CN115524639A - Hollow coil cable fault detection system based on MEMS sensor - Google Patents
Hollow coil cable fault detection system based on MEMS sensor Download PDFInfo
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Abstract
The invention discloses an air-core coil cable fault detection system based on an MEMS sensor, which comprises the MEMS sensor and a data analysis unit; the MEMS sensor is arranged in an induced magnetic field of the power transmission conductor, and a signal of which the time differentiation of the current of the power transmission conductor is in direct proportion is measured through electromagnetic induction; the data analysis unit is communicated with the MEMS sensor and is used for carrying out data processing on the sampling signal of the MEMS sensor so as to obtain a parameter value of the power transmission conductor and obtain parameter information measured by the MEMS sensor; and according to the waveform relation of the parameter values and the parameter information, carrying out fault detection on the power transmission conductor. The invention does not need to use an iron core, so as to solve at least one of ferromagnetic resonance and magnetic saturation nonlinearity problems caused by the iron core.
Description
Technical Field
The invention relates to the technical field of cable fault diagnosis, in particular to an air core coil cable fault detection system based on an MEMS sensor.
Background
At present, the most common Current Transformer used in a power distribution network is the traditional CT, the measurement and protection data of the power frequency Current of a power system cannot directly obtain large Current data from a power transmission line of the power distribution network, and the large Current of the power transmission line needs to be converted into small Current suitable for measurement according to a certain transformation ratio by using the Current Transformer (CT for short) to obtain the large Current.
The traditional CT forms a magnetic flux induction loop by an iron core, due to the saturation nonlinear characteristic of the iron core, the measuring range of the traditional CT is limited to a great extent, the linearity is good in the saturation range of the iron core, and when the measured current exceeds the range, the measured value of the traditional CT is distorted and wrong greatly. Meanwhile, the traditional CT is a nonlinear inductive element, when high-frequency nonlinear periodic current interference occurs in power frequency current of a power distribution network, nonlinear saturation occurs in an iron core, so that supersaturation of excitation of the iron core cannot correctly reflect the increase of current, great error is caused, and the measurement range of the current transformer is limited. When the nonlinear periodic current is too large, the magnetic flux in the iron core is changed rapidly, and if the iron core has too large stabbing pain, the output current is seriously distorted, and serious consequences are caused. When the magnetic saturation type secondary waveform relay protection circuit is used as relay protection, secondary waveforms are seriously distorted due to magnetic saturation, reaction is delayed or relay misoperation is caused, and great influence is caused on a power system. In addition, the traditional electromagnetic current transformer has a narrow working frequency band due to the limitation of magnetic saturation of an iron core, is difficult to meet the requirements of modern measurement and protection, and is easy to generate electromagnetic resonance in a high-frequency stage, so that secondary equipment is damaged. In the use process of the traditional electromagnetic current transformer, the output current of the secondary side is 5A and 1A analog quantity output, the secondary side of the transformer is required to be incapable of being opened, otherwise, danger or injury can be caused to equipment and a person, the secondary side cannot be directly connected with networked and intelligent equipment, product digitization and intellectualization are difficult to realize, and the requirement of an intelligent power grid is difficult to meet.
In addition, the problems of overlarge size, overhigh installation and maintenance cost, outstanding safety and the like of the electromagnetic current transformer are more and more prominent in the intelligent automation process of the power system, and the traditional current transformer cannot be compatible with digital intelligent equipment and is not beneficial to the development of the power system.
Disclosure of Invention
The invention aims to provide an air-core coil cable fault detection system based on an MEMS sensor, which does not need to use an iron core and solves at least one of ferromagnetic resonance and magnetic saturation nonlinearity problems caused by the iron core.
In order to achieve the above object, the present invention provides a system for detecting a fault of an air-core coil cable based on an MEMS sensor, comprising an MEMS sensor and a data analysis unit;
the MEMS sensor is arranged in an induced magnetic field of the power transmission conductor, and a signal which is proportional to the differential of the current of the power transmission conductor to time is measured through electromagnetic induction;
the data analysis unit is communicated with the MEMS sensor and is used for carrying out data processing on the sampling signal of the MEMS sensor so as to obtain a parameter value of the power transmission conductor and obtain parameter information measured by the MEMS sensor; and according to the waveform relation between the parameter values and the parameter information, carrying out fault detection on the power transmission conductor.
Further, the process of performing data analysis on the MEMS sensor sampling signal by the data analysis unit includes: when the power transmission conductor has a short-circuit fault, the instantaneous expression of the short-circuit current is obtained as follows:
theta is the initial angle of the short-circuit current, I m Amplitude of steady-state value of rated short-circuit current, T 1 And the time constant is the primary time constant, ω t is the included angle between the coil plane and the magnetic field, ω is the angular frequency, and t is the time.
Further, the short-circuit current includes: a first non-periodic component and a first periodic component; when the initial angle θ is zero, the first aperiodic component is maximum, and its expression is:
further, laplace transform is performed on the first aperiodic component expression of the short-circuit current:
a plurality of s and a plurality of s 2 As a parameter of the Laplace transform, ω 2 And multiplying the angular frequency by a transfer function G(s) to obtain a second output voltage, wherein the expression of the second output voltage is as follows:
further, the second output voltage expression is subjected to inverse Laplace transformation to obtain a third output voltage, and the expression is as follows:
a, B, C, D, E are constant coefficients determined by circuit parameters, and T is the equivalent frequency of the second aperiodic component; when the power distribution network has a short-circuit fault, the third output voltage comprises five components, wherein two terms are second periodic components, and the other three terms are second non-periodic components.
Further, the expression of the second periodic component is:
The expression of the second aperiodic component is:
when T → ∞ is reached, the dc component of the third output voltage finally decays to 0, the rate of decay and the first time constant T of the third output voltage 1 And the cut-off frequency omega of the integrator T =1/T related; when the equivalent frequency of the second aperiodic component is much greater than the upper cut-off frequency of the integrator, the coefficients E, D of the second aperiodic component are much smaller than the periodic component coefficient C, resulting in:
further, the parameter value includes a current value.
Further, the parameter information includes a first output voltage and an output dc component curve, a waveform of the first output voltage is used for reflecting a periodic component and a non-periodic component, and when the waveform is similar to a waveform of the transient current, it indicates that the MEMS sensor has good transient characteristics.
Further, the MEMS sensor is an air-core coil current transformer.
Further, the air-core coil current transformer comprises a top-layer air-core coil current transformer and a bottom-layer air-core coil current transformer; and the top-layer hollow coil current transformer and the bottom-layer hollow coil current transformer are sequentially connected in series to obtain the hollow coil current transformer.
Compared with the prior art, the invention has the following beneficial effects by adopting the technical scheme:
according to the invention, the data analysis unit is used for carrying out data processing on the MEMS sensor sampling signal to obtain the current value of the power transmission conductor and the waveform relation between the first output voltage measured by the MEMS sensor and the output direct-current component curve, so that the fault detection is carried out on the power transmission conductor without using an iron core, at least one of ferromagnetic resonance and magnetic saturation nonlinearity problems caused by the iron core is solved, and the measurement effect is ensured;
the MEMS sensor is arranged in the induced magnetic field of the transmission conductor, no direct electrical connection is needed, contact measurement is not needed, when the MEMS sensor goes wrong, secondary equipment cannot be endangered, no influence can be caused on the line of the transmission conductor, and meanwhile, the functional effect can be achieved through a small size, so that the cost can be reduced.
Drawings
FIG. 1 is a diagram of a power conductor versus the ambient magnetic field of an air coil cable fault detection system based on MEMS sensors in accordance with an embodiment of the present invention;
FIG. 2 is a flow chart of an air core coil cable fault detection system based on MEMS sensors in accordance with an embodiment of the present invention;
FIG. 3 is a graph of the input current of a hollow-coil current transformer in the MEMS sensor based hollow-coil cable fault detection system in accordance with an embodiment of the present invention;
FIG. 4 is a graph of the voltage response output of a hollow-coil current transformer in a MEMS sensor based hollow-coil cable fault detection system in accordance with an embodiment of the present invention;
FIG. 5 is a diagram of the relationship between an air coil current transformer and a power conductor of an air coil cable fault detection system based on MEMS sensors in accordance with an embodiment of the present invention;
fig. 6 is a schematic diagram of an overall structure of an air-core coil current transformer of an air-core coil cable fault detection system based on a MEMS sensor according to an embodiment of the present invention.
Detailed Description
While the MEMS sensor based air coil cable fault detection system of the present invention will now be described in greater detail with reference to the schematic drawings wherein there is shown a preferred embodiment of the invention, it is to be understood that those skilled in the art can modify the invention herein described while still achieving the advantageous effects of the invention. Accordingly, the following description should be construed as broadly as possible to those skilled in the art and not as limiting the invention.
The invention is more particularly described in the following paragraphs with reference to the accompanying drawings by way of example. Advantages and features of the present invention will become apparent from the following description and from the claims. It is to be noted that the drawings are in a very simplified form and are not to precise scale, which is merely for the purpose of facilitating and distinctly claiming the embodiments of the present invention.
As shown in fig. 1 to 3, the present embodiment provides a system for detecting a fault of an air-core coil cable based on a MEMS sensor, which includes the MEMS sensor and a data analysis unit; the MEMS sensor is arranged in an induced magnetic field of the power transmission conductor, and a signal of which the time differentiation of the current of the power transmission conductor is in direct proportion is measured through electromagnetic induction; the data analysis unit is communicated with the MEMS sensor and is used for carrying out data processing on the sampling signal of the MEMS sensor so as to obtain a parameter value of the power transmission conductor and obtain parameter information measured by the MEMS sensor; and according to the waveform relation between the parameter values and the parameter information, carrying out fault detection on the power transmission conductor.
In order to avoid endangering the secondary equipment and thus affecting the line of the power transmission conductor, in this embodiment, specifically, the MEMS sensor is installed in the induced magnetic field of the power transmission conductor, and a signal proportional to the time derivative of the power transmission conductor current is measured through electromagnetic induction, thereby ensuring the line stability of the power transmission conductor.
Meanwhile, in order to avoid the nonlinear problem caused by the iron core, in this embodiment, specifically, the data analysis unit is in communication with the MEMS sensor and is configured to perform data processing on the MEMS sensor sampling signal, so as to obtain a parameter value of the power transmission line and obtain parameter information measured by the MEMS sensor; and according to the waveform relation between the parameter values and the parameter information, fault detection is carried out on the power transmission conductor without using an iron core, so that at least one of ferromagnetic resonance and magnetic saturation nonlinearity problems caused by the iron core is solved, and the measurement effect is ensured.
Wherein the parameter value comprises a current value. And, the parameter information includes a first output voltage and an output dc component curve, the waveform of the first output voltage is used to reflect a periodic component and a non-periodic component, and when the waveform is similar to the waveform of the transient current, as shown in fig. 3, it is demonstrated that the MEMS sensor has good transient characteristics. Because the power system is subjected to various sudden disturbances or faults during operation, the parameter values and the parameter information are changed greatly, and the parameter values and the parameter information are changed in the transient process, so that the changes can be recorded by good transient characteristics for fault monitoring.
Theoretically, for a varying magnetic field around the power current of a transmission line of a power system, as known from ampere's loop law, the line integral of the magnetic induction along any closed loop is equal to the product of the algebraic sum of all currents contained in the closed loop and the magnetic permeability. Thus, the region of the power transmission conductor has
Wherein B represents the magnetic induction intensity at the line element in the space of the power transmission conductor, dl represents the length of a differential line element on the periphery of the conductor, alpha is the included angle between the magnetic field and the line element direction, i represents the current flowing in the current-carrying conductor, mu 0 represents the magnetic permeability in vacuum, and mu 0 ＝4π·10 -7 H/m。
When the power frequency current varies sinusoidally, a magnetic field B is generated around the varying current in the conductor, based on
The magnetic field generated by current of power transmission conductor to any point in space can be obtained
Where r is the distance from the centre of the power conductor to the measurement point and I (t) is the current through the conductor. Magnetic flux phi through the conductor at a distance r from the power conductor
From the law of electromagnetic induction, a conductor located in a magnetic field generates an induced voltage of
Order to
Then
It is known that the differential of the power conductor current is proportional to the voltage of the conductor in the magnetic field of the surrounding space. Therefore, theoretically, the current of the power transmission conductor can be obtained by performing integral solution on the induced electromotive force of the conductor in the magnetic field.
In this embodiment, when a short-circuit fault occurs in the power transmission conductor, the instantaneous expression of the short-circuit current is obtained as follows:
theta is the initial angle of the short-circuit current, I m Amplitude of steady-state value of rated short-circuit current, T 1 And omega t is an included angle between the coil plane and the magnetic field, omega is angular frequency, and t is time.
Further, the short-circuit current includes: a first non-periodic component and a first periodic component; when the initial angle θ is zero, the first aperiodic component is largest, and its expression is:
performing Laplace transformation to obtain:
a plurality of s and a plurality of s 2 As a parameter of the Laplace transform, ω 2 And multiplying the angular frequency by a transfer function G(s) to obtain a second output voltage, wherein the expression is as follows:
When the power distribution network has a short-circuit fault, the third output voltage comprises five components, wherein two of the five components are second periodic components, and the other three components are second aperiodic components.
Specifically, the expression of the second periodic component is:
furthermore, ti =60ms was established again by the Fcn module in the simulation toolkit Simulink using MATLAB,
f =50Hz primary short-circuit current, the transient current expression is as follows:
referring to fig. 4, in another embodiment, the MEMS sensor is an air-core coil current transformer, and the number of the air-core coil current transformers may be four. Specifically, the air-core coil current transformer comprises a top-layer air-core coil current transformer and a bottom-layer air-core coil current transformer; the top layer hollow coil current transformer and the bottom layer hollow coil current transformer are sequentially connected in series to obtain the hollow coil current transformer, and the hollow coil current transformer can be any one of a circle, an ellipse, a rectangle and a square. In addition, the number of the air-core coil current transformers in the embodiment of the invention may also be other, for example, more than one, and those skilled in the art may flexibly select the number according to actual needs.
Because the hollow coil current transformer is internally provided with no iron core, the hollow coil current transformer is easily interfered by an external magnetic field. The interference magnetic field can be decomposed into two components which are parallel to the plane direction and perpendicular to the direction of the air-core coil current transformer.
Specifically, when the interference magnetic field is parallel to the plane of the air coil current transformer, the direction of the magnetic field is parallel to each turn of the coil of the independent air coil current transformer, and the magnetic flux passing through the air coil current transformer is zero, so that induced electromotive force is not generated.
In addition, when the interference magnetic field is perpendicular to the plane of the air-core coil current transformer, specifically, when the external magnetic field is a uniform magnetic field, the magnetic fluxes passing through the air-core coil current transformer are equal in magnitude, the induced electromotive forces generated by the air-core coil current transformer are equal in magnitude and opposite in direction, and the induced electromotive forces generated by the air-core coil current transformer due to reverse series connection are mutually offset. Therefore, theoretically, the air-core coil current transformer can well resist the interference of an external uniform magnetic field.
In addition, when an external magnetic field is uneven, the external interference current of the air-core coil current transformer is taken as an example for analysis, the air-core coil current transformer is composed of a top-layer air-core coil current transformer and a bottom-layer air-core coil current transformer which are the same in structure and size, therefore, the current in any direction in the plane of the air-core coil current transformer can generate a magnetic field vertical to the air-core coil current transformer, and the interference current in any direction can be selected outside the air-core coil current transformer for development and analysis. When the interference current i0 is located at one side of the air coil current transformer, the distances of the interference current to each independent air coil current transformer are da, db, dc and dd respectively. The length of the outer layer of the hollow coil current transformer is a, the width of the outer layer of the hollow coil current transformer is b, and the turn pitch of the outer layer of the hollow coil current transformer is c. According to electromagnetic induction, the magnetic flux passing through any one turn k (k =0,1,2 \8230; n + 1) of the wire in the air coil current transformer is as follows:
because air core coil current transformer concatenates in proper order by top layer air core coil current transformer and bottom layer air core coil current transformer and constitutes, the total magnetic flux that interference current i (t) produced in air core coil current transformer A is:
similarly, according to the electromagnetic induction theory, the magnetic flux passing through each independent air core coil current transformer in B, C and D is as follows:
because the air-core coil current transformer is formed by sequentially connecting the top-layer air-core coil current transformer and the bottom-layer air-core coil current transformer in series, the total magnetic flux generated by the interference current in the air-core coil current transformer is
Φ(t)＝2[Φ A (t)-Φ B (t)-Φ C (t)+Φ D (t)]。
The power system is generally a three-phase circuit, and the anti-interference capability of the three-phase circuit is considered, and electromagnetic field software is firstly utilized to analyze the distribution condition of a magnetic field around a transmission conductor. And establishing a magnetic field simulation model of the power transmission conductor by using electromagnetic field simulation software AnsoftMaxwell. The distance between the transmission conductor and the ground is 20m, the magnetic field change from the edge of the conductor to the height of 1m is obtained, the center of the internal magnetic field of the conductor is zero, the internal magnetic field linearly changes from the center to the surface of the conductor, the surface of the conductor is the position with the maximum magnetic field, and the two sides of the conductor are symmetrically distributed. The magnetic field is gradually attenuated from the surface edge of the power transmission conductor to infinity outside, the magnetic field is already attenuated to minimum at the position of 0.5m of the power transmission conductor, and the magnetic field intensity conductor is close to zero, so that the area of 0.5m outside the power transmission conductor can be regarded as a uniform magnetic field or a magnetic field with little change and close to 0. The requirements between the three-phase lines in the power system are much greater than the above-mentioned requirements. The magnetic field of adjacent power lines acting on the air coil current transformer can therefore be regarded as a uniform magnetic field or a zero magnetic field, the induction effect on the air coil current transformer is 0, no response is output, and it is therefore not necessary to take into account the region (h) remote from the power line>0.5 m) of the power transmission line, only the area (h) close to the power transmission line needs to be considered<0.5 m). The method is analyzed and verified by utilizing ANSOFTMAXWELL magnetic field analysis software, the width a of the coil of the air-core coil current transformer is 30mm, the distances da from interference current to the edge of the coil of the air-core coil current transformer are set to be 10cm, db and dc are both 13cm, dd are set to be 16cm, the external interference current is obtained, when the external interference current is far away from the coil of the air-core coil current transformer, the generated magnetic flux is less, the interference on the coil of the air-core coil current transformer is lower, and meanwhile, the influence of the interference magnetic field is lower and lower when the number of turns of the coil of the air-core coil current transformer is less. Considering the influence of external interference current on a coil of the hollow coil current transformer, wherein the external interference current can be decomposed into horizontal and vertical currents, the interference current of the infinite long lead is 15cm away from the coil of the hollow coil current transformer, the interference current in the simulation model is introduced with 10A current, and the magnetic flux which penetrates through the four regions is calculated to be phi A ＝3.405×10 -8 Wb,Φ B ＝3.918×10 -8 Wb,Φ C ＝3.405×10 -8 Wb,Φ D ＝1.917×10 -8 Wb
The total magnetic flux can be calculated as Δ Φ 2 ＝-4.002×10 -8 Wb
There is a relative error calculation formula:
the error can be obtained under two conditions
It is thus understood that the anti-series structure can suppress the influence of a part of the disturbance magnetic field. The influence of an external interference magnetic field on the hollow coil current transformer is mainly the influence of a magnetic field generated by near-end interference current, most power transmission conductors are overhead wires in the operation of a power system, and the distance between the power transmission conductors is far greater than the distance between the power transmission conductors and a near conductor measuring area, so that the measuring precision can be achieved as long as the distance between the power transmission conductors and the interference current is kept enough in the measuring process.
In this embodiment, when a fault of a cable needs to be detected, it is only necessary to install an air-core coil current transformer in an induced magnetic field of a power transmission conductor, measure a signal in which a differential of a current of the power transmission conductor with respect to time is proportional through electromagnetic induction, and perform data processing on a sampled signal of the air-core coil current transformer through a data analysis unit, so as to obtain a current value of the power transmission conductor and a first output voltage and an output dc component curve measured by an MEMS sensor, and to perform fault detection on the power transmission conductor according to the current value of the power transmission conductor and a waveform relationship between the first output voltage and the output dc component curve measured by the MEMS sensor.
Through the whole set of hollow coil cable fault detection system based on the MEMS sensor, an iron core is not required, so that at least one of ferromagnetic resonance and magnetic saturation nonlinearity problems caused by the iron core is solved, and the measurement effect is ensured.
In summary, the air-core coil cable fault detection system based on the MEMS sensor provided by the present invention has the following advantages:
according to the invention, the data analysis unit is used for carrying out data processing on the MEMS sensor sampling signal to obtain the current value of the power transmission conductor and the waveform relation between the first output voltage measured by the MEMS sensor and the output direct-current component curve, so that the fault detection is carried out on the power transmission conductor without using an iron core, at least one of ferromagnetic resonance and magnetic saturation nonlinearity problems caused by the iron core is solved, and the measurement effect is ensured;
the MEMS sensor is arranged in the induced magnetic field of the transmission conductor, no direct electrical connection is needed, contact measurement is not needed, when the MEMS sensor goes wrong, secondary equipment cannot be endangered, no influence can be caused on the line of the transmission conductor, and meanwhile, the functional effect can be achieved through a small size, so that the cost can be reduced.
It will be apparent to those skilled in the art that various changes and modifications may be made in the present invention without departing from the spirit and scope of the invention. Thus, if such modifications and variations of the present invention fall within the scope of the claims of the present invention and their equivalents, the present invention is also intended to include such modifications and variations.
Claims (10)
1. An air core coil cable fault detection system based on an MEMS sensor is characterized by comprising the MEMS sensor and a data analysis unit;
the MEMS sensor is arranged in an induced magnetic field of the power transmission conductor, and a signal which is proportional to the differential of the current of the power transmission conductor to time is measured through electromagnetic induction;
the data analysis unit is communicated with the MEMS sensor and is used for carrying out data processing on the sampling signal of the MEMS sensor so as to obtain a parameter value of the power transmission conductor and obtain parameter information measured by the MEMS sensor; and according to the waveform relation between the parameter values and the parameter information, carrying out fault detection on the power transmission conductor.
2. The MEMS sensor-based air coil cable fault detection system of claim 1, wherein the process of data analysis of the MEMS sensor sampled signal by the data analysis unit comprises: when the power transmission conductor has a short-circuit fault, obtaining an instantaneous expression of a short-circuit current as follows:
theta is the initial angle of the short-circuit current, I m Amplitude of steady-state value of rated short-circuit current, T 1 And omega t is an included angle between the coil plane and the magnetic field, omega is angular frequency, and t is time.
3. The MEMS sensor-based air coil cable fault detection system of claim 2, wherein the short circuit current comprises: a first non-periodic component and a first periodic component; when the initial angle θ is zero, the first aperiodic component is largest, and its expression is:
4. the MEMS sensor-based air core coil cable fault detection system of claim 3 wherein the first aperiodic component expression for the short circuit current is Laplace transformed:
a plurality of s and a plurality of s 2 As a parameter of the Laplace transform, ω 2 And multiplying the angular frequency by a transfer function G(s) to obtain a second output voltage, wherein the expression is as follows:
5. the MEMS sensor-based air core coil cable fault detection system of claim 4, wherein the second output voltage expression is inverse Laplace transformed to a third output voltage, and wherein the expression is:
a, B, C, D, E are constant coefficients determined by circuit parameters, and T is the equivalent frequency of the second aperiodic component; when the power distribution network has a short-circuit fault, the third output voltage comprises five components, wherein two of the five components are second periodic components, and the other three components are second non-periodic components.
6. The MEMS sensor-based air coil cable fault detection system of claim 5, wherein the second periodic component is expressed by:
The expression of the second aperiodic component is:
when T → ∞ is reached, the dc component of the third output voltage finally decays to 0, the rate of decay and the first time constant T of the third output voltage 1 And cut-off frequency omega of the integrator T =1/T related; when the equivalent frequency of the second aperiodic component is much greater than the upper cut-off frequency of the integrator, the coefficients E, D of the second aperiodic component are much smaller than the periodic component coefficient C, resulting in:
7. the MEMS sensor-based air coil cable fault detection system of claim 1, wherein the parameter value comprises a current value.
8. The MEMS sensor-based air coil cable fault detection system of claim 1, wherein the parameter information includes a first output voltage and an output dc component curve, wherein a waveform of the first output voltage is used to reflect a periodic component and a non-periodic component, and when the waveform is similar to a waveform of the transient current, the MEMS sensor has good transient characteristics.
9. The MEMS sensor-based air coil cable fault detection system of claim 1, wherein the MEMS sensor is an air coil current transformer.
10. The MEMS sensor-based air coil cable fault detection system of claim 9 wherein the air coil current transformers comprise a top layer air coil current transformer and a bottom layer air coil current transformer; and the top-layer hollow coil current transformer and the bottom-layer hollow coil current transformer are sequentially connected in series to obtain the hollow coil current transformer.
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