Patent Publication Number: US-2020286681-A1

Title: Current transformer

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of International Application No. PCT/CN2018/115588, filed on Nov. 15, 2018, which claims priority to Chinese Patent Application No. 201711196326.6, filed on Nov. 25, 2017. The disclosures of the aforementioned applications are hereby incorporated by reference in their entireties. 
    
    
     TECHNICAL FIELD 
     This application relates to an electronic element, and in particular, to a current transformer. 
     BACKGROUND 
     A photovoltaic power generation system is a power generating system that directly converts solar energy into electric energy by using a solar cell. Direct current arcing may occur in a process of generating power by using the photovoltaic power generation system. Usually, an arcing detection apparatus (AFCI) is used to identify whether there is direct current arcing in the photovoltaic power generation system. To eliminate the risk, the AFCI needs to accurately capture a differential-mode-current noise signal generated in an arcing process. 
     In a circuit, a primary-side winding of a current transformer is serially connected to a power line on which a current needs to be measured, and a current that flows through the primary-side winding may be determined based on a detected current that flows through a secondary-side winding of the current transformer, and further the current that needs to be measured is determined. The current transformer may be configured to detect a differential mode current and a common mode current in the power line. However, in an actual circuit, there are both the differential mode current and the common mode current in the power line. Because the differential mode current and the common mode current flow through the same line, the common mode current affects the differential mode current output from the current transformer. It is difficult to independently measure the differential mode current, and a captured differential mode current is not accurate enough. 
     SUMMARY 
     This application provides a current transformer with a simple structure, so that a differential-mode-current signal and a common-mode-current signal can be accurately collected, to independently measure a differential mode current and a common mode current. 
     According to a first aspect, an embodiment of this application provides a current transformer applicable to an arcing detection apparatus. The current transformer is configured to detect a differential-mode-current signal. The current transformer includes a magnetic core, a first primary-side winding, a second primary-side winding, and a first secondary-side winding. The magnetic core includes a first closed magnetic circuit and a second closed magnetic circuit. The first closed magnetic circuit is formed by connecting a first magnetic core structure and a second magnetic core structure, the second closed magnetic circuit is formed by connecting the first magnetic core structure and a third magnetic core structure, and the first magnetic core structure is a magnetic core structure common to the first closed magnetic circuit and the second closed magnetic circuit. The first primary-side winding passes through the first closed magnetic circuit, and the second primary-side winding passes through the second closed magnetic circuit. The first secondary-side winding is wound around the first magnetic core structure. 
     When the current transformer is connected to a circuit to detect a differential mode current in the circuit, if a first current that flows through the first primary-side winding and a second current that flows through the second primary-side winding are differential mode currents, a first magnetic flux and a second magnetic flux are superimposed; or if the first current that flows through the first primary-side winding and the second current that flows through the second primary-side winding are common mode currents, the first magnetic flux and the second magnetic flux cancel each other out. The first magnetic flux is a magnetic flux that is generated in the first magnetic core structure when the first current passes through the first primary-side winding, and the second magnetic flux is a magnetic flux that is generated in the first magnetic core structure when the second current passes through the second primary-side winding. 
     In one embodiment, the first primary-side winding and the second primary-side winding may be respectively serially connected to two lines on which the differential mode currents need to be measured. When the first current that flows through the first primary-side winding and the second current that flows through the second primary-side winding are differential mode currents, a positive direction of the first magnetic flux generated by the first current in the first magnetic core structure is the same as a positive direction of the second magnetic flux generated by the second current in the first magnetic core structure, and the first magnetic flux and the second magnetic flux are superimposed, to increase magnetic fluxes generated by the differential mode currents. When the first current that flows through the first primary-side winding and the second current that flows through the second primary-side winding are the common mode currents, a positive direction of the first magnetic flux generated by the first current in the first magnetic core structure is opposite to a positive direction of the second magnetic flux generated by the second current in the first magnetic core structure, and the first magnetic flux and the second magnetic flux cancel each other out, to suppress magnetic fluxes generated by the common mode currents. Because the magnetic fluxes generated by the common mode currents are suppressed, magnetic fluxes in the first magnetic core structure are mainly the magnetic fluxes generated by the differential mode currents. A change of a magnetic flux in the first magnetic core structure enables the first secondary-side winding wound around the first magnetic core structure to generate an induced current. Therefore, a current output by the first secondary-side winding may be used to feed back the differential mode current. 
     In some possible embodiments, the second magnetic core structure may be a “U”-shaped magnetic core structure, an arc-shaped magnetic core structure, an angle-shaped magnetic core structure, or the like. The third magnetic core structure may be a “U”-shaped magnetic core structure, an arc-shaped magnetic core structure, an angle-shaped magnetic core structure, or the like. A shape and a size of the second magnetic core structure and a shape and a size of the third magnetic core structure are not limited in this application. 
     In one embodiment, the second magnetic core structure and the third magnetic core structure are magnetic core structures symmetric to each other, and the second magnetic core structure and the third magnetic core structure have a same size and a same shape. 
     In a possible implementation, the second magnetic core structure and the third magnetic core structure have a same magnetic resistance, and both the second magnetic core structure and the third magnetic core structure have a comparatively small magnetic resistance. 
     In one embodiment, the current transformer may further include a second secondary-side winding. A first winding of the second secondary-side winding is wound around the second magnetic core structure, and a second winding of the second secondary-side winding is wound around the third magnetic core structure. The first winding of the second secondary-side winding and the second winding of the second secondary-side winding are serially connected. 
     When the first current that flows through the first primary-side winding and the second current that flows through the second primary-side winding are differential mode currents, a positive direction of a third magnetic flux generated by the first current in the second magnetic core structure and the third magnetic core structure is opposite to a positive direction of a fourth magnetic flux generated by the second current in the second magnetic core structure and the third magnetic core structure, and the third magnetic flux and the fourth magnetic flux cancel each other out, to suppress the magnetic fluxes generated by the differential mode currents. When the first current that flows through the first primary-side winding and the second current that flows through the second primary-side winding are common mode currents, a positive direction of a third magnetic flux generated by the first current in the second magnetic core structure and the third magnetic core structure is the same as a positive direction of a fourth magnetic flux generated by the second current in the second magnetic core structure and the third magnetic core structure, and the third magnetic flux and the fourth magnetic flux are superimposed, to increase the magnetic fluxes generated by the common mode currents. Therefore, a current output by the second secondary-side winding may be used to feed back the common mode current. That is, the differential mode current and the common mode current may be independently measured by using the first secondary-side winding and the second secondary-side winding. 
     According to a second aspect, an embodiment of this application provides another current transformer applicable to an apparatus that needs to detect a common mode current. The current transformer is configured to detect the common mode current. The current transformer includes a first primary-side winding, a second primary-side winding, and a second secondary-side winding. A magnetic core includes a first closed magnetic circuit and a second closed magnetic circuit. The first closed magnetic circuit is formed by connecting a first magnetic core structure and a second magnetic core structure, the second closed magnetic circuit is formed by connecting the first magnetic core structure and a third magnetic core structure, and the first magnetic core structure is a magnetic core structure common to the first closed magnetic circuit and the second closed magnetic circuit. The first primary-side winding passes through the first closed magnetic circuit, and the second primary-side winding passes through the second closed magnetic circuit. A first winding of the second secondary-side winding is wound around the second magnetic core structure, a second winding of the second secondary-side winding is wound around the third magnetic core structure, and the first winding of the second secondary-side winding and the second winding of the second secondary-side winding are serially connected. 
     When the current transformer is connected to a circuit to detect the common mode current in the circuit, if a first current that flows through the first primary-side winding and a second current that flows through the second primary-side winding are differential mode currents, a third magnetic flux and a fourth magnetic flux are superimposed; or if the first current that flows through the first primary-side winding and the second current that flows through the second primary-side winding are common mode currents, the third magnetic flux and the fourth magnetic flux cancel each other out. The third magnetic flux is a magnetic flux that is generated in the second magnetic core structure and the third magnetic core structure when the first current passes through the first primary-side winding, and the fourth magnetic flux is a magnetic flux that is generated in the second magnetic core structure and the third magnetic core structure when the second current passes through the second primary-side winding. 
     In one embodiment, the first primary-side winding and the second primary-side winding may be respectively serially connected to two lines on which the common mode currents need to be measured. When the first current that flows through the first primary-side winding and the second current that flows through the second primary-side winding are the differential mode currents, a positive direction of the third magnetic flux generated by the first current in the second magnetic core structure and the third magnetic core structure is opposite to a positive direction of the fourth magnetic flux generated by the second current in the second magnetic core structure and the third magnetic core structure, and the third magnetic flux and the fourth magnetic flux cancel each other out, to suppress the magnetic fluxes generated by the differential mode currents. When the first current that flows through the first primary-side winding and the second current that flows through the second primary-side winding are the common mode currents, the positive direction of the third magnetic flux generated by the first current in the second magnetic core structure and the third magnetic core structure is the same as a positive direction of a magnetic flux generated by the second current in the second magnetic core structure and the third magnetic core structure, and the third magnetic flux and the fourth magnetic flux are superimposed. For currents that pass through the two power lines, one current may be a sum of common mode currents with a same value, and the other current may be a difference between differential mode currents with a same value. Because magnetic fluxes generated by the differential mode current are suppressed, a magnetic flux in the second magnetic core structure and a magnetic flux in the third magnetic core structure are mainly magnetic fluxes generated by the common mode currents. A change of the magnetic flux in the second magnetic core structure enables the first winding of the second secondary-side winding wound around the second magnetic core structure to generate an induced current, and a change of the magnetic flux in the third magnetic core structure enables the second winding of the second secondary-side winding wound around the third magnetic core structure to generate the induced current. Therefore, a current output by the second secondary-side winding may be used to feed back the common mode current. 
     In some possible embodiments, the second magnetic core structure may be a “U”-shaped magnetic core structure, an arc-shaped magnetic core structure, an angle-shaped magnetic core structure, or the like. The third magnetic core structure may be a “U”-shaped magnetic core structure, an arc-shaped magnetic core structure, an angle-shaped magnetic core structure, or the like. A shape and a size of the second magnetic core structure and a shape and a size of the third magnetic core structure are not limited in this application. 
     In one embodiment, the second magnetic core structure and the third magnetic core structure are magnetic core structures symmetric to each other, and the second magnetic core structure and the third magnetic core structure have a same size and a same shape. 
     In one embodiment, the second magnetic core structure and the third magnetic core structure have a same magnetic resistance, and both the second magnetic core structure and the third magnetic core structure have a comparatively small magnetic resistance. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       To describe technical solutions in the embodiments of this application more clearly, the following describes the accompanying drawings required for the embodiments in this application. 
         FIG. 1  and  FIG. 2  show some prior-art solutions of current transformers; 
         FIG. 3  is a schematic diagram of a circuit formed by connecting two primary-side windings of a current transformer to a circuit according to an embodiment of this application; 
         FIG. 4  is a schematic structural diagram of a current transformer according to an embodiment of this application; 
         FIG. 5  is a schematic diagram of a direction of a magnetic flux generated in a magnetic core when an electrified current that flows through a first primary-side winding and an electrified current that flows through a second primary-side winding have a same direction; 
         FIG. 6  is a schematic diagram of a direction of a magnetic flux generated in a magnetic core when an electrified current that flows through a first primary-side winding and an electrified current that flows through a second primary-side winding have opposite directions; 
         FIG. 7  is a schematic structural diagram of another current transformer according to an embodiment of this application; 
         FIG. 8  is a schematic structural diagram of still another current transformer according to an embodiment of this application; and 
         FIG. 9  is schematic diagrams of several shapes of a magnetic core of a current transformer according to an embodiment of this application. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     The following describes technical solutions in the embodiments of this application with reference to the accompanying drawings in the embodiments of this application. 
     In some current solutions, to independently measure a differential mode current, one positive primary-side winding and one negative primary-side winding are usually disposed in a current transformer to suppress a common mode current.  FIG. 1  and  FIG. 2  show some prior-art solutions of current transformers for detecting a differential mode current. 
     A first solution is shown in  FIG. 1 . A current transformer includes only a magnetic core and a secondary-side winding. When the current transformer is connected to a circuit, a first cable through which a positive current passes is forward inserted, and a second cable through which a negative current passes is reversely inserted. In this solution, if the first cable and the second cable are wrongly inserted in directions, a differential mode current cannot be independently measured. 
     A second solution is shown in  FIG. 2 . A current transformer includes a magnetic ring, a first primary-side winding, a second primary-side winding, and a secondary-side winding. The first primary-side winding, the second primary-side winding, and the secondary-side winding are all wound around the magnetic ring. Lead-out pins of the first primary-side winding are crossed, and lead-out pins of the second primary-side winding are not crossed. In this solution, because the lead-out pins of the first primary-side winding need to be crossed, the first primary-side winding needs to be bent, and this is difficult in processing. 
     In the solution of this application, a first primary-side winding and a second primary-side winding respectively pass through two different closed magnetic circuits of a magnetic core without bending the primary-side winding, and such a structure is simple. When a current transformer is connected to a circuit, only one lead-out pin of a first primary-side winding and one lead-out pin of a second primary-side winding need to be connected to the circuit, so that a differential mode current can be independently measured without threading. 
     The solution in this application may be applied to a circuit in which a differential mode current and/or a common mode current need/needs to be independently detected. For example, the solution may be applied to an arcing detection apparatus. A circuit formed by connecting two primary-side windings of the current transformer in this application to a circuit may be shown in  FIG. 3 .  FIG. 3  is a schematic diagram of the circuit formed by connecting the two primary-side windings of the current transformer to the circuit according to an embodiment of this application. As shown in the diagram, the two primary-side windings of the current transformer are respectively connected to two parallel power lines (L 1  and L 2 ). I 1  and I 2  have a same value but have opposite directions, and I 1  and I 2  are differential mode currents in the two power lines. I 3  and I 4  have a same direction, and I 3  and I 4  are common mode currents in the two power lines. A secondary-side winding of the current transformer in this application is connected to a current detection circuit. One end of the secondary-side winding of the current transformer is grounded, and the other end of the current transformer is connected to a detection apparatus. For example, the other end of the current transformer is connected to a galvanometer, or the secondary-side winding of the current transformer is connected to a pin of a processor, or the like. 
       FIG. 4  is a schematic structural diagram of a current transformer according to an embodiment of this application. As shown in the diagram, a current transformer  10  includes a magnetic core  11 , a first primary-side winding  12 , a second primary-side winding  13 , and a first secondary-side winding  14 . 
     The magnetic core  11  includes a first closed magnetic circuit  201  and a second closed magnetic circuit  202 . The first closed magnetic circuit  201  is formed by connecting a first magnetic core structure  111  and a second magnetic core structure  112 , the second closed magnetic circuit  202  is formed by connecting the first magnetic core structure  111  and a third magnetic core structure  113 , and the first magnetic core structure  111  is a magnetic core structure common to the first closed magnetic circuit  201  and the second closed magnetic circuit  202 . The first primary-side winding  12  passes through the first closed magnetic circuit  201 . The second primary-side winding  13  passes through the second closed magnetic circuit  202 . The first secondary-side winding  54  is wound around the first magnetic core structure  111 . 
     When the current transformer  10  is connected to a circuit to detect electrified currents of two parallel power lines (for example, the first power line L 1  and the second power line L 2  in  FIG. 3 ), for positive directions and negative directions of magnetic fluxes that are generated in the magnetic cores by electrified currents that flow through the first primary-side winding and the second primary-side winding, refer to  FIG. 5  and  FIG. 6 . The magnetic flux is a scalar. In this application, the positive direction and the negative direction of the magnetic flux are related to a magnetic field direction of a magnetic field that passes through a cross section of the magnetic core. The direction of the magnetic field is the positive direction of the magnetic flux. 
       FIG. 5  is a schematic diagram of a direction of a magnetic flux generated in a magnetic core when an electrified current that flows through a first primary-side winding and an electrified current that flows through a second primary-side winding have a same direction. As shown in  FIG. 5 , a direction of the electrified current i 1  that flows through the first primary-side winding and a direction of the electrified current i 2  that flows through the second primary-side winding are both perpendicular to paper outward. The i 1  generates a first magnetic field, the i 2  generates a second magnetic field, and both the first magnetic field and the second magnetic field have a counter-clockwise direction. When the magnetic field passes through a cross section of a first magnetic core structure, a magnetic field direction of the first magnetic field is upward, and a positive direction of a first magnetic flux Φ 1  generated by the i 1  in the first magnetic core structure is upward; and a magnetic field direction of the second magnetic field is downward, and a positive direction of a second magnetic flux Φ 2  generated by the i 2  in the first magnetic core structure is downward. 
       FIG. 6  is a schematic diagram of a direction of a magnetic flux generated in a magnetic core when an electrified current that flows through a first primary-side winding and an electrified current that flows through a second primary-side winding have opposite directions. As shown in  FIG. 6 , a direction of an electrified current i 1  that flows through the first primary-side winding is perpendicular to paper inward, and a direction of an electrified current i 2  that flows through the second primary-side winding is perpendicular to paper outward. The i 1  generates a first magnetic field, and the first magnetic field has a clockwise direction. The i 2  generates a second magnetic field, and the second magnetic field has a counter-clockwise direction. When the magnetic field passes through a cross section of a first magnetic core structure, a magnetic field direction of the first magnetic field is downward, that is, a positive direction of a first magnetic flux Φ 1  generated by the i 1  in the first magnetic core structure is downward; and a magnetic field direction of the second magnetic field is downward, that is, a positive direction of a second magnetic flux Φ 2  generated by the i 2  in the magnetic core structure is downward. 
     It can be learned from  FIG. 5  and  FIG. 6  that when the electrified current that flows through the first primary-side winding and the electrified current that flows through the second primary-side winding are currents with a same direction, magnetic fluxes generated by the two electrified currents in the first magnetic core structure have different directions: a position direction and a negative direction, and the magnetic fluxes cancel each other out. When the electrified current that flows through the first primary-side winding and the electrified current that flows through the second primary-side winding are current with different directions, magnetic fluxes generated by the two electrified currents in the first magnetic core structure have a same direction: a positive direction or a negative direction, and the magnetic fluxes are superimposed. 
     The following uses an example to describe a case in which the magnetic fluxes generated by the electrified currents in the magnetic core are in the first magnetic core structure. It is assumed that the first primary-side winding and the second primary-side winding of the current transformer shown in  FIG. 4  are connected to the circuit in  FIG. 3 . There are the following several cases for a first current I 5  that flows through the power line L 1  and a second current I 5  that flows through the power line L 2 . In a first case, the I 5  and the I 6  are currents with a same value but in opposite directions. In a second case, the I 5  and the I 6  are currents with a same value and in a same direction. In a third case, the I 5  and the I 6  are currents with different values and in a same direction. 
     In the first case, it is assumed that both the I 5  and the I 6  are 2 A, a current direction of the I 5  is perpendicular to paper inward, and a current direction of the I 6  is perpendicular to paper outward. When the second magnetic core structure and the third magnetic core structure have a same a magnetic permeability, a first magnetic flux Φ 1  generated by the I 5  in the first magnetic core structure is equal to a second magnetic flux Φ 2  generated by the I 6  in the first magnetic core structure. Because both a positive direction of the first magnetic flux Φ 1  and a positive direction of the second magnetic flux Φ 2  are downward, a total magnetic flux Φ generated by the I 5  and the I 6  in the second magnetic core structure is Φ=Φ 1 +Φ 2 = 2 Φ 1 = 2 Φ 2 . 
     In the second case, it is assumed that both the I 5  and the I 6  are 2 A, and the current direction of the I 5  and the current direction of the I 6  are perpendicular to paper outward. When the second magnetic core structure and the third magnetic core structure have a same magnetic permeability, a first magnetic flux Φ 1  generated by the Φ 5  in the first magnetic core structure is equal to a second magnetic flux I 2  generated by the I 6  in the first magnetic core structure. Because a positive direction of the first magnetic flux Φ 1  is upward, and a positive direction of the second magnetic flux Φ 2  is downward, a total magnetic flux Φ generated by the I 5  and the I 6  in the second magnetic core structure is Φ=Φ 1 −Φ 2 =0. 
     In the third case, it is assumed that the I 5  is 3 A, the I 6  is 1 A, and the current direction of the I 5  and the current direction of the I 6  are perpendicular to paper outward. The current I 5  may be decomposed into a 2 A-current i 51  with a current direction perpendicular to paper outward and a 2 A-current i 52  with a current direction perpendicular to paper outward. The current I 6  is decomposed into a 2 A-current i 61  with a current direction perpendicular to paper outward and a 1 A-current i 62  with a current direction perpendicular to paper inward. When the second magnetic core structure and the third magnetic core structure have a same magnetic permeability, a magnetic flux Φ 51  generated by the current i 51  in the first magnetic core is equal to a magnetic flux Φ 61  generated by the current i 61  in the first magnetic core. A magnetic flux Φ 52  generated by the current i 52  in the first magnetic core is equal to a magnetic flux Φ 62  generated by the current i 62  in the first magnetic core. Because a positive direction of the magnetic flux Φ 51  is upward, a positive direction of the magnetic flux Φ 61  is downward, a positive direction of the magnetic flux Φ 52  is upward, and a positive direction of the flux Φ 62  is upward, a total magnetic flux Φ generated by the I 5  and the I 6  in the second magnetic core structure is Φ=Φ 51 −Φ 61 +Φ 52 +Φ 62 = 2 Φ 52 = 2 Φ 62 . 
     It can be learned from the foregoing that when the first current that flows through the first primary-side winding and the second current that flows through the second primary-side winding are the currents with the same value but in the opposite directions, the magnetic flux generated by the first current in the first magnetic core structure and the magnetic flux generated by the second current in the first magnetic core structure are superimposed. When the first current that flows through the first primary-side winding and the second current that flows through the second primary-side winding are the currents with the same value and in the same direction, the magnetic flux generated by the first current in the second magnetic core structure and the magnetic flux generated by the second current in the second magnetic core structure cancel each other out. When the first current that flows through the first primary-side winding and the second current that flows through the second primary-side winding are currents with different values, the two currents are equal to a group of currents with a same value and in a same direction and a group of currents with different values and in a same direction. Magnetic fluxes generated by the currents with the same value and in the same direction in the first magnetic core structure are superimposed, and magnetic fluxes generated by currents in the first magnetic core structure with different values and in the same direction cancel each other out. In the two parallel power lines, the currents with the same value and in the same direction are the differential mode currents. Therefore, magnetic fluxes in the first magnetic core structure are mainly magnetic fluxes generated by the differential mode currents that pass through the first primary-side winding and the second primary-side winding. An induced current generated by the first secondary-side winding wound around the first magnetic core structure is an induced current corresponding to the differential mode current. That is, a current detected by the first secondary-side winding is a differential mode current, so that the differential mode current is independently measured. 
       FIG. 7  is a schematic structural diagram of another current transformer according to an embodiment of this application. As shown in the diagram, a current transformer  30  includes a magnetic core  31 , a first primary-side winding  32 , a second primary-side winding  33 , and a second secondary-side winding  34 . 
     The magnetic core  31  includes a first closed magnetic circuit  401  and a second closed magnetic circuit  402 . A first magnetic core structure  311  and a second magnetic core structure  312  are connected to constitute the first closed magnetic circuit  401 , and the first magnetic core structure  311  and a third magnetic core structure  313  are connected to constitute the second closed magnetic circuit  402 . The first magnetic core structure  311  is a magnetic core structure common to the first closed magnetic circuit  401  and the second closed magnetic circuit  402 . The first primary-side winding  32  passes through the first closed magnetic circuit  401 , and the second primary-side winding  33  passes through the second closed magnetic circuit  402 . A first winding  341  of the second secondary-side winding  34  is wound around the second magnetic core structure  312 , a second winding  342  of the second secondary-side winding  34  is wound around the third magnetic core structure  313 , and the first winding  341  and the second winding  342  are serially connected. 
     As shown in  FIG. 5 , the direction of the electrified current i 1  that flows through the first primary-side winding and the direction of the electrified current i 2  that flows through the second primary-side winding are both perpendicular to paper outward. The i 1  generates the first magnetic field, and the i 2  generates the second magnetic field. When the magnetic field passes through a cross section of the second magnetic core structure, the magnetic field direction of the first magnetic field is downward, and a positive direction of a third magnetic flux Φ 31  generated by the i 1  in the second magnetic core structure is downward; and a magnetic field direction of the second magnetic field is downward, and a fourth magnetic flux Φ 41  generated by the i 2  in the second magnetic core structure is downward. When the magnetic field passes through a cross section of the third magnetic core structure, the magnetic field direction of the first magnetic field is upward, and a positive direction of a third magnetic flux Φ 32  generated by the i 1  in the third magnetic core structure is upward; and the magnetic field direction of the second magnetic field is upward, and a positive direction of a fourth magnetic flux Φ 42  generated by the i 2  in the second magnetic core structure is upward. 
     As shown in  FIG. 6 , the direction of the electrified current i 1  that flows through the first primary-side winding is perpendicular to paper inward, and the direction of the electrified current i 2  that flows through the second primary-side winding is perpendicular to paper outward. The i 1  generates the first magnetic field, and the i 2  generates the second magnetic field. When the magnetic field passes through a cross section of the second magnetic core structure, the magnetic field direction of the first magnetic field is upward, and a positive direction of a fourth magnetic flux Φ 41  generated by the i 2  in the second magnetic core structure is downward. When the magnetic field passes through a cross section of the third magnetic core structure, the magnetic field direction of the first magnetic field is downward, and a positive direction of a third magnetic flux Φ 32  generated by the i 1  in the third magnetic core structure is downward; and the magnetic field direction of the second magnetic field is upward, and a positive direction of a fourth magnetic flux Φ 42  generated by the i 2  in the second magnetic core structure is upward. 
     It can be learned from  FIG. 5  and  FIG. 6  that when the electrified current that flows through the first primary-side winding and the electrified current that flows through the second primary-side winding are currents with a same direction, magnetic fluxes generated by the two electrified currents in the second magnetic core structure have a same direction: a positive direction or a negative direction; magnetic fluxes generated in the third magnetic core structure also have a same direction: a positive direction or a negative direction; and the magnetic fluxes are superimposed in the second magnetic core structure and the magnetic fluxes are superimposed in the third magnetic core structure. When the electrified current that flows through the first primary-side winding and the electrified current that flows through the second primary-side winding are currents with different directions, magnetic fluxes generated by the two electrified currents in the second magnetic core structure have different directions: a positive direction and a negative direction; magnetic fluxes generated in the third magnetic core structure have different directions: a positive direction and a negative direction; and the magnetic fluxes cancel each other out in the second magnetic core structure and the magnetic fluxes cancel each other out in the third magnetic core structure. 
     Because the differential mode currents have opposite directions and a same value, and the common mode currents have a same direction. Magnetic fluxes generated by the differential mode currents in the second magnetic core structure cancel each other out, and magnetic fluxes generated by the common mode currents in the second magnetic core structure are superimposed. The magnetic fluxes in the second magnetic core structure are magnetic fluxes generated by the common mode currents passing through the first primary-side winding and the second primary-side winding. An induced current generated by the first winding of the second secondary-side winding wound around the second magnetic core structure is an induced current corresponding to the common mode currents. Magnetic fluxes generated by the differential mode currents in the third magnetic core structure cancel each other out, and magnetic fluxes generated by the common mode currents in the third magnetic core structure are superimposed. The magnetic fluxes in the third magnetic core structure are mainly magnetic fluxes generated by the common mode currents passing through the first primary-side winding and the second primary-side winding. An induced current generated by the second winding of the second secondary-side winding in the third magnetic core structure is an induced current corresponding to the common mode currents. The first winding and the second winding are serially connected, and a current detected by the second secondary-side winding is a common mode current, so that the common mode current can be independently measured. 
     In some possible embodiments, further, with reference to the two solutions of the current transformers in  FIG. 4  and  FIG. 7 , one current transformer may be configured to detect both the differential mode current and the common mode current. 
       FIG. 8  is a schematic structural diagram of still another current transformer according to an embodiment of this application. As shown in the diagram, a current transformer  50  includes a magnetic core  51 , a first primary-side winding  52 , a second primary-side winding  53 , a first secondary-side winding  54 , and a second secondary-side winding  55 . 
     The magnetic core  51  includes a first closed magnetic circuit  601  and a second closed magnetic circuit  602 . The first closed magnetic circuit  601  is formed by connecting a first magnetic core structure  511  and a second magnetic core structure  512 , the second closed magnetic circuit  602  is formed by connecting the first magnetic core structure  511  and a third magnetic core structure  513 , and the first magnetic core structure  511  is a magnetic core structure common to the first closed magnetic circuit  601  and the second closed magnetic circuit  602 . The first primary-side winding  52  passes through the first closed magnetic circuit  601 , and the second primary-side winding  53  passes through the second closed magnetic circuit  602 . The first secondary-side winding is wound around the first magnetic core structure  511 . A first winding  551  of the second secondary-side winding  55  is wound around the second magnetic core structure  512 , a second winding  552  of the second secondary-side winding  55  is wound around the third magnetic core structure  513 , and the first winding  551  and the second winding  552  are serially connected. 
     In one embodiment, the foregoing describes the positive direction and the negative direction of the magnetic fluxes generated by the electrified currents that flow through the first primary-side winding and the second primary-side winding in the first magnetic core structure, the second magnetic core structure, and the third magnetic core structure. Details are not described herein again. It can be learned from the foregoing content that the current detected by the first secondary-side winding is the differential mode current, so that the differential mode current can be independently measured; and the current detected by the second secondary-side winding is the common mode current, so that the common mode current can be independently measured. That is, the current transformer in this embodiment of this application can independently measure both the common mode current and the differential mode current. 
     The foregoing describes structures of several current transformers in this application. The following describes some possible structures and features of a magnetic core of the current transformer. 
     In this application, the first primary-side winding, the second primary-side winding, the first secondary-side winding, and/or the second secondary-side winding are/is insulated from each other. The first primary-side winding, the second primary-side winding, the first secondary-side winding, and/or the second secondary-side winding are/is constituted by a conductive coil and an insulation layer that is surrounded by the coil. The first secondary-side winding and/or the second secondary-side winding respectively have more coil turns than the first primary-side winding and the second primary-side winding. For example, the first primary-side winding and the second primary-side winding each have one turn, and the first secondary-side winding and/or the second secondary-side winding each have five turns. In a specific implementation, a quantity of turns and a turn ratio of the first primary-side winding, the second primary-side winding, the first secondary-side winding, and/or the second secondary-side winding may be designed depending on a specific requirement. This is not limited in this application. 
     In one embodiment, the first magnetic core structure, the second magnetic core structure, and the third magnetic core structure are made of a soft magnetic material for magnetic line transmission in a magnetic circuit. The soft magnetic material includes but is not limited to a soft iron, a soft magnetic alloy, a ferrite material, and a silicon steel sheet. To ensure that the first magnetic core structure, the second magnetic core structure, and the third magnetic core structure have a same magnetic permeability, the first magnetic core structure, the second magnetic core structure, and the third magnetic core structure may have a same soft magnetic material. 
     In a possible implementation, the second magnetic core structure and the third magnetic core structure may be magnetic core structures that have some same features. For example, the second magnetic core structure and the third magnetic core structure have a same size and a same shape, or the second magnetic core structure and the third magnetic core structure have a same magnetic resistance, or a magnetic permeability of the second magnetic core structure is the same as a third magnetic permeability. 
     In another embodiment, the second magnetic core structure and the third magnetic core structure may be two completely same magnetic core structures. Herein, that the second magnetic core structure is completely the same as the third magnetic core structure means that the size, the shape, the magnetic resistance, the magnetic permeability, and the like of the second magnetic core structure are the same as those of the third magnetic core structure. 
     In some possible embodiments, the second magnetic core structure may be a “U”-shaped magnetic core structure, an arc-shaped magnetic core structure, an angle-shaped magnetic core structure, or the like. The third magnetic core structure may be a “U”-shaped magnetic core structure, an arc-shaped magnetic core structure, an angle-shaped magnetic core structure, or the like. A shape and a size of the second magnetic core structure and a shape and a size of the third magnetic core structure are not limited in this application. 
     When the second magnetic core structure is the same as the third magnetic core structure, the magnetic core of the current transformer may be shown in  FIG. 9 . Both a second magnetic core structure and a third magnetic core structure of a magnetic core  01  are “U”-shaped magnetic core structures. A second magnetic core structure and a third magnetic core structure of a magnetic core  02  are angle-shaped magnetic core structures, and both a second magnetic core structure and a third magnetic core structure of a magnetic core  03  are arc-shaped magnetic core structures. 
     It should be noted that, on the premise that a process condition is met, a magnetic resistance of a first closed magnetic circuit and a magnetic resistance of a second closed magnetic circuit are reduced as much as possible, to increase a common-mode/differential-mode suppression capability of a current transformer. In an optional implementation, a magnetic resistance of the first magnetic core structure may be reduced, so as to reduce the magnetic resistance of the first closed magnetic circuit and the magnetic resistance of the second closed magnetic circuit; or a magnetic resistance of the second magnetic core structure and a magnetic resistance of the third magnetic core structure may be reduced, so as to reduce the magnetic resistance of the first closed magnetic circuit and the magnetic resistance of the second closed magnetic circuit. 
     In the embodiments of this application, when the current transformer is configured to detect the differential mode current, the current transformer may cancel the magnetic flux generated by the common mode current out, eliminate interference of the common mode current, and accurately collect a differential-mode-current signal. When the current transformer is configured to detect the common mode current, the current transformer may cancel the magnetic flux generated by the differential mode current out, eliminate interference of the differential mode current, and accurately collect a differential-mode-current signal. The current transformer in this application may independently measure the differential mode current and the common mode current. 
     The foregoing descriptions are merely specific implementations of the present invention, but are not intended to limit the protection scope of the present invention. Any variation or replacement readily figured out by a person skilled in the art within the technical scope disclosed in the present invention shall fall within the protection scope of the present invention. Therefore, the protection scope of the present invention shall be subject to the protection scope of the claims.