Distribution network fault current limiting method based on hybrid distribution transformer

Abstract

The invention discloses a distribution network fault current limiting method based on a hybrid distribution transformer. The method comprises the following steps: detecting load current in the hybrid distribution transformer; when no fault occurs, the secondary winding current of the series transformer is controlled, so that the primary side voltage of the series transformer is compensated to be 0, the equivalent impedance of the series transformer connected into a main loop is 0, and the series transformer has no influence on the normal operation of the system; after a fault occurs, the fault detection link is delayed, the phase and the amplitude of the current of the secondary side of the series transformer are controlled to remain unchanged by default during the delay period, and the fault current is limited to a certain extent; when it is detected that the load current is larger than a set threshold value, it is determined that a fault occurs, and the amplitude and phase of the secondary winding current of the series transformer are controlled through a converter in the hybrid distribution transformer, so that the primary side voltage of the series transformer is not zero, namely, current-limiting voltage is generated, and reverse voltage is provided for a main circuit to suppress fault current. According to the invention, fault ride-through of the hybrid distribution transformer under the ground fault can be realized.

Description

Technical Field

[0001] The invention relates to the fields of power electronic commutation and fault current limiting, and in particular to a distribution network fault current limiting method based on a hybrid distribution transformer. Background Art

[0002] As the "nerve endings" of the power grid, the distribution network connects the main power grid and thousands of households. Under the construction of the new power system, the distribution network plays the role of an energy allocation hub, and the mission of development and construction is of great responsibility. At present, the distribution transformer in the distribution network is still the most important electrical equipment for voltage conversion and power distribution, but it lacks power quality regulation and the ability to actively control distributed power sources and loads as an energy allocation hub. Traditional power transformers have high reliability and strong overload and short-circuit bearing capacity. But at the same time, the no-load loss is large; when a fault occurs on the load side, the fault cannot be isolated, which leads to the expansion of the fault. The hybrid distribution transformer combines the advantages of the two devices by combining the power frequency transformer and the power electronic device, avoiding the problems existing when each is used separately, and provides a feasible idea for the power conversion and power quality management of the distribution network under the background of the construction of the new power system. The fault ride-through technology of the domestic hybrid distribution transformer mainly focuses on the voltage recovery under the fault on the power supply side, and there is little research on the active current limiting technology under the fault on the load side, and there is still a research gap in the research on the sustainable operation technology under the converter fault. Summary of the invention

[0003] Purpose of the invention: The purpose of the present invention is to provide a distribution network fault current limiting method based on a hybrid distribution transformer.

[0004] Technical solution: The distribution network fault current limiting method based on a hybrid distribution transformer of the present invention comprises:

[0005] Detect the load current in the hybrid distribution transformer; when no fault occurs, the load current is less than the set threshold, and the secondary winding current of the series transformer is controlled so that the primary side voltage of the series transformer is compensated to 0, and the equivalent impedance of the series transformer connected to the main circuit is 0, and the series transformer has no effect on the normal operation of the system;

[0006] After a fault occurs, the fault detection link is delayed for a period of time. During the delay period, the phase and amplitude of the current on the secondary side of the series transformer are controlled to remain unchanged by default, and the fault current is limited to a certain extent;

[0007] When it is detected that the load current is greater than the set threshold, a fault is determined to have occurred. The amplitude and phase of the secondary winding current of the series transformer are controlled by the converter in the hybrid distribution transformer so that the primary side voltage of the series transformer is not zero, that is, a current limiting voltage is generated to provide a reverse voltage to the main circuit to suppress the fault current.

[0008] Furthermore, when no fault occurs, the equivalent impedance Z of the series transformer connected to the main circuit is e for:

[0009]

[0010] Among them, L s1 , L s2 are the primary and secondary self-inductances of the series transformer, M s is the mutual inductance of the series transformer, I1 is the primary winding current of the series transformer, I2 is the secondary winding current of the series transformer, ω is the angular frequency, and j is the imaginary unit.

[0011] Furthermore, the equivalent impedance Z of the series transformer connected to the main circuit is e-2 for:

[0012]

[0013] Among them, Z1 is the line impedance, Z2 is the load impedance, L s1 is the self-inductance of the primary side of the series transformer, ω is the angular frequency, and j is the imaginary unit.

[0014] Furthermore, when a fault is detected, the equivalent impedance Z of the series transformer connected to the main circuit e-3 for:

[0015]

[0016] Where Z1 is the line impedance, L s1 is the self-inductance of the primary side of the series transformer, ω is the angular frequency, j is the imaginary unit, M s is the series transformer mutual inductance, Z2 is the load impedance, U s It is an AC voltage source equivalent to the secondary winding of a three-winding transformer connected to the primary winding of a series transformer.

[0017] Furthermore, a virtual three-phase current method is used to detect the load current, including:

[0018] The load current is taken as the A-phase current of the virtual three-phase current. The A-phase current is delayed by 60° to obtain the anti-phase current of the C-phase current. The B-phase current is calculated based on the A-phase current and the C-phase current to construct a virtual three-phase current. The fundamental amplitude is obtained by performing dq0 transformation on the constructed virtual three-phase current. The sudden change of the fundamental amplitude is used as a sign of a short-circuit fault.

[0019] Further, the secondary winding current of the series transformer is realized by coordinating the control of the AC / DC converter and the DC / AC converter in the hybrid distribution transformer through the PWM controller;

[0020] The AC / DC converter adopts a double closed-loop decoupled proportional-integral control structure with a voltage outer loop and a current inner loop, thereby controlling the DC side capacitor voltage to remain at a reference value, and absorbing or emitting reactive power to complete reactive power compensation for the grid by controlling the output reactive current;

[0021] The current control method of the DC / AC converter adopts fixed switching frequency PWM current control. The PWM carrier frequency remains unchanged. * The current error Δi=i is calculated by adding the feedback current i * -i, after being compared with the PWM carrier by the PI regulator, generates a PWM signal to drive the DC / AC converter.

[0022] Furthermore, the amplitude and phase of the current limiting voltage are dynamically adjusted according to the type of fault; the fault current is detected through the fault detection link, different fault types are determined according to the size of each phase current, and then the secondary winding current reference value of the series transformer is set according to the fault type.

[0023] The system corresponding to the method includes:

[0024] A detection unit is used to detect the load current in the distribution network. If the load current is greater than a set threshold, a fault occurs; otherwise, no fault occurs;

[0025] The first control unit is used to control the secondary winding current of the series transformer when no fault occurs, so that the primary side voltage of the series transformer is compensated to 0, the equivalent impedance of the series transformer connected to the main circuit is 0, and the series transformer has no effect on the normal operation of the system;

[0026] The delay control unit is used for the moment when a fault occurs. The fault detection link is delayed for a period of time. During the delay period, the phase and amplitude of the current on the secondary side of the series transformer are controlled to remain unchanged by default, and the fault current is limited to a certain extent.

[0027] The fault current limiting unit is used to control the amplitude and phase of the secondary winding current of the series transformer through the converter in the hybrid distribution transformer after detecting a fault, so that the primary side voltage of the series transformer is not zero, that is, to generate a current limiting voltage and provide a reverse voltage to the main circuit to suppress the fault current.

[0028] An electronic device for storing and executing the method, the device comprising:

[0029] A memory storing executable program code;

[0030] a processor coupled to the memory;

[0031] The processor calls the executable program code stored in the memory to execute the steps of the distribution network fault current limiting method based on a hybrid distribution transformer.

[0032] A computer-readable storage medium for storing and executing the method, wherein the computer-readable storage medium stores computer instructions, and when the computer instructions are called, they are used to execute the steps of the distribution network fault current limiting method based on a hybrid distribution transformer.

[0033] Beneficial effects: Compared with the prior art, the significant technical effects of the present invention are: taking power from one of the secondary windings of the hybrid distribution transformer as the power source for fault current limiting, no additional topology is required, and the fault current limiting function does not contain resistance, which helps to reduce heat loss during current limiting operation, and by actively controlling the current output by the converter, while suppressing the large fault current, its steady-state level can also be regulated, which can achieve fault ride-through of the hybrid distribution transformer under a ground fault. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] Figure 1 It is a control block diagram of the method of the present invention;

[0035] Figure 2 This is a schematic diagram of the control circuit of the present invention;

[0036] Figure 3 It is the control block diagram of AC / DC converter;

[0037] Figure 4 It is a single-phase topology diagram of the voltage compensation current limiting link;

[0038] Figure 5 It is the single-phase equivalent diagram of the voltage compensation current limiting link;

[0039] Figure 6 This is the fixed switching frequency PWM current control principle diagram;

[0040] Figure 7 It is a simulation curve diagram of the present invention; wherein (a) is a simulation curve of a phase current in the main circuit, and (b) is a simulation curve of the voltage of the series transformer. DETAILED DESCRIPTION

[0041] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0042] like Figure 1 and Figure 2As shown, the hybrid distribution transformer includes: a three-winding transformer, an AC / DC converter, a DC / AC converter and a series transformer, the primary winding 1 of the three-winding transformer is connected to the grid, a secondary winding 3 is connected to the AC side of the AC / DC converter, the DC side of the AC / DC converter is connected to the DC side of the DC / AC converter, the AC side of the DC / AC converter is connected to the secondary winding of the series transformer, and the primary winding of the series transformer is connected in series between another secondary winding 2 of the three-winding transformer and the load; at the same time, a bypass switch is connected in series between the primary winding of the series transformer and the secondary winding 2 of the three-winding transformer, a bypass switch and an LC filter circuit (composed of an inductor L1 and a capacitor C1) are connected in series between the secondary winding 3 of the three-winding transformer and the AC side of the AC / DC converter, a DC capacitor C is connected in parallel between the DC side of the AC / DC converter and the DC side of the DC / AC converter, and an LC filter circuit (composed of an inductor L2 and a capacitor C2) is connected in series between the AC side of the DC / AC converter and the secondary winding of the series transformer.

[0043] The AC / DC converter and the DC / AC converter are converters with two DC sides connected back to back. They are two three-phase two-level converters with the same structure. Their DC sides are connected in series with a DC capacitor in parallel in the middle. The AC / DC converter realizes the function of a static VAR generator (SVG), and the DC / AC converter plays the role of voltage compensation current limiting.

[0044] The distribution network fault current limiting method based on the hybrid distribution transformer described in the present invention controls the voltage of the series transformer by controlling the AC / DC converter and the DC / AC converter, and achieves fault crossing without the need for additional circuits, that is, realizes the voltage compensation type current limiting function. The principle of voltage compensation type current limiting is as follows: when no fault occurs, the secondary winding current of the series transformer is controlled so that the primary side voltage of the series transformer is compensated to 0, and the impedance of the series transformer equivalently connected to the main circuit is 0, and the series transformer has no effect on the normal operation of the system; when a fault occurs, the fault detection link will not detect the fault immediately, but will delay for a period of time, during which the current on the secondary side of the series transformer remains unchanged, so the fault current will be suppressed to a certain extent; after the fault is detected, the amplitude and phase of the secondary winding current of the series transformer are controlled, and a voltage opposite to the AC power supply is provided on the primary side of the series transformer to suppress the fault current. Specifically including:

[0045] The AC / DC converter adopts a double closed-loop decoupled proportional integral control structure with a voltage outer loop and a current inner loop, thereby controlling the DC side capacitor voltage to maintain at a reference value, and absorbing or emitting reactive power to complete reactive power compensation for the power grid by controlling the output reactive current. Figure 3 shown.

[0046] The single-phase topology of the voltage compensation current limiting link is as follows: Figure 4As shown, where U s is the equivalent AC voltage source of the secondary winding 2 of the three-winding transformer, Z1 is the line impedance, Z2 is the load impedance, L s1 , L s2 are the primary and secondary self-inductances of the series transformer, M s is the mutual inductance of the series transformer, I1 is the primary winding current of the series transformer, and I2 is the secondary winding current of the series transformer; U dc is the DC side voltage of the DC / AC converter.

[0047] Ignoring the resistance loss of the series transformer, since the grid side of the DC / AC converter presents a controlled power supply characteristic, it is simplified to a controllable voltage source U p ,like Figure 5 shown.

[0048] When the system is operating normally, the current and voltage satisfy the following equation:

[0049] U s =I1(Z1+Z2)+jωL s1 I1-jωM s I2 (1)

[0050] U p =jωM s I1-jωL s2 I2 (2)

[0051] Where ω is the angular frequency and j is the imaginary unit; control the secondary winding current I2 of the series transformer so that jωL s1 I1-jωM s I2=0, so the primary side voltage U1 of the series transformer is compensated to 0, and the equivalent impedance Z of the series transformer connected to the main circuit e for:

[0052]

[0053] The series transformer has no effect on the normal operation of the system. The secondary winding current of the series transformer under this condition is expressed as formula (4), where k is the mutual inductance coupling coefficient of the series transformer,

[0054]

[0055] Combining equations (2) and (4), we can get the secondary voltage of the series transformer at this time: If the mutual inductance coupling coefficient k is not 1, the voltage on the secondary side of the series transformer is not 0, which means that the DC / AC converter will provide a certain power to the secondary side coil of the series transformer. Its value can be expressed as:

[0056]

[0057] Among them, S p The power supplied by the DC / AC converter to the secondary coil of the series transformer.

[0058] It can be seen from formula (5) that when the system is operating normally, the DC / AC converter will provide a certain value of pure inductive reactive power to the secondary coil of the series transformer, and its value is related to the square of the mutual inductance coupling coefficient k.

[0059] When a short circuit occurs in the system, the load impedance Z2 is short-circuited, and the primary winding current of the series transformer will rise from I1 to I 1f , at this time the main circuit satisfies the formula:

[0060] U s =I 1f Z1+jωL s1 I 1f -jωM s I2 (6)

[0061] The voltage on the primary side of the series transformer will rise to U 1f , the equivalent impedance Z presented by the series transformer e-1 It can be expressed as:

[0062]

[0063] From formula (7), we can know that after a fault occurs, the equivalent impedance Z can be controlled by adjusting the amplitude and phase of I2. e-1 , thereby changing the size of the fault current and achieving the purpose of current limiting.

[0064] If the control I2 remains unchanged, it can be known from formula (4):

[0065]

[0066] Combining equations (7) and (8), the current limiting impedance at this time is The power delivered by the DC / AC converter is

[0067] If the amplitude of I2 remains unchanged and the phase is adjusted, according to equation (6), the steady-state value of the fault current can be expressed as By adjusting I2 and U s The angle α is 360 degrees phase modulation. When α = 90°, jωM s I2 and U s Completely reverse, at this time, in addition to the current limiting inductor L s1 , and equivalently connected to the amplitude of ωM s I2 is a reverse voltage source, which makes the current limiting effect optimal. Assume that I2 after phase modulation satisfies the relationship jωMs I2=-cU s , substituting it into equation (7), the current limiting impedance when α = 90° is: The power delivered by the DC / AC converter is Because jωM s I2 and U2 are inversely related, c is a positive scalar constant, and its value is

[0068] Therefore, the value of I2 can be controlled when no fault occurs, so that the voltage across the primary winding of the series transformer is 0, thereby isolating the impact of the current limiting link on the main circuit. When a ground fault occurs, at that moment, due to a certain delay in fault detection, the control method remains unchanged, and the phase and amplitude of I2 also remain unchanged. From the above analysis, it can be seen that at this time, there is a certain equivalent impedance on the series transformer, and the main circuit current I1 is reduced to a certain extent. When the fault detection link detects a fault, the current limiting link switches to the mode of adjusting the I2 phase and adjusts it to α = 90°. At this time, jωM s I2 and U2 are in opposite directions, which is equivalent to adding a reverse voltage source in the main circuit. The current limiting effect is best and can effectively limit the size of the fault current.

[0069] When the self-inductance of the primary and secondary coils of the series transformer is determined, the higher the mutual inductance coupling coefficient k, the smaller the initial compensation current, which helps to reduce the capacity of the power supply connected to the secondary side. In addition, increasing k can also reduce the output power of the converter during normal system operation and current limiting operation. The level of k does not affect the steady-state current limiting impedance, and thus does not affect the current limiting effect.

[0070] The voltage compensation current limiting function, due to the isolation of the series transformer, prevents the fault current from passing through the power electronic devices, avoids the direct impact of the fault current, and enhances the flexibility of the converter power design. At the moment of the short-circuit fault, the current injected on the secondary side of the series transformer maintains the previous compensation value unchanged, and the device will automatically operate in the current limiting mode, and the current limiting link responds without delay. After the amplitude and phase of the injected current are adjusted, it will switch to a more efficient current limiting mode, and the current limiting capability will be further enhanced. The current limiting can act repeatedly and cooperate with the system reclosing. The series transformer always operates in a non-saturated state, avoiding the harmonics caused by the magnetic circuit saturation and ensuring the linearity of the current limiting impedance.

[0071] The current control method of the DC / AC converter is as follows: Figure 6 As shown, fixed switching frequency PWM current control is adopted, and the PWM carrier frequency remains unchanged. * The current error Δi=i is calculated by adding the feedback current i *-i, after passing through the PI regulator, is compared with the PWM carrier to generate a PWM signal to drive the converter. The deviation between the given current and the feedback current is compared with the fixed-frequency PWM carrier after passing through the proportional-integral amplifier to generate a switching signal, thereby forming a control system that minimizes the deviation and avoids fluctuations in the switching frequency.

[0072] Furthermore, the fault detection algorithm can automatically trigger current limiting, so the time margin requirement for fault detection is relatively low, so the requirement for rapidity can be appropriately relaxed. It is possible to consider using single-phase current as a reference signal and construct a virtual three-phase current from it. A For example, the specific operation method is: A Delay 60° to get -i C , by i B =-i A -i C Find i B , and then from the single-phase current i A Virtually construct the three-phase current i A 、i B 、i C The fundamental amplitude is obtained by performing dq0 transformation on the constructed virtual three-phase current, and the sudden change of the fundamental amplitude is used as a sign of short-circuit fault.

[0073] Analyze the general situation, set the effective value of the fundamental current in normal operation to I, the initial phase to zero, and the effective value of the nth harmonic of the disturbance signal to I n , the initial phase is θ n , and according to the exponential law Attenuation, then

[0074] When a short circuit occurs, the current phase and amplitude will change, and in addition to the periodic AC component, there will also be an exponentially decaying DC component. Assume that the phase change is α f , the amplitude jumps to I f , the DC component is (T n is the time constant of the fault circuit), and still contains the previous high-frequency components, the fault current can be expressed as

[0075] For fault current i Af (t) is subjected to narrow bandpass filtering to obtain its fundamental component, which is then used as a reference current signal to transform the virtual three-phase current, as shown in the following formula:

[0076]

[0077]

[0078] Among them, I df ,I qf ,I 0f are the d-axis, q-axis, and 0-axis components after the virtual three-phase current transformation, and P is the abc-dq0 transformation. df and I qf The measured and calculated values ​​are known quantities, and the fundamental amplitude and phase jump of the fault current are obtained as formula (11). The sudden change of the fundamental amplitude is taken as the main sign of short-circuit fault, and a threshold I is set. T If the fundamental amplitude is greater than or equal to I T , it is judged to be a fault, otherwise the system is considered to be in normal operation.

[0079]

[0080] The detection algorithm based on dq0 transformation itself has good real-time performance, but the introduction of virtual three-phase causes an inherent delay of 1 / 6 of the power frequency cycle in fault detection.

[0081] The system corresponding to the method includes:

[0082] A detection unit is used to detect the load current in the distribution network. If the load current is greater than a set threshold, a fault occurs; otherwise, no fault occurs;

[0083] The first control unit is used to control the secondary winding current of the series transformer when no fault occurs, so that the primary side voltage of the series transformer is compensated to 0, the equivalent impedance of the series transformer connected to the main circuit is 0, and the series transformer has no effect on the normal operation of the system;

[0084] The delay control unit is used for the moment when a fault occurs. The fault detection link is delayed for a period of time. During the delay period, the phase and amplitude of the current on the secondary side of the series transformer are controlled to remain unchanged by default, and the fault current is limited to a certain extent.

[0085] The fault current limiting unit is used to control the amplitude and phase of the secondary winding current of the series transformer through the converter in the hybrid distribution transformer after detecting a fault, so that the primary side voltage of the series transformer is not zero, that is, to generate a current limiting voltage and provide a reverse voltage to the main circuit to suppress the fault current.

[0086] An electronic device for storing and executing the method, the device comprising:

[0087] A memory storing executable program code;

[0088] a processor coupled to the memory;

[0089] The processor calls the executable program code stored in the memory to execute the steps of the distribution network fault current limiting method based on a hybrid distribution transformer.

[0090] A computer-readable storage medium for storing and executing the method, wherein the computer-readable storage medium stores computer instructions, and when the computer instructions are called, they are used to execute the steps of the distribution network fault current limiting method based on a hybrid distribution transformer.

[0091] Simulation experiment:

[0092] This embodiment builds Figure 2 The fault current limiting simulation model based on the hybrid distribution transformer is shown in FIG. Wherein, the line impedance Z1 = (0.19 + 2.16j) Ω, the load impedance is Z2 = (0.19 + 2.16j) Ω, the frequency f = 50 Hz, and the self-inductance of the series transformer is L s1 =L s2 =10mH, mutual inductance is M s =9mH, DC capacitor C = 2000μF, LC filter parameters are L2 = 6mH, C2 = 30μF. When t = 0.1s, a three-phase ground short circuit fault occurs, and the simulation results are Figure 7 As shown in (a) and (b). When no fault occurs, the main circuit current is consistent with that when no current limiting link is installed, and the primary voltage of the series transformer is approximately equal to 0, indicating that the current limiting link has no effect on the main circuit, which is consistent with the theoretical analysis. When a three-phase grounding fault occurs, the main circuit current without the current limiting link quickly rises to about 175A, while the current peak with the current limiting link is only about 80A, a decrease of about 53%. The stabilized current drops from 100A to 35A, a decrease of about 65%. At this time, the voltage phase of the series transformer is opposite to the voltage phase of the hybrid distribution transformer on the main circuit, which is equivalent to providing a reverse voltage source to limit the overcurrent. The simulation proves that the proposed fault current limiting method based on the hybrid distribution transformer can have a good current limiting effect, which is equivalent to compensating for the voltage drop caused by the short circuit fault, and helps the hybrid distribution transformer to achieve fault isolation and active support under power grid faults.

Claims

1. A distribution network fault current limiting method based on a hybrid distribution transformer, characterized in that: include: Detect the load current in the hybrid distribution transformer; when no fault occurs, the load current is less than the set threshold, and the secondary winding current of the series transformer is controlled so that the primary side voltage of the series transformer is compensated to 0, and the equivalent impedance of the series transformer connected to the main circuit is 0, and the series transformer has no effect on the normal operation of the system; After a fault occurs, the fault detection link is delayed for a period of time. During the delay period, the phase and amplitude of the current on the secondary side of the series transformer are controlled to remain unchanged by default, and the fault current is limited to a certain extent; When it is detected that the load current is greater than the set threshold, a fault is determined to have occurred. The amplitude and phase of the secondary winding current of the series transformer are controlled by the converter in the hybrid distribution transformer so that the primary side voltage of the series transformer is not zero, that is, a current limiting voltage is generated to provide a reverse voltage to the main circuit to suppress the fault current.

2. The distribution network fault current limiting method based on hybrid distribution transformer according to claim 1 is characterized in that: When no fault occurs, the equivalent impedance Z of the series transformer connected to the main circuit e for: Among them, L s1 , L s2 are the primary and secondary self-inductances of the series transformer, M s is the mutual inductance of the series transformer, I1 is the primary winding current of the series transformer, I2 is the secondary winding current of the series transformer, ω is the angular frequency, and j is the imaginary unit.

3. The distribution network fault current limiting method based on hybrid distribution transformer according to claim 1 is characterized in that: During the delay period, the equivalent impedance Z of the series transformer connected to the main circuit e-2 for: Among them, Z1 is the line impedance, Z2 is the load impedance, L s1 is the self-inductance of the primary side of the series transformer, ω is the angular frequency, and j is the imaginary unit.

4. The distribution network fault current limiting method based on hybrid distribution transformer according to claim 1 is characterized in that: When a fault is detected, the equivalent impedance Z of the series transformer connected to the main circuit e-3 for: Where Z1 is the line impedance, L s1 is the self-inductance of the primary side of the series transformer, ω is the angular frequency, j is the imaginary unit, M s is the series transformer mutual inductance, Z2 is the load impedance, U s It is an AC voltage source equivalent to the secondary winding of a three-winding transformer connected to the primary winding of a series transformer.

5. The distribution network fault current limiting method based on hybrid distribution transformer according to claim 1 is characterized in that: Use the virtual three-phase current method to detect the load current, including: The load current is taken as the A-phase current of the virtual three-phase current. The A-phase current is delayed by 60° to obtain the anti-phase current of the C-phase current. The B-phase current is calculated based on the A-phase current and the C-phase current to construct a virtual three-phase current. The fundamental amplitude is obtained by performing dq0 transformation on the constructed virtual three-phase current. The sudden change of the fundamental amplitude is used as a sign of a short-circuit fault.

6. The distribution network fault current limiting method based on hybrid distribution transformer according to claim 1 is characterized in that: The secondary winding current of the series transformer is realized by coordinating the AC / DC converter and the DC / AC converter in the hybrid distribution transformer through the PWM controller; The AC / DC converter adopts a double closed-loop decoupled proportional-integral control structure with a voltage outer loop and a current inner loop, thereby controlling the DC side capacitor voltage to remain at a reference value, and absorbing or emitting reactive power to complete reactive power compensation for the grid by controlling the output reactive current; The current control method of the DC / AC converter adopts fixed switching frequency PWM current control. The PWM carrier frequency remains unchanged. * The current error Δi=i is calculated by adding the feedback current i * -i, after being compared with the PWM carrier by the PI regulator, generates a PWM signal to drive the DC / AC converter.

7. A distribution network fault current limiting method based on a hybrid distribution transformer according to claim 1, characterized in that: The amplitude and phase of the current limiting voltage are dynamically adjusted according to the type of fault; the fault current is detected through the fault detection link, and different fault types are judged according to the size of each phase current, and then the secondary winding current reference value of the series transformer is set according to the fault type.

8. A distribution network fault current limiting system based on a hybrid distribution transformer, characterized in that: include: A detection unit is used to detect the load current in the distribution network. If the load current is greater than a set threshold, a fault occurs; otherwise, no fault occurs; The first control unit is used to control the secondary winding current of the series transformer when no fault occurs, so that the primary side voltage of the series transformer is compensated to 0, the equivalent impedance of the series transformer connected to the main circuit is 0, and the series transformer has no effect on the normal operation of the system; The delay control unit is used for the moment when a fault occurs. The fault detection link is delayed for a period of time. During the delay period, the phase and amplitude of the current on the secondary side of the series transformer are controlled to remain unchanged by default, and the fault current is limited to a certain extent. The fault current limiting unit is used to control the amplitude and phase of the secondary winding current of the series transformer through the converter in the hybrid distribution transformer after detecting a fault, so that the primary side voltage of the series transformer is not zero, that is, to generate a current limiting voltage and provide a reverse voltage to the main circuit to suppress the fault current.

9. An electronic device, characterized in that: The device comprises: A memory storing executable program code; a processor coupled to the memory; The processor calls the executable program code stored in the memory to execute the steps of the distribution network fault current limiting method based on a hybrid distribution transformer as described in any one of claims 1 to 7.

10. A computer-readable storage medium, characterized in that: The computer-readable storage medium stores computer instructions, and when the computer instructions are called, they are used to execute the steps of the distribution network fault current limiting method based on a hybrid distribution transformer as described in any one of claims 1 to 7.

Images