safe operation and maintenance of circuit breakers and switchgear

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Safe Operation and Maintenance of Circuit Breakers and

Switchgear


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!

• Do not install more switchgear than is absolutely required

• No more functionally than is necessary

Economics dictates:


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 XFaults after X will not affect loads between X and Y• However, Y must be set to trip in a longertime


Local and distant faults

Fault at A

Waveform at switch


Local and distant faults• 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


Local and distant faults

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-closerOvercurrent 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


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safe operation and maintenance of circuit breakers and switchgear

Engineering