CONCEPTUAL DESIGN OF POWER CIRCUIT BREAKER USING MICROMECHANICAL SWITCHES

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CONCEPTUAL DESIGN OF POWER CIRCUIT BREAKER USING
MICROMECHANICAL SWITCHES

• George G Karady, Gerald T Heydt, Neil Shah
• Arizona State University, Tempe, AZ, USA

PSERC
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• MOTIVATION
• No major change in CB design in many years
• Large moving components and size
• Need for vacuum or SF 6 enclosure
• No synchronous switching
• Application of electronics components and MEMS
  switches allow miniaturization and zero current
  switching

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• Micro-switch based
• Circuit Breaker Concept

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Conceptual circuit diagram for an ac circuit
breaker

• Circuit breaker contains two switches
• Positive switch operates in the positive cycle
• Negative switch operates in the negative cycle

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• Switching string assembly with several strings
  connected in parallel. (Positive switch only)
• Voltage rating is increased by switching
  additional units in series
• Current rating is increased by switching
  additional units in series

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• Switching string operation

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Illustration of circuit breaker closing.
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Illustration of current interruption.
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• Operation of Switching Strings Connected in
  Parallel

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PSPICE simulation of circuit interruption
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• Equivalent circuit of a switching string
• Closed switches equivalent is the contact
  resistance
• Open switches equivalent is the diode voltage

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Equivalent circuit modeling the non- simultaneous
operation of the switches.
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• Current distributions when all string except one
  is turned on with one millisecond delay.
• A. Inductive load current
• B. The closing of all switches in string 1
  eliminated the diodes and inserted the contact
  resistances
• C. Simultaneously the current of the other two
  strings reduced to zero, because the diodes
  become reverse biased.

Simulation of switch closing
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• Current distribution during current interruption.
  
• A). String current when one string is turned off
  with 1 msec delay.
• B) String current when all strings except one is
  turned off with 1 msec delay.
• The short circuit current is interrupted with a
  half cycle

Simulation of switch opening
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• Current injection circuit for interruption of DC
  current.

• DC current interruption requires current
  injection
• Charge capacitor produces current oscillation.
• During the negative cycle the switches are opened
• At zero crossing the diodes interrupts the current

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• Circuit with MEMS switch

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MEMS Switch
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MEMS Switch
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• New latching type switch become available
• The switch has two position ON or OFF
• The switch has an operating coil
• Short positive or negative pulse change switch
  position
• The problem is that the insulation has to
  withstand 7.2 kV between the contact and the
  magnet.

• Voltage 400V
• Current 100mA
• Resistance 0.5 ohm
• Insulation 400V
• Operating coil current 100mA, 500 usec

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• A diode and grading resistance is connected in
  parallel with the switch
• The switching coil is supplied by a microwave
  power supply
• A antenna provided short duration 945Mz signal is
  rectified by the tunnel diode and capacitor
• The obtained DC signal operates the switch
• The 95kV BIL is achieved by a distance of 30
  inches

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• Five building blocks are connected in series to
  form a switching unit
• The operating coils are connected in series to
  meet with the 400V insulation level
• A microwave power supply provides dc operating
  current for the series connected coils
• A high power (1W) microwave transmitter provides
  the short duration square wave signal
• The antenna picks up the signal
• The signal is rectified. This provides dc current
  to the operating coil

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Switching chain assembly
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Circuit breaker arrangement
Positive switch
Negative switch
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AC circuit breaker concept
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• System Realization

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Reduced scale circuit breaker
Control solenoid
Switching string
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The technical data of the developed small scale
circuit breakerRated current 8 A steady
stateInterruption current 50 A for a half
cycleRated voltage 4000 VBIL 95
kVNumber of switches in series in a single
string 10Number of strings in parallel 8
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• Research work needed
• Analysis of steady state current distribution in
  a large swathing string assembly using
  equivalent circuit
• Analysis of transient current distribution during
  turn on using SPICE
• Analysis of transient current distribution during
  turn off using SPICE
• Analysis of turn on and off overvoltages
• Reliability analyses Safe operation time?
  Required number of units ?

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• Research work needed
• Building and testing MEMS switch assembly 5 x 6
  matrix
• Development of Microwave pulse generation for
  turn on and off.
• Development of circuit breaker control (on and
  off signal generation using and or logic
  components)
• Building and testing of a MEMS based circuit
  breaker
• Development and analysis of DC current
  interruption concept using current injection
• Testing the DC injection method using the
  mechanical switch assembly
• Testing the mechanical circuit breaker

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• Conclusions
• The study proved that the micro-switched based
  medium voltage circuit breaker is feasible.
• It offers small size, zero current switching and
  interruption of short circuit current within a
  half cycle.

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• The specific results are
• Development of novel concept for CBs using
  switching matrix and switching string.
• Development of a method to analyze the effect of
  none simultaneous operation of switches in a
  switching string assembly.
• Reliability analysis of switching matrix.
• Building of a proof of principles switching
  string assembly to experimentally proof the
  validity of the concept.
• Proposal for development of a new type of MEMS
  device and the specification of the new device.
• Development of a novel analytical model for the
  reliability analysis of the switching matrix.

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• FUTURE WORK
• Finalization the analytical technique for
  operation of large switching matrixes,
• Improvement of reliability analysis and
• Testing the proof of principle switching
  assembly.
• Detailed design of the MEMS based switch
• Implementation of the educational objective

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• Acknowledgement
• The authors would like to acknowledge the support
  of NSF and the Navy.
• The authors thank to Prof B. Kim of ASU and
• Graduate students Mr. Neil Shah, Daniel S. James
  II and Rahim Kasim for their contribution.