Cem hunter

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Cem hunter

  1. 1. DC Open Series Fault considerations forDistributed GenerationμGrid Architecture <br />Hunter Blake Estes<br />The University of Texas at Austin<br />Department of Electrical & Computer Engineering<br />Center for Electromechanics (CEM)<br />Focus: Energy Systems<br />Adviser(s): Dr. Alexis Kwasinski, Dr. Robert Hebner<br />
  2. 2. DC μGrid – Open Series Faults<br />One major concern regarding the safety of DC μGrids are fault conditions.<br />Open series faults occur when the circuit pathway is interrupted, either intentionally (as in a breaker) or unintentionally (as in a loose or severed cable). <br />For an AC, 60 Hz waveform, upon a fault condition, the maximum time an arc should sustain itself (excluding re-strikes) is only half a cycle, as the periodic voltage source waveform will reach a 0V crossing and propagation should cease. Moreover, for each half-cycle, since the waveform is symmetric about the peaks, the average time period for a continued arc path is ¼ the period, or 4.167 ms.<br />For DC architectures, open series faults initially seem more concerning since there is no zero crossing. If a current branch is interrupted, an arc will initially form to sustain current. However, there will be a transition period until such time as the arc dissipates and current ceases to flow.<br />The focus of this research will be to experimentally determine the effects open series DC faults have in terms of local and system disturbances, in relation to AC.<br />
  3. 3. Experimental Approach<br /><ul><li>Experimental μGrid:
  4. 4. 3φ AC source panel variac or transformer  passive 6 diode bridge rectifier (DBR) </li></ul> “open series fault” mechanism  R-L load<br />(Note: For AC testing, the DBR rectifier was removed from the circuit.)<br /><ul><li>Study DC (280 – 750V) vs. AC systems of “quasi-equivalent” parameters
  5. 5. Monitor AC & DC currents, voltages, arc fault transients, power dissipated, </li></ul> bus disturbances, duration, re-strikes, loading effects<br /><ul><li>5 to 50 μs data capture rates, 12 bit resolution
  6. 6. Goals are DC arc modeling, DC breaker designs, DC arc fault detection, fault clearing strategies, & safety recommendations</li></li></ul><li>AC Arc Fault Transients<br />Visually, AC open series faults did not produce much of a sustained arc, as it dissipated quickly upon reaching a zero crossing. In some cases though, re-strikes were observed. (see above)<br />However, very fast transient “spikes” were often observed relating to gap voltage & current. In some cases, these levels were concerning. In the chart above, voltage spikes reached 1000V for a 219 Vrms(310Vpeak) waveform.<br />Additionally, AC tests of equivalent parameters often produced different results based upon when contacts were opened in relation to sinusoidal waveforms. As such, tests were not repeatable.<br />
  7. 7. DC Arc Fault Transients<br />In the case of DC open series faults, a strong arc was established and often persisted for several seconds.<br />Locally, the arc appears very chaotic in nature, even vaporizing copper contact materials at high current levels (175A).<br />During this time, current flow slowly began to diminish while gap voltage increased in almost a mirror-image waveform. Ultimately, the arc collapsed and current fell to zero, with the entire bus voltage now applied across the arc gap.<br />However, no fast-acting transient spikes were observed.<br />
  8. 8. Conclusions<br />From a system standpoint, AC open series faults are alarming due to transient “spikes” seen in both the current and gap voltage waveforms (di/dt & dv/dt)<br />These spikes could propagate throughout the power system and be electrically malign to power apparatus<br />AC architectures could mitigate this through the use of series impedance or shunt capacitance, but the former is unwanted while the latter increases cost<br />From a local standpoint, DC open series faults seem concerning for they are more persistent than AC. As such, they may be considered mechanically malign & hazardous to personnel, equipment, and processes.<br />However, because no fast-acting transients were observed during open series faults, DC faults may be considered electrically benign from a system perspective.<br />In the event of a DC open series fault, if it could be detected and isolated from a microgrid through the use of power electronics, unaffected branches would continue to remain operational. <br />For this reason, the lack of fast-acting transients during open series faults may be seen as an advantage of DC architectures.<br />

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