Dr Darren Woodhouse

Grounding system engineers and power utility owners have a duty of care to ensure that substation grounding systems continue to meet their safety performance requirements over the operational lifetime of the installation. Changes to power system or third-party assets associated with an installation, as well as environmental factors such as temperature or moisture level fluctuations, can cause variation in a grounding system's performance. To perform a grounding system assessment, engineers must select a suitable set of fault scenarios on which to base their assessments. However, the relative risk level of hazards produced by a ground fault (GF) depends on that proportion of the fault current that passes to ground through the local grounding of the installation and the duration of the event. The consideration of all hazards posed by a single installation is thus physically and theoretically an expensive proposition, particularly for installations with complex grounding systems. An electronic recording system is proposed to capture the characteristics of power system ground potential rise (GPR) events during GFs. This system allows the failure of critical grounding system components to be highlighted as indicated by a substantial change in the measured impedance. The system also establishes the GPR event frequency, allowing the probabilistic nature of GFs associated with an installation to be assessed. This paper provides an explanation of the GPR monitoring rationale and a methodology for implementing such a system, and typical results are shown from trials undertaken in recent years.

The prevalent method used to determine the Ground Potential Rise (GPR) of an earthing or grounding system is to perform a Fall of Potential (FOP) test undertaken during an grounding system current injection test. Determining the test GPR requires analysis of FOP test data due to the FOP response's non-conservative asymptotic behaviour. This paper assesses achievable tolerance by FOP test analysis including recognised and alternative methods. Susceptibility to voltage and distance measurement noise is examined by application to an grounding system model to establish minimum error bounds and method estimation variation on GPR estimates. The applicability of the FOP test 'rule of thumb' termination condition, to take three to four readings beyond the 'knee' of the FOP response, is also examined. In particular, the question of whether appropriate information is gathered when this condition is met is discussed.

Grounding systems are critical plant for the correct operation of major power installations, particularly major substations. As the average age of major substations in a utility's franchise increases, the likelihood that an assessment of the condition of a grounding system will be initiated. Whether or not the grounding system needs to be assessed in line with the substations age is often debated. However, the evidence is that the driving factors behind an assessment of a grounding system's aging or condition, is dependent on factors different to that of the remainder of the plant. Assessing the condition of a ground requires the execution of an appropriate regime of tests, conducted over the lifetime of the ground, and in some cases over the lifetime of the associated HV asset. Maximizing the knowledge gleaned from the information contained in these measurements is the art of the specialist, which distinguishes them from then infrequent practitioner.

Earthing by its nature is particularly open to speculation, opinion and contention. One such point of contention is whether primary or backup clearing times should be used in the preparation of appropriate safety criteria. As humans become increasingly susceptible to ventricular fibrillation the longer the heart current duration, the use of back up clearing times often leads to significantly lower allowable touch voltages (and higher associated costs in achieving compliance). On the other hand using primary protection clearing times for the protection of people, while using back up clearing times for the protection of equipment, seems ill balanced. This paper investigates and presents the case for and against the use of primary clearing times, including a quantified risk based assessment.

A probabilistic model for predicting the expected touch voltage hazard profiles at HV/MV substations is presented. The model uses readily available asset information to analyse the MV network fed from the HV/MV substation. Probabilistic profiling of the expected magnitude, frequency and duration of typical ground faults on a given distribution network, provides quantitative information for use in determining the actual risk profile for the assets. This in turn helps asset owners demonstrate that they meet their duty of care in relation to ground fault related hazards. The model is applied to two example substations, generating a probabilistic profile for ground fault current, ground potential rise and the expected hazard ratio of expected touch voltages in relation to IEEE80 safety criteria across the range of simulated faults. The model output profiles for each HV/MV substation are then compared against profiles derived from seven years of recorded ground potential rise events during real ground faults.

In this paper the authors build on the findings of previous works to explore the risk due to step voltage hazards. Of particular interest is a correction to work published by Wang. The outcome is significant as it appears to indicate that the Wang result was significantly conservative and the corrected results are somewhat confronting. The same calculation framework is used to derive step voltage time curves which represent a constant probability of ventricular fibrillation and can be used as a component of the risk level assessment. Again the results were found to be confronting, motivating the authors to explore other physiological effects outside of ventricular fibrillation for establishing appropriate levels of risk for step voltages. The outcome of that work has produced a novel recommendation for a establishing step voltage safety criteria and an insight into further research which will further clarify the risk posed by step voltages.

Earthing systems must be designed to provide adequate performance and safety for the life of the asset. However, a major issue encountered with older substations in Australia is where MEN encroachment related voltage hazards occur. Touch voltages between the substation EPR and the relatively low potential of the encroaching MEN are difficult to correct with traditional earthing methods. Newer substations can also incur such encroachments, particularly when established in rural growth areas. Earthing designs under such circumstances may therefore be required to be dynamic and scalable. This paper presents two case studies in which the innovative use of the oft-thought traditional counterpoise conductor has contributed to effective management of EPR through a number of stages of development of the substation and surrounding infrastructure. The paper will focus on utilising counterpoise conductors in the management of EPR at both 'Brownfield' and 'Greenfield' substations by considering transfer hazards, touch voltage hazards, compliance with telecommunications assets and commonly ignored mutual coupling benefits between fault current carrying conductors and the counterpoise. The issues, concepts and opportunities discussed may offer additional strategies to those from various industries with a role in either the detailed design, commissioning testing and specification and ongoing management of earthing systems.

One of the most exotic concepts in the field of earthing is the ability of the earth to conduct electricity. Its sheer size means that the impedances produced by the passage of current through the soil are very low. Large earthing systems in low soil resistivity areas can produce very small earthing system impedances. What is not as well appreciated is that an overhead conductor using an earth return path to complete a circuit, such as is the case during a phase to earth fault, incurs an additional impedance due to the passage of current through the earth. This paper explores the analogous condition of current flow in a homogeneous conductor and the distribution of current within that conductor due to the impact of skin effect. The earth fault current distribution is constrained by the magnetic forces produced by the current in the earth and the excitation current, being the insulated earth fault current carrying conductor, located either above the ground or within the earth. The earth fault currents distribute in a manner unexpected by many, but with direct impact on transmission line easements and the manner in which transmission impedances are calculated.

Since the publication of the earthing guideline, ENA EG-0, the electrical industry response has been a mix of enthusiasm, resistance and compulsion to accept and implement the safety criteria methods proposed. In the large part it has been acknowledged as a positive improvement as the most thorough and modern treatment of earthing system risk analysis. While the risk based safety methods proposed in EG-0 appear functional with regard the assessment of substation primary faults, there is little doubt that assessment of Zone-Substation-Secondary risks appear higher than previously thought, particularly when compared to traditional methods. This paper proposes a method of modelling and evaluating a zone substation distribution network in order to better calculate the earthing risk zone substation assets create for individuals living within the influence of the installations. Moreover, the methods proposed give light to a possible solution to the conundrum surrounding any earthing system risk analysis where multiple fault scenarios exist.

While continuity based testing is a relatively well accepted method for assessing the integrity of a grounding grid, no specific continuity based test has distinguished itself with practitioners at large. Why this continues to be the case is not completely understood but it is likely a combination of ignorance of more effective procedures and the availability of better instrumentation. This paper reviews the various continuity based methods used for integrity testing and the characteristics required of a test procedure to make it effective at assessing integrity. This is considered primarily due to a lack of review of the various methods and instrumentation available to perform integrity testing. Accurate measurement of resistance in the electrically noisy environment of a power substation is a difficult task. In the case of grounding grid measurements, not only are the noise levels quite high but the resistances to be measured are relatively small, and the absolute significance of a specific measurement can be obscure. In practice the context of any given measurement is crucial to determining the significance of the measurement. While continuity testing can be performed at any power installation this paper concentrates specifically on the integrity assessment of a major substation. Most power utility substations are of a reasonable size, typically a few thousand square meters of real estate, which usually defines the extent of the grounding system and hence the extent of the integrity testing. This paper identifies the obstacles to performing an accurate measurement of resistance in the noisy environment of a power substation, establishing measurement targets and specific instances of poor integrity.

A current trend in the construction of substations, at least in Australia, is to use underground cable circuits to exit substations due to the high density that underground circuits can be packed. In the already electrically congested volume in the immediate vicinity of the substation this construction practice may be the only way of supplying the number of necessary circuits. The significant cost differential between equivalent overhead and underground lines motivates utilities to construct and use overhead lines as soon as the electrical plant density becomes sufficiently sparse for their construction to be feasible. Further to this, ongoing pressure on power utilities, through increasing land prices and town planning statutes, has forced the adoption of technologies such as GIS to facilitate the use of ever smaller substation footprints. In turn these factors have led to the rife use of HV transition points as a means of providing the necessary circuit density across the substation field interface to connect the circuits terminating in the substation to the field. Little do the substation designers appear to realise the challenges transition points pose to the earthing engineer and why they are the silent earthing system adversaries to modern style HV power reticulation methods. This paper demonstrates why these asset locations become earthing 'hotspots' and then discusses some methods that might equip the designer to manage the prevailing risks. Our conclusion, with regard construction and maintenance practices for lines, is that when lines are designed in ignorance of these effects the risk level can quickly increase 10 fold.

Safety criteria for earthing related hazards have historically been derived using deterministic processes. To quantify the risk associated with these hazards the current trend is to base safety criteria on probabilistic processes. This paper examines how the selection of probability distribution functions used to derive the criteria can impact the outcome, particularly as these events are low probability with high consequence.

Surge arrester specification is of vital importance for the protection of any underground cable system. This paper outlines additional considerations for arrester specification in specially bonded cable systems. In particular, the paper addresses the effect of earth potential rise on arresters in mid point bonded extensions to fully crossbonded cable sections, under a single line to ground fault conditions. A steady state model has been developed to determine the standing steady state voltages impressed on the arreseters in the mid point bonded section. The model has been compared with simulations in EMTP-RV and verified using field tests.

The task of determining the condition of the earthing in the Upper Tumut generation system, was undertaken as part of the Snowy Mountains Hydro Electric Authority's (SMHEA) safety risk assessment and asset condition monitoring programme. The testing programme [1][4] to ascertain performance under earth fault and lightning conditions had to overcome considerable physical difficulties as well as the restrictions of 'close' proximity injection loops. The application of software [2], test instrumentation and testing procedures developed within Australia in collaboration between Energy Australia, Newcastle University, and SMHEA, to obtain real solutions are described in this paper. Also discussed are condition assessment processes that complement the current injection testing programme. This paper also provides a summary of the minimum requirements of an earthing system injection test to satisfactorily assess the condition of complex electrical power system installations.

High resistivity surfacing materials such as crushed rock and bitumen, are often used to limit electric shock hazards for utility operators and maintenance staff, as well as the public. Certain safety codes also make allowance for the likelihood that utility operators and the public will be wearing footwear. This paper evaluates the performance of series impedance insulation used as earthing system safety mitigation measures. Data gathered as part of a detailed R&D programme, undertaken by the Safearth Engineered Solution team within Energy Australia, is discussed with respect to the performance of surface layers and footwear, and their effect upon electrical installation safety profiles under earthfault conditions. Guidelines for improved performance specifications are developed based upon test results and application requirements.

Mobile telephone base stations sited beneath or adjacent to HV towers require significant risk management consideration. The paper examines the level of shock hazard associated with people moving near a HV tower, and determines the sensitivity of the risk to a range of parameters which might be expected to have significant impact upon the various shock scenarios. High resistivity surfacing materials, such as crushed rock and bitumen, are often used to limit electric shock hazards for utility operators and maintenance staff, as well as the public. This paper evaluates the performance of series impedance insulation used as a earthing system safety mitigation measure. Investigations have been made into the impact of probabilistic aspects of the shock scenario. Interference theory and Monte Carlo simulation techniques are used to statistically appraise the actual level of safety afforded by various safety criteria when compared with the benchmark body current criteria.