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Designing of HV Power Substation and Layout
electrical-engineering-portal
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Introduction
2
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Substation Earthing Calculation Methodology (Earthing Materials)
4
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Different Layouts for Substations (single busbar, mesh, 1 1/2 cb)
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6
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Components of a Substation (cbs, cts, isolators, insulation, transformers etc
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This allows for the
control of power flows in the network and general switching operations for maintenance purposes
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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
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The earthing system also guarantees equipotential bonding such that there are no dangerous potential gradients
developed in the substation
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Touch Voltage: This is the difference in potential between the surf ace potential and the potential at
an Earthed equipment whilst a man is standing and touching the earthed structure
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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
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Mesh Voltage: This is the maximum touch voltage that is developed inthe mesh of the earthing grid
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A grid layout with particular conductors is then analyzed to determine the
effective substation earthing resistance, from which the earthing voltage is calculated
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Additionally, it is necessary to ensure a sufficient margin such that expansion of the system is catered for
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These tests are best performed in dry
weather such that conservative resistivity readings are obtained
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Conductors
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Bare copper conductor is usually used for the substation earthing grid
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25-0
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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
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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
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3
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These rods are usually made of solid copper, or
copper clad steel
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Switchyard Fence
Earthing: The switchyard fence earthing practices are possibleand are used by different utilities
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Extend the substation earth grid 0
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5m beyond the fence perimeter
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2
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This earthing rod system is not coupled to the main substation earthing grid
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In an ideal substation all circuits and equipment would be duplicated such that following a fault, or during
maintenance, a connection remains available
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Methods have been adopted to achieve a compromise between complete security of supply and capital
investment
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Category 2 – Short outage is necessary to transfer the load to an alternative circuit for maintenance or
fault conditions
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Category 4 – Loss of the entire substation due to fault or maintenance
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With this design, there is an ease of
operation of the substation
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Additionally there is the
facility to support the economical
operation of future feeder bays
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Each circuit is protected by its own
circuit breaker and hence plant
outage does not necessarily result
in loss of supply
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A fault on the feeder or transformer
Single- busbar substation layout
circuit breaker causes loss of the
transformer and feeder circuit, one
of which may be restored after isolating the faulty circuit breaker
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A fault on the bus section circuit breaker causes complete shutdown of the substation
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A busbar fault causes loss of one transformer and one
feeder
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Maintenance of one busbar section or isolator will cause the temporary outage of two circuits
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Maintenance of a feeder or transformer circuit breaker involves loss of the circuit
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Introduction of bypass isolators between busbar and circuit isolator allows circuit breaker maintenance
facilities without loss of that circuit
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The characteristics of such a substation are as follows
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Circuit breakers may be maintained
without loss of supply or protection, and no additional bypass facilities are required
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Breaker faults will involve the loss of a maximum of
two circuits
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One and a half Circuit Breaker layout
The layout of a 1 1/2 circuit breaker substation is shown in the schematic below
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Thus, 1 1/2 circuit breakers
protect 1 feeder
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There is the additional cost of the
circuit breakers together with the
complex arrangement
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It is possible to operate any one
pair of circuits, or groups of pairs of
circuits
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There is a very high security
against the loss of supply
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Spatial Separation
1
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2
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3
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4
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The limits of this work section, or maintenance zone, may be the ground or a platform from which the man
works
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The provision of a section
clearance
2
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A section clearance is
composed of a the reach of a man,
taken as 8 feet, plus an earth
clearance
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HENCE:
Separation by earthed barrier = Earth
Clearance + 50mm for barrier + Earth
Clearance
Separation by section clearance = 2
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The height of the platform is taken as 1
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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
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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
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Between the terminals of the bus bar isolator and their connections
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Between the terminals of the circuit breaker and their connections
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Between the terminals of the feeder isolator and their connections
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Circuit Breakers
There are two forms of open circuit breakers:
1
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2
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The form of circuit breaker influences the way in which the circuit breaker is accommodated
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1
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An added advantage is that in indoor substations, there is the reduction in the
height of the building
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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
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Additionally such a mounting is
economical since at least two insulators per phase are still needed to support the fixed circuit breaker plug
contacts
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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
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Current Transformers
CT’s may be accommodated in one of six manners:
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1
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2
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3
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4
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5
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6
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In all except the second of the list, the CT’s occupy incidental space and do not affect the size of the layout
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Accommodation of CT’s over
isolator bushings, or bushings through walls or roofs, is usually confined to indoor substations
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The design of isolators is closely related to the design of substations
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Should be able to withstand forces on it due to its situation
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Should be corona free at rated voltage
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Should need the minimum number of supporting insulators
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The most suitable material for the conductor system is copper or aluminium
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In an effort to make the conductor ideal,
three different types have been utilized, and these include:
Flat surfaced Conductors
Stranded Conductors
Tubular Conductors
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Insulation
Insulation security has been rated very highly among the aims of good substation design
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Increased
creepage length, resistance glazing, insulation greasing and line washing have been used with varying degrees
of success
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Auto transformers can offer
advantage of smaller physical size and reduced losses
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n
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b
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f
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: Oil immersed, oil circulation forced
o
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b
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For economy of service roads,
transformers are located on one side of a substation, and the connection to switchgear is by bare conductors
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Hence, the transformer is usually located around a sump used to collect the excess oil
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Overhead Line Terminations
Two methods are used to terminate overhead lines at a substation
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Tensioning conductors to substation structures or buildings
2
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The choice is influenced by the height of towers and the proximity to the substation
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1 m

66kV – 110kV

6
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7 m

Greater than 165kV

7

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