Investigating Ways to Prevent Electrical Arc Flash

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  1. 1. Investigating Ways to Prevent Electrical Arc Flash
  2. 2. 2 Summary Introduction Background Experimental configuration Experimental results Conclusions Future research Image source: http://arcflashadvisory.com/index-3.html Image source: http://solarprofessional.com/ar ticles/design- installation/calculating-dc-arc- flash-hazards-in-pv-systems
  3. 3. 3 Introduction Intense heat High temperature (>3000o C) Pressure waves (203 kPa at 0.6 m) Copper vapour Hot air-rapid expansion Intense light (IR & UV radiation) Sound waves Molten and vaporized metal Shrapnel/debris (~1000 km/h ) Flames Smoke Toxic gases
  4. 4. 4 Arc Flash Severity Arc current and voltage (arc power) Arc duration Conductors gap Working distance X/R electrical system ratio Installation type/layout Earthing/grounding system Other factors such as: Gas nature/state Conductors material Equipment age/condition Environment
  5. 5. 5 Comprehensive System to Manage Arc Flash Hazards Arc flash hazard analysis: Hazard identification Risk reduction measures Safety labeling Personnel training PPE Mitigation methods to reduce: Arc incident energy Arc incident impact
  6. 6. 6 Background: From Arc to Arc Flash Electrical Discharges Extremely dangerous when they appear unexpectedly flashovers and arcing faults Measureable electrical parameters V, I and V-I characteristic Arc Discharge Continuous plasma discharge Series or parallel Non-axisymmetric/free burning or axisymmetric/stabilized Time dependent arc impedance resistive, inductive and capacitive parts Initiation i. Insulation breakdown/flashover (mainly gas dielectrics) ii. Conducting object introduction Vs. Spark short living transient plasma discharge
  7. 7. 7 Arc Flash Initiation Focus on arc onset to increase our understanding about how an arc evolves into an arc flash event Experimental investigation on short time scale arcs (sparks) Evaluation of the electrical characteristics of the transient plasma discharge Possible mitigation means of the risk of arc initiation under a given set of conditions
  8. 8. 8 Experimental Configuration Charging resistor 25 M Discharging resistor 25 M Capacitor 1.9 nF or 1.0 nF Limiting resistor 1 Current viewing resistor 25 m 22 pF 1 nF HV probe (1000:1) Spark gap 0.1 1 mm Attenuation 40 dB (50 coaxial cable) Oscilloscope Normal laboratory conditions: Temperature (20 oC) & Pressure (1 atm) Atmospheric Air Density (1.204 kg/m3)
  9. 9. 9 Experimental Configuration 1 //22 pF 25 m//1 nF Earth plane 50 cable output HV busbar Spark gap Grounded pin
  10. 10. 10 Experimental Configuration Focus on the initiation: Electrical properties of spark/arc may help to mitigate arc flash in a wider context Accurate measurements with challenging conditions: High level of the current and very fast rise time Small experimental rig dimensions in order to limit total inductance Custom designed current shunt method: directly electrically inserted in the circuit Very fast rise time response and measuring accuracy V and I measurements Vspark, Ispark, Rspark and Espark accurate calculations Inductance of the current viewing resistor according to the manufacturer datasheet ~ 0.7 nH Theoretical calculated inductance of the circuit branch that includes the spark gap ~ 90 nH
  11. 11. 11 Experimental Results Typical current & voltage acquired waveforms 0.5 mm gap length & 1.9 nF capacitor
  12. 12. 12 Experimental Results An approach which allows separation of the resistive and inductive voltage drop was developed in order to make precise calculations of the energy delivered in the spark. 0.7 nH 25 m V dt dI LRI(t)V(t) 0.1 mm & 1.91 nF
  13. 13. 13 Experimental Results Typical current, voltage and energy calculated waveforms 0.5 mm gap length & 1.9 nF capacitor
  14. 14. 14 Experimental Results Typical current, voltage and energy calculated waveforms 0.5 mm gap length & 1.0 nF capacitor
  15. 15. 15 Experimental Results Temporal development of spark energy 1.9 nF capacitor
  16. 16. 16 Experimental Results Temporal development of spark energy 1.0 nF capacitor
  17. 17. 17 Experimental Results Typical dynamic spark resistance and equivalent spark resistance 0.5 mm gap length & 1.9 nF capacitor
  18. 18. 18 Experimental Results Typical dynamic spark resistance and equivalent spark resistance 0.5 mm gap length & 1.0 nF capacitor
  19. 19. 19 Experimental Results Breakdown voltage and spark current for both capacitors 1.9 and 1.0 nF capacitors
  20. 20. 20 Experimental Results Spark energy for 1.9 and 1.0 nF capacitors
  21. 21. 21 Conclusions Spark (short living transient plasma discharge): an early time process in the arc Gap length increase: spark current increases and dissipation energy intensifies Electrical energy stored in the system increase: energy dissipation and spark current increase Geometrical characteristics of the electrical system & electrical energy stored in the circuit physically close to the arc: This affects arc initiation and should be taken into consideration during the design phase of the electrical installation. Future investigations Arc power determination during the transition from transient to steady-state phases and a comparison between them Energy partition of the electrical energy delivered in the spark/arc
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