AC circuit breaker with series connected switches George G Karady.
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Transcript of AC circuit breaker with series connected switches George G Karady.
AC circuit breaker with series connected switches
• CB’s protect power system• Short circuit triggers the CB• CB interrupts the inductive short circuit current• After few cycle of arcing the current is interrupted at
zero crossing
Network 1 Network 2
CB 1 CB 2
Short circuit
AC circuit breaker with series connected switches
Equivalent circuit• Supply is represented by Thevenin equivalent• Network open circuit voltage and short circuit
impedance• Zshort = 1.8 ohm Vopen = 7.2 kV• Short circuit with zero fault impedance
AC
Network Switch Shortcircuit
Znet
Zsnub
AC circuit breaker with series connected switches
Equivalent circuit• CB with switches connected in series. • Snabber impedances are connected in parallel with
each switch• Typical values :Capacitor = 0.1microF, Resistance =
10kohm. Total impedance Zsnub = 10k + 26ki ohm
AC
Network Switch Shortcircuit
Znet
Zsnub
AC circuit breaker with series connected switches
• The switches turn off in sequence• The voltage across the switch that turns off first is the largest
AC
Network Switch Shortcircuit
Znet
Zsnub
AC circuit breaker with series connected switches
• The equivalent circuit is:
ShortcircuitAC
NetworkVswitch
ZnetZsnub
Vnet
Openfirst
The voltage across the switch that turns off first is:
AC circuit breaker with series connected switches
• Voltage across the switch turns off first is:
Ishort 4000A Vnet_.ln 7.2kV
Znet i7.2kV
4kA Znet 1.8i
Zsnub 10k1
i 0.1 F 2 60 Hz Zsnub 10 26.526i k
Vswitch_1 Vnet_.ln
Zsnub
Zsnub Znet Vswitch_1 7.2 10
3 0.161i V
• Voltage across the switch is close to the supply voltage
Rectifier with thyristors connected in series supplies a load
Rectifier supplies a load AC
Network LoadRectifier
AC
Network LoadRectifier
AC
NetworkLoad
Rectifier
Znet
Zsnub
Zload
Current path in the positive cycle• Snubber is connected in parallel with each diode
Rectifier with thrystors connected in series supplies a load
• Each thrystor is protected by a snubber connected in parallel• The current is determined by the load impedance• The voltage across the diode that turns off first is:
AC
Network
Znet Zsnub Zload
Xrect
Vnet
VdiodeOff
Rectifier with thrystors connected in series supplies a load
Ishort 4000A Vnet_.ln 7.2kV
Znet i7.2kV
4kA Znet 1.8i
Zsnub 10k1
i 0.1 F 2 60 Hz Zsnub 10 26.526i k
Xrectifier i 2 60 Hz 0.1 H Xrectifier 37.699i
Sload 3 7.2kV 400 A( ) Sload 8.64 103 kV A
Zload7.2kV
400A Zload 18
Vswitch_1 Vnet_.ln
Zsnub
Zsnub Znet Zload Xrectifier Vswitch_1 7.208kV
AC circuit breaker with series connected switches
Switching sequence• A diode and a snubber circuit is connected in parallel
with each switches• In the positive cycle switch 1 turns off,• The current is transferred to the diodes and
interrupted at zero crossing • Switch 2 turns off in the negative cycle to prevent
conduction
Switch 1 Switch 2