The Basics of Substation Grounding: Parts of the Grounding System

One of the vital aspects of the protection of people and equipment in electrical substations is the provision of an adequate grounding system. The grounding system interconnects the equipment neutrals, equipment housings, lightning masts, surge arresters, overhead ground wires, and metallic structures, placing them at earth’s potential.

 

 

The subject of grounding systems in substations made up of a network of conductors interconnecting the metallic parts of equipment and structures, and an arrangement of buried conductors providing an electrical connection to the earth, has long been studied..

Many workers involved in the various applications of electricity — lighting, electromechanical conversion, telecommunications, process control, information technology, biomedical equipment, and more — do not have a keen understanding of the purpose and design procedures of a grounding network.

The objective of this article, rather than presenting procedures to design a grounding grid, is to create discussion surrounding the need for and purpose of a grounding system. With these elements clearly defined, it will be possible to understand the design procedures with due analysis to each particular case.

 

The Need for a Grounding System in the Substation

The matter of grounding systems in substations is vital. The main functions of a grounding system are:

• Provide the neutrals of generators, transformers, capacitors, and reactors a connection to the earth

• Offer a low impedance path to the earth for the currents coming from ground faults, lightning rods, surge arresters, gaps, and related devices

• Limit the potential differences that appear between the substation metallic objects or structures, and the ground potential rise (GPR), due to the flow of ground currents; they may pose a danger to equipment and personnel

• Improve the operation of the protective relay scheme to clear ground faults

• Increase the reliability and availability of the electrical system

• Allow the grounding of de-energized equipment during maintenance

 

Parts of the Substation’s Grounding System

Substation safety requires the grounding and bonding of all exposed metal parts. The metallic structures, generators, transformer tanks, circuit breakers, switchboards, switches, metal walkways, steelwork of buildings, fences, instrument transformer secondaries, capacitors, lightning arresters, surge arresters, and reactors must be grounded. With proper grounding, things that are touching or standing on the ground nearby to any of this equipment will not receive a shock if an electric conductor arcs to or comes in contact with them.

A substation grounding system has two well-defined parts — the grounding network and the connection to the earth.

 

The Grounding Network

The grounding network contains the conductors responsible for offering a low impedance path between the equipment frames or metallic structures and the connection to the earth. This network should have high reliability because the breaking of a ground connection can cause safe equipment to become dangerous.

The usual practice is connecting the equipment frames and metallic structures individually to the ground electrode–with copper conductors or straps–to:

• Minimize the number of equipment disengaged from the ground when, by accident, one of the connections breaks

• Circulate the ground-fault current through a predetermined circuit. If the ground-fault current flows through random paths, there is a risk that they lack the thermal capacity and mechanical strength to carry the current, risking people, damaging equipment, and causing fires

 

Figure 1 shows a typical grounding network. In the illustration, each piece of equipment has two links — to the earth and the grounding conductor.

 

Figure 1 Grounding network. Image courtesy of Prof. J. H. Briceño.

 

The two links provide dependable circuits for the return of ground-fault current. The connection to the grounding conductor is optional; it lessens the risk when the connection to earth does not guarantee proper surface potential gradients. When used, most of the fault current will return through the conductor, reducing the potential gradients on the surface of the ground.

The equipment 4, located at another substation, has a separate connection to the earth. By using the grounding conductor, the ground connections of the two substations work in parallel; this is generally beneficial as it reduces the return of current through the ground, lessening the surface potential gradients.

Without the grounding conductor, all ground-fault current from equipment 4 will return through the earth. The connection to the earth in both substations should have low impedance, so that the ground-fault current magnitude will be large enough to activate the overcurrent protection system, clearing the fault, and the generated surface potential gradients will be safe.

Equipment frames and steel structures may be used as a path to earth if their conductivity–including the joints–is equivalent to the required conductor or strap. Examples are the connection of surge arresters to the transformer tank and the overhead ground wires and lightning masts–extending upward from the top– attached to the substation steel structure.

The following are recommendations for the design and construction of the grounding network:

• Compute the magnitude and duration of the most severe ground-fault current to select the size of the conductors, straps, and connectors. The conductors, straps, and connectors must have sufficient thermal and mechanical capacity to resist fusing and withstand the electromechanical stresses produced during failure–for the time that the protection scheme will allow the current to flow. Additionally, they should not lose their electrical properties over time. 

• Avoid creating random loops or circuits for the return of ground-fault current. Do this by attaching each piece of equipment to the earth or the ground conductor

• Minimize the separation between the grounding conductors and their associated phase conductors, to reduce the ground-circuit reactance

• Analyze the return paths of the ground-fault current when there is associated equipment located in another substation, with a separate connection to the earth. It could happen that some return paths cannot carry the ground-fault current, such as cable shield and armor

• Extend the grounding network to all island structures within the substation

 

The Connection to the Earth

There are three main methods to connect a substation grounding network to the earth:

• Radial
• Ring
• Grid

 

The radial system consists of one or more grounding electrodes with connections to each device in the substation. It is the most economical, but the least satisfactory because, when a ground fault occurs, it produces enormous surface potential gradients.

The ring system consists of a conductor placed around the area occupied by the substation equipment and structures and connected to each one by short links. It is an economical and efficient system that reduces the significant distances of the radial system. The surface potential gradients decrease because the ground-fault current travels through several prearranged paths.

The grid system is usual. It consists of a grid of horizontally arranged copper conductors, embedded a little below grade, and connected to the substation equipment and metallic structures; grounding rods can be added to reach layers of lower resistivity at a greater depth. This system is the most effective but also the most expensive.

 

The Grid System

The primary purpose of a grounding grid is to equalize the potential gradients above the grid, protecting people and equipment.

Under ground-fault conditions, the portion of the fault current flowing from the earth to the grid or vice versa triggers a rise of the ground potential above the grid–with respect to remote earth. This event is the ground potential rise. Numerically, the ground potential rise is equal to the product of the grid resistance times the maximum grid current.

If the people inside and around the substation can tolerate the ground potential rise, the grounding grid is safe.

Assuming a 2 Ω ground resistance, a 5,000 A ground-fault current  — which might be more — would cause a ground potential rise of 10,000 V during the ground fault. This voltage drop could injure people and damage equipment in the substation. Frequently, getting a low resistance is difficult; for this reason, it is not practical to design only for a safe ground potential ground on the substation, mainly when comprising large ground-fault currents.

The grid is capable of controlling the surface potential gradients at each point inside the substation. Although the grid will not reduce the grounding resistance by much, all the surfaces will have nearly the same potential as the equipment and metallic structures.

In almost no substation can a single grounding electrode have the necessary conductivity and thermal capacity to handle the ground-fault current. But if several electrodes are installed and connected to metallic structures, to equipment housings, and the neutrals of electrical machines, the result will be a grounding grid. By burying the grid in a good resistivity soil, a suitable grounding system can be obtained.

The grounding grid should cover as much ground as possible in the substation, including an area outside the fence. The conductors will be laid in parallel, trying to maintain a uniform spacing along the rows of equipment and structures in the substation. This arrangement will simplify the connections.

The length of conductor, spacings, and the total area of the grid, to achieve acceptable surface potential gradients, will depend on the particular context of the substation.

Places with a high concentration of fault currents, such as the neutrals of generators, power transformers, and grounding transformers, are critical, requiring reinforcements such as more conductors and larger sizes. In areas frequented by operators, it is customary the use of grounding mats. Grounding mats are solid metallic plates or metal gratings, placed above the grounding grid, where workers place their feet when operating equipment. This practice will keep the potential gradients low in those spots.

 

A Review of the Parts of the Grounding System

The subject of grounding electrical substations is under continuous research.

Many workers in the electrical area have insufficient knowledge about substation grounding, even though this is of ​​vital importance since the safety of people and equipment depends on it.

A substation grounding system has two main parts: the grounding network and the connection to the earth. The grounding network bonds all equipment frames and metallic structures in the substation, while the connection to the earth is the interface between the electrical system and the earth.

There are three methods to connect a substation to the earth: radial, ring, and grid.

The grid is the most effective system, although the most expensive. It is a lattice of copper conductors placed below grade and connected to the substation frame and equipment. 

The grid equalizes the surface potential gradients, protecting people and equipment.