What is DISTRIBUTION SUBSTATIONS

DISTRIBUTION SUBSTATIONS

SUBSTATION & FEEDER 

Substations transform the electrical energy to a different voltage and transfer electrical energy from one line to another. Usually planners try to locate a substation near the center of load.
A feeder is an electrical distribution circuit fed from a single source point: through a breaker or a fuse at the substation.
It operates at the primary distribution voltage and delivers power to an assigned area. Together, the feeders emanating from a substation serve all the loads and cover all the areas assigned to that particular substation.
This area should be an approximate circle, polygon or hexagon and the substation should be located approximately at the central point in cases where the load is more or less uniform. Some times there are constraints of geography – river, canal, forest etc. or just poor planning in the past which can cause an exception to this rule. A feeder consists of a single route, leaving the substation which branches with spurs. Feeders are planned by starting from the substation with the main trunk portion of the largest economical conductor and generally follow streets, roads, highways and property boundaries.

DISTRIBUTION SUBSTATION

The distribution substation is the convenient point for the control and protection of the distribution network. A typical substation may have the following equipment: 1. Power Transformer(s)
 2. Circuit Breakers
3. Disconnecting Switches And Isolators
 4. Station Buses
 5. Current Limiting Reactors
 6. Shunt Reactors
 7. Current Transformers
 8. Potential Transformers
 9. Capacitor Voltage Transformers.
10.Coupling Capacitors
11.Series Capacitors
 12.Shunt Capacitors
13.Grounding System
14.Lightning Arresters And/Or Gaps
15.Line Traps
16.Protective Relays
17.Station Batteries
18. And Other Apparatus

SUBSTATION BUS SCHEMES 

The electrical and physical arrangements of the switching and busing at the sub-transmission voltage level are determined by the selected substation scheme (or diagram). On the other hand, the selection of a particular substation scheme is based upon safety, reliability, economy, simplicity, and other considerations. The most commonly used substation bus schemes are:
1. Single bus scheme
 2. Double bus – double breaker (or double main) scheme
3. Main – and – transfer bus scheme
 4. Double bus – single breaker scheme 5. Ring bus scheme
 6. Breaker – and – a half scheme

Single Bus Scheme

Advantage: Lowest cost.
Disadvantages:
Failure of bus or any circuit breaker results in shutdown of entire substation.
 Difficult to do any maintenance.
Bus cannot be extended without completely de energizing substation.
Can be used only where loads can be interrupted or have other supply arrangements.

Double Bus – Double Breaker

Advantages:
Each circuit has two dedicated                              
breakers.
Has flexibility in permitting
feeder circuits to be connected
to either bus.
Any breaker can be taken out
of service for maintenance.
High reliability.
Disadvantages:
Most expensive.
Would lose half the circuits for
breaker failure if circuits are
not connected to both buses.

Main – And – Transfer

Advantages:
Low initial cost and ultimate                                          
cost.
Any breaker can be taken out for
service or maintenance.
Potential devices may be used on
the main bus for relaying.
Disadvantages:
Requires one extra breaker for
the bus tie.
Switching is somewhat
complicated when maintaining a
breaker.
Failure of bus or any circuit
breaker results in shutdown of
entire substation.

Double Bus – Single Breaker                 

Advantages:
Permits some flexibility with two
operating buses.
Either main bus may be isolated for
maintenance.
Circuit can be transferred readily from
one bus to the other by use of bus –tie
breaker and bus selector disconnect
switches.
Disadvantages:
One extra breaker is required for the bus tie.
Four switches are required per circuit.
Bus protection scheme may cause loss of substation when it operates if all
circuits are connected to that bus.
High exposure to bus faults.
Line breaker failure takes all circuits connected to that bus out of service.
Bus – tie breaker failure takes entire substation out of service.

Ring Bus Scheme                                                                       

Advantages:
Low initial and ultimate cost.                                                    

Flexible operation for breaker
maintenance without interrupting
load.
Requires only one breaker per circuit.
Does not use main bus.
Each circuit is fed by two breakers.
All switching is done with breakers.
Disadvantages:
If a fault occurs during a breaker maintenance period, the ring can be separated into
two sections.
Automatic reclosing and protective relaying circuitry rather complex.
If a single set of relays is used, the circuit must be taken out of service to maintain
the relays. (common to all schemes.)
Requires potential device on all circuits since there is no definite potential reference
point. These devices may be required in all cases for synchronizing, live line, or
voltage indication.
Breaker failure during a fault on one of the circuits causes loss of one additional
circuit owing to operation of breaker – failure relaying.

Breaker – And – a Half Scheme

Advantages:                                                                                   

Most flexible operation.
High reliability.
Breaker failure of bus side breaker removes
only one circuit from service.
All switching is done with breakers.
Simple operation; no disconnect switching
required for normal operation.
Either main bus can be taken out of service
at any time for maintenance.
Bus failure does not remove any feeder
circuits from service.
Disadvantages:
1½ breaker per circuit.
Relaying and automatic reclosing are
somewhat involved since the middle breaker
must be responsive to either of its
associated circuits.