33 kV Gas Insulated Switchgear Part 1

Posted on Posted in Electrical, Study Material

33 kV Gas Insulated Switchgear

In Underground metro stations, there are two 33 kV auxiliary substations in each station. These substations have GIS type switchgear installation, separate earth mats and METs, a switching room or feeder post for ROCS charging.


I tried to keep a short, crisp summary of a typical 33 kV GIS switchgear which might help in working in a 33kV substation.


The insulating gas for this switchgear would  be SF6 Sulphur Hexafluoride, at a temperature 20 °C at 0.05 MPa.

Bus Rating would be 33 kV GIS, 25 kA for 3 sec, Single bus bar 800A, @ 35°C to 50°C (refer technical specification table below for more technical details)



Switchgear IEC 62271-200 / EN 62271-200
Switchgear IEC 60694 / EN 60694
Behaviour in the event of internal faults IEC 62271-200 / EN 62271-200
Three-position disconnector and disconnector IEC 62271-102 / EN 62271-102
Busbar earthing switch IEC 62271-102 / EN 62271-102
Circuit-breaker IEC62271-100/EN 62271-100
Current transformer IEC 60044-1 / EN 60044-1
Voltage transformer IEC 60044-2 / EN 60044-2
Voltage detection systems IEC 61243-5
Protection against accidental contact, foreign objects and water IEC 60529 / EN 60529
Installation HD 637 S1



Technical Parameters
Busbar System Single busbar
Electrical Data:
Rated voltage 36 kV
Rated operating voltage 33 kV
Rated frequency [Hz] 50
Rated power-frequency withstand voltage 70 kV
Rated lightning impulse withstand voltage 170 kV
Rated short-time withstand current 25 kA
Rated short-circuit duration 3 s
Rated peak current 62.5kA
Rated operating current busbar 800 A
Degree of Protection:
Main circuits IP 65
Drives IP 3X
Cable connection compartment IP 3X
Low voltage cabinet IP 4X
Auxiliary Voltage:
Control 48 V DC
Motor 48 V DC
Protection system 48 V DC
Remote control 48 V DC
Socket/lighting/heating 230 V AC
IAC Classification acc. to IEC 62271-200
Classification IAC AFL
Internal arc 25 kA 1 s
Cubicle width max. 800mm
Cubicle depth maximum 2000 mm
Cubicle height max. 2500 mm
Pressure relief duct Not Required
SF6 pressure control IDIS
Voltage Indication System IVIS
Control panel mechanical
Mech. Operation Closed door operation


Design aspects

* Now we have the technical parameters, now the client might need these design features which makes the switchgear more handy.


  • It should be type tested and pre-fabricated metal construction
  • It should have VCBs, three position disconnector (On-Off-Earth)
  • Bus Bar should be enclosed in SF6 Gas
  • Modular single panels, which can be combined to produce the specified switchgear configuration.
    • Basic Module – For VCB
    • Floor Pan Module – For wide range of cable connections, couplers and CTs (toroidal)
    • Cable compartment – A metal enclosed in air insulated clad with enough space for CBs, three position connector, mechanical interlocks and auxiliary switches. All the switching devices should be outside of insulation area for ease of access.

Insulation Details

  • CB compartment should be Gas insulated
  • Bus bar compartment should be Gas insulated
  • Cable connection/ transformer clad compartment are to be air insulated with individual pressure release valve


  • A mechanical interface which is clearly visible without opening the front door to be provided. This interface includes all mechanical/ panel and interrogating interlocks.
  • All ON/ OFF of CBs, isolator, earth switch, manual spring charge of CBs should be operable without opening the front door
  • Mechanical mimic should be on the front door

Bus Bar Connections

  • The bus bar connection should be designed in way that while changing the panel no other adjacent panels need to moved or opened
  • Bus bar coupling between adjacent panels should have minimum electrical sealing joints.

Gas filled compartment

  • The gas filled compartments must be maintenance free for the entire operating time at site, no gas work must be needed even there is a change in center panel or extending the switchgear in future
  • All live parts like VCBs, three position connector and main bus bar should be capsuled in SF6 filled enclosure

Circuit Breakers

  • Three pole VCB with maintenance free interrupters is to be installed horizontally in the gas compartment
  • It should be spring operated, motor charged and manually closing mechanism with three pole simultaneous operations. The closing mechanism should operate independent of the hand lever
  • The ON/ OFF position should be clearly visible from the cubicle
  • The spring charging time of the motor must not exceed 20 sec in case of a VCB
  • The continuous current rating of the breaker should not be less than 630 A, short circuit current rating should be as per technical parameters mentioned above.
  • Interlocking system and two density alarm for gas with remote and local indicators should be there
  • Spring charge indicator should be provided


Vacuum Circuit Breaker must be equipped with

  • Spring stored energy mechanism with motor auto-reclosing
  • Mechanical Open/ Close buttons
  • Shunt release and closing solenoid
  • Operating cycle counter
  • Auxiliary switch with 6 NO and 6 NC
  • Spring charge indicator

Isolator and Earthing Switch

  • Motor operated with emergency manual operation
  • Visibility and operation should be operable with front door closed

Instrumentation Transformers

Current Transformers

  • CTs used must be Toroidal type, installed outside the gas compartment


Outgoing Feeder Variant Incomer Feeder Variant
Toroidal type Current Transformer Toroidal type Current Transformer
CT Primary Current: 400-200 A CT Primary Current: 400-200 A
CT Secondary Current: 1-1A CT secondary Current: 1-1-1A
Core 1: Cl-0.2 / 15 VA Core 1: Cl-0.2 / 15 VA
Core 2: Cl-5P20 / 15 VA Core 2: Cl-5P20 / 15 VA
Core 3: Cl-5P20 / 15 VA Core 3: Cl-5P20 / 15 VA

CT Console Box

The Box should be made out of CRCA steel duly painted having 3mm thickness suitable for outdoor type & complying with IP: 55.

The details size of the Box is as follows:

Height = 900mm Width = 450mm Breadth = 300mm

3 rows of disconnecting type terminal blocks of Elmex / Connect well make, each row shall have minimum 15nos TB (should continuously carry minimum 20A & dia of the stud 6mm) with end plates.

Copper Earth strips inside the Box of size 25x3mm thickness.

Drainage Cannopy should be provided.



Insulation – 33kV Bus PT (IVT) must be air insulated

Voltage protection (Bus VTs shall be Part of Feeder Protection relay)

  • Overvoltage (59)
  • Under voltage (27)

–       Frequency Relay (81 O/U)

Outgoing Feeder (Separate Relays to be provided for Trafo Differential & Over Current & Earth fault)

Differential protection (For Transformer Feeders) in addition to the above

  • Differential protection for transformer (87T)

–       Restricted earth-fault for transformer (87N)

Other protections and related functions (Part of Numerical Relay)

  • Lock-out (86)

–       Trip circuit supervision (95)


Protection and control requirements in a 33KV GIS Switchgear

Following functions shall be available in the Protection Relay Incomer & Outgoing Feeders


Current protection (Directional & Non-directional feeder Protection)


  • Over current instantaneous (50)
  • Over current IDMT (51)
  • Earth fault instantaneous (50N)
  • Earth fault IDMT (51N)
  • Directional Earth fault IDMT (67)
  • Directional over Current IDMT (67)
  • Auto Reclosure – 4 Shots




All tests shall be carried out according to relevant IEC standards.


Tests shall be carried out according to IEC requirements. The following minimum tests apply:

  • Wiring and function tests
  • Equipment verification tests
  • Low voltage circuit insulation test
  • High voltage power frequency test


Notification for factory tests along with list of proposed tests shall be submitted as required.



The site tests shall include the following:

Power frequency withstand test (at 80% of the rated power frequency withstand voltage)

Insulation resistance

Functional test of the fully installed and wired equipment delivered.


Technical Specification of the entire unit

Sl. No.
33 kV GIS (Cubicle type)
 Standard Applicable  IEC-62271-100 / IEC-62271-200
Enclosure - Tank
Stainless steel
Enclosure - Panel
Nominal System Voltage
33 kV
Highest System Voltage
36 kV
No. of phases and frequency
3ph. 50 Hz
Busbar material
Bus Color code
System Earthing
Solidly earthed
Circuit Breaker Rating
Continuous Current Rating at 50 Deg C
Short Circuit Rating
25 kA
Short Circuit duration
3 sec
Internal Arc Rating
Internal Arc Duration
1 sec
Rated making Current
As per IEC-62271
Operating duty
O-0.3sec-CO-3 minutes -CO
Leakage in gas compartment
Less than 0.2%
Busbar rating
Outgoing feeder rating
Power Frequency Withstand voltage
70 kV for 1 minute
Impulse micro sec
170 kV
Control Voltage
48 V DC
Spring charge motor voltage
48 V DC
CT Ratio
Secondary Current 1A (Ratio
during detail engineering)
PT ratio -STAR/ STAR/ Open delta
(33//3) / (.11//3) / (. 11/3)
Aux. Contacts
6 NO + 6 NC
XLPE Cables as specified
XLPE Cables as specified
Degree of protection (HV equipment)
IP – 65 for Gas Compartment



Leave a Reply

Your email address will not be published. Required fields are marked *