Zhihong Fu

Break-Point Diagnosis of Grounding Grids Using Transient Electromagnetic Apparent Resistivity Imaging

Summary form only given. Faults in the grounding grid are hidden danger to the safety of power system and cause significant economic loss every year. This paper presents a new means to locate faults in grounding grids using the transient electromagnetic imaging method. The method compares the distribution characteristics of the equivalent resistivity of underground covered by grids for no fault case and those with break points. We describe why and how the proposed method can be used for break point diagnosis of grounding grids. Simulation was performed using Ansoft FEM package, and a field experiment was conducted to verify the effectiveness of the proposed method. This study provides a practical means for break point diagnosis with no requirement of power shut down or soil excavation. Moreover, the proposed method does not require the design drawings of grounding grids.

A Novel Method for Fault Diagnosis of Grounding Grids

Faults of grounding grids are threat to the safety of power system and cause significant economic loss every year in China. Accurate detection of the faults will alleviate these detrimental effects. This paper presents a novel approach using the transient electromagnetic method for fault diagnosis of grounding grids. The induced magnetic intensity is collected by receiver coils in the form of electromotive force. After inversion calculation, the contour map of equivalent resistivity of underground covered by grids is obtained. The faults due to corrosion or break can be accurately located from the distribution characteristics of equivalent resistivity maps. The influence of the fault of different degrees, material of grounding conductor, and soil resistivity on distribution of induced magnetic intensity to fault diagnosis will be analyzed in detail. The effect of the underground pipeline on fault diagnosis of grounding grids is also investigated. Simulation was performed, and a field experiment was conducted to verify the effectiveness of the proposed method.

Current Pulse Generation for Transient Electromagnetic Applications

Abstract In this article, a novel circuit topology of near rectangular current pulse generation suitable for the transient electromagnetic (TEM) applications is proposed. Satisfactory TEM applications, in general, rely upon a TEM transmitter circuit acting as a current excitation source to produce a high-quality primary magnetic field. This requires that the generated current pulse be of short turn-off delay time, with highly linear falling edge, and fast rising edge. The generation of near rectangular bipolar current pulses is very challenging for high-current high-power applications. The proposed TEM transmitter circuit can be considered as a voltage stabilizing clamp current pulse shaper. Its operation principles are described and guidelines for optimal component selection are also provided. Simulation results show that the proposed circuit demonstrates superior performance compared to other TEM transmitter circuit topologies, such as the improved RCD snubber, IEDD, and the quasi-resonant rectifying circuit.

Transient process and optimal design of receiver coil for small‐loop transient electromagnetics

Distributed parameters of the receiver coils greatly affect transient electromagnetic signals over short time periods, causing a delay in the signals effective sampling time and the loss of shallow exploration information. This paper investigates the influence of transient process on apparent resistivity calculation and analyses the relations between the error of apparent resistivity and receiver coil design. We find that, under the same effective area, different radii of the receiver coils lead to different levels of impact on the estimation of the apparent resistivity. An optimization model is proposed to determine the optimal receiver coil size that gives rise to the smallest estimation error of the apparent resistivity. The relationship between the optimal radius and the effective areas is developed, which serves as a guideline for the optimal receiver coil design. The results may provide a useful means for improving the accuracy of the small loop transient electromagnetic instrumentation for shallow-depth mapping.

Configuration Detection of Substation Grounding Grid Using Transient Electromagnetic Method

Optimizing the grounding grid configuration is an effective approach to improve the performance of grounding grid. On the other hand, configuration of grounding grid is necessary for some defect diagnosis method to detect the faults existing in grounding conductors. This paper employs the transient electromagnetic (TEM) method to draw the configuration of a substation grounding grid. The surface magnetic field is recorded by receiver coils above the grounding grid. Based on the regular distribution characteristics of a recorded magnetic field, the grounding conductor is accurately located from the curve of magnetic intensity. The complete process of configuration detection of the grounding grid is described in detail. After fast inversion calculation of the recorded magnetic field, the equivalent resistivity is obtained. The configuration of the grounding grid will be clearly and accurately shown in the contour map and three-dimensional (3-D) surface map of equivalent resistivity. Model study is performed, and a field test is also conducted. The results are in good agreement to demonstrate the effectiveness of TEM method for configuration detection of the substation grounding grid.

Bipolar Steep Pulse Current Source for Highly Inductive Load

A bipolar pulse current source is proposed for a highly inductive load. The current source exploits a noninductive capacitor to store or release the energy back to and from inductive load during switching on or off of power inverter. A boost module is designed at the dc side of inverter to supplement the required energy gap so that edge steepness of the load current can be improved. The capacitor and boost elements are configured based on the analysis of capacitor impact on the load current and boost inductor current. The stability of load current can be guaranteed by the multiple driving pulses generated from closed-loop control of boost switch, regardless of load changes or initial pulse width offset. An experiment prototype is built to generate a flat-top 20 A current with the 200 μs rise/fall time for an inductive load of 2 mH or higher. Due to the steep rising and falling edges, the inductive load can realize high-frequency pulse currents. Moreover, the steep pulse current source can excite strong magnetic field, which make it especially useful for the inductive coils of high-power geophysical transient electromagnetic method (TEM) transmitter.

Break point diagnosis of grounding grids using transient electromagnetic apparent resistivity imaging

Faults in the grounding grid are a hidden danger to the safety of power systems and cause significant economic loss every year. This paper presents a new means to locate faults in grounding grids using the transient electromagnetic imaging method. The method compares the distribution characteristics of the equivalent resistivity of the underground covered by grids for the no-fault case and those with break points. We describe why and how the proposed method can be used for the break-point diagnosis of grounding grids. Simulation was performed using the Ansoft finite-element method package, and a field experiment was conducted to verify the effectiveness of the proposed method. This study provides a practical means for break-point diagnosis with no requirement of power shut down or soil excavation. Moreover, the proposed method does not require the design drawings of grounding grids.

Research on homogeneous magnetic field source of cube four-turn coil

A standard source of homogeneous magnetic field is essential to calibrating magnetic measurement equipment. The magnetic field distribution generated by cube four-turn coil device which is taken as the standard magnetic field source is deduced theoretically. Simulation was performed using ANSYS Maxwell software, and the results agree with the theoretical calculation very well. We discuss the homogeneity of magnetic field generated by cube coil device in axial and vertical axis direction, and results show that it can produce magnetic field of high homogeneity in a considerable area around the device center. For cube four-turn coil device with 1 m side, the bias ratio of magnetic induction intensity is within 0.25% in the central 0.2m×0.2m×0.4m cuboid. Compared with Helmholtz coil, the cube four-turn coil device has significant advantages on both homogeneity and field size as standard homogeneous field generating devices.

Service Life Estimation for the Small- and Medium-Sized Earth Grounding Grids

Service lifetime estimation for earth grounding grids as a problem or a task faced by the industry is presented. A mathematical model based on Freschis semianalytical model to determine the performance of the earth grounding grid at a certain year after being buried is proposed. The mathematical model can also explain the performance degradation of the earth grounding grid in principle and mathematically. According to the model, a scheme to estimate the service life of the earth grounding grid is proposed. A case study is given to illustrate how the scheme works. If the estimated service life of an earth grounding grid is not satisfactory, some measures, such as cathodic protection, should be considered to extend the service lifetime of the earth grounding grid.

A novel transient electromagnetic measuring system

Transient electromagnetic measuring system, a kind of geophysics detecting devices, still have some technological problems unsolved in high current steep pulse transmitting, high precision synchronous control, large dynamic range signal gathering, etc., This paper proposes a novel transient electromagnetic measuring system(WTEM) with some technological improvements, including: (1) applying PocketPC and Bluetooth wireless technology in receivers to improve the system performance and reduces the receivers scale; (2) constructing a steep pulse transmitting circuit to output high current pulse with high linearity falling edge, short shut-off time and fast rising edge, finally improving the surficial resolving power; (3) proposing a GPS synchronous controller with high synchronous precision, in which TCXO working with GPS corporately, recovering the short-time invalidation of GPS module. In the project of detecting the oil pipeline corrosion, indoor and field tests have been done to WTEM system. The results indicate that WTEM possesses fine performances, short withdrawing time from saturation status, high synchronous precision and short turn-off time of transmitting current, etc..