Current-limiting inductor design method and system of transformer-free controller

Abstract

The invention discloses a method and system for designing a current-limiting inductor of a transformer-free type controller, and relates to the technical field of power electronics, and the method comprises the steps: constructing a topological graph of a transformer-free type multi-terminal flexible interconnection controller, and carrying out the parameter design of a port filter inductor; when any line breaks down, equivalent circuits of all lines are constructed, and inductance parameter design is carried out; and setting the port protection action current. According to the method, the short-circuit current of the transformerless multi-terminal flexible interconnection controller in a fault state can be effectively reduced by designing proper port filter inductance parameters, so that the safety and the stability of a system are improved. By setting the protection action current, the relay protection can still act correctly after the short-circuit current is limited by the current-limiting inductor, meanwhile, maloperation of a non-fault port is avoided, and the reliability of the relay protection is improved.

Description

Technical Field

[0001] The invention relates to the technical field of power electronics, and in particular to a current-limiting inductor design method and system for a transformerless controller. Background Art

[0002] Due to the advancement of science and technology and economic development, the demand for electricity is increasing day by day. The large-scale access of new energy sources such as wind power generation, photovoltaics and energy storage has made the structural complexity of the power system increasingly complex. The interconnection of regional power grids has improved the reliability of power supply, formed a huge power supply network, and enhanced the ability to optimize the allocation of power resources. However, this has also brought many problems, such as uneven distribution of system power flow and insufficient reactive support. However, the existing power system control methods mainly rely on measures such as adjusting generator output, transformer tap switches and line disconnection, which are difficult to meet the needs of the growing new power grid. Therefore, these problems have seriously affected the safe and stable operation of the power system.

[0003] In recent years, researchers have proposed unified power flow controllers (UPFCs) to solve problems such as uneven power flow distribution and insufficient reactive power support in power transmission systems. Compared with building new transmission lines, UPFCs can make full use of the remaining capacity of each AC transmission line by controlling power flow, greatly reducing the required cost. However, due to the large size and high cost of traditional UPFCs, their commercial applications are limited, so a new type of UPFC is urgently needed to solve existing problems. Some researchers have proposed a transformerless multi-terminal flexible interconnection controller. The transformerless multi-terminal flexible interconnection controller adopts a series-parallel structure, and both the series and parallel parts use cascade multilevel inverters (CMIs). The transformerless multi-terminal flexible interconnection controller not only eliminates the cost and volume problems caused by the power frequency transformer in UPFC, but also enables it to be connected to a higher voltage level power grid by using a modular structure. The series part can realize the decoupling control of the active and reactive power flows of the connected ports, and is easy to expand the transmission line, which is referred to as the power flow control module in the following text. The parallel part provides node voltage support and reactive power compensation for the system, which is referred to as the cascaded multilevel converter in the following text. However, there is no power frequency transformer for electrical isolation. When short-circuit faults occur in multiple AC transmission lines, the short-circuit current of the non-fault line will flow into the fault line, causing the short-circuit current of the fault line of the transformerless multi-terminal flexible interconnection controller to increase sharply, and eventually cause irreversible damage to the device, which reduces the operational reliability of the transformerless multi-terminal flexible interconnection controller. This phenomenon may cause potential damage to the device and the power grid. Summary of the invention

[0004] In view of the above-mentioned problems, the present invention is proposed.

[0005] Therefore, the problem to be solved by the present invention is that when short-circuit faults occur in multiple AC transmission lines in the existing method, the short-circuit current of the non-fault line will flow into the fault line, causing the short-circuit current of the fault line of the transformerless multi-terminal flexible interconnection controller to increase sharply, eventually causing irreversible damage to the device.

[0006] To solve the above technical problems, the present invention provides the following technical solutions: a method for designing a current-limiting inductor for a transformerless controller, which includes constructing a transformerless multi-terminal flexible interconnection controller topology diagram and designing port filter inductor parameters; when any line fails, constructing an equivalent circuit of all lines and designing inductor parameters; and setting the port protection action current.

[0007] As a preferred solution of the current limiting inductor design method of a transformerless controller described in the present invention, wherein: the topology diagram includes three transformers, each transformer constitutes one phase, and there are three lines, namely line 1, line 2 and line 3; when designing the port filter inductor parameters, the suppression effect of the filter inductor parameters on the current ripple of the switching frequency is considered, and the inductance size is related to the switching frequency and the output voltage of the power flow control module. Based on the single-phase inverter parameter design method, the equivalent filter inductor design at the port outlet of the power flow control module is performed, and the formula is expressed as follows:

[0008]

[0009] Among them, V Cn represents the output voltage of the power flow control module, L represents the equivalent filter inductance, T sw represents the switching period, ΔI max Indicates the current ripple.

[0010] As a preferred solution of the current limiting inductor design method of the transformerless controller described in the present invention, the port filter inductor parameter design also includes, after determining the maximum current ripple allowed by the system, calculating the inductance size expressed as:

[0011]

[0012] The peak value of the current ripple is limited to 5% to 10% of the rated current amplitude. Let the current ripple peak be 10% of the rated current amplitude, and the lower limit of the port inductance value is expressed as:

[0013]

[0014] As a preferred solution of the current-limiting inductor design method of the transformerless controller described in the present invention, the inductor parameter design includes, when a three-phase short circuit occurs in line 1, an equivalent circuit design is performed on the bus voltages connected to the three lines, which is expressed as:

[0015]

[0016] in, It represents the bus voltage connected to lines 1, 2, and 3. V1, V2, and V3 represent the rated voltages of the high-voltage sides of the three transformers. The subscripts T1, T2, and T3 represent the first, second, and third transformers respectively. P represents active power, R represents resistance, Q represents reactive power, X represents reactance, and j represents an imaginary unit.

[0017] As a preferred solution of the current limiting inductor design method of the transformerless controller described in the present invention, the inductor parameter design also includes, when a three-phase short circuit occurs in line 1, equivalent power is obtained by performing equivalent operation on line 2 and line 3. The equivalent impedance Z from the equivalent power source to the PCC point of line 1 eq23 Respectively expressed as,

[0018]

[0019] Among them, Z L2 It is represented by the equivalent impedance of the line from the outlet of the second transformer of line 2 to the PCC point, Z L3 It is represented by the equivalent impedance of the line from the outlet of the third transformer of line 3 to the PCC point, L c is the port inductance, ω is the angular frequency, Z T2 It is the impedance value of the second transformer converted to the high voltage side, Z T3 It is expressed as the impedance value of the third transformer converted to the high voltage side; the short-circuit current of line 1, line 2 and line 3 and the PCC point voltage of line 2 and line 3 are expressed as,

[0020]

[0021] in, It is expressed as the short-circuit current of lines 1, 2, and 3. It is expressed as the equivalent power supply of feeder 2 and feeder 3 relative to the short-circuit point, Z eq23 Expressed as equivalent power Equivalent impedance to the 1PCC point of the feeder, Z k23 It is expressed as the equivalent impedance from the feeder 1PCC point to the short-circuit point, It is expressed as the PCC point voltage of lines 2 and 3; by combining the equivalent circuits, we can finally obtain the maximum short-circuit current of lines 1, 2 and 3 and the effective value of the PCC point voltage of lines 2 and 3 when three-phase short circuit occurs.

[0022] As a preferred solution of the current limiting inductor design method of the transformerless controller described in the present invention, the port protection action current is set according to two principles. The first principle is to set it according to the maximum load current of the device; the second principle is to set it according to the short-circuit current of the worst case of the non-fault port. The calculation according to the two principles is expressed as:

[0023]

[0024] Among them, I act1 ,I act2 It is expressed as the current adjusted based on the first and second principles, Expressed as reliability coefficient, K MS Expressed as the motor self-starting coefficient, K re Expressed as the return factor of the relay, I k2max , I k3max They are respectively expressed as the maximum short-circuit current of the two non-fault lines.

[0025] As a preferred solution of the current limiting inductor design method of a transformerless controller described in the present invention, wherein: the setting of the port protection action current also includes, based on the sensitivity check of the III-stage current protection, to meet the port protection sensitivity, the sensitivity coefficient K sen It needs to be greater than or equal to 1.3, which is expressed as,

[0026]

[0027] Among them, I k Expressed as the actual short-circuit current, I act It is expressed as the relay operating current after setting.

[0028] Another object of the present invention is to provide a current limiting inductor design system for a transformerless controller, which can design the current limiting inductor according to fault conditions.

[0029] To solve the above technical problems, the present invention provides the following technical solutions: a system for a current-limiting inductor design method for a transformerless controller, comprising: a filter inductor parameter design module, a fault equivalent module and a setting module; the filter inductor parameter design module constructs a transformerless multi-terminal flexible interconnection controller topology diagram and performs port filter inductor parameter design; when any line fails, the fault equivalent module constructs an equivalent circuit of all lines and performs inductance parameter design; the setting module sets the port protection action current.

[0030] A computer device comprises a memory and a processor, wherein the memory stores a computer program, and the processor implements the steps of the current-limiting inductor design method of a transformerless controller as described above when executing the computer program.

[0031] A computer-readable storage medium stores a computer program, which, when executed by a processor, implements the steps of the current-limiting inductor design method for a transformerless controller as described above.

[0032] The beneficial effect of the present invention is that the present invention can effectively reduce the short-circuit current of the transformerless multi-terminal flexible interconnection controller under a fault state by designing appropriate port filter inductance parameters, thereby improving the safety and stability of the system.

[0033] When a short circuit occurs, the designed inductance parameters help the device to recover quickly or achieve fault ride-through after the fault, maintaining continuous power supply to the power system.

[0034] The present invention ensures that the relay protection can still operate correctly after the current limiting inductor limits the short-circuit current by setting the protection action current, and avoids the false operation of the non-fault port, thereby improving the reliability of the relay protection. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the following briefly introduces the drawings required for describing the embodiments. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without creative work. Among them:

[0036] Figure 1 This is a flow chart of a current-limiting inductor design method for a transformerless controller in Example 1.

[0037] Figure 2 This is a topology diagram of a transformerless multi-terminal flexible interconnection controller according to a current-limiting inductor design method for a transformerless controller in Example 1.

[0038] Figure 3 This is an equivalent circuit diagram of a current-limiting inductor design method for a transformerless controller in Example 1 when line 1 fails.

[0039] Figure 4 This is a simulation diagram of the voltage at the port of line 1 when line 2 fails in a current-limiting inductor design method for a transformerless controller in Example 2.

[0040] Figure 5This is a simulation diagram of the voltage at the port of line 2 when line 2 fails in a current-limiting inductor design method for a transformerless controller in Example 2.

[0041] Figure 6 This is a simulation diagram of the voltage at the port of line 3 when line 2 fails in a current-limiting inductor design method for a transformerless controller in Example 2.

[0042] Figure 7 This is a current simulation diagram of line 1 when line 2 fails in a current-limiting inductor design method for a transformerless controller in Example 2.

[0043] Figure 8 This is a current simulation diagram of line 2 when line 2 fails in a current-limiting inductor design method for a transformerless controller in Example 2.

[0044] Fig. 9 This is a current simulation diagram of line 3 when line 2 fails in a current-limiting inductor design method for a transformerless controller in Example 2.

[0045] Fig.10 This is a module structure diagram of a current-limiting inductor design system for a transformerless controller in Example 3. DETAILED DESCRIPTION

[0046] In order to make the above-mentioned objects, features and advantages of the present invention more obvious and easy to understand, the specific implementation methods of the present invention are described in detail below in conjunction with the accompanying drawings.

[0047] In the following description, many specific details are set forth to facilitate a full understanding of the present invention, but the present invention may also be implemented in other ways different from those described herein, and those skilled in the art may make similar generalizations without violating the connotation of the present invention. Therefore, the present invention is not limited to the specific embodiments disclosed below.

[0048] Example 1, reference Figure 1-Figure 3 , which is the first embodiment of the present invention, and which provides a method for designing a current-limiting inductor of a transformerless controller, including: Figure 1 As shown:

[0049] S1. Construct a transformerless multi-terminal flexible interconnection controller topology diagram and design the port filter inductor parameters.

[0050] The present invention ensures that the current limiting inductor of the device port meets the basic filtering performance requirements. Secondly, its parameter design should also meet two requirements. The first is to ensure the sensitivity of the protection action at the outlet circuit breaker, and the second is to ensure that the voltage at the non-fault port is higher than 80% of the system rated voltage under the worst short-circuit condition. According to the above current limiting inductor design principle, the short-circuit current of the transformerless unified power flow controller under the fault state is reduced, and the device operation control recovery or fault ride-through capability after the device fault is improved.

[0051] The topology diagram of transformerless multi-terminal flexible interconnection controller is as follows: Figure 2 As shown, the topology includes three transformers, each transformer constitutes one phase, and there are three lines in total, namely line 1, line 2 and line 3. When designing the parameters of the port filter inductor, the main consideration is its suppression effect on the current ripple of the switching frequency. The inductance size is related to the switching frequency and the output voltage of the power flow control module. The equivalent filter inductance design at the port outlet of the power flow control module can be expressed as, referring to the single-phase inverter parameter design method,

[0052]

[0053] Among them, V Cn represents the output voltage of the power flow control module, L represents the equivalent filter inductance, T sw represents the switching period, ΔI max Indicates the current ripple.

[0054] After determining the maximum current ripple allowed by the system, the inductance is calculated as:

[0055]

[0056] In general, the peak value of the current ripple should be limited to 5% to 10% of the rated current amplitude. Let the peak-to-peak value of the current ripple be 10% of the rated current amplitude, then the inductance size is,

[0057]

[0058] The above formula gives the lower limit of the port inductance value. If the inductance is too large, the fault current will be too small, affecting the sensitivity of the protection action. Therefore, it should be designed to be less than a certain value to ensure the sensitivity of the device protection action.

[0059] S2. When any line fails, construct equivalent circuits of all lines and design inductance parameters.

[0060] Figure 3This is the equivalent circuit when line 1 fails. When a three-phase short circuit occurs in the system, the short-circuit current is the largest, which also leads to the lowest voltage at the non-fault port. Therefore, the worst three-phase short circuit is analyzed. At this time, the bus voltages of the three lines are respectively,

[0061]

[0062] in, It represents the bus voltage connected to lines 1, 2, and 3. V1, V2, and V3 represent the rated voltages of the high-voltage sides of the three transformers. The subscripts T1, T2, and T3 represent the first, second, and third transformers respectively. P represents active power, R represents resistance, Q represents reactive power, X represents reactance, and j represents an imaginary unit.

[0063] Equivalence of line 2 and line 3 to obtain equivalent power supply The equivalent impedance Z from the equivalent power source to the PCC point of line 1 eq23 Respectively expressed as,

[0064]

[0065] Among them, Z L2 It is represented by the equivalent impedance of the line from the outlet of the second transformer of line 2 to the PCC point, Z L3 It is represented by the equivalent impedance of the line from the outlet of the third transformer of line 3 to the PCC point, L c is the port inductance, ω is the angular frequency, Z T2 It is the impedance value of the second transformer converted to the high voltage side, Z T3 It represents the impedance value of the third transformer converted to the high voltage side.

[0066] The short-circuit current of line 1, line 2 and line 3 and the PCC point voltage of line 2 and line 3 are expressed as,

[0067]

[0068] in, It is expressed as the short-circuit current of lines 1, 2, and 3. It is expressed as the equivalent power supply of feeder 2 and feeder 3 relative to the short-circuit point, Z eq23 Expressed as equivalent power Equivalent impedance to the 1PCC point of the feeder, Z k23 It is expressed as the equivalent impedance from the feeder 1PCC point to the short-circuit point, It is expressed as the PCC point voltage of lines 2 and 3.

[0069] By combining the above equivalent circuits, we can finally obtain the maximum short-circuit current of line 1, line 2 and line 3 when the three-phase short circuit occurs, as well as the effective value of the PCC point voltage of line 2 and line 3. Obviously, if the inductance is larger, the voltage drop at the PCC point of the non-fault port will be smaller, so the voltage drop requirement can still be met. The subsequent inductance parameter design only needs to make it meet the sensitivity of the port protection action.

[0070] S3. Adjust the port protection action current.

[0071] For the protection applied to the port of the transformerless multi-terminal flexible interconnection controller, it does not need to cooperate with other relay protection devices in the system, and can achieve instantaneous fast action and quickly disconnect the device from the fault line, which is beneficial to the device safety and fault crossing. In addition, the current flowing through the protection is the current of the line connected to each port of the device, and does not involve the protection range. Therefore, its action current can be set according to the setting principle of the III-stage time-limited overcurrent protection of the three-stage current protection to avoid the maximum load current, thereby greatly improving its sensitivity and ensuring that it can still operate correctly after the current limiting inductor limits the short-circuit current. However, due to the energy interaction of the port, the non-fault line will also flow through the short-circuit current. If its value is greater than the action current under the above setting principle, the protection at the non-fault port may be malfunctioned and cannot meet the reliability requirements of the relay protection. From the above analysis, it can be seen that when a short circuit occurs, the non-fault line also feeds the short-circuit current to the fault line at the same time, and the short-circuit current of the fault line where the circuit breaker should trip correctly is even worse. Therefore, based on the above analysis and system operation requirements, the device port protection action current setting method proposed in this paper is as follows:

[0072] (1) Setting according to the maximum load current of the device; (2) Setting according to the short-circuit current under the worst condition of the non-fault port; Under the two setting principles, the larger value is taken as the action current of the device port protection. This protection action current is mainly for phase-to-phase short circuits in the system. For ground faults, the zero-sequence voltage and zero-sequence current can be considered for judgment. Since the maximum short-circuit current at the non-fault port is avoided, this action current setting method can automatically realize the fault line selection function while meeting the fault judgment sensitivity requirements for the fault line, that is, instantly tripping the fault line, while the non-fault line is reliably locked.

[0073] The port protection action current of the transformerless multi-terminal flexible interconnection controller is calculated according to two setting principles:

[0074]

[0075] Among them, I act1 ,I act2 It is expressed as the current adjusted based on the first and second principles, Expressed as reliability coefficient, K MSExpressed as the motor self-starting coefficient, K re Expressed as the return factor of the relay, I k2max , I k3max They are respectively expressed as the maximum short-circuit current of the two non-fault lines.

[0076] Refer to the sensitivity check of the current protection of stage III. To meet the port protection sensitivity, the sensitivity coefficient should be greater than or equal to 1.3, expressed as:

[0077]

[0078] Among them, I k Expressed as the actual short-circuit current, I act It is expressed as the relay operating current after setting.

[0079] According to all the above relationships, the inductance value can be obtained by reverse solution.

[0080] Example 2, reference Figure 4-Figure 9 , which is the second embodiment of the present invention, and which is different from the first embodiment in that: a current-limiting inductor design method for a transformerless controller also includes, in order to verify and explain the technical effects adopted in the method, comparing the test results with scientific argumentation means to verify the real effect of the method.

[0081] Take the transformerless multi-terminal flexible interconnection controller connecting three transmission lines as an example. Assuming that line 1 is a balanced line, after a three-phase short-circuit fault occurs, the device is cut off by TBS. The maximum value of the current limiting inductance is calculated using the above method. Compared with the case where only the filter inductance is considered, after the current limiting inductance is designed, the short-circuit current of each transmission line will be further reduced, and the voltage drop at the PCC point will be less than 20%, which improves the device operation control recovery or fault ride-through capability after the device fails. The experimental data are shown in Table 1.

[0082] Table 1: Simulation parameters

[0083]

[0084] The specific simulation results are as follows Figure 4-Figure 9 The simulation results show that the addition of the current limiting inductor not only increases the port voltage of the non-fault line so that it does not drop below 80% of the rated voltage, but also further reduces the short-circuit current.

[0085] Example 3, reference Fig.10, which is the third embodiment of the present invention, and is different from the previous two embodiments in that: a system for a current-limiting inductor design method for a transformerless controller includes a filter inductor parameter design module 100, a fault equivalent module 200 and a setting module 300; the filter inductor parameter design module 100 constructs a transformerless multi-terminal flexible interconnection controller topology diagram and performs port filter inductor parameter design; when any line fails, the fault equivalent module 200 constructs an equivalent circuit of all lines and performs inductance parameter design; the setting module 300 sets the port protection action current.

[0086] If the functions are implemented in the form of software functional units and sold or used as independent products, they can be stored in a computer-readable storage medium. Based on this understanding, the technical solution of the present invention, or the part that contributes to the prior art or the part of the technical solution, can be embodied in the form of a software product. The computer software product is stored in a storage medium, including several instructions for a computer device (which can be a personal computer, a server, or a network device, etc.) to perform all or part of the steps of the methods described in each embodiment of the present invention. The aforementioned storage medium includes: U disk, mobile hard disk, read-only memory (ROM, Read-Only Memory), random access memory (RAM, Random Access Memory), disk or optical disk, etc., which can store program codes.

[0087] The logic and / or steps represented in the flowchart or otherwise described herein, for example, can be considered as an ordered list of executable instructions for implementing logical functions, and can be embodied in any computer-readable medium for use by an instruction execution system, device or apparatus (such as a computer-based system, a system including a processor, or other system that can fetch instructions from an instruction execution system, device or apparatus and execute instructions), or in conjunction with such instruction execution systems, devices or apparatuses. For the purposes of this specification, "computer-readable medium" can be any device that can contain, store, communicate, propagate or transmit a program for use by an instruction execution system, device or apparatus, or in conjunction with such instruction execution systems, devices or apparatuses.

[0088] More specific examples of computer-readable media (a non-exhaustive list) include the following: an electrical connection with one or more wires (electronic device), a portable computer disk case (magnetic device), a random access memory (RAM), a read-only memory (ROM), an erasable and programmable read-only memory (EPROM or flash memory), an optical fiber device, and a portable compact disk read-only memory (CDROM). In addition, the computer-readable medium may even be a paper or other suitable medium on which the program is printed, since the program may be obtained electronically, for example, by optically scanning the paper or other medium, followed by editing, deciphering or, if necessary, processing in another suitable manner, and then stored in a computer memory.

[0089] It should be understood that the various parts of the present invention can be implemented by hardware, software, firmware or a combination thereof. In the above-mentioned embodiments, a plurality of steps or methods can be implemented by software or firmware stored in a memory and executed by a suitable instruction execution system. For example, if implemented by hardware, as in another embodiment, it can be implemented by any one of the following technologies known in the art or their combination: a discrete logic circuit having a logic gate circuit for implementing a logic function for a data signal, a dedicated integrated circuit having a suitable combination of logic gate circuits, a programmable gate array (PGA), a field programmable gate array (FPGA), etc.

[0090] It should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention rather than to limit it. Although the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that the technical solutions of the present invention may be modified or replaced by equivalents without departing from the spirit and scope of the technical solutions of the present invention, which should all be included in the scope of the claims of the present invention.

Claims

1. A method for designing a current-limiting inductor for a transformerless controller, characterized in that: include, Construct a transformerless multi-terminal flexible interconnection controller topology diagram and design the port filter inductor parameters; When any line fails, an equivalent circuit of all lines is constructed to design the inductance parameters; Adjust the port protection action current.

2. The method for designing a current-limiting inductor for a transformerless controller according to claim 1, wherein: The topology diagram includes three transformers, each transformer constitutes one phase, and there are three lines, namely line 1, line 2 and line 3; When designing the port filter inductance parameters, the suppression effect of the filter inductance parameters on the current ripple of the switching frequency is considered. The inductance size is related to the switching frequency and the output voltage of the power flow control module. Based on the single-phase inverter parameter design method, the equivalent filter inductance design at the port outlet of the power flow control module is performed. The formula is expressed as follows: Among them, V Cn represents the output voltage of the power flow control module, L represents the equivalent filter inductance, T sw represents the switching period, ΔI max Indicates the current ripple.

3. The method for designing a current-limiting inductor for a transformerless controller according to claim 2, wherein: The port filter inductor parameter design also includes, after determining the maximum current ripple allowed by the system, calculating the inductance value to be expressed as: The peak value of the current ripple is limited to 5% to 10% of the rated current amplitude. Let the current ripple peak be 10% of the rated current amplitude, and the lower limit of the port inductance value is expressed as:

4. The method for designing a current-limiting inductor for a transformerless controller as claimed in claim 3, characterized in that: The inductance parameter design includes, when a three-phase short circuit occurs in line 1, an equivalent circuit design is performed on the bus voltages connected to the three lines, and the equivalent circuit design is expressed as: in, It represents the bus voltage connected to lines 1, 2, and 3. V1, V2, and V3 represent the rated voltages of the high-voltage sides of the three transformers. The subscripts T1, T2, and T3 represent the first, second, and third transformers respectively. P represents active power, R represents resistance, Q represents reactive power, X represents reactance, and j represents an imaginary unit.

5. The method for designing a current-limiting inductor for a transformerless controller as claimed in claim 4, characterized in that: The inductance parameter design also includes, when a three-phase short circuit occurs in line 1, performing an equivalent operation on line 2 and line 3 to obtain an equivalent power supply. The equivalent impedance Z from the equivalent power source to the PCC point of line 1 eq23 Respectively expressed as, Among them, Z L2 It is represented by the equivalent impedance of the line from the outlet of the second transformer of line 2 to the PCC point, Z L3 It is represented by the equivalent impedance of the line from the outlet of the third transformer of line 3 to the PCC point, L c is the port inductance, ω is the angular frequency, Z T2 It is the impedance value of the second transformer converted to the high voltage side, Z T3 It is expressed as the impedance value of the third transformer converted to the high voltage side; The short-circuit current of line 1, line 2 and line 3 and the PCC point voltage of line 2 and line 3 are expressed as, in, It is expressed as the short-circuit current of lines 1, 2, and 3. It is expressed as the equivalent power supply of feeder 2 and feeder 3 relative to the short-circuit point, Z eq23 Expressed as equivalent power Equivalent impedance to the 1PCC point of the feeder, Z k23 It is expressed as the equivalent impedance from the feeder 1PCC point to the short-circuit point, It is represented by the PCC point voltage of lines 2 and 3; By combining the equivalent circuits, we can finally obtain the maximum short-circuit currents of line 1, line 2 and line 3 and the effective values ​​of the PCC point voltages of line 2 and line 3 during a three-phase short circuit.

6. The method for designing a current-limiting inductor for a transformerless controller as claimed in claim 5, characterized in that: The setting of the port protection action current includes two principles. The first principle is to set it according to the maximum load current of the device; the second principle is to set it according to the short-circuit current under the worst condition of the non-fault port. The calculation based on the two principles is expressed as: Among them, I act1 ,I act2 It is expressed as the current adjusted based on the first and second principles, Expressed as reliability coefficient, K MS Expressed as the motor self-starting coefficient, K re Expressed as the return factor of the relay, I k2max , I k3max They are respectively expressed as the maximum short-circuit current of the two non-fault lines.

7. The method for designing a current-limiting inductor for a transformerless controller according to claim 6, wherein: The setting of the port protection action current also includes checking the sensitivity of the current protection based on the III section. To meet the port protection sensitivity, the sensitivity coefficient K sen It needs to be greater than or equal to 1.3, which is expressed as, Among them, I k Expressed as the actual short-circuit current, I act It is expressed as the relay operating current after setting.

8. A system using a current-limiting inductor design method of a transformerless controller as claimed in any one of claims 1 to 7, characterized in that: It comprises a filter inductance parameter design module (100), a fault equivalent module (200) and a setting module (300); The filter inductor parameter design module (100) constructs a transformer-free multi-terminal flexible interconnection controller topology diagram and performs port filter inductor parameter design; When any line fails, the fault equivalent module (200) constructs equivalent circuits of all lines and performs inductance parameter design; The setting module (300) sets the port protection action current.

9. A computer device comprising a memory and a processor, wherein the memory stores a computer program, wherein: When the processor executes the computer program, the processor implements the steps of a method for designing a current-limiting inductor of a transformerless controller according to any one of claims 1 to 7.

10. A computer-readable storage medium having a computer program stored thereon, characterized in that: When the computer program is executed by a processor, the steps of a method for designing a current-limiting inductor of a transformerless controller according to any one of claims 1 to 7 are implemented.

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