A GIS carries the same ground fault current as a conventional substation and requires the same low grounding impedance, but occupies 10 % to 25 % of the land. Its metal enclosures carry induced currents of significant magnitude that must be confined to defined paths. For that reason the grounding is designed together with the manufacturer, and the contact between enclosures is also reviewed.
Short answer
A gas-insulated substation (GIS) must have the same low grounding impedance as a conventional one, because it carries the same magnitude of ground fault current. But it has three particularities: it occupies only 10 % to 25 % of the land of a conventional substation, its metal enclosures carry induced currents that must follow defined paths, and switching operations and internal faults generate very high frequency transients that reach the ground. IEEE Std 80-2013 devotes clause 10 to this and recommends joint work between the manufacturer and the user. This guide does not repeat what a GIS is: for that see GIS substation.
What changes compared with a conventional substation
- Less area. With a surface of 10 % to 25 % of that of a conventional substation, it can be hard to achieve a good ground with conventional methods alone.
- Enclosures carrying current. The metal enclosures of the GIS carry induced currents of significant magnitude, which must be confined to specific paths. For that reason special attention is paid to the bonding of the enclosures, and the manufacturer’s recommendations are usually the ones that govern.
- Very high frequency transients. A breakdown of the gas insulation, between the contacts of a switching device or during a fault, can generate very fast transients that couple into the grid. The standard states that, in general, they must be considered in the complete grounding design. They cause ground potential rises of large magnitude and very short duration and are a source of electromagnetic interference.
Enclosures and circulating currents
The effectiveness of the enclosure shielding is determined by its impedance, which governs the circulation of the induced currents. There are two designs:
| Design | How it works | Status |
|---|---|---|
| Continuous enclosure | The sections of one phase are bonded together, and the three phases are connected to each other with short connections at both ends. The enclosure current almost equals that of the inner conductor, in the opposite direction, and cancels much of the external magnetic field | The usual one |
| Non-continuous enclosure | The sections are insulated from each other; there is no external return, so unequal voltages and irregular local currents appear | Considered less advantageous and not used by the industry today |
In the continuous design, under asymmetrical faults, the dc component is not shielded and produces an external voltage drop. The general design procedure is in substation grounding design with IEEE 80. The voltages that arrive outside the enclosure are covered in transferred voltage and transferred potentials.
To limit the effects of circulating currents, the standard requires that:
- all enclosures normally operate at ground voltage level;
- there be no significant voltage differences between enclosure sections, and neither the support structures nor the grounding be affected by the flow of current;
- the power cable shields be connected to ground by connections separate from the enclosures, with an air gap or insulating elements at the cable terminations; those elements can discharge with the very fast transients and the design must consider the consequences;
- the enclosure return current not flow through current transformers mounted on them.
Frequent bonding and frequent grounding of the enclosures is the best solution to limit hazardous touch and step voltages within the GIS area. As an additional measure, conductive platforms, or ground mats, bonded to the GIS structures and grounded are used.
Connection to the grid: manufacturer and user
Normally the manufacturer defines what the main ground bus is and what the user must do to connect it to the substation ground. Enough documentation is needed so that no connection between the bus and the grid interferes with the enclosure current path.
| Who | What they provide |
|---|---|
| Manufacturer | Bonds between subassemblies to have safe gradients; accessible connectors, resistant to electromagnetic forces and able to carry the maximum fault current without overheating; pads or connectors with at least two paths to ground; procedures for joining dissimilar metals, for example copper with aluminum enclosures |
| User | Fault current sources, expected magnitudes and durations; joint review of all connections: neutrals, discharges, external arresters, relays, structures, cable shields and safe step and touch voltages outside |
Care is also needed that excessive currents are not induced in neighboring structures or reinforcing steel, and that no current loops form through other equipment, such as transformers or conventional switchgear. Special care is needed where the enclosure grounding is interrupted: at the connection with the transformer and at the interface with conventional equipment. The connection with cables and transformers is explained in GIS interfaces with cables and transformers.
Foundation
A monolithic reinforced concrete slab helps as an auxiliary grounding device. If the floor is continuous, it is advisable to bond its steel mesh to the main ground bus, so that the enclosures and the structure steel are at approximately the same potential. Since the rebars are much closer to each other than the conductors of a typical grid, the potential at the floor surface is more uniform. The foundation can act as an auxiliary electrode if the current discharge does not damage the concrete.
Touch criterion in the GIS
The manufacturer usually designs the equipment and does the calculations of enclosure voltages and currents in a fault, but the user must verify that the complete installation is safe. The standard points out a paradox: a large ground connection is not always the best, because the circulating currents of the enclosures must also be taken into account, knowing where they will flow and to what extent it is preferable for them to do so. There are no single answers on how much current circulating through the ground connections is too much, and for that reason some manufacturers deliver a main ground bus and others only mark connection points.
For the check, the standard gives these recommendations:
- In an internal fault with return through earth, the resistive and inductive voltage drop adds to the soil touch voltage. This is generally not needed for faults external to the GIS.
- In an external line-to-ground fault, the voltages induced on the enclosure are reviewed for a hand-to-hand contact between metals; the step and touch voltages at the soil surface are calculated the same as in a conventional installation.
- For the voltages induced by external faults, analyzing the nearby fault is enough, because remote ones cause fewer problems.
How we do it at Tiza Group
The grounding design of a GIS requires the manufacturer’s data, the fault current and the soil model. At Tiza Group we develop it together with the manufacturer and the user, within the design of high and extra-high voltage substations. Learn about our engineering design and studies service or write to us. We deliver it remotely, worldwide.
Related guides
- IEEE 80: Safety in Substation Grounding
- GIS Substation: SF6, Testing and Common Failures
- GIS Cable and Transformer Interfaces (IEC 62271)
- Grounding Grid Materials and Connections
- Substation Grounding Grid: Design and Testing
References
- IEEE Std 80-2013, IEEE Guide for Safety in AC Substation Grounding, clauses 10.1 to 10.9.
Frequently asked questions
Why is GIS grounding different from that of a conventional substation?
Because the metal enclosures carry significant induced currents, which must be confined to specific paths, and because a GIS occupies 10 % to 25 % of the area of a conventional substation, which makes it hard to obtain a good ground with conventional methods alone. In addition, breakdown of the gas insulation generates very high frequency transients that couple into the grid.
Who defines what the main ground bus of a GIS is?
Normally the manufacturer, who also specifies what the user must do to connect the GIS to the substation grid. The user provides the fault sources, their magnitudes and durations, and helps review the planned connections.
Can the foundation of a GIS be used as part of the grounding?
Yes, as an auxiliary electrode, provided the current discharge does not damage the concrete through local heating or gradual erosion of the bonds between concrete and steel. Connecting the floor reinforcing steel to the ground bus leaves the enclosures and the foundation steel at approximately the same potential.