O&M of HV Switchgear Types of Operating Principles

Types of Operating Principles

  Bulk Oil

  Minimum or Small Oil

  SF6

  Vacuum

Bulk Oil Circuit Breaker

  Three interrupter poles housed in a common metal tank filled with insulating oil.

 Commonly called the DEAD TANK design.

  Bulky, thick steel walls provided to withstand the tremendous gas and oil pressure created within the arc chamber by the arc during short circuit.

  The interrupter, solely depends on the turbulent flow of oil and gas bubbes to quench the arc, is known as a “Plain break Interrupter”.

Bulk Oil Circuit Breaker (cont’d)

  The insulating oil   Helps to cool and quench the arc   Insulates live parts from the tank

  An example:   Reyrolle LMT2 6.6kV switchgear (4 panels) would need 1

drum of oil for replacement of oil.

  The Circuit Breaker can be supplied with solenoid (eg. Reyrolle A5T) or spring mechanism for closing and tripping.

Pictures of Reyrolle C-series BOV and Reyrolle LMT2 BOV

Ring Main Unit – Typical Features

 Normal Current – 800A, voltage 22kV & below

  Available in bulk oil, vacuum and SF6 versions

 No relay protection, only earth fault indicator

  Single stroke manual closing; available also in motor charged version

  “Idiot proof” interlocks

 Usually dead-tank, sealed and suitable for outdoor installation.

Ring Main Unit – Load Break Switch

  Features   Quick-Acting-Fault-Making Switch   No interrupter

 Capabilities   Making: Up to rated short circuit making current   Breaking: Only rated normal current

Ring Main Unit – Fuse Switch or Switch Fuse

  Features   Quick-Acting-Fault-Making Switch   Current limiting fuse link and isolator

 Capabilities   Making: Up to rated short circuit making current   Breaking: Up to fuse rating

Small Oil Volume (SOV) Circuit Breaker  Oil is purely used for arc quenching and NOT for

insulation

  Insulation is provided by the dielectric material of the pole.

  In general, replacement of oil in SOV circuit breaker is recommended after every fault interruption.

Small Oil Volume (SOV) Circuit Breaker  During the opening

operation, the piston assembly effectively causes oil rush into the arc quenching device to expedite arc extinction

  Example: CalorEmag ZE4

Properties of SF6 (Sulphur Hexafluoride)  Colourless, odourless, non-toxic, non-flammable

  Five times heavier than air

 Chemically inert up to 150 degC.

 Decomposes into SF4, SF2, S2, F2, S, F etc at high temperature

 No carbon component

 Good di-electric strength, good arc quenching medium

Types of SF6 Circuit Breaker  Single Pressure Puffer

  Uniform pressure in SF6 gas compartment; gas compressed and discharged through puffer during circuit interruption.

  Double Pressure Puffer

  High and Low pressure SF6 gas compartments; high pressure gas discharged through puffer into low pressure region during circuit interruption.

  Rotating Arc Quencher

  Magnetic coil assists quenching of arc during circuit interruption.

Single Pressure Puffer

Types of SF6 Circuit Breaker  Construction of SF6 circuit breakers of late, adopt

the same structure of a small oil volume circuit breaker in that the three interrupter poles are housed in insulated “live” poles.

Eg Y-SF6 22kV circuit breaker

  The poles are filled with pressurised SF6 (1-5 bars) supplied from a common reservoir

Any gas leakage can be monitored through a pressure gauge.

Vacuum Circuit Breaker   Vacuum interrupters employ no insulant like oil or gas in

its arc quenching device.

 No maintenance

  Require relatively smaller mechanical energy for contact closing and opening.

  Interrupting device:   A pair of flat contacts housed in a completely sealed bottle

kept at a pressure of 10-3 mm of mercury.   With absence of any conducting material, contact gap kept

as small as 15mm.

Vacuum Circuit Breaker   Arc extinction:

After a natural current zero, the remaining metal vapour condenses on the shields of the vacuum bottle and the dielectric strength builds up rapidly across the parted contacts to prevent a RESTRIKE.

The uniquely high dielectric strength of vacuum ensures that the current is almost always interrupted on the first current zero encountered.

Result: CURRENT CHOPPING.

If improper control => OVERVOLTAGE (2 – 3 times normal voltage) OR cause damage to transformer insulation.

Vacuum Circuit Breaker   Advantage as compared to other circuit

breakers:   Very short contact travel   Elimination of clumsy drive rod and piston

system

 Movement of the contact within the bottle is permitted by the use of stainless steel bellows at the base of the bottle.

  Size of vacuum bottle is determined by external factors:   Requirements on creepage clearance   Basic Impulse Level

Gas Insulated Switchgear   IEC 517

Metal enclosed switchgear in which the insulation is obtained, at least partially, by an insulating gas other than air at atmospheric pressure.

  Features   Constant environment   Filled with SF6 gas at 1-1.5 bars   Vacuum interrupter   Metal-clad or metal enclosed   Compact

Gas Insulated Switchgear Problem #1  Compact CT Chamber

Switchgear can be compact because it is designed with static relay protection system in mind – smaller relay mounting less CT burden requirements and smaller size & weight o CT.

With the use of electromechanical relays for distribution switchgear, the size of Current Transformer will definitely be large.

An example can be found in Siemens 22kV GIS

Gas Insulated Switchgear Problem #2  Compact cable chamber and special terminations

Switchgear are all designed with pre-moulded plug-in type of cable termination. As termination plug is short, its reliability depends very much on the care exercised by the jointers in adhering to the correct dimensions and cleanliness of the terminations.

Also the switchgear is designed for single core cable system. As such, termination of cable plug is more difficult for our three-core cable system.

Gas Insulated Switchgear Operating Mechanisms  Manual- or motor-charged spring operating mechanisms

(VCB)

 Manual or electrical operating system (VCB) Open/Close circuit breaker at high speed

CB ON CB OFF Isolator OFF Ready-to- Earthed

Earthed ON

Gas Insulated Switchgear Operating Mechanisms   3PDS (Three-position disconnector system with closed/

open/earthing positions) is operated manually (for Meiden GIS) or electrically (for Siemens GIS)

  Perform status indications.

8 Three-position switch

Switching positions -- a.  Closed b.  Opened c.  Ready-to-earth

Siemens GIS

Gas Insulated Switchgear Construction   Barrel Type

  Single phase encapsuated   No possibility of phase to phase fault   More O-ring seals

 Drum & Cubicle Type   3 phases encapsualted   Phase to phase fault possible   Less compartment, less possibility of leaks   Less space of terminating cables

Barrel type