Novel single-stage current source type direct current power flow controller
By employing a novel single-stage current source DC power flow controller, which utilizes partial power flow and ZCS soft-switching technology, the power loss and system complexity issues of existing DC power flow controllers are resolved, achieving efficient and reliable power flow control suitable for complex power grid environments.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- SHANGHAI JIAOTONG UNIV
- Filing Date
- 2024-10-28
- Publication Date
- 2026-05-01
AI Technical Summary
Existing DC power flow controllers suffer from problems such as high power loss, high system complexity, and insufficient control accuracy, which are particularly difficult to solve effectively in high power transmission and complex power grid environments.
A novel single-stage current source DC power flow controller is adopted. Through the design of half-bridges on the primary and secondary sides of the transformer, some power can flow directly, reducing the amount of power transmitted by the transformer. Combined with ZCS soft-switching capability and duty cycle modulation, bidirectional regulation and precise control of power flow are achieved.
It significantly reduces transformer losses, improves system efficiency and reliability, possesses stability and adaptability in complex power grid environments, reduces system costs and complexity, and achieves precise power flow control.
Smart Images
Figure CN121966281A_ABST
Abstract
Description
Technical Field
[0001] This invention relates to the technical field of DC power flow control, and more specifically, to a novel single-stage current source type DC power flow controller. Background Technology
[0002] As the proportion of distributed renewable energy and energy storage in DC power distribution systems continues to rise, the importance of DC power grids as the core network connecting various distributed energy sources is becoming increasingly prominent. Distributed DC systems, represented by photovoltaic-storage DC systems, DC fast charging stations, and DC data centers, are growing rapidly worldwide, and the interconnection and efficient operation of these systems have become urgent technical challenges to be solved.
[0003] Power flow control in DC power grids is one of the key technologies for achieving stable operation and optimized energy distribution in DC systems. Existing DC power flow control technologies mainly include two basic approaches: controlling line resistance and controlling DC voltage. Based on these two approaches, various DC power flow controllers have been developed, including variable series resistors, DC transformers, and series voltage source type power flow controllers.
[0004] However, existing DC power flow controller technology has several shortcomings:
[0005] DC transformer type power flow controller: Although this type of controller can achieve effective power flow control, some power needs to be transmitted through a DC transformer, resulting in significant power loss, especially in high-power transmission scenarios.
[0006] Series voltage source power flow controllers: These controllers require an external power supply to provide or absorb power, increasing system complexity and cost. Furthermore, the reliability and stability of the external power supply directly affect the overall system performance.
[0007] Variable resistance power flow controller: This method adjusts power flow by changing the line resistance, but it also suffers from significant power loss and is difficult to achieve precise control of power flow.
[0008] Therefore, a new technical solution is needed to improve the above-mentioned technical problems. Summary of the Invention
[0009] In view of the shortcomings of the prior art, the purpose of this invention is to provide a novel single-stage current source type DC power flow controller.
[0010] According to the present invention, a novel single-stage current-source DC power flow controller includes a transformer primary-side half-bridge, a transformer, and a transformer secondary-side half-bridge. The transformer primary-side half-bridge is formed by parallel connection of a bridge arm composed of two capacitors and a bridge arm composed of two MOSFETs. The transformer secondary-side half-bridge is formed by parallel connection of two identical bridge arms composed of an inductor and two MOSFETs connected in series. This controller can achieve bidirectional power flow regulation between DC grids. The controller draws energy from a first DC grid and outputs a voltage V. f It is connected in series in a DC line.
[0011] Preferably, by adjusting V f The value of V enables bidirectional power flow regulation between DC power grids; the capacitor voltage is constant, i.e., V C1 =V C2 =V grid1 / 2 .
[0012] Preferably, the DC power flow controller has two operating modes: forward operation and reverse operation; switches S1 and S2 use narrow pulse modulation waveforms to achieve current commutation in the transformer; switches S3-S6 use duty cycle modulation; and the output voltage V is controlled by adjusting the duty cycle of switches S3-S6. o Controlled by the current; in forward power flow mode, switches S5 and S6 are always on, while switches S3 and S4 are modulated by duty cycle; in reverse power flow mode, switches S3 and S4 are always on, while switches S5 and S6 are modulated by duty cycle.
[0013] Preferably, the time period t0-t1:
[0014] At time t0, switch S1 begins to conduct; V p for At this point, the leakage inductance current begins to increase, the current flowing through switch S6 begins to decrease, and the current flowing through switch S5 begins to increase. The leakage inductance current, the current flowing through switch S5, and the current flowing through switch S6 are as follows:
[0015]
[0016]
[0017]
[0018] During the time interval t1-t2: At time t1, the current flowing through switch S6 decreases to 0; then the current in switch S6 begins to increase in the opposite direction. At time t2, S1 and S4 are turned off simultaneously, and the current flowing through switch S6 increases to its maximum. The leakage inductance current, the current flowing through switch S5, and the current in switch S6 are expressed by equations (4), (5), and (6):
[0019]
[0020]
[0021]
[0022] Preferably, the time period t2-t3:
[0023] At time t2, V p Become The leakage inductance current begins to decrease, and the currents flowing through both switching transistors S5 and S6 decrease; at time t3, the current flowing through switching transistor S6 decreases to 0; the leakage inductance current, the currents flowing through switching transistors S5 and S6 are expressed by equations (7), (8), and (9):
[0024]
[0025]
[0026]
[0027] The time period t3-t4: At time t3, the current flowing through switch S6 becomes 0; during this time period, no current flows through switch S6; the leakage inductance current, the current flowing through switch S5, and the current flowing through switch S6 are expressed by equations (10)(11)(12):
[0028]
[0029] i S5 = I line (11)
[0030] i S6 =0(12).
[0031] Preferably, the time period t4-t5:
[0032] At time t4, switch S4 turns on; the current flowing through switch S6 increases, and the current flowing through switch S5 and L... k The current decreases; the leakage inductance current and the current flowing through switch S5 and switch S6 are expressed by equations (13)(14)(15):
[0033]
[0034]
[0035]
[0036] The time period t5-t6: At time t5, the leakage inductance current decreases to 0; during this time period, no current flows through the leakage inductance, V p The leakage inductance current and the current flowing through switch S5 and switch S6 are expressed by equations (16)(17)(18):
[0037] i Lk =0 (16)
[0038]
[0039]
[0040] Preferably, when the switch S4 is turned on, the voltage across the inductor L1 is: (V grid1 -V grid2 When switch S4 is turned off, the voltage across inductor L1 is: When the duty cycle of switch S4 is d during one cycle, the relationship between the mains voltage on side 1 and the mains voltage on side 2 can be derived through the volt-second balance across inductor L1 as follows:
[0041]
[0042] Simplifying, we get:
[0043]
[0044] Similarly, in reverse operation mode, the output voltage V o The relationship with side 1 of the DC grid is as follows:
[0045]
[0046] Preferably, the power transmitted from side 1 of the DC grid to side 2 of the DC grid is related to the voltage difference between the two ends and the voltages on both sides of the DC grid, that is:
[0047]
[0048] V in equations (20) and (21) o With V grid1 Substituting the relationships, we get:
[0049]
[0050] The transmission power of a line is related to the transformer turns ratio. Different forward and reverse transmission powers are set to design the transformer turns ratio.
[0051] Preferably, when the primary current switches between positive and negative, the voltage across the inductor changes only when the current on the bridge arm of switching transistors S5, S7, S6, or S8 drops to 0. Therefore, a duty cycle loss occurs during the period when the leakage inductance current switches between positive and negative; dloss is the lost duty cycle.
[0052]
[0053] If the duty cycle for lost data is set to be less than 5%, then:
[0054]
[0055] In forward operation mode, switches S3 and S4 are turned off with zero current, and in reverse operation mode, switches S5 and S6 are turned off with zero current. Therefore, before the switches are turned off, the diode connected in parallel with the switches in reverse direction is turned on, meaning the current in the switches is reversed.
[0056]
[0057] dtmin is the duty cycle of switching transistors S1 and S2; combined with equations (25) and (26), the range of values for leakage inductance Lk is determined.
[0058] Preferably, when the voltage across inductor L1 is (V grid1 -V o When ), at dT s The voltage across the time inductor L1 is (V) grid1 -V o The ripple value of the line current is:
[0059]
[0060] The current transmitted through the line is:
[0061]
[0062] The ripple rate of a line current is the ratio of the ripple value to the average value of the line current.
[0063]
[0064] The design line current ripple rate is less than 5%, and the values of inductors L1 and L2 are equal.
[0065] Compared with the prior art, the present invention has the following beneficial effects:
[0066] 1. This invention employs a partial power transmission topology, allowing most of the power to flow directly from one side of the DC grid to the other, while only a small portion of the power is transmitted through a transformer. This design eliminates the need for the transformer to withstand system-level voltage, thereby significantly reducing transformer losses. Simultaneously, the overall efficiency of the converter is greatly improved due to the reduction in power conversion stages.
[0067] 2. The controller of the present invention has ZCS (Zero Current Switching) soft switching capability, which means that when the switching transistor switches states, it can perform switching action under zero current conditions, thereby reducing the power loss caused by switching action; this feature not only improves the energy efficiency of the system, but also reduces the working stress of the switching transistor and extends the service life of the equipment.
[0068] 3. The output current of this invention can be adjusted independently without relying on the line resistance value. This feature enables the controller to maintain stable performance even when the line resistance varies greatly. This feature improves the adaptability and reliability of the controller in complex power grid environments.
[0069] 4. The controller of this invention possesses bidirectional power flow regulation capability, enabling precise power flow control in both forward and reverse flow conditions. This feature expands the controller's application range and meets the needs of DC power grids under various operating modes. It employs a relatively simple circuit topology, reducing power conversion stages and required equipment, thereby lowering the overall system cost and complexity. This characteristic makes the controller more economical and efficient in practical applications. Simulation verification through building a controller model in a simulation environment demonstrates the feasibility of the technical solution of this invention, and the simulation results are consistent with the theoretical analysis. This provides strong support for the further development and practical application of the controller. Attached Figure Description
[0070] 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:
[0071] Figure 1 This is a schematic diagram of the DC power flow controller topology of the present invention;
[0072] Figure 2 This is a schematic diagram of the modulation waveform of the power flow controller of the present invention;
[0073] Figure 3 This is a working modal diagram of the present invention;
[0074] Figure 4 This is a schematic diagram of the forward power flow simulation results of this invention;
[0075] Figure 5 This is a schematic diagram of the power flow reverse simulation results of the present invention;
[0076] Figure 6 This is a schematic diagram of the simulation results of the forward current control of power flow in low resistance according to the present invention;
[0077] Figure 7 This is a schematic diagram of the simulation results of the reverse current control of power flow in low resistance according to the present invention. Detailed Implementation
[0078] 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 protection scope of the present invention.
[0079] This invention proposes a novel single-stage current-source DC power flow controller. This controller employs a partial power flow method, significantly reducing power loss by minimizing power transmission through the transformer. Simultaneously, the controller possesses ZCS (Zero Current Switching) soft-switching capability, further reducing losses during power conversion. Furthermore, the controller's output current can be independently adjusted, independent of changes in line resistance, improving system adaptability and reliability. This invention aims to address the shortcomings of existing DC power flow controllers in terms of power loss, system complexity, and control accuracy, providing a new technical solution for the efficient operation and optimized energy distribution of DC power grids.
[0080] This invention innovatively proposes a power flow controller topology scheme with partial power transmission to address the bidirectional power flow regulation requirements of interconnected DC systems. Compared with existing technologies, this invention has the following characteristics: the power flow partially passes through the transformer, reducing transformer losses; the power flow controller has ZCS soft-switching capability, reducing power conversion losses; and the output current can be independently adjusted regardless of the line resistance value, making it suitable for applications with large variations in line resistance.
[0081] Figure 1 The DC power flow controller topology shown mainly consists of a transformer primary-side half-bridge, a transformer, and a transformer secondary-side half-bridge. The transformer primary-side half-bridge is formed by parallel connection of a bridge arm consisting of two capacitors and a bridge arm consisting of two MOSFETs. An inductor and two MOSFETs connected in series form a single bridge arm on the secondary side; two identical single bridge arms are connected in parallel to form the secondary bridge arm. This topology draws power from the DC grid 1 side, and the output voltage Vf is connected in series in the DC line. For this topology, a large amount of energy flows from the DC grid 1 side to the DC grid 2 side via the transformer secondary side, and a small amount of energy flows from the DC grid 1 side to the DC grid 2 side via the transformer. Therefore, the losses caused by the transformer are relatively small, making it a highly efficient topology.
[0082] Power flow regulation can be achieved by adjusting the value of Vf. To simplify the calculation of its power characteristics, it is assumed that the capacitor voltage is approximately constant, i.e., VC1 = VC2 = Vgrid1 / 2.
[0083] The DC power flow controller operates in two modes: forward and reverse. The modulation methods for both modes are similar. Switches S1 and S2 use narrow pulse modulation waveforms to achieve current commutation in the transformer. Switches S3-S6 use duty cycle modulation. The output voltage Vo is controlled by adjusting the duty cycle of switches S3-S6. Figure 2 The modulation method of the switching transistor and some key circuit waveforms are given. For example... Figure 2 As shown, in the forward power flow operation mode, switches S5 and S6 are always on, while switches S3 and S4 use duty cycle modulation. In the reverse power flow operation mode, switches S3 and S4 are always on, while switches S5 and S6 use duty cycle modulation. Due to the similarity between the forward and reverse power flow operation modes, only the specific working principle of the forward power flow operation mode will be analyzed.
[0084] t0-t1 time period: At time t0, S1 starts to conduct. V p for At this point, the leakage inductance current begins to increase, the current flowing through S6 begins to decrease, and the current flowing through S5 begins to increase. The leakage inductance current and the currents flowing through S5 and S6 are given by the following formulas:
[0085]
[0086]
[0087]
[0088] During the time interval t1-t2: At time t1, the current flowing through S6 decreases to 0. Afterwards, the current through S6 begins to increase in the opposite direction. At time t2, S1 and S4 are simultaneously turned off, and the current flowing through S6 increases to its maximum. The leakage inductance current and the current flowing through S5 and S6 can be expressed by equations (4), (5), and (6):
[0089]
[0090]
[0091]
[0092] The time interval t2-t3: At time t2, V p Become The leakage inductance current begins to decrease, and the currents flowing through S5 and S6 also decrease. At time t3, the current flowing through S6 decreases to 0. The leakage inductance current and the currents flowing through S5 and S6 can be expressed by equations (7), (8), and (9):
[0093]
[0094]
[0095]
[0096] The time interval t3-t4: At time t3, the current flowing through S6 becomes 0. During this time interval, no current flows through S6. The leakage inductance current and the current flowing through S5 and S6 can be approximately expressed by equations (10)(11)(12):
[0097]
[0098] i S5 =I line (11)
[0099] i S6 =0 (12)
[0100] During the time interval t4-t5: At time t4, switch S4 is turned on. The current flowing through S6 increases, while the current flowing through S5 and Lk decreases. The leakage inductance current and the current flowing through S5 and S6 can be approximately expressed by equations (13)(14)(15):
[0101]
[0102]
[0103]
[0104] The time period t5-t6: At time t5, the leakage inductance current decreases to 0. During this time period, no current flows through the leakage inductance, V p The leakage inductance current and the current flowing through S5 and S6 can be approximately expressed by equations (16)(17)(18):
[0105] i Lk =0 (16)
[0106]
[0107]
[0108] When switch S4 is turned on, the voltage across inductor L1 is: (V grid1 -V grid2When switch S4 is turned off, the voltage across inductor L1 is: Let the duty cycle of S4 during one cycle be d. Based on the volt-second balance across inductor L1, the relationship between the mains voltage side 1 and the mains voltage side 2 can be derived as follows:
[0109]
[0110] Simplifying, we get:
[0111]
[0112] Similarly, in reverse operation mode, the relationship between the output voltage Vo and the DC grid side 1 can be obtained as follows:
[0113]
[0114] like Figure 1 In the DC power flow controller shown, parameter design is required for the transformer turns ratio n, transformer leakage inductance Lk, and inductors L1 and L2. The transformer turns ratio is designed based on the circuit's regulation capability. The leakage inductance value is determined by the zero-current turn-off conditions of switches S5 and S6 and the duty cycle loss phenomenon. The range of values for inductors L2 and L1 can be determined by limiting the ripple rate of the line current.
[0115] The power transmitted from side 1 of the DC grid to side 2 of the DC grid is related to the voltage difference between the two ends and the voltages on both sides of the DC grid, that is:
[0116]
[0117] Substituting the relationship between Vo and Vgrid1 in equations (20) and (21) into the equations, we get:
[0118]
[0119] It is evident that the transmission power of a line is related to the transformer turns ratio. By setting different forward and reverse transmission powers, the transformer turns ratio can be designed.
[0120] When the primary current switches between positive and negative, the voltage across the inductor changes only when the current on bridge arms S5, S7, or S6, S8 drops to 0. Therefore, a duty cycle loss occurs during the period when the leakage inductance current switches between positive and negative. Let dloss be the lost duty cycle.
[0121]
[0122] If the duty cycle for lost data is set to be less than 5%, then:
[0123]
[0124] In order to achieve zero-current turn-off of S3 and S4 in forward operation mode and zero-current turn-off of S5 and S6 in reverse operation mode, the diode connected in parallel with the switch in reverse direction is turned on before the switch is turned off, that is, the current in the switch is reversed.
[0125] but:
[0126]
[0127] dtmin is the duty cycle of the switching transistors S1 and S2. Combining equations (25) and (26), the range of values for the leakage inductance Lk can be determined.
[0128] Taking the forward transmission mode as an example, when the voltage across inductor L1 is (V grid1 -V o When ), at dT s The voltage across the time inductor L1 is (V) grid1 -V o The ripple value of the line current is:
[0129]
[0130] The current transmitted through the line is:
[0131]
[0132] The ripple rate of a line current is the ratio of the ripple value to the average value of the line current.
[0133]
[0134] The design line current ripple rate is less than 5%, the values of inductors L1 and L2 are equal, and the reverse power flow mode is similar to the forward power flow mode. The range of values of L1 and L2 can be restricted by (29).
[0135] To verify the feasibility of the proposed DC power flow controller, a simulation environment was built using Plesc. Figure 1 The DC power flow controller model shown is illustrated in Table 1, and its circuit parameters are as follows:
[0136] Based on the single-stage current source type DC power flow controller topology proposed in this invention and actual engineering requirements, the rated power of the DC power flow controller is set to 450W.
[0137] Table 1 Simulation Circuit Parameters
[0138]
[0139] Simulation verification under forward power flow conditions: The duty cycle of switches S1 and S2 is set to 0.1, and the duty cycle of switches S3 and S4 is set to 0.8 to verify the theoretical analysis. The primary side voltage Vp, output voltage Vo, leakage inductance current iLk, inductor 1 current iL1, inductor 2 current iL2, and line current waveform iline are shown below. Figure 4 As shown, the simulation results are consistent with the theoretical analysis, fully verifying the correctness of the theoretical analysis.
[0140] like Figure 4 As shown, under the above simulation conditions, the power transmitted from DC grid 1 to DC grid 2 is about 450W, and the output voltage Vo is about 105V. By adjusting the duty cycle of S3 and S4, the output voltage can be further adjusted, thereby regulating the power flow of the line.
[0141] Simulation verification under reverse power flow conditions: The duty cycle of switches S1 and S2 is set to 0.1, and the duty cycle of switches S3 and S4 is set to 0.8 to verify the theoretical analysis. The primary side voltage Vp, output voltage Vo, leakage inductance current iLk, inductor 1 current iL1, inductor 2 current iL2, and line current waveform iline are shown below. Figure 5 As shown, the simulation results are consistent with the theoretical analysis, fully verifying the correctness of the theoretical analysis.
[0142] like Figure 5 As shown, under the above simulation conditions, the power transmitted from DC grid 1 to DC grid 2 is approximately -450W, and the output voltage Vo is approximately 106V. By adjusting the duty cycle of S3 and S4, the output voltage can be further adjusted, thereby regulating the power flow of the line.
[0143] Those skilled in the art will understand that, besides implementing the system and its various devices, modules, and units provided by this invention in the form of purely computer-readable program code, the same functions can be achieved entirely through logical programming of the method steps, making the system and its various devices, modules, and units of this invention function in the form of logic gates, switches, application-specific integrated circuits, programmable logic controllers, and embedded microcontrollers. Therefore, the system and its various devices, modules, and units provided by this invention can be considered as a hardware component, and the devices, modules, and units included therein for implementing various functions can also be considered as structures within the hardware component; alternatively, the devices, modules, and units for implementing various functions can be considered as both software modules implementing the method and structures within the hardware component.
[0144] 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 novel single-stage current source type DC power flow controller, characterized in that, The system includes a primary-side half-bridge, a transformer, and a secondary-side half-bridge. The primary-side half-bridge is composed of a bridge arm consisting of two capacitors connected in parallel and a bridge arm consisting of two MOSFETs connected in parallel. The secondary-side half-bridge is composed of two identical bridge arms consisting of an inductor and two MOSFETs connected in series, connected in parallel. This controller enables bidirectional power flow regulation between DC power grids. The controller draws power from a first DC power grid and outputs a voltage V. f It is connected in series in a DC line.
2. The novel single-stage current source type DC power flow controller according to claim 1, characterized in that, By adjusting V f The value of V enables bidirectional power flow regulation between DC power grids; the capacitor voltage is constant, i.e., V C1 =V C2 =V grid1 / 2 .
3. The novel single-stage current source type DC power flow controller according to claim 1, characterized in that, The DC power flow controller operates in two modes: forward and reverse. Switches S1 and S2 employ narrow pulse modulation waveforms to achieve current commutation in the transformer. Switches S3-S6 use duty cycle modulation. By controlling the duty cycle of switches S3-S6, the output voltage V is controlled. o Controlled by the current; in forward power flow mode, switches S5 and S6 are always on, while switches S3 and S4 are modulated by duty cycle; in reverse power flow mode, switches S3 and S4 are always on, while switches S5 and S6 are modulated by duty cycle.
4. The novel single-stage current source type DC power flow controller according to claim 3, characterized in that, The time period t0-t1: At time t0, switch S1 begins to conduct; V p for At this point, the leakage inductance current begins to increase, the current flowing through switch S6 begins to decrease, and the current flowing through switch S5 begins to increase. The leakage inductance current, the current flowing through switch S5, and the current flowing through switch S6 are as follows: During the time interval t1-t2: At time t1, the current flowing through switch S6 decreases to 0; then the current in switch S6 begins to increase in the opposite direction. At time t2, S1 and S4 are turned off simultaneously, and the current flowing through switch S6 increases to its maximum. The leakage inductance current, the current flowing through switch S5, and the current in switch S6 are expressed by equations (4), (5), and (6):
5. The novel single-stage current source type DC power flow controller according to claim 4, characterized in that, The time period t2-t3: At time t2, V p Become The leakage inductance current begins to decrease, and the currents flowing through both switching transistors S5 and S6 decrease; at time t3, the current flowing through switching transistor S6 decreases to 0; the leakage inductance current, the currents flowing through switching transistors S5 and S6 are expressed by equations (7), (8), and (9): The time period t3-t4: At time t3, the current flowing through switch S6 becomes 0; during this time period, no current flows through switch S6; the leakage inductance current, the current flowing through switch S5, and the current flowing through switch S6 are expressed by equations (10)(11)(12): i S5 =I line (11) i S6 ＝0(12)。 6. The novel single-stage current source type DC power flow controller according to claim 5, characterized in that, The time period t4-t5: At time t4, switch S4 is turned on; The current flowing through switch S6 increases, and the current flowing through switch S5 and L... k The current decreases; the leakage inductance current and the current flowing through switch S5 and switch S6 are expressed by equations (13)(14)(15): The time period t5-t6: At time t5, the leakage inductance current decreases to 0; during this time period, no current flows through the leakage inductance, V p The leakage inductance current and the current flowing through switch S5 and switch S6 are expressed by equations (16)(17)(18): i Lk ＝0(16) 7. The novel single-stage current source type DC power flow controller according to claim 6, characterized in that, When switch S4 is turned on, the voltage across inductor L1 is: (V grid1 -V grid2 When switch S4 is turned off, the voltage across inductor L1 is: When the duty cycle of switch S4 is d during one cycle, the relationship between the mains voltage on side 1 and the mains voltage on side 2 can be derived through the volt-second balance across inductor L1 as follows: Simplifying, we get: Similarly, in reverse operation mode, the output voltage V o The relationship with side 1 of the DC grid is as follows:
8. The novel single-stage current source type DC power flow controller according to claim 7, characterized in that, The power transmitted from side 1 of the DC grid to side 2 of the DC grid is related to the voltage difference between the two ends and the voltages on both sides of the DC grid, that is: V in equations (20) and (21) o With V grid1 Substituting the relationships, we get: The transmission power of a line is related to the transformer turns ratio. Different forward and reverse transmission powers are set to design the transformer turns ratio.
9. The novel single-stage current source type DC power flow controller according to claim 8, characterized in that, When the primary current switches between positive and negative, the voltage across the inductor changes only when the current on the bridge arm of switching transistors S5, S7, S6, or S8 drops to 0. Therefore, a duty cycle loss occurs during the period when the leakage inductance current switches between positive and negative; dloss is the lost duty cycle. If the duty cycle for lost data is set to be less than 5%, then: In forward operation mode, switches S3 and S4 are turned off with zero current, and in reverse operation mode, switches S5 and S6 are turned off with zero current. Therefore, before the switches are turned off, the diode connected in parallel with the switches in reverse direction is turned on, meaning the current in the switches is reversed. dtmin is the duty cycle of switching transistors S1 and S2; combined with equations (25) and (26), the range of values for leakage inductance Lk is determined.
10. The novel single-stage current source type DC power flow controller according to claim 8, characterized in that, When the voltage across inductor L1 is (V grid1 -V o When ), at dT s The voltage across the time inductor L1 is (V) grid1 -V o The ripple value of the line current is: The current transmitted through the line is: The ripple rate of a line current is the ratio of the ripple value to the average value of the line current. The design line current ripple rate is less than 5%, and the values of inductors L1 and L2 are equal.