© 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