Substation earthing: special considerations for GIs substationsTerry Irwin, J.Lopez-Roldan (VA Tech Reyrolle) 1. G I s in contrast to AIS. The use of compressed SF, as an insulating medium has led t o the development of compact gas-insulated substation (CIS) technology (figure 1). GIs, having many advantages over conventional air-insulated substations (AIS), have been receiving wide application. However, this alternate technology has inevitably lead t o a different set of problems t o resolve. in the case of substation earthing, we can discern three major aspects of the GIS substation design which need a different approach t o those used in AIS. 1. The use of a 10 times better insulating gas makes it possible t o design a much more compact substation. This also means a significant reduction in the grounded area of the substation. 2. This 'compact' design means the phase conductors are much closer than in AIS and with metal enclosures, for gas containment, electromagnetically induced currents appear in the earthing system. 3. Compressed SF6 gas insulation facilitates small dielectric clearances in the GIS. As a result breakdown occurs rapidly in the nanosecond range. The rapid collapse of voltage results in the generation of very fast travelling wave transients which propagate throughout the CIS. The coupling of these transients with the earthing system provokes a transient ground potential rise (TGPR). 2. Reduced ground area The area occupied by a GIS substation is typically only 10-25% of that of the equivalent air insulated installation. Normally, with an AIS, a single uninsulated copper loop laid around the perimeter of the site with cross connections t o pick up the individual items of equipment, will provide a sufficiently low resistance electrode. However, the smaller area occupied by a CIS means that the size of the main earth loop will be smaller and therefore the total amount of conducting path will also be smaller. The possible solutions t o reduce the earth electrode resistance are (1): 0 0 High density grid: frequent and short connections from the switchgear elements t o the earth grid. This reduces the TGPR in the GIS and contributes t o reduce the total earth electrode resistance, but not in direct proportion t o the additional length. Connection t o the reinforced concrete mat: connecting the reinforcing steel mesh and structural steel t o the earth grid will reduce the total earth electrode resistance. However this is complicated and it has t o be Authorized licensed use limited to: UNIVERSITY TEKNOLOGI MALAYSIA. Downloaded on February 26,2010 at 03:00:18 EST from IEEE Xplore. Restrictions apply. e. The results confirmed a high percentage of current circulating through the enclosure (in the range from 50 t o 85% of the 2000 A of the primary current). 4. steel supports. Fast Transients Overvoltages and TGPR At the beginning of the GIS technology. arcing between the grounded enclosures and other grounded components which are indicative of much higher potentials.5m diameter. earth shunt connections) etc. Measurements have been performed in a Reyrolle 420 kV substation using a portable current transformer (CT). The induced currents in the enclosure can reach 90% of the value of the primary busbar current and they circulate in opposite direction which reduces the total magnetic field outside thle enclosure. In contrast t o these relatively low potentials.5). Induced currents. the earth electrode resistance is s t i l l high.done in a way that avoids problems of overheating and damage of the reinforced structure due t o excessive circulating currents. Use of deep driven ground rods: If. e. Restrictions apply.4. Gas Insulated Substations have a grounded outer sheath enclosing the high voltage inner conductor. then the use of dee!p driven ground rods will be required. The accuracy of the measurement system was first checked in the laboratory which showed less than 5% error wich was considered t o be adequate for the proposed measurements. The Rogowski coil was wrapped around various earthing connections in the GIS. were routinely observed during breakdown in HV tests or Authorized licensed use limited to: UNIVERSITY TEKNOLOGI MALAYSIA. the grounding design was based in the classical approach of limiting the power frequency enclosure potentials t o safe levels based on the maximum expected fault-current conditions. This consisted of a 0. Depending on the current circulating through the bus-bars there will be a significant electromagnetic field surrounding the enclosures (figure 2). earth straps. ladders etc. after the above methods have been applied. It was also found that a high level of circulating current (up t o a 50%) was present in the inter-phase copper earth straps which shunt the individual phase enclosures. . Downloaded on February 26. unlike conventional equipment whose closest ground is the earth's surface. an integrator and a digital voltmeter.g. (2. inter-phase shunts. grounded chambers. At the same time the phase separation is muc:h smaller. Thle alternating variation of this magnetic field induces currents in the grounded enclosure and other metallic parts in the substation such as steel structures.2010 at 03:00:18 EST from IEEE Xplore. flexible Rogowski coil. inter-phase enclosure connections and ground connections (i. 3. 3. Because of the low operating speed of the disconnector.2010 at 03:00:18 EST from IEEE Xplore.7 m from Vs) : 172 kV (33% attenuation by T of bushin9). 2. Downloaded on February 26. the aidSF6 termination is modelled as a junction of three transmission lines each with i t s own surge impedance: 1. Authorized licensed use limited to: UNIVERSITY TEKNOLOGI MALAYSIA. the bus enclosure t o ground transmission line. One part of the incident wave is reflected back into the bus duct.5 Vs = 263 kV 0 Arrives a t bushing A (17. 0 TGPR a t bushing A: 45 kV (26% V input t o bushing) 0 Maximum TGPR measured in substation building: 3 kV Some of the reported experiences of TGPR in GIS are : 0 TGPR is of relatively high magnitude but lasts only microseconds with frequencies t o above 30MHz.e. each closing and opening operation will produce tens of pre-strikes and re-strikes (figure 3). Just t o give an idea of the order of magnitude of the TGPR these are the results of the measurements performed in a 525 kV GIS by Ontario-Hydro (6): Voltage across contacts during disconnector closing: 526 kV(Vs) 0 Voltage propagate through GIS bus : 0.during normal disconnector operation. Restrictions apply. the overhead line-to-ground transmission line. 0 Destruction of electronic components in secondary equipment and temporary measurement equipment used for commissioning etc. Inadvertent operation of protective devices. The reasons for this TGPR were the specific characteristics of the breakdown in SF6 producing overvoltages with a very fast rise time of 5-20 ns and a t the same time the specific coaxial arrangement of the bus-duct which happened to be very good for the transmission of these MHz range voltage surges. the internal coaxial GIS bus duct. another part continues through the overhead line-to-ground transmission line and the rest forms the TGPR a t the bushing-enclosure junction (6). 0 Personal safety : no injuries reported but possible dangerous reactions against the spark or tingling when working in the GIS .. Sparking in air. This TGPR is soon attenuated by the enclosure ground straps which act as other transmission lines. between earth straps in close proximity. When the travelling wave arrives a t the aidSF6 termination. . the vertical bushing and the overhead line define external surge impedances which allow the incident travelling wave t o "refract" out. between the grounded parts of the system i. Each pre-strike generates a fast transient wave of half the value of the voltage across the contacts which is transmitted in both directions away from the pre-strike point (3). In figure 4. An exhaustive research was done t o understand the mechanism of this particular TGPR in CIS. TSakakibara. straight and low inductance connections to the earth grid contribute t o reduce the TGPR. References: 1. ELECTRA No. 5. At these points the enclosures are separated bly an insulating spacer and the associated earth strap connections are often too long and too inductive for effective grounding of high frequency transient potentials. Vol 3. 2. The effects of the TGPR produced by fast transients overvoltages must also be considered and the associated impact on high frequency earthing techniques especially a t enclosure discontinuities. ' I IEEE Guide for Safety in A Substation Grounding" C 3. K. "Grounding of Gas Insulated Substations". July/August 1975. Miwa. 4.Dick. T. PAS-94. December 1993.Lopez-Roldan. ANSI / IEEE Std 80-1986. J. "Design. Simulation and Testing of an EHV Metal-Enclosed Disconnector". Consideration must be given t o induced currents which may cause overheating in the earthing system even under normal load flow conditions. Authorized licensed use limited to: UNIVERSITY TEKNOLOGI MALAYSIA. cable sealing end:s. . Dodds and H. N.Nurse.Yoshida. C. T. together with the classical approach of limiting the power frequency enclosure potentials t o safe levels based on the maximum expected fault-current conditions. "Earthing of GIs-An application guide". IEEE Transactions on Power Delivery. Boggs and G. "Transient Ground Potential Rise in Gas Insulated Substations-Experimental Studies". T. No 10. No4.lrwin.Ebden and J. J. it is also necessary t o be aware of the specific problems related to the GIS design. E. 520. Therefore significant voltages of several kV develop across the insulated flange which may cause sparking in the surrounding air. Vol. October 1982. IEEE Transactions on Power Apparatus and Systems.Spindle. Vol. such as metal oxide varistors (MOV) is highly advisable. No 4. "Distribution of induced grounding current in large-capacity GIS using multipoint grounding system". London 1999. Restrictions apply. 151.Short. then connecting the enclosure [via short earth straps] t o the building structure when crossing the walls will help t o attenuate the TGPR inside the substation. special care must be taken when dealing with discontinuities in the gas enclosure as encountered with an external CT.Fujimoto. PAS-101. 6.Hansson. S. October 1986. S. Finally. Summary When designing the grounding of a CIS.2010 at 03:00:18 EST from IEEE Xplore. 5. transformer connections etc. 6.Ford. PWRD-1. In this case the use of surge suppressers. Terasaka and I. IEEE Transactions on Power Apparatus and Systems. If the GIS is inside a building. Downloaded on February 26. Lewis. Eleventh International Symposium in High Voltage Engineering. N) PLAN SHEATH OF BUS Figure. Downloaded on February 26. Pre-strikes during a disconnector closing operation OVERHEAD LINE S R CONDUCTOR OF BUS *?-9z2%zwA~~ GR0T.2010 at 03:00:18 EST from IEEE Xplore.4. GIs double bus-bar section view CB: Circuit Breaker D: Disconnectors ME: Metal Enclosure BB: BusBars CT: Current Transfomers VT: Voltage Transformers S: Steel structures Figure 2. Transmission line model of the TGPR in the air/SF6termination (6) Authorized licensed use limited to: UNIVERSITY TEKNOLOGI MALAYSIA. . Magnetic flux density distribution around the three phase enclosures in a GIS bus-duct Figure 3. Restrictions apply.Figurel.