1. Prediction of Small Current Interruption Performance for EHV Gas Circuit Breaker 2006. 11. 29 Hong-Kyu, Kim Korea Electrotechnology Research Insitutie

2. Contents Introduction Cold Gas Flow Analysis Prediction of Small Current Interruption Performance  Empirical Method  Streamer Theory Example Optimal Design to Improve Interruption Performance

3. Introduction Summary of Research :  Development of simulation tool to predict small current interruption performance (SCIP) for EHV GCB.  Verification of developed tool through test.  Shape optimization to improve SCIP.  Methodology Electric field analysis : FEM (Finite element method) Gas flow analysis : FVFLIC (Finite volume Fluid in Cell) method Calculation of breakdown voltage - Empirical method - Streamer theory

4. Principle of Large Current Interruption

5. Concept of small current interruption  Because interrupting current is small (tens of or hundreds of A), the arc is extinguished right after contact separation.  Between the electrodes, almost double the network voltage is applied.  Due to the high voltage, the possibility of re-strike, or dielectric breakdown becomes high.  This means small current interruption is closely related to the dielectric characteristics between the electrodes. Withstanding voltage is smaller than applied voltage, there is a high possibility of dielectric breakdown.

6. Comparison of TRV and Vbd Vbd < TRV There is a period of Vbd Re-strike *TRV : transient recovery voltage *Vbd : dielectric breakdown voltage TRV

7. Prediction of small current interruption performance Electric Field Analysis Cold Gas Flow Analysis Electric Field Intensity (E) Density (  ) Breakdown Voltage V bd = V bd (E,  ) Vbd > TRV ? Success Failure Yes No

8. Gas Flow Analysis Things to be considered  Moving boundary  Supersonic flow (mach number >1)  Complicated geometry from the view of CFD

9. Governing Equation  Conservation of mass, momentum and energy  Axisymmetric Equation Mass Momentum Energy Z-direction R-direction

10. CFD scheme ; FVFLIC (Finite volume fluid in cell) method  Unstructured grid can be used.  Shock wave can be considered.  Computational cost is very cheap compared to other commercial CFD software. Calculation time is less than one hour with the grid number of 20,000 and stroke 200[mm]. Commercial S/W requires at least a few hours under the same analysis condition.

11. Electric Field Analysis Consideration of Moving Parts  Moving pars are automatically translated with respect to stroke.  For each position, electric field intensity is calculated using FEM. Potential Distribution

12. Prediction of Small current Interruption Performance Empirical Method  Breakdown voltage is predicted using the empirical formulation as follows :   : gas density  E : electric field intensity a, b : constant

13. Streamer Theory  Breakdown voltage is predicted as follows based on the Streamer theory. (E/N)* : critical E/N = 3.56E-15 [Vcm 2 ] for SF 6 E : electrical field intensity when the voltage difference is 1[V] N : number of particles per volume V ap : applied voltage (TRV)

14. Exmaples Test model : 145kV GCB  Puffer and Hybrid type Puffer type Hybrid type

15. Testing condition  Voltage is increased until breakdown occurs. (Initial f value is f =0.7) TRV waveform in analysis t 0 : arcing time

16. Comparison of TRV and Vbd  Puffer Type (a) Vpeak = 327 kV, T arc = 3.4 ms (b) Vpeak = 371 kV, T arc = 3.14 ms V bd > V ap -> Success (test)V bd Re-strike (test)

17. Prediction Results  Prediction Index  Criterion F bd > 0 : Success, F db < 0 : Restrike Hybrid Type V peak [kV]T arc [ms] Testing Result F bd [%] (Empirical) F bd [%] (Streamer) 3261.79 ○ 11.91.7 3511.94×3.5-5.1 3341.92 ○ 8.5-1.2 ○ : No restrike, × : Restrike

18. V peak [kV]T arc [ms] Testing Result F bd [%] (Empirical) F bd [%] (Streamer) 3273.40 ○ 10.20.1 3452.70×1.4-7.7 3713.14×-4.3-11.6 3473.29 ○ 3.1-6.3 3743.25×-4.4-13.1 Puffer Type ○ : No restrike, × : Restrike

19. Discussion  When both methods predicted the failure, re-strike occurred in the test.  Although empirical method predicted as success and streamer method predicted as failure in some cases, the relative index error is less than 10% compared to the test.  If we design the GCB model both methods predict as success, the improved GCB model with higher interruption performance can be obtained.

20. Optimal Design of GCB Shape  By nozzle shape optimization of GCB, we aim to get the improved GCB model with higher small current interruption capability.  As an optimization tool, sequential meta-modeling technique with Kriging model and Evolution Strategy is employed.  Definition of objective function V bd : withstanding voltage [V] V ap : transient recovery voltage [V] Maximize {F bd }

21. Optimization Result Old model Improved model T arc [ms]V peak [kV] Testing Result Old1.94351× Improved1.6372O Nozzle shape ○ : No restrike × : Restrike

22. Conclusion  Development of simulation tool to predict small current interruption performance (SCIP) for EHV GCB.  SCIP is predicted using prediction index which is the relative difference of Vbd and TRV.  Both empirical and streamer method show satisfactory result with the prediction error less than 10%.  By combining the SCIP tool with optimization technique, improved GCB model with higher interruption capability is designed and verified by the test.