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

Network LoadRectifier

|

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