Power System Configurations.

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Switchyards Design Considerations

Fault level 

  • Expressed as MVA or kA and duration of 1-3 sec. 
  • Depends upon source impedance ie distances from the power sources 
  • All CBs to be rated for Fault level 
  • Earthing switches for making the fault level 
  • All busbars, bus supports to be designed to withstand mechanical forces due to short circuits. 

Safety Clearance & Creepage Distance

  • As per voltage level among live parts & earth 
  • Shortest path taken on surface by leakage current. 

Grounding & Lightning Protection

  • Grid/Mesh is formed and burried in ground 
  • Earthing Electrodes (pipe/rod/plates) are connected to mesh 
  • Shield wires to protect against direct lightning strokes 

Safety Interlocks in Control schemes

  • Isolator Open/Close possible only if CB is open. 
  • E/S closing possible only when isolator is closed.etc.

Double Main & Transfer Bus Arrangement (DMT)

Almost all PGCIL 220kV substations use this arrangement

  • 245kV Switchyard Layout

Almost all PGCIL 220kV substations use this arrangement

  • One and a Half Breaker Arrangement
Almost all PGCIL 765kv,400kV substations use this arrangemen
  • Three Main & Transfer Bus Arrangement
Practiced in Europe, Germany
  • Ring or Mesh System
  • Power Line Carrier Coupling

Switchyards Design features

Design Aspects 

  • Single line diagram 
  • Busbar Arrangement 
  • Fault level 
  • Type of construction 
  • Creepage & Clearance 
  • Grounding & Lightning Protection 
  • Safety Interlocks 
  • Other Miscellaneous items e.g lighting , civil

Considerations for Choice of Busbar Arrangement

  • Importance of Substation 
  • Reliability 
  • Redundancy 
  • Economics and availability of finance 
  • Availability of Space and Right Of Way for approaching lines 
  • Future Expansion

Switching Devices 

 Ratings
 Level
 Isolator
 Switch
 Circuit Breaker
Current Carrying
 Load
 Yes
 Yes
 Yes
 Fault
 Yes
 Yes
 Yes
  Current Making
 Load
 No
 Yes
 Yes
 Fault
 No
 No
 Yes
  
Current Breaking 
 Load
 No
 Yes
 Yes
 Fault
 No
 No
 Yes
  
  • Isolator is rated only for carrying current, 
  • Make/Break small line charging current.
  • OPEN/CLOSE operation done under ZERO current condition.

Power System Configurations

Power System Structure


  • Transmission circuits handle/route largest power , interconnect various power stations & feed to load centres 
  • Subtransmission circuits feed to distribution substations & bulk consumers within a geographical area. 
  • Distribution circuits fed from distribution substations feed domestic , medium size industries and commercial consumers.

Requirement for planning of the T&D Network configuration.

  • Voltage levels , Amount of Power Transfer, Length of transmission lines, Fault level 
  • Importance of the consumers / customers 
  • Clearance times, Stability