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Comparison of circuit breaker stress related to back to back switching

Comparison of circuit breaker stress related to back to back switching. Helmut Heiermeier 14. October 2007. Background. During the revision of ANSI C37 06 higher values for inrush currents have been introduced These higher values increase the stress for the circuit breaker dramatically

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Comparison of circuit breaker stress related to back to back switching

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  1. Comparison of circuit breaker stress related to back to back switching Helmut Heiermeier 14. October 2007

  2. Background During the revision of ANSI C37 06 higher values for inrush currents have been introduced These higher values increase the stress for the circuit breaker dramatically Furthermore it is questioned whether such values exists or can be tested in Labs.

  3. Performed calculations The stress on circuit breaker contacts is a function of the applied energy ( current integral) to these contacts A comparison has been made between the cummulated energy experienced during a type test for 100% terminal fault and the cumulated energy during a back to back cap bank test Following cases have been calculated Case1 550 kV 63 kA 550 kV 40 kA 550 kV nom current 4000 A 550 kV nom current above 4000 A 550 kV old ANSI Case2 245 kV 63 kA 245 kV 40 kA 245 kV nom current below 2000 A 245 kV nom current 2000A and above 245 kV old ANSI

  4. Basis for calculations Terminal fault: Min arcing time 10ms 5 operations BTB test prearcing time 3 ms 120 closings at voltage peak Old ANSI Prearcing time 3ms 48 operations

  5. Graphical representation

  6. Conclusion The new inrush current values gives a stress to circuit breaker contacts which is several times higher than the stress occuring during type tests The new inrush current values gives a stress to circuit breaker contacts which is several times higher than the expected short circuit current interrupting capability ( even for electrical endurance )

  7. Questions The standard as it is written should give statistical confidence that the breaker is able to do it‘s job for a high nr. of operations If such high inrush currents really exists, how frequent are breakers operated under such conditions? Can cap banks really cope with such high currents since these currents create accelerated aging on the capacitors If the standard asks for such values, how and where to test it? Most labs have big problems to reach these values Why and how is the inrush related to the nominal current capability?

  8. Recomendation If such high inrush currents really exists Limit the nr of permissible operations under these conditions Limit the inrush currents by means of current limiting reactors Use controlled switching to gain contact life Use a special developed switching device Point out that these stresses are extreme cases not intended for daily uses

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