© 2018 SEL and Southern States, LLC

Circuit Breaker Ratings –A Primer for Protection Engineers

Bogdan KasztennySchweitzer Engineering Laboratories

andJoe Rostron

Southern States, LLC

Typical Fault Clearing Operation

Relay Time CB Mechanical Time Arc Time

Typical Fault Clearing Operation

Contact Parting Time Arc Time

Fault Clearing Timing DiagramFault

Initiation

Relay Operating Time

Time

(Release Time)

Breaker Actuation

Opening Time(Mechanical Time)

Primary Contacts Parting

Arcing Time

Final Arc Extinction

CB Interrupting Time

Contact Parting Time

Fault Clearing Time

• Higher current → more plasma → more difficult current interruption

• DC component increases the current• Breakers need extra margin to break

asymmetrical currents• Today, breaker standards ask for

asymmetrical rating under reference dc conditions

Asymmetrical Breaker Rating

Combining AC and DC Components

AC Component

Cur

rent

Time

DC Component

Arcing Time

DC

Contact Parting Time

Short Circuit Current

Peak Current

Zero Crossing

Combining AC and DC Components

AC Component

Cur

rent

Time

DC Component

Arcing Time

DC

Contact Parting Time

Short Circuit Current

Peak Current

Zero Crossing

• RMS current defines asymmetrical CB rating

• RMS current combines ac RMS with dc at contact parting time

Combining AC and DC Components

S =2

1 + 2DC%100

2

AC Component

Cur

rent

Time

DC Component

Arcing Time

DC

Contact Parting Time

Short Circuit Current

Peak Current

Zero Crossing

S-FactorExtra Margin for Asymmetrical Currents

0 20 40 60 80 1001

1.1

1.2

1.3

1.4

1.5

1.6

1.7

1.8

DC Level, %

S-F

acto

r

60

70

80

90

100

50

Asy

mm

etric

al R

atin

g / S

ymm

etric

al R

atin

g

• DC depends on t and TDC

• tPART = tRELAY + tMECH

• Standard dc reference condition TDC = 2.71 cycle (X/R = 17)

Relay operating time (tRELAY = 0.5 cycle)

DC Component Decays With Time

DC%(t) = 100% � e−t

TDC

Required S-FactorDepends on Relay Time and CB Mechanical Time

0 20 40 60 80 1001

1.1

1.2

1.3

1.4

1.5

1.6

1.7

1.8

Contact Parting Time, ms

S-F

acto

r

2

34

Reference Relay Time(0.5 Cycle)

Parting Time (Cycles)Mechanical

Time

• Elimination of interposing and lockout relays• Naturally fast operating principles Switch-onto-fault and stub-bus

Bus differential

Unrestrained transformer differential

• New line protection principles

Relay Operating Times Are Improving

• Based on fast incremental quantities TD32 (directional, 1.5 ms)

TD21 (distance, 2−5 ms)

• Based on traveling waves TW32 (directional, 0.1 ms)

TW87 (differential, 1−3 ms)

New Line Protection Principles

• Large processing power• 1 MHz sampling• 0.1 ms processing• Low-latency fiber channels (0.9 ms / 100 mi)• Solid-state outputs (10 µs)

Ultra-High-Speed Line Protective Relays

Field Case 1: TW87 Operates in 0.9 ms

Field Case 2: TD21 Operates in 1.8 ms

Field Case 3: POTT Operates in 2.2 ms

CB Derating for Arbitrary Relay Time

IRATED = S � ISYM = ISYM 1 + 2 e−tPARTTDC

2

Required margin for asymmetrical rating for a given contact parting time

Contact parting time includes relay operating time and breaker mechanical time

IRATED = ISYM 1 + 2 e−tREL+tMECH

TDC

2

CB Derating for Arbitrary Relay Time

For the standard relay time of 0.5 cycle

For an arbitrary relay time

I )RATED(0.5cycle = ISYM 1 + 2 e−t0.5+tMECH

TDC

2

I )RATED(tREL = ISYM 1 + 2 e−tREL+tMECH

TDC

2

CB Derating for Arbitrary Relay Time

We introduce the R ratio

R =I )RATED(tREL

I )RATED(0.5cycle=

1 + 2 e−tREL+tMECH

TDC

2

1 + 2 e−t0.5+tMECH

TDC

2

R < 1 – loss of rating compared with 0.5 cycleR > 1 – gain in rating compared with 0.5 cycle

CB Derating for Fast TrippingStandard X/R of 17 (45 ms)

0 1 2 3 4 5 6 7 8 992

93

94

95

96

97

98

99

100

101

Relay Operating Time, ms

Asy

mm

. Rat

ing

Rel

ativ

e to

0.5

cyc

Rel

ay T

ime,

%

13 ms(2-cycle CB)

30 ms(3-cycle CB)

63 ms(5-cycle CB)

tMECH

CB Derating for Fast TrippingHigh X/R of 37.7 (100 ms)

0 1 2 3 4 5 6 7 8 996

97

98

99

100

101

Relay Operating Time, ms

Asy

mm

. Rat

ing

Rel

ativ

e to

0.5

cyc

Rel

ay T

ime,

%

13 ms(2-cycle CB)

30 ms(3-cycle CB)

63 ms(5-cycle CB)

tMECH

CB Derating for Fast TrippingLow X/R of 9.4 (25 ms)

0 1 2 3 4 5 6 7 8 992

93

94

95

96

97

98

99

100

101

Relay Operating Time, ms

Asy

mm

. Rat

ing

Rel

ativ

e to

0.5

cyc

Rel

ay T

ime,

%

13 ms(2-cycle CB)

30 ms(3-cycle CB)

63 ms(5-cycle CB)

tMECH

“Slow” and “Fast” TrippingGains and Loses in Margin

5 10 15 20 25 3090

95

100

105

110

115

120

Relay Operating Time, ms

Asy

mm

. Rat

ing

Rel

ativ

e to

0.5

cyc

Rel

ay T

ime,

%

13 ms(2-cycle CB)

tMECH

30 ms(3-cycle CB)

63 ms(5-cycle CB)

• Relay misoperation Can actuate CB at any time

• Evolving faults Fault current may start after CB actuation

Can lead to “negative trip times”

Other Cases of “Fast Tripping”

Evolving Fault Field Case

Fault Inception

Trip

Relay Operating Time?• 12 ms after fault current in the B phase• 8 ms before fault current in the C phase

• Worst-case scenario Caused by evolving faults or relay misoperations

DC = 100% → S = 1.7 (70% margin required)

• Dismissing worst-case scenario Low-probability

Circuit Breaker ApplicationWorst-Case Scenario vs. BF Protection

Series-Compensated LinesExponential DC Decay Does Not Apply

• Ultra-high-speed tripping calls for small increase in asymmetrical CB rating over the 0.5 cycle reference Fast breakers, low X/R 7% Slower breaker, large X/R 3%

• Breakers are applied with customary 20% margins

• CB practitioners routinely derate breakers

Conclusions