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:
- Category 1: No outage is necessary within the substation for either maintenance or fault conditions.
- Category 2: Short outage is necessary to transfer the load to an alternative circuit for maintenance or fault conditions.
- Category 3: Loss of a circuit or section of the substation due to fault or maintenance.
- Category 4: Loss of the entire substation due to fault or maintenance.
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.
- Each circuit is protected by its own circuit breaker and hence plant outage does not necessarily result in loss of supply.
- A fault on the feeder or transformer circuit breaker causes loss of the transformer and feeder circuit, one of which may be restored after isolating the faulty circuit breaker.
- A fault on the bus section circuit breaker causes complete shutdown of the substation. All circuits may be restored after isolating the faulty circuit breaker.
- A busbar fault causes loss of one transformer and one feeder. Maintenance of one busbar section or isolator will cause the temporary outage of two circuits.
- Maintenance of a feeder or transformer circuit breaker involves loss of the circuit.
- Introduction of bypass isolators between busbar and circuit isolator allows circuit breaker maintenance facilities without loss of that circuit.
The characteristics of such a substation are as follows.
- Operation of two circuit breakers is required to connect or disconnect a circuit, and disconnection involves opening of a mesh.
- Circuit breakers may be maintained without loss of supply or protection, and no additional bypass facilities are required.
- Busbar faults will only cause the loss of one circuit breaker. Breaker faults will involve the loss of a maximum of two circuits.
- generally, not more than twice as many outgoing circuits as infeeds are used in order to rationalise circuit equipment load capabilities and ratings.
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:
- There is the additional cost of the circuit breakers together with the complex arrangement.
- It is possible to operate any one pair of circuits, or groups of pairs of circuits.
- There is a very high security against the loss of supply.
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
- Earth Clearance: this is the clearance between live parts and earthed structures, walls, screens and ground.
- Phase Clearance: this is the clearance between live parts of different phases.
- Isolating Distance: this is the clearance between the terminals of an isolator and the connections thereto.
- Section Clearance: this is the clearance between live parts and the terminals of a work section. The limits of this work section, or maintenance zone, may be the ground or a platform from which the man works.
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.
- A section clearance is composed of a the reach of a man, taken as 8 feet, plus an earth clearance.
- For the voltage at which the earth clearance is 8 feet, the space required will be the same whether a section clearance or an earthed barrier is used.
Separation by section clearance = 2.44m + Earth clearance
- For vertical clearances it is necessary to take into account the space occupied by the equipment and the need for an access platform at higher voltages.
- The height of the platform is taken as 1.37m below the highest point of work.
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
- Together with maintenance zoning, the separation, by isolating distance and phase clearances, of the substation components and of the conductors interconnecting them constitute the main basis of substation layouts.
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.
- Ground Mounting and Plinth Mounting: the main advantages of this type of mounting are its simplicity, ease of erection, ease of maintenance and elimination of support structures. An added advantage is that in indoor substations, there is the reduction in the height of the building. A disadvantage however is that to prevent danger to personnel, the circuit breaker has to be surrounded by an earthed barrier, which increases the area required.
- Retractable Circuit Breakers: these have the advantage of being space saving due to the fact that isolators can be accommodated in the same area of clearance that has to be allowed between the retractable circuit breaker and the live fixed contacts. Another advantage is that there is the ease and safety of maintenance. Additionally such a mounting is economical since at least two insulators per phase are still needed to support the fixed circuit breaker plug contacts.
- Suspended Circuit Breakers: at higher voltages tension insulators are cheaper than post or pedestal insulators. With this type of mounting the live tank circuit breaker is suspended by tension insulators from overhead structures, and held in a stable position by similar insulators tensioned to the ground. There is the claimed advantage of reduced costs and simplified foundations, and the structures used to suspend the circuit breakers may be used for other purposes.
- Over Circuit Breaker bushings or in pedestals.
- In separate post type housings.
- Over moving bushings of some types of insulators.
- Over power transformers of reactor bushings.
- Over wall or roof bushings.
- Over cables.
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:
- Space Factor
- Insulation Security
- Standardisation
- Ease of Maintenance
- Cost
- Horizontal Isolation types
- Vertical Isolation types
- Moving Bushing types
Conductor Systems
An ideal conductor should fulfil the following requirements:
- Should be capable of carrying the specified load currents and short time currents.
- Should be able to withstand forces on it due to its situation. These forces comprise self weight, and weight of other conductors and equipment, short circuit forces and atmospheric forces such as wind and ice loading.
- Should be corona free at rated voltage.
- Should have the minimum number of joints.
- Should need the minimum number of supporting insulators.
- Should be economical.
- Flat surfaced Conductors
- Stranded Conductors
- Tubular Conductors
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:
- o.n.: Oil immersed, natural cooling
- o.b.: Oil immersed, air blast cooling
- o.f.n.: Oil immersed, oil circulation forced
- o.f.b.: Oil immersed, oil circulation forced, air blast cooling
Overhead Line Terminations
Two methods are used to terminate overhead lines at a substation.
- Tensioning conductors to substation structures or buildings
- Tensioning conductors to ground winches.
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