Compressed SF6 gas used in MV and HV switchgears as an insulating medium has led to the development of compact gas-insulated substation (CIS) technology (figure 1). GIS  , having many advantages over often used and conventional air-insulated substations (AIS) , have been receiving wide application.However, this alternate technology has inevitably lead to a different set of problems to resolve. in the case of substation earthing, we can discern three major aspects of the GIS substation design which need a different approach to those used in AIS.

1. The use of a 10 times better insulating gas makes it possible to 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).

Figure 1 - GIS double bus-bar section view

Explanation:

CB: Circuit BreakerD: DisconnectorsME: Metal EnclosureBB: BusBarsCT: Current TransfomersVT: Voltage TransformersS: Steel structures

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Reduced ground areaThe area occupied by a GIS substation is typically only 10-25% of that of the equivalent air insulated instal lation.

Normally, with an AIS, a single uninsulated copper loop laid around the perimeter of the site with cross connections to 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 to reduce the earth electrode resistance are (1): High density grid: frequent and short connections from the switchgear

elements to the earth grid. This reduces the TGPR in the GIS and contributes to reduce the total earth electrode resistance, but not in direct proportion to the additional length.

Connection to the reinforced concrete mat: connecting the reinforcing steel mesh and structural steel to the earth grid will reduce the total earth electrode resistance. However this is complicated and it has to be done in a way that avoids problems of overheating and damage of the reinforced structure due to excessive circulating currents.

Use of deep driven ground rods: If, after the above methods have been applied, the earth electrode resistance is still high, then the use of dee!p driven ground rods will be required.

Induced currentsGas Insulated Substations have a grounded outer sheath enclosing the high voltage inner conductor, unlike conventional equipment whose closest ground is the earth’s surface. At the same time the phase separation is muc:h smaller.

Depending on the current circulating through the bus-bars there will be a significant electromagnetic field surrounding the enclosures (figure 2). Thle alternating variation of this magnetic field induces currents in the grounded enclosure and other metallic parts in the substation such as steel structures, inter-phase enclosure connections and ground connections (i.e. earth shunt connections) etc. (2,4,5).

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.

Figure 2 - Magnetic flux density distribution around the three phase enclosures in a GIS bus-duct

Measurements have been performed in a Reyrolle 420 kV substation using a portable current transformer (CT). This consisted of a 0.5m diameter, flexible Rogowski coil, an integrator and a digital voltmeter. The accuracy of the measurement system was first checked in the laboratory which showed less than 5% error wich was considered to be adequate for the proposed measurements.

The Rogowski coil was wrapped around various earthing connections in the GIS, e.g. grounded chambers, earth straps, inter-phase shunts, steel supports, ladders etc. The results confirmed a high percentage of current circulating through the enclosure (in the range from 50 to 85% of the 2000 A of the primary current).

It was also found that a high level of circulating current (up to a 50%) was present in the inter-phase copper earth straps which shunt the individual phase enclosures.

Fast Transients Overvoltages and TGPR

At the beginning of the GIS technology, the grounding design was based in the classical approach of limiting the power frequency enclosure potentials to safe levels based on the maximum expected fault-current conditions.

In contrast to these relatively low potentials, arcing between the grounded enclosures and other grounded components which are indicative of much higher potentials, were routinely observed during breakdown in HV tests or during normal disconnector operation. An exhaustive research was done to understand the mechanism of this particular TGPR in CIS.