Introduction
In power systems, circuit breakers are used to switch electrical equipment and networks under normal and fault conditions. The primary function of a circuit breaker is to interrupt the flow of current (load or short circuit) by opening its contacts and thereby isolating the switched parts of the system. The design and working of a circuit breaker depend on its application and voltage rating. Moulded case technology (with air at atmospheric pressure) is used at low voltages (< 1000 V), whereas air-blast and vacuum circuit breakers are prevalent at medium voltages (< 72 kV). SF6 circuit breakers are normally used for high voltage systems (> 72 kV).
Disconnecting a Load Interrupting a Fault
SF6 Gas
SF6 circuit breakers utilize Sulphur Hexa Fluoride (SF6) gas as the surrounding insulating medium for extinguishing the arc established between the moving breaker contacts. It has many favourable characteristics that make it an ideal insulation candidate in modern high voltage switchgear:
- High dielectric strength (about 8 to 9 times greater than air at 5 bar pressure).
- Electronegative in nature i.e. catch free electrons forming heavy ions with low mobility that prevents an avalanche type of breakdown.
- Good thermal heat transfer capability, high ionization energy and low dissociation temperature resulting in excellent arc extinction properties.
- Very high electrical conductivity at elevated temperatures proving low arc voltage
- Colourless, odourless, inert and non-poisonous.
Some of the disadvantages of SF6 gas are its high cost, its tendency to form corrosive metal fluorides during arcing and the fact that it is a greenhouse gas.
SF6 Circuit Breaker Types
At present, SF6 circuit breakers can be classified into two major categories:
- Dead tank – enclosure at ground potential.
- Live tank – enclosure at line potential.
Dead tank designs can offer greater short circuit current breaking capability and seismic rating but are relatively bulky (require more SF6 gas), whereas live tank designs are modular and more compact.
Live Tank Circuit Breaker
Dead Tank Circuit Breaker
There are also several different ways in which the electrical current (and resulting arc) in an SF6 circuit breaker can be interrupted. These types include: double pressure (now obsolete), single pressure (also called puffer), self-blast (where arc energy supports pressure build up in arc chamber), rotating arc (arc rotates electro-dynamically in cool background gas) and double motion technology (with two moving contacts). In addition, the drive mechanism of the circuit breaker can be of the hydraulic or spring-loaded type.
For the remainder of this article, we will look in more detail at the basic assembly and working of an SF6 gas insulated circuit breaker that uses single pressure (puffer technology) with a spring loaded operating mechanism; this is the most widely used type in the high voltage industry.
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Construction and Main Components
A complete assembly of a single pressure puffer type SF6 circuit breaker consists of the following parts:
Interrupter Unit
The separation of circuit breaker contacts, quenching of the resulting arc and interruption of current takes place in the interrupter unit. It houses two sets of contacts which are commonly called the ‘main’ or ‘normal current carrying contacts’ and ‘arcing contacts’. Both types of these contact sets have one stationary contact whilst the other contact is able to move. Current carriers (providing connection to the external circuit breaker terminals) are connected to the stationary and moving main contacts. The tips of all circuit breaker contacts are coated with a copper-tungsten arc-resistant material.
The main body of the interrupter (which is filled with SF6 gas) contains a moving puffer cylinder that can axially slide upward and downward along the contacts. There is one stationary piston inside the cylinder which is fixed with other stationary parts of the SF6 circuit breaker, in such a way that it cannot change its position during the movement of the cylinder. A nozzle is located at the opening of the cylinder.
Main Components of SF6 Circuit Breaker Interrupter
Insulating Stack
The interrupter unit is vertically mounted on top of an insulating stack which is made up of a hollow insulator encapsulating the drive rod which connects the mechanical operating mechanism of the circuit breaker to the moving contacts housed within the interrupter. Depending on the voltage rating of the system, the insulating stack may be a single piece, or multiple segments mechanically coupled in series. Just like any other insulator, it provides adequate line-to-ground dry arcing and creepage distance to prevent flashovers associated with transient overvoltages and ambient pollution. The complete circuit breaker unit is usually fixed on a steel structure which secures it to an embedded concrete foundation.
Mechanical Operating Mechanism
The driving apparatus provides the kinetic energy required to open and close the circuit breaker contacts. It consists of a set of opening and closing springs that are charged manually, or with the help of a small electric motor.
Control Panel
The control cubicle communicates between the circuit breaker mechanical operating mechanism, system protection (relays) and supervisory devices. It can be configured for either ‘remote’ or ‘manual’ operating setting.
245-kV (Single Interrupter) Live Tank SF6 Circuit Breaker
Additional Components
At extra high voltage (usually 380-kV and above), due to manufacturing economics and design requirements, the SF6 circuit breaker may have notable differences in construction and may also feature additional components:
- Instead of a single interrupter unit, two or more interrupter units are connected in series (and mounted horizontally onto the insulating stack). For such circuit breakers grading capacitors (C) are connected across the interrupters to equalize the voltage across them.
- For transmission line switching applications, these circuit breakers may be equipped with pre-insertion resistors (PIR) to dampen high magnitudes of switching transient overvoltages. These PIRs (normally 300 to 600 ohms) are connected in parallel with the CB main contacts. They are inserted in the circuit for a specified time interval (8 to 12ms) before main breaker contacts are closed.
Different Configurations of SF6 Circuit Breaker
- The external connecting terminals are fitted with grading rings to ensure that the electric field stresses at the terminal surface do not exceed corona onset
550-kV (Two Interrupters in Series) Live Tank SF6 Circuit Breaker
How SF6 Circuit Breakers Work
Normal Condition
In the normal condition, the circuit breaker contacts are closed and current flows from one contact carrier to the other via the main contacts and the sliding puffer cylinder.
Circuit Breaker Opening Operation
When the circuit breaker control panel receives an opening command (to clear a fault or disconnect part of a network), it sends a signal to the trip coil of the mechanical operating mechanism, which in turn releases the latch holding the charged opening spring. As the opening spring discharges, it pulls the drive rod (connected to the interrupter) in a linear direction, which causes the moving contacts and puffer cylinder to move downwards.
The movement of the puffer cylinder against the stationary piston leads to a decrease in the puffer cylinder’s internal volume, which causes compression of the SF6 gas inside the cylinder. Due to contact overlap, gas compression starts before any contacts open. As the downward movement continues, the main contacts separate and the current commutates to the arcing contacts which are still in the closed position (due to their physically longer construction). During the course of further opening, the arcing contacts start to separate and an arc is established between them.
Operation of a Puffer Type SF6 Circuit Breaker
As the arc flows it blocks the flow of SF6 gas through the nozzle to some extent. Thus, the gas pressure in the puffer cylinder continues to increase. When the sinusoidal current waveform approaches zero, the arc becomes relatively weak and the pressurized SF6 gas inside the puffer cylinder flows axially (through nozzle) over the arc length. This blast of SF6 gas removes the thermal energy in the contact gap and reduces the degree of ionization (electrical conductivity) such that the arc is extinguished.
When the arc is interrupted, transient recovery voltage (TRV) starts to appear across the contacts; the opening speed of the circuit breaker contacts should be fast enough to create an adequate contact separation distance to withstand this voltage stress. In case the contact gap’s dielectric strength is lower than TRV stress, the arc will be re-established in a phenomenon which is commonly called circuit breaker re-ignition or re-strike.
Circuit Breaker Closing Operation
During the circuit breaker closing sequence, the closing coil releases the energy of the closing spring which causes the contacts to move towards each other, ultimately bringing them to their normal closed position. At the same time, SF6 gas is redrawn into the puffer cylinder making the circuit breaker ready for the next operation.
Whilst closing, a circuit breaker can sometimes experience an event known as pre-strike. As the contacts move towards each other during closing, the contact gap’s dielectric strength decreases. At some point, the voltage stress across the contact gap exceeds its dielectric strength, thus producing a ‘pre-strike ‘arc which bridges the contacts.
Design Requirements
In addition to the general characteristics normally associated with an electrical switch i.e. low contact resistance when closed and almost perfect insulation in open condition, a high voltage SF6 circuit breaker needs to fulfil additional design requirements. Amongst these, the most pertinent ones are briefly described below:
- Rated current – the current carrying parts of the circuit breaker should be able to carry the maximum anticipated load current without exceeding temperature rise
- Rated short circuit breaking current – the circuit breaker should have the ability to interrupt the maximum short circuit current of the network (typical standardized values are 25, 40 and 63 kA).
- Rated voltage and insulation level – the insulation of the circuit breaker externally, and across the contacts, should be able to withstand specified magnitudes of low frequency and transient overvoltages.
- Re-strikes – during capacitive current breaking, the probability of re-strike (re-ignition) between circuit breaker contacts should be very low.
- Mechanical endurance – the circuit breaker should be able to undergo a large number of operations (2,000 to 10,000) with very limited maintenance.
- Opening time – to achieve adequate clearance between contacts and in order to withstand TRV, a circuit breaker needs to open an extremely fast speed (in the order of 40 to 60 ms).
- Rated operating sequence – the circuit breaker is required to satisfactory complete a specified operating sequence. For high speed reclosing circuit breakers, a typical sequence is: O-0.3sec-CO-3min-CO (where O and C represent opening and closing respectively, and 0.3sec and 3min are time delays).
- Switching duty – during the course of its service life, the circuit breaker should be able to successfully undertake a variety of switching (interruption) duties that most notably include: terminal and short line faults, transformer and reactor switching, and capacitive current interruption. In terms of demands on the circuit breaker, these different types of switching duties vary the magnitude of current and TRV that a circuit breaker is required to withstand.