Research on Accurate Fault Location of Small Current Grounding System

Based on the principles of single-end fault ranging and signal injection method, a method of identifying fault distance parameters based on disturbance signal injection is proposed. The simulation calculation results verify that the method can effectively exclude the influence of line distribution capacitance on fault location accuracy while bearing a large range of transition resistance, and the impact of the arc extinguishing coil can be avoided by adjusting the signal injection module opening and closing time, which has high practical value in engineering.


Introduction
Locating faults quickly and accurately is of great significance to improve the safety of the power grid and the reliability of the power supply and reduce the loss caused by power outages [1].The existing fault location methods can be theoretically divided into the traveling wave method, fault analysis method, and artificial intelligence algorithm [2].
In [3], a single-end electrical fault ranging method is proposed based on the RL model, which overcomes the defects of the traditional single-end ranging method influenced by the system impedance, but for long-distance and large-capacity transmission lines, ranging accuracy is affected by its distributed capacitance.In [4], the current distribution coefficient is considered as an imaginary number, establishes the real and imaginary part equations of the measured voltage and current at the protection installation after the fault, improves the single-end fault location accuracy by using multi-time section information, and obtains the fault point location by damped least squares solution.
In this paper, we introduce an active localization method by using perturbation signal injection.The method uses power electronics to inject pulse signals with different frequency bands into the faulty line under the corresponding control strategy [5].Based on the transient changes generated by the dynamic components of the line, the frequency response of the faulty line at multiple frequencies can be obtained, which is conducive to the identification of line parameters [6].By analyzing the sequence network model of the system at the time of signal injection, the parameter identification equation of the fault distance is derived [7], and the numerical analysis of the parameter identification equation is obtained by using the least squares method to finally obtain the distance between the measurement point and the fault location.

Fault location principle
The schematic diagram of single-phase fault location based on disturbance signal injection is shown in Figure 1, which consists of a data acquisition unit and a signal injection device.The signal injection device consists of an independent AC power supply and thyristor.If a single-phase ground fault occurs at point F of phase A, the data acquisition unit detects the voltage reduction of phase A. After the signal injection device receives the wire selection signal, the thyristor is turned on at a certain trigger angle to make the ungrounded system form a complete circuit and artificially create a short-time single-phase complete ground circuit, thus generating a strong disturbance signal.The signal injection device can be changed by changing the thyristor trigger angle to form a voltage and current pulse with controlled amplitude and phase.At the moment of thyristor conduction, the AC power supply injects amplitude and phase-controlled pulse signal to the fault line, and this pulse has obvious harmonic components after chopping by power electronics.In general, the overhead line capacitance value is much greater than the fault transition resistance.To simplify the analysis, we ignore the impact of ground capacitance on the injected signal, and the following analysis of the disturbance signal injection expression can be obtained: According to this model, the differential equation can be written as: We can get: where  is the line impedance angle,  is the trigger angle, and  is the thyristor conduction angle.

Characteristic analysis of disturbance signal
The strength of the injected disturbance signal is related to parameters, such as thyristor conduction angle, grounding resistance, and fault distance.Since the location of the fault in the line and the fault grounding resistance has large randomness, we should ensure that the injected disturbance signal does not affect the stable operation of the system, but also meets the strength of the injected signal in the most undesirable situation.Figure 3 shows the strength of the injected signal at different thyristor breaking angles.

Figure 3. Signal injection intensity under different opening angles of thyristor
According to the requirements of relay protection of the power system, to avoid over the fixed time overcurrent protection action current and the provisions in the most unfavorable circumstances (metallic grounding and line bus exit at the occurrence of grounding short circuit), disturbance injection current and line synthetic current RMS value does not exceed 1.15-1.25 times the maximum load current.The generated perturbation signal contains rich spectral characteristics after chopping by power electronics.Compared with the medium-resistance method and the high-frequency signal injection method in which a single frequency and steady-state characteristics are injected, the perturbed signal spectral characteristics are more obvious, which is easy to do with the line load fluctuation signal to distinguish, and are not affected by external interference sources and line distribution capacitance shunt.

   
To verify that the transient response spectrum generated by the injected perturbation signal is continuous at each location of the system, a simulation was built according to Figure 2, and a fast Fourier transform of the perturbation signal was performed to obtain the spectral characteristics of the perturbation signal, as shown in Figure 4. ) One of the principles on which this paper is based is that the spectrum of the transient response generated by the injected disturbance signal is continuous at all parts of the system.Thus, for any given frequency signal (DC component, fundamental, and each harmonic), the corresponding fault equation can be given in the fault complex sequential network.For an injected signal with determined amplitude and frequency, the transient corresponding generated by this signal in the system depends only on the system parameters, ground resistance, and fault distance.According to the above analysis of the strength, spectral characteristics, and phase of the injected disturbance signal, the disturbance signal due to its unique waveform and the location of its appearance, its signal energy, and signal energy distribution, can be selected as a characteristic quantity for signal detection.The controllable signal contains sufficient harmonic information, and according to the fault sequence network diagram, the parameter identification equation between harmonic voltage, harmonic current, and path harmonic impedance is established, which provides feasibility for the identification of fault distance and grounding resistance.

Fault modeling
The following figure shows the single-phase ground fault sequence network diagram of the small current grounding system.Since there are more dynamic components in the line, it will be difficult to solve the differential equations if they are written.Therefore, by using the Laplace transform, the line time domain differential equation into the frequency domain function under the equation is solved.Its corresponding frequency domain function under the fault composite sequence network diagram is shown in Figure 5.
By further combining and simplifying the equations, we can obtain: We can obtain the final equation: ) where  ,  are the fault distance parameters and the transition resistance to be identified.According to the principle of least squares, the sum of squares of errors is set to be minimum: In this parameter identification, there are two data to be identified.By the symmetric component method, the three-phase high-disturbance harmonic signal is symmetrically decomposed by sampling twice arbitrarily, and the positive, negative, and zero-sequence components of the corresponding harmonic components are obtained and substituted to obtain the parameters to be identified.

Simulation results
Simulation conditions: To fully verify the robustness of the disturbance signal injection method, the reliability of the simulation results is set for different fault point locations (fault point locations are selected at 0.1, 5, 10, 15, and 20 km from the first section), and different transition resistance at the grounding point (0, 50, 100, 500, 1, 000, and 2, 000 Ω).  1, this method can be used to calculate the fault distance for the single-phase grounding fault of a small current grounding system with different fault distances.2, the conclusion can be drawn: the fault location method is not affected by the transition resistance, but there is a certain error in the numerical identification effect of the transition resistance.

Conclusion
Based on the principle of single terminal fault location detection and signal injection method, a method using disturbance signal injection is proposed in this paper.The frequency spectrum responses of the system under various harmonic components are obtained by injecting the disturbance signals with controllable amplitude and distinct characteristics into the system.The least-square modeling is completed for the single-phase grounding complex sequence network, and the harmonic voltage, harmonic current, and harmonic impedance of each sequence network are combined to reduce the parameters to be identified, and the parameter identification of the line is converted to the identification of the fault distance and grounding resistance.The least square method in the parameter identification technology is used to realize the calculation of the fault location of the small current grounding system.The method has the characteristics of strong resistance to transition resistance and high-ranging accuracy.

Figure 1 .
Figure 1.Disturbance signal injection to single-phase fault location principle diagram 2.2.Perturbation signal model analysis The single-phase disturbance signal injection loop is shown in Figure 2. The plan is to turn on the thyristor at the trigger angle  to briefly connect the fault point with the disturbance signal injection device circuit.

Figure 2 .
Figure 2. Simplified model of perturbation signal injection

Figure 5 .
Figure 5. System sequence network diagram It is assumed that the capacitance and inductance of the line belong to zero state response.The operator equation is based on the relationship between the component pairs in the graph.

Table 1 .
Parameter identification results at different fault distances (

Table 2 .
Parameter identification results in different values of transition resistance ( * 10  )