Low-current grounding discrimination method based on phase current break variable transient analysis

Abstract

The invention discloses a low-current discrimination method based on transient analysis of phase current abrupt change, which is used in a power distribution network system with a neutral point not effectively grounded and mainly solves the problem of discrimination of single-phase grounding faults in the system. The method is characterized in that three-phase current of a line is collected, a zero-sequence current half-wave effective value of the line is calculated in real time, and when the zero-sequence current half-wave effective value is larger than a set threshold, a judgment program is started. A judgment program firstly obtains a break variable of a three-phase current sampling value, the break variable can be considered to be a fault current, and then a half-wave effective value of the phase current break variable is calculated; the criterion is that the fault phase is used as a provider of the fault current, the half-wave effective value of the phase current break variable of the fault phase of the fault line is certainly greater than the sum of the half-wave effective values of the current break variables of the other two phases by considering that a plurality of lines exist in the power distribution network, and a half cycle wave is continuously judged; if the condition is always satisfied, the single-phase grounding line and phase can be determined. The method only judges half cycle, belongs to transient analysis, is short in duration time, can effectively avoid interference caused by arc suppression coil grounding, and is rapid and reliable.

Description

Technical Field

[0001] The present invention is a small current grounding judgment method based on transient analysis of phase current mutation, and specifically relates to a judgment method for single-phase grounding faults in a distribution network system with ineffective neutral point grounding. Background Art

[0002] At present, most distribution networks adopt the operation mode of non-effective grounding of the neutral point, including no grounding of the neutral point, grounding through a resistor and grounding through an arc suppression coil. This kind of non-effective grounding of the neutral point generates a small fault current when a single-phase grounding occurs, so it is also called a small current grounding system. The relatively mature methods for judging small current grounding are: the zero-sequence reactive power direction method, which judges by comparing the phase of the zero-sequence current and the zero-sequence voltage. The zero-sequence current of the fault line lags the zero-sequence voltage by 90°, and the zero-sequence current of the non-fault line leads the zero-sequence voltage by 90°. This method is only applicable to distribution networks with ungrounded neutral points. In the case of grounding through an arc suppression coil, the arc suppression coil current will change the direction of the zero-sequence current of the fault line. This method must also overcome the influence of current distortion; the zero-sequence active power direction method requires a resistor to be connected in parallel to the arc suppression coil to produce The zero-sequence current of the fault line lags the zero-sequence voltage by approximately 160°, while the zero-sequence current of the non-fault line leads the zero-sequence voltage by 90°. This method is significantly affected by CT imbalance, line length, and transition resistance. The "false active current component" caused by three-phase capacitor imbalance also significantly impacts the active component algorithm. The transient waveform first half-wave method uses the polarity of the zero-sequence voltage and current of the fault line opposite within the first half-wave of the transient. This method is unaffected by arc suppression coils, but it is highly waveform-dependent and may misjudge in certain complex environments. The above analysis shows that due to the low grounding current in small-current grounding faults, the complex operating environment, and the presence of various interference factors, existing methods may produce errors.

[0003] To address the difficulty in determining single-phase grounding faults in distribution networks with non-effectively grounded neutral points, the present invention proposes a small current grounding identification method based on transient analysis of phase current mutations. Starting from the source of the fault, only the three-phase currents of the line are sampled. The fault phase serves as the provider of the fault current. The half-wave effective value of the phase current mutation of the fault phase of the fault line must be greater than the sum of the half-wave effective values of the current mutations of the other two phases. This method is simple, effective, and adaptable to various environments. Summary of the Invention

[0004] In order to solve the problem of accurately judging when single-phase grounding occurs in a distribution network with non-effective neutral point grounding, the present invention provides a small current grounding judgment method based on transient analysis of phase current mutation.

[0005] In order to achieve the above object, the technical solution adopted by the present invention is:

[0006] A method for identifying a small current ground fault based on transient analysis of phase current mutations includes the following steps:

[0007] (1) The three-phase current of the line is sampled in real time, and the obtained sampling sequence is stored in the sampling buffer. The buffer stores at least 3 cycles of data. In this case, the number of sampling points per cycle is 80.

[0008] (2) Calculate the half-wave effective value of the zero-sequence current I0. If I0 is greater than the threshold value I0set (depending on the specific line), it is determined that a single-phase grounding fault has occurred in the distribution network, and the program proceeds to the next step.

[0009] (3) The current sampling value minus the sampling value 2 cycles ago is used as the phase current mutation amount to obtain the sampling sequence of the three-phase current mutation amount. The half-wave effective value of the three-phase current mutation amount is calculated based on the sequence.

[0010] (4) Continuously execute step (3) to obtain the half-wave effective value sequence of the three-phase current mutation amount

[0011] dI a (k), dI b (k), dI c (k), k=1,2,3,...,N; N=40.

[0012] (5) For each k value, k = 1, 2, 3, ..., N; N = 40;

[0013] Do the following:

[0014] If dI a (k)>(dI b (k)+dI c (k)), counter countA increases by 1;

[0015] If dI b (k)>(dI a (k)+dI c (k)), counter countB increases by 1;

[0016] If dI c (k)>dI a (k)+dI b (k)), the counter countC is incremented by 1.

[0017] (6) Determine whether the line is faulty

[0018] If countA == N, then phase A of the line is grounded;

[0019] If countB == N, then the line B is grounded;

[0020] If countC == N, then phase C of the line is grounded;

[0021] If none of the above conditions are met, the line is normal.

[0022] The beneficial effects of the present invention are:

[0023] (1) Simple calculation: The present invention only needs to introduce the three-phase current of the line and judge the occurrence of the fault according to the change of the current, without many complicated calculations;

[0024] (2) Strong anti-interference ability: The present invention adopts the sudden change of phase current and uses the current sampling point minus the sampling point 2 cycles ago, thus eliminating the noise interference in the signal;

[0025] (3) High reliability: The present invention adopts a continuous judgment point method in the calculation process, and it is necessary to judge half a cycle. Only when all the judged points meet the conditions and the zero-sequence current is greater than the set value, a conclusion is made, which fully considers various unfavorable factors;

[0026] (4) Strong real-time performance: The judgment process of the present invention is completed within half a cycle after the zero-sequence current crosses the threshold, and the response time is within 20ms, which can give a signal very shortly after the fault occurs. DETAILED DESCRIPTION

[0027] The embodiments of the present invention are specifically described below:

[0028] The present invention only needs to collect the three-phase current of the line, and implements the A / D sampling rate of 80 points per cycle. The specific steps are as follows:

[0029] (1) Data collection and storage:

[0030] The three-phase current of the line is sampled in real time, and the obtained sampling sequence is stored in the sampling buffer. The buffer stores at least 3 cycles of data; the zero-sequence current sampling value is the sum of the three-phase currents;

[0031] The sequence is ia(n), ib(n), ic(n), i0(n), n=1, 2, 3..., 240.

[0032] (2) Startup judgment:

[0033] Calculate the half-wave effective value of zero-sequence current: N=40.

[0034] If I0>I 0set , set the startup flag and the program enters the following steps.

[0035] I 0setDepending on the specific line, the effects of three-phase unbalanced line load and distributed capacitance should be avoided.

[0036] (3) Calculation of mutation amount:

[0037] The current current sampling value minus the sampling value 2 cycles ago is used as the phase current mutation amount to obtain the sampling sequence of the three-phase current mutation amount. The half-wave effective value of the three-phase current mutation amount is calculated based on the sequence.

[0038] Mutation sequence:

[0039] dia(n)=ia(n)-ia(n-160), dib(n)=ib(n)-ib(n-160), dic(n)=ic(n)-ic(n-160)

[0040] Half-wave effective value of phase current mutation:

[0041]

[0042] (4) Obtain the half-wave effective value sequence of the phase current mutation amount:

[0043] Continuously execute step (3) to obtain the half-wave effective value sequence of the three-phase current mutation quantity,

[0044] dI a (k), dI b (k), dI c (k), k=1,2,3,...,N; N=40.

[0045] (5) Compare the three-phase current mutation amount:

[0046] For each k value, k = 1, 2, 3, ..., N; N = 40;

[0047] Do the following:

[0048] If dI a (k)>(dI b (k)+dI c (k)), counter countA increases by 1;

[0049] If dI b (k)>(dI a (k)+dI c (k)), counter countB increases by 1;

[0050] If dI c (k)>dI a (k)+dI b (k)), the counter countC is incremented by 1.

[0051] (6) Determine the fault:

[0052] If countA == N, then phase A of the line is grounded;

[0053] If countB == N, then the line B is grounded;

[0054] If countC == N, then phase C of the line is grounded;

[0055] If none of the above conditions are met, the line is normal.

[0056] The basic principles, main features, and advantages of the present invention are shown and described above. Those skilled in the art should understand that the present invention is not limited to the foregoing embodiments. The foregoing embodiments and descriptions are merely illustrative of the principles of the present invention. Various changes and modifications may be made to the present invention without departing from the spirit and scope of the present invention. Such changes and modifications are intended to fall within the scope of the present invention. The scope of protection claimed in the present invention is defined by the appended claims and their equivalents.

Claims

1. A method for identifying small current grounding based on transient analysis of phase current mutation, characterized in that The following steps are involved: (1) The three-phase current of the line is sampled in real time, and the obtained sampling sequence is stored in the sampling buffer. The buffer stores at least 3 cycles of data. In this case, the number of sampling points per cycle is 80. (2) Calculate the half-wave effective value of the zero-sequence current I0. If I0 is greater than the threshold value I0set (depending on the specific line), it is determined that a single-phase grounding fault has occurred in the distribution network, and the program proceeds to the next step. (3) The current sampling value minus the sampling value 2 cycles ago is used as the phase current mutation amount to obtain the sampling sequence of the three-phase current mutation amount. The half-wave effective value of the three-phase current mutation amount is calculated based on the sequence. (4) Continuously execute step (3) to obtain the three-phase current mutation half-wave effective value sequence dI a (k), dI b (k), dI c (k), k=1,2,3,...,N; N=40. (5) For each k value, k = 1, 2, 3, ..., N; N = 40; Do the following: If dI a (k)>(dI b (k)+dI c (k)), counter countA increases by 1; If dI b (k)>(dI a (k)+dI c (k)), counter countB increases by 1; If dI c (k)>dI a (k)+dI b (k)), the counter countC is incremented by 1. (6) Determine whether the line is faulty If countA == N, then phase A of the line is grounded; If countB == N, then the line B is grounded; If countC == N, then phase C of the line is grounded; If none of the above conditions are met, the line is normal.

2. A method for identifying a small current ground fault based on transient analysis of phase current mutation according to claim 1, characterized in that Based on the three-phase current sampling values, the half-wave effective value of the three-phase current mutation is calculated. If the half-wave effective value of the current mutation of a phase is greater than the sum of the other two phases and lasts for half a cycle, it can be determined that a single-phase grounding fault has occurred in the line.

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