A grid-connected inverter multi-machine parallel and off-grid starting control method

Abstract

The application discloses a kind of network type inverter multi-machine parallel and off-grid starting control method, comprising: starting control ring according to received external Cmd instruction, first inverter carries out the selection of starting mode;First inverter determines starting mode, and according to the control strategy of starting mode operation;The starting control ring of second inverter selects starting mode according to received external Cmd instruction;Second inverter enables PWM module, and the rated voltage established in common connection point is tracked first inverter to realize synchronization;Repeat the first two steps until the rated voltage of all inverters realizes synchronization, executes the steps such as closing operation.The application can realize that inverter cluster is switched between multiple stages by flag bit, the control strategy in different states is independent of each other, realizes the modularization decoupling of starting process, eliminates the response lag problem of traditional linear control logic.

Description

Technical Field

[0001] This invention belongs to the field of power electronic control, specifically relating to a method for controlling the parallel and off-grid start-up of multiple grid-connected inverters. Background Technology

[0002] With the increasing penetration of renewable energy, the role of multiple inverters operating in parallel in microgrid support and ancillary services is becoming increasingly critical. However, they face severe challenges during initial startup, especially in grid-connected / off-grid switching and off-grid cold start scenarios: During the off-grid parallel synchronization phase, the lack of a voltage reference and insufficient pre-synchronization during multi-unit parallel operation (synchronization before paralleling, i.e., the voltage amplitude and phase between the inverter to be connected in parallel and the inverter being paralleled are not completely synchronized) can cause phase / amplitude / frequency deviations, threatening IGBT safety and system stability; During the grid-connected / off-grid mode switching process, insufficient dynamic coordination can easily lead to voltage and frequency fluctuations, power oscillations, or short-term power outages, resulting in decreased power quality and protection malfunctions. Therefore, there is an urgent need for an efficient, robust, adaptable, and communication-dependent multi-unit coordinated grid-connected / off-grid startup method. Summary of the Invention

[0003] This invention is proposed to overcome the shortcomings of the existing technology, and its purpose is to provide a method for controlling the parallel and off-grid start-up of multiple grid-connected inverters.

[0004] This invention is achieved through the following technical solution:

[0005] A method for controlling the parallel and off-grid start-up of multiple grid-connected inverters includes the following steps:

[0006] S1. The start-up control loop of the first inverter selects the start-up mode according to the received external Cmd (Command Prompt) command;

[0007] S2. After the first inverter determines the start-up mode, it operates according to the control strategy of the start-up mode. The first inverter raises the output voltage to the rated voltage according to the set voltage rise rate to establish the rated voltage of the common connection point.

[0008] S3. The start-up control loop of the second inverter selects the start-up mode according to the received external Cmd command;

[0009] S4. The second inverter enables its PWM module to track the rated voltage established by the first inverter at the point of common coupling, thereby achieving synchronization.

[0010] S5. Repeat steps S3 and S4 until the rated voltage of all inverters is synchronized. Perform the closing operation. After closing, the parallel start is completed. After the parallel start is confirmed to be successful (parallel connection is blocked), the status flag (flag V) will be switched from the start-incomplete state (operatingStatusFlag=0) to the start-complete state (operatingStatusFlag=1).

[0011] In the above technical solution, the start-up control loop is placed between the power outer loop control and the voltage and current inner loop control, and achieves seamless coordination of parallel start-up and active voltage support for off-grid units through a hierarchical finite state machine.

[0012] In the above technical solution, the selection of the startup mode is based on the inverter operating state transition model of the hierarchical finite state machine. The inverter operating state transition model includes a startup state layer, a normal state layer and a fault state layer. The current state of the inverter operating state transition model is indicated by the state flag operatingStatusFlag.

[0013] The startup status layer is startupStatus, which serves as the startup logic execution layer of the inverter. The startup status layer includes gridMode and offGridMode modes, and the mode of the startup status layer can be selected through the external Cmd command.

[0014] The normal state layer is normalStatus, which is used for the continuous running state of the system after the startup state is completed. When the system detects that the startup state is completed, the system will enter the continuous running state in the normal state layer.

[0015] The fault status layer is faultStatus, which is the processing layer for fault events that occur during the inverter startup process. When an error occurs during the startup command issuance or an anomaly occurs during the startup process, the process jumps to the fault status layer for processing. The startup command is Power_Status_Cmd≠1∩2.

[0016] In the above technical solution, the specific method for selecting the startup mode is as follows:

[0017] S11. After receiving the external Cmd command, the inverter currently waiting to be started enters the start mode selection. The fault state switching flag of the other inverters that have not been started is initialized, and the inverters that have been started maintain their original state.

[0018] S12. After the inverter fault state switching flag is initialized, the inverter to be started first selects the start mode through flag IV and runs in gridMode or offGridMode of the start state layer. Then, the current running status output by flag V is used to determine whether the start process is complete. If the start is not complete, the system controls flag I to re-enter the start state layer and re-select gridMode or offGridMode of the start state until the start is complete.

[0019] S13. If the inverter has finished starting, after the startupStatus state layer ends, a startup status completion flag will be output. The status judgment will be entered through flag V to determine the operating status of the next step. After the startup status is determined, the system control will jump to the normal status layer normalStatus through flag II.

[0020] S14. If a fault occurs during inverter startup, the current fault status is output through different fault flags in flag V under startupStatus. After status judgment, the system control jumps to faultStatus under fault status through flag III.

[0021] In the above technical solution, the inverter fault state switching flags include flag I, flag II, flag III, flag IV, and flag V;

[0022] The flag I is the start-up running status indicator flag finishFlag=0;

[0023] The flag II is a normal operation status indicator flag finishFlag=1;

[0024] The flag III is a fault operation status indication flag finishFlag=2;

[0025] The flag IV is the startup mode command flag Power_Status_Cmd. When Power_Status_Cmd=2, it runs in gridMode mode; when Power_Status_Cmd=1, it runs in offGridMode mode; when Power_Status_Cmd≠1∩2, the command was issued incorrectly.

[0026] Flag V is the status flag for whether the startup is complete. OperatingStatusFlag=0 indicates that the startup is not complete; operatingStatusFlag=1 indicates that the startup is complete; and operatingStatusFlag=2 indicates an abnormal status.

[0027] In the above technical solution, the control strategy under the offGridMode mode of the startup state layer is as follows:

[0028] (1)

[0029] (2)

[0030] In formula (1): U droopRef The reference value for the reactive power loop voltage is given, in V; U Ramp This represents an intermediate variable value for voltage rise, in V; K The voltage rise slope is expressed in V / s. T s The system control cycle is expressed in seconds (s). U N Rated voltage, unit: V;

[0031] In formula (2): U deltaPre This is the pre-synchronization voltage compensation value, in V; ω deltaPre This is the pre-synchronization angular frequency compensation value, in rad / s; P transmit_Cmd For transmitting control commands; P switch_Cmd This is a closing control command;

[0032] In offGridMode mode, the circuit breaker is closed, the inverter wave generation is disabled, the voltage establishment state enables controllable ramp control, the point of common coupling voltage is constructed, the output value of the grid-connected pre-synchronization compensation value is set to zero, and the synchronization function is disabled.

[0033] In the above technical solution, the control strategy in the gridMode mode of the startup state layer is as follows:

[0034] (3)

[0035] In formula (3): U droopRef The reference value for the reactive power loop voltage is given, in V; U deltaPre This is the pre-synchronization voltage compensation value, in V; ω deltaPre This is the pre-synchronization angular frequency compensation value, in rad / s; U N Rated voltage, unit: V; U delta These are the calculated values ​​for pre-synchronization voltage compensation. ωdelta This is the calculated value for pre-synchronization angular frequency compensation; P transmit_Cmd For transmitting control commands; P switch_Cmd This is a closing control command;

[0036] In gridMode mode, the rated voltage setpoint control is enabled in voltage setup mode. The output value of the pre-synchronization stage is assigned to the synchronization compensation value as the control input of the next stage, and the synchronization function is enabled.

[0037] In the above technical solution, the control strategy under the normal status layer (normalStatus) is as follows:

[0038] (4)

[0039] In equation (4): U droopRef The reference value for the reactive power loop voltage is given, in V; U deltaPre This is the pre-synchronization voltage compensation value, in V; ω deltaPre This is the pre-synchronization angular frequency compensation value, in rad / s; P transmit_Cmd For transmitting control commands; P switch_Cmd This is a closing control command; U N Rated voltage, unit: V;

[0040] In the normalStatus state, the reactive voltage loop voltage is controlled according to the rated setpoint, the synchronization loop is disabled, the inverter startup process ends, the waveform control is enabled, the circuit breaker is closed, the startup loop is disconnected, and the synchronization loop is disconnected.

[0041] In the above technical solution, the control strategy under the fault status layer (faultStatus) is as follows:

[0042] (5)

[0043] In formula (5): U droopRef The reference value for the reactive power loop voltage is given, in V; U deltaPre This is the pre-synchronization voltage compensation value, in V; ω deltaPre This is the pre-synchronization angular frequency compensation value, in rad / s; P transmit_Cmd For transmitting control commands; P switch_Cmd This is a closing control command;

[0044] Under faultStatus, the reactive voltage loop voltage control output is zero, the synchronization loop is disabled, the waveform control is disabled, and the circuit breaker is tripped.

[0045] In the above technical solution, the condition for entering the offGridMode state is as follows: the system receives the start mode instruction Power_Status_Cmd=1 to select the control strategy of offGridMode mode (off-grid start logic), first sends the closing instruction through the closing flag Pre_Switch_Cmd=1, then judges whether the closing feedback flag Pre_Switch_Feedback satisfies equation (6), if it satisfies the off-grid closing timeout offGridSwitchTime timer is cleared, then the wave transmission control flag Pwm_Transmit_Cmd is controlled to enable wave transmission through equation (7), and controls according to equation (1). U droopRef If the voltage rises and equation (6) is not satisfied, the closing timeout offGridSwitchTime timer will start counting.

[0046] (6);

[0047] (7);

[0048] The conditions for entering gridMode state are: the system receives the startup mode command Power_Status_Cmd=2 and selects the gridMode mode control strategy (grid-connected startup logic). First, the transmission control flag Pwm_Transmit_Cmd is used to control the transmission of waves through formula (7), and then the grid-connected operation begins. U droopRef = U N Then, determine whether the synchronous closing flag Pre_Switch_Flag satisfies equation (8). If it does not satisfy equation (8) and the synchronization threshold of equation (9) is also not satisfied, the synchronization timeout preSwitchTime timer starts counting and the presynchronization delay preSyncTime timer is cleared. If equation (8) is not satisfied but equation (9) is satisfied, determine whether the delay timer preSyncTime satisfies equation (10). If it is satisfied, control the closing by using the closing flag Pre_Switch_Cmd=1 and clear the preSyncTime timer. If it is not satisfied, the delay timer preSyncTime starts counting. When equation (8) is satisfied, determine whether equation (6) is satisfied. If it is satisfied, clear the grid-connected closing timeout gridSwitchTime timer. Otherwise, the gridSwitchTime timer starts counting.

[0049] (8);

[0050] (9);

[0051] (10);

[0052] In the above formula: U TH This is the voltage synchronization threshold, in V. ω TH This is the angular frequency synchronization threshold, in rad / s;

[0053] The conditions for entering the faultStatus condition are as follows: When the system receives the start mode command Power_Status_Cmd≠1∩2, the command is issued incorrectly, and the current operating cycle directly jumps to the faultStatus condition; when the offGridSwitchTime timer timeout of the offGridSwitchTime timeout satisfies equation (11), the preSwitchTime timer timeout of the synchronization timeout satisfies equation (12), and the gridSwitchTime timeout of the gridSwitchTime timeout satisfies equation (13), the operating status flag operatingStatusFlag is output in the current step of the offGridMode mode after the offGridSwitchTime timer satisfies the condition; when the gridSwitchTime timer and the preSwitchTime timer satisfy the condition, the operating status flag operatingStatusFlag is output in the current step of the gridMode mode; when the abnormal flag satisfies equation (14), the system controls the next step of the system to jump to the faultStatus condition after status judgment.

[0054] (11);

[0055] (12);

[0056] (13);

[0057] (14);

[0058] The conditions for entering the normalStatus state include the offGridMode mode condition and the gridMode mode condition. The offGridMode mode condition is: In offGridMode mode, when the closing feedback flag Pre_Switch_Feedback satisfies equation (6), the control voltage... U droopRef Satisfying equation (1) U droopRef ≥ UN When the offGridSwitchTime timer for off-grid closing does not satisfy equation (11); the gridMode mode conditions are: in gridMode mode, the synchronization timeout preSwitchTime timer does not satisfy equation (12), the synchronization closing flag Pre_Switch_Flag satisfies equation (8), and the closing feedback flag Pre_Switch_Feedback satisfies equation (6); when the offGridMode mode conditions or gridMode mode conditions are satisfied, the offGridMode mode or gridMode mode outputs the operation completion flag operatingStatusFlag in the current step. When the operation completion flag satisfies equation (15), the system controls the next step to jump to the normal status layer normalStatus after status judgment.

[0059] (15).

[0060] The beneficial effects of this invention are:

[0061] This invention provides a multi-unit parallel and off-grid start-up control method for grid-connected inverters. It is a multi-unit parallel collaborative start-up method based on state switching, applicable to multi-unit grid-connected parallel and off-grid parallel start-up. This invention inserts a finite state machine control loop into the existing controller power control loop. By setting system-level state transition conditions, the inverter cluster can switch between multiple stages through flag bits, such as normal operation with load access, grid-connected and off-grid start-up, system abnormality or operation abnormality fault state. At the same time, the control strategies in different states are independent of each other.

[0062] This invention employs a layered design for different operating states, achieving modular decoupling of the startup process (system layer → mode layer → execution layer), thus eliminating the response lag problem of traditional linear control logic. Attached Figure Description

[0063] Figure 1 This is a schematic diagram of the inverter's operating state switching principle in this invention;

[0064] Figure 2 This is a schematic diagram of the inverter parallel start-up control system of Embodiment 1 of the present invention;

[0065] Figure 3 It is the parallel start-up line voltage of inverters PCS1# and PCS2# in Embodiment 1 of the present invention. U AB , U BC Waveform diagram;

[0066] Figure 4This is a waveform diagram of the closing command, waveform sending command, voltage setting and operating status flag during the parallel startup of inverters PCS1# and PCS2# in Embodiment 1 of the present invention.

[0067] For those skilled in the art, other related figures can be obtained from the above figures without any creative effort. Detailed Implementation

[0068] To enable those skilled in the art to better understand the technical solution of the present invention, the technical solution of the present invention will be further described below with reference to the accompanying drawings and specific embodiments.

[0069] Example 1

[0070] A method for controlling the parallel and off-grid start-up of multiple grid-connected inverters, characterized by the following steps:

[0071] S1. The start-up control loop selects the start-up mode (off-grid start-up mode) for the first inverter based on the received external Cmd command.

[0072] S2. After the first inverter determines the start-up mode, it operates according to the control strategy of the start-up mode. The first inverter raises the output voltage to the rated voltage according to the set voltage rise rate to establish the rated voltage of the common connection point.

[0073] S3. The start-up control loop of the second inverter selects the start-up mode (off-grid parallel start-up mode) based on the received external Cmd command.

[0074] S4. The second inverter enables the PWM module to track the rated voltage established by the first inverter at the point of common coupling to achieve synchronization.

[0075] S5. Repeat steps S3 and S4 until the rated voltage of all inverters is synchronized. Perform the closing operation and lock out the parallel connection. The status flag is switched from 0 to 1, that is, "finishFlag=0" is switched to "finishFlag=1".

[0076] The startup control loop is located between the power outer loop and the voltage and current inner loop, and achieves seamless coordination of parallel startup and active voltage support for off-grid units through a state transition mechanism; the power outer loop is a VSG control loop.

[0077] The selection of the off-grid start-up mode is based on the inverter operating state transition model of the hierarchical finite state machine. The inverter operating state transition model includes a start-up state layer, a normal state layer, and a fault state layer, and the current state is indicated by operatingStatusFlag.

[0078] The startup status layer is startupStatus, which serves as the main execution layer for the inverter's startup logic. It includes gridMode and offGridMode modes, and the two modes are selected by external instructions.

[0079] The normal state layer is normalStatus, which is used for the continuous running state of the system after the startup process is completed. When the system detects that the startup process is completed, the system will enter the continuous state in the normal state layer.

[0080] The fault status layer is called faultStatus, which is the processing layer for fault events that occur during the inverter startup process. When an error occurs during the startup command issuance or an anomaly occurs during the startup process, the process jumps to the fault status layer for processing.

[0081] The operational logic block diagram of the inverter when selecting the off-grid start-up mode is as follows: Figure 1 As shown, the specific steps include:

[0082] S11. After receiving the external Cmd command, the inverter enters the start mode selection, and the fault state switching flag of the inverter that has not started is initialized.

[0083] S12. After the inverter fault state switching flag is initialized, the starting mode is first selected by flag IV and the operation is in gridMode or offGridMode mode of the startup state layer. Then, the current running status is output by flag V to judge the status and determine whether the startup process is completed. If the startup is not completed, the system control will continue to enter the startup state layer startupStatus through flag I to continue running until the startup is completed.

[0084] S13. If the inverter has finished starting up, the startup status layer startupStatus will output a startup status completion flag after the startup status layer ends. The system will then enter the status judgment through flag V to determine the operating status for the next step. After the startup status is determined to be complete, the system will control the jump to the normal status layer normalStatus through flag II.

[0085] S14. If a fault occurs during inverter startup, the current fault status is output through different fault flags in flag V under startupStatus. After status judgment, the system control jumps to faultStatus under fault status through flag III.

[0086] The inverter fault state switching flags include flag I, flag II, flag III, flag IV, and flag V;

[0087] Flag I is the start-up running status indicator flag (finishFlag=0);

[0088] Flag II is a normal operation status indicator (finishFlag=1);

[0089] The flag III is a fault operation status indication flag (finishFlag=2);

[0090] The flag IV is the on-grid / off-grid start-up mode command (Power_Status_Cmd). When Power_Status_Cmd=2, it runs in gridMode; when Power_Status_Cmd=1, it runs in offGridMode; when Power_Status_Cmd≠1∩2, the command was issued incorrectly.

[0091] Flag V is the start-up incomplete status flag (operatingStatusFlag=0); operatingStatusFlag=1 is the grid-connected or off-grid start-up complete status flag; operatingStatusFlag=2 includes abnormal states such as closing timeout and synchronization timeout that occurred during the start-up.

[0092] The control strategy under the offGridMode mode of the startup state layer is as follows:

[0093] (1);

[0094] (2)

[0095] In formula (1): U droopRef The reference value for the reactive power loop voltage is given, in V; U Ramp This represents an intermediate variable value for voltage rise, in V; K The voltage rise slope is expressed in V / s. T s The system control cycle is expressed in seconds (s). U N Rated voltage, unit: V;

[0096] In formula (2): U deltaPre This is the pre-synchronization voltage compensation value, in V; ω deltaPre This is the pre-synchronization angular frequency compensation value, in rad / s; P transmit_Cmd For transmitting control commands; P switch_Cmd This is a closing control command;

[0097] The actual operation process is as follows: In offGridMode, the circuit breaker is closed, the inverter's waveform generation is disabled, the voltage establishment state is enabled with controllable voltage ramp control to build the point of common voltage, the output value of the grid-connected pre-synchronization compensation value is set to zero, and the synchronization function is disabled; through control... K The value controls the rate of rise of the starting voltage, effectively reducing the oscillation problem that may be caused by the inverter starting voltage rising too quickly.

[0098] The specific working process when running in the offGridMode state is as follows: Figure 2 (Control logic in the control block diagram): The control loop connects the pre-synchronization loop and the VSG control loop through the start-up control loop; the start-up control loop selects the off-grid start-up mode according to the received external Cmd command, and controls the reactive voltage setpoint of the VSG control loop in the start-up state. U droopRef According to the slope K The slope is raised from zero voltage to the rated value and then maintained at the rated value; the pre-synchronization voltage compensation value in off-grid condition. U deltaPre Pre-synchronization angular frequency compensation value ω deltaPre Set the value to zero and input the above input quantity into the VSG control loop. ω It outputs the angular velocity of the virtual synchronous machine rotor. U It is the voltage amplitude of the reactive power control output of the virtual synchronous machine. J and D p These are the rotor inertia coefficient and damping coefficient of the virtual synchronous machine; the active frequency loop angular frequency setpoint ω. ref and output virtual synchronous machine rotor angular velocity ω After comparison, by frequency modulation coefficient K p ( Figure 2 In the control block diagram K p ) operation, superimposing active power setpoint P ref and active feedback value P g After passing through the virtual inertia and damping stages, the virtual synchronous machine rotor angular velocity is output. ω Reactive voltage loop voltage setpoint U 0 and the voltage amplitude of the reactive power control output of the virtual synchronous machine U Comparison, through voltage regulation coefficient K q Calculation, superimposing reactive power setpoint Q ref and reactive power feedback value Q g The PI controller generates the virtual synchronous machine reactive power control output voltage amplitude.U . Figure 2 In the figure, “1 / s” represents the integral element of the transfer function.

[0099] The control strategy under the gridMode mode of the startup state layer is as follows:

[0100] (3)

[0101] In formula (3): U droopRef The reference value for the reactive power loop voltage is given, in V; U deltaPre This is the pre-synchronization voltage compensation value, in V; ω deltaPre This is the pre-synchronization angular frequency compensation value, in rad / s; U N Rated voltage, unit: V; U delta These are the calculated values ​​for pre-synchronization voltage compensation. ω delta This is the calculated value for pre-synchronization angular frequency compensation; P transmit_Cmd For transmitting control commands; P switch_Cmd This is a closing control command;

[0102] In gridMode, the rated voltage setpoint control is enabled in the voltage setup state. The output value of the pre-synchronization link is assigned to the synchronization compensation value as the control input of the next link. The synchronization function is enabled to ensure that the voltage phase angle can track the grid-side voltage amplitude and phase angle or the voltage amplitude and phase angle constructed by the parallel inverter when the inverter starts up.

[0103] The specific working process under the gridMode state is as follows: a unified control architecture is implemented for both off-grid parallel startup and grid-connected startup processes, and the two modes adopt equivalent design in the core control links; the startup process is executed in stages; the first stage realizes the steady-state operation of the inverter and builds the common point voltage, and the second stage completes the inverter's tracking control of the common point voltage phase;

[0104] Specifically, the process involves: first, collecting the common point / grid-side voltage, and then generating a pre-synchronization voltage compensation value via a pre-synchronization loop. U deltaPre Pre-synchronization angular frequency compensation value ω deltaPre The input is sent to the startup control loop; the startup control loop selects the startup mode based on the received external Cmd command, and provides the reactive voltage setpoint to the VSG loop according to the operating status. U droopRef Set to start at rated voltage, pre-synchronization voltage compensation value U deltaPrePre-synchronization angular frequency compensation value ω deltaPre Maintain synchronization enable until a closing feedback signal is received; the above three input quantities are input to the VSG loop. ω It is the rotor angular velocity output by the virtual synchronous machine; the active frequency loop angular frequency setpoint. ω ref and angular frequency feedback value ω After comparison, by frequency modulation coefficient K p Calculation, superposition of active power setpoint P ref and active feedback value P g After passing through the virtual inertia and damping stages, the virtual synchronous machine rotor angular velocity is output. ω Reactive voltage loop voltage setpoint U droopRef and voltage feedback value U Comparison, through voltage regulation coefficient K q Calculation, superimposing reactive power setpoint Q ref and reactive power feedback value Q g The PI controller generates the virtual synchronous machine reactive power control output voltage amplitude. U .

[0105] After startupStatus completes, the reactive power control output voltage amplitude and active power control output rotor angular velocity of the virtual synchronous machine in the power loop are used as reference values ​​for the voltage loop input. Then, after passing through the voltage-current loop PI controller and undergoing dq / αβ coordinate transformation, the control voltage in the αβ coordinate system is generated. U αβ After being modulated by SVPWM, a modulation signal is generated to drive the IGBT to operate.

[0106] The control strategy under the normal status layer (normalStatus) is as follows:

[0107] (4)

[0108] In equation (4): U droopRef The reference value for the reactive power loop voltage is given, in V; U deltaPre This is the pre-synchronization voltage compensation value, in V; ω deltaPre This is the pre-synchronization angular frequency compensation value, in rad / s; P transmit_Cmd For transmitting control commands; P switch_Cmd This is a closing control command;U N Rated voltage, unit: V;

[0109] In the normalStatus state, the reactive voltage loop voltage is controlled according to the rated setpoint, the synchronization loop is disabled, the inverter startup process ends, the waveform control is enabled, the circuit breaker is closed, the startup loop is disconnected, and the synchronization loop is disconnected, ensuring normal waveform control is performed after the inverter startup is completed.

[0110] The control strategy under the fault status layer (faultStatus) is as follows:

[0111] (5)

[0112] In formula (5): U droopRef The reference value for the reactive power loop voltage is given, in V; U deltaPre This is the pre-synchronization voltage compensation value, in V; ω deltaPre This is the pre-synchronization angular frequency compensation value, in rad / s; P transmit_Cmd For transmitting control commands; P switch_Cmd This is a closing control command;

[0113] Under faultStatus, the reactive voltage loop voltage control output is zero, the synchronization loop is disabled, the waveform control is disabled, and the circuit breaker is tripped to ensure normal shutdown during inverter faults.

[0114] The conditions for entering the offGridMode state are as follows: The system receives the start command Power_Status_Cmd=1 and selects the control strategy of offGridMode mode. First, it sends a closing command through the closing flag Pre_Switch_Cmd=1. Then, it judges whether the closing feedback flag Pre_Switch_Feedback satisfies equation (6). If it satisfies the offgrid closing timeout, the offGridSwitchTime timer is cleared. Then, it controls the wave transmission control flag Pwm_Transmit_Cmd to enable wave transmission through equation (7), and controls the wave transmission according to equation (1). U droopRef If the voltage rises and equation (6) is not satisfied, the closing timeout offGridSwitchTime timer will start counting.

[0115] (6);

[0116] (7);

[0117] The conditions for entering gridMode state are: the system receives the startup command Power_Status_Cmd=2 and selects the gridMode control strategy. First, the transmission control flag Pwm_Transmit_Cmd is set to control the transmission through formula (7), and then the transmission is carried out under grid-connected operation. U droopRef = U N Then, determine whether the synchronous closing flag Pre_Switch_Flag satisfies equation (8). If it does not satisfy equation (8) and the synchronization threshold of equation (9) is also not satisfied, the synchronization timeout preSwitchTime timer starts counting and the presynchronization delay preSyncTime timer is cleared. If equation (8) is not satisfied but equation (9) is satisfied, determine whether the delay timer preSyncTime satisfies equation (10). If it is satisfied, control the closing by using the closing flag Pre_Switch_Cmd=1 and clear the preSyncTime timer. If it is not satisfied, the delay timer preSyncTime starts counting. When equation (8) is satisfied, determine whether equation (6) is satisfied. If it is satisfied, clear the grid-connected closing timeout gridSwitchTime timer. Otherwise, the gridSwitchTime timer starts counting.

[0118] (8)

[0119] (9);

[0120] (10);

[0121] In the above formula: U TH This is the voltage synchronization threshold, in V. ω TH This is the angular frequency synchronization threshold, in rad / s;

[0122] The conditions for entering the faultStatus state are as follows: when the system receives the start mode command Power_Status_Cmd≠1∩2, the command is issued incorrectly, and the current running cycle directly jumps to the faultStatus state; when the offGridSwitchTime timeout of the offGridSwitchTime timer satisfies equation (11), the synchronization timeout of the preSwitchTime timer satisfies equation (12), and the gridSwitchTime timeout of the gridSwitchTime timer satisfies equation (13), the current cycle outputs the operatingStatusFlag in offGridMode mode after the offGridSwitchTime timer satisfies the conditions; and the current cycle outputs the operatingStatusFlag in gridMode mode after the gridSwitchTime timer and the preSwitchTime timer satisfy the conditions.

[0123] When the abnormal flag satisfies equation (14), the system will jump to faultStatus in the next step after the status is judged.

[0124] (11);

[0125] (12);

[0126] (13);

[0127] (14);

[0128] The condition for entering the normalStatus condition is: in the offGridMode state, when the closing feedback flag Pre_Switch_Feedback satisfies equation (6), the control voltage... U droopRef Satisfying equation (1) U droopRef ≥ U NWhen the offGridSwitchTime timer does not satisfy equation (11) in gridMode state, the synchronization timeout preSwitchTime timer does not satisfy equation (12), the synchronization closing flag Pre_Switch_Flag satisfies equation (8), and the closing feedback flag Pre_Switch_Feedback satisfies equation (6); when the above triggering conditions are met, the current step of the offGridMode and gridMode states will output the operation completion flag operatingStatusFlag. When the flag satisfies equation (15), the system will jump to normalStatus in the next step after status judgment.

[0129] (15);

[0130] Using a 1250kW inverter as an example, this explanation focuses on a dual-unit parallel off-grid start-up scenario, with the system's AC line voltage set to 690V. Figure 3 The figure shows the grid-side line voltage during startup. U AB , U BC The waveform shows that PCS1# starts first at 0.2s, and raises the output voltage to the rated value in 2s according to the set voltage rise rate. After establishing the common point voltage, it runs stably for a period of time. At 3s, PCS2# starts, synchronously tracking the voltage amplitude and phase established by the common point of connection PCS1# and closing synchronously at the rated voltage. After closing, the two units achieve stable parallel operation.

[0131] Figure 4 Showing with Figure 3 The corresponding timing waveforms for each state of the two inverter systems during startup include the closing command, PWM enable signal, voltage setpoint, and operating status flags. For example... Figure 4 As can be seen in (a), during the startup process, PCS1#, as the off-grid startup pre-control unit, first outputs the closing command and activates the PWM drive, and then executes the voltage ramp-up program to build the bus voltage. When the voltage reaches the rated value... U N When this happens, its running status flag changes from 0 to 1; for example... Figure 4 As can be seen in (b), PCS2#, as the control unit after parallel startup, first enables the PWM module to track the rated voltage established by PCS1# at the point of common coupling to achieve synchronization. After completing voltage synchronization, it performs a closing operation, and the status flag after parallel connection is locked switches from 0 to 1 synchronously. Through the analysis of... Figure 3 Voltage waveform characteristics and Figure 4 The collaborative analysis of the state flag transition process shows that the execution logic and results are consistent, which confirms the effectiveness and technical feasibility of this technical solution at the implementation level.

[0132] In this embodiment, a hierarchical start-up control module is installed between the power outer loop control and the voltage and current inner loop control of a traditional grid-connected inverter. Through a state transition mechanism, seamless coordination of parallel start-up and active voltage support for off-grid units are achieved. While retaining the traditional dual-loop control structure, the embedding of the hierarchical start-up control module ensures the continuity of the grid-connected and off-grid switching process and realizes the precise support function of off-grid units as synchronous voltage sources.

[0133] It should be noted that, unless otherwise specified, the embodiments and features described in the present invention can be combined with each other.

[0134] The applicant declares that the above description is only a specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Those skilled in the art should understand that any changes or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention fall within the protection and disclosure scope of the present invention.

Claims

1. A method for controlling the parallel and off-grid start-up of multiple grid-connected inverters, characterized in that: Includes the following steps: S1. The start-up control loop of the first inverter selects the start-up mode according to the received external Cmd command; S2. After the first inverter determines the start-up mode, it operates according to the control strategy of the start-up mode. The first inverter raises the output voltage to the rated voltage according to the set voltage rise rate to build the rated voltage of the common connection point. S3. The start-up control loop of the second inverter selects the start-up mode according to the received external Cmd command; S4. The second inverter enables its PWM module to track the rated voltage established by the first inverter at the point of common coupling, thereby achieving synchronization. S5. Repeat steps S3 and S4 until the rated voltage of all inverters is synchronized. Perform the closing operation. After closing, the parallel start is completed. After the parallel start is confirmed to be successful, the status flag will be switched from the start-incomplete state to the start-complete state. The startup control loop is positioned between the power outer loop control and the voltage and current inner loop control, and achieves seamless coordination of parallel startup and active voltage support for off-grid units through a hierarchical finite state machine. The selection of the startup mode is based on the inverter operating state transition model of the hierarchical finite state machine. The inverter operating state transition model includes a startup state layer, a normal state layer, and a fault state layer. The current state of the inverter operating state transition model is indicated by the state flag operatingStatusFlag.

2. The multi-unit parallel and off-grid start-up control method for grid-connected inverters according to claim 1, characterized in that: The startup status layer is startupStatus, which serves as the startup logic execution layer of the inverter. The startup status layer includes gridMode and offGridMode modes, and the mode of the startup status layer is selected by receiving external Cmd commands. The normal state layer is normalStatus, which is used for the continuous running state of the system after the startup state is completed. When the system detects that the startup state is completed, the system will enter the continuous running state in the normal state layer. The fault status layer is called faultStatus, which is the processing layer for fault events that occur during the inverter startup process. When an error occurs during the startup command issuance or an anomaly occurs during the startup process, the process jumps to the fault status layer for processing.

3. The multi-unit parallel and off-grid start-up control method for grid-connected inverters according to claim 1, characterized in that: The specific method for selecting the startup mode is as follows: S11. After receiving the external Cmd command, the inverter currently waiting to be started enters the start mode selection, and the fault status switching flag of the other inverters that have not been started is initialized. S12. After the inverter fault state switching flag is initialized, the inverter to be started first selects the start mode through flag IV and runs in gridMode or offGridMode of the start state layer. Then, the current running status output by flag V is used to determine whether the start process is complete. If the start is not complete, the system controls flag I to re-enter the start state layer and re-select gridMode or offGridMode of the start state until the start is complete. S13. Inverter startup complete. After startupStatus ends, output startup status completion flag. Use flag V to determine the status and the operating status for the next step. After the startup status is determined, the system controls the jump to normal status through flag II. S14. If a fault occurs during inverter startup, the current fault status is output through different fault flags in flag V under startupStatus. After status judgment, the system control jumps to faultStatus under fault status through flag III.

4. The multi-unit parallel and off-grid start-up control method for grid-connected inverters according to claim 3, characterized in that: The inverter fault state switching flags include flag I, flag II, flag III, flag IV, and flag V; The flag I is the start-up running status indicator flag finishFlag=0; The flag II is a normal operation status indicator flag finishFlag=1; The flag III is a fault operation status indication flag finishFlag=2; The flag IV is the startup mode command flag Power_Status_Cmd. When Power_Status_Cmd=2, it runs in gridMode mode; when Power_Status_Cmd=1, it runs in offGridMode mode; when Power_Status_Cmd≠1∩2, the command was issued incorrectly. The flag V is the status flag indicating whether the startup is complete; operatingStatusFlag=0 indicates that the startup is not complete. operatingStatusFlag=1 indicates a startup completion status; operatingStatusFlag=2 indicates an abnormal status.

5. The multi-unit parallel and off-grid start-up control method for grid-connected inverters according to claim 2, characterized in that: The control strategy for the offGridMode mode of the startup state layer is as follows: （1） （2） In formula (1): U droopRef The reference value for the reactive power loop voltage is given, in V; U Ramp This represents an intermediate variable value for voltage rise, in V; K The voltage rise slope is expressed in V / s. T s The system control cycle is expressed in seconds (s). U N Rated voltage, unit: V; In formula (2): U deltaPre This is the pre-synchronization voltage compensation value, in V; ω deltaPre This is the pre-synchronization angular frequency compensation value, in rad / s; P transmit_Cmd For transmitting control commands; P switch_Cmd This is a closing control command.

6. The multi-unit parallel and off-grid start-up control method for grid-connected inverters according to claim 2, characterized in that: The control strategy for the gridMode mode of the startup state layer is as follows: （3） In formula (3): U droopRef The reference value for the reactive power loop voltage is given, in V; U deltaPre This is the pre-synchronization voltage compensation value, in V; ω deltaPre This is the pre-synchronization angular frequency compensation value, in rad / s; U N Rated voltage, unit: V; U delta These are the calculated values ​​for pre-synchronization voltage compensation. ω delta This is the calculated value for pre-synchronization angular frequency compensation; P transmit_Cmd For transmitting control commands; P switch_Cmd This is a closing control command.

7. The multi-unit parallel and off-grid start-up control method for grid-connected inverters according to claim 2, characterized in that: The control strategy for the normal status layer (normalStatus) is as follows: （4） In equation (4): U droopRef The reference value for the reactive power loop voltage is given, in V; U deltaPre This is the pre-synchronization voltage compensation value, in V; ω deltaPre This is the pre-synchronization angular frequency compensation value, in rad / s; P transmit_Cmd For transmitting control commands; P switch_Cmd This is a closing control command; U N This is the rated voltage, measured in volts (V).

8. The multi-unit parallel and off-grid start-up control method for grid-connected inverters according to claim 2, characterized in that: The control strategy for the fault status layer (faultStatus) is as follows: （5） In equation (5): U droopRef The reference value for the reactive power loop voltage is given, in V; U deltaPre This is the pre-synchronization voltage compensation value, in V; ω deltaPre This is the pre-synchronization angular frequency compensation value, in rad / s; P transmit_Cmd For transmitting control commands; P switch_Cmd This is a closing control command.

9. The multi-unit parallel and off-grid start-up control method for grid-connected inverters according to claim 4, characterized in that: The conditions for entering the offGridMode state are as follows: The system receives the start mode command Power_Status_Cmd=1 and selects the control strategy of offGridMode mode. First, it sends a closing command through the closing flag Pre_Switch_Cmd=1. Then, it judges whether the closing feedback flag Pre_Switch_Feedback satisfies equation (6). If it satisfies the offgrid closing timeout, the offGridSwitchTime timer is cleared. Then, the wave transmission control flag Pwm_Transmit_Cmd is enabled to control wave transmission through equation (7), and the control is performed according to equation (1). U droopRef If the voltage rises and equation (6) is not satisfied, the closing timeout offGridSwitchTime timer will start counting. （6）； （7）； The conditions for entering gridMode state are: the system receives the startup mode command Power_Status_Cmd=2 to select gridMode mode control strategy, firstly using equation (7) to control the transmission control flag Pwm_Transmit_Cmd to control the transmission, and then operating in grid-connected mode. U droopRef = U N Then, determine whether the synchronous closing flag Pre_Switch_Flag satisfies equation (8). If it does not satisfy equation (8) and the synchronization threshold of equation (9) is also not satisfied, the synchronization timeout preSwitchTime timer starts counting and the presynchronization delay preSyncTime timer is cleared. If equation (8) is not satisfied but equation (9) is satisfied, determine whether the delay timer preSyncTime satisfies equation (10). If it is satisfied, control the closing by using the closing flag Pre_Switch_Cmd=1 and clear the preSyncTime timer. If it is not satisfied, the delay timer preSyncTime starts counting. When equation (8) is satisfied, determine whether equation (6) is satisfied. If it is satisfied, clear the grid-connected closing timeout gridSwitchTime timer. Otherwise, the gridSwitchTime timer starts counting. （8）； （9）； （10）； In the above formula: U TH This is the voltage synchronization threshold, in V. ω TH This is the angular frequency synchronization threshold, in rad / s; U deltaPre This is the pre-synchronization voltage compensation value, in V; ω deltaPre This is the pre-synchronization angular frequency compensation value, in rad / s; The conditions for entering the faultStatus state are as follows: when the system receives the start mode command Power_Status_Cmd≠1∩2, the command is issued incorrectly, and the current running cycle directly jumps to the faultStatus state; when the offGridSwitchTime timeout of the offGridSwitchTime timer satisfies equation (11), the synchronization timeout of the preSwitchTime timer satisfies equation (12), and the gridSwitchTime timeout of the gridSwitchTime timer satisfies equation (13), the current cycle outputs the operatingStatusFlag in offGridMode mode after the offGridSwitchTime timer satisfies the conditions; and the current cycle outputs the operatingStatusFlag in gridMode mode after the gridSwitchTime timer and the preSwitchTime timer satisfy the conditions. When the abnormal flag satisfies equation (14), the system will jump to faultStatus in the next step after the status is judged. （11）； （12）； （13）； （14）； The conditions for entering the normalStatus state include the offGridMode mode condition and the gridMode mode condition. The offGridMode mode condition is: In offGridMode mode, when the closing feedback flag Pre_Switch_Feedback satisfies equation (6), the control voltage... U droopRef Satisfying equation (1) U droopRef ≥ U N When the offGridSwitchTime timer for off-grid closing does not satisfy equation (11); the gridMode mode conditions are: in gridMode mode, the synchronization timeout preSwitchTime timer does not satisfy equation (12), the synchronization closing flag Pre_Switch_Flag satisfies equation (8), and the closing feedback flag Pre_Switch_Feedback satisfies equation (6); when the offGridMode mode conditions or gridMode mode conditions are satisfied, the offGridMode mode or gridMode mode outputs the operation completion flag operatingStatusFlag in the current step. When the operation completion flag satisfies equation (15), the system controls the next step to jump to the normal status layer normalStatus after status judgment. （15）。

Images