A wind and tide integrated power generation system grid-connected current control method

Abstract

The application discloses a wind and tide integrated power generation system grid-connected current control method, comprising the following steps: obtaining the DC bus voltage of a public DC bus and the grid-connected current of a grid-side inverter, determining the grid-connected current reference value, and determining the current error signal according to the grid-connected current reference value and the grid-connected current; determining the modulation ratio according to the reference voltage of the grid-side inverter and the DC bus voltage, and determining the corresponding resonance gain according to the modulation ratio; adjusting the current error signal by using a discrete domain proportional integral resonance controller configured with the resonance gain, the discrete domain proportional integral resonance controller is obtained by pre-distortion compensation on the target resonance frequency and bilinear transformation on the continuous domain proportional integral resonance controller after pre-distortion compensation; generating a driving signal according to the grid-side inverter control voltage instruction, and controlling the grid-side inverter to output the grid-connected current to the power grid. The application is suitable for the grid-connected control of the wind and tide integrated power generation system.

Description

Technical Field

[0001] This invention belongs to the field of grid-connected control technology for new energy power generation, specifically relating to a grid-connected current control method for an integrated wind and tidal power generation system. Background Technology

[0002] Offshore wind and tidal energy are both important marine renewable energy sources. my country's coastal waters and adjacent sea areas possess relatively abundant offshore wind and tidal energy resources. Integrating offshore wind and tidal power generation can fully utilize marine space, improve the utilization rate of marine renewable energy, and reduce dependence on traditional fossil fuels. For integrated wind and tidal power generation systems, the wind power generation unit and the tidal power generation unit are typically connected to a common DC bus after being converted by a turbine-side converter, and then uniformly connected to the grid by a grid-side inverter, thus forming a power generation system structure with multiple input sources, DC-side aggregation, and unified grid-side output.

[0003] However, both wind speed and tidal current speed at sea are time-varying and fluctuating. When the output power of the wind power generation unit and the tidal current generation unit changes, the fluctuations in multi-source input power will be transmitted to the grid-side inverter through the common DC bus, increasing the difficulty of stabilizing the DC bus voltage and controlling the grid-connected current. Especially during the switching between operating conditions such as dual-source wind and tidal operation, single-wind operation, and single-tidal current operation, the system is prone to problems such as DC bus voltage fluctuations, grid-connected current distortion, power factor decline, and increased low-order harmonic content, affecting the grid-connected power quality and operational stability of the integrated wind and tidal power generation system.

[0004] Currently, grid-connected inverters for new energy typically employ a dual-closed-loop control structure consisting of an outer loop for DC bus voltage and an inner loop for grid-connected current. Traditional proportional-integral (PI) controllers can achieve zero steady-state error regulation of DC quantities in a synchronous rotating coordinate system, but they lack frequency-selective compensation capabilities for specific frequency harmonics, making it difficult to effectively suppress characteristic low-order harmonics such as the 5th and 7th harmonics generated under complex operating conditions in integrated wind and solar power systems. While proportional-resonant controllers can provide high gain at the target frequency to compensate for specific frequency harmonics, continuous-domain resonant controllers require discretization in digital control systems. Directly using the traditional bilinear transform method for discretization can easily lead to the discrete-domain resonant center frequency deviating from the target harmonic frequency, thereby reducing the actual harmonic suppression effect.

[0005] Furthermore, the operating status of the integrated wind and tidal power generation system varies with changes in wind speed, tidal current velocity, input power, grid-connected current, and DC bus voltage. While a fixed resonant gain can achieve a certain harmonic suppression effect under specific operating conditions, it may lead to a decrease in harmonic suppression capability or insufficient system stability margin when the system operating status changes.

[0006] Existing technology CN102983588A discloses a photovoltaic grid-connected inverter system based on a grid-connected control algorithm. It employs a dual-closed-loop control structure consisting of an outer loop for DC bus voltage control and an inner loop for grid-connected current control. A proportional-integral resonant controller is used in the corresponding control loops to suppress the second harmonic in the DC bus voltage and the second harmonic negative-sequence component in the grid-connected current. This technology is primarily designed for grid-connected inverter control under single-energy input conditions for photovoltaic arrays. Its control object, input power variation patterns, and operating modes differ from multi-source power generation systems where wind power generation units and tidal current generation units share a common DC bus. Therefore, it is difficult to directly adapt to fluctuations in multi-source input power and the switching between dual-source operation, single-wind operation, and single-tidal current operation. Meanwhile, the resonant regulation in this technology mainly targets second harmonics or negative sequence components, failing to specifically address the 5th and 7th characteristic low-order harmonics that easily appear in grid-connected current under multi-source power fluctuation conditions. The resonant coefficients used are pre-set fixed parameters; when the DC bus voltage, grid-connected current, and inverter operating status change significantly, fixed resonant parameters cannot simultaneously ensure harmonic suppression capability and control system stability. Furthermore, although this technology uses a digital signal processor for control, its disclosed content does not specifically address the potential resonant center frequency shift that may occur during the digital discretization process of the continuous-domain resonant controller. Therefore, under the combined influence of the sampling period and the target resonant frequency, the actual resonant frequency of the digital controller may deviate from the predetermined compensation frequency, thus affecting the suppression effect of specific harmonics.

[0007] Therefore, there is an urgent need for a grid-connected current control method that can adapt to the complex operating conditions of integrated wind and tidal power generation systems, while taking into account harmonic suppression performance, digital control accuracy, and system stability. Summary of the Invention

[0008] The purpose of this invention is to overcome the defects of the prior art and provide a grid-connected current control method for an integrated wind and tidal power generation system.

[0009] The objective of this invention can be achieved through the following technical solutions: This invention provides a grid-connected current control method for an integrated wind and tidal power generation system, comprising the following steps: The actual value of the DC bus voltage of the common DC bus in the wind-sun integrated power generation system and the actual value of the grid-connected current of the grid-side inverter are obtained. The grid-connected current reference value is determined based on the actual value of the DC bus voltage and the DC bus voltage reference value. The current error signal is determined based on the grid-connected current reference value and the actual value of the grid-connected current. The modulation ratio is determined based on the reference voltage of the grid-side inverter and the actual value of the DC bus voltage, and the resonant gain corresponding to the current operating state is determined based on the modulation ratio. The current error signal is adjusted by a discrete domain proportional-integral resonant controller configured with the resonant gain to obtain the grid-side inverter control voltage command. The discrete domain proportional-integral resonant controller is obtained by pre-distortion compensation of the target resonant frequency and bilinear transformation of the pre-distortion compensated continuous domain proportional-integral resonant controller. The target resonant frequency corresponds to the fifth and seventh harmonics in the grid-connected current. The grid-side inverter is driven by generating a drive signal based on the grid-side inverter control voltage command, thereby controlling the grid-side inverter to output grid-connected current to the grid.

[0010] Furthermore, the step of obtaining the actual value of the DC bus voltage of the common DC bus and the actual value of the grid-connected current of the grid-side inverter, determining the grid-connected current reference value based on the actual value of the DC bus voltage and the DC bus voltage reference value, and determining the current error signal based on the grid-connected current reference value and the actual value of the grid-connected current specifically includes: The DC bus voltage reference value is compared with the actual DC bus voltage value to obtain the DC bus voltage deviation, and the DC bus voltage deviation is input into the voltage outer loop proportional-integral controller to obtain the d-axis current reference value; Set the q-axis current reference value to zero. The grid-connected current reference value includes the d-axis current reference value and the q-axis current reference value. By adopting a grid voltage-oriented control method, the d-axis of the synchronous rotating coordinate system is aligned with the grid voltage vector, and the actual value of the grid-connected current is transformed to obtain the actual current component on the d-axis and the actual current component on the q-axis. The d-axis current reference value is compared with the actual d-axis current component to obtain the d-axis current error signal, and the q-axis current reference value is compared with the actual q-axis current component to obtain the q-axis current error signal. The current error signal includes the d-axis current error signal and the q-axis current error signal.

[0011] Furthermore, determining the modulation ratio based on the reference voltage of the grid-side inverter and the actual value of the DC bus voltage specifically includes: Obtain the reference voltage vector of the grid-side inverter and determine the reference voltage amplitude of the reference voltage vector; The modulation ratio is determined based on the relative relationship between the reference voltage amplitude and the actual value of the DC bus voltage, and the modulation ratio is used as a characterizing quantity of the current operating state of the wind and tidal power generation system. The modulation ratio is updated when the output power of the wind power generation unit, the output power of the tidal power generation unit, the actual value of the grid-connected current, or the actual value of the DC bus voltage changes.

[0012] Furthermore, determining the resonant gain corresponding to the current operating state based on the modulation ratio specifically includes: Establish a current inner loop stability analysis model corresponding to the discrete domain proportional-integral resonant controller, and establish discrete domain state variables and discrete domain state space equations based on the current inner loop stability analysis model; Based on the Lyapunov stability condition, the stability of the inner current loop under different resonant gains is determined, and the range of resonant gain values ​​that meet the stability requirements of the inner current loop is obtained. Multiple candidate resonant gains corresponding to different operating states are selected from the range of stable values ​​of the resonant gain.

[0013] Furthermore, the step of selecting multiple candidate resonant gains corresponding to different operating states from the stable range of the resonant gain specifically includes: The range of the modulation ratio is divided into multiple modulation ratio operating intervals, and a corresponding candidate resonant gain is configured for each modulation ratio operating interval from the range of stable resonant gain values, thereby establishing a correspondence between the modulation ratio operating interval and the candidate resonant gain. Obtain the current modulation ratio, determine the modulation ratio operating range to which the current modulation ratio belongs, and determine the candidate resonant gain corresponding to the modulation ratio operating range to which the current modulation ratio belongs as the resonant gain corresponding to the current operating state; The current operating states include the dual-source wind and tidal power generation state where both the wind power generation unit and the tidal power generation unit are operating simultaneously, the single-wind operating state where the wind power generation unit is operating and the tidal power generation unit is out of operation, and the single-tidal power generation state where the tidal power generation unit is operating and the wind power generation unit is out of operation.

[0014] Furthermore, determining the candidate resonant gain corresponding to the modulation ratio operating range to which the current modulation ratio belongs as the resonant gain corresponding to the current operating state specifically includes: A first switching threshold and a second switching threshold are set between two adjacent modulation ratio operating ranges, wherein the first switching threshold is higher than the second switching threshold, so as to form a hysteresis range between two adjacent modulation ratio operating ranges. When the current modulation ratio increases and reaches the first switching threshold, the currently used resonant gain is switched to a candidate resonant gain corresponding to the adjacent modulation ratio operating range in the direction of modulation ratio increase. When the current modulation ratio decreases and reaches the second switching threshold, the currently used resonant gain is switched to a candidate resonant gain corresponding to the adjacent modulation ratio operating range in the direction of modulation ratio decrease. When the current modulation ratio is within the hysteresis interval, the currently used resonant gain remains unchanged.

[0015] Furthermore, the continuous-domain proportional-integral resonant controller includes a proportional control branch, an integral control branch, and a resonant compensation branch, and the transfer function of the continuous-domain proportional-integral resonant controller is expressed as: in, The transfer function of the continuous-domain proportional-integral resonant controller is represented by . Represents a variable in the complex frequency domain; Indicates proportional gain; Indicates integral gain; This represents the resonant gain; Indicates the resonant bandwidth parameter; Indicates the target resonant frequency; The target resonant frequency is expressed as: in, The target resonant frequency is used to compensate for the sixth fundamental frequency component of the fifth and seventh harmonics in the three-phase stationary coordinate system in the synchronous rotating coordinate system.

[0016] Furthermore, the step of obtaining the discrete-domain proportional-integral resonant controller by pre-distortion compensation of the target resonant frequency and bilinear transformation of the pre-distortion compensated continuous-domain proportional-integral resonant controller specifically includes: Set the sampling period of the digital controller, and determine the pre-distortion equivalent frequency parameters based on the target resonant frequency and the sampling period; Based on the predistortion equivalent frequency parameters, a predistortion compensation bilinear transformation relationship is constructed, which is expressed as follows: in, This represents the pre-distortion equivalent frequency parameter determined based on the target resonant frequency and the sampling period; Represents discrete-domain variables; Substituting the pre-distortion compensation bilinear transformation relationship into the resonance compensation branch of the continuous domain proportional-integral resonant controller, a discrete domain resonance compensation branch is obtained. The discrete domain resonance compensation branch is then combined with the discretized proportional control branch and integral control branch to obtain the discrete domain proportional-integral resonant controller.

[0017] Furthermore, the discrete-domain proportional-integral resonant controller is implemented using a differential recursive form, specifically including: The transfer function of the discrete domain proportional-integral resonant controller is rearranged into a standard difference recursive form to obtain the first output recursive coefficient, the second output recursive coefficient, and the first to third error input coefficients. Calculate the controller output at the current sampling time: in, Indicates the first The controller output at each sampling time; and They represent the first The sampling time and the first sampling time The controller output at each sampling time; Indicates the first Current error signal at each sampling time; and They represent the first The sampling time and the first sampling time Current error signal at each sampling time; and These represent the first output recursive coefficient and the second output recursive coefficient, respectively. , and These represent the first to third error input coefficients, respectively. The d-axis current error signal and the q-axis current error signal are respectively used as the differential recursive form of current error signal for calculation to obtain the d-axis control voltage command and the q-axis control voltage command, and the grid-side inverter control voltage command is composed of the d-axis control voltage command and the q-axis control voltage command.

[0018] Furthermore, the step of generating a drive signal for the grid-side inverter based on the grid-side inverter control voltage command, and controlling the grid-side inverter to output grid-connected current to the grid, specifically includes: Obtain the d-axis control voltage command and q-axis control voltage command of the grid-side inverter in the synchronous rotating coordinate system; Perform coordinate inverse transformation on the d-axis control voltage command and the q-axis control voltage command to obtain the three-phase reference voltage used to control the grid-side inverter; The three-phase reference voltage is pulse-width modulated to generate a drive signal for controlling the on / off switching of the power switching devices of the grid-side inverter; The drive signal is applied to the power switching device of the grid-side inverter, causing the grid-side inverter to output grid-connected current according to the grid-side inverter control voltage command.

[0019] Compared with the prior art, the present invention has the following advantages: (1) In existing integrated wind and tidal power generation systems, the output power of both the wind power generation unit and the tidal power generation unit is volatile. When switching between dual-source wind and tidal operation, single wind operation, and single tidal operation, the common DC bus is prone to power imbalance, which in turn causes DC bus voltage fluctuation and grid-connected current tracking deviation. This invention determines the grid-connected current reference value based on the actual value of the DC bus voltage and the DC bus voltage reference value, and further determines the current error signal based on the grid-connected current reference value and the actual value of the grid-connected current. This enables the DC bus voltage regulation and grid-connected current control to form a corresponding double closed-loop control relationship, thereby reducing the continuous accumulation of multi-source input power fluctuations on the common DC bus, improving the grid-side inverter's ability to follow input power changes, and reducing the magnitude of DC bus voltage overshoot and drop during operating condition switching.

[0020] (2) Existing proportional-integral controllers lack sufficient selective gain for specific frequency harmonics, making it difficult to effectively suppress the fifth and seventh harmonics generated by the integrated wind and tidal power generation system during multi-source power fluctuations and operating condition switching. While increasing the overall controller gain to enhance harmonic suppression capabilities can amplify noise and reduce system stability margin, this invention employs a proportional-integral resonant controller with resonant gain to adjust the current error signal. The target resonant frequency corresponds to the sixth fundamental frequency component formed by the fifth and seventh harmonics in a synchronous rotating coordinate system, enabling the controller to provide higher gain near the target frequency without significantly increasing control gain in other frequency bands. This not only allows for selective compensation of the fifth and seventh harmonics, reducing the total harmonic distortion rate of the grid-connected current, but also avoids the parameter coupling and increased computational load caused by setting multiple resonant compensation stages separately, improving the sinusoidal nature and symmetry of the grid-connected current waveform.

[0021] (3) When existing continuous domain resonant controllers are implemented in digital controllers, if ordinary bilinear transformation is directly used for discretization, the discrete domain resonant center frequency is prone to deviate from the target resonant frequency due to nonlinear mapping of the frequency axis. For the sixth fundamental frequency components corresponding to the fifth and seventh harmonics, even if a certain frequency shift occurs, the target harmonic may fall outside the effective high-gain range of the controller, resulting in inconsistency between theoretical design and actual digital control effect. Before the bilinear transformation, this invention performs pre-distortion compensation based on the target resonant frequency and sampling period, and obtains a discrete domain proportional-integral resonant controller based on the frequency relationship after pre-distortion compensation, so that the resonant center frequency of the discrete domain controller is consistent with or approximately consistent with the target resonant frequency. This can reduce the influence of sampling period and discretization method on the resonant position, so that the digital controller can still effectively compensate for the target harmonic in actual operation, and improve the consistency of the control method under different digital control platforms and different sampling conditions.

[0022] (4) Existing control methods using fixed resonant gain typically only achieve good control performance at a specific operating point. When the output power, DC bus voltage, and grid-connected current of the wind power generation unit or tidal current generation unit change, the fixed resonant gain may be too large under light load conditions, increasing the oscillation tendency of the current inner loop, or too small under heavy load or operating condition switching conditions, resulting in insufficient target harmonic suppression. This invention determines the modulation ratio based on the reference voltage of the grid-side inverter and the actual value of the DC bus voltage, and uses the modulation ratio as a characterizing quantity of the current operating state, and then determines the corresponding resonant gain based on the modulation ratio. Since the modulation ratio reflects the relative relationship between the reference output voltage demand and the available DC voltage, it can comprehensively characterize the input power change, DC bus voltage fluctuation, and the voltage utilization of the grid-side inverter. Therefore, compared with adjusting parameters only based on wind speed, tidal current velocity, or the start-stop status of the energy unit, it can more directly reflect the actual operating state of the grid-side inverter, enabling the resonant gain to adapt to different operating conditions such as dual-source wind and tidal current operation, single wind operation, and single tidal current operation, taking into account both harmonic suppression capability and dynamic response performance.

[0023] (5) Existing adaptive gain methods, if they only adjust the resonant gain according to the operating state without considering the stability boundary of the discrete-domain current inner loop, may reduce the system stability margin while increasing the harmonic compensation intensity. This invention establishes a stability analysis model of the discrete-domain proportional-integral resonant controller, determines the stable range of the resonant gain based on the discrete-domain state-space equation and Lyapunov stability conditions, and configures candidate resonant gains for different modulation ratio operating intervals within this stable range, so that the dynamic adjustment of the resonant gain is always constrained by the stability conditions. This avoids oscillation or modulation saturation caused by excessive resonant gain, maintains the stability of the current inner loop when the system operating state changes, and makes the trade-off between harmonic suppression performance and system stability no longer dependent on a single fixed parameter.

[0024] (6) Existing partition parameter switching methods are easily affected by sampling noise, DC bus ripple, and instantaneous fluctuations in grid-connected current when the state variable is near the interval boundary, resulting in frequent switching of resonant gain between adjacent parameters. This invention sets different rising and falling switching thresholds between adjacent modulation ratio operating intervals to form hysteresis intervals, and keeps the current resonant gain unchanged when the modulation ratio is within the hysteresis interval, thereby reducing parameter jitter and the resulting sudden changes in control voltage command, and improving the continuity and stability of grid-connected current control.

[0025] (7) If existing continuous domain controllers are directly operated in real time on a digital control platform, the computational load is large, and additional control delay may be introduced due to the increased program execution time. This invention reorganizes the discrete domain proportional-integral resonant controller into a differential recursive form, and calculates the current controller output based on the current error signal and the controller output of the previous two time points. This facilitates real-time control in a digital signal processor or microcontroller through a fixed number of multiplication and addition operations, reducing the impact of computational delay on the accuracy of resonance compensation.

[0026] (8) The present invention adopts grid voltage directional control mode and sets the q-axis current reference value to zero, which can reduce the reactive power exchange between the wind and tidal power generation system and the grid, avoid reactive current and harmonic current from jointly occupying the current capacity of the grid-side inverter, thereby improving the system power factor and reserving a larger control margin for active power output and harmonic compensation. Attached Figure Description

[0027] Figure 1 This is a schematic diagram of the integrated wind and tidal power generation system according to an embodiment of the present invention; Figure 2 This is a schematic diagram of the grid-connected dual closed-loop control structure of the integrated wind and tidal power generation system according to an embodiment of the present invention; Figure 3 This is a flowchart of the discretization process of the proportional-integral resonant controller based on pre-distortion compensation according to an embodiment of the present invention. Figure 4 This is a flowchart of the adaptive configuration of resonant gain based on modulation ratio according to an embodiment of the present invention; Figure 5 This is a schematic diagram of the resonant gain hysteresis switching module according to an embodiment of the present invention. Detailed Implementation

[0028] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort should fall within the scope of protection of the present invention.

[0029] like Figure 1As shown, this embodiment provides a grid-connected current control method for an integrated wind and tidal power generation system. The integrated wind and tidal power generation system includes a wind power generation unit, a tidal power generation unit, a generator-side converter, a common DC bus, a grid-side inverter, a filter inductor, and a power grid. It may further include a wind turbine generator control unit, a tidal power generator control unit, a grid-connected control unit, a voltage sensor, a current sensor, and a drive circuit. The wind power generation unit and the tidal power generation unit constitute a multi-source input section, the common DC bus constitutes an energy collection section, and the grid-side inverter constitutes a unified grid-connected output section.

[0030] exist Figure 1 In the specific structure shown, the wind power generation unit includes a wind turbine and a permanent magnet synchronous generator (PMSG) driven by the wind turbine. The power flow generation unit includes a power flow turbine and a PMSG driven by the power flow turbine. The two PMSG generators are connected to their respective generator-side converters, which convert the frequency-converted AC power output from the generators and connect it to the common DC bus. A DC support capacitor can be installed on the common DC bus to buffer the instantaneous power difference between the wind power generation unit and the power flow generation unit. The DC side of the grid-side inverter is connected to the common DC bus, and the AC side is connected to the grid via a filter inductor, used to convert the electrical energy in the common DC bus into three-phase AC power that meets grid connection requirements.

[0031] The aforementioned permanent magnet synchronous generator is merely one example. In another specific embodiment, at least one of the wind power generation unit and the tidal power generation unit can be an asynchronous generator, an electrically excited synchronous generator, or other generator suitable for variable speed operation, and connected to a common DC bus via a matched full-power machine-side converter. When using a permanent magnet synchronous generator, the system has the advantages of compact structure and high low-speed power generation efficiency; when using other generators, the applicable scenarios of the integrated wind and tidal power generation system can be expanded according to equipment cost, maintenance conditions, and rated power range.

[0032] In this embodiment, the wind turbine generator control unit and the tidal current generator control unit respectively control their corresponding generator-side converters to achieve generator torque control, speed control, or maximum power point tracking; the grid-connection control unit controls the grid-side inverter to maintain the voltage of the common DC bus and regulate the grid-connection current. The generator-side control methods of the wind turbine generator unit and the tidal current generator unit do not limit the grid-connection current control method of this invention. As long as the two energy sources are connected to the same common DC bus after power conversion, the grid-side inverter can uniformly execute the grid-connection control of this embodiment.

[0033] During system operation, wind speed and tidal current velocity change over time, and the output power of the wind power generation unit and the tidal current generation unit also changes accordingly. When the sum of the two input power sources is unbalanced with the output power of the grid-side inverter, the power difference is absorbed or released by the DC support capacitor of the common DC bus, thus manifesting as a change in the DC bus voltage. Therefore, this embodiment uses a dual closed-loop control structure consisting of an outer loop of DC bus voltage and an inner loop of grid-connected current to convert the DC-side power balance requirement into a grid-connected current reference value, and the grid-side inverter is controlled by the inner loop of grid-connected current to track this grid-connected current reference value.

[0034] like Figure 2 As shown, the grid-connected control unit collects the actual value of the DC bus voltage of the common DC bus, the three-phase grid-connected current at the output side of the grid-side inverter, and the three-phase grid voltage. In one specific sampling method, the DC bus voltage, three-phase grid-connected current, and three-phase grid voltage are all sampled synchronously within the same pulse width modulation period. The sampling time is set in the center region of the carrier waveform to reduce sampling ripple caused by power switching operations. Each analog quantity can undergo anti-aliasing filtering before entering the digital controller, and is then subjected to amplitude limiting, zero drift correction, and outlier removal within the digital controller. Synchronous sampling helps maintain the timing correspondence between the DC bus voltage, grid-connected current, and reference voltage, avoiding deviations in modulation ratio judgment and current error calculation caused by asynchronous sampling.

[0035] The outer loop of the DC bus voltage compares the preset DC bus voltage reference value with the actual DC bus voltage value to obtain the DC bus voltage deviation: in, This indicates the DC bus voltage deviation. This indicates the reference value for the DC bus voltage. Indicates time The corresponding actual value of the DC bus voltage. The reference value of the DC bus voltage can be preset according to the rated DC voltage of the grid-side inverter, the allowable modulation range and the grid voltage level, or it can be slowly adjusted within the allowable range according to the power status of the two generation units.

[0036] The DC bus voltage deviation input voltage outer loop proportional-integral controller, and the voltage outer loop proportional-integral controller output d-axis current reference value, can be expressed in the continuous domain as follows: in, This indicates the reference value for the d-axis current. The Laplace transform of the DC bus voltage deviation This represents the voltage outer loop proportional gain. This represents the voltage outer loop integral gain. This represents a variable in the complex frequency domain. The outer loop voltage proportional term is used to improve the response speed when the DC bus voltage changes, and the integral term is used to eliminate steady-state voltage deviation.

[0037] In digital control implementation, upper limits for the amplitude and rate of change of the d-axis current reference value can be set, and an anti-integral saturation circuit can be implemented for the voltage outer loop integral state. When switching between dual-source (wind and solar) and single-source operation, this limiting and anti-integral saturation treatment can prevent excessively large d-axis current reference values ​​from being generated by the DC bus voltage deviation in a short period of time, reducing the probability of overcurrent and modulation saturation in the grid-side inverter. This treatment is an engineering implementation limitation of the voltage outer loop and does not change the fundamental relationship in which the grid-connected current reference value is determined by the DC bus voltage deviation.

[0038] To reduce reactive power exchange between the integrated wind and solar power generation system and the grid, this embodiment sets the q-axis current reference value to zero, i.e. Therefore, the grid-connected current reference value includes the d-axis current reference value. and q-axis current reference value In another specific implementation, when the grid dispatch requires the system to provide reactive power support, the q-axis current reference value can also be determined by the reactive power command or the AC voltage regulation command; in this case, the subsequent current error calculation, proportional-integral resonant control, pre-distortion discretization, and resonant gain adaptive configuration processes remain unchanged. Setting the q-axis current reference value to zero is beneficial for improving the power factor, while setting the q-axis current reference value according to the reactive power command can extend the system's voltage support capability.

[0039] The grid-connected current inner loop adopts a grid voltage-oriented control method. The grid-connected control unit obtains the grid phase angle based on the three-phase grid voltage. and the fundamental angular frequency of the power grid The d-axis of the synchronous rotating coordinate system is aligned with the grid voltage vector. The grid phase angle can be obtained through a phase-locked loop, or through an orthogonal signal generator, a frequency-locked loop, or other synchronization methods that can track the grid fundamental phase.

[0040] In one coordinate transformation method, the three-phase grid-connected current , and Transform it into the actual d-axis current component according to the following formula. and q-axis actual current component : in, , and These represent the three-phase grid-connected currents, and These represent the actual current components along the d-axis and q-axis in the synchronously rotating coordinate system, respectively. This represents the electrical angle of the synchronous rotating coordinate system relative to the three-phase stationary coordinate system. For a three-phase three-wire system, the zero-axis component may not need to be introduced separately; for a system with a neutral line, the zero-axis component can be added based on the above transformation.

[0041] The d-axis current reference value and the q-axis current reference value are compared with the corresponding actual current components to obtain the d-axis current error signal and the q-axis current error signal, respectively. in, and They represent the first The d-axis current error signal and q-axis current error signal at each sampling time. and They represent the first The d-axis current reference value and q-axis current reference value at each sampling time. and They represent the first The actual current components along the d-axis and q-axis at each sampling time. These d-axis current error signals and q-axis current error signals together constitute the current error signal described in the claims.

[0042] In this embodiment, both the d-axis and q-axis grid-connected current inner loops employ proportional-integral (PI) resonant controllers. The PI resonant controller includes a proportional control branch, an integral control branch, and a resonant compensation branch. The proportional control branch improves the dynamic response speed when the current error signal changes, the integral control branch achieves zero steady-state error adjustment of the fundamental current component in a synchronously rotating coordinate system, and the resonant compensation branch increases the controller gain near the target resonant frequency to selectively compensate for characteristic low-order harmonics.

[0043] The transfer function of the continuous-domain proportional-integral resonant controller is expressed as: in, Let represent the transfer function of a continuous-domain proportional-integral resonant controller. This represents the proportional gain of the inner current loop. This represents the inner loop integral gain of the current. Indicates the resonant gain. This represents the bandwidth parameter of the resonant compensation branch. Indicates the target resonant frequency. Bandwidth parameter. Used to determine the effective compensation range of the resonant compensation branch near the target resonant frequency; When the frequency is small, the frequency selectivity is strong. When the value is large, the tolerance for small deviations in the grid frequency is higher, but the impact on non-target frequency components also increases accordingly.

[0044] In integrated wind and solar power systems, the fifth and seventh harmonics are prone to appear in the grid-connected current during multi-source power fluctuations and operational state switching. In a three-phase stationary coordinate system, the fifth harmonic is a negative-sequence component, and the seventh harmonic is a positive-sequence component. After coordinate transformation with synchronous rotation at the grid fundamental frequency, they appear as a negative six-fold fundamental frequency component and a positive six-fold fundamental frequency component, respectively. The second-order resonant compensation branch has symmetrical gain characteristics at both the positive and negative target resonant frequencies; therefore, the same target resonant frequency can be used to simultaneously compensate for the fifth and seventh harmonics. The target resonant frequency is set as follows: in, This represents the fundamental angular frequency of the power grid. When the fundamental frequency of the power grid is 50 Hz, the target resonant frequency corresponds to 300 Hz. When the fundamental frequency of the power grid changes within an allowable range, the target resonant frequency can be updated synchronously based on the fundamental angular frequency of the power grid obtained by the phase-locked loop, or it can be fixed according to the rated fundamental frequency. The former method is more adaptable to weak power grids or frequency fluctuation scenarios, while the latter method is simpler to implement.

[0045] The d-axis current error signal and the q-axis current error signal are input to the proportional-integral resonant controller respectively to obtain the d-axis controller output. and q-axis controller output .like Figure 2 As shown, in one specific control method, grid voltage feedforward and filter inductor cross-coupling compensation can be added to the controller output. Taking the grid-connected current flowing from the grid-side inverter to the grid as the positive direction, the grid-side inverter control voltage command can be expressed as: in, and These represent the d-axis control voltage command and the q-axis control voltage command, respectively. and These represent the d-axis and q-axis components of the grid voltage in a synchronous rotating coordinate system, respectively. and These represent the equivalent resistance and filter inductance of the grid-side filter branch, respectively. This represents the grid angular frequency used in the coordinate transformation. Since the d-axis is aligned with the grid voltage vector, under steady-state conditions... Approaching zero. It should be noted that the sign of the cross-coupling term can be adjusted accordingly when the grid current's positive direction or the Park transformation sign convention differs. Adding grid voltage feedforward and cross-coupling compensation can reduce the coupling between the d-axis and q-axis, making the parameter tuning of the proportional-integral resonant controller more direct; in scenarios with limited control bandwidth and hardware resources, some feedforward or decoupling terms can also be omitted.

[0046] like Figure 3 As shown, to enable the proportional-integral resonant controller to be deployed on digital signal processors, microcontrollers, or other digital control platforms, this embodiment employs a pre-distortion-compensated bilinear transform method for discretization. First, a continuous-domain proportional-integral resonant controller model is established, with the proportional gain as the input. Integral gain Resonant gain Target resonant frequency Resonance bandwidth parameters and the fundamental angular frequency of the power grid Then set the sampling period of the digital controller. The sampling period can be the same as the pulse width modulation period, or it can be an integer fraction of the pulse width modulation period, but the current error update, controller recursion, and modulation command update should maintain a defined timing relationship.

[0047] Ordinary bilinear transformation introduces a nonlinear mapping between the continuous-domain and discrete-domain frequency axes. When the target resonant frequency is not negligible relative to the sampling frequency, directly applying ordinary bilinear transformation may cause the discrete-domain resonant center frequency to deviate from the intended target resonant frequency. Therefore, before the bilinear transformation, pre-distortion equivalent frequency parameters are calculated based on the target resonant frequency and the sampling period: in, This represents the pre-distortion equivalent frequency parameter. Indicates the target resonant frequency. This represents the sampling period. This expression maps the target resonant frequency in the continuous domain to the same digital frequency in the discrete domain through a bilinear transformation, thereby compensating for the frequency distortion caused by the ordinary bilinear transformation.

[0048] Construct the bilinear transformation relationship for predistortion compensation based on the predistortion equivalent frequency parameters: in, Represents a complex frequency variable in the continuous domain. Represents discrete domain variables, The unit delay operator represents one sampling period. This represents the pre-distortion equivalent frequency parameter. In another equivalent notation, we can first use... The ordinary bilinear transformation is then used, and the original target resonance parameter is replaced with the pre-distorted continuous domain resonance parameter; the two methods have equivalent effects when the target frequency matching conditions are the same.

[0049] Substituting the above bilinear transformation relationship for pre-distortion compensation into the continuous domain resonance compensation branch, we can obtain the discrete domain resonance compensation branch. For ease of explanation, let the resonance compensation branch be... After substituting the values, it can be rearranged as follows: in: The transfer function of the discrete-domain resonant compensation branch is given. , and This represents the denominator coefficient of the discrete-domain resonant compensation branch. This form indicates that the resonant gain... It mainly affects the molecular coefficient, while , , and Together, they determine the pole locations and frequency selection characteristics of the discrete-domain resonant branch.

[0050] The proportional control branch is still represented as after discretization. The integral control branch can be implemented using a bilinear integral form: in, This represents the discrete-domain integral control branch. Subsequently, the proportional control branch, the discrete-domain integral control branch, and the discrete-domain resonant compensation branch are combined to obtain the discrete-domain proportional-integral-resonant controller. In another specific implementation, the integral control branch can also employ backward Euler discretization, which has the advantages of simple program structure and numerical stability; when using the bilinear integral form, the frequency characteristics of the continuous domain and the discrete domain correspond more smoothly. Regardless of the integral discretization method used, the resonance compensation branch employs pre-distortion compensation targeting the target resonant frequency.

[0051] Discrete-domain proportional-integral resonant controllers can be implemented using direct, cascaded, or branch-parallel program structures. In a direct recursive implementation corresponding to the claim, the discrete-domain controller is rearranged into a standard-difference recursive form to obtain the first output recursive coefficients. Second output recursive coefficient and the first to third error input coefficients , And $b_2, and calculate the controller output at the current sampling time according to the following formula: in, Indicates the first The controller output at each sampling time. and These represent the controller outputs at the previous sampling time and the two previous sampling times, respectively. Indicates the first Current error signal at each sampling time, and These represent the current error signals at the previous sampling time and the two previous sampling times, respectively. , , , and The result is obtained by normalizing the transfer function of the discrete-domain proportional-integral resonant controller. The above recursive calculation is then performed on the d-axis current error signal and the q-axis current error signal respectively to obtain the corresponding d-axis controller output and q-axis controller output.

[0052] In another implementation, the proportional control branch, integral control branch, and resonant compensation branch each maintain their own discrete states, and the outputs of the three branches are summed at each sampling time. This branch-parallel implementation facilitates separate setting of integral and resonant branch limiting, and also allows for online updates of the resonant gain. Without changing the states of other branches, the direct recursive implementation requires fewer multiply-accumulate operations, making it suitable for controllers with limited storage resources. Both implementation methods can be based on... Figure 3 The process shown obtains the grid-side inverter control voltage command.

[0053] When the digital controller starts up, the historical current error signal and historical controller output can be initialized to zero. Alternatively, after the grid-side inverter completes grid synchronization, the controller state can be smoothly initialized using the current current error and feedforward voltage. When the resonant gain switches, it is preferable to maintain the continuity of the integral state and historical output, or to keep the controller output before and after the switch continuous through state mapping, so as to reduce the instantaneous impact of parameter switching on the grid-side inverter control voltage command.

[0054] Furthermore, this embodiment establishes a current inner-loop stability analysis model corresponding to the discrete-domain proportional-integral resonant controller. This model can include the filter inductor current state, the internal state of the proportional-integral resonant controller, the calculation delay state, and the necessary DC bus small-signal state. The discrete-domain closed-loop model near the selected operating point is represented as: in, Indicates the first The discrete-domain state vector at each sampling time. Indicates resonant gain and modulation ratio The relevant closed-loop state matrix, Represents the input matrix, This represents the reference input or external disturbance. The state vector can be selected as a combination of d-axis and q-axis currents, integral states, resonant branch states, and states with a delay calculated over one sampling period, depending on the analysis precision.

[0055] For a given resonant gain and operating state, when a symmetric positive definite matrix exists... satisfy: The corresponding discrete-domain closed-loop system then satisfies the Lyapunov stability condition. Wherein, This represents the current resonant gain and the closed-loop state matrix under the current operating state. express The transpose of the matrix, Let Lyapunov be the matrix. By calculating point-by-point within the preset resonant gain range, using a binary search, or solving linear matrix inequalities, the stable range of resonant gain values ​​that meet the stability requirements can be obtained. The candidate resonant gains are all selected from this stable range, so that the adaptive adjustment of the resonant gain is subject to the discrete domain closed-loop stability constraint.

[0056] The stable operating range can be obtained offline during the controller design phase and stored in the grid-connected control unit, or it can be calculated separately for multiple typical DC bus voltages, grid power, and grid impedance conditions. When actual operating parameters fall between adjacent typical operating points, a more conservative stable operating range can be used, or the candidate resonant gain can be interpolated before performing stability margin verification. Offline solving reduces the amount of online computation; multi-operating-point analysis reduces the limitation of a single rated operating point model on the adaptability to complex operating conditions.

[0057] like Figure 4 As shown, this embodiment uses the modulation ratio of the grid-side inverter as a characterizing quantity of the current operating state of the integrated wind and tidal power generation system. First, the reference voltage amplitude is determined according to the d-axis control voltage command and the q-axis control voltage command: in, This represents the magnitude of the reference voltage vector for the grid-side inverter. and These represent the d-axis control voltage command and the q-axis control voltage command, respectively. When using a normalization method with the DC bus half-voltage as the base value, the modulation ratio can be expressed as: in, Indicates the current modulation ratio. This represents the actual value of the DC bus voltage. In another definition method using the maximum phase voltage in the linear region of space vector pulse width modulation as the base value, the modulation ratio can also be expressed as... Both definitions are used to characterize the relative relationship between the reference voltage requirement and the available DC bus voltage. When implementing them, the modulation ratio definition, operating range threshold, and corresponding relationship table should be unified to avoid mixing different normalized base values ​​in the same control program.

[0058] When the output power of the wind power generation unit, the output power of the power tidal current generation unit, the grid-connected current, or the DC bus voltage changes, the d-axis current reference value and the grid-side inverter control voltage command change accordingly, affecting the modulation ratio. Accordingly, the modulation ratio not only reflects whether the energy unit is in operation, but also comprehensively reflects the impact of DC bus voltage level, grid current magnitude, grid voltage, and filter inductor voltage drop on the voltage utilization of the grid-side inverter. Compared with selecting parameters solely based on wind speed, power flow velocity, or energy unit start-stop signals, using the modulation ratio can more directly characterize the actual control margin of the grid-side inverter.

[0059] Based on the discrete-domain stability analysis results, the control system divides the modulation ratio range into multiple modulation ratio operating intervals, and configures candidate resonant gains for each operating interval from the stable range of resonant gain values. Let the first... Each operating range satisfies: in, and They represent the first The lower and upper thresholds of each operating range, Indicates the number of operating intervals. Figure 4 In The current modulation ratio corresponding to this embodiment Each operating range corresponds to a candidate resonant gain. , to This allows for the establishment of a correspondence table between the modulation ratio operating range and the candidate resonant gain.

[0060] The candidate resonant gains are not required to increase or decrease monotonically according to the modulation ratio, but can be determined comprehensively based on the target harmonic amplitude, closed-loop stability margin, modulation saturation margin, and noise sensitivity. In one specific configuration, the fifth and seventh harmonic contents are first measured or simulated at each typical modulation ratio operating point, and then the minimum resonant gain that meets the harmonic suppression requirements and retains the preset stability margin is selected from the stable range; this method avoids selecting unnecessarily large resonant gains in pursuit of local harmonic attenuation. In another configuration, a comprehensive evaluation quantity can be constructed based on the harmonic amplitude and stability margin at each operating point, and the candidate resonant gain with the optimal comprehensive evaluation quantity can be selected to balance power quality and dynamic stability.

[0061] When the integrated wind and solar power generation system is running, the grid-connected control unit calculates the current modulation ratio in real time. The system determines the operating range to which the current operating range belongs and uses the candidate resonant gain corresponding to that operating range as the current resonant gain. The resonant gain can be updated in every control cycle or at a parameter update frequency lower than the sampling frequency of the current inner loop. Using a lower parameter update frequency and applying appropriate low-pass processing to the modulation ratio can reduce the impact of switching ripple and sampling noise on the operating range determination; current error control is still performed in every sampling cycle and will not reduce the basic control bandwidth of the current inner loop.

[0062] like Figure 5 As shown, to avoid frequent switching of the resonant gain caused by fluctuations in the modulation ratio near the boundary of the operating range, this embodiment sets up a hysteresis switching module between two adjacent modulation ratio operating ranges. , to For the adjacent first... The first operating range and the first Each operating range is configured with an upward switching threshold. and drop switching threshold ,and As the modulation ratio gradually increases from a low value and reaches the rising switching threshold, the resonant gain changes from... Switch to When the modulation ratio gradually decreases from a high value and reaches the descent switching threshold, the resonant gain changes from... Switch to When the modulation ratio is between two thresholds, the resonant gain before switching remains unchanged.

[0063] In a smoother implementation, upon reaching the switching condition, the resonant gain is not immediately replaced by a step change. Instead, it transitions from the current candidate resonant gain to the target candidate resonant gain within a preset transition time using a ramp or first-order filtering method. This approach further reduces abrupt changes in control voltage commands and is suitable for scenarios with large grid-side inverter capacity or significant differences in resonant gains. Direct hysteresis switching requires less computation, making it suitable for resource-constrained digital control platforms; the smooth transition method improves current continuity during parameter switching.

[0064] In one specific operating mode, when the wind power generation unit and the tidal current generation unit operate simultaneously, the system is in a dual-source operation state. Both generation units jointly input power to the common DC bus. The grid-connected control unit generates a d-axis current reference value based on the DC bus voltage deviation, calculates the modulation ratio based on the current reference voltage amplitude and the actual DC bus voltage value, and then selects the corresponding resonant gain according to the modulation ratio operating range. Thus, the proportional-integral resonant controller compensates for the fifth and seventh harmonics in the grid-connected current while ensuring the stability of the discrete-domain current inner loop.

[0065] In another specific operating mode, when the wind speed decreases to a level where the wind turbine generator cannot generate power normally, or when the wind turbine experiences a planned shutdown or the corresponding generator-side converter goes out of service, the wind turbine generator stops inputting power to the common DC bus, while the power flow generator continues to operate, and the system enters a single power flow operation state. At this time, the input power of the common DC bus, the d-axis current reference value, the grid-side inverter reference voltage, and the modulation ratio may all change. The grid-connected control unit redetermines the resonant gain based on the new modulation ratio operating range, instead of directly specifying a fixed gain based solely on the wind turbine generator's shutdown signal, thus ensuring that the control parameters correspond to the actual operating state of the grid-side inverter.

[0066] In another specific operating mode, when the power flow velocity decreases to a level where the power flow generator unit cannot generate power normally, or when the power flow generator stops operating or the corresponding generator-side converter is out of service, the power flow generator unit stops inputting power to the common DC bus, while the wind power generator unit continues to operate, and the system enters single-wind operation mode. The grid-connected control unit also dynamically selects the resonant gain based on the current modulation ratio and uses a pre-distortion compensated discrete-domain proportional-integral resonant controller to generate grid-side inverter control voltage commands, ensuring the system maintains stable grid-connected output.

[0067] When switching between dual-source (wind and tide) operation, single-wind operation, and single-trough operation, the control system can use the energy unit's operating signals for status recording and protection. However, the actual selection of the resonant gain is still based on the modulation ratio and its hysteresis range. In this way, even if there is a brief time difference between the energy unit's start / stop signal and the actual changes in the common DC bus voltage and grid-connected current, the resonant gain can be adjusted according to the grid-side reference voltage requirements and the available DC bus voltage, reducing the increase in harmonics and parameter mismatches in the initial stage of operation state switching.

[0068] During the above operation, the d-axis control voltage command and the q-axis control voltage command undergo coordinate inverse transformation to obtain the three-phase reference voltage. The coordinate inverse transformation uses the same grid phase angle as the aforementioned current coordinate transformation. This ensures that the control voltage vector is synchronized with the grid. The three-phase reference voltage is then input to the modulation module, which can use space vector pulse width modulation, sinusoidal pulse width modulation, or other modulation methods suitable for three-phase grid-side inverters to generate drive signals for each power switching device of the grid-side inverter.

[0069] When using space vector pulse width modulation (SVM), the grid-connected control unit generates the switching duty cycle of each bridge arm based on the sector and duration of the reference voltage vector, and can set dead-time compensation, minimum pulse width limit, and over-modulation limit. The drive signal is applied to the power switching devices of the grid-side inverter via the drive circuit, causing the grid-side inverter to output a three-phase AC voltage; the output voltage is filtered by an inductor to form a three-phase grid-connected current that meets grid connection requirements. SVM has high DC voltage utilization and is suitable for scenarios with high power density requirements; sinusoidal pulse width modulation has a simpler implementation structure and is suitable for scenarios with limited control resources.

[0070] In a non-limiting example of parameters, when the fundamental frequency of the power grid is 50 Hz, the fundamental angular frequency of the power grid is... for rad / s, target resonant frequency for rad / s, corresponding to 300 Hz. The digital controller can be a combination of DSP, MCU, FPGA and processor, or other real-time control platforms. The sampling period should ensure the target resonant frequency is significantly lower than the Nyquist frequency, while allowing time for current sampling, recursive calculations, and duty cycle updates. Specifically... , , The candidate resonant gain, modulation ratio threshold, and hysteresis width can be determined through small-signal modeling, discrete-domain stability analysis, simulation, and prototype testing.

[0071] Through the above implementation, the outer loop of the DC bus voltage forms a grid-connected current reference value based on the voltage change of the common DC bus, while the inner loop of the grid-connected current forms a current error signal based on the grid-connected current reference value and the actual grid-connected current. The proportional-integral resonant controller, oriented towards the fifth and seventh harmonics, provides selective gain near the target resonant frequency. Pre-distortion compensation bilinear transform reduces the resonant center frequency offset caused by digital discretization. The resonant gain configuration based on modulation ratio, discrete domain stability analysis, and hysteresis switching allows the controller to adapt to different power levels and operating states. The coordinated operation of these components reduces the total harmonic distortion rate of the grid-connected current under multi-source power fluctuations and operating condition switching conditions, improves the waveform quality of the grid-connected current, and maintains good DC bus voltage stability, power factor, and continuous grid connection capability.

[0072] The specific embodiments of the present invention have been described above. It should be understood that the present invention is not limited to the specific embodiments described above. Those skilled in the art can make changes or substitutions to the generator type, generator-side converter method, coordinate transformation symbol, integral branch discretization method, modulation ratio normalization method, number of operating intervals, resonant gain transition method, and pulse width modulation method within the scope defined by the claims. Where there is no conflict, the technical features in the various embodiments of this application can be combined arbitrarily.

[0073] If the aforementioned functions are implemented as software functional units and sold or used as independent products, they can be stored in a computer-readable storage medium. Based on this understanding, the technical solution of this invention, or the part that contributes to the prior art, or a part of the technical solution, can be embodied in the form of a software product. This computer software product is stored in a storage medium and includes several instructions to cause a computer device (which may be a personal computer, server, or network device, etc.) to execute all or part of the steps of the methods described in the various embodiments of this invention. The aforementioned storage medium includes various media capable of storing program code, such as USB flash drives, portable hard drives, read-only memory (ROM), random access memory (RAM), magnetic disks, or optical disks.

[0074] The above description is merely a specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any person skilled in the art can easily conceive of various equivalent modifications or substitutions within the technical scope disclosed in the present invention, and these modifications or substitutions should all be covered within the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be determined by the scope of the claims.

Claims

1. A grid-connected current control method for an integrated wind and tidal power generation system, characterized in that, Includes the following steps: The actual value of the DC bus voltage of the common DC bus in the wind-sun integrated power generation system and the actual value of the grid-connected current of the grid-side inverter are obtained. The grid-connected current reference value is determined based on the actual value of the DC bus voltage and the DC bus voltage reference value. The current error signal is determined based on the grid-connected current reference value and the actual value of the grid-connected current. The modulation ratio is determined based on the reference voltage of the grid-side inverter and the actual value of the DC bus voltage, and the resonant gain corresponding to the current operating state is determined based on the modulation ratio. The current error signal is adjusted by a discrete domain proportional-integral resonant controller configured with the resonant gain to obtain the grid-side inverter control voltage command. The discrete domain proportional-integral resonant controller is obtained by pre-distortion compensation of the target resonant frequency and bilinear transformation of the pre-distortion compensated continuous domain proportional-integral resonant controller. The target resonant frequency corresponds to the fifth and seventh harmonics in the grid-connected current. The grid-side inverter is driven by generating a drive signal based on the grid-side inverter control voltage command, thereby controlling the grid-side inverter to output grid-connected current to the grid.

2. The grid-connected current control method for an integrated wind and tidal power generation system according to claim 1, characterized in that, The process of obtaining the actual value of the DC bus voltage of the common DC bus and the actual value of the grid-connected current of the grid-side inverter, determining the grid-connected current reference value based on the actual value of the DC bus voltage and the DC bus voltage reference value, and determining the current error signal based on the grid-connected current reference value and the actual value of the grid-connected current specifically includes: The DC bus voltage reference value is compared with the actual DC bus voltage value to obtain the DC bus voltage deviation, and the DC bus voltage deviation is input into the voltage outer loop proportional-integral controller to obtain the d-axis current reference value; Set the q-axis current reference value to zero. The grid-connected current reference value includes the d-axis current reference value and the q-axis current reference value. By adopting a grid voltage-oriented control method, the d-axis of the synchronous rotating coordinate system is aligned with the grid voltage vector, and the actual value of the grid-connected current is transformed to obtain the actual current component on the d-axis and the actual current component on the q-axis. The d-axis current reference value is compared with the actual d-axis current component to obtain the d-axis current error signal, and the q-axis current reference value is compared with the actual q-axis current component to obtain the q-axis current error signal. The current error signal includes the d-axis current error signal and the q-axis current error signal.

3. The grid-connected current control method for an integrated wind and tidal power generation system according to claim 1, characterized in that, The step of determining the modulation ratio based on the reference voltage of the grid-side inverter and the actual value of the DC bus voltage specifically includes: Obtain the reference voltage vector of the grid-side inverter and determine the reference voltage amplitude of the reference voltage vector; The modulation ratio is determined based on the relative relationship between the reference voltage amplitude and the actual value of the DC bus voltage, and the modulation ratio is used as a characterizing quantity of the current operating state of the wind and tidal power generation system. The modulation ratio is updated when the output power of the wind power generation unit, the output power of the tidal power generation unit, the actual value of the grid-connected current, or the actual value of the DC bus voltage changes.

4. The grid-connected current control method for an integrated wind and tidal power generation system according to claim 1, characterized in that, The step of determining the resonant gain corresponding to the current operating state based on the modulation ratio specifically includes: Establish a current inner loop stability analysis model corresponding to the discrete domain proportional-integral resonant controller, and establish discrete domain state variables and discrete domain state space equations based on the current inner loop stability analysis model; Based on the Lyapunov stability condition, the stability of the inner current loop under different resonant gains is determined, and the range of resonant gain values ​​that meet the stability requirements of the inner current loop is obtained. Multiple candidate resonant gains corresponding to different operating states are selected from the range of stable values ​​of the resonant gain.

5. The grid-connected current control method for an integrated wind and tidal power generation system according to claim 4, characterized in that, The selection of multiple candidate resonant gains corresponding to different operating states from the stable range of the resonant gain specifically includes: The range of the modulation ratio is divided into multiple modulation ratio operating intervals, and a corresponding candidate resonant gain is configured for each modulation ratio operating interval from the range of stable resonant gain values, thereby establishing a correspondence between the modulation ratio operating interval and the candidate resonant gain. Obtain the current modulation ratio, determine the modulation ratio operating range to which the current modulation ratio belongs, and determine the candidate resonant gain corresponding to the modulation ratio operating range to which the current modulation ratio belongs as the resonant gain corresponding to the current operating state; The current operating states include the dual-source wind and tidal power generation state where both the wind power generation unit and the tidal power generation unit are operating simultaneously, the single-wind operating state where the wind power generation unit is operating and the tidal power generation unit is out of operation, and the single-tidal power generation state where the tidal power generation unit is operating and the wind power generation unit is out of operation.

6. The grid-connected current control method for an integrated wind and tidal power generation system according to claim 5, characterized in that, The step of determining the candidate resonant gain corresponding to the modulation ratio operating range to which the current modulation ratio belongs as the resonant gain corresponding to the current operating state specifically includes: A first switching threshold and a second switching threshold are set between two adjacent modulation ratio operating ranges, wherein the first switching threshold is higher than the second switching threshold, so as to form a hysteresis range between two adjacent modulation ratio operating ranges. When the current modulation ratio increases and reaches the first switching threshold, the currently used resonant gain is switched to a candidate resonant gain corresponding to the adjacent modulation ratio operating range in the direction of modulation ratio increase. When the current modulation ratio decreases and reaches the second switching threshold, the currently used resonant gain is switched to a candidate resonant gain corresponding to the adjacent modulation ratio operating range in the direction of modulation ratio decrease. When the current modulation ratio is within the hysteresis interval, the currently used resonant gain remains unchanged.

7. The grid-connected current control method for an integrated wind and tidal power generation system according to claim 1, characterized in that, The continuous-domain proportional-integral resonant controller includes a proportional control branch, an integral control branch, and a resonant compensation branch. The transfer function of the continuous-domain proportional-integral resonant controller is expressed as: in, The transfer function of the continuous-domain proportional-integral resonant controller is represented by . Represents a variable in the complex frequency domain; Indicates proportional gain; Indicates integral gain; This represents the resonant gain; Indicates the resonant bandwidth parameter; Indicates the target resonant frequency; The target resonant frequency is expressed as: in, The target resonant frequency is used to compensate for the sixth fundamental frequency component of the fifth and seventh harmonics in the three-phase stationary coordinate system in the synchronous rotating coordinate system.

8. The grid-connected current control method for an integrated wind and tidal power generation system according to claim 7, characterized in that, The process of obtaining the discrete-domain proportional-integral (PII) resonant controller by pre-distorting the target resonant frequency and performing a bilinear transformation on the pre-distorted continuous-domain PPII controller specifically includes: Set the sampling period of the digital controller, and determine the pre-distortion equivalent frequency parameters based on the target resonant frequency and the sampling period; Based on the predistortion equivalent frequency parameters, a predistortion compensation bilinear transformation relationship is constructed, which is expressed as follows: in, This represents the pre-distortion equivalent frequency parameter determined based on the target resonant frequency and the sampling period; Represents discrete-domain variables; Substituting the pre-distortion compensation bilinear transformation relationship into the resonance compensation branch of the continuous domain proportional-integral resonant controller, a discrete domain resonance compensation branch is obtained. The discrete domain resonance compensation branch is then combined with the discretized proportional control branch and integral control branch to obtain the discrete domain proportional-integral resonant controller.

9. The grid-connected current control method for an integrated wind and tidal power generation system according to claim 8, characterized in that, The discrete-domain proportional-integral resonant controller is implemented using a differential recursive approach, specifically including: The transfer function of the discrete domain proportional-integral resonant controller is rearranged into a standard difference recursive form to obtain the first output recursive coefficient, the second output recursive coefficient, and the first to third error input coefficients. Calculate the controller output at the current sampling time: in, Indicates the first The controller output at each sampling time; and They represent the first The sampling time and the first sampling time The controller output at each sampling time; Indicates the first Current error signal at each sampling time; and They represent the first The sampling time and the first sampling time Current error signal at each sampling time; and These represent the first output recursive coefficient and the second output recursive coefficient, respectively. , and These represent the first to third error input coefficients, respectively. The d-axis current error signal and the q-axis current error signal are respectively used as the differential recursive form of current error signal for calculation to obtain the d-axis control voltage command and the q-axis control voltage command, and the grid-side inverter control voltage command is composed of the d-axis control voltage command and the q-axis control voltage command.

10. The grid-connected current control method for an integrated wind and tidal power generation system according to claim 1, characterized in that, The step of generating a drive signal for the grid-side inverter based on the grid-side inverter control voltage command, and controlling the grid-side inverter to output grid-connected current to the grid, specifically includes: Obtain the d-axis control voltage command and q-axis control voltage command of the grid-side inverter in the synchronous rotating coordinate system; Perform coordinate inverse transformation on the d-axis control voltage command and the q-axis control voltage command to obtain the three-phase reference voltage used to control the grid-side inverter; The three-phase reference voltage is pulse-width modulated to generate a drive signal for controlling the on / off switching of the power switching devices of the grid-side inverter; The drive signal is applied to the power switching device of the grid-side inverter, causing the grid-side inverter to output grid-connected current according to the grid-side inverter control voltage command.

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