Substation Construction- Ground Grids Flashcards
A substations grounding reliability is critical for ?
Fast clearing of faults out on the line or inside the substation yard
A substations reliability is dependent on ? and must be built in to improve the overall safety and reliability of the electrical system
good grounding
If the earth is a high-resistant earth, what can be done to ensure a low resistance is used in grounding a substation?
- Ground wells may have to be bored several hundred feet until the required earth is reached.
- Ground rods longer than 10 feet may be used.
- Soil additives and treatments may be used to increase soil conductivity by reducing the resistivity of the soil. Soil treatment is dependent on an ongoing testing program because treatments will wash out in time.
- High earth resistivity may also be seasonal because of the earth’s drying out or becoming frozen. Supplementing the grid conductors with deeply placed electrodes is one method to achieve the required resistivity.
Increasing the area of the ground grid is much more effective at reducing the grid resistance than make it denser.
True
Substation ground grids normally consist of ? buried ?
bare stranded 4/0 copper cable
18 inches below the soil.
Why is copper used as the material for the ground grid?
Copper is commonly used for substation ground grids. In addition to its high conductivity, copper has the advantage of being resistant to most underground corrosion
In grounding, what are mechanical pressure-type connectors?
Pressure-type connections produce a mechanical bond between the conductor and the connector. This connection holds the conductors in place or squeezes them together, providing surface-to-surface contact with the exposed strands
In grounding, what are exothermically welded connectors?
Exothermic welding fuses the conductors together to form a molecular bond between all strands of the conductors.
Of the two connectors, pressure-type is susceptible to installation errors, oxidation, and corrosion as the connection ages.
True
All fault current sources such as surge arresters, capacitor banks or coupling capacitors, transformers, and, where appropriate, machine neutrals, lighting, and power circuits should also be connected to the grid.
true
All above-ground conductive metal parts that might accidentally become energized must be grounded to the grid.
true
The wire or straps used to ground above-ground equipment should be kept to the shortest possible length and they should be formed to conform to the equipment’s foundation to minimize exposure to mechanical damage.
True
Substation fences should be grounded to ground rods installed ?
one foot inside the fence at corner posts, gate posts, and at posts on each side of an overhead transmission or distribution line crossing.
Soils with high conductivity have the ability to dissipate power surges. A type of soil with this property is ? .
clay
The resistivity of the soil determines the ? in the ground grid.
spacing between cables
The grounding network for a transmission substation is typically made from bare copper in a grid pattern with conductors spaced about ? apart.
10’
If the earth has a high resistance, ? may be used to reach an earth of the required low resistance.
ground rods longer than 10 feet
An ongoing testing program is required when ? are used as a method to lower the resistance of a substation grounding grid.
soil additives
The most effective method of reducing the resistance of a ground grid is by ? .
increasing the grid area
Having a grid that bonds structures, equipment, and fencing reduces the risk of ? between different elements.
potential differences
The minimum conductor size for a distribution substation ground grid is typically ? copper.
1/0
The grid network for a transmission substation is typically made from ? bare copper.
4/0
The final substation surfacing material is a ? layer of crushed stone.
3” to 6”
For below grade connections, ? connections are the preferred type.
exothermic welded
Because of the failure possibility during a high fault current event, ? ground grid connections are not allowed.
soldered
When a fault occurs in a substation or on one of the circuits feeding in or out of the substation, ? .
a voltage rise in the earth and ground grid occurs
An advantage of using ? to cover the grounding grid is the extra protection provided by the high resistance properties
crushed stone
An advantage of using ? to cover the grounding grid is the extra protection provided by the high resistance properties
prefabricated copper-clad steel mesh
