Detection of sub-cycle, self-clearing faults

Details Detection of sub-cycle, self-clearing faults Detection of sub-cycle, self-clearing faults

1999-02-12

2001-03-06

Lieu, Julie (Department: 2736)

Communications: electrical

Condition responsive indicating system

Specific condition

C340S654000, C340S660000, C324S500000, C361S071000

Reexamination Certificate

active

06198401

ABSTRACT:

TECHNICAL FIELD
The invention relates to detecting faults in power circuits, and more particularly to the detection of sub-cycle, self-clearing faults that clear too quickly to trip a circuit breaker.
BACKGROUND
An AC power source produces a potential difference between its output terminals. Conventionally, the potential of the terminals is measured with respect to electrical ground. In typical power distribution systems, the AC frequency is 60 Hz.
To ensure personal safety and prevent property damage, overcurrent protection is provided on the AC line, particularly on high voltage lines. For example, a circuit breaker may be placed in a line to interrupt the electrical current in the line if the current exceeds some predetermined setting. A circuit breaker is an electrically-actuated switch that opens to stop current flow. Opening of the switch, also referred to as “tripping”, may be in response to thermal or electromagnetic conditions in the circuit. After the circuit breaker opens, it may be reset or closed to allow current to again flow through the line.
SUMMARY
Some faults, referred to as self-clearing faults, are too short in duration to be detected by traditional detection techniques.
An apparatus for detecting such self-clearing faults in an AC power circuit carrying an electrical current includes a fast element which detects an occurrence of a fault within one cycle of initiation, detects clearing of the fault within one cycle of the clearing, and detects a duration of the fault to within one cycle or less. The apparatus further includes a self-clear element which compares the duration of the fault to a predetermined portion of a cycle of the electrical current to determine if the fault constitutes a self-clearing fault.
Embodiments may include one or more of the following features. For example, to determine if a fault has occurred, the fast element measures a value of the electrical current and compares the value to one or more reference values, such as high and low threshold values. The high and low threshold values may be calculated every several cycles or before the fault occurs, and may be based on one or more parameters associated with operation of an analog-to-digital conversion hardware and process used in measuring the current value and converting it to a digital value. Such parameters include a gain of a filter of the analog-to-digital converter, a calibration factor of the analog-to-digital converter, and a DC offset of the analog-to-digital converter.
To determine if the fault is self-clearing, the apparatus also may include an element which determines if the fault begins near a voltage peak.
The apparatus may compare a time of the fault with a time of one or more previous faults to determine a frequency of fault occurrences. The apparatus may then be configured to sound an alarm or open a circuit breaker in the AC circuit if the fault frequency exceeds a predetermined value.
The apparatus may also include a counter which records a number of fault occurrences. The apparatus may be configured to either sound an alarm or open a circuit breaker in the AC circuit if the number exceeds a predetermined value, or to increment the counter when the number is less than or equal to the predetermined value.
Other features and advantages will be apparent from the following description, including the drawings, and from the claims.


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