Short-circuit protection method and short-circuit protection device for a transformer

Abstract

The application provides a short-circuit protection method and device for a transformer. The short-circuit protection method comprises: obtaining output voltages of a plurality of output lines of at least one winding on a low-voltage side of the transformer; performing real-time calculation and short-circuit judgment according to the output voltages, and sending a protection signal when the short-circuit is judged. The short-circuit protection device comprises: a voltage obtaining unit for obtaining output voltages of a plurality of output lines of at least one winding on a low-voltage side of the transformer; a short-circuit judgment unit for performing real-time calculation and short-circuit judgment according to the output voltages, and sending a protection signal when the short-circuit is judged; and a controllable circuit breaker arranged on a high-voltage side of the transformer and connected with the short-circuit judgment unit, for cutting off the connection between the transformer and a power grid after receiving the protection signal. The application provides a short-circuit protection method and device based on voltage characteristics, realizes short-circuit protection of a three-phase transformer through line voltage detection, greatly reduces the protection cost, and improves the protection reliability.

Description

Technical Field

[0001] This invention relates to the field of power electronics technology, and in particular to a method and apparatus for short-circuit protection of a transformer. Background Technology

[0002] In power systems, short circuit faults are a common and serious type of circuit fault. If timely and effective protective measures are not taken when a short circuit fault occurs, it can lead to serious accidents, threatening property and personal safety. Therefore, short circuit protection is an indispensable function.

[0003] In power systems, short-circuit protection on the low-voltage side of a three-phase transformer is usually achieved by connecting a circuit breaker in series at the low-voltage output distribution terminal for hardware protection, or by connecting a current sensor in series to collect current for software protection.

[0004] In most scenarios, if a short circuit fault occurs in the downstream power distribution line, the current flowing through the circuit breaker will suddenly increase to several to tens of times the rated current. Therefore, the circuit breaker will trip and disconnect the line to achieve short circuit protection.

[0005] In systems with multiple secondary windings, the requirements for transformer short-circuit protection are quite unique. Using the same protection method based on current circuit breakers is not suitable. One reason is that systems with multiple secondary windings significantly increase protection costs; another is that the location of the short circuit affects reliability. If the short-circuit current does not flow through the downstream circuit breaker, the main transformer cannot be effectively protected.

[0006] For example, in a 2.4MW charging system, the main transformer is a 2500kVA phase-shifting transformer, drawing power from the 12.5kV medium-voltage grid. This power is then supplied to multiple chargers via a phase-shifting transformer with 49 low-voltage windings, while also providing input isolation and reducing total harmonic distortion (THD). The transformer's short-circuit impedance is approximately 7%. If a short circuit occurs in the low-voltage winding, a short-circuit current exceeding 12 times the rated current will be generated, along with significant electrodynamic and heating effects. If protection is not implemented promptly, this could potentially damage the entire transformer.

[0007] For the aforementioned charging systems or multi-output winding power distribution systems, using current-based protection methods will have the following drawbacks:

[0008] (1) For a transformer with N low-voltage side windings, hardware protection requires N three-phase circuit breakers. When the system requires software protection and reports short-circuit faults, 3*N current sensors are also required. When N is large, the detection and protection cost will increase significantly.

[0009] (2) In practical engineering applications, current sensors and circuit breakers are generally not installed at the output position of the transformer, but at the power distribution end of the equipment. If a short circuit occurs inside the transformer or in the line between the output end and the circuit breaker, the short circuit current will not pass through the circuit breaker, and this situation will not be detected and therefore cannot be protected.

[0010] Therefore, there is an urgent need to provide a new method and device for transformer short-circuit protection that can effectively reduce protection costs and improve protection reliability. Summary of the Invention

[0011] The purpose of this invention is to provide a transformer short-circuit protection method and device, which can effectively reduce protection costs and improve protection reliability.

[0012] To achieve the above objectives, the present invention provides a short-circuit protection method for a transformer, characterized in that it includes: acquiring the output voltage of multiple output lines of at least one winding on the low-voltage side of the transformer; performing real-time calculation and short-circuit judgment based on the output voltage, and issuing a protection signal when a short circuit is determined.

[0013] In one embodiment of the present invention, the steps of real-time calculation and short-circuit judgment based on the output voltage include: calculating the line voltage between any two output lines among the plurality of output lines based on the output voltage to obtain a plurality of line voltages; and judging a short circuit when the change of the plurality of line voltages conforms to a short-circuit change characteristic.

[0014] In one embodiment of the present invention, the short-circuit variation characteristic includes an amplitude characteristic, wherein the amplitude of one of the plurality of line voltages becomes less than or equal to a first characteristic value, the amplitude of the others of the plurality of line voltages becomes greater than or equal to a second characteristic value, and the first characteristic value is less than the second characteristic value.

[0015] In one embodiment of the present invention, the method for short circuit judgment includes: (a) when the amplitude of one of the plurality of line voltages is less than or equal to the first characteristic value, an abnormality is considered and short circuit judgment is initiated; (b) continue to judge whether the amplitude of the other of the plurality of line voltages is greater than or equal to the second characteristic value; (c) if both of the above (a) and (b) are always satisfied within a detection window time, then a short circuit fault is determined, wherein the detection window time is greater than or equal to one cycle time of the alternating current of the transformer.

[0016] In one embodiment of the present invention, the short-circuit variation characteristic further includes a phase characteristic, wherein the phase difference between two of the plurality of line voltages satisfies a phase characteristic value.

[0017] In one embodiment of the present invention, the method for short circuit judgment includes: (a) when the amplitude of one of the plurality of line voltages is detected to be less than or equal to the first characteristic value, an abnormality is considered and short circuit judgment is initiated; (b) it is further determined whether the amplitude of the other of the plurality of line voltages is greater than or equal to the second characteristic value; (c) by zero-crossing detection, the time difference of the same-direction zero-crossing moments of the other of the plurality of line voltages is obtained, and the phase difference is calculated accordingly, and it is determined whether the phase difference satisfies the phase characteristic value; (d) if both of the above (a) and (b) are always satisfied within a detection window time, and (c) is satisfied once, then a short circuit fault is determined, wherein the detection window time is greater than or equal to one cycle time of the alternating current of the transformer.

[0018] In one embodiment of the present invention, the phase feature value is set to an angle range of 160° to 180°.

[0019] In one embodiment of the present invention, the winding is a Y-shaped winding, a delta winding, or an extended delta winding.

[0020] In one embodiment of the present invention, the winding is a Y-type winding, and the first characteristic value is 0, the second characteristic value is 0.87*Uo, where Uo is the rated voltage amplitude; or, the first characteristic value is Uo*5%, the second characteristic value is 0.87*Uo*80%, where Uo is the rated voltage amplitude.

[0021] In one embodiment of the present invention, the winding is an extended delta winding, and the first characteristic value is 0.38*Uo, the second characteristic value is 0.99*Uo, where Uo is the rated voltage amplitude; or, the first characteristic value is 0.38*Uo*120%, the second characteristic value is 0.99*Uo*80%, where Uo is the rated voltage amplitude.

[0022] In one embodiment of the present invention, the protection signal is sent to a controllable circuit breaker located on the high-voltage side of the transformer, and the controllable circuit breaker disconnects the transformer from the power grid after receiving the protection signal.

[0023] In one embodiment of the present invention, the output voltage is sampled by a voltage sensor.

[0024] In one embodiment of the present invention, the short-circuit variation characteristic further includes the phasor sum of the plurality of line voltages always being 0.

[0025] To achieve the above objectives, the present invention further provides a short-circuit protection device for a transformer, characterized in that it includes: a voltage acquisition unit for acquiring the output voltage of multiple output lines of at least one winding on the low-voltage side of the transformer; a short-circuit judgment unit for performing real-time calculation and short-circuit judgment based on the output voltage, and issuing a protection signal when a short circuit is judged; and a controllable circuit breaker, disposed on the high-voltage side of the transformer and connected to the short-circuit judgment unit, for disconnecting the transformer from the power grid after receiving the protection signal.

[0026] In another embodiment of the present invention, the voltage acquisition unit is a voltage sensor.

[0027] In another embodiment of the present invention, the short circuit determination unit is also connected to the upper-level system and reports a short circuit fault to the upper-level system when a short circuit is determined.

[0028] In another embodiment of the present invention, the step of the short circuit judgment unit performing real-time calculation and short circuit judgment based on the output voltage includes: calculating the line voltage between any two output lines among the plurality of output lines based on the output voltage to obtain a plurality of line voltages; and judging a short circuit when the change of any one of the plurality of line voltages conforms to a short circuit change characteristic.

[0029] In another embodiment of the present invention, the short-circuit variation characteristic includes an amplitude characteristic, wherein the amplitude of one of the plurality of line voltages becomes less than or equal to a first characteristic value, the amplitude of the others of the plurality of line voltages becomes greater than or equal to a second characteristic value, and the first characteristic value is less than the second characteristic value.

[0030] In another embodiment of the present invention, the method for short circuit judgment by the short circuit judgment unit includes: (a) when the amplitude of one of the plurality of line voltages is less than or equal to the first characteristic value, it is considered to be abnormal and short circuit judgment is started; (b) continue to judge whether the amplitude of the other of the plurality of line voltages is greater than or equal to the second characteristic value; (c) if the above two conditions (a) and (b) are always satisfied within a detection window time, it is judged to be a short circuit fault, wherein the detection window time is greater than or equal to one cycle time of the alternating current of the transformer.

[0031] In another embodiment of the invention, the short-circuit variation characteristic further includes a phase characteristic, wherein the phase value of one of the plurality of line voltages becomes a phase characteristic value.

[0032] In another embodiment of the present invention, the method for short-circuit judgment by the short-circuit judgment unit includes: (a) when the amplitude of one of the plurality of line voltages is detected to be less than or equal to the first characteristic value, an abnormality is considered and short-circuit judgment is initiated; (b) further judging whether the amplitude of the other of the plurality of line voltages is greater than or equal to the second characteristic value; (c) by zero-crossing detection, obtaining the time difference of the same-direction zero-crossing moments of the other of the plurality of line voltages, calculating the phase difference accordingly, and judging whether the phase difference satisfies the phase characteristic value; (d) if both of (a) and (b) above are always satisfied within a detection window time, and (c) is satisfied once, then a short-circuit fault is determined, wherein the detection window time is greater than or equal to one cycle time of the alternating current of the transformer.

[0033] In another embodiment of the present invention, the phase feature value is set to an angle range of 160° to 180°.

[0034] In another embodiment of the present invention, the winding is a Y-shaped winding, a delta winding, or an extended delta winding.

[0035] In another embodiment of the present invention, the winding is a Y-type winding, and the first characteristic value is 0, the second characteristic value is 0.87*Uo, where Uo is the rated voltage amplitude; or, the first characteristic value is Uo*5%, the second characteristic value is 0.87*Uo*80%, where Uo is the rated voltage amplitude.

[0036] In another embodiment of the present invention, the winding is an extended delta winding, and the first characteristic value is 0.38*Uo, the second characteristic value is 0.99*Uo, where Uo is the rated voltage amplitude; or, the first characteristic value is 0.38*Uo*120%, the second characteristic value is 0.99*Uo*80%, where Uo is the rated voltage amplitude.

[0037] In another embodiment of the invention, the short-circuit variation characteristic further includes the phasor sum of the plurality of line voltages always being 0.

[0038] This invention provides a short-circuit protection method and device based on voltage characteristics. It realizes short-circuit protection of three-phase transformers through line voltage detection, which greatly reduces protection costs and improves protection reliability.

[0039] Compared to existing current detection methods, the advantages of this invention are:

[0040] (1) If only used for short-circuit protection, voltage sensors (voltage divider type) are less expensive than current sensors. This can save significant costs for multi-winding output applications. If the downstream equipment (e.g., charger) has a real-time mains voltage sampling function, this function can be used directly, thus eliminating the need for a voltage sensor. For example, the mains voltage sampling data of the charger can be used directly for short-circuit protection.

[0041] (2) Where existing current detection methods cannot provide protection, the voltage detection method of the present invention can provide protection, regardless of the location of the short circuit.

[0042] (3) The overcurrent tripping speed of a circuit breaker is related to the magnitude of the current. In some cases, the tripping speed may be slow when the protection threshold is approached, increasing the system risk. The voltage detection method of this invention is determined by software, and the protection time and sensitivity are controllable and stable.

[0043] Additional aspects and advantages of the invention will be set forth in part in the description which follows, and in part will be apparent from the description, or may be learned by practice of the invention. Attached Figure Description

[0044] The above and other features and advantages of the present invention will become more apparent from a detailed description of exemplary embodiments thereof with reference to the accompanying drawings.

[0045] Figure 1A This is a schematic diagram of the magnetic circuit analysis when a three-phase transformer is operating normally.

[0046] Figure 1B This is the phasor diagram of a three-phase transformer under normal operating conditions.

[0047] Figure 2A This is a schematic diagram of the magnetic circuit analysis when a short circuit occurs in the S phase of a three-phase transformer.

[0048] Figure 2B This is the phasor diagram of a three-phase transformer when a short circuit occurs in the S phase.

[0049] Figure 3 This is a schematic flowchart of the short-circuit protection method for a three-phase transformer according to the present invention;

[0050] Figure 4 This is a schematic diagram of the short-circuit protection device for a three-phase transformer according to the present invention;

[0051] Figures 5A to 5H The present invention relates to the connection structure of various types of windings of a three-phase transformer;

[0052] Figures 6A to 6D These are examples of different types of windings of the three-phase transformer of the present invention experiencing line-to-line short circuits.

[0053] Figures 7A to 7GThese are examples of different types of windings in the three-phase transformer of the present invention experiencing phase short circuits or partial winding short circuits.

[0054] Figure 8A The three-phase transformer of the present invention is as follows Figure 6A The diagram shows the voltage change waveforms between the three output lines before and after a short circuit occurs on the RS line of the Y-type winding.

[0055] Figure 8B and Figure 8C These are the phasor diagrams before and after the short circuit;

[0056] Figure 8D A schematic diagram for detecting the output voltage of the output line of the short-circuited winding to determine a short circuit;

[0057] Figure 9 This is a schematic diagram showing the voltage amplitude change when a secondary winding on the low-voltage side of the three-phase transformer of the present invention is energized, and the output voltages of the three output lines decrease asynchronously (due to the influence of the Y capacitor in the downstream equipment).

[0058] Figure 10A The three-phase transformer of the present invention is as follows Figure 7B The diagram shows the voltage change waveform of the line voltage between the three output lines before and after a short circuit occurs in the extended winding section on phase R of the extended delta winding.

[0059] Figure 10B and Figure 10C These are the phasor diagrams before and after the short circuit;

[0060] Figure 10D A schematic diagram for detecting the output voltage of the output line of the short-circuited winding to determine a short circuit;

[0061] Figure 11 for Figure 10D The illustrated embodiment is a signal timing diagram. Detailed Implementation

[0062] Exemplary embodiments will now be described more fully with reference to the accompanying drawings. However, these exemplary embodiments can be implemented in many forms and should not be construed as limited to the embodiments set forth herein; rather, they are provided so that the invention will be thorough and complete, and the concept of the exemplary embodiments will be fully conveyed to those skilled in the art. The same reference numerals in the drawings denote the same or similar structures, and therefore their detailed description will be omitted.

[0063] In describing the elements / components / etc. described and / or illustrated herein, the terms “a,” “an,” “the,” “the,” and “at least one” are used to indicate the presence of one or more elements / components / etc. The terms “comprising,” “including,” and “having” are used to indicate an open-ended inclusion and to mean that additional elements / components / etc. may exist in addition to those listed. Relative terms, such as “upper” or “lower,” may be used in the embodiments to describe the relative relationship of one component of the icon to another component. It is understood that if the device of the icon is flipped so that it is upside down, the component described as being on the “upper” side will become the component on the “lower” side. Furthermore, the terms “first,” “second,” etc., in the claims are used only as illustrative marks and are not intended to limit the number of objects to which they apply.

[0064] This invention primarily provides a short-circuit protection method and device based on line voltage. When a short circuit occurs inside a transformer, the characteristics of the three line voltages of the output winding will change. Therefore, by detecting the RMS (instantaneous) value of the line voltage, a short circuit can be identified and protection implemented without requiring detection via a current path. Furthermore, by analyzing the amplitude and phase of the line voltage phasor characteristics, the type of different short circuits can be accurately determined.

[0065] like Figure 1A The diagram illustrates the magnetic circuit of a three-phase transformer during normal operation. The three-phase transformer has an iron core, with R-phase, S-phase, and T-phase windings wound on three core columns. The magnetic circuits of these windings are shown below. Figure 1A As shown. When the three-phase transformer is operating normally, the component φ of the resultant magnetic flux generated by all winding currents on the three core columns. R φ S φ T If they differ by 120° and have the same amplitude, such as Figure 1B As shown, the induced electromotive force on the winding satisfies:

[0066]

[0067] Therefore, the terminal voltage on the winding and the magnetic flux surrounding the winding change in the same way over time.

[0068] Furthermore, according to the principle of magnetic flux continuity:

[0069]

[0070] The total magnetic flux at the magnetic circuit junction P is 0, such as Figure 1A As shown, the sum of the three-phase components of the magnetic flux is 0:

[0071] φ R +φ S +φT =0.

[0072] like Figure 2A The diagram illustrates the magnetic circuit of a three-phase transformer when the S-phase winding is short-circuited. When the S-phase winding is short-circuited, the winding coil forms a closed loop with very low impedance, inducing a very large loop current, and the terminal voltage approaches zero. Simultaneously, the loop short-circuit current in the winding generates a magnetic flux φ that is analogous to the primary side magnetic flux. Pri Magnetic flux φ in opposite directions Short ,like Figure 2A As shown, the combined magnetic flux of the S phase is close to 0.

[0073] Furthermore, the φ of the magnetic circuit intersection node P S If one component is reduced to 0, the other components must still satisfy:

[0074] φ R +φ T =0;

[0075] That is, φ R φ T It becomes two components that are 180° apart and have the same amplitude, such as Figure 2B As shown. The phase voltage also changes accordingly, and the change in the combined line voltage is related to the winding connection type, but it always satisfies that the phasor sum is 0.

[0076] like Figure 3 As shown, the transformer short-circuit protection method 100 of the present invention mainly includes:

[0077] Step S101: Obtain the output voltage of multiple output lines of at least one winding on the low-voltage side of the transformer;

[0078] Step S102: Perform real-time calculation and short-circuit judgment based on the output voltage, and issue a protection signal when a short circuit is detected.

[0079] In this invention, step S102 involves calculating the line voltage between any two output lines from the plurality of output lines based on the output voltage, thereby obtaining a plurality of line voltages. Furthermore, a short circuit is determined when the change in the plurality of line voltages conforms to a short-circuit change characteristic. The short-circuit change characteristic may include an amplitude characteristic; for example, the amplitude of one of the plurality of line voltages becomes less than or equal to a first characteristic value, and the amplitudes of the others become greater than or equal to a second characteristic value, where the first characteristic value is less than the second characteristic value. In other embodiments, the short-circuit change characteristic may also include a phase characteristic; for example, the phase difference between two of the plurality of line voltages satisfies a phase characteristic value. The phase characteristic value may be set to an angle range, for example, 160° to 180°. The short-circuit change characteristic may further include the phasor sum of the plurality of line voltages always being 0.

[0080] like Figure 4 As shown, the short-circuit protection device for the transformer of the present invention can be applied to a power system, which may include, for example, a power grid 10, a three-phase transformer 20, power equipment 30, an upstream system 40, and a short-circuit protection device 50. The primary winding 21 of the three-phase transformer 20 is located on the high-voltage side and draws power from the power grid 10, while the secondary winding 22 is located on the low-voltage side and includes multiple secondary windings to supply power to the downstream power equipment 30. Figure 4 In the illustrated embodiment, the primary winding 21 may be, for example, a Y-shaped primary winding, and the secondary windings 22 may include, for example, an extended delta secondary winding 221 and a Y-shaped secondary winding 222. The figure only shows the short-circuit protection device 50 for one of the Y-shaped secondary windings 222, but it is understood that the short-circuit protection device 50 may also be provided for other secondary windings.

[0081] In this invention, the short-circuit protection device 50 may include, for example, a voltage acquisition unit 51, a short-circuit judgment unit 52, and a controllable circuit breaker 53.

[0082] The voltage acquisition unit 51 can be, for example, a voltage sensor, used to acquire the output voltage of multiple output lines of at least one winding on the low-voltage side of the transformer 30. For example, the output voltage of the R, S, and T output lines of the Y-type secondary winding 222 can be sampled in real time by the voltage sensor, and the instantaneous value of the output voltage obtained in real time can be sent to the short-circuit judgment unit 52. In other embodiments, the voltage sensing function of the downstream power equipment 40 can also be used directly to acquire the output voltage of each output line.

[0083] The short-circuit detection unit 52 can be, for example, an MCU with short-circuit protection function, used to perform real-time calculation and short-circuit detection based on the output voltage, and to issue a protection signal PS (Protection Signal) when a short circuit is detected. For example, the short-circuit detection unit 52 can calculate the line voltage between any two output lines from a plurality of output lines based on the output voltage to obtain a plurality of line voltages; and when the change of the plurality of line voltages conforms to a short-circuit change characteristic, it is determined to be a short circuit. In this invention, when a short circuit occurs in the line, the characteristics of the line voltage between the three output lines will change, the MCU will detect this change and start short-circuit detection. If the change of line voltage conforms to a short-circuit change characteristic, it is determined to be a short circuit, and the MCU will immediately issue a protection signal PS to the controllable circuit breaker 53 for short-circuit protection. At the same time, the short-circuit detection unit 52 can also be connected to the upper-level system 40 and report a short-circuit fault FR (Fault Report) to the upper-level system 40 when a short circuit is detected.

[0084] The controllable circuit breaker 53 is installed on the high-voltage side of the transformer 30 and connected to the short-circuit judgment unit 52. It is used to disconnect the transformer 30 from the power grid 10 upon receiving the protection signal PS, thereby providing short-circuit protection to downstream components. In other embodiments, the controllable circuit breaker 53 may also be other protective shut-off mechanisms.

[0085] In some embodiments, the controllable circuit breaker can also be installed in each secondary winding, and its connection relationship and operating principle are similar to those of the controllable circuit breaker 53, which will not be described again here. In other embodiments, controllable circuit breakers can be installed in both the primary and secondary windings to increase reliability.

[0086] Figures 5A-5H The connection structures of various types of windings in the three-phase transformer of the present invention are shown respectively. Among them, Figure 5A , Figure 5B The connection structure of the Y-type winding is shown, and Figure 5B for Figure 5A The deformation. Figure 5C , Figure 5D The connection structure of the delta winding is shown, which consists of three windings connected in a triangle. Figure 5D for Figure 5C The deformation. Figure 5E , Figure 5F The diagram illustrates a negative connection structure for an extended delta winding, which consists of three delta windings and three extended windings, wherein the three delta windings are connected in a triangle, and... Figure 5F for Figure 5E The deformation. Figure 5G , Figure 5H The diagram illustrates a connection structure for a positive connection of extended delta windings, which consists of three delta windings and three extended windings, wherein the three delta windings are connected to form a triangle, and Figure 5H for Figure 5G The deformation.

[0087] Figures 6A to 6D The following examples illustrate different scenarios of line-to-line short circuits occurring in different types of windings of the three-phase transformer of the present invention. Figure 6A This illustrates a scenario where a line-to-line short circuit occurs in a Y-type winding, for example, the RS line in the figure is short-circuited. Figure 6B This illustrates a scenario where a line-to-line short circuit occurs in a delta winding, such as the RS line in the figure being short-circuited. Figure 6C This illustrates a scenario where a line-to-line short circuit occurs in the negative connection of the extended delta winding, for example, the RS line in the figure is short-circuited. Figure 6D This illustrates a scenario where a line-to-line short circuit occurs in the positive connection of the extended delta winding, for example, the RS line in the figure is short-circuited.

[0088] Figures 7A to 7G The following examples illustrate different scenarios where phase short circuits or partial winding short circuits occur in different types of windings of the three-phase transformer of the present invention. Figure 7A The diagram illustrates a scenario where a phase short circuit occurs in a Y-type winding, such as the R phase in the figure. Figure 7B This illustrates a short circuit in the negative connection portion of an extended delta winding, such as the short circuit in the extended winding portion of phase R in the figure. Figure 7C This illustrates a short circuit in the negative connection portion of the extended delta winding, for example, a short circuit occurs in the delta winding portion of phase R in the figure. Figure 7D This illustrates a scenario where a phase short circuit occurs in the negative connection of the extended delta winding, for example, phase R in the figure is short-circuited. Figure 7E This illustrates a situation where a short circuit occurs in the extended delta winding, specifically in the positive connection portion of the winding, such as the extended winding portion of phase R in the figure. Figure 7F This illustrates a situation where a short circuit occurs in the positive connection portion of the extended delta winding, for example, the delta winding portion of phase R in the figure experiences a short circuit. Figure 7G This illustrates a scenario where a phase short circuit occurs in the positive connection of the extended delta winding, for example, phase R in the figure is short-circuited.

[0089] Example 1: Detection and protection of line-to-line short circuits in Y-type windings

[0090] The following will be based on... Figure 6A Taking the case of a short circuit in the RS line of the Y-type winding shown as an example, the short circuit protection method of a preferred embodiment of the present invention will be described in detail.

[0091] like Figure 6A As shown, when a short circuit occurs on the RS line of the Y-type winding, the voltage change waveforms between the three output lines before and after the short circuit are as follows: Figure 8A As shown, for example, after the RS short circuit moment, the line voltage Vrs between RS becomes 0, and the line voltages Vst, Vtr and phase between other output lines will all change.

[0092] The phasor diagrams corresponding to the period before and after the short circuit are as follows: Figure 8B , Figure 8C As shown.

[0093] Furthermore, the line voltage change characteristics before and after the short circuit conform to Table 1:

[0094]

[0095] Table 1

[0096] As shown in Table 1 above, the following characteristics are observed when there is a line-to-line short circuit:

[0097] (1) When one line voltage (Vrs) becomes 0, the amplitudes of the other two line voltages (Vst, Vtr) decrease to 0. (87%)

[0098] (2) The phase difference between the two line voltages (Vst, Vtr) becomes 180°;

[0099] (3) The sum of the phasors of the three line voltages always satisfies 0.

[0100] In this invention, for the case of a short circuit between lines of Y-type windings and other types of windings, the amplitude and phase change characteristics of the three line voltages of the short-circuited winding all conform to the above-mentioned change characteristics. It can be summarized that all line-to-line short circuits will exhibit the change characteristic of "the line voltage of the short circuit becomes 0, while the other line voltages still have a certain value". Therefore, the basic short-circuit protection logic can be set according to this change characteristic.

[0101] The method for detecting and judging the line voltage of the short-circuit winding is as follows: Figure 8D As shown:

[0102] (a) When the amplitude of a line voltage is detected to be less than or equal to the first characteristic value VL, an abnormality is considered to be present, and short-circuit judgment is initiated.

[0103] (b) Continue to determine whether the amplitude of one of the other two line voltages is greater than or equal to the second characteristic value VH;

[0104] (c) If both conditions (a) and (b) are always met within the detection window T for short circuit judgment, then it is judged as a short circuit fault.

[0105] Through the above judgment process, short-circuit protection can be achieved by detecting line voltage.

[0106] In this embodiment, the method for setting the first feature value VL and the second feature value VH is as follows:

[0107]

[0108] Table 2

[0109] Based on the line voltage variation characteristics summarized above, the line voltage amplitude characteristics in Example 1 are shown in Table 2, where the rated voltage amplitude is Uo. The first characteristic value VL is the minimum line voltage after a short circuit (0), and the second characteristic value VH is the maximum line voltage after a short circuit (0.87 * Uo). In other embodiments, to ensure a certain margin, the first characteristic value VL can be set to Uo * 5%, and the second characteristic value VH can be set to 0.87 * Uo * 80%.

[0110] To ensure the reliability of short-circuit detection, the detection window time T must not be less than one cycle of the transformer's AC power (i.e., greater than or equal to one cycle of the transformer's AC power). If a faster protection speed is not required, the detection window time T can be appropriately increased to improve the accuracy of short-circuit detection.

[0111] The short-circuit protection method in Example 1 can reliably detect short-circuit faults. However, in certain special cases, non-short-circuit conditions may be detected as short-circuit conditions, such as... Figure 9 As shown, when one of the secondary windings on the low-voltage side of a three-phase transformer is de-energized, the three line voltages drop asynchronously (due to the influence of the Y capacitor in the downstream equipment). At this time, the voltage amplitude change characteristics are similar to short-circuit characteristics, which may trigger short-circuit protection. In this situation, the system should continue to operate and short-circuit protection measures should not be taken. Therefore, to improve the accuracy of short-circuit detection, a phase detection mechanism for the line voltage will be added in the following embodiments to reliably achieve short-circuit protection while preventing false protection.

[0112] Example 2: Detection and protection of partial short circuits in extended delta windings.

[0113] The following will be based on... Figure 7B Taking the example of a short circuit occurring in the extended winding portion of the R phase of the extended delta winding shown, the short circuit protection method of another preferred embodiment of the present invention will be described in detail.

[0114] like Figure 7B As shown, the extended delta phase-shifting winding shown is actually composed of three delta windings and three extension windings, and the turns ratio of the delta windings and extension windings determines the phase shift angle α. If a short circuit occurs in the extension winding section on phase R, the changes in the three line voltages before and after the short circuit are as follows: Figure 10A As shown, the phase shift angle α = -7.5°.

[0115] The phasor diagrams corresponding to the period before and after the short circuit are as follows: Figure 10B , 10C As shown.

[0116] Furthermore, the changes in line voltage before and after the short circuit conform to Table 3:

[0117]

[0118] Table 3

[0119] As shown in Table 3 above, some of the windings exhibit the following changes when short-circuited:

[0120] (1) The magnitude changes of the three line voltages are related to the phase shift angle α, as shown in Table 3;

[0121] (2) There are two line voltages with a phase difference of approximately 180°.

[0122] (3) The sum of the phasors of the three line voltages always satisfies 0.

[0123] In this invention, for a partial short circuit in the extended delta phase-shifting winding, the change in line voltage conforms to the aforementioned characteristics. Furthermore, for the delta winding and the Y-shaped winding, which can be considered extended delta windings with phase shift angles α of 30° and 0° respectively, the line voltage change during a phase short circuit can also be summarized by the aforementioned characteristics: namely, satisfying the amplitude characteristic of (1) and the phase characteristic of (2). Therefore, based on these two factors, a highly accurate short-circuit protection logic setting can be implemented.

[0124] The logical determination method for amplitude characteristics is the same as in Example 1.

[0125] The logical determination of phase characteristics needs to be completed within the detection window time T, so it can be achieved through zero-crossing detection (ZCD).

[0126] The method for detecting and judging the line voltage of the short-circuit winding is as follows: Figure 10D As shown:

[0127] (a) When a line voltage amplitude is detected to be less than the first characteristic value VL, it is considered to be abnormal and short circuit judgment is initiated;

[0128] (b) Continue to determine whether the amplitude of one of the other two line voltages is greater than the second characteristic value VH;

[0129] (c) By detecting the zero-crossing points, the time difference Δt between the two line voltages at their simultaneous zero-crossing points ZCD0 and ZCD1 can be obtained, and the phase difference can be calculated. Determine whether the phase difference satisfies a phase characteristic value.

[0130] (d) If both (a) and (b) are always satisfied within the detection window time T for short circuit judgment, and (c) is satisfied once, then it is judged as a short circuit fault.

[0131] The above judgment process can accurately determine short-circuit faults and prevent false protection situations that may occur with a single voltage amplitude detection method.

[0132] In this embodiment, the method for setting the feature value is as follows:

[0133]

[0134] Table 4

[0135] According to the line voltage variation characteristics summarized above, the amplitude characteristics and phase characteristics of the line voltage in Embodiment 2 are shown in Table 4. The rated voltage amplitude is Uo. The first eigenvalue VL is the minimum value of the line voltage after short circuit, which is 0.38*Uo, and the second eigenvalue VH is the maximum value of the line voltage after short circuit, which is 0.99*Uo. To ensure a certain margin, the first eigenvalue VL can be set to 0.38*Uo*120%, and the second eigenvalue VH can be set to 0.99*Uo*80%. Preferably, the phase eigenvalue is set to an angle range, and the angles within this range are considered to be ≈180°, such as 160° - 180°. Among them, Δt is obtained through zero-crossing detection and converted to If it can be considered that the phase eigenvalue is satisfied.

[0136] To ensure the reliability of short-circuit judgment, the setting of the detection window time T shall not be less than one cycle time of the alternating current of the transformer (i.e., greater than or equal to one cycle time of the alternating current of the transformer). If a fast protection speed is not required, the duration of the detection window time T can be appropriately increased to improve the accuracy of short-circuit judgment.

[0137] In the present invention, different short-circuit types correspond to different eigenvalues. If multiple short-circuit types need to be protected simultaneously, the union of different eigenvalues can be taken.

[0138] As Figure 11 shown, it shows the signal timing of this embodiment:

[0139] (1) Before time t0, the system is normal and the three line voltages are rated values;

[0140] (2) A short circuit occurs at time t0, the amplitude of one line voltage decreases to VL, the short-circuit protection program detects an abnormality, and starts the short-circuit judgment program. The detection window time is T;

[0141] (3) At time t1, a positive zero-crossing of one line voltage is detected and denoted as ZCD0;

[0142] (4) At time t2, a positive zero-crossing of another line voltage is detected and denoted as ZCD1;

[0143] (5) Before time t3, there is always one line voltage amplitude < VL and another line voltage amplitude > VH

[0144] (6) At time t3, the detection window time T arrives, that is, t3 - t0 = T, and the program stops detecting;

[0145] (7) At time t3, the judgment program believes that the amplitude characteristics are satisfied, and If the phase characteristics are met, a short circuit is detected, and a short circuit protection signal PS is issued. The short circuit protection signal PS flips to a high level, and at the same time, the short circuit protection signal PS triggers hardware protection, disconnecting the input line. The three line voltages drop to 0 after t3.

[0146] This invention provides a short-circuit protection method and apparatus based on voltage characteristics. It realizes short-circuit protection of three-phase transformers through line voltage detection, which greatly reduces protection costs and improves protection reliability.

[0147] Compared to existing current detection methods, the advantages of this invention are:

[0148] (1) If only used for short-circuit protection, voltage sensors (voltage divider type) are less expensive than current sensors. This can save significant costs for multi-winding output applications. If the downstream equipment (e.g., charger) has a real-time mains voltage sampling function, this function can be used directly, thus eliminating the need for a voltage sensor. For example, the mains voltage sampling data of the charger can be used directly for short-circuit protection.

[0149] (2) Where existing current detection methods cannot provide protection, the voltage detection method of the present invention can provide protection, regardless of the location of the short circuit.

[0150] (3) The overcurrent tripping speed of a circuit breaker is related to the magnitude of the current. In some cases, the tripping speed may be slow when the protection threshold is approached, increasing the system risk. The voltage detection method of this invention is determined by software, and the protection time and sensitivity are controllable and stable.

[0151] (4) It has a wide range of applications. Through parameter setting, various short circuit types can be protected and located. The software protection method has strong scalability. It can use the sampled line voltage data to expand other detection and abnormal protection, such as PLL, OVP, and UVP.

[0152] Exemplary embodiments of the present invention have been specifically illustrated and described above. It should be understood that the present invention is not limited to the disclosed embodiments; rather, the present invention is intended to cover various modifications and equivalent arrangements contained within the spirit and scope of the appended claims.

Claims

1. A short-circuit protection method for a transformer, characterized in that, The method comprises: obtaining output voltages of a plurality of output lines of at least one winding of a low-voltage side of a transformer; calculating line voltages between any two of the plurality of output lines according to the output voltages to obtain a plurality of line voltages, and determining a short circuit when a change of the plurality of line voltages meets a short circuit change characteristic, and sending a protection signal when the short circuit is determined, the short circuit change characteristic comprises an amplitude characteristic, wherein an amplitude of one of the plurality of line voltages becomes less than or equal to a first characteristic value, and amplitudes of the other of the plurality of line voltages become greater than or equal to a second characteristic value, and the first characteristic value is less than the second characteristic value, the short circuit change characteristic further comprises a phase characteristic, wherein a phase difference between the two of the plurality of line voltages meets a phase characteristic value, and the method for determining a short circuit comprises: (a) when it is detected that the amplitude of one of the plurality of line voltages is less than or equal to the first characteristic value, considering that there is an abnormality, and starting to determine a short circuit; (b) continuing to determine whether the amplitude of one of the other of the plurality of line voltages is greater than or equal to the second characteristic value; (c) obtaining a time difference of same-direction zero-crossing time points of the other of the plurality of line voltages by zero-crossing point detection, and calculating a phase difference according to the time difference, and determining whether the phase difference meets the phase characteristic value; (d) if the above (a) and (b) are always met within a detection window time, and (c) is met once, then a short circuit fault is determined, wherein the detection window time is greater than or equal to one cycle of alternating current of the transformer.

2. The short-circuit protection method according to claim 1, characterized in that The phase characteristic value is set as an angle range, and the angle range is 160°-180°.

3. The short-circuit protection method according to claim 1, characterized in that The winding is a Y-type winding or a delta winding or an extended delta winding.

4. The short-circuit protection method according to claim 3, characterized in that The winding is a Y-type winding, and the first characteristic value is 0, and the second characteristic value is 0.87*Uo, wherein Uo is a rated voltage amplitude; or the first characteristic value is Uo*5%, and the second characteristic value is 0.87*Uo*80%, wherein Uo is a rated voltage amplitude.

5. The short-circuit protection method according to claim 3, characterized in that, The winding is an extended delta winding, and the first characteristic value is 0.38*Uo, and the second characteristic value is 0.99*Uo, wherein Uo is a rated voltage amplitude; or the first characteristic value is 0.38*Uo*120%, and the second characteristic value is 0.99*Uo*80%, wherein Uo is a rated voltage amplitude.

6. The short-circuit protection method according to claim 1, characterized in that The protection signal is sent to a controllable circuit breaker located at a high-voltage side of the transformer, and the controllable circuit breaker cuts off a connection between the transformer and a power grid after receiving the protection signal.

7. The short-circuit protection method according to claim 1, characterized in that, The output voltages are sampled by a voltage sensor.

8. The short-circuit protection method according to claim 1, characterized in that The short circuit change characteristic further comprises a phasor of the plurality of line voltages and always being 0.

9. A short-circuit protection device for a transformer, characterized in that The method comprises: a voltage obtaining unit configured to obtain output voltages of a plurality of output lines of at least one winding of a low-voltage side of a transformer; a short circuit determining unit configured to calculate line voltages between any two of the plurality of output lines according to the output voltages to obtain a plurality of line voltages, and determine a short circuit when a change of the plurality of line voltages meets a short circuit change characteristic, and send a protection signal when the short circuit is determined. A controllable breaker is arranged at a high voltage side of the transformer and connected with the short circuit judging unit, for cutting off the connection between the transformer and the power grid after receiving the protection signal, The short circuit change characteristic comprises an amplitude characteristic, wherein the amplitude of one of the plurality of line voltages becomes less than or equal to a first characteristic value, the amplitudes of the other of the plurality of line voltages become greater than or equal to a second characteristic value, and the first characteristic value is less than the second characteristic value, The short circuit change characteristic further comprises a phase characteristic, wherein the phase difference of the two of the plurality of line voltages satisfies a phase characteristic value, and the method for short circuit judgment of the short circuit judging unit comprises: (a) when the amplitude of one of the plurality of line voltages is detected to be less than or equal to the first characteristic value, considering that there is an abnormality and starting short circuit judgment; (b) continuing to judge whether the amplitude of one of the other of the plurality of line voltages is greater than or equal to the second characteristic value; (c) obtaining the time difference of the same direction zero-crossing time of the other of the plurality of line voltages through zero-crossing point detection, and calculating the phase difference according to the time difference, and judging whether the phase difference satisfies the phase characteristic value; (d) if the above (a) and (b) are always satisfied within a detection window time, and (c) is satisfied once, the short circuit fault is determined, wherein the detection window time is greater than or equal to one cycle of the alternating current of the transformer.

10. The short circuit protection device of claim 9, wherein, The voltage acquisition unit is a voltage sensor.

11. The short circuit protection device of claim 9, wherein The short circuit judging unit is further connected with a superior system, and reports the short circuit fault to the superior system when the short circuit is judged.

12. The short circuit protection device of claim 9, wherein, The phase characteristic value is set as an angle range, and the angle range is 160°-180°.

13. The short circuit protection device of claim 9, wherein The winding is a Y-type winding or a delta winding or a extended delta winding.

14. The short circuit protection device of claim 13, wherein, The winding is a Y-type winding, and the first characteristic value is 0, and the second characteristic value is 0.87*Uo, wherein Uo is the rated voltage amplitude; or the first characteristic value is Uo*5%, and the second characteristic value is 0.87*Uo*80%, wherein Uo is the rated voltage amplitude.

15. The short circuit protection device of claim 13, wherein, The winding is an extended delta winding, and the first characteristic value is 0.38*Uo, and the second characteristic value is 0.99*Uo, wherein Uo is the rated voltage amplitude; or the first characteristic value is 0.38*Uo*120%, and the second characteristic value is 0.99*Uo*80%, wherein Uo is the rated voltage amplitude.

16. The short circuit protection device of claim 9, wherein, The short circuit change characteristic further comprises that the phasor of the plurality of line voltages is always 0.

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