Substation grounding details

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Substation Grounding

The grounding connections provided to substation equipment and structures fall under two categories, namely

a. Safety Grounds

b. System Grounds

System ground is normally for neutral grounding and safety ground is for equipment grounding. Minimum conductor size for equipment safety grounding.

All safety ground termination shall be made directly on to the ground grid. All system ground shall be terminated on to a ground rod interconnected to the grounding grid.

SOIL RESISTIVITY MEASUREMENT

Measurement

·      A number of measuring techniques are described in detail in ANSI/IEEE 8. The Wenner's four-pin method as described in ANSI/IEEE 8shall be used for measurement of soil resistivity. As many readings as required for various spacing and depth, in all the eight directions, sufficient to model the soil shall be carried out.

·      Soil resistivity readings shall normally be taken under dry conditions, during Summer months, if possible, However the same shall not affect the project’s schedule.

·      Fill up soil resistivity shall be carried by soil modeling in laboratories on samples dried to 2% moisture content after compaction.

·      Soil resistivity measurement shall also be carried out before and after fill up and compaction of soil at site.

Interpretation of Test Results

For soils with resistivity value less than 500 Ohm, if the difference between the highest and the lowest readings are within 30 % then the soil can be considered as uniform soil. For soils with resistivity value greater than 500 Ohm, if the difference between the highest and the lowest readings are within 20 % then the soil can be considered as uniform soil. For uniform soils, the mean value shall be considered as soil resistivity value. In case of wide variations in field readings, computer software alone shall be used to simulate two-layer model or multi-layer soil model. Two-layer soil models are good approximation of many soil structures, while multi-layer soil models may be used for more complex soil conditions. Software shall be based on IEEE-80.

 

Backfill Material

Backfill material shall have possibly the same soil resistivity or better than that of the original soil. In case of considerable backfill the soil resistivity shall be taken after completion of the backfill compaction. The same shall be used for grounding calculations. In case of delay of backfill activity at site the estimated value of resistivity of the backfill material or that of the existing soil whichever is higher shall be used for grounding calculations.

SELECTION OF GROUNDING CONDUCTOR MATERIAL, SIZE AND JOINTS

Basic Requirements

• ·           Copper material shall be used for grounding.
• ·           Soft drawn, stranded copper shall be used for the ground grid conductors.
• ·           The conductor shall be round shaped for maximum cross-sectional contact with the ground. In coastal zone with low soil resistivity, tinned copper conductor shall be used. Copper-clad steel shall be used for ground rods.
• ·    Each element of the ground system (including grid proper, connecting ground leads, and electrodes) shall be so designed that it shall :
• ·       Resist fusing and deterioration of electric joints under the most adverse combination of fault-current magnitude and fault duration to which it might be subjected.
• ·           Be mechanically rugged to a high degree, especially in locations exposed to physical damage.
• ·     Have sufficient conductivity so that it will not contribute substantially to dangerous local potential differences.

Equipment grounding details

Steel Structures and Switch Racks

Switch racks and every steel structure that supports insulators or electrical equipment shall be grounded by means of bolted connections at two (2) diagonally opposite legs. Equipment mounted on steel supporting structures shall have separate grounding conductors. The pigtail ground conductor shall be supported on the structure at 1.0 meter intervals by clamps. Casting pigtail conductor inside the steel structure concrete foundation is not acceptable.

Fences / Gates

If space permits a perimeter ground conductor shall be laid which follows the

fence line and the gate in any position (open or close) and the fence then shall be bonded electrically at corner posts, gate posts and every alternate line post. The gates shall be bonded to the gateposts with a flexible copper cable or braid.

The barbed wire on the top of the SSD (Safety and Security Directive) type

fence/boundary wall, if applicable, shall be bonded to the grounding grid.

Cables

Metallic cable sheaths shall be effectively grounded by connecting a flexible braid to the sheath to eliminate dangerous induced voltages to ground.

Control Cables

Metallic sheath of control cables shall be grounded at both ends to the grounding grid via ground busbar in the cubicle.

Power Cables

a. Sheath of Power cables rated 69kV to 380kV shall be grounded directly from one end and through SVL(Sheath Voltage Limiters) from the other

b. Grounding of sheath of single core cables rated for 34.5kV and 13.8kV shall be from One end only.

c. Sheath of three core cables rated for 13.8kV shall be grounded at both ends.

 If ring type CTs are installed on power cables, the grounding of sheath shall be done such that the sheath current to ground will not influence CT secondary current.

Instrument Cables

Instrument cables carrying analog or digital signals shall have their metallic screening grounded at one point by means of PVC insulated grounding wire connected to separate instrument ground bar which is insulated from cubicle ground.

Signal Cables

All signal cables used in telemetering and communications shall have their shield grounded at one end only to reduce interference from stray sources.

Cable Tray System

Cable tray system shall be grounded with bare copper conductor of 50mm² size at

both ends and shall be bonded across gaps including expansion gaps

Substation Buildings

Substation buildings shall be encircled by a grounding conductor.

Reinforcement bars of the substation buildings and equipment foundation in the yard shall be connected to the main grounding grid at least at two diagonally opposite points.

For grounding of the electrical apparatus installed inside substation

buildings two separate exposed copper conductors/strips of size per manufacturer recommendation, each connected to the grounding grid at two (2) different points shall be laid. The grounding grid shall be laid inside the substation buildings and

it shall be connected to the main grid outside the buildings, at minimum two points.

Metal building(s)

·         Metal buildings shall be grounded at each substructure column with a minimum size of l20mm² bare copper conductor.

·         Angle irons installed on indoor trenches to support the metallic covers shall also be grounded at both ends.

·         Metallic doors in substation buildings shall be grounded with a flexible copper cable or braid.

HVAC

All air conditioning ducts inside the control building(s) shall be grounded at both ends and cross bonded at all joints and across the non-metallic duct connecting Air Handling Unit (AHU).

Grounding of control panels, Operating Mechanism Housing, Box, etc and other equipments associated with HVAC shall be done

Metallic Conduits

All metallic conduits shall be connected to the grounding grid at each manhole or at terminating points by using a conductor size of 50 mm². Conduits terminating in metal junction boxes shall be grounded by means of grounding studs or brazed connections. Where several conduits or junction boxes are located adjacent to each other, an adequately sized solid wire shall be used to interconnect the boxes. It shall be connected to grounding system at one single point.

Circuit Breakers and Disconnect Switches

·         All circuit breakers and disconnect switches shall be grounded at two diagonally opposite corners from two separate points of the grounding grid. Further  grounding switch blades of Disconnect Switch shall be directly grounded to grounding grid.

·         Good electrical connection shall be maintained between the steel structure and any bolted accessories mounted on it.

·         Operating Handles for Outdoor Switches as a large percentage of fatal accidents from voltage gradients are associated with manual operating handles of disconnect switches, etc.

·         A metal grounding plate or mat (operating platform), shall be placed where the operator must stand on it to operate the device. The operating handles shall be grounded by connecting a ground conductor (preferably flexible wire, braid strap) from the vertical operating pipe to the supporting structure, then continuing another stranded ground conductor to the switch operating platform. It is reiterated that the operating handle and the platform shall not be directly connected to the grounding grid but instead both connected to the support structure which in turn shall be connected to the grounding grid at least at two diagonally opposite points.

Terminal Transmission Tower Grounding

Terminal transmission towers located adjacent to the substation shall be connected to

the substation grounding grid at two diagonally opposite points. The shield wire shall

be connected to the tower structure, which in turn is connected to the grounding grid.

 Lightning Masts

Metal lightning masts shall have one safety ground.

Ring Main Unit (RMU)

The RMU inside the substation, if applicable, shall have two safety ground connections.

Oil Tanks and Oil /Water Pipings

All oil tanks shall be grounded at two points with bolted cable connections to two

different points of the grounding grid. Oil piping shall be grounded at intervals of

12m. Runs shorter than l2m shall be grounded at least at two points. Water piping

shall be connected to the grounding system at all service points. In addition, two

copper conductors of adequate size, shall be connected to the main water pipe from two separate points of the grounding grid.

Metal Clad Switchgear

Metal Clad switchgear shall have two safety grounds connected to the switchgear grounding bus. Withdrawable circuit breakers and PTs shall be provided with reliable connection to the ground bus. Grounding via the roller wheels and the rail is not acceptable.

Grounding of Lighting Equipment

Grounding of the lighting fixtures, lamp holders, lamps, receptacles and metal poles supporting lighting fixtures shall be per Article 250 and 410 of NEC (NFPA 70).

Portable Equipment

Portable electrical equipment shall be grounded in accordance with the

applicable requirements of Articles 250 of the NEC (NFPA 70).

Temporary Grounding

All the components used for temporary protective system shall meet IEEE Std. 1246,

“Guide for Temporary Protective Grounding System Used in Substations”.

Instruments, Relays and Meters

Instruments, meters and relays shall be grounded in accordance with the requirements of the NEC, Articles 250-120 to 126 and Articles 170 to 178.

Equipment Requiring both Safety and System Grounds

All operating grounds shall have their connections made to the grounding rods, which in turn shall be connected to the grounding grid.

Power Transformers

·         Power transformer tanks shall be safety grounded at two points diagonally opposite to each other. These connections shall be made from two different points of the grounding grid.

·         A separate system ground shall be provided for the neutral of the transformer by means of two (2) stranded copper wires. The neutral copper wire shall be sized for the system fault level.

·         The neutral grounding wires shall be insulated from the transformer tank by support insulators mounted on the tank wall and shall be connected to the grounding grid directly.

·         Independently mounted radiator bank and LPOF/XLPE cable termination

·         boxes shall be separately grounded at two diagonally opposite locations.

·         Tertiary windings and stabilizing windings shall be grounded per IEC60076-3, Annexure B.

Instrument Transformers

·         Potential and current transformers shall have their metal cases grounded.

·         The grounding terminal of the potential transformers shall be connected to the grounding grid.

·         The neutral point of the secondary connections of potential and current transformers shall be grounded to the ground grid in the control/relay room instead of switchyard to reduce the transient overvoltages.

·         Other requirements of instruments transformer grounding shall be per IEEE C57.13.3, “Guide for Grounding of Instrument Transformer Secondary Circuits and Cases”.

Surge Arresters

·         Where surge counter and/leakage current indicating meters are installed, a 5 kV insulated cable shall be used between arrester ground terminal and surge counter. The surge monitor's ground terminal shall be connected to the ground grid via two (2) 240 mm² stranded copper conductors.

·         The system ground conductor shall be as short as possible, free of sharp bends, and shall not be installed in metallic conduit. In addition, ground rods shall be driven adjacent to the arrester connection to the grounding grid to provide the lowest ground grid resistance at this point.

Station Auxiliary Transformer

Station auxiliary transformer shall be safety grounded at two locations diagonally opposite. One system ground shall be directly connected to the neutral wye connected windings that are to be solidly grounded.

Shunt Capacitors

Shunt capacitors are considered safety grounded when mounted on a metal structure

that is connected to the grounding grid. One system ground conductor shall be connected to the grounding grid when the capacitors are to be connected in a grounded star configuration.

Coupling Capacitor Voltage Transformers (CCVTs)

The grounding terminal and neutral point of secondary connections of CCVT shall be connected to the grounding grid similar to potential transformer as