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Mitigation OF Undesired Auto Recloser for Inrush Current in Distribution System
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Copyright © MCEC and SEL 2014
Mitigation of Undesired Operation of Recloser Controls Due to
Distribution Line Inrush
Lee AyersMid-Carolina Electric Cooperative
Larry WrightSchweitzer Engineering Laboratories, Inc.
Agenda
• Reasoning for changing recloser style
• Experience with tripping due to inrush
• Factors affecting inrush
• Solutions to tripping during inrush
Example MCEC Distribution Circuit
70 A 3R
1R35 A
1R50 A
30 A
Three-Phase Recloser
Single-Phase Recloser
Hydraulic Recloser Clearing CurvesTwo Fast A and Two Slow B Curves
.61.21.82.43.66
1218243660
120
240360600
1,200
Time (cycles)
1 2 3 4 56 10 20 30 50Current (multiples of pickup)
.01
.02
.03
.04
.06.1
.2
.3
.5
1
235
10
20
Tim
e (s
econ
ds)
CB
A
Fast Curve Clears Temporary Faults
• Saves fuses
• Reduces outages
Historical Data
• 90% of faults clear on fast curve
• 5% of faults clear on slow curve
• 5% of faults proceed to lockout
Aging Recloser Fleet
• Reclosers were at least 20 years old
• One manufacturer was out of business
• Insufficient stock was available
Solid Dielectric Recloser With Microprocessor-Based Control
Reasons for Selecting New Reclosers
• Maintenance costs
• Coordination
• Safety
• Environmental concerns
• Testing
• Additional advantages
µP Control Operate Curve
.01
.02
.03
.05
.2
.3
.5
1
235
10
20
Tim
e (s
econ
ds)
.1
.5 2 3 5 10 20 40Multiples of Pickup
.8 1 100
3
6
10
30
60
150
300
600
Time (cycles)
A (101)
B(117)
C(133)
Initial Experience With µP Control
• More than 150 reclosers (35 A and 50 A) changed out
• Many event reports received
• Noticed unintended operations on fast curve when power restored
First Recloser Trip – Raw Event
Transformer Fuse Coordination
.01
.02
.03
.04
.05
.07
.1
.2
.3
.4
.5
10 20 50 100 200 500 1,000 5,000Current (A)
Tim
e (s
econ
ds)
Recloser A Curve
1.5X Fuse
Second Recloser Trip – Raw Event
Is This Normal?
“There is an argument that a recloser rarely closes in and holds on second fast operations
due to inrush current.”– McGraw Edison Power Systems Division, “Overcurrent Protection for
Distribution Systems – Seminar Notes,” May 1984
Is This Acceptable?No. Why Unnecessarily Blink
Customer Power?
Transformer Inrush
Inrush Current
Residual Flux
Total FluxSaturation
Density
Supply Voltage
t = 0 Steady-State Flux
One Method to Secure Relays Change Settings
• Change curve shape
• Raise pickup
• Raise time dial
Testing Other Fast Curves
10 20 50 100 200 500 1,000 5,000 10,000Current (A)
.01
.02
.03
.05
.07.1
.2
.3
.5
.71
2357
10
Tim
e (s
econ
ds) 4
20T
RN
17
A (TD = 2)
1
Test Results
Curve Trip4 NoR NoN No17 No1 Yes
A (TD = 2) Yes
Disadvantages of Changing Settings
• Slows protection
• Is difficult to quantify distribution line inrush♦ EMTP♦ Real-time digital simulation
Inrush Current
Cycles
Prim
ary
Cur
rent
(A)
-50
0
50
100
150
200
250
300
0 1 2 3 4 5 6 7
Second Harmonic
Cycles
Cycles1 2 3 4 5 6 7
1 2 3 4 5 6 7Prim
ary
Cur
rent
(A)
0
50
100P
erce
ntag
e of
Fund
amen
tal
0
20
40
60
80
Fundamental Frequency MagnetizationSecond-Harmonic Magnetization
Second-Harmonic PercentageSecond-Harmonic Block Threshold
First Recloser Trip – Raw Event
First Recloser Trip Harmonics
60 120 180 240 300 360 420 480 540 600Frequency
85.0
87.5
90.0
92.5
95.0
97.5
100.0
Per
cent
of F
unda
men
tal
Second Recloser Trip – Raw Event
Second Recloser Trip Harmonics
60 120 180 240 300 360 420 480 540 600Frequency
01020P
erce
nt o
f Fun
dam
enta
l
30405060708090
100
A Better, Easier Solution: Second-Harmonic Blocking
Second-Harmonic Blocking Logic
PU
DO
0.05 • Nominal Current Rating
Second-Harmonic Blocking Pickup Setting
Second-Harmonic Blocking Enabled
I Second Harmonic
Second-Harmonic Blocking Torque-Control Setting
Second-Harmonic Blocking Output
Second-Harmonic Blocking Timer
–+
+
_
I Second Harmonic •100I
I
First Event With Second-Harmonic Blocking
Second Event With Second-Harmonic Blocking
Incorrect Second-Harmonic Blocking
• Step change in fundamental frequency♦ Fourier transform♦ Short-lived blocking (0.25 cycles)
• CT saturation♦ Has low fault current and low X / R ratio♦ Is backed up by slow curve
Substation Recloser Inrush Event
CT Excitation Curve
0 20 40 60 80 1000
20
40
60
Current (mA)
Vol
tage
(V)
14.4 V
Path Forward
• Install three-phase 70 A reclosers with single-pole tripping
• Implement second-harmonic blocking on reclosers and feeder relays
• Perform coordination studies again
Questions?