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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:


  • 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.


http://www.eng.uwi.tt/depts/elec/staff/alvin/ee35t/notes/Single-Busbar.jpg


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.


Mesh Substation


The general layout for a full mesh substation is shown in the schematic below.
http://www.eng.uwi.tt/depts/elec/staff/alvin/ee35t/notes/Full-mesh.jpg
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.
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