Optimization design method for liquid metal flow limiter

Abstract

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

Description

Technical Field

[0001] The present invention relates to the technical field of power switches, and particularly to an optimized design method for a liquid metal current limiter. Background Art

[0002] With the development of the national economy and the increase of power consumption load, the capacity of medium and high voltage DC transmission and distribution systems is increasing day by day. As a result, the short-circuit current level of the power grid continues to increase, posing a huge hidden danger to the safe and stable operation of the power system. Moreover, with the growing energy storage market, the application of DC transmission lines is becoming more and more widespread. Different from AC systems, when a short-circuit current occurs in a DC transmission system, there is no zero-crossing point of the current in the system, so it is difficult to extinguish the arc. At present, the breaking and arc-extinguishing capabilities of DC circuit breakers in medium and high voltage systems do not meet the increasing demand for short-circuit current capacity. Therefore, scholars have proposed to use a fault current limiter in combination with a DC circuit breaker. When a short-circuit fault occurs in the power system, first, the current limiter is used to limit the short-circuit current to the current level that the circuit breaker can break, and then the DC circuit breaker operates to break the line, thereby achieving the fault breaking of the power system. Using this method can effectively reduce the design requirements of DC circuit breakers and improve the safe and stable operation ability of the power system.

[0003] A liquid metal current limiter is a new type of current limiter developed using the magnetostriction 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, so that it is subjected to a contraction force in a specially designed insulated current-carrying hole and breaks. The generated arc can limit the fault current to the level that the circuit breaker can break. It has the characteristics of self-adaptive detection and current limiting, fast response time, reusable, strong current limiting ability, low conduction resistance, etc., and can be applied to intelligent power grids such as energy routers and new energy microgrids.

[0004] CN104091717A discloses a new type of self-powered liquid metal current limiter and method. It uses a current-carrying hole to conduct, and a movable insulating extraction plate is used to stretch the arc at the moment of breaking to play a role in quickly limiting the current. However, if faster current limiting response and deeper current limiting depth are to be achieved, it is necessary to reduce the cross-sectional area of the current-carrying hole, which increases the hidden danger of conduction resistance. In addition, the movement direction of the movable insulating extraction plate caused by air pressure is not necessarily vertically upward. If there is a horizontal impact component, the structural stability will be greatly 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 make the liquid metal break. Although it can play an effective role in current limiting, the external current detection device and the electric mechanism will reduce the operation stability of the current limiter. At the same time, increasing the signal transmission of the detected current and the response of the electric mechanism will also increase the current limiting response time of the entire system and reduce the current limiting efficiency.

[0006] Therefore, at present, the liquid metal current limiter still has the characteristics of too large on-resistance, long response time, and poor current limiting ability. And for the magnetic contraction breaking of liquid metal, if a faster response time and higher current limiting efficiency are to be achieved, the diameter of the liquid metal column in the contraction area needs to be reduced, but this will increase the on-impedance of the circuit, thereby increasing the loss. Therefore, the on-resistance and the response time are in a contradictory relationship.

[0007] Based on this, CN109995006A discloses a liquid metal fault current limiter and its current limiting method. When a normal current passes through, a main current-carrying hole with a larger aperture is used for conduction, which can reduce its on-resistance. When the detection device detects the coming of a fault current, pulse discharge is generated through an external circuit to make the auxiliary hole with a smaller aperture arc first, and then trigger the large current-carrying hole to arc, which plays a role in improving the response time. However, the discharge circuit where the small hole is located needs to monitor the current in the line in real time. When a line fault occurs, first the detection device identifies the fault, then transmits the fault signal to the externally connected pre-charged capacitor, inductor, and thyristor. After triggering the thyristor to conduct, a pulse current is generated in the circuit, and finally, the liquid metal in the small hole is broken and arced by this pulse current. Compared with the self-driven contraction breaking under the same small hole, the arc-breaking process of this small hole is too cumbersome and does not effectively improve the response time of the entire system. And excessive dependence on external device mechanisms will also reduce the operating stability of the current limiter system. Summary of the Invention

[0008] In view of the deficiencies and defects existing in the prior art, the present invention provides an optimized design method for a liquid metal current limiter, which utilizes the magnetic contraction physical characteristics of liquid metal and solves the contradiction between on-resistance and current limiting response time through a specially designed current-carrying hole structure, thereby achieving the reduction of rated current-carrying conduction loss while increasing its current limiting efficiency.

[0009] The object of the present invention can be achieved by the following technical solutions:

[0010] An optimized design method for 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 the design specifications and parameters, conduct current limiting experiments on the current-carrying partition at different current levels to determine the relationship curve family A between the aperture and the pre-arcing time, i the relationship curve B between the aperture and the on-resistance, and the relationship curve C between the opening height of different current-carrying partition holes and the pre-arcing time;

[0013] S3. According to the design specifications and parameters, refer to the curve family A i, curve B, determine the relationship curve A between the aperture diameter and the pre-arc time under the short-circuit current I I , the aperture diameter r of the main hole of the minimum required current-carrying partition a ;

[0014] Refer to curve A I , and clarify the pre-arc time T I ; Conduct a current-limiting experiment in combination with the design specifications and parameters to clarify the current-limiting depth η under the short-circuit current I I ;

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

[0016] S5, design the current-carrying partition, add a secondary hole with a smaller aperture diameter, which can play the role of reducing the conduction resistance, shortening the pre-arc time, and increasing the current-limiting depth;

[0017] S6, place the current-carrying partition in the cavity of the liquid metal current limiter.

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

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

[0020] S5-1, along the horizontal line direction of the center of the main hole of the current-carrying partition, add a secondary hole with an aperture diameter smaller than r a , and the aperture diameter is r b ;

[0021] S5-2, obtain the current I of the liquid metal in the secondary hole when it conducts b ;

[0022] S5-3, find the relationship curve between the aperture diameter r i and the pre-arc time when the current level is I b in A b , and then clarify the pre-arc time T Ib ; Conduct a current-limiting experiment in combination with the design specifications and parameters to clarify the current-limiting depth η under the current I b ; Ib ;

[0023] S5-4, judge the relationship between T Ib and T I : if T Ib ≥ T I , then reduce rb , perform step S5-5; if T Ib <T I , then perform step S5-6;

[0024] S5-5, determine whether the reduced r b meets the design specifications and parameters: if not, perform step S5-7; if so, jump to step S5-2;

[0025] S5-6, determine whether the pre-arcing time T Ib and the current limiting depth η Ib meet the design specifications and parameters: if so, jump to S6; if not, reduce r b , perform step S5-5;

[0026] S5-7, increase the opening height of the auxiliary hole;

[0027] Determine the pre-arcing time after increasing the auxiliary hole according to curve C; conduct a current limiting experiment in combination with the design specifications and parameters to obtain the current limiting depth;

[0028] Judge whether the two meet the design specifications and parameters: if so, jump to S6; if not, repeat step 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 the short-circuit current I and I b .

[0031] The beneficial technical effects of the present invention: Utilize the magnetic constriction effect of liquid metal itself to achieve self-adaptive current limiting, and regulate the conduction resistance, response time and current limiting depth by designing the flow-through partition structure; for the current liquid metal current limiter with high on-state loss and low current limiting efficiency, large-aperture flow-through holes can be used to reduce the on-state resistance, and small-aperture flow-through holes can be used to shorten the response time and improve the current limiting efficiency; ultimately achieve the effect of restricting the expected short-circuit current without an external excitation device, and improve the system operation efficiency and stability. Description of the Drawings

[0032] Figure 1 is the overall flow chart of the present invention.

[0033] Figure 2 is the structural schematic diagram of the liquid metal current limiter of the present invention.

[0034] Figure 3 is the relationship diagram between the aperture of the flow-through partition and the pre-arcing time of current limiting under different currents in the embodiment of the present invention.

[0035] Figure 4It is a relationship diagram between the aperture of the flow-through partition and the on-resistance in the embodiment of the present invention.

[0036] Figure 5 It is a relationship diagram between the opening height of the flow-through partition and the pre-arcing time in the embodiment of the present invention.

[0037] Figure 6 It is a waveform diagram of the current-limiting experiment that does not meet the design requirements in the embodiment of the present invention.

[0038] Figure 7 It is a schematic structural diagram of the flow-through partition after adding auxiliary holes in the embodiment of the present invention.

[0039] Figure 8 It is a waveform diagram of the current-limiting experiment that meets the design requirements after adding auxiliary holes in the embodiment of the present invention.

[0040] Figure 9 It is a schematic structural diagram of the flow-through partition after adjusting the height of the auxiliary hole in the embodiment of the present invention.

[0041] Figure 10 It is a waveform diagram of the current-limiting experiment that meets the design requirements after adjusting the height of the auxiliary hole in the embodiment of the present invention.

[0042] Reference numerals in the drawings: 1 is a copper electrode, 2 is the outer shell structure of the liquid metal current limiter, 3 is the flow-through partition, 4 is the liquid metal, 5 is the flow-through hole, 5-1 is the main flow-through hole, and 5-2 is the auxiliary flow-through hole. Specific embodiments

[0043] In order to make the objectives, technical solutions and advantages of the present invention clearer, the present 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 only used to explain the present invention and do not limit the present invention.

[0044] Embodiment 1: Refer to the attached Figure 1 and the attached Figure 2 , an optimized design method for a liquid metal current limiter, includes the following steps:

[0045] S1, determining the design specifications and parameters of the liquid metal current limiter: short-circuit current 2 kA, maximum pre-arcing time 2 ms, minimum current-limiting depth 50%, maximum on-resistance 2 mΩ, minimum aperture of the flow-through partition 1 mm, and number of partitions 5.

[0046] S2, based on the design specifications and parameters, respectively conduct current-limiting experiments on liquid metal current limiters with different apertures of the flow-through partition at current levels of 2 kA, 1.5 kA, 1 kA, and 0.5 kA, and determine the relationship curve family A between different apertures and the pre-arcing time at current level i i .

[0047] The current-limiting experiment is carried out in a high-voltage laboratory. A short-circuit current of 2 kA required is applied to the liquid metal current limiter by a current generating device. Each through-flow partition aperture size corresponds to an arcing time before. By replacing the through-flow partitions with different apertures, a family of relationship curves A can be obtained. i , and the experimental results can be referred to in the appendix Figure 3 .

[0048] After placing the through-flow partitions with different apertures in the current limiter and filling the liquid metal 4, the resistance at both ends of the current limiter is measured, and the relationship curve B between the aperture and the conduction resistance can be obtained. Since the surface tension of the liquid metal 4 is relatively large, it is not in a completely filled state in the through-flow hole 5. Therefore, the theoretical calculation formula R = ρL / S cannot be used for calculation, and the measured data can be referred to in the appendix Figure 4 .

[0049] In addition, under the condition of a 2-kA current, experiments are carried out for different opening heights of the through-flow partitions, and the relationship curve C between the opening height and the arcing time before can be obtained, as shown in the appendix Figure 5 .

[0050] S3. According to the design specifications and parameters, refer to the appendix Figure 3 to determine the relationship curve A between the aperture and the arcing time before under a 2-kA short-circuit current 2k ; according to the conduction resistance threshold of 2 mΩ, refer to the appendix Figure 4 to determine the required minimum aperture r of the main hole 5-1 of the through-flow partition a to be 2.4 mm;

[0051] According to the main hole aperture of 2.4 mm and curve A 2k , clarify the arcing time before T of the current limiter breaking I to be 2.7 ms; for this through-flow partition, a current-limiting breaking experiment is carried out. During the experiment, the current in the liquid metal 4 flowing through the through-flow hole 5-1 increases rapidly. Due to its own contraction effect, the liquid column of the liquid metal 4 in the through-flow hole 5-1 is broken and arcs. The arcing voltage suppresses the climb of the short-circuit current, so it plays a role in current limiting. By comparing the peak value of the current after current limiting with the magnitude of the applied short-circuit current, the current-limiting depth can be obtained, specifically: (1 - peak value of the current after current limiting / expected short-circuit current) * 100%. The experimental results show that the current-limiting depth η I is 37.2%, and the results are as shown in the appendix Figure 6 .

[0052] S4. After comparison, it is found that T I is higher than the design parameter of 2 ms and η I is lower than the design parameter of 50%. Therefore, the current limiter under this design parameter does not meet the design requirements, and step S5 is carried out.

[0053] S5. Design the through-flow partition, refer to the appendix Figure 7 :

[0054] Perform step S5-1, and add a secondary hole 5-2 with a hole diameter r of 2 mm along the horizontal line of the center of the original main hole 5-1 of the flow partition plate. Since the secondary hole 5-2 is in parallel with the main hole 5-1, the on-resistance will be lower, which can meet the on-design requirement of less than 2 mΩ. b For step S5-2 to S5-3, when current passes through the parallel small holes, the magnitude of the current flowing through each small hole is related to its resistance. There is I

[0055] : I a : I b = R b : R a , as can be seen from the appendix Figure 4 , when the secondary hole 5-2 is 2 mm, the on-resistance is 3 mΩ. Therefore, according to the proportional relationship, the currents I a : I b = 3:2. Therefore, I a is 1.2 kA, and I b is 0.8 kA. In the appendix Figure 3 , find the corresponding current level and the aperture of the partition plate, and the pre-arcing time T Ib of the secondary hole 5-2 can be obtained as 4.2 ms.

[0056] From step S5-4, it can be known that T Ib ≥ T I , that is, the secondary hole 5-2 cannot play the role of shortening the pre-arcing time. Therefore, reduce the aperture of the secondary hole 5-2 to 1.2 mm.

[0057] From step S5-5, it can be known that the aperture of the secondary hole 5-2 of the flow partition plate, which is 1.2 mm, is not less than the design specifications and parameters of 1 mm. Therefore, execute steps S5-2 to S5-3, and the pre-arcing time 1.9 ms < T I and the current-limiting depth of 52.2% are obtained. The waveform diagram of the current-limiting experiment is as shown in the appendix Figure 8 .

[0058] From step S5-6, it can be known that the pre-arcing time T Ib and the current-limiting depth η Ib both meet the design requirements. Then place the flow partition plate in the liquid metal current limiter cavity to complete the design of the liquid metal current limiter.

[0059] Embodiment 2: In step S1, determine the design specifications and parameters of the liquid metal current limiter as follows: short-circuit current 2 kA, maximum pre-arcing time 1.5 ms, minimum current-limiting depth 50%, maximum on-resistance 2 mΩ, and minimum aperture of the flow partition plate 1 mm.

[0060] Referring to Embodiment 1, proceed to step S5-5. At this time, even if the aperture diameter of the auxiliary hole 5-2 is reduced to the design threshold of 1 mm, the corresponding pre-arcing time of 1.7 ms still does not meet the design requirements. Therefore, step S5-7 needs to be carried out;

[0061] Carry out step S5-7. Refer to the appendix Figure 4 and the appendix Figure 9 , and increase the opening height of the current-carrying hole 5-2 from the initial 13 mm to 14 mm. Specifically, when the opening height is increased, the height difference between the liquid metal 4 level in the current limiter cavity and the current-carrying hole 5-2 becomes smaller. Therefore, the volume of the liquid metal 4 above the current-carrying hole 5-2 becomes smaller, and the resulting gravitational effect is reduced. So when a short-circuit current comes, the liquid column in the current-carrying hole 5-2 can be more easily interrupted, and thus the pre-arcing time can be shortened. Refer to the appendix Figure 10 , after increasing the opening height to 14 mm, the pre-arcing time is shortened from 1.7 ms to 1.4 ms. The current-limiting experiment results show that the current-limiting depth becomes 53.7%, meeting the design requirements. Place the current-carrying partition in the liquid metal current limiter cavity to complete the design of the liquid metal current limiter.

[0062] Furthermore, if in step S5-3, the relationship curve between the aperture r i and the pre-arcing time at the current level of I b cannot be found in A b , then additional experiments can be carried out for the current-carrying partition with this aperture: Use the current-carrying partition with the aperture r b to conduct a current-limiting experiment at the current level of I b to clarify the pre-arcing time T Ib and the current-limiting depth η Ib under this condition.

[0063] The above embodiments are descriptions of the specific implementation manners of the present invention, rather than limitations on the present invention. Those skilled in the relevant technical fields can make various transformations 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. An optimized design method for a liquid metal current limiter, characterized in that, It includes the following steps: S1. Determine the design specifications and parameters of the liquid metal current limiter; S2. Based on the design specifications and parameters, conduct current-limiting experiments on the flow-through baffle at different current levels to determine the family of relationship curves A between the aperture diameter and the pre-arc time, the relationship curve B between the aperture diameter and the on-resistance, and the relationship curve C between the opening height of different flow-through baffles and the pre-arc time at current level i. i And the relationship curve C between the opening height of different flow-through baffles and the pre-arc time; S3. According to the design specifications and parameters, refer to curve family A i and curve B to determine the relationship curve A between the aperture diameter and the pre-arc time under the short-circuit current I I and the required minimum aperture diameter r of the main hole of the current-carrying partition board a ; Reference curve A I , to determine the pre-arc time T I ; combined with the design specifications and parameters, conduct a current-limiting experiment to determine the current-limiting depth η at the short-circuit current I I ; S4. According to the data obtained in the previous step, judge whether the liquid metal current limiter meets the design specifications and parameters: if not, go to step S5; if it meets, the liquid metal current limiter can be put into use and go to step S6; S5. Design the current-carrying partition board, add a secondary hole with a smaller aperture, which plays a role in reducing the conduction resistance, shortening the pre-arcing time, and increasing the current-limiting depth; S6. Place the current-carrying partition board in the cavity of the liquid metal current limiter.

2. The optimized design method of a liquid metal current limiter according to claim 1, characterized in that The design specifications and parameters in the step S1 specifically include: the maximum on-resistance R d , short-circuit current I, minimum current-limiting depth η, number of partitions, maximum pre-arcing time T, and minimum through-flow partition aperture diameter.

3. The optimized design method of a liquid metal current limiter according to claim 1, characterized in that The specific steps for designing the secondary hole with a smaller aperture in S5 are as follows: S5-1, add auxiliary holes with a diameter smaller than r along the horizontal line of the center of the main hole of the flow partition plate a , and the diameter of the auxiliary hole is r b ; S5-2, obtain the current I of the liquid metal in the auxiliary hole when it is conducting b ; S5-3, at A i Find the relationship curve between the aperture r b and the pre-arc time when the current level is I b , and then clarify the pre-arc time T Ib ; Conduct a current-limiting experiment in combination with the design specifications and parameters to clarify the current-limiting depth η b at current I Ib ; S5-4, Determine T Ib and T I 's relationship: If T Ib ≥T I , then decrease r b , and proceed to step S5-5; If T Ib <T I , then proceed to step S5-6; S5-5, Determine the reduced r b Whether it meets the design specifications and parameters: If not, proceed to step S5-7; if it meets, jump to step S5-2; S5-6, determine the pre-arc time T Ib and the current-limiting depth η Ib to see if they meet the design specifications and parameters: if so, jump to S6; if not, decrease r b and perform step S5-5; S5-7. Increase the opening height of the secondary hole; Determine the pre-arcing time after increasing the opening of the secondary hole according to curve C; Conduct a current-limiting experiment in combination with the design specifications and parameters to obtain the current-limiting depth; Judge whether the two meet the design specifications and parameters: if they meet, jump to S6; if not, repeat step S5-7 until the design requirements are met.

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

5. The optimized design method of a liquid metal current limiter according to claim 3, characterized in that, The current level i includes the short-circuit current I and I b .

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