A Power Electronic On-Load Tap Changer and Method Applicable to Power Transformers

Abstract

The present invention belongs to the field of power transformers, and discloses a power electronic on-load tap-changer and method applicable to power transformers. In the non-switching state, this switch is connected through a mechanical switch. The power electronic switch is completely electrically isolated from the main circuit, does not bear the stage voltage, and also avoids damage to the power electronic switch caused by overvoltage generated by system operation faults; 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 tap-changer is realized, and the load current is ensured to be continuous. During the switching process, each power electronic switch conducts and turns off only once, simplifying the switching process, reducing the probability of faults, and improving the operation stability; a current-limiting element and a lightning arrester are used to protect the power electronic switch, which can ensure the safety of the power electronic switch during operation and in case of faults.

Description

Technical Field

[0001] The present invention belongs to the field of power transformers, specifically relates to the field of on-load tap-changers for power transformers, and particularly relates to a power electronic on-load tap-changer and method applicable to power transformers. 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 continuous load current, so as to maintain power supply safety and the stability of the power system.

[0003] The on-load tap-changing scheme based on a mechanical tap-changer not only has a slow operating speed, but also may generate a large arc during the tap-changing process, affecting the service life of the transformer, resulting in a large amount of maintenance work and cost, and severely restricting the role of the on-load tap-changing of the transformer. At present, although the technical performance of the mechanical on-load tap-changer has been greatly improved, there are still three problems that are difficult to solve with mechanical contacts: First, an arc is generated during the switching process of the tap contacts; Second, the tap-changing response speed is slow, the efficiency is low, and there is a lag of 100 milliseconds or even several seconds; Third, the mechanical transmission structure is complex, the failure rate is high, and the amount of maintenance is large. The power electronic on-load tap-changing technology applies power electronic devices to the structure of the on-load tap-changer of a power transformer to achieve the purpose of arc-free tap-changing, and the switching frequency of the power electronic devices can reach 2 kHz and above, while the response speed of the mechanical switch is in the millisecond level. Therefore, the power electronic switch has a significant advantage in terms of response speed. The core of the power electronic on-load tap-changing technology is power electronic devices. High-voltage and large-capacity power electronic on-load tap-changers put forward higher requirements for their performance indicators such as voltage level and current-carrying capacity; where large-capacity means that the rated capacity of the tap-changer is above 1 MVA; at present, high-voltage power electronic on-load tap-changers have not been applied in engineering; moreover, the existing power electronic on-load tap-changers of power transformers cannot take into account the safety during the switching process and the stability of the system connected to the transformer while achieving arc-free tap-changing, continuous load current during the tap-changing process, and ensuring the switching speed, resulting in poor switching stability and certain power supply safety hazards.

[0004] It can be seen that the existing power electronic on-load tap-changers of power transformers cannot ensure the safety during the switching process and the stability of the system connected to the transformer, resulting in poor switching stability and certain power supply safety hazards. Summary of the Invention

[0005] The present invention provides a power electronic on-load tap-changer and method applicable to power transformers, aiming to solve the technical problems of existing power electronic on-load tap-changers for power transformers, which cannot ensure the safety of the switching process and the stability of the system connected to the transformer, resulting in poor switching stability and certain power supply safety hazards. By using the present on-load tap-changer, the safety of the power electronic switch during operation and in case of faults is ensured.

[0006] To achieve the above object, the present invention adopts the following technical content:

[0007] A power electronic on-load tap-changer applicable to a power transformer includes a first module and a second module;

[0008] The first module includes a first moving contact, a first mechanical switch, a first power electronic switch, and a second power electronic switch;

[0009] The second module includes a second moving contact, a second mechanical switch, a third power electronic switch, and a fourth power electronic switch;

[0010] The first moving contact, the first power electronic switch, and the second power electronic switch are sequentially connected in series between the first tap and the output terminal of the transformer; after the first power electronic switch and the second power electronic switch are connected in series, they are connected in parallel with the first mechanical switch;

[0011] The second moving contact, the third power electronic switch, and the fourth power electronic switch are sequentially connected in series between the second tap and the output terminal of the transformer; after the third power electronic switch and the fourth power electronic switch are connected in series, they are connected in parallel with the second mechanical switch;

[0012] Wherein, the first tap and the second tap are adjacent taps;

[0013] 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;

[0014] Current limiting elements are respectively connected in parallel at both ends of the second power electronic switch and the fourth power electronic switch, and the current limiting elements are used to protect the switching state during the circuit switching process.

[0015] Further, the current limiting element includes a first reactor and a first current limiting resistor connected in series and a second reactor and a second current limiting resistor connected in series; wherein, the series-connected first reactor and first current limiting resistor are connected in parallel to both ends of the second power electronic switch; the series-connected second reactor and second current limiting resistor are connected in parallel to both ends of the fourth power electronic switch.

[0016] Further, it is characterized in that the first power electronic switch includes a first IGBT and a second IGBT with their emitters connected; a first power diode is connected in parallel with the first IGBT, and a second power diode is connected in parallel with the second IGBT; a resistor and a capacitor connected in series are connected in parallel with the series-connected first IGBT and second IGBT;

[0017] Wherein, the first IGBT and the second IGBT are respectively composed of one IGBT device or multiple IGBT devices connected in series.

[0018] Further, the third power electronic switch has the same structure as the first power electronic switch.

[0019] 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;

[0020] Wherein, the first IGCT and the second IGCT are respectively composed of one IGCT device or multiple IGCT devices connected in series.

[0021] Further, the fourth power electronic switch has the same structure as the second power electronic switch.

[0022] Further, the voltage threshold of the first lightning arrester is less than the blocking voltage of the first power electronic switch; the voltage threshold of the second lightning arrester is less than the blocking voltage of the third power electronic switch.

[0023] Further, lightning arresters are respectively connected in parallel at both ends of the second power electronic switch and the fourth power electronic switch.

[0024] A on-load voltage regulation method applicable to a power transformer, based on the above-mentioned power electronic on-load voltage regulation switch applicable to a power transformer, includes:

[0025] Step 1: Connect the second moving contact to the second tap of the transformer, and the second mechanical switch, the third power electronic switch, and the fourth power electronic switch are all in the off state. The load current flows from the first moving contact through the first mechanical switch and out through the output terminal;

[0026] Step 2: Close the first power electronic switch and the second power electronic switch in sequence. After the first mechanical switch is bypassed, disconnect the first mechanical switch. The load current flows through the first power electronic switch and the second power electronic switch and out through the output terminal;

[0027] Step 3: Disconnect the second power electronic switch. 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 out through the output terminal;

[0028] Step 4: Close the third power electronic switch, and a circulating current is formed between the first tap and the second tap;

[0029] Step 5: Open 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 terminal;

[0030] Step 6: Close the fourth power electronic switch, and the load current flows through the third power electronic switch and the fourth power electronic switch, and flows out through the output terminal;

[0031] Step 7: Close the second mechanical switch, and sequentially open the fourth power electronic switch and the third power electronic switch. The load current flows through the second moving contact and the second mechanical switch, and flows out through the output terminal;

[0032] Step 8: Disconnect the connection between the first moving contact and the first tap.

[0033] Further, in Step 3, the second power electronic switch is opened, 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 terminal;

[0034] In Step 5, the first power electronic switch is opened, 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 terminal.

[0035] Compared with the prior art, the present invention has the following beneficial effects:

[0036] The present invention provides a power electronic on-load tap-changer applicable to a power transformer. This switch is connected through a mechanical switch in the non-switching state. The power electronic switch is completely electrically isolated from the main circuit, does not bear the stage voltage, and also avoids damage to the power electronic switch caused by overvoltage generated by system operation faults; through the combined control of two mechanical moving contacts, two mechanical switches, and four power electronic switches, arc-free switching of the power electronic tap-changer is realized, and the continuity of the load current is ensured. During this process, each power electronic switch is only turned on and off once, simplifying the switching process, reducing the probability of faults, and improving the operation stability; current-limiting elements and lightning arresters are used to protect the power electronic switch, which can ensure the safety of the power electronic switch during operation and in case of faults; this switch adopts a completely symmetrical modular design, making the production process, assembly, testing, operation, and maintenance after commissioning more convenient.

[0037] Preferably, in the present invention, the current-limiting element includes a series-connected reactor and a current-limiting resistor, which can limit the overcurrent during the circuit switching process and in case of faults, protect the power electronic switch from damage, and improve the durability and reliability of the switch.

[0038] Preferably, in the present invention, both the first power electronic switch and the third power electronic switch adopt a first IGBT and a second IGBT with their emitters connected, and a power diode is connected in parallel. At the same time, a resistor and a capacitor are connected in series for protection, which improves the response speed and stability of the switch and is applicable to application scenarios with high voltage and large current.

[0039] Preferably, in the present invention, both the second power electronic switch and the fourth power electronic switch adopt a first IGCT and a second IGCT connected in reverse parallel, which improves the load-carrying capacity and stability of the switch and is applicable to application scenarios requiring higher rated voltage and current.

[0040] Preferably, in the present invention, a design is adopted in which the voltage threshold of the lightning arrester is less than the blocking voltage of the power electronic switch, which can protect the switch from damage under overvoltage conditions and improves the safety and reliability of the switch.

[0041] Preferably, in the present invention, lightning arresters are respectively connected in parallel at both ends of the second power electronic switch and the fourth power electronic switch, which further enhances the overvoltage protection ability of the switch during the switching process and improves the safety of the system.

[0042] The present invention also provides an on-load tap-changing method applicable to a power transformer. Based on the above-mentioned power electronic on-load tap-changing switch applicable to a power transformer, this method can, by precisely controlling the closing and opening sequence of the switch, ensure that in the most serious fault situation, if a power electronic switch that has been opened is damaged due to overvoltage, and the probability of damage to the other three power electronic switches due to structural reasons or lightning arrester protection is very small, so this switching can be unaffected, guarantee the power supply safety of the power transformer and the system stability, achieve a smooth switching from the first tap to the second tap, avoid voltage and current shocks during the switching process, and improve the stability and reliability of the power system.

[0043] Preferably, in the present invention, through the limiting effects of the reactor and the current-limiting resistor, the overcurrent during the switching process is further reduced, the safety of the switch and related equipment is protected, and the stability and reliability of the entire voltage regulation process are improved. Description of the Drawings

[0044] Figure 1 It is a schematic structural diagram of a power electronic on-load tap-changing switch applicable to a power transformer provided by an embodiment of the present invention;

[0045] Figure 2 It is a schematic diagram of a power electronic on-load tap-changing switch applicable to a power transformer provided by an embodiment of the present invention in the non-tap-changing stage;

[0046] Figure 3Schematic diagram of a power electronic on-load tap-changer applicable to a power transformer in the tap-changing stage provided by an embodiment of the present invention;

[0047] Figure 4 Schematic diagram of the structure of the first power electronic switch and the third power electronic switch of a power electronic on-load tap-changer applicable to a power transformer provided by an embodiment of the present invention;

[0048] Figure 5 Schematic diagram of the structure of the second power electronic switch and the fourth power electronic switch of a power electronic on-load tap-changer applicable to a power transformer provided by an embodiment of the present invention;

[0049] Figure 6 Schematic diagram of the first tap-changing process of a power electronic on-load tap-changer applicable to a power transformer provided by an embodiment of the present invention;

[0050] Figure 7 Schematic diagram of the second tap-changing process of a power electronic on-load tap-changer applicable to a power transformer provided by an embodiment of the present invention;

[0051] Figure 8 Schematic diagram of the third tap-changing process of a power electronic on-load tap-changer applicable to a power transformer provided by an embodiment of the present invention;

[0052] Figure 9 Schematic diagram of the fourth tap-changing process of a power electronic on-load tap-changer applicable to a power transformer provided by an embodiment of the present invention;

[0053] Figure 10 Schematic diagram of the fifth tap-changing process of a power electronic on-load tap-changer applicable to a power transformer provided by an embodiment of the present invention;

[0054] Figure 11 Schematic diagram of the sixth tap-changing process of a power electronic on-load tap-changer applicable to a power transformer provided by an embodiment of the present invention;

[0055] Figure 12 Schematic diagram of the seventh tap-changing process of a power electronic on-load tap-changer applicable to a power transformer provided by an embodiment of the present invention;

[0056] Figure 13 Schematic diagram of the eighth tap-changing process of a power electronic on-load tap-changer applicable to a power transformer provided by an embodiment of the present invention.

[0057] Reference numerals:

[0058] 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;

[0059] 100. Transformer; 200. On-load tap-changer;

[0060] 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 implementation mode

[0061] In order to make the technical problems, technical solutions and beneficial effects solved by the present invention clearer and more understandable, the following specific embodiments are used to further elaborate on the present invention in detail. 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.

[0062] To make the objectives, 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 with reference to the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are some, but not all, of the embodiments of the present invention. Generally, the components of the embodiments of the present invention described and illustrated in the accompanying drawings here can be arranged and designed in various different configurations.

[0063] Therefore, the following detailed description of the embodiments of the present invention provided in the drawings is not intended to limit the scope of the claimed present invention, but merely represents selected embodiments of the present invention. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts fall within the scope of protection of the present invention.

[0064] It should be noted that similar reference numerals and letters indicate similar items in the following drawings. Therefore, once an item is defined in one drawing, it does not need to be further defined and explained in subsequent drawings.

[0065] In the description of the embodiments of the present invention, it should be noted that if terms such as "upper", "lower", "horizontal", "inner", etc. indicate the orientation or positional relationship based on the orientation or positional relationship shown in the drawings, or the orientation or positional relationship in which the invention product is usually placed during use, it is only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore cannot be construed as a limitation of the present invention. In addition, terms such as "first", "second", etc. are only used for distinguishing descriptions and cannot be construed as indicating or implying relative importance.

[0066] In addition, if the term "horizontal" appears, it does not mean that the component is required to be absolutely horizontal, but it can be slightly inclined. For example, "horizontal" only means that its direction is more horizontal relative to "vertical", and does not mean that the structure must be completely horizontal, but it can be slightly inclined.

[0067] In the description of the embodiments of the present invention, it should also be noted that unless otherwise clearly specified and limited, if terms such as "set", "installed", "connected", "connected" are 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 directly connected, or indirectly connected through an intermediate medium, and it can be the communication inside two elements. For those of ordinary skill in the art, the specific meanings of the above terms in the present invention can be understood according to specific situations.

[0068] Embodiment 1

[0069] As mentioned in the background art, for on-load tap-changers of high-voltage and large-capacity power transformers, Chinese Patent Publication No. CN114783744A discloses a power electronic switching switch for on-load tap-changers, which mainly has the following problems:

[0070] First, when the transformer is operating normally, that is, in the non-switching state, since the power electronic switch is connected in parallel with the mechanical switch, its two ends will always bear the stage voltage, and the overvoltage generated by the system fault will damage the power electronic switch.

[0071] Second, during the switching process, when one branch power electronic switch is conducting and the other branch switch is off, the two ends of the off power electronic switch need to bear the sum of the stage 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, the voltages borne by the two power electronic switches are equal. As the voltage level of the power electronic switch increases, more power electronic devices need to be connected in series, that is, the cost is increased and the reliability is reduced.

[0072] Third, in the switched 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 in parallel and open, and the same overvoltage will be applied across their terminals, increasing the risk of simultaneous damage. Once damaged, the switching fails, leading to the expansion of the fault.

[0073] For another example, Chinese Patent Publication No. CN113077980A discloses a on-load tap-changer for a high-voltage power transmission transformer and its control method, which mainly has the following problems:

[0074] First, in the mechanical part, in addition to the four mechanical switches, there is also a tap selector to make contact with the corresponding tap. As described in the background art, the response speed of the mechanical switch is slower than that of the power electronic switch, which increases the overall voltage regulation time and reduces the efficiency.

[0075] Second, the mechanical switch and the power electronic switch need to cooperate during the entire switching process. Due to the large differences in the working mechanisms and action times of the mechanical switch and the power electronic switch, it further increases the difficulty of controlling the time required for the entire switching process.

[0076] Third, during the switching process, when one power electronic switch is closed and the other is open, and one end of the open power electronic switch is connected to another tap, the voltage across the turned-off power electronic switch needs to withstand the sum of the step voltage and the voltage across the current-limiting resistor. In high-voltage levels, the higher the voltage level of the power electronic switch, the more power electronic devices need to be connected in series, which means higher costs and lower reliability. Once another power electronic switch is damaged, if one end of this power electronic switch is connected to the switching target tap, the switching fails, leading to the expansion of the fault.

[0077] Fourth, during the switching process, if a fault occurs and a short-circuit current flows through the loop of one of the closed power electronic switches, the power electronic switch on the other branch is open, and the voltage across its terminals needs to withstand the step voltage plus the voltage generated by the short-circuit current across the current-limiting resistor, that is, the overvoltage. Once another power electronic switch is damaged, if one end of this power electronic switch is connected to the switching target tap, the switching fails, leading to the expansion of the fault.

[0078] It can be seen that the existing on-load tap-changer of the power electronic power transformer, while achieving arc-free voltage regulation, continuous load current during the voltage regulation process, and ensuring the 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 potential safety hazards.

[0079] To solve the above problems, this embodiment provides a power electronic on-load tap-changer applicable to power transformers. By using this tap-changer, arc-free voltage regulation of the on-load tap-changer of the power transformer can be achieved, the load current is continuous during the voltage regulation process, the switching speed is increased, and at the same time, the technical requirements for operation under high voltage levels are fully considered to ensure the safety of the switching process and the stability of the system connected to the transformer.

[0080] The following technical terms are now explained:

[0081] IGBT (Insulated Gate Bipolar Transistor), that is, insulated gate bipolar transistor, is a composite fully controlled voltage-driven power semiconductor device composed of BJT (bipolar junction transistor) and MOSFET (insulated gate field effect transistor). IGBT combines the advantages of high current density of BJT, high input impedance, small drive power, fast switching speed, and small switching loss of MOSFET, and has characteristics such as a wide safe operating area, high short-circuit withstand ability, and low noise.

[0082] IGCT (Integrated Gate-Commutated Thyristor) is an integrated gate-commutated thyristor, which is a high-voltage and 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 characteristics such as fast switching speed, small switching loss, strong current-carrying capacity, strong short-circuit withstand ability, good thermal stability, and simple control circuit.

[0083] As Figure 1 shown, this embodiment provides a power electronic on-load tap-changer applicable to power transformers. This tap-changer mainly consists 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 voltage of the power transformer.

[0084] In this embodiment, a series structure of two power electronic switches is adopted between each moving contact and the output terminal, with one being connected in parallel with a lightning arrester and the other being connected in parallel with a current-limiting element (reactor and current-limiting resistor); by controlling the on-off sequence of the four power electronic switches and coordinating with the on-off of the mechanical switch, arc-free, fast, and safe switching of the transformer tap is completed, and a completely symmetric modular structure design is adopted.

[0085] As Figure 1 shown, this embodiment provides a power electronic on-load tap-changer applicable to power transformers. The specific structure and connection relationship are as follows:

[0086] The power electronic on-load tap-changer includes a first module and a second module; from the perspective of facilitating product design, the first module and the second module with exactly the same structure can be adopted.

[0087] Among them, 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.

[0088] The second module includes 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.

[0089] The first tap of the transformer 100 is connected to the first mechanical switch 7 through the first moving contact 5, and the first mechanical switch 7 is connected in parallel with the series-connected first power electronic switch 1 and second power electronic switch 2.

[0090] A first lightning arrester 9 is connected in parallel with the first power electronic switch 1, and a current-limiting element for protecting the switching state during the circuit switching process is connected in parallel with the second power electronic switch 2.

[0091] The second tap connected to the first tap is connected to the second mechanical switch 8 through the second moving contact 6, and the second mechanical switch 8 is connected in parallel with the series-connected third power electronic switch 3 and fourth power electronic switch 4.

[0092] A second lightning arrester 10 is connected in parallel with the third power electronic switch 3, and a current-limiting element for protecting the switching state during the circuit switching process is also connected in parallel with the fourth power electronic switch 4; the current-limiting element includes a reactor and a current-limiting resistor, realizing the protection of the second power electronic switch 2 and the fourth power electronic switch 4.

[0093] In this embodiment, when the transformer is operating normally, that is, in the non-tap-changing stage, when the first moving contact 5 is connected to a certain tap of the transformer ( Figure 2 taking the N tap as an example), the states of each switch are as follows: the first mechanical switch 7 is closed, the first moving contact 5 is connected to a certain tap (the first tap) of the transformer 100, and the remaining moving contacts, mechanical switches, and power electronic switches are all in the off state.

[0094] In this stage, as Figure 2As shown, during the normal operation stage of the transformer 100, a certain tap is connected to one of the moving contacts (the first moving contact 5 or the second moving contact 6) of the on-load tap-changer 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 corresponding tap of the transformer 100. The electrical part of the entire on-load tap-changer 200 does not bear the step voltage and load current during normal operation, avoiding faults in the transformer body or power system during normal operation and generating overvoltage to damage the on-load tap-changer 200 of the power electronics.

[0095] When the transformer 100 enters the voltage regulation stage, the voltage regulation process of the on-load tap-changer 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).

[0096] At the beginning of the voltage regulation process, connect the second moving contact 6 to the N + 1 tap; turn on the first power electronic switch 1 and the second power electronic switch 2 in sequence. During the turn-on process, the first reactor 11 can reduce the rate of change of current with time and avoid damage to the power electronic switch caused by too large di / dt. Similarly, the second reactor 12 also has the same function. After the first mechanical switch 7 is short-circuited, disconnect the first mechanical switch 7. The load current transfers from the first mechanical switch 7 to the first power electronic switch 1 and the second power electronic switch 2.

[0097] Then disconnect the second power electronic switch 2, that is, the load current transfers from the second power electronic switch 2 to its parallel branch.

[0098] At this stage, as Figure 3 shown, the third power electronic switch 3 needs to bear the sum of the step voltage and the voltages across the first reactor 11 and the first current-limiting resistor 13. In a high-voltage system, it can reach several times the step voltage (approximate value, the actual value is determined by the voltage level of the transformer), and reaches the maximum value during the entire voltage regulation process. Therefore, the third power electronic switch 3 is the most vulnerable to damage, and a measure of connecting a second lightning arrester 10 in parallel at its two ends is taken for protection. Similarly, in the case of changing from N + 1 to N tap, the first power electronic switch 1 also bears the maximum voltage, so protection is also required, and a first lightning arrester 9 is specifically used for protection. The fourth power electronic switch 4 does not bear high voltage because it is connected in parallel with the second reactor 12 and the second current-limiting resistor 14. Similarly, in the case of changing from N + 1 to N tap, the power electronic switch 2 also does not bear high voltage. In the figure, ΔU represents the step voltage and Is represents the load current.

[0099] In this embodiment, if a short-circuit current occurs in the system fault transformer at this stage, it 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, in the case of switching from the N+1 tap to the N tap, an 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 due to overvoltage.

[0100] In this embodiment, at this stage, if the second power electronic switch 2 is damaged due to overvoltage caused by paralleling the first reactor 11 and the first current-limiting resistor 13, it does not affect the switching process. Similarly, in the case of switching from the N+1 tap to the N tap, the damage of the fourth power electronic switch 4 does not affect this switching, ensuring the safety of transformer power supply and the stable operation of the system in case of faults.

[0101] 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 100N tap and the N+1 tap of the transformer are connected, generating a circulating current. Since the loop contains the first reactor 11 and the second reactor 12 as well as the first current-limiting resistor 13 and the second current-limiting resistor 14, it can prevent the 100N tap and the N+1 tap of the transformer from being short-circuited.

[0102] Next, the first power electronic switch 1 is turned off, and the load current flows out from the tap N+1, the third power electronic switch 3, the second reactor 12 and the second current-limiting resistor 14, via the output terminal X.

[0103] 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 disconnected in sequence, and the connection between the first moving contact 5 and the tap N is disconnected, completing a voltage regulation process.

[0104] In this embodiment, each mechanical switch only operates once (turns off or on) and each power electronic switch only turns on and off once during the whole process. The cooperation between the power electronic switch and the mechanical switch only occurs after the first mechanical switch 7 is turned off and before the second mechanical switch 8 is turned on.

[0105] This embodiment provides a power electronic on-load tap-changer applicable to power transformers. 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, the first power electronic switch 1 and the third power electronic switch 3, and the second power electronic switch 2 and the fourth power electronic switch 4 in the above process.

[0106] As Figure 4 shown, in this embodiment, the first power electronic switch 1 and the third power electronic switch 3 have the same structure, both consisting of IGBTs and power diodes. Among them, the first IGBT 301 and the second IGBT 302 are connected in common emitter, and each IGBT should have at least one IGBT device in series. At least one first power diode 303 and one second power diode 304 are connected in parallel in opposite directions 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 RC snubber circuit, which is connected in parallel at both ends of the series-connected first IGBT 301 and second IGBT 302 to protect the circuit and prevent the power electronic devices from being damaged due to too fast voltage change.

[0107] In this embodiment, the first power electronic switch 1 and the third power electronic switch 3 can also adopt the IGCT structure; as Figure 5 shown, in the structure based on IGCT, it is composed of two anti-parallel first IGCTs 307 and second IGCTs 308; and each IGCT should have no less than one IGCT device in series.

[0108] In this embodiment, the second power electronic switch 2 and the fourth power electronic switch 4 have the same structure, both consisting of IGCTs. As Figure 5 shown, in the structure based on IGCT, it is composed of two anti-parallel first IGCTs 307 and second IGCTs 308; and each IGCT should have no less than one IGCT device in series.

[0109] In this embodiment, the second power electronic switch 2 and the fourth power electronic switch 4 can also adopt the IGBT structure; as Figure 5 shown, in the structure based on IGBT, the first IGBT 301 and the second IGBT 302 are connected in common emitter, and each IGBT should have at least one IGBT device in series; at least one first power diode 303 and one second power diode 304 are connected in parallel in opposite directions 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 RC snubber circuit, which is connected in parallel at both ends of the series-connected first IGBT 301 and second IGBT 302 to protect the circuit and prevent the power electronic devices from being damaged due to too fast voltage change.

[0110] In this embodiment, in order to enhance the over-voltage protection ability of the switch during the switching process and improve the safety of the system, a group of lightning arresters are connected in parallel to both the second power electronic switch 2 and the fourth power electronic switch 4.

[0111] This embodiment also provides a working method for a power electronic on-load tap-changer applicable to a power transformer, that is, a tap-changing method. Taking the on-load tap-changer switching from the Nth tap to the N+1th tap as an example, the entire tap-changing process is described as follows:

[0112] First step, connect the second moving contact 6 to the N+1th tapping, as Figure 6 shown; after completion, since all the mechanical switches and power electronic switches in the branch connected by the second moving contact 6 are in the off state, there is no current in the part connected by 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 tap-changer through the output terminal X. The arrow direction in the figure is the direction of the moving contact relative to the transformer. If it is switched from the N+1th tapping to the Nth tapping, the direction is opposite.

[0113] Second step, close the first power electronic switch 1 and the second power electronic switch 2 in sequence, as Figure 7 shown; after the first mechanical switch 7 is bypassed, disconnect the first mechanical switch 7. 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 step voltage, Is represents the load current, and the arrow represents the current path and direction.

[0114] Third step, disconnect the second power electronic switch 2, as Figure 8 shown. At this time, the load current changes to flow out through the first reactor 11 and the first current-limiting resistor 13 connected in parallel with the second power electronic switch 2.

[0115] Fourth step, close the third power electronic switch 3; as Figure 9 shown. At this time, a circulating current is formed between the two tappings, and the transformer windings between the two tappings N and N+1 are connected through the first reactor 11 and the second reactor 12, and the first current-limiting resistor 13 and the second current-limiting resistor 14 to avoid short-circuit between the tappings.

[0116] Fifth step, disconnect the first power electronic switch 1, as Figure 10 shown. At this time, the load current flows out through the tapping N+1, the third power electronic switch 3, the second reactor 12 and the second current-limiting resistor 14, and through the output terminal X.

[0117] Sixth step, close the fourth power electronic switch 4, as Figure 11 shown. At this time, the load current changes to flow out through the fourth power electronic switch 4 connected in parallel with the second reactor 12 and the second current-limiting resistor 14.

[0118] Step 7: Close the second mechanical switch 8, as Figure 12 shown, and then disconnect the fourth power electronic switch 4 and the third power electronic switch 3 in sequence; the load current flows through the second mechanical switch 8 and flows out from terminal X.

[0119] Step 8: Disconnect the connection between the first moving contact 5 and the tap N, as Figure 13 shown, and the arrow direction is the direction in which the moving contact moves relative to the transformer; if it is switched from tap N + 1 to tap N, the direction is opposite.

[0120] This embodiment provides a power electronic on-load tap-changer applicable to power transformers, with the following characteristics:

[0121] First, it can effectively protect the power electronic switches of high voltage levels:

[0122] Through the analysis of the switching process, lightning arresters are connected in parallel to protect the power electronic switches that are operating under harsh conditions (i.e., need to withstand higher voltages during operation) and are at risk of overvoltage.

[0123] In view of the characteristics of power electronic devices being sensitive to the rate of change of current, reactors (including the first reactor 11 and the second reactor 12) are installed on the branches of the current-limiting resistors (including the first current-limiting resistor 13 and the second current-limiting resistor 14) to reduce di / dt; further, introducing reactors in the switching circuit can also reduce the losses on the current-limiting resistors during the parallel stage of two taps.

[0124] Second, through the structural design of the combination of power electronic switches, reactors (including the first reactor 11 and the second reactor 12), and current-limiting resistors (including the first current-limiting resistor 13 and the second current-limiting resistor 14), the safe operation of the switching process is ensured.

[0125] When the power electronic switch protected by the lightning arrester in one group of series-connected power electronic switches conducts, the power electronic switches connected in parallel with the reactors (including the first reactor 11 and the second reactor 12) and the current-limiting resistors (including the first current-limiting resistor 13 and the second current-limiting resistor 14) turn off, and the other group of series-connected power electronic switches are all in the off state and connected to the switching target tap. If a system fault occurs at this moment and a short-circuit current flows through the tap-changer, causing damage to the power electronic switches connected in parallel with the reactors and the current-limiting resistors, it will not affect the switching process, ensuring the power supply safety of the power transformer and the system stability.

[0126] Third, a streamlined mechanical part structure.

[0127] Simplify the coordination between the mechanical switch and the power electronic switch. During the key switching step (when both tap changers of the transformer are connected to the main circuit), all use power electronic switches to improve the switching speed and reduce the uncontrollable problems caused by the coordination between the mechanical switch and the power electronic switch. Further, as mentioned above, the situation where the maximum overvoltage occurs during the system failure appears during the switching process of the power electronic switch. Since the speed of the power electronic switch is significantly better than that of the mechanical switch, the probability of failure is reduced.

[0128] Fourth, when the on-load tap-changer is in the non-operating state, no power electronic switch is connected to the main circuit, and the power electronic device does not bear the step voltage, realizing complete electrical isolation of the electrical part and avoiding damage to the power electronic device caused by long-term bearing of the step voltage and overvoltage generated during system failures.

[0129] Fifth, the tap-changer adopts a completely symmetrical modular design, which provides convenience for the production process, assembly and testing of the formed product, as well as the later operation and maintenance.

[0130] In summary, the present invention provides a power electronic on-load tap-changer and method applicable to power transformers, which have the following advantages compared with the existing tap-changing measures:

[0131] First, this switch is connected through the mechanical switch in the non-switching state. The power electronic switch is completely electrically isolated from the main circuit, does not bear the step voltage, and also avoids damage to the power electronic switch caused by the overvoltage generated during system operation failures. Second, through the combined control of two mechanical moving contacts, two mechanical switches, and four power electronic switches, arc-free switching of the power electronic tap-changer is achieved, and the load current is ensured to be continuous. During this process, each power electronic switch only conducts and turns off once; the cooperation between the mechanical switch and the power electronic switch only needs to be carried out twice, that is, the switching process is simplified, the probability of failure is reduced, and the operation stability is improved. Third, in normal operation and in case of failure, the method of using reactors and lightning arresters is adopted to protect the power electronic switch, ensuring the safety of the power electronic switch during operation and in case of failure. Fourth, the structure and control sequence adopted by this switch, even in the most severe failure situation, if the already disconnected power electronic switch is damaged due to overvoltage, the other three power electronic switches are protected by structural reasons or lightning arresters, so the probability of damage is very small, and thus this switching is not affected, ensuring the power supply safety of the power transformer and the system stability. Finally, this switch adopts a completely symmetrical modular design, making the production process, assembly, testing, operation and maintenance after commissioning more convenient; adopting this switch takes into account the safety of the switching process and the stability of the system connected to the transformer, and can realize arc-free voltage regulation, continuous load current during the voltage regulation process, ensure the switching speed, thereby improving the stability and operation efficiency of the entire power system.

[0132] The above embodiments are only one of the implementation manners capable of implementing the technical solution of the present invention. The scope of protection required by the present invention is not only limited by this embodiment, but also includes any changes, substitutions and other implementation manners that are easily conceivable by any person skilled in the art within the technical scope disclosed by the present invention.

Claims

1. A on-load voltage regulation method applicable to a power transformer, characterized in that, Based on a power electronic on-load tap-changer, the power electronic on-load tap-changer includes a first module and a second module; The first module includes 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 includes 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 terminal of the transformer (100); after the first power electronic switch (1) and the second power electronic switch (2) are connected in series, they are 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 terminal of the transformer (100); after the third power electronic switch (3) and the fourth power electronic switch (4) are connected in series, they are connected in parallel with the second mechanical switch (8); Wherein, the first tap and the second tap are adjacent taps; A first lightning arrester (9) is connected in parallel at both ends of the first power electronic switch (1); a second lightning arrester (10) is connected in parallel at both ends of the third power electronic switch (3); A current limiting element is connected in parallel at both ends of the second power electronic switch (2) and the fourth power electronic switch (4), and the current limiting element is used to protect the switching state during the circuit switching process; The on-load tap-changing method includes: Step 1: Connect the second moving contact (6) to the second tap of the transformer (100), and the second mechanical switch (8), the third power electronic switch (3), and the fourth power electronic switch (4) are all in the off state. The load current flows from the first moving contact (5) through the first mechanical switch (7) and out through the output terminal; Step 2: Close the first power electronic switch (1) and the second power electronic switch (2) in sequence. After the first mechanical switch (7) is bypassed, disconnect the first mechanical switch (7). The load current flows through the first power electronic switch (1) and the second power electronic switch (2) and out through the output terminal; Step 3: Disconnect the second power electronic switch (2). 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 out through the output terminal; Step 4: Close the third power electronic switch (3), and a circulating current is formed between the first tap and the second tap; Step 5: Disconnect the first power electronic switch (1). The load current flows 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 out through the output terminal; Step 6: Close the fourth power electronic switch (4). The load current flows through the third power electronic switch (3) and the fourth power electronic switch (4) and out through the output terminal; Step 7: Close the second mechanical switch (8), and then disconnect the fourth power electronic switch (4) and the third power electronic switch (3) in sequence. The load current flows through the second moving contact (6) and the second mechanical switch (8), and flows out through the output terminal. Step 8: Disconnect the connection between the first moving contact (5) and the first tap.

2. The on-load voltage regulation method applicable to a power transformer according to claim 1, characterized in that The current limiting element includes 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 series-connected first reactor (11) and first current limiting resistor (13) are connected in parallel across both ends of the second power electronic switch (2); the series-connected second reactor (12) and second current limiting resistor (14) are connected in parallel across both ends of the fourth power electronic switch (4).

3. The on-load voltage regulation method applicable to a power transformer according to claim 1, characterized in that, The first power electronic switch (1) includes a first IGBT (301) and a second IGBT (302) with their emitters connected; a first power diode (303) is connected in parallel with the first IGBT (301), and a second power diode (304) is connected in parallel with the second IGBT (302); a resistor (305) and a capacitor (306) connected in series are connected in parallel with the series-connected first IGBT (301) and second IGBT (302). Wherein, the first IGBT (301) and the second IGBT (302) are respectively composed of one IGBT device or multiple IGBT devices connected in series.

4. The on-load voltage regulation method applicable to a 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 on-load voltage regulation method applicable to a power transformer according to claim 1, wherein The second power electronic switch (2) includes a first IGCT (307) and a second IGCT (308), and the first IGCT (307) and the second IGCT (308) are connected in reverse parallel. Wherein, the first IGCT (307) and the second IGCT (308) are respectively composed of one IGCT device or multiple IGCT devices connected in series.

6. The on-load voltage regulation method applicable to a 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 on-load voltage regulation method applicable to a power transformer according to any one of claims 1-6, 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); the voltage threshold of the second lightning arrester (10) is less than the blocking voltage of the third power electronic switch (3).

8. The on-load voltage regulation method applicable to a power transformer according to claim 1, wherein Lightning arresters are respectively connected in parallel across both ends of the second power electronic switch (2) and the fourth power electronic switch (4).

9. The on-load voltage regulation method applicable to a power transformer according to claim 1, characterized in that In Step 3, disconnect the second power electronic switch (2), 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 terminal. In Step 5, disconnect the first power electronic switch (1), 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 terminal.

Images