AC and DC Power Circuit Breakers: Effective System Protection for Traction Power Systems

Historically, electrical engineers have had only two choices when it came to specifying power circuit breakers: oil-filled or air.  Today, with established technologies for insulating and interrupting methods, the application of circuit breakers is sometimes an afterthought. Although 2kV-38kV class of AC air circuit breakers has virtually disappeared from the scene, they still have a prominent place in the 250V-2000V class of equipment, particularly in AC and DC traction power systems.

Keep in mind when standards were established for testing and rating of AC and DC circuit breakers, they may not have had your specific application in mind, permitting your implementation to “slip through the cracks,” resulting in unforeseen (or unforeseeable) problems including equipment malfunction.

AC Circuit Breaker Operation

AC breakers create challenges when high fault currents with high X/R ratios (i.e. proportion of system reactance divided by system resistance) occur on the AC system.  Consequently, low fault currents on an AC system are of no concern since an AC breaker enjoys the benefit of a zero current crossing during the AC cycle.  During these zero crossings, the arc energy is dissipated sufficiently, and the breaker establishes a successful open circuit during the next voltage peak. 

DC Circuit Breaker Operation

DC electrical systems handle fault interruptions differently.  The maximum fault current that a DC breaker can withstand is as much of a concern as it is for an AC breaker.  However, for a DC breaker, the L/R ratio (i.e. rate of rise time constant) also plays an integral role. A DC circuit breaker performs well for high fault currents with low L/R values.  We attribute this to the strong forces that play a part in pushing an electric arc up into the magnetic blow-off quenching zone.  However, higher L/R values and low fault currents become difficult for a DC breaker to interrupt.  This aspect is attributed to having no zero current crossings together with the weak magnetic forces that would otherwise push the persistent arc into the quenching zone.  In extreme cases of low fault current, the DC breaker may fail to trip.

Precise short circuit study models are particularly important for industrial power systems such as traction power and rail electrification.  For accurate analysis, you should obtain realistic X/R parameters for AC systems and realistic L/R parameters for DC systems from either trusted data or an experienced engineer.  At each faulted bus, you should perform an accurate short circuit analysis to flag AC breakers which have been subjected to X/R ratios and fault currents outside of their published test ranges.  Similarly, for DC breakers, you should perform an accurate short circuit analysis to ensure that it suits the equivalent L/R rise times at each faulted bus.

To ensure effective system protection for your AC or DC traction power system, you should analyze your electrical system and specify the appropriate circuit breaker.  We make specific recommendations to improve reliability which can save you money.  If you would like to know more, feel free to call us at (240) 582-3900 or email us at info@helioselectric.net.  

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