Electrical Substations

Electrical Substations

Various Scheme used 
1) Single bus Scheme
2) Sectionalized single bus scheme
3) Double bus double breaker scheme
4) Double bus half breaker Scheme or One and half breaker scheme
5) Two main and one auxiliary bus scheme
6) One main and one auxiliary bus scheme

7) Ring bus scheme


Double Bus Half Breaker Scheme
In electrical substation or Switchyard various Bus Bar arrangements are used as it is a common point for various incoming and out going circuits. The most used scheme for high voltage high power switch yard such as a 220 KV or 400 KV is "Double Bus half breaker arrangement" or more commonly know as "Half breaker arrangement" (Fig 1)due to many advantages of this scheme. Schematic diagram of this arrangement is shown below, see it carefully 


One and half breaker scheme
Fig 1

It consist of (Fig 2)
1)two main buses (Main Bus-I and Main Bus-II),
2)Three bays in each Diagonal
3)Each bay consist two isolators or disconnect switch at both end along with earth switch, a circuit breaker at middle  and other auxiliaries like CT, LA etc.
Bay and diagonal in a substation arrangement
Fig 2

To understand complete process we will see all conditions step by step.

Case1; Normal working condition (Fig 3)
All circuit  breakers and isolators are in  closed condition
So the both of incoming lines form generating plant and outgoing lines  of transmission system are remain in contact with both the two busses through circuit breakers.

Fig 3
Case2; Fault on Main Bus-I Fig 4
  •   Sensing elements will send tripping signal to CB-A of Diagonal 1 and CB-A of Diagonal 2 to get opened and disconnect Main Bus-I form the system.
  •   Even though Main Bus-I got out from the system, the power continues form remaining CBs in to the Main Bus-II and evacuate through outgoing lines.
Fig 4

  •   To isolate the faulty part perfectly form the system, isolators of that part must be opened (Fig 5).
  •   To check for the fault and to do repair work on faulty bus it must be earthed through earth switches to drain out completely the remaining electrical charge on the bus (Fig 5).
  •   During maintenance work on Bus-1 the personals are safely isolated form the remain part of the system (Fig 5). 
Fig 5

  • After done with the maintenance or repair work then to connect the bus again in to the system first its earthling switches must be open Fig 6. 
Fig 6
  •   Then isolators associated with it must be close first before circuit breakers Fig 7.
Fig 7

• At last the circuit breakers must be closed to compete the process Fig 8

Fig 8

Case3; Fault on any Breaker Fig 9
 
During normal working if there is a fault on any breaker the remaining breakers continues power transfer   
Example; Fault on CB-B of Diagonal 2 maintain continuity through CB-A and CB-C of Diagonal 2.
• After trouble-shooting and repair the faulty Bay can be again put in to the service by following proper switching sequence  as followed in the previous case.

Fig 9

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