Preventive maintenance of substation equipment and transmission lines

Gas insulated substation

The main advantage of Gas Insulated Substation (GIS) is its high reliability and also compactness which has direct influence on land requirement, land cost, environmental considerations, etc. The initial equipment cost of GIS is usually higher than that of conventional Air Insulated Substation (AIS).

The advantage and life cycle cost analysis are generally considered before deciding for GIS. The land area required for a GIS substation is in the order of 10% to 20% of that for an AIS substation considering the switchgear bay.

The saving in overall land area depends very much on the specific voltage level and the connection to transformers, reactors and incoming and outgoing lines. If the substation is connected to overhead lines, then space will have to be allocated for towers and droppers which might reduce the total land saving.

GIS also is considered for severe environment conditions, where saline pollution near coastal areas or industrial pollution requiring regular cleaning of insulators and corrosion of metallic components and electrical joints. GIS being totally enclosed units shall be immune to these severities.

GIS is also adopted when substation is to be installed at very high altitudes or very low temperatures or seismic considerations and hydro stations.

Maintenance of GIS

Before taking up the maintenance of GIS, recommended safety rules from the manufacturer are required to be adhered to. Some of them are listed below but, it is recommended to integrate with recommendations of manufacturer of GIS.

Recommendation #1

The maintenance programme and time based intervals specified/no. of operations whichever is earlier to form the basis of maintenance.

Recommendation #2

Whenever maintenance is taken up, it is essential to:

1. Employ the authorized personal
2. Define and discuss in advance the maintenance to be performed and the relative hazards. Proper formatted record sheets to be prepared.
3. Use parts only supplied by Original Equipment Manufacturers (OEM).
4. It is necessary to identify the equipment which is required to be maintained. Ensure that it is in deenergized/degassed condition.
5. It is essential to make sure that the equipment is earthed on all sides of the work-zone.
6. The work-zone should be barricaded and operator should have necessary protective clothing and recommended safety devices.
7. It is required to be ensured that necessary maintenance equipment such as slings, platforms, scaffoldings and electrical equipments/tools are in proper shape.

Conditions Monitoring of GIS

Generally GIS requires no or very little maintenance and monitoring the SF6 gas pressure and quality is considered sufficient.

Manufacturer’s instructions are to be followed for special tests, if any, for that particular make of GIS substation.

SF6 Gas

As SF6 gas is used in all chambers of GIS the monitoring of pressure and quality is of the importance. As per IEC 62271-203 the leakage rate from any single compartment of GIS to atmosphere and between compartments shall not exceed 0.5% per year for the service life of the equipment.

The pressure inside a GIS may vary from the rated filling pressure level due to different service conditions. Pressure increase due to temperature and leakage between compartments may impose additional mechanical stresses.

Pressure decrease due to leakage may reduce he insulation properties.