Gas Insulated Swichgear ( GIS) - GENERAL EARTHING INFORMATION

Earthing and return current conductors

Earthing conductors

Earthing conductors interconnect the GIS enclosure at numerous points with the earthing net (multipoint earthing). Being a component of the earthing system they effectively help to avoid dangerous touch voltages during a short circuit, and to keep HF transient overvoltages on a sufficiently low level during normal service. To meet these requirements they shall have low impedances at 50/60 Hz as well as in the HF range.

The Gas Insulated Switchgear (GIS)

·         The Gas Insulated Switchgear (GIS) shall be a complete unit comprised of main equipment, all control, monitoring, protection, measuring and auxiliary devices and systems including interconnections, all termination

·         Joints at interfaces, mechanical linkages, couplings, SF6 gas piping, cabling, wiring, grounding materials etc. required for proper and satisfactory operation.
·         The GIS shall be of compact and modular design with individual equipment modules connected together to form a complete assembly. However, combined disconnect switch and grounding switch, cable termination, PT, exit bus duct and CT (with cores located within gas compartment) shall be in separate gas tight compartment. If, CT is installed in breaker gas compartment; insulation class for CT shall be class F. of either single-phase enclosure type or three-phase enclosure type for voltage levels of 230kV and below. of single-phase enclosure type for voltage level of 380kV. Installed on suitable pads or supporting frames/structures with provision for leveling, including fasteners to foundation, which shall also be included in the supply. Support-structures shall be designed and fabricated to withstand the short circuit forces of the ground fault current.
·         GIS shall having expansion joints and flexible connections, where several enclosures are connected in the longitudinal direction, such as main bus. Expansion joints provided for installation alignment shall be locked in place when alignment is complete. Expansion joints for compensation of thermal expansion and erection tolerances shall have
·         the means to preserve mechanical integrity of the enclosure and the plug-in contacts for the conductor. Provided with galleries, stairways, movable platform or walkways for accessing the equipment above two meters for maintenance and operation.
·          All structures, movable platform, galleries, stairways and walkways shall have grounding provision and shall conform to the relevant Occupational Safety and Health Administration (OSHA) regulations.
·         For single-phase type GIS, creation of single gas zone by external piping connection between the phases is not acceptable. Also, external piping connection (with normally open valve) to create common gas zone (between same phase) in single-phase type GIS is not acceptable.
·         The GIS enclosure shall be safe to touch and fully ensure operational security and personnel safety under all normal and fault conditions with the maximum allowed induced voltage in the enclosure of 65 volts.
·         The switchgear and all its components and accessories shall be designed for minimum maintenance during service. The manufacturer shall state the minimum interval between minor inspections (which will be restricted to visual checking and adjustments of external parts only) and major inspection/overhaul, including refilling or replenishment of gas and cleaning of the contaminant or filter in the circuit breaker chamber(s).
·         Suitable openings shall be provided in the circuit breaker, disconnect and grounding switch modules for major inspection/adjustments.
·         All motors shall be in accordance with IEC 60034. The bearings and other such parts shall be permanently lubricated for the entire service life. Padlocks shall be made of stainless steel or brass.
The GIS interface points shall be carefully coordinated with other equipment such as overhead lines, cables, transformers, reactors, capacitor banks, etc., supplied by other manufacturers in order to ensure full compatibility. The design criteria for outdoor

SF6 gas (Part 1)

• Since 1960, SF6 has been used as arc quenching and insulating medium for high and medium voltage switchgear systems. The favourable electrotechnical, chemical and physical characteristics of the gas have considerably influenced the development of the switchgear technology. SF6 is an alternative to other conventional insulating and quenching media such as e.g. oil, and air. The use of SF6 gas considerably increases, in some applications, the efficient utilisation of resources in energy transmission and distribution with respect to technology, finances and personnel [1]. At the same time SF6 in comparison to oil reduces the risk of hazard (e.g. fire, explosion) to personnel and environment. An overall evaluation considering all ecological, economic, safety and technological aspects has proven that SF6 is still an excellent choice as insulating medium [2]. The existing SF6 technology in the field of energy transmission and

The Gas Insulated Switchgear (GIS)

·         The Gas Insulated Switchgear (GIS) shall be a complete unit comprised of main equipment, all control, monitoring, protection, measuring and auxiliary devices and systems including interconnections, all termination

·         Joints at interfaces, mechanical linkages, couplings, SF6 gas piping, cabling, wiring, grounding materials etc. required for proper and satisfactory operation.
·         The GIS shall be of compact and modular design with individual equipment modules connected together to form a complete assembly. However, combined disconnect switch and grounding switch, cable termination, PT, exit bus duct and CT (with cores located within gas compartment) shall be in separate gas tight compartment. If, CT is installed in breaker gas compartment; insulation class for CT shall be class F. of either single-phase enclosure type or three-phase enclosure type for voltage levels of 230kV and below. of single-phase enclosure type for voltage level of 380kV. Installed on suitable pads or supporting frames/structures with provision for leveling, including fasteners to foundation, which shall also be included in the supply. Support-structures shall be designed and fabricated to withstand the short circuit forces of the ground fault current.
·         GIS shall having expansion joints and flexible connections, where several enclosures are connected in the longitudinal direction, such as main bus. Expansion joints provided for installation alignment shall be locked in place when alignment is complete. Expansion joints for compensation of thermal expansion and erection tolerances shall have     the means to preserve mechanical integrity of the enclosure and the plug-in contacts for the conductor. Provided with galleries, stairways, movable platform or walkways for accessing the equipment above two meters for maintenance and operation.
·          All structures, movable platform, galleries, stairways and walkways shall have grounding provision and shall conform to the relevant Occupational Safety and Health Administration (OSHA) regulations.
·         For single-phase type GIS, creation of single gas zone by external piping connection between the phases is not acceptable. Also, external piping connection (with normally open valve) to create common gas zone (between same phase) in single-phase type GIS is not acceptable.
·         The GIS enclosure shall be safe to touch and fully ensure operational security and personnel safety under all normal and fault conditions with the maximum allowed induced voltage in the enclosure of 65 volts.
·         The switchgear and all its components and accessories shall be designed for minimum maintenance during service. The manufacturer shall state the minimum interval between minor inspections (which will be restricted to visual checking and adjustments of external parts only) and major inspection/overhaul, including refilling or replenishment of gas and cleaning of the contaminant or filter in the circuit breaker chamber(s).
·         Suitable openings shall be provided in the circuit breaker, disconnect and grounding switch modules for major inspection/adjustments.
·         All motors shall be in accordance with IEC 60034. The bearings and other such parts shall be permanently lubricated for the entire service life. Padlocks shall be made of stainless steel or brass.
·         The GIS interface points shall be carefully coordinated with other equipment such as overhead lines, cables, transformers, reactors, capacitor banks, etc., supplied by other manufacturers in order to ensure full compatibility. The design criteria for outdoor portion of the GIS bus duct leading to SF6-to-Air Overhead Line Termination / SF6-to-Oil Transformer Termination shall be as mentioned in clause 4.2.2 and in the data schedule.
·         All modules of the switchgear and components of the same rating and construction, which may need to be replaced, shall be strictly interchangeable.
·         To facilitate transport and handling, lifting eyes or other suitable attachments shall be provided with each GIS module.
·         Each module of switchgear, consisting of individual elements intended to be directly connected together, shall be constructed as a transportable assembly suitable for shipping and transportation without being dismantled.
·         All new gaskets, sealants and desiccants for permanent sealing of all field joints and all access covers, removed during assembly, shall also be provided.
·         The GIS shall be designed per IEC 62271-203 or ANSI/IEEE C37.122 standard. The enclosure shall be capable of sustaining without damage all mechanical, electrical and thermal shocks that may occur in service during normal and fault conditions, including pressure effects of internal fault arc current of specified short circuit level and time. The enclosure assembly, material and design shall be such as to minimize induced electrical losses and heating effects which could occur in service under normal and fault conditions.
·         Joints and couplings between dissimilar metals shall be avoided to prevent galvanic corrosion.
·         All supporting steel work shall be hot-dip galvanized
·         A partition separating a compartment filled with insulating gas from a neighboring compartment such as a cable box, filled with liquid, shall not show any leakage affecting the dielectric properties of the two media.
·         All welding in GIS shall conform to Section IX of ASME code or equivalent. The number of welds shall be kept to a minimum. At least 10% of all welds must be subject to non-destructive testing by X-ray or ultrasound and all records of these must be made available.
·         The construction and thickness of the GIS enclosure and air storage tank of the pneumatic operating mechanism shall conform to ANSI/ASME Boiler and Pressure Vessel Code, and ANSI/ASME B31.1 or equivalent.
·         All current carrying parts shall be made of electrolytic grade copper or aluminum alloy. All interconnecting sections of current transferring parts shall be silver-plated.
·         All piping for SF6 gas, hydraulic and pneumatic operating mechanism including their fittings shall be made of copper, brass or stainless steel.
·         All external connectors and terminal pads shall be made of copper or aluminum with tin-plating and designed per NEMA CC1 and NEMA CC3. Terminal pads shall have 4 holes.
·         Rated interrupting current of the auxiliary contacts shall be not less than 10Adc.
·         The specific ratings of the GIS equipment shall be as specified in the data schedule.
·         The switchgear shall be capable of carrying the specified rated current continuously at the design ambient and specified ambient conditions  without temperature rise of various parts exceeding the limits stated in IEC 60694 or equivalent ANSI standards. The maximum
·         All equipment and components of the switchgear including bus support insulators shall be free of partial discharges when operated at the rated voltage. The partial discharge intensity at ll0% rated voltage shall not exceed 10 pC with a maximum background level of 5pC.
·         Future alterations and extensions can be undertaken in either direction by GIS manufacturers other than the original manufacturer.
·         Maintenance or removal of a disconnect switch or circuit breaker will not involve removal from service of adjacent disconnect switch or main bus sections.
·         Separate GIS bus link module shall be provided at both ends of the GIS to avoid modification of the existing bus for future extension of the GIS buses by GIS manufacturers other than the original manufacturer.
·         Adequate clearances around GIS shall allow free movement of SF6 gas service cart.
·         Two maintenance grounding switches shall be provided across each circuit breaker to measure the contact resistances of circuit breaker at site.
·         Sufficient clearances around bushing for cable HV AC testing shall be provided.
Adequate facilities shall be made available for neat storage of HV AC cable-test bushing in the substation