Fault location and characterization in AC and DC power systems

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2012-12

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Abstract

The focus of this research is on identification, location, interruption, characterization and overall management of faults in conventional AC distribution systems as well as isolated MVDC power systems. The primary focus in AC distributions systems is on identifying and locating underground cable faults using voltage and current waveforms as the input data. Cable failure process is gradual and is characterized by a series of single-phase sub-cycle incipient faults with high arc voltage. They often go undetected and eventually result in a permanent fault in the same phase. In order to locate such incipient cable faults, a robust yet practical algorithm is developed taking into account the fault arc voltage. The algorithm is implemented in the time-domain and utilizes power quality monitor data to estimate the distance to the fault in terms of the line impedance. It can be applied to locate both sub-cycle as well as permanent faults. The proposed algorithm is evaluated and proved out using field data collected from utility distribution circuits. Furthermore, this algorithm is extended to locate evolving faults on overhead distribution lines. Evolving faults are faults beginning in one phase of a distribution circuit and spreading to another phase after a few cycles. The algorithm is divided into two parts, namely, the single line-to-ground portion of the fault and the line-to-line-to-ground portion of the fault. For the single line-to-ground portion of the fault, the distance to the fault is estimated in terms of the loop or self-reactance between the monitor and the fault. On the other hand, for the line-to-line-to-ground and line-to-line portion of the fault the distance is estimated in terms of the positive-sequence reactance. The secondary focus of fault management in AC distribution systems is on identifying fault cause employing voltage and current waveform data as well as meteorological information. As the first step, unique characteristics of cable faults are examined along with methods to identify such faults with suitable accuracy. These characteristics are also used to distinguish underground cable faults from other overhead distribution line faults. The overhead line faults include tree contact, animal contact and lightning induced faults. Waveform signature analysis, wavelet transforms and arc voltages during the fault event are used for fault cause identification and classification. A statistical based classification methodology to identify fault cause is developed by utilizing promising characteristics. Unlike the AC system infrastructure which is already in place, the DC system considered in this document is that of a notional electric ship. The nature of DC current, with the absence of a current zero as well as the presence of power electronic devices influencing the current behavior, makes interrupting DC fault currents challenging. As a part of this research an innovative DC fault interruption scheme is proposed for rectifier- fed MVDC systems. A fault at the terminals of a phase-controlled rectifier results in a high magnitude current impulse caused by the filter capacitor discharging into the fault resistance. It is proposed to use a series inductor to limit the magnitude of this current impulse. The addition of the inductor results in an underdamped series RLC circuit at the output terminals of the rectifier which causes the fault current to oscillate about zero. Furthermore, it is proposed to utilize a conventional AC circuit breaker to interrupt this fault current by exploiting the zero crossings resulting from the oscillations. Using the proposed scheme for the example case, the peak fault current magnitude as well as the interruption time is significantly reduced.

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