detection point where the current difference is larger than the set threshold and the next current detection point.

[0016] In a preferred embodiment, determining the topology combination of the controllable voltage source based on the distribution network characteristics includes:

[0017] Connecting each line of the distribution network to a semi-physical simulation voltage source topology model, wherein the semi-physical simulation voltage source topology model outputs current control range data corresponding to a plurality of voltage source topology structure combinations;

[0018] Select the voltage source topology combination corresponding to the current control data with the widest current control range.

[0019] In a preferred embodiment, it also includes:

[0020] measuring the real-time phase and real-time amplitude of the controllable voltage source in real time;

[0021] Subtracting the real-time phase and real-time amplitude measured in real time from the preset phase and preset amplitude to obtain a phase tracking error and an amplitude tracking error;

[0022] The phase tracking error and amplitude tracking error are sent as feedback signals to the hysteresis comparator, which is used to compare the tracking error with the upper and lower thresholds to adjust the phase and amplitude of the controllable voltage.

[0023] In a preferred embodiment, it also includes:

[0024] The controllable voltage source is modulated by carrier phase shifting.

[0025] A second embodiment of the present invention provides a fault detection device for a high-resistance grounding fault in a distribution network, wherein the distribution network includes a plurality of distribution lines, including:

[0026] a compensation current injection module, which, if a high-resistance grounding fault occurs in the distribution network and the fault current is greater than a set threshold, injects a compensation current into the distribution network through a controllable voltage source until the fault current drops to a near-zero range;

[0027] a reduction operation module, which gradually reduces the injected compensation current after a set interval, and if the system damping rate changes and the fault grounding current is less than a set threshold, gradually reduces the output voltage amplitude of the controllable voltage source;

[0028] A fault detection module determines a fault line or locates a fault point according to current characteristics of the distribution network during a curtailment operation; wherein the topological structure combination of the controllable voltage source is determined based on the characteristics of the distribution network.

[0029] In a preferred embodiment, the fault detection device further comprises:

[0030] The compensation current generating module generates the compensation current according to the system damping rate when the distribution network is operating normally.

[0031] In a preferred embodiment, the current characteristics of the distribution network include the current values ​​of each line; the reduction operation module is specifically used to determine whether the difference between the current values ​​of any two distribution lines after each reduction is greater than a set threshold. If so, the distribution line with the larger current among the two distribution lines is the fault line.

[0032] In a preferred embodiment, the current characteristics of the distribution network include the current values ​​before and after the compensation current is injected into each current detection point on each line; the fault detection module includes:

[0033] A current difference calculation unit calculates the current difference before and after the compensation current is injected into each current detection point on each line;

[0034] a detection point search unit for searching for a current detection point where the current difference is greater than a set threshold, and searching for a next current detection point adjacent to the current detection point where the current difference is greater than the set threshold along the current direction on the power distribution line;

[0035] The positioning unit locates the fault point of the high-resistance grounding fault between the current detection point where the current difference is greater than the set threshold and the next current detection point if the current difference corresponding to the next current detection point is less than the set threshold.

[0036] In a preferred embodiment, it also includes:

[0037] a model input module, connecting each line of the distribution network to a semi-physical simulation voltage source topology model, wherein the semi-physical simulation voltage source topology model outputs current control range data corresponding to a plurality of voltage source topology structure combinations;

[0038] The topology structure combination selection module selects the voltage source topology structure combination corresponding to the current control data with the widest current control range.

[0039] In a preferred embodiment, it also includes:

[0040] A real-time measurement module for measuring the real-time phase and real-time amplitude of the controllable voltage source in real time;

[0041] A tracking module is configured to obtain a phase tracking error and an amplitude tracking error by subtracting the real-time phase and real-time amplitude measured in real time from a preset phase and a preset amplitude;

[0042] The hysteresis comparison module sends the phase tracking error and amplitude tracking error as feedback signals to the hysteresis comparator, and uses the hysteresis comparator to compare the tracking error with the upper and lower thresholds to adjust the phase and amplitude of the controllable voltage.

[0043] In a preferred embodiment, it also includes:

[0044] The carrier modulation module adopts carrier phase shift to modulate the controllable voltage source.

[0045] It can be seen from the above technical solution that the present invention provides a fault detection method and device for a high-resistance grounding fault in a distribution network. First, when a high-resistance grounding fault occurs in the distribution network and the fault current is greater than a set threshold, the injected compensation current is gradually reduced. If the system damping rate changes and the fault grounding current is less than the set threshold, the output voltage amplitude of the controllable voltage source is gradually reduced. Finally, the fault line is determined or the fault point is located according to the current characteristics of the distribution network during the reduction operation. The topological structure combination of the controllable voltage source of the present invention is determined based on the characteristics of the distribution network, which can achieve full current compensation of the fault current, quickly extinguish the fault arc, and prevent the fault from expanding. At the same time, in the case of a high-resistance grounding fault, the fault current can be appropriately increased to achieve accurate positioning of the fault point. BRIEF DESCRIPTION OF THE DRAWINGS

[0046] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0047] Figure 1 This is one of the flow charts of the fault detection method for a high-resistance grounding fault in a distribution network according to an embodiment of the present invention.

[0048] Figure 2 Schematic diagram of fault current control principle in an embodiment of the present invention.

[0049] Figure 3 Schematic diagram of a simplified analysis circuit for controlling fault current using a controllable voltage source in an embodiment of the present invention.

[0050] Figure 4 Schematic diagram of controlling fault current and zero-sequence current of fault line by a controllable voltage source in an embodiment of the present invention.

[0051] Figure 5 Schematic diagram of a distribution network in an embodiment of the present invention.

[0052] Figure 6 This is one of the flow charts of the fault detection method for a high-resistance grounding fault in a distribution network according to an embodiment of the present invention.

[0053] Figure 7 Schematic diagram of the EMTP-ATP simulation model in an embodiment of the present invention.

[0054] Figure 8 This is a schematic diagram of the basic inverter unit and control method in the embodiment of the present application.

[0055] Figure 9 This is one of the schematic diagrams of the traditional multi-level inverter topology structure in the embodiment of the present invention.

[0056] Figure 10 This is the second schematic diagram of a traditional multi-level inverter topology structure in an embodiment of the present invention.

[0057] Figure 11 This is the third schematic diagram of the traditional multi-level inverter topology structure in the embodiment of the present invention.

[0058] Figure 12 Schematic diagram of a carrier phase-shift modulation method of a cascaded multi-level inverter in an embodiment of the present invention.

[0059] Figure 13 2 is a characteristic diagram of a hysteresis comparator in an embodiment of the present invention.

[0060] Figure 14 FIG. 1 is a schematic diagram of implementing two-level hysteresis control using a hysteresis comparator in an embodiment of the present invention.

[0061] Figure 15 This is one of the schematic diagrams of implementing three-level hysteresis control using a hysteresis comparator in an embodiment of the present invention.

[0062] Figure 16 This is the second schematic diagram of implementing three-level hysteresis control using a hysteresis comparator in an embodiment of the present invention.

[0063] Figure 17 This is one of the module schematic diagrams of the fault detection device for high-resistance grounding fault in the distribution network according to an embodiment of the present invention.

[0064] Figure 18 This is the second module schematic diagram of the fault detection device for high-resistance grounding fault in the distribution network according to an embodiment of the present invention.

[0065] Figure 19 In the embodiment of the present invention Figure 17 Schematic diagram of the specific structure of the fault detection module. DETAILED DESCRIPTION

[0066] To make the objectives, technical solutions, and advantages of the embodiments of the present invention more clear, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the accompanying drawings of the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. All other embodiments obtained by ordinary technicians in this field based on the embodiments of the present invention without making any creative efforts shall fall within the scope of protection of the present invention.

[0067] A first embodiment of the present invention provides a method for detecting a high-resistance ground fault in a distribution network, wherein the distribution network includes a plurality of distribution lines, such as Figure 1 Shown, including:

[0068] S1: If a high-resistance ground fault occurs in the distribution network and the fault current is greater than a set threshold, a compensation current is injected into the distribution network through a controllable voltage source until the fault current drops to a near-zero range;

[0069] S2: after a set interval, gradually reducing the injected compensation current; if the system damping rate changes and the fault grounding current is less than a set threshold, gradually reducing the output voltage amplitude of the controllable voltage source;

[0070] S3: determining a fault line or locating a fault point according to current characteristics of the distribution network during the curtailment operation; wherein the topological structure combination of the controllable voltage source is determined based on the characteristics of the distribution network.

[0071] This aspect provides a fault detection method for a high-resistance grounding fault in a distribution network. First, when a high-resistance grounding fault occurs in the distribution network and the fault current is greater than a set threshold, the injected compensation current is gradually reduced. If the system damping rate changes and the fault grounding current is less than the set threshold, the output voltage amplitude of the controllable voltage source is then gradually reduced. Finally, the fault line is determined or the fault point is located based on the current characteristics of the distribution network during the reduction operation. The topological structure combination of the controllable voltage source of the present invention is determined based on the characteristics of the distribution network, which can achieve full current compensation of the fault current, quickly extinguish the fault arc, and prevent the fault from expanding. At the same time, in the case of a high-resistance grounding fault, the fault current can be appropriately increased to achieve accurate positioning of the fault point.

[0072] It can be understood that the high-resistance grounding fault of the present invention, in theory, high resistance should be understood as 1000 ohms and above. Under actual working conditions, high-resistance faults ranging from several thousand ohms to more than 10,000 ohms are real. Although the proportion of high-resistance faults in grounding faults is small, their harm is greater.

[0073] In addition, it can be understood that the near-zero interval of the present invention can be understood as an interval range close to the zero value, that is, close to 0. For example, 0.0001 is close to 0, so the near-zero interval can be considered as (-0.0001-0.0001). Obviously, the near-zero interval is determined according to the industry. In some high-tech industries, the gap between 0.0001 and 0 is very large, but in the power industry, the gap between 0.0001 and 0 can be ignored. Of course, the above 0.0001 is only used as an example. Ordinary technicians in this field can know the range of the near-zero interval of different power systems based on experience.

[0074] The schematic diagram of the full current compensation device is as follows: Figure 2 As shown, the active inverter device is connected between the neutral point and the ground, which is equivalent to a controllable current source with an output current of I. The three-phase power supply voltage is determined by E A 、E B 、E C Indicates that U0 represents the neutral point voltage, r0 and C0 represent the resistance and capacitance of each phase to ground respectively. Assuming that the single-phase grounding fault occurs in phase C, the voltage at the fault phase power supply is U C , the voltage at the fault point is U f , the fault phase current is I C , the line impedance is Z, the voltage drop from the fault phase power supply to the fault point is ΔU, and the ground fault transition resistance is R f .

[0075] like Figure 2 As shown in the figure, when a ground fault in the distribution network is detected, the required injection current is generated by calculating the system damping rate during normal operation of the distribution network. The fault ground current is adjusted by injecting current through the PWM active inverter. The injection current is controlled in a closed loop to compensate for the reactive component, active component and harmonic component in the fault current, reducing the fault current to near zero and achieving 100% arc extinguishing of the instantaneous ground fault.

[0076] It can be understood that the controllable voltage source is placed in the active inverter, and the input current of the active inverter can be adjusted by adjusting the output voltage of the controllable voltage source.

[0077] In this aspect, for the full current compensation device of the active inverter, the system containing the controllable voltage source can be simplified to obtain a simplified circuit, such as Figure 3As shown, the current system has a total of n routes, where the system's single-phase distributed impedance to ground is Z1, Z2, ... Zn, Z f is the grounding impedance when a single-phase grounding fault occurs on phase A of a certain line. Point N is the system neutral point, the voltage on it is recorded as (UN), U0 is the controllable voltage source, Z0 is the internal resistance of the controllable voltage source, and Ua, Ub, and Uc are three-phase voltage sources.

[0078] According to the node voltage method, the equation is written and the system neutral point voltage can be expressed as:

[0079]

[0080] where Z d =Z1 / / Z2 / / … / / Z n , that is, the system's single-phase distributed impedance to ground.

[0081] The three-phase currents are:

[0082] I a =(U N +U a ) / Z d +(U N +U a ) / Z f

[0083] I b =(U N +U b ) / Z d

[0084] I c =(U N +U c ) / Z d (2)

[0085] The fault point current is:

[0086] I g =(U N +U a ) / Z f (3)

[0087] Since the current output by the controllable voltage source is:

[0088] I os =I a +I b +I c =3U N / Z d +I g (4)

[0089] In summary, the current at the fault point can be calculated using the following formula:

[0090] I g =I os -3U N / Z d (5)

[0091] According to the above formula, the output voltage of the controlled voltage source can control the system neutral point voltage, thereby controlling the current at the fault point. Therefore, when a single-phase ground fault occurs in the system, adjusting the voltage of the controlled voltage source can reduce the fault current to near zero, achieving 100% arc extinguishing for transient ground faults and preventing the expansion of permanent ground faults. Adjusting the voltage of the controlled voltage source can also accurately select and locate the fault line.

[0092] In an optional embodiment, before injecting the compensation current into the distribution network through the controllable voltage source, the fault detection method further includes:

[0093] The compensation current is generated according to a system damping rate during normal operation of the power distribution network.

[0094] In this embodiment, specifically, in the arc suppression coil compensated grounding power grid, the distribution network usually increases the system damping rate by connecting a damping resistor in series at the neutral point to prevent the occurrence of resonant overvoltage, such as Figure 5 As shown, R0 is the neutral point series resistance, Rs, Rt, and Rw are the line leakage resistances, and Cs, Ct, and Cw are the line-to-ground capacitances. According to the definition of damping rate: the ratio of the three-phase conductance to ground to the capacitive susceptance, and since conductance and susceptance determine the active and reactive components in the system, the damping rate is:

[0095]

[0096] Among them, G0 is the neutral point conductance, which is the reciprocal of the series resistance, that is: G0 = 1 / R0; 2Gs, Gt, Gw are the line leakage conductance, which are the reciprocal of the leakage resistance, that is: Gs = 1 / Rs; Gt = 1 / Rr; Gw = 1 / Rw; Bs, Bt, Bw are the line-to-ground susceptance, which are not equal in practice, and their relationship with the capacitance is: Bs = ωCs; Bt = ωCt; Bw = ωCw.

[0097] It can be understood that in the present invention, the set value, set difference and set threshold can all be reasonably configured according to the inventive concept of this aspect. For example, the set value is 110V, the set difference is 5V, and the set threshold is 8. The present invention does not limit this.

[0098] In an optional embodiment, the current characteristics of the distribution network include current values ​​of each line; and determining the fault line according to the current characteristics of the distribution network during the curtailment operation in step S3 includes:

[0099] It is determined whether the difference between the current values ​​of any two distribution lines after each power reduction is greater than a set threshold. If so, the distribution line with the larger current among the two distribution lines is the fault line.

[0100] Specifically, the reduction operation process can be implemented through iterative operations. For example, the step of determining the fault line in step S3 includes:

[0101] An iterative operation is performed to reduce the output voltage amplitude of the controllable voltage source by a set difference value, and determine whether the difference between the zero-sequence current values ​​of any two distribution lines is greater than a set threshold value. If not, the step of reducing the output voltage amplitude of the controllable voltage source by the set difference value is repeated until at least one of the zero-sequence current differences of each distribution line is greater than the set threshold value; then, the distribution line with the larger zero-sequence current among the two lines whose difference is greater than the set threshold value is determined, and the distribution line determined is the fault line.

[0102] In this embodiment, by gradually reducing the voltage amplitude, a certain buffer can be formed for the distribution network, avoiding the problem of instability of the distribution network caused by excessive reduction. In addition, by gradually reducing the voltage amplitude, when at least one of the zero-sequence current differences is greater than the set threshold, it can be seen that the zero-sequence current of the fault line is significantly different from the zero-sequence current of the non-fault line, thereby eliminating misjudgment caused by interference.

[0103] In addition, in an optional embodiment, the current characteristics of the distribution network include the current values ​​before and after the compensation current is injected into each current detection point on each line; the fault point is located according to the current characteristics of the distribution network during the curtailment operation in step S3, such as Figure 6 Shown, including:

[0104] S31: Calculate the current difference before and after the compensation current is injected into each current detection point on each line;

[0105] S32: searching for a current detection point where the current difference is greater than a set threshold, and searching for the next current detection point adjacent to the current detection point where the current difference is greater than the set threshold along the current direction on the power distribution line;

[0106] S33: If the current difference corresponding to the next current detection point is smaller than the set threshold, the fault point of the high-resistance grounding fault is located between the current detection point where the current difference is larger than the set threshold and the next current detection point.

[0107] Specifically, in this embodiment, according to Figure 4After a fault occurs at CT1 and CT2 before and after the ground fault, and before the full current compensation device operates, all line-to-ground capacitances inject capacitive current into the fault point. Therefore, the closer the fault line detection point is to the fault point, the higher the zero-sequence current. After the full compensation device operates, the injection of inductive current causes the zero-sequence current at the upstream detection point (CT1) to change from capacitive to inductive, resulting in a significant change. However, the zero-sequence current at the downstream detection points (CT2, CT3) and the non-fault line detection points changes very little. Therefore, through the above steps, the zero-sequence current at the upstream detection point changes significantly, while the zero-sequence current at the downstream detection points changes slightly. Simultaneously, the zero-sequence voltage at the non-fault line detection points changes slightly. This allows for a reasonable threshold to be set. When the difference between the current and the current is greater than the set threshold, this detection point can be identified as the upstream detection point of the fault point. This allows for continued feature analysis of the downstream detection points, which should show a smaller change, thus confirming the high-resistance fault point.

[0108] Furthermore, the topological structure combinations of the controllable voltage source in the embodiment of the present invention include multiple types. The topological structure combinations of the controllable voltage source determined based on the characteristics of the distribution network include:

[0109] S01: connecting each line of the distribution network to a semi-physical simulation voltage source topology model, wherein the semi-physical simulation voltage source topology model outputs current control range data corresponding to a plurality of voltage source topology structure combinations;

[0110] S02: Selecting a voltage source topology structure combination corresponding to the current control data with the widest current control range.

[0111] In this embodiment, the realization of high-power controllable voltage source mainly relies on power electronics technology, and its basic topology is as follows: Figure 7 shown.

[0112] The basic inverter unit uses SPWM technology to reduce the output voltage harmonics through high-frequency triangular carrier. The basic inverter unit and control method are as follows Figure 8 shown.

[0113] To ensure inverter output voltage quality, reduce output voltage ripple, and lower output voltage harmonic content, multilevel inverters are the primary structure in applications such as DC transmission, photovoltaic inverters, active power filters, flexible transmission, and high-voltage, high-power motors. The main advantages of multilevel inverters are low electromagnetic interference, high efficiency, and high output voltage and power quality.

[0114] There are three main types of traditional multi-level inverter topologies: diode clamped, flying capacitor, and cascade. Figures 9 to 11. For multi-level inverters, the more levels there are, the more devices are required, and the more complex the modulation circuit and control strategy are. At the same time, problems such as large system size and high cost are more significant. The core of the main circuit topology problem is how to reduce the number of devices with the same number of levels, and vice versa. In recent years, a variety of hybrid topologies and new topologies have emerged, but their essence is still a mixture or deformation of the three basic topologies mentioned above. Based on the characteristics of the present invention, the topology with the best control effect and the lowest cost can be compared and selected.

[0115] Due to the influence of changes in system network parameters, frequency offset, imbalance, etc., the phase and amplitude of the controllable voltage source output voltage need to quickly track these changes and adjust the output voltage in real time.

[0116] In a preferred embodiment, the phase voltage instantaneous value and phase angle automatic tracking technology includes:

[0117] S014: Modulating the controllable voltage source using carrier phase shifting.

[0118] In this embodiment, the cascade multilevel inverter generally adopts carrier phase shift modulation, and its basic control method is as follows: Figure 12 shown.

[0119] In a preferred embodiment, the phase voltage instantaneous value and phase angle automatic tracking technology further includes:

[0120] S011: measuring the real-time phase and real-time amplitude of the controllable voltage source in real time;

[0121] S012: Subtracting the real-time phase and real-time amplitude measured in real time from the preset phase and preset amplitude to obtain a phase tracking error and an amplitude tracking error;

[0122] S013: Sending the phase tracking error and the amplitude tracking error as feedback signals to the hysteresis comparator, and using the hysteresis comparator to compare the tracking error with upper and lower thresholds to adjust the phase and amplitude of the controllable voltage.

[0123] like Figure 13As shown in the figure, the tracking error between the measured value and the residual value is fed into the hysteresis comparator as a feedback signal in real time. The hysteresis comparator's characteristics are then used to control the feedback signal within the hysteresis band, thereby achieving good tracking results. While the switching frequency under hysteresis control is not fixed, hysteresis control is widely used in multilevel inverters, demonstrating advantages such as ease of implementation, good dynamic characteristics, independence from load parameters, and excellent stability. Its essence is threshold control of the tracking error, typically defined as the difference between a reference value and the actual output value. The tracking error is controlled within a hysteresis band with zero as the neutral value and upper and lower thresholds as the boundaries. By comparing the tracking error with the upper and lower thresholds, the switching state at the next moment is selected according to the established control logic to ensure that the actual output tracks the set value.

[0124] like Figure 14 , a hysteresis comparator is used to implement two-level hysteresis control. The definition of the hysteresis comparator is as follows:

[0125] if input≥B,output=1,else if input≤-B,output=0

[0126] The hysteresis comparator has two outputs, 1 and 0. According to its definition, when the hysteresis comparator output is 1, it corresponds to the inverter output +1 level, and when the output is 0, it corresponds to the inverter output -1 level.

[0127] Further, such as Figure 15 and Figure 16 There is also three-level hysteresis control. This method introduces four-wheel drive width and increases the 0-level tendency, making the control voltage output smoother.

[0128] The device of the present invention adopts carrier phase shift modulation technology and hysteresis control technology to quickly track and control the voltage amplitude and phase, thereby ensuring the stability of the output voltage and good dynamic characteristics.

[0129] It can be seen that the present invention proposes a wide-range precise control method for ground fault current in distribution network based on controllable voltage source, which can realize full current compensation of fault current, quickly extinguish fault arc and prevent fault expansion, and at the same time, appropriately increase fault current in the case of high-resistance ground fault, so as to realize accurate positioning of fault point.

[0130] A second embodiment of the present invention provides a fault detection device for a high-resistance ground fault in a distribution network, wherein the distribution network includes a plurality of distribution lines, such as Figure 17 Shown, including:

[0131] A second embodiment of the present invention provides a fault detection device for a high-resistance grounding fault in a distribution network, wherein the distribution network includes a plurality of distribution lines, including:

[0132] A compensation current injection module 1 is configured to inject a compensation current into the distribution network through a controllable voltage source if a high-resistance ground fault occurs in the distribution network and the fault current is greater than a set threshold, until the fault current drops to a near-zero range;

[0133] A reduction operation module 2 is configured to gradually reduce the injected compensation current after a set interval, and if the system damping rate changes and the fault grounding current is less than a set threshold, gradually reduce the output voltage amplitude of the controllable voltage source;

[0134] The fault detection module 3 determines the fault line or locates the fault point according to the current characteristics of the distribution network during the curtailment operation; wherein the topological structure combination of the controllable voltage source is determined based on the characteristics of the distribution network.

[0135] The present invention provides a fault detection device for a high-resistance grounding fault in a distribution network. By providing a compensation current injection module, a reduction operation module, and a fault detection module, in specific use, first, when a high-resistance grounding fault occurs in the distribution network and the fault current is greater than a set threshold, the injected compensation current is gradually reduced. If the system damping rate changes and the fault grounding current is less than the set threshold, the output voltage amplitude of the controllable voltage source is then gradually reduced. Finally, the fault line is determined or the fault point is located based on the current characteristics of the distribution network during the reduction operation. The topological structure combination of the controllable voltage source of the present invention is determined based on the characteristics of the distribution network, which can achieve full current compensation of the fault current, quickly extinguish the fault arc, and prevent the fault from expanding. At the same time, in the case of a high-resistance grounding fault, the fault current can be appropriately increased to achieve accurate location of the fault point.

[0136] like Figure 2 As shown in the figure, when a ground fault in the distribution network is detected, the required injection current is generated by calculating the system damping rate during normal operation of the distribution network. The fault ground current is adjusted by injecting current through the PWM active inverter. The injection current is controlled in a closed loop to compensate for the reactive component, active component and harmonic component in the fault current, reducing the fault current to near zero and achieving 100% arc extinguishing of the instantaneous ground fault.

[0137] At the same time, combining formulas (1) to (5), it can be seen that the output voltage of the controllable voltage source can control the neutral point voltage of the system, thereby achieving control of the current at the fault point. Therefore, when a single-phase grounding fault occurs in the system, by adjusting the voltage of the controllable voltage source, the fault current can be reduced to near zero, thereby achieving 100% arc extinguishing of the instantaneous grounding fault and preventing the permanent grounding fault from expanding. The controllable voltage source voltage can also be adjusted to achieve accurate line selection and location of the fault.

[0138] In a preferred embodiment, Figure 18 , the fault detection device further includes:

[0139] The compensation current generating module 011 generates the compensation current according to the system damping rate when the distribution network operates normally.

[0140] In this embodiment, specifically, in the arc suppression coil compensated grounding power grid, the distribution network usually increases the system damping rate by connecting a damping resistor in series at the neutral point to prevent the occurrence of resonant overvoltage, such as Figure 5 As shown, R0 is the neutral point series resistance, Rs, Rt, and Rw are the line leakage resistances, and Cs, Ct, and Cw are the line-to-ground capacitances. According to the definition of damping rate: the ratio of the three-phase conductance to ground to the capacitive susceptance, and since conductance and susceptance determine the active and reactive components in the system, the damping rate is:

[0141]

[0142] Among them, G0 is the neutral point conductance, which is the reciprocal of the series resistance, that is: G0 = 1 / R0; 2Gs, Gt, Gw are the line leakage conductance, which are the reciprocal of the leakage resistance, that is: Gs = 1 / Rs; Gt = 1 / Rr; Gw = 1 / Rw; Bs, Bt, Bw are the line-to-ground susceptance, which are not equal in practice, and their relationship with the capacitance is: Bs = ωCs; Bt = ωCt; Bw = ωCw.

[0143] In a preferred embodiment, the current characteristics of the distribution network include the current values ​​of each line; the reduction operation module is specifically used to determine whether the difference between the current values ​​of any two distribution lines after each reduction is greater than a set threshold. If so, the distribution line with the larger current among the two distribution lines is the fault line.

[0144] In this embodiment, by gradually reducing the voltage amplitude, a certain buffer can be formed for the distribution network, avoiding the problem of instability of the distribution network caused by excessive reduction. In addition, by gradually reducing the voltage amplitude, when at least one of the zero-sequence current differences is greater than the set threshold, it can be seen that the zero-sequence current of the fault line is significantly different from the zero-sequence current of the non-fault line, thereby eliminating misjudgment caused by interference.

[0145] In a preferred embodiment, the current characteristics of the distribution network include the current values ​​before and after the compensation current is injected into each current detection point on each line; Figure 19 As shown, the fault detection module 3 includes:

[0146] The current difference calculation unit 31 calculates the current difference before and after the compensation current is injected into each current detection point on each line;

[0147] A detection point search unit 32 searches for a current detection point where the current difference is greater than a set threshold, and searches for the next current detection point adjacent to the current detection point where the current difference is greater than the set threshold along the current direction on the power distribution line;

[0148] The locating unit 33 locates the fault point of the high-resistance grounding fault between the current detection point where the current difference is greater than the set threshold and the next current detection point if the current difference corresponding to the next current detection point is less than the set threshold.

[0149] Specifically, in this embodiment, according to Figure 4 After a fault occurs at CT1 and CT2 before and after the ground fault, and before the full current compensation device operates, all line-to-ground capacitances inject capacitive current into the fault point. Therefore, the closer the fault line detection point is to the fault point, the higher the zero-sequence current. After the full compensation device operates, the injection of inductive current causes the zero-sequence current at the upstream detection point (CT1) to change from capacitive to inductive, resulting in a significant change. However, the zero-sequence current at the downstream detection points (CT2, CT3) and the non-fault line detection points changes very little. Therefore, through the above steps, the zero-sequence current at the upstream detection point changes significantly, while the zero-sequence current at the downstream detection points changes slightly. Simultaneously, the zero-sequence voltage at the non-fault line detection points changes slightly. This allows for a reasonable threshold to be set. When the difference between the current and the current is greater than the set threshold, this detection point can be identified as the upstream detection point of the fault point. This allows for continued feature analysis of the downstream detection points, which should show a smaller change, thus confirming the high-resistance fault point.

[0150] In a preferred embodiment, it also includes:

[0151] a model input module, connecting each line of the distribution network to a semi-physical simulation voltage source topology model, wherein the semi-physical simulation voltage source topology model outputs current control range data corresponding to a plurality of voltage source topology structure combinations;

[0152] The topology structure combination selection module selects the voltage source topology structure combination corresponding to the current control data with the widest current control range.

[0153] In this embodiment, the realization of high-power controllable voltage source mainly relies on power electronics technology, and its basic topology is as follows: Figure 7 shown.

[0154] The basic inverter unit uses SPWM technology to reduce the output voltage harmonics through high-frequency triangular carrier. The basic inverter unit and control method are as follows Figure 8 shown.

[0155] In order to achieve the inverter output voltage quality, reduce the output voltage ripple, and reduce the output voltage harmonic content, in applications such as power DC transmission, photovoltaic inverters, active power filters, flexible transmission, high-voltage and high-power motors, the main structure is all implemented using multi-level inverters. The advantages of multi-level inverters are mainly low electromagnetic interference, high efficiency, and high output voltage power quality. There are three main traditional multi-level inverter topologies: diode clamped, flying capacitor, and cascade. Figures 9 to 11 . As for multi-level inverters, the more levels there are, the more components are required, and the more complex the modulation circuit and control strategy are. At the same time, problems such as large system size and high cost are more significant. The core of the main circuit topology problem is how to reduce the number of components with the same number of levels, and vice versa. In recent years, a variety of hybrid topologies and new topologies have emerged, but their essence is still a mixture or deformation of the three basic topologies mentioned above. According to the characteristics of this application, the topology with the best control effect and the lowest cost is compared and selected, and the corresponding control algorithm is developed accordingly.

[0156] Due to changes in system network parameters, frequency offset, and imbalance, the phase and amplitude of the controllable voltage source's output voltage must quickly track these changes and adjust the output voltage in real time. The various control strategies and algorithms studied above guide the design and development of a device for precise control of ground fault current over a wide range using a controllable voltage source, and incorporate these control algorithms into the device.

[0157] In a preferred embodiment, it also includes:

[0158] A real-time measurement module for measuring the real-time phase and real-time amplitude of the controllable voltage source in real time;

[0159] A tracking module is configured to obtain a phase tracking error and an amplitude tracking error by subtracting the real-time phase and real-time amplitude measured in real time from a preset phase and a preset amplitude;

[0160] The hysteresis comparison module sends the phase tracking error and amplitude tracking error as feedback signals to the hysteresis comparator, and uses the hysteresis comparator to compare the tracking error with the upper and lower thresholds to adjust the phase and amplitude of the controllable voltage.

[0161] In a preferred embodiment, it also includes:

[0162] The carrier modulation module adopts carrier phase shift to modulate the controllable voltage source.

[0163] In this embodiment, the cascade multilevel inverter generally adopts carrier phase shift modulation, and its basic control method is as follows: Figure 12 shown.

[0164] like Figure 13As shown in the figure, the tracking error between the measured value and the residual value is fed into the hysteresis comparator as a feedback signal in real time. The hysteresis comparator's characteristics are then used to control the feedback signal within the hysteresis band, thereby achieving good tracking results. While the switching frequency under hysteresis control is not fixed, hysteresis control is widely used in multilevel inverters, demonstrating advantages such as ease of implementation, good dynamic characteristics, independence from load parameters, and excellent stability. Its essence is threshold control of the tracking error, typically defined as the difference between a reference value and the actual output value. The tracking error is controlled within a hysteresis band with zero as the neutral value and upper and lower thresholds as the boundaries. By comparing the tracking error with the upper and lower thresholds, the switching state at the next moment is selected according to the established control logic to ensure that the actual output tracks the set value.

[0165] like Figure 14 , a hysteresis comparator is used to implement two-level hysteresis control. The definition of the hysteresis comparator is as follows:

[0166] if input≥B,output=1,else if input≤-B,output=0

[0167] The hysteresis comparator has two outputs, 1 and 0. According to its definition, when the hysteresis comparator output is 1, it corresponds to the inverter output +1 level, and when the output is 0, it corresponds to the inverter output -1 level.

[0168] Further, such as Figure 15 and Figure 16 There is also three-level hysteresis control. This method introduces four-wheel drive width and increases the 0-level tendency, making the control voltage output smoother.

[0169] The device of the present invention adopts carrier phase shift modulation technology and hysteresis control technology to quickly track and control the voltage amplitude and phase, thereby ensuring the stability of the output voltage and good dynamic characteristics.

[0170] It can be seen that the present invention proposes a wide-range precise control method for ground fault current in distribution network based on controllable voltage source, which can realize full current compensation of fault current, quickly extinguish fault arc and prevent fault expansion, and at the same time, appropriately increase fault current in the case of high-resistance ground fault, so as to realize accurate positioning of fault point.

[0171] Specific embodiments are used in the present invention to illustrate the principles and implementation methods of the present invention. The description of the above embodiments is only used to help understand the method of the present invention and its core ideas. At the same time, for those skilled in the art, according to the ideas of the present invention, there will be changes in the specific implementation methods and application scopes. In summary, the content of this specification should not be understood as limiting the present invention.

Claims

1. A method for detecting a high-resistance ground fault in a distribution network, wherein the distribution network comprises a plurality of distribution lines, characterized in that: include: If a high-resistance ground fault occurs in the distribution network and the fault current is greater than a set threshold, a compensation current is injected into the distribution network through a controllable voltage source until the fault current drops to a near-zero range; After a set interval, the injected compensation current is gradually reduced, and if the system damping rate changes and the fault grounding current is less than a set threshold, the output voltage amplitude of the controllable voltage source is gradually reduced; Determining the fault line or locating the fault point according to the current characteristics of the distribution network during the curtailment operation; wherein the topological structure combination of the controllable voltage source is determined based on the characteristics of the distribution network; The method of determining the topological structure combination of the controllable voltage source based on the characteristics of the distribution network includes: Connecting each line of the distribution network to a semi-physical simulation voltage source topology model, wherein the semi-physical simulation voltage source topology model outputs current control range data corresponding to a plurality of voltage source topology structure combinations; Select the voltage source topology combination corresponding to the current control data with the widest current control range to achieve full current compensation of the fault current; The current characteristics of the distribution network include the current value of each line; and determining the fault line according to the current characteristics of the distribution network during the curtailment operation includes: It is determined whether the difference between the current values ​​of any two distribution lines after each power reduction is greater than a set threshold. If so, the distribution line with the larger current among the two distribution lines is the fault line.

2. The method for detecting a high-resistance ground fault in a distribution network according to claim 1, wherein: Before injecting the compensation current into the distribution network through the controllable voltage source, the fault detection method further includes: The compensation current is generated according to a system damping rate during normal operation of the power distribution network.

3. The method for detecting a high-resistance ground fault in a distribution network according to claim 1, wherein: The current characteristics of the distribution network include the current values ​​before and after the compensation current is injected into each current detection point on each line; Locating the fault point based on the current characteristics of the distribution network during the curtailment operation, including: Calculate the current difference before and after the compensation current is injected into each current detection point on each line; Searching for a current detection point where the current difference is greater than a set threshold, and searching for the next current detection point adjacent to the current detection point where the current difference is greater than the set threshold along the current direction on the power distribution line; If the current difference corresponding to the next current detection point is smaller than the set threshold, the fault point of the high-resistance grounding fault is located between the current detection point where the current difference is larger than the set threshold and the next current detection point.

4. The method for detecting a high-resistance grounding fault in a distribution network according to claim 1, wherein: Also includes: measuring the real-time phase and real-time amplitude of the controllable voltage source in real time; Subtracting the real-time phase and real-time amplitude measured in real time from the preset phase and preset amplitude to obtain a phase tracking error and an amplitude tracking error; The phase tracking error and amplitude tracking error are sent as feedback signals to the hysteresis comparator, which is used to compare the tracking error with the upper and lower thresholds to adjust the phase and amplitude of the controllable voltage.

5. The method for detecting a high-resistance grounding fault in a distribution network according to claim 1, wherein: Also includes: The controllable voltage source is modulated by carrier phase shifting.

6. A fault detection device for a high-resistance ground fault in a distribution network, wherein the distribution network comprises a plurality of distribution lines, characterized in that: include: a compensation current injection module, which, if a high-resistance grounding fault occurs in the distribution network and the fault current is greater than a set threshold, injects a compensation current into the distribution network through a controllable voltage source until the fault current drops to a near-zero range; a reduction operation module, which gradually reduces the injected compensation current after a set interval, and if the system damping rate changes and the fault grounding current is less than a set threshold, gradually reduces the output voltage amplitude of the controllable voltage source; a fault detection module for determining a fault line or locating a fault point according to current characteristics of the distribution network during a curtailment operation; wherein the topological structure combination of the controllable voltage source is determined based on the characteristics of the distribution network; Wherein, the fault detection device for high-resistance grounding fault in the distribution network further includes: a model input module, connecting each line of the distribution network to a semi-physical simulation voltage source topology model, wherein the semi-physical simulation voltage source topology model outputs current control range data corresponding to a plurality of voltage source topology structure combinations; A topology combination selection module selects the voltage source topology combination corresponding to the current control data with the widest current control range to achieve full current compensation of the fault current; Among them, the current characteristics of the distribution network include the current value of each line; the reduction operation module is specifically used to determine whether the difference between the current values ​​of any two distribution lines after each reduction is greater than a set threshold. If so, the distribution line with the larger current among the two distribution lines is the fault line.

7. The fault detection device for high-resistance grounding fault in the distribution network according to claim 6, characterized in that: The fault detection device further comprises: The compensation current generating module generates the compensation current according to the system damping rate when the distribution network is operating normally.

8. The fault detection device for high-resistance grounding fault in the distribution network according to claim 6, characterized in that: The current characteristics of the distribution network include the current values ​​before and after the compensation current is injected into each current detection point on each line; The fault detection module includes: A current difference calculation unit calculates the current difference before and after the compensation current is injected into each current detection point on each line; a detection point search unit for searching for a current detection point where the current difference is greater than a set threshold, and searching for a next current detection point adjacent to the current detection point where the current difference is greater than the set threshold along the current direction on the power distribution line; The positioning unit locates the fault point of the high-resistance grounding fault between the current detection point where the current difference is greater than the set threshold and the next current detection point if the current difference corresponding to the next current detection point is less than the set threshold.

9. The fault detection device for high-resistance grounding fault in the distribution network according to claim 6, characterized in that: Also includes: A real-time measurement module for measuring the real-time phase and real-time amplitude of the controllable voltage source in real time; A tracking module is configured to obtain a phase tracking error and an amplitude tracking error by subtracting the real-time phase and real-time amplitude measured in real time from a preset phase and a preset amplitude; The hysteresis comparison module sends the phase tracking error and amplitude tracking error as feedback signals to the hysteresis comparator, and uses the hysteresis comparator to compare the tracking error with the upper and lower thresholds to adjust the phase and amplitude of the controllable voltage.

10. The fault detection device for high-resistance grounding fault in a distribution network according to claim 6, characterized in that: Also includes: The carrier modulation module adopts carrier phase shift to modulate the controllable voltage source.

Images