Substation Construction- Ground Grids

Question 1

A substations grounding reliability is critical for ?

Answer 1

Fast clearing of faults out on the line or inside the substation yard

Question 2

A substations reliability is dependent on ? and must be built in to improve the overall safety and reliability of the electrical system

Answer 2

good grounding

Question 3

If the earth is a high-resistant earth, what can be done to ensure a low resistance is used in grounding a substation?

Answer 3

1. Ground wells may have to be bored several hundred feet until the required earth is reached.
2. Ground rods longer than 10 feet may be used.
3. 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.
4. 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.

Question 4

Increasing the area of the ground grid is much more effective at reducing the grid resistance than make it denser.

Answer 4

True

Question 5

Substation ground grids normally consist of ? buried ?

Answer 5

bare stranded 4/0 copper cable
18 inches below the soil.

Question 6

Why is copper used as the material for the ground grid?

Answer 6

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

Question 7

In grounding, what are mechanical pressure-type connectors?

Answer 7

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

Question 8

In grounding, what are exothermically welded connectors?

Answer 8

Exothermic welding fuses the conductors together to form a molecular bond between all strands of the conductors.

Question 9

Of the two connectors, pressure-type is susceptible to installation errors, oxidation, and corrosion as the connection ages.

Answer 9

True

Question 10

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.

Answer 10

true

Question 11

All above-ground conductive metal parts that might accidentally become energized must be grounded to the grid.

Answer 11

true

Question 12

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.

Answer 12

True

Question 13

Substation fences should be grounded to ground rods installed ?

Answer 13

one foot inside the fence at corner posts, gate posts, and at posts on each side of an overhead transmission or distribution line crossing.

Question 14

Soils with high conductivity have the ability to dissipate power surges. A type of soil with this property is ? .

Answer 14

clay

Question 15

The resistivity of the soil determines the ? in the ground grid.

Answer 15

spacing between cables

Question 16

The grounding network for a transmission substation is typically made from bare copper in a grid pattern with conductors spaced about ? apart.

Answer 16

10’

Question 17

If the earth has a high resistance, ? may be used to reach an earth of the required low resistance.

Answer 17

ground rods longer than 10 feet

Question 18

An ongoing testing program is required when ? are used as a method to lower the resistance of a substation grounding grid.

Answer 18

soil additives

Question 19

The most effective method of reducing the resistance of a ground grid is by ? .

Answer 19

increasing the grid area

Question 20

Having a grid that bonds structures, equipment, and fencing reduces the risk of ? between different elements.

Answer 20

potential differences

Question 21

The minimum conductor size for a distribution substation ground grid is typically ? copper.

Answer 21

1/0

Question 22

The grid network for a transmission substation is typically made from ? bare copper.

Answer 22

4/0

Question 23

The final substation surfacing material is a ? layer of crushed stone.

Answer 23

3” to 6”

Question 24

For below grade connections, ? connections are the preferred type.

Answer 24

exothermic welded

Question 25

Because of the failure possibility during a high fault current event, ? ground grid connections are not allowed.

Answer 25

soldered

Question 26

When a fault occurs in a substation or on one of the circuits feeding in or out of the substation, ? .

Answer 26

a voltage rise in the earth and ground grid occurs

Question 27

An advantage of using ? to cover the grounding grid is the extra protection provided by the high resistance properties

Answer 27

crushed stone

Question 28

An advantage of using ? to cover the grounding grid is the extra protection provided by the high resistance properties

Answer 28

prefabricated copper-clad steel mesh