Dr Audih alfaoury
1
Sub-stations are important part of power system.
At many places in the line of the power system(T&D) it may
be
necessary to change some characteristic such as
(voltage, frequency, p.f. etc.) of electric supply. This is
accomplished by suitable apparatus called sub-station.
The following are the important points must take in
consideration while laying out a sub-station :
( I ) It should be located at a proper site. As far as possible, it
should be located at the centre of the load.
(II) It should provide safe and reliable arrangement. For
safety, and operation.
(III) It should provide clearances for maintenance and
facilities for carrying out repairs.
(IV) It should be easily operated and maintained.
(V) It should involve minimum capital cost.
According to design feature ,the sub-station has many
components (e.g. circuit breakers, switches, fuses,
instruments etc.) which ensure continuous and reliable
service, the sub-stations are classified as :
(i) Pole-mounted sub-station
(ii) Underground sub-station
(iii) Outdoor sub-station
(iv) Indoor sub-station
Pole-Mounted Sub-Station
Low tension oil
circuit breaker
Underground sub-station
Underground Sub-Station:
The underground sub-station requires more careful
consideration than other types of sub-stations, the following
points must be kept in view:
(i) The size of the station should be as minimum as possible.
(ii)Should be reasonable access for equipment and
personnel.
(iii)Should be provision(secured) for emergency lighting and
protection against fire.
(iv) Should be good ventilation.
(v)Should be provision for remote indication of the rise in
temperature so that H.V. supply can be disconnected.
(vi) The transformers, switches and fuses should be air
cooled.
Symbols for Equipment in Sub-Stations
1- Security of supply:
Security of supply may be considered in terms of the effect
of the loss of supply ( arising from fault conditions or from
outages due to maintenance).
The British Code for the design of high voltage open
terminal substations BS 7354 categorizes substation service
continuity as:
Category1: No outage necessary.
Category2 : Short outage necessary to transfer the load to
an alternative circuit for maintenance or fault conditions.
Category3: Loss of a circuit or section.
Category4: Loss of substation.
Mesh arrangement
disconnector
CB1
CB3
CB2
(all busbars, circuit breakers (CB)and disconnectors
must be capable of carrying the combined loads of
both transformers and line circuit power transfers).
3-Normal operation is with the bypass
disconnectors or optional circuit
breaker open so that both transformers
are not disconnected for a single
transformer fault.
Three switch mesh
By pass
An arrangement known as a three
switch mesh.
1-only three circuit breakers controlling
four circuits.
2-Any circuit breaker may be
maintained at any time without
disconnecting that circuit.
4-In case of a fault on one transformer circuit, with
disconnects the healthy transformer circuit can continue the
supply without affecting the feeders.
5- A fault on the bus section circuit breaker causes complete
substation shutdown until isolated and power restored.
A full mesh is composed from three switch arrangement for
multiple circuit sub-stations is the full mesh layout as shown
in Fig.
section circuit breaker
Full mesh
1- Operation of two circuit breakers is required to connect or
disconnect a circuit and disconnection involves opening
the mesh.
2- Line or transformer circuit disconnectors may then be
used to isolate the particular circuit and the mesh
reclosed.
3- Circuit breakers may be maintained without loss of supply .
(the particular circuit may be fed from an alternative route
around the mesh).
4- Busbar faults will only cause the loss of one circuit.
Ring
The ring busbar offers increased security compared to the
single busbar arrangement. A typical scheme which would
occupy more space than the single busbar arrangement is
shown in Fig.
(I) Category 1 :
within the substation for maintenance or fault the 1 and 1⁄2
breaker scheme under maintenance conditions in the circuit
breaker area is used .
Isolator
Circuit Breaker
Isolator
feeder
Bypass isolator for circuit
breaker maintenance
GIS 132kV substation double switchyard arrangement indoor
The circuit breaker bypass facilities and security of the mesh
arrangement coupled with some of the flexibility of the double
busbar scheme.
This scheme is used at important high voltage sub-stations
and large generating.
1- Additional costs of circuit breakers are involved together
with complex protection arrangements.
2- It is possible to operate with any one pair of circuits, or
group of pairs of circuits separated from the remaining
circuits.
3- The circuit breakers and other system components must be
rated for the sum of the load currents of two circuits.
4- High security against loss of supply.
5- High cost
(II) Category2
The double busbar arrangement consists of two bus-bars, a
“main” bus-bar and a “spare” bus-bar. Is the most popular
open terminal outdoor substation throughout the world. It has
the flexibility to allow the grouping of circuits onto separate
busbars with facilities for transfer from one busbar to another
for maintenance or operational reasons. A typical double
busbar with transfer busbar arrangement is shown in Fig.
1-When circuit breakers are
under maintenance the
protection is arranged to
trip
the
bus
coupler
breaker.
2-The system is considered
to offer less flexibility
buscoupler
Double busbar scheme with bypass disconnector and buscoupler switch
bypass disconnector
bus-coupler
busbar
1. Each circuit may be connected to either busbar using the
busbar selector disconnectors. On-load busbar selection
may be made using the bus-coupler circuit breaker.
2. Motorized busbar selector disconnectors may be used to
reduce the time to reconfigure the circuit arrangements.
3. Busbar and busbar disconnector maintenance may be
carried out without loss of supply to more than one circuit.
4. The use of circuit breaker bypass isolator facilities is not
considered to offer substantial benefits since modern
circuit breaker maintenance times are short and in highly
interconnected systems alternative feeder arrangements
are normally possible.
5. A variant on the scheme uses a ‘wrap around’ busbar
layout arrangement as shown in Fig. in order to reduce the
length of the substation.
(III) Category 3 -Single busbar scheme is use
Simple to operate, in figure we illustrates a five circuit
breaker single busbar arrangement with four feeder
circuits, one bus section and ten disconnectors.
1. Each circuit is protected by
bus section
its own circuit breaker and
hence plant outage does not
necessarily result in loss of
supply.
2. A fault on a 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.
bus section
O.C.B oil circuit breaker
C.T current transformer
L.A lightning arrester
P.T. Power transformer
3. A fault on a bus section circuit breaker causes complete
shutdown of the sub-station. All circuits may be restored after
isolating the faulty circuit breaker .
4. A busbar fault causes loss of one transformer and one
feeder. Maintenance of one busbar section or disconnector
will cause the temporary outage of two circuits.
5. Maintenance of a feeder or transformer circuit breaker
involves loss of that circuit. The introduction of bypass
isolators between the busbar and circuit isolator as in( Fig.a)
.allows
circuit
breaker
maintenance without loss of
the circuit. Bypass may also
be obtained by using a
disconnector on the outgoing ways between two
switchgear bays (Fig. b).
Fig. b
Fig.a
(IV) Category 4
Loss of substation : Single bus bar without by pass section in
busbar ,this arrangement is used
the radial
busbar
decreased security compared to the double
busbar
arrangement since no alternative power flow routes are
available. A typical scheme which would occupy less space
than the other arrangements and is cheaper schemes.
A fault on a 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
2 Extendibility :The design should allow for future extendibility. Adding bays
of switchgear to a substation is normally possible ,as well as
addition of overhead line or cable feeder bays are required
then busbar disconnectors may be installed at the outset
(known as ‘skeleton bays’). The use of gas insulated
switchgear (GIS) not permit to add any future extension
work, but an open terminal switchyard arrangement allows
the user a choice of switchgear for future extension work.
3- Maintainability
The design must take into account the electricity supply
company system planning and operations procedures
together with a knowledge of reliability and maintenance
requirements for the proposed substation equipment.
Portable earthing points and earthing switch/interlock
requirements will also need careful consideration.
Similarly standard minimum clearances for safe working and
access to equipment with safety clearances.
Clearances and ground clearances based on British practice
(BS7354) are given in table
The figures illustrate diagrammatically the clearances
required between the different items of substation equipment
4 Operational flexibility
Two transformer substation operation with the facility to take
out of service one and restore to service, without loss of
supply would be a normal design consideration.
In general a multiple busbar arrangement will provide greater
flexibility than a ring busbar.
5 Protection arrangements
The design must allow for the protection of each system
element by adding suitable CT locations to ensure
overlapping of protection zones. The number of CB that
require to be tripped, the type of protection and extent and
type of mechanical or electrical interlocking must be
considered.
6 Short circuit limitations
In order to keep fault levels down parallel connections
should be avoided. The system split is required.
Multi-busbar arrangements with sectioning allow the system
to be split or connected through a fault limiting reactor. It is
also possible to split a system using circuit breakers in a
mesh or ring type substation layout.
7 Land area
The cost land in a densely populated area is considerable.
Therefore the compact substation design is use of indoor
gas insulated switchgear (GIS) substation designs or by
using such configurations as the transformer feeder
substation layout.
8 Cost
A satisfactory cost comparison between different substation
layout designs is extremely difficult because of the differences
in performance and maintainability.
plant, structural, civil and space costs, etc..
Diagram of 66/11 kV Sub-Station
That part of power system which distributes electric power
for local use is known as distribution system. It generally
consists of feeders, distributors and the small cable.
There is no definite line between transmission and
distribution according to voltage or bulk capacity. In general,
the a.c. distribution system is the electrical system between
the step-down substation fed by the transmission system
and the consumers. The a.c. distribution system is classified
into:
a) primary distribution system.
which operates at voltages higher than general utilization
and handles large electrical energy. The most commonly
used primary distribution voltages are 11 kV, 6·6kV and 3·3
kV. The, primary distribution is carried out by 3-phase, 3-wire
system.
primary distribution system
b) secondary distribution system.
It is that part of a.c. distribution system which includes the
range of voltages for ultimate consumer. The secondary
distribution employs 400/230 V, 3-phase, 4-wire system.
The substations are situated near the consumers’ localities
and contain step-down transformers. At each distribution
substation, the voltage is stepped down to 400V and power
is delivered by 3-phase,4-wire a.c. system. The voltage
between any two phases is 400 V and between any phase
and neutral is 230V. The single phase domestic loads are
connected between any one phase and the neutral,
whereas 3-phase 400 V motor loads are connected across
3-phase lines directly.
secondary distribution system
Connection Schemes of Distribution System
1. Radial System:
In this system, separate feeders radiate from a single
substation and feed the distributors at one end only.
(a) The consumers suffering from
any fault on the feeder which cuts
off supply who are on the side of
the fault away from the substation.
(b) The consumers at the distant
end would be subjected of voltage
fluctuations .
(c)) Due to these limitations, this
system is used for short distances
only.
2-Ring main system:
In this system, the distribution transformers form a loop.
The loop circuit starts from the substation bus-bars, makes a
loop through the area to be served, and returns to the
substation. The ring main system has the following
advantages:
(a) There are less voltage fluctuations at consumer’s
terminals.
(b) The system is very reliable as each distributor (is fed via
two feeders).
Design Considerations in Distribution System
Design of distributors requires careful consideration.
(i) Feeders: A feeder is designed from the point of view of its
current carrying capacity but voltage drop consideration is
not important.
(ii) Distributors. A distributor is designed from the point of
view of the voltage drop in it (± 6% of rated value). The size
and length of the distributor should be such that voltage at
the consumer’s terminals is within the permissible limits.
That’s all…………………Thanks