Thursday, March 31, 2011

Arc Extinction in SF6 Circuit Breaker

5. Description of different Circuit Breaker
5.4  Sulphur Hexafluoride (SF6) Circuit Breaker and SF6 Insulated Metalclad switchgear
5.4.3 Arc Extinction in SF6 Circuit Breaker
       High voltage circuit breaker with SF6 gas as the insulation and quenching medium have been in use throughout the world for more than 30 years. This gas is particularly suitable because of its high dielectric strength and thermal conductivity.
       The current interruption process in a high voltage circuit breaker is a complex matter due to simultaneous interaction of several phenomena. When the circuit breaker contacts separate, an electric arc will be established, and current will continue to flow through the arc. Interruption will take place at an instant when the alternating current reaches zero.
       When a circuit breaker is tripped in order to interrupt a short circuit current, the contact parting can start anywhere in the current loop. After the contacts have parted mechanically, the current will flow between the contacts through an electric arc, which consists of a core of extremely hot gas with a temperature of 5,000 to 20.000 K. This column of gas is fully ionized (plasma) and has an electrical conductivity comparable to that of carbon. When the current approaches zero, the arc diameter will decrease. with the cross section approximately proportional to the current. In the vicinity of zero passage of current, the gas has been cooled down to around 2.000 K and will no longer be ionized plasma, nor will it be electrically conducting.
       Tow physical requirements (regimes) are involved :
  • Thermal regime : The hot arc channel has to be cooled down to a temperature low enough that it ceases to be electrically conducting.
  • Dielectric regime : After the arc extinction, the insulating medium between the contacts must withstand the rapidly increasing recovery voltage. This recovery voltage has a transient component (transient recovery voltage, TRV) caused by the system when current is interrupted.
        If either of these two requirements is not met, the current will continue to flow for another half cycle, until the next current zero is reached. It is quite normal for a circuit breaker to interrupt the short circuit current at the second or even third current zero after contact separation.
Thermal regime
       The thermal regime is especially critical at short line fault interruption. The circuit parameters directly affecting this regime are the rate of decrease of the current to be interrupted (di/dt) and the initial rate of rise of the transient recovery voltage (dv/dt)  immediately after current zero. The higher the values of either of these two parameters, the more severe the interruption is. A high value of (di/dt) results a hot arc with a large amount of stored energy at current zero, which makes interruption more difficult. High values of (dv/dt) will result in an increase of the energy to the post arc current.
       These exists a certain inertia in the electrical conductivity of the arc. When the current approaches zero, these is still a certain amount of electrical conductivity left in the arc path. This gives rise to what is called a "post-arc current". With amplitude up to a few A. Whether or not interruption is going to by successful is determined by a race between the cooling effect and the energy input in the arc path by the transient recovery voltage. When the scales of the energy balance tip in favor of the energy input the circuit breaker will fail thermally. The thermal interruption regime for SF6 circuit breakers corresponds to the period of time starting some µs before current zero, until extinguishing of the post arc current, a few µs after current zero.

Dielectric regime
       When the circuit breaker has successfully passed the thermal regime, the transient recovery voltage (TRV) between the contacts rises rapidly and will reach a high value. For example, in a single unit 245 KV circuit breaker, the contact gap may be stressed by 400 KV or more 70 to 200 µs after the current zero. 
       In the dielectric regime the extinguishing/ isolating medium is longer electrically conducting, but it still has a much higher temperature than the ambient. This reduces the voltage withstand capacity of the contact gap. The stress on the circuit breaker depends on the rate of rise and the amplitude of the TRV.
       The withstand capability of the contact gap must always higher than the transient recovery voltage otherwise a dielectric re-ignition will occur (dielectric failure). This requires an extremely high dielectric withstand capability of the gas, which is still rather hot and therefore has low density.
       The arc extinction process, in SF6 CB, is different from that in air blast CB. During the arcing period, SF6 gas is blown axially along the arc. The gas removes from the arc by axial convection and radial dissipation. As a result, the arc diameter reduces during the decreasing mode of the current wave. The diameter becomes small during current zero and the arc is extinguished.
       Due to its electro negativity and low arc time constant, the SF6 gas regains its dielectric strength rapidly after the final current zero, the rate of rise of dielectric strength is very high and the time constant is very small.
       The arc extinguishing properties of SF6 gas was pointed out in 1953. The research pointed out that SF6 is a remarkable medium for arc extinction. The arc extinguishing properties are improved by moderate rates of forced gas flow through the arc space. 
       Plain break contacts drawn apart, (AC arcs), in SF6 can interrupt about 100 times more current than in air at given voltage.
       The basic requirement in arc extinction is not primarily the dielectric strength, but high rate of recovery of dielectric strength. In SF6 gas, the electrons get attached with molecules to become ions. Thereby, the dielectric strength is quickly regained. Problems connected with current chopping are therefore minimized. 
       In SF6 CB, The gas is made to flow from a high pressure zone to a low pressure zone through a convergent-divergent nozzle. The mass flow is a function of the nozzle-throat diameter, the pressure ratio, and the time of flow. the nozzle is located such that the flow of gas covers the arc. The gas flow attains almost  supersonic speed in the divergent portion of the nozzle., thereby the gas takes away the heat from the periphery of the arc, causing reduction in the diameter of the arc. Finally, the arc diameter becomes almost zero at current zero and the arc is extinguished. The arc space is filled with fresh SF6gas and the dielectric strength of the contact space is rapidly recovered due to the electro-negativity of the gas. The single flow pattern (Fig. 23a) and double flow pattern (Fig. 23b) are used for arc extinguishing in single-pressure puffer type and double-pressure type SF6 circuit breakers.

(a)  Single axial flow pattern

(b)  Double axial flow pattern
Figure 23 Arc extinction in gas flow circuit breakers (Gas flows from high pressure to low pressure)

single pressure puffer type circuit breaker, with single flow quenching medium
  • When breaker is fully closed, the pressure in the puffer cylinder is equal to that outside the cylinder.
  • During opening stroke, puffer cylinder and moving contact tube start moving.
  • Gas gets compressed within puffer cylinder.
  • After a certain travel, contact separates and arc is drawn.
  • However, compressed gas flows from higher pressure to lower pressure through the nozzle.
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