Mitigation OF Undesired Auto Recloser for Inrush Current in Distribution System

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?