New energy low-frequency sending-out networking system

Abstract

The invention relates to the technical field of flexible low-frequency power transmission, and provides a new energy low-frequency transmission networking system. The system comprises a new energy field, an onshore new energy low-frequency pooling station, a main electric energy conversion circuit and a power frequency alternating current power grid which are connected in sequence, and further comprises a plurality of slave electric energy conversion circuits connected with the main electric energy conversion circuit in parallel; wherein the land new energy low-frequency collection station collects new energy low-frequency electric energy output by a new energy field; the main electric energy conversion line and the slave electric energy conversion line convert the new energy low-frequency electric energy into power grid power-frequency electric energy and transmit the power grid power-frequency electric energy to a power-frequency alternating-current power grid; in the electric energy conversion process, a main station AC-AC frequency converter of the main electric energy conversion line obtains the reference output power of each slave station AC-AC frequency converter according to the total output power of the system and the capacity margin of each slave station AC-AC frequency converter; and the slave station AC-AC frequency converter of the slave electric energy conversion line receives the reference output power correspondingly sent by the master station AC-AC frequency converter so as to adjust the output power of the slave station AC-AC frequency converter. The system can ensure fair distribution of power.

Description

Technical Field

[0001] This application relates to the field of flexible low-frequency power transmission technology, and in particular to a new energy low-frequency power transmission network system. Background Technology

[0002] With the acceleration of energy transition, the development of new energy sources is characterized by large-scale, offshore, and clustered development. Traditional high-voltage AC and high-voltage DC transmission technologies have significant limitations in the transmission of new energy power. Low-frequency transmission grid systems for new energy sources, through their characteristics such as low-frequency reactance reduction, converter decoupling, and grid fault tolerance, have become the main means of adapting to the transmission of new energy from offshore locations.

[0003] However, in the case of multiple AC-AC inverters in the low-frequency transmission network system of new energy, the current static power allocation strategy for distributing power among multiple AC-AC inverters may lead to uneven power distribution, affecting the dynamic response capability of the low-frequency transmission network system of new energy to fluctuations in new energy output. Summary of the Invention

[0004] Therefore, it is necessary to provide a new energy low-frequency transmission network system to address the aforementioned technical problems.

[0005] On the one hand, this application provides a new energy low-frequency transmission network system, the system including a new energy field, an onshore new energy low-frequency collection station, a main power conversion line and an industrial frequency AC power grid connected in sequence; the system also includes multiple secondary power conversion lines connected in parallel with the main power conversion line;

[0006] The onshore low-frequency energy collection station is used to collect the low-frequency electrical energy output by the new energy field.

[0007] The main power conversion line and the slave power conversion line are used to convert the new energy low-frequency power into power grid frequency power and transmit it to the power frequency AC power grid.

[0008] In the process of converting the low-frequency power of the new energy source into the power frequency power of the grid, the main station AC-AC converter of the main power conversion line is used to evaluate the capacity margin of each slave station AC-AC converter; based on the total output power of the system and the capacity margin of each slave station AC-AC converter, the reference output power of each slave station AC-AC converter is obtained and sent to each slave station AC-AC converter accordingly.

[0009] The slave AC-AC inverter of the power conversion line is used to receive the reference output power sent by the master AC-AC inverter and adjust its own output power according to the reference output power.

[0010] In one embodiment, the new energy field includes at least one of an offshore low-frequency wind farm and an offshore photovoltaic station; the offshore low-frequency wind farm includes a permanent magnet direct-drive wind turbine; the offshore photovoltaic station includes a photovoltaic array, a controller, a battery pack, and an inverter, the inverter including a DC inverter or an AC inverter.

[0011] In one embodiment, the master station AC-AC inverter obtains the reference output power of each slave station AC-AC inverter based on the total system output power and the capacity margin of each slave station AC-AC inverter, and sends it to each slave station AC-AC inverter accordingly, including:

[0012] The master station AC-AC inverter obtains the capacity ratio coefficient of each slave station AC-AC inverter based on the capacity margin of each slave station AC-AC inverter;

[0013] The master station AC-AC inverter obtains the slave station's total output power based on the system's total output power and its own actual output power;

[0014] The master station AC-AC inverter obtains the reference output power of each slave station AC-AC inverter based on the capacity ratio coefficient of each slave station AC-AC inverter and the total output power of the slave station, and sends it to each slave station AC-AC inverter accordingly.

[0015] In one embodiment, the main power conversion line further includes a low-frequency transformer and a power frequency transformer; the low-frequency transformer, the main station AC-AC converter, and the power frequency transformer are connected in sequence.

[0016] In one embodiment, the power conversion line further includes a low-frequency transformer and a power frequency transformer; the low-frequency transformer, the slave station AC-AC converter, and the power frequency transformer are connected in sequence.

[0017] In one embodiment, the low-frequency transformer between the main power conversion line and the slave power conversion line is connected on the high-voltage side via a low-frequency switch and a low-frequency overhead line; the low-frequency switch performs coordinated control of the active power output between the main station AC-AC converter and the slave station AC-AC converter according to the instructions output by the voltage margin controller.

[0018] In one embodiment, the master station AC-AC inverter and the slave station AC-AC inverter are modular multilevel matrix inverters.

[0019] In one embodiment, the low-frequency side of the master station AC-AC inverter adopts a constant voltage and constant frequency control mode, and the low-frequency side of the slave station AC-AC inverter adopts a constant active power and constant reactive power control mode.

[0020] In one embodiment, the master station AC-AC inverter is used to detect the inter-station voltage phase difference between the master station AC-AC inverter and the slave station AC-AC inverter, and generate a compensation signal based on the inter-station voltage phase difference; and suppress inter-station circulating current based on the compensation signal and the constant voltage and constant frequency control mode.

[0021] In one embodiment, the slave AC-AC inverter is used to acquire the compensation signal generated by the master AC-AC inverter, and generate a phase correction term based on the compensation signal; and suppress inter-station circulating current based on the phase correction term and the constant active power and constant reactive power control modes.

[0022] The aforementioned renewable low-frequency power transmission network system comprises, in sequence, a renewable energy field, an onshore renewable low-frequency collection station, a main power conversion line, and a power frequency AC grid. The system also includes multiple slave power conversion lines connected in parallel with the main power conversion line. The onshore renewable low-frequency collection station is used to collect renewable low-frequency electrical energy output from the renewable energy field. The main power conversion line and slave power conversion lines are used to convert renewable low-frequency electrical energy into power frequency electrical energy for the grid and transmit it to the power frequency AC grid. During the conversion of renewable low-frequency electrical energy into power frequency electrical energy, the master AC-AC converter of the main power conversion line is used to assess the capacity margin of each slave AC-AC converter. Based on the total system output power and the capacity margin of each slave AC-AC converter, the reference output power of each slave AC-AC converter is obtained and sent to each slave AC-AC converter accordingly. The slave AC-AC converters of the slave power conversion lines are used to receive the reference output power sent by the master AC-AC converter and adjust their own output power accordingly. The master AC-AC inverter in this application obtains the reference output power of each slave AC-AC inverter based on the total output power of the system and the capacity margin of each slave AC-AC inverter, and sends it to each slave AC-AC inverter accordingly. The slave AC-AC inverters adjust their own output power according to the reference output power. This can ensure fair power distribution and prevent power overload, and improve the dynamic response capability of the new energy low-frequency power transmission network system to fluctuations in new energy output. Attached Figure Description

[0023] To more clearly illustrate the technical solutions in the embodiments of this application or related technologies, the drawings used in the description of the embodiments of this application or related technologies will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other related drawings can be obtained based on these drawings without creative effort.

[0024] Figure 1 This is a structural diagram of a low-frequency power transmission network system for new energy sources in one embodiment;

[0025] Figure 2This is a schematic diagram of another structure of a low-frequency power transmission network system for new energy sources in one embodiment;

[0026] Figure 3(a) is a simulation result of the voltage margin controller output command in simulation experiment one of the embodiments;

[0027] Figure 3(b) shows the simulation results of the active power output of a gigawatt-level large-scale new energy field in one embodiment of the simulation experiment.

[0028] Figure 3(c) shows the simulation results of the active power output of the master station AC-AC converter and the active power output of the slave station AC-AC converter in one embodiment of the simulation experiment 1.

[0029] Figure 3(d) is a simulation result of the effective value of the low-frequency side voltage of the main station AC-AC converter in one embodiment of simulation experiment 1;

[0030] Figure 4(a) is a simulation result of the voltage margin controller output command in simulation experiment 2 of one embodiment;

[0031] Figure 4(b) shows the simulation results of the active power output of the gigawatt-level large-scale new energy field in one embodiment of simulation experiment 2;

[0032] Figure 4(c) shows the simulation results of the active power output of the master station AC-AC converter and the active power output of the slave station AC-AC converter in simulation experiment 2 of one embodiment;

[0033] Figure 4(d) is a simulation result of the effective value of the low-frequency side voltage of the main station AC-AC converter in simulation experiment 2 of one embodiment;

[0034] Figure 5(a) is a simulation result of the voltage margin controller output command in simulation experiment three of one embodiment;

[0035] Figure 5(b) shows the simulation results of the active power output of a gigawatt-level large-scale new energy field in one embodiment of simulation experiment three;

[0036] Figure 5(c) shows the simulation results of the active power output of the master station AC-AC converter and the slave station AC-AC converter in simulation experiment 3 of one embodiment;

[0037] Figure 5(d) is a simulation result of the effective value of the low-frequency side voltage of the main station AC-AC converter in simulation experiment 3 of one embodiment. Detailed Implementation

[0038] To make the objectives, technical solutions, and advantages of this application clearer, the following detailed description is provided in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the scope of this application.

[0039] It should be noted that the terms "comprising" and "having," and any variations thereof, as used in this application, are intended to cover non-exclusive inclusion. The term "multiple" as used in this application refers to two or more. The term "and / or" as used in this application refers to one of the solutions, or any combination of multiple solutions.

[0040] In one exemplary embodiment, such as Figure 1 As shown, a new energy low-frequency power transmission network system is provided. This system includes a new energy power plant, an onshore new energy low-frequency collection station, a main power conversion line, and a power frequency AC grid, connected sequentially. The system also includes multiple slave power conversion lines connected in parallel with the main power conversion line. The onshore new energy low-frequency collection station is used to collect the new energy low-frequency electrical energy output from the new energy power plant. The main power conversion line and slave power conversion lines are used to convert the new energy low-frequency electrical energy into power frequency electrical energy for the grid and transmit it to the power frequency AC grid. During the conversion of new energy low-frequency electrical energy into power frequency electrical energy, the master AC-AC converter of the main power conversion line is used to evaluate the capacity margin of each slave AC-AC converter. Based on the total system output power and the capacity margin of each slave AC-AC converter, a reference output power is obtained for each slave AC-AC converter and sent accordingly. The slave AC-AC converters of the slave power conversion lines are used to receive the reference output power sent by the master AC-AC converter and adjust their own output power according to the reference output power.

[0041] New energy power plants can be gigawatt (GW) level large-scale new energy power plants.

[0042] The main power conversion line may include a master station AC-AC frequency converter, and the slave power conversion line may include slave station AC-AC frequency converters. The rated capacity of the master station AC-AC frequency converter and each slave station AC-AC frequency converter is different.

[0043] In the new energy low-frequency transmission network system, the new energy field is connected to the onshore new energy low-frequency collection station through low-frequency switches and low-frequency submarine cables; the other end of the onshore new energy low-frequency collection station is connected to multiple low-frequency overhead lines through low-frequency switches, and the multiple low-frequency overhead lines are respectively connected to the main power conversion line and multiple secondary power conversion lines; the other end of the main power conversion line and multiple secondary power conversion lines are respectively connected to the industrial frequency AC power grid.

[0044] During the operation of the new energy low-frequency transmission grid system, the new energy field outputs new energy low-frequency power, the onshore new energy low-frequency collection station collects the new energy low-frequency power, and the main power conversion line and the slave power conversion line convert the new energy low-frequency power into power grid frequency power and transmit it to the power frequency AC grid.

[0045] In the process of converting low-frequency renewable energy into power grid frequency energy, the main station AC-AC converter of the main power conversion line can monitor the total output power P of the renewable energy low-frequency transmission network system in real time through measurement on the low-frequency side of the main station AC-AC converter or by external dispatch. total The system also assesses the capacity margin of each slave AC-AC inverter. The master AC-AC inverter can obtain the reference output power of each slave AC-AC inverter based on the total output power of the system and the capacity margin of each slave AC-AC inverter, and send the reference output power of each slave AC-AC inverter to each slave AC-AC inverter accordingly.

[0046] Each slave AC-AC inverter on the power conversion line can receive the reference output power sent by the master AC-AC inverter, and perform local tracking based on the reference output power to adjust its own output power, thereby achieving adaptive power sharing and avoiding power imbalance, voltage drift and circulating current problems caused by traditional static allocation.

[0047] In the aforementioned new energy low-frequency power transmission network system, the master station AC-AC inverter obtains the reference output power of each slave station AC-AC inverter based on the total output power of the system and the capacity margin of each slave station AC-AC inverter, and sends it to each slave station AC-AC inverter accordingly. The slave station AC-AC inverters adjust their own output power according to the reference output power. This can ensure fair power distribution and prevent power overload, thereby improving the dynamic response capability of the new energy low-frequency power transmission network system to fluctuations in new energy output.

[0048] In one embodiment, the new energy field includes at least one of an offshore low-frequency wind farm and an offshore photovoltaic station; the offshore low-frequency wind farm includes a permanent magnet direct-drive wind turbine; the offshore photovoltaic station includes a photovoltaic array, a controller, a battery pack, and an inverter, the inverter including a DC inverter or an AC inverter.

[0049] New energy farms include at least one of offshore low-frequency wind farms and offshore photovoltaic stations. In addition, other types of new energy farms can be added or expanded according to actual needs.

[0050] Offshore low-frequency wind farms can use permanent magnet direct-drive wind turbines. If the actual scenario requires multiple offshore low-frequency wind farms with different rated capacities, it is necessary to consider issues such as internal circulation within the farm.

[0051] Offshore photovoltaic (PV) stations consist of PV arrays, controllers, battery banks, and DC inverters (or AC inverters). If the actual scenario requires multiple offshore PV stations with different rated capacities, it is necessary to consider issues such as internal circulation current.

[0052] In this embodiment, the new energy field includes at least one of offshore low-frequency wind farms and offshore photovoltaic stations, which can efficiently develop high-quality offshore energy, increase the proportion of clean energy, and ensure energy supply security.

[0053] In one embodiment, the master cycloconverter obtains the reference output power of each slave cycloconverter according to the total output power of the system and the capacity margin of each slave cycloconverter, and sends it to each slave cycloconverter correspondingly. The specific steps are as follows: The master cycloconverter obtains the capacity proportionality coefficient of each slave cycloconverter according to the capacity margin of each slave cycloconverter; The master cycloconverter obtains the total output power of the slave stations according to the total output power of the system and its own actual output power; The master cycloconverter obtains the reference output power of each slave cycloconverter according to the capacity proportionality coefficient of each slave cycloconverter and the total output power of the slave stations, and sends it to each slave cycloconverter correspondingly.

[0054] The master cycloconverter can obtain the capacity proportionality coefficient k (0 < k ≤ 1) of each slave cycloconverter according to the capacity margin of each slave cycloconverter. The higher the capacity margin of the slave cycloconverter, the larger the capacity proportionality coefficient k of the slave cycloconverter.

[0055] The master cycloconverter can obtain the total output power P of the new energy low-frequency transmission networking system in real time by measuring on the low-frequency side of the master cycloconverter or through external dispatching. total , and can obtain the total output power P of the slave stations according to the difference between the total output power P of the system total and its own actual output power P master . total - P master .

[0056] The master cycloconverter can obtain the reference output power of each slave cycloconverter according to the product of the capacity proportionality coefficient of each slave cycloconverter and the total output power of the slave stations, as shown in Equation (1).

[0057] (1)

[0058] Where, represents the reference output power of each slave cycloconverter, represents the capacity proportionality coefficient of each slave cycloconverter, represents the total output power of the system, represents the actual output power of the master cycloconverter.

[0059] The master cycloconverter can send the reference output power of each slave cycloconverter to each slave cycloconverter correspondingly.

[0060] In this embodiment, the master station AC-AC inverter obtains the capacity ratio coefficient of each slave station AC-AC inverter based on the capacity margin of each slave station AC-AC inverter; the master station AC-AC inverter obtains the total output power of the slave stations based on the total output power of the system and its own actual output power, so as to obtain the reference output power of each slave station AC-AC inverter and send it to each slave station AC-AC inverter accordingly; this can ensure fair power distribution and prevent power overload, make full use of the dual-sided control characteristics of the AC-AC inverter, and improve the dynamic response capability of the new energy low-frequency power transmission network system to the fluctuation of new energy output.

[0061] In one embodiment, the main power conversion line also includes a low-frequency transformer and a power frequency transformer; the low-frequency transformer, the main station AC-AC converter, and the power frequency transformer are connected in sequence.

[0062] In the main power conversion line, the low-frequency transformer is connected to the main station AC-AC converter, and the main station AC-AC converter is connected to the power frequency transformer through the power frequency switch.

[0063] The core function of the low-frequency transformer is to connect the low-frequency collection station of onshore new energy with the low-frequency side of the AC-AC converter of the main station. It can bridge the voltage difference between the new energy side and the low-frequency transmission side, ensure equipment safety and system anti-interference capability, and adjust the short-circuit impedance to match the equivalent impedance of the new energy side with the impedance of the main power conversion line, improve power transmission efficiency, and enable the new energy low-frequency power to be efficiently connected to the main power conversion line.

[0064] The main station AC-AC converter is the core conversion unit of the new energy low-frequency transmission grid system. It can realize bidirectional conversion between the two frequencies of new energy low-frequency power and power grid power frequency power, and is a key link to realize the interconnection between new energy low-frequency grid connection and power frequency grid.

[0065] The core function of the power frequency transformer is to connect the power frequency side of the main station AC-AC converter to the power frequency AC power grid. It can bridge the voltage difference between the main station AC-AC converter and the power frequency AC power grid, ensuring that the power frequency energy converted by the main station AC-AC converter meets the grid connection requirements.

[0066] In one embodiment, the power conversion line further includes a low-frequency transformer and a power frequency transformer; the low-frequency transformer, the station AC-AC converter, and the power frequency transformer are connected in sequence.

[0067] In the power conversion line, the low-frequency transformer is connected to the slave station AC-AC converter, and the slave station AC-AC converter is connected to the power frequency transformer through the power frequency switch.

[0068] The core function of the low-frequency transformer is to connect the low-frequency collection station of onshore new energy with the low-frequency side of the AC-AC converter of the slave station. It can bridge the voltage difference between the new energy side and the low-frequency transmission side, ensure equipment safety and system anti-interference capability, and adjust the short-circuit impedance to match the equivalent impedance of the new energy side with the impedance of the power conversion line, improve power transmission efficiency, and enable the new energy low-frequency power to be efficiently connected to the power conversion line.

[0069] The station-based AC-AC converter is the core conversion unit of the new energy low-frequency transmission grid system. It can realize bidirectional conversion between the two frequencies of new energy low-frequency power and power grid frequency power, and is a key link to realize the interconnection between new energy low-frequency grid connection and power grid frequency.

[0070] The core function of a power frequency transformer is to connect the power frequency side of the slave AC-AC converter to the power frequency AC power grid. It can bridge the voltage difference between the slave AC-AC converter and the power frequency AC power grid, ensuring that the power frequency energy converted by the slave AC-AC converter meets the grid connection requirements.

[0071] In one embodiment, the low-frequency transformer between the main power conversion line and the slave power conversion line is connected on the high-voltage side via a low-frequency switch and a low-frequency overhead line; the low-frequency switch performs coordinated control of the active power output between the main station AC-AC converter and the slave station AC-AC converter according to the instructions output by the voltage margin controller.

[0072] The low-frequency switch can coordinate the active power output between the master AC-AC converter and the slave AC-AC converter without communication, based on the instructions output by the voltage margin controller.

[0073] Among them, the voltage margin controller can be added to the outer loop controller of the master station AC-AC converter and each slave station AC-AC converter. Based on the voltage characteristics of the low-frequency side of the master station AC-AC converter and each slave station AC-AC converter, the constraint conditions shown in Equation (2) are established. That is, when the voltage d-axis component of the low-frequency side of the master station AC-AC converter and each slave station AC-AC converter is greater than the maximum value of the voltage margin or less than the minimum value of the voltage margin, the inter-station power coordination is started.

[0074] (2)

[0075] in, This represents the d-axis component of the voltage on the low-frequency side of the AC-AC converter at the master station and each slave station. This represents the maximum voltage margin. This represents the minimum voltage margin.

[0076] The voltage constraints mentioned above can be used for power coordination between power plants. The new energy power plant can monitor the status of the AC-AC converters at the master station and each slave station in real time through the voltage constraints, and output corresponding control commands to control the output active power of the new energy power plant.

[0077] Taking a master AC-AC inverter and a slave AC-AC inverter connected in parallel as an example, the control commands of the voltage margin controller can be set to 0, 1, 2, and 3. Control command 0 indicates that both the master and slave AC-AC inverters are functioning normally, and the renewable energy plant is generating active power normally. Control command 1 indicates that the master AC-AC inverter is faulty, the low-frequency switch is open, and the renewable energy plant's active power output is reduced to half of its normal value. Control command 2 indicates that the slave AC-AC inverter is faulty, the low-frequency switch is open, and the renewable energy plant's active power output is reduced to half of its normal value. Control command 3 indicates that both the master and slave AC-AC inverters are faulty, and the renewable energy plant needs to stop generating active power.

[0078] In addition, the low-frequency side voltage of the master station AC-AC inverter and each slave station AC-AC inverter can be detected in real time according to the above constraints, so as to realize the communication-free switching from the slave station to the master station. That is, the low-frequency side control mode of the slave station AC-AC inverter is switched from constant active power and constant reactive power control mode to constant voltage and constant frequency control mode.

[0079] In this embodiment, the low-frequency transformer between the main power conversion line and the slave power conversion line is connected on the high-voltage side via a low-frequency switch and a low-frequency overhead line. The low-frequency switch coordinates the active power output between the main station AC-AC converter and the slave station AC-AC converter according to the instructions output by the voltage margin controller, thereby ensuring the normal operation of the new energy low-frequency transmission network system.

[0080] In one embodiment, the master station AC-AC inverter and the slave station AC-AC inverter are modular multilevel matrix inverters.

[0081] The master station AC-AC converter and the slave station AC-AC converter can adopt modular multilevel matrix converters (M3C) with corresponding rated active capacity according to the actual needs of the scenario. The modular multilevel matrix converter has excellent power quality, independent control capability and low harmonic characteristics. It can realize bidirectional AC-AC power conversion without intermediate DC link, which can meet the operation requirements of large capacity, low harmonic and high reliability of new energy low frequency transmission grid system.

[0082] In one embodiment, the low-frequency side of the master station AC-AC inverter adopts a constant voltage and constant frequency control mode, while the low-frequency side of the slave station AC-AC inverter adopts a constant active power and constant reactive power control mode.

[0083] The main station AC-AC inverter adopts a constant voltage and constant frequency control mode (which can be called constant Vf control mode) on the low-frequency side, serving as a balancing node in the new energy low-frequency transmission grid system to maintain the voltage and frequency stability of the new energy low-frequency transmission grid system. Here, V represents voltage and f represents frequency.

[0084] The AC-AC converter at the slave station adopts a constant active power and constant reactive power control mode (which can be called constant PQ control mode) on the low-frequency side. Here, P represents active power and Q represents reactive power.

[0085] In practical applications, the alternating frequency converter at the station can also adopt other control modes besides constant voltage and constant frequency control modes on the low-frequency side, depending on actual needs.

[0086] In one embodiment, the master station AC-AC inverter is used to detect the inter-station voltage phase difference between the master station AC-AC inverter and the slave station AC-AC inverter, and generate a compensation signal based on the inter-station voltage phase difference; and suppress inter-station circulating current based on the compensation signal and a constant voltage and constant frequency control mode.

[0087] The circulating current between the master station AC-AC inverter and the slave station AC-AC inverter can be suppressed by using the low-frequency side phase difference compensation algorithm.

[0088] The master AC-AC inverter can use phase-locked loop (PLL) technology to acquire the synchronization signal between its own low-frequency side voltage and the low-frequency side voltage of the slave AC-AC inverter, thus obtaining the inter-station voltage phase difference δ between the master and slave AC-AC inverters. It can then generate a compensation signal Δθ based on this phase difference δ, and superimpose this compensation signal Δθ onto the control reference of its low-frequency side constant voltage and constant frequency control modes to suppress inter-station circulating current. Specifically, the compensation signal Δθ is superimposed onto the voltage phase reference components of the constant voltage and constant frequency control modes to correct the phase angle of the voltage vector.

[0089] In this embodiment, the master station AC-AC inverter detects the inter-station voltage phase difference between the master station AC-AC inverter and the slave station AC-AC inverter to generate a compensation signal. Based on the compensation signal and the constant voltage and constant frequency control mode, the inter-station circulating current is suppressed, which can ensure the stable transmission of power in the new energy low-frequency transmission network system.

[0090] In one embodiment, the slave AC-AC inverter is used to acquire the compensation signal generated by the master AC-AC inverter and generate a phase correction term based on the compensation signal; and suppress inter-station circulating current based on the phase correction term and constant active power and constant reactive power control modes.

[0091] The master AC-AC inverter can send the generated compensation signal Δθ to the slave AC-AC inverter. The slave AC-AC inverter can obtain the compensation signal Δθ generated by the master AC-AC inverter and generate a phase correction term based on the compensation signal Δθ. The phase correction term can be incorporated into the constant active power and constant reactive power control modes on its low-frequency side, thereby ensuring the current phasor alignment between the master AC-AC inverter and the slave AC-AC inverter.

[0092] In this embodiment, the slave AC-AC inverter can generate a phase correction term based on the compensation signal generated by the master AC-AC inverter. Based on the phase correction term and the constant active power and constant reactive power control modes, the inter-station circulating current is suppressed, which can ensure the current phasor alignment between the master AC-AC inverter and the slave AC-AC inverter, and can guarantee the stable transmission of power in the new energy low-frequency power transmission grid system.

[0093] To better understand the above method, the following describes in detail an application embodiment of the new energy low-frequency transmission network system of this application.

[0094] The initial purpose of low-frequency AC transmission technology was primarily to address the demand for power generation and transmission from low-speed hydro turbines, increasing transmission capacity by reducing the transmission frequency. Early low-frequency transmission systems used frequency multiplier transformers to achieve third-order frequency conversion. However, with the development of power electronics technology, modular multilevel converters (such as modular multilevel matrix converters) have become the core AC / AC conversion equipment in low-frequency transmission systems due to their excellent power quality, independent control capabilities, and low harmonic characteristics.

[0095] Compared to traditional power frequency high-voltage AC transmission, low-frequency transmission can significantly suppress the charging current of submarine cables, extending the applicable distance of AC transmission at sea. Compared to high-voltage direct current transmission, low-frequency AC systems require only one onshore converter station, offering significant advantages in cost, reliability, and operation and maintenance. Especially in multi-terminal offshore wind power grid connection scenarios, low-frequency transmission systems can circumvent the technical bottlenecks and fault clearing challenges of DC circuit breakers in flexible DC transmission, inheriting mature control and protection strategies from AC systems, and possessing greater networking flexibility and operational reliability.

[0096] Compared to traditional megawatt (MW) level renewable energy low-frequency transmission network systems, gigawatt-level large-scale renewable energy low-frequency transmission network systems face the challenge of a significant leap in power scale. This requires modular multilevel matrix inverters (MMIs) with higher capacity support to achieve efficient low-frequency networking and power transmission. However, simply increasing the capacity of MPIs often encounters multiple bottlenecks, including increased voltage stress on submodules, more complex thermal management, and a heavier computational burden on control algorithms. If these problems are not innovatively solved, they will lead to decreased efficiency and soaring costs in gigawatt-level large-scale renewable energy low-frequency transmission network systems, and limit the dynamic response capabilities of renewable energy clusters. With the continuous expansion of offshore renewable energy scale and increasingly scarce transmission channel resources, developing renewable energy low-frequency transmission network systems suitable for gigawatt-level applications has significant technical and economic benefits.

[0097] To address the aforementioned issues, this embodiment provides a gigawatt-level low-frequency transmission network system for new energy (which can be referred to as a gigawatt-level large-scale new energy low-frequency transmission network system), providing important technical support for the efficient consumption of large-scale new energy in coastal areas.

[0098] The new energy low-frequency transmission network system provided in this embodiment is as follows: Figure 2 As shown, it includes a gigawatt-scale large-scale new energy power plant, multiple low-frequency submarine cables, an onshore new energy low-frequency collection station, multiple low-frequency overhead lines, multiple AC-AC frequency converters (one of which is the master station AC-AC frequency converter, and the others are slave station AC-AC frequency converters), multiple sets of low-frequency switches, multiple sets of power frequency switches, a power frequency AC power grid, as well as multiple low-frequency transformers and multiple power frequency transformers.

[0099] The gigawatt-scale large-scale renewable energy plant is connected to the onshore renewable energy low-frequency collection station via low-frequency switches and low-frequency submarine cables. The other end of the onshore renewable energy low-frequency collection station is connected to multiple low-frequency overhead lines via low-frequency switches. Each low-frequency overhead line is connected to a low-frequency transformer at the other end. One of these low-frequency transformers is connected to the power frequency AC grid via a main station AC-AC converter, a power frequency switch, and a power frequency transformer. The other low-frequency transformers are connected to the power frequency AC grid via slave station AC-AC converters, power frequency switches, and power frequency transformers. The low-frequency transformers between the main station AC-AC converter and the slave station AC-AC converters are connected on the high-voltage side via low-frequency switches and low-frequency overhead lines.

[0100] The low-frequency transformer, the main station AC-AC converter, and the power frequency transformer connected in sequence in the line where the main station AC-AC converter is located can be called the main power conversion line; the low-frequency transformer, the slave station AC-AC converter, and the power frequency transformer connected in sequence in the line where the slave station AC-AC converter is located can be called the slave power conversion line.

[0101] Under the coordinated control of the power station, the low-frequency electricity generated by the gigawatt-scale renewable energy power plant is transmitted to the onshore renewable energy low-frequency collection station via its respective low-frequency transformers and submarine cables. The onshore renewable energy low-frequency collection station transmits the low-frequency electricity from the gigawatt-scale renewable energy power plant to multiple onshore low-frequency overhead lines via low-frequency switches. The low-frequency electricity from the renewable energy power plant on each low-frequency overhead line is stepped down by low-frequency switches and low-frequency transformers to the low-frequency side of the main station AC-AC converter and the low-frequency side of the slave station AC-AC converter. After dynamic power distribution and inter-station circulating current suppression by the main station AC-AC converter, the main station AC-AC converter and the slave station AC-AC converter convert the distributed low-frequency renewable energy into grid power frequency electricity, which is then transmitted to the power frequency AC grid via power frequency switches and power frequency transformers.

[0102] A gigawatt-scale large-scale renewable energy plant typically consists of an offshore low-frequency wind farm and an offshore solar power station. Other types of renewable energy plants can be added as needed based on specific application scenarios. Offshore low-frequency wind farms can utilize permanent magnet direct-drive wind turbines. If the application scenario requires multiple offshore low-frequency wind farms with different rated capacities, issues such as internal circulation currents within the farm must be carefully considered. Offshore solar power stations consist of photovoltaic arrays, controllers, battery banks, and DC inverters (or AC inverters). If the application scenario requires multiple offshore solar power stations with different rated capacities, issues such as internal circulation currents within the station must be carefully considered.

[0103] The master station AC-AC converter and the slave station AC-AC converter can be modular multilevel matrix converters (M3C) with corresponding rated active capacity according to the actual needs of the scenario.

[0104] One of the multiple AC-AC inverters needs to act as the master AC-AC inverter, while the others are slave AC-AC inverters. The master AC-AC inverter uses a constant voltage and constant frequency control mode (which can be called constant Vf control mode) on the low-frequency side. This means that voltage V and frequency f are used as control targets, serving as a balancing node in the renewable energy low-frequency power transmission network system to maintain voltage and frequency stability. The slave AC-AC inverters use a constant active power and constant reactive power control mode (which can be called constant PQ control mode) on the low-frequency side. Here, P represents active power and Q represents reactive power. In practical applications, the slave AC-AC inverters can also use other control modes besides constant voltage and constant frequency control modes on the low-frequency side, depending on actual needs.

[0105] If the actual scenario requires two or more AC-AC inverters with different rated capacities, then dynamic power distribution between AC-AC inverter stations and suppression of inter-station circulating current must be considered.

[0106] The main station cycloconverter can obtain the capacity ratio coefficient k (0 < k ≤ 1) of each slave station cycloconverter according to the capacity margin of each slave station cycloconverter. The higher the capacity margin of the slave station cycloconverter, the larger the capacity ratio coefficient k of the slave station cycloconverter.

[0107] The main station cycloconverter can obtain the total system output power P of the new energy low-frequency transmission network system in real time by measuring at the low-frequency side of the main station cycloconverter or through external dispatching. total According to the total system output power P total and its own actual output power P master The difference between them can be used to obtain the total output power P of the slave stations total -P master .

[0108] The main station cycloconverter can obtain the reference output power of each slave station cycloconverter according to the product of the capacity ratio coefficient of each slave station cycloconverter and the total output power of the slave stations, as shown in Equation (1).

[0109] (1)

[0110] Where represents the reference output power of each slave station cycloconverter, represents the capacity ratio coefficient of each slave station cycloconverter, represents the total system output power, represents the actual output power of the main station cycloconverter.

[0111] The main station cycloconverter can send the reference output power of each slave station cycloconverter to each slave station cycloconverter correspondingly. Each slave station cycloconverter of each slave power conversion line can receive the reference output power sent by the main station cycloconverter correspondingly, and perform local tracking according to the reference output power, adjust its own output power, achieve adaptive power sharing, ensure fair power distribution and prevent overload, and avoid problems such as uneven power, voltage drift and circulating current caused by traditional static distribution. The bilateral control characteristics of the cycloconverter can be fully utilized to improve the dynamic response ability of the new energy low-frequency transmission network system to the fluctuations of new energy output.

[0112] The circulating current between the main station cycloconverter and the slave station cycloconverter can be suppressed according to the low-frequency side phase difference compensation algorithm.

[0113] The master AC-AC inverter can use phase-locked loop (PLL) technology to acquire the synchronization signal between its own low-frequency side voltage and the low-frequency side voltage of the slave AC-AC inverter, thus obtaining the inter-station voltage phase difference δ between the master and slave AC-AC inverters. Based on this phase difference δ, a compensation signal Δθ can be generated and superimposed on the control reference of its low-frequency side constant voltage and constant frequency control modes to suppress inter-station circulating current. Specifically, the compensation signal Δθ is superimposed on the voltage phase reference component of the constant voltage and constant frequency control modes to correct the phase angle of the voltage vector.

[0114] The low-frequency switch can coordinate the active power output between the master AC-AC converter and the slave AC-AC converter without communication, based on the instructions output by the voltage margin controller.

[0115] Among them, the voltage margin controller can be added to the outer loop controller of the master station AC-AC converter and each slave station AC-AC converter. Based on the voltage characteristics of the low-frequency side of the master station AC-AC converter and each slave station AC-AC converter, the constraint conditions shown in Equation (2) are established. That is, when the voltage d-axis component of the low-frequency side of the master station AC-AC converter and each slave station AC-AC converter is greater than the maximum value of the voltage margin or less than the minimum value of the voltage margin, the inter-station power coordination is started.

[0116] (2)

[0117] in, This represents the d-axis component of the voltage on the low-frequency side of the AC-AC converter at the master station and each slave station. This represents the maximum voltage margin. This represents the minimum voltage margin.

[0118] The voltage constraints mentioned above can be used for power coordination between power plants. The new energy power plant can monitor the status of the AC-AC converters at the master station and each slave station in real time through the voltage constraints, and output corresponding control commands to control the output active power of the new energy power plant.

[0119] Taking a master AC-AC inverter and a slave AC-AC inverter connected in parallel as an example, the control commands of the voltage margin controller can be set to 0, 1, 2, and 3. Control command 0 indicates that both the master and slave AC-AC inverters are functioning normally, and the renewable energy plant is generating active power normally. Control command 1 indicates that the master AC-AC inverter is faulty, the low-frequency switch is open, and the renewable energy plant's active power output is reduced to half of its normal value. Control command 2 indicates that the slave AC-AC inverter is faulty, the low-frequency switch is open, and the renewable energy plant's active power output is reduced to half of its normal value. Control command 3 indicates that both the master and slave AC-AC inverters are faulty, and the renewable energy plant needs to stop generating active power.

[0120] In addition, the low-frequency side voltage of the master station AC-AC inverter and each slave station AC-AC inverter can be detected in real time according to the above constraints, so as to realize the communication-free switching from the slave station to the master station. That is, the low-frequency side control mode of the slave station AC-AC inverter is switched from constant active power and constant reactive power control mode to constant voltage and constant frequency control mode.

[0121] In summary, the new energy low-frequency transmission network system provided in this embodiment can realize the long-distance collection and transmission of large-scale offshore new energy, and can improve the new energy absorption capacity and grid operation efficiency.

[0122] To verify the correctness of the new energy low-frequency transmission network system provided in this embodiment, a simulation model with two AC-AC converters connected in parallel can be built in the power system electromagnetic transient simulation software. The new energy low-frequency power generated by the gigawatt-level large-scale new energy field is transmitted to the onshore collection station via a low-frequency transformer and submarine cable, and then led to the overhead line via a low-frequency switch. After being stepped down, it is connected to the low-frequency side of the master station AC-AC converter and the low-frequency side of the slave station AC-AC converter. Finally, it is converted into power frequency power by the master station AC-AC converter and the slave station AC-AC converter and connected to the power frequency AC grid via a power frequency transformer.

[0123] Specifically, the low-frequency side of the AC-AC inverter at the slave station adopts constant active power and constant reactive power control mode, while the power frequency side adopts stator module capacitor voltage and constant reactive power control mode. The low-frequency side of the AC-AC inverter at the master station adopts constant voltage and constant frequency control mode, while the power frequency side adopts stator module capacitor voltage and constant reactive power control mode. A gigawatt-scale large-scale renewable energy plant can use five 1GW equivalent renewable energy plants to simulate the power output characteristics of offshore low-frequency wind farms and offshore photovoltaic stations.

[0124] Simulation Experiment 1: At 5s, the active power output of a gigawatt-level large-scale renewable energy plant is increased from 3GW to 5GW, and at 8s, it is reduced to 2.5GW.

[0125] Figures 3(a)-3(d) show the simulation results of the dynamic characteristics of the new energy low-frequency transmission network system with two AC-AC inverters in parallel. As shown in Figure 3(a), during operation, the voltage margin controller output command is 0, indicating that both the master and slave AC-AC inverters are operating normally. Figure 3(b) shows that the active power output of the gigawatt-level large-scale new energy power plant increases from 3GW to 5GW in 5 seconds, and then decreases to 2.5GW after 3 seconds, demonstrating excellent dynamic response. As shown in Figure 3(c), since the active power output command of the slave AC-AC inverter is 2.5GW, the slave AC-AC inverter maintains 2.5GW of active power transmission during the active power adjustment period of the gigawatt-level large-scale new energy power plant. The master AC-AC inverter responds well to changes in the active power of the new energy power plant, transmitting the difference in active power between the gigawatt-level large-scale new energy power plant and the slave AC-AC inverter. Figure 3(d) shows that the effective value of the low-frequency side voltage of the main station AC-AC inverter is maintained at around 400kV, supporting the voltage and frequency of the new energy low-frequency transmission grid system.

[0126] Simulation Experiment 2: At 5 seconds, the main station AC-AC converter is set to malfunction, making it difficult to support the voltage and frequency of the new energy low-frequency transmission network system.

[0127] Figures 4(a)-4(d) show the coordination simulation results under the fault of the main station AC-AC inverter. Figure 4(a) shows that the voltage margin controller output command switches from 0 to 1 at 5s, indicating that the main station AC-AC inverter station has failed. As shown in Figure 4(b), after receiving command 1, the gigawatt-level large-scale renewable energy plant reduces its active power output from 5GW to 2.5GW. Figure 4(c) shows that after the fault of the main station AC-AC inverter station, the active power transmission drops to 0, while the slave station AC-AC inverter station, after receiving the abnormal control command, switches from constant active power and constant reactive power control mode to constant voltage and constant frequency control mode, thereby ensuring the overall stability of the renewable energy low-frequency transmission network system and the normal transmission of active power from the gigawatt-level large-scale renewable energy plant. As shown in Figure 4(d), the effective value of the low-frequency side voltage of the slave station AC-AC inverter station is maintained at around 400kV, and the voltage fluctuates within the normal range at the moment of control mode switching.

[0128] Simulation Experiment 3: At 5 seconds, a fault is set in the slave station AC-AC inverter station, making it difficult to transmit active power normally.

[0129] Figures 5(a)-5(d) show the coordination simulation results under a fault in the slave AC-AC inverter. As shown in Figure 5(a), the voltage margin controller output command switches from 0 to 2 at 5 seconds, indicating a fault in the slave AC-AC inverter station. Figure 5(b) shows that after receiving command 2, the gigawatt-level large-scale renewable energy plant reduces its active power output from 5GW to 2.5GW. As shown in Figure 5(c), after the slave AC-AC inverter station faults, the active power transmission drops to 0, while the master AC-AC inverter station maintains normal operation, transmitting 2.5GW of active power. Figure 5(d) shows that the effective value of the low-frequency side voltage of the master AC-AC inverter station remains around 400kV, indicating that the voltage remains stable even when the slave AC-AC inverter station fails.

[0130] In summary, the renewable low-frequency power transmission network system provided in this embodiment can stably transmit renewable low-frequency power from gigawatt-scale renewable energy plants to the industrial frequency AC grid, thereby improving renewable energy absorption capacity and grid operating efficiency. Simulation results with two AC-AC converters in parallel further verify the correctness of the renewable low-frequency power transmission network system in this embodiment. Furthermore, corresponding coordinated control measures are proposed for both master and slave station faults. Finally, simulation results with two M3C converters in parallel further verify the correctness of the renewable low-frequency power transmission network system.

[0131] It should be understood that although the steps in the flowcharts of the embodiments described above are shown sequentially according to the arrows, these steps are not necessarily executed in the order indicated by the arrows. Unless explicitly stated herein, there is no strict order restriction on the execution of these steps, and they can be executed in other orders. Moreover, at least some steps in the flowcharts of the embodiments described above may include multiple steps or multiple stages. These steps or stages are not necessarily completed at the same time, but can be executed at different times. The execution order of these steps or stages is not necessarily sequential, but can be performed alternately or in turn with other steps or at least some of the steps or stages in other steps. It is understood that the steps in different embodiments can be freely combined as needed, and all non-contradictory solutions formed by such combinations are within the scope of protection of this application.

[0132] It should be noted that the user information (including but not limited to user device information, user personal information, etc.) and data (including but not limited to data used for analysis, data stored, data displayed, etc.) involved in this application are all information and data authorized by the user or fully authorized by all parties, and the collection, use and processing of the relevant data must comply with relevant regulations.

[0133] Those skilled in the art will understand that all or part of the processes in the methods of the above embodiments can be implemented by a computer program instructing related hardware. The computer program can be stored in a non-volatile computer-readable storage medium. When executed, the computer program can include the processes of the embodiments of the above methods. Any references to memory, database, or other media used in the embodiments provided in this application can include at least one of non-volatile memory and volatile memory. Non-volatile memory can include read-only memory (ROM), magnetic tape, floppy disk, flash memory, optical memory, high-density embedded non-volatile memory, resistive random access memory (ReRAM), magnetic random access memory (MRAM), ferroelectric random access memory (FRAM), phase change memory (PCM), graphene memory, etc. Volatile memory can include random access memory (RAM) or external cache memory, etc. By way of illustration and not limitation, RAM can take many forms, such as Static Random Access Memory (SRAM) or Dynamic Random Access Memory (DRAM). The databases involved in the embodiments provided in this application may include at least one type of relational database and non-relational database. Non-relational databases may include, but are not limited to, blockchain-based distributed databases. The processors involved in the embodiments provided in this application may be general-purpose processors, central processing units, graphics processing units, digital signal processors, programmable logic devices, quantum computing-based data processing logic devices, artificial intelligence (AI) processors, etc., and are not limited to these.

[0134] The technical features of the above embodiments can be combined in any way. For the sake of brevity, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this application.

[0135] The embodiments described above are merely illustrative of several implementation methods of this application, and while the descriptions are specific and detailed, they should not be construed as limiting the scope of this patent application. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of this application, and these all fall within the protection scope of this application. Therefore, the protection scope of this application should be determined by the appended claims.

Claims

1. A new energy low-frequency transmission network system, characterized in that, The system includes a new energy field, an onshore new energy low-frequency collection station, a main power conversion line, and an industrial frequency AC power grid connected in sequence; the system also includes multiple secondary power conversion lines connected in parallel with the main power conversion line. The onshore new energy low-frequency collection station is used to collect the new energy low-frequency electrical energy output by the new energy field; The main power conversion line and the slave power conversion line are used to convert the new energy low-frequency power into grid power frequency power and transmit it to the grid power frequency AC power grid. In the process of converting the low-frequency power of the new energy source into the power frequency power of the grid, the main station AC-AC converter of the main power conversion line is used to evaluate the capacity margin of each slave station AC-AC converter; based on the total output power of the system and the capacity margin of each slave station AC-AC converter, the reference output power of each slave station AC-AC converter is obtained and sent to each slave station AC-AC converter accordingly. The slave AC-AC inverter of the power conversion line is used to receive the reference output power sent by the master AC-AC inverter and adjust its own output power according to the reference output power.

2. The system according to claim 1, characterized in that, The new energy field includes at least one of offshore low-frequency wind farm and offshore photovoltaic station; the offshore low-frequency wind farm includes permanent magnet direct-drive wind turbine; the offshore photovoltaic station includes photovoltaic array, controller, battery pack and inverter, the inverter includes DC inverter or AC inverter.

3. The system according to claim 1, characterized in that, The master station AC-AC inverter obtains the reference output power of each slave station AC-AC inverter based on the total output power of the system and the capacity margin of each slave station AC-AC inverter, and sends it to each slave station AC-AC inverter accordingly, including: The master station AC-AC inverter obtains the capacity ratio coefficient of each slave station AC-AC inverter based on the capacity margin of each slave station AC-AC inverter; The master station AC-AC inverter obtains the slave station's total output power based on the system's total output power and its own actual output power; The master station AC-AC inverter obtains the reference output power of each slave station AC-AC inverter based on the capacity ratio coefficient of each slave station AC-AC inverter and the total output power of the slave station, and sends it to each slave station AC-AC inverter accordingly.

4. The system according to claim 1, characterized in that, The main power conversion line also includes a low-frequency transformer and a power frequency transformer; the low-frequency transformer, the main station AC-AC converter, and the power frequency transformer are connected in sequence.

5. The system according to claim 4, characterized in that, The power conversion line also includes a low-frequency transformer and a power frequency transformer; the low-frequency transformer, the slave station AC-AC converter, and the power frequency transformer are connected in sequence.

6. The system according to claim 5, characterized in that, The low-frequency transformer between the main power conversion line and the slave power conversion line is connected on the high-voltage side via a low-frequency switch and a low-frequency overhead line; the low-frequency switch performs coordinated control of the active power output between the main station AC-AC converter and the slave station AC-AC converter according to the instructions output by the voltage margin controller.

7. The system according to claim 1, characterized in that, The master station AC-AC inverter and the slave station AC-AC inverter are modular multilevel matrix inverters.

8. The system according to claim 7, characterized in that, The low-frequency side of the master station AC-AC inverter adopts a constant voltage and constant frequency control mode, while the low-frequency side of the slave station AC-AC inverter adopts a constant active power and constant reactive power control mode.

9. The system according to claim 8, characterized in that, The master station AC-AC inverter is used to detect the inter-station voltage phase difference between the master station AC-AC inverter and the slave station AC-AC inverter, and generate a compensation signal based on the inter-station voltage phase difference; based on the compensation signal and the constant voltage and constant frequency control mode, it suppresses inter-station circulating current.

10. The system according to claim 9, characterized in that, The slave station AC-AC inverter is used to acquire the compensation signal generated by the master station AC-AC inverter, and generate a phase correction term based on the compensation signal; and suppress inter-station circulating current based on the phase correction term and the constant active power and constant reactive power control modes.

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