Current source resonance type line-to-line direct current power flow controller topology and frequency conversion control method

Abstract

The invention provides a current source resonance type inter-line direct current power flow controller topology and a frequency conversion control method. The current source resonance type inter-line direct current power flow controller topology comprises a first current source type H bridge, a second current source type H bridge, an isolation transformer and a resonance capacitor, each H bridge serves as a port and is composed of four bidirectional switches, and each bidirectional switch comprises two power MOSFETs with common source electrodes connected in series; the turn ratio of the primary side to the secondary side of the isolation transformer is 1: 1; the two resonant capacitors and the leakage inductor of the isolation transformer form a CLC resonant circuit; wherein the direct current side of the first current source type H bridge is connected with a first direct current line, and the alternating current side is connected with the primary side of the isolation transformer through the CLC resonance circuit; the direct-current side of the second current source type H bridge is connected with a second direct-current line, and the alternating-current side is connected with the secondary side of the isolation transformer through the CLC resonance circuit; during operation, the first current source type H bridge and the second current source type H bridge invert direct current of a direct current line into alternating current in a square wave form and inject the alternating current into the CLC resonance circuit.

Description

Technical Field

[0001] This invention relates to the field of electrical engineering, and more specifically, to a current source resonant type inter-line DC power flow controller topology and frequency conversion control method. Background Technology

[0002] In recent years, DC distribution systems have received widespread attention due to their advantages such as high transmission efficiency, strong controllability, and no need for reactive power management. Simultaneously, with the rapid development of key DC equipment such as DC transformers and DC circuit breakers, the feasibility of constructing multi-terminal DC distribution networks has been increasingly enhanced. In multi-terminal DC grids, power flow control lacks sufficient freedom, and the power flow on some lines cannot be actively controlled, posing a risk of exceeding limits and affecting the safe operation of the system. To address this challenge, the concept of the Inter-Line DC Power Flow Controller (IDCPFC) has been proposed. The IDCPFC is a two-port device, with each port connected in series with two adjacent DC lines, enabling flexible power flow switching between the two lines. DC distribution networks equipped with IDCPFCs can significantly improve their operational flexibility and safety.

[0003] In terms of IDCPFC topology design, existing topologies all rely on DC-side energy storage capacitors to indirectly control line power flow by adjusting their voltage. On the one hand, energy storage capacitors significantly increase the size of the IDCPFC and reduce its power density. On the other hand, the capacitive energy in the energy storage capacitors is highly susceptible to resonance with the inductive energy of the line, which not only weakens the stability of the equipment during normal operation but also causes oscillations in the line current under transient conditions (such as startup, mode switching, and fault response), affecting the operational safety of the DC grid system.

[0004] Reference 1: Qiu Peng, Wu Junjian, Lu Yi, et al. Research on novel multi-port DC power flow controller and its control strategy [J]. Zhejiang Electric Power, 2024, 43(07):86-93. DOI:10.19585 / j.zjdl.202407010. This reference designs an inter-line DC power flow controller using an MMC topology, relying on the DC capacitor submodule in the MMC to achieve power flow control. Its drawbacks are: 1) The DC capacitor is bulky, limiting the power density and application scenarios of the device; 2) The rapid switching of the DC capacitor will introduce high-frequency voltage and current ripple in the line, causing disturbance to the power grid; 3) The presence of the DC capacitor seriously weakens the dynamic characteristics of the device and may cause stability problems.

[0005] Reference 2: Zhong Xu, Zhu Miao, Chi Yongning, et al. Construction and Implementation of Composite DC Power Flow Controller [J]. Proceedings of the CSEE, 2020, 40(02):444-456. DOI:10.13334 / j.0258-8013.pcsee.181771. This reference introduces an external auxiliary power supply into the inter-line DC power flow controller, which can further improve the control degree of the device and realize active power flow regulation of the two lines. However, this topology requires a non-polar capacitor with a high capacitance value to be connected in series in the line, and relies on the DC voltage across its ends to indirectly realize the regulation of the line power flow. This design will not only significantly increase the size of the device, but also reduce its operational reliability and cause transient oscillation problems. Summary of the Invention

[0006] To address the shortcomings of existing technologies, the purpose of this invention is to provide a current source resonant type inter-line DC power flow controller topology and frequency conversion control method.

[0007] The current source resonant type inter-line DC power flow controller topology provided by the present invention includes: a first current source H-bridge, a second current source H-bridge, an isolation transformer, and two resonant capacitors; Each H-bridge serves as a port, and each H-bridge consists of four bidirectional switches, each containing two common-source power MOSFETs connected in series. The primary-to-secondary turns ratio of the isolation transformer is 1:1; The two resonant capacitors and the leakage inductance of the isolation transformer form a CLC resonant circuit. The first current source H-bridge has its DC side connected to a first DC line and its AC side connected to the primary side of the isolation transformer via the CLC resonant circuit; the second current source H-bridge has its DC side connected to a second DC line and its AC side connected to the secondary side of the isolation transformer via the CLC resonant circuit. During operation, the first current source H-bridge and the second current source H-bridge invert the DC current of the DC line into an AC current in the form of a square wave and inject it into the CLC resonant circuit.

[0008] Preferably, the topology is installed in a three-terminal ring DC distribution network, which includes three voltage source converters (VSCs) and three DC lines. The first and second VSCs operate in constant power control mode, and the third VSC operates in constant voltage control mode. The first and second DC lines are connected in series with the DC sides of the first and second current source H-bridges, respectively. One end of the third DC line is connected to the DC-side output bus of the third VSC, and the other end is connected to a common connection point, which is the junction point of the three DC lines, forming a ring network.

[0009] Preferably, based on the average value of the first DC line current under steady state. Average current of the second DC line and transmission power The operating area is divided into eight octaves, where when , When both are greater than 0, it operates in octet I or octet V; In octave I, >0, the first port operates in frequency modulation mode, each bidirectional switch operates in four-step commutation switching mode, the second port operates in rectification mode, and its switch is normally open or normally closed to form an uncontrolled rectifier bridge. In octet V, <0, the second port operates in frequency modulation mode, each bidirectional switch operates in four-step commutation switching mode, the first port operates in rectification mode, and its switch is normally open or normally closed to form an uncontrolled rectifier bridge.

[0010] Preferably, the current ratio is defined. When working in octet I, When working in octet V, ;in , These are the average values ​​of the currents in the first DC line and the second DC line, respectively; the current ratio... It is also expressed as the amplitude of the AC current on the first port. Second port AC side current amplitude The ratio, in octet I, In the octet V, .

[0011] Preferably, the CLC resonant circuit has two characteristic frequencies. and The calculation formula is:

[0012]

[0013] in, This is the capacitance value of the resonant capacitor. This is the leakage inductance of the isolation transformer.

[0014] Preferably, the switching frequency is defined as Normalized frequency Then the characteristic admittance is:

[0015] The quality factor is:

[0016] in, This is the equivalent load conductance converted to the AC side.

[0017] Preferably, the equivalent load conductance The calculation formula is:

[0018]

[0019] in, This is the equivalent load conductance on the DC side.

[0020] Preferably, in the phasor domain, the CLC resonant circuit obtains the Norton equivalent capacitance through Norton equivalent transformation. Norton equivalent current source The calculation formula is: .

[0021] Preferably, the current ratio The expression is:

[0022] in, The imaginary unit, Angular frequency, It is the resonant angular frequency.

[0023] The frequency conversion control method for the current source resonant type inter-line DC power flow controller topology provided by the present invention includes: Step 1: Obtain the reference value and feedback value of the first DC line current; Step 2: Calculate the error between the reference value and the feedback value; Step 3: Process the error using a PI controller to generate an action signal; Step 4: Subtract the frequency feedforward value from the absolute value of the action signal to obtain the normalized switching frequency; Step 5: Determine the operating limit based on the sign of the action signal. If the action signal is negative, operate in limit I; if the action signal is positive, operate in limit V. Step 6: Generate the corresponding switch drive signal based on the normalized switching frequency and operating limit.

[0024] Compared with the prior art, the present invention has the following beneficial effects: (1) This solution adopts a current source type bridge topology, which eliminates the need for DC-side capacitors in the traditional IDCPFC topology, thus significantly reducing the device size of IDCPFC. (2) The novel CLC resonant circuit proposed in this scheme has excellent power flow regulation characteristics and can achieve wide-range regulation of the current ratio between the two lines within a narrow frequency range. (3) The frequency feedforward-based frequency conversion control method proposed in this scheme not only has a fast start-up characteristic, but also realizes the smooth and seamless switching of the device between different operating limits, giving it excellent dynamic characteristics. Attached Figure Description

[0025] Other features, objects, and advantages of the present invention will become more apparent from the following detailed description of non-limiting embodiments with reference to the accompanying drawings: Figure 1 It is a CLC-IDCPFC topology; Figure 2 It is a three-terminal DC distribution network containing CLC-IDCPFC; Figure 3 This is the internal AC equivalent loop of CLC-IDCPFC; Figure 4 Operating limits and modulation strategies for CLC-IDCPFC; Figure 5 A phasor domain model of the internal AC loop of CLC-IDCPFC; Figure 6 The phasor domain Norton equivalent circuit for the internal AC loop of CLC-IDCPFC; Figure 7 Current ratio M With normalized frequency f n and quality factor Q The change curve; Figure 8 The frequency feedforward closed-loop control strategy is adopted for CLC-IDCPFC; Figure 9 During the CLC-IDCPFC startup process f n Transient waveform; Figure 10 Transient waveforms of various electrical quantities during the CLC-IDCPFC startup process; Figure 11 The steady-state waveform of CLC-IDCPFC operating at limit V; Figure 12 During the CLC-IDCPFC operating condition switching process f n Transient waveform; Figure 13 Transient waveforms of various electrical quantities during the CLC-IDCPFC operating condition switching process; Figure 14The steady-state waveform of CLC-IDCPFC operating at limit I. Detailed Implementation

[0026] The present invention will now be described in detail with reference to specific embodiments. These embodiments will help those skilled in the art to further understand the present invention, but do not limit the invention in any way. It should be noted that those skilled in the art can make several changes and improvements without departing from the concept of the present invention. These all fall within the scope of protection of the present invention.

[0027] Example To address the negative impact of DC-side capacitors on device size and dynamic characteristics in existing IDCPFC topologies, this invention proposes a novel current-source resonant IDCPFC topology (CLC-IDCPFC) based on a novel CLC resonant circuit and its frequency conversion control strategy. This application scheme has the following characteristics: (1) A novel IDCPFC topology architecture without DC-side capacitors is proposed for the first time. This topology does not require indirect control of line power flow through DC-side capacitors, but instead adopts a current-source bridge topology to achieve direct power flow control. This architecture can significantly reduce the device size of IDCPFC.

[0028] (2) A novel CLC-type resonant circuit is proposed. This circuit can achieve wide-range regulation of the current ratio of the two lines within a narrow frequency range and has excellent power flow control characteristics.

[0029] (3) For CLC-IDCPFC, a frequency feedforward-based variable frequency control strategy is proposed. This strategy not only has good start-up characteristics, but also enables smooth switching of the device between different operating conditions, giving it excellent dynamic performance.

[0030] The present invention provides a current source resonant inter-line DC power flow controller topology and its frequency conversion control strategy. The technical solution mainly includes an IDCPFC topology architecture based on a current source bridge topology, the design and characteristic analysis of a novel CLC resonant circuit, and a feedforward frequency conversion control strategy for CLC-IDCPFC.

[0031] A. CLC-IDCPFC Topology and System Wiring The topology of CLC-IDCPFC is as follows: Figure 1 As shown, it consists of two current-source H-bridges, one isolation transformer with a primary-to-secondary turns ratio of 1:1, and two resonant capacitors. Each H-bridge is designated as one port and consists of four bidirectional switches. Each bidirectional switch contains two common-source power MOSFETs (metal-oxide-semiconductor field-effect transistors) connected in series. The capacitance values ​​of the two resonant capacitors are both [value missing]. C rThey are related to the leakage inductance of the isolation transformer. L r To form a CLC resonant circuit. v o1 and v o2 These are the instantaneous DC-side output voltage values ​​for ports 1 and 2, respectively. i s1 and i s2 These are the instantaneous AC current values ​​at ports 1 and 2, respectively, which appear as square waves in actual operation. v cr1 and v cr2 These are the instantaneous voltage values ​​of the two resonant capacitors, respectively. i Lr This is the instantaneous value of the transformer input current. P t The average power transferred from port 1 to port 2 under steady-state conditions is a constant value. During CLC-IDCPFC operation, the DC current of the line is inverted into a square-wave AC current by the H-bridge and injected into the CLC resonant circuit, such as... Figure 3 As shown. Power is transferred between the two ports via internal AC components, effectively achieving power transfer between two DC lines. If port 1 transfers power to port 2, then port 1 operates in inverter mode, and port 2 operates in rectification mode. P t >0; If port 2 transmits power to port 1, then port 2 operates in inverter mode and port 1 operates in rectifier mode. P t <0.

[0032] The CLC-IDCPFC is installed in a three-terminal ring DC distribution network, and the overall system wiring is as follows: Figure 2 As shown. This distribution network includes three voltage source converters (VSCs) and three DC lines. VSC 1 and VSC 2 operate in constant power control mode, and their output power is respectively... P 1 and P 2. VSC 3 operates in constant voltage control mode, and the voltage of its DC-side output bus is: V 3. The instantaneous values ​​of the currents in lines 1, 2, and 3 are denoted as follows: i 1. i 2 and i 3. Its reference direction has already been... Figure 2 It is marked in the middle.

[0033] B. Modulation strategy of CLC-IDCPFC CLC-IDCPFC can handle complex power flow regulation scenarios and has the ability to operate under multiple conditions. Denote the average value of i 1 and i 2 as I 1, I 2. Taking I 1, I 2 and P t as the coordinate axes to construct a space coordinate system, the operating region of CLC-IDCPFC can be divided into 8 octants, as shown in Figure 4 . It should be noted that in the DC loop network discussed in this scheme, since Line 1 and Line 2 are the main lines for power transmission, if the current in any line is opposite to the reference direction, a circulating current will be generated in the system, which is not allowed. Therefore, this scheme only discusses the I 1 and I 2 are both greater than 0, that is, corresponding to the octants I and V shown in Figure 4 . In octant I, P t >0, Port 1 operates in the frequency modulation mode, and each bidirectional switch operates in the "four-step commutation" switching mode, which can be equivalent to an ideal switch in the analysis. Port 2 operates in the rectification mode, T 1a ~T 4a are always on, T 1b ~T 4b are always off, and the antiparallel diodes of T 1b ~T 4b are used to form an uncontrolled rectifier bridge. In octant V, P t <0, Port 2 operates in the frequency modulation mode, and each bidirectional switch operates in the "four-step commutation" switching mode, which can be equivalent to an ideal switch in the analysis. Port 1 operates in the rectification mode, S 1a ~S 4a are always on, S 1b ~S 4b are always off, and the antiparallel diodes of S 1b ~S 4b are used to form an uncontrolled rectifier bridge.

[0034] C. Key parameters of CLC-IDCPFC The following takes the operating conditions corresponding to octant I as an example to analyze the operating principle of CLC-IDCPFC. Since Port 2 operates in uncontrolled rectification at this time, the voltage and current on its AC side are strictly in phase, so it can be equivalent to a resistive load port. The equivalent conductance of its load can be calculated as follows: (1) where G dc is the equivalent load conductance on the DC side,G ac This is the equivalent load conductance referred to the AC side. Definition M This is the current ratio of the CLC-IDCPFC. If the device operates at limit I, then... M = I 2 / I 1; If the device operates in quadrant V, M = I 1 / I 2. Meanwhile, since the turns ratio of the isolation transformer is 1, M It can also be used i s1 and i s2 The amplitude (denoted as) I s1 and I s2 To represent this. In hexagram I, there is... M = I s2 / I s1 In hexagram V, there is M = I s1 / I s2 .

[0035] The proposed CLC resonant circuit is defined with the following parameters. This resonant circuit has two characteristic frequencies, denoted as _____. f r1 and f r2 Their calculation formulas are as follows: (2) Let the switching frequency of the device be . f s Define normalized frequency f n = f s / f r1 Define the characteristic admittance of a resonant circuit. Y r and quality factor Q as follows: (3) D. The principle and characteristics of the novel CLC resonant circuit The characteristics of the CLC resonant circuit are analyzed in the phasor domain below. By using the fundamental frequency approximation, each electrical quantity is characterized and calculated using phasors of the switching frequency, which can efficiently and accurately characterize the principle characteristics of the CLC resonant circuit. Figure 5This is a phasor domain model of the internal AC circuit of the CLC-IDCPFC. I s1 For the AC side current phasor of port 1, V cr1 and V cr2 Let be the voltage phasor of the resonant capacitor. I Lr Let be the phasor of the transformer input current. Port 2 is modeled as the load conductance according to equation (1). G ac Its current phasor is I s2 Since the transformer turns ratio is 1, no additional calculation is needed for the primary and secondary side parameters. Figure 5 The circuit shown can be transformed using Norton's equivalent transformation to obtain the following: Figure 6 The circuit shown. The Norton equivalent capacitance is... C nor Norton equivalent current source I nor The following can be calculated: (4) Furthermore, regarding Figure 6 The current ratio expression of CLC-IDCPFC can be obtained by applying the voltage divider formula to the Norton equivalent circuit shown, as shown in equation (5).

[0036] (5) The current ratio can be plotted based on equation (5). M The change curve, such as Figure 7 As shown. Based on Figure 7 The following three operating characteristics of the proposed CLC resonant circuit can be summarized: Feature 1: This circuit can operate within a narrow frequency range (1 < 1). f n <2) Achieves a wide range of current ratio variations. This indicates that the CLC-IDCPFC proposed in this scheme has good power flow control characteristics and can meet all possible power flow control requirements with only a narrow operating frequency band.

[0037] Feature 2: This circuit in f n = f r2 There is a fixed gain point at this point. At this point, the current ratio... M Unaffected by external electrical parameters, it remains constant at 1. This indicates that in actual operation, the CLC-IDCPFC can operate at this fixed gain point under open-loop control, achieving highly robust and evenly distributed power flow control between the two lines.

[0038] Feature 3:M The maximum value of Q The design is quite sensitive. If... Q If the value is too low, the current gain capability of the rectifier port to the inverter port will be significantly affected. Therefore, to ensure the wide operating range capability of the CLC-IDCPFC, factors such as stress and volume must be comprehensively considered, and the parameters of the resonant components must be designed reasonably.

[0039] E. Variable Frequency Control Strategy For CLC-IDCPFC, this solution designs a frequency feedforward-based variable frequency control strategy, the block diagram of which is shown below. Figure 8 As shown. The current in line 1 is the control target, and its reference value is... I 1,ref The feedback value is i 1. The error between the reference value and the feedback value is used to generate an action signal via a PI controller. Subtracting 1.5 times the frequency feedforward from the absolute value of the action signal yields the normalized switching frequency. f n The sign of the action signal determines the operating threshold of the CLC-IDCPFC. A negative action signal indicates CLC-IDCPFC operates in threshold I, while a positive signal indicates it operates in threshold V. Subsequently, a corresponding switching drive signal is generated according to the modulation strategy described above. It should be noted that the additional feedforward value not only enables the device to quickly reach steady state during startup but also ensures smooth and seamless switching between different operating thresholds.

[0040] To verify the effectiveness of the CLC-IDCPFC topology and its control strategy proposed in this invention, according to Figure 2 The three-terminal ring DC distribution network shown is modeled on the PLECS simulation platform. The parameters of the DC distribution network are shown in Table 1. The circuit parameters of the CLC-IDCPFC are set as follows: C r =7.95 mF, L r =7.95 mH. Under this design, f r1 =20 kHz, Y r =1.

[0041] Table 1 DC Distribution Network Parameters

[0042] Example 2 This example tests the startup characteristics of CLC-IDCPFC. t =0.15s ago, the CLC-IDCPFC device was in bypass mode, at which time the steady-state current values ​​of lines 1 and 2 were 91.76A and 104.78A, respectively.t At 0.15s, CLC-IDCPFC is started, and its control target is set to... I 1,ref =120A. Under this control objective, the CLC-IDCPFC should operate at limit V. During startup... f n transient waveforms such as Figure 9 As shown. f n Starting from its feedforward value of 1.5, it gradually converges to the steady-state operating value of 1.2905 after a transient process of approximately 260 ms. The waveforms of various electrical quantities during startup are as follows: Figure 10 As shown in the figure, the line current smoothly changes to the control-set operating point without transient oscillations. Simultaneously, the amplitudes of the various electrical quantities within the CLC-IDCPFC do not exhibit significant overshoot. This indicates that the CLC-IDCPFC has excellent startup performance and can quickly, accurately, and smoothly control the line power flow.

[0043] The steady-state simulation waveforms of the internal electrical quantities of CLC-IDCPFC are as follows: Figure 11 As shown, the voltage / current of the resonant capacitor / inductor exhibits a good sine wave, indicating that the above analysis based on the fundamental frequency approximation is quite accurate. Port 2, as the inverter port, has an AC current phase that lags behind that of port 1. The voltage amplitudes of the resonant capacitor are approximately 110V and 200V, respectively, far less than the system's rated voltage of 10kV. This demonstrates that the proposed CLC-IDCPFC only needs to be calibrated according to a small fraction of the system's rated power, offering significant economic advantages.

[0044] Example 3 This example tests the characteristics of CLC-IDCPFC when switching between different operating limits. t =0.4s ago, the system was operating stably at limit V according to the control command in Example 1. t At 0.4s, the control target switches to I 1,ref =70A. Under the new control objective, CLC-IDCPFC should operate in limit I. During limit switching... f n transient waveforms such as Figure 12 As shown, the transient waveforms of each electrical quantity are as follows: Figure 13 As shown, the steady-state waveforms of each electrical quantity after the operating condition switch are as follows: Figure 14 As shown. First, f n The value gradually increases to the upper limit of 1.5. During this process, the action signal remains negative, and the CLC-IDCPFC continues to modulate according to the heddle V. When f nWhen the value reaches 1.5, the action signal changes from negative to positive, and the CLC-IDCPFC enters the modulation mode of the first octet. At this time... f n The voltage gradually decreased to a new steady-state operating point of 1.2642. The transient process of the operating condition switching was approximately 200ms, during which the line current and the electrical quantities within the CLC-IDCPFC did not show significant overshoot. Therefore, the control performance designed in this scheme is excellent, enabling seamless and balanced switching of the device between different operating limits.

[0045] Those skilled in the art will understand that, in addition to implementing the system, apparatus, and their modules provided by this invention in purely computer-readable program code, the same program can be implemented in the form of logic gates, switches, application-specific integrated circuits, programmable logic controllers, and embedded microcontrollers by logically programming the method steps. Therefore, the system, apparatus, and their modules provided by this invention can be considered a hardware component, and the modules included therein for implementing various programs can also be considered structures within the hardware component; alternatively, modules for implementing various functions can be considered both software programs implementing the method and structures within the hardware component.

[0046] Specific embodiments of the present invention have been described above. It should be understood that the present invention is not limited to the specific embodiments described above, and those skilled in the art can make various changes or modifications within the scope of the claims, which do not affect the essence of the present invention. Unless otherwise specified, the embodiments and features described in this application can be arbitrarily combined with each other.

Claims

1. A current source resonant type inter-line DC power flow controller topology, characterized in that, include: The system consists of a first current-source H-bridge, a second current-source H-bridge, an isolation transformer, and two resonant capacitors. Each H-bridge serves as a port, and each H-bridge consists of four bidirectional switches, each containing two common-source power MOSFETs connected in series. The primary-to-secondary turns ratio of the isolation transformer is 1:1; The two resonant capacitors and the leakage inductance of the isolation transformer form a CLC resonant circuit. The first current source H-bridge has its DC side connected to a first DC line and its AC side connected to the primary side of the isolation transformer via the CLC resonant circuit; the second current source H-bridge has its DC side connected to a second DC line and its AC side connected to the secondary side of the isolation transformer via the CLC resonant circuit. During operation, the first current source H-bridge and the second current source H-bridge invert the DC current of the DC line into an AC current in the form of a square wave and inject it into the CLC resonant circuit.

2. The current source resonant type inter-line DC power flow controller topology according to claim 1, characterized in that, The topology is installed in a three-terminal ring DC distribution network, which includes three voltage source converters (VSCs) and three DC lines. The first and second VSCs operate in constant power control mode, and the third VSC operates in constant voltage control mode. The first and second DC lines are connected in series with the DC sides of the first and second current source H-bridges, respectively. One end of the third DC line is connected to the DC-side output bus of the third VSC, and the other end is connected to a common connection point, which is the junction point of the three DC lines, forming a ring network.

3. The current source resonant type inter-line DC power flow controller topology according to claim 1, characterized in that, Based on the average value of the first DC line current under steady state Average current of the second DC line and transmission power The operating area is divided into eight octaves, where when , When both are greater than 0, it operates in octet I or octet V; In octave I, >0, the first port operates in frequency modulation mode, each bidirectional switch operates in four-step commutation switching mode, the second port operates in rectification mode, and its switch is normally open or normally closed to form an uncontrolled rectifier bridge. In the octet V, <0, the second port operates in frequency modulation mode, each bidirectional switch operates in four-step commutation switching mode, the first port operates in rectification mode, and its switch is normally open or normally closed to form an uncontrolled rectifier bridge.

4. The current source resonant type inter-line DC power flow controller topology according to claim 3, characterized in that, Define current ratio When working in octet I, When working in octet V, ;in , These are the average values ​​of the currents in the first DC line and the second DC line, respectively; the current ratio... It is also expressed as the amplitude of the AC current on the first port. Second port AC side current amplitude The ratio, in octet I, In the octet V, .

5. The current source resonant type inter-line DC power flow controller topology according to claim 4, characterized in that, The CLC resonant circuit has two characteristic frequencies. and The calculation formula is: in, This is the capacitance value of the resonant capacitor. This is the leakage inductance of the isolation transformer.

6. The current source resonant type inter-line DC power flow controller topology according to claim 5, characterized in that, Define the switching frequency as Normalized frequency Then the characteristic admittance is: The quality factor is: in, This is the equivalent load conductance converted to the AC side.

7. The current source resonant type inter-line DC power flow controller topology according to claim 6, characterized in that, The equivalent load conductance The calculation formula is: in, This is the equivalent load conductance on the DC side.

8. The current source resonant type inter-line DC power flow controller topology according to claim 7, characterized in that, In the phasor domain, the CLC resonant circuit obtains the Norton equivalent capacitance through Norton equivalent transformation. Norton equivalent current source The calculation formula is: 。 9. The current source resonant type inter-line DC power flow controller topology according to claim 8, characterized in that, The current ratio The expression is: in, The imaginary unit, Angular frequency, It is the resonant angular frequency.

10. A frequency conversion control method based on the current source resonant inter-line DC power flow controller topology according to any one of claims 1 to 9, characterized in that, include: Step 1: Obtain the reference value and feedback value of the first DC line current; Step 2: Calculate the error between the reference value and the feedback value; Step 3: Process the error using a PI controller to generate an action signal; Step 4: Subtract the frequency feedforward value from the absolute value of the action signal to obtain the normalized switching frequency; Step 5: Determine the operating limit based on the sign of the action signal. If the action signal is negative, operate in limit I; if the action signal is positive, operate in limit V. Step 6: Generate the corresponding switch drive signal based on the normalized switching frequency and operating limit.

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