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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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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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