Poboljšani metodi za merenje sigurnosnih karakteristika sistema uzemljenja u visokonaponskim postrojenjima

Abstract

The grounding system has an important role to maintain safe and reliable operation of highvoltage (HV) power facility (usually substation). The main task of a grounding system is to discharge maximum ground fault current expected into the soil, without exceeding safety limits for touch and step voltages within the substation and the surrounding area. It is desirable that the substation grounding provides a near zero resistance to remote earth. The prevailing practice is to install a grounding grid of horizontal ground electrodes (buried bare copper conductors) supplemented by a number of vertical ground rods connected to the grid, and by a number of equipment grounding mats and interconnecting cables. The grounding grid provides a common ground for the electrical equipment and for all metallic structures at the substation. It also limits the surface potential gradient. Grounding is traditionally modeled as a pure resistance. This is a good approximation at low frequencies, but as the frequency gets higher, inductance starts to play an important role. National and international standards give detailed guidelines for grounding system design, as well as for testing, under power frequency conditions, but only limited recommendations for high frequency and transient conditions are available. The main performance metrics of the grounding system are: grounding grid integrity, grounding system impedance, and touch and step voltage. The performance metrics should be periodically tested. The goal is to perform the test when the substation is in-service (to avoid interruption of service and costly downtime). Due to conductive interference in the grounding system (usually several amperes, up to about 15 A), caused by power frequency, its harmonics and background noise, the active HV substation is an extremely challenging environment for the measurement on grounding system. This is further compounded by the impact of very low impedance of the grounding system (typically less than 0.5 Ω). Essential data to calculate safety risks in advance is often not available. Measurements are therefore an essential supplement to the theory. Unlike the theoretical approaches, empirical approaches are not widely available in the literature due to the arduous work involved, the difficulty to obtain permissions, and safety restrictions. There is a continual interest in scientific community and standardization bodies to improve the measurement of performance metrics of the grounding system. The drawbacks of actual measurement methods are related to less or more subjectivity and requirements for high level of test current (up to 100 A AC and up to 300 A DC). In this thesis, in order to avoid mentioning drawbacks, we elaborated and experimentally verified the efficient methods. The overall contribution of this thesis is the substantial enhancement of the grounding system measurement by eliminating the current restrictions found in the IEEE standards 80 and 81, respectively. The original works, that constitute the core of this thesis, have been published in referring publications. This thesis is divided into three parts. Part I includes Chapters 2 and 3. Part II includes Chapters 4 to 7. Part III contains Chapter 8. Part I gives an extensive review of published literature related to the analytical and simulation investigations of earth electrode systems. Part I also analyzes the existing methods for measurement performance metrics of the grounding system. Critical review of the constraints inherent in existing methods was given. Part II includes original contributions of the thesis: new method for measurement of grounding grid integrity (Ch.4); an effective method (so called FSM) for eliminating the effect of conductive interference at power frequency (Ch. 5); new method for grounding system impedance measurement (so called FOP-FSM) (Ch.6); new method for measurement of touch and step voltage (so called FEM-FSM) (Ch.7). Our experiment-driven verification approach of the proposed measurement methods is based on totally 49 power facilities (voltage range: 110 kV, 220 kV and 400 kV) in Serbia. Part III gives an extensive review of thesis results. Part III also proposes and elaborates directions for further automation of the proposed measurement methods.