Abstract:
A core-saturated superconductive fault current limiter and a control method of the fault current limiter. The fault current limiter includes a superconductive magnet ( 2 ), a core ( 4 ), an AC winding ( 5 ), a cryostat system, a monitor system ( 7 ) and a DC control system ( 6 ). The output of the DC control system ( 6 ) is connected to the two terminals of the superconductive magnet ( 2 ). The DC control system ( 6 ) is also connected to the monitor system ( 7 ). The core ( 4 ) has an unequal section core structure. The control method includes: controlling the current which is flowing through the superconductive magnet ( 2 ) for limiting the fault current in the power net ( 1 ) in the case of a short circuit fault event.

Description:
FIELD OF THE INVENTION 
     This invention relates to a core-saturated superconductive fault current limiter, which has a superconductive magnet, a reactance system that is an iron core and an AC winding, a cryostat system, and a monitoring system, and further includes a DC control system. The DC control system is connected to the superconductive magnet at both ends, and connected to the monitoring system through a data cable. The iron core of the reactance system adopts the iron core structure with unequal cross-section. This invention can automatically regulate the magnitude of the current in the superconductive magnet during the stable operation of the protected power grid lines, and can automatically identify the fault when a short-circuit fault event occurs at the protected power grid positions. It can greatly improve the current limiting efficiency of the current limiter by cutting off the current in the superconductive magnet such that all iron cores of the limiter are engaged in limiting the fault current in the power grid. At the same time, it reduces the effect of magnetic leakage on iron cores by varying the structure, shape and size of the iron cores to redistribute the magnetic potential on the iron cores. In the case that excitation magnetic field stays unchanged or decreasing, the designed saturation of the iron cores can be realized. 
     The present invention is mainly applied for short-circuit current protection in power transmission, distribution networks and power apparatuses, especially for short-circuit current protection in networks with high voltage or super high voltage. 
     BACKGROUND OF THE INVENTION 
     With the development of superconductor technology, it becomes feasible to install superconductive fault current limiters in a power grid used to limit the short-circuit fault current. At present, among all kinds of superconductive fault current limiters developed all over the world, the core-saturated superconductive fault current limiter is most desirable due to the following properties: incorporating detection, initiation of limiting action and current limitation together, and no quench of the superconductivity in the process of current limiting. 
     The traditional core-saturated superconductive fault current limiter is composed of four parts: an iron core group, an AC winding, a superconductive magnet (namely a superconductive winding), and a direct current (DC) power supply. The superconductive magnet provides excitation for two parallel iron cores. Two AC coils connected in series are wound on two iron cores respectively to cancel the magnetic fields generated in the center column so as to minimize the effect of the AC inductance voltage on the DC superconductive windings. When the current limiter operates normally, the DC excitation causes the iron cores to be in the deep saturation. Iron cores produce small inductive impedance in the AC winding so that there is no effect on the power grid. In the state of the fault current limiting, the super high short-circuit current drives one iron core out of saturation in a half-wave and the magnetic field in the other iron core increases to realize the current limiting by a single iron core (the reactor in the enhanced magnetism state is not engaged in the current limiting). This is so-called passive fault current limiting. Although the passive fault current limiting can indeed limit the fault current, it has the following obvious short-comings: 1) it does not fully utilize all of the iron cores for limiting the fault current, thus demands heavier iron cores and larger size of the AC winding to produce the desired current limiting effects; 2) the DC side has to be subject to high inductive voltage during the state of the fault current limiting; and 3) the DC power supply must be a constant power supply. Otherwise, the efficiency of current liming will be reduced if there is interference in the power supply. 
     Furthermore, for the superconductive fault current limiters on a high voltage grid, AC windings are at high voltage level while iron cores and superconductive magnets are at low voltage level. Safe insulation distances are required between AC windings, between AC windings and the iron cores, and between superconductive magnets. For this reason, the loose coupling structure shown in  FIG. 5  is generally adopted, that is, superconductive magnet  2  and the AC winding  5  are on different iron core posts of the same iron core window. Since the iron cores operate in the nonlinear segment of the B-H magnetization curve of iron core materials for a certain time period or all time periods, which falls in the range of saturation or deep saturation. Therefore, in the loose coupling structure, magnetic leakage is inevitable. This phenomenon becomes aggravated as the degree of saturation increases.  FIG. 6  shows the schematic diagram of the rectangular-shape iron core excitation of prior art. The iron core part wound by the superconductive magnet  2  is called as an excitation segment  41 . The iron core part wound by the AC winding  5  is called as a working segment  42 . The other part connected with the excitation segment  41  and the working segment  42  is called as a conduction segment  43 . When the excitation current in the superconductive magnet  2  is low, the magnetic field in the iron cores is weak. The iron cores are in the unsaturated state, with lower magnetic leakage flux. The magnetic flux in the working segment  42  and the excitation segment  41  are approximately same. If the excitation current is increased, the magnetic leakage increases. Because the magnetic leakage increases, the ratio of magnetic potential on the excitation segment  41  to the total magnetic potential produced by windings is also increasing. As a result, the magnetic potential on the working segment  42  reduces relative to that on the excitation segment  41 . It is needed to increase excitation current to reach a certain degree of saturation, and it is difficult to reach a certain degree of saturation even with increased excitation current. 
     Consequently, the traditional core-saturated superconductive fault current limiter has been considered as a current limiting technology with higher cost, heavier weight and the required power supply technology is harder to implement and is impractical in reality. 
     SUMMARY OF THE INVENTION 
     The purpose of this invention is to overcome the deficiencies and shortcomings of existing technologies. A core-saturated superconductive fault current limiter and its control method are provided. The power electronic control technology is used to turn passive current limiting into active current limiting. At the same time, the effect of magnetic leakage on iron cores is reduced by varying the structure, shape and size of the iron cores, to redistribute the magnetic potential on the iron cores. In the case that excitation magnetic potential stays unchanged or decreasing, the designed saturation of the iron cores can be realized. The core-saturated superconductive fault current limiter according to the present invention has small size, high current limiting efficiency, rapid recovering properties, and no inductive over high voltage damage etc and has better applicability. 
     To achieve the above purpose, this invention provides a core-saturated superconductive fault current limiter, which comprises a superconductive magnet, a reactance system that is an iron core and an AC winding, a cryostat system, a monitoring system, that is characterized by further including a DC control system and the DC control system is connected to the superconductive magnet at both ends, and connected with the monitoring system through a data cable. The iron core of the reactance system adopts the iron core structure with unequal cross-section. 
     The DC control system comprises: 
     a DC power supply module, which has an output terminal; 
     a magnetic energy absorbing module connected with the output terminal of the DC power supply module, which consists of an absorption branch and a follow current branch. The absorption branch is connected with the superconductive magnet in series, in which the current flows in single direction. The follow current branch is connected in parallel with the series circuit that consists of the absorption branch and the superconductive magnet, and its current flows in a direction opposite to that in the absorption branch; 
     a control module, which connects the state data lines of the controllable power electronic switches in the power mutual inductor of the power grid, DC power supply module, and the magnetic energy absorbing module through signal lines, and which receives electrical measurement signals in the power grid, receives and transmits state signals and control signals of controllable power electronic components in the DC control system, identifies the operation state of the power grid and controls the operation state of the DC power supply module and the magnetic energy absorbing module. 
     The DC power supply in the DC power supply module is the DC power supply provided by rectifying a single-phase or three-phase AC, or the DC power supply provided by dry batteries, or secondary batteries. 
     The DC power supply provided by rectifying a single-phase or three-phase AC includes one or more controllable power electronic switches that realize controllable output voltage and regulable output constant current. 
     The absorption branch consists of controllable power electronic switches being connected in parallel with magnetic energy absorbing elements. 
     The power electronic switches in the DC power supply module and the magnetic energy absorbing module are selected from one of Insulated Gate Bipolar Transistors (IGBT), Gate Turn-Off thyristors (GTO) and Integrated Gate-Commutated Thyristor (IGCT). 
     The magnetic energy absorbing elements in the absorption branch are one or a combination of some among Metal Oxide Varistor (MOV), Gas Discharge Tube (GDT), Solid Discharge Tube (SDT), or Transient Voltage Suppressor (TVS). 
     The follow current branch is composed of one or more diodes. 
     The iron core of the reactance system adopts the iron core structure with unequal cross-section. The iron core structure with unequal cross-section is a closed-shape iron core, including an excitation segment, a working segment and a conduction segment. Wherein, the ratio of the cross-section area of the excitation segment iron core to that of the working segment iron core falls between 1.01 and 10, and the ratio of the cross-section area of the excitation segment iron core to that of the conduction segment iron core falls between 1 and 10. 
     The closed-shape iron core can be a ring or polygon. 
     The number of sides in the polygon iron core may be 4˜20. 
     The excitation segment is wound by superconductive windings, the working segment is wound by AC windings, and the conduction segment connects the excitation segment and the working segment. 
     The cross-section area of the conduction segment iron core is bigger than that of the working segment iron core, and less than or equal to that of the excitation segment iron core. 
     The shape of the cross section of the excitation segment iron core post is different from that of the working segment iron core post. 2˜6 said excitation segment iron core posts are compactly assembled into an assembled center column around the center for 360°. There is provided an assembled insulation plate between every two adjacent excitation segment iron core posts. The outer contour of the assembled center column&#39;s section can have any one of the following shapes: circular, ellipse, runway, and rectangle or the like. 
     In the cross-section shape of the excitation segment iron core post, a circular arc of the outer contour of the assembled center column&#39;s section is located at the inner side of the rectangular-shape iron core. A wedge-shape is formed at the outer side of the rectangular-shape iron core. The other two sides are parallel straight sides. 
     The vertex angle of the wedge shape is 60°, 90°, 120° or 180°. 
     The superconductive magnet is connected in parallel with a voltage-clamp coil. 
     The voltage-clamp coil is a single copper, aluminum, or silver coil winding or a single resistor, or is a copper/aluminum coil winding connected with a resistor in series. 
     The ratio between the number of turns of the superconductive magnet and that of the voltage-clamp coil is equal to or bigger than 20. 
     To achieve the above purpose, the invention also provides a control method for the core-saturated superconductive fault current limiter. The method comprises the following steps: 
     1) The control module identifies the operation state of the power grid by processing electrical measurement signals received from the power mutual inductor in the power grid. The operation states include: normal operation state, short-circuit fault state and short-circuit fault recovery state; 
     2) When the control module identifies that the power grid operates in the normal operation state, it controls the controllable power electronic switches of the magnetic energy absorbing module to set it in a non-magnetic energy absorbing state. It also controls the controllable power electronic switches of the DC power supply module to set it in the low-voltage constant DC output state; 
     3) When the DC power supply module is in the low-voltage constant DC output state, the control module regulates the magnitude of the current in the superconductive magnet to match the load of the power grid, according to the load amount of the power grid. Then return to step 1). This cycle will be maintained until the operation state of the power grid changes; 
     4) When the control module identifies that the power grid is in the short-circuit fault state, it controls the controllable power electronic switches in the magnetic energy absorbing module to set it in the magnetic energy absorbing state. It also controls the controllable power electronic switches of the DC power supply module to interrupt its DC output. Then return to step 1). This cycle will be maintained until the operation state of the power grid changes; 
     5) When the control module identifies that the power grid is in the fault recovery state, it controls the controllable power electronic switches in the magnetic energy absorbing module to set it in the non-magnetic energy absorbing state. It also controls the controllable power electronic switches of the DC power supply module to set it in high-voltage DC output state. Then return to step 1). This cycle will be maintained until the operation state of the power grid changes. 
     The present invention has the following beneficial technical effects: the core-saturated superconductive fault current limiter proposed in this invention turns the passive current limiting into active current limiting, and thus improves the efficiency of current limiting compared with the traditional technology; the effect of magnetic leakage on iron cores is reduced by varying the structure, shape and size of the iron cores, to cancel the disadvantage caused by the magnetic leakage that is produced because of the iron cores entering saturation sate, so as to redistribute the magnetic potential on the iron cores. In the case that excitation magnetic potential stays unchanged or decreasing, the designed saturation of the iron cores can be realized. The current limiter according to this invention has better applicability in terms of technology and economics. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an overall schematic circuit diagram of the core-saturated superconductive fault current limiter according to the present invention. 
         FIG. 2  is a schematic diagram of the DC control system. 
         FIG. 3  is a schematic diagram of an application embodiment of the DC control system according to the present invention. 
         FIG. 4  shows a flow chart of steps of the control method for the DC control system according to the present invention. 
         FIG. 5  is a schematic diagram of the structure of a loose coupling six-iron core structure of prior art. 
         FIG. 6  shows a schematic diagram of the rectangular-shape iron core excitation of prior art. 
         FIGS. 7 ,  8  and  9  show schematic diagrams of the iron core with unequal cross-section according to the present invention respectively, wherein AC windings are wound on different positions. 
         FIG. 10  is a schematic diagram of the cross section of an assembled center column that is formed by compactly assembling six excitation segment iron core posts. 
         FIG. 11  shows the magnified schematic diagram of the section of one wedge-shaped excitation segment iron core post. 
         FIG. 12  shows the front view of the iron core with unequal cross-section according to the present invention. 
         FIG. 13  shows the top view of the iron core with unequal cross-section according to the present invention. 
         FIG. 14  shows a schematic diagram of the structure of a voltage-clamp coil in an embodiment. 
         FIG. 15  shows another schematic diagram of the structure of a voltage-clamp coil. 
     
    
    
     
         
         
           
               1  Power grid 
               2  Superconductive magnet 
               3  Load in the power grid 
               4  Iron core 
               5  AC winding 
               6  DC control system 
               7  Monitoring system 
               8  Power mutual inductor 
               9  Cryogenic system 
               10  Assembled insulation plate 
               11  Voltage-clamp coil 
               12  Resistor 
               41  Excitation segment 
               42  Working segment 
               43  Conduction segment 
               411  A Circular Arc 
               412  Wedge shape 
               413  Parallel straight side 
               414  Parallel straight side 
               61  DC power supply module 
               62  Magnetic energy absorbing module 
               63  Control module 
               621  Absorption branch 
               622  Follow current branch 
           
         
       
    
     DETAILED DESCRIPTION 
     In the following description, specific implementations and embodiments are illustrated in conjunction with the drawings. 
       FIG. 1  shows a schematic circuit diagram of a core-saturated superconductive fault current limiter according to the present invention. This invention provides a core-saturated superconductive fault current limiter, which is connected to the power supply in the power grid  1 , and includes a superconductive magnet  2 , an iron core  4  and an AC winding  5 , and also includes a DC control system  6 . The DC control system  6  is connected to the superconductive magnet  2  at both ends, and connected with a monitoring system  7  through a data cable. 
     As shown in  FIG. 2  of a schematic circuit diagram of the DC control system, the DC control system  6  includes: 
     a DC power supply module  61 , which has an output terminal; 
     a magnetic energy absorbing module  62  connected with the output terminal of the DC power supply module  61 , which is composed of an absorption branch  621  and a follow current branch  622 . The absorption branch  621  is connected to the superconductive magnet  2  in series, wherein the current flows in one direction; the follow current branch  622  is connected in parallel with the series circuit that consists of the absorption branch  621  and the superconductive magnet  2 , and its current flows in a direction opposite to that in the absorption branch  621 ; 
     a control module  63 , which connects the state data cable of the controllable electronic switches in the power mutual inductor  8  of the power grid, DC power supply module  61  and the magnetic energy absorbing module  62  through signal lines, receives electrical measurement signals in the power grid  1 , and receives and transmits state signals and control signals of the controllable electronic components in the DC control system  6 , identifies the operation state of the power grid  1  and controls the operation state of the DC power supply module  61  and the magnetic energy absorbing module  62 . 
     As shown in  FIG. 3 , the DC power supply in the DC power supply module  61  is a DC power supply provided by rectifying a single-phase or three-phase AC, wherein the DC power supply module  61  uses a controllable power electronic switch Q 1 , an inductance coil L 1 , a capacitor C 2  and a diode D 2  to control output voltage, and uses a controllable power electronic switch Q 3  to realize the output constant current regulation. When the current limiter operates normally, the DC power supply module  61  outputs a lower DC voltage, to maintain the excitation current in the superconductive magnet  2 . When the current limiter is in the recovery state, the DC power supply module  61  outputs a higher DC forcing voltage, so that the magnetic field in the superconductive magnet  2  can reach the excitation state rapidly. 
     The absorption branch  621  consists of a controllable power electronic switch Q 2  connected in parallel with a magnetic energy absorbing component Rm. When the superconductive magnet  2  operates in the excitation state, the absorption branch  621  exhibits connectivity, corresponding to the short-circuit state. When the superconductive magnet  2  exits from the excitation state, the absorption branch  621  disconnects and absorbs the magnetic energy in the superconductive magnet  2 , so as to limit the voltage across the superconductive magnet  2  to a safe level. 
     The magnetic energy absorbing component Rm in the absorption branch  621  is one or a combination of some among Metal Oxide Varistor (MOV), Gas Discharge Tube (GDT), Solid Discharge Tube (SDT), Transient Voltage Suppressor (TVS). 
     The follow current branch  622  is made up of one or more diodes D 1 . In the follow branch  622 , the current flows in a single direction opposite to the direction of the current in the superconductive magnet  2 . Hence, the follow branch  622  may be one or more diodes, which are connected in series, in parallel, or any combination of series-parallel connections, and may also be controllable electronic switches. 
     The control module  63  can control the operation state of the DC power supply module  61  and magnetic energy absorbing module  62  respectively, according to the operation strategy of the current limiter.  FIG. 4  shows a flow chart of steps of the control method for the DC control system, comprising the following steps: 
     1) The control module  63  identifies the operation state of the power grid by processing the electrical measurement signals received from the power mutual inductor  8  in the power grid. The operation state includes three distinctive states: normal operation state, short-circuit fault state, and short-circuit fault recovery state. 
     2) When the control module  63  identifies that the power grid is operating in the normal operation state, it controls the controllable electronic switches in the magnetic energy absorbing module  62  to set it in a non-magnetic energy absorbing state. It also controls the controllable electronic switches of the DC power supply module  61  to set it in the low-voltage constant DC output state. 
     3) When the DC power supply module  61  is in the low-voltage constant DC output state, the control module  63  regulates the current magnitude in the superconductive magnet  2  to match the load of the power grid, according to the load amount of the power grid. Then return to step 1). This cycle will be maintained until the operation state of the power grid changes. 
     4) When the control module  63  identifies that the power grid is in the short-circuit fault state, it controls the controllable power electronic switches in the magnetic energy absorbing module  62  to set it in the magnetic energy absorbing state; and it also controls the controllable electronic switches of the DC power supply module  61 , to interrupt the DC output. Then return to step 1). This cycle will be maintained until the operation state of the power grid changes. 
     5) When the control module  63  identifies that the power grid is in the fault recovery state, it controls the controllable power electronic switches in the magnetic energy absorbing module  62  to set it in the non-magnetic energy absorbing state; and it also controls the power electronic switches of the DC power supply module  61  to set it in a high-voltage DC output state. Then return to step 1). This cycle will be maintained until the operation state of the power grid changes. 
       FIG. 7  is a schematic diagram of one exemplary embodiment of the invention, which shows a rectangular-shape iron core structure with unequal cross-section and wherein, the superconductive magnet  2  and the AC winding  5  are separately wound on two opposite sides. It provides an iron core structure with unequal cross-section for working at the magnetic unsaturated region. The iron core is a closed-shape iron core, including an excitation segment  41 , a working segment  42 , and a conduction segment  43 . Wherein, the ratio of the cross-section area of the excitation segment  41  iron core to that of the working segment  42  iron core falls between 1.01 and 10, and the preferred ratio falls between 1.2 and 3. The ratio of the cross-section area of the excitation segment  41  iron core to that of the conduction segment  43  iron core falls between 1 and 10, and the preferred ratio falls between 1 and 3. The cross-section area of the conduction segment  43  iron core is bigger than that of the working segment  42  iron core, and less than or equal to that of the excitation segment  41  iron core. As shown in  FIG. 7 , in this embodiment, the cross-section area of the conduction segment  43  iron core is less than that of the excitation segment  41  iron core. The excitation segment  41  is wound by the superconductive magnet  2 , the working segment  42  is wound by the AC winding  5 , and the conduction segment  43  connects the excitation segment  41  and the working segment  42 . 
       FIG. 8  and  FIG. 9  are schematic diagrams of two exemplary embodiments of this invention, which shows a rectangular-shape iron core structure with unequal cross-section, and wherein, the superconductive magnet  2  and the AC winding  5  are separately wound on two adjacent sides. The iron core is a closed-shape iron core, including an excitation segment  41 , a working segment  42 , and a conduction segment  43 . Wherein, the ratio of the cross-section area of the excitation segment  41  iron core to that of the working segment  42  iron core falls between 1.01 and 10, and the preferred ratio falls between 1.2 and 3. The ratio of the cross-section area of the excitation segment  41  iron core to that of the conduction segment  43  iron core falls between 1 and 10, and the preferred ratio falls between 1 and 3. The cross-section area of the conduction segment  43  iron core is bigger than that of the working segment  42  iron core, and less than or equal to that of the excitation segment  41  iron core. As shown in  FIGS. 8 and 9 , in this embodiment, the cross-section area of the conduction segment  43  iron core is less than that of the excitation segment  41  iron core. The excitation segment  41  is wound by the superconductive magnet  2 , the working segment  42  is wound by the AC winding  5 , and the conduction segment  43  connects the excitation segment  41  and the working segment  42 . 
     As shown in  FIG. 10 , six said excitation segment iron core posts are compactly assembled together into an assembled center column around the center for 360°. There is provided an assembled insulation plate  10  between every two adjacent excitation segment iron core posts  41 . The outer contour of the combined center column&#39;s cross-section is circular. 
       FIG. 11  shows the magnified schematic diagram of one wedge-shaped section of the assembled center column formed with six compactly assembled excitation segment iron core posts. By this example, in the cross-section shape of the excitation segment iron core post  41 , a circular arc  411  of the outer contour of the assembled center column&#39;s section is located at the inner side of the rectangular-shape iron core. A wedge shape  412  is formed at the outer side of the rectangular-shape iron core. And the other two sides are parallel straight sides  413 , 414 . 
       FIG. 14  shows the schematic diagram of the structure about the superconductive magnet  2  connected in parallel with a voltage-clamp coil  11 , wherein the voltage-clamp coil&#39;s number of turns is far smaller than that of the superconductive magnet. Thus, the equivalent number of turns of the parallel structure composed of the superconductive magnet and the voltage-clamp coil is significantly decreased compared with that of the superconductive magnet, so that the induction voltage taken by the superconductive magnet is reduced. When the superconductive magnet is powered, the current provided by the DC constant power supply only passes through the superconductive magnet, due to its null resistance, and thus the parallel voltage-clamp coil does not affect the normal excitation function of the superconductive magnet.  FIG. 15  shows the other schematic diagram of the structure about the superconductive magnet  2  connected in parallel with a voltage-clamp coil  11 . According to specific requirements, the voltage-clamp coil may also be connected in series with a resistor R 12  first, and then connected in parallel with the superconductive magnet. 
     As follows, the core-saturated superconductive fault limiter DC control system and the iron core structure with unequal cross-section will be further illustrated in conjunction with a practical embodiment. 
       FIG. 3  shows a detailed circuit diagram of the DC control system for a 35 kV superconductive fault current limiter. Wherein, the DC power supply module  61  uses a three phase full bridge to rectify three phase 380V AC and then outputs a DC current with the voltage of 550V. The DC power supply module  61  uses the controllable power electronic switch Q 1 , the inductance coil L 1 , the capacitor C 2  and the diode D 2  to control its output voltage, and then realizes the constant current regulation through the controllable power electronic switch Q 3 . 
     In this exemplary embodiment, the detailed configuration, principle and function of the DC control system of the core-saturated superconductive fault current limiter are as follows: 
     1) DC Power Supply Module  61   
     In this embodiment, a pulsed DC is output by rectifying a single phase or three phase AC through a power electronic switch rectifier. The harmonic waves in the power supply output are removed by a filter capacitor C 1  that is connected in parallel at the rear. 
     The output rear of the DC power supply is connected in parallel with a switch BOAST booster circuit, which is composed of an inductor L 1 , a power electronic switch Q 1 , a diode D 2 , a filter/energy storage capacitor C 2 . The magnitude of the output voltage can be controlled through a PWM power electronic switch Q 1  to realize the controllable voltage. 
     When the current limiter is operating normally, the DC power supply module  61  outputs a lower voltage by pulse-width modulating the duty factor of the power electronic switch Q 1  through the PWM, to maintain the excitation current in the superconductive magnet  2 . When the current limiter operates in the recovery state, the DC power supply module  61  outputs a higher DC forcing voltage by pulse-width modulating the duty factor of the power electronic switch Q 1  through the PWM to cause the magnetic field in the superconductive magnet  2  to reach the excitation state rapidly. 
     The power electronic switch Q 3  is used to regulate and switch off the output of the DC power supply, which is controlled by the control module  63 . It uses a PWM to pulse-width modulate and control the duty factor of the power electronic switch Q 3  so as to adjust the current value in the superconductive magnet  2 . 
     The power electronic switches Q 1  and Q 3  can be composed of one or more Insulated Gate Bipolar Transistors (IGBT), Gate Turn-Off thyristors (GTO) or Integrated Gate-Commutated Thyristor (IGCT), which are connected in series or in parallel. 
     2) Magnetic Energy Absorbing Module  62   
     The magnetic energy absorbing module  62  consists of the absorption branch  621  and the follow current branch  622 . The absorption branch  621  is connected with the superconductive magnet  2  in series, and in this branch the current flows in a single direction. The follow current branch  622  connects in parallel with the series circuit that consists of the absorption branch  621  and the superconductive magnet  2 , and its current flows in the opposite direction to that in the absorption branch  621 . 
     According to  FIG. 3 , the absorption branch  621  consists of the power electronic switch Q 2  connected in parallel with a MOV Rm, and is connected in series with the superconductive magnet  2  in one circuit. 
     The component group composed of the power electronic switch Q 1  and the MOV Rm connected in parallel includes at least one power electronic switch and MOV Rm connected in parallel or a plurality of power electronic switches and MOV Rms connected in parallel. The MOV Rm in the absorption branch may be one of high energy MOV, GDT, SDT, TVS and the like. 
     According to  FIG. 3 , the follow current branch  622  is made up of one diode D 1 . In the follow current branch  622 , the current flows in an opposite direction to the current in the superconductive magnet  2 , used for follow current. 
     According to the requirements, the follow current branch  622  may also consist of a plurality of diodes, which are connected in series, in parallel, or in any combination of series-parallel connections. Moreover, the diode D 1  may also be replaced by controllable power electronic components. 
     When the current limiter exists from the excitation state, it sends a signal to the power electronic switch Q 3  as shown in  FIG. 2-2  to switch off the output of the power supply. At this time, the superconductive magnet  2  forms follow current through diodes D 1  and Q 2 . Switching off the Q 3  and Q 2  simultaneously generates a transient high voltage at two ends of the superconductive magnet  2 , which is applied across the Q 2 . At this point, the MOV Rm discharges and clamps the voltage on the two ends of the superconductive magnet  2  at the same time. Thus the magnetic energy in the superconductive magnet  2  is released, while the power electronic switch Q 2  is protected from the high voltage damage. 
     3) Control Module  63   
     The control module  63  as shown by the dotted line block in  FIG. 3 , is connected to the data cable of the power electronic components of the power mutual inductor  8  in the power grid, DC power supply module  61 , and the magnetic energy absorbing module  62  through the data control lines. It is used for processing the received data, and controls the operation state of the power electronic components in the DC power supply module  61  and the magnetic energy absorbing module  62 . 
       FIG. 12  shows a front view of the embodiment of a three phase current limiter that is composed of six rectangular-shape iron cores. The six iron cores shown in the figure are all rectangular-shape (quadrangle) or the like. It includes an excitation segment  41 , a working segment  42 , and a conduction segment  43 . Wherein, the ratio of the cross-section area of the excitation segment  41  iron core to that of the working segment  42  iron core falls between 1.01 and 10, and the preferred ratio falls between 1.2 and 3. In this embodiment, the ratio is approximately 2. The ratio of the cross-section area of the excitation segment  41  iron core to that of the conduction segment  43  iron core falls between 1 and 10, and the preferred ratio falls between 1 and 3. In this embodiment, the ratio is 1. In addition, the cross-section area of the conduction segment  43  iron core is bigger than that of the working segment  42  iron core, and less than or equal to that of the excitation segment  41  iron core. As shown in  FIG. 12 , in this embodiment, the cross-section area of the conduction segment  43  iron core is equal to that of the excitation segment  41  iron core. 
       FIG. 13  shows a top view of the embodiment of a three phase current limiter, which is composed of six rectangular-shape iron cores. The excitation segment  41  iron core post has different cross-section shape from the working segment  42  iron core post. Six said excitation segment iron core posts are compactly assembled into an assembled center column around the center for 360°. There is provided an assembled insulation plate  10  between every two adjacent excitation segment iron core posts  41 . The outer contour of the assembled center column&#39;s cross-section is circular. The wedge-shaped excitation segment iron core post  41  with six-iron core structure has a vertex angle of 60°. 
     The present invention adopts power electronic control technology to turn a superconductive fault current limiter into an active current limiter, and thus enhances the current limiting efficiency. Moreover, it also utilizes the iron core structure with unequal cross-section so that the working iron core can reach the saturation required in the design. In addition, the bias magnetic potential provided by the superconductive winding is lower than that in the prior art, thus significantly reduces the size of the iron cores.