Hybrid distribution transformer low-voltage turn-on over-current protection device and protection strategy combined with current limiter

Abstract

The invention provides a hybrid distribution transformer low-voltage turn-on overcurrent protection device and protection strategy combined with a current limiter, and the device comprises a hybrid distribution transformer which is composed of a power frequency electromagnetic part and a high-frequency power electronic part, and the power frequency electromagnetic part comprises a power transformer and a series transformer. The high-frequency power electronic part comprises converters which are connected back to back; the saturated iron core fault current limiter is integrated in a main loop of the hybrid distribution transformer, presents low impedance during normal operation, and is switched to a high impedance state through iron core desaturation to limit fault current during fault; the bypass switch module is connected to the input side of the high-frequency power electronic part, and when the fault current exceeds a set threshold value, the bypass switch module is conducted to bypass and isolate the high-frequency power electronic part within less than or equal to 1ms, and the continuous power supply of the power-frequency electromagnetic part is maintained; and the fault isolation device is configured at the two ends of the hybrid distribution transformer, and performs breaking operation in response to the current setting value and the low voltage setting value after current limiting so as to realize fault area isolation.

Description

Technical Field

[0001] The invention belongs to the technical field of electrical equipment, and particularly relates to a low-voltage starting over-current protection device and protection strategy for a hybrid distribution transformer combined with a current limiter. Background Art

[0002] With the wide application of new energy (such as solar energy, wind energy, etc.), the operation mode of the distribution network has changed significantly. The access of new energy not only brings the advantages of power diversity and distributed generation, but also puts forward higher requirements for the voltage, current and power quality control of the power grid. In order to meet this demand, the hybrid distribution transformer has emerged. The hybrid distribution transformer combines the advantages of traditional distribution transformers and power electronic converters, and has multiple functions such as power transmission, power quality governance and reactive power compensation. It can not only support the transmission of traditional electric energy, but also regulate voltage and current, filter out harmonics, and compensate reactive power, significantly improving the power quality of the power grid and maintaining the stability of the power grid.

[0003] However, with the large-scale access of new energy, sudden short-circuit faults may occur in the power grid, resulting in a rapid increase in fault current. Since the hybrid distribution transformer involves power electronic converters and has strong power processing capabilities, the current may increase rapidly in the event of a fault, making the breaking capacity of traditional circuit breakers unable to effectively respond. On the one hand, when the current is too large, the breaking speed of the circuit breaker cannot catch up with the growth speed of the current. Especially in extreme situations such as short circuits, the current quickly reaches the rated value of the circuit breaker, resulting in protection delay or failure. On the other hand, the electromagnetic part usually has a large current-carrying capacity, while the power electronic part has low tolerance to overcurrent. Once an overcurrent occurs, the power electronic devices may be quickly damaged, leading to the failure of the hybrid distribution transformer.

[0004] Therefore, for hybrid distribution transformers, there is an urgent need for a new protection device and protection strategy that can quickly limit the current when the current is too large, ensure that the circuit breaker can operate in the separable range in time, thereby preventing the hybrid distribution transformer from being damaged and improving the reliability of the distribution system. Summary of the Invention

[0005] Aiming at the defects and deficiencies existing in the prior art (such as the insufficient response speed of traditional circuit breakers, the vulnerability of power electronic components, and the low protection sensitivity in the scenario of new energy grid connection, etc.), the invention provides a low-voltage starting over-current protection device and protection strategy for a hybrid distribution transformer combined with a current limiter. To achieve that when a fault occurs, the amplitude of the current can be quickly limited, so that the circuit breaker can disconnect the circuit in the safe current range in time, and the power electronic part of the hybrid distribution transformer system can be quickly bypassed when a fault occurs, thereby ensuring the safe operation of the hybrid distribution transformer and the entire power system.

[0006] The protection device consists of two parts: the hybrid distribution transformer body and the current limiter. A bypass switch is added to the traditional hybrid distribution transformer topology. When a short-circuit fault occurs and the current exceeds the threshold of the thyristor, the thyristor will conduct, isolating the high-frequency power electronics part, and the power frequency electromagnetic part will continue to supply power. The current limiter selects a saturated-core fault current limiter with advantages such as self-response to faults, fast response speed, repeatable switching, and high reliability.

[0007] By combining the hybrid distribution transformer with the current limiter, it can effectively solve the problems of excessive fault current that may occur during the grid connection of new energy and the operation of the hybrid distribution transformer, the inability of traditional circuit breakers to cut off the current in a timely manner, and the coordinated protection of the power electronics part and the electromagnetic part of the hybrid distribution transformer. By introducing the current limiter, when a fault occurs, it can quickly limit the amplitude of the current, enabling the circuit breaker to disconnect the circuit in a timely manner within the safe current range, and the power electronics part of the hybrid distribution transformer system can be quickly bypassed during a fault, thus ensuring the safe operation of the hybrid distribution transformer and the entire power system.

[0008] This solution achieves efficient coordinated protection through the following innovative designs: Integrated topology design: Embed the saturated-core fault current limiter (SCFCL) into the main circuit of the hybrid distribution transformer. The power frequency electromagnetic part and the high-frequency power electronics part operate in coordination. The SCFCL quickly switches to a high-impedance state through a double-core de-saturation mechanism (reverse series AC windings and DC excitation), limiting the peak value of the 10kV system fault current from 28kA to 8.2kA, and the current limiting coefficient reaches 70%. Fast bypass and breaking mechanism: Add a thyristor bypass module. When the fault current exceeds the threshold, it conducts and isolates the high-frequency power electronics part within ≤1ms, maintaining the continuous power supply of the power frequency electromagnetic part to avoid damage to devices such as IGBTs. At the same time, in combination with the circuit breaker, it completes the three-stage coordinated protection of "current limiting → bypass → breaking" within ≤17ms. Low-voltage start overcurrent protection: Through the dual-trigger logic of the low-voltage setting value (70% of the rated voltage) and the overcurrent setting value, it accurately identifies high-impedance short-circuit faults in the new energy grid connection scenario, significantly improving the protection sensitivity and reliability.

[0009] This solution solves the problems of response delay of traditional protection devices, vulnerability of power electronics components, and load power supply interruption, and is applicable to the complex grid environment with distributed energy access, providing full-process fast protection for hybrid distribution transformers and ensuring the safe and stable operation of the system.

[0010] The technical solution specifically adopted by the present invention to solve its technical problems is: A low-voltage start overcurrent protection device for a hybrid distribution transformer combined with a current limiter, comprising: Hybrid distribution transformer: It is composed of a power frequency electromagnetic part and a high-frequency power electronics part. The power frequency electromagnetic part includes a power transformer and a series transformer, and the high-frequency power electronics part includes converters connected back-to-back; Saturated core fault current limiter: Integrated in the main circuit of the hybrid distribution transformer, it presents a low impedance during normal operation and switches to a high impedance state through core de-saturation to limit the fault current during a fault; Bypass switch module: Connected to the input side of the high-frequency power electronics part, when the fault current exceeds the set threshold, it conducts within ≤1 ms to bypass and isolate the high-frequency power electronics part, maintaining the continuous power supply of the power frequency electromagnetic part; Fault isolation device: Configured at both ends of the hybrid distribution transformer, it performs a breaking operation in response to the current setting value and low voltage setting value after current limiting to achieve fault area isolation.

[0011] Furthermore, its protection strategy includes the following coordinated control stages: Current limiting stage: After detecting a fault, the saturated core fault current limiter switches to a high impedance state within ≤1 ms, limiting the fault current to the safe breaking range of the fault isolation device; Bypass stage: Trigger the bypass switch to conduct within ≤5 ms after current limiting is completed, cut off the high-frequency power electronics part, and maintain the power supply of the power frequency electromagnetic part; Breaking stage: Judge whether the current after current limiting reaches the setting value. If it is satisfied, trip the circuit breaker within ≤10 ms.

[0012] Furthermore, the fault isolation device is a circuit breaker.

[0013] Furthermore, the fast response bypass module is composed of thyristors.

[0014] Furthermore, the current setting value is calculated based on the rated current of the transformer, reliability coefficient, and return coefficient.

[0015] Furthermore, the low voltage setting value is 70% of the rated voltage.

[0016] Furthermore, the reliability coefficient of the current setting value is 1.2 - 1.3, and the return coefficient is 0.85 - 0.95.

[0017] Furthermore, the saturated core fault current limiter is of a double-core structure, including an AC winding and a DC excitation circuit connected in reverse series.

[0018] Furthermore, the saturated core fault current limiter limits the amplitude of the first peak wave of the fault current to the safe breaking range of the circuit breaker through a double-core de-saturation mechanism.

[0019] Compared with the prior art, the present invention and its preferred solutions at least include the following beneficial effects: Full-process rapid collaborative protection: Through the three-stage collaborative strategy of "limiter action - power electronic bypass - circuit breaker tripping", the fault response time is significantly shortened. Before the circuit breaker trips, the fault current is quickly suppressed and sensitive components are isolated, avoiding equipment damage or system instability caused by the delay of a single action in traditional protection; Balancing the protection of power electronic components and power supply continuity: Adopting a fast bypass mechanism to isolate the high-frequency power electronic part, ensuring continuous power supply for the power frequency electromagnetic part. This not only prevents power electronic devices from being damaged due to overcurrent but also maintains uninterrupted operation of the load, solving the contradiction between protection action and power supply reliability in traditional solutions; Improving adaptability to complex scenarios: Through the collaborative triggering logic of low-voltage startup and overcurrent setting, the recognition sensitivity to complex working conditions such as high-impedance faults and voltage fluctuations in new energy grid-connected scenarios is enhanced, reducing the risk of misjudgment; Optimizing current limiting ability and system compatibility: Based on the dynamic impedance switching characteristics of the saturated core limiter, the current peak value is quickly limited during a fault, while avoiding additional losses to the normal operation of the system, ensuring the long-term stable operation of the device.

[0020] With the large-scale access of new energy, when a short-circuit fault occurs, the current is large and the growth rate is too fast. Traditional circuit breakers cannot cut off the current in time, which will cause the failure of power electronic devices and even hybrid transformer devices. The protection device and protection strategy provided by the present invention can quickly limit the amplitude of the current when a fault occurs by introducing a limiter, enabling the circuit breaker to disconnect the circuit in a safe current range in time, and the power electronic part of the hybrid distribution transformer system can be quickly bypassed when a fault occurs, thus ensuring the safe operation of the hybrid distribution transformer and the entire power system. Description of the Drawings

[0021] The present invention will be further described in detail below in conjunction with the drawings and specific embodiments: Figure 1 It is the topological structure diagram of the traditional hybrid distribution transformer device; Figure 2 It is the topological structure diagram of the hybrid distribution transformer after adopting the protection device in the embodiment of the present invention; Figure 3 It is the basic working characteristic diagram of the fault limiter in the embodiment of the present invention; Figure 4 It is the schematic diagram of the two-core saturated core fault limiter in the embodiment of the present invention; Figure 5 It is the current waveform diagram of the SCFCL 10kV simulation current limiting effect in the embodiment of the present invention; Figure 6 It is the schematic diagram of the low-voltage startup overcurrent protection of the hybrid distribution transformer in the embodiment of the present invention; Figure 7Flow chart of the collaborative protection strategy of the protection device in the embodiment of the present invention; Figure 8 Timing diagram of the collaborative protection strategy in the embodiment of the present invention; Figure 9 Circuit diagram of the fault occurrence in the embodiment of the present invention; Figure 10 Structural diagram of the 10kV hybrid distribution transformer in the embodiment of the present invention. Detailed implementation manners

[0022] To make the features and advantages of the present invention more obvious and understandable, specific embodiments are hereinafter given and described in detail as follows: It should be noted that the following detailed description is exemplary and is intended to provide further explanation of the present application. Unless otherwise specified, all technical and scientific terms used in this specification have the same meaning as commonly understood by those of ordinary skill in the technical field to which this application belongs.

[0023] It should be noted that the terms used herein are only for describing specific embodiments and are not intended to limit the exemplary embodiments according to the present application. As used herein, unless the context clearly indicates otherwise, the singular form is also intended to include the plural form. In addition, it should be understood that when the terms "comprising" and / or "including" are used in this specification, they indicate the presence of features, steps, operations, devices, components, and / or combinations thereof.

[0024] The purpose of the embodiment of the present invention is to provide a hybrid distribution transformer overcurrent protection device and protection strategy combined with a current limiter to solve the problems of excessive fault current that may occur during the grid connection of new energy and the operation of the hybrid distribution transformer, the inability of traditional circuit breakers to cut off the current in time, and the coordination of protection between the power electronic part and the electromagnetic part of the hybrid distribution transformer. By introducing a current limiter, when a fault occurs, the amplitude of the current can be quickly limited, so that the circuit breaker can disconnect the circuit in time within a safe current range, and the power electronic part of the hybrid distribution transformer system can be quickly bypassed when a fault occurs, ensuring the safe operation of the hybrid distribution transformer and the entire power system.

[0025] The hybrid distribution transformer protection device combined with a current limiter is mainly divided into two parts: a hybrid distribution transformer and a current limiter.

[0026] As a preferred solution of this embodiment, the typical topology of the hybrid distribution transformer includes a power frequency electromagnetic part and a high-frequency power electronic part. The power frequency electromagnetic part includes a power transformer and a series transformer, which convert the AC voltage and current of one system into the AC voltage and current of another system through electromagnetic induction at power frequency. The high-frequency power electronic part includes two back-to-back connected power electronic converters, one of which is connected in series with the power frequency part and the other is connected in parallel with the power frequency part to achieve series voltage control and parallel current control. A bypass switch is added to the traditional topology of this device. When a short-circuit fault occurs and the current exceeds the threshold of the thyristor, the thyristor will conduct, isolating the high-frequency power electronic part, and the electromagnetic part will continue to supply power.

[0027] As a preferred solution of this embodiment, a saturated core fault current limiter (SCFCL) with advantages such as fault self-response, fast response speed, repeatable switching, and high reliability is selected as the current limiter. During normal operation of the system, the impedance value presented by the current limiter in the system is very small and will not affect the normal operation of the system. When a fault occurs, the current limiter can act quickly to achieve a sudden increase in impedance and effectively limit the fault current. When the fault is cleared, the current limiter can quickly return to a small impedance again.

[0028] As a preferred solution of this embodiment, for this device, since it needs to effectively handle complex scenarios such as distributed energy access, high-impedance faults, and load fluctuations, a low-voltage startup overcurrent protection combined with a saturated core current limiter is adopted. This protection has higher reliability and sensitivity compared to traditional overcurrent protection and can handle the situation where the fault current is too large and the traditional circuit breaker cannot cut off the current in time.

[0029] As a preferred solution of this embodiment, the cooperative protection strategy adopted by this device is divided into three stages: rapid action of the current limiter, power electronic bypass, and circuit breaker tripping. The cooperative protection strategy can avoid damage to power electronic devices due to overcurrent or voltage mutation while ensuring uninterrupted operation of the load. It can also isolate the entire device from the power grid when the current exceeds the set value to ensure the safety of the device and the system.

[0030] Figure 1 Shown is the typical topology of the hybrid distribution transformer, which includes a power frequency electromagnetic part and a high-frequency power electronic part. The power frequency electromagnetic part includes a power transformer and a series transformer, which convert the AC voltage and current of one system into the AC voltage and current of another system through electromagnetic induction at power frequency. The high-frequency power electronic part includes two back-to-back connected power electronic converters, one of which is connected in series with the power frequency part and the other is connected in parallel with the power frequency part to achieve series voltage control and parallel current control.

[0031] Since the power frequency electromagnetic part usually has a large current-carrying capacity, while the high-frequency power electronics part itself has a low tolerance to overcurrent. Once an overcurrent occurs, the power electronic devices may be quickly damaged. Therefore, the solution provided in this embodiment adds a bypass switch to the traditional topology, such as Figure 2 shown. When a short-circuit fault occurs and the current exceeds the thyristor threshold, the thyristor will conduct, isolating the high-frequency power electronics part, and the electromagnetic part continues to supply power. The inverter is sensitive to overcurrent, and rapid action can avoid damage to devices such as IGBTs. Therefore, the conduction time of the bypass switch thyristor should be controlled within 1 ms to limit the rise of the short-circuit current within an acceptable range. Since the closing time of a traditional mechanical circuit breaker to establish a reliable current path exceeds 10 ms, far exceeding the formation time of the first peak of the short-circuit current, it is a more reasonable solution to add a bypass switch to carry the current during the fault.

[0032] For the current limiter part, a saturated core fault current limiter (SCFCL) with advantages such as self-response to faults, fast response speed, repeatable switching, and high reliability is selected. The basic operating characteristics of the fault current limiter are as Figure 3 shown. During normal system operation, the impedance value presented by the current limiter in the system is very small and will not affect the normal operation of the system; when a fault occurs, the current limiter can act quickly, realizing a sudden increase in impedance and playing an effective role in limiting the fault current; when the fault is cleared, the current limiter can quickly return to a small impedance again.

[0033] Figure 4 shows the typical structure of a two-core saturated core type fault current limiter. The AC windings 1 and 2 are connected in series to the network, and the polarities of the two AC windings are opposite. 3 and 4 are double cores. The DC excitation circuit consists of a DC excitation power supply and a DC winding 5. The DC excitation current generates a downward DC excitation magnetomotive force in the iron core, making the two iron cores in a deep saturation state during normal system operation. The equivalent inductance of the AC winding presents a very small inductance to the outside. After a fault occurs, the system current Iac on the AC winding rises, generating an AC magnetomotive force in the left and right iron cores sufficient to cancel the DC excitation magnetomotive force. Therefore, within one cycle of the fault current, the two iron cores alternately desaturate. The magnetic permeability of the desaturated iron core is very large, and the equivalent inductance of the AC winding on it also increases to limit the short-circuit current.

[0034] Figure 5 compares the fault current waveforms of the system before and after installing the SCFCL. It can be seen from the figure that the amplitude of the first peak of the system fault current is reduced from 28 kA to 8.2 kA by the SCFCL, and the current limiting coefficient reaches 70%, proving that the SCFCL can be applied to high voltage level occasions and has a good current limiting effect.

[0035] Figure 6 It shows the principle of overcurrent protection with low-voltage startup for a hybrid distribution transformer. Among them, QF1 and QF2 are the circuit breakers at both ends of the hybrid distribution transformer, and SCFCL is a saturated-core fault current limiter. When the fault current exceeds the current setting value and the voltage value is less than the voltage setting value, then QFI and QF2 are tripped to isolate the entire hybrid distribution transformer from the line.

[0036] Setting of its operating current: After using a low-voltage relay, the setting value of the current relay no longer needs to consider the maximum load that may occur when parallel-operated transformers are removed or motors self-start, but is set to be greater than the rated current of the transformer. Setting of the operating voltage: (1) The current setting value is set to reliably avoid the maximum load; (2) The low-voltage value is set to avoid the voltage during motor self-start.

[0037] The formulas for setting the current and voltage are as follows: Where Iset is the setting current, Krel is the reliability coefficient (generally taken as 1.2 - 1.3), Kre is the return coefficient (generally taken as 0.85 - 0.95), I N is the rated current of the transformer, Uset is the setting voltage, U N is the rated voltage of the transformer.

[0038] Figure 7 It shows the strategy flow chart of the coordinated protection of the protection device, which is divided into the following stages: 1. Fast action stage of the current limiter When an external fault occurs and the current increases, the control unit immediately sends a trigger signal to the current limiter. The current limiter switches to a high-impedance state to suppress the amplitude of the fault current and limit it within the safe breaking range of the circuit breaker. The current limiter acts prior to the circuit breaker to ensure that the current during breaking is within the safe threshold.

[0039] 2. Bypass stage of the power electronics part After the current limiter completes current limiting, it is judged whether the current of the bypass switch exceeds the threshold. If it exceeds, the control unit triggers the bypass circuit (thyristor module) to remove the power electronics part (AC / DC converter) from the main circuit. The power frequency electromagnetic part continues to supply power through the bypass path to ensure uninterrupted operation of the load and avoid damage to the power electronics devices due to overcurrent or voltage mutation.

[0040] 3. Circuit breaker breaking stage After the bypass stage ends, it is judged whether the current value after current limiting reaches the setting value of the overcurrent protection with low-voltage startup. If it reaches, a circuit breaker breaking instruction is generated. The circuit breaker completes the breaking operation to completely isolate the hybrid distribution transformer from the fault area.

[0041] Figure 8 The timing diagram for the collaborative protection strategy is as follows: Fault detection → Current limiter operation (0 - t1): ≤1 ms Current limiting operation → Current limiting completion (t1 - t2): ≤5 ms Current limiting completion → Bypass switch conduction (t2 - t3): ≤1 ms Bypass switch conduction → Circuit breaker sectioning completion (t3 - t4): ≤10 ms Full - process response time (0 - t4): ≤17 ms, significantly faster than the traditional protection scheme (>50 ms).

[0042] Current suppression ability: The fault current is limited below the breaking capacity of the traditional circuit breaker.

[0043] System reliability: Through collaborative control and fast bypass, it ensures zero damage to power electronic devices and continuous power supply to the electromagnetic part.

[0044] Figure 9 It is the circuit diagram of the fault occurrence. Figure 10 It is the structure diagram of a certain 10 kV hybrid distribution transformer with a rated voltage of 10 kV and a capacity of 2 MW. According to the capacity and rated voltage, the effective value of the rated current can be calculated as 115.5 A. Considering the extreme case of a 20% voltage drop, the rated current of the main transformer needs to increase by 20%, which is 138.6 A. When the turns ratio of the inverter - side winding to the series - side winding of the series transformer is 3:1, the effective value of the rated current of the inverter - side winding becomes 38.5 A. At this time, the amplitude of the current output by the inverter is calculated as 94.2 A. At this time, the setting value of the low - voltage over - current protection of the hybrid distribution transformer is: It should be noted that unless otherwise defined, the technical terms or scientific terms used in this invention should have the ordinary meaning understood by those with ordinary skills in the field to which this invention belongs. The "first", "second" and similar words used in this invention do not indicate any order, quantity or importance, but are only used to distinguish different components. Words such as "including" or "comprising" mean that the elements or objects appearing before this word cover the elements or objects listed after this word and their equivalents, without excluding other elements or objects. Words such as "connected" or "linked" are not limited to physical or mechanical connections, but may include electrical connections, whether direct or indirect. "Up", "down", "left", "right", etc. are only used to represent relative position relationships. When the absolute position of the object being described changes, the relative position relationship may also change accordingly.

[0045] As described above, it is only the preferred embodiment of the present invention, and it is not a limitation to the present invention in other forms. Any person skilled in the art may use the technical content disclosed above to make changes or modifications into equivalent embodiments with equivalent changes. However, as long as it does not depart from the technical solution content of the present invention, any simple modification, equivalent change and modification made to the above embodiments based on the technical essence of the present invention still belong to the protection scope of the technical solution of the present invention.

[0046] The present invention is not limited to the above best implementation mode. Anyone inspired by the present invention can obtain other various forms of a low-voltage starting overcurrent protection device and protection strategy for a hybrid distribution transformer with a combined current limiter. All equivalent changes and modifications made according to the scope of the patent application of the present invention shall fall within the coverage of the present invention.

Claims

1. A low-voltage starting over-current protection device for a hybrid distribution transformer combined with a current limiter, characterized in that, include: Hybrid distribution transformer: It consists of an industrial frequency electromagnetic part and a high frequency power electronic part. The industrial frequency electromagnetic part includes a power transformer and a series transformer. The high frequency power electronic part includes a back-to-back connected converter. Saturated core fault current limiter: integrated in the main circuit of the hybrid distribution transformer, presents low impedance during normal operation, and switches to a high impedance state through core desaturation to limit fault current in case of a fault; Bypass switch module: connected to the input side of the high-frequency power electronic part, when the fault current exceeds the set threshold, it is turned on within ≤1ms to bypass and isolate the high-frequency power electronic part, and maintain continuous power supply to the power frequency electromagnetic part; Fault isolation device: It is configured at both ends of the hybrid distribution transformer and performs disconnection operation in response to the current setting value and low voltage setting value after current limiting to isolate the fault area.

2. A hybrid distribution transformer low voltage start-up overcurrent protection device combined with a current limiter according to claim 1, characterized in that: Its protection strategy includes the following coordinated control stages: Current limiting stage: after a fault is detected, the saturated iron core fault current limiter switches to a high impedance state within ≤1ms to limit the fault current to the safe disconnection range of the fault isolation device; Bypass stage: After the current limiting is completed, the bypass switch is triggered to turn on within ≤5ms, the high-frequency power electronic part is cut off, and the power supply of the industrial frequency electromagnetic part is maintained; Breaking stage: Determine whether the current reaches the set value after current limiting. If so, the circuit breaker will be tripped within ≤10ms.

3. A low-voltage starting over-current protection device for a hybrid distribution transformer combined with a current limiter according to claim 1, characterized in that: The fault isolation device is a circuit breaker.

4. A low-voltage starting overcurrent protection device for a hybrid distribution transformer combined with a current limiter according to claim 1, characterized in that: The fast response bypass module is composed of thyristors.

5. A low-voltage starting overcurrent protection device for a hybrid distribution transformer combined with a current limiter according to claim 1, characterized in that: The current setting value is calculated based on the transformer rated current, reliability factor and return factor.

6. A low-voltage starting overcurrent protection device for a hybrid distribution transformer combined with a current limiter according to claim 1, characterized in that: The low voltage setting value is 70% of the rated voltage.

7. A low-voltage starting over-current protection device for a hybrid distribution transformer combined with a current limiter according to claim 5, characterized in that: The reliability coefficient of the current setting value is 1.2~1.3, and the return coefficient is 0.85~0.

95.

8. A low-voltage starting overcurrent protection device for a hybrid distribution transformer combined with a current limiter according to claim 1, characterized in that: The saturated iron core fault current limiter is a double iron core structure, comprising an AC winding and a DC excitation circuit connected in reverse series.

9. The low-voltage starting overcurrent protection device for a hybrid distribution transformer combined with a current limiter according to claim 1, characterized in that: The saturated iron core fault current limiter limits the first peak wave amplitude of the fault current to within the safe disconnecting range of the circuit breaker through a double iron core desaturation mechanism.

Images