Patent Application
20130027162
The present disclosure relates to current transformers. Moreover, it relates to miniature neutral toroidal current transformers.
Current transformers (CT) are can be used for measuring of electric currents and can be implemented in a variety of systems. One example of such systems can include power distribution systems, meters and protective relays. When current in a circuit is too high to directly apply to measuring instruments or measuring sensors, a current transformer produces a reduced current that is proportional to the current in the circuit, which can be conveniently connected to measuring and recording instruments. A current transformer can also isolate the measuring instruments or sensors from what can be considered a high voltage in the monitored circuit.
Similarly to other transformers, a current transformer has a primary winding, a core, and a secondary winding. The current flowing in the primary winding produces a magnetic field in the core, which then induces a current in the secondary winding circuit. A primary objective of current transformer design is to ensure that the primary and secondary circuits are efficiently coupled, so that the secondary current bears an accurate relationship to the primary current.
The most common design of CT consists of a length of wire wrapped many times around a silicon steel ring passed over the circuit being measured. Shapes and sizes can vary depending on the end user or switchgear manufacturer. Examples of low voltage single ratio metering current transformers can be ring type or plastic molded case. High-voltage current transformers can be mounted on porcelain bushings to insulate them from ground. Some CT configurations can be configured to slip around the bushing of a high-voltage transformer or circuit breaker.
The CT core is constructed of ferromagnetic materials, which tend to “saturate” at a predetermined level that can be dictated by, for example, the core material and the core dimensions. The saturation point is where any further increase in magnetizing field force (H) does not result in a proportional increase in magnetic flux density (B) or any more increase in the secondary current resulting from this magnetic flux change. Consequently, the CT secondary waveform can become highly distorted and of relatively low magnitude.
According to a first aspect, a current transformer is described, the current transformer comprising: a substantially toroidal core having a height, and a width; a secondary winding wrapped about the height and the width of the core, the secondary winding being a wire having a diameter and extending around an entire circumference of the core; wherein a diameter of the center of the core is equal to or greater than a diameter of a largest conductive wire and insulation surrounding a conductive wire adapted to pass through a center of the core, wherein the core saturates when a primary current flowing through the conductive wire exceeds a set primary current, and wherein the current transformer is adapted to convert the primary current to a secondary current induced by the secondary winding, the second current being a function of a number of turns of the secondary winding and the primary current, and wherein the current transformer is configured to be a neutral current transformer associated with a ground fault circuit breaker.
According to a second aspect, a method of making a compact current transformer is described, the method comprising: providing a taped substantially toroidal core having a height, and a width, the toroidal core being made of a material having a permeability; providing a secondary winding wrapped about the height and the width of the core, the secondary winding being a wire having a diameter and extending around an entire circumference of the substantially toroidal core; and optimizing a set saturation point of the toroidal core by optimizing the permeability of the material of the core, the diameter of the wire of the secondary winding, and a cross-sectional area of the toroidal core, the cross sectional area being a product of the height and the width of the substantially toroidal core.
The accompanying drawings, which are incorporated into and constitute a part of this specification, illustrate one or more embodiments of the present disclosure and, together with the description of example embodiments, serve to explain the principles and implementations of the disclosure.
In an example embodiment as shown in
In addition to the internal current transformers (30), the circuit interrupter can be associated with an additional current transformer (see
Now referring particularly to the neutral current transformer shown in
The toroidal shaped core (400) allows for the passage of a conductive wire through a center (608) of the core (400), and the core (400) can be mounted in the casing (402) as described in previous paragraphs.
In one embodiment of the present disclosure, the core of the neutral current transformer can be configured by initially determining the largest expected diameter of the conductive wire, including any insulation on the conductive wire that will pass through the toroidal neutral current transformer. Such diameter can become the minimum inner diameter of the core, thereby ensuring that the largest expected conductor is able to fit through the toroidal core. Additionally, the desired physical finished dimensions (e.g., height, width) of the current transformer are also determined. Then the desired ratio of the primary current to the secondary current, as well as the magnitudes of the primary current and the secondary current are also determined. Once the desired inner diameter, current ratios and magnitudes are established, a plurality of mathematical formulas and equations can be used to calculate other parameters to optimize the core.
One such optimization parameter is the saturation point of the core. The saturation point is reached when a further increase in the field intensity (H) ceases to cause an increase in flux density (B), where the relationship between the field intensity (H) and flux density (B) can be expressed by:
B=μH, (1)
where (μ) is the permeability of the core material. A higher permeability of the ferromagnetic material can result in greater flux density. The flux density (B) can also be expressed as:
B=Niμ/2(π)r, (2)
where (N) is the turns ratio of the current transformer and (i) is the magnetizing current. The average radius (r), of the core can be express as:
r=(r2+r1)/2, (3)
where (r2) is the inner radius, and (r1) is the outer radius of the core. Therefore, the field intensity (H) can be represented as:
H=Ni/2(π)r, (4)
and a cross-sectional area of the core (A) can be expressed as:
A=h(r1−r2)/2, (5)
where h is the height of the core. Finally, the total flux density Φ can be expressed as:
Φ=BA. (6)
By way of the above described equations, the core can be configured to produce a desired flux density without reaching or exceeding a predetermined saturation point. Factors such as the core material (μ), cross-sectional area of the core (A), and the size of the wire used as the secondary winding can have a significant effect on the saturation of the core. For example, it can be seen from the equations above that a smaller wire size can result in a higher resistance and thus decrease the saturation point, or decreasing the cross-sectional area of the core to decrease the saturation point. The term “turns ratio” in a transformer is typically defined as the ratio of the number of turns of the secondary winding to the primary winding. However, in the exemplary current transformer, the number of turns of the primary windings is one. Therefore, the turns ratio as used in herein in the present disclosure is intended to mean the number of turns of the secondary winding.
In some embodiments, the mathematical equations can computed automatically by a computer configured perform such calculations. According to an exemplary method, a macro can be programmed in a spreadsheet such that a user can input desired parameters such as the core material (μ), cross-sectional area of the core (A), and the size of the wire used as the secondary winding to obtain a first order design. By inputting such parameters, a user can determine if such combination of parameters meet the desired specification (e.g., does not exceed the desired saturation parameters). Additionally, the macro can be configured to also calculate parameters such as wound wire resistance, stacking factor (e.g., amount of space a wire needs for winding), expected saturation curves, power dissipated, and estimated finished physical dimensions.
In the exemplary configuration as described in the present disclosure, the core of the neutral current transformer is smaller than the current transformers internal to the circuit breaker. Accordingly, saturation will occur at a lower current for the smaller core. By way of example and not of limitation, the neutral current transformer of the present disclosure is configured such that the core will not saturate until more than approximately 200% of the expected current is sensed. However, other saturation points are also possible.
When the conductive wire that passes through the core (400) of the neutral current transformer carries current, the wire induces a magnetic field, which is sensed by the secondary windings on the taped core of the neutral current transformer. The neutral current transformer is configured to down convert the current passing through the wire to a lower current which can be fed to the circuit interrupter, according to a predetermined ratio (e.g., 225 A:60 mA, 160 A:60 mA, 80 A:60 mA). The ratio refers to the ratio of a primary current to a secondary current, and the ratio of the neutral current transformer is configured to match the ratio of the current transformers internal to the circuit interrupters.
In the exemplary embodiment of the present disclosure, the secondary winding on the taped core is configured to produce 60 mA of secondary current from a primary current of 225 A. By way of example and not of limitation, a 225 A to 60 mA ratio neutral current transformer can have 3750 turns of wire (e.g., winding) to configure such ratio. In another embodiment, the secondary winding on the taped core can comprise 2667 turns of wire to provide the same 60 mA of secondary current for a primary current of 160 A. In yet another embodiment, the secondary winding on the taped core can comprise 1333 turns of wire to again produce the same 60 mA of secondary current for a primary current of 80 A in an 80 A to 60 mA ratio current transformer. Other ratios and combinations of the number of turns of wire are also possible.
As shown in
It should be noted that the particular arrangement of components shown in the figures are non-limiting examples and that other arrangements, within the scope of the invention, are possible. For example, although discussed as separate components, it is contemplated that the hardware override circuit (72) and the microprocessor (70) can be implemented as a single device, such as and without limitation, an integrated circuit (not shown). Alternatively, a wide range of analog and/or digital circuits (not shown) may be employed. Furthermore, any suitable number of power lines or phases may be employed.
The examples set forth above are provided to give those of ordinary skill in the art a complete disclosure and description of how to make and use the embodiments of the miniature neutral current transformer of the disclosure, and are not intended to limit the scope of what the inventors regard as their disclosure. Modifications of the above-described modes for carrying out the disclosure may be used by persons of skill in the art, and are intended to be within the scope of the following claims. All patents and publications mentioned in the specification may be indicative of the levels of skill of those skilled in the art to which the disclosure pertains. All references cited in this disclosure are incorporated by reference to the same extent as if each reference had been incorporated by reference in its entirety individually.
It is to be understood that the disclosure is not limited to particular methods or systems, which can, of course, vary. It is also to be understood that the terminology used herein is for the purpose of describing particular embodiments only, and is not intended to be limiting. As used in this specification and the appended claims, the singular forms “a,” “an,” and “the” include plural referents unless the content clearly dictates otherwise. The term “plurality” includes two or more referents unless the content clearly dictates otherwise. Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which the disclosure pertains.
A number of embodiments of the disclosure have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the present disclosure. Accordingly, other embodiments are within the scope of the following claims.
