Hybrid excitation type saturated core ac fault current limiter

Abstract

This invention relates to the field of power system fault protection technology, and particularly to a hybrid excitation type saturated iron core AC fault current limiter, comprising: a closed-loop 9-limb iron core, which adopts a three-phase three-limb equivalent structure and has a permanent magnet embedded inside; first and second AC coil groups, both wound on the closed-loop 9-limb iron core, with one end of the first AC coil group connected to one end of the AC power supply group and the other end connected to one end of the second AC coil group; first and second DC coil groups, both wound on the closed-loop 9-limb iron core, with one end of the first DC coil group connected to the positive terminal of the DC power supply group and the other end connected to one end of the second DC coil group, and the other end of the second DC coil group connected to the negative terminal of the DC power supply group; and a transient component energy discharge circuit group, one end of which is connected to the other end of the second AC coil group and the other end of which is connected to the other end of the AC power supply group. This effectively reduces the impact of the DC component of the short-circuit current on the normal operation of the fault current limiter.

Description

Technical Field

[0001] This invention relates to the field of power system fault protection technology, and in particular to a hybrid excitation type saturated core AC fault current limiter, also known as a three-phase transient energy drain type saturated core fault current limiter (TTEDFCL). Background Technology

[0002] However, with the expansion of new power grids and the increase in power generation capacity, the probability of system short-circuit faults and the level of fault currents have surged. Short-circuit problems in three-phase systems are mainly classified into four types: single-phase-to-ground short circuits, two-phase-to-ground short circuits, two-phase-to-phase short circuits, and three-phase short circuits. In actual high-voltage AC transmission technology (HVAC) systems, the transient fault current consists of a superposition of a steady-state component (Short circuit current steady component, SCSC) and a transient component (Short circuit current transient component, SCTC). Furthermore, the presence of SCTC can increase the breaking burden on circuit breakers and cause structural damage, as well as DC bias in transformers and protection malfunctions. This poses a threat to the stability and safety of the power grid. Therefore, limiting the short-circuit current in HVAC systems has become a significant challenge.

[0003] Saturated Core Fault Current Limiter (SCFCL) is the most effective measure for limiting short-circuit current in HVAC systems, offering significant advantages such as fast response, high reliability, and a large impedance ratio. However, in limiting the total short-circuit current containing SCTC, while SCFCL can limit the amplitude of SCTC, it increases the time constant of SCTC and may even exacerbate the zero-point drift of the current. Furthermore, the superimposed short-circuit current from SCTC may cause the SCFCL core on the desaturated side to enter a counter-clockwise saturation state, making electromagnetic coupling between the AC and DC coils difficult and weakening the current-limiting effect. Even worse, the DC fault flux generated by SCTC can cause the permanent magnet to demagnetize counter-clockwise, potentially leading to irreversible permanent demagnetization, severely threatening the stability of the permanent magnet and affecting the lifespan of the SCFCL. Summary of the Invention

[0004] This invention provides a hybrid excitation type saturated iron core type AC fault current limiter, which can effectively reduce the impact of the DC component of short-circuit current on the normal operation of the fault current limiter.

[0005] This invention provides a hybrid excitation type saturated iron core AC fault current limiter, comprising: a closed-loop 9-limb iron core, wherein the closed-loop 9-limb iron core adopts a three-phase three-limb equivalent structure and has a permanent magnet embedded inside; a first AC coil group and a second AC coil group, wherein the first AC coil group and the second AC coil group are wound on the closed-loop 9-limb iron core, and the first end of the first AC coil group is connected to the first end of the target AC power supply group, and the second end of the first AC coil group is connected to the first end of the second AC coil group; a first DC coil group and a second DC coil group, wherein the first DC coil group and the second DC coil group are wound on the closed-loop 9-limb iron core, and the first end of the first DC coil group is connected to the positive terminal of the target DC power supply group, the second end of the first DC coil group is connected to the first end of the second DC coil group, and the second end of the second DC coil group is connected to the negative terminal of the target DC power supply group; and a transient component energy dissipation circuit group, wherein the first end of the transient component energy dissipation circuit group is connected to the second end of the second AC coil group, and the second end of the transient component energy dissipation circuit group is connected to the second end of the AC power supply group.

[0006] Optionally, the closed-loop 9-column core includes a left outer air gap core column, a first left inner core column, a second left inner core column, a third left inner core column, a middle core column, a third right inner core column, a second right inner core column, a first right inner core column, and a right outer air gap core column arranged at intervals. The upper and lower sides of the left outer air gap core column, the first left inner core column, the second left inner core column, the third left inner core column, the middle core column, the third right inner core column, the second right inner core column, the first right inner core column, and the right outer air gap core column are respectively provided with an upper transverse yoke and a lower transverse yoke. The permanent magnet is embedded in the middle of the middle core column.

[0007] Optionally, both the first AC coil group and the second AC coil group include three AC coils. Each first AC coil is wound around the upper end of the first left inner core, the second left inner core, and the third left inner core, and each second AC coil is wound around the upper end of the third right inner core, the second right inner core, and the first right inner core.

[0008] Optionally, both the first DC coil group and the second DC coil group include three DC coils. Each first DC coil is wound around the lower end of the first left inner core, the second left inner core, and the third left inner core, and each second DC coil is wound around the lower end of the third right inner core, the second right inner core, and the first right inner core.

[0009] Optionally, the first left inner core, the second left inner core, and the third left inner core are wound counterclockwise by AC / DC coils, while the third right inner core, the second right inner core, and the first right inner core are wound in the same direction by AC / DC coils.

[0010] Optionally, the transient component energy dissipation circuit group includes three transient component energy dissipation circuits, each of which includes a first switching transistor, a second switching transistor, a DC blocking capacitor, and a dissipation resistor.

[0011] The first end of the first switch and the second switch connected in parallel counterclockwise is connected to the second end of their corresponding second AC coil. The second end of the first switch and the second switch connected in parallel counterclockwise is connected to the first end of the energy-dissipating resistor. The second end of the energy-dissipating resistor is connected to the second end of its corresponding AC power supply. The first end of the DC blocking capacitor is connected to the second end of its corresponding second AC coil. The second end of the DC blocking capacitor is connected to the second end of its corresponding AC power supply.

[0012] Optionally, the target AC power supply group includes three AC power supplies, with the first end of each AC power supply connected to its corresponding first AC coil, and the second end of each AC power supply connected to the second end of its corresponding transient component energy dissipation circuit.

[0013] Optionally, the target DC power supply group includes three DC power supplies, with the positive terminal of each DC power supply connected to the first end of its corresponding first DC coil, and the negative terminal of each DC power supply connected to the second end of its corresponding second DC coil.

[0014] Optionally, the magnetic flux generated by the permanent magnet is clockwise in the left side core of the closed-loop 9-pillar iron core and counterclockwise in the right side core of the closed-loop 9-pillar iron core.

[0015] Optionally, the first left inner core column and the first right inner core column are in the same phase A, the second left inner core column and the second right inner core column are in the same phase B, and the third left inner core column and the third right inner core column are in the same phase C. Each AC coil and each DC coil are connected in series in the inner core column of the same phase. The AC magnetomotive force and the DC magnetomotive force are in the same direction in one side column and in the counterclockwise direction in the other side column.

[0016] The hybrid excitation type saturated iron core type AC fault current limiter proposed in this invention has a small inductance value under normal conditions, which has no impact on the normal operation of the DC system and can be used as a substitute for conventional smoothing reactors. When a short circuit fault occurs in the high-voltage AC power system, the fault current surges, and the impedance of the current limiter automatically increases, suppressing the four types of short circuit currents in the three phases, limiting the amplitude of the transient component of the short circuit current and accelerating the decay of the transient component of the short circuit current, thereby effectively reducing the impact of the DC component of the short circuit current on the normal operation of the fault current limiter.

[0017] Additional aspects and advantages of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. Attached Figure Description

[0018] The above and / or additional aspects and advantages of the present invention will become apparent and readily understood from the following description of the embodiments taken in conjunction with the accompanying drawings, wherein:

[0019] Figure 1 This is a schematic diagram of a hybrid excitation type saturated iron core type AC fault current limiter according to an embodiment of the present invention;

[0020] Figure 2 This is a topology diagram of a hybrid excitation type saturated iron core type AC fault current limiter provided according to an embodiment of the present invention;

[0021] Figure 3 This is a schematic diagram of the equivalent magnetic circuit model of a hybrid excitation type saturated iron core type AC fault current limiter according to an embodiment of the present invention;

[0022] Figure 4 The above is a schematic diagram of the equivalent circuit model of a hybrid excitation type saturated iron core AC fault current limiter according to an embodiment of the present invention, wherein (a) is the overall equivalent circuit diagram, (b) is the circuit model of the hybrid excitation type saturated iron core AC fault current limiter under normal conditions, (c) is the circuit model of the hybrid excitation type saturated iron core AC fault current limiter under the positive half-cycle of the fault, (d) is the circuit model of the hybrid excitation type saturated iron core AC fault current limiter under the negative half-cycle of the fault, and (e) is the circuit model of the hybrid excitation type saturated iron core AC fault current limiter under the fault recovery state;

[0023] Figure 5The magnetic induction intensity distribution diagram of a hybrid excitation type saturated iron core AC fault current limiter according to an embodiment of the present invention is shown in the following diagrams: (a) is a steady-state magnetic field cloud diagram of the hybrid excitation type saturated iron core AC fault current limiter; (b) is a cross-sectional view of the hybrid excitation type saturated iron core AC fault current limiter; (c) is a single-phase short-circuit grounding magnetic field cloud diagram of the hybrid excitation type saturated iron core AC fault current limiter; (d) is a two-phase grounding short-circuit magnetic field cloud diagram of the hybrid excitation type saturated iron core AC fault current limiter; (e) is a two-phase phase-to-phase short-circuit magnetic field cloud diagram of the hybrid excitation type saturated iron core AC fault current limiter; and (f) is a three-phase grounding short-circuit magnetic field cloud diagram of the hybrid excitation type saturated iron core AC fault current limiter.

[0024] Figure 6 A schematic diagram of the magnetic induction intensity characteristics of the core of a hybrid excitation type saturated iron core AC fault current limiter according to an embodiment of the present invention;

[0025] Figure 7 The above is a comparison diagram of the current limiting effect of a hybrid excitation type saturated iron core type AC fault current limiter on four types of faults in a three-phase system according to an embodiment of the present invention. Among them, (a) is the current limiting effect diagram of a single-phase short circuit fault, (b) is the current limiting effect diagram of a two-phase ground short circuit fault, (c) is the current limiting effect diagram of a phase-to-phase short circuit fault, and (d) is the current limiting effect diagram of a three-phase short circuit fault.

[0026] Figure 8 The present invention provides a schematic diagram of the RC branch current of a hybrid excitation type saturated iron core type AC fault current limiter, wherein (a) is a waveform diagram of the RC branch current during a single-phase short circuit fault, (b) is a waveform diagram of the RC branch current during a two-phase ground short circuit fault, (c) is a waveform diagram of the RC branch current during a phase-to-phase short circuit fault, and (d) is a waveform diagram of the RC branch current during a three-phase short circuit fault. Detailed Implementation

[0027] Embodiments of the present invention are described in detail below, examples of which are illustrated in the accompanying drawings, wherein the same or similar reference numerals denote the same or similar elements or elements having the same or similar functions throughout. The embodiments described below with reference to the accompanying drawings are exemplary and intended to explain the present invention, and should not be construed as limiting the present invention.

[0028] The following describes an embodiment of the hybrid excitation type saturated iron core type AC fault current limiter with reference to the accompanying drawings.

[0029] Figure 1 This is a schematic diagram of a hybrid excitation type saturated iron core type AC fault current limiter provided in an embodiment of the present invention.

[0030] like Figure 1As shown, the hybrid excitation type saturated iron core AC fault current limiter includes: a closed-loop 9-column iron core, a first AC coil group, a second AC coil group, a first DC coil group, a second DC coil group, and a transient component energy discharge circuit group.

[0031] The closed-loop 9-column iron core adopts a three-phase, three-column equivalent structure and has embedded permanent magnets. The first and second AC coil groups are wound on the closed-loop 9-column iron core, with the first end of the first AC coil group connected to the first end of the target AC power supply group, and the second end of the first AC coil group connected to the first end of the second AC coil group. The first and second DC coil groups are wound on the closed-loop 9-column iron core, with the first end of the first DC coil group connected to the positive terminal of the target DC power supply group, the second end of the first DC coil group connected to the first end of the second DC coil group, and the second end of the second DC coil group connected to the negative terminal of the target DC power supply group. The first end of the transient component energy dissipation circuit group is connected to the second end of the second AC coil group 3, and the second end of the transient component energy dissipation circuit group is connected to the second end of the AC power supply group. When any of the following faults occurs in the target power system: single-phase ground fault, two-phase ground fault, two-phase phase-to-phase fault, and three-phase ground fault, the sum of the AC flux of the closed-loop 9-column iron core is not zero, and the transient component energy dissipation circuit group is used to dissipate the transient component of the short-circuit current.

[0032] Specifically, such as Figure 1 and 2 As shown, the closed-loop 9-column core includes, from left to right, a left outer air gap core column, a first left inner core column, a second left inner core column, a third left inner core column, a middle core column, a third right inner core column, a second right inner core column, a first right inner core column, and a right outer air gap core column. The upper and lower sides of the left outer air gap core column, the first left inner core column, the second left inner core column, the third left inner core column, the middle core column, the third right inner core column, the second right inner core column, the first right inner core column, and the right outer air gap core column are respectively provided with an upper transverse yoke and a lower transverse yoke. A permanent magnet is embedded in the middle of the middle core column. The permanent magnet can be made of neodymium iron boron, a rare earth permanent magnet material.

[0033] Furthermore, each of the first AC coil group and the second AC coil group includes three AC coils. Each first AC coil is wound counterclockwise on the upper ends of the first left inner core column, the second left inner core column, and the third left inner core column, respectively. Each second AC coil is wound in the same direction on the upper ends of the third right inner core column, the second right inner core column, and the first right inner core column, respectively.

[0034] Furthermore, each of the first and second DC coil groups includes three DC coils. Each first DC coil is wound counterclockwise around the lower ends of the first left inner core, the second left inner core, and the third left inner core, respectively. Each second DC coil is wound in the same direction around the lower ends of the third right inner core, the second right inner core, and the first right inner core, respectively.

[0035] Furthermore, the transient component energy dissipation circuit group includes three transient component energy dissipation circuits. Each transient component energy dissipation circuit includes a first switching transistor D1, a second switching transistor D2, a DC blocking capacitor C, and a dissipation resistor R.

[0036] The first end of the first switch D1 and the second switch D2 connected in parallel counterclockwise is connected to the second end of the corresponding second AC coil. The second end of the first switch D1 and the second switch D2 connected in parallel counterclockwise is connected to the first end of the energy leakage resistor R. The second end of the energy leakage resistor R is connected to the second end of the corresponding AC power supply. The first end of the DC blocking capacitor C is connected to the second end of the corresponding second AC coil. The second end of the DC blocking capacitor C is connected to the second end of the corresponding AC power supply.

[0037] Furthermore, the target AC power supply group includes three AC power supplies, each with its first terminal connected to a line inductor L. line and line resistance R line Connected to its corresponding first AC coil, the second terminal of each AC power supply is connected through a load resistor R. load It is connected to the second terminal of the corresponding transient component energy dissipation circuit.

[0038] Furthermore, the target DC power supply group includes three DC power supplies, each with its positive terminal connected to a current-limiting inductor L. f The first end of the corresponding first DC coil is connected, and the negative terminal of each DC power supply is connected to the second end of the corresponding second DC coil.

[0039] Furthermore, the magnetic flux generated by the permanent magnet is clockwise in the left side of the closed-loop 9-pillar iron core and counterclockwise in the right side of the closed-loop 9-pillar iron core.

[0040] Furthermore, the first left inner core column and the first right inner core column are in the same phase A, the second left inner core column and the second right inner core column are in the same phase B, and the third left inner core column and the third right inner core column are in the same phase C. Each AC coil and each DC coil are connected in series in the inner core column of the same phase. The AC magnetomotive force and the DC magnetomotive force are in the same direction in one side column and in the counterclockwise direction in the other side.

[0041] It should be noted that in the above-mentioned short-circuit current DC component energy-dissipating fault current limiter, the core column is in a critical saturation state under normal grid conditions, that is, at the inflection point of the BH curve of the ferromagnetic material, in order to ensure the speed of core column desaturation during faults.

[0042] According to the above description, the embodiment of the present invention adds a short-circuit current DC component discharge branch to the AC circuit of a traditional hybrid excitation type saturated iron core AC fault current limiter by series electrical coupling. When any of the faults, such as single-phase ground fault, two-phase ground fault, two-phase phase-to-phase fault, and three-phase ground fault, occurs in the target power system, the A, B, and C phase iron cores in the closed-loop 9-column iron core alternately desaturate, and conduct the first and second switching transistors in each transient component discharge circuit to discharge the transient component in the short-circuit current.

[0043] The working principle of the hybrid excitation type saturated iron core type AC fault current limiter proposed in the embodiments of the present invention is as follows:

[0044] like Figure 1 and 2 As shown, under normal operating conditions, the hybrid-excitation type saturated iron-core AC fault current limiter, based on the magnetization characteristics of the iron core material, maintains the dual iron-core columns in a deep saturation state through the generated constant magnetic flux. Under this condition, the equivalent inductance of the winding remains stable, neither affecting the steady-state operation of the AC system nor hindering the flexible adjustment of static reserve parameters. It also functions as a replacement for conventional smoothing impedance devices. When an anomaly occurs in the target power system, the sudden increase in fault current will excite a magnetomotive force with the opposite polarity to the permanent magnet field, causing the dual iron-core columns to alternately exit the saturation state. This magnetic state transition triggers two key responses: firstly, the impedance characteristics of the current-limiting device are autonomously enhanced; secondly, the energy-dissipating branch is rapidly activated through the thyristor trigger circuit. This dual-action mechanism not only effectively suppresses the peak fault current containing a DC component but also significantly accelerates the dissipation process of the transient DC component.

[0045] The specific working process of the three-phase transient energy-dissipating type magnetic saturation short-circuit fault current limiter is as follows:

[0046] (1) The excitation direction of the DC power supply and the magnetomotive force of the permanent magnet are both in the direction of the ground normal to supplement the DC bias magnetomotive force. Under normal system conditions, the cores of phases A, B, and C are deeply saturated. The rated current of the system is small, and the AC flux of the side column only produces small fluctuations, which will not cause the side column to desaturate. At the same time, the thyristors D1 and D2 in the circuit are turned off, and the current flows through capacitor C. The compensation capacitor C can offset part of the line inductive reactance, so that the output inductance of the hybrid excitation type saturated iron core type AC fault current limiter TTEDFCL is very small, which will not affect the system operation stability and is the basis for the subsequent current limiting impedance change multiple. Under normal conditions and subsequent three-phase symmetrical short circuit, the sum of the AC flux of each side of the iron core is 0. The AC flux of the iron core column and the healthy phase on the same side form a magnetic circuit, reaching the state of "magnetic balance". Therefore, the left and right sides of TTEDFCL can each form a magnetic circuit.

[0047] (2) When a fault occurs, the current in the fault phase surges, and the AC magnetomotive force of the fault phase cancels the DC magnetomotive force. The iron core of the fault phase desaturates alternately. At this time, the AC and DC windings are fully electromagnetically coupled, which is equivalent to a single-phase transformer. The current-limiting inductor L f The AC circuit connection serves as a current limiter. When an asymmetrical short-circuit fault occurs, such as a single-phase ground fault, a two-phase short circuit, or a two-phase ground fault, the sum of the AC flux in each core is not zero. Taking a phase A fault as an example, the phase A fault current surges, and the AC flux is much greater than that of phases B and C. A small portion of the AC flux closes through the side posts of phases B and C, causing slight distortion in the currents of phases B and C. However, due to the very short fault duration, the distortion has a negligible impact on the power system. At the moment of the fault, thyristors D1 and D2 are simultaneously triggered and turned on. The transient component of the short-circuit current, SCTC, flows through the energy-draining resistor R, reducing the SCTC time constant and accelerating the decay of the transient short-circuit current.

[0048] In other words, the working process of the three-phase transient energy-dissipating type magnetic saturation short-circuit fault current limiter is as follows:

[0049] (1) In each half cycle, the DC flux of one of the three-phase left working iron core and the three-phase right working iron core is in the same direction as the AC flux, while the DC flux of the other three-phase working iron core is in the opposite direction to the AC flux.

[0050] (2) When the power system is in normal condition, the grid operating current is small, and the DC magnetomotive force generated by the DC excitation power supply and the permanent magnet has an absolute advantage over the magnetomotive force generated by the first AC winding and the second AC winding. During normal operation, the three-phase left working iron core and the three-phase right working iron core are saturated by the combined influence of the DC excitation power supply and the permanent magnet. By adjusting the size of the DC excitation power supply, the saturation degree of the soft magnetic material can be changed, thereby controlling the permeability and smoothly adjusting the AC steady-state output inductance of the fault current limiter to achieve power flow control.

[0051] (3) When a short circuit fault occurs in the power system, the fault current of the system exceeds the rated current. The magnetic flux generated by the first AC winding or the second AC winding is opposite to the magnetic flux generated by the DC excitation power supply and the permanent magnet, causing the left working iron core and the right working iron core of the fault phase to quickly exit saturation alternately. After exiting saturation, the permeability of the working iron core increases, which leads to an increase in the inductance of the AC winding. The AC winding on the working iron core that has exited saturation is electromagnetically coupled with the DC winding. The current limiting inductor in the peripheral circuit of the DC winding will be connected in series with the AC side, and the external inductance of the fault current limiter increases, thereby limiting the short circuit current in a complete cycle.

[0052] (4) When a short circuit fault occurs in the power system, the AC magnetic flux flows through the left outer air gap support, the right outer air gap support, the three-phase left working iron core and the right working iron core, reducing the fault magnetic flux passing through the permanent magnet, which greatly reduces the eddy current loss in the permanent magnet.

[0053] (5) After the fault is cleared, the fault current decreases, the power system returns to normal operation, and the overall impedance value of the fault current limiter decreases.

[0054] The hybrid excitation type saturated iron core type AC fault current limiter proposed in this invention will be further explained below through a specific embodiment.

[0055] like Figure 3 As shown, all components in the magnetic circuit are linear. Therefore, the magnetic circuit parameters can be analyzed using the superposition theorem and the law of electromagnetic induction. By superimposing the AC and DC magnetic fluxes, the magnitude of the magnetic flux flowing through each branch in steady state can be obtained as follows:

[0056] (1)

[0057] In the formula, This represents the magnetic flux of the three-phase iron core on the left. This represents the magnetic flux of the three-phase iron core on the right. For air gap reluctance, For the coercivity of permanent magnets, The length of the permanent magnet. This refers to the number of turns in the DC winding. It is the DC excitation current. For saturated iron core reluctance, It is a permanent magnet reluctance. For the number of turns of the AC winding, For fault phase current, , , , , and Calculate the simplification coefficients for the denominator of the equation. For the magnetic flux of the permanent magnet branch, It represents the magnetic flux of the air gap side column.

[0058] (1) Normal state magnetic circuit analysis of three-phase TTEDFCL

[0059] The magnetic reluctance of each working core in the TTEDFCL system is R under normal operating conditions. es Therefore, the inductance of each phase AC winding can be calculated. for:

[0060] (2)

[0061] Because the saturation reluctance is very large, the output inductance of the AC coil is very small under normal conditions. The TTEDFCL does not affect the normal power flow distribution of the system and has almost no impact on the system stability.

[0062] (2) Current-limited magnetic circuit analysis of three-phase TTEDFCL

[0063] Among three-phase short-circuit faults, single-phase ground faults have the highest probability of occurrence. Taking phase A ground fault as an example, the main consideration is that the time to reach steady state after the short circuit is 3-5 AC cycles. Therefore, the equivalent magnetic circuit can still be regarded as a linear magnetic circuit, satisfying the magnetic circuit superposition theorem.

[0064] At this time, within the full cycle of the short-circuit current, the A, B, and C phase working cores of the TTEDFCL alternately desaturate, and the AC and DC coils on the desaturated working cores undergo electromagnetic coupling, limiting the current inductor L... f Current limiting is applied upon connection. Due to the desaturation of the A-phase core, a fault current is induced in the DC excitation circuit. The current in the A-phase DC excitation circuit is no longer a steady-state constant current I. D It is not the fault current I, but rather the fault current formed by the superposition of the short-circuit current and the original DC excitation current. kD The number of desaturated terminals varies under different fault conditions. Each of the three-phase core terminals can independently maintain a deeply saturated or unsaturated state. Based on the equivalent circuits of the AC and DC windings of each phase, and neglecting the leakage inductance of the DC winding, the inductance presented by TTEDFCL in the faulty k-phase is... for:

[0065] (3)

[0066] in, For the unsaturated iron core permeability, S is the permeability of the saturated iron core. eq l is the equivalent area of ​​the magnetic circuit. eq1 and l eq2 These are the equivalent AC and DC magnetic circuit lengths, respectively.

[0067] In formula (3), the last term is much larger in magnitude than the first term, therefore the impedance change mainly depends on the current-limiting inductor L.f The size can be adjusted by L. f To improve the current limiting effect of the three-phase system.

[0068] like Figure 4 As shown in (a)-(e), taking the current flow path inside terminals 1 and 2 as an example, when the system is running normally, the iron core is deeply saturated, and the excitation impedance Z mA1 Z mA2 The system current flows through the excitation impedance branch, and the leakage impedance Z of the AC coil is relatively small. A1 Z A2 At this point, the current is very small, so the impact of the current limiter on the system is negligible. When a fault occurs, cores I and VI desaturate, and the AC coils, DC excitation coils wound on them, and the core form a transformer, connecting the DC excitation circuit to the system. Current-limiting inductor L... f It is connected to the system to limit short-circuit current.

[0069] The output impedance expression of TTEDFCL is shown in formula (4):

[0070] (4)

[0071] In the formula, For TTEDFCL output inductor, The angular frequency of the power supply. It is a DC blocking capacitor. Imaginary factor For energy dissipation resistor, For time, This is the moment of short circuit.

[0072] It can be obtained that the output impedance of TTEDFCL is related to the discharge resistor R and the current-limiting inductor L. FCL It is related to the compensation capacitor C.

[0073] This embodiment establishes a 500kV / 3kA finite element simulation model. The magnetic induction intensity distribution during normal operation of the TTEDFCL is as follows: Figure 5 As shown. At this time, the core B is greater than 2.0T and is in a deep saturation state.

[0074] like Figure 5 As shown in (a) and (b), the six columns of phases A, B, and C are all in deep saturation. At this time, the permeability of each core is very small, and the TTEDFCL exhibits low impedance in the system, having little impact on the normal operation of the system. Figure 5 As shown in (c)-(f), it can be observed that when a short-circuit fault occurs in the four scenarios, the three-phase cores A, B, and C desaturate alternately. At this time, the permeability of the desaturated core increases, electromagnetic coupling occurs between the AC and DC coils, and the thyristor turns on, L... fWhen connected to the system, the TTEDFCL presents a high impedance in the system, which helps to limit transient current.

[0075] When a fault occurs, such as Figure 5 As shown in (c)-(f), the short-circuit current level continuously surges. The AC magnetomotive force generated by the AC coil, superimposed with SCTC, begins to cancel the DC excitation generated by the DC coil and PM, causing the two cores to alternately desaturate throughout the cycle (gradually falling below the magnetic induction knee point Bs=1.9T). The AC and DC coils on the desaturated side are fully coupled, equivalent to a single-phase transformer, while L... f The connection makes the SEDFCL body exhibit high impedance characteristics to the outside world, so as to limit the transient short-circuit fault current.

[0076] Taking Case 1 as an example, the waveform of the three-phase core magnetic induction intensity B of the TEDFCL is as follows: Figure 6 As shown, with the surge in short-circuit current, core I demagnetization reaches its minimum at the peak of the positive half-cycle. Compared to Cases 1-3, due to the lack of a reasonable inversion process for magnetic circuit parameters, the demagnetizing flux generated by the short-circuit current in Cases 1-2 is 2-4 times greater. The AC-side demagnetizing flux not only cancels out the mixed excitation bias flux but also has a "margin." The larger fault flux places core I in a reverse saturation state (greater than the reverse magnetic induction knee point Bs=1.9T), at -2.03T and -2.07T respectively. During this stage, core I cannot fully couple the AC and DC coils, and the current-limiting inductor cannot be connected in series with the AC side of the HVAC system to limit the short-circuit current, thus weakening the current-limiting capability. Subsequently, in each full cycle, as the short-circuit current decays, the distribution characteristics of B also show a decaying trend. Cases 3-5, after the design process, avoid the reverse saturation process of core I.

[0077] like Figure 7 As shown in (a)-(b), before t=50ms, the three-phase current waveforms of TTEDFCL and without TTEDFCL are basically the same, with an effective value of 2.32kA, which verifies that TTEDFCL does not affect the normal operation of the HVAC system in steady state.

[0078] like Figure 7As shown in (a), when a short circuit occurs in phase A of the system without a TTEDFCL, the maximum short-circuit current (first peak) is 77.80 kA. After the system transitions to a new steady state, the short-circuit current amplitude is 44.47 kA. After installing a TTEDFCL without an EDB, the maximum short-circuit current and the new steady-state amplitude in phase A are 59.73 kA and 31.81 kA, respectively, with transient and steady-state current limiting coefficients of 76.77% and 71.53%, respectively. Taking Case 1 as an example, after installing a TTEDFCL with an EDB, the maximum short-circuit current and the new steady-state amplitude in phase A are 54.78 kA and 24.79 kA, respectively, with transient and steady-state current limiting coefficients of 70.41% and 55.75%, respectively. The currents in phases B and C do not change significantly.

[0079] like Figure 7 As shown in (b), when a two-phase-to-ground short circuit occurs in the system without a TTEDFCL, the maximum value and new steady-state amplitude of the phase A short-circuit current are 77.80 kA and 44.47 kA, respectively, and the maximum value and new steady-state amplitude of the phase B short-circuit current are 46.29 kA and 44.66 kA, respectively. After installing a TTEDFCL without EDB, the maximum value and new steady-state amplitude of the phase A short-circuit current are 61.09 kA and 34.32 kA, respectively, with transient current limiting coefficients and steady-state current limiting coefficients of 78.52% and 77.18%, respectively; the maximum value and new steady-state amplitude of the phase B short-circuit current are 37.49 kA and 34.15 kA, respectively, with transient current limiting coefficients and steady-state current limiting coefficients of 80.99% and 76.47%, respectively. Taking Case 1 as an example, after adding the TTEDFCL with EDB, the maximum short-circuit current and the new steady-state amplitude of phase A are 54.56kA and 24.94kA, respectively, with transient current limiting coefficients and steady-state current limiting coefficients of 70.41% and 55.75%, respectively; the maximum short-circuit current and the new steady-state amplitude of phase B are 32.94kA and 24.80kA, respectively, with transient current limiting coefficients and steady-state current limiting coefficients of 71.16% and 71.53%, respectively. The phase C current, however, shows little change.

[0080] like Figure 7As shown in (c), when a phase-to-phase short circuit occurs between phases A and B in a system without a TTEDFCL, since the current waveforms of phases A and B are the same but in opposite directions, only the current limiting effect of phase A is analyzed. The maximum short-circuit current and the new steady-state amplitude of phase A are 57.36kA and 39.58kA, respectively. After installing a TTEDFCL without EDB, the maximum short-circuit current and the new steady-state amplitude of phase A are 43.42kA and 25.07kA, respectively, with transient current limiting coefficients and steady-state current limiting coefficients of 75.70% and 63.34%, respectively. Taking Case 1 as an example, after installing a TTEDFCL with EDB, the maximum short-circuit current and the new steady-state amplitude of phase A are 40.69kA and 22.26kA, respectively, with transient current limiting coefficients and steady-state current limiting coefficients of 70.94% and 56.24%, respectively. Therefore, the situation of phase B is the same as that of phase A, while the current of phase C does not change much.

[0081] like Figure 7 As shown in (d), when a three-phase short circuit occurs in the system without TTEDFCL, the maximum value and new steady-state amplitude of the phase A short circuit current are 77.80kA and 44.47kA, respectively; the maximum value and new steady-state amplitude of the phase B short circuit current are 46.29kA and 44.66kA, respectively; and the maximum value and new steady-state amplitude of the phase C short circuit current are 73.74kA and 44.37kA, respectively. After installing a TTEDFCL without EDB, the maximum short-circuit current and the new steady-state amplitude of phase A are 60.53kA and 34.02kA, respectively, with transient and steady-state current limiting coefficients of 77.80% and 76.50%, respectively; the maximum short-circuit current and the new steady-state amplitude of phase B are 37.56kA and 33.76kA, respectively, with transient and steady-state current limiting coefficients of 81.14% and 75.59%, respectively; and the maximum short-circuit current and the new steady-state amplitude of phase C are 57.03kA and 33.01kA, respectively, with transient and steady-state current limiting coefficients of 77.34% and 74.40%, respectively. Taking Case 1 as an example, after adding the TTEDFCL with EDB, the maximum short-circuit current and the new steady-state amplitude of phase A are 54.09kA and 24.33kA, respectively, with transient current limiting coefficients and steady-state current limiting coefficients of 69.52% and 54.71%, respectively; the maximum short-circuit current and the new steady-state amplitude of phase B are 33.10kA and 25.09kA, respectively, with transient current limiting coefficients and steady-state current limiting coefficients of 71.51% and 56.18%, respectively; and the maximum short-circuit current and the new steady-state amplitude of phase C are 51.53kA and 24.59kA, respectively, with transient current limiting coefficients and steady-state current limiting coefficients of 69.88% and 55.42%, respectively.

[0082] like Figure 8As shown in (a)-(d) of the diagram, the capacitor branch current of the TTEDFCL is mainly composed of the steady-state component of short-circuit current (SCSC), with a low SCTC content. Before the fault occurs, the bidirectional thyristor is turned off, and the rated current of the system flows only through the capacitor branch. After the fault occurs, the thyristor is triggered to conduct, and the transient process of the C branch current reaches a new steady state after slight oscillations. The decay process of SCTC is not constant but time-varying, and its magnitude is related to the rate of change of SCTC. After several cycles, this part of SCTC decays to 0, while SCSC changes steadily. The resistor branch current of the TTEDFCL mainly consists of SCTC and a small amount of SCSC. In steady state, the thyristor is turned off, and there is no current in the R branch. After a short circuit, the thyristor conducts, and the DC current in the R branch increases rapidly. The short-circuit current surges instantaneously and then rapidly oscillates and decays. After several cycles, the SCTC content is greatly consumed, finally reaching a new steady state with a low SCSC content.

[0083] In summary, the hybrid excitation type saturated iron core type AC fault current limiter proposed according to the embodiments of the present invention has the following beneficial effects:

[0084] (1) Using a saturated iron core and permanent magnet to generate variable reactance, this method has a better effect on fault current compared with traditional reactors;

[0085] (2) The topology with added energy dissipation branch has strong effectiveness and rationality, and has little impact on the current limiting performance of the traditional hybrid excitation type saturated iron core type AC fault current limiter;

[0086] (3) During normal operation, it can play the role of series compensation, which can improve the static stability and limit transmission power of the system, increase the static stability reserve coefficient, and ensure the stability of the AC transmission system.

[0087] (4) A bidirectional thyristor detection and control system is added so that the current limiter can achieve automatic and rapid response throughout the entire working process;

[0088] (5) Compared with the traditional magnetically saturated iron core type fault current limiter, it can make the DC component of the fault current in the AC system decay faster and alleviate the negative impact of the DC component of the fault current on various power equipment.

[0089] In the description of this specification, the references to terms such as "one embodiment," "some embodiments," "example," "specific example," or "some examples," etc., indicate that a specific feature, structure, material, or characteristic described in connection with that embodiment or example is included in at least one embodiment or example of the present invention. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples. Moreover, without contradiction, those skilled in the art can combine and integrate the different embodiments or examples described in this specification, as well as the features of different embodiments or examples.

[0090] Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of indicated technical features. Thus, a feature defined as "first" or "second" may explicitly or implicitly include at least one of that feature. In the description of this invention, "N" means at least two, such as two, three, etc., unless otherwise explicitly specified.

Claims

1. A hybrid excitation type saturated iron core type AC fault current limiter, characterized in that, include: A closed-loop 9-column iron core is provided, which adopts a three-phase three-column equivalent structure and has a permanent magnet embedded inside. The closed-loop 9-column iron core includes a left outer air gap core column, a first left inner core column, a second left inner core column, a third left inner core column, a middle core column, a third right inner core column, a second right inner core column, a first right inner core column, and a right outer air gap core column, which are arranged at intervals. The upper and lower sides of the left outer air gap core column, the first left inner core column, the second left inner core column, the third left inner core column, the middle core column, the third right inner core column, the second right inner core column, the first right inner core column, and the right outer air gap core column are respectively provided with an upper transverse yoke and a lower transverse yoke. The permanent magnet is embedded in the middle of the middle core column. A first AC coil group and a second AC coil group are wound on the closed-loop 9-column iron core. The first end of the first AC coil group is connected to the first end of the target AC power supply group, and the second end of the first AC coil group is connected to the first end of the second AC coil group. Each of the first and second AC coil groups includes three AC coils. Each first AC coil is wound on the upper end of the first left inner core column, the second left inner core column, and the third left inner core column. Each second AC coil is wound on the upper end of the third right inner core column, the second right inner core column, and the first right inner core column. A first DC coil group and a second DC coil group are wound on the closed-loop 9-column iron core. The first end of the first DC coil group is connected to the positive terminal of the target DC power supply group, the second end of the first DC coil group is connected to the first end of the second DC coil group, and the second end of the second DC coil group is connected to the negative terminal of the target DC power supply group. Each of the first and second DC coil groups includes three DC coils. Each first DC coil is wound on the lower end of the first left inner core column, the second left inner core column, and the third left inner core column. Each second DC coil is wound on the lower end of the third right inner core column, the second right inner core column, and the first right inner core column. A transient component energy dissipation circuit group, wherein the first terminal of the transient component energy dissipation circuit group is connected to the second terminal of the second AC coil group, and the second terminal of the transient component energy dissipation circuit group is connected to the second terminal of the AC power supply group.

2. The hybrid excitation type saturated iron core type AC fault current limiter according to claim 1, characterized in that, The first left inner core, the second left inner core, and the third left inner core are wound counterclockwise by AC / DC coils, while the third right inner core, the second right inner core, and the first right inner core are wound in the same direction by AC / DC coils.

3. The hybrid excitation type saturated iron core type AC fault current limiter according to claim 1, characterized in that, The transient component energy dissipation circuit group includes three transient component energy dissipation circuits. Each transient component energy dissipation circuit includes a first switching transistor, a second switching transistor, a DC blocking capacitor, and an energy dissipation resistor. The first end of the first switch and the second switch connected in parallel counterclockwise is connected to the second end of their corresponding second AC coil. The second end of the first switch and the second switch connected in parallel counterclockwise is connected to the first end of the energy-dissipating resistor. The second end of the energy-dissipating resistor is connected to the second end of its corresponding AC power supply. The first end of the DC blocking capacitor is connected to the second end of its corresponding second AC coil. The second end of the DC blocking capacitor is connected to the second end of its corresponding AC power supply.

4. The hybrid excitation type saturated iron core type AC fault current limiter according to claim 3, characterized in that, The target AC power supply group includes three AC power supplies. The first end of each AC power supply is connected to its corresponding first AC coil, and the second end of each AC power supply is connected to the second end of its corresponding transient component energy dissipation circuit.

5. The hybrid excitation type saturated iron core type AC fault current limiter according to claim 1, characterized in that, The target DC power supply group includes three DC power supplies. The positive terminal of each DC power supply is connected to the first end of its corresponding first DC coil, and the negative terminal of each DC power supply is connected to the second end of its corresponding second DC coil.

6. The hybrid excitation type saturated iron core type AC fault current limiter according to claim 1, characterized in that, The magnetic flux generated by the permanent magnet is clockwise in the left side core of the closed-loop 9-column iron core and counterclockwise in the right side core of the closed-loop 9-column iron core.

7. The hybrid excitation type saturated iron core type AC fault current limiter according to claim 1, characterized in that, The first left inner core column and the first right inner core column are in the same phase A, the second left inner core column and the second right inner core column are in the same phase B, and the third left inner core column and the third right inner core column are in the same phase C. Each AC coil and each DC coil are connected in series in the inner core column of the same phase. The AC magnetomotive force and the DC magnetomotive force are in the same direction in one side column and in the counterclockwise direction in the other side column.

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