1 Study SF 6 Thermal Plasma generated during/after power interruption Final Project for Introduction to Plasma Processing Instructor: Prof. Kasra Etemadi STUDENT: Hosny, Ahmed A. EE403/503, Fall 2005

2 Outline  Introduction  Plasma characteristics  Advantages and Disadvantages of SF 6  Residual SF 6 plasma species  Gas Insulated Switchgear (GIS)  Conclusion  Future work  References Hosny, Ahmed

3 What is Circuit Breaker (CB)?  CB is one of the most essential safety mechanisms in electric networks.  Interrupt short circuit currents (~ 60kA, low pf ~ 0.1).  Fault clearance time is very important for power system stability and avoid equipment damage for itself and the rest of the network.  CB can be characterized by, short circuit current, Rated voltage, … Figure 1: A schematic of Circuit Breaker Conditions Electric Sub- circuit A Circuit Breaker (CB) ON/Off Electric Sub- circuit B V, I Hosny, Ahmed

4 Plasma Elements… Impulse Voltage, Transient Recovery Voltage (TRV) Coupling Gas ( SF 6 ) Breakdown Manmade Plasmas Extinguish Hosny, Ahmed Figure 2: A plot shows plasma elements.

5 Plasma Parameters… Debye Length [m] Average Electron Energy, [eV] Electron Number density, [cm -3 ] Thermonuclear Plasmas Electron Beam Glow Discharge Arc Discharge Solid MHD Generator Ionosphere Solar Corona Interstellar Gas Gaseous Nebulae 1 cm 1 m 1  m 1 Å 1 nm 1 mm P = 100 Hz P = 10 KHz P = 1 MHz P = 100 MHz P = 10 GHz P = 1 THz Plasma Frequency Hosny, Ahmed

6 Plasma … Power Source Transient Recovery Voltage (TRV)  TRV is the voltage that builds up across a circuit breaker after the interruption of a fault current.  It consists of oscillations of lumped elements and of traveling waves.  It stresses the circuit breaker contacts and depends on the type and location of the fault in addition to the CB it self. Figure 3: (a) Single-phase equivalent circuit, (b) Transient recovery voltage [10] Hosny, Ahmed

7 Arc Quenching Mechanism Figure 4: A Schematic representation of the puffer interrupter indicating some important physical processes. [2] Hosny, Ahmed

8  SF 6 has a dielectric strength of about two to three times that of air.  It is nontoxic.  It is nonflammable.  It is noncorrosive; it doesn’t react with other materials because it is inert gas. However, when it is heated to C it decomposes and its decomposition products react with other materials.  It exhibits excellent properties for arc quenching. So, it used as an interrupting medium in circuit breakers instead of air or oil. Pros & Cons of SF 6 Hosny, Ahmed

9 SF 6 Circuit Breaker Figure 6: SF6 Circuit breaker, 36kV, 4000A, SC 50kA. [12] Figure 5 : A plot of the thermal conductivity of SF 6 and N 2 [11] Hosny, Ahmed

10 Residual SF 6 Plasma Species  Ionization  Associative detachment  Dissociative attachment Table 1: Particle densities of residual SF 6 plasma at 3000 K, 10 5 Pa [1] Hosny, Ahmed

11 Electron Velocity Distribution Function Hosny, Ahmed Normalization Factor Energy loss due to collisions Collision frequency

12 Advantages of Gas-insulated switchgear (GIS) are :  Compact size.  Totally isolated from the atmospheric conditions such as air pollution, high temperature, snow, etc.  High degree of reliability and safety precaution.  Easy to install.  SF 6 has a dielectric strength much higher than air which is the insulated gas for conventional switchgear type. SF 6 CB applications … GIS Figure 7: Gas-insulated substations (the picture shows a typical example) are very compact in size and reliable in operation Hosny, Ahmed

13 Conclusion  The primary Cause of high transient over-voltage is the generation of multiple re-ignitions during the interrupting period by some types of CB. This TRV are the most likely cause of CB damage.  The design of CB can be determined using the thermal flow characteristics near current zero.  The critical field strength for the breakdown of the residual plasma has been found to be proportional to the pressure and is equal to 2.0V/(m.Pa), which is only ~ (1/45)th of that of SF 6 at room temperature. Hosny, Ahmed

14 Future Work  Further study on calculation of TRV and post-arc current just after current zero.  Advanced arc model and measurement techniques, which can support the physical phenomena in CBs.  Study the theory of positive corona in SF 6 due to impulse voltage. Hosny, Ahmed

15 References 1.J.D. Yan, M.T.C. Fang and Q.S. Liu, “Dielectric Breakdown of a Residual SF6 Plasma at 3000K under Diatomic Equilibrium”, IEEE Transaction on Dielectrics and Electrical Insulation, Vol. 4 No.1, February D.W. Shimmin and et al, “Transient Pressure Variations in SF 6 Puffer Circuit breakers”, Applied Physics, 23 (1990) pp P.H. Schavemaker and L. Van der Sluis, “ The influence of the Topology of Test Circuits on the Interrupter Performance of Circuit Breakers”, IEEE Transaction on Power Delivery, Vol. 10, No. 4, October M. T. C. Fang and M. Y. Shent, “A comparative study of two computational methods for the simulation of discharge development in SF 6 ”, Appl. Phys. 28 (1995) Z. Ma and et al, “ An Investigation of Transient Over voltage Generation when switching high voltage shunt reactors by SF 6 circuit Breaker”, IEEE Transaction on Power Delivery, Vol. 13, No. 2, April Jong-Chul Lee and Youn J. Kim, “ Numerical Modeling of SF 6 thermal plasma generated during the switching process”, Science Direct, Elsevier, 2005, pp Hosny, Ahmed

16 References (Cont.) 7.Richard Morrow, “ Theory of Positive Corona in SF6 Due to a Voltage Impulse”, IEEE Transaction on Plasma Science, Vol. 19, No. 2, April J. D. Yan, M. T. C. Fang and Q. S. Liu, “Dielectric Breakdown of a Residual SF6 Plasma at 3000 K under Diatomic Equilibrium”, IEEE Transaction on Dielectrics and Electric Insulation, Vol. 4, N0. 1, February Gerd Duning and Manfred Lindmayer,” Plasma Density Decay of Vacuum Discharge After Current Zero”, IEEE Transaction on Plasma Science, Vol. 27, No. 4, August Mazen Abdel-Salam and et al, High-Voltage Engineering: Theory and Practice, Marcel Dekker, Inc., New York, s_.htm. s_.htm 12. global/seitp/seitp328.nsf&v=9AAC720001&e=us&c=C1256CCB004E3 ABBC125699F E. global/seitp/seitp328.nsf&v=9AAC720001&e=us&c=C1256CCB004E3 ABBC125699F E Hosny, Ahmed