power circuit breakers

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Power Circuit Breakers

Theory and Operation

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AgendaBreaker fundamentals– Types of Breakers– Ratings– Design Tests

Application & 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 BreakersWaits for current zero to extinguish arc

Typical interrupting times 3-8 cycles

Attempts to prevent arc re-ignition (The interrupter must build up dielectric faster than the recovery voltage builds up)

Commutes plasma by removing energy which is in the form of heat

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Power Circuit Breakers

The ability of the breaker to interrupt fault current is determined by two things:– the magnitude of the fault current– the magnitude and rate of rise of the voltage

across the contacts after the current goes out (both 60 hertz and transient voltages)

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InterruptersArcing Contacts and medium (oil or gas) flow

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Oil Interrupters:

I

Current zeros

Recoveryvoltage

t

Arc goes out

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Types of Breakers:

OilAir blastAir magneticSF6 gasVacuumOthers

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Circuit Breaker Year of Manufacture

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20

40

60

80

100

120

140

1923

1949

1954

1959

1964

1969

1974

1979

1984

1989

1994

1999

Year

Num

ber airblast

vacuumairmagSF6OCB

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Interrupting MediumsOil – 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

Air Blast– Stored air is used to blow out the arc and cool

contacts– Very loud operation

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InterruptersOil

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InterruptersOil

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Oil Breaker (Westinghouse 345G)

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Oil Breaker(McGraw Edison CG38)

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Interrupting Mediums

Air 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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InterruptersAir Magnetic

Arcing Contacts

InsulatingFins

MagneticField

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InterruptersSF6 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

ContinuousInterruptingClose & latch – momentaryVoltage ratingsSwitching scenarios

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Power Circuit Breaker Ratings

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Power Circuit Breaker RatingsContinuous (load current)…typically 600,1200, 2000 or 3000 amperesANSI 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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Interrupting Rating (fault current)…typically 20, 31.5, 40, or 63 kilo-amperesANSI C37.04 Definition: The highest value of the symmetrical component of the three-phase, short-circuit current in rms amps that the circuit breaker shall be required to interrupt 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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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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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 RatingsClose 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 RatingsRated Maximum Voltage is the highest rmsphase-to-phase voltage for which the circuit breaker is designed.The upper limit for operation.Typically 15kV, 84kV (25kV, 38kV, 46kV and 69kV), 145kV, 245kV, 362kV and 550kV

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Power Circuit Breaker RatingsBasic Impulse Level (BIL) ,expressed in kV, is the maximum electrical impulse level (lightning strike) that the breaker can be exposed to and not flashover either internally or externally.Actual design BIL does not reflect the maximum impulse that can be generated because the magnitude of the impulse depends on the location of the strike.The designed BIL reflects the insulation coordination practices used in the design of electrical systems (Source: High Voltage Circuit Breakers: Design and Applications)

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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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The ability of the breaker to interrupt fault current is determined by the magnitude of the fault current and also by the rate of rise of the voltage across the breaker contacts as it opensTransient Recovery Voltage (TRV): is the transient voltage that appears across the contacts 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 rise Crest recovery

voltage

TRV

fault

TRVbus = 1 - cos t/sqrtLC

bus line

Time in hundreds of microseconds

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Standard C37.011 defines the TRV envelope that breakers must meet for rated current (Maximum TRV per standards is 2.4 per unit of rated breaker voltage)

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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 when the transient recovery voltage is analyzed for both scenarios:

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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

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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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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.


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1200pf, 3000pf and 12,000pf are typical values for external bushing capacitors.

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Operating Duty Cycle: A series of circuit breaker operations and time delays.Defined by ANSI as:O-15 SEC-CO-3 MIN.-CO.The first time delay can be reduced to 0.3 seconds for breakers designed for high-speed reclosing.0.3 seconds is the mechanical resetting time.

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A derating factor R is applied to OCBsbased on their reclosing duty.This does not apply to SF6 breakers.

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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/RStandards assume X/R of 17

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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 Circuit Breakers

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Switching Surges:

Z0= 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:

Z0= L/CRc

Rc

Z0 V0

Z0

(Rc + Z0)V0 = VS

bypass

VS

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Z0 V0

Z0 (Rc + Z0)

V0R first close

V0B second close

Rc > Z0

V0R first close

V0B second close

Rc < Z0

Rc

V0R first close = V02 second close

Rc = Z0

2.0 P.U.

2.0 P.U.

2.0 P.U.Optimum closing resistor size = surge impedance

Closing resistors:

V0 = VS

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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

power circuit breakers

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