An alternating current excitation system no-load voltage building and pre-synchronization grid-connection control method and system

Abstract

This invention discloses a method and system for no-load voltage build-up and pre-synchronization grid connection control of an AC excitation system. The method includes: acquiring the amplitude and phase of the grid voltage in real time via a phase-locked loop when the speed of the doubly-fed induction motor reaches the synchronous speed range; detecting the rotor mechanical position and calculating the slip angle using an encoder; extracting the fundamental component and correcting the phase of the stator-side no-load voltage; setting a target value for the stator voltage based on grid voltage-oriented control and calculating the target rotor current; constructing an inner-loop control loop for the rotor current, calculating the feedforward decoupling term and performing phase compensation; superimposing an outer-loop control loop for the stator voltage, and correcting the rotor current through closed-loop adjustment to eliminate the deviation between the stator-side no-load voltage and the grid voltage; and closing the grid connection circuit breaker and switching the control mode when the preset synchronization conditions are met. The dual-closed-loop control structure achieves precise no-load voltage build-up and smooth pre-synchronization grid connection of the excitation system, improving the stability and reliability of the grid connection process.

Description

Technical Field

[0001] This invention relates to the field of power system relay protection technology, specifically to a method and system for no-load voltage building-up and pre-synchronization grid connection control of an AC excitation system. Background Technology

[0002] In recent years, with the rapid development of new energy power generation technologies, the penetration rate of renewable energy sources such as wind power and solar power in the power grid has been continuously increasing. However, these new energy power generation systems exhibit characteristics such as low inertia, low short-circuit ratio, high impedance, and weak damping, making it difficult for traditional grid-following control strategies to meet their operational needs under weak grid conditions. Against this backdrop, variable-speed pumped storage systems, as a new type of energy storage-power generation system, offer important support in peak shaving, valley filling, and frequency regulation due to their advantages such as wide power regulation range and high speed. However, in the actual operation of variable-speed pumped storage systems, especially in no-load voltage build-up and pre-synchronization control under power generation conditions, many challenges still exist.

[0003] Traditional control methods typically employ a stator voltage outer loop amplitude-phase control strategy, directly adjusting the rotor current to control the rotor voltage amplitude and phase, thus achieving synchronization with the power grid. This method has significant drawbacks: First, the physical meaning of the phase loop is not clearly defined, leading to a lack of theoretical basis for control parameter design, often relying on trial and error based on experience, resulting in low debugging efficiency and difficulty in ensuring optimal parameters. Second, due to the neglect of detailed modeling of the system's dynamic characteristics, traditional methods are insufficient in analyzing the coupling relationship between the rotor-side converter excitation voltage and the stator-side no-load voltage, making them susceptible to factors such as sampling delay, modulation error, and dq-axis coupling at low switching frequencies, resulting in insufficient stator voltage tracking accuracy.

[0004] Furthermore, existing technologies often employ low-pass filtering to extract the fundamental component of the stator voltage. However, if the phase shift effect of the filter is not effectively compensated, a fixed phase difference may exist between the stator voltage and the grid voltage, further reducing the reliability of pre-synchronization. These problems can lead to voltage fluctuations during the no-load voltage build-up phase, prolonged pre-synchronization time, and even inrush currents at the moment of grid connection, threatening equipment safety.

[0005] Therefore, there is an urgent need for a control strategy based on a precise dynamic model to address the shortcomings of existing methods in terms of modeling clarity, scientific parameter design, and anti-interference capabilities, thereby ensuring smooth system startup and achieving high-precision pre-synchronization grid connection. Summary of the Invention

[0006] In order to address the technical deficiencies in existing technologies, such as the lack of clear physical meaning in the stator voltage outer loop amplitude and phase control strategy, insufficient stator voltage tracking accuracy, and low pre-synchronization reliability, this invention provides an AC excitation system for no-load voltage build-up and pre-synchronization grid connection control.

[0007] To solve the above problems, the present invention is implemented according to the following technical solution:

[0008] In a first aspect, the present invention provides a method for no-load voltage build-up and pre-synchronous grid connection control of an AC excitation system, comprising the following steps: when the rotational speed of the doubly-fed induction motor reaches the synchronous speed range, the amplitude and phase of the grid voltage are acquired in real time through a phase-locked loop; the mechanical position of the doubly-fed induction motor rotor is detected using an encoder, and the slip angle is calculated in combination with the number of motor pole pairs; based on the slip angle, the rotor current is converted from a stationary coordinate system to a synchronous rotating coordinate system with the synchronous phase as the reference; the fundamental component of the stator-side no-load voltage is extracted and the phase is corrected to obtain the corrected stator-side no-load voltage phase; based on... The grid voltage-oriented control system sets a target value for the stator voltage and calculates the target rotor current based on the corrected phase of the stator-side no-load voltage. An inner-loop control loop for the rotor current is constructed, and the feedforward decoupling term is calculated using the rotor voltage equation. Phase compensation is then applied to the control signal. An outer-loop control loop for the stator voltage is superimposed, and the rotor current is corrected through closed-loop regulation to eliminate amplitude and phase deviations between the stator-side no-load voltage and the grid voltage. When the stator-side no-load voltage and the grid voltage meet preset synchronization conditions, the grid-connected circuit breaker is closed, and the control mode is switched to a grid-connected generation control mode based on stator voltage orientation.

[0009] In conjunction with the first aspect, the present invention provides a first specific implementation of the first aspect, wherein the specific steps of extracting the fundamental component and correcting the phase of the stator-side open-circuit voltage to obtain the corrected phase of the stator-side open-circuit voltage include: performing low-pass filtering on the stator-side open-circuit voltage to extract the fundamental component; and compensating for the phase shift introduced by the low-pass filtering at the fundamental frequency.

[0010] In conjunction with the first aspect, the present invention provides a second specific implementation of the first aspect, wherein the phase compensation is obtained by real-time calculation of the phase response of the filter transfer function.

[0011] In conjunction with the first aspect, the present invention provides a third specific embodiment of the first aspect, wherein the inner loop transfer function of the rotor current is: ;in, and These are the parameters for the rotor current PI controller. The transfer function is the delay function caused by sampling and modulation. The self-inductance between the equivalent two-phase windings of the rotor in a synchronously rotating coordinate system. The resistance of the rotor.

[0012] In conjunction with the first aspect, the present invention provides a fourth specific implementation of the first aspect, wherein the stator voltage outer loop transfer function is: ;in, and These are the parameters for the stator voltage PI controller. Let be the inner loop transfer function of the rotor current. The stator synchronous velocity angular frequency, This refers to the mutual inductance between the coaxial equivalent windings of the stator and rotor in a synchronous rotating coordinate system.

[0013] In conjunction with the first aspect, the present invention provides a fifth specific implementation of the first aspect, specifically, the deviation between the stator voltage amplitude and the grid voltage amplitude is less than a preset amplitude threshold; the deviation between the stator voltage phase and the grid voltage phase is less than a preset phase threshold.

[0014] Secondly, the present invention also provides an AC excitation system no-load voltage building-up and pre-synchronization grid-connection control system, comprising: a phase-locked loop module for real-time acquisition of the amplitude and phase of the grid voltage; an encoder module for detecting the mechanical position of the doubly-fed induction motor rotor; a coordinate transformation module for converting the rotor current from a stationary coordinate system to a synchronous rotating coordinate system; a phase compensation module for extracting the fundamental component and correcting the phase of the stator-side no-load voltage to obtain the corrected stator-side no-load voltage phase; a calculation module for setting a target value of the stator voltage based on grid voltage-oriented control and calculating the target rotor current in combination with the corrected stator-side no-load voltage phase; a rotor current inner loop control module for constructing a rotor current inner loop control circuit, calculating the feedforward decoupling term through the rotor voltage equation, and performing phase compensation on the control signal; a stator voltage outer loop control module for superimposing the stator voltage outer loop control circuit, and correcting the rotor current through closed-loop adjustment to eliminate the amplitude and phase deviation between the stator-side no-load voltage and the grid voltage; and a grid-connection control module for closing the grid-connection circuit breaker and switching the control mode when the synchronization condition is met.

[0015] In conjunction with the second aspect, the present invention provides a first specific implementation of the second aspect, wherein the output of the stator voltage outer loop control module is used as the reference value of the rotor current inner loop control module to form a dual closed-loop control structure.

[0016] In conjunction with the second aspect, the present invention provides a second specific implementation of the second aspect. Specifically, after closing the grid-connected circuit breaker, the grid-connected control module switches the control mode to a vector control mode based on stator voltage orientation to realize grid-connected power generation operation.

[0017] Compared with the prior art, the beneficial effects of the present invention are:

[0018] By adding an outer voltage loop control, better grid voltage tracking performance is achieved, solving the problem that it is difficult to obtain good pre-synchronization effect by simply relying on rotor current commands. By considering the phase shift angle of the low-pass filter in the dq transformation of the stator voltage extraction stage, the problem of a fixed phase difference between the stator voltage and the grid voltage is avoided. Through the design of the rotor current inner loop PI controller and the dq axis feedforward decoupling component, the influence of factors such as sampling, modulation, parameter errors, and dq axis coupling at low switching frequencies is effectively compensated. The overall control scheme achieves good tracking of the stator voltage and the grid voltage during DFIM no-load start-up, providing a reliable guarantee for subsequent grid-connected operation. Attached Figure Description

[0019] The specific embodiments of the present invention will be further described in detail below with reference to the accompanying drawings, wherein:

[0020] Figure 1 This is the DFIM system before no-load startup of the present invention.

[0021] Figure 2 This is a block diagram of the no-load voltage build-up control of the AC excitation system for grid voltage orientation according to the present invention.

[0022] Figure 3 This is a schematic diagram of the switching of the pre-synchronization control and grid-connected operation control loop of the present invention. Detailed Implementation

[0023] The preferred embodiments of the present invention will be described below with reference to the accompanying drawings. It should be understood that the preferred embodiments described herein are for illustration and explanation only and are not intended to limit the present invention. Example 1

[0024] like Figures 1-3 As shown, a method for no-load voltage build-up and pre-synchronization grid connection control of an AC excitation system includes the following steps:

[0025] Step S1: When the speed of the doubly fed induction motor reaches the synchronous speed range, the amplitude and phase of the grid voltage are obtained in real time through the phase-locked loop.

[0026] Specifically, once the system determines that the motor has approached or exceeded 90% of its synchronous speed, the control system immediately starts the grid-side converter. The grid-side converter adopts a three-phase voltage-source PWM converter topology, with its input directly connected to the grid voltage. Electrical connection to the grid is achieved through a grid-side filter inductor (the inductance value is selected based on the grid voltage level and the converter's power level). The grid-side converter control employs a dual-loop control strategy: an outer loop for DC bus voltage and an inner loop for grid current. At startup, the outer loop for DC bus voltage outputs a current reference value based on the deviation between the preset target DC bus voltage value and the current actual DC bus voltage (initial value close to zero) after passing through a PI regulator. The inner loop for grid current compares this current reference value with the actual grid current (initial value close to zero) and generates a PWM pulse width modulation signal via a current regulator. This signal controls the switching of the grid-side converter's transistors, enabling rapid rise of the grid current and tracking of the grid voltage. Simultaneously, a phase-locked loop (PLL) circuit acquires the amplitude and phase of the grid voltage in real time. The phase-locked loop (PLL) receives the three-phase voltage signal from the grid, which is then transformed by Clark to obtain the α and β phase voltage components. This is further transformed by Park to the dq rotating coordinate system to extract the amplitude and phase information of the grid voltage. The dynamic response parameters of the PLL (such as loop bandwidth) are optimized based on the grid voltage fluctuation characteristics and the transient characteristics of the motor startup process. This ensures that within a grid voltage fluctuation range of ±10% and a frequency variation of ±0.5Hz, the PLL can complete phase locking within 5-10 grid cycles, providing a precise synchronous phase signal for the current control of the grid-side converter. Through the dual closed-loop control of the grid-side converter and the precise phase locking of the PLL, the grid-side converter can quickly raise and stabilize the DC bus voltage to the target value, providing a stable DC power supply for the normal operation of the rotor-side converter and subsequent excitation control.

[0027] Step S2: Use an encoder to detect the mechanical position of the doubly fed induction motor rotor, calculate the slip angle in combination with the number of motor pole pairs, and convert the rotor current from the stationary coordinate system to the synchronous rotating coordinate system based on the synchronous phase according to the slip angle.

[0028] Specifically, an encoder is used to detect the mechanical angular position of the doubly-fed induction motor rotor. The encoder resolution is typically between 1024 and 4096 lines. The pulse signal output by the encoder is transmitted to the motor control system via an interface circuit. In the control system, the mechanical position angle of the rotor is determined by counting the pulse signal output by the encoder and combining it with the number of pole pairs p of the motor. The calculation formula is as follows:

[0029] Where N is the number of pulses output by the encoder. It is the total number of pulses of the encoder.

[0030] Meanwhile, the phase-locked loop module provides a synchronization phase angle With rotor mechanical position angle The difference between them constitutes the slip angle. The calculation formula is: Based on the calculated slip angle The rotor current is transformed from a stationary coordinate system to a synchronous rotating coordinate system based on the synchronous phase. During this transformation, two orthogonal components of the rotor current are first sampled using a current sensor. and Construct the coordinate transformation matrix:

[0031]

[0032] This transformation matrix converts the rotor current component in the stationary coordinate system into the DC component in the synchronous rotating coordinate system. and :

[0033]

[0034] Converted rotor current component and It can be used to implement vector control of doubly fed induction motors.

[0035] Step S3: Extract the fundamental component and correct the phase of the stator side open-circuit voltage to obtain the corrected phase of the stator side open-circuit voltage.

[0036] Step S301: Perform low-pass filtering on the stator side no-load voltage to extract the fundamental component; compensate for the phase shift introduced by the low-pass filtering at the fundamental frequency.

[0037] When a doubly-fed induction motor is operating under no-load conditions, the stator voltage is generated by AC excitation induction on the rotor side. At this time, the stator current is relatively small, and the motor's operating characteristics are relatively simple. To accurately obtain the fundamental component of the stator-side no-load voltage, the following steps are typically employed: Real-time sampling of the three-phase stator voltage is performed using a voltage sensor. The sampled signal is filtered and amplified by a signal conditioning circuit to remove harmonics and interference from the rotor switching frequency, ensuring signal purity. The sampled three-phase voltage signal is then converted into a voltage signal in a two-phase stationary coordinate system. This is typically achieved using the Clarke transform, as shown in the following formula: ;

[0038] in, , , It is the stator three-phase voltage. , It is the voltage in a two-phase stationary coordinate system.

[0039] Bandpass or low-pass filters are used to protect the fundamental frequency of the power grid. The center frequency of the filter is set to the fundamental frequency, and the bandwidth is selected appropriately to filter out harmonics and other interference components. The signal after filtering is the fundamental component. After extracting the fundamental component, phase correction is required to ensure the accuracy of the stator-side no-load voltage phase. The specific implementation of this step is as follows:

[0040] Phase-locked loop (PLL) technology is used to perform phase tracking on the extracted fundamental voltage signal. The PLL adjusts the phase of the output signal in real time using a phase detector, a bandpass or low-pass filter, and a voltage-controlled oscillator (VCO) to ensure it is in phase with the input signal. The phase detector compares the phase difference between the input signal and the VCO output signal; the bandpass or low-pass filter filters this phase difference; and the VCO adjusts its output frequency and phase based on the filtered signal. The phase difference between the extracted fundamental voltage signal and a reference phase of the grid voltage is calculated. This reference phase is typically provided by the PLL module to ensure phase consistency with the grid voltage. The phase difference can be calculated using the arctangent function, as shown in the following formula: ;

[0041] in, , It is the component of the fundamental voltage in a two-phase stationary coordinate system.

[0042] Based on the calculated phase difference, the fundamental voltage signal is phase-corrected. The corrected phase should be consistent with the phase of the grid voltage to ensure synchronous operation of the motor. The corrected voltage phase can be expressed by the following formula: ;

[0043] in, It is the extracted fundamental voltage phase. It is the calculated phase difference.

[0044] Step S4: Based on grid voltage directional control, set the target value of stator voltage and calculate the target rotor current by combining the corrected stator side no-load voltage phase.

[0045] Specifically, Grid Voltage Oriented Control (GVOC) is a commonly used control strategy designed to ensure that the stator-side no-load voltage of a doubly-fed induction motor is synchronized with the grid voltage. Its core idea is to align the fundamental component of the stator voltage with the fundamental component of the grid voltage, thereby achieving stable motor operation. The fundamental component of the grid voltage is acquired in real time through a phase-locked loop (PLL) module. The PLL samples the three-phase grid voltage, extracting the grid fundamental frequency and phase to ensure synchronization. The fundamental voltage phase angle provided by the PLL serves as the reference for directional control. Based on the motor's operating requirements, a target value for the stator voltage is set. This typically includes both amplitude and phase. The target amplitude is usually set to the rated value of the grid voltage to ensure the motor operates at its rated voltage. The target phase is consistent with the fundamental phase of the grid voltage, ensuring synchronization between the stator-side no-load voltage and the grid voltage.

[0046] Based on the corrected stator-side no-load voltage phase, the target rotor current is calculated to achieve precise motor control. The specific steps are as follows:

[0047] The stator voltage of a doubly-fed induction motor can be described by the following equation: ;

[0048] in, , These are the d-axis and q-axis components of the stator flux linkage, respectively. , These are the d-axis and q-axis components of the stator voltage, respectively. The resistance of the stator; The stator synchronous speed angular frequency is defined. The fundamental component of the stator-side no-load voltage is extracted, and its phase is corrected to ensure consistency with the grid voltage fundamental. The corrected stator-side no-load voltage phase is used for subsequent control calculations. The target rotor current is determined based on the fundamental component of the stator-side no-load voltage and the motor model.

[0049] Specifically, the control system uses the grid voltage as the orientation reference to set the target value of the stator voltage. During the no-load voltage build-up phase, the target value of the stator voltage amplitude is set to the amplitude of the grid voltage, and the target value of the phase is set to the phase of the grid voltage.

[0050] In the dq synchronous rotating coordinate system, the flux linkage equation of DFIM is:

[0051] ;

[0052] in: , These are the d-axis and q-axis components of the stator flux linkage, respectively. , These are the d-axis and q-axis components of the rotor current, respectively. The mutual inductance between the coaxial equivalent windings of the stator and rotor in the dq coordinate system; , These are the self-inductances between the equivalent two-phase windings of the stator and rotor in the dq coordinate system, respectively.

[0053] According to the voltage equation and flux linkage equation of a doubly-fed induction motor, the stator voltage and rotor current have the following relationship in a synchronous rotating coordinate system:

[0054] ;

[0055] in: , , , These are the d-axis and q-axis components of the stator and rotor voltages, respectively. , These are the resistances of the stator and rotor, respectively; The stator synchronous velocity angular frequency, The slip angular frequency, ω is the rotor electrical angular frequency.

[0056] like Figure 1 As shown, under no-load conditions, the stator-side circuit is open, therefore = = 0, therefore the flux linkage equation of DFIM becomes:

[0057] ;

[0058] The voltage equation under no-load conditions is obtained as follows:

[0059] ;

[0060] When the grid voltage is oriented, the grid voltage The steady-state components in the dq coordinate system are: , The control objective for no-load pressure build-up is to make... ; The target rotor current is obtained as follows: .

[0061] Step S5: Construct the inner loop control circuit of rotor current, calculate the feedforward decoupling term through the rotor voltage equation, and perform phase compensation on the control signal.

[0062] Specifically, in order to eliminate the cross-coupling effect in rotor current control and improve control accuracy and dynamic performance, a feedforward decoupling term is calculated based on the rotor voltage equation to eliminate coupling factors in rotor current control, improve control accuracy, and ensure that the phase and amplitude of the rotor current accurately track the target value.

[0063] The rotor voltage equation of a doubly-fed induction motor is as follows:

[0064] ;

[0065] in, and These are the d and q components of the rotor voltage; It is the rotor resistance; It is the rotor inductance; It is the slip angular frequency.

[0066] Based on the rotor voltage equation, a feedforward decoupling term is calculated to eliminate cross-coupling effects:

[0067] ;

[0068] The phase compensation is obtained by calculating the phase response of the filter transfer function in real time. Specifically, phase compensation is usually achieved by designing a phase lead-lag compensator. The transfer function of the compensator, i.e., the inner loop transfer function of the rotor current, can be expressed as:

[0069] ;

[0070] in, and These are the parameters for the rotor current PI controller. The transfer function is the delay function caused by sampling and modulation. The self-inductance between the equivalent two-phase windings of the rotor in a synchronously rotating coordinate system. The resistance of the rotor.

[0071] Step S6: Superimpose the stator voltage outer loop control circuit, and adjust the rotor current through closed-loop regulation to eliminate the amplitude and phase deviation between the stator side no-load voltage and the grid voltage.

[0072] Specifically, the stator voltage outer loop outputs a rotor current correction command via a PI regulator based on the amplitude and phase deviation between the stator-side no-load voltage and the grid voltage. The parameters of the PI regulator are designed according to the stator voltage adjustment range and dynamic response characteristics. The stator voltage outer loop control loop and the rotor current inner loop control loop work together to correct the rotor current through closed-loop regulation, gradually eliminating the amplitude and phase deviation between the stator-side no-load voltage and the grid voltage.

[0073] The outer loop transfer function of the stator voltage is: ;

[0074] in, and These are the parameters for the stator voltage PI controller. Let be the inner loop transfer function of the rotor current. The stator synchronous velocity angular frequency, This refers to the mutual inductance between the coaxial equivalent windings of the stator and rotor in a synchronous rotating coordinate system.

[0075] Specifically, the outer loop control of the stator voltage uses a PI controller, taking the deviation between the stator voltage dq component and its target value as input, and the output as the correction amount for the inner loop of the rotor current:

[0076] ;

[0077] ;

[0078] in, , These are the parameters for the outer loop PI controller of the stator voltage. and It is the rotor current correction value output by the outer loop PI controller of the stator voltage. It is a reference value for the q-axis component of the stator voltage, usually set to the amplitude of the mains voltage. ; It is the actual measured value of the q-axis component of the stator voltage; It is the reference value for the d-axis component of the stator voltage, and is usually set to zero to achieve grid voltage directional control; It is the actual measured value of the d-axis component of the stator voltage.

[0079] The correction amount of the inner loop of the rotor current is used to adjust the rotor current reference value, thereby eliminating the amplitude and phase deviation of the stator side no-load voltage.

[0080] The corrected rotor current reference value is:

[0081] ;

[0082] ;

[0083] in, This is the corrected reference value for the d-axis component of the rotor current. This is the corrected reference value for the q-axis component of the rotor current. This is the initial reference value for the d-axis component of the rotor current. This is the correction amount for the d-axis component of the rotor current. The initial reference value for the q-axis component of the rotor current is also based on the motor's control objectives. This is the correction amount for the q-axis component of the rotor current.

[0084] This dual closed-loop control structure can effectively compensate for stator voltage deviations caused by factors such as motor parameter errors and measurement errors, ensuring that the amplitude and phase of the stator voltage are consistent with the grid voltage under no-load conditions.

[0085] Step S7: When the stator side no-load voltage and the grid voltage meet the preset synchronization conditions, close the grid-connected circuit breaker and switch the control mode to the grid-connected power generation control mode based on stator voltage orientation.

[0086] The amplitude, phase, frequency, and phase sequence of the stator-side no-load voltage and grid voltage are monitored in real time. When the deviation between the stator voltage amplitude and the grid voltage amplitude is less than a preset amplitude threshold, the deviation between the stator voltage phase and the grid voltage phase is less than a preset phase threshold, the frequency deviation is within the allowable range, and the phase sequence is correct, the synchronization condition is deemed met. A control signal is issued to close the grid-connected circuit breaker, connecting the doubly-fed induction motor to the grid. The circuit breaker uses a fast-response vacuum circuit breaker to ensure rapid closing at the instant the synchronization condition is met, reducing the inrush current at the moment of grid connection. The control mode is switched from the no-load voltage building-up and pre-synchronization control mode to the grid-connected generation control mode based on stator voltage orientation. The specific process is as follows:

[0087] The control system's switching logic shifts from focusing on stator voltage establishment and synchronization condition monitoring to a stator voltage-oriented vector control logic to maintain stable motor power generation. New stator voltage and current reference values ​​are set, and active and reactive power reference values ​​are adjusted according to grid demand and motor load conditions to achieve maximum power output and stable grid support. Parameters related to grid-connected power generation control, such as integrators and filters, are reset and initialized to ensure accurate startup of the power generation control mode. In stator voltage-oriented vector control mode, the control system monitors stator voltage and current in real time, dynamically adjusting the output voltage and frequency of the rotor-side converter to ensure that the motor's output power matches grid parameters, achieving efficient energy conversion and stable transmission. At the moment of grid connection, grid current and motor parameters are monitored in real time. When abnormal current or motor faults are detected, the grid circuit breaker is immediately disconnected to protect the motor and grid. After grid connection, parameters such as grid voltage, stator current, active power, and reactive power are continuously monitored. Based on grid dispatch instructions and load changes, the motor's operating status is adjusted in real time to ensure efficient and stable operation.

[0088] Example 2

[0089] An AC excitation system no-load voltage build-up and pre-synchronization grid connection control system, comprising:

[0090] A grid-side converter module is used to establish a constant DC bus voltage.

[0091] Specifically, the grid-side converter module adopts a three-phase full-bridge IGBT power converter structure with a rated capacity of 30kW and a switching frequency of 5kHz. This module uses PWM modulation technology to control the on / off state of the power switches, achieving energy conversion between AC and DC. The grid-side converter module employs a voltage-oriented control strategy, establishing a stable DC bus voltage through closed-loop control. The grid-side converter module also features power factor regulation, adjusting the phase of the grid-side current according to system requirements to achieve unity power factor operation or reactive power compensation.

[0092] A phase-locked loop (PLL) module is used to acquire the amplitude and phase of the grid voltage in real time.

[0093] The encoder module is used to detect the mechanical position of the rotor of a doubly fed induction motor.

[0094] The coordinate transformation module is used to transform the rotor current from the stationary coordinate system to the synchronous rotating coordinate system.

[0095] Specifically, the coordinate transformation module includes a three-phase / two-phase transformation unit and a stationary / rotating coordinate transformation unit. The three-phase / two-phase transformation unit performs the Clarke transformation from three-phase current to two-phase stationary coordinate system current. The stationary / rotating coordinate transformation unit performs the Park transformation from two-phase stationary coordinate system current to synchronously rotating coordinate system current. The trigonometric functions used in the transformation process are implemented using a lookup table method to improve calculation efficiency. The coordinate transformation module is synchronized with the control cycle of the current control loop.

[0096] The phase compensation module is used to extract the fundamental component and correct the phase of the stator side open-circuit voltage to obtain the corrected stator side open-circuit voltage phase.

[0097] Specifically, the phase compensation module includes a bandpass or low-pass filter and a phase correction unit. The bandpass or low-pass filter is used to extract the fundamental component of the stator-side open-circuit voltage. The phase correction unit compensates for the phase delay introduced by the bandpass or low-pass filter at the fundamental frequency. The phase compensation module also includes an adaptive mechanism that automatically adjusts the compensation angle according to changes in the actual grid frequency, ensuring accurate compensation for phase delay even when the grid frequency fluctuates.

[0098] The calculation module is used to set the target value of the stator voltage based on grid voltage-oriented control, and calculate the target rotor current by combining the corrected stator-side no-load voltage phase.

[0099] The rotor current inner loop control module is used to construct the rotor current inner loop control circuit. It calculates the feedforward decoupling term through the rotor voltage equation and performs phase compensation on the control signal.

[0100] Specifically, the rotor current inner-loop control module consists of a PI controller, a feedforward decoupling unit, and a phase lead compensator. The feedforward decoupling unit calculates the cross-coupling terms based on the rotor voltage equation, achieving decoupling between the d-axis and q-axis control. The phase lead compensator is implemented using a digital filter, and its transfer function is... ;in, and These are the parameters for the rotor current PI controller. The transfer function is the delay function caused by sampling and modulation. The self-inductance between the equivalent two-phase windings of the rotor in a synchronously rotating coordinate system. This represents the rotor's resistance. It provides phase lead within the system bandwidth to compensate for phase lag in the system. The rotor current inner loop control module ensures a rapid response to changes in rotor current within its control cycle.

[0101] The stator voltage outer loop control module is used to superimpose the stator voltage outer loop control circuit, and correct the rotor current through closed-loop adjustment to eliminate the amplitude and phase deviation between the stator side no-load voltage and the grid voltage.

[0102] Specifically, the stator voltage outer loop control module consists of a PI controller and a voltage comparator. The voltage comparator calculates the deviation between the stator voltage dq component and its target value, which serves as the input to the PI controller. The output of the stator voltage outer loop control module is used as the reference value correction for the rotor current inner loop control module, forming a dual closed-loop control structure. The control cycle of the stator voltage outer loop control module is one order of magnitude slower than that of the rotor current inner loop, ensuring system stability.

[0103] The grid-connected control module is used to close the grid-connected circuit breaker and switch the control mode when the synchronization conditions are met.

[0104] Specifically, the grid-connected control module includes a synchronization condition detection unit, a circuit breaker control unit, and a mode switching unit. The synchronization condition detection unit monitors the amplitude and phase difference between the stator voltage and the grid voltage in real time. When the preset synchronization condition is met and the duration exceeds a preset value, it determines that grid connection operation can proceed. The circuit breaker control unit generates a circuit breaker closing signal to control the operation of the grid-connected circuit breaker. The mode switching unit is responsible for switching the control mode before and after grid connection, from no-load voltage build-up control to grid-connected generation control. The grid-connected control module also includes protection functions, capable of quickly disconnecting the grid-connected circuit breaker to protect equipment safety when abnormal conditions are detected.

[0105] In a preferred embodiment, the output of the stator voltage outer loop control module serves as the reference value for the rotor current inner loop control module, forming a dual closed-loop control structure. This control structure can effectively compensate for stator voltage deviations caused by factors such as motor parameter errors and measurement errors, ensuring that the amplitude and phase of the stator voltage are consistent with the grid voltage under no-load conditions.

[0106] In a preferred embodiment, after closing the grid-connected circuit breaker, the grid-connected control module switches the control mode to a stator voltage-oriented vector control mode to achieve grid-connected power generation operation. In grid-connected power generation mode, the control system uses the stator voltage as the orientation reference, controls the active power by adjusting the d-axis component of the rotor current and controls the reactive power by adjusting the q-axis component of the rotor current, thereby achieving grid-connected power generation operation of the doubly-fed induction motor.

[0107] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any way. Therefore, any modifications, equivalent changes, and alterations made to the above embodiments based on the technical essence of the present invention without departing from the scope of the present invention shall still fall within the scope of the present invention.

Claims

1. A method for no-load voltage build-up and pre-synchronization grid connection control of an AC excitation system, characterized in that, Includes the following steps: When the speed of the doubly fed induction motor reaches the synchronous speed range, the amplitude and phase of the grid voltage are obtained in real time through a phase-locked loop; The mechanical position of the doubly fed induction motor rotor is detected by an encoder, and the slip angle is calculated by combining the number of pole pairs of the motor. Based on the slip angle, the rotor current is converted from the stationary coordinate system to the synchronous rotating coordinate system with the synchronous phase as the reference. The fundamental component of the stator side open-circuit voltage is extracted and its phase is corrected to obtain the corrected phase of the stator side open-circuit voltage. Based on grid voltage-oriented control, a target value for the stator voltage is set, and the target rotor current is calculated by combining the corrected stator-side no-load voltage phase. Construct an inner loop control circuit for rotor current, calculate the feedforward decoupling term through the rotor voltage equation, and perform phase compensation on the control signal; The stator voltage outer loop control circuit is superimposed, and the rotor current is adjusted and corrected through closed-loop regulation to eliminate the deviation in amplitude and phase between the stator side no-load voltage and the grid voltage; When the stator side no-load voltage and the grid voltage meet the preset synchronization conditions, the grid-connected circuit breaker is closed, and the control mode is switched to the grid-connected power generation control mode based on stator voltage orientation.

2. The method for no-load voltage build-up and pre-synchronization grid connection control of an AC excitation system according to claim 1, characterized in that, The specific steps for extracting the fundamental component and correcting the phase of the stator-side no-load voltage to obtain the corrected phase of the stator-side no-load voltage include: Low-pass filtering is applied to the stator-side open-circuit voltage to extract the fundamental component; Compensation for the phase shift introduced by low-pass filtering at the fundamental frequency.

3. The method for no-load voltage build-up and pre-synchronization grid connection control of an AC excitation system according to claim 1, characterized in that, The phase compensation is obtained by calculating the phase response of the filter transfer function in real time.

4. The method for no-load voltage build-up and pre-synchronization grid connection control of an AC excitation system according to claim 1, characterized in that, The inner loop transfer function of the rotor current is: ； in, and These are the parameters for the rotor current PI controller. The transfer function is the delay function caused by sampling and modulation. The self-inductance between the equivalent two-phase windings of the rotor in a synchronously rotating coordinate system. The resistance of the rotor.

5. The method for no-load voltage build-up and pre-synchronization grid connection control of an AC excitation system according to claim 1, characterized in that, The outer loop transfer function of the stator voltage is: ； in, and These are the parameters for the stator voltage PI controller. Let be the inner loop transfer function of the rotor current. The stator synchronous velocity angular frequency, This refers to the mutual inductance between the coaxial equivalent windings of the stator and rotor in a synchronous rotating coordinate system.

6. The method for no-load voltage build-up and pre-synchronization grid connection control of an AC excitation system according to claim 1, characterized in that, The deviation between the stator voltage amplitude and the grid voltage amplitude is less than the preset amplitude threshold; The deviation between the stator voltage phase and the grid voltage phase is less than the preset phase threshold.

7. A control system for no-load voltage build-up and pre-synchronization grid connection of an AC excitation system, characterized in that, include: A phase-locked loop (PLL) module is used to acquire the amplitude and phase of the grid voltage in real time. Encoder module, used to detect the mechanical position of the rotor of a doubly fed induction motor; The coordinate transformation module is used to transform the rotor current from the stationary coordinate system to the synchronous rotating coordinate system; The phase compensation module is used to extract the fundamental component and correct the phase of the stator side open-circuit voltage to obtain the corrected stator side open-circuit voltage phase. The calculation module is used to set the target value of the stator voltage based on grid voltage-oriented control, and calculate the target rotor current by combining the corrected stator-side no-load voltage phase. The rotor current inner loop control module is used to construct the rotor current inner loop control circuit, calculate the feedforward decoupling term through the rotor voltage equation, and perform phase compensation on the control signal. The stator voltage outer loop control module is used to superimpose the stator voltage outer loop control circuit, and correct the rotor current through closed-loop adjustment to eliminate the amplitude and phase deviation between the stator side no-load voltage and the grid voltage. The grid-connected control module is used to close the grid-connected circuit breaker and switch the control mode when the synchronization conditions are met.

8. The AC excitation system no-load voltage build-up and pre-synchronization grid connection control system according to claim 7, characterized in that: The output of the stator voltage outer loop control module serves as the reference value for the rotor current inner loop control module, forming a dual closed-loop control structure.

9. The AC excitation system no-load voltage build-up and pre-synchronization grid connection control system according to claim 7, characterized in that: After closing the grid-connected circuit breaker, the grid-connected control module switches the control mode to a vector control mode based on stator voltage orientation to achieve grid-connected power generation operation.

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