celli – it – main session 2 – paper 700

Frankfurt (Germany), 6-9 June 2011

CELLI – IT – Main Session 2 – Paper 700

Extending Switching Reclosing Timeto Reduce Interruptionsin Distribution Networks

G. Celli, E. Ghiani, F. Pilo and S. Tedde

Department of Electrical and Electronic Engineering University of Cagliari

ITALY



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.



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)



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.



Auto-reclosing

ProtectionLineRelay(LR)

Instantof fault

Relayoperating

time

LRoperates

CBOperating time

Fault Duration Dead time

Closingtime

Closingcircuit

energizedCB fully(re)closed

System disturbance time - Short interruption if <1s

Reclosing delay - Dead TimeAuto ReclosingRelay (RR)

Circuit Breaker(CB)

LR resets

CBFullyopen

Reclaim Time

HV/MV

MV distribution lines

MV LineCircuit Breaker

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)



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.



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.

1dg tG g t

dt Classical Cassie and Mayr fault arc model

516505 10 V

2.15e

d e aRMS

Tv t t T l

I

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.



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.




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.

0 20 40 60 80 100 1200

0.5

1

CFOS

V [kV]

F(V)

CFOA1CFOA2CFOA3

T0 = 293°K

T1 = 323°K

T2 = 400°KT3 = 600°K

T0

T1

Tarc




00

0.5

1

CFOS V [kV]

F(V)

Steady state Insulation Recovery

CharacteristicF(V)

Increasing Auto-reclosing

Dead TimetDT

Switching Surge Frequency

Distribution G(V)

Transient Insulation Recovery

CharacteristicsF(V,tDT)

Decreasing Flashover Risks

RFO(tDT)

Vmin Vmax

By enlarging auto-reclosing dead time, tDT

better recovery of the dielectric strength

reduced risk of flashover:

max

min

,V

FO DT DT

V

R t G V F V t dV



Case Study

Rr Xr

132 / 20 kV

SwRSwI

Rc XLc

MV Sw

Variable Location

Variable load

Rohl

Variable load

XCc

Fault

Variable load

Variable loadRc XLc

XLohl

XCc

XCohl

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, …



Transient Faults ReductionTABLE I

REDUCTION OF TRANSIENT FAULTS WITH ENLARGEMENT OF RECLOSING TIME DELAY

% of reignitions

Single Phase fault % of reignitions

Double Phase Fault

Reclosing Time delay

Fault in free air Isolated Neutral

Fault inside of MV cell

Fault inside of MV cell

0.4s Ref. case

0.03% 33.5% 83.75%

0.5 s 0.01% 9.56% 63.76%

0.6 s - 1.40% 56.71%

0.7 s - 0.12% 43.60%

0.8 s - 0.08% 42.45%

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.



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.

celli – it – main session 2 – paper 700

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