Safe Operation and Maintenance of Circuit Breakers and Switchgear

Safe Operation and Maintenance
of Circuit Breakers and
Switchgear
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Topics
• Single line diagrams
• Active and passive components
• Circuit breaker utilization
• Forms of medium voltage switchgear
• Basic circuit breaker design
• Auto re-closing

Single line diagrams
•Typical example:
high voltage distribution network
•Single line diagram
•Three phase nature of circuit is
ignored

Active and Passive Components
• ACTIVE components (continuously in use) =
transformers, cables, overhead lines and
metering equipment
• PASSIVE components (function only when
required to) = switchgear including breakers,
links, disconnects, earth switches

Circuit Breaker Utilization
• In a perfect world we wouldn’t need switchgear!
Economics dictates:
• Do not install more switchgear than is
absolutely required
• No more functionally than is necessary

Circuit Breaker Utilization
• Two most common forms of medium voltage
switchgear are automatic circuit breakers and
non-automatic, load breaking, fault making
switches.
• Main function of a circuit breaker is to
automatically interrupt fault current and to
close onto a fault and thereby make fault
current.

Fuse switch in cross section
Note:
If used to feed a transformer
all three phases must be
disconnected (three phase
disconnector) when a single
phase trips

Basic Circuit Breaker Design
• Primary purpose =
to interrupt fault current
automatically

Supplementary circuit breaker
• If time graded protection were used at X
Faults after X will not affect loads between
X and Y
• However, Y must be set to trip in a longer
time

Local and distant faults
Fault at A
Waveform at
switch

• Distant fault shown at A, circuit impedance, mainly resistance, acts
to limit the fault current and bring the power to a value close to
unity.
• As the voltage and current are in phase, the voltage across the open
contacts of the circuit breaker is low at the time of current zero,
hence the probability of arc extinction is high.

Local and distant faults contd..
Fault at B
Waveform at
switch

Close in Fault
• Fault at B – generator reactance limits If and causes pf to be low ~
0,15
• As the contacts open the current and voltage are almost 90 degrees out
of phase
• therefore as the contacts open the voltage across the contacts is high at
the time of current zero – probability of RE-STRIKING is increased

Current Interruption
Therefore with all circuit breakers current interruption (and arc extinction)
does not always occur on the first current zero crossing the following factors
influence this:
• design of breaker
• magnitude of the fault current
• power factor
Several current cycles may occur before interruption is achieved

Current Chopping
• Current chopping is the interruption of the current before or
after the natural current zero crossing
• This can cause severe over-voltages on the system and
damage due to over stressing of insulation levels
• This is an acute problem when loads are low as in:
• transformer magnetising currents
• capacitive line charging currents

Variation of arc voltage with time
• During arcing the voltage across the open contacts is known as the arc
voltage
• At the instant of current zero the voltage rises to the peak value of the re-striking
transient voltage which oscillates at a frequency dependant on
system resonant frequency
• This resonant voltage is dampened by the system impedances

Effects of different clearance times on
three-phase fault
• Blue phase clears first, other two phases still arcing
• The voltage across the first contacts to clear will be 50% greater than the peak
line to neutral voltage

RRRV
•Rate of Rise of Recovery Voltage greatly affects circuit breaker performance
• Some designs are particularly susceptible to re-striking voltage (Air Blast)
• In Air blast (axial blast designs) breakers resistors are switched in during the arc
interruption process

Auto Re-closing
• Many of the faults that occur will be transient in
nature (over 90% of all faults are earth faults)
• These are caused by branches blowing across lines,
insulators flashing over etc.
• Controlling circuit breaker (at R in next figure) to
re-close automatically after a fault occurs

Re-closed overhead line network
Using an Auto-recloser
here
improves the
reliability of
supply

Typical auto re-close sequence
Dead-time
Reclose
Time delayed trip
allows time for a
fuse to blow

Successful implementation of auto reclosers
• Can be used on overhead lines only
• If cables and lines are combined – use breakers on cable
sections
• Check grading with fuses on transformers and spurs to
ensure successful reclosing operations

Pole mounted auto-re-closer (at A)
Cabled network
– no recloser
Recloser on
overhead lines

Pole mounted, oil filled re-closer
Overcurrent coils

Vacuum interrupter in SF6
auto-re-closer
• Maintenance is lower than
oil filled
• Replacement of battery
every 5 years
• Elastomeric bushings are
more resistant to damage
• Control box normally
situated at ground level

DO YOU WANT TO KNOW MORE?
If you are interested in further training or information,
please visit:
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Safe Operation and Maintenance of Circuit Breakers and Switchgear

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Safe Operation and Maintenance of Circuit Breakers and Switchgear