frankfurt (germany), 6-9 june 2011 celli – it – main session 2 – paper 700 extending switching...
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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
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.
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)
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.
Frankfurt (Germany), 6-9 June 2011
CELLI – IT – Main Session 2 – Paper 700
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)
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.
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.
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.
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.
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.
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
Frankfurt (Germany), 6-9 June 2011
CELLI – IT – Main Session 2 – Paper 700
Reignition Arc Model
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
Frankfurt (Germany), 6-9 June 2011
CELLI – IT – Main Session 2 – Paper 700
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, …
Frankfurt (Germany), 6-9 June 2011
CELLI – IT – Main Session 2 – Paper 700
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.
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.