Gas-insulated substations (GIS) are electrical substations where high-voltage electrical equipment is enclosed in a sealed environment filled with insulating gas, such as sulphur hexafluoride (SF6), to minimize size and enhance safety.
Need of GIS- Gas Insulated Substation
- Space efficiency: GIS allow for compact installations in urban areas or locations with limited space availability.
- Enhanced safety: Encapsulation of high-voltage components reduces the risk of electrical hazards and protects against environmental factors.
- Reduced environmental impact: SF6 gas insulation minimizes greenhouse gas emissions and prevents atmospheric pollution.
- High reliability: Sealed enclosures prevent moisture ingress and ensure stable operation in harsh environmental conditions.
- Lower maintenance requirements: GIS systems require less maintenance compared to airinsulated substations due to reduced exposure to external elements.
- Increased security: Protection against vandalism and unauthorized access is improved with sealed enclosures and advanced monitoring systems.
Essential Parts of GIS
Busbar, Circuit Breaker, Disconnector (line or bus), Earthing switch (line or bus), Current transformer (feeder / bus), Voltage transformer (feeder / bus), Feeder Disconnector, Feeder Earthing switch , Lightning / Surge Arrester, Cable termination, Control Panel
Why higher cost of GIS compared to Conventional Substation?
Reason behind higher cost of GIS substation:
In the GIS system, all the live components are enclosed in a grounded metal enclosure, and then the whole system is housed in a chamber full of gas, which results in the increase in cost.
Whole substation is assembled in closed constructed building, which increases the cost.
Care should be taken that no dust particles enter into the live compartments which results in flashovers. So sealed compartments are required, which results in the increase in cost.
SF6 gas pressure must be monitored in each compartment, reduction in the pressure of the SF6 gas in any module results in flash overs. The monitoring system results in the increase in cost.
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 and drawback of GIS
Advantages of GIS substation
Space required is less i.e. 1/10th of conventional substation
GIS are very safe & operating personnel are protected by the earthed metal enclosure.
Most reliable compared to air insulated substation.
Weight is very less due to aluminium enclosure used.
No pollution.
Dielectric strength of SF6 Gas is 3 times the dielectric strength of air.
Time of erection is less as assembled parts are used.
Drawbacks of GIS Substation
High initial investment: Gas-insulated substation (GIS) involves significant capital expenditure due to the specialized equipment and technology required.
Limited space flexibility: GIS have less flexibility for expansion or modifications due to the compact design and fixed infrastructure.
Maintenance challenges: Maintenance of GIS equipment can be complex and costly due to the need for specialized personnel and equipment.
Environmental concerns: SF6 is a strong greenhouse gas with a high global warming potential. Leakage or improper handling of SF6 during installation, maintenance, or decommissioning can contribute to environmental pollution.
Transportation difficulties: Transporting GIS components to remote or inaccessible locations can be challenging due to their size, weight, and delicate nature.
Limited expertise: GIS technology requires specialized knowledge and expertise for design, installation, and maintenance.
Maintenance schedule of GIS substation
Visual Inspection Plan:
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 maintenance Plan:
Minor 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:
– Verification of SF6 by-product and impurity content (SO2 and moisture, in situations
when chambers are not equipped with absorbers)
– Verification of SF6 pressures (density)
– Find any SF6 leakages (in case of alarms since the last verification)
– Verification of SF6 gas purity
– Verify proper operation of pressure switches, in the case of hydraulic mechanism use
– Verification of SF6 density relay operations
– Verification of control and alarm functions
– Verify the correct alignment and operation of position indicators
– Note down and verify circuit breakers operating times (from auxiliary switches) Exercise
the circuit breakers and switching elements.
Major maintenance Plan:
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:
– Lubrication of different linkages and drives
– Replacement of gaskets and absorbers when chambers are opened
– Record and verification of travel curves for circuit breakers
– Opening and verification of the switching elements if they have reached the limits suggested by the GIS manufacturers
– 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.
– Overhaul of devices is required when it has reached its end-of-life. Typically, this is determined based on the suggestions and end user experience. Nevertheless, an overhaul operation asks for the expertise of the original equipment provider, while the other inspections can typically be completed by the user, provided that adequate training has been given by the GIS manufacturer. The conditions of the tools and devices used for maintenance, such as the gas-recovery cart, have also to be carefully verified.
Causes of fire in GIS substation
Particle or moisture contamination inside the compartment causes flash overs.
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.
The SF6 can be easily exploded into different decomposition gas products when subjected to electrical discharge, such as electric arc, spark.
Negligence of maintenance causes minor faults to develop and causing fire.
Short circuit and too much overloading, which causes heating and ultimately fire.
Protective devices not in good working condition, fail to detect fault, causing fire.
Failure of protective relaying circuit causes faults to persist and ultimately fire.
Unskilled persons employed for sensitive circuits cannot maintain it properly and so faults can be developed ultimately resulting fire.