Circuit Breaker Ratings – A Primer for Protection Engineers Bogdan Kasztenny Schweitzer Engineering Laboratories and Joe 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 Diagram

Asymmetrical Breaker Rating 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 SEL DMC asset 1545

Combining AC and DC Components

Combining AC and DC Components

Combining AC and DC Components RMS current defines asymmetrical CB rating RMS current combines ac RMS with dc at contact parting time S= 2 1+ 2 DC% 100 2

S-Factor Extra Margin for Asymmetrical Currents

DC Component Decays With Time DC%(t)=100%∙ e − t T DC 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)

Required S-Factor Depends on Relay Time and CB Mechanical Time

Relay Operating Times Are Improving 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

New Line Protection Principles 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) SEL DMC asset

Ultra-High-Speed Line Protective Relays Large processing power 1 MHz sampling 0.1 ms processing Low-latency fiber channels (0.9 ms / 100 mi) Solid-state outputs (10 ms)

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 Required margin for asymmetrical rating for a given contact parting time I RATED =S∙ I SYM = I SYM 1+2 e − t PART T DC 2 Contact parting time includes relay operating time and breaker mechanical time I RATED = I SYM 1+2 e − t REL + t MECH T DC 2

CB Derating for Arbitrary Relay Time For the standard relay time of 0.5 cycle I RATED(0.5cycle = I SYM 1+2 e − t 0.5 + t MECH T DC 2 For an arbitrary relay time I RATED( t REL = I SYM 1+2 e − t REL + t MECH T DC 2

CB Derating for Arbitrary Relay Time We introduce the R ratio R= I RATED( t REL I RATED(0.5cycle = 1+2 e − t REL + t MECH T DC 2 1+2 e − t 0.5 + t MECH T DC 2 R < 1 – loss of rating compared with 0.5 cycle R > 1 – gain in rating compared with 0.5 cycle

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

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

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

“Slow” and “Fast” Tripping Gains and Losses in Margin

Other Cases of “Fast Tripping” Relay misoperation Can actuate CB at any time Evolving faults Fault current may start after CB actuation Can lead to “negative trip times” SEL DML asset 1545

Evolving Fault Field Case Trip Fault Inception Relay Operating Time? 12 ms after fault current in the B phase 8 ms before fault current in the C phase

Circuit Breaker Application Worst-Case Scenario Caused by evolving faults or relay misoperations DC = 100% → S = 1.7 (70% margin required) Low-probability case

Series-Compensated Lines Exponential DC Decay Does Not Apply

Conclusions 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