1. Frankfurt (Germany), 6-9 June 2011 CELLI – IT – Main Session 2 – Paper 700 Extending Switching Reclosing Time to Reduce Interruptions in Distribution Networks G. Celli, E. Ghiani, F. Pilo and S. Tedde Department of Electrical and Electronic Engineering University of Cagliari ITALY

2. Frankfurt (Germany), 6-9 June 2011 CELLI – IT – Main Session 2 – Paper 700 Introduction Performance-based rates (PBRs) are regulatory orders that reward utilities for good reliability and penalize them for poor reliability. Large deployment of network automation with reclosing practices has been adopted by distributors to achieve the reliability performance goals Italian example: order 333/07 provides for payment to each MV customer which suffers a number of long unplanned interruptions per year higher than a threshold.

3. Frankfurt (Germany), 6-9 June 2011 CELLI – IT – Main Session 2 – Paper 700 Introduction The reward and penalty mechanism is based on specific reliability indicators:  SAIDI  SAIFI  MAIFI Implementation of Multiple Auto Reclose Operation Long unplanned interruption (> 3 min) Short unplanned interruption (> 1 s and < 3 min)

4. Frankfurt (Germany), 6-9 June 2011 CELLI – IT – Main Session 2 – Paper 700 Goal of the paper  investigate the possible benefits of the extension of the switching reclosing time delay before the first fast reclosure.  develope a probabilistic fault arc reignition model in order to estimate these benefits.

5. Frankfurt (Germany), 6-9 June 2011 CELLI – IT – Main Session 2 – Paper 700 Auto-reclosing Several faults on overhead lines are temporary. Most of them can be successfully eliminated adopting a fast auto-reclosing of the circuit breaker. Minimum dead time adopted is 300 ms (reasonable deionization time)

6. Frankfurt (Germany), 6-9 June 2011 CELLI – IT – Main Session 2 – Paper 700 Auto-reclosing Effectiveness of high speed auto-reclosing depends on the origins of the temporary fault.  adverse weather conditions, saline deposit on insulators or switching overvoltages,  lightning strikes followed by several subsequent strokes,  contact with an external object (like a bird or a dead wood). Moreover, the DSOs have experienced some faults inside metal enclosed electrical installations that, starting as single- phase-to-ground fault, evolve to double phase.

7. Frankfurt (Germany), 6-9 June 2011 CELLI – IT – Main Session 2 – Paper 700 Fault Arc In order to simulate the complete transient behaviour of a fault in distribution networks with auto-reclosing, the fault arc model and the dielectric strength recovery model have to be implemented. Classical Cassie and Mayr fault arc model The stationary arc conductance, G, depends on instantaneous arc current, arc voltage, arc resistance, and the instantaneous arc length. Typically, this model is in relationship with an arc reignition model after every zero crossing of the arc current.

8. Frankfurt (Germany), 6-9 June 2011 CELLI – IT – Main Session 2 – Paper 700 Reignition Arc Model The previous reignition model is valid only immediately after the arc interruption (self-extinguish conditions). A new reignition model has been developed by correlating the insulation recovery characteristics with the temperature variation in the zone surrounding the fault for arcs in order to take into account the flashover probability variation.

9. Frankfurt (Germany), 6-9 June 2011 CELLI – IT – Main Session 2 – Paper 700 Reignition Arc Model A thermal model has been implemented in the EMTP program to represent the temperature rise in the surroundings of the fault arc and the subsequent cooling when the arc is interrupted.

10. Frankfurt (Germany), 6-9 June 2011 CELLI – IT – Main Session 2 – Paper 700 Reignition Arc Model By enlarging auto- reclosing dead time,  t DT better recovery of the dielectric strength reduced risk of flashover:

11. Frankfurt (Germany), 6-9 June 2011 CELLI – IT – Main Session 2 – Paper 700 Case Study The probabilistic fault and reignition arc models has been implemented with the commercial package EMTP-RV. Simplified 20 kV the test network A Monte Carlo simulation has been performed: fault position, fault resistance, fault arc, environmental conditions, …

12. Frankfurt (Germany), 6-9 June 2011 CELLI – IT – Main Session 2 – Paper 700 Transient Faults Reduction 400 ms is sufficient for single-phase to ground fault in free air. An important reignition fault reduction has been observed for faults inside MV cells, bigger moving from 400 ms to 600 ms.

13. Frankfurt (Germany), 6-9 June 2011 CELLI – IT – Main Session 2 – Paper 700 Conclusions A new tool for studying the auto-reclosing operation in MV distribution network has been developed, which permits investigating the effects of changing the reclosing time. Increasing the auto-reclosing dead time up to 600  800 ms reduces the probability of arc reignition especially for transient faults inside metal enclosed electrical installation. The impact of the increased reclosing time on the severity of voltage dips is minimal since the equipment is already susceptible for shorter voltage dips.