D. Guo

Proposal for Probabilistic Risk Assessment in Grounding Systems and Its Application to Transmission Substations

An approach to probabilistic risk assessment of electrical system grounding is proposed. The method uses all significant factors that affect the risk of electrocution at substations and takes into account their probabilistic nature. The approach implements an accurate statistical description of IEC479-1 fibrillation and body impedance data, and it uses detailed computer simulations of the modeled grounding system to provide safety voltage distributions that take into account the individuals presence at a site as a random variable. Variation in the power system fault level is accounted for, and extensive data of actual system fault clearance time are included. It is proposed that the probabilistic risk assessment is utilized as a second stage of the grounding system assessment when the first-stage deterministic analysis requires expensive or impractical mitigation. Implementation of the second stage probabilistic risk assessment yields a measure of individual risk. This is then benchmarked against industry-accepted “as low as reasonably practicable” values to determine whether investment in mitigation is required. To illustrate the applicability of the proposed approach, the probabilistic risk assessment is applied to a practical case study of a transmission substation.

Controlled Large-Scale Tests of Practical Grounding Electrodes—Part I: Test Facility and Measurement of Site Parameters

This paper describes the establishment of a large-area outdoor experimental facility at the Dinorwig pumped-storage power station in North Wales for investigating the performance of practical grounding systems. The facility itself and arrangements of test electrodes and measurement equipment are described, providing a background for subsequent test methodologies. In addition, tests to determine the variation in the water and reservoir bed resistivities have been undertaken, resulting in a survey data set that better informs experimental work and simulations of vertical ground rod and grid electrodes. It is proposed that this facility may also be used to calibrate grounding meters and for validating computational modelling techniques. In a complementary paper, results of low-voltage dc, ac, and impulse tests of varying magnitudes are described, and experimental results are compared with computed values obtained from numerical models.

Controlled Large-Scale Tests of Practical Grounding Electrodes—Part II: Comparison of Analytical and Numerical Predictions With Experimental Results

This paper presents tests carried out at an outdoor experimental facility, described in the companion Part 1 paper, to investigate the performance of practical grounding systems. Here, the results of low-voltage dc, ac, and impulse tests performed on rod and grid electrodes are described, and the measured quantities are compared with computed values obtained from numerical models. Measured ground resistance and impedance at low frequency (including power frequency) showed reasonable agreement with simple standard formulae and computational models, but revealed a significant falloff with current magnitude in the range often used for practical testing of high-voltage grounding systems. This may have implications for the specifications of grounding test equipment and extrapolation of measured ground resistance/impedance at low-voltage/current to values representative of realistic fault currents. A frequency dependence of ground impedance was also measured. Specifically, a fall in impedance over a frequency range up to 100 kHz is not generally accounted for in grounding models.

Experimental investigation into the performance of large-scale earthing electrodes

Analytical and numerical simulation techniques have been developed for the calculation of earth resistance/impedance and to estimate the potential distribution in the vicinity of earth electrodes. However, very little literature is available on experimental validation of these calculation techniques. To address this, a programme of experimental tests on various earth electrodes has been carried out at the lower water reservoir of a hydro pumped-storage power station in North Wales. In this paper, the earthing test facility at Dinorwig power station is described including the details of the experimental set up. The results from experimental tests on a 5mx5m earth grid, immersed in water and energised are under ac, dc and impulse, are presented. The values of measured earth resistance/impedance and water surface potential distributions are compared with those obtained from analytical calculations and detailed numerical computer simulations.

Large-scale earthing test facilities at Dinorwig power station

In this paper, experimental set and initial test results are described of low voltage DC, AC and impulse tests on a rod electrode immersed in a water reservoir of a large hydro power station. A detailed water resistivity survey indicates largely uniform temperature and resistivity throughout the reservoir. Calculations and simulations of the rod electrode under DC and power frequency AC show reasonable agreement with experimental results. Further investigations are required to understand better the behaviour of the test system under high frequency and impulse conditions.

Wireless measurement system for a large-scale grounding electrode test facility

An evaluation of the impedance of ground electrodes under dc, ac or impulse conditions requires reliable and accurate measurement of the current and earth potential rise (EPR) at the point of injection. This is commonly achieved in wired systems by arranging current-injection and measurement circuits in quadrature, but such wide-area test arrangements are not always possible due to prohibitive environmental conditions or site access restrictions. This problem has been encountered in establishing a large-scale test facility in the lower lake of the Dinorwig pumped-storage power station in Llanberis, North Wales. The geography of the site restricts the location of both current source and data acquisition equipment to a common location on the lake shore, with all wired circuits confined to a narrow corridor along a floating pontoon. Simulations have shown that wired systems arranged in this way lead to significant measurement error due to circuit coupling, increasing with both the frequency and magnitude of the injected current. This paper presents work undertaken to evaluate alternative non-wired measurement systems for application to high current testing of ground electrodes in harsh environments. Technical details of the developed long-range wireless data transmission system are discussed and field data acquired from high-voltage impulse tests at the Cardiff University earthing facility at Llanrumney fields are presented, showing good agreement with both simulation and measured ground resistance data acquired using conventional wired systems.

Change in pattern of void-type PD with ageing

The measurement and assessment of partial discharge (PD) activity is used both for condition assessment of high voltage plant (by operators) and for quality control (by manufacturers). Except for the online circumstances, the measurement usually lasts for a short period of time while higher than working stress is applied. The resulting pattern of discharge data, when compared with a knowledge base of PD patterns of different types of defects is a crucial factor in diagnosing the condition of the insulation system in the plant.

Application of an analysis technique to characterise impulse response of grounding systems

Transient response plays a key role in the evaluation of the performance of grounding systems and for the protection of electrical installations under lightning strikes. The frequency spectrum of the lightning impulse contains harmonics components up to the megahertz range. The measured transient response of grounding systems under test may be distorted by spurious high frequency interference in the acquired signals, which presents challenges for the accurate analysis of high frequency performance of such systems. In this paper, the high frequency performance of a rod electrode is investigated based on measurements of its transient response under impulse energisation. A practical method is implemented to eliminate the high frequency noise in the measured voltage and current shapes, which allows a frequency domain analysis based Fast Fourier Transforms.

Earthing requirements for HVDC systems

Following the recent developments in the field of power electronic valves, High Voltage Direct Current (HVDC) systems have become more attractive in power systems, and are widely recognised as a suitable tool for future power systems, especially in bulk power transmission over long distances. Grounding electrodes are an important part with significant roles in the operation of the HVDC systems. In this paper, first a brief review of different types of HVDC configuration is presented. Essential requirements in the design and operation of the HVDC grounding system are then presented and discussed. In the relevant studies, environmental impacts of HVDC electrodes, e.g. step voltage, effect on the buried metallic structures, DC magnetic bias of power transformers and mitigation methods are highlighted. Finally, a basic modelling of different physical configurations of HVDC electrode resistance is performed.

Assessing electrocution risks in transmission substations using probabilistic criteria

The following work investigates the risk of electrocution from earth potential rise at transmission substations and nearby third party locations as a result of a power system earth fault. Current earthing standards use a deterministic approach where the risk assessment considers a set of worst-case conditions. However, it is recognised that most of the parameters involved in calculating the earth potential rise and associated safety voltages are probabilistic by nature. A probabilistic technique has been developed to determine the risk of electrocution of an individual under power system earth fault conditions. The so determined individual risk can be compared with accepted health & safety risk levels, employing the ALARP principle (as low as reasonably practicable). This paper presents improvements to the developed probabilistic technique by introducing defined risk zones with specific probability of presence and introducing exclusion zones where there is virtually no touch voltage risk. The improved technique is applied to a practical case study electrical installation.