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8/18/2019 20699779 Power Circuit Breakers
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Power CircuitBreakers
Theory and Operation
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Agenda Breaker fundamentals
– Types of Breakers
– Ratings
– Design Tests
Applicat ion & Operations – Application Considerations
– Operation Considerations
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Introduction
What does a breaker do?
Circuit interruptionTypes of breakers
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Power Circuit Breakers
Waits for current zero to extinguish arc
Typical interrupting times 3-8 cycles
At tempts to prevent arc re-ignition (Theinterrupter must bui ld up dielectric fasterthan the recovery voltage builds up)
Commutes plasma by removing energywhich is in the form of heat
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Power Circuit Breakers
The ability of the breaker to interrupt faultcurrent is determined by two things: – the magnitude of the fault current
– the magnitude and rate of rise of the voltageacross the contacts after the current goes out(both 60 hertz and transient voltages)
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Interrupters Arcing Contacts and medium (oil or gas)
flow
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Oil Interrupters:
I
Current zeros
Recovery
voltage
t
Arc goes out
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Types of Breakers:
Oil
Air blast
Air magnetic
SF6 gas
Vacuum
Others
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Circuit Breaker Year of Manufacture
0
20
40
60
80
100
120
140
1 9 2 3
1 9 4 9
1 9 5 4
1 9 5 9
1 9 6 4
1 9 6 9
1 9 7 4
1 9 7 9
1 9 8 4
1 9 8 9
1 9 9 4
1 9 9 9
Year
N u m
b e r airblast
vacuum
airmag
SF6
OCB
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Interrupting Mediums Oil
– Mineral oil is used to extinguish the arc and to
insulate the live parts to ground
– Generally free breathing
– May use a single tank or multiple tanks
Ai r Blast
– Stored air is used to blow out the arc and coolcontacts
– Very loud operation
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Interrupters
Oil
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Interrupters Oil
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Oil Breaker (Westinghouse 345G)
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Oil Breaker
(McGraw Edison CG38)
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Interrupting Mediums
Ai r Magnetic
– A magnetic field is place perpendicular to the
plane of the arc which drives the arc into insulating
fins
– Sensitive to moisture
– Compact
SF6 (Sulfur Hexa-flouride gas)
– Multi-pressure or puffer operation
– High speed
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Interrupters Air Magnetic
Arcing Contacts
Insulating
Fins
Magnetic
Field
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Interrupters
SF6 Gas
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Puffer Interrupter
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Self-Blast Interrupter
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Gas Breaker (ABB PM)
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Gas Breaker (Westinghouse SP)
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Gas Breaker (ABB PM)
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Interrupting Mediums
Vacuum – Contacts incased in a sealed bottle which is under
vacuum
– Compact
Others – Gas mixtures (N2, SF6 etc.)
– Water – Solid State
– Super-conductor???
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Vacuum
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Vacuum (GE PVDB-1)
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Ratings
Continuous
Interrupting
Close & latch – momentary
Voltage ratings
Switching scenarios
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Power Circuit Breaker Ratings
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Power Circuit Breaker Ratings
Continuous (load current)…typically
600,1200, 2000 or 3000 amperes
ANSI C37.04 Definition: The established limit
of current in rms amps at rated frequency that
it shall be required to carry continuously
without exceeding temperature limits of its
components.
Select a breaker with a Continuous Current
rating greater than the peak load current.
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Power Circuit Breaker Ratings
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Power Circuit Breaker Ratings
Interrupting Rating (fault current)…typically20, 31.5, 40, or 63 kilo-amperes
ANSI C37.04 Definition: The highest value ofthe symmetrical component of the three-phase, short-circuit current in rms amps thatthe circuit breaker shall be required tointerrupt at rated maximum voltage and on
the standard operating duty. Select a breaker with a fault duty greater than
the maximum available fault current.
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Power Circuit Breaker Ratings
Cannot forget X/R!
Standards assume X/R < 17.
What do you do if X/R > 17?
Delay opening.
Increase kA interrupting requirements
for the breaker.
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Power Circuit Breaker Ratings
K Factor: Applies only to OCBs.
Defined as the ratio of the rated maximum voltage to
the lower limit of the range of operating voltage.
The interrupting capabilities are inversely proportional
to the operating voltage (i.e. a reduced voltage
results in an increase in the current interrupting
capability).
Reference Engineering Manual Section 2 SI 4.0 – Application Guideline – AC High Voltage Circuit
Breakers Based on Short Circuit Capability Criteria.
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Power Circuit Breaker Ratings
Close and Latch rating is expressed as
a multiple of the rated short-circuit
current.
The maximum current for which the
breaker will close and latch….stay
closed.
2.6 x rated short circuit current
(Interrupting not Momentary).
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Power Circuit Breaker Ratings
Rated Maximum Voltage is the highest rmsphase-to-phase voltage for which the circuitbreaker is designed.
The upper limit for operation.
Typically 15kV, 84kV (25kV, 38kV, 46kV and69kV), 145kV, 245kV, 362kV and 550kV
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Power Circuit Breaker Ratings Basic Impulse Level (BIL) ,expressed in kV, is
the maximum electrical impulse level(lightning strike) that the breaker can beexposed to and not flashover either internallyor externally.
Actual design BIL does not reflect themaximum impulse that can be generatedbecause the magnitude of the impulsedepends on the location of the strike.
The designed BIL reflects the insulationcoordination practices used in the design ofelectrical systems (Source: High VoltageCircuit Breakers: Design and Applications)
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Power Circuit Breaker Ratings
A circuit breaker is a tie between two
networks
Source Load
Source Load
TRV
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Fault: faultV=0
TRV
v
t
Fault
i
Breaker opens
2 per unitTransient Recovery
Voltage
z
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Power Circuit Breaker Ratings
The ability of the breaker to interrupt faultcurrent is determined by the magnitude of thefault current and also by the rate of rise ofthe voltage across the breaker contacts asit opens
Transient Recovery Voltage (TRV): is thetransient voltage that appears across thecontacts while interrupting a faulted circuit.
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Transient
Recovery
Voltage:
t
V
TRVline = 1 - cos t/sqrtLC
Total TRV = bus - line
Initial Rate
of riseCrest recovery
voltage
TRV
fault
TRVbus = 1 - cos t/sqrtLC
bus line
Time in hundreds of microseconds
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Power Circuit Breaker Ratings
Standard C37.011 defines the TRV envelopethat breakers must meet for rated current(Maximum TRV per standards is 2.4 per unitof rated breaker voltage)
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Power Circuit Breaker Ratings
Some breakers were able to clear bus
faults with no problem but were unable
to interrupt faults a short distance out on
the line even though the fault current
was less than the bus fault.
The cause of this becomes clear whenthe transient recovery voltage is
analyzed for both scenarios:
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Power Circuit Breaker Ratings
Short Line Fault: A fault that occurs a
relatively short distance downstream
from the circuit breaker on its load side.
Also known as a kilometric fault since
this is generally considered to be the
critical distance for maximum severity of
the recovery voltage. Allegheny specifies full nameplate
interrupting for a 90% short line fault
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Vbus
TRV= Vbus - Vline
TRV
Vbus Vline
Short Line Fault:
Vline
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t
V
Fault i
Very steep rate of rise for TRV caused by
the short length of the transmission line
TRV
46
t
V
Fault i
Adding shunt capacitance
attenuates the rise
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TRV= Vbus - Vline
TRV
Vbus Vline
Short Line Fault: Insert bushing
shunt capacitance
on line side
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Bushing shunt
capacitance
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Power Circuit Breaker Ratings
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Short Line Fault:
– SF6 breakers up to 50 kA don’t require external
shunt bushing capacitors.
– Most breakers rated at 63 kA require external
shunt bushing capacitors to meet Short Line Fault
requirements of the standards.
– Mainly breakers above 100 kV.
Power Circuit Breaker Ratings
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Power Circuit Breaker Ratings
1200pf, 3000pf and 12,000pf are typical
values for external bushing capacitors.
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Power Circuit Breaker Ratings
Operating Duty Cycle: A series of circuitbreaker operations and time delays.
Defined by ANSI as:
O-15 SEC-CO-3 MIN.-CO.
The first time delay can be reduced to0.3 seconds for breakers designed for
high-speed reclosing. 0.3 seconds is the mechanical resetting
time.
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Power Circuit Breaker Ratings
A derating factor R is applied to OCBs
based on their reclosing duty.
This does not apply to SF6 breakers.
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Power Circuit Breaker Ratings
Asymmetrical Current Rating: Maximum
RMS current, at rated frequency,
including DC component against which
the breaker is required to operate.
Function of the system X/R
Standards assume X/R of 17
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Power Circuit Breaker Ratings
Momentary Rating:
– Short time current rating
– The current, equal to the rated symmetrical
short circuit current, that the breaker is
required to carry for a specified time.
– Time = Allowable Tripping Delay
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Application of CircuitBreakers
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Switching
Surges:
Z 0= L/C
Voltage doubles when closing in on an
open line = 2 P.U. at open line terminal
Assume that High Speed Re-closing traps a negative 1 P.U. charge on the
line. Then when the breaker re-closes the maximum voltage at the open end
can approach a maximum of 3.5 - 4.0 P.U. for multiple reflections depending
on damping (R):
Trapped charge = -1.0 P.U.
3.5 P.U.
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Breaker Applications
Closing resistors are used to reduce
the switching surge created at the far
end of a transmission line when it is
energized
– Once breaker is closed the resistor is
removed from the circuit.
– Not designed to handle continuous load
current or fault current.
– AP uses closing resistors on EHV breakers
– Typical value: 400-450 ohms.
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Closing resistors:
Z 0= L/C
Rc
Rc
Z 0 V 0
Z 0
(Rc + Z 0)V 0 = V S
bypass
V S
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Z 0 V 0
Z 0(Rc + Z 0)
V 0R first close
V 0B second close
Rc > Z0
V 0R first close
V 0B second close
Rc < Z0
Rc
V 0R first close = V 02 second close
Rc = Z0
2.0 P.U.
2.0 P.U.
2.0 P.U.Optimum closing
resistor size =
surge impedance
Closing resistors:
V 0 = V S
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Breaker Applications
Grading capacitors.
– Applied on circuit breakers with more than one
gap per pole
– Grading capacitors are applied across the
contacts of each interrupter to divide the recovery
voltage during interruption
– Insures that the first gap to interrupt does not see
the whole TRV
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500kv PM breaker
Grading capacitors