Patent Application
20100148907
Embodiments of the invention relate to current transformers for measuring a differential current transmitted by at least two conductors, calibration methods for such current transformers, and electrical monitoring systems using such current transformers.
Current transformers are devices used to scale large primary currents to smaller and more easy to measure secondary currents for use in metering and protective relaying in the electrical power industry. A differential current transformer, for example, may be used for measurement of leakage current in an insulation breakdown monitoring system. To improve sensitivity in such current transformers, there is a need for improved current transformers.
Briefly, in accordance with one aspect disclosed herein, a current transformer comprises a magnetic core having a closed central opening, at least two conductors extending through the central opening and positioned symmetrically within the magnetic core, and at least one set of winding turns wound around the core in a balanced configuration with respect to the at least two conductors.
In accordance with another aspect disclosed herein, a calibration method for a current transformer is provided. The current sensor comprises a core with a central opening, at least two conductors extending through the central opening, and a winding on the core. The calibration method comprises transmitting currents through the at least two conductors, obtaining a measured differential current through the winding, and changing a spatial relative position of the at least two conductors and turns of winding with respect to the core to obtain a target reading.
In accordance with still another aspect disclosed herein, an electric monitoring system comprises a current transformer. The current sensor comprises a magnetic core having a closed central opening, at least two conductors of the system to be measured extending through the central opening and positioned symmetrically within the magnetic core, and at least one set of winding turns wound around the core in a balanced configuration with respect to the at least two conductors. The electric monitoring system further comprises a processing module for receiving a measured differential current from the at least one set of winding turns and monitoring the measured differential current.
In accordance with still another aspect disclosed herein, an insulation condition monitoring method for a rotating electric machine is provided. The method comprises calibrating a current transformer by transmitting currents with the same current amplitude and reversed direction through two conductors extending through a central opening of a magnetic core; receiving a measured differential current from a winding on the core; and changing a spatial relative position of the at least two conductors and turns of the of winding with respect to the core to obtain a target reading. The method further comprises measuring a first set of values for an instantaneous differential current using the calibrated current transformer and an instantaneous phase voltage during operation of the machine, calculating a second set of values for a phasor current and a phasor voltage based upon the first set of values of the instantaneous differential current and the instantaneous phase voltage, respectively; calculating an angular relationship between the phasor current and phasor voltage; and determining the insulation condition based on the angular relationship.
These and other features, aspects, and advantages of the present invention will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein:
Referring to
A differential current transformer can be used for measurement of leakage current in an insulation breakdown monitoring system. The currents carried in the first and second conductors 14 and 16 are large and the leakage current measured by the plurality winding turns 18 is very small. As the first and second conductors 14 and 16 and the winding turns 18 are often randomly positioned with respect to each other and with respect to the core 12, the differential magnetic fluxes picked up by individual winding turns 18 at different locations are different and result in error in the measured differential current from the winding turns 18.
Embodiments of the invention relate to a current transformer for measuring a differential current between currents transmitted on at least two conductors. The current transformer comprises a core with a closed central opening, the at least two conductors extending through the closed central opening, and a plurality of windings wound on the core. The at least two conductors represent as a primary winding of the current transformer. Magnetic flux generated by the at least two conductors induces a current on the plurality of windings, and accordingly, the windings represent as a secondary winding of current transformer, and a reading of the current from the windings is a measured “differential current” of the currents carried on at least two conductors. The windings and the at least two conductors are arranged in the core in a balanced configuration. The balanced configuration is advantageous because for improving sensitivity in differential current sensing.
Embodiments of the invention relate to a calibration method for a current transformer to measure a differential current between currents transmitted on at least two conductors. The method comprises extending the at least two conductors through a central opening of a core, winding a plurality of winding turns on the core, transmitting currents through the at least two conductors such that an ideal differential current of the currents is zero, receiving a measured differential current by reading of the winding turns, and changing spatial relative position of the at least to conductors and the windings with respect to the core to obtain a target reading. In one embodiment the target reading comprises the smallest obtainable target reading. In another embodiment, the target reading comprises a reading smaller than a preset value.
Embodiments of the invention additionally relate to electrical monitoring systems and methods using current transformers with the balanced configuration or comprising the calibration method for current transformers. The monitoring systems may include, for example, insulation monitoring systems or ground fault or arc fault detection systems for multi-phase motors, generators, and transformers.
In certain embodiments, the current transformer further includes at least two reference lines respectively extending through the center point of the central opening and a corresponding conductor. In another embodiment, the current transformer comprises a number N of sets of winding turns, and each set of winding turns is centered on a corresponding reference. Thus, windings and conductors are in a balanced configuration in the central opening of the core.
In certain embodiments, current transformers each comprise one or more locking mechanisms for securing windings and/or conductors at the balanced configuration. In certain embodiments, a locking mechanism for windings may comprise a permanent mechanism such as adhesives and banding, or a removable mechanism such as brackets or clamps. In some embodiments, locking mechanisms for conductors may comprise plates or blocks with centering holes. Such plates and/or blocks may be internal or external to the core.
Referring to
In one embodiment, central opening 28 comprises first and second reference lines 46 and 48. First reference line 46 extends through center point 36 and first conductor 30, and second reference line 48 extends through center point 36 and the second conductor 32. In one embodiment, the first and second reference lines 46 and 48 are in line with each other. In other embodiments, such as shown in
In one embodiment, core 26 has a symmetric configuration about at least one of the phantom lines 38 and 40. In certain embodiments, core 26 may be circular, rectangular, square, or a combination thereof. In certain embodiments, core 26 comprises a laminated magnetic steel, a solid magnetic steel, or a sintered magnetic alloy. The overall core diameter may range from 1 cm to 100 cm, for example.
Referring to
Referring to
In one embodiment, differential current is measured at terminals 33 and 35 which are electrically coupled to one of the sets of first and second winding turns 42 and 44, and the first and second sets of winding turns are electrically connected to each other via electrical couplers 78. In another embodiment, electrical couplers 78 may comprise the same wire used for the winding turns.
Referring to
In one embodiment, central opening 28 comprises first, second, and third reference lines 62, 64, and 66, which are spaced from one another by 120 degrees. First reference line 62 extends through center point 36 and first conductor 50, second reference line 64 extends through center point 36 and the second conductor 52, and third reference line 64 extends through center point 36 and the second conductor 54.
Referring to
Referring to
In one embodiment, a calibration method for a current transformer, comprises providing at least two conductors through a central opening of a core and a winding on the core, transmitting currents through the at least two conductors, receiving a measured differential current by reading of the windings, and changing spatial relative position of the at least two conductors and the windings with respect to the core to obtain a target reading. The target reading which may be a minimum reading or a reading smaller than a preset value. In one embodiment, at least one of the conductors is moved in a circumferential direction for obtaining the targeted reading. In another embodiment, wherein the windings are wound in a moveable manner, individual turns of the windings are moved along the core to obtain the targeted reading. In still another embodiment, the core is moved in an up-down direction, or in a radial direction to obtain the targeted reading.
The signals from the sensors 84 and 86, measured by the data acquisition system 88, are applied to a processing module 90. Module 90 will typically include hardware circuitry and software for performing computations indicative of insulation condition as described below. Module 90 may thus include a range of circuitry types, such as, a microprocessor based module, and application-specific or general purpose computer, programmable logic controller, or even a logical module or code within such a device. The module 90 is configured to convert the values for the instantaneous differential current and the instantaneous phase voltage to respective values for phasor current and phasor voltage. The processing module 90 further calculates an angular relationship between phasor current and phasor voltage and generates an output based on the calculated angular relationship, as an indication of insulation condition. A memory module 92 is used for storing the output generated from the processing module 90. The same, or a different memory module may also store programming code, as well as parameters and values required for the calculations made by the processing module 90. An indicator module 94 compares the output of the processing module 90 to a predetermined threshold value and generates an indication signal 96 based on the comparison. In general, the indication signal 96 may provide a simple status output, or may be used to activate or set a flag, such as an alert when the output of the processing module 90 exceeds the threshold value, indicating that the insulation is in need of attention or will be in need of attention based upon its current state or a trend in its state. The method of calculating insulation condition from the measured differential current and phasor voltage may be of the type described by Younsi et al., U.S. Pub. No. 2005/0218906A1, the disclosure of which is incorporated herein by reference.
The current sensors are differential current sensors configured to generate feedback signals representative of instantaneous differential current through each machine winding. Similarly, the voltage sensors are adapted to measure the instantaneous phase voltage across the machine windings and corresponding neutral point. Output from the sensors is provided to the data acquisition system 88, and there through, to the processing module 90. As discussed below, based upon these sensed parameters, processing module 90 evaluates the condition of insulation of the machine windings.
In one embodiment, the current sensors each comprises a current transformer as described above with reference to
In one embodiment, the current sensor comprises a current transformer as described above with reference to
While only certain features of the invention have been illustrated and described herein, many modifications and changes will occur to those skilled in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention.
