EE35T - Substation Design and Layout

The First Step in designing a Substation is to design an Earthing and Bonding System.

Earthing and Bonding

The function of an earthing and bonding system is to provide an earthing system connection to which transformer neutrals or earthing impedances may be connected in order to pass the maximum fault current. The earthing system also ensures that no thermal or mechanical damage occurs on the equipment within the substation, thereby resulting in safety to operation and maintenance personnel. The earthing system also guarantees eqipotential bonding such that there are no dangerous potential gradients developed in the substation.

In designing the substation, three voltage have to be considered.

1. Touch Voltage: This is the difference in potential between the surface potential and the potential at an earthed
                              equipment whilst a man is standing and touching the earthed structure.

2. Step Voltage: This is the potential difference developed when a man bridges a distance of 1m with his feet
                           while not touching any other earthed equipment.

3. Mesh Voltage: This is the maximum touch voltage that is developed in the mesh of the earthing grid.
 

Substation Earthing Calculation Methodology

Calculations for earth impedances and touch and step potentials are based on site measurements of ground resistivity and system fault levels. A grid layout with particular conductors is then analysed to determine the effective substation earthing resistance, from which the earthing voltage is calculated.

In practice, it is normal to take the highest fault level for substation earth grid calculation purposes. Additionally, it is necessary to ensure a sufficient margin such that expansion of the system is catered for.

To determine the earth resistivity, probe tests are carried out on the site. These tests are best performed in dry weather such that conservative resistivity readings are obtained.

Earthing Materials

1. Conductors: Bare copper conductor is usually used for the substation earthing grid. The copper bars themselves
                         usually have a cross-sectional area of 95 square millimetres, and they are laid at a shallow depth
                         of 0.25-0.5m, in 3-7m squares. In addition to the buried potential earth grid, a separate above ground
                         earthing ring is usually provided, to which all metallic substation plant is bonded.

2. Connections: Connections to the grid and other earthing joints should not be soldered because the heat generated
                          during fault conditions could cause a soldered joint to fail. Joints are usually bolted, and in this case, the
                          face of the joints should be tinned.

3. Earthing Rods: The earthing grid must be supplemented by earthing rods to assist in the dissipation of earth fault
                             currents and further reduce the overall substation earthing resistance. These rods are usually made of
                             solid copper, or copper clad steel.

4. Switchyard Fence
               Earthing: The switchyard fence earthing practices are possible and are used by different utilities. These are:

                                (i) Extend the substation earth grid 0.5m-1.5m beyond the fence perimeter. The fence is then
                                     bonded to the grid at regular intervals.
                                (ii) Place the fence beyond the perimeter of the switchyard earthing grid and bond the fence to its
                                     own earthing rod system. This earthing rod system is not coupled to the main substation earthing
                                     grid.
 

Layout of Substation

The layout of the substation is very important since there should be a Security of Supply. In an ideal substation all circuits and equipment would be duplicated such that following a fault, or during maintenance, a connection remains available. Practically this is not feasible since the cost of implementing such a design is very high. Methods have been adopted to achieve a compromise between complete security of supply and capital investment. There are four categories of substation that give varying securities of supply:


Different Layouts for Substations

Single Busbar

The general schematic for such a substation is shown in the figure below.

With this design, there is an ease of operation of the substation. This design also places minimum reliance on signalling for satisfactory operation of protection. Additionally there is the facility to support the economical operation of future feeder bays.

Such a substation has the following characteristics.

Mesh Substation

The general layout for a full mesh substation is shown in the schematic below.

The characteristics of such a substation are as follows.

One and a half Circuit Breaker layout

The layout of a 1 1/2 circuit breaker substation is shown in the schematic below.

The reason that such a layout is known as a 1 1/2 circuit breaker is due to the fact that in the design, there are 9 circuit breakers that are used to protect the 6 feeders. Thus, 1 1/2 circuit breakers protect 1 feeder. Some characteristics of this design are:


Principle of Substation Layouts

Substation layout consists essentially in arranging a number of switchgear components in an ordered pattern governed by their function and rules of spatial separation.

Spatial Separation
 

Separation of maintenance zones

Two methods are available for separating equipment in a maintenance zone that has been isolated and made dead.

1. The provision of a section clearance
2. Use of an intervening earthed barrier

The choice between the two methods depends on the voltage and whether horizontal or vertical clearances are involved.

HENCE:

Separation by earthed barrier = Earth Clearance + 50mm for barrier + Earth Clearance

Separation by section clearance = 2.44m + Earth clearance


Establishing Maintenance Zones

Some maintenance zones are easily defined and the need for them is self evident as is the case of a circuit breaker. There should be a means of isolation on each side of the circuit breaker, and to separate it from adjacent live parts, when isolated, either by section clearances or earth barriers.
 

Electrical Separations

There are at least three such electrical separations per phase that are needed in a circuit:

1. Between the terminals of the busbar isolator and their connections.
2. Between the terminals of the circuit breaker and their connections.
3. Between the terminals of the feeder isolator and their connections.
 

Components of a Substation

The substation components will only be considered to the extent where they influence substation layout.

Circuit Breakers

There are two forms of open circuit breakers:
1. Dead Tank - circuit breaker compartment is at earth potential.
2. Live Tank - circuit breaker compartment is at line potential.

The form of circuit breaker influences the way in which the circuit breaker is accommodated. This may be one of four ways.

Current Transformers

CT's may be accommodated in one of six manners:

In all except the second of the list, the CT's occupy incidental space and do not affect the size of the layout. The CT's become more remote from the circuit breaker in the order listed above. Accommodation of CT's over isolator bushings, or bushings through walls or roofs, is usually confined to indoor substations.
 

Isolators

These are essentially off load devices although they are capable of dealing with small charging currents of busbars and connections. The design of isolators is closely related to the design of substations. Isolator design is considered in the following aspects:

Some types of isolators include:


Conductor Systems

An ideal conductor should fulfil the following requirements:

The most suitable material for the conductor system is copper or aluminium. Steel may be used but has limitations of poor conductivity and high susceptibility to corrosion.

In an effort to make the conductor ideal, three different types have been utilized, and these include:


Insulation

Insulation security has been rated very highly among the aims of good substation design. Extensive research is done on improving flashover characteristics as well as combating pollution. Increased creepage length, resistance glazing, insulation greasing and line washing have been used with varying degrees of success.
 

Power Transformers

EHV power transformers are usually oil immersed with all three phases in one tank. Auto transformers can offer advantage of smaller physical size and reduced losses. The different classes of power transformers are:

Power transformers are usually the largest single item in a substation. For economy of service roads, transformers are located on one side of a substation, and the connection to switchgear is by bare conductors. Because of the large quantity of oil, it is essential to take precaution against the spread of fire. Hence, the transformer is usually located around a sump used to collect the excess oil.

Transformers that are located and a cell should be enclosed in a blast proof room.
 

Overhead Line Terminations

Two methods are used to terminate overhead lines at a substation.

The choice is influenced by the height of towers and the proximity to the substation.

The following clearances should be observed:
 
 
VOLTAGE LEVEL
MINIMUM GROUND CLEARANCE
less than 66kV
6.1m
66kV - 110kV
6.4m
110kV - 165kV
6.7m
greater than 165kV
7.0m