Substation and its Equipments

Substation & Its Equipments:

  • An electrical substation is an assembly of electrical components including bus bar, switchgears, power transformers & axillaries.
  • A substation receives electrical power from generating station via incoming transmission line & delivers electrical power via outgoing transmission lines.
  • Substations are integral part of power system & form important link between the generating stations, transmission system, distribution system & load point.

Functions of The Substation:

  • The main function of substation is to step-down the voltage level of the bulk power transmitted from a central station.
  • Supply of electrical power within specified voltage & frequency limits.
  • Supply of required electrical power to all the consumers.  
  • Supply of electrical power to all the consumers with shortest possible fault duration.
  • Supply of electrical power to the consumers with a optimum efficiency of plant & network.

Essential Features of Substation:

A typical substation has the following essential features:
  • Outdoor switchyard having bus bar arrangement.
  • Low voltage switchgear, medium voltage switchgear & control room building.
  • Office building.
  • Road & rail tracks for transporting equipment.
  • Incoming line tower & outgoing line towers/ cables.
  • Stores.
  • Maintenance workshops
  • Auxiliary power supply scheme, protection system.
  • Battery room & low voltage DC supply system.
  • Fire fighting system.
  • Station earthing system.
  • Lightning protection system, overhead shielding.
  • Drainage system.
  • Substation lighting system.
  • Fence & gate, security system.
  • Water system for earthing & drinking.

Substation Equipments:

  • Bus bars
  • Surge Arrestors (Lightning Arrestors)
  • Isolators (Disconnecting Switch) with or without Earth Switch
  • Earthing Switches
  • Current Transformers
  • Voltage Transformers
  • Circuit Breakers
  • Power Transformers
  • Shunt Reactors
  • Shunt Capacitors
  • Marshalling Box Kiosks
  • Metering Panels, Control & Relay Panels
  • Low Voltage AC Switchgears
  • Station Earthing System
  • Lightning Mast
  • Power Cables
  • Control Cables
  • Insulators
  • Carrier Equipment
  • Station Battery/ DC System
  • Fire Extinguishers/ Fire Fighting Equipments
  • Auxiliary Transformer
  • Nitrogen Injection Fire Extinguishing System

Substation Equipments & Its Functions:

BUS BAR: Various incoming & outgoing circuits (transmission lines) are connected to the bus bar.
  • Bus bar receives power from incoming circuits & delivers power to outgoing circuits.
  • Bus bars are used to carry very large currents or to distribute current to multiple device within switchgear or equipment.

Bus bars are of two types:
  • Rigid Aluminum tubular bus bars supported on post insulators.
  • Flexible ACSR or Aluminum stranded bus bars supported from two ends by strain insulators
  • For rigid aluminium bus bar, the following are the sizes commonly used for various voltages:
    33 kV – 50 mm size
    66 kV – 63 mm size
    110 kV – 75 mm size
    220 kV – 75 mm size
    400 kV – 100 mm size
  • For Flexible ACSR or Aluminum stranded bus bar, following are the conductors & its sizes:
    Moose conductor ACSR – 54/ 3.53 mm (No. of Aluminium strand/ dia in mm), 7/ 3.53 mm (steel used for reinforcement) --- For 66 kV.
    Falcon conductor ACSR – 54/ 4.539 mm for Aluminium, 19/ 2.616 mm for steel --- For 220 kV/ 400 kV.
    Drake ACSR conductor – 26/ 4.442 mm for Aluminium, 7/ 3.454 mm for steel --- For 33 kV & 11 kV

Surge Arrestors (Lightning Arrestors):

  • Surge arrestor discharges the over voltage surge to earth & protect the equipment insulation from switching surges & lightning surges.
  • These are connected between phase conductor & ground, gapped arrestors & gapless zinc oxide arrestors are of two major types. Zinc oxide material exhibits low resistance at high voltage & high resistance at low voltage.

Isolators (Disconnecting Switches):

  • Isolators or disconnecting switches can be used for disconnecting a circuit under no current condition.
  • They are generally installed along with the circuit breaker.
  • An isolator can be opened after opening of the circuit breaker.
  • It can be opened or closed under no current condition & provides isolation of circuits for maintenance purpose.
  • They are named according to the circuit they break.
    Bus isolator without earth switch
    Line isolator without earth switch
    Line isolator with earth switch
    Bypass isolator without earth switch
    Transformer isolator without earth switch

  • Isolators are of two types
    Double Break Type
    Single Break Type
  • The operation of isolator may be manual or remote (power operated)
  • Isolators will have an auxiliary switch for connection of indication lamp or for interlocking.
  • Isolators are of two types:
    Double Break Type
    Single Break Type
  • The operation of isolator may be manual or remote (power operated)
  • Isolators will have an auxiliary switch for connection of indication lamp or for interlocking.
  • The followings are the different voltage level isolators used in KPTCL:
    For 220 kV – 1250 amps, 31.5 KA/ 40 KA for 3 seconds short time rating remote, electrically cum manually operated double break type disconnects with solid core insulators.

    Upright with earth switch LPH 5750 mm.
    Upright without earth switch LPH 5750 mm.
    Upright without earth switch LPH 8250 mm.
    Under hung without earth switch Mounting height 12850 mm

  • For 110 kV- 1250 Amps, 31.5kA double break:
    Upright with Earth switch LPH 4600 mm.
    Upright without Earth switch LPH 4600 mm.
    Upright without Earth switch LPH 6350 mm.
    Under hung/ bypass without earth switch mounting height 1000 mm.
  • For 66 kV- 1250/800 Amps, 31.5 kA:
    Upright with Earth switch LPH 4250 mm.
    Upright without Earth switch LPH 4250 mm.
    Upright without Earth switch LPH 5500 mm.

For 33 kV Isolator: :

33 kV, 800 Amps, 25 kA Upright with/ without earth switch LPH 3700 mm.

Earthing Switches

  • These are used to discharging any induced voltage on the circuit to earth for safety.

Current Transformers: :

  • Current transformers are used for stepping down current for measurement, protection and control. A sample of current is required for indicating current in ammeters, kwh & kw in Trivector meter.
  • Current transformer should have minimum 2 secondaries, one for metering and other for protection for transmission lines.
  • For power transformer, current transformer should have 5 secondaries i.e. for
    a) metering
    b) differential protection c) back up protection
    d) bus bar protection
    e) directional protection.

  • The followings are the CTs used for different voltage levels:
    1. For 220 kV class – 800-600-400-300/1-1-1A+800/1-1A
    2. For 110 kV class –
    a.600-400/1-1-1A for 100 MVA for Transformer
    b.400-200/1-1-1A for Line
    3. For 66 kV class –
    a.1600-1000/1-1-1A for 150 MVA Transformer.
    b.1000-600/1-1-1A for 100 MVA Transformer.
    c.400-200/1-1-1A for Lines.

Voltage Transformer (Potential Transformer):

  • These are used for stepping down voltage for measurement, protection and control.
  • Step down voltage to low values necessary for:

    Indication of voltage
    Metering of supply or exchange of energy
  • The rated step down secondary voltage as specified as 110 V line-line or 110/√3 line to earth
  • Voltage transformers with 2 secondaries are necessary. The second secondary will be used for directional earth fault protection.

Circuit Breakers::

  • Circuit breakers are required for switching during normal & abnormal operating conditions and to interrupt short-circuit currents.
  • They are located near every switching point, at the both ends of every protected zone.
  • Circuit breakers are required for carrying continuously maximum current of the system.
  • When closed on fault it should not get damaged.

Power Transformer:

  1. To step-up or step-down AC voltage and to transfer electrical power from one voltage level to another.
  2. Tap changers are used for voltage control.
  3. Usually transformers for outdoor use are oil filled.
  4. For large capacity, 3 single phase units are used to form the 3 phase bank.

Shunt Reactors:

  • These are used for long Extra High Voltage transmission lines to control voltage and compensate shunt capacitance of the line during low load periods.

Shunt Capacitance:

  • For compensating reactive power of lagging power factor, to improve power factor.
  • Switched in during heavy loads, switched off during low loads.
  • Static capacitors are used for producing reactive power.

Marshalling Box:

  • To mount monitoring instrument, control equipment and to provide access to various transducers.
  • Control & protective cables are laid between marshalling box.
  • It is located in switch yard & corresponding indoor control panels.

Metering Panels, Control & Relay Panels:

  • To house various measuring instruments, control instruments & protective relays.
  • Control cables are laid between switchyard equipment & these panels.
  • Types of Control & Relay panels:
    a.Simplex type – relays & control & indication equipments on the front of the panel.
    b.Duplex type – control & indication on the front and protective equipments (relays) on rear side of the panel.

Low Voltage AC Switchgears:

To provide AC power to auxiliaries, station lighting system and are located inside switchgear building.
  • To provide low earth resistance path for discharging current from surge arrestor.
  • For providing path for neutral to ground currents and for earth fault current protection.
  • Earth mat provided in every electrical substation below the ground level to meet the above requirements.

Lightning Mast:

  • To protect substation equipment from direct lightning strokes.
  • Lightning masts are located in outdoor yards.

  • Power Cables:

    • Used for power flow, single core or three core cables PVC insulated or XLPE insulated.
      The followings are the XLPE type power cables used in KPTCL:
      3x95 Sq. mm
      3x240 Sq. mm
      3x400 Sq. mm
      1x1000 Sq. mm

    Control Cables:

    • These are generally multi-core cables of either PVC insulated or shielded cables used for protection, control & measurement.
      The followings are the core & its sizes:
      2 core – 2.5/6/10 Sq. mm
      4 core – 2.5/4/6/10/16 Sq. mm
      10 core – 2.5 Sq. mm
      14 core – 2.5 Sq. mm
      19 core – 4 Sq. mm
      1 core – 150 Sq. mm


    • Insulator used to insulate live conductor.
    • String insulator of tension or suspension and post insulator and solid core insulator are made of porcelain and glass.
    • String of string insulators/ suspension insulators are made of disc insulators 90 kN and 120 kN.

    Carrier Equipments:

    • Used for protection and communication signaling, voice communication, protection signaling, control and monitoring signals.
    • The PLCC panel is located in control room.
    • The wave trap is connected to the transmission line and is usually mounted above CVT or on a separate structure.

    Station Battery/ DC System:

    • To meet emergency requirement of tripping and closing of breakers independently.
    • To meet annunciation, lighting and indication requirement as well as protective relays.
    • The DC system is divided into 3 parts:
      a.Storage Battery
      b.Charging Equipment
      c.Distribution Board
    • For 33kV, 66kV, 110 kV substations, 110 Volts DC is used and 100 Ah capacity cells are used.
    • For 220 kV and above, 250 Volts DC is used and 400 Ah capacity cells are used (For 220 kV and above standby battery set is installed).

    Charging Equipment:

    • It is required for initial charging.
    • Quick charging/ boost charging after the battery has discharged under heavy loads.
    • Trickle charging.
    • The battery charger unit consists of Trickle charger, float charger and boost charger.

    DC Distribution Board:

    • Used for distribution of DC supply for various circuits.
    • Batteries are to be installed in a separate battery room which is well ventilated, with acid resistant floor and wall surfaces.
    • The cells are mounted on wooden stand which is insulated from floor and each cell is insulated from the stand.

    Fire Extinguishers/ Fire Fighting Equipments:

    • These equipments are used in the event of flash over of substation equipments to extinguish fire and to avoid catching of fire to neighboring equipments.
    • These equipments are installed at outdoor yard near to the electrical equipments such as Transformers, CTs, PTs, Breakers and also are installed in control room, battery room and repair bay.
    • The followings are the different types of fire extinguishers used
      a. Foam type Extinguisher of capacity 45 & 9 liters
      b. Water type (Gas pressure type) of capacity 9 liters
      c. Carbon dioxide type
      d. Dry chemical powder type Extinguisher

    Auxiliary Transformer:

    • Required for lighting outdoor yard and for office and control room building.
    • And power supply to compressor motor of the circuit breaker.

    Nitrogen Injection Fire Extinguishing System:

    • It is used to protect the transformer from the fire.
    • Nitrogen gas is used in this system.
    • This system is installed in outdoor yard nearer to the transformer.

    Move to Top