A flexible interconnection system for a distribution area for suppressing circulating current

Abstract

The application discloses a kind of for inhibiting loop current's transformer area flexible interconnection system, wherein, first flexible interconnection device and second flexible interconnection device include DC common mode inductance, parallel four-phase bridge arm, first controller and second controller, two DC common mode inductances are connected to the positive and negative poles of four-phase bridge arm respectively and are respectively as positive and negative DC side port, the midpoint of four-phase bridge arm is respectively as four-phase AC side port, the control pole of the first three-phase bridge arm in four-phase bridge arm is respectively connected to first controller, the control pole of the last phase bridge arm is respectively connected to second controller;First, second flexible interconnection device is connected by positive and negative DC side port, and first, second flexible interconnection device is connected by four-phase AC side port four phase lines of the distribution transformer of first, second distribution transformer area. The application can realize transformer area flexible interconnection without isolation, and solve the technical problem that ground loop current is too large when isolation transformer is cancelled.

Description

Technical Field

[0001] This invention relates to a flexible interconnection system for power distribution transformers to suppress circulating currents, belonging to the field of power electronics technology. Background Technology

[0002] With the introduction of the "dual carbon" target and the implementation of policies such as the construction of rooftop distributed photovoltaic systems across counties, the distribution network is experiencing a large-scale integration of distributed energy sources and charging piles, among other random power sources (loads). Distribution substations are characterized by high overload rates, variable operating conditions, and uneven load rates, posing significant challenges to power supply reliability. Without expanding the capacity of distribution substations, using power electronic devices for interconnection and coordination is a feasible solution.

[0003] Currently, most flexible interconnection devices for transformer substations in the industry use a three-arm VSC topology, with the neutral (N) line led out from the midpoint of the DC support capacitor. Because the N line current is uncontrollable, circulating current is generated on the common ground line during transformer substation interconnection. Existing technology involves adding an isolation transformer to the flexible interconnection system. While this solves the circulating current problem, the presence of the isolation transformer increases the system size, reduces efficiency, and raises costs. Summary of the Invention

[0004] The purpose of this invention is to overcome the shortcomings of the prior art and provide a flexible interconnection system for transformer substations to suppress circulating current. This system can achieve flexible interconnection of transformer substations without isolation transformers and solve the technical problem of excessive ground wire circulating current caused by the elimination of isolation transformers.

[0005] To achieve the above objectives, the present invention is implemented using the following technical solution:

[0006] This invention provides a flexible interconnection system for suppressing circulating current in power distribution areas, comprising a first power distribution area, a first flexible interconnection device, a second flexible interconnection device, and a second power distribution area. The first and second flexible interconnection devices have identical structures and each includes a DC common-mode inductor, parallel four-phase bridge arms, a first controller, and a second controller. The two DC common-mode inductors are respectively connected to the positive and negative terminals of the four-phase bridge arms, serving as positive and negative DC side ports. The midpoints of the four-phase bridge arms serve as four-phase AC side ports. The control terminals of the first three phases of the four-phase bridge arms are respectively connected to the corresponding first controller, and the control terminal of the last phase of the four-phase bridge arm is respectively connected to the corresponding second controller. The second controller includes a parallel PI controller and a quasi-resonant controller. The first and second flexible interconnection devices are connected via the positive and negative DC side ports. The first flexible interconnection device is connected to the four phase lines of the distribution transformer in the first power distribution area via the four-phase AC side ports, and the second flexible interconnection device is connected to the four phase lines of the distribution transformer in the second power distribution area via the four-phase AC side ports.

[0007] Optionally, each phase of the four-phase bridge arm adopts a two-level topology or a three-level topology.

[0008] Optionally, in the two-level topology, each phase arm includes a first power transistor and a second power transistor connected in series. The connection point of the first power transistor and the second power transistor serves as the midpoint of the corresponding phase arm, and the bases of the first power transistor and the second power transistor serve as the control electrode of the corresponding phase arm. The collectors of the first power transistors in each phase arm are connected together and serve as the positive electrode of the four-phase arm, and the emitters of the second power transistors in each phase arm are connected together and serve as the negative electrode of the four-phase arm.

[0009] Optionally, a supporting capacitor is also connected in parallel on the four-phase bridge arm.

[0010] Optionally, the four-phase AC side port is also connected to an AC side filter, which includes an L-type filter, an LC-type filter, and an LCL-type filter.

[0011] Optionally, both the first and second distribution transformer areas are 380V AC distribution transformer areas, and the neutral point of their distribution transformers is grounded.

[0012] Optionally, during low-frequency circulating current suppression, the center frequency of the quasi-resonant controller in the second controller is consistent with the frequency of the low-frequency component. If there are multiple low-frequency components, a corresponding number of quasi-resonant controllers are connected in parallel.

[0013] Optionally, during high-frequency circulating current suppression, the inductance value of the DC common-mode inductor is proportional to the voltage values ​​at the positive and negative DC side ports.

[0014] Compared with the prior art, the beneficial effects achieved by the present invention are as follows:

[0015] This invention provides a flexible interconnection system for power distribution areas to suppress circulating currents. A DC common-mode inductor is placed on both the positive and negative DC buses. For the high-frequency component of the ground current circulating through both buses, the high-frequency characteristics of the inductor significantly increase the impedance in the loop, thereby suppressing the high-frequency circulating current. The difference between the collected neutral (N) line current and the reference current is calculated by a PI controller and a quasi-resonant controller. The superimposed outputs serve as the control signal for the phase bridge arm corresponding to the N line current. The PI controller can track the DC component well, and the quasi-resonant controller can track the AC component well, thus suppressing low-frequency circulating currents. In summary, the interconnection system provided by this invention enables flexible interconnection of power distribution areas without isolation transformers, saving on the size and cost of the flexible interconnection system and improving its efficiency. Attached Figure Description

[0016] Figure 1This is a schematic diagram of a flexible interconnection system for suppressing circulating current provided in Embodiment 1 of the present invention;

[0017] Figure 2 This is a schematic diagram illustrating the working principle of the second controller provided in Embodiment 1 of the present invention;

[0018] Figure 3 This is a schematic diagram comparing ground current circulation with and without the high-frequency circulating current suppression provided in Embodiment 1 of the present invention;

[0019] Figure 4 This is a schematic diagram comparing ground current circulation with and without low-frequency circulation suppression provided in Embodiment 1 of the present invention. Detailed Implementation

[0020] The present invention will be further described below with reference to the accompanying drawings. The following embodiments are only used to more clearly illustrate the technical solution of the present invention, and should not be used to limit the scope of protection of the present invention.

[0021] Example 1:

[0022] like Figure 1 As shown, this embodiment of the invention provides a flexible interconnection system for suppressing circulating current in a distribution substation, including a first distribution substation, a first flexible interconnection device, a second flexible interconnection device, and a second distribution substation. The first and second flexible interconnection devices have the same structure and both include a DC common-mode inductor and parallel four-phase bridge arms. The two DC common-mode inductors are respectively connected to the positive and negative terminals of the four-phase bridge arms and serve as positive and negative DC side ports, respectively. (When suppressing high-frequency circulating current, the inductance value of the DC common-mode inductor is proportional to the voltage value of the positive and negative DC side ports.) The midpoints of the four-phase bridge arms serve as four-phase AC side ports (a, b, c, n). The first and second flexible interconnection devices are connected through the positive and negative DC side ports. The first flexible interconnection device is connected to the four phase lines (A, B, C, N) of the distribution transformer in the first distribution substation through the four-phase AC side ports. The second flexible interconnection device is connected to the four phase lines of the distribution transformer in the second distribution substation through the four-phase AC side ports.

[0023] Each phase arm in the four-phase bridge arm adopts a two-level topology or a three-level topology; in this embodiment Figure 1 Taking a two-level topology as an example, each phase bridge arm includes a first power transistor and a second power transistor connected in series. The connection point of the first power transistor and the second power transistor serves as the midpoint of the corresponding phase bridge arm, and the bases of the first power transistor and the second power transistor serve as the control electrode of the corresponding phase bridge arm. The collectors of the first power transistors in each phase bridge arm are connected together and serve as the positive electrode of the four-phase bridge arm, and the emitters of the second power transistors in each phase bridge arm are connected together and serve as the negative electrode of the four-phase bridge arm.

[0024] The control terminals of the first three phases of the four-phase bridge arm are respectively connected to the corresponding first controller, and the control terminal of the last phase of the four-phase bridge arm is respectively connected to the corresponding second controller. The second controller includes a parallel PI controller and a quasi-resonant controller; for example Figure 2 As shown, the parameter k in the PI controller p The parameter k is set to 2. i The parameter k in the quasi-resonant controller is set to 5. r The tuning is set to 20, and the center frequency ω r Set to 150Hz, control bandwidth ω c The whole position is 10 rad / s, where s is the Laplace parameter. During low-frequency circulating current suppression, the center frequency ω of the quasi-resonant controller in the second controller... r If the frequency of the low-frequency component is consistent with that of the low-frequency component, then a corresponding number of quasi-resonant controllers are connected in parallel.

[0025] Specifically, a supporting capacitor C is connected in parallel on each of the four-phase bridge arms, and an AC-side filter is also connected to the four-phase AC side port. The AC-side filter includes an L-type filter, an LC-type filter, and an LCL-type filter. In this embodiment... Figure 1 Taking the L-type filter as an example, the inductance value of each inductor in the L-type filter is 1mH.

[0026] Specifically, both the first and second distribution transformer areas are 380V AC transformer areas, and the neutral point of their distribution transformers is grounded.

[0027] The working principle of this implementation is as follows:

[0028] (1) High-frequency circulating current suppression is achieved using a DC common-mode inductor:

[0029] By placing the DC common-mode inductor on both the positive and negative DC buses (the connection line between the positive and negative DC side ports), the high-frequency components of the ground loop current flow through both the positive and negative DC buses simultaneously. Utilizing the inductor's high-frequency characteristics, this significantly increases the impedance in the loop, thereby suppressing the high-frequency circulating current. In this embodiment, the DC common-mode inductor is a wire-wound type with an inductance of 2mH and a rated current of 333A. Figure 3 As shown in (a), without a DC common-mode inductor, the asynchronous carrier waves in the controllers of the first and second flexible interconnect devices cause inconsistent states of the power electronic switches in the four-phase bridge arms, resulting in a high-frequency circulating current on the ground wire of the flexible interconnect system. When the carrier waves are 180° out of phase, the peak value of the high-frequency circulating current on the ground wire is approximately 30A. When the high-frequency circulating current suppression method provided by this invention is used, and a DC common-mode inductor is placed on the positive and negative DC buses, as shown in (a), Figure 3 As shown in (b), the peak value of the high-frequency circulating current of the ground wire is only about 5A, which proves the effectiveness of the method proposed in this invention.

[0030] (2) Low-frequency circulating current suppression is achieved by using the fourth bridge arm connected to the N line:

[0031] First, the current of the N-line on the flexible interconnect device is collected. Second, the reference current of the fourth phase bridge arm is set to 0. Then, the collected N-line current i n The difference between the current and the reference current is fed into the second controller, where it is processed by both the PI controller and the quasi-resonant controller. The sum of the outputs is then used as the control signal d for the fourth phase bridge arm. n In this embodiment, when the fourth phase bridge arm is not used to control the N-line current, due to the large number of unbalanced and nonlinear loads in the distribution substation, low-frequency circulating currents will be generated on the N-line and ground line. The spectrum components mainly include DC components, fundamental frequency components, and third harmonic components. This embodiment takes the third harmonic component as an example. Figure 4 As shown in (a), the peak circulating current is approximately 30A; after adopting the low-frequency circulating current suppression method provided by this invention, as... Figure 4 As shown in (b), the peak circulation value is approximately 8A, demonstrating the effectiveness of the method proposed in this invention.

[0032] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the technical principles of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A flexible interconnection system for suppressing circulating current in a transformer area, characterized by, The first flexible interconnection device and the second flexible interconnection device have the same structure and each include two DC common-mode inductors, four-phase bridge arms in parallel, a first controller, and a second controller, one end of each of the two DC common-mode inductors of the first flexible interconnection device is connected to the positive and negative poles of the four-phase bridge arms thereof, and the other end of each of the two DC common-mode inductors is connected to the positive and negative poles of the four-phase bridge arms of the second flexible interconnection device through the two DC common-mode inductors of the second flexible interconnection device; The midpoints of the four-phase bridge arms of the first flexible interconnection device and the second flexible interconnection device are used as four-phase alternating-current side ports thereof, the control poles of the first three-phase bridge arms of the four-phase bridge arms are connected to the corresponding first controllers, the control poles of the last phase bridge arm of the four-phase bridge arms are connected to the corresponding second controllers, the second controller includes a PI controller and a quasi-resonant controller in parallel, and the first flexible interconnection device is connected to four phase lines of a power distribution transformer of the first power distribution area through the four-phase alternating-current side ports thereof, and the second flexible interconnection device is connected to four phase lines of a power distribution transformer of the second power distribution area through the four-phase alternating-current side ports thereof.

2. The flexible interconnection system for suppressing circulating current of a transformer area according to claim 1, wherein, Each phase bridge arm of the four-phase bridge arms adopts a two-level topology or a three-level topology.

3. The flexible interconnection system for suppressing circulating current of a transformer area according to claim 2, characterized in that, In the two-level topology, each phase bridge arm includes first and second power tubes in series, the connection point of the first and second power tubes is used as the midpoint of the corresponding phase bridge arm, and the bases of the first and second power tubes are used as the control poles of the corresponding phase bridge arm; the collectors of the first power tubes of each phase bridge arm are connected together and used as the positive pole of the four-phase bridge arms, and the emitters of the second power tubes of each phase bridge arm are connected together and used as the negative pole of the four-phase bridge arms.

4. The flexible interconnection system for suppressing circulating current of a transformer area according to claim 1, wherein, The four-phase bridge arms are further connected in parallel with support capacitors.

5. The flexible interconnection system for suppressing circulating current of a transformer area according to claim 1, wherein, The four-phase alternating-current side ports of the first flexible interconnection device and the second flexible interconnection device are each connected with an alternating-current side filter, and the alternating-current side filter includes an L-type filter, an LC-type filter, and an LCL-type filter.

6. The flexible interconnection system for suppressing circulating current of a transformer area according to claim 1, wherein, The first power distribution area and the second power distribution area are both 380V alternating-current areas, and the neutral points of the power distribution transformers thereof are grounded.

7. The flexible interconnection system for suppressing circulating current of a transformer area according to claim 1, wherein In low-frequency circulating current suppression, the center frequency of the quasi-resonant controller in the second controller is consistent with the frequency of the low-frequency component, and if there are multiple low-frequency components, a corresponding number of quasi-resonant controllers are connected in parallel.

8. The flexible interconnection system for suppressing circulating current of a transformer area according to claim 1, wherein, In high-frequency circulating current suppression, the inductance value of the DC common-mode inductor is in a positive proportional relationship with the voltage value of the positive and negative DC side ports.

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