Transition loop current limiting and overvoltage absorption structure of bridge arm type on-load voltage regulating device

Abstract

The utility model provides a kind of transition loop current limiting and overvoltage absorption structure of bridge arm type on-load voltage regulating device, comprising: first thermistor element and the first end of first thermistor element connects bridge arm input common node, second end is connected with the first end of corresponding transition switch, the second end of transition switch is connected with corresponding output bus;First varistor component, connect between bridge arm input common node and upper output bus, or respectively parallel in the two ends of each switch in upper bridge arm switch group;Second varistor component, connect between bridge arm input common node and lower output bus, or respectively parallel in the two ends of each switch in lower bridge arm switch group.The utility model utilizes thermistor element self-adapting to limit transition loop circulation, utilizes varistor component to quickly absorb operating overvoltage, while solve the problem of excessive circulation and overvoltage in voltage regulating switching process.

Description

Technical Field

[0001] This utility model belongs to the field of on-load tap changer technology for power equipment, specifically relating to a current limiting and overvoltage absorption structure for the transition circuit of a bridge-arm type on-load tap changer. Background Technology

[0002] Bridge-arm on-load tap changers are widely used in power transformer voltage regulation. They combine different potentials of multi-tap regulating windings through upper and lower bridge-arm switch groups, enabling reactive or active voltage regulation at multiple tap levels while the transformer is under load. During the switching between adjacent tap levels, to ensure uninterrupted current on the load side, a first transition switch K1 is typically installed between the common input node of the bridge arm and the upper output bus, and a second transition switch K2 is installed between the common input node of the bridge arm and the lower output bus. These two transition switches are briefly activated during the switching process to form a temporary closed current path, ensuring the continuity of the load current during the switching period.

[0003] At the instant that transition switches K1 or K2 are turned on, a voltage difference exists between the newly connected tap and the old tap because the bridge arm switch corresponding to the old tap has not yet been completely disconnected. This voltage difference is typically an integer multiple of the voltage step size U. This voltage difference forms a driving potential through the transition switch circuit, causing a circulating current to flow through the transition switch and its connected circuit. When the leakage reactance of the multi-tap voltage regulating winding is small or the overlapping conduction time of the switches during the switching process is long, the amplitude of this circulating current may reach a large level, generating significant thermal stress and electrodynamic impact on the contacts of the transition switch, accelerating contact wear, and in severe cases, even causing contact welding. At the same time, during the opening and closing process of the bridge arm switch and the transition switch, due to the presence of inductive components in the circuit, including the leakage inductance of the multi-tap voltage regulating winding and the distributed inductance of the connecting lines, the magnetic field energy stored in the inductive components at the instant of switch opening will be converted into an operational overvoltage, superimposed on both ends of the switch contacts and the output bus. The amplitude of this operational overvoltage may exceed the insulation withstand level of the bridge arm switch and the transition switch, causing damage to the switching devices and their insulation structure.

[0004] In existing technologies, to address the problem of excessive circulating current in the transition circuit, measures such as increasing the rated current-carrying capacity of the transition switching switch or shortening the overlap conduction time of the switch are typically adopted. However, increasing the current-carrying capacity of the switch leads to an increase in the cost and size of the switching devices, while shortening the overlap conduction time places extremely high demands on the control accuracy of the switch action sequence, significantly increasing the difficulty of engineering implementation. Regarding the problem of switching overvoltage, existing protection schemes generally install surge arresters on the high-voltage or low-voltage side of the transformer. However, the installation location of the surge arresters is relatively far from the bridge arm switch, resulting in limited overvoltage suppression effect across the bridge arm switch contacts, making it difficult to effectively protect against transient overvoltages generated during switch opening. Therefore, existing technologies struggle to simultaneously suppress circulating current in the transition circuit and effectively absorb switching overvoltages without excessively increasing equipment costs and complexity. Utility Model Content

[0005] To address the aforementioned problems in the existing technology, this utility model provides a current-limiting and overvoltage absorption structure for the transition circuit of a bridge-arm type on-load tap changer. The technical problem to be solved by this utility model is achieved through the following technical solution: This utility model provides a transition circuit current limiting and overvoltage absorption structure for a bridge-arm type on-load tap changer, applied in a bridge-arm type on-load tap changer. The bridge-arm type on-load tap changer includes a multi-tap tap regulating winding 1, a bridge-arm type on-load tap changer switch, and a transition closed circuit structure. The bridge-arm type on-load tap changer switch includes an upper bridge-arm switch group and a lower bridge-arm switch group. The output terminals of each switch in the upper bridge-arm switch group are connected to the upper output bus 7a, and the output terminals of each switch in the lower bridge-arm switch group are connected to the lower output bus 7b. The transition closed circuit structure includes a first transition switching switch 4a and a second transition switching switch 4b. The transition circuit current limiting and overvoltage absorption structure includes: The first thermistor element 10a has a first end for connecting to the bridge arm input common node 2, and its second end is connected to the first end of the first transition switch 4a. The second end of the first transition switch 4a is used to connect to the upper output bus 7a. The second thermistor element 10b has a first end for connecting to the bridge arm input common node 2, and its second end is connected to the first end of the second transition switch 4b. The second end of the second transition switch 4b is used to connect to the lower output bus 7b. The first varistor component 11a is connected between the common input node 2 of the bridge arm and the upper output bus 7a, or is connected in parallel to both ends of each switch in the upper bridge arm switch group. The second varistor assembly 11b is connected between the common input node 2 of the bridge arm and the lower output bus 7b, or is connected in parallel to both ends of each switch in the lower bridge arm switch group.

[0006] Beneficial effects: First, this invention connects a thermistor element in series in the transition switching circuit. By utilizing its positive temperature coefficient characteristic, the resistance value is adaptively increased when the circulating current passes through the transition circuit to limit the amplitude of the circulating current. No external control circuit is required. The structure is simple and the response is fast. It effectively protects the transition switching contacts from the impact of large currents and extends the service life of the contacts.

[0007] Secondly, this utility model connects a varistor assembly in parallel between the input common node of the bridge arm and the output bus or across the two ends of the bridge arm switch. When the operating overvoltage generated at the moment the bridge arm switch or transition switch is turned off exceeds the varistor voltage, it quickly absorbs the overvoltage energy and clamps the voltage to a safe level, effectively protecting the bridge arm switch, transition switch and its insulation structure from overvoltage damage.

[0008] Third, the thermistor element and the varistor assembly of this utility model provide protection against two different transient threats: transient loop circulating current and operating overvoltage, forming a complete transient loop protection system. The two protection functions are independent of each other and do not interfere with each other.

[0009] Fourth, the current limiting and overvoltage absorption structure of the transition circuit of the bridge arm type on-load tap changer provided by this utility model is directly set near the transition closed circuit and the bridge arm switch, which is closest to the protected device, has a fast response speed and good suppression effect, and is superior to the traditional protection scheme of setting up surge arresters at the far end of the high voltage side or low voltage side of the transformer.

[0010] Fifth, during steady-state operation, the current limiting and overvoltage absorption structure of the transition circuit of the bridge arm type on-load tap changer provided by this utility model ensures that the thermistor element is in a low-resistance state with no current flowing through it, and the varistor component is in a high-resistance state with minimal leakage current. It does not participate in the voltage regulation circuit, does not increase the steady-state operation loss of the device, and does not affect the normal voltage regulation function of the bridge arm type on-load tap changer.

[0011] The present invention will be further described in detail below with reference to the accompanying drawings and embodiments. Attached Figure Description

[0012] Figure 1 This is a schematic diagram of the current limiting and overvoltage absorption structure of the transition circuit of a bridge-arm type on-load tap changer provided by this utility model. Detailed Implementation

[0013] The present invention will be further described in detail below with reference to specific embodiments, but the implementation of the present invention is not limited thereto.

[0014] This utility model specifically relates to a current limiting and overvoltage absorption structure for the transition circuit of a bridge arm type on-load tap changer. By connecting a positive temperature coefficient thermistor element in series in the transition switching circuit and connecting a zinc oxide varistor component in parallel between the bridge arm input common node and the output bus, the circulating current in the transition circuit and the absorption of operating overvoltage are achieved during the tap changer switching process.

[0015] like Figure 1 As shown, this utility model provides a current limiting and overvoltage absorption structure for the transition circuit of a bridge-arm type on-load tap changer, applied in a bridge-arm type on-load tap changer. The bridge-arm type on-load tap changer includes a multi-tap tap regulating winding 1, a bridge-arm type on-load tap changer switch, and a transition closed loop structure. The bridge-arm type on-load tap changer switch includes an upper bridge-arm switch group and a lower bridge-arm switch group. The output terminals of each switch in the upper bridge-arm switch group are connected to the upper output bus 7a, and the output terminals of each switch in the lower bridge-arm switch group are connected to the lower output bus 7b. The transition closed loop structure includes a first transition switching switch 4a and a second transition switching switch 4b. The transition circuit current limiting and overvoltage absorption structure includes: The first thermistor element 10a has a first end for connecting to the bridge arm input common node 2, and its second end is connected to the first end of the first transition switch 4a. The second end of the first transition switch 4a is used to connect to the upper output bus 7a. The second thermistor element 10b has a first end for connecting to the bridge arm input common node 2, and its second end is connected to the first end of the second transition switch 4b. The second end of the second transition switch 4b is used to connect to the lower output bus 7b. The first varistor component 11a is connected between the common input node 2 of the bridge arm and the upper output bus 7a, or is connected in parallel to both ends of each switch in the upper bridge arm switch group. The second varistor assembly 11b is connected between the common input node 2 of the bridge arm and the lower output bus 7b, or is connected in parallel to both ends of each switch in the lower bridge arm switch group.

[0016] The parallel connection of the zinc oxide varistor assembly is not limited to the bridge arm input common node 2 and the output bus. It can also be set in parallel at both ends of each bridge arm switch to provide more targeted overvoltage protection for a single bridge arm switch.

[0017] The multi-tap voltage regulating winding 1 is located on the voltage regulating side of the transformer and has at least three taps along the winding direction, designated as the first tap, the second tap, and the third tap. The voltage difference between adjacent taps is a preset voltage step size. The common lead of each tap is combined to form the bridge arm input common node 2. The upper bridge arm switch group includes a first upper bridge arm switch 3a, a second upper bridge arm switch 3b, and a third upper bridge arm switch 3c. The input terminals of each upper bridge arm switch are electrically connected to the corresponding tap of the multi-tap voltage regulating winding 1, and the output terminals of each upper bridge arm switch are connected to the upper output bus 7a. The lower bridge arm switch group includes a first lower bridge arm switch 3d, a second lower bridge arm switch 3e, and a third lower bridge arm switch 3f. The input terminals of each lower bridge arm switch are electrically connected to the corresponding tap of the multi-tap voltage regulating winding 1, and the output terminals of each lower bridge arm switch are connected to the lower output bus 7b.

[0018] refer to Figure 1 ,exist Figure 1 The first transition switch is designated as 4a, and the second transition switch is designated as 4b. The first upper bridge arm switch is designated as 3a, the second upper bridge arm switch as 3b, and the third upper bridge arm switch as 3c; the first lower bridge arm switch is designated as 3d, the second lower bridge arm switch as 3e, and the third lower bridge arm switch as 3f; the first thermistor element is designated as 10a, and the second thermistor element as 10b; the first varistor assembly is designated as 11a, and the second varistor assembly as 11b.

[0019] In an optional embodiment of this utility model, the first thermistor element 10a and the second thermistor element 10b are respectively installed on the incoming side of the first transition switch 4a and the second transition switch 4b near the bridge arm input common node 2, or installed on the outgoing side of the first transition switch 4a and the second transition switch 4b near the upper output bus 7a or the lower output bus 7b.

[0020] The series connection position of the first thermistor element 10a and the second thermistor element 10b is not limited to the side of the input line of the transition switch near the common node of the bridge arm input. It can also be set on the side of the output line of the transition switch near the output bus, as long as it is connected in series with the transition switch.

[0021] In an optional embodiment of this utility model, there is one first varistor component 11a and one second varistor component 11b. The first varistor component 11a is connected in parallel between the bridge arm input common node 2 and the upper output bus 7a; The second varistor component 11b is connected in parallel between the bridge arm input common node 2 and the lower output bus 7b.

[0022] In this embodiment, the first varistor component 11a and the second varistor component 11b are located between the bridge arm input common node 2 and the output bus.

[0023] In an optional embodiment of this utility model, there are three of each of the first varistor component 11a and the second varistor component 11b. The first upper bridge arm switch 3a, the second upper bridge arm switch 3b and the third upper bridge arm switch 3c are all connected in parallel with one of the first varistor components 11a;

[0024] The first lower bridge arm switch 3d, the second lower bridge arm switch 3e, and the third lower bridge arm switch 3f are all connected in parallel with a second varistor component 11b.

[0025] In this embodiment, each bridge arm switch is connected in parallel at both ends to provide more targeted overvoltage protection for a single bridge arm switch.

[0026] In an optional embodiment, the first thermistor element 10a and the second thermistor element 10b are PTC thermistor elements or are replaced with power resistors with fixed resistance values; the PTC thermistor element has a cold resistance value of 3Ω to 50Ω at an ambient temperature of 25°C, and the Curie temperature is set between 80°C and 120°C, or set to be 20K to 30K higher than the highest ambient temperature inside the device.

[0027] The first thermistor element 10a and the second thermistor element 10b can be PTC thermistors, which are positive temperature coefficient thermistors, preferably made of barium titanate-based ceramic semiconductor. The PTC thermistor element can be disc-shaped, columnar, or stacked, with its diameter or effective cross-sectional area determined by its rated current carrying capacity, and its thickness or number of series plates determined by its rated voltage. The cold resistance of the PTC thermistor element at 25°C is preferably 3Ω~50Ω, and its Curie temperature is preferably set between 80°C and 120°C. In applications with higher internal ambient temperatures, the Curie temperature can also be selected to be 20K~30K higher than the highest internal ambient temperature. The rated voltage of the PTC thermistor element is not lower than the maximum power frequency voltage that may occur between the bridge arm input common node and the corresponding output bus. Its short-time current carrying capacity and short-time energy tolerance are determined based on the maximum expected transient circulating current, the duration of a single switching operation, and the allowable temperature rise. When the current flowing through it causes its temperature to exceed the Curie temperature, the resistance increases dramatically, by several orders of magnitude, thereby adaptively limiting the amplitude of the current flowing through it.

[0028] The function of the PTC thermistor element is as follows: after the transition switch 4a or 4b is turned on, when the voltage difference between the old and new taps drives the circulating current through the transition circuit, the temperature of the PTC thermistor element rises due to the Joule heating effect of the circulating current, and the resistance value increases sharply, thereby adaptively limiting the amplitude of the circulating current and protecting the contacts of the transition switch from large current impact.

[0029] The first thermistor element 10a and the second thermistor element 10b can also be replaced with a power resistor with a fixed resistance value. However, the fixed resistance resistor does not have adaptive current limiting characteristics and its resistance value needs to be preset according to the maximum circulating current amplitude. During normal steady-state operation, the transition switches 4a and 4b are in the open state, and the normal load current mainly flows through the bridge arm switch corresponding to the current position, without passing through the transition switch and its series-connected thermistor element. Therefore, under the normal timing of this scheme, if the fixed resistor is used as a replacement element, it usually only carries current for a short time during the switching transition and does not bear the long-term load current. However, the fixed resistor does not have the adaptive resistance increase characteristic of the PTC. If the transition circuit is turned on for a long time due to control failure, mechanical jamming, or special operating strategy, the fixed resistor will generate continuous power consumption and voltage drop, which poses risks of temperature rise, efficiency, and insulation. Therefore, it can be replaced but is not preferred.

[0030] In a specific embodiment of this utility model, the first varistor component 11a and / or the second varistor component 11b is a zinc oxide varistor component, a silicon carbide varistor, a gas discharge tube, or a transient voltage suppression diode.

[0031] The zinc oxide varistor component is a zinc oxide (ZnO) based metal oxide varistor, which is circular or square in shape and is composed of multiple zinc oxide varistor chips connected in series or parallel, encapsulated in an insulating shell. The zinc oxide varistor component exhibits nonlinear voltage-current characteristics: under normal operating voltage, its resistance is extremely high, on the order of megaohms, and its leakage current is extremely small, equivalent to an open circuit; when the voltage across its terminals exceeds the varistor voltage (operating voltage), the resistance decreases sharply, enabling it to absorb overvoltage energy as heat in a very short time, on the order of nanoseconds.

[0032] The varistor component selected in this invention only needs to meet the requirements of overvoltage protection for bridge-arm type on-load tap changers in terms of response speed and energy absorption capacity.

[0033] In a specific embodiment of this utility model, the first varistor component 11a and the second varistor component 11b are composed of multiple varistor sheets connected in series or in parallel and encapsulated in an insulating shell; the varistor voltage of the first varistor component 11a is set to be higher than the peak value of the normal operating voltage between the bridge arm input common node 2 and the upper output bus 7a, and lower than the insulation withstand voltage of each switch in the upper bridge arm switch group and the first transition switching switch 4a; the varistor voltage of the second varistor component 11b is set to be higher than the peak value of the normal operating voltage between the bridge arm input common node 2 and the lower output bus 7b, and lower than the insulation withstand voltage of each switch in the lower bridge arm switch group and the second transition switching switch 4b.

[0034] The varistor voltage of the first varistor component 11a is set to be higher than the peak value of the normal operating voltage between the bridge arm input common node and the upper output bus, but lower than the insulation withstand voltage of the bridge arm switch and the transition switching. The varistor voltage setting principle of the second varistor component 11b is the same as that of the first varistor component 11a.

[0035] In a specific embodiment of this utility model, the voltage values ​​presented by the first varistor component 11a and the second varistor component 11b when they pass a 1mA DC current are taken as 1.2 to 1.5 times the peak value of the normal operating voltage between the corresponding nodes, and the ratio of their residual voltage under the nominal inrush current to the voltage value under the 1mA DC current is not greater than 1.8.

[0036] Preferably, the DC 1mA varistor voltage U1mA of the first varistor component 11a is taken as 1.2 to 1.5 times the peak value of the normal operating voltage between the corresponding nodes, the residual voltage ratio is not greater than 1.8, and its residual voltage under the nominal inrush current is lower than the withstand voltage of the protected switch and insulation structure. The varistor voltage setting principle of the second zinc oxide varistor component 11b is the same. The nominal discharge current and energy absorption capacity of the zinc oxide varistor component are determined according to the voltage level of the bridge arm on-load tap changer, the expected operating overvoltage energy, and the safety margin.

[0037] The function of the first varistor component 11a and the second varistor component 11b is as follows: at the instant the bridge arm switch or transition switch is turned off, when the operating overvoltage generated by the inductive element in the circuit is superimposed between the bridge arm input common node 2 and the output bus, the zinc oxide varistor component quickly changes from a high resistance state to a low resistance state, absorbs the overvoltage energy and dissipates it in the form of heat, clamps the voltage between the bridge arm input common node 2 and the output bus below a safe level, and protects the bridge arm switch, transition switch and their insulation structure from overvoltage damage.

[0038] In a specific embodiment of this utility model, a preset air gap is maintained between the first thermistor element 10a, the second thermistor element 10b and the corresponding first varistor assembly 11a, the second varistor assembly 11b, or a heat-resistant insulating partition is provided.

[0039] The first thermistor element 10a, the second thermistor element 10b, and the corresponding zinc oxide varistor assembly maintain electrical clearances and creepage distances that meet the voltage level requirements of the device. Preferably, in a 10kV device, the air gap between the thermistor element and the zinc oxide varistor assembly is not less than 30mm; in a 35kV device, the air gap between them is not less than 50mm.

[0040] Furthermore, when the PTC thermistor element and the zinc oxide varistor assembly are arranged adjacent to each other or when the installation space is limited, a heat-resistant insulating partition or heat-insulating baffle is set between them to reduce the impact of short-term heating of the PTC thermistor element on the temperature rise, leakage current and volt-ampere characteristics of the zinc oxide varistor assembly.

[0041] The working principle and dynamic working process of this utility model are divided into steady-state operation, transition switching switch conduction and circulating current limiting process, and operation overvoltage absorption process.

[0042] The steady-state operating conditions are as follows: When the bridge-arm type on-load tap changer is in steady-state voltage regulation operation, the bridge arm switch corresponding to the current gear is in the closed conducting state, and the load current flows through the conducting mechanical bridge arm switch. Transition switches 4a and 4b are both in the open state, and no current flows through the thermistor elements 10a and 10b; they are in a low-resistance state at room temperature. The voltage across the varistor components 11a and 11b is the normal operating voltage, lower than their varistor voltage, and they are in a high-resistance state with minimal leakage current, not affecting the normal voltage regulation function of the bridge-arm type on-load tap changer.

[0043] The transition switch activation and circulating current limiting process is as follows: When the on-load tap changer of the boom type switches between adjacent tap changer positions, taking the boom switch 3a being open in the old position and the boom switch 3b being closed in the new position as an example: Before the bridge arm switch 3a is opened, the transition switch 4a is turned on first, so that the bridge arm input common node 2 forms a temporary closed loop with the upper output bus through the thermistor element 10a and the transition switch 4a, providing a continuous current path for the load.

[0044] During the overlapping conduction period after the transition switch 4a is turned on but before the bridge arm switch 3a is fully turned off, a voltage difference exists between the first tap (corresponding to 3a) and the common input node of the bridge arm. This voltage difference drives a circulating current through the loop: "first tap → bridge arm switch 3a → upper output bus → transition switch 4a → thermistor element 10a → bridge arm input common node → first tap". At this time, the thermistor element 10a is connected in series in this circulating current loop. The circulating current flowing through the thermistor element 10a generates Joule heat, causing the temperature of the thermistor element 10a to rise. When the temperature of the thermistor element 10a exceeds its Curie temperature, the resistance value of the thermistor element 10a increases sharply, thereby effectively limiting the amplitude of the circulating current and preventing excessive circulating current from causing thermal stress and electrodynamic shock to the contacts of the transition switch 4a and the bridge arm switch 3a.

[0045] When the first tap 3a is completely disconnected, the circulating current circuit is cut off, and there is no longer any circulating current flowing through the thermistor element 10a. The thermistor element 10a gradually cools down and returns to a low-resistance state at room temperature, preparing for the next voltage regulation switch.

[0046] The overvoltage absorption process is as follows: At the instant the bridge arm switch 3a is opened or the transition switch 4a is opened, the magnetic field energy stored in the inductive components (leakage inductance of the multi-tap voltage regulating winding, line inductance, etc.) in the circuit is released, generating an operational overvoltage between the bridge arm input common node and the upper output bus.

[0047] When the overvoltage exceeds the varistor voltage of the first zinc oxide varistor component, the first zinc oxide varistor component quickly changes from a high resistance state to a low resistance state. The overvoltage energy is dissipated as heat through the first zinc oxide varistor component. The voltage between the bridge arm input common node and the upper output bus is clamped at the residual voltage level of the first zinc oxide varistor component (slightly higher than the varistor voltage), protecting the bridge arm switch, transition switch and its insulation structure from overvoltage damage.

[0048] After the overvoltage energy is dissipated, the voltage between the bridge arm input common node and the upper output bus returns to the normal operating voltage level, and the first zinc oxide varistor component returns to the high resistance state.

[0049] The current limiting process of the thermistor element 10b in the transition switching switch 4b circuit on the lower bridge arm side, and the overvoltage absorption process of the varistor assembly 11b, are the same as the working process on the upper bridge arm side.

[0050] See Table 1, which is a sample numerical table.

[0051]

[0052] Table 1 shows that the selection principles and space arrangement requirements for thermistor elements and varistor assemblies for 10kV and 35kV voltage level devices are as follows: In 10kV voltage level devices, the recommended range for the cold resistance of the PTC element at an ambient temperature of 25℃ is 3Ω to 20Ω, and the Curie temperature is preferably set between 80℃ and 100℃. Its rated voltage should not be lower than the maximum power frequency voltage that may occur in the transition circuit, and its short-time current carrying capacity and energy tolerance need to be checked based on the maximum expected transition circulating current and the duration of a single switch. For the corresponding varistor assembly, its DC 1mA varistor voltage U1mA should be 1.2 to 1.5 times the peak value of the normal operating voltage between the corresponding nodes, the residual voltage ratio should not be greater than 1.8, and the nominal discharge current (8 / 20μs waveform) should be selected from 5kA to 10kA. At the same time, the air gap between the PTC element and the MOV assembly should not be less than 30mm. This configuration is suitable for occasions with small tap voltage steps and low switching energy.

[0053] In 35kV voltage level installations, the cold resistance range of PTC elements is increased to 10Ω to 50Ω, and the Curie temperature is adjusted accordingly to 100℃ to 120℃, while the rated voltage and short-time withstand capability verification principles remain unchanged. The DC 1mA varistor voltage U1mA of the zinc oxide varistor assembly is still selected at 1.2 to 1.5 times the peak normal operating voltage, and the residual voltage ratio remains no greater than 1.8. However, the nominal discharge current is increased to 10kA to 20kA to accommodate higher operating overvoltage energy. Simultaneously, the air gap requirement between the PTC element and the MOV assembly is increased to no less than 50mm to cope with application scenarios with larger tap voltage steps and more severe operating overvoltage energy. Furthermore, at both voltage levels, if the PTC element and MOV assembly are arranged adjacent to each other due to limited installation space, it is recommended to add a heat-resistant insulating partition or heat-insulating baffle between them to reduce the adverse effects of PTC short-time heating on MOV temperature rise, leakage current, and volt-ampere characteristics.

[0054] It is worth noting that the terms "first" and "second" in this utility model are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of indicated technical features. Therefore, a feature defined as "first" or "second" may explicitly or implicitly include one or more of that feature. In the description of this utility model, "a plurality of" means two or more, unless otherwise explicitly specified.

[0055] The above description, in conjunction with specific preferred embodiments, provides a further detailed explanation of the present invention. It should not be construed that the specific implementation of the present invention is limited to these descriptions. For those skilled in the art, various simple deductions or substitutions can be made without departing from the concept of the present invention, and all such modifications and substitutions should be considered within the protection scope of the present invention.

Claims

1. A current-limiting and overvoltage absorption structure for a transition circuit of a bridge-arm type on-load tap changer, applied in a bridge-arm type on-load tap changer, the bridge-arm type on-load tap changer comprising a multi-tap tap regulating winding (1), a bridge-arm type on-load tap changer switch, and a transition closed loop structure, the bridge-arm type on-load tap changer switch comprising an upper bridge-arm switch group and a lower bridge-arm switch group, the output terminals of each switch in the upper bridge-arm switch group being connected to the upper output bus (7a), and the output terminals of each switch in the lower bridge-arm switch group being connected to the lower output bus (7b), the transition closed loop structure comprising a first transition switching switch (4a) and a second transition switching switch (4b), characterized in that, The transition circuit current limiting and overvoltage absorption structure includes: The first thermistor element (10a) has a first end for connecting to the bridge arm input common node (2) and a second end for connecting to the first end of the first transition switch (4a), the second end of which is used to connect to the upper output bus (7a). The second thermistor element (10b) has a first end for connecting to the bridge arm input common node (2) and a second end for connecting to the first end of the second transition switch (4b), the second end of which is used to connect to the lower output bus (7b). The first varistor assembly (11a) is connected between the common input node (2) of the bridge arm and the upper output bus (7a), or is connected in parallel to both ends of each switch in the upper bridge arm switch group; The second varistor assembly (11b) is connected between the common input node (2) of the bridge arm and the lower output bus (7b), or is connected in parallel to both ends of each switch in the lower bridge arm switch group.

2. The current limiting and overvoltage absorption structure of the transition circuit of the bridge arm type on-load tap changer according to claim 1, characterized in that, The first thermistor element (10a) and the second thermistor element (10b) are respectively installed on the incoming side of the first transition switch (4a) and the second transition switch (4b) near the bridge arm input common node (2), or installed on the outgoing side of the first transition switch (4a) and the second transition switch (4b) near the upper output bus (7a) or the lower output bus (7b).

3. The current limiting and overvoltage absorption structure of the transition circuit of the bridge arm type on-load tap changer according to claim 1, characterized in that, There is one first varistor component (11a) and one second varistor component (11b). The first varistor assembly (11a) is connected in parallel between the bridge arm input common node (2) and the upper output bus (7a); The second varistor assembly (11b) is connected in parallel between the bridge arm input common node (2) and the lower output bus (7b).

4. The current limiting and overvoltage absorption structure of the transition circuit of the bridge arm type on-load tap changer according to claim 1, characterized in that, The multi-tap voltage regulating winding (1) is located on the voltage regulating side of the transformer and has at least three taps along the winding direction. The voltage difference between adjacent taps is a preset voltage step size. The common lead of each tap is combined to form the bridge arm input common node (2). The upper bridge arm switch group includes a first upper bridge arm switch (3a), a second upper bridge arm switch (3b) and a third upper bridge arm switch (3c). The input terminal of each upper bridge arm switch is electrically connected to the corresponding tap of the multi-tap voltage regulating winding (1), and the output terminal of each upper bridge arm switch is connected to the upper output bus (7a). The lower bridge arm switch group includes a first lower bridge arm switch (3d), a second lower bridge arm switch (3e) and a third lower bridge arm switch (3f). The input terminal of each lower bridge arm switch is electrically connected to the corresponding tap of the multi-tap voltage regulating winding (1), and the output terminal of each lower bridge arm switch is connected to the lower output bus (7b).

5. The current limiting and overvoltage absorption structure of the transition circuit of the bridge arm type on-load tap changer according to claim 4, characterized in that, There are three of each of the first varistor component (11a) and the second varistor component (11b). The first upper bridge arm switch (3a), the second upper bridge arm switch (3b) and the third upper bridge arm switch (3c) are all connected in parallel with one of the first varistor components (11a). The first lower bridge arm switch (3d), the second lower bridge arm switch (3e), and the third lower bridge arm switch (3f) are all connected in parallel with a second varistor component (11b).

6. The current limiting and overvoltage absorption structure of the transition circuit of the bridge arm type on-load tap changer according to claim 1, characterized in that, The first thermistor element (10a) and the second thermistor element (10b) are PTC thermistor elements or can be replaced by power resistors with fixed resistance values; the PTC thermistor element has a cold resistance value of 3Ω to 50Ω at an ambient temperature of 25°C, and the Curie temperature is set between 80°C and 120°C, or set to be 20K to 30K higher than the highest ambient temperature inside the device.

7. The current limiting and overvoltage absorption structure of the transition circuit of the bridge arm type on-load tap changer according to claim 1, characterized in that, The first varistor assembly (11a) and the second varistor assembly (11b) are composed of multiple varistor sheets connected in series or in parallel and encapsulated in an insulating shell. The varistor voltage of the first varistor assembly (11a) is set to be higher than the peak value of the normal operating voltage between the bridge arm input common node (2) and the upper output bus (7a) and lower than the insulation withstand voltage of each switch in the upper bridge arm switch group and the first transition switch (4a). The varistor voltage of the second varistor assembly (11b) is set to be higher than the peak value of the normal operating voltage between the bridge arm input common node (2) and the lower output bus (7b) and lower than the insulation withstand voltage of each switch in the lower bridge arm switch group and the second transition switch (4b).

8. The current limiting and overvoltage absorption structure of the transition circuit of the bridge arm type on-load tap changer according to claim 7, characterized in that, The voltage values ​​presented by the first varistor component (11a) and the second varistor component (11b) when they pass a 1mA DC current are taken as 1.2 to 1.5 times the peak value of the normal operating voltage between the corresponding nodes, and the ratio of their residual voltage under the nominal impulse current to the voltage value under the 1mA DC current is not greater than 1.

8.

9. The current limiting and overvoltage absorption structure of the transition circuit of the bridge arm type on-load tap changer according to claim 1, characterized in that, The first thermistor element (10a), the second thermistor element (10b) and the corresponding first varistor assembly (11a) and second varistor assembly (11b) maintain a preset air gap or are provided with heat-resistant insulating partitions.

10. The current limiting and overvoltage absorption structure of the transition circuit of the bridge arm type on-load tap changer according to claim 1, characterized in that, The first varistor component (11a) and / or the second varistor component (11b) are zinc oxide varistor components, silicon carbide varistors, gas discharge tubes or transient voltage suppression diodes.

Images