The following embodiments of the invention in conjunction with the present invention of the embodiment will be clearly and completely described, obviously, the described embodiments are merely part of embodiments of the present invention rather than all embodiments. Based on the embodiments in the present invention, all other embodiments obtained by those of ordinary skill in the art without creative labor are in the scope of the present invention.
 It should be noted that the features of the present invention in the present invention may be combined with each other in the case of an unable conflict.
 The present invention will be further described below in conjunction with specific embodiments, but is not limited as the invention.
 In this embodiment, in a conventional manner magnetically coupled magnetic saturation DC left flow restrictor, the right outer main branch core winding wound string added a magnetic coupling branch, and showed the tight coupling state, thereby reducing the DC breaker (DCcircuit breaker, DCCB) overvoltage energy absorption achieved by using a secondary short-circuit fault energy. Simulation and experimental results show that the present embodiment can effectively limit the fault current DC power, shorten the fault current fall time, while reducing the DC circuit breaker and the energy absorption to achieve the second overvoltage and short-circuit fault energy utilization.
 This embodiment is realized by the following technical solution, magnetically saturated core fault current limiter of the DC secondary active current limiting, the core comprises two columns, two upper and lower transverse yokes, two NdFeB, two a winding coil DC main branch, two branches coupled winding coil, coupled to a plurality of sub-modules composed of parallel branches. Two permanent magnets embedded in the spatial inverse horizontal upper and lower portions of the yoke. DC main branch windings wound in the left and right core column conducting direct current system. Two coupled windings wound on the two branches of the DC main winding peripheral branches, it was the tight coupling state, and the branch connection coupling. Two coupled branches is less than two turns of the winding coil winding coil DC main branch. Coupling each branch comprises a sub-module storage capacitor, energy absorption resistance, mechanical switches, three thyristors.
 Working principle of this embodiment is that: in the normal operating case, the magnetic saturation characteristic of the ferromagnetic element, the magnetic flux of the permanent magnets work in the left column core saturation, the inductance of the coil at this time to maintain a predetermined value, the DC system no effect on normal operation, may alternatively conventional smoothing reactor; fault state, the fault current increases, the magnetomotive force in the opposite direction of the fault current is generated with the permanent magnet, the magnetic flux is reduced such that the core desaturation, this flow limit automatic impedance becomes large, effective to inhibit increase in the steepness of the fault current. Fault current decreases quickly to a short circuit current by means of magnetic coupling charge storage capacitor, this time greatly reduces the DC breaker and overvoltage energy absorption, wherein simultaneously by adjusting the conduction angle of the thyristor of two, to target charging capacitor is charged, the failure to achieve a secondary energy utilization.
 The method of limiting the DC magnetically saturated core fault current limiter may be of the secondary active current limiting embodiment of the present embodiment includes:
 Under normal operation, power is small operating current, the permanent magnets bias coil magnetomotive force than the absolute dominance, so that during normal operation the core left column, a right column by core saturation of the permanent magnets; the magnetic saturation of the core when approximately equal to the conductivity of air permeability, and therefore, when the system is operating normally, the normal impedance of the flow restrictor is small, and the same two total inductance of the DC main leg coil and the high voltage DC smoothing reactors, the system normal operation has no effect. At the same time as the core magnetic saturation, the branch coupling system will not fluctuate current conduction.
 When a failure occurs, the system DC current rapidly increases, due to the opposite direction of the DC magnetic flux of the main permanent magnets and the winding leg, such that about two column cores rapid desaturation; permeability core out of saturation after the rapid rise, resulting in DC main branch winding inductance value is larger, short circuit fault current is limited. Back pressure generated in the direction of the fault current is decreasing, the flow restrictor with a thyristor VT 01 , VT 03 , VT 11 , VT 13 , ... VT n1 , VT n3 Same direction on-voltage, is coupled in parallel with the main leg magnetically coupled to a DC branch, the branch fault current flowing into the coupling capacitor C connected in parallel to all submodules 1 , C 2 , ... C n Charging, over-voltage DC circuit breaker is reduced, the energy absorption, while storing large amounts of energy. If a permanent fault in the circuit breaker reclosing, the conducting thyristor VT 02 , VT 12 , VT 22 , ... VT n2 , VT n2 All the sub-modules in parallel capacitance C 1 , C 2 , ... C n Discharged through the thyristor in series, then the stored energy can be used for current limitation, greatly improve the current limiting performance. If the circuit breaker reclosing the fault has disappeared, the system has returned to normal, the mechanical switch S is turned on all the sub-modules in parallel 1 S 2... s n All the sub-modules in parallel capacitance C 1 , C 2 , ... C n Via a resistor R 1 R 2 , ... R n Meanwhile discharge, the stored energy is consumed.
 After the fault, the fault current rapidly decreases, this time the saturated core fault current limiter HVDC converter operation state has returned to the normal operation state of the line, the fault current limiter and the overall resistance value decreases rapidly, will not normal operation of the system impact.
 In the case of implementation, if Figure 5 , The magnetically saturated core fault current limiter of the DC secondary current limitation of the DC main branch comprising a first winding, a second winding of the DC main branch, the first branch is coupled winding, a second winding coupled branch coupling branch, a first permanent magnet and a second permanent magnet; mouth-shaped core is a solid structure, the core comprising a left column and a right column core, respectively located on upper and lower ends and a lower transverse yoke transverse yoke, the transverse yoke embedded in the middle a first permanent magnet, a second lower transverse yoke of the permanent magnet embedded in the middle; a first winding and a second DC main branch DC main branch winding core are wound in the left column and the right column of the core, and connected in series with a DC power the first coupling and the second coupling winding branch leg windings wound tightly coupled manner to the outside of the main branch and the second winding of the main winding current branch a first DC coupled and branch connection; as Figure 5 , The coupling of n sub-branch module SM 1 , SM 2 , … SM n Connected in parallel, n is a positive integer. Flow restrictor access transmission line runs, the DC magnetic flux through the core to form a loop, a first, a second iron core magnetic flux is also generated by the permanent magnet excitation circuit configuration, the two magnetic flux act on the entire core columns.
 Further, each sub-branch coupling module SM n Includes storage capacitor C n The first, second, third thyristor VT n1 , VT n2 , VT n3 , A mechanical switch S n Energy-absorbing resistor R n. Wherein the first, second, third flow from thyristor action by controlling the thyristor on and off to control the charge of each sub-module in parallel and series discharge mode. The mechanical switch is turned on with the absorbing resistor, capacitor residual energy release. like Figure 5A Indicated.
 Further, the iron core core columns left and right core limb section may be circular, oval or rectangular. The present embodiment uses a rectangular, such as Figure 5 , The left and right core limb is rectangular, cross-sectional area are equal, equal in length; the upper transverse yoke, the transverse yoke length and cross-sectional area are equal; said first permanent magnet, a second permanent magnet in its cross-sectional area equal to the cross-sectional area of the yoke, and the same thickness of the two permanent magnets, the size of exactly the same size of the structure, which is to ensure the symmetry of the magnetic structure. Around core leg cross-sectional area smaller than the cross-sectional area of the permanent magnet, which is to ensure the normal operation of the system, the permanent magnets can make the core magnetic saturation, thereby retaining the flow restrictor small inductor running.
Further, the first and second permanent magnets are made of NdFeB, is a superior performance NdFeB rare earth permanent magnet material, its advantages are: (1) high magnetic properties; coercivity iron oxide is equivalent to 5 to 10 times the body of permanent magnet material, 5 to 15 times the alnico permanent magnetic material; rich (2) resources, low price; main material iron 2/3, 1/3 neodymium rare earth material, resources relative abundance; (3) good mechanical properties, cutting and drilling may be performed.
 like Figure 5 , The coupling legs of the present embodiment comprises n sub-module SM 1, SM 2,… SM n Parallel composition. Each sub-module SM n Includes storage capacitor C n The first, second, third thyristor VT n1 , VT n2 , VT n3 , A mechanical switch S n Energy-absorbing resistor R n. Mechanical switches S n And energy-absorbing resistor R n Series, and the storage capacitor C and then n In parallel, a first thyristor VT n1 The second thyristor VT n2 Parallel, negative electrode and the storage capacitor C n Series, the third thyristor VT n3 The positive electrode and the storage capacitor C n Series. In which the energy storage capacitor C n It requires high energy storage capacity; thyristor require a higher current-carrying capacity and the forward reverse withstand voltage performance. A first winding and a second DC main branch leg DC main winding (primary side) of the coil turns is greater than the first coupling and the second coupling winding branch branches winding (secondary side) of the coil turns, and each of the power the electronic device are located in the secondary side of the flow restrictor, at a relatively low pressure side, to reduce the cost of the insulation. For ease of understanding, it will be equal to the number of sub-modules 3 as an example.
 In this embodiment, the left and right columns in the critical core saturation in the normal power state, i.e. BH curve at the inflection point of the ferromagnetic material, to ensure that when failure left and right core limb desaturation speed, so that the flow restrictor can quickly becomes large inductor current limit.
 The operation of this embodiment is: equivalent circuit diagram of a magnetically saturated core fault current limiter of the DC secondary current limitation of the DC breakers connected in series as Image 6 , The DC current in the coil induces a magnetic field direction of the permanent magnet opposite to the magnetic field; and a coil flux path generated by the permanent magnets, such as Figure 5 Shown, the magnetic flux generated by the permanent magnet and the coil through both the left and right two core limb, in normal operation, since the operating current I grid dc Small, much smaller than in terms of the magnetic flux generated by the permanent magnets, so that during normal operation the left and right core saturation of the permanent magnet by column; due permeability at saturation of the core is approximately equal to the air permeability, and therefore, when the system during normal operation, the normal impedance of the flow restrictor is small, after a reasonable design allows the two branches of the main winding coincides DC smoothing reactors and a total inductance HVDC system used, the normal operation of the system without Influence. At this time, since the magnetic saturation in the core, the branch can not flow restrictor coupled in parallel into the DC magnetic coupling through the main branch, the flow path of current flow restrictor as Figure 7A Indicated.
 When a fault occurs, the fault current rising phase, the fault current in the DC offset flux coil produced a first biasing magnetic flux, generated by the second permanent magnet, such that the left and right core columns rapid desaturation, after the core exits saturation permeability rapidly increased, resulting in the first and second main current branch winding inductance value rapidly becomes large, short circuit fault current effectively limited. All this time, since none of the thyristor is turned on, coupling the branch is still not turned on, the flow path of current flow restrictor as Figure 7b Indicated.
 When the operation of the DC circuit breaker, fault current descent phase, the left and right core is still in the back column saturation, since a reverse voltage across the flow restrictor, case 3 is turned on first thyristor VT submodules 01 , VT 11 , VT 21 And a third thyristor VT 03 , VT 13 , VT 23 , Backpressure same direction on the direction of the flow thereof through the flow restrictor, so the flow path of the coupling leg is turned on, this time as the flow of current in Figure 7C Indicated. DC fault currents magnetically coupled by way of a storage capacitor C 1 , C 2 And C 3 They are charged in parallel. After the charging is completed rapidly, the flow restrictor winding current drops to zero, saturation recovery column around the core, the overall resistance value of the flow restrictor decreases rapidly to a smoothing reactor size, it does not affect the normal operation of the system, this limit current flow path as in the current Figure 7D Indicated. Fault current flowing into the coupling branch of the flow restrictor, greatly reduces the DC circuit breaker and the energy absorption during overvoltage.
 3 submodules storage capacitor C 1 , C 2 And C 3 After the charging is completed, the DC circuit breaker reclosing operation, if no fault is detected still exists, the conduction 3 submodules mechanical switch S 1 S 2 S 3 3 submodules by absorbing resistor R 1 R 2 R 3 Energy flow path, this time as the flow of current in Figure 7e Indicated. Detected fault persists if the DC circuit breaker reclosing, the second thyristor is turned on VT 3 submodules 02 , VT 12 , VT 22 , The flow path to limit current flow as Figure 7e Indicated. Capacitor C 1 , C 2 , C 3 Series discharge starts to play the role of the current limitation, will further enhance the performance of the second flow restrictor. After a second DC breaker breaking operation, removal of the fault current.
 Full work can magnetically saturated core fault current limiter of the DC secondary current limitation flowchart as Figure 8 Indicated.
 In this embodiment, the current limiting effect of three restrictor (SAFCL, TFCL, 100mH smoothing reactor) of such Figure 9 Indicated. A magnetic saturation of the core may be a DC fault current limiter (Secondary active limiting faultcurrent limiter, SAFCL) of the secondary active current limiting and current limiting performance of conventional DC magnetic saturation of the core flow restrictor (traditionalfault current limiter, TFCL) quite strong in 100mH smoothing reactor 23.1%. Its second current limiting performance is stronger than the conventional magnetic core saturation current flow restrictor 33.2% stronger than the smoothing reactor 100mH 48.6%. When three kinds of the flow restrictor with the use of the DC circuit breaker, the absorption of energy such as a DC breaker Figure 10 Shown, when magnetically saturated core fault current limiter of the DC secondary current limitation mating suction DC breaker can be reduced as compared to 72.7% for the first time. When used with a DC breaker than conventional magnetic saturation of the core flow restrictor second DC energy absorption decreased 92% when compared with the ratio of smoothing reactors, the second energy absorption is reduced 71.4%. When three kinds of the flow restrictor with the DC circuit breaker used, such as over-voltage on the DC circuit breaker Figure 11 Indicated. When magnetically saturated core fault current limiter of the DC secondary current limitation fit, over-voltage of the DC circuit breaker is reduced as compared to 7.4% for the first time. When used with a DC breaker than conventional magnetic saturation of the core flow restrictor DC overvoltage second reduced 18.4%, and the ratio with the smoothing reactor, the second energy absorption is reduced 14.1%.
 Only for the first DC breaker breaking, the magnetically saturated core fault current limiter of the DC secondary current limitation compared to the conventional DC magnetic saturation of the core flow restrictor with the DC circuit breaker, reducing the over-voltage circuit breaker peak 11.9%, 71.1% reduced energy absorption. Compared to the smoothing reactor with the DC circuit breaker, the circuit breaker reduces the overvoltage peak of 17.7%, 40.5% reduced energy absorption.
 Finally, the design may be a 220V capacity magnetic saturation current limitation of the secondary core fault current limiter current prototype, results of the experiment Figure 12 As shown in seen from the figure, the fault current limiter according to the present embodiment includes a secondary active current limiting capability, which second limiting capacity compared to the ability to upgrade the first flow restrictor 26%.
 In this embodiment, the secondary magnetically active current limiting of saturated core fault current limiter capable of exhibiting excellent current limiting performance at the time of breaking the first DC breaker, circuit breakers greatly reduce energy absorption, and can overvoltage peak reduced to some extent. When the DC breaker while reclosing to permanent fault current limitation, the second lift limiting performance while further reducing the electrical stresses second DC breaker of breaking.
 Above is merely a preferred embodiment of the present invention, and not intended to limit the scope of embodiments of the present invention, those skilled in the art, who should be able to use the description appreciated that the present invention is made and equivalents obvious variations the resulting solutions are intended to be included within the scope of the present invention.