A fast-switching superconducting fault current limiter transformer and its working method

Abstract

The present invention discloses a fast-switching superconducting fault current limiting transformer and its working method. The superconducting fault current limiting transformer includes a cryogenic dewar, a high-voltage winding, a low-voltage winding, lead terminals of a superconducting winding, a first, a second and a third fast switch; the high-voltage winding and the low-voltage winding are immersed in a cooling liquid in the cryogenic dewar and are led out through tapping heads; each phase of the low-voltage winding is divided into two parts, namely a main current-carrying winding and a main current-limiting winding. The main current-carrying winding is connected in series with the first fast switch through a tapping head, and the main current-limiting winding is connected in series with the second fast switch through a tapping head. The branches where the main current-carrying winding and the main current-limiting winding are located are in parallel; there is a set of mutually insulated lead terminals at intervals along the low-voltage winding; the third fast switch is placed in the gas area at the upper part of the dewar and is connected to the lead terminals or the neutral position of the low-voltage winding. Under a short-circuit fault, by the action of the third fast switch, the length of the superconducting strip cut off when the first fast switch trips is changed, and the current-limiting impedance of the superconducting winding is increased to achieve variable-impedance current limiting.

Description

Technical Field

[0001] The present invention relates to the technical fields of superconducting technology, transformer technology and switchgear technology, and particularly relates to a superconducting transformer device that uses a fast switch to change the winding topology of a superconducting transformer to achieve voltage transformation and multi-topology variable impedance current limiting. Background Art

[0002] With the continuous development of the economy, the electricity demand is increasing continuously, the scale of the power grid is getting larger and larger, the requirement for transmission capacity is also getting larger, and at the same time, the short-circuit current will also increase continuously, endangering the system safety. Based on the zero-resistance characteristic of the superconducting state and the resistance characteristic during normal operation of superconducting materials, and combined with the current-carrying capacity of superconducting materials dozens of times that of traditional conductors, superconducting transformers and superconducting current-limiting transformers can increase the electric energy transmission density and greatly reduce the volume compared with conventional transformers. At the same time, they can also provide effective current limitation by converting the superconducting state of superconducting materials into the normal state from the beginning of a fault without any control system, and have excellent development prospects.

[0003] At present, some scholars at home and abroad have carried out relevant research on superconducting transformers and superconducting current-limiting transformers. However, the current relevant research mainly focuses on small-capacity superconducting transformers, which have low capacity and voltage levels, low current-limiting ability, non-adjustable current-limiting range, relatively low volume and weight advantages compared with power transformers of the same level, and relatively high costs, without economic advantages. Limited by the costs of superconducting materials and refrigeration systems, when the capacity exceeds 10 MVA, superconducting transformers have more obvious economic advantages than conventional power transformers of the same capacity.

[0004] Therefore, superconducting current-limiting transformers with large capacity, high current-limiting rate, high normal operation impedance and fast recovery ability are an important future development direction. However, there is currently little research on superconducting current-limiting transformers with large capacity and high current-limiting rate. Problems such as the fixed current-limiting impedance of existing superconducting current-limiting transformers, all windings losing normal operation and current limiting during a fault current and being difficult to recover need to be solved. Summary of the Invention

[0005] In view of the fact that the existing superconducting transformers and superconducting fault current limiters cannot well achieve the function of fault current limiting, the present invention proposes a fast-switching type superconducting fault current limiter transformer and its working method. The present invention uses different connection methods and operating states of fast switches to change the topological structure and current limiting impedance of the superconducting transformer winding, realizing the variable impedance current limiting function. The opening of the remaining short-circuit current after current limiting is achieved through another fast switch outside the Dewar, clearing the fault of the superconducting fault current limiter transformer. When the superconducting fault current limiter transformer automatically recloses, the winding that is disconnected first has a low degree of quench and fast recovery, and can be connected to the winding that is disconnected first to meet the requirements of the automatic reclosing of the superconducting fault current limiter transformer. After the remaining superconducting windings are fully recovered, they are connected again. The two fast switches outside the Dewar can simultaneously meet the variable impedance current limiting function and the current opening function during normal and fault conditions.

[0006] To achieve the above object, the present invention is implemented by the following technical solutions:

[0007] A fast-switching type superconducting fault current limiter transformer, comprising a cryogenic Dewar 1, a high-voltage winding 2, a low-voltage winding 3, lead terminals of the low-voltage winding, a first fast switch 5, a second fast switch 6, and a third fast switch 7; the low-voltage winding 3 is a superconducting winding, and the high-voltage winding 2 and the low-voltage winding 3 are immersed in the cooling liquid in the cryogenic Dewar 1 and led out through a tapping 4; each phase of the low-voltage winding 3 is divided into a main current-carrying winding and a main current-limiting winding. The main current-carrying winding is connected in series with the first fast switch 5 through the tapping 4, the main current-limiting winding is connected in series with the second fast switch 6 through the tapping 4, and the branches where the main current-carrying winding and the main current-limiting winding are located are in parallel; the low-voltage winding 3 has a set of mutually insulated lead terminals at a preset distance; the third fast switch 7 is placed in the gas region above the cryogenic Dewar and is connected to the lead terminals of the low-voltage winding or a neutral position 8.

[0008] Preferably, the connection method of the high-voltage winding 2 is star or delta, the winding material is a copper winding or a superconducting winding, and when using a superconducting winding, the same topological structure as the low-voltage winding 3 is adopted, and the variable impedance is realized through the action of the fast switch.

[0009] Preferably, the low-voltage winding 3 is composed of multiple superconducting tapes connected in series and parallel, and superconducting tapes with the same or different critical currents and quench impedances are selected according to the current limiting rate requirement for winding. The winding connection method is star or delta; the superconducting winding conductor is yttrium barium copper oxide YBCO or bismuth strontium calcium copper oxide BSCCO.

[0010] Preferably, the number of superconducting tape parallel strands in the main current-carrying winding is more than that in the main current-limiting winding. The main current-carrying winding has a strong current-carrying capacity and a small current-limiting impedance, and is mainly used for carrying current; the main current-limiting winding has a weak current-carrying capacity and a large current-limiting impedance, and is mainly used for fault current limiting; or according to actual needs, the part with fewer superconducting tape parallel strands can be used as the main current-limiting winding, and the part with more superconducting tape parallel strands can be used as the main current-carrying winding; when the third fast switch 7 is located at the outermost lead terminal 6, all the main current-carrying windings are transformed by voltage. When the third fast switch 7 operates to the neutral position 8, the main current-carrying winding is completely cut off when the first fast switch 5 opens, and at this time the current-limiting rate is the largest; the main current-limiting winding is transformed by voltage for a long time and limits the current in case of a short-circuit fault.

[0011] Preferably, the lead terminals of the low-voltage winding are a set of terminals led out at preset intervals of the superconducting winding, or at the joints of the tape according to the tape connection method and the current-limiting rate requirements. The material of the terminal lead wire is a superconducting material or a metal conductor.

[0012] The working method of the fast-switch type superconducting fault current limiter transformer includes the following steps:

[0013] Step 1: During normal operation, the first fast switch 5 and the second fast switch 6 are both closed, the third fast switch 7 is located at the neutral position 8, and all superconducting windings are carrying current and transforming voltage; when performing opening and closing operations under normal conditions, the first fast switch 5 and the second fast switch 6 open or close simultaneously to complete the normal opening and closing operations.

[0014] Step 2: When a short-circuit fault occurs, the superconducting winding quickly loses its superconductivity and initially limits the short-circuit current. The third fast switch 7 has the following three action modes: a) The third fast switch 7 moves to the outermost lead terminal, and at the same time makes the first fast switch 5 open, cutting off the part of the main current-carrying winding in series with it, increasing the current-limiting impedance; b) The third fast switch 7 operates to a certain internal lead terminal, and at the same time makes the first fast switch 5 open, cutting off the part of the main current-carrying winding in series with it, and the current-limiting impedance is even larger; c) The third fast switch 7 is located at the neutral position 8. After the first fast switch 5 opens, the entire main current-carrying winding is cut off. At this time, the current-limiting impedance is the largest and the current-limiting rate is the highest.

[0015] Step 3: After the short-circuit current is limited to the preset range, the second fast switch 6 opens to cut off the remaining short-circuit current, clear the short-circuit fault, and at the same time the third fast switch 7 operates back to the neutral position 8 to prepare for the next closing.

[0016] Step 4: When the superconducting fault current limiter transformer has an automatic reclosing requirement, since the first-disconnected part of the main current-carrying winding has a low degree of quench and a fast recovery rate, and at the same time, the later-disconnected part of the main current-carrying winding can enhance heat dissipation with the help of the first-disconnected part and the recovery speed is accelerated. When reclosing, the main current-carrying winding is connected to meet the automatic reclosing requirement of the superconducting fault current limiter transformer; after the main current-limiting winding is completely restored to the superconducting state, the third fast switch 7 is connected to the circuit.

[0017] Currently, similar patents include: A method for adaptive current protection of a fast-switching variable impedance transformer, application number 201610868221.X. In this patent, a reactor is connected in series with the transformer, which is a combined electrical appliance of the reactor and the transformer. By opening and closing the parallel switch of the reactor, the reactor is put into and cut off to achieve normal lossless current conduction and fault variable impedance current limiting. However, in this solution, the transformer is a conventional transformer, with relatively higher losses compared to superconducting transformers, and at the same time, the volume and weight are also relatively larger.

[0018] A winding structure of a superconducting fault current limiter transformer, application number 202010375883.X. This patent proposes an optimized structure for the winding and connection method of the superconducting winding to enhance heat dissipation and shorten the recovery time after the superconducting winding quenches. However, during the current-limiting process of this superconducting fault current limiter transformer, the current-limiting impedance of the transformer winding remains fixed and it is difficult to adapt to various short-circuit conditions.

[0019] Based on the premise of not changing the amount of superconducting conductor used in the present application, through the action of the fast switch, part of the winding is cut off during short-circuit current, increasing the current-limiting impedance of the winding, realizing variable impedance current limiting, and realizing the fault clearing function after current limiting through the action of the fast switch. At the same time, in the present application, by cutting off different windings in sequence, the recovery time of the first-cut superconducting winding is shortened, realizing the rapid recovery of the superconducting winding.

[0020] Compared with the prior art, the beneficial effects achieved by the present invention are:

[0021] Due to the ingenious application of the fast switch, compared with the existing superconducting fault current limiter transformer, without changing the amount of superconducting winding used, the current-limiting impedance can be increased to N times the impedance of the existing superconducting fault current limiter transformer (N≥1, N is the number of parallel superconducting windings on the low-voltage side), greatly improving the current-limiting depth and avoiding the use of current-limiting reactors. At the same time, due to different connection and action methods of the fast switch, different current-limiting impedances can be realized, meeting the hierarchical adaptive current limiting of different short-circuit conditions.

[0022] The fast switch is used as the circuit breaker on the transformer side. Its opening time is much shorter than that of the power system circuit breaker, usually ≤20 ms. And due to the superconducting current-limiting effect, the current interrupted is the short-circuit current after current-limiting, effectively extending the switch life. There is no need for the power system to use an additional circuit breaker anymore. The functions of superconducting current-limiting and the fast fault clearing of the vacuum switch significantly reduce the impact of the short-circuit current in the power system on the transformer, improve the reliability of the power system, and effectively extend the service life of power equipment.

[0023] During a short-circuit fault, the superconducting windings on the low-voltage side of a conventional superconducting current-limiting transformer will all quench and conduct current for a long time for current-limiting. Its recovery time can reach several seconds due to the inherent characteristics of superconductivity, which cannot meet the requirements of the system's automatic fast reclosing. There is no good solution to this problem at present. In this application, the fast switch is used to first cut off the superconducting windings with a low degree of quenching or without quenching, reducing the superconducting recovery time to less than 1 / 10 of the original. Therefore, it can be quickly connected through the fast switch during reclosing, thus meeting the system reclosing requirements and breaking through the problem that the superconducting quench is difficult to quickly recover and cannot meet the requirements of the system's millisecond-level reclosing.

[0024] Compared with a conventional power transformer, by using superconducting tapes, the volume and weight of the superconducting current-limiting transformer are reduced by more than 40%, and at the same time, the transformer efficiency is as high as 99%. Due to the fast current-limiting characteristics and large current-limiting impedance, by designing multiple levels of current-limiting impedance to reserve the current-limiting impedance value for the increasing trend of short-circuit current in the future, it can meet the growth of short-circuit current in the future without replacing the superconducting current-limiting transformer and other equipment in the system. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 It is the overall topological structure diagram of the present invention.

[0026] Figure 2 It is the topological structure diagram of phase a on the low-voltage side of the present invention.

[0027] Figure 3 It is the topological structure diagram of the circuit of phase a on the low-voltage side of the present invention under different operating states of the fast switch.

[0028] Figure 4 It is the topological structure diagram of the existing AC power system.

[0029] Figure 5 It is the topological structure diagram of the AC power system after using the combined electrical apparatus of the present invention. DETAILED IMPLEMENTATION METHOD

[0030] The following will further describe the present invention in detail in conjunction with the accompanying drawings and specific embodiments.

[0031] As Figure 1As shown in the figure, a fast-switching superconducting fault current limiting transformer of the present invention includes a cryogenic dewar 1, a high-voltage winding 2, a low-voltage winding 3, lead terminals of the superconducting winding (two lead terminals 9 and 10 in this embodiment), a first fast switch 5, a second fast switch 6, and a third fast switch 7; the low-voltage winding 3 is a superconducting winding, and the high-voltage winding 2 and the low-voltage winding 3 are immersed in the cooling liquid in the cryogenic dewar 1 and led out through a tap 4; each phase of the low-voltage winding 3 is divided into a main current-carrying winding and a main current-limiting winding. The main current-carrying winding is connected in series with the first fast switch 5 through the tap 4, and the main current-limiting winding is connected in series with the second fast switch 6 through the tap 4. The branches where the main current-carrying winding and the main current-limiting winding are located are in parallel; there is a set of mutually insulated lead terminals (9, 10) at intervals on the low-voltage winding 3; the third fast switch 7 is placed in the gas area at the upper part of the dewar and can be connected to the lead terminals (9, 10) of the low-voltage winding or the neutral position 8.

[0032] As Figure 2 shown, under normal conditions, the first fast switch 5 and the second fast switch 6 are both closed, and the third fast switch 7 is in the neutral position 8. At this time, all windings are in current-carrying and voltage-changing states. The circuit on the low-voltage side can be opened and closed by the combined action of the first fast switch 5 and the second fast switch 6. At the same time, through the different connection states of the lead terminals (9, 10) or the neutral position 8 of the third fast switch 7, when the first fast switch 5 is disconnected, the length of the cut-off winding can be continuously increased, so as to realize the multi-stage change of the current-limiting impedance of the superconducting winding, and enable the superconducting fault current limiting transformer to achieve better current-limiting ability under different short-circuit currents. In the short-circuit condition, according to the detected short-circuit current, the third fast switch 7 is actuated to the appropriate lead terminals (9, 10) or does not act in the neutral position 8, and then the first fast switch 5 is disconnected to cut off a part of the winding, reducing the number of parallel windings, increasing the current-limiting impedance and current-limiting rate after the winding quenches. After the short-circuit current is limited to a certain range, the second fast switch 6 is disconnected to cut off the remaining short-circuit current, remove the fault, and at the same time, the third fast switch 7 is actuated to the neutral position 8 to complete the reset. In the case of reclosing, the winding connected to the fast switch that is disconnected first passes through a small short-circuit current amplitude, bears the short-circuit current for a short time, quenches and recovers quickly or does not quench. Therefore, reclosing can be achieved by reconnecting the first fast switch 5 (or the second fast switch 6) that is disconnected first.

[0033] In Figure 3The circuit topology diagrams show the low-voltage side a-phase fast switch in different operating states. In the figure, (a) is the circuit structure when the third fast switch 7 is in the neutral position 8 and the first fast switch 5 is open; (b) is the circuit structure when the third fast switch 7 is in the neutral position 8 and the second fast switch 6 is open; (c) is the circuit structure when the third fast switch 7 is at the outer terminal 9 and the first fast switch 5 is open; (d) is the circuit structure when the third fast switch 7 is at the outer terminal 9 and the second fast switch 6 is open; (e) is the circuit structure when the third fast switch 7 is at the inner terminal 10 and the first fast switch 5 is open; (f) is the circuit structure when the third fast switch 7 is at the inner terminal 2(10) and the second fast switch 6 is open.

[0034] Figure 4 The existing AC power system topology diagram shows that the upper-level power system bus and the lower-level bus are connected by switches, transformers, and reactors, and circuit breakers are connected to both the high-voltage and low-voltage sides of the transformer.

[0035] Figure 5 The AC power system topology diagram after using the fast-switch type superconducting fault current limiter transformer of the present invention shows that by comparing the two, it can be found that after using the variable impedance topology structure of the present application on the low-voltage side winding, the use of circuit breakers and current limiting reactors can be reduced, which is beneficial to reducing the floor area occupied by AC system equipment.

Claims

1. A fast-switching superconducting fault current limiting transformer, characterized in that: The described fast-switching superconducting fault current limiting transformer includes a cryogenic dewar (1), a high-voltage winding (2), a low-voltage winding (3), lead terminals of the low-voltage winding, a first fast switch (5), a second fast switch (6), and a third fast switch (7); the low-voltage winding (3) is a superconducting winding, and the high-voltage winding (2) and the low-voltage winding (3) are immersed in the cooling liquid in the cryogenic dewar (1) and led out through a tapping (4); each phase of the low-voltage winding (3) is divided into a main current-carrying winding and a main current-limiting winding. The main current-carrying winding is connected in series with the first fast switch (5) through the tapping (4), and the main current-limiting winding is connected in series with the second fast switch (6) through the tapping (4). The branches where the main current-carrying winding and the main current-limiting winding are located are in parallel; there is a set of mutually insulated lead terminals at every preset distance on the low-voltage winding (3); the third fast switch (7) is placed in the upper gas region of the cryogenic dewar and is connected to the lead terminals of the low-voltage winding or the neutral position. The working method of the described fast-switching superconducting fault current limiting transformer includes the following steps: Step 1: During normal operation, both the first fast switch (5) and the second fast switch (6) are closed, and the third fast switch (7) is at the neutral position (8). All superconducting windings are conducting current and transforming voltage; when performing opening and closing operations under normal conditions, the first fast switch (5) and the second fast switch (6) can complete normal opening and closing operations by opening or closing simultaneously. Step 2: When a short-circuit fault occurs, the superconducting winding quickly loses its superconductivity and initially limits the short-circuit current. The third fast switch (7) has the following three action modes: a) The third fast switch (7) moves to the outermost lead terminal, and at the same time, the first fast switch (5) is opened to cut off the part of the main current-carrying winding in series with it, increasing the current-limiting impedance; b) The third fast switch (7) moves to a certain internal lead terminal, and at the same time, the first fast switch (5) is opened to cut off the part of the main current-carrying winding in series with it, and the current-limiting impedance is even larger. c) The third fast switch (7) is at the neutral position (8). After the first fast switch (5) is opened, the entire main current-carrying winding is cut off. At this time, the current-limiting impedance is the largest and the current-limiting rate is the highest. Step 3: After the short-circuit current is limited to the preset range, the second fast switch (6) is opened to cut off the remaining short-circuit current, clear the short-circuit fault, and at the same time, the third fast switch (7) moves back to the neutral position (8) to prepare for the next closing. Step 4: When the superconducting fault current limiting transformer has an automatic reclosing requirement, since the part of the main current-carrying winding that is disconnected first has a low degree of loss of superconductivity and a fast recovery speed, and at the same time, the part of the main current-carrying winding that is disconnected later can enhance heat dissipation with the help of the part that is disconnected first, the recovery speed is accelerated. When reclosing, the main current-carrying winding is connected to meet the automatic reclosing requirement of the superconducting fault current limiting transformer. After the main current-limiting winding is completely restored to the superconducting state, it is then connected to the circuit by the third fast switch (7).

2. The fast-switching superconducting fault current limiting transformer according to claim 1, characterized in that: The connection mode of the described high-voltage winding (2) is star or delta. The winding material is a copper winding or a superconducting winding. When using a superconducting winding, the same topological structure as the low-voltage winding (3) is adopted, and the variable impedance is achieved through the action of the fast switch.

3. The fast-switching superconducting fault current limiting transformer according to claim 1, characterized in that: The low-voltage winding (3) is composed of multiple superconducting tapes connected in series and parallel, and superconducting tapes with the same or different critical currents and quench impedances are selected for winding according to the required current limiting rate. The winding connection method is star or delta; the superconducting winding conductor uses yttrium barium copper oxide YBCO or bismuth strontium calcium copper oxide BSCCO.

4. The fast-switching superconducting fault current limiting transformer according to claim 1, characterized in that: The number of parallel superconducting tapes in the main current-carrying winding is more than that in the main current-limiting winding. The main current-carrying winding has strong current-carrying capacity and small current-limiting impedance, and is mainly used for carrying current; the main current-limiting winding has weak current-carrying capacity and large current-limiting impedance, and is mainly used for fault current limiting. Or according to actual needs, the part with fewer parallel superconducting tapes is used as the main current-limiting winding, and the part with more parallel superconducting tapes is used as the main current-carrying winding; when the third fast switch (7) is located at the outermost lead terminal (6), all the current in the main current-carrying winding changes voltage. When the third fast switch (7) operates to the neutral position (8), the main current-carrying winding is completely cut off when the first fast switch (5) opens. At this time, the current limiting rate is the largest; the main current-limiting winding changes voltage by carrying current for a long time and limits current in case of a short-circuit fault.

5. The fast-switching superconducting fault current limiting transformer according to claim 1, characterized in that: The lead terminals of the low-voltage winding are a set of terminals led out from the superconducting winding at preset intervals, or at the joints of the tapes according to the tape connection method and the requirements of the current limiting rate. The material of the terminal lead wire uses superconducting material or metal conductor.

6. The fast-switching superconducting fault current limiting transformer according to claim 1, characterized in that: The operation of the third fast switch (7) can short-circuit different groups of terminals, so that the length of the cut-off tape changes after the first fast switch (5) or the second fast switch (6) operates to open, and the current-limiting impedance after quenching is changed step by step. The third fast switch (7) uses a power electronic switch, a fast vacuum switch or a liquid nitrogen switch.

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