CN115877134A - Medium-voltage cable latent fault oriented trigger detection and identification method - Google Patents
Medium-voltage cable latent fault oriented trigger detection and identification method Download PDFInfo
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Abstract
The invention discloses a method for triggering, detecting and identifying latent faults of medium-voltage cables, which comprises the following steps: calculating the change quantity of the effective value of the zero sequence current of the medium-voltage cable; judging whether the variation of the effective value of the zero-sequence current of the medium-voltage cable is larger than a variation threshold value or not, and if so, calculating the effective value of each phase current and the effective value of each phase voltage; otherwise, judging that no latent fault exists currently; acquiring a fault phase; calculating the starting time and the ending time corresponding to the out-of-limit part of the zero-sequence current, judging whether load change exists before and after the time period, and if so, judging other faults; otherwise, calculating an initial angle of the fault phase voltage, judging whether the initial angle of the fault phase voltage is in a latent fault phase angle threshold interval, and if so, judging the fault as a latent fault; otherwise, judging as other faults. The invention realizes the early detection of the latent fault of the medium-voltage cable. The method has important significance for improving the power supply reliability.
Description
Technical Field
The invention relates to the field of cable fault detection, in particular to a trigger detection and identification method for medium-voltage cable latent faults.
Background
Latent fault of medium voltage cable: the fault duration is usually 1/4 cycle or longer, voltage characteristics under different neutral point grounding modes are similar, but the amplitude difference of current is larger.
The existing method mainly utilizes a preventive test when cable fault diagnosis is carried out, but the preventive test needs to be carried out when power failure occurs, power supply continuity is affected, partial discharge and temperature need to be monitored on site by a special sensor, the installation, operation and maintenance cost is high, the monitoring means is widely applied to the high-voltage transmission cable, however, the medium-voltage cable is wide in points and wide in range, the problems of direct burial or sewage soaking and the like exist, the application of the state perception means based on-site monitoring is limited, and therefore, the fault identification technology and device which are good in economy, high in practicability and sensitive in detection need to be researched aiming at various fault defects of the medium-voltage cable urgently.
For an early fault trigger detection algorithm, early fault disturbance is recorded based on zero sequence voltage amplitude out-of-limit, although part of early fault disturbance is detected based on the characteristics, analysis finds that the algorithm in the existing waveform recording device cannot guarantee sensitive detection of early fault and detection omission is very likely because the selected characteristic quantity cannot be matched with all types of grounding type early fault characteristics. In recent years, highly sensitive disturbance detection algorithms based on waveform distribution characteristics, similarity calculation and the like have been proposed in the art, but the algorithms have high requirements on hardware and are rarely applied in practice. Because of the constraints of the hardware computing power of the field monitoring device, the detection algorithm needs to fully consider the early fault characteristics and balance rapidity and sensitivity. The early fault detection algorithm can be divided into methods based on waveform characteristics, models and machine learning, but the methods are directed at a neutral point direct grounding system and mainly utilize the characteristic that the fault current is increased instantly during early faults, while the medium-voltage distribution network in China mainly adopts small-current grounding, and for the system, an early fault detection algorithm based on humanoid concept learning is proposed.
Disclosure of Invention
Aiming at the defects in the prior art, the method for triggering, detecting and identifying the latent fault of the medium-voltage cable solves the problem that the latent fault of the medium-voltage cable is difficult to detect.
In order to achieve the purpose of the invention, the invention adopts the technical scheme that:
the method for triggering, detecting and identifying the latent fault of the medium-voltage cable comprises the following steps:
s1, calculating the change quantity of the effective value of the zero sequence current of the medium-voltage cable;
s2, judging whether the variation of the effective value of the zero-sequence current of the medium-voltage cable is larger than a variation threshold value or not, and if so, entering a step S3; otherwise, judging that no latent fault exists at present;
s3, calculating effective values of all phase currents and effective values of all phase voltages;
s4, obtaining a fault phase by comparing the effective current value and the effective voltage value of each phase;
s5, calculating the starting time and the ending time corresponding to the out-of-limit part of the zero-sequence current, judging whether load change exists before and after the time period, and if so, judging that other faults exist; otherwise, entering step S6;
s6, calculating an initial angle of a fault phase voltage, judging whether the initial angle of the fault phase voltage is in a latent fault phase angle threshold interval, and if so, judging the fault as a latent fault; otherwise, judging as other faults.
Further, the specific method of step S1 includes the following substeps:
s1-1, setting the sliding step length of sampling to be 1/4 period, and setting the length of a calculation window to be 1/2 period. The number of sampling points in each period is set to be 128;
s1-2, according to a formula:
obtaining zero sequence current effective value I at sampling point k of medium voltage cable rmsx (k) (ii) a Wherein N is b The total number of sampling points of the half-wave signal; n is a sampling sequence; i.e. i x Representing a zero sequence current sampling signal;
s1-3, taking the effective value of the first half wave as the zero sequence current I under the normal operation condition 0_normal ；
S1-4, according to a formula:
ΔI 0 ＝I rmsx (k)-I 0_normal
obtaining the effective value change delta I of the zero sequence current of the medium-voltage cable 0 。
Further, the specific method of step S4 is:
and taking the phase corresponding to the maximum value of the three-phase current effective value and the minimum value of the three-phase voltage effective value as a fault phase.
Further, the specific method for calculating the initial angle of the fault phase voltage in step S6 is as follows:
acquiring a voltage sampling signal of a cycle before a fault according to the initial time corresponding to the out-of-limit part of the zero sequence current;
according to the formula:
obtaining the initial angle of the fault phase voltage
Wherein a is 1 And b 1 Is an intermediate parameter; n is the number of sampling points in one period; v k Sampling a kth sampling value in the voltage sampling signal of a cycle before the fault; and pi is the circumferential ratio.
Further, the threshold interval of the latent fault phase angle is 80 to 100 °, and 260 to 280 °.
The invention has the beneficial effects that: the method does not need to rely on a large number of data samples, comprehensively considers the disturbance change rule of phase voltage, current and zero sequence voltage and current as the criterion characteristic, and has the advantages of simple used characteristic quantity and less calculated quantity. Aiming at a small current grounding system, the method closely combines the early fault characteristics of the system according to the requirements of trigger detection and a fault detection algorithm on sensitivity, rapidity and accuracy, simultaneously considers the fault variation characteristics of phase voltage/current and zero sequence voltage/current, realizes the early detection of the latent fault of the medium-voltage cable, and has important significance for timely finding the early fault (instantaneous fault) in the small current grounding system, reducing unplanned power failure and improving the power supply reliability.
Drawings
FIG. 1 is a schematic flow diagram of the process;
FIG. 2 is a schematic diagram of voltage effective value calculation results under different calculation windows and sliding step length conditions;
FIG. 3 is a diagram illustrating the calculation results of voltage effective values at different sampling frequencies;
fig. 4 is a schematic diagram of the detection of the start-stop time of a latent fault.
Detailed Description
The following description of the embodiments of the present invention is provided to facilitate the understanding of the present invention by those skilled in the art, but it should be understood that the present invention is not limited to the scope of the embodiments, and it will be apparent to those skilled in the art that various changes may be made without departing from the spirit and scope of the invention as defined and defined in the appended claims, and all matters produced by the invention using the inventive concept are protected.
As shown in fig. 1, the method for detecting and identifying a trigger facing a latent fault of a medium voltage cable includes the following steps:
s1, calculating the change quantity of the effective value of the zero-sequence current of the medium-voltage cable;
s2, judging whether the variation of the effective value of the zero-sequence current of the medium-voltage cable is larger than a variation threshold value or not, and if so, entering a step S3; otherwise, judging that no latent fault exists at present;
s3, calculating effective values of all phase currents and effective values of all phase voltages;
s4, obtaining a fault phase by comparing the effective current value and the effective voltage value of each phase;
s5, calculating the starting time and the ending time corresponding to the out-of-limit part of the zero-sequence current, judging whether load change exists before and after the time period, and if so, judging that other faults exist; otherwise, entering step S6;
s6, calculating an initial angle of a fault phase voltage, judging whether the initial angle of the fault phase voltage is in a latent fault phase angle threshold interval, and if so, judging the fault as a latent fault; otherwise, judging as other faults.
The specific method of step S1 includes the following substeps:
s1-1, setting the sliding step length of sampling to be 1/4 period, and setting the length of a calculation window to be 1/2 period. The number of sampling points in each period is set to be 128;
s1-2, according to a formula:
obtaining zero sequence current effective value I at sampling point k of medium voltage cable rmsx (k) (ii) a Wherein N is b The total sampling point number of the half-wave signal is calculated; n is a sampling sequence; i.e. i x Representing a zero sequence current sampling signal;
s1-3, taking the effective value of the first half wave as the zero sequence current I under the normal operation condition 0_normal ；
S1-4, according to a formula:
ΔI 0 ＝I rmsx (k)-I 0_normal
obtaining the effective value change delta I of the zero sequence current of the medium-voltage cable 0 。
In the specific implementation process, the key parameters for realizing trigger detection of the method comprise the calculation window length, the sliding step length, the sampling frequency and the algorithm starting point. When the zero sequence current/voltage effective value is used for trigger detection, the selection of each parameter meets a certain condition, so that a better trigger detection effect can be achieved, and the parameters are listed one by one as follows to provide reference for parameter setting of a trigger detection algorithm:
(1) selection of sliding step size and calculation window
FIG. 2 analyzes the voltage effective value calculation results (voltage with 1/4 cycle duration) under different calculation windows and sliding step conditions. As can be seen from the figure, the residual voltage obtained under the parameter condition of the half-wave calculation window is lower, and the calculation accuracy is obviously better than that of the full-wave calculation window in this point. Therefore, the following conditions are satisfied when selecting the sliding step size and calculating the window length:
A. the sliding step length is less than or equal to the length of the calculation window;
B. calculating the window length which can not be less than 1/4 period, and generally selecting half wave and full wave;
C. when the calculation window is selected to be 1/4 period, the sliding step size should be selected to be 1/4 period.
In summary, the method can select the sliding step length as 1/4 period, and the calculation window length as 1/2 period.
(2) Selection of sampling frequency
Fig. 3 analyzes the influence of different sampling frequencies on the voltage effective value calculation result under the condition of sliding the half-wave calculation window point by point. It can be seen that when the number N of sampling points per cycle is 8, 16, 32, and 64, respectively, the voltage effective value calculation result of the point-by-point half-wave effective value algorithm is more influenced than when N =128, 256, 512, and 1024. Table 1 records the lowest value of the effective voltage values at different sampling frequencies. When N is larger than or equal to 128, the minimum value of the effective voltage value obtained by each calculation basically tends to be stable. Therefore, the method sets the number of sampling points N per period to 128, so that the algorithm rapidity can be improved, and the accuracy can be ensured.
Table 1: lowest value of effective voltage value under different sampling frequencies
(3) Influence of the starting point of the algorithm
The starting point of the algorithm is set as the first voltage zero crossing point, namely, the calculation effect is optimal when the phase angle is 0 degrees. However, when the sliding step is 1/4 of a cycle, no matter a half-wave or full-wave calculation window is adopted, the influence of the selection of the starting point of the algorithm on the calculation result is not obvious; when the sliding step is 1/2 cycle and the calculation window is full wave, the influence of the starting point of the algorithm is not large. The choice of starting point for the algorithm can therefore be neglected in case the sliding step and the calculation window are chosen correctly.
When the fault is a latent fault, there is a condition of "self-clearing", that is, after the fault is eliminated, the voltage/current operation state returns to normal, so the fault has a fault ending time except for a starting time, and if the fault ending time does not exist, the fault is not a latent fault.
a. The fault initial sampling point criterion is as follows:
I 0_rms (n start -1)<I th2 &&I 0_rms (n start )>I th2
wherein n is start For the fault starting sampling point, I 0_rms Is an effective value of zero sequence current, and a fault sampling point n start The corresponding time is the starting time t of the fault start 。I th2 Is the threshold value of the zero sequence current effective value.
b. And (3) judging the sampling point at the end of the fault:
I 0_rms (n end -1)>I th2 &&I 0_rms (n end )<I th2
wherein n is end For sampling points at the end of the fault, I 0_rms Is an effective value of zero sequence current, and a fault sampling point n end The corresponding time is the fault end time t end . The detection diagram of the start-stop time of the latent fault is shown in fig. 4.
c. Calculating the duration of the failure Δ t break ：
Δt break ＝t end -t start
When making standardDetermining the starting point t of a fault start And then, FFT analysis is carried out by utilizing the cycle wave before the fault, and the fault initial phase angle is calculated according to the voltage waveform data
Assuming that the voltage sampling signal is V (t), it is expanded into a Fourier series form:
wherein m is a natural number, a m 、b m Amplitude of the cosine and sine terms of the respective harmonic, a 1 、b 1 The amplitudes of the cosine and sine, respectively, of the fundamental component, taking into account ω 1 t =2k pi/N, N being the number of sampling points in one cycle, a 1 、b 1 Can be expressed as:
wherein V k Is the k-th sampled value of the voltage signal at the fault initial phase angle of the voltage
Can be expressed as:
it should be noted that the sampling point calculated by the above formula is the failure start sampling point n start The first N sampling points of (1), i.e. the sampling signal of the cycle before the fault occurs.
Since a latent fault occurs at the time of a voltage peak, if the fault is a latent fault, the fault initial phase angle
Should the peak time (i.e., around 90 ° or 270 °) be approached, the latent faultTwo endpoint values of the phase angle threshold interval->
And
may take between 80 deg. and 100 deg. (as well as 260 deg. and 280 deg.).
In conclusion, the method does not need to rely on a large number of data samples, comprehensively considers the disturbance change rule of the phase voltage, the current and the zero sequence voltage and the current as the criterion characteristic, and has the advantages of simple used characteristic quantity and less calculated quantity. Aiming at a small current grounding system, the method closely combines the early fault characteristics of the system according to the requirements of trigger detection and a fault detection algorithm on sensitivity, rapidity and accuracy, simultaneously considers the fault variation characteristics of phase voltage/current and zero sequence voltage/current, realizes the early detection of the latent fault of the medium-voltage cable, and has important significance for timely finding the early fault (instantaneous fault) in the small current grounding system, reducing unplanned power failure and improving the power supply reliability.
Claims (5)
1. A trigger detection and identification method for medium-voltage cable latent faults is characterized by comprising the following steps:
s1, calculating the change quantity of the effective value of the zero-sequence current of the medium-voltage cable;
s2, judging whether the variation of the effective value of the zero-sequence current of the medium-voltage cable is larger than a variation threshold value or not, and if so, entering a step S3; otherwise, judging that no latent fault exists at present;
s3, calculating an effective value of each phase current and an effective value of each phase voltage;
s4, obtaining a fault phase by comparing the effective current value and the effective voltage value of each phase;
s5, calculating the starting time and the ending time corresponding to the zero-sequence current out-of-limit part, judging whether load change exists before and after the time period, and if so, judging other faults; otherwise, entering step S6;
s6, calculating an initial angle of a fault phase voltage, judging whether the initial angle of the fault phase voltage is in a latent fault phase angle threshold interval, and if so, judging the fault as a latent fault; otherwise, judging as other faults.
2. The medium voltage cable latent fault oriented trigger detection and identification method according to claim 1, wherein the specific method of step S1 comprises the following sub-steps:
s1-1, setting the sliding step length of sampling to be 1/4 period, and setting the length of a calculation window to be 1/2 period. The number of sampling points in each period is set to be 128;
s1-2, according to a formula:
obtaining zero sequence current effective value I at sampling point k of medium voltage cable rmsx (k) (ii) a Wherein N is b The total number of sampling points of the half-wave signal; n is a sampling sequence; i.e. i x Representing a zero sequence current sampling signal;
s1-3, taking the effective value of the first half wave as the zero sequence current I under the normal operation condition 0_normal ；
S1-4, according to a formula:
ΔI 0 ＝I rmsx (k)-I 0_normal
obtaining the effective value change delta I of the zero sequence current of the medium-voltage cable 0 。
3. The method for detecting and identifying the medium voltage cable latent fault-oriented trigger according to claim 1, wherein the specific method of step S4 is:
and taking the phase corresponding to the maximum value of the three-phase current effective value and the minimum value of the three-phase voltage effective value as a fault phase.
4. The method for triggering detection and identification of a latent fault in a medium voltage cable according to claim 1, wherein the specific method for calculating the initial angle of the fault phase voltage in step S6 is as follows:
acquiring a voltage sampling signal of a cycle before a fault according to the initial time corresponding to the out-of-limit part of the zero sequence current;
according to the formula:
obtaining the initial angle of the fault phase voltage
Wherein a is 1 And b 1 Is an intermediate parameter; n is the number of sampling points in one period; v k Sampling a value of the kth sampling in the voltage sampling signal of the cycle before the fault; and pi is the circumferential ratio.
5. The method for the detection and identification of the triggering of a latent fault in a medium voltage cable according to claim 4, characterized in that the latent fault phase angle threshold interval is 80 ° to 100 ° and 260 ° to 280 °.
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| CN117233471A (en) * | 2023-11-09 | 2023-12-15 | 四川大学 | Medium-voltage distribution network branch line-collision fault detection method based on contact impedance gradual change characteristic |
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| CN117233471A (en) * | 2023-11-09 | 2023-12-15 | 四川大学 | Medium-voltage distribution network branch line-collision fault detection method based on contact impedance gradual change characteristic |
| CN117233471B (en) * | 2023-11-09 | 2024-01-23 | 四川大学 | Medium-voltage distribution network branch line-collision fault detection method based on contact impedance gradual change characteristic |
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