DC current limiter and DC fault protection system

Abstract

The invention provides a DC current limiter and a DC fault protection system, and the DC current limiter comprises a first current limiting inductor which is connected with an input end; the first end of the solid resistance assembly is connected with the first current-limiting inductor; the second current-limiting inductor is respectively connected with the second end and the output end of the solid reactor assembly; the current switching circuit comprises a first switching branch which is bridged between the input end and the second end and comprises a first diode and a first resistor which are connected in series; wherein the cathode of the first diode is connected with the input end; the second switching branch is bridged between the output end and the first end and comprises a second diode and a second resistor which are connected in series; wherein the cathode of the second diode is connected with the first end; wherein in the first working state, the first diode and the second diode are reversely biased and cut off; and in the second working state, the first diode and the second diode are positively biased to be conducted.

Description

Technical Field

[0001] This invention relates to the field of current fault protection technology, and in particular to a DC current limiter and a DC fault protection system. Background Technology

[0002] With the rapid development of high-voltage flexible direct current (HVDC) transmission technology, its advantages such as high stability, strong flexibility, and low loss have led to its widespread application in long-distance, high-capacity power transmission. However, flexible DC systems exhibit inherent characteristics of "low impedance and low inertia," which means that once a DC-side short-circuit fault occurs, the fault current will rise sharply to more than ten times the rated current in a very short time (usually a few milliseconds), posing a severe threat and impact on key equipment such as converters, cables, and circuit breakers in the entire system.

[0003] In related technologies, a common approach is to use inductive elements to suppress the rapid rise of current in the early stages of a fault by using a large equivalent inductance. However, after the fault current is limited by the inductive element and enters the decay phase after being interrupted by the circuit breaker, the current limiting device may still maintain a high equivalent inductance, thereby suppressing the decay rate of the fault current, prolonging the duration of the fault current, increasing the energy absorption requirements of the DC circuit breaker and its surge arrester, and increasing the risk of overcurrent stress on the power electronic devices in the system. Summary of the Invention

[0004] This invention provides a DC current limiter and a DC fault protection system to address the shortcomings of existing technologies where, after the fault current is limited by inductive elements and enters the decay stage through circuit breaker interruption, the current limiting device may still maintain a high equivalent inductance, thereby suppressing the decay rate of the fault current and increasing the risk of overcurrent stress on power electronic devices in the system.

[0005] This invention provides a DC current limiter, comprising an input terminal and an output terminal, including a first current-limiting inductor connected to the input terminal; a solid-state reactance component, the first terminal of which is connected to the first current-limiting inductor; a second current-limiting inductor connected to the second terminal and the output terminal of the solid-state reactance component; and a current switching circuit, comprising: a first switching branch connected between the input terminal and the second terminal, including a first diode and a first resistor connected in series; wherein the cathode of the first diode is connected to the input terminal; and a second switching branch connected between the output terminal and the first terminal, including a second diode and a second resistor connected in series; wherein the cathode of the second diode is connected to the first terminal; wherein, in a first operating state, the first diode and the second diode are reverse-biased and cut off; and in a second operating state, the first diode and the second diode are forward-biased and conduct.

[0006] According to the DC current limiter provided by the present invention, the first diode and the second diode are fast recovery diodes.

[0007] According to the DC current limiter provided by the present invention, the resistance values ​​of the first resistor and the second resistor are equal.

[0008] The DC current limiter provided by the present invention further includes an iron core; the iron core includes: a first side post, a middle post, and a second side post arranged in parallel; a permanent magnet disposed on an iron yoke for connecting the middle post and the second side post; the first current limiting inductor includes a first current limiting winding wound on the first side post; the second current limiting inductor includes a second current limiting winding wound on the first side post; wherein the first current limiting winding and the second current limiting winding are adapted to generate magnetic flux opposite to the direction of the permanent magnet.

[0009] According to the DC current limiter provided by the present invention, the solid reactance component includes a first decoupling winding and a second decoupling winding connected in series and wound on the second side post; wherein the magnetic flux generated by the first decoupling winding and the second decoupling winding are opposite in direction and equal in magnitude.

[0010] According to the DC current limiter provided by the present invention, the middle column is provided with an air gap.

[0011] According to the DC current limiter provided by the present invention, the first current limiting winding and the second current limiting winding are configured such that the magnetomotive force generated is less than the magnetomotive force generated by the permanent magnet.

[0012] According to the DC current limiter provided by the present invention, the magnetic flux generated by the first current limiting winding and the second current limiting winding is equal in magnitude.

[0013] The present invention also provides a DC fault protection system, comprising: a DC current limiter as described in any of the preceding claims; a DC circuit breaker connected to the input terminal of the DC current limiter, adapted to perform a breaking action when the DC current meets a preset condition; wherein the DC current limiter enters a second operating state in response to the breaking action of the DC circuit breaker.

[0014] According to the DC fault protection system provided by the present invention, the DC circuit breaker includes a surge arrester; the surge arrester is connected in parallel across the two ends of the breaking unit of the DC circuit breaker to conduct when the DC circuit breaker performs a breaking action, thereby limiting the breaking overvoltage.

[0015] The DC current limiter and DC fault protection system provided by this invention are in a first operating state when a fault current is input. The first and second diodes are reverse-biased and cut off, and the fault current flows through the first current-limiting inductor, the solid reactance component, and the second current-limiting inductor, thereby limiting the rise of the fault current. When the DC circuit breaker performs an interruption action, causing a change in the voltage relationship at the DC current limiter ports, and consequently forward-biasing the first and second diodes, the DC current limiter enters a second operating state. The first switching branch and the second switching branch are connected, transforming the DC current limiter from a series equivalent state to a parallel equivalent state. Two loop current paths are formed inside the current limiter, causing the fault current to be shunted in these loop current paths and dissipated through the first and second resistors, thereby accelerating the attenuation of the fault current and shortening its duration. Attached Figure Description

[0016] To more clearly illustrate the technical solutions in this invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of this invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0017] Figure 1 This is a circuit diagram of a DC current limiter according to an embodiment of the present invention; Figure 2 This is a schematic diagram of a DC current limiter in a first operating state according to an embodiment of the present invention; Figure 3 This is a schematic diagram of a DC current limiter in a second operating state according to an embodiment of the present invention; Figure 4 This is a schematic diagram of a DC current limiter according to an embodiment of the present invention; Figure 5 This is a schematic diagram showing the calculated inductance of the first current-limiting winding; Figure 6 This is a schematic diagram showing the calculated inductance of the first decoupling winding; Figure 7 This diagram illustrates the comparison of the fault current limiting effects of three different current limiting devices. Detailed Implementation

[0018] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions of this invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of this invention. All other embodiments obtained by those skilled in the art based on the embodiments of this invention without creative effort are within the scope of protection of this invention.

[0019] In related technologies, after the fault current is limited by inductive elements and enters the decay stage through the circuit breaker, the current limiting device may still maintain a high equivalent inductance, which increases the risk of power electronic devices in the system being subjected to overcurrent stress.

[0020] In view of this, the present invention provides a DC current limiter.

[0021] Figure 1 This is a circuit diagram of a DC current limiter according to an embodiment of the present invention; Figure 2 This is a schematic diagram of a DC current limiter in a first operating state according to an embodiment of the present invention; Figure 3 This is a schematic diagram of a DC current limiter in its second operating state according to an embodiment of the present invention.

[0022] like Figure 1-3 As shown, the DC current limiter has an input terminal and an output terminal, including a first current limiting inductor L1, a second current limiting inductor L2, a solid reactance component, and a current switching circuit.

[0023] The first current-limiting inductor L1 is connected to the input terminal. A solid-state reactance assembly has its first terminal connected to the first current-limiting inductor L1. The second current-limiting inductor L2 is connected to both the second terminal and the output terminal of the solid-state reactance assembly.

[0024] The current switching circuit includes: a first switching branch connected between the input terminal and the second terminal, comprising a first diode D1 and a first resistor R connected in series. D1 In this circuit, the cathode of the first diode D1 is connected to the input terminal. The second switching branch, bridging the output and first terminals, includes a second diode D2 and a second resistor R connected in series. D2 In this configuration, the cathode of the second diode D2 is connected to the first terminal.

[0025] In the first operating state, the first diode D1 and the second diode D2 are reverse biased and cut off; in the second operating state, the first diode D1 and the second diode D2 are forward biased and conduct.

[0026] According to an embodiment of the present invention, when the current input to the DC current limiter is in a normal state or when a fault current is input to the input terminal, the DC current limiter is in a first operating state. At this time, the first diode D1 and the second diode D2 are reverse biased and cut off, and the current flows through the first current limiting inductor L1, the solid reactance component, and the second current limiting inductor L2 in sequence. In the event of a fault current, due to the sudden increase in the input current, the first current limiting inductor L1 and the second current limiting inductor L2 can provide inductive impedance to suppress the rapid change of current and limit the rise of the fault current. When the fault current at the input terminal begins to decay, the first current limiting inductor L1 and the second current limiting inductor L2, in order to suppress the decay of the current, cause the DC current limiter to enter a second operating state. The first diode D1 and the second diode D2 are forward biased and conduct, thereby generating two parallel internal circulating currents i. a and i b , where i a The current flows along the path of the first current-limiting inductor L1 - solid reactance assembly - first switching branch, i b The current flows along the path of the second current-limiting inductor L2 - the second switching branch - the solid reactance assembly, during which it passes through the first resistor R. D1 Second resistor R D2 The dissipation effect causes the internal circulating current to dissipate rapidly, accelerating the decay of the current and shortening the duration of the fault current.

[0027] Preferably, the inductance of the first current-limiting inductor and the second current-limiting inductor can be set to be the same, and the resistance of the first switching branch and the second switching branch can be the same, so that the current values ​​of the two internal circulating currents are the same.

[0028] According to embodiments of the present invention, a fixed inductance component is used to provide a substantially constant inductive impedance to a DC current limiter, so that the equivalent inductance of the DC current limiter is predictable under different operating conditions. For example, the fixed inductance component may be an air-core inductor or an iron-core reactor that does not saturate within a preset current range through an air gap design. In addition, the fixed inductance component may also be composed of multiple inductors connected in series or in parallel to obtain a predetermined equivalent fixed inductance value.

[0029] According to an embodiment of the present invention, the resistance value of each resistor is selected based on the expected fault current level and the thermal capacity of the device, so as to suppress the circulating peak current and reduce thermal stress while ensuring the energy dissipation rate.

[0030] In one illustrative embodiment, the first diode and the second diode are fast recovery diodes to enable the diodes to have sufficient reverse voltage withstand and current carrying capacity.

[0031] In one illustrative embodiment, the first resistor and the second resistor have equal resistance values. This ensures that the equivalent impedances of the two switching branches remain symmetrical, thereby making the internal circulating current more evenly distributed and reducing the electrothermal stress on individual branch devices, thus improving the reliability of the device.

[0032] Figure 4 This is a schematic diagram of a DC current limiter according to an embodiment of the present invention.

[0033] like Figure 4 As shown, the DC current limiter also includes an iron core. The iron core includes a first side post, a middle post, and a second side post arranged in parallel. A permanent magnet is disposed on a yoke used to connect the middle post and the second side post. The first current-limiting inductor includes a first current-limiting winding wound on the first side post. The second current-limiting inductor includes a second current-limiting winding wound on the first side post. The first and second current-limiting windings are adapted to generate magnetic flux opposite to the direction of the permanent magnet.

[0034] According to an embodiment of the present invention, a constant bias magnetomotive force can be provided by the permanent magnet disposed on the iron core. Under normal current input, the magnetomotive force of the permanent magnet is dominant, so that the iron core is in a magnetic saturation state and each current-limiting winding exhibits low inductance; when a fault current is input, the reverse magnetomotive force generated by the fault current cancels the permanent magnet bias, so that the iron core desaturates and the inductance of each current-limiting winding increases sharply, thereby achieving current limiting. Preferably, the core can be made of neodymium iron boron material.

[0035] Preferably, the inductance generated by the first current-limiting winding and the second current-limiting winding is equal.

[0036] According to an embodiment of the present invention, under normal current input conditions, the equivalent inductance L of each current-limiting winding is... DC_N It can be calculated using its self-inductance and mutual inductance:

[0037] Among them, L 1_N L 2_N and M 12_N These are the self-inductance of the first current-limiting winding, the self-inductance of the second current-limiting winding, and the mutual inductance of the first and second current-limiting windings, respectively.

[0038] In one illustrative embodiment, the solid-state reactive assembly includes a first decoupling winding L3 and a second decoupling winding L4 connected in series and wound on a second side post. The magnetic flux generated by the first decoupling winding L3 and the second decoupling winding L4 are opposite in direction and equal in magnitude.

[0039] According to an embodiment of the present invention, the first decoupling winding L3 and the second decoupling winding L4 are wound on the second side post with the same number of turns and opposite winding directions to generate magnetic fluxes of opposite directions and equal magnitude.

[0040] According to an embodiment of the present invention, the mutual inductance between each decoupling winding and each current-limiting winding satisfies the following relationship:

[0041] Among them, M 13 M is the mutual inductance between the first current-limiting winding and the first decoupling winding. 14 M is the mutual inductance between the first current-limiting winding and the second decoupling winding. 23 M is the mutual inductance between the second current-limiting winding and the first decoupling winding. 24 This refers to the mutual inductance between the second current-limiting winding and the second decoupling winding.

[0042] With the above constraints, power deconstruction can be achieved, so that under normal current and fault current current limiting conditions, the first and second decoupling windings connected in series can be externally equivalent to a fixed inductor, the value of which is basically not affected by the core saturation degree.

[0043] According to an embodiment of the present invention, the external equivalent inductance of the first current-limiting winding and the second current-limiting winding is... L WD for:

[0044] in, L 3. L 4 and M 34 These are the self-inductance of the first decoupling winding, the self-inductance of the second decoupling winding, and the mutual inductance of the first decoupling winding and the second decoupling winding, respectively.

[0045] In one illustrative embodiment, the central column is provided with an air gap to improve the magnetic circuit's resistance to saturation.

[0046] In one illustrative embodiment, the first current-limiting winding and the second current-limiting winding are configured to generate a magnetomotive force less than that generated by the permanent magnet.

[0047] According to an embodiment of the present invention, the magnetomotive force of the permanent magnet and the magnetomotive force of the current-limiting winding can satisfy the following equation:

[0048] As a saturation safety factor, It is the magnetomotive force of the permanent magnet. The magnetomotive force of the current-limiting winding; magnetomotive force of permanent magnet for:

[0049] in, For the coercivity of permanent magnets, The thickness is the permanent magnet.

[0050] The magnetomotive force of the current-limiting winding is :

[0051] N DC and I DC These represent the number of turns of the current-limiting winding and the rated current of the system, respectively.

[0052] In one illustrative embodiment, the magnetic flux generated by the first current-limiting winding and the second current-limiting winding is equal in magnitude, so that the equivalent inductance change characteristics of the first DC inductor and the second DC inductor tend to be consistent, thereby achieving a more balanced current-limiting effect during the current-limiting stage, and facilitating the symmetrical distribution of internal circulating current in the second operating state, thus improving the reliability of the device.

[0053] For example, under normal current input conditions, the iron core is saturated, and the DC current limiter is equivalent to a medium-inductance L-type current limiter. mid =L DC_N +L WD It acts as a smoothing reactor, and the current switching circuit is cut off due to the reverse bias of the diode.

[0054] When a fault current is input, the iron core desaturates, and at this time the inductance L of the current-limiting winding... DC_F Significantly increased, the DC current limiter is equivalent to a high-inductance L-state high =L DC_F +L WD This suppresses the rise of fault current. The current switching circuit is still cut off due to the reverse bias of the diode.

[0055] When the DC circuit breaker upstream of the DC current limiter opens, the fault current decreases, and the current switching circuit automatically turns on due to the reverse voltage. At this time, the external equivalent inductance drops sharply to a low inductance state L. low Its value is mainly determined by the decoupling winding inductance L WD The dominant factor, calculated using the following formula:

[0056] The aforementioned DC current limiter exhibits a dual acceleration effect during the fault current clearing process. The first acceleration is the equivalent inductance of the DC current limiter, defined as the rapid drop from a high inductance state to a low inductance state during the fault current limiting process:

[0057] The second acceleration occurs because a portion of the fault current is diverted to the internal loop and dissipated by resistors in the switching branch, further accelerating the decay of the total current in the system. This is defined as the expected value of the ratio of the original fault current to the current actually required to be discharged through the circuit breaker after diversion. The relationship can be simplified as follows:

[0058] Among them, i DC The DC current flowing through the system during the phase when the fault current is cleared; the resistance R of the resistor in the switching branch. D The smaller the value, the more significant the diversion acceleration effect; t represents time.

[0059] Another aspect of the present invention provides a DC fault protection system, such as the DC current limiter and DC circuit breaker provided in any of the preceding claims. The DC circuit breaker is connected to the input terminal of the DC current limiter and is adapted to perform a breaking action when the DC current meets a preset condition. The DC current limiter enters a second operating state in response to the breaking action of the DC circuit breaker.

[0060] According to an embodiment of the present invention, the DC fault protection system further includes a surge arrester connected in parallel across the two ends of the breaking unit of the DC circuit breaker to conduct when the DC circuit breaker performs a breaking action, thereby limiting the breaking overvoltage.

[0061] Figure 5 This is a schematic diagram showing the calculated inductance of the first current-limiting winding; Figure 6 This is a schematic diagram showing the calculated inductance of the first decoupling winding.

[0062] To illustrate the effects of the DC current limiter provided by this invention, the following numerical simulations are used for explanation.

[0063] Inductance Calculation and Verification: The self-inductance of the first current-limiting winding can be calculated by the ratio of its voltage to current change rate, i.e.:

[0064] Where u1 is the voltage across the first current-limiting winding, i1 is the current flowing through the first current-limiting winding, and di1 / dt is the rate of change of the current flowing through the first current-limiting winding.

[0065] In the simulation, by measuring the real-time changes in voltage and current across the current-limiting winding, the inductance values ​​under normal operation and fault conditions can be accurately extracted. The simulation results are as follows: Figure 5 As shown, the equivalent inductance of the DC current limiter is 100mH during normal operation, and it rapidly rises to 520mH after the fault occurs, which is consistent with the theoretical expectation. Meanwhile, from Figure 6As can be seen, the inductance value of the decoupling winding remained constant throughout the entire fault process, with almost no change, fully verifying its function as a "solid reactance component" and the correctness of the decoupling design from the current-limiting winding. Its equivalent inductance value stabilized at approximately 10.13 mH in the simulation.

[0066] Furthermore, a high-voltage flexible DC transmission system model was built to verify the coordination performance of the DC current limiter and the DC circuit breaker.

[0067] Comparative analysis of traffic limiting effects: Figure 7 This diagram illustrates the comparison of the fault current limiting effects of three different current limiting devices. Since the impedance of the 100mH smoothing reactor remains constant, its current limiting effect is the worst during a fault, with the peak fault current reaching 13.68kA. Similarly, the reactor has no effect on suppressing the current decrease during a fault, and the current rise rate is essentially the same. For the TFCL, it is clear that the current rise rate is suppressed during a fault, with a peak fault current of 4.33kA. However, when the current decreases, the TFCL maintains a high inductance state, significantly prolonging the fault current duration during the surge arrester's energy discharge process. Even when the peak current is much smaller than that of the reactor, the current clearing duration is even longer.

[0068] In comparison, the current limiter of this invention achieves an inductance value comparable to that of a traditional magnetic saturation current limiter during the fault current rise phase, with a peak fault current of 4.62 kA. However, during the fault clearing phase, the current decrease rate is significantly improved compared to the former two, and the fault current can be almost completely discharged within 0.5 ms.

[0069] Quantitative analysis of current limiting effect: Simulation recorded the rate of rise of fault current. After installing the DC current limiter of this invention, the initial rate of rise of fault current (di / dt| t=0+ The rate of increase is significantly reduced. This rate of increase is related to the system voltage U. DC The equivalent inductance L of the DC current limiter high The relationship is approximated as:

[0070] Among them, L line This represents the inductance value of the faulty line in the simulation. In the simulation, the high inductance state L... high The rate of current rise under low inductance is much lower than that under low inductance L. mid The flow restriction effect has been quantitatively demonstrated.

[0071] Circulating current energy dissipation and circuit breaker stress reduction: Here, it is assumed that the resistance of the first resistor and the second resistor is R. D During the fault current clearing phase, the internal circulating current i a i b In resistor R D Energy consumption ERD It can be obtained through simulation integration:

[0072] The peak value of the fault current and the rate of its decrease directly determine the energy that the surge arrester needs to discharge. This invention's current limiter discharges energy extremely quickly while limiting the current rise, demonstrating a significant advantage in this aspect. Simulation data shows that E RD This accounts for a considerable proportion of the total fault energy. This portion of energy is dissipated by the current limiter itself, rather than passing through the surge arrester of the DC circuit breaker, thereby directly reducing the energy absorption requirements of the surge arrester and verifying the effectiveness of this invention in protecting the circuit breaker.

[0073] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A DC current limiter, comprising an input terminal and an output terminal, characterized in that, include: A first current-limiting inductor is connected to the input terminal; A solid-state reactive component, wherein the first end of the solid-state reactive component is connected to the first current-limiting inductor; The second current-limiting inductor is connected to the second terminal and the output terminal of the solid-state reactor component, respectively; The current switching circuit includes: The first switching branch, connected between the input terminal and the second terminal, includes a first diode and a first resistor connected in series; wherein the cathode of the first diode is connected to the input terminal; The second switching branch, connected between the output terminal and the first terminal, includes a second diode and a second resistor connected in series; wherein the cathode of the second diode is connected to the first terminal; In the first operating state, the first diode and the second diode are reverse biased and cut off; in the second operating state, the first diode and the second diode are forward biased and conduct.

2. The DC current limiter according to claim 1, characterized in that, The first diode and the second diode are fast recovery diodes.

3. The DC current limiter according to claim 1, characterized in that, The resistance values ​​of the first resistor and the second resistor are equal.

4. The DC current limiter according to any one of claims 1-3, characterized in that, It also includes the iron core; The iron core includes: The first side column, the middle column, and the second side column are arranged in parallel. A permanent magnet is disposed on the yoke used to connect the central column and the second side column; The first current-limiting inductor includes a first current-limiting winding wound on the first side post; The second current-limiting inductor includes a second current-limiting winding wound on the first side post; The first current-limiting winding and the second current-limiting winding are adapted to generate magnetic flux in the opposite direction to that of the permanent magnet.

5. The DC current limiter according to claim 4, characterized in that, The solid-state component includes a first decoupling winding and a second decoupling winding connected in series and wound on the second side post; The magnetic flux generated by the first decoupling winding and the second decoupling winding are opposite in direction and equal in magnitude.

6. The DC current limiter according to claim 4, characterized in that, The central column is provided with an air gap.

7. The DC current limiter according to claim 4, characterized in that, The first current-limiting winding and the second current-limiting winding are configured such that the magnetomotive force they generate is less than the magnetomotive force generated by the permanent magnet.

8. The DC current limiter according to claim 4, characterized in that, The magnetic flux generated by the first current-limiting winding and the second current-limiting winding is equal.

9. A DC fault protection system, characterized in that, include: The DC current limiter as described in any one of claims 1-8; A DC circuit breaker, connected to the input terminal of the DC current limiter, is suitable for performing a breaking action when the DC current meets preset conditions; The DC current limiter enters a second operating state in response to the opening action of the DC circuit breaker.

10. The DC fault protection system according to claim 9, characterized in that, The DC circuit breaker includes a surge arrester; The surge arrester is connected in parallel across the two ends of the breaking unit of the DC circuit breaker to conduct when the DC circuit breaker performs a breaking action, thereby limiting the breaking overvoltage.

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