DC Open Series Fault considerations for

Distributed GenerationμGrid Architecture

Hunter Blake EstesThe University of Texas at AustinDepartment of Electrical & Computer EngineeringCenter for Electromechanics (CEM)Focus: Energy SystemsAdviser(s): Dr. Alexis Kwasinski, Dr. Robert Hebner

DC μGrid – Open Series Faults

One major concern regarding the safety of DC μGrids are fault conditions.

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).

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.

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.

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.

Experimental Approach Experimental μGrid:

3φ AC source panel variac or transformer passive 6 diode bridge rectifier (DBR) “open series fault” mechanism R-L load (Note: For AC testing, the DBR rectifier was removed from the circuit.)

Study DC (280 – 750V) vs. AC systems of “quasi-equivalent” parameters Monitor AC & DC currents, voltages, arc fault transients, power dissipated,

bus disturbances, duration, re-strikes, loading effects 5 to 50 μs data capture rates, 12 bit resolution Goals are DC arc modeling, DC breaker designs, DC arc fault detection, fault clearing

strategies, & safety recommendations

Experiment 3

ALPSpassive rectifier50 A

Circuit breaker

Oldenburgload bank

Series DC fault(type 1: 0.1 in/sectype 2: 9.8 m/s2)

AC

DC

208 [VAC] 280 [VDC]

PSCL 208 VAC

supply

DCcable

70 A 4 2.3 6.2 mHZ j L Variac

0-208 [VAC]

AC Arc Fault Transients

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)

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.

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.

DC Arc Fault Transients

In the case of DC open series faults, a strong arc was established and often persisted for several seconds.

Locally, the arc appears very chaotic in nature, even vaporizing copper contact materials at high current levels (175A).

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.

However, no fast-acting transient spikes were observed.

Conclusions 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)

These spikes could propagate throughout the power system and be electrically malign to power apparatus

AC architectures could mitigate this through the use of series impedance or shunt capacitance, but the former is unwanted while the latter increases cost

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.

However, because no fast-acting transients were observed during open series faults, DC faults may be considered electrically benign from a system perspective.

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.

For this reason, the lack of fast-acting transients during open series faults may be seen as an advantage of DC architectures.