A new energy grid-connected topology based on a resolver, a control system and a method

Abstract

The application provides a new energy grid-connected topology based on a resolver, a control system and a method, which comprise an inverter (1), a resolver (2) and a grid-connected switch (3), wherein the AC side wiring terminal of the inverter (1) is led out from the three-phase bridge arm midpoint to be connected to the primary side three-phase wiring terminal of the resolver (2); the primary side wiring terminal of the resolver (2) is connected to the inverter (1), and the secondary side wiring terminal is connected to the grid-connected switch (3); and the grid-connected switch (2) is connected in series between the resolver (2) and the power grid. The technical problems of poor electromagnetic oscillation suppression effect, lack of inertia support, difficult grid-connected system design, low power system operation safety, high technology use cost and small short-circuit current during short-circuit fault of the existing new energy grid-connected technology are solved.

Description

Technical Field

[0001] This invention belongs to the field of new energy grid connection technology, specifically relating to a new energy grid connection topology, control system and method based on a rotary transformer. Background Technology

[0002] To protect the natural environment and address climate change, my country has set the goal of "peaking carbon and achieving carbon neutrality." To achieve this goal, clean new energy sources, represented by wind power and solar power, will increasingly be connected to the power grid. However, currently, new energy sources are connected to the grid via power electronic converters. The high proportion of power electronic devices can induce electromagnetic oscillations in the power system, potentially damaging grid-connected equipment, jeopardizing grid stability, and causing major accidents. Furthermore, as the installed capacity of traditional synchronous generators decreases, the rotational inertia in the power system is also decreasing, making it difficult to provide inertial support for the grid, which can also lead to grid stability issues. Therefore, there is an urgent need for a grid connection scheme to achieve friendly grid connection of new energy sources and distributed power sources, ensuring the safe and stable operation of the power system.

[0003] In 2009, a subsynchronous resonance (20Hz) occurred in Texas, USA, caused by the interaction between a doubly-fed induction generator (DFIG) wind turbine cluster and a series compensation line, resulting in turbine disconnection from the grid and damage to the crowbar circuit. Since 2011, multiple instances of subsynchronous resonance (3-10Hz) have occurred in Guyuan, Hebei, my country, involving the interaction between DFIG wind turbine clusters and a series compensation line, causing turbine disconnection from the grid and abnormal transformer vibration. In 2015, a subsynchronous / supersynchronous resonance (SCR) occurred in Hami, Xinjiang, my country, involving a direct-drive wind turbine cluster and a weak power grid, exhibiting oscillation components across multiple frequency ranges, triggering torsional vibration in the shaft systems of three thermal power units and causing them to trip. These cases demonstrate that electromagnetic oscillations induced by a high proportion of power electronic equipment in power systems differ from the low-frequency and subsynchronous electromechanical oscillations involving traditional generators. ESCRs are characterized by a wide oscillation frequency range and complex oscillation mechanisms, posing significant safety hazards to power systems. The mechanism analysis of this new type of electromagnetic oscillation mainly adopts the impedance analysis method. It is believed that the power electronic equipment such as grid-connected converters, LC or LCL filters and the power grid work together to form an equivalent LC resonant circuit in the whole system. Once the equivalent LC resonant circuit is formed, there is a resonant point. When a component or device in the system provides negative damping at the resonant frequency, the equivalent LC resonant circuit will oscillate, causing power system instability. The control structure, control parameters, phase-locked loop (PLL), and grid short-circuit ratio of grid-connected converters all affect the impedance characteristics of the equivalent circuit. For example, the paper "Research on Impedance Adaptive Dual-Mode Control of High-Penetration New Energy Power Generation Grid-Connected Inverter" (Li Ming, Hefei University of Technology), published in December 2020, proves that when grid voltage feedforward control is adopted, a positive feedback channel related to grid impedance is introduced, which may cause harmonic oscillations. The paper "Analysis of the Influence of Reactive Power Control on the Stability of Grid-Connected Converters under Weak Grid" (Xu Luyao et al., Automation of Electric Power Systems), published in August 2019, proves that the reactive power outer loop of the converter will affect the impedance characteristics of the system, thereby affecting the system stability. The paper "Research on the Mechanism and Stability Criterion of Machine-Grid Electrical Oscillation of Voltage Source Type Grid-Connected Converter" (Zhang Chen et al., Journal of Electrical Engineering), published in June 2017, analyzes the RLC equivalent circuit of the "machine-grid system" and concludes that the PLL bandwidth, current loop bandwidth, and grid short-circuit ratio all affect the damping of the RLC equivalent circuit, causing changes in system stability.

[0004] Regarding the prevention and control of electromagnetic oscillations caused by power electronic devices in power systems, the 2019 publication "Mitigating subsynchronous control interaction in wind power systems: Existing techniques and open challenges" (Renewable and Sustainable Energy Reviews, 2019, 108: 330-346) proposes installing direct-drive wind turbines and constructing hybrid wind farms in doubly-fed induction generator (DFIG) wind farms to improve system damping. The 2020 publication "Analysis and mitigation of SSCI in DFIG systems with experimental validation" (IEEE Transactions on Energy) further supports this approach. Conversion, 2020, 35(2): 714-723, IEEE Transactions on Energy Conversion, 2020, 35(2): 714-723) By optimizing the control parameters of the grid-connected converter, the system damping at the resonant frequency is increased, thereby suppressing electromagnetic oscillations in the doubly fed wind turbine system; The literature "Resonance Problem and Active Suppression Method of Flexible DC Transmission System" (Tang Xin et al., Proceedings of the CSEE) published in 2014 proposes a virtual resistance control method for flexible DC transmission systems, which can suppress electromagnetic oscillations. The invention patent application number CN202110501964.4, entitled "A Method for Suppressing Grid-Connected Resonance of String Photovoltaic Cluster Inverters Based on Hybrid Damping," includes the following steps: Step 1: Analyzing the resonance mechanism and characteristics of the grid-connected inverter using a mathematical model to obtain the resonance frequency of the photovoltaic power generation system; Step 2: Introducing capacitor current feedback and grid-connected voltage proportional feedforward as active damping in the inverter current loop based on the resonance frequency of the photovoltaic power generation system; Step 3: Adding a second-order RLC resonance suppression circuit as passive damping based on the capacitor current feedback circuit and grid-connected voltage proportional feedforward control strategy in Step 2, and setting the second-order RLC resonance suppression circuit using the resonance frequency of the photovoltaic power generation system.In summary, current methods for suppressing electromagnetic oscillations caused by power electronic devices in power systems primarily focus on altering the system's equivalent impedance. However, the factors influencing the system's equivalent impedance are multifaceted. Controlling one factor makes it difficult to maintain the equivalent impedance at the desired value when other factors change. Furthermore, changes in the resonant frequency may prevent the suppression of electromagnetic oscillations. Therefore, methods that suppress electromagnetic oscillations by altering the system's equivalent impedance cannot fundamentally solve the problem.

[0005] To address the issue that the rotational inertia of the power grid decreases after new energy sources are connected to the grid, thus failing to effectively support the grid frequency, virtual synchronous generator control technology is currently widely used to simulate the rotor inertia of traditional synchronous generators. However, virtual synchronous generator control technology requires an energy storage system, which increases costs.

[0006] Furthermore, when a short-circuit fault occurs, power electronic converters do not exhibit a subtransient process similar to that of motors. The initial short-circuit current is lower, and due to higher internal reactance and current closed-loop control, the steady-state short-circuit current also decreases rapidly. Therefore, the amplitude and duration of the short-circuit current when a power electronic converter is connected to the grid are smaller than when a motor is connected. Moreover, because power electronic converters have limited overcurrent capacity, it is generally stipulated that the short-circuit current provided by a power electronic converter should not exceed twice its rated current. An excessively small short-circuit current may prevent protection devices from being activated, posing challenges to short-circuit relay protection in the power system.

[0007] In summary, existing technologies that modify the system's equivalent impedance cannot fundamentally suppress electromagnetic oscillations caused by power electronic devices and increase the difficulty of system design; using virtual synchronous generator control technology to simulate rotating inertia increases costs; and the short-circuit current provided by power electronic converters is relatively small, posing challenges to short-circuit protection of the power system. Existing technologies suffer from several technical problems, including poor electromagnetic oscillation suppression, lack of inertia support, difficulties in grid-connected system design, low power system operational safety, high technology usage costs, and relatively small short-circuit current during short-circuit faults. Summary of the Invention

[0008] The technical problem to be solved by this invention is how to solve the existing technical problems of new energy grid connection technology, such as poor electromagnetic oscillation suppression effect, lack of inertia support, difficulty in grid connection system design, low power system operation safety, high technology use cost and small short circuit current during short circuit faults.

[0009] The present invention solves the above-mentioned technical problems by adopting the following technical solution: a new energy grid-connected topology based on a rotary transformer includes: an inverter (1), a rotary transformer (2) and a grid-connected switch (3).

[0010] The AC side terminals of the inverter (1) are led out from the midpoint of the three-phase bridge arm to connect to the three-phase terminals of the primary side of the rotary transformer (2);

[0011] The primary side of the rotary transformer (2) is connected to the inverter (1), and the secondary side is connected to the grid-connected switch (3).

[0012] The grid-connected switch (3) is connected in series between the rotary transformer (2) and the power grid.

[0013] This invention uses a dual three-phase motor as a rotary transformer and proposes a new renewable energy grid connection scheme based on this. The motor stator of this invention is directly connected to the grid, possessing the same overload capacity as thermal power units, sufficient voltage and frequency deviation tolerance, and the ability to provide sufficiently large short-circuit current. Similar to traditional thermal power grid connection, this invention connects the motor to the grid. Because the motor itself has a filtering function, no LC or LCL filter is needed, eliminating the capacitor components in the renewable energy grid connection system. The equivalent LC resonant circuit cannot be formed, suppressing electromagnetic oscillations at the source.

[0014] In a more specific technical solution, the inverter (1) is a three-phase bridge inverter, which includes power switching devices IGBTs.

[0015] In a more specific technical solution, the DC side of the inverter (1) is connected to the new energy power generation system to transmit electrical energy to the power grid through the new energy power generation system.

[0016] In a more specific technical solution, the new energy power generation system includes wind power and photovoltaic energy.

[0017] In the grid-connected topology proposed in this invention, the motor connection port is connected to the power grid in the same way as the traditional synchronous generator connection to the power grid, with the same grid connection port. The inverter in the proposed grid-connected topology is also present in commonly used new energy grid-connected solutions, so the proposed grid-connected topology can be plugged and used, which is beneficial for the transformation and upgrading of existing new energy power plants such as wind power and photovoltaic power.

[0018] In a more specific technical solution, the rotary transformer (2) includes no less than two sets of stator windings, and the stator winding connection method includes: star connection and neutral point isolation. The rotary transformer (2) has rotational inertia when it is working.

[0019] This invention introduces the rotational inertia of a dual three-phase motor into the power system, ensuring the inertia support of the power system. Compared with the currently commonly used renewable energy grid connection schemes that use power electronic converters, this invention introduces the rotational inertia into the power system through a rotary transformer, avoiding the problem of decreasing power system inertia under high renewable energy penetration and enhancing the stability of the power system.

[0020] In a more specific technical solution, the stator winding includes a first stator winding and a second stator winding. The first stator winding is used as the primary side of the rotary transformer (2) and connected to the inverter (1). The second stator winding is used as the secondary side of the rotary transformer (2) and connected to the power grid through the grid-connected switch (3). The electrical angle between the first stator winding and the second stator winding includes a 30° electrical angle difference and a 60° electrical angle difference.

[0021] This invention, through the optimized design of a dual-winding motor, achieves voltage level matching, avoiding the need for a step-up transformer in conventional converter grid connection schemes, and without significantly increasing hardware costs. The rotary transformer, with its two sets of stator windings and isolated neutral points, provides the same isolation function as a traditional transformer, ensuring electrical insulation between the power grid and the new energy power generation system. This effectively prevents further escalation of the accident in the event of a fault on one side, enhancing the safety of the power system.

[0022] In a more specific technical solution, the rotary transformer (2) adopts a dual three-phase asynchronous motor or a dual three-phase synchronous motor.

[0023] In a more specific technical solution, the rotary transformer (2) is connected to the relay protection device in the power grid to provide a short-circuit current to the relay protection device, the magnitude of which is adapted to the relay protection device.

[0024] Compared to existing grid-connected technologies, power electronic converters, when experiencing short-circuit faults, lack the subtransient process of motors and possess higher internal reactance and current closed-loop control. Therefore, the amplitude and duration of short-circuit current when connected to the grid with a power electronic converter are smaller than when a motor is connected. Furthermore, due to the limited overcurrent capacity of power electronic converters, it is generally stipulated that the short-circuit current provided by a power electronic converter should not exceed twice its rated current. Excessively small short-circuit currents may prevent protection devices from activating, posing challenges to short-circuit relay protection in power systems. This invention inherits the characteristics of traditional thermal power unit grid systems. Without altering the analysis and design theories of the power system, direct grid connection of a motor can provide a sufficiently large short-circuit current to activate relay protection devices.

[0025] In a more specific technical solution, a control system for a new energy grid-connected topology based on a rotary transformer includes:

[0026] The given power calculation module is used to calculate the grid-connected active power setpoint based on the detected grid voltage amplitude v and frequency f. and reactive power setpoint When the grid frequency f is lower than the preset standard value, the active power setpoint is increased. When the voltage amplitude v is lower than the preset standard value, the reactive power setpoint is increased. ;

[0027] A current calculation module is provided to calculate the current based on the given active power value. and the given reactive power value Calculate the given value of the d-axis current of the primary side of the rotary transformer (2) in the synchronously rotating dq coordinate system. and q-axis current setpoint The given current calculation module is connected to the given power calculation module;

[0028] A current regulator is used to perform closed-loop control of the d-axis and q-axis currents of the primary side of the rotary transformer (2) to detect the obtained three-phase currents of the primary side. , , The d-axis current is obtained through Clark and Park transforms. q-axis current , to the given value of the d-axis current and the given q-axis current value The comparison is used to output the d-axis voltage setpoint via the deviation value current regulator. and q-axis voltage setpoint The current regulator is connected to the given current calculation module;

[0029] The coordinate transformation module includes three coordinate transformations: Clark transformation, Park transformation, and inverse Park transformation. The Clark transformation is used to transform the detected current in the three-phase stationary coordinate system of the primary side of the rotary transformer. , , Converted to current in a two-phase stationary α-β coordinate system , Park transformation is used to transform , Converted to current in synchronous rotating dq coordinate system , The Park inverse transform is used to convert the given d-axis voltage value and the q-axis voltage setpoint Converted to voltage setpoint in a two-phase stationary α-β coordinate system , The coordinate transformation module is connected to the current regulator;

[0030] PWM modulation module, used to adjust the voltage according to the given value. , Calculate the operating signals of the three-phase bridge arm switching transistors of the output inverter (1). , , The output voltage of the inverter (1) is controlled to control the primary current of the rotary transformer (2) to track a given value. The PWM modulation module is connected to the conversion module.

[0031] The inverter (1) is an actuator in the control system. It is a three-phase bridge inverter composed of power switching devices IGBTs. The inverter operates according to the action signals of the three-phase bridge arm switching transistors. , , Controlling the on and off of the power switching transistors adjusts the output voltage of the inverter (1), thereby controlling the primary current of the rotary transformer (2);

[0032] The rotary transformer (2) is a controlled object in the control system. It is a dual three-phase motor. The rotary transformer (2) includes no less than two sets of stator windings. The stator windings are respectively connected to the inverter (1) and the power grid. The stator windings are all connected in a star configuration and are isolated by a neutral point. The rotary transformer (2) has rotational inertia when it is working, which is used to provide inertia support for the power system.

[0033] The two ends of the grid-connected switch (3) are connected to the rotary transformer (2) and the power grid, respectively. The inverter (1) is connected to the new energy power generation system to control the grid-connected and off-grid status of the new energy power generation system by opening and closing the grid-connected switch (3).

[0034] In more specific technical solutions, the methods for grid-connected control of the control system include:

[0035] S1. Based on the detected grid voltage amplitude v and frequency f, the grid-connected active power setpoint is calculated. and reactive power setpoint When the grid frequency f is lower than the preset standard value, the active power setpoint is increased. When the voltage amplitude v is lower than the preset standard value, the reactive power setpoint is increased. ;

[0036] S2, based on the given active power value and the given reactive power value Calculate the given value of the d-axis current of the primary side of the rotary transformer (2) in the synchronously rotating dq coordinate system. and q-axis current setpoint ;

[0037] S3. Perform closed-loop control on the d-axis current and q-axis current of the primary side of the rotary transformer (2) to detect the obtained three-phase current of the primary side. , , The d-axis current is obtained through Clark and Park transforms. q-axis current Wherein, the d-axis current The q-axis current These are the excitation current component and torque current component of the primary stator winding of the rotary transformer, respectively. After coordinate transformation, the two components can be independently controlled in the synchronously rotating dq coordinate system. The d-axis current... The q-axis current With respect to the given d-axis current value and the given q-axis current value The current deviation value is obtained by comparison, and the current regulator outputs the d-axis voltage setpoint based on the current deviation value. and q-axis voltage setpoint ;

[0038] S4. The given value of the d-axis voltage and the q-axis voltage setpoint Perform the inverse Park transformation to obtain the voltage setpoint in the two-phase stationary α-β coordinate system. , ;

[0039] S5. According to the given voltage value , The operation signal of the three-phase bridge arm switch of the output inverter (1) is calculated and the output voltage of the inverter (1) is controlled accordingly to control the primary current of the rotary transformer (2) to track the given value.

[0040] S6. Using the rotational inertia of the rotary transformer (2) during operation, provide inertia support for the power system;

[0041] S7. The grid connection and off-grid status of the new energy power generation system are controlled by opening and closing the grid connection switch (3).

[0042] Compared with the prior art, this invention has the following advantages: This invention uses a dual three-phase motor as a rotary transformer and proposes a new renewable energy grid connection scheme based on this. Since the motor itself has a filtering function, there is no need for an LC or LCL filter, eliminating the capacitor components in the renewable energy grid connection system. The equivalent LC resonant circuit cannot be formed, thus suppressing electromagnetic oscillations from the source.

[0043] In the grid-connected topology proposed in this invention, the motor connection port is connected to the power grid in the same way as the traditional synchronous generator connection to the power grid, with the same grid connection port. The inverter in the proposed grid-connected topology is also present in commonly used new energy grid-connected solutions, so the proposed grid-connected topology can be plugged and used, which is beneficial for the transformation and upgrading of existing new energy power plants such as wind power and photovoltaic power.

[0044] Compared to the currently commonly used grid connection scheme for new energy sources via power electronic converters, this invention introduces rotational inertia into the power system through a rotary transformer. This avoids the problem of decreasing power system inertia under high penetration rates of new energy sources, ensuring the inertia support of the power system and enhancing its stability.

[0045] This invention, through the optimized design of a dual-winding motor, achieves voltage level matching, avoiding the need for a step-up transformer in conventional converter grid connection schemes, and without significantly increasing hardware costs. The rotary transformer, with its two sets of stator windings and isolated neutral points, provides the same isolation function as a traditional transformer, ensuring electrical insulation between the power grid and the new energy power generation system. This effectively prevents further escalation of the accident in the event of a fault on one side, enhancing the safety of the power system.

[0046] Compared to existing grid-connected technologies where power electronic converters require a short-circuit current amplitude and duration to trigger relay protection devices during short-circuit faults, direct grid connection of motors can provide a sufficiently large short-circuit current to activate the relay protection devices. Furthermore, this invention inherits the characteristics of traditional thermal power unit grid systems, without altering the analysis and design theories of power systems, allowing the use of mature traditional theories to analyze and design renewable energy grid-connected systems.

[0047] Furthermore, the grid-connected control strategy of this invention can achieve independent control of grid-connected reactive power and active power, and can adjust grid-connected reactive power and active power according to changes in grid voltage and frequency, providing strong support for grid voltage and frequency. When the grid frequency fluctuates, the grid-connected control strategy of this invention completes frequency adjustment by controlling the inverter, while traditional thermal power plants adjust the frequency by adjusting the intake air volume of the steam turbine. The inverter, composed of power electronic devices, has a much faster response speed than the steam turbine. In the short time that the rotary transformer uses its rotational inertia to maintain the grid frequency, the inverter can complete the adjustment to support the grid frequency, greatly enhancing the emergency response capability of the power system. This invention solves the technical problems existing in current new energy grid-connected technologies, such as poor electromagnetic oscillation suppression, lack of inertia support, difficulty in grid-connected system design, low power system operation safety, high technology usage cost, and small short-circuit current during short-circuit faults. Attached Figure Description

[0048] Figure 1 This is a schematic diagram of the new energy grid connection scheme based on rotary transformer proposed in this invention;

[0049] Figure 2 This is a schematic diagram of two sets of stator windings with a phase difference of 30°.

[0050] Figure 3 This is a schematic diagram of two sets of stator windings with a phase difference of 60°.

[0051] Figure 4 This is a schematic diagram of the rotor rotation angular frequency of a dual three-phase asynchronous motor;

[0052] Figure 5 This is a schematic diagram of the electromagnetic torque of a dual three-phase asynchronous motor;

[0053] Figure 6 This is a schematic diagram of the three-phase current on the primary side of a rotary transformer;

[0054] Figure 7 This is a schematic diagram of the three-phase current on the secondary side of a rotary transformer;

[0055] Figure 8 This is a schematic diagram of grid-connected active power;

[0056] Figure 9 This is a schematic diagram of grid-connected reactive power. Detailed Implementation

[0057] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, 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.

[0058] Example 1

[0059] like Figure 1 As shown, a new energy grid-connected topology based on a rotary transformer includes an inverter 1, a rotary transformer 2, and a grid-connected switch 3.

[0060] Inverter 1 is a three-phase bridge inverter composed of power switching devices IGBT. Its DC side is powered by new energy sources such as wind power and photovoltaics. The AC side connection is led out from the midpoint of the three-phase bridge arm and connected to the three-phase connection of the primary side of the rotary transformer 2. By controlling inverter 1, the primary side current of rotary transformer 2 can be controlled, thereby controlling the grid-connected active power and reactive power.

[0061] Rotary transformer 2 is a dual three-phase motor, which can be either a dual three-phase asynchronous motor or a dual three-phase synchronous motor. Although rotary transformer 2 is essentially a motor, through appropriate control strategies, it does not generate electromagnetic torque during operation, but only performs energy transmission, functioning equivalent to a transformer. When rotary transformer 2 is operating, its rotor has rotational inertia. Introducing this rotational inertia into the power system can provide inertial support for the power system, improving system stability.

[0062] like Figure 2 and Figure 3 As shown, the rotary transformer 2 includes two sets of stator windings. The two sets of stator windings can be phased by 30° or 60° electrical degrees. The two stator structures are as follows: Figure 2 As shown, both sets of stator windings are star-connected with neutral point isolation. One set of stator windings serves as the primary side of the rotary transformer 2, connected to inverter 1, while the other set serves as the secondary side of the rotary transformer 2, connected to the power grid via grid-connected switch 3. This achieves physical isolation between the power grid and the new energy power generation system. Furthermore, by utilizing the electromagnetic induction between the two sets of stator windings and controlling the primary side current, the power transmitted from the secondary side to the grid can be controlled. Since the motor itself has a filtering function, no LC or LCL filter is needed, eliminating the capacitor components in the new energy grid-connected system. The equivalent LC resonant circuit cannot be formed, thus suppressing electromagnetic oscillations at the source.

[0063] The two ends of the grid-connected switch 3 are connected to the rotary transformer 2 and the power grid, respectively, and are used to control the grid-connected and off-grid status of the new energy power generation system.

[0064] The present invention also provides a control system for the above-mentioned renewable energy grid-connected topology based on a rotary transformer, comprising:

[0065] The given power calculation module has its grid voltage amplitude v and frequency f input terminals connected to the grid voltage amplitude v and frequency f output terminals of the grid-connected switch 3, respectively, to calculate the grid-connected active power setpoint based on the detected grid voltage amplitude v and frequency f. and reactive power setpoint When the grid frequency f is lower than the preset standard value, the active power setpoint is increased. When the voltage amplitude v is lower than the preset standard value, the reactive power setpoint is increased. ;

[0066] Given a current calculation module, its input terminal is connected to the active power given value of the given power calculation module. Output terminal and the given reactive power value The output terminal is connected to the given active power value. and the given reactive power value Calculate the given value of the d-axis current of the primary side of the rotary transformer (2) in the synchronously rotating dq coordinate system. and q-axis current setpoint ;

[0067] The Clark converter module has three inputs: the three-phase currents of the primary winding of the rotary transformer 2. , , The output is the α-axis current value in the two-phase stationary α-β coordinate system. and β-axis current value The Park converter module has two inputs: the α-axis current value output by the Clark converter module. and β-axis current value The output is the d-axis current value in the synchronously rotating dq coordinate system. and q-axis current value ;

[0068] The positive input of the first subtractor is the d-axis current given value output by the given current calculation module. The negative input is the d-axis current value output by the Park transform module. The positive input of the second subtractor is the q-axis current given value output by the given current calculation module. The negative input is the q-axis current value output by the Park transform module. The subtractor is used to compare the d-axis and q-axis currents. , With a given value , The deviation between the two is sent to the current regulator, which is used to perform closed-loop control of the d-axis and q-axis currents.

[0069] The inputs to the two current regulators are the current deviations between the outputs of the first and second subtractors of the d-axis currents and their setpoints, respectively. Based on these current deviations, the current regulators output the setpoints for the d-axis and q-axis voltages. , ;

[0070] The Park inverse transformer module has two inputs, namely the d-axis voltage setpoint output by the current regulator. and q-axis voltage setpoint d-axis and q-axis voltage setpoints , After the inverse Park transformation, the voltage setpoint in the two-phase stationary α-β coordinate system is obtained. , ;

[0071] The PWM modulation module has two input values, which are the voltage setpoints in the two-phase stationary α-β coordinate system output by the Park inverse transform module. , According to the voltage setpoint in the two-phase stationary α-β coordinate system , The operating signals of the three-phase bridge arm switches of inverter 1 were calculated. , , And output to inverter 1;

[0072] The inverter 1 will respond according to the action signal of the three-phase bridge arm switching transistor. , , Controlling the on and off of the power switching transistors, thereby controlling the output voltage of the inverter 1;

[0073] The rotary transformer 2 is connected to the inverter 1 with the first stator winding as the primary side of the rotary transformer (2) and the second stator winding as the secondary side of the rotary transformer (2) through the grid-connected switch (3). The output voltage of the inverter 1 is adjusted to control the primary current of the rotary transformer 2 to track the given value, thereby controlling the amount of active and reactive power delivered to the grid.

[0074] The control strategy corresponding to the aforementioned new energy grid-connected topology control system based on a rotary transformer is as follows: The setpoints for active and reactive power connected to the grid are calculated based on the detected grid voltage amplitude v and frequency f. , When the grid frequency f is lower than the standard value, increase the given value of active power. When the voltage amplitude v is lower than the standard value, increase the given value of reactive power. Then, based on the given values ​​of active and reactive power... , Calculate the given values ​​of the d-axis and q-axis currents of the primary side of rotary transformer 2 in the synchronously rotating dq coordinate system. , Closed-loop control is performed on the d-axis and q-axis currents of the primary side of rotary transformer 2, and the detected three-phase currents of the primary side are obtained. , , The d-axis and q-axis currents are obtained after Clark and Park transformations. , Then with the given value , Compare the deviation values ​​to the output d-axis and q-axis voltage setpoints of the current regulator. , ; , The voltage setpoint in the two-phase stationary α-β coordinate system is obtained through the inverse Park transformation. , Then the PWM modulation module according to , The operating signals of the three-phase bridge arm switches of inverter 1 are calculated to control the output voltage of inverter 1, which in turn controls the primary current of the rotary transformer 2 to track the given value. This control strategy enables independent control of grid-connected reactive power and active power, and can adjust grid-connected reactive power and active power according to changes in grid voltage and frequency, providing strong support for grid voltage and frequency.

[0075] like Figures 2 to 4 As shown, this embodiment uses a dual three-phase asynchronous motor with a stator winding phase difference of 30° electrical angle as the rotary transformer, and a simulation model was built in the MATLAB / Simulink simulation environment. The relevant parameters of the rotary transformer are as follows: number of pole pairs 2, stator resistance 1.5. The stator mutual inductance is 0.12H, the stator leakage inductance is 0.0085H, the equivalent leakage mutual inductance of the two sets of stator windings is 0.0026H, and the rotor resistance is 0.56 ohms. The rotor leakage inductance is 0.0085H, and the moment of inertia is 0.049. When the active power and reactive power setpoints are set to 2000W and 1000W respectively, the simulation results are as follows: Figures 3-8 As shown, the rotor rotation angular frequency of the dual three-phase asynchronous motor is such that the motor reaches synchronous speed in about 0.2s, the electromagnetic torque becomes 0, the start-up is completed, and it enters the rotary transformer working mode.

[0076] like Figure 5 and Figure 6 As shown, the three-phase currents on the primary side and the three-phase currents on the secondary side of the rotary transformer are sinusoidal and smooth, and the current frequencies on both sides are the same as the grid frequency of 50Hz.

[0077] like Figures 7 to 9 As shown, after the dual three-phase motor enters the rotary transformer working mode, the active power and reactive power quickly track the given value, and stably deliver energy to the power grid.

[0078] In summary, this invention uses a dual three-phase motor as a rotary transformer and proposes a new renewable energy grid connection scheme based on this. Since the motor itself has a filtering function, no LC or LCL filter is needed, eliminating the capacitor components in the renewable energy grid connection system. This prevents the formation of an equivalent LC resonant circuit, suppressing electromagnetic oscillations at the source. In the grid connection topology proposed in this invention, the motor's connection port is connected to the grid in the same way as a traditional synchronous generator, with the same grid connection port.

[0079] The inverter in the proposed grid-connected topology also exists in the commonly used new energy grid-connected solutions. Therefore, the proposed grid-connected topology can be plugged and used, which is beneficial for the transformation and upgrading of existing new energy power plants such as wind power and photovoltaic power. This invention introduces the rotational inertia of the dual three-phase motor into the power system, ensuring the inertia support of the power system.

[0080] Compared to the commonly used renewable energy grid connection schemes that use power electronic converters, this invention introduces rotational inertia into the power system through a rotary transformer, avoiding the problem of decreasing power system inertia under high renewable energy penetration and enhancing the stability of the power system.

[0081] This invention, through the optimized design of a dual-winding motor, achieves voltage level matching, avoiding the need for a step-up transformer in conventional converter grid connection schemes, and without significantly increasing hardware costs. The rotary transformer, with its two sets of stator windings and isolated neutral points, provides the same isolation function as a traditional transformer, ensuring electrical insulation between the power grid and the new energy power generation system. This effectively prevents further escalation of the accident in the event of a fault on one side, enhancing the safety of the power system.

[0082] Compared to the short-circuit current amplitude and duration used by power electronic converters in traditional technologies to trigger relay protection devices during short-circuit faults, this invention inherits the characteristics of traditional thermal power unit grid systems. Without changing the analysis and design theory of the power system, it can provide a sufficiently large short-circuit current to trigger relay protection devices by directly connecting the motor to the grid.

[0083] Furthermore, the grid-connected control strategy of this invention can achieve independent control of grid-connected reactive power and active power, and can adjust grid-connected reactive power and active power according to changes in grid voltage and frequency, providing strong support for grid voltage and frequency. When the grid frequency fluctuates, the grid-connected control strategy of this invention completes frequency adjustment by controlling the inverter, while traditional thermal power plants adjust the frequency by adjusting the intake air volume of the steam turbine. The inverter, composed of power electronic devices, has a much faster response speed than the steam turbine. In the short time that the rotary transformer uses its rotational inertia to maintain the grid frequency, the inverter can complete the adjustment to support the grid frequency, greatly enhancing the emergency response capability of the power system. This invention solves the technical problems existing in current new energy grid-connected technologies, such as poor electromagnetic oscillation suppression, lack of inertia support, difficulty in grid-connected system design, low power system operation safety, high technology usage cost, and small short-circuit current during short-circuit faults.

[0084] The above embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit it. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A control system for a new energy grid-connected topology based on a rotary transformer, characterized in that, The new energy grid-connected topology based on a rotary transformer includes: an inverter (1), a rotary transformer (2), and a grid-connected switch (3). The AC side terminals of the inverter (1) are led out from the midpoint of the three-phase bridge arm to connect to the three-phase terminals of the primary side of the rotary transformer (2); The primary side of the rotary transformer (2) is connected to the inverter (1), and the secondary side is connected to the grid-connected switch (3). The rotary transformer (2) includes no less than two sets of stator windings. The stator winding connection method includes: star connection and neutral point isolation. The rotary transformer (2) has rotational inertia when it is working. The stator winding of the rotary transformer includes a first stator winding and a second stator winding. The first stator winding is used as the primary side of the rotary transformer (2) and connected to the inverter (1). The second stator winding is used as the secondary side of the rotary transformer (2) and connected to the power grid through the grid-connected switch (3). The electrical angle between the first stator winding and the second stator winding includes a 30° electrical angle difference and a 60° electrical angle difference. The grid-connected switch (3) is connected in series between the rotary transformer (2) and the power grid; The control system includes: The given power calculation module is used to calculate the grid-connected active power setpoint based on the detected grid voltage amplitude v and frequency f. and reactive power setpoint When the grid frequency f is lower than the preset standard value, the active power setpoint is increased. When the voltage amplitude v is lower than the preset standard value, the reactive power setpoint is increased. ; A current calculation module is provided to calculate the current based on the given active power value. and the given reactive power value Calculate the given value of the d-axis current of the primary side of the rotary transformer (2) in the synchronously rotating dq coordinate system. and q-axis current setpoint The given current calculation module is connected to the given power calculation module; A current regulator is used to perform closed-loop control of the d-axis and q-axis currents of the primary side of the rotary transformer (2) to detect the obtained three-phase currents of the primary side. , , The d-axis current is obtained through Clark and Park transforms. q-axis current , to the given value of the d-axis current and the given q-axis current value The comparison is used to output the d-axis voltage setpoint via the deviation value current regulator. and q-axis voltage setpoint The current regulator is connected to the given current calculation module; The coordinate transformation module includes three coordinate transformations: Clark transformation, Park transformation, and inverse Park transformation. The Clark transformation is used to transform the detected current in the three-phase stationary coordinate system of the primary side of the rotary transformer. , , Converted to current in a two-phase stationary α-β coordinate system , Park transformation is used to transform , Converted to current in synchronous rotating dq coordinate system , The Park inverse transform is used to convert the given d-axis voltage value and the q-axis voltage setpoint Converted to voltage setpoint in a two-phase stationary α-β coordinate system , The coordinate transformation module is connected to the current regulator; PWM modulation module, used to adjust the voltage according to the given value. , Calculate the operating signals of the three-phase bridge arm switching transistors of the output inverter (1). , , The output voltage of the inverter (1) is controlled to control the primary current of the rotary transformer (2) to track a given value. The PWM modulation module is connected to the conversion module. The inverter (1) is an actuator in the control system, and operates according to the action signals of the three-phase bridge arm switching transistors. , , Controlling the on and off of the power switching transistors adjusts the output voltage of the inverter (1), thereby controlling the primary current of the rotary transformer (2); The rotary transformer (2) is a controlled object in the control system. It is a dual three-phase motor. The rotary transformer (2) includes no less than two sets of stator windings. The stator windings are respectively connected to the inverter (1) and the power grid. The stator windings are all connected in a star configuration and are isolated by a neutral point. The rotary transformer (2) has rotational inertia when it is working, which is used to provide inertia support for the power system. The two ends of the grid-connected switch (3) are connected to the rotary transformer (2) and the power grid, respectively. The inverter (1) is connected to the new energy power generation system to control the grid-connected and off-grid status of the new energy power generation system by opening and closing the grid-connected switch (3).

2. The control system according to claim 1, characterized in that, The inverter (1) is a three-phase bridge inverter, which includes power switching devices IGBTs.

3. The control system according to claim 1, characterized in that, The DC side of the inverter (1) is connected to the new energy power generation system to transmit electrical energy to the power grid through the new energy power generation system.

4. The control system according to claim 1, characterized in that, The new energy power generation system includes wind power and photovoltaic energy.

5. The control system according to claim 1, characterized in that, The rotary transformer (2) is a dual three-phase asynchronous motor or a dual three-phase synchronous motor.

6. The control system according to claim 1, characterized in that, The rotary transformer (2) is connected to the relay protection device in the power grid to provide a short-circuit current to the relay protection device, the magnitude of which is adapted to the relay protection device.

7. A method for grid-connected control using the control system described in any one of claims 1-6, characterized in that, include: S1. Based on the detected grid voltage amplitude v and frequency f, the grid-connected active power setpoint is calculated. and reactive power setpoint When the grid frequency f is lower than the preset standard value, the active power setpoint is increased. When the voltage amplitude v is lower than the preset standard value, the reactive power setpoint is increased. ; S2, based on the given active power value and the given reactive power value Calculate the given value of the d-axis current of the primary side of the rotary transformer (2) in the synchronously rotating dq coordinate system. and q-axis current setpoint ; S3. Perform closed-loop control on the d-axis current and q-axis current of the primary side of the rotary transformer (2) to detect the obtained three-phase current of the primary side. , , The d-axis current is obtained through Clark and Park transforms. q-axis current Wherein, the d-axis current The q-axis current These are the excitation current component and torque current component of the primary stator winding of the rotary transformer, respectively. After coordinate transformation, the two components can be independently controlled in the synchronously rotating dq coordinate system. The d-axis current... The q-axis current With respect to the given d-axis current value and the given q-axis current value The current deviation value is obtained by comparison, and the current regulator outputs the d-axis voltage setpoint based on the current deviation value. and q-axis voltage setpoint ; S4. The given value of the d-axis voltage and the q-axis voltage setpoint Perform the inverse Park transformation to obtain the voltage setpoint in the two-phase stationary α-β coordinate system. , ; S5. According to the given voltage value , The operation signal of the three-phase bridge arm switch of the output inverter (1) is calculated and used to control the output voltage of the inverter (1) so as to control the primary current of the rotary transformer (2) to track the given value. S6. Using the rotational inertia of the rotary transformer (2) during operation, provide inertia support for the power system; S7. The grid connection and off-grid status of the new energy power generation system are controlled by opening and closing the grid connection switch (3).

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