A high-resistance single-phase ground fault diagnosis method

Abstract

The application discloses a high-resistance single-phase grounding fault diagnosis method, obtains bus voltage and feeder zero sequence current, and calculates the amplitude; sets a zero sequence overcurrent high fixed value according to the maximum capacitance current of the line, sets a zero sequence overcurrent low fixed value according to the unbalanced current and zero drift current of the system; and judges whether the single-phase grounding fault of the feeder occurs or not through a high-resistance grounding detection criterion. The application uses the floating threshold zero sequence overcurrent technology to realize the high-resistance grounding detection, is not affected by the polarity of the zero sequence voltage, can sensitively detect the high-resistance grounding fault, and has simple setting and setting methods, and can be used for preventing the cable burning and personal injury accidents in the process of continuously running after the high-resistance grounding of the small-resistance / small-current grounding system.

Description

TECHNICAL FIELD

[0001] The application belongs to the field of relay protection of power systems, and particularly relates to a high-resistance single-phase grounding fault diagnosis method. BACKGROUND

[0002] When a high-resistance single-phase grounding fault occurs in a 6-66kV system, neither a neutral point grounding system through a small resistance nor a neutral point grounding system through a small resistance can accurately identify the fault. When a high-resistance grounding fault occurs in the system, the fault characteristics are not obvious, and the fault cannot be removed for a long time, which threatens the personal safety of passing pedestrians and even seriously affects the reliability of power supply. At present, considering the strong damping effect of high-resistance grounding, the fault detection of high-resistance grounding mainly takes the steady-state quantity criterion as the main basis. The zero sequence voltage of the small current grounding system is started, and when the high resistance grounding occurs, the zero sequence voltage is very low and cannot start the line selection due to the reasons such as the failure of the arc suppression coil damping resistance to be short-circuited and the large arc suppression coil detuning. The sampled zero sequence overcurrent of the small resistance grounding system needs to avoid the capacitive current during the metallic grounding, and the high resistance grounding cannot act. In the past, scholars have proposed to use zero sequence voltage ratio to brake overcurrent protection, which has complex principles, needs to set the inflection point voltage and the brake coefficient, and has complex setting calculation, which is not conducive to the promotion and use on site. The detection sensitivity of high resistance grounding is only 1000Ω, and the situation that the zero sequence voltage is inaccurate when the PT is broken is not considered. When the primary PT is broken, the protection will refuse to act, and when the secondary PT is broken, the protection will malfunction. SUMMARY

[0003] In order to solve the technical problems mentioned in the background art, the application provides a high-resistance single-phase grounding fault diagnosis method, which has high sensitivity and simple setting calculation.

[0004] In order to achieve the above technical purpose, the technical scheme of the application is as follows:

[0005] A high-resistance single-phase grounding fault diagnosis method, comprising the following steps:

[0006] Obtaining bus voltage and feeder zero sequence current and calculating the amplitude;

[0007] Setting a zero sequence overcurrent high value I 0 high setting according to the maximum capacitive current of the line, and setting a zero sequence overcurrent low value I 0 low setting according to the system unbalanced current and zero drift current;

[0008] Judging whether a single-phase grounding fault occurs in the feeder through a high-resistance grounding detection criterion;

[0009] The high-resistance grounding detection criterion is as follows:

[0010]

[0011] Wherein, I0 is the collected zero sequence current of the interval, U0 is the collected system zero sequence voltage, U 0 metal is the system zero sequence voltage when the metallic grounding occurs.

[0012] Further, the overcurrent setting method is as follows:

[0013] The zero sequence overcurrent high setting value I 0 high setting = K r × I c , K r is a reliability coefficient, I c is the capacitive current of the line to the ground when the metallic grounding occurs on the busbar;

[0014] The zero sequence overcurrent low setting value I 0 low setting is set according to the avoidance of unbalanced current and zero drift current.

[0015] Further, according to the collected busbar voltage, it is judged whether the PT disconnection exists, if the PT disconnection exists, the formula (2) in the high resistance grounding detection criterion is replaced by the following formula (3):

[0016] I0>I 0 delay setting (3)

[0017] Wherein, I 0 delay setting is the delay zero sequence overcurrent setting value; if there is no upper and lower line cooperation requirement, any formula (1) or (3) meeting the high resistance grounding detection criterion is considered to occur single-phase grounding fault; if there is upper and lower line cooperation requirement, only formula (1) is used for fault judgment.

[0018] Further, the delay zero sequence overcurrent setting value I 0 delay setting is set according to the avoidance of the maximum zero sequence current of the non-fault line when the fault line zero sequence current is I 0 high setting , and the setting value of the PT disconnection is I r . Wherein, R r is the neutral point grounding resistance of the system.

[0019] The beneficial effects brought by the above technical scheme are:

[0020] When the high resistance grounding occurs, the zero sequence voltage is low, the zero sequence current threshold is automatically reduced according to the zero sequence voltage of the application, and the zero sequence current threshold only needs to avoid the capacitive current of the line when the high resistance grounding occurs, so the sensitivity is very high. The application only needs to set two setting value items by the user, the zero sequence high setting value setting is consistent with the setting time limit zero sequence overcurrent protection setting value in the existing protection device, and the zero sequence low setting value only needs to avoid the zero sequence unbalanced current, so the setting calculation workload is small, the realization is simple, and the on-site large-scale popularization and application are facilitated. BRIEF DESCRIPTION OF DRAWINGS

[0021] Figure 1This is a flowchart of the method of the present invention;

[0022] Figure 2 This is a diagram showing the action curve of the present invention;

[0023] Figure 3 This is a schematic diagram for calculating the grounding point current Ig during a single-phase grounding event.

[0024] Figure 4 This is the Thevenin equivalent circuit diagram for a single-phase ground fault. Detailed Implementation

[0025] The technical solution of the present invention will be described in detail below with reference to the accompanying drawings.

[0026] This invention designs a method for diagnosing high-resistivity single-phase grounding faults, such as... Figure 1 As shown, obtain the bus voltage and feeder zero-sequence current, and calculate their amplitudes; set the zero-sequence overcurrent high setting value I based on the maximum line capacitive current. 0 high setting Set the zero-sequence overcurrent low setting value I based on the system unbalanced current and zero drift current. 0 low setting ;

[0027] The high-resistance grounding detection criterion is used to determine whether a single-phase grounding fault has occurred in the feeder.

[0028] The high-resistance grounding detection criterion is as follows:

[0029]

[0030] Where I0 is the zero-sequence current of this interval, U0 is the zero-sequence voltage of the system, and U 0 metal This is the zero-sequence voltage of the system when a metallic ground fault occurs.

[0031] like Figure 2 As shown, when the zero-sequence current of the line meets the above conditions and the timing value of the trip delay timer is greater than the set value, the output trips.

[0032] Equation (1) is used for grounding protection with low transition resistance. When the fault line current is low when the high resistance grounding is low, the current of the line is lower than the capacitance current of the line when the metallic grounding is low. Therefore, the sensitivity of Equation (1) cannot meet the requirements. Therefore, Equation (2) is used for high resistance grounding detection.

[0033] If a disconnection is found, a PT disconnection alarm signal is output to notify the operators for handling. The system's three-phase voltage is collected to determine whether a PT disconnection exists. If a PT disconnection is found, the above equation (2) is replaced with equation (3):

[0034] I0>I 0 delay setting (3)

[0035] In the formula, I 0 delay settingFor time-delay zero-sequence overcurrent fixed value, according to the maximum zero-sequence current of non-fault line at I 0 high setting 000, the trip time of formula (3) increases the breaker action delay time relative to formula (1), and if there is no upper and lower line coordination requirement, either formula (1) or (3) meeting the requirement is considered as the occurrence of ground fault outlet trip. If there is upper and lower line coordination requirement, the delay time cannot be increased, and the ground detection criterion is only used for ground protection by formula (1).

[0036] The scheme only needs to set two fixed values, and the setting method is: 0 high setting = K r × I c , K rr is a reliability coefficient, generally set to 1.2 or more, I c is the line capacitance current, that is, the capacitive current of the line to ground when the line is grounded; I 0 low setting is the zero-sequence overcurrent low fixed value, which is set to avoid unbalanced current and zero drift current, and is generally set to 1.5 A of primary current; the fixed time limit zero-sequence overcurrent fixed value is set according to the maximum zero-sequence current of non-fault line at I 0 high setting 000, the fixed value of PT disconnection The fixed value is automatically calculated by the device, without user setting.

[0037] The wiring diagram of the line of the neutral point grounded through a small resistance system is as follows Figure 3 , which can be a 10kV grounding system, containing four lines: line 1, line 2, line 3, wherein line 3 has a transition resistance grounding fault, A, B and C are three-phase different circuits, R r is the neutral point grounding resistance, R g is the grounding fault transition resistance, the neutral point grounding resistance R r is generally 10 ohms in a 10kV system.

[0038] Let the bus zero-sequence voltage be Ignore the impedance of the grounding transformer, then the three-phase voltage of the 10kV bus is offset by the neutral point voltage, respectively According to Kirchhoff's current law, there is formula (4):

[0039]

[0040] In formula (4), is the sum of three-phase currents of the power supply, is the sum of three-phase currents of each line.

[0041] When the three-phase parameters are symmetrical and the load is symmetrical, the sum of three-phase currents of the power supply branch and each non-fault line is calculated as That is, the three-phase-to-ground capacitive current is Where j is the imaginary unit, j 2 =-1, ω is the angular velocity corresponding to the power frequency, which for a 50Hz power frequency is ω = 2π × 50, and C is the single-phase-to-ground capacitance of the line, the same below. For faulty lines, in addition to the three-phase-to-ground capacitance current... There is also a transition resistance R due to grounding faults. g The generated ground current, assuming a ground fault occurs at a distance l from the busbar, with reactance per unit length of x, then taking phase A ground fault as an example, R g The generated ground current is: from Substituting the above result into equation (4), it transforms into equation (5):

[0042]

[0043] Based on the above analysis, for non-faulty lines, the zero-sequence current is The ground capacitive current generated under the action:

[0044]

[0045] In equation (6), n is the sequence number of the non-faulty line, corresponding to Figure 2 n = 1, 2, C n Let be the capacitance value of the nth line.

[0046] like Figure 4 The Thevenin equivalent circuit shown below, for a C-phase ground fault,

[0047]

[0048]

[0049] In the formula, ∑jXc represents the sum of the system's capacitance to ground. Since the system is a low-resistance grounded system, ∑jXc... C >>R r ,so

[0050]

[0051]

[0052] For a faulty circuit, the zero-sequence current is the three-phase-to-ground capacitance current. With ground fault transition resistance R g The sum of the generated ground currents:

[0053]

[0054] Because of jX Cn jX Cn >>R r ,

[0055] 10kV small resistance grounding system R r Generally 10 ohms, line-to-ground capacitance is generally a few hundred ohms, jX Cn >> R r Fault line I0 is more than 10 times the non-fault line.

[0056] The electromagnetic transient simulation software (PSCAD, Power Systems Computer Aided Design) software is used to simulate the system in Figure 4 The voltage level is 10.5kV, the neutral grounding resistance is 10 ohms, the transition resistance is selected in the range of 0-4000 ohms, the zero sequence voltage at the 10kV bus and the zero sequence current flowing through each distribution line are calculated. 0 high setting Set to 25A, I 0 low setting Set to 1A, the corresponding secondary values are 0.5A, 0.02A, the line capacitance current is 16A, and the secondary is 0.32A.

[0057] In the extreme case of a single line-to-ground capacitance current of 50A, the relationship between the transition resistance size and U0 and I0, the fault line and the non-fault line I0 difference is more than 10 times. Table 1 below is the simulation result, Rg is the primary value, U0, I0 is the value transformed to secondary by voltage transformer and zero sequence current transformer, the voltage transformer transformation ratio is 10kV / 100V, and the zero sequence current transformer (CT) transformation ratio is 50 / 1.

[0058] Table 1

[0059]

[0060] The above simulation results are consistent with the theoretical formula derivation. In the above embodiment, due to the small fault quantity characteristics of high resistance grounding fault, higher precision is required, special zero sequence CT can be used as much as possible, but the polarity of special zero sequence CT is difficult to check. In the above embodiment, the zero sequence current is accessed by special zero sequence CT, the amplitude criterion is used, and the polarity of the zero sequence CT can be self-adapted, which ensures the precision requirement of high resistance grounding fault judgment.

[0061] The non-fault line is mainly the line-to-ground capacitance current, according to the above analysis of small current grounding system high resistance grounding and small resistance grounding system single-phase grounding, the line-to-ground capacitance current is much smaller than the fault point-to-ground current, so the zero sequence current in the fault line is obviously larger than that in the non-fault line.

[0062] The above example makes a detailed introduction to the selection basis of the preset condition, and the specific application is based on the above preset condition. The criterion is not affected by the polarity of the zero sequence CT, and the polarity of the special zero sequence CT does not need to be checked. The influence of the ground capacitance current can be well excluded, the sensitivity is high, the special zero sequence CT and the open delta voltage are adopted, the characteristics of small fault characteristic quantity of high resistance grounding can be adapted to the characteristics of the need for accurate measurement, and the maximum detection transition resistance capacity can reach 4000 ohms.

[0063] Through the above examples, high resistance grounding faults can be accurately detected, and the phenomenon of misjudgment can be avoided. When a high resistance grounding fault occurs in a distribution line, it can be cut off in time. Not only can the step voltage or contact voltage generated by the distribution line falling to the ground harm the passing pedestrians, but also the upper level grounding transformer can be prevented from being affected by the fault, and the reliability of power supply is improved.

[0064] The embodiments are only for illustrating the technical idea of the present application, and cannot limit the protection scope of the present application. Any modification made according to the technical idea of the present application on the basis of the technical scheme falls within the protection scope of the present application.

Claims

1. A high impedance single-phase ground fault diagnostic method characterized by, The method comprises the following steps: obtaining bus voltage and feeder zero sequence current and calculating the amplitude; According to the line maximum capacitance current, set the zero sequence overcurrent high fixed value According to the system unbalance current and zero drift current, set the zero sequence overcurrent low fixed value ; judging whether a single-phase grounding fault occurs in the feeder through a high-resistance grounding detection criterion; the high-resistance grounding detection criterion is as follows: ， wherein, is the acquired zero sequence current of the interval, is the acquired system zero sequence voltage, is the system zero sequence voltage when a metallic ground occurs; wherein, according to the collected bus voltage, it is judged whether there is a PT disconnection situation, if there is a PT disconnection, formula (2) in the high-resistance grounding detection criterion is replaced by the following formula (3): ， wherein, is the time delay zero sequence overcurrent fixed value; if there is no upper and lower line matching requirement, any formula (1) or (3) in the high resistance ground detection criterion is considered to occur single-phase ground fault; if there is an upper and lower line matching requirement, only formula (1) is used for fault judgment; Wherein, the delay zero sequence over-current fixed value According to the fault line zero sequence current in The non-fault line maximum zero sequence current setting, PT broken line fixed value Wherein The system neutral point grounding resistance.

2. The high-impedance single-phase ground fault diagnostic method of claim 1, wherein, The overcurrent setting method is as follows: zero sequence overcurrent high setting value , is the reliability coefficient, is the capacitive current of the line to ground when a metallic ground occurs on the busbar; Zero sequence overcurrent low setting Set to avoid unbalanced current and zero drift current.

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