Flexible low frequency-voltage source converter based hybrid ac-dc transmission system and control method thereof

Abstract

The application discloses a flexible low-frequency-flexible direct-current hybrid power transmission system and a control method thereof. The hybrid power transmission system comprises a receiving-end converter station, a low-frequency alternating-current new energy field station and a high-frequency direct-current new energy field station. The power conversion unit of the receiving-end converter station adopts a modular design. Each power module is combined through series / parallel connection to form a power-frequency alternating-current port, a low-frequency alternating-current port and a high-voltage direct-current port of the converter station. Each port is connected with a power-frequency main network, a low-frequency new energy field station and a direct-current new energy field station through a power transmission line. The receiving-end converter station is controlled through multi-level control of a system level, a bridge arm level and a module level to realize voltage stability of the low-frequency alternating-current port and the high-voltage direct-current port, current sinusoidalization of the power-frequency alternating-current port and direct-current bus voltage stability of each power module. The application realizes coordinated operation of the flexible low-frequency and the flexible direct-current two power transmission modes and is suitable for efficient sending of different distance new energy field stations. Furthermore, the application realizes integrated integration of different types of converter stations.

Description

Technical Field

[0001] This invention belongs to the field of flexible power transmission technology, specifically a flexible low-frequency-flexible DC hybrid power transmission system and its control method. Background Technology

[0002] Traditional high-voltage AC transmission systems face numerous challenges in handling the intermittency and volatility of new energy sources, including low power transmission efficiency, poor stability, and large transmission losses over long distances.

[0003] To address this, flexible high-voltage direct current (HVDC) transmission technology has emerged. This technology uses voltage source converters (VSCs) to achieve bidirectional power flow and independent control of active and reactive power, offering advantages such as fast response and flexible control. It has been widely applied in offshore wind farm integration and inter-regional interconnection. HVDC is particularly suitable for long-distance transmission of renewable energy because it can significantly reduce transmission losses and improve capacity utilization in long-distance lines, achieving efficient voltage conversion and fault isolation through topologies such as modular multilevel converters (MMCs). However, in short-distance renewable energy aggregation scenarios, HVDC systems have significant limitations, such as the need for high-cost DC cables and conversion equipment, leading to excessive equipment investment and maintenance costs. Furthermore, in multi-terminal DC systems, DC fault isolation and power coordination control still need optimization to avoid a decline in overall system efficiency. In existing technologies, although MMC has become the mainstream topology, its integrated design in hybrid transmission scenarios is not yet mature, especially when combined with low-frequency AC, lacking effective voltage stabilization and power balancing mechanisms. These problems have hindered the efficient transmission of new energy sources and the intelligent transformation of the power grid. There is an urgent need for a new hybrid power transmission scheme to integrate the advantages of flexible DC and low-frequency AC to achieve more efficient energy transmission.

[0004] Low-frequency AC transmission, as an emerging technology, reduces line impedance and increases transmission capacity and distance by lowering the transmission frequency (e.g., 16.7Hz or 20Hz), making it particularly suitable for accessing nearby renewable energy power plants. Compared to traditional power frequency AC, low-frequency AC significantly reduces cable loss and reactor size, making it suitable for wind power aggregation on offshore platforms or in remote areas. In short-distance transmission, low-frequency AC systems simplify aggregation network design, reduce conversion stages, thereby lowering overall costs and improving efficiency. However, existing low-frequency AC systems face high costs for long-distance applications, such as the need for longer low-frequency lines and larger isolation equipment, leading to reduced economic viability. Furthermore, when mixed with DC systems, interface compatibility issues are prominent, such as low voltage conversion efficiency, difficulties in harmonic suppression, and complex multi-port power coordination control.

[0005] Traditional methods often employ independent converter stations for low-frequency to power frequency or DC conversion, resulting in large footprints, redundant equipment, and high maintenance costs. While low-frequency aggregation and networking at renewable energy power plants simplifies transformer design, the lack of seamless integration with high-voltage DC can easily lead to voltage fluctuations and power imbalances. Summary of the Invention

[0006] To address the problems existing in the prior art, this invention provides a flexible low-frequency-flexible DC hybrid power transmission system and its control method, making it applicable to large-scale power transmission scenarios of new energy power plants at different distances. This enables integrated design of receiving-end converter stations under different power transmission modes, thereby improving the economic efficiency of system operation.

[0007] Therefore, the present invention adopts the following technical solution.

[0008] In a first aspect, the present invention provides a flexible low-frequency-flexible DC hybrid power transmission system, which includes a receiving-end converter station, a low-frequency AC renewable energy power station, and a high-voltage DC renewable energy power station.

[0009] The receiving-end converter station has a power frequency AC port, a low frequency AC port, and a high voltage DC port. The power conversion unit of the receiving-end converter station consists of 9 bridge arms. Each bridge arm contains N power modules. Each power module contains 2 AC interfaces and 2 DC interfaces. The DC interfaces are generated by the isolated DC-DC converter inside the power module. The AC interfaces and DC interfaces of the N power modules of each bridge arm are cascaded to form the total AC interface and DC interface of each bridge arm. The total AC interface of each bridge arm forms a one-to-one connection between the power frequency AC port and the low frequency AC port of the receiving-end converter station. The total DC ports of each bridge arm are connected in parallel to form the high voltage DC port.

[0010] The low-frequency AC new energy power station is connected to the low-frequency AC port of the receiving-end converter station via a low-frequency AC transmission line.

[0011] The aforementioned high-voltage direct current (HVDC) renewable energy power station is connected to the HVDC port of the receiving-end converter station via a HVDC transmission line.

[0012] Furthermore, the power module consists of 3 full-bridge units, 2 DC support capacitors and 1 high-frequency transformer, and each full-bridge unit consists of 4 fully controlled switches forming 2 bridge arms;

[0013] The DC buses of the first and second full-bridge units are connected in parallel to the two ends of the first DC support capacitor. The midpoints of the two arms of the first full-bridge unit form two AC interfaces. The midpoints of the two arms of the second full-bridge unit are connected to the primary side of the high-frequency transformer. The midpoints of the two arms of the third full-bridge unit are connected to the secondary side of the high-frequency transformer. The DC bus of the third full-bridge unit is connected in parallel to the two ends of the second DC support capacitor and forms two DC interfaces of the power module. The second full-bridge unit, the high-frequency transformer, and the third full-bridge unit constitute an isolated DC-DC converter.

[0014] Furthermore, in the aforementioned low-frequency AC renewable energy power station, each renewable energy unit adopts a low-frequency AC aggregation and grid-based voltage boosting operation mode; the aforementioned high-voltage DC renewable energy power station adopts a variable-frequency AC aggregation and grid-based operation mode, which is converted to high-voltage DC through a modular multilevel converter at the sending end.

[0015] Furthermore, the sending-end modular multilevel converter controls the amplitude and frequency of its three-phase AC voltage. f dco , where frequency f dco Based on real-time power calculations of the new energy power plants:

[0016]

[0017] in, f gn The rated frequency for the high-voltage direct current (HVDC) renewable energy power station network. p dco This refers to the real-time AC power of the sending-end modular multilevel converter (MSC). m Let be the order of the polynomial fitting. for p dco The i The coefficient of the power term.

[0018] In a second aspect, the present invention provides a control method for the above-mentioned flexible low-frequency-flexible DC hybrid transmission system, comprising:

[0019] Low-frequency AC port voltage-current dual closed-loop control is used to generate common-mode component reference values ​​for the three bridge arm voltages of each low-frequency AC output phase.

[0020] Bridge arm level capacitor voltage balance control is used to generate current reference values ​​for the power frequency AC ports corresponding to the three bridge arms of each low-frequency AC output phase.

[0021] Power frequency AC port current closed-loop control is used to generate differential mode component reference values ​​for each bridge arm voltage;

[0022] The capacitor voltage equalization control between the power modules of each bridge arm is used to generate the switch drive signal on the AC interface side of the power module;

[0023] High-voltage DC port voltage closed-loop control is used to generate DC interface voltage reference values ​​for each power module in each bridge arm;

[0024] Each power module generates the switching drive signal for the isolated DC-DC converter through high-frequency modulation.

[0025] Furthermore, the specific implementation steps of the low-frequency AC port voltage-current dual closed-loop control are as follows:

[0026] S11) Collect the three-phase output voltage of the low-frequency AC port, collect the AC interface current of the 9 bridge arms, and calculate the three-phase output current of the low-frequency AC port.

[0027] S12) Perform Clarke and Park coordinate transformations on the three-phase output voltage and three-phase output current to obtain the d-axis and q-axis components of the output voltage and the output current. The phase angle used in the Park coordinate transformation is determined by the set low-frequency transmission frequency. f LF The result is obtained by integration;

[0028] S13) In the dq coordinate system, the output voltage and output current are controlled by a dual closed loop to generate the dq axis components of the common mode voltage reference values ​​of the 9 bridge arm AC interfaces.

[0029] S14) Perform Park inverse transformation on the dq axis components of the common-mode voltage reference value to obtain the three-phase common-mode voltage reference value.

[0030] Furthermore, the specific implementation steps of bridge arm stage capacitor voltage balance control are as follows:

[0031] S21) Divide the 9 bridge arms into three groups, each group corresponding to the 3 bridge arms of each low-frequency AC port output phase;

[0032] S22) For x The group of phases, x =U, V, W, collect the DC bus voltages of N power modules in each bridge arm, and sum them to obtain the total DC bus voltage of each bridge arm;

[0033] S23) Perform three-phase conversion on the sum of the DC bus voltages of each bridge arm. The coordinate transformation of the coordinate system yields the transformed sum of DC bus voltages;

[0034] S24) Perform closed-loop control on the sum of the transformed DC bus voltages to generate a bridge arm power reference value;

[0035] S25) Calculate based on bridge arm power reference value x Reference values ​​for the positive sequence d-axis component, positive sequence q-axis component, negative sequence d-axis component, and negative sequence q-axis component of the power frequency AC current of the three bridge arms in the dq coordinate system;

[0036] S26) Construct the reference value obtained from S25) x Group power frequency AC current in Reference values ​​in a coordinate system;

[0037] S27) Perform an inverse Clarke transformation on the reference value obtained in S26) to obtain the current reference value of the power frequency AC port in the three-phase stationary coordinate system.

[0038] Furthermore, the specific implementation steps of the closed-loop control of the power frequency AC port current are as follows:

[0039] S31) x For the corresponding three bridge arms, calculate the differential mode component of the bridge arm current to obtain the three-phase current of the power frequency AC port;

[0040] S32) Perform closed-loop control on the three-phase currents respectively to generate... x Reference values ​​for the differential mode components of the three bridge arm voltages.

[0041] Furthermore, the specific implementation steps for capacitor voltage balancing control among the power modules of each bridge arm are as follows:

[0042] S41) Calculate the total reference voltage value of each bridge arm AC interface;

[0043] S42) For each bridge arm, the nearest level approximation method is used to generate the full-bridge converter switching drive signal corresponding to the AC interface of each power module.

[0044] Furthermore, the specific implementation steps of the high-voltage DC port voltage closed-loop control are as follows: collect the voltage of the high-voltage DC port, perform closed-loop control on it to generate voltage reference values ​​for the DC interfaces of each power module.

[0045] Furthermore, the specific implementation steps for each power module to generate the switching drive signal for the isolated DC-DC converter through high-frequency modulation are as follows:

[0046] S61) Sample the DC interface output voltage of each power module and perform closed-loop control to generate the phase shift angle of the secondary side of the isolation converter relative to the primary side;

[0047] S62) Generate complementary drive pulses for both full bridges at a fixed switching frequency. With the input side as a reference, the output side is phase-delayed / leaded according to the phase shift angle, and a dead time is set.

[0048] Compared with the prior art, the technical solution adopted in this invention has the following beneficial effects:

[0049] 1. This invention achieves coordinated operation of flexible low-frequency and flexible DC power transmission modes through the design of modular power conversion units. It is applicable to the efficient transmission of new energy power plants at different distances, avoids the redundant configuration of traditional independent converter stations, and significantly reduces system complexity.

[0050] 2. This invention adopts an integrated design, combining the power frequency AC port, low frequency AC port and high voltage DC port in series / parallel, which greatly reduces the footprint, equipment investment and maintenance costs of the receiving-end converter station, and improves the economy and feasibility of engineering deployment.

[0051] 3. This invention introduces a multi-level control strategy, including system-level voltage-current dual closed loop, bridge arm-level capacitor voltage balancing, and module-level high-frequency modulation, to ensure stable voltage at low-frequency AC ports and high-voltage DC ports, sinusoidal current at power frequency AC ports, and balanced DC bus voltages of each power module, thereby improving system stability and anti-disturbance capability.

[0052] 4. This invention adopts a low-frequency AC aggregation network and a variable frequency AC aggregation mode on the new energy power station side, which optimizes the power conversion efficiency, reduces conversion losses, and achieves electrical isolation through an isolated DC-DC converter, thereby enhancing the system's fault isolation and safety performance. Attached Figure Description

[0053] To more clearly illustrate the technical solutions in the embodiments of the present invention, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0054] Figure 1 This is a schematic diagram of the flexible low-frequency-flexible DC hybrid power transmission system of the present invention;

[0055] Figure 2 This is a topology diagram of the power module in this invention;

[0056] Figure 3 This is a schematic diagram of the low-frequency AC new energy power station in this invention;

[0057] Figure 4 This is a schematic diagram of the structure of the high-voltage direct current renewable energy power station in this invention;

[0058] Figure 5 The frequency of the sending end MMC AC side in a specific embodiment of the present invention f dcoWith real-time power p dco Relationship diagram;

[0059] Figure 6 This is a flowchart of the control method for the flexible low-frequency-flexible DC hybrid power transmission system of the present invention;

[0060] Figure 7 This is a simulation result diagram of the flexible low-frequency-flexible DC hybrid power transmission system of the present invention connected to a high-voltage DC new energy power station;

[0061] Figure 8 This is a simulation result diagram of the flexible low-frequency-flexible DC hybrid power transmission system of the present invention connected to a low-frequency AC new energy power station. Detailed Implementation

[0062] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0063] In one embodiment of the present invention, a flexible low-frequency-flexible DC hybrid power transmission system (hereinafter referred to as the hybrid power transmission system) is provided, such as... Figure 1 As shown, the system includes a receiving-end converter station, a low-frequency AC transmission line, a low-frequency AC renewable energy power plant, a high-voltage DC transmission line, and a high-voltage DC renewable energy power plant. The receiving-end converter station provides power frequency AC ports, low-frequency AC ports, and high-voltage DC ports. The power conversion unit of the receiving-end converter station consists of nine bridge arms, each containing N power modules, where N is determined according to the system voltage level; for example, in typical applications, N ranges from 100 to 400. Each power module contains two AC interfaces and two DC interfaces. The DC interfaces are generated from the DC bus of the power module through an isolated DC-DC converter. The AC and DC interfaces of the N power modules in each bridge arm are cascaded to form the total AC and DC interfaces of that bridge arm. The total AC interfaces of each bridge arm form a one-to-one connection between the power frequency AC and low-frequency AC ports of the receiving-end converter station, and the total DC ports of each bridge arm are connected in parallel to form the high-voltage DC port. Low-frequency AC renewable energy power plants are connected to the low-frequency AC port of the receiving-end converter station via low-frequency AC transmission lines, while high-voltage DC renewable energy power plants are connected to the high-voltage DC port of the receiving-end converter station via high-voltage DC transmission lines.

[0064] The circuit structure of the power module is as follows: Figure 2 As shown, it consists of 3 full-bridge units (first full-bridge unit FB1, second full-bridge unit FB2, and third full-bridge unit FB3) and 2 DC support capacitors (first DC support capacitor C).dc1 Second DC support capacitor C dc2 It consists of a full-bridge unit and a high-frequency transformer T. Each full-bridge unit consists of four fully controlled switches (such as IGBTs) forming two bridge arms. The DC buses of the first full-bridge unit FB1 and the second full-bridge unit FB2 are connected in parallel to the first DC support capacitor C. dc1 At both ends, the midpoints of the two arms of the first full-bridge unit FB1 form two AC interfaces; the midpoints of the two arms of the second full-bridge unit FB2 are connected to the primary side of the high-frequency transformer T; the midpoints of the two arms of the third full-bridge unit FB3 are connected to the secondary side of the high-frequency transformer T; and the DC bus of the third full-bridge unit FB3 is connected in parallel to the second DC support capacitor C. dc2 The two ends form two DC interfaces for the power module. The second full-bridge unit FB2, the high-frequency transformer T, and the third full-bridge unit FB3 constitute an isolated DC-DC converter. The turns ratio of the high-frequency transformer T is designed according to isolation requirements, such as 1:1 or 1:2. The first DC support capacitor C... dc1 Second DC support capacitor C dc2 The typical capacity is 5-10mF, and the withstand voltage rating is 1-2kV.

[0065] In the aforementioned low-frequency AC renewable energy power stations, each renewable energy unit (such as a wind turbine) adopts a low-frequency AC collection and grid-connected voltage boosting operation mode, such as... Figure 3 As shown, the collection frequency f LF Typically 16.7Hz or 20Hz, the voltage is stepped up by a low-frequency transformer and then connected to a low-frequency AC transmission line. The aforementioned high-voltage DC renewable energy power station adopts an AC aggregation and networking operation mode, such as... Figure 4 As shown, the DC voltage is converted to high voltage DC through a sending-end modular multilevel converter (sending-end MMC). The number of sending-end MMC sub-modules is determined according to the DC voltage. For example, in a ±500kV system, the number of sub-modules is 200-300.

[0066] In one embodiment of the present invention, the AC aggregation network operating frequency of the high-voltage direct current (HVDC) renewable energy power station is variable to optimize the power capture efficiency and transmission stability of the renewable energy units. This mode controls the three-phase voltage amplitude and frequency on its AC side through the sending-end MMC. f dco This enables dynamic adjustment of the aggregation network. Specifically, the control system of the sending-end MMC collects the power on the AC side in real time. p dco And calculate the frequency according to the preset polynomial model. f dco :

[0067]

[0068] in, fgn The rated frequency for the high-voltage direct current (HVDC) renewable energy power station network. p dco For the real-time AC power of the sending-end MMC, m Let be the order of the polynomial fitting. for p dco The i The coefficient of the power term. Rated frequency. f gn The typical setting is 50Hz or 60Hz, depending on the design capacity of the wind farm, for example, in offshore wind farm applications. f gn It can be set to 50Hz to match the European power grid standard. The order m of the polynomial fitting is selected based on the system complexity and accuracy requirements, typically ranging from 2 to 4, to balance computational complexity and control accuracy. For example, when m When = 2, the polynomial form is: f dco = f gn + × p dco + , where the coefficient and The coefficients are determined through offline optimization or real-time adaptive algorithms. In practice, the coefficients can be obtained by fitting historical power data and simulation models, for example, by using the least squares method to fit the power-frequency optimization curve. Figure 5 Typical f dco and p dco Relationship diagram p dco This mode utilizes current and voltage sampling, along with instantaneous power theory calculations. It ensures that the frequency is reduced at low power levels to minimize losses, and increased at high power levels to enhance transmission capacity, thereby improving overall efficiency.

[0069] This invention also provides a control method for a flexible low-frequency-flexible DC hybrid transmission system, such as... Figure 6As shown, the following controls are implemented: low-frequency AC port voltage-current dual closed-loop control to generate common-mode component reference values ​​for the three bridge arm voltages of each low-frequency AC output phase; bridge arm capacitor voltage balancing control to generate current reference values ​​for the power frequency AC ports corresponding to the three bridge arms of each low-frequency AC output phase; power frequency AC port current closed-loop control to generate differential-mode component reference values ​​for each bridge arm voltage; capacitor voltage equalization control between power modules in each bridge arm to generate switching drive signals for the AC interface side of the power modules; high-voltage DC port voltage closed-loop control to generate DC interface voltage reference values ​​for each power module in each bridge arm; and each power module generates a switching drive signal for the isolated DC-DC converter through high-frequency modulation. The control method is implemented using a digital signal processor (DSP), with a sampling frequency of up to 10kHz.

[0070] Specifically, the implementation steps of the low-frequency AC port voltage-current dual closed-loop control are as follows:

[0071] S11) Collect the three-phase output voltages of the low-frequency AC port U, V, and W. u oU , u oV and u oW Collect AC interface currents from 9 bridge arms i AU , i BU , i CU , i AV , i BV , i CV , i AW , i BW , i CW Calculate the three-phase output current at the low-frequency AC port. i oU , i oV , i oW :

[0072]

[0073] S12) Three-phase output voltage u oU , u oV and u oW and three-phase output current i oU, i oV and i oW Perform Clarke and Park coordinate transformations to obtain the d-axis component of the output voltage. u od and q-axis components u oq d-axis component of output current i od and q-axis components i oq The phase angle used in the Park coordinate transformation is determined by the set low-frequency transmission frequency. f LF The points are obtained through integration. f LF Typical values ​​are 16.67 Hz or 20 Hz.

[0074] S13) In the dq coordinate system, perform dual closed-loop control on the output voltage and output current to generate the dq-axis components of the common-mode voltage reference values ​​for the nine bridge arm AC interfaces. and ,Right now:

[0075]

[0076]

[0077] Among them, superscript Indicates a reference value. G ou ( s For voltage outer loop controllers (such as PI controllers, proportional coefficient) k p =1, integral coefficient k i =100), G oi ( s ) is a current inner-loop controller (such as a PI controller, k p =50, k i =500);

[0078] S14) dq-axis component of the common-mode voltage reference value and Perform an inverse Park transform to obtain the three-phase common-mode voltage reference value. , , The phase angle used for coordinate transformation is the same as that in S12).

[0079] Specifically, the implementation steps of the bridge arm capacitor voltage balance control are as follows:

[0080] S21) Divide the 9 bridge arms into three groups, each group corresponding to the 3 bridge arms of each low-frequency AC port output phase;

[0081] S22) For x ( x =U, V, W) phase group, collect the DC bus voltage of N power modules of each bridge arm. u dcAxi , u dcBxi , u dcCxi subscript i ( i =1, 2, 3, ..., N) represents the module number. The total DC bus voltage of each bridge arm is obtained by summing the values ​​of each module number. u dcAx , u dcBx , u dcCx ,For example u dcAx =Σ u dcAxi ;

[0082] S23) u dcAx , u dcBx , u dcCx Three-phase to Coordinate transformation of the coordinate system yields , , ,Right now:

[0083]

[0084] S24) , , Perform closed-loop control to generate bridge arm power reference values. , , :

[0085]

[0086] Among them, superscript Indicates a reference value. N dc ( s () is a notch filter, used to filter out the AC component in various deviations and retain the DC component; G dc (s This is a DC bus voltage controller. A typical one... N dc ( s The expression for ) is:

[0087]

[0088] In the formula, f i This is the power frequency input frequency. That is, in this embodiment, N dc In 2 f i 2 f LF , f i - f LF , f i + f LF The gain at these frequencies is zero, which can completely filter out harmonic components at those frequencies. Typical G dc ( s ) is a PI controller, such as k p =0.5, k i =50;

[0089] S25) Calculated x Reference values ​​of the positive sequence d-axis components of the power frequency AC current of the three bridge arms in the dq coordinate system Reference values ​​for positive sequence q-axis components (Generally set to 0), negative sequence d-axis component reference value Reference values ​​for negative q-axis components :

[0090]

[0091] in, U im This refers to the amplitude of the power frequency AC voltage.

[0092] S26) construct x Group power frequency AC current in Reference values ​​in coordinate system and :

[0093]

[0094] In the formula, iThe phase of phase A of the power frequency AC input voltage is obtained using a three-phase phase-locked loop algorithm;

[0095] S27) Perform the Clarke inverse transform to obtain the current reference value of the power frequency AC port in the three-phase stationary coordinate system:

[0096]

[0097] Specifically, the implementation steps of the closed-loop control of the power frequency AC port current are as follows:

[0098] S31) x For the corresponding three bridge arms, calculate the differential mode component of the bridge arm current to obtain the three-phase current at the power frequency AC port:

[0099]

[0100] S32) Perform closed-loop control on the three-phase currents respectively to generate... x Reference values ​​for the differential mode components of the three bridge arm voltages:

[0101]

[0102] In the formula, G ii ( s This is a current controller for the power frequency AC port. Since this controller is implemented in a stationary coordinate system, a typical... G ii ( s This is a proportional-resonant controller, with a proportional parameter. k p =50, resonance control parameters are: k r =500, center frequency is 2 f i .

[0103] Specifically, the implementation steps for capacitor voltage balancing control among the power modules of each bridge arm are as follows:

[0104] S41) Calculate and obtain the total voltage reference value of each bridge arm AC interface:

[0105]

[0106] S42) For each bridge arm, the nearest level approximation method is used to generate the full-bridge converter switching drive signal corresponding to the AC interface of each power module, where the switching frequency is typically 100-200Hz.

[0107] Specifically, the implementation steps of the high-voltage DC port voltage closed-loop control are as follows:

[0108] S51) Collect the voltage at the high-voltage DC port. u hvdc Closed-loop control is used to generate voltage reference values ​​for the DC interfaces of each power module:

[0109]

[0110] In the formula, G v ( s ) is a voltage controller for a high-voltage DC port (such as a PI controller, k p =0.2, k i =20).

[0111] Specifically, the specific implementation steps for each power module to generate the switching drive signal for the isolated DC-DC converter through high-frequency modulation are as follows:

[0112] S61) Sample the DC interface output voltage of each power module Closed-loop control is applied to generate the phase shift angle of the secondary side of the isolation converter relative to the primary side. :

[0113]

[0114] in, G ( s ) is a DC interface output voltage controller (such as a PI controller, k p =0.1, k i =10);

[0115] S62) Generate complementary drive pulses for both full bridges at a fixed switching frequency (e.g., 20kHz). With the input side as a reference, the output side is phase-delayed / leaded according to the phase shift angle, and a dead time (e.g., 200ns) is set.

[0116] In this embodiment, simulation results demonstrate that the hybrid transmission system exhibits good steady-state and dynamic characteristics when connected to different types of renewable energy power plants. For high-voltage direct current (HVDC) renewable energy power plants, the steady-state waveform at the sending end is as follows: Figure 7 (a) and Figure 7 As shown in (b), both the AC voltage and AC current are three-phase symmetrical sinusoids, exhibiting excellent waveform quality. When the system voltage experiences a 50% drop, as... Figure 7 (c) and Figure 7As shown in (d), the AC voltage and current quickly recover to stability after a brief dynamic adjustment in the early stages of the disturbance, demonstrating the system's strong dynamic response capability. Furthermore, as... Figure 7 (e) and Figure 7 As shown in (f), the active power absorbed by the system can quickly follow the power changes of the DC renewable energy power station; the capacitor voltage of the converter submodule remains within the allowable range during steady state and disturbances, with a small voltage ripple amplitude, and can quickly recover to a stable state under control, indicating that the adopted control strategy is effective in submodule voltage equalization and ripple suppression. For low-frequency AC renewable energy power stations, the steady-state waveforms of the power frequency and low-frequency voltages are as follows: Figure 8 (a) and Figure 8 As shown in (b), its power frequency voltage and low-frequency current are both three-phase symmetrical sinusoids, exhibiting excellent waveform quality. When the system power drops by 50%, as... Figure 8 (c) and Figure 8 As shown in (d), the AC currents on the low-frequency side and the power frequency side undergo a short-time transient process and then quickly recover and stabilize in the new operating state; as Figure 8 (e) and Figure 8 As shown in (f), the active power absorbed by the system can quickly respond to changes in the power of low-frequency renewable energy power plants, while the reactive power remains at zero. The capacitor voltage of the converter submodule remains within the allowable range during both steady state and disturbance processes, with a small voltage ripple amplitude. It can also quickly recover to a stable state under control, further verifying the excellent performance of the adopted control strategy in terms of submodule voltage equalization and ripple suppression.

[0117] The foregoing has shown and described the basic principles, main features, and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The embodiments and descriptions in the specification are merely illustrative of the principles of the invention. Various changes and modifications can be made to the invention without departing from its spirit and scope, and all such changes and modifications fall within the scope of the claimed invention.

Claims

1. A flexible low-frequency-flexible DC hybrid power transmission system, characterized in that, This includes receiving-end converter stations, low-frequency AC new energy power stations, and high-voltage DC new energy power stations; The receiving-end converter station has a power frequency AC port, a low frequency AC port, and a high voltage DC port. The power conversion unit of the receiving-end converter station consists of 9 bridge arms. Each bridge arm contains N power modules. Each power module contains 2 AC interfaces and 2 DC interfaces. The DC interfaces are generated by the isolated DC-DC converter inside the power module. The AC interfaces and DC interfaces of the N power modules of each bridge arm are cascaded to form the total AC interface and DC interface of each bridge arm. The total AC interface of each bridge arm forms a one-to-one connection between the power frequency AC port and the low frequency AC port of the receiving-end converter station. The total DC ports of each bridge arm are connected in parallel to form the high voltage DC port. The low-frequency AC new energy power station is connected to the low-frequency AC port of the receiving-end converter station via a low-frequency AC transmission line. The aforementioned high-voltage direct current (HVDC) renewable energy power station is connected to the HVDC port of the receiving-end converter station via a HVDC transmission line.

2. The flexible low-frequency-flexible DC hybrid transmission system as described in claim 1, characterized in that, The power module consists of three full-bridge units, two DC support capacitors, and one high-frequency transformer. Each full-bridge unit consists of four fully controlled switches forming two bridge arms. The DC buses of the first and second full-bridge units are connected in parallel to the two ends of the first DC support capacitor. The midpoints of the two arms of the first full-bridge unit form two AC interfaces. The midpoints of the two arms of the second full-bridge unit are connected to the primary side of the high-frequency transformer. The midpoints of the two arms of the third full-bridge unit are connected to the secondary side of the high-frequency transformer. The DC bus of the third full-bridge unit is connected in parallel to the two ends of the second DC support capacitor and forms two DC interfaces of the power module. The second full-bridge unit, the high-frequency transformer, and the third full-bridge unit constitute an isolated DC-DC converter.

3. The flexible low-frequency-flexible DC hybrid transmission system as described in claim 1, characterized in that, In the low-frequency AC renewable energy power station, each renewable energy unit adopts a low-frequency AC collection and networking voltage boosting operation mode; the high-voltage DC renewable energy power station adopts a variable frequency AC collection and networking operation mode, which is converted to high-voltage DC through a modular multilevel converter at the sending end.

4. The control method for the flexible low-frequency-flexible DC hybrid transmission system according to any one of claims 1-3, characterized in that, include: Low-frequency AC port voltage-current dual closed-loop control is used to generate common-mode component reference values ​​for the three bridge arm voltages of each low-frequency AC output phase. Bridge arm level capacitor voltage balance control is used to generate current reference values ​​for the power frequency AC ports corresponding to the three bridge arms of each low-frequency AC output phase. Power frequency AC port current closed-loop control is used to generate differential mode component reference values ​​for each bridge arm voltage; The capacitor voltage equalization control between the power modules of each bridge arm is used to generate the switch drive signal on the AC interface side of the power module; High-voltage DC port voltage closed-loop control is used to generate DC interface voltage reference values ​​for each power module in each bridge arm; Each power module generates the switching drive signal for the isolated DC-DC converter through high-frequency modulation.

5. The control method as described in claim 4, characterized in that, The specific implementation steps of low-frequency AC port voltage-current dual closed-loop control are as follows: S11) Collect the three-phase output voltage of the low-frequency AC port, collect the AC interface current of the 9 bridge arms, and calculate the three-phase output current of the low-frequency AC port. S12) Perform Clarke and Park coordinate transformations on the three-phase output voltage and three-phase output current to obtain the d-axis and q-axis components of the output voltage and the d-axis and q-axis components of the output current. The phase angle used in the Park coordinate transformation is obtained by integrating the set low-frequency transmission frequency. S13) In the dq coordinate system, the output voltage and output current are controlled by a dual closed loop to generate the dq axis components of the common mode voltage reference values ​​of the 9 bridge arm AC interfaces. S14) Perform Park inverse transformation on the dq axis components of the common-mode voltage reference value to obtain the three-phase common-mode voltage reference value.

6. The control method as described in claim 4, characterized in that, The specific implementation steps of bridge arm capacitor voltage balance control are as follows: S21) Divide the 9 bridge arms into three groups, each group corresponding to the 3 bridge arms of each low-frequency AC port output phase; S22) For x The group of phases, x =U, V, W, collect the DC bus voltages of N power modules in each bridge arm, and sum them to obtain the total DC bus voltage of each bridge arm; S23) Perform three-phase conversion on the sum of the DC bus voltages of each bridge arm. The coordinate transformation of the coordinate system yields the sum of the DC bus voltages of each bridge arm after the transformation; S24) Perform closed-loop control on the sum of the transformed DC bus voltages of each bridge arm to generate a bridge arm power reference value; S25) Calculate based on bridge arm power reference value x Reference values ​​for the positive sequence d-axis component, positive sequence q-axis component, negative sequence d-axis component, and negative sequence q-axis component of the power frequency AC current of the three bridge arms in the dq coordinate system; S26) Construct the component reference values ​​obtained from S25) x Group power frequency AC current in Reference values ​​in a coordinate system; S27) Perform an inverse Clarke transformation on the reference value obtained in S26) to obtain the current reference value of the power frequency AC port in the three-phase stationary coordinate system.

7. The control method as described in claim 4, characterized in that, The specific implementation steps of closed-loop control of power frequency AC port current are as follows: S31) x The three corresponding bridge arms, x =U, V, W, calculate the differential mode component of the bridge arm current, and obtain the three-phase current of the power frequency AC port; S32) Performs closed-loop control on the three-phase currents at the power frequency AC port to generate... x Reference values ​​for the differential mode components of the three bridge arm voltages.

8. The control method as described in claim 4, characterized in that, The specific steps for capacitor voltage equalization control among the power modules of each bridge arm are as follows: S41) Calculate the total reference voltage value of each bridge arm AC interface; S42) For each bridge arm, the nearest level approximation method is used to generate the full-bridge converter switching drive signal corresponding to the AC interface of each power module.

9. The control method as described in claim 4, characterized in that, The specific implementation steps of the high-voltage DC port voltage closed-loop control are as follows: collect the voltage of the high-voltage DC port, perform closed-loop control on it to generate voltage reference values ​​for the DC interfaces of each power module.

10. The control method as described in claim 4, characterized in that, The specific implementation steps for each power module to generate the switching drive signal for the isolated DC-DC converter through high-frequency modulation are as follows: S61) Sample the DC interface output voltage of each power module and perform closed-loop control to generate the phase shift angle of the secondary side of the isolation converter relative to the primary side; S62) Generate complementary drive pulses for both full bridges at a fixed switching frequency. With the input side as a reference, the output side is phase-delayed / leaded according to the phase shift angle, and a dead time is set.

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