4.Circuit-Breaker Model Validation
Measured voltage and current waveforms of a 125A circuit-breaker operating in both a DC high-current test facility and a distributed power system rack were used to validate the model when operating in the magnetic region. Operation in the thermal region is not shown, as this mode is typically of secondary importance when investigating over-current protection in telecommunications power systems.
Figures 5 and 6 show simulated waveforms of the circuit-breaker model operating in circuits with equivalent characteristics to the test circuits used to obtain the measured waveforms shown in Figures 2 and 3 respectively.
Simulated results show quite good agreement with measured results considering the complex physical arcing process that takes place during circuit-breaker operation. The major area of discrepancy is the overvoltage transient generated as the arc extinguishes. It should be noted that minor waveform variations have been observed with repeated tests under the same test conditions.
The circuit-breaker model has assisted Telstra’s power system designers to analyse the operation of a 125A circuit-breaker operating in a Telstra distributed power system battery rack. The measured current waveform of a circuit-breaker interrupting a short circuit from the 48V battery string negative (active) output terminal to the rack frame is shown in Figure 7. No voltage waveforms were taken in this test. Figure 8 shows simulated waveforms of the circuit-breaker model operating in a circuit with equivalent characteristics to that used to obtain the measured waveform shown in Figure 7. Again, quite good agreement is obtained between the simulated and measured current waveform.
In summary, this article describes a new circuit-breaker model that accurately represents circuit-breaker behaviour in DC distribution systems over a wide range of over-currents. The circuit breaker model can be coupled with other distribution component models [1-2] to simulate the protection performance in telecommunications DC distribution systems. The development of this circuit-breaker model, and other previously published modelling work, has provided Telstra power system designers with a useful design and analysis tool.
 T.P.Robbins, “Fuse model for over-current protection simulation of DC distribution systems”, In Proceedings of the Conference INTELEC, 1993, pp.336-340.
 T.P.Robbins, “Battery model for over-current protection simulation of DC distribution systems”, In Proceedings of the Conference INTELEC, 1994, pp.307-314.