A method for suppressing secondary harmonic current in a bipolar direct current system

Abstract

The application discloses a kind of bipolar direct current system secondary harmonic current suppression method, including the following steps: step one, by modulus transformation, establish the symmetrical component model of bipolar microgrid of AC-DC interconnected containing three active bridge direct current flow controller;Step two, based on the symmetrical component model of bipolar microgrid of AC-DC interconnected containing TAB-PFC, establish the voltage and current double-loop controller of full-bridge converter embedded with improved wave trap, increase the secondary harmonic impedance of TAB-PFC by the voltage and current double-loop controller of FBC embedded with improved wave trap, inhibit bipolar direct current system secondary harmonic current.

Description

Technical Field

[0001] This invention relates to the field of harmonic current suppression in bipolar DC systems, specifically a method for suppressing second harmonic current in bipolar DC systems. Background Technology

[0002] Typically, DC microgrids are interconnected to mitigate the impact of weather-related fluctuations in renewable energy output, reduce energy storage demand and operating costs, and increase reliability. Furthermore, DC microgrids can be connected to AC mains or AC microgrids via grid-interface converters (GICs) to flexibly achieve power balancing and energy trading. In small-scale DC microgrids, single-phase bidirectional converters can be used as GICs. When a single-phase GIC is connected to the grid and operates at unity power factor, a second harmonic will inevitably appear in the DC bus voltage. Under unbalanced voltage, this harmonic will also be generated on the DC side of a three-phase GIC. The presence of this second harmonic current will increase the instantaneous value of the transformer current in the distributed generation converter, increase core losses, and even lead to magnetic saturation. Moreover, if the distributed generation converter implements soft switching, the instantaneous value of the inductor current will be lower than the average value at the trough of the second harmonic current, affecting the implementation of soft switching and thus increasing switching losses. If the second harmonic current flows into the line, all loads along the line will suffer the aforementioned risks, affecting the normal operation of each load. Currently, the problem of second harmonic current is addressed through hardware and control solutions. Hardware solutions include increasing bus capacitance, using different GIC topologies, and installing active power decoupling circuits. The aim is to eliminate bus voltage ripple and reduce second harmonic current in the converter through hardware circuitry. However, these methods rely on prior knowledge of the controlled system to achieve high performance, are significantly affected by the system's operating mode, and require more expensive DC bus voltage or hardware modifications. Summary of the Invention

[0003] The purpose of this invention is to overcome the shortcomings of the prior art and provide a method for suppressing second harmonic current in a bipolar DC system, comprising the following steps: Step 1: Establish a symmetrical component model of an AC / DC interconnected bipolar microgrid containing TAB-PFC through modulus transformation; Step 2: Based on the symmetrical component model of the AC / DC interconnected bipolar microgrid with TAB-PFC, establish an FBC voltage and current dual-loop controller with an embedded improved notch filter. The FBC voltage and current dual-loop controller with an embedded improved notch filter increases the second harmonic impedance of TAB-PFC and suppresses the second harmonic current of the bipolar DC system.

[0004] Furthermore, the aforementioned method of establishing a symmetrical component model of an AC / DC interconnected bipolar microgrid containing TAB-PFC through modulus transformation includes: connecting the symmetrical component circuit model of TAB-PFC, the symmetrical component circuit model of the load, and the symmetrical component circuit model of the transmission line in the AC / DC interconnected bipolar microgrid to obtain the symmetrical component model of the AC / DC interconnected bipolar microgrid.

[0005] Furthermore, a state-space model of TAB-PFC is performed, and its large-signal model is shown in the following equation. In the formula: T s The sampling frequency; in, In the formula: L p , L n These are the positive and negative filter inductors, respectively. C p and C n These are the positive and negative filter capacitors, respectively. d p , d n These represent the duty cycles of the positive and negative electrodes of the TAB-PFC, respectively. After symmetric component transformation, and assuming R Lp = R Ln = R L , L p = L n = L , C p = C n = C The large-signal model of the symmetric component of TAB-PFC is shown in the following equation: in A m and B m As shown in the following formula: In the formula: iL0 , i L1 These represent the common-mode and differential-mode components of the output filter inductor current, respectively, for the positive and negative terminals. v k0 and v k1 These are the common-mode and differential-mode components of the positive and negative output voltages of the TAB-PFC, respectively. i 0、 i 1 represents the common-mode and differential-mode components of the current in the positive and negative transmission lines, respectively. v g0 , v g1 These are the common-mode and differential-mode components of the TAB-PFC input voltage, representing the positive and negative terminals, respectively. d 0、 d 1 represents the common-mode component and differential-mode component of the duty cycle of the positive and negative electrodes of the TAB-PFC, respectively.

[0006] Furthermore, the symmetrical component model of the load obtained through modulus transformation is as follows: v ld1 and v ld0 These represent the differential-mode and common-mode components of the positive and negative voltages on the load side, respectively. i ld1 and i ld0 These represent the differential-mode and common-mode components of the positive and negative load currents, respectively. Y 1d , I 1d These are the equivalent differential-mode (common-mode) admittance and differential-mode current source of the positive and negative loads, respectively, via Norton equivalent.

[0007] Furthermore, the aforementioned FBC voltage-current dual-loop controller with embedded improved notch filter includes: An inductor current closed loop is introduced on the basis of single voltage loop control to form a voltage and current dual closed loop control. A notch filter is embedded in the control loop to increase the second harmonic impedance of TAB-PFC. The improved notch filter expression is as follows: In the formula: ω c 2 is the characteristic angular frequency of the notch filter. ξ 1 represents the notch depth, 2 represents the notch depth. ξ 2 represents the filter bandwidth. α The deviation factor is greater than 1. The beneficial effects of this invention are: by modulus transformation, a symmetrical component model of an AC / DC interconnected bipolar microgrid containing TAB-PFC is derived; based on the symmetrical component model, a dual-loop control block diagram of FBC voltage and current with an embedded improved notch filter is proposed, and the second harmonic impedance of TAB-PFC is increased to suppress the second harmonic current of the bipolar DC system. Attached Figure Description

[0008] Figure 1 This is a flowchart illustrating a method for suppressing second harmonic current in a bipolar DC system. Figure 2 A simplified schematic diagram of a bipolar microgrid structure with TAB-PFC AC / DC interconnection; Figure 3 A schematic diagram of the lumped parameter model of a bipolar DC microgrid transmission line; Figure 4 A schematic diagram of the equivalent modulus circuit of a bipolar DC microgrid transmission line; Figure 5 A schematic diagram of the equivalent circuit model of the load; Figure 6 Schematic diagram of the symmetrical component model of the load; Figure 7 Symmetric component model of AC / DC interconnected bipolar microgrid with TAB-PFC. Detailed Implementation

[0009] The technical solution of the present invention will be further described in detail below with reference to the accompanying drawings, but the scope of protection of the present invention is not limited to the following description.

[0010] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention; that is, the described embodiments are only a part of the embodiments of the invention, and not all of them. The components of the embodiments of the invention described and shown in the accompanying drawings can generally be arranged and designed in various different configurations.

[0011] Therefore, the following detailed description of the embodiments of the invention provided in the accompanying drawings is not intended to limit the scope of the claimed invention, but merely to illustrate selected embodiments of the invention. All other embodiments obtained by those skilled in the art based on the embodiments of the invention without inventive effort are within the scope of protection of the invention. It should be noted that relational terms such as "first" and "second" are used only to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations.

[0012] Furthermore, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or apparatus. Without further limitation, an element defined by the phrase "comprising one..." does not exclude the presence of other identical elements in the process, method, article, or apparatus that includes said element.

[0013] The features and performance of the present invention will be further described in detail below with reference to embodiments.

[0014] like Figure 1 As shown, specifically, it includes the following steps: S1: Derive the symmetrical component model of a bipolar microgrid with AC / DC interconnection and TAB-PFC by modulus transformation; S2: Based on the symmetrical component model, a dual-loop control block diagram for FBC voltage and current with an embedded improved notch filter is proposed to increase the second harmonic impedance of TAB-PFC to suppress the second harmonic current of the bipolar DC system.

[0015] The symmetrical component model of the AC / DC interconnected bipolar microgrid containing TAB-PFC, as described in step S1, is derived through modulus transformation: By using modulus transformation, an equivalent decoupled circuit model of a bipolar microgrid can be obtained. This simplifies the analysis of bipolar microgrids and makes it easier to understand and study their steady-state and transient characteristics. Furthermore, because the symmetrical component method can decouple the transmission line model, many control methods or analysis theories originally proposed for unipolar microgrids can be more easily extended to bipolar microgrids.

[0016] ① Symmetric component model of TAB-PFC A simplified schematic diagram of an AC / DC interconnected bipolar microgrid structure with TAB-PFC is shown below. Figure 2 As shown in the figure v sp , v sn These are the positive and negative power supply voltages, respectively. v gp , v gn These represent the positive and negative input voltages of the TAB-PFC, respectively. Q1~Q8 are the switching transistor numbers. i Lp , i Ln These are the output filter inductor currents for the positive and negative terminals, respectively. L fp , L fn andC fp , C fn The output filter inductor and capacitor are for the positive and negative terminals, respectively. i p , i n These represent the currents of the positive and negative poles of the transmission line, respectively. v kp and v kn These are the positive and negative output voltages of the TAB-PFC, respectively. L lp , L ln , R lp , R ln These represent the inductance and resistance of the positive and negative transmission lines, respectively. L lnu , R lnu These are the neutral line inductance and resistance, respectively. C busp , C busn These are the positive and negative bus capacitors, respectively. v ldp , v ldn These are the positive and negative voltages on the load side, respectively. Y ldp , I ldp , Y ldn , I ldn These are the positive and negative loads via Norton's equivalent admittance and current source, respectively.

[0017] State-space modeling of TAB-PFC is performed, and its large-signal model is shown in equation (1). (1) In the formula: T s The sampling frequency.

[0018] in, (2) (3) In the formula: L p , L n These are the positive and negative filter inductors, respectively. C p and Cn These are the positive and negative filter capacitors, respectively. d p , d n These represent the duty cycles of the positive and negative electrodes of the TAB-PFC, respectively.

[0019] After symmetric component transformation, and assuming R Lp = R Ln = R L , L p = L n = L , C p = C n = C Then the symmetric component large signal model of TAB-PFC is shown in equation (4), where A m and B m As shown in equations (5) and (6).

[0020] (4) (5) (6) In the formula: i L0 , i L1 These represent the common-mode and differential-mode components of the output filter inductor current, respectively, for the positive and negative terminals. v k0 and v k1 These are the common-mode and differential-mode components of the positive and negative output voltages of the TAB-PFC, respectively. i 0、 i 1 represents the common-mode and differential-mode components of the current in the positive and negative transmission lines, respectively. v g0 , v g1 These are the common-mode and differential-mode components of the TAB-PFC input voltage, representing the positive and negative terminals, respectively. d 0、 d 1 represents the common-mode component and differential-mode component of the duty cycle of the positive and negative electrodes of the TAB-PFC, respectively.

[0021] ② Symmetrical component model of the line The transmission line model of a bipolar DC microgrid expressed in terms of lumped parameters is as follows: Figure 3As shown in the figure v p1 , v n1 These are the positive and negative power supply voltages, respectively. i p1 , i n1 These are the positive and negative pole transmission line currents, respectively. C lp , C ln These are the positive and negative capacitors of the transmission line, respectively. i p2 , i n2 These are the positive and negative load-side currents, respectively. v p2 , v n2 These are the positive and negative load-side voltages, respectively. R lne , L lne , i ne1 , i ne2 These are the neutral resistance, inductance, line current, and load-side current, respectively. R lp =R ln =R lne =R l ，L lp =L ln =L lne = L l ，C lp =C ln =C lne =C l .

[0022] The equivalent circuit of the symmetrical component of the circuit is obtained by modulus change, such as... Figure 4 As shown in the figure v 01 , v 11 These are the common-mode and differential-mode components of the positive and negative power supply side voltages, respectively. i 01 , i 11These are the common-mode and differential-mode components of the transmission line currents for the positive and negative poles, respectively. L 10 , L 11 and C 10 , C 11 These represent the common-mode and differential-mode components of the output filter inductors and capacitors, respectively, for the positive and negative terminals. v 02 , v 12 These are the common-mode and differential-mode components of the voltage on the positive and negative load sides, respectively.

[0023] ③ Symmetrical component model of load The equivalent circuit model of the load is as follows: Figure 5 As shown in the figure v ldp and v ldn These represent the positive and negative voltages on the load side, respectively. i ldp and i ldn These represent the positive and negative load currents, respectively. The symmetrical component model of the load is obtained through modulus transformation, as shown below. Figure 6 As shown in the figure v ld1 and v ld0 These represent the differential-mode and common-mode components of the positive and negative voltages on the load side, respectively. i ld1 and i ld0 These represent the differential-mode and common-mode components of the positive and negative load currents, respectively. Y 1d , I 1d These are the equivalent differential-mode (common-mode) admittance and differential-mode current source of the positive and negative loads, respectively, via Norton equivalent.

[0024] By connecting the symmetrical component circuit models of the TAB-PFC section, load section, and transmission line section in the obtained AC / DC interconnected bipolar microgrid, the symmetrical component model of the AC / DC interconnected bipolar microgrid can be obtained, such as... Figure 7 As shown in the figure V s This is the DC bus voltage. I ld0 , I ld1These are the equivalent common-mode and differential-mode current sources, respectively. This modulus circuit model transforms the bipolar system into two parts: common-mode and differential-mode components. If the bipolar system is perfectly balanced, the common-mode and differential-mode circuits are completely decoupled, which is of great significance for the characteristic analysis and control system design of AC / DC interconnected bipolar microgrids. The symmetric component model described in step S2 proposes a dual-loop control block diagram for FBC voltage and current with an embedded improved notch filter, increasing the second harmonic impedance of TAB-PFC to suppress the second harmonic current of the bipolar DC system. Figure 7 In this system, the common-mode and differential-mode circuits are completely decoupled and structurally identical. Therefore, the controller design for the TAB-PFC modulus circuit is the same for both the differential-mode and common-mode models, and the analysis is based on the common-mode circuit. The block diagram of the designed FBC voltage-current dual-loop control with embedded improved notch filter is shown below. Figure 7 As shown in the figure V kpref This is the positive reference voltage. V kpref The negative reference voltage is T, and T is the conversion module that converts the polar component to the modulus component. V k1ref This is the differential mode reference voltage. V k0ref This is the common-mode reference voltage. G cv1 ( s () is a differential mode voltage regulator. G cv0 ( s This is a common-mode voltage regulator. i L1ref This is the differential mode reference current. i L0ref For common-mode reference current, G ci1 ( s This is a differential mode current regulator. G ci0 ( s ) is a common-mode current regulator, T -1 The module is an inverse conversion module that converts the modulus components into polar components; PWM is a pulse width modulation module.

[0025] To effectively reduce the second harmonic current on the line side and increase the closed-loop impedance amplitude on the line side, an inductor current closed loop is introduced on the basis of single voltage loop control, forming a voltage and current dual closed-loop control. Simultaneously, a notch filter is embedded in the control loop to increase the second harmonic impedance of the TAB-PFC. To simultaneously achieve fast dynamic response and good stability margin, the improved notch filter expression used in this invention is as follows: (7) In the formula: ω c 2 is the characteristic angular frequency of the notch filter. ξ 1 represents the notch depth, 2 represents the notch depth. ξ 2 represents the filter bandwidth. α The deviation factor is greater than 1.

[0026] The above description is merely a preferred embodiment of the present invention. It should be understood that the present invention is not limited to the forms disclosed herein and should not be construed as excluding other embodiments. It can be used in various other combinations, modifications, and environments, and can be altered within the scope of the concept described herein through the above teachings or related technologies or knowledge. Modifications and variations made by those skilled in the art that do not depart from the spirit and scope of the present invention should be within the protection scope of the appended claims.

Claims

1. A method of suppressing secondary harmonic currents in a bipolar direct current system, characterized by, Includes the following steps: Step 1: Establish a symmetrical component model of an AC / DC interconnected bipolar microgrid containing TAB-PFC through modulus transformation; Step 2: Based on the symmetrical component model of the AC / DC interconnected bipolar microgrid with TAB-PFC, establish an FBC voltage and current dual-loop controller with an embedded improved notch filter. The FBC voltage and current dual-loop controller with an embedded improved notch filter increases the second harmonic impedance of TAB-PFC and suppresses the second harmonic current of the bipolar DC system. The symmetrical component circuit model of TAB-PFC in the aforementioned AC / DC interconnected bipolar microgrid symmetrical component model includes: The state-space model of TAB-PFC is shown in the following equation: In the formula: T s The sampling frequency; v kp and v kn These are the positive and negative output voltages of the TAB-PFC, respectively. i p , i n These are the currents of the positive and negative poles of the transmission line, respectively. v gp , v gn These are the positive and negative input voltages of the TAB-PFC, respectively. i Lp , i Ln These are the output filter inductor currents for the positive and negative terminals, respectively. in, In the formula: L p , L n These are the positive and negative filter inductors, respectively. C p and C n These are the positive and negative filter capacitors, respectively. d p , d n These represent the duty cycles of the positive and negative electrodes of the TAB-PFC, respectively. R Lp , R Ln These are the resistances of the positive and negative poles of the transmission line, respectively. After symmetric component transformation, and assuming R Lp = R Ln = R L , L p = L n = L , C p = C n = C The symmetrical component circuit model of TAB-PFC is shown in the following equation: in A m and B m As shown in the following formula: In the formula: i L0 , i L1 These represent the common-mode and differential-mode components of the output filter inductor current, respectively, for the positive and negative terminals. v k0 and v k1 These are the common-mode and differential-mode components of the positive and negative output voltages of the TAB-PFC, respectively. i 0、 i 1 represents the common-mode and differential-mode components of the current in the positive and negative transmission lines, respectively. v g0 , v g1 These are the common-mode and differential-mode components of the TAB-PFC input voltage, representing the positive and negative terminals, respectively. d 0、 d 1 represents the common-mode component and differential-mode component of the duty cycle of the positive and negative electrodes of the TAB-PFC, respectively; The aforementioned FBC voltage-current dual-loop controller with embedded improved notch filter includes: An inductor current closed loop is introduced on the basis of single voltage loop control to form a voltage and current dual closed loop control. An improved notch filter is embedded in the control loop to increase the second harmonic impedance of TAB-PFC. The expression of the improved notch filter is as follows: In the formula: ω c 2 is the characteristic angular frequency of the notch filter. ξ 1 represents the notch depth, 2 represents the notch depth. ξ 2 represents the filter bandwidth. α The deviation factor is greater than 1.

2. The method for suppressing second harmonic current in a bipolar DC system according to claim 1, characterized in that, The aforementioned method of establishing a symmetrical component model of an AC / DC interconnected bipolar microgrid containing TAB-PFC through modulus transformation includes: connecting the symmetrical component circuit model of TAB-PFC, the symmetrical component circuit model of the load, and the symmetrical component circuit model of the transmission line in the symmetrical component model of the AC / DC interconnected bipolar microgrid to obtain the symmetrical component model of the AC / DC interconnected bipolar microgrid.

3. The method for suppressing second harmonic current in a bipolar DC system according to claim 2, characterized in that, The symmetrical component circuit model of the load includes: v ld1 and v ld0 These represent the differential-mode components and common-mode components of the positive and negative voltages on the load side, respectively. i ld1 and i ld0 These represent the differential-mode components and common-mode components of the positive and negative load currents, respectively. Y ld , I ld These are the positive and negative loads via Norton's equivalent differential-mode admittance and differential-mode current source, respectively. Y ldp , I ldp The load between the positive bus and the neutral line is represented by the Norton equivalent admittance and current source, respectively. Y ldn , I ldn The load between the negative bus and the neutral line is transmitted via Norton's equivalent admittance and current source, respectively. v ldp This is the voltage between the positive busbar and the neutral line; v ldn This is the voltage between the negative busbar and the neutral line.

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