20699779 power circuit breakers

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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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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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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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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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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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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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 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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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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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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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 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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A derating factor R is applied to OCBs

based 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/R

Standards 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 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

20699779 power circuit breakers

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