Electricity: measuring and testing – Measuring – testing – or sensing electricity – per se – With coupling means
Reexamination Certificate
2002-05-21
2003-05-13
Le, N. (Department: 2858)
Electricity: measuring and testing
Measuring, testing, or sensing electricity, per se
With coupling means
C324S115000, C324S11700H, C324S522000, C324S547000
Reexamination Certificate
active
06563296
ABSTRACT:
BACKGROUND
This invention relates to methods and apparatus for detecting a time-varying current. In particular, the invention detects a transient current produced by a variety of physical mechanisms, e.g., a pulse of current carried on an electrical conductor, with a better signal-to-noise ratio than the existing current sensors provide.
One important application of the apparatus of the invention relates to the detection of a sudden localized redistribution of charge at a defective point of an insulation which is under high electric stress, e.g., electric power cables, high voltage transformers. Such a redistribution of charge leads to an intermittent arcing that signals the presence of a fault. The term fault, as used herein, refers both to an incipient fault which may not cause an immediate failure but may eventually lead to a failure, and also to a complete failure of the system. It is well-known that partial discharge events are common precursors to many significant failure modes in a variety of high voltage devices.
The early detection of an incipient fault through monitoring partial discharge events can prevent the occurrence of a complete failure which may result in considerable inconvenience and financial loss. For example, the failure of high voltage devices of a utility company such as transformers and high voltage distribution cables, especially during a critical period, can lead to enormous inconvenience for the customers and financial loss for the company.
The energy associated with a partial discharge event is typically extremely small. This energy is particularly small when the defect that gives rise to the partial discharge is at its early stages of development. Given the desirability of detecting defects at such early stages, it is evident that having a sensor that detects a pulse of current with a high signal-to-noise ratio is extremely desirable.
U.S. Pat. No. 5,075,629 discloses a device for detecting a partial discharge in a transformer. In particular, this patent discloses an antenna, constructed of a single solid core with a conducting wire wrapped around it, that is placed inside the transformer to detect emission of an electromagnetic wave produced by the transformer as a result of the occurrence of a partial discharge. The center of the conducting wire wrapped around the single core is grounded to produce two coils. The time-varying magnetic field of the electromagnetic wave induces two voltages in the coils that provide the input signals for a differential amplifier. The output of the amplifier signals the occurrence of a partial discharge.
The antenna disclosed in the '629 patent has a solid core that does not permit the passage of a cable therethrough. Thus, the apparatus can not be connected directly through a cable to the transformer to receive a current pulse that a partial discharge produces. Accordingly, it relies on a weaker mode of coupling, i.e., detection of the electromagnetic wave emanated from the transformer. This mode of coupling imposes the further limitation that the apparatus can not be employed outside the transformer because high voltage transformers are typically shielded by enclosures.
Some prior art techniques relate to finding the location of a partial discharge event so as to allow correction of the defect giving rise to the discharge. For example, U.S. Pat. No. 5,530,364 discloses an apparatus for detecting the location of an incipient fault in an insulated cable. In particular, the apparatus of the invention scans the surface of the cable by physically moving two separate axially spaced sensors, disposed adjacent to the surface, over the cable. A partial discharge event produces a current pulse through the cable which, in turn, produces electrical pulses at each of the two sensors. These two pulses are added to produce a resultant signal whose magnitude reaches a maximum if the partial discharge occurs at a point midway between the two sensors, thus indicating the location of the discharge.
The energy produced by a partial discharge event in a high voltage device is typically extremely small. Accordingly, it is imperative that the system designed for detecting such events provide a means of coupling to the device that has a minimal loss and also a detection means that provide a high signal-to-noise ratio. In addition, periodic monitoring of a device for partial discharges requires a system that can be easily connected to and disconnected from the device. Many prior art systems suffer from a number of limitations related to the above-mentioned desirable features that the present invention seeks to remedy. For example, the use of the apparatus of '629 patent not only employs a weak mode of coupling to the transformer but also requires opening up a transformer to place the antenna within it. The apparatus of the '364 patent is essentially a timing system for signaling the arrival of two pulses at the sensors, and does not provide new techniques for improving the signal-to-noise ratio of the detected pulses.
It should be understood that the desirability of detecting transient currents with a high signal-to-noise ratio is not limited to monitoring of partial discharges. Another possible application, for example, relates to monitoring a beam of electrons or ions used in implantation systems.
Accordingly, it is an object of the present invention to detect a transient current carried by a wire with an improved signal-to-noise ratio.
It is another object of the invention to detect a pulse of current or a beam of charged particles with an improved signal-to-noise ratio.
It is yet another object of the invention to provide a current sensor that can be easily calibrated.
It is yet another object of the invention to provide a current sensor with a broad-band and/or selectable frequency response.
It is yet another object of the invention to provide an apparatus for monitoring partial discharges in high voltage devices with an improved signal-to-noise ratio.
The invention is next described in connection with illustrated embodiments. It is obvious to those skilled in the art that various modifications to the embodiment can be made without departing from the scope and the spirit of the invention.
SUMMARY OF THE INVENTION
The invention detects a time-varying current by employing a plurality of generally toroidal coils with separate windings that are placed in close proximity of each other such that they respond to a current signal, which passes through the central aperture of the toroids, at substantially the same time. In particular, the current induces a plurality of voltages across these coils where each induced voltage indicates the detection of the current. The invention also allows combining the induced voltages, i.e., summing and/or subtracting the voltages, to produce a resultant signal that signifies the existence of the time-varying current. A single coil of toroidal construction is sometimes referred to as a Rogowski coil.
It is well understood in physics that a changing magnetic flux within a coil induces a voltage across it. Furthermore, a current due to moving charged particles produces a surrounding magnetic field. Accordingly, a changing magnetic flux, associated with a current pulse, through the closely-spaced coils of the present invention induces a plurality of voltages across them. The spacings between the coils are selected such that each coil responds to substantially the same magnetic field. In addition, each coil is electrically insulated from the others so that each induced voltage represents an independent response to the same current. The induced voltages can be either utilized individually or can be combined in various ways, described more fully below, to produce a combined signal.
One aspect of the invention relates to selecting at least one coil to have either a winding of opposite polarity with respect to another coil or to have a winding of similar polarity but reverse output connections with respect to the other coil. Two such coils are herein referred to as having opposite polarities with re
Deb Anjan K.
Engellenner Thomas J.
Nutter & McClennen & Fish LLP
Smith Geoffrey S.
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