Circuit Breaker Selection 1

8/12/2019 Circuit Breaker Selection 1

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Circuit Breaker Selection

(Section 10.5)


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AC Circuit Breakers

A circuit breaker is a mechanical switch capable

of interrupting fault currents and of reclosing.

Classified as low voltage circuit breakers (< 1500

V) OR power circuit breakers (>1500 V)

Different types depending on the medium in

which the arc is elongated (air, oil, SF6, vacuum..)


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AC Circuit Breakers


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Reclosing of circuit breakers

Most faults are temporary and self clearing.

It is assumed that whatever caused the fault

has disintegrated.

In EHV systems the standard practice is to

reclose only once.

Multiple shot reclosing in EHV systems maylead to transient stability problems


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Circuit breaker selection

Modern circuit breaker standards are based onsymmetrical interrupting current.

Only the symmetrical fault current at the point of

location is calculated and a circuit breaker withsymmetrical interrupting capability equal to the

calculated current is chosen.

Can handle asymmetrical fault current if dc offset is

not too large Power circuit breakers with a 2 cycle interrupting

time are designed to handle asymmetrical currents

up to 1.4 times the symmetrical interrupting

capability


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Fault current components


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SubtransientPeriod

Steady StatePeriod

TransientPeriod


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Fault current calculation

Period Fault Current Generator

Reactance

Synchronous

Motor

Reactance

Induction

Motor

reactance

( 50 hp)

Subtranient

period

Subtransient

currentOR

Asymmetrical

current

XdG Xdsm Xdsm

Transient

period

Transient

current

OR

Symmetrical

current

XdG Xdsm

= 1.5Xdsm

Xdsm

= ???Xdsm


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E/X method for circuit breaker

selection

If X/R is > 15, the DC offset may not have

decayed sufficiently and a method for

correcting the calculated fault current to

account for the DC and AC time constants

needs to be applied.

If X/R is unknown, the calculated fault current

should not be more than 80% of the breakers

interrupting capability.


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Application Guide for AC High Voltage Circuit

Breakers rated on a Symmetrical Current Basis


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Application Guide for AC High Voltage Circuit

Breakers rated on a Symmetrical Current Basis


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Voltage ratings

Rated maximum voltage: Designated the maximum

rms line-line operating voltage.

Rated low frequency withstand voltage: The

maximum rms line to line voltage the circuitbreaker can withstand without insulation damage.

Rated impulse withstand voltage: The maximum

crest voltage of a voltage pulse with standard riseand delay times that the breaker insulation can

withstand.


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Voltage ratings

Rated voltage range factor K: The range of

voltage for which the symmetrical interrupting

capability times the operating voltage is

constant.


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Current ratings

Rated continuous current: The maximum rmscurrent that the breaker can carry continuouslywhile closed without heating.

Rated short circuit current: The maximum rmssymmetrical current that the breaker can safelyinterrupt at rated maximum voltage.

Rated momentary current: The maximum rmsasymmetrical current that the breaker canwithstand while closed without damage.


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Current ratings

Rated interrupting time: The time in cycles from

the instant the trip coil is energized to the

instant the fault current is cleared.

Rated interrupting MVA: For a three phase

circuit breaker, this is 3times the rated

maximum voltage in kV times the rated short

circuit current in kA.


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Symmetrical interrupting capability of

a 69 kV class breaker


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Symmetrical interrupting current increases

from rated short circuit current I = 19 kA at

rated maximum voltage Vmax= 72.5 kV to Imax=

KI = (1.21)(19) = 23 kA at an operating voltage

of Vmin= (

) =

72.5

1.21= 60 kV.

For any operating voltage V between Vmin

and

Vmaxthe symmetrical interrupting current is(72.5)(19)


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For any operating voltage V below Vmin the

symmetrical interrupting current remains

constant at Imax= KI = 23 kA


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Example 1

The calculated symmetrical fault is 17 kA at a

three phase bus where the operating voltage

voltage is 64 kV. The X/R ratio at the is unknown.

Select a circuit breaker from the given tables forthis bus.


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Example 1

Consider the next highest available standardvoltage from the table. Therefore consider a 69kV class breaker.

At 64 kV, the symmetrical interrupting capabilityof the breaker is

(72.5)(19)

64= 21.5 kA

If X/R is unknown, the calculated fault current

should not be more than 80% of the breakersinterrupting capability. We need to do thischeck


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Example 1

Calculated fault current = 17 kA

Breakers interrupting capability = 21.5 kA

Therefore

17

21.5= 79.1 % < 80%

So the calculated fault current is not more than

80% of the breakers interrupting capabilitytherefore the chosen 69 kV class breaker issuitable for the application.


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Example 2

A 69 kV circuit breaker has a voltage range factor

K of 1.21, a continuous current rating of 1.2 kA

and a short circuit current rating of 19 kA at a

maximum rated voltage of 72.5 kV. Determinethe maximum symmetrical interrupting

capability of the breaker and determine the

voltage range at which the symmetricalinterrupting current is lower than the maximum

symmetrical interrupting current


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Example 2

Maximum symmetrical interrupting capability

= K x rated short circuit current

= 1.21(19)

= 23 kA

Vmin= (

) =

72.5

1.21= 60 kV.

Therefore the voltage range at which the

symmetrical interrupting current is lower than themaximum symmetrical interrupting current is 60 kV

72.5 kV


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Tutorial Exercises


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Tutorial Exercises


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Tutorial Exercises

Circuit Breaker Selection 1

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