Safety Performance Evaluation of Fence Grounding Configurations in High Voltage Installations

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Safety Performance Evaluation of Fence Grounding Configurations in High Voltage Installations Zacharias G. Datsios1, Pantelis N. Mikropoulos2, Andreas Teneketzoglou3 and Dimitrios Tzikas4 High Voltage Laboratory, School of Electrical & Computer Engineering, Faculty of Engineering, Aristotle University of Thessaloniki, Thessaloniki 541 24, Greece 1 zdatsios@auth.gr, 2pnm@eng.auth.gr, 3ante@ee.auth.gr, 4dctzikas@auth.gr

Abstract- The design of the grounding configuration for the metal fence of a high voltage installation is important as the outside perimeter of the fence is accessible to the general public. In this work the safety provided by several fence grounding techniques commonly used in high voltage installations is evaluated for a 150/20 kV air insulated substation, a 400 kV step-up GIS substation and a large scale photovoltaic power station with the aid of grounding analysis software. A safe and cost-efficient fence grounding design depends on ground fault characteristics, soil conditions, installation area, distance between the fence and grounding grid as well as on the size and geometry of the latter. Index Terms--fence grounding, photovoltaic power stations, safety, substations, touch and step voltages.

I.

INTRODUCTION

A well designed grounding system of a high voltage installation should afford protection to persons against the danger of critical electric shock [1], [2]. Thus, the grounding system design should ensure that, in case of the most dangerous ground fault, the arising touch and step voltages are limited to values lower than the corresponding allowable voltage limits. When a ground fault occurs, the grounding grid, therefore also all grounded metal structures, are elevated to the same potential, called ground potential rise (GPR). Metal structures which are not connected to the grounding grid attain a potential due to coupling through the ground and, therefore, may pose a threat to persons. Hence, safety should also be evaluated for such structures located inside or in the vicinity of the installation area, e.g. security lighting and CCTV posts, pipes, rails and fences. Most commonly, high voltage installations such as substations, power plants and industrial facilities are protected by fences. The design of the grounding configuration for the metal fence of a high voltage installation is essential as the outside perimeter of the fence is accessible to the general public; this is even more so where the installation is located in a residential or urban area. Furthermore, it is well known that metal fences with large lengths could transfer high potentials to areas away from the grounding grid of the installation. This is common where the grid only covers small parts of the whole installation area, e.g. power plants and industrial facilities; in such cases measures against dangerous potentials transferred through the fence should be considered as well. Several papers have been published regarding the safety provided along fences against

potentials arising due to ground faults [3]-[5] as well as due to coupling effects [6]-[10]. According to the IEEE Std 80-2000 [1] and its recent revision [2], grounding of a metal fence which encloses an installation having energized electrical conductors or equipment can be achieved either by connecting the fence to the installation grounding grid or by utilizing a separate grounding configuration. If the latter design is selected, it must be ensured that the fence cannot be accidentally connected to the grounding grid, e.g. via metal pipes or cable sheaths. Otherwise, in case of a ground fault the fence will attain the ground potential rise of the grid and high touch and step voltages could appear along its length. In this paper, fence grounding techniques commonly used in high voltage installations are evaluated for a 150/20 kV air insulated substation, a 400 kV GIS step-up substation and a large scale photovoltaic power station. These installations differ in area, ground fault characteristics and distance between fence and grounding grid. The safety provided by several fence grounding techniques is assessed with the aid of grounding analysis software [11]. It is shown that a safe and cost-efficient fence grounding design depends on ground fault characteristics, soil conditions, installation area, distance between the fence and grounding grid as well as on the size and geometry of the latter. II. 150/20 kV AIR INSULATED SUBSTATION The 150/20 kV air insulated substation under study (Fig. 1) encompasses an area of approximately 11500 m2. The most dangerous ground fault in this substation is a 150 kV single phase ground fault with a symmetrical current, If, of 30 kA. The fault current division factor, Sf, is 0.8. The decrement factor, Df, is calculated 1.0313 for a fault duration, tf, of 0.5 s and a X/R ratio of 10. Hence, the maximum grid current, IG, is calculated 24.75 kA. Based on soil resistivity measurements, in accordance with [12] and [13], it was found that a uniform soil model with a resistivity of 87.1 ホゥm represents satisfactorily the actual soil conditions in the installation area. Table I shows the allowable touch, Etouch70, and step, Estep70, voltage limits for the evaluated substation calculated according to the IEEE Std [1], [2], as influenced by surface material thickness, hs, and resistivity, マ《. These limits, referring to a shock current duration of 0.5 s and body weight of 70 kg, are also retained for the area beyond the outer boundaries of the substation, which is located in a rural area.


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