Time domain reflectometry techniques in grounding electrodes fault detection

Abstract

A grounding system is an integral part of any electrical network, especially in substations, power plants, and industrial operations. Faults in grounding systems pose a serious threat to compromising power equipment protection and worker safety. A large number of vertical electrodes are used to construct a grounding network, and breakpoints in these electrodes due to soil-induced corrosion are one of the prime reasons for faults. This thesis proposes, for the first time, the use of time domain reflectometry (TDR) techniques for fault detection in vertical grounding electrodes. First, an approach based on a rod insertion method is presented. Here, a secondary electrode is inserted adjacent to the grounding electrode while a fast risetime pulse is continuously applied to excite a transverse electromagnetic (TEM) wave that propagates along the electrodes and reflects from the breakpoint. The reflected signals for different depths of the secondary electrode are analyzed to identify the location and severity of the breakpoint. Second, a single wire time domain reflectometry (SW-TDR) technique is developed and shown to be a viable method for grounding electrode fault detection. In SW-TDR, a fast rise-time pulse is injected onto the single conductor grounding electrode primarily exciting transverse magnetic (TM) mode surface wave propagation. The surface wave propagates along the electrode and is reflected at any impedance mismatch such as a fault in the electrode. The mismatch location and severity of the fault can be identified using the reflected signal waveform. Expressions for the fields of the surface wave supported by a finite-length single electrode in a lossy medium are presented. To demonstrate the feasibility of the proposed techniques, a full-wave simulation approach is used to evaluate the wideband input impedance of both a single grounding electrode and a configuration with grounding and inserted electrodes. FFT is then applied to obtain the corresponding TDR responses. The results show the proposed TDR techniques are capable of detecting faults for a wide range of soil conductivity for a system bandwidth of 300MHz. Experimental methods have been developed for both the rod insertion and single wire TDR techniques. The electrode dimensions, soil electrical parameters of the ground media, and input pulse bandwidth are scaled to maintain the same transient pulse propagation characteristics as in the full-scale numerical analysis. Both full-scale numerical simulations and scale-model measurements demonstrate that full breakpoints and partially damaged regions can be identified along with the fault severity and type. In the full-scale simulation case, a maximum deviation of 6.5% from the theoretical estimation is observed for the rod insertion method, and 4% for the single-wire TDR method. The experimental results show a deviation of less than 7% from both the theoretical estimation and numerical simulations in determining the fault location using the rod insertion approach, and a deviation of 9% for the single-wire TDR approach.