CIRCUIT BREAKERS E. Chikuni

Circuit Breakers Mechanical Switches capable of making and breaking currents under normal circuit conditions and for a specified time under abnormal conditions, specifically under short circuit conditions. They are designated by the medium in which interruption occurs, e.g. air-blast, oil, vacuum, SF6. They are rated by voltage, insulation level, current, interrupting capacity, transient recovery voltage, interrupting time and trip delay.

Brief History Knife Switches. Introduction of Springs for Quicker Action. Arcing Contacts & Main Contacts Arching Chutes

Air Circuit Breaker

Oil Circuit Breakers Since Oil very effective medium in quenching. Improvements since include oil blast, deion grid (forcing the arc electromagnetically into a narrow insulated path). OCBs are being used for system voltages up to 330kV

Air-Blast Circuit Breakers Since An oil-less arc, stronger, faster than oil. Air velocity approaching the speed of sound. Can interrupt an arc in 2 cycles (40ms), including the time to energize the trip coil. Indoor ratings up to 38kV, Outdoor up to 800kV Used e.g. generator breakers up to 42 kA & higher, as furnace breaker Continuous ratings up to 4000A Operating pressures up to 25 bar

SF6 Circuit Breakers Since Utilizes the excellent arc quenching and insulating properties of the gas SF6 Dead Tank designs incorporated in GIS substations up to 800kV GIS substations are more compact and environmentally friendly than outdoor substations. Operating pressure about 3 bars

SF6 Circuit Breakers Since Utilizes the excellent arc quenching and insulating properties of the gas SF6 Dead Tank designs incorporated in GIS substations up to 800kV GIS substations are more compact and environmentally friendly than outdoor substations.

SF6 Circuit Breakers Several characteristics of SF6 circuit breakers can explain their success: Simplicity of the interrupting chamber which does not need an auxiliary breaking chamber; Autonomy provided by the puffer technique; The possibility to obtain the highest performance, up to 63 kA, with a reduced number of interrupting chambers; Short break time of 2 to 2.5 cycles; High electrical endurance, allowing at least 25 years of operation without reconditioning; Possible compact solutions when used for GIS or hybrid switchgear; Integrated closing resistors or synchronized operations to reduce switching over-voltages; Reliability and availability; Low noise levels.

SF6 Breaker Principle

SF6 Puffer Principle

SF6 Contacts

SF6 Installation

Vacuum Circuit Breakers More recent development. Used up to 33kV but units for over 220kV service have been constructed. The contacts are built in an evacuated enclosure. During operation a metallic arc forms but is quickly extinguished during an ionic re-combination process. This is attributable to the excellent di- electric properties of vacuum. The time involved is in the order of microseconds.

Vacuum Circuit Breaker Construction

Example of a MV SF6 CB

Application MV overhead distribution Switch: PM6 MV/LV distribution substation Ringmaster RN2 Heavy ind. substation Withdrawable: YSF6 Withdrawable YSF6 Main priority substation Medium priority substation Withdrawable: YSF6 Fixed: GENIE Medium ind. substation Withdrawable: YSF6 Fixed: GENIE Medium ind./comm.substation Fixed indoor: GENIE Fixed outdoor: CE6:CE2/SE6/MU HV/MV 132/33kV 132/11kV MV/MV 132/11kV 11/11kV MV/LV 11kV/415V O/HEAD UTILITYCOMMERCIALINDUSTRY

FIXED PORTION Relay Cabinet Busbars Cable box CT’s Circuit Breaker Truck Circuit position

FIXED PORTION Relay Cabinet Busbars Cable box CT’s Circuit Breaker Truck Circuit earth position

FIXED PORTION Relay Cabinet Busbars Cable box CT’s Additional busbar earthing device Busbar earth position

FIXED PORTION Duplicate busbar option Relay Cabinet Upper Busbars Cable box CT’s Relay Cabinet Upper Busbars Cable box CT’s Lower Busbars

Voltage transformer Isolateable and fused - reliability Fully interlocked -operator safety Circuit and busbar connected - network flexibility Cast resin design - maintenance free

Moving portion Horizontal isolation and withdrawal - ease of operation Technology SF6 puffer - low switching surges 3 phase cast resin moulding with minimum number of seals - proven reliability Low standing gas pressure - integrity Electrical pressure switches - constant monitoring of medium

Puffer technique CB open initiation - main contacts separated - overpressure created by piston

Puffer technique Arcing contacts separated - Arc struck between arcing contacts - SF6 over pressure forced into nozzle -Arc interrupted at first current zero

Puffer technique Circuit Breaker open -Contacts in fully open position - Gas pressure equalised

Interrupting medium SF 6 : universal technology Long service life. Switching under difficult conditions. High installation flexibility. Very low maintenance. No surges, smooth breaking. Insensitivity to the environment.

Endurance characteristic Current interrupted (A) Number of operations 20kA 25kA 31.5kA 40kA

Cable box Cable entry above & below - installation flexibility Dyscon cable termination's - operator safety Neon circuit indication - low cost voltage presence

Busbars Fully encapsulated system - installation reliability High conductivity copper conductors A, 2500A rated Padlockable shutters - operator safety

Construction Metalclad design Compact design. Modular construction. Encapsulated MV system Spring loaded busbar and circuit shutters