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Introduction:
Sub-stations are important part of power system ,its
responsible to transfer some characteristic such as (voltage,
frequency, p.f. etc.) of electric supply.
The important points must take in consideration while laying
out a sub-station ,the substation should be:
Located at a proper site ( At the load centre ).
Provide safe and reliable arrangement.
Safe clearances.
Easily operated and maintained.
Minimum capital cost.
The sub-station has many components (e.g. circuit breakers,
switches, fuses, instruments etc.) which ensure continuous
and reliable service.
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Low tension oil circuit breaker
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The sub-stations are classified as :
1- Pole-mounted sub-station
2-Underground sub-station
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Underground Sub-Station:
The underground sub-station requires more careful
consideration than other types of sub-stations:
(i) The size of the station should be as minimum as possible.
(ii) Reasonable access for equipment and personnel.
(iii) Lighting secured and protection against fire.
(iv) Good ventilation.
(v) Remote indication of the rise in temperature so H.V.
supply can be disconnected.
(vi) The transformers, switches and fuses should be air
cooled.
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3- Outdoor sub-station
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4- Indoor sub-station
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Symbols for Equipment in Sub-Stations
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1- Security of supply:
Security of supply (reliability) : In terms of the effect of the loss
of supply. The British Code for the design of high voltage open
terminal substations BS 7354 categorizes substation service
as the following categories:
Category 1: No outage (maximum security of supply).
Category 2 : Short outage to transfer the load to an alternative
circuit in case of maintenance or fault conditions.
Category 3: Loss of a circuit or section.
Category 4: Loss of substation.
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Three switch mesh
Mesh arrangement (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. **Note: (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 and optional circuit
breaker open so in case of
maintenance or fault we transfer using
optional breaker.
CB1
CB2 CB3
disconnector
By
pa
ss
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4-In case of a fault on one transformer circuit, we can disconnect,
and the healthy transformer circuit continue the supply without
affecting the feeders.
5-A fault on the bus section circuit breaker causes complete
substation shutdown until isolate the fault and power restored.
full mesh circuit:
Is composed from three switch's arrangement for multiple
circuit sub-stations is the full mesh layout as shown in Fig.
Full mesh
section circuit breaker
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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 .
4- Busbar faults will only cause the loss of one circuit.
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Ring circuit:
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.
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Category 1 :
For maximum security of supply within the substation for
maintenance or fault , the ( one and half ) 1 and 1⁄2 circuit
breaker scheme is used .
Bypass isolator for circuit breaker maintenance
Circuit Breaker
Isolator
feeder
Isolator
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GIS 132kV substation double switchyard arrangement indoor
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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 the 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
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Category2
Tow schemes are Used :
1-Double busbar arrangement with transfer busbar
arrangement . This arrangement consists of:
Two bus-bars, a “main” bus-bar and a “spare” bus-bar. It’s
the most popular open terminal outdoor substation. Which
offering the flexibility to allow the grouping of circuits onto
separate busbars to transfer one busbar to another for
maintenance .
-When circuit breakers are
under maintenance the
protection is arranged to
trip the bus coupler
breaker.
-The system less flexibility.
bus-
coupler
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2- Double busbar scheme with bypass disconnector and bus-
coupler switch
Each circuit may be connected to either busbar using the
busbar selector disconnectors.
On-load busbar selection may be done using the bus-coupler
circuit breaker.
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ass d
isco
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bu
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ou
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busbar
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- Motorized busbar selector disconnectors may be used to
reduce the time to reconfigure the circuit arrangements.
- Busbar and busbar disconnector maintenance may be
carried out without loss of supply to more than one circuit.
- 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.
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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.
- Each circuit is protected by
its own circuit breaker.
- A fault on a feeder or
transformer circuit breaker
causes loss of the
transformer and feeder
circuit which may be
restored after isolating the
faulty circuit breaker.
bus section
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bus section
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O.C.B oil circuit breaker C.T current transformer L.A lightning arrester P.T. Power transformer
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- 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 .
- 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.
- 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 out-going
ways between two switchgear
bays (Fig. b). Fig.a Fig. b
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Category 4
Single bus bar without by pass section in busbar is used.
- Radial busbar arrangement is used this decreased security
compared to the double busbar arrangement since no
alternative power flow routes are available.
- This scheme occupy less space than the
other arrangements and is cheaper.
- A fault on a feeder or transformer circuit breaker
causes loss of the transformer and feeder circuit.
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2 Extendibility :-
The design should allow for future extendibility (bays known
as ‘skeleton bays’ for switchgear ,overhead line or cable
feeder.
Note :Gas insulated switchgear (GIS) not permit to add any
future extension work.
In 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 of
reliability and maintenance requirements for the proposed
substation equipment.
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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 in the next slide.
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This figure illustrate clearances required between the different items of substation equipment
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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.(two transformers in parallel)
- Multiple busbar arrangement will provide greater flexibility
than a ring busbar.
5 Protection arrangements:
- Schemes must allow for the protection of each system
element by adding suitable CT locations to ensure
overlapping of protection zones(see figure 1&2 ).
- 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.
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Figure 1 Overlapping of protection zones. Figure 2.
6 Short circuit limitations:
-Parallel connections should be avoided. (In order to keep fault levels down).
-The system split is required using multi-busbar arrangements
with sectioning, to 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..
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Diagram of 66/11 kV Sub-Station 35 Dr Audih
Distribution system is a part of power system which
distributes electric power for local users. It’s generally
consists of feeders, distributors and the small long cables.
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.
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primary distribution system
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It is that part of a.c. distribution system which includes the
range of voltages for ultimate consumers. The secondary
distribution employs 400/230 V, 3-phase, 4-wire system.
The substations (step-down transformers) are situated near
the consumers. At each distribution substation, the voltage is
stepped down to 400VLL and power is delivered by 3-phase,4-
wire a.c. system.
The voltage between any phase and neutral (single phase
domestic loads) is 230V..
b) Secondary distribution system.
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secondary distribution system
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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.
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2-Ring main system:
In this system, the distribution
transformers form a loop .
The loop circuit starts from the
substation to make a loop
through the area to be served,
and returns to the substation.
The ring main system has the
following advantages:
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(a) less voltage fluctuations at consumer’s terminals.
(b)The consumers are feds via two feeders).
(c) High efficiency than radial systems.
(d) Low power loss compare with radial systems.
(d) Low voltage drop compare with radial systems.
Some considerations in distribution systems
Distribution systems required careful consideration such as :
(i) Feeders: The current carrying capacity of a feeder is
considerate , but voltage drop consideration is not
important.
(ii)Distributors. The voltage drop effected distributors is
(± 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
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