A method for optimizing design of a liquid metal current limiter

Abstract

The application relates to the technical field of power switches and discloses a liquid metal current limiter optimization design method, which realizes self-adaptive current limiting by using the magnetic shrinkage effect of the liquid metal itself, realizes the regulation and control of on-state resistance, response time and current limiting depth by designing a through-flow partition plate structure; for the liquid metal current limiter with high on-state loss and low current limiting efficiency, a large-aperture through-flow hole can be used to reduce the on-state resistance, and a small-aperture through-flow hole can be used to shorten the response time and improve the current limiting efficiency; finally, the limitation effect on the expected short-circuit current can be realized without an external excitation device, and the system operation efficiency and stability are improved.

Description

Technical Field

[0001] This invention relates to the field of power switch technology, and in particular to an optimized design method for a liquid metal current limiter. Background Technology

[0002] With the development of the national economy and the increase in electricity load, the capacity of medium- and high-voltage direct current (DC) transmission and distribution systems is increasing daily, leading to a continuous rise in short-circuit current levels and posing a significant threat to the safe and stable operation of the power system. Furthermore, with the growing energy storage market, the application of DC transmission lines is becoming increasingly widespread. Unlike AC systems, DC transmission systems lack a zero-crossing point when short-circuit currents occur, making arc extinguishing difficult. Currently, the breaking and arc-extinguishing capabilities of DC circuit breakers in medium- and high-voltage systems do not meet the ever-increasing demand for short-circuit current capacity. Therefore, scholars have proposed using fault current limiters in conjunction with DC circuit breakers. When a short-circuit fault occurs in the power system, the current limiter first limits the short-circuit current to the current level that the circuit breaker can break, and then the DC circuit breaker operates to disconnect the line, thereby achieving fault disconnection of the power system. Using this method can effectively reduce the design requirements of DC circuit breakers and improve the safe and stable operation capability of the power system.

[0003] Liquid metal current limiters are a new type of current limiter developed using the magnetic contraction effect of liquid metal. When a short-circuit current occurs, the large current flowing through the liquid metal will cause the Lorentz force on the liquid metal to increase rapidly, thereby causing it to be subjected to contraction force in the specially designed insulated current passage and break. The generated arc can limit the fault current to the level that the circuit breaker can break. It has the characteristics of adaptive detection current limiting, fast response time, reusability, strong current limiting capability, and low conduction resistance. It can be applied to smart grid and new energy microgrid products such as energy routers.

[0004] CN104091717A discloses a novel self-powered liquid metal current limiter and method. It utilizes a flow-through orifice for conduction and a movable insulating plate to lengthen the arc at the moment of disconnection, achieving rapid current limiting. However, to achieve a faster current-limiting response and a deeper current-limiting depth, the cross-sectional area of ​​the flow-through orifice needs to be reduced, thus increasing the potential for increased conduction resistance. Furthermore, the movement direction of the movable insulating plate caused by air pressure is not necessarily vertically upward; if a horizontal impact force component exists, the stability of the structure will be significantly reduced.

[0005] The liquid metal current limiting methods disclosed in CN107507746A and CN106356237 use an external short-circuit current detection device and an external electromagnetic repulsion mechanism to interrupt the liquid metal. While these methods can effectively limit the current, the external current detection device and electric mechanism reduce the operational stability of the current limiter. Furthermore, the increased signal transmission for the detected current and the response of the electric mechanism also increase the overall system's current limiting response time, thus reducing the current limiting efficiency.

[0006] Therefore, current liquid metal current limiters still suffer from excessive on-resistance, long response time, and poor current-limiting capability. Furthermore, for liquid metal magnetic contraction breaking, to achieve a faster response time and higher current-limiting efficiency, the diameter of the liquid metal column in the contraction region needs to be reduced, but this increases the on-resistance of the circuit, thus increasing losses. Therefore, on-resistance and response time are contradictory.

[0007] Based on this, CN109995006A discloses a liquid metal fault current limiter and its current limiting method. When normal current flows, the main current-passing orifice with a larger aperture is used to reduce its on-resistance. When the detection device detects a fault current, a pulse discharge is generated through an external circuit, causing the smaller aperture secondary orifice to arc preferentially, thereby triggering the arcing of the main current-passing orifice and improving the response time. However, the discharge circuit containing the smaller orifice requires real-time monitoring of the current in the circuit. When a circuit fault occurs, the detection device first identifies the fault, then transmits the fault signal to the external pre-charge capacitor, inductor, and thyristor. After triggering the thyristor to conduct, a pulse current is generated in the circuit, and finally, this pulse current excites the liquid metal in the smaller orifice to interrupt and arc. Compared to self-driven contraction interruption under the same orifice size, this orifice interruption and arcing process is too cumbersome and does not effectively improve the overall system response time. Furthermore, excessive reliance on external devices reduces the operational stability of the current limiter system. Summary of the Invention

[0008] This invention addresses the shortcomings and defects of existing technologies by providing an optimized design method for liquid metal current limiters. By utilizing the magnetic contraction physical properties of liquid metal and employing a specially designed flow passage structure, the contradiction between conduction resistance and current limiting response time is resolved, thereby increasing current limiting efficiency while reducing rated current conduction loss.

[0009] The objective of this invention can be achieved through the following technical solutions:

[0010] A method for optimizing the design of a liquid metal current limiter includes the following steps:

[0011] S1, Determine the design specifications and parameters of the liquid metal current limiter;

[0012] S2, based on design specifications and parameters, conducts current-limiting experiments on the flow-through baffle at different current levels to determine the family of curves A relating the orifice diameter to the arc pre-arc time at current level i. i Curve B shows the relationship between aperture and conduction resistance, and curve C shows the relationship between the opening height of different flow-through baffles and the arc pre-time.

[0013] S3, based on design specifications and parameters, refer to curve family A. iCurve B determines the relationship between orifice diameter and pre-arc time under short-circuit current I. I The minimum required diameter r of the main orifice of the flow baffle a ;

[0014] Reference curve A I Determine the pre-arc time T I ; Conduct current limiting experiments based on design specifications and parameters to determine the current limiting depth η under short-circuit current I. I ;

[0015] S4. Based on the data obtained in the previous step, determine whether the liquid metal current limiter meets the design specifications and parameters: if it does not meet the specifications, proceed to step S5; if it does meet the specifications, the liquid metal current limiter can be put into use, and proceed to step S6.

[0016] S5, the flow baffle is designed with an additional secondary hole with a smaller diameter, which reduces the on-resistance, shortens the arc pre-time, and increases the current limiting depth;

[0017] S6, place the flow baffle in the liquid metal flow restrictor cavity.

[0018] Preferably, the design specifications and parameters in step S1 specifically include: the maximum on-resistance R. d Short-circuit current I, minimum current limiting depth η, number of baffles, maximum pre-arc time T, minimum diameter of current-carrying baffle.

[0019] Preferably, the specific steps for designing a smaller secondary hole in S5 are as follows:

[0020] S5-1, Along the horizontal line of the center of the main hole of the flow baffle, add holes with a diameter smaller than r. a The secondary hole has a diameter of r. b ;

[0021] S5-2, calculate the current I of the liquid metal in the secondary hole when it is conducting. b ;

[0022] S5-3, in A i Find the current level I b Time aperture r b The relationship curve between the arc advance time and the arc advance time T is used to clarify the arc advance time T. Ib Conduct current-limiting experiments based on design specifications and parameters to determine the current I. b The current limiting depth η Ib ;

[0023] S5-4, Determine T Ib With T I Relationship: If T Ib ≥T I Then reduce rb Proceed to step S5-5; if T Ib <T I Then proceed to steps S5-6;

[0024] S5-5, Determine the reduced r b Does it meet the design specifications and parameters? If not, proceed to step S5-7; if it does, skip to step S5-2.

[0025] S5-6, Determine the pre-arc time T Ib and current limiting depth η Ib Does the design specification and parameters meet the requirements? If yes, proceed to S6; otherwise, decrease r. b Proceed to step S5-5;

[0026] S5-7, Adjust the opening height of the secondary hole;

[0027] The arc time before the secondary orifice is determined based on curve C; a flow-limiting experiment is conducted in conjunction with the design specifications and parameters to obtain the flow-limiting depth;

[0028] Determine whether the two meet the design specifications and parameters: if they do, proceed to S6; if they do not, repeat steps S5-7 until the design requirements are met.

[0029] Preferably, the aperture specification design threshold in step S5-5 is 1 mm.

[0030] Preferably, the current level i includes short-circuit currents I and I0. b .

[0031] The beneficial technical effects of this invention are as follows: Adaptive current limiting is achieved by utilizing the magnetic contraction effect of liquid metal itself; the conduction resistance, response time, and current limiting depth are controlled by designing a flow-through baffle structure; for current liquid metal current limiters with high conduction losses and low current limiting efficiency, a large-diameter flow-through orifice can be used to reduce conduction resistance, and a small-diameter flow-through orifice can be used to shorten the response time and improve current limiting efficiency; ultimately, the desired short-circuit current can be limited without an external excitation device, improving system operating efficiency and stability. Attached Figure Description

[0032] Figure 1 This is the overall flowchart of the present invention.

[0033] Figure 2 This is a schematic diagram of the liquid metal flow limiter structure of the present invention.

[0034] Figure 3 This is a graph showing the relationship between the aperture of the flow-through baffle and the time before the current-limiting arc under different currents in an embodiment of the present invention.

[0035] Figure 4This is a diagram showing the relationship between the aperture of the flow-through baffle and the conduction resistance in an embodiment of the present invention.

[0036] Figure 5 This is a diagram showing the relationship between the opening height of the flow baffle and the arc pre-arc time in an embodiment of the present invention.

[0037] Figure 6 The waveform diagram is shown in the current limiting experiment of the embodiment of the present invention, which does not meet the design requirements.

[0038] Figure 7 This is a schematic diagram of the flow baffle structure after adding a secondary hole in an embodiment of the present invention.

[0039] Figure 8 The waveform diagram of the current limiting experiment in this embodiment of the invention, after adding a secondary orifice, meets the design requirements.

[0040] Figure 9 This is a schematic diagram of the flow baffle structure after adjusting the height of the secondary hole in an embodiment of the present invention.

[0041] Figure 10 This is a waveform diagram of a current-limiting experiment in an embodiment of the present invention, showing that the height of the secondary orifice meets the design requirements.

[0042] Reference numerals: 1 is copper electrode, 2 is liquid metal flow limiter shell structure, 3 is flow baffle, 4 is liquid metal, 5 is flow hole, 5-1 is main flow hole, and 5-2 is secondary flow hole. Detailed Implementation

[0043] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and do not limit the scope of the invention.

[0044] Example 1: Refer to Appendix Figure 1 and appendix Figure 2 A method for optimizing the design of a liquid metal current limiter includes the following steps:

[0045] S1, determine the design specifications and parameters of the liquid metal current limiter: short-circuit current 2kA, maximum arc pre-arc time 2ms, minimum current limiting depth 50%, maximum conduction resistance 2mΩ, minimum flow barrier aperture 1mm, number of barriers 5.

[0046] S2, based on design specifications and parameters, current-limiting experiments were conducted on liquid metal current limiters with different flow-passing baffle orifice diameters at current levels of 2kA, 1.5kA, 1kA, and 0.5kA, respectively, to determine the family A of curves showing the relationship between different orifice diameters and arc pre-arc time at current level i. i .

[0047] The current limiting experiment was conducted in a high-voltage laboratory. A short-circuit current of 2kA was applied to the liquid metal current limiter using a current generator. Each aperture size of the flow-passing baffle corresponds to a pre-arc time. By replacing the flow-passing baffles with different aperture sizes, a family of relationship curves A can be obtained. i The experimental results can be found in the appendix. Figure 3 .

[0048] By placing flow-passing baffles of different orifice diameters into the flow restrictor and filling it with liquid metal 4, the resistance across the flow restrictor is measured, thus obtaining the relationship curve B between the orifice diameter and the conduction resistance. Because the surface tension of liquid metal 4 is relatively high, it is not completely filled in the flow-passing orifice 5; therefore, the theoretical calculation formula R = ρL / S cannot be used for calculation. Actual measured data can be found in the appendix. Figure 4 .

[0049] Furthermore, experiments were conducted under a 2kA current condition for different current-passing baffle opening heights, and the relationship curve C between the opening height and the arc pre-arc time was obtained, as shown in the attached figure. Figure 5 .

[0050] S3, according to design specifications and parameters, refer to the appendix. Figure 3 Determine the relationship curve between aperture and pre-arc time under a 2kA short-circuit current. 2k Based on the on-resistance threshold of 2mΩ, refer to the appendix. Figure 4 Determine the minimum required aperture r of the main orifice 5-1 of the flow baffle. a It is 2.4mm;

[0051] Based on the main hole diameter of 2.4mm and curve A 2k Clearly define the time T before the current limiter breaks the arc. I The time was 2.7 ms. A current-limiting and breaking experiment was conducted on this flow-through baffle. During the experiment, the current in the liquid metal 4 flowing through the flow-through orifice 5-1 increased rapidly. Due to its own contraction effect, the liquid column of liquid metal 4 in the flow-through orifice 5-1 broke and arced. The arcing voltage suppressed the rise of the short-circuit current, thus playing a current-limiting role. The current-limiting depth can be obtained by comparing the peak current after current limiting with the magnitude of the applied short-circuit current, specifically: (1 - peak current after current limiting / expected short-circuit current) * 100%. Experimental results show that the current-limiting depth η under this condition... I The result was 37.2%, as shown in the attached figure. Figure 6 As shown.

[0052] S4, after comparison, T was found I 2ms higher than the design parameters and η I The current limiter is 50% lower than the design parameters, therefore the current limiter under these design parameters does not meet the design requirements, proceed to step S5.

[0053] S5, Design the flow baffle, refer to the appendix. Figure 7 :

[0054] Proceed to step S5-1, increasing the aperture r along the horizontal line of the center of the original main hole 5-1 of the flow baffle. b The secondary hole 5-2 is 2mm in diameter. Since the secondary hole 5-2 is connected in parallel with the main hole 5-1, the on-resistance will be lower, which can meet the on-resistance design requirement of less than 2mΩ.

[0055] In steps S5-2 to S5-3, when current flows through the parallel orifices, the magnitude of the current flowing through each orifice is related to its resistance, i.e., I0 a :I b =R b :R a , by appendix Figure 4 It can be seen that when the secondary hole 5-2 is 2mm, the conduction resistance is 3mΩ. Therefore, the current I in the two holes can be calculated according to the proportional relationship. a :I b =3:2, therefore I a For 1.2kA, I b It is 0.8kA, in the attached Figure 3 By finding the corresponding current level and the aperture of the separator, the arc pre-arc time T of the secondary hole 5-2 can be obtained. Ib It takes 4.2ms;

[0056] By performing step S5-4, we can determine T Ib ≥T I Since the secondary hole 5-2 cannot shorten the arc pre-time, the diameter of the secondary hole 5-2 is reduced to 1.2mm.

[0057] As shown in step S5-5, the diameter of the secondary hole 5-2 of the flow baffle, 1.2 mm, is not less than the design specification and parameter 1 mm. Therefore, by executing steps S5-2 to S5-3, the arc pre-arc time of 1.9 ms < T is obtained. I The current limiting depth is 52.2%, and the waveform diagram of the current limiting experiment is attached. Figure 8 As shown;

[0058] As can be seen from steps S5-6, the arc pre-arc time T Ib and current limiting depth η Ib If all requirements are met, the flow baffle is placed in the liquid metal flow limiter cavity to complete the design of the liquid metal flow limiter.

[0059] Example 2: In step S1, the design specifications and parameters of the liquid metal current limiter are determined as follows: short-circuit current 2kA, maximum arc pre-arc time 1.5ms, minimum current limiting depth 50%, maximum conduction resistance 2mΩ, and minimum flow baffle aperture 1mm.

[0060] Referring to Example 1, when proceeding to step S5-5, even if the diameter of the secondary hole 5-2 is reduced to the design threshold of 1mm, its corresponding arc pre-time of 1.7ms still does not meet the design requirements, so step S5-7 needs to be performed.

[0061] Proceed to steps S5-7, referring to the appendix. Figure 4 and appendix Figure 9 The opening height of the flow passage 5-2 was increased from the initial 13mm to 14mm. Specifically, when the opening height is increased, the height difference between the liquid metal 4 in the flow limiter cavity and the flow passage 5-2 becomes smaller. Therefore, the volume of liquid metal 4 above the flow passage 5-2 becomes smaller, resulting in a reduction in the gravitational effect. Consequently, when a short-circuit current occurs, the liquid column in the flow passage 5-2 can be broken more easily, thus shortening the arc pre-current time. (See attached diagram.) Figure 10 After increasing the opening height to 14mm, the arc pre-arc time was shortened from 1.7ms to 1.4ms. The current limiting test results showed that the current limiting depth became 53.7%, which met the design requirements. The flow baffle was placed in the liquid metal current limiter cavity to complete the design of the liquid metal current limiter.

[0062] Furthermore, if it is not possible to achieve A in step S5-3 i Find the current level I b Time aperture r b The relationship curve between the orifice diameter and the time before the arc can be used to conduct supplementary experiments for flow baffles of this orifice diameter: using an orifice diameter r b The flow baffle in I b A current-limiting experiment was conducted at the current level to determine the arc pre-arc time T under this condition. Ib and current limiting depth η Ib .

[0063] The above embodiments are descriptions of specific implementations of the present invention, and not limitations thereof. Those skilled in the art can make various modifications and changes without departing from the spirit and scope of the present invention to obtain corresponding equivalent technical solutions. Therefore, all equivalent technical solutions should be included in the patent protection scope of the present invention.

Claims

1. A method for optimizing the design of a liquid metal current limiter, characterized in that, Includes the following steps: S1, Determine the design specifications and parameters of the liquid metal current limiter; S2, based on design specifications and parameters, conducts current-limiting experiments on the flow-through baffle at different current levels to determine the family of curves A relating the orifice diameter to the arc pre-arc time at current level i. i Curve B shows the relationship between aperture and conduction resistance, and curve C shows the relationship between the opening height of different flow-through baffles and the arc pre-time. S3, based on design specifications and parameters, refer to curve family A. i Curve B determines the relationship between orifice diameter and pre-arc time under short-circuit current I. I The minimum required diameter r of the main orifice of the flow baffle a ; Reference curve A I Determine the pre-arc time T I ; Conduct current limiting experiments based on design specifications and parameters to determine the current limiting depth η under short-circuit current I. I ; S4. Based on the data obtained in the previous step, determine whether the liquid metal current limiter meets the design specifications and parameters: if it does not meet the specifications, proceed to step S5; if it does meet the specifications, the liquid metal current limiter can be put into use, and proceed to step S6. S5, the flow baffle is designed with an additional secondary hole with a smaller diameter, which reduces the on-resistance, shortens the arc pre-time, and increases the current limiting depth; S6, Place the flow baffle in the liquid metal flow restrictor cavity; The specific steps for designing a smaller secondary hole in S5 are as follows: S5-1, Along the horizontal line of the center of the main hole of the flow baffle, add holes with a diameter smaller than r. a The secondary hole has a diameter of r. b ; S5-2, calculate the current I of the liquid metal in the secondary hole when it is conducting. b ; S5-3, in A i Find the current level I b Time aperture r b The relationship curve between the arc advance time and the arc advance time T is used to clarify the arc advance time T. Ib Conduct current-limiting experiments based on design specifications and parameters to determine the current I. b The current limiting depth η Ib ; S5-4, Determine T Ib With T I Relationship: If T Ib ≥T I Then reduce r b Proceed to step S5-5; if T Ib <T I Then proceed to steps S5-6; S5-5, Determine the reduced r b Does it meet the design specifications and parameters? If not, proceed to step S5-7; if it does, skip to step S5-2. S5-6, Determine the pre-arc time T Ib and current limiting depth η Ib Does the design specification and parameters meet the requirements? If yes, proceed to S6; otherwise, decrease r. b Proceed to step S5-5; S5-7, Adjust the opening height of the secondary hole; Determine the arc inlet time after adjusting the secondary hole based on curve C; A current-limiting experiment was conducted based on the design specifications and parameters to determine the current-limiting depth; Determine whether the two meet the design specifications and parameters: if they do, proceed to S6; if they do not, repeat steps S5-7 until the design requirements are met.

2. The optimized design method for a liquid metal current limiter according to claim 1, characterized in that, The design specifications and parameters in step S1 specifically include: the maximum on-resistance R. d Short-circuit current I, minimum current limiting depth η, number of baffles, maximum pre-arc time T, minimum diameter of current-carrying baffle.

3. The optimized design method for a liquid metal current limiter according to claim 1, characterized in that, The aperture specification design threshold in step S5-5 is 1 mm.

4. The optimized design method for a liquid metal current limiter according to claim 1, characterized in that, The current level i includes short-circuit currents I and I0. b .

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