Power electronic on-load tap changer and method suitable for power transformer

Abstract

The invention belongs to the field of power transformers, and discloses a power electronic on-load tap changer and method suitable for a power transformer, the switch is accessed through a mechanical switch in a non-switching state, the power electronic switch and a main circuit are completely electrically isolated, stage voltage is not borne, and the on-load tap changer is suitable for the power transformer. The damage to the electronic power switch caused by overvoltage generated by a system operation fault is also avoided; through the combined control of the two mechanical moving contacts, the two mechanical switches and the four power electronic switches, the non-arc switching of the power electronic voltage regulating switch is realized, the load current is ensured to be continuous, each power electronic switch is only switched on and off once in the switching process, the switching process is simplified, the fault probability is reduced, and the operation stability is improved; the current-limiting element and the lightning arrester are adopted to protect the electronic power switch, so that the safety of the electronic power switch in the operation process and under the condition that a fault occurs can be ensured.

Description

Technical Field

[0001] The present invention belongs to the field of power transformers, specifically to the field of on-load voltage regulators for power transformers, and in particular to a power electronic on-load voltage regulator suitable for power transformers and a method thereof. Background Art

[0002] The function of the on-load tap changer of a power transformer is to adjust the voltage of the transformer while ensuring the continuity of the load current, so as to maintain power supply safety and the stability of the power system.

[0003] The on-load voltage regulation scheme based on mechanical tap changers not only has a slow action speed, but also may generate large arcs during the voltage regulation process, which affects the service life of the transformer, resulting in a large maintenance workload and cost, which seriously restricts the on-load voltage regulation of the transformer. At present, although the technical performance of mechanical on-load tap changers has been greatly improved, there are three problems that are difficult to solve due to mechanical contacts: first, the tap contacts generate arcs during the switching process; second, the voltage regulation response speed is slow and the efficiency is low, with a lag of 100 milliseconds or even several seconds; third, the mechanical transmission structure is complex, the failure rate is high, and the maintenance workload is large. The power electronic on-load voltage regulation technology is to apply power electronic devices to the on-load voltage regulation switch structure of the power transformer to achieve the purpose of arc-free voltage regulation, and the switching frequency of the power electronic devices can reach 2kHz or above, while the response speed of the mechanical switch is in the millisecond level. Therefore, the power electronic switch has a significant advantage in response speed. The core of power electronic on-load voltage regulation technology is power electronic devices. High-voltage and large-capacity power electronic on-load voltage regulators have higher requirements on their voltage level and current carrying capacity and other performance indicators; large capacity refers to the rated capacity of the voltage regulator being above 1MVA; currently, high-voltage power electronic on-load voltage regulators have not been applied in engineering projects; and existing power electronic on-load voltage regulators for power transformers, while achieving arc-free voltage regulation, continuous load current during the voltage regulation process, and ensuring switching speed, cannot take into account the safety of the switching process and the stability of the system to which the transformer is connected, resulting in poor switching stability and certain power supply safety hazards.

[0004] It can be seen that the existing power electronic power transformer on-load tap changer cannot guarantee the safety of the switching process and the stability of the system to which the transformer is connected, resulting in poor switching stability and certain power supply safety risks. Summary of the invention

[0005] The present invention provides a power electronic on-load voltage regulating switch and method suitable for a power transformer, so as to solve the technical problems that the existing power electronic on-load voltage regulating switch for a power transformer cannot ensure the safety of a switching process and the stability of a system connected to the transformer, resulting in poor switching stability and certain potential safety hazards in power supply. The on-load voltage regulating switch is adopted to ensure the safety of the power electronic switch during operation and in the event of a fault.

[0006] In order to achieve the above object, the present invention adopts the following technical contents: A power electronic on-load voltage regulating switch suitable for a power transformer, comprising a first module and a second module; The first module includes a first moving contact, a first mechanical switch, a first power electronic switch and a second power electronic switch; The second module includes a second moving contact, a second mechanical switch, a third power electronic switch and a fourth power electronic switch; The first moving contact, the first power electronic switch and the second power electronic switch are sequentially arranged in series between the first tap and the output end of the transformer; the first power electronic switch and the second power electronic switch are connected in series and then connected in parallel with the first mechanical switch; The second moving contact, the third power electronic switch and the fourth power electronic switch are sequentially arranged in series between the second tap and the output end of the transformer; the third power electronic switch and the fourth power electronic switch are connected in series and then connected in parallel with the second mechanical switch; Wherein, the first tap and the second tap are adjacent taps; A first lightning arrester is connected in parallel at both ends of the first power electronic switch; a second lightning arrester is connected in parallel at both ends of the third power electronic switch; Two ends of the second power electronic switch and the fourth power electronic switch are respectively connected in parallel with a current limiting element, and the current limiting element is used to protect the switching state during the circuit switching process.

[0007] Furthermore, the current limiting element includes a first inductor and a first current limiting resistor connected in series and a second inductor and a second current limiting resistor connected in series; wherein the first inductor and the first current limiting resistor connected in series are connected in parallel to both ends of the second power electronic switch; and the second inductor and the second current limiting resistor connected in series are connected in parallel to both ends of the fourth power electronic switch.

[0008] Further, it is characterized in that the first power electronic switch includes a first IGBT and a second IGBT connected to each other at the emitter; the first IGBT is connected in parallel with a first power diode, and the second IGBT is connected in parallel with a second power diode; the first IGBT and the second IGBT connected in series are connected in parallel with a resistor and a capacitor connected in series; The first IGBT and the second IGBT are respectively composed of one IGBT device or a plurality of IGBT devices connected in series.

[0009] Furthermore, the third power electronic switch has the same structure as the first power electronic switch.

[0010] Further, the second power electronic switch includes a first IGCT and a second IGCT, and the first IGCT and the second IGCT are connected in reverse parallel; The first IGCT and the second IGCT are respectively composed of one IGCT device or a plurality of IGCT devices connected in series.

[0011] Furthermore, the fourth power electronic switch has the same structure as the second power electronic switch.

[0012] Furthermore, a voltage threshold of the first lightning arrester is less than a blocking voltage of the first power electronic switch; and a voltage threshold of the second lightning arrester is less than a blocking voltage of the third power electronic switch.

[0013] Furthermore, lightning arresters are respectively connected in parallel at both ends of the second power electronic switch and the fourth power electronic switch.

[0014] A method for on-load voltage regulation applicable to a power transformer, based on the above-mentioned power electronic on-load voltage regulation switch applicable to the power transformer, comprises: Step 1: Connect the second moving contact to the second tap of the transformer, the second mechanical switch, the third power electronic switch and the fourth power electronic switch are all in the off state, and the load current flows from the first moving contact through the first mechanical switch and out through the output end; Step 2: close the first power electronic switch and the second power electronic switch in sequence, and open the first mechanical switch after the first mechanical switch is bypassed, so that the load current flows through the first power electronic switch and the second power electronic switch and flows out through the output end; Step 3: disconnect the second power electronic switch, and the load current flows through the first power electronic switch and the current limiting element connected in parallel with the second power electronic switch, and flows out through the output end; Step 4: closing the third power electronic switch, so that a circulating current is formed between the first tap and the second tap; Step 5: disconnect the first power electronic switch, and the load current flows through the second moving contact, the third power electronic switch, and the current limiting element connected in parallel with the fourth power electronic switch, and flows out through the output end; Step 6: closing the fourth power electronic switch, the load current flows through the third power electronic switch and the fourth power electronic switch, and flows out through the output end; Step 7: close the second mechanical switch, and open the fourth power electronic switch and the third power electronic switch in sequence, and the load current flows through the second moving contact and the second mechanical switch and flows out through the output end; Step 8: Disconnect the first moving contact from the first tap.

[0015] Furthermore, in step three, the second power electronic switch is turned off, and the load current flows through the first power electronic switch, the first reactor and the first current limiting resistor, and flows out through the output end; In step five, the first power electronic switch is disconnected, and the load current flows through the second moving contact, the third power electronic switch, the second reactor and the second current limiting resistor, and flows out through the output end.

[0016] Compared with the prior art, the present invention has the following beneficial effects: The present invention provides a power electronic on-load voltage regulating switch suitable for power transformers. The switch is connected through a mechanical switch in a non-switching state. The power electronic switch is completely electrically isolated from the main circuit, does not bear the step voltage, and avoids damage to the power electronic switch caused by overvoltage caused by system operation failure. Through the combined control of two mechanical moving contacts, two mechanical switches, and four power electronic switches, arc-free switching of the power electronic voltage regulating switch is realized, and the load current is guaranteed to be continuous. In this process, each power electronic switch is only turned on and off once, which simplifies the switching process, reduces the probability of failure, and improves the operation stability. The power electronic switch is protected by a current limiting element and a lightning arrester, which can ensure the safety of the power electronic switch during operation and in the event of a failure. The switch adopts a completely symmetrical modular design, which makes the production process, assembly, testing, and operation and maintenance after commissioning more convenient.

[0017] Preferably, in the present invention, the current limiting element includes a series-connected reactor and a current limiting resistor, which can limit overcurrent during circuit switching and in the event of a fault, protect the power electronic switch from damage, and improve the durability and reliability of the switch.

[0018] Preferably, in the present invention, the first power electronic switch and the third power electronic switch both use a first IGBT and a second IGBT connected in emitter, and a power diode is connected in parallel, and a series resistor and capacitor are used for protection, which improves the response speed and stability of the switch and is suitable for high voltage and high current application scenarios.

[0019] Preferably, in the present invention, the second power electronic switch and the fourth power electronic switch both use the first IGCT and the second IGCT connected in reverse parallel, which improves the load-bearing capacity and stability of the switch and is suitable for application scenarios requiring higher rated voltage and current.

[0020] Preferably, in the present invention, the voltage threshold of the arrester is designed to be lower than the blocking voltage of the power electronic switch, which can protect the switch from damage in the event of overvoltage, thereby improving the safety and reliability of the switch.

[0021] Preferably, in the present invention, lightning arresters are connected in parallel at both ends of the second power electronic switch and the fourth power electronic switch, respectively, to further enhance the overvoltage protection capability of the switches during the switching process and improve the safety of the system.

[0022] The present invention also provides an on-load voltage regulation method applicable to a power transformer. Based on the above-mentioned power electronic on-load voltage regulating switch applicable to the power transformer, the method can achieve, by accurately controlling the closing and opening sequence of the switch, that in the most serious fault situation, if the power electronic switch that has been disconnected is damaged due to overvoltage, the probability of damage to the other three power electronic switches due to structural reasons or lightning arrester protection is very small, so that the current switching is not affected, ensuring the power supply safety of the power transformer and the stability of the system, realizing smooth switching from the first tap to the second tap, avoiding voltage and current shocks during the switching process, and improving the stability and reliability of the power system.

[0023] Preferably, in the present invention, the limiting effect of the reactor and the current-limiting resistor further reduces the overcurrent during the switching process, protects the safety of the switch and related equipment, and improves the stability and reliability of the entire voltage regulation process. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 A schematic diagram of the structure of a power electronic on-load tap-changing switch applicable to a power transformer provided in an embodiment of the present invention; Figure 2 A schematic diagram of a power electronic on-load voltage regulating switch applicable to a power transformer in a non-voltage regulating stage provided by an embodiment of the present invention; Figure 3 A schematic diagram of a power electronic on-load voltage regulating switch applicable to a power transformer in a voltage regulating stage provided by an embodiment of the present invention; Figure 4 A schematic structural diagram of a first power electronic switch and a third power electronic switch of a power electronic on-load tap-changing switch applicable to a power transformer provided by an embodiment of the present invention; Figure 5 A schematic structural diagram of a second power electronic switch and a fourth power electronic switch of a power electronic on-load tap-changing switch applicable to a power transformer provided by an embodiment of the present invention; Figure 6 A schematic diagram of a first voltage regulation process of a power electronic on-load voltage regulating switch applicable to a power transformer provided in an embodiment of the present invention; Figure 7 A schematic diagram of a second voltage regulation process of a power electronic on-load tap-changing switch applicable to a power transformer provided in an embodiment of the present invention; Figure 8 A schematic diagram of a third voltage regulation process of a power electronic on-load voltage regulating switch applicable to a power transformer provided in an embodiment of the present invention; Fig. 9 A schematic diagram of a fourth voltage regulation process of a power electronic on-load tap-changing switch applicable to a power transformer provided by an embodiment of the present invention; Fig.10 A schematic diagram of a fifth voltage regulation process of a power electronic on-load tap-changing switch applicable to a power transformer provided in an embodiment of the present invention; Fig.11 A schematic diagram of a sixth voltage regulation process of a power electronic on-load voltage regulating switch applicable to a power transformer provided by an embodiment of the present invention; Fig.12 A schematic diagram of the seventh voltage regulation process of a power electronic on-load tap-changing switch applicable to a power transformer provided in an embodiment of the present invention; Fig.13 A schematic diagram of the eighth voltage regulation process of a power electronic on-load tap-changing switch suitable for a power transformer provided in an embodiment of the present invention.

[0025] Reference numerals: 1. First power electronic switch; 2. Second power electronic switch; 3. Third power electronic switch; 4. Fourth power electronic switch; 5. First moving contact; 6. Second moving contact; 7. First mechanical switch; 8. Second mechanical switch; 9. First lightning arrester; 10. Second lightning arrester; 11. First reactor; 12. Second reactor; 13. First current limiting resistor; 14. Second current limiting resistor; 100, transformer; 200, on-load voltage regulator; 301, first IGBT; 302, second IGBT; 303, first power diode; 304, second power diode; 305, resistor; 306, capacitor; 307, first IGCT; 308, second IGCT. DETAILED DESCRIPTION

[0026] In order to make the technical problems, technical solutions and beneficial effects solved by the present invention more clearly understood, the present invention is further described in detail in the following specific embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0027] In order to make the purpose, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments. Generally, the components of the embodiments of the present invention described and shown in the drawings here can be arranged and designed in various different configurations.

[0028] Therefore, the following detailed description of the embodiments of the present invention provided in the accompanying drawings is not intended to limit the scope of the invention claimed for protection, but merely represents selected embodiments of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0029] It should be noted that similar reference numerals and letters denote similar items in the following drawings, and therefore, once an item is defined in one drawing, further definition and explanation thereof is not required in subsequent drawings.

[0030] In the description of the embodiments of the present invention, it should be noted that if the terms "upper", "lower", "horizontal", "inner", etc. indicate an orientation or positional relationship based on the orientation or positional relationship shown in the drawings, or the orientation or positional relationship in which the product of the invention is usually placed when in use, it is only for the convenience of describing the present invention and simplifying the description, and does not indicate or imply that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore cannot be understood as a limitation on the present invention. In addition, the terms "first", "second", etc. are only used to distinguish the description, and cannot be understood as indicating or implying relative importance.

[0031] In addition, if the term "horizontal" appears, it does not mean that the component must be absolutely horizontal, but can be slightly tilted. For example, "horizontal" only means that its direction is more horizontal than "vertical", which does not mean that the structure must be completely horizontal, but can be slightly tilted.

[0032] In the description of the embodiments of the present invention, it is also necessary to explain that, unless otherwise clearly specified and limited, the terms "set", "install", "connect", and "connect" should be understood in a broad sense, for example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be a direct connection, or it can be indirectly connected through an intermediate medium, or it can be the internal connection of two components. For ordinary technicians in this field, the specific meanings of the above terms in the present invention can be understood according to specific circumstances.

[0033] Example 1 As mentioned in the background technology, for high voltage, large capacity power transformer on-load tap changer, Chinese patent publication number CN114783744A discloses a power electronic switch for on-load tap changer, which mainly has the following problems: First, when the transformer is operating normally, that is, in the non-switching state, since the power electronic switch is connected in parallel to the mechanical switch, its two ends will always be subjected to the step voltage, and the overvoltage caused by the system failure will damage the power electronic switch.

[0034] Second, during the switching process, when one branch power electronic switch is turned on and the other branch switch is turned off, the two ends of the turned-off power electronic switch need to withstand the sum of the level voltage and the voltage across the current limiting resistor (R1 or R2). Since the two power electronic switches on the branch are connected in parallel and open circuit, the two power electronic switches withstand the same voltage. As the voltage level of the power electronic switch increases, more power electronic devices need to be connected in series, which increases the cost and reduces the reliability.

[0035] Third, in the switching state, if a fault occurs and a short-circuit current flows through one of the branches, the two power electronic switches on the other branch are connected in parallel and open-circuited, and both ends will be subjected to the same overvoltage, increasing the risk of simultaneous damage. Once damaged, the switching fails, causing the fault to expand.

[0036] For example, Chinese patent publication No. CN113077980A discloses an on-load tap changer for a high-voltage transmission transformer and a control method thereof, which mainly has the following problems: First, the mechanical part, in addition to the four mechanical switches, also includes a tap selector to contact the corresponding tap. As described in the background technology, the response speed of the mechanical switch is slower than that of the power electronic switch, which increases the entire voltage regulation time and reduces the efficiency.

[0037] Second, the mechanical switch and the power electronic switch need to cooperate during the entire switching process. Since the working mechanism and action time of the mechanical switch and the power electronic switch are very different, it is more difficult to control the time required for the entire switching process.

[0038] Third, during the switching process, when one power electronic switch is closed and the other power electronic switch is open, and one end of the open power electronic switch is connected to another tap, the two ends of the closed power electronic switch need to withstand the sum of the stage voltage and the voltage across the current limiting resistor at a high voltage level. The higher the voltage level of the power electronic switch, the more power electronic devices need to be connected in series, which means that the cost increases and the reliability decreases. Once the other power electronic switch is damaged, if one end of the power electronic switch is connected to the switching target tap, the switching fails, causing the fault to expand.

[0039] Fourth, during the switching process, if a fault occurs and a short-circuit current is generated, it flows through one of the closed power electronic switch loops, and the power electronic switch on the other branch is open. Its two ends need to withstand the voltage generated by the stage voltage superimposed on the short-circuit current at both ends of the current limiting resistor, that is, overvoltage. Once the other power electronic switch is damaged, if one end of the power electronic switch is connected to the switching target tap, the switching will fail, causing the fault to expand.

[0040] It can be seen that the existing power electronic on-load tap changer of power transformer, while achieving arc-free voltage regulation, continuous load current during the voltage regulation process, and ensuring switching speed, cannot take into account the safety of the switching process and the stability of the system connected to the transformer, resulting in poor switching stability and certain safety hazards.

[0041] In order to solve the above problems, this embodiment provides a power electronic on-load tap-changer suitable for a power transformer. By adopting this switch, arc-free voltage regulation of the on-load tap-changer of the power transformer can be achieved, load current is continuous during the voltage regulation process, and switching speed is improved. At the same time, the technical requirements for operation under high voltage level conditions are fully considered to ensure the safety of the switching process and the stability of the system to which the transformer is connected. The following technical terms are explained: IGBT (Insulated Gate Bipolar Transistor), also known as insulated gate bipolar transistor, is a composite fully controlled voltage-driven power semiconductor device composed of BJT (bipolar transistor) and MOSFET (insulated gate field effect transistor). IGBT combines the advantages of BJT's high current density and MOSFET's high input impedance, low drive power, fast switching speed, and low switching loss, and has the characteristics of wide safe operating area, high short-circuit withstand capability, and low noise.

[0042] IGCT (Integrated Gate-Commutated Thyristor) is an integrated gate-commutated thyristor, which is a high-voltage, high-power, semi-controlled power electronic device. IGCT combines the advantages of GTO (gate turn-off thyristor) and IGBT (insulated gate bipolar transistor), and has the characteristics of fast switching speed, low switching loss, strong current capacity, strong short-circuit resistance, good thermal stability, and simple control circuit.

[0043] like Figure 1 As shown, this embodiment provides a power electronic on-load voltage regulating switch suitable for power transformers. The switch is mainly composed of two mechanical contacts, two mechanical switches, four power electronic switches, two groups of lightning arresters, two reactors and two current limiting resistors to achieve on-load regulation of the power transformer voltage.

[0044] In this embodiment, two power electronic switches are connected in series between each moving contact and the output end, one of which is connected in parallel with a lightning arrester, and the other is connected in parallel with a current limiting element (a reactor and a current limiting resistor); by controlling the on-off timing of the four power electronic switches and coordinating the on-off of the mechanical switch, arc-free, fast and safe switching of the transformer tap is achieved, and a completely symmetrical modular structure design is adopted.

[0045] like Figure 1 As shown, this embodiment provides a power electronic on-load tap-changing switch applicable to a power transformer, and the specific structure and connection relationship are as follows: The power electronic on-load tap-changing switch comprises a first module and a second module. In order to facilitate product design, the first module and the second module having completely the same structure can be used.

[0046] The first module includes a first moving contact 5, a first mechanical switch 7, and a first power electronic switch 1 and a second power electronic switch 2 connected in series.

[0047] The second module comprises a second moving contact 6 , a second mechanical switch 8 , and a third power electronic switch 3 and a fourth power electronic switch 4 connected in series.

[0048] The first tap of the transformer 100 is connected to the first mechanical switch 7 via the first moving contact 5 , and the first mechanical switch 7 is connected in parallel with the first power electronic switch 1 and the second power electronic switch 2 which are connected in series.

[0049] The first power electronic switch 1 is connected in parallel with a first lightning arrester 9 , and the second power electronic switch 2 is connected in parallel with a current limiting element for protecting the switching state during the circuit switching process.

[0050] The second tap connected to the first tap is connected to the second mechanical switch 8 via the second moving contact 6, and the second mechanical switch 8 is connected in parallel with the third power electronic switch 3 and the fourth power electronic switch 4 which are connected in series.

[0051] The third power electronic switch 3 is connected in parallel with a second lightning arrester 10, and the fourth power electronic switch 4 is also connected in parallel with a current limiting element for protecting the switching state during circuit switching; the current limiting element includes an inductor and a current limiting resistor to protect the second power electronic switch 2 and the fourth power electronic switch 4.

[0052] In this embodiment, when the transformer is operating normally, that is, in the non-voltage regulation stage, the first moving contact 5 is connected to a tap of the transformer ( Figure 2 In the case of the N connector in FIG. 1 , the states of the switches are: the first mechanical switch 7 is closed, the first moving contact 5 is connected to a tap (the first tap) of the transformer 100, and the remaining moving contacts, mechanical switches and power electronic switches are all in the disconnected state.

[0053] At this stage, if Figure 2 As shown, in the normal operation stage of the transformer 100, a tap is connected to one of the moving contacts (the first moving contact 5 or the second moving contact 6) of the on-load voltage regulator 200, and the load current flows through the mechanical switch (the first mechanical switch 7 or the second mechanical switch 8), and the rest are not connected. There is a step voltage between the other moving contact and the tap corresponding to the transformer 100, and the electrical part of the entire on-load voltage regulator 200 does not bear the step voltage and load current during normal operation, thereby avoiding the transformer body or the power system from failing during normal operation, and the overvoltage that causes damage to the power electronics of the on-load voltage regulator 200.

[0054] When the transformer 100 enters the voltage regulation stage, the voltage regulation process of the on-load tap-changing switch 200 is as follows (at this time, it is assumed that the current position of the first mechanical switch 7 is connected to the tap N of the transformer 100).

[0055] At the beginning of the voltage regulation process, the second moving contact 6 is connected to the N+1 tap; the first power electronic switch 1 and the second power electronic switch 2 are turned on in sequence. During the turning-on process, the first reactor 11 can reduce the rate of change of the current over time to prevent damage to the power electronic switch caused by excessive di / dt. Similarly, the second reactor 12 has the same function. After the first mechanical switch 7 is short-circuited, the first mechanical switch 7 is disconnected. The load current is transferred from the first mechanical switch 7 to the first power electronic switch 1 and the second power electronic switch 2.

[0056] Then the second power electronic switch 2 is turned off, that is, the load current is transferred from the second power electronic switch 2 to its parallel branch.

[0057] At this stage, if Figure 3 As shown, the third power electronic switch 3 needs to withstand the sum of the step voltage and the voltage across the first reactor 11 and the first current limiting resistor 13. In the high-voltage system, it can reach several times the step voltage (approximate value, the actual value is determined by the transformer voltage level), and reaches the maximum value in the entire voltage regulation process. Therefore, the third power electronic switch 3 is most vulnerable to damage, and the second lightning arrester 10 is connected in parallel at both ends for protection. Similarly, when switching from N+1 to N taps, the first power electronic switch 1 also withstands the maximum voltage, so it also needs to be protected, specifically using the first lightning arrester 9 for protection. The fourth power electronic switch 4 will not bear high voltage due to the parallel connection of the second reactor 12 and the second current limiting resistor 14. Similarly, when switching from N+1 to N taps, the power electronic switch 2 will not bear high voltage. In the figure, ΔU represents the step voltage, and Is represents the load current.

[0058] In this embodiment, at this stage, if a short-circuit current occurs in the transformer due to a system fault, the current will flow into the first moving contact 5 through the tap N, pass through the first power electronic switch 1, the first reactor 11 and the first current limiting resistor 13, and flow out from the output terminal X. The third power electronic switch 3 needs to bear the sum of the step voltage and the voltage generated by the short-circuit current on the first reactor 11 and the first current limiting resistor 13. In a high-voltage system, it can reach several times the normal operating voltage (approximate value, the actual value is determined by the transformer voltage level and the power system to which the transformer is connected). Similarly, when switching from N+1 to N taps, overvoltage will also appear on the first power electronic switch 1. Based on this fact, lightning arresters are installed on the first power electronic switch 1 and the third power electronic switch 3, and the voltage threshold is set to be less than the blocking voltage of the first power electronic switch 1 and the third power electronic switch 3 to prevent the first power electronic switch 1 and the third power electronic switch 3 from being damaged by overvoltage.

[0059] In this embodiment, at this stage, if the second power electronic switch 2 is damaged due to the overvoltage of the parallel first reactor 11 and the first current limiting resistor 13, the switching process is not affected. Similarly, in the case of switching from N+1 to N taps, the damage of the fourth power electronic switch 4 does not affect the current switching, ensuring the transformer power supply safety and stable operation of the system in the event of a fault.

[0060] Next, the third power electronic switch 3 is turned on, and the second reactor 12 and the second current limiting resistor 14 are connected; at this moment, the transformer 100N joint and the N+1 joint are connected, and a circulating current is generated. Since the loop includes the first reactor 11 and the second reactor 12 and the first current limiting resistor 13 and the second current limiting resistor 14, a short circuit between the transformer 100N joint and the N+1 joint can be prevented.

[0061] Next, the first power electronic switch 1 is turned off, and the load current flows out through the output terminal X from the tap N+1, the third power electronic switch 3, the second reactor 12 and the second current limiting resistor 14.

[0062] Next, the fourth power electronic switch 4 is closed, and finally the second mechanical switch 8 is closed. Then, the fourth power electronic switch 4 and the third power electronic switch 3 are opened in sequence, and the connection between the first moving contact 5 and the tap N is disconnected, completing a voltage regulation process.

[0063] In this embodiment, during the whole process, each mechanical switch only operates once (off or on), and the power electronic switch only opens and closes once. The cooperation between the power electronic switch and the mechanical switch only occurs after the first mechanical switch 7 is opened and before the second mechanical switch 8 is closed.

[0064] This embodiment provides a power electronic on-load tap changer suitable for a power transformer. Due to the completely symmetrical design, the operation sequence of switching from the N+1 tap to the N tap is to interchange the first mechanical switch 7 and the second mechanical switch 8 in the above process, interchange the first power electronic switch 1 and the third power electronic switch 3, and interchange the second power electronic switch 2 and the fourth power electronic switch 4.

[0065] like Figure 4 As shown, in this embodiment, the first power electronic switch 1 and the third power electronic switch 3 have the same structure, both of which are composed of IGBTs and power diodes, wherein the first IGBT 301 and the second IGBT 302 are connected in common emitter, and each IGBT should have at least one IGBT device connected in series. At least one first power diode 303 and a second power diode 304 are connected in reverse parallel at both ends to play the role of protection and freewheeling. The resistor 305 and the capacitor 306 are connected in series to form a resistance-capacitance absorption loop, which is connected in parallel at both ends of the first IGBT 301 and the second IGBT 302 connected in series to protect the circuit and prevent the voltage from changing too fast and damaging the power electronic device.

[0066] In this embodiment, the first power electronic switch 1 and the third power electronic switch 3 may also adopt an IGCT structure; Figure 5 As shown, the IGCT-based structure is composed of two reverse-parallel first IGCTs 307 and second IGCTs 308; and each IGCT should have no less than one IGCT device connected in series.

[0067] In this embodiment, the second power electronic switch 2 and the fourth power electronic switch 4 have the same structure and are both composed of IGCT. Figure 5 As shown, the IGCT-based structure is composed of two reverse-parallel first IGCTs 307 and second IGCTs 308; and each IGCT should have no less than one IGCT device connected in series.

[0068] In this embodiment, the second power electronic switch 2 and the fourth power electronic switch 4 may also adopt an IGBT structure; Figure 5 As shown, in the IGBT-based structure, the first IGBT 301 and the second IGBT 302 are connected to the common emitter, and each IGBT should have at least one IGBT device connected in series; at least one first power diode 303 and a second power diode 304 are connected in parallel at both ends in reverse direction to play the role of protection and freewheeling. The resistor 305 and the capacitor 306 are connected in series to form a resistance-capacitance absorption loop, which is connected in parallel at both ends of the first IGBT 301 and the second IGBT 302 connected in series to protect the circuit and prevent the voltage from changing too fast and damaging the power electronic device.

[0069] In this embodiment, in order to enhance the overvoltage protection capability of the switch during the switching process and improve the safety of the system, a group of lightning arresters is connected in parallel to the second power electronic switch 2 and the fourth power electronic switch 4 .

[0070] This embodiment also provides a working method of a power electronic on-load voltage regulating switch applicable to a power transformer, that is, a voltage regulating method. The entire voltage regulating process is described by taking the on-load voltage regulating switch switching from the Nth tap to the N+1th tap as an example. The specific steps are as follows: The first step is to connect the second moving contact 6 to the N+1th tap, such as Figure 6 As shown; after completion, since the mechanical switches and power electronic switches of the branch connected to the second moving contact 6 are all in the disconnected state, there is no current in the part connected to the second moving contact 6. The load current flows from the first moving contact 5 through the first mechanical switch 7 and flows out of the voltage regulating switch through the output terminal X. The direction of the arrow in the figure is the direction of movement of the moving contact relative to the transformer. If it is switched from the N+1 tap to the N tap, the direction is opposite.

[0071] In the second step, the first power electronic switch 1 and the second power electronic switch 2 are closed in sequence. Figure 7 As shown; after the first mechanical switch 7 is bypassed, the first mechanical switch 7 is disconnected, and at this time the load current passes through the first power electronic switch 1 and the second power electronic switch 2 and flows out through the output terminal X; in the figure, ΔU represents the level voltage, Is represents the load current, and the arrow represents the current path and direction.

[0072] The third step is to disconnect the second power electronic switch 2. Figure 8 As shown, at this time, the load current flows out through the first reactor 11 and the first current limiting resistor 13 connected in parallel with the second power electronic switch 2 .

[0073] Step 4: close the third power electronic switch 3; Fig. 9 As shown, a circulating current is formed between the two taps at this time, and the transformer winding between the two taps N and N+1 is connected through the first inductor 11 and the second inductor 12 and the first current limiting resistor 13 and the second current limiting resistor 14 to avoid short circuit between the taps.

[0074] Step 5: disconnect the first power electronic switch 1, such as Fig.10 As shown, at this time, the load current flows out through the output terminal X via the tap N+1, the third power electronic switch 3, the second inductor 12 and the second current limiting resistor 14.

[0075] Step 6: close the fourth power electronic switch 4, such as Fig.11 As shown, at this time, the load current flows out through the fourth power electronic switch 4 connected in parallel with the second inductor 12 and the second current limiting resistor 14 .

[0076] Step 7: Close the second mechanical switch 8. Fig.12 As shown, the fourth power electronic switch 4 and the third power electronic switch 3 are then disconnected in sequence; the load current flows through the second mechanical switch 8 and flows out of the X terminal.

[0077] Step 8: disconnect the first moving contact 5 from the tap N. Fig.13 As shown, the direction of the arrow is the direction in which the moving contact moves relative to the transformer; if it is switched from the N+1 tap to the N tap, the direction is opposite.

[0078] This embodiment provides a power electronic on-load tap-changing switch applicable to a power transformer, which has the following characteristics: First, it can effectively protect high-voltage power electronic switches: After analyzing the switching process, power electronic switches with harsh operating conditions (i.e., they need to withstand higher voltages during operation) and the risk of overvoltage are protected by parallel lightning arresters.

[0079] In view of the fact that power electronic devices are sensitive to the rate of change of current, reactors (including the first reactor 11 and the second reactor 12) are installed on the current limiting resistor (including the first current limiting resistor 13 and the second current limiting resistor 14) branch to reduce di / dt; further, introducing the reactor in the switching circuit can also reduce the loss on the current limiting resistor in the parallel connection stage of the two taps.

[0080] Second, the safe operation of the switching process is ensured by the structural design of the power electronic switch and the combination of the reactor (including the first reactor 11 and the second reactor 12) and the current limiting resistor (including the first current limiting resistor 13 and the second current limiting resistor 14).

[0081] When the power electronic switch protected by the lightning arrester in one group of power electronic switches connected in series is turned on, the power electronic switches of the parallel reactor (including the first reactor 11 and the second reactor 12) and the current limiting resistor (including the first current limiting resistor 13 and the second current limiting resistor 14) are turned off, and the other group of power electronic switches connected in series are all in the off state and connected to the switching target tap, if a system failure occurs at this moment and a short-circuit current flows through the voltage regulating switch, causing damage to the power electronic switch connected in parallel with the reactor and the current limiting resistor, it will not affect the switching process, thereby ensuring the power supply safety of the power transformer and the stability of the system.

[0082] Third, streamlined mechanical structure.

[0083] Simplify the coordination between mechanical switches and power electronic switches. In the key switching steps (both transformer taps are connected to the main circuit), all power electronic switches are used to increase the switching speed and reduce the uncontrollable problems caused by the coordination between mechanical switches and power electronic switches. Furthermore, as mentioned above, the maximum overvoltage caused by system failure occurs during the switching process of power electronic switches. Since the speed of power electronic switches is significantly better than that of mechanical switches, the probability of failure is reduced.

[0084] Fourth, when the on-load tap-changing switch is not in operation, no power electronic switch is connected to the main circuit, and the power electronic devices are not subjected to the step voltage, so that the electrical part is completely isolated to avoid damage to the power electronic devices due to long-term exposure to the step voltage and overvoltage caused by system failure.

[0085] Fifth, the voltage regulating switch adopts a completely symmetrical modular design, which provides convenience for the production process, assembly and testing of the product, as well as the subsequent operation and maintenance.

[0086] In summary, the present invention provides a power electronic on-load voltage regulating switch and method applicable to a power transformer, which has the following advantages over existing voltage regulating measures: First, the switch is connected through a mechanical switch in the non-switching state, and the power electronic switch is completely electrically isolated from the main circuit, does not bear the step voltage, and avoids the overvoltage caused by the system operation failure to damage the power electronic switch; secondly, through the combined control of two mechanical moving contacts, two mechanical switches, and four power electronic switches, the arc-free switching of the power electronic voltage regulating switch is realized, and the load current is guaranteed to be continuous. In this process, each power electronic switch is only turned on and off once; the mechanical switch and the power electronic switch only need to cooperate twice, which simplifies the switching process, reduces the probability of failure, and improves the operation stability. In addition, in normal operation and fault conditions, the power electronic switch is protected by a reactor and a lightning arrester to ensure the safety of the power electronic switch during operation and in the event of a fault. Thirdly, the structure and control sequence adopted by this switch, even in the most serious fault situation, if the disconnected power electronic switch is damaged due to overvoltage, the other three power electronic switches are protected by lightning arresters due to structural reasons, so the probability of damage is very small, so that this switching can be unaffected, ensuring the power supply safety of the power transformer and the stability of the system. Finally, this switch adopts a completely symmetrical modular design, which makes the production process, assembly, testing, operation and maintenance after commissioning more convenient; the use of this switch takes into account the safety of the switching process and the stability of the system connected to the transformer, and can achieve arc-free voltage regulation, continuous load current during the voltage regulation process, and ensure switching speed, thereby improving the stability and operating efficiency of the entire power system.

[0087] The above embodiment is only one of the implementation methods that can realize the technical solution of the present invention. The scope of protection claimed by the present invention is not limited only to this embodiment, but also includes changes, replacements and other implementation methods that can be easily thought of by any technician familiar with the technical field within the technical scope disclosed by the present invention.

Claims

1. A power electronic on-load tap-changing switch suitable for a power transformer, characterized in that: comprising a first module and a second module; The first module comprises a first moving contact (5), a first mechanical switch (7), a first power electronic switch (1) and a second power electronic switch (2); The second module comprises a second moving contact (6), a second mechanical switch (8), a third power electronic switch (3) and a fourth power electronic switch (4); The first moving contact (5), the first power electronic switch (1) and the second power electronic switch (2) are sequentially connected in series between the first tap and the output end of the transformer (100); the first power electronic switch (1) and the second power electronic switch (2) are connected in series and then connected in parallel with the first mechanical switch (7); The second moving contact (6), the third power electronic switch (3) and the fourth power electronic switch (4) are sequentially connected in series between the second tap and the output end of the transformer (100); the third power electronic switch (3) and the fourth power electronic switch (4) are connected in series and then connected in parallel with the second mechanical switch (8); Wherein, the first tap and the second tap are adjacent taps; Two ends of the first power electronic switch (1) are connected in parallel with a first lightning arrester (9); two ends of the third power electronic switch (3) are connected in parallel with a second lightning arrester (10); Two ends of the second power electronic switch (2) and the fourth power electronic switch (4) are respectively connected in parallel with a current limiting element, the current limiting element being used to protect the switching state during the circuit switching process.

2. The power electronic on-load tap-changing switch suitable for power transformer according to claim 1, characterized in that: The current limiting element comprises a first reactor (11) and a first current limiting resistor (13) connected in series, and a second reactor (12) and a second current limiting resistor (14) connected in series; wherein the first reactor (11) and the first current limiting resistor (13) connected in series are connected in parallel to both ends of the second power electronic switch (2); and the second reactor (12) and the second current limiting resistor (14) connected in series are connected in parallel to both ends of the fourth power electronic switch (4).

3. The power electronic on-load tap-changing switch suitable for power transformer according to claim 1, characterized in that: The first power electronic switch (1) comprises a first IGBT (301) and a second IGBT (302) whose emitters are connected to each other; the first IGBT (301) is connected in parallel with a first power diode (303), and the second IGBT (302) is connected in parallel with a second power diode (304); the first IGBT (301) and the second IGBT (302) connected in series are connected in parallel with a resistor (305) and a capacitor (306) connected in series; The first IGBT (301) and the second IGBT (302) are respectively composed of one IGBT device or a plurality of IGBT devices connected in series.

4. The power electronic on-load tap-changing switch suitable for power transformer according to claim 3, characterized in that: The third power electronic switch (3) has the same structure as the first power electronic switch (1).

5. The power electronic on-load tap-changing switch suitable for power transformer according to claim 1, characterized in that: The second power electronic switch (2) comprises a first IGCT (307) and a second IGCT (308), and the first IGCT (307) and the second IGCT (308) are connected in reverse parallel; The first IGCT (307) and the second IGCT (308) are respectively composed of one IGCT device or a plurality of IGCT devices connected in series.

6. The power electronic on-load tap-changing switch suitable for power transformer according to claim 5, characterized in that: The fourth power electronic switch (4) has the same structure as the second power electronic switch (2).

7. The power electronic on-load tap-changing switch applicable to a power transformer according to any one of claims 1 to 6, characterized in that: The voltage threshold of the first lightning arrester (9) is less than the blocking voltage of the first power electronic switch (1); and the voltage threshold of the second lightning arrester (10) is less than the blocking voltage of the third power electronic switch (3).

8. The power electronic on-load tap-changing switch suitable for power transformer according to claim 1, characterized in that: Lightning arresters are respectively connected in parallel at both ends of the second power electronic switch (2) and the fourth power electronic switch (4).

9. A method for on-load voltage regulation applicable to a power transformer, characterized in that: The power electronic on-load tap-changing switch applicable to a power transformer according to any one of claims 1 to 8 comprises: Step 1: The second moving contact (6) is connected to the second tap of the transformer (100), the second mechanical switch (8), the third power electronic switch (3) and the fourth power electronic switch (4) are all in the disconnected state, and the load current flows from the first moving contact (5) through the first mechanical switch (7) and out through the output end; Step 2: closing the first power electronic switch (1) and the second power electronic switch (2) in sequence, and opening the first mechanical switch (7) after the first mechanical switch (7) is bypassed, so that the load current flows through the first power electronic switch (1) and the second power electronic switch (2) and flows out through the output end; Step 3: disconnecting the second power electronic switch (2), so that the load current flows through the first power electronic switch (1) and the current limiting element connected in parallel with the second power electronic switch (2), and flows out through the output end; Step 4: closing the third power electronic switch (3), so that a circulating current is formed between the first tap and the second tap; Step 5: disconnecting the first power electronic switch (1), causing the load current to flow through the second moving contact (6), the third power electronic switch (3) and the current limiting element connected in parallel with the fourth power electronic switch (4), and then out through the output end; Step 6: closing the fourth power electronic switch (4), so that the load current flows through the third power electronic switch (3) and the fourth power electronic switch (4) and flows out through the output end; Step 7: closing the second mechanical switch (8), and opening the fourth power electronic switch (4) and the third power electronic switch (3) in sequence, so that the load current flows through the second moving contact (6) and the second mechanical switch (8), and flows out through the output end; Step 8: Disconnect the connection between the first moving contact (5) and the first tap.

10. The on-load voltage regulation method applicable to a power transformer according to claim 9, characterized in that: In step three, the second power electronic switch (2) is disconnected, and the load current flows through the first power electronic switch (1), the first reactor (11) and the first current limiting resistor (13), and flows out through the output end; In step five, the first power electronic switch (1) is disconnected, and the load current flows through the second moving contact (6), the third power electronic switch (3), the second reactor (12) and the second current limiting resistor (14), and flows out through the output end.

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