Generally GIS requires no or very little maintenance and monitoring the SF6 gas pressure and quality is considered sufficient.
For maintenance of the GIS, regular inspections, Routine scheduled maintenance and overhaul maintenance are specified by the manufactures.
The maintenance to be carried out and their periodicity is indicated in the “Maintenance Schedule”.
Manufacturer’s instructions are to be followed for special tests, if any, for that particular make of GIS substation.
In GIS substation some of the equipment like Bushings, Surge Arresters, Transformers shall be provided outside the GIS area. Condition monitoring of these equipment is to be carried out as followed for AIS substation equipment.
Typical Maintenance processes GIS manufacturers provide end users with suggested maintenance plans. These plans can insignificantly differ between manufacturers but the basic principles are as follows:
Visual Verification
On a frequent basis (few times a year), it is suggested to complete a visual inspection of all GIS devices. The equipment does not require de-energization. The objective of this inspection is to verify that there is no sign of unexpected wear or equipment disoperation. Common operations completed during this inspection are:
Examine compressor run times and adequate operation for pneumatic systems. In the case of spring operators conduct a visual inspection for any defects.
Verify oil pressure and tightness.
Note down switching equipment operations using the operation counters.
Record and verify SF6 density using meters or installed probes.
Verify adequate functioning of low voltages devices.
Minor Verification
This verification can be completed every 5–10 years on GIS devices but the verification can also depend on a number of operations of switching elements. The objective is to verify the adequate operation of all switching elements. For this, the corresponding equipment has to be de-energized.
Laboratory assessment of the gas may assist in identifying unusual wear, insulator defects or other problems due to arcing or partial discharge and can be repaired before it degenerates to an unexpected major fault. This maintenance procedure does not demand opening gas chambers. Common operations completed during this inspection are:
1.Verification of SF6 by-product and impurity content (SO2 and moisture, in situations when chambers are not equipped with absorbers)
2.Verification of SF6 pressures (density)
3.Find any SF6 leakages (in case of alarms since the last verification)
4.Verification of SF6 gas purity
5.Verify proper operation of pressure switches, in the case of hydraulic mechanism use
6.Verification of SF6 density relay operations
7.Verification of control and alarm functions
8.Verify the correct alignment and operation of position indicators
9.Note down and verify circuit breakers operating times (from auxiliary switches) Exercise the circuit breakers and switching elements
Major Verification
This verification can be completed every 15–20 years but it strongly depends on the number of operations of switching equipment. Typically, major verifications are more condition-based than time-based maintenance. Opening of some chambers may be needed during such verifications. In addition to the tasks completed during minor verifications, the common operations completed during major inspections are:
1.Lubrication of different linkages and drives
2.Replacement of gaskets and absorbers when chambers are opened.
3.Record and verification of travel curves for circuit breakers.
4.Opening and verification of the switching elements if they have reached the limits suggested by the GIS manufacturers.
5.Overhaul of the hydraulic mechanism with oil, filter, and switches replacement plus maintenance on the rams and drive mechanisms. Inspection of the circuit breaker interrupter mechanism including nozzles and contacts.
Repairing SF6 Gas Leakage
SF6 gas leakage is an important concern on numerous levels including the environmental effects, degradation of the GIS insulation system integrity and gas cost. Releases of SF6 are also becoming reportable incidents in some countries due to the atmospheric greenhouse gas effects.
In GIS installations most leaks are discovered during the initial assembly and are related to flange mis-alignment, pinched O-rings or gaskets and dirty or corroded surfaces. Leaks are also assigned to wrongly installed by-pass piping, loose flange nuts, poor gas density gauge mounting, and similar instrumentation adjustments.
Safety practices during maintenance of GIS substation
1) The installation and commissioning should be carried out by qualified and authorized personnel.
2) Define and discuss in advance the maintenance to be performed and the relative hazards.
3) Use parts only supplied by Original Equipment Manufacturer (OEM).
4) Treat everything or each equipment is live
5) It is essential to make sure that the equipment is to be earthed on all sides of work zones.
6) Check that the personnel operating the apparatus have instruction manual with them.
7) Dis-connector switch can be operated only when VCB is off.
8) At a time, Dis-connector switch can be operated either electrically or mechanically but not simultaneously.
9) Use of overalls, jumpers and coats having metal buttons, metal strap and similar metal fittings should be avoided.
10) Do not wear suspenders and arm bands with metal buckles or other metal parts. These might come in close proximity to live parts and cause serious, if not fatal, injury.
11) Think carefully before you act:
i) Make sure you are right, watch out for the other man and make sure he is right.
ii) Never speak to any person working upon live mains or apparatus, doing the work is aware of your presence.
12) Dangerous Areas: When working in areas which contain or may contain live mains and danger notice plates, barriers, rails or other guarding arrangement for the working area. Do not store materials within high voltage enclosures or low voltage areas.
13) The internal parts need cleaning during periodic maintenance under a clean and dry environment.
14) Working in Damp Situations: Extra precautions should be taken when working in abnormally damp area.
Causes of fire in GIS substation
1) Particle or moisture contamination inside the compartment causes flash overs.
2) A leak in gas-insulated equipment means there is less gas to protect it from the effects of electrical arcs. The results are lost signals, short circuits, malfunctions, and, ultimately equipment failure that compromise safety and operations.
3) The SF6 can be easily exploded into different decomposition gas products when subjected to electrical discharge, such as electric arc, spark.
4) Negligence of maintenance causes minor faults to develop and causing fire.
5) Short circuit and too much overloading, which causes heating and ultimately fire.
6) Protective devices not in good working condition, fail to detect fault, causing fire.
7) Failure of protective relaying circuit causes faults to persist and ultimately fire.
8) Unskilled persons employed for sensitive circuits cannot maintain it properly and so faults can be developed ultimately resulting fire.
Application of High-Speed Earthing Switch in Gas Insulated Substation (GIS)
- The high speed ground switches can be furnished for single pole or group operated applications to provide grounding for inspection, maintenance, repair or replacement of other substation equipment such as transformers, capacitor banks, circuit breakers, circuit switchers, etc.
- High speed earthing switch have the additional capability of closing an energized conductor, creating a short circuit without receiving significant damage to the switch or the enclosure.
- High speed earthing switches are used to ground various active elements of the substation, such as transmission lines, transformer banks and main bus etc.
- In GIS facilities high speed earth switches are used to initiate protective relay functions. They are typically, not used to ground circuit breakers or voltage transformers.
- High speed earthing switches are designed & tested to interrupt electrostatically induced capacitive currents & electromagnetically induced inductive currents occurring in de-energized transmission lines in parallel / close proximity to energized transmission lines.
- High speed earthing switches can also remove DC trapped charges on a transmission line.
Advantages of GIS Substation
1. It occupies very less space (1/10th) compared to ordinary substations. Hence gas insulated substations are most preferred where area for substation is small. (e.g.: In crowded cities).
2. It is very safe as the earthed metal enclosure makes for a safe working environment for the attending personnel.
3. Most reliable compared to air insulated substations, because the number of outages due to the faults are very less.
4. Initial high investment is required for installation but the cost can be comparable for the less maintenance, reliable and safe operation against conventional substations hence economical.
5. An extremely careful selection of materials, an expedient design and a high standard of manufacturing quality assures long service life with practically no maintenance requirements.
6. Low weight due to aluminum enclosure, leads to low cost foundations and buildings.
7. Can be assembled at the shop and modules can be commissioned in the plant easily.
8. As gas insulated substations are enclosed type of substations very Lesly impacted by pollution, outside disturbances etc.
9. The over voltages while closing and opening line, cables motors capacitors etc. are low hence less impacted by such ON and OFF operations.
10. As in gas insulated substations, SF6 gas is used in the circuit-breaker unit for arc quenching. This type of breaker can interrupt current without over-voltages and with minimum acing time. Contacts have long life and the breaker is maintenance free.
11. As gas pressure (4 kg/cm2) is relatively low so not poses serious leakage problems.