EMC Philosophy applied to Design the Grounding Systems for Gas Insulation Switchgear (GIS) Indoor Substation Marcos Telló Department of Electrical Engineering Pontifical Catholic University of Rio Grande do Sul - PUCRS Porto Alegre, Brazil tello@ee.pucrs.br Guilherme A. D. Dias, Daniel S. Gazzana, Roberto C. Leborgne Department of Electrical Engineering Federal University of Rio Grande do Sul - UFRGS Porto Alegre, Brazil gaddias@terra.com.br, dgazzana@ece.ufrgs.br, rcl@ece.ufrgs.br Arturo S. Bretas, Department of Electrical and Computer Engineering University of Florida, Gainesville, USA arturo@ece.ufl.edu Abstract This paper presents the EMC (Electromagnetic Compatibility) philosophy applied to design the grounding systems of GIS Insulated Substation - GIS indoor substation. The grounding systems are characterized by the following subsystems: Main Grounding grid (60 Hz Grounding System), Equipotential Grounding grid (HF Grounding system) and Lightning Grounding system. The real case measurements for 100 MVA, 69/13.8 kv GIS indoor distribution substation indicated that adopted EMC philosophy is appropriate because the maximum measured potential difference among the various grounding systems of GIS indoor substation was approximately zero volts and the maximum measured touch voltage was less than 1.4 volts. Keywords Electromagnetic Compatibility, GIS Grounding System, Indoor substation I. INTRODUCTION GIS indoor substations are usually installed in urban sites because the required area of GIS is smaller than the area occupied by a conventional substation (air insulated substation - AIS). In addition, GIS presents reduced environment impact, increases reliability and increases the safety of workers and people walking near a substation during a ground fault in the power system. GIS manufacturers have their own recommendations on how the equipment must be grounded. Generally, the design of GIS grounding system is characterized by the existence of two systems: a system responsible for carrying short circuit currents (60 Hz or Main Grounding system) and a system related to the existence of very fast transients occurring due to the switching operation of GIS, whose frequencies are of the order of MHz (Equipotential or HF High frequency Grounding system). In addition, there is the lightning protection system, which is usually connected along the steel rebars in the concrete used to dissipate the high frequency currents from lightning (Lightning Grounding system). It is important to state that the Equipotential Grounding system and Lightning Grounding system do not need to be designed to support the fault current. The very high frequencies from the GIS switching can produce high potential differences on grounding system wires, which can produce dangerous rises in the grounding potential. In addition, the radiated electromagnetic field created by the GIS switching can lead to increase transient overvoltage in secondary circuits of GIS. Therefore, the design of Equipotential Grounding system requires a low-inductance of grounding system for GIS equipment. In this context, the paper describes the design of the grounding systems for GIS indoor substations. II. DESIGN OF THE GIS INDOOR GROUNDING SYSTEM The design of GIS indoor grounding systems can be divided into three parts: a) Main Grounding system (also called 60 Hz Grounding system), b) Equipotential Grounding system (also called HF Grounding system) and c) Lightning Grounding system. Fig. 1 shows the adopted philosophy of GIS indoor substation grounding systems. Fig. 1. Philosophy of GIS indoor substation grounding systems. Considering Fig. 1, the substation building has three floors. For example, the Cables Room is on the ground floor. The 978-1-4799-7993-6/15/$31.00 2015 IEEE
GIS Room and Power Transformer Room are located on the first floor and the HV Cubicles Room, Relay Room and Control Room are on the second floor. The Main Grounding system (60 Hz Grounding system) of the substation is placed in the soil and is made of cooper. However, considering that on all floors of the building there are devices that dissipate short circuit currents, the conductors of the Main Grounding system are embedded in the concrete floor throughout the building floors. In many cases the copper conductors of the Main Grounding grid go down along the sides of the columns of the substation building and in other situations the copper conductors go down inside of the building columns. Fig. 2 shows an example of the Main Grounding system of an indoor substation. The Lightning Protection System is usually embedded in the structural concrete that characterizes the columns of substation buildings and is made of galvanized steel. This system is responsible for dissipating the lightning currents. Fig.4. shows the Lightning Protection system and a respective connection with the conductors of the Main Grounding system. Fig. 2. Main Grounding System (60Hz Grounding system). The Equipotential Grounding system is made of galvanized steel meshes in a grid configuration of 10 x 10 cm 2 (see Fig. 2) and is embedded under the concrete floor. The connection of the Equipotential Grounding system between the floors of the building is made of galvanized steel (steel rebars) in the concrete. The wires of the Equipotential Grounding system are welded to form an electrically conductive network. The purpose of the Equipotential Grounding grid is to reduce the effects of very fast transients and to control the potential on the floors (touch and step voltages) due to ground fault. Fig. 3 shows the Equipotential Grounding system (HF Grounding system). Fig. 4. Lightning Grounding system of indoor substation. Each grounding system (Main, Equipotential and Lightning Grounding System) is directly connected to the Main Grounding grid installed into the soil in several points. To avoid the corrosion of the steel conductors, the connection of the Main Grounding system is made of cooper conductors. III. CASE STUDY: INDOOR SUBSTATION This section describes the main stages of a grounding system project in an indoor substation. The general concept adopted is characterized by the fact that the grounding system must limit the effect of gradient potential under normal and fault conditions. From this point of view most GIS manufacturers consider that the equipment is adequately grounded when the potential difference between metal parts of GIS and other metallic structures does not exceed 65-130 V during the occurrence of fault [1]. This voltage interval corresponds to fault times ranging from 0.8 s to 3.2 s if a 50 kg criterion is used, and ranging from 1.46 s to 5.8 s considering a 70 kg body. Fig. 5 shows the contact potential (touch voltage) limits for metal-to-metal contact as function of time. Fig. 3. Equipotential Grounding system of indoor substation. Fig. 5. Touch voltage limits for metal-to-metal contact [1].
After establishing the general concept, it is possible to start the grounding grid design. A. Main Grounding grid The design of the Main Grounding grid for GIS indoor substation is the same as for AIS. However, it is important to point out that the Main Grounding grid is a reference to the potentials which will arise inside the building. In other words, considering that the Equipotential Grounding system is connected to the Main Grounding system, the maximum gradient potentials under normal and fault conditions inside the building are controlled by the Equipotential Grounding system. Therefore when the fault current is dissipated by the Main Grounding grid, the expected voltage difference between such grid and the Equipotential Grounding grid is small, ensuring safety inside the building. Fig. 6 shows the layout of the Main Grounding system and the correspondent Equipotential contours, respectively. 03 copper bars 120 mm 2 grounding GIS and HV cubicles embedded in concrete surface do (~ 5 10 cm) bonded with main grounding grid (buried in soil) is extremities Green area with floor mat 10 cm x 10 cm embedded in concrete surface (~ 5 10 cm) Fig. 6. Main Grounding grid layout and Equipotential contours, respectively. The methodology used to design the Main Grounding grid is described in [2]. For the specific example of Fig.6, the total line-to-ground fault current is 24.54 ka and the resulting current that flows through the grounding grid is 1.76 ka. B. Equipotential Grounding Grid According to [3] the occurrence of very fast transients is typical in GIS. The high frequencies of such transitories propagate as electromagnetic waves within the GIS and the return current flows into the enclosure. In the transition points from the GIS to HV cables, the current flow is interrupted and at these points the very fast transients are induced into the grounding system. As a consequence, high potential differences occur on earth wires of short lengths which can produce dangerous rises in the earth potential. Therefore, in addition to the Main Grounding grid (Grounding in 60Hz), it is necessary to use the Equipotential Grounding grid to control the effects of very fast transients that occur when the switching operations of the GIS happen. The very fast transients may cause high frequency disturbances, which can be induced into the grounding system. The installation of metal grid grounding system (Equipotential grounding grid) minimizes the impedance of the grounding system and provides an area of Equipotential surface. Fig. 7 shows an overview of the Equipotential grounding grid installed into the concrete floor. Fig. 7. Equipotential Grounding grid (size 10x10 cm 2 ). Fig. 7 shows galvanized steel reinforcing meshes with mesh size of 10cm x 10cm, used as Equipotential Grounding grid (HF Grounding grid). The wires of the Equipotential Grounding grid are welded to form an electrically conductive network. Ideally the Equipotential Grounding system would be a flat metal plate, but this type of grounding is rather expensive. A grid with thinner wires and smaller meshes is less effective than the flat plate, but is less expensive. The question that arises is: how to compare the performance of a flat plate and a metal grid? Applying the methodology of [4] it is possible to compare the mesh grid impedance to the metal plate impedance. Fig. 8 shows the ratio of the Zgrid over the Zplate for low frequency obtained in a ground plate (base value) under similar conditions. Fig. 8. Ratio Zgrid/Zplate for low frequency. Fig. 8 shows that the grid with mesh size of 10cm x 10cm is the configuration that best approaches the performance of
the plate (Zplate is the base value of the pu system). It is important to state that the Equipotential Grounding grid is also responsible for controlling the potential occurring on different floors (touch and step voltages) due to ground fault. C. Lightning Grounding Grid According to [5] the behavior of grounding systems under lightning discharge conditions governs the degree of protection provided by the grounding systems. There are many models that can be used to analyze the grounding grids response under lightning discharge. The effective area (or effective radius) of the grid is an important parameter to keep in mind when designing an optimum grounding system. The concept of effective area of grid indicates that the impulse impedance dissipates along the grid area before reaching the other end. Therefore, only a part of the grid is effective to scatter the lightning discharge current. Using the analytical methodology described in [5], it is possible to determine the value of the impulse impedance and, in consequence, the effective area of the grounding grid. The effective area is characterized by an equivalent circle, which has a radius called the effective radius. According to [5], the extremity of the effective area is considered up to the point where the grounding impedance at the injected point decreases to a value below 3% of the final value of impulse impedance. The final value of impulse impedance means that an increase in the area of the ground grid does not alter significantly the value of the impulse impedance. On the other hand [6] indicates that the effective area is achieved when the grounding impedance value is equal to the low frequency resistance. The concept of effective area is based on grounding grid impedance at the injected point. Considering the grounding grid shown in Fig. 6, the ground conductors are installed in a soil where the surface layer has a resistivity ρ 1 equal 500 Ωm and the second layer presents a resistivity ρ 2 of 5,000 Ωm. The depth of the first layer of soil is 8.6 m. The conductors of the grounding grid are laid under the substation building with depth of 0.5 m from ground level. Applying the methodology described in [5], Fig. 9 shows the grid impulse impedance and effective radius considering lightning discharge with several times (0-10 µs) wavefronts. The lightning discharge is applied in the center and in the corner of the grounding grid. radius is greater than the radius of the grid, the whole grid is effective in dissipating the lightning current. Otherwise, if the effective radius is smaller than the radius of the grounding grid, then only a part of the grid is effective in dissipating the lightning current. Based on numerical simulations [7], Fig. 10 presents the potential on the soil surface generated by a lightning reaching the grounding system. In the begin of the transitory the potential reaches high values (Fig. 10 left). After 100μs the voltage on the surface is approximately Zero Volts (Fig. 10 right) leads the system to the steady state. Fig. 10. Potential on soil surface generated by lightning discharge. Additionally, applying the methodology presented in [7]- [8], the designed grounding systems is safety for human beings, considering the expected lightning discharge for the region where the substation is located. IV. MEASUREMENTS TO EVALUATE THE GROUNDING SYSTEMS The Fall of Potential Method (FOP) was applied to evaluate the performance of the grounding systems. Fig. 11 shows the FOP method arrangement. The OMICRON model CPC100 was used as DC current source. Fig. 11. FOP Method arrangement to measure the GIS indoor substation. Fig. 12 shows the GIS room. In this figure the points P36 to P40 are some examples of points where the voltage difference between two points, one on the floor and other in equipment (touch voltage), were measured. The maximum touch voltage measured inside the indoor substation was 1.4 Volts. The measurements between the main ground grid and equipotential grounding grid were evaluated and the voltage difference between these grounding systems is approximately zero volts. (a) Fig. 9. (a) Impulse Impedance and (b) Effective Radius. The radius of circle having an area equal to grounding grid area is 13.56m (radius of the grid). Thus, when the effective (b)
The Equipotential Grounding system has two functions: to reduce the effects of very fast transients and to control the potential on floors (touch and step voltages) due to ground fault. The actual practice is to design the Main Grounding grid considering the short circuit current. It is important to have in mind that after the Main Grounding grid has been designed it is necessary to evaluate the impulse impedance. By knowing the impulse impedance, it is possible to evaluate the degree of lightning protection using the value of the impulse impedance to estimate the effective area of the grid. The measurements carried out in GIS indoor substations indicate that the potential difference between metal parts of the GIS and other metallic structures are in accordance with the criteria established by the GIS manufacturers (65-130V), and the human body safe limits of potential differences inside the building under fault condition are satisfied. Fig.12.Touch voltage measurement in GIS room. V. CONCLUSIONS The design of the grounding systems for GIS indoor substation is characterized by the existence of three grounding grids: the Main Grounding system (60Hz), the Equipotential Grounding system (HF) and the Lightning Grounding system. Therefore all electrical installation is indoor and considering that all building has it structure in steel, there is an intrinsic connection between the various grounding systems. The Equipotential Grounding grid and the Lightning Grounding grid are connected to the Main Grounding grid, which is installed into the soil according to Fig. 1. The Main Grounding system must dissipate the power frequency short circuit current and the lightning current. In our particular case the Main Grounding system works as a reference for the potentials inside the building, which means: during a fault the potential of the Main Grounding grid and the Equipotential Grounding grid grow in relation to the remote earth, but the difference potential between these grids is not significant. In addition, the touch and step voltages within a mesh of the Equipotential Grounding grid is smaller than the tolerable touch voltage. Measurements carried out in one particular GIS indoor substation indicate that the maximum touch voltage measured was 1.4 Volts and the potential difference among the various grounding systems was approximately Zero Volts. ACKNOWLEDGMENT The authors would like to thank CAPES, Ministry of Education of Brazil and CEEE-D Utility for the facilities offered during the development of this work. REFERENCES [1] IEEE Guide for safety in AC Substation Grounding IEEE Std. 80-2000. [2] Heppe, R. J., Computation of Potential at Surface Above an Energized Grid or Other Electrode. IEEE Transactions on Power Apparatus and Systems, Vol. PAS-98, No. 6, pp. 1978-1989, November 1979. [3] SF6 Switchgear, type F35-72.5kV, F35-145kV, F35-170kV- Recommendation for earthing and shielding of primary switchgear ALSTOM. [4] Gupta, B. R., Singh, V. K., Impulse Impedance of Rectangular Grounding Grid. IEEE Transactions on Power delivery, No. 1, pp. 214-217, January 1992. [5] Hyltén-Cavallius, N. R., Giao, T. N., Floor Net Used as ground Return in High-Voltage Test Areas. IEEE Transactions on Power Apparatus Systems, Vol. PAS-88, No. 7, pp. 996-1004, July 1969. [6] Grcev, L., Lightning Surge Efficiency of Grounding Grids. IEEE Transactions on Power delivery, Vol. 26, pp. 1692-1699, February 2011. [7] D. S. Gazzana, A. S. Bretas, G. A. D. Dias, M. Telló, D. W. P. Thomas, C. Christopoulos, The Transmission line modeling method to represent the soil ionization phenomenon in grounding systems, IEEE Transactions on Magnetics, vol. 50, n. 2, p. 505-508, February, 2014. [8] D. S. Gazzana, A. S. Bretas, G. A. D. Dias, M. Telló, D. W. P. Thomas, C. Christopoulos, A Study of Human Safety Against Lightning Considering the Grounding System and the Evaluation of the Associated Parameters, Electric Power Systems Research, vol. 113, p. 88-94, August, 2014.