New energy grid-forming control system and method

Abstract

A new energy grid-forming control system and a method. The system comprises a power instruction calculation module, the power instruction calculation module comprising a grid-following control module. The power command calculation module is used for determining actually measured active power and actually measured reactive power. When the control system switches from grid-following control to grid-forming control, the grid-following control module is used for: on the basis of a difference value between reference active power and the actually measured active power and a difference value between reference reactive power and the actually measured reactive power, regulating a current increment of a current loop for grid-following control until the difference value between the reference active power and the actually measured active power and the difference value between the reference reactive power and the actually measured reactive power both meet a third convergence condition and, after the difference value between the reference active power and the actually measured active power and the difference value between the reference reactive power and the actually measured reactive power both have met the third convergence condition, reducing the current increment to 0 on the basis of a third step length.

Description

FIELD

[0001] The present disclosure relates to the technical field of renewable energy, and in particular to a renewable energy grid-forming control system and method. BACKGROUND

[0002] As renewable energy power generation equipment is integrated into the power grid on a large scale, the proportion of physical rotors inherent to synchronous generators gradually decreases, which externally manifests as a reduction in the system's disturbance rejection capability. The lack of rotors leads to a reduction in inertia within the power system, and the capability to withstand wide-frequency-domain oscillations and absorb harmonics for the power system is also significantly reduced. Furthermore, unlike conventional generators, in which the electromotive force is regulated through an excitation mean and the phase angle is regulated through rotor motion, renewable energy-based power electronic generation and consumption equipment that rely on multiple control loops increase the risks of frequency and voltage fluctuations and flicker. The combination and superimposition of different switching devices further deteriorate the power quality in the system, posing significant crises and challenges for large-scale renewable energy grid integration.

[0003] To mitigate the above-mentioned grid-connection issues of renewable energy power generation equipment, a grid-forming control strategy based on a grid-forming converter may be applied in the power system. Compared with a grid-following control strategy represented by vector control, with the grid-forming control strategy, the stability of the converter is greatly improved. Relying on virtual rotor equations and excitation units, configurable inertia and damping are provided for the power grid, ensuring the grid adaptability of renewable energy grid connection, and making it possible for large-scale, multi-source, heterogeneous renewable energy converters to operate simultaneously in parallel in the power grid.

[0004] However, when switching from grid-following control to grid-forming control, issues such as mutual oscillations between multiple energy storage converters due to frequency differences and the resulting uneven power distribution are prone to occur. SUMMARY

[0005] In view of this, the following technical solutions are disclosed in the present disclosure.

[0006] In a first aspect of the present disclosure, a renewable energy grid-forming control system is provided, which includes a mechanical equation module, an excitation equation module, a power command calculation module, and a grid-following control module.

[0007] The power command calculation module is configured to determine a measured active power and a measured reactive power.

[0008] The excitation equation module is configured to, when the renewable energy grid-forming control system switches from grid-forming control to grid-following control: regulate a voltage increment of the excitation equation module based on a reference voltage and a transformed voltage until a difference between the reference voltage and the transformed voltage satisfies a first convergence condition, and reduce, after the difference between the reference voltage and the transformed voltage satisfies the first convergence condition, the voltage increment to 0 stepwise by a first step size, where the transformed voltage is obtained by performing Park Transformation on a three-phase voltage of a renewable energy station controlled by the renewable energy grid-forming control system.

[0009] The mechanical equation module is configured to, when the renewable energy grid-forming control system switches from the grid-forming control to the grid-following control: regulate an angular velocity increment of the mechanical equation module based on a first phase angle and a second phase angle until a difference between the first phase angle and the second phase angle satisfies a second convergence condition, and reduce, after the difference between the first phase angle and the second phase angle satisfies the second convergence condition, the angular velocity increment stepwise by a second step size to be less than or equal to a second threshold, where the first phase angle is a phase angle of an output terminal of a grid-following control unit, and the second phase angle is a phase angle of an output terminal of a grid-forming control unit.

[0010] The grid-following control module is configured to, when the renewable energy grid-forming control system switches from the grid-following control to the grid-forming control: regulate a current increment of a current loop for the grid-following control based on a difference between a reference active power and the measured active power, and a difference between a reference reactive power and the measured reactive power, until both the difference between the reference active power and the measured active power and the difference between the reference reactive power and the measured reactive power satisfy a third convergence condition, and reduce, after both the difference between the reference active power and the measured active power and the difference between the reference reactive power and the measured reactive power satisfy the third convergence condition, the current increment to 0 stepwise by a third step size.

[0011] In a second aspect of the present disclosure, a renewable energy grid-forming control method is provided, including: when a renewable energy grid-forming control system switches from grid-forming control to grid-following control, regulating a voltage increment of an excitation equation module based on a reference voltage and a transformed voltage until a difference between the reference voltage and the transformed voltage satisfies a first convergence condition, and reducing, after the difference between the reference voltage and the transformed voltage satisfies the first convergence condition, the voltage increment to 0 stepwise by a first step size, where the transformed voltage is obtained by performing Park Transformation on a three-phase voltage of a renewable energy station controlled by the renewable energy grid-forming control system; when the renewable energy grid-forming control system switches from the grid-forming control to the grid-following control, regulating an angular velocity increment of a mechanical equation module based on a first phase angle and a second phase angle until a difference between the first phase angle and the second phase angle satisfies a second convergence condition, and reducing, after the difference between the first phase angle and the second phase angle satisfies the second convergence condition, the angular velocity increment stepwise by a second step size to be less than or equal to a second threshold, where the first phase angle is a phase angle of an output terminal of a grid-following control unit, and the second phase angle is a phase angle of an output terminal of a grid-forming control unit; and when the renewable energy grid-forming control system switches from the grid-following control to the grid-forming control, regulating a current increment of a current loop for the grid-following control based on a difference between a reference active power and a measured active power, and a difference between a reference reactive power and a measured reactive power, until both the difference between the reference active power and the measured active power and the difference between the reference reactive power and the measured reactive power satisfy a third convergence condition, and after both the difference between the reference active power and the measured active power and the difference between the reference reactive power and the measured reactive power satisfy the third convergence condition, reducing the current increment to 0 stepwise by a third step size.

[0012] The beneficial effects of the present solutions are as follows.

[0013] When switching from the grid-following control to the grid-forming control, the current increment of the current loop for the grid-following control is regulated based on the difference between the reference active power and the measured active power and the difference between the reference reactive power and the measured reactive power until both the difference between the reference active power and the measured active power and the difference between the reference reactive power and the measured reactive power satisfy the third convergence condition, and after both the difference between the reference active power and the measured active power and the difference between the reference reactive power and the measured reactive power satisfy the third convergence condition, the current increment is stepwise reduced to 0 by the third step size. Through this switching manner, smooth switching from the grid-following control to the grid-forming control is achieved, meeting the requirements of low impact, no disturbance, and consistent power before and after switching. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] In order to more clearly illustrate the technical solutions in the embodiments of the present disclosure or in the prior art, the drawings required for the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only embodiments of the present disclosure. For those of ordinary skill in the art, other drawings may be obtained based on the provided drawings without creative efforts.

[0015] FIG. 1 is a block diagram showing a control strategy of an existing grid-forming control system;

[0016] FIG. 2 is a schematic structural diagram of a renewable energy grid-forming control system according to an embodiment of the present disclosure;

[0017] FIG. 3 shows schematic structural diagrams of some modules according to an embodiment of the present disclosure;

[0018] FIG. 4 shows flowcharts of switching between grid-following control and grid-forming control according to an embodiment of the present disclosure;

[0019] FIG. 5 is a schematic structural diagram of a fault identification module according to an embodiment of the present disclosure;

[0020] FIG. 6 is a block diagram showing power angle stability control before and after a fault according to an embodiment of the present disclosure;

[0021] FIG. 7 is a block diagram showing dynamic admittance regulation control for a fault recovery process according to an embodiment of the present disclosure;

[0022] FIG. 8 is a block diagram showing multi-machine parallel control of multiple grid-forming converters without communication according to an embodiment of the present disclosure;

[0023] FIG. 9 is a flowchart of a black-start according to an embodiment of the present disclosure;

[0024] FIG. 10 is a flowchart of anti-islanding detection and protection according to an embodiment of the present disclosure; and

[0025] FIG. 11 is a flowchart of a renewable energy grid-forming control method according to an embodiment of the present disclosure. DETAILED DESCRIPTION

[0026] The technical solutions in the embodiments of the present disclosure are clearly and completely described below in conjunction with the drawings in the embodiments of the present disclosure. Obviously, the described embodiments are only a part of the embodiments of the present disclosure, not all of the embodiments. Based on the embodiments in the present disclosure, all other embodiments obtained by those of ordinary skill in the art without creative efforts fall within the protection scope of the present disclosure.

[0027] Reference is made to FIG. 1, which is a block diagram of a control strategy of an existing grid-forming control system. It can be seen that compared to the common grid-following control, grid-forming control abandons a phase-locked loop that follows the power grid, and instead adopts an excitation link that actively regulates a machine terminal voltage and a virtual rotor link that maintains power angle stability, thereby enabling the control link to have autonomous synchronization similar to that of a synchronous generator and externally exhibiting the frequency and voltage regulation capabilities of the synchronous generator.

[0028] However, the existing grid-forming control strategy still has the following defects.

[0029] First, in the power system, especially in high-proportion renewable energy power systems, a short-circuit fault is the most common and severe type of grid fault. Similar to a synchronous machine, at the moment of grid voltage sag and recovery caused by the short-circuit fault, rotor and excitation links of a grid-forming converter actively maintain the stability of the internal electromotive force. This inherent characteristic causes the converter to output an extremely large current to maintain the voltage level, especially at the moment of voltage sag and recovery, the drastic change in module current may lead to overcurrent. In other words, the grid-forming converter is prone to overcurrent during the short-circuit fault, which may further cause device damage.

[0030] Second, in operating conditions where a large-scale power outage of the power grid requires a power station to drive local loads, or where the grid load fluctuates greatly, the grid quality is poor, and there are factors such as grid instability or even grid collapse, requiring an energy storage system to regulate the grid quality, smooth the load fluctuations, and ensure islanding operation, the energy storage system is generally required to be configured at a megawatt (MW) level or even up to dozens of MWs. When an isolated power grid experiences issues such as grid collapse, the MW-level or even tens-of-MW-level energy storage system is required to switch to off-grid operation synchronously and quickly, or to serve as an emergency power supply to provide station service power for a certain period to implement the startup of the generator set.

[0031] Third, the key to the energy storage system entering off-grid operation is switching from a grid-following mode to a grid-forming mode, and the current and voltage during the switching process should be smooth without obvious impact. During a black-start process of a renewable energy power generation system, dozens or even hundreds of energy storage converters are required to smoothly and quickly switch to the off-grid operation state or complete the zero-start voltage boost of the transformer to establish the off-grid voltage. In this process, mutual oscillations between multiple machines due to frequency differences and the resulting uneven power distribution are prone to occur.

[0032] Fourth, in a grid-forming control system, when multiple grid-forming converters operate in parallel, systemic oscillations are prone to occur. The main reason for the oscillations is the inconsistency of parameters of the grid-forming converters. The main influencing parameters include an inertia coefficient, an active power droop coefficient, and a line reactance. Communication means are usually adopted to assign and adjust control parameters, which has significant problems in terms of cost and stability.

[0033] Fifth, the islanding effect refers to a state where the system continues to supply power to a certain portion of lines in a de-energized grid when the grid loses voltage. Once an unintentional islanding occurs, the voltage and frequency in the island are no longer controlled by the grid, which may further lead to damage to user equipment on the power line, interfere with the grid recovery process, and bring danger to maintenance personnel. Therefore, how to detect and protect against the islanding effect in the grid-forming control system is also extremely important.

[0034] In response to the above issues, a renewable energy grid-forming control system is provided according to an embodiment of the present disclosure to address at least one of the above issues.

[0035] Reference is made to FIG. 2, which is a schematic structural diagram of a renewable energy grid-forming control system according to the embodiment.

[0036] The system includes a mechanical equation module, an excitation equation module, a power command calculation module, and a grid-following control module.

[0037] The power command calculation module is configured to determine a measured active power and a measured reactive power.

[0038] The excitation equation module is configured to, when the renewable energy grid-forming control system switches from grid-forming control to grid-following control: regulate a voltage increment of the excitation equation module based on a reference voltage and a transformed voltage until a difference between the reference voltage and the transformed voltage satisfies a first convergence condition, and reduce, after the difference between the reference voltage and the transformed voltage satisfies the first convergence condition, the voltage increment to 0 stepwise by a first step size, where the transformed voltage is obtained by performing Park Transformation on a three-phase voltage of a renewable energy station controlled by the renewable energy grid-forming control system.

[0039] The mechanical equation module is configured to, when the renewable energy grid-forming control system switches from the grid-forming control to the grid-following control: regulate an angular velocity increment of the mechanical equation module based on a first phase angle and a second phase angle until a difference between the first phase angle and the second phase angle satisfies a second convergence condition, and reduce, after the difference between the first phase angle and the second phase angle satisfies the second convergence condition, the angular velocity increment stepwise by a second step size to be less than or equal to a second threshold, where the first phase angle is a phase angle of an output terminal of a grid-following control unit, and the second phase angle is a phase angle of an output terminal of a grid-forming control unit.

[0040] The grid-following control module is configured to, when the renewable energy grid-forming control system switches from the grid-following control to the grid-forming control: regulate a current increment of a current loop for the grid-following control based on a difference between a reference active power and the measured active power, and a difference between a reference reactive power and the measured reactive power, until both the difference between the reference active power and the measured active power and the difference between the reference reactive power and the measured reactive power satisfy a third convergence condition, and reduce, after both the difference between the reference active power and the measured active power and the difference between the reference reactive power and the measured reactive power satisfy the third convergence condition, the current increment to 0 stepwise by a third step size.

[0041] Reference is made to (1) of FIG. 3, which is a control structure diagram of the mechanical equation module according to an embodiment of the present disclosure. The structure outside the dashed rectangular box is a control structure of a mechanical equation in the existing grid-forming control system, which will not be elaborated upon, and the structure inside the dashed rectangular box is a newly added control structure based on the existing mechanical equation in this embodiment.

[0042] As shown in the figure, through the mechanical equation module in this embodiment, the phase angle of the output terminal of the grid-following (GFL) control unit (which may be a grid-following converter), i.e., 9gfl in the figure, and the phase angle of the output terminal of the grid-forming (GFM) control unit (which may be a grid-forming converter), i.e., 0gfm in the figure, are obtained. The two phase angles are processed based on a proportional integral (PI) amplifier to obtain a phase difference Aro, and this phase difference is determined as the angular velocity increment ro_add, that is, assigning Aro to ro_add, i.e., ro_add=Aro. The phase angle of the output terminal of the grid-forming control unit is regulated based on the angular velocity increment until the difference between the first phase angle and the second phase angle satisfies the second convergence condition.

[0043] The second convergence condition may be that an absolute value of the difference between the first phase angle and the second phase angle is less than a certain threshold, for example, less than 0.01, which is expressed as |0gfl-0gfm|<O.O1.

[0044] After the difference between the first phase angle and the second phase angle satisfies the second convergence condition, the mechanical equation module reduces the angular velocity increment stepwise by the second step size to be less than or equal to the second threshold.

[0045] The second step size is set to 0.01, and the second threshold is 0.0001.

[0046] That is, after the difference between the first phase angle and the second phase angle satisfies the second convergence condition, the mechanical equation module repeatedly reduces the angular velocity increment according to N_add N_add-0.0l, where o_add in the left side of the equals sign is a value after reduction, and N_add in the right side of the equals sign is a value before reduction. After each update, it is determined whether ®_add<=0.0001 is satisfied. If ro_add<=0.0001 is satisfied, the mechanical equation module stops reducing the angular velocity increment; and if w_add<=0.0001 is not satisfied, the mechanical equation module continues to reduce the angular velocity increment according to the aforementioned equation.

[0047] The operating principle of the above mechanical equation module may be seen in the flowchart of switching from grid-forming control to grid-following control shown in (1) of FIG. 4, where c>_add An indicates assigning the calculated phase difference to the angular velocity increment ®_add.

[0048] The first phase angle is obtained through a conventional phase-locked loop module shown in FIG. 2. The conventional phase-locked loop module has a structure as shown in (2) of FIG. 3.

[0049] Reference is made to (3) of FIG. 3, which is a control block diagram of the excitation equation module in this embodiment. The structure outside the dashed rectangular box is a control structure of an excitation equation in the existing grid-forming control system, which will not be elaborated upon, and the structure inside the dashed rectangular box is a newly added control structure based on the existing excitation equation in this embodiment.

[0050] It can be seen that through the excitation equation module in this embodiment, the reference voltage Uref and the transformed voltage Ud are inputted into the newly added proportional integral amplifier to obtain a voltage difference Au, and then the voltage difference is assigned to the voltage increment U_add of the excitation equation module (i.e., U_add=Au). The reference voltage is regulated based on the voltage increment until the difference between the reference voltage and the transformed voltage satisfies the first convergence condition.

[0051] The transformed voltage Ud is obtained by performing Park Transformation on the three-phase voltage Uabc of the renewable energy station controlled by the control system through the conventional phase-locked loop module shown in (2) of FIG. 3.

[0052] The first convergence condition may be that an absolute value of the difference between the reference voltage Uref and the transformed voltage Ud is less than a certain threshold, for example, less than 1, that is, this first convergence condition is expressed as |Uref-Ud|<1.

[0053] The first step size is set to 0.1, and a first threshold is 0.01.

[0054] That is, after the first convergence condition is satisfied, the excitation equation module repeatedly reduces the voltage increment according to U_add=U_add-0.1, where U_add in the left side of the equals sign is a value after reduction, and U_add in the right side of the equals sign is a value before reduction. After each update, it is determined whether U_add<=0.01 is satisfied. If U_add<=0.01 is satisfied, the excitation equation module stops reducing the voltage increment. At this point, the excitation equation module directly sets the voltage increment to be 0, i.e., U_add=0. If U_add<=0.01 is not satisfied, the excitation equation module continues to reduce the voltage increment according to the aforementioned equation.

[0055] If it is not required to switch from grid-forming control to grid-following control, the excitation equation module may directly set the voltage increment to be 0, i.e., U_add=0.

[0056] The operating principle of the above excitation equation module may be seen in the flowchart of switching from grid-forming control to grid-following control shown in (1) of FIG. 4.

[0057] The structure of the grid-following control module may be seen in the structure inside the dashed rectangular box in (4) of FIG. 3. When the control system switches from the grid-following control to the grid-forming control, the current increment of the current loop for the grid-following control is regulated based on the difference between the reference active power Pref and the measured active power P, and the difference between the reference reactive power Qref and the measured reactive power Q, until both the difference between the reference active power and the measured active power and the difference between the reference reactive power and the measured reactive power satisfy the third convergence condition.

[0058] Specifically, the reference active power Pref and the measured active power P are calculated through PI, so as to obtain an active power difference AP, and the reference reactive power Qref and the measured reactive power Q are calculated through PI, so as to obtain a reactive power difference AQ. The active power difference is assigned to a first current increment AId, and the reactive power difference is assigned to a second current increment AIq, i.e., AId=AP, and AIq=AQ.

[0059] After regulating the current increment in the above manner, a first current Idref and a second current Iqref of the current loop are correspondingly adjusted, thereby affecting the reference active power Pref and the reference reactive power Qref.

[0060] After each regulation of the current increment, it is determined whether the third convergence condition is satisfied at this moment.

[0061] The third convergence condition may be that the active power difference is less than or equal to 10 kilowatts (kW), and the reactive power difference is less than or equal to 10kW, i.e., Pref-P<=10kW and Qref-Q<=10kW.

[0062] If the third convergence condition is not satisfied, the current increment continues to be regulated in the aforementioned manner. If the third convergence condition is satisfied, the current increment is reduced to 0 stepwise by the third step size.

[0063] The third step size is 1. The manner for reducing the current increment to 0 stepwise by the third step size is as follows.

[0064] The current current increment is decreased by the third step size. Specifically, the first current increment and the second current increment are decreased according to AId=AId-1 and AIq=AIq-1, where AId and AIq in the left sides of the equals signs are the current increments after reduction, and AId and AIq in the right sides of the equals signs are the current increments before reduction.

[0065] It is determined whether the decreased current increment is less than or equal to a third threshold. Third thresholds corresponding to different current increments may be different. For example, the third threshold corresponding to the first current increment is 100A, and the third threshold corresponding to the second current increment is 10A, and then it is determined whether the decreased current increments are less than or equal to the third thresholds, that is, it is determined whether AId<=100A and AIq<=10A are satisfied.

[0066] If the decreased current increment is less than or equal to the third threshold, the current increment is set to be 0. Specifically, the second current increment is set to 0, and the voltage increment corresponding to the first current increment is set to 0, i.e., AIq=0 and AVd=0.

[0067] If the decreased current increment is greater than the third threshold, the step of decreasing the current increment by the third step size is performed repeatedly until the decreased current increment is less than or equal to the third threshold.

[0068] In an embodiment, if it is not required to switch from the grid-following control to the grid-forming control, the grid-following control module directly sets the second current increment to 0, and sets the voltage increment corresponding to the first current increment to 0, i.e., AIq=0 and AVd=0.

[0069] The operating principle of the above grid-following control module may be seen in the flowchart of switching from the grid-following control to the grid-forming control shown in (2) of FIG. 4.

[0070] Through the control principles of the above modules, the requirements of low impact, no disturbance, and consistent power before and after switching may be achieved during switching between the grid-following control and the grid-forming control.

[0071] In an embodiment, to achieve mutual switching between the grid-following control and the grid-forming control, a small number of switchable grid-following / grid-forming power generation units are configured in a renewable energy station where all power generation units adopt grid-following control. In this way, the operational capability of the station under weak grid conditions can be enhanced and instability problems can be avoided.

[0072] As shown in FIG. 2, the system in the embodiment further includes a fault identification module.

[0073] The fault identification module is configured to determine whether a power grid to which the renewable energy station is connected is in a short-circuit trigger state or a short-circuit recovery state.

[0074] The power command calculation module is configured to perform, when the power grid is in the short-circuit trigger state or the short-circuit recovery state, power angle compensation according to the state of the power grid.

[0075] The fault identification module outputs a fault mode signal to indicate whether the power grid to which the renewable energy station is connected is in the short-circuit trigger state and the short-circuit recovery state. Reference is made to FIG. 5, which is a schematic structural diagram of the fault identification module in this embodiment. LPF in the figure represents a low-pass filter.

[0076] The core of the fault identification module is an SR flip-flop. The SR flip-flop outputs different fault mode signals based on a value of an S bit (a voltage fault signal bit) and a value of an R bit (a current fault signal bit), in conjunction with a truth table of the SR flip-flop shown in Table 1. Table 1 R S Q Fault Mode Signal 0 0 Hold State No Change 0 1 1 Short-Circuit Trigger State 1 0 0 Short-Circuit Recovery State 1 1 Unstable Forbidden

[0077] Based on the structure shown in FIG. 5, the fault identification module performs per-unit conversion on a voltage error. If the voltage error is less than 7% of a rated voltage, the voltage fault signal bit may be set to 1; and if the voltage error is greater than or equal to 7% of the rated voltage, the voltage fault signal bit may be set to 0.

[0078] The fault identification module performs per-unit conversion on an active current error and a reactive current error. If the active current error is greater than 125% of a rated current, or the reactive current error is greater than 125% of the rated current, the current fault signal bit may be set to 1. If the active current error is not greater than 125% of the rated current, and the reactive current error is not greater than 125% of the rated current, the current fault signal bit may be set to 0.

[0079] The rated voltage UdN and the rated current IN are both rated values of the grid-forming converter used for grid-forming control.

[0080] When it is determined that the power grid is in the short-circuit trigger state or the short-circuit recovery state, the power command calculation module may, according to the control block diagram shown in FIG. 6, adopt different processing methods for the virtual admittance, the reference voltage, and the reference power upon fault entry and fault exit, so as to perform power angle compensation and voltage fine-tuning, thereby achieving both controllable limitation of current impact upon fault entry and smooth current transition upon fault exit.

[0081] P' is determined according to the following equation (1). P' =      + Q'2 CD

[0082] Q' is determined according to the following equation (2). r (V-|VPCC|)DU + QS; Upu > 0.9 Q' = j 2S„ew (1 - Vpu );        0.9 > Upu > 0.5 (2) V        ^new»        UpU < 0.5

[0083] As shown in FIG. 2, the system in this embodiment further includes a virtual admittance dynamic current-limiting module and a dynamic admittance regulation module.

[0084] The dynamic admittance regulation module is configured to regulate an admittance value of the virtual admittance dynamic current-limiting module when the power grid is in the short-circuit recovery state, so as to suppress a current outputted by a grid-forming converter.

[0085] In an embodiment, the dynamic admittance regulation module may regulate the admittance value of the virtual admittance dynamic current-limiting module according to the control block diagram shown in FIG. 7.

[0086] Regulating the admittance value of the virtual admittance dynamic current-limiting module may prevent the active current from climbing due to power angle hysteresis during the output process, causing overcurrent, and at the same time prevent the inertia superposition caused by pressure difference and phase lag due to sampling deviation during voltage recovery. Through dynamic damping correction, the large current at this time is suppressed until the voltage is completely restored and the mechanical link returns to a steady state.

[0087] In an embodiment, as shown in FIG. 2, the system further includes a harmonic (circulating current) extraction module and an auxiliary current loop module.

[0088] The harmonic (circulating current) extraction module is configured to: detect, in a real-time manner, a current and a voltage at a point of common coupling of a plurality of grid-forming converters connected in parallel with each other, to obtain an Nth-order harmonic of the current (i.e., ipcc shown in FIG. 8) and an Nth-order harmonic of the voltage (i.e., upcc shown in FIG. 8); convert the Nth-order harmonic of the current into a voltage regulation quantity (i.e., uhdq shown in FIG. 2) through a PI control loop, and input the voltage regulation quantity into the virtual admittance dynamic current-limiting module to suppress a voltage loop of the virtual admittance dynamic current-limiting module; and convert the Nth-order harmonic of the voltage into a current regulation quantity (i.e., ihdq shown in FIG. 2) through the PI control loop, and input the current regulation quantity into the auxiliary current loop module to suppress a harmonic circulating current through the auxiliary current loop module.

[0089] N is an integer determined according to the modulation method, capacitance parameters, and inductance parameters.

[0090] The harmonic (circulating current) extraction module may process the inputted Nth-order harmonic of the current and Nth-order harmonic of the voltage according to the control block diagram shown in FIG. 8 to obtain the corresponding voltage regulation quantity and current regulation quantity.

[0091] The role of adding the harmonic (circulating current) extraction module and the auxiliary current loop module is as follows.

[0092] In the absence of communication lines, multi-machine parallel operation is similar to voltage source parallel operation. Since it is difficult to meet the fully synchronized state, the grid-forming converter is most prone to circulating current phenomena in the case of multi-machine parallel operation, especially at the moment of grid faults. When a short-circuit fault occurs in the system, the voltage at the point of common coupling drops rapidly. However, due to factors such as sampling time and control feedback time, the system regulation has a certain delay and lag, and the output voltage of the converter cannot be adjusted quickly, which results in this large voltage difference being borne by a filter and a line impedance. The converter may withstand transient impacts and steady-state overload currents containing non-periodic direct-current offset components and periodic fault alternating-current components. A deeper grid voltage drop indicates a greater impact current experienced by the system, which may even reach about seven times the current during normal system operation. Unlike synchronous generators, which have an overcurrent capability of 6-8p.u., a fault current of a grid-forming grid-connected converter should be limited to within 1.5-3p.u. of a normal grid-connected operating current. Particularly after the converter becomes highly power-electronic, the capability to withstand overcurrent is severely limited, and fault overcurrent may pose a serious threat to the safe and stable operation of the power grid.

[0093] The harmonic (circulating current) extraction module and the auxiliary current loop module may convert the harmonic current into the voltage regulation quantity through the PI control loop and introduce the voltage regulation quantity into the voltage loop for suppression; and convert the harmonic voltage into the current regulation quantity through the PI control loop and introduce the current regulation quantity into the current loop for suppression, thereby achieving the effect of suppressing the harmonic circulating current and reducing fault overcurrent.

[0094] In an embodiment, the system further includes a voltage accumulation module and a grid-following phase-locked loop module.

[0095] Referring to FIG. 9, the power command calculation module is further configured to: close, after receiving a black-start command, a direct-current circuit breaker to perform direct-current pre-charging; close an alternating-current circuit breaker after the direct-current pre-charging is completed, and start, in response to closing the alternating-current circuit breaker, the grid-following phase-locked loop module in parallel to detect a frequency and a phase voltage peak in a real-time manner; start the mechanical equation module in a case that the measured active power is zero, and determine a control phase of the mechanical equation module; and start the excitation equation module in a case that the measured reactive power is zero.

[0096] The voltage accumulation module is configured to accumulate a feedforward voltage of the excitation equation module to a set voltage.

[0097] The power command calculation module is further configured to: when a frequency deviation of the excitation equation module is less than 0.1Hz, and a voltage peak of the excitation equation module is greater than or equal to the set voltage, fix the feedforward voltage of the excitation equation module to the set voltage to end a black-start process. The set voltage is represented by Uo, and the voltage peak of the excitation equation module is represented by Ud.

[0098] If the frequency deviation of the excitation equation module is not less than 0.1Hz, or the voltage peak of the excitation equation module is less than the set voltage, the step of starting the mechanical equation module in a case that the measured active power is zero, and determining the control phase of the mechanical equation module is performed until the frequency deviation of the excitation equation module is less than 0.1Hz and the voltage peak of the excitation equation module is greater than or equal to the set voltage.

[0099] From the perspective of an inverter controller, based on a virtual synchronous generator control framework comprising a power-frequency loop, a reactive power-voltage loop, and a generator stator voltage model, a zero-start boost parallel black-start strategy is provided in the present disclosure, which addresses the issues of how to avoid current impact during the voltage establishment process and how to realize sequential startup of two grid-forming converters with VSG control in the existing parallel black-start of grid-forming converters.

[0100] In an embodiment, referring to FIG. 2, the system further includes an anti-islanding module. As shown in FIG. 10, the anti-islanding module performs anti-islanding protection in the following manner: detecting whether the renewable energy station satisfies an anti-islanding detection condition; in response to detecting that the renewable energy station satisfies the anti-islanding detection condition, performing a 75th-order reactive harmonic disturbance, and detecting in a real-time manner whether a 75th-order voltage harmonic generated by the 75th-order reactive harmonic disturbance is greater than 5%; if the 75th-order voltage harmonic is greater than 5%, performing anti-islanding protection; if the 75th-order voltage harmonic is not greater than 5%, performing a 45th-order reactive harmonic disturbance, and detecting in a real-time manner whether a 45th-order voltage harmonic generated by the 45th-order reactive harmonic disturbance is greater than 5%; if the 45th-order voltage harmonic is greater than 5%, performing the anti-islanding protection; and if the 45th-order voltage harmonic is not greater than 5%, exiting the anti-islanding detection.

[0101] In an embodiment, as shown in FIG. 10, the anti-islanding module detects whether the renewable energy station satisfies the anti-islanding detection condition by: detecting a harmonic variation range of the renewable energy station in a real-time manner; if the harmonic variation range does not continuously exceed 2% within an anti-islanding duration, determining that the renewable energy station does not satisfy the anti-islanding detection condition; if the harmonic variation range continuously exceeds 2% within the anti-islanding duration, determining whether the renewable energy station satisfies a high / low voltage ride-through condition; if the renewable energy station satisfies the high / low voltage ride-through condition, determining that the renewable energy station does not satisfy the anti-islanding detection condition; and if the renewable energy station does not satisfy the high / low voltage ride-through condition, determining that the renewable energy station satisfies the anti-islanding detection condition.

[0102] The harmonic variation range continuously exceeding 2% within the anti-islanding duration means that the harmonic variation range is greater than 2%, and this state (that is, the state that the harmonic variation range is greater than 2%) lasts for a duration reaching a preset anti-islanding duration. The anti-islanding duration may be set according to various parameters of the renewable energy station and is not limited. Correspondingly, if the harmonic variation range is not greater than 2%, or the duration that the harmonic variation range is greater than 2% is less than the anti-islanding duration, it is determined that the harmonic variation range does not continuously exceed 2% within the anti-islanding duration.

[0103] The islanding effect refers to a self-sufficient power generation system formed by a grid-connected system that fails to detect the de-energized state in time and continues to supply power to the grid when the grid experiences a trip due to factors such as grid faults or maintenance outages, forming a connection with the load alone. The photovoltaic islanding effect may bring a series of hazards to system-related equipment and staff. A reliable and accurate islanding detection method is the basis for grid-connected inverters to realize the anti-islanding function.

[0104] In this embodiment, by means of reactive harmonic injection, it is possible to more accurately detect whether the islanding effect occurs in the renewable energy station, thereby enabling timely countermeasures to be taken when the islanding effect occurs.

[0105] A renewable energy grid-forming control method is further provided according to an embodiment of the present disclosure. Referring to FIG. 11, the method includes: S1101, when a renewable energy grid-forming control system switches from grid-forming control to grid-following control, regulating a voltage increment of an excitation equation module based on a reference voltage and a transformed voltage until a difference between the reference voltage and the transformed voltage satisfies a first convergence condition, and reducing, after the difference between the reference voltage and the transformed voltage satisfies the first convergence condition, the voltage increment to 0 stepwise by a first step size, where the transformed voltage is obtained by performing Park Transformation on a three-phase voltage of a renewable energy station controlled by the renewable energy grid-forming control system; S1102, when the renewable energy grid-forming control system switches from the grid-forming control to the grid-following control, regulating an angular velocity increment of a mechanical equation module based on a first phase angle and a second phase angle until a difference between the first phase angle and the second phase angle satisfies a second convergence condition, and reducing, after the difference between the first phase angle and the second phase angle satisfies the second convergence condition, the angular velocity increment stepwise by a second step size to be less than or equal to a second threshold, where the first phase angle is a phase angle of an output terminal of a grid-following control unit, and the second phase angle is a phase angle of an output terminal of a grid-forming control unit; and S1103, when the renewable energy grid-forming control system switches from the grid-following control to the grid-forming control, regulating a current increment of a current loop for the grid-following control based on a difference between a reference active power and a measured active power, and a difference between a reference reactive power and a measured reactive power, until both the difference between the reference active power and the measured active power and the difference between the reference reactive power and the measured reactive power satisfy a third convergence condition, and after both the difference between the reference active power and the measured active power and the difference between the reference reactive power and the measured reactive power satisfy the third convergence condition, reducing the current increment to 0 stepwise by a third step size.

[0106] In an embodiment, the reducing the current increment to 0 stepwise by the third step size includes: decreasing the current increment by the third step size; determining whether the decreased current increment is less than or equal to a third threshold; setting, if the decreased current increment is less than or equal to the third threshold, the current increment to be 0; and performing, if the decreased current increment is greater than the third threshold, the step of decreasing the current increment by the third step size repeatedly until the decreased current increment is less than or equal to the third threshold.

[0107] In an embodiment, the method further includes: determining whether a power grid to which the renewable energy station is connected is in a short-circuit trigger state or a short-circuit recovery state; and performing, when the power grid is in the short-circuit trigger state or the short-circuit recovery state, power angle compensation based on the state of the power grid.

[0108] In an embodiment, the method further includes: regulating, when the power grid is in the short-circuit recovery state, an admittance value of a virtual admittance dynamic current-limiting module to suppress a current outputted by a grid-forming converter.

[0109] In an embodiment, the method further includes: detecting, in a real-time manner, a current and a voltage at a point of common coupling of a plurality of grid-forming converters connected in parallel with each other, to obtain an Nth-order harmonic of the current and an Nth-order harmonic of the voltage; converting the Nth-order harmonic of the current into a voltage regulation quantity through a proportional integral (PI) control loop, and inputting the voltage regulation quantity into the virtual admittance dynamic current-limiting module to suppress a voltage loop of the virtual admittance dynamic current-limiting module; and converting the Nth-order harmonic of the voltage into a current regulation quantity through the PI control loop, and inputting the current regulation quantity into an auxiliary current loop module to suppress a harmonic circulating current through the auxiliary current loop module.

[0110] In an embodiment, the method further includes: closing, after receiving a black-start command, a direct-current circuit breaker to perform direct-current pre-charging; closing an alternating-current circuit breaker after the direct-current pre-charging is completed, and in response to closing the alternating-current circuit breaker, starting a grid-following phase-locked loop module in parallel to detect a frequency and a phase voltage peak in a real-time manner; starting the mechanical equation module in a case that the measured active power is zero, and determining a control phase of the mechanical equation module; starting the excitation equation module in a case that the measured reactive power is zero; accumulating a feedforward voltage of the excitation equation module to a set voltage; and when a frequency deviation of the excitation equation module is less than 0.1Hz, and a voltage peak of the excitation equation module is greater than or equal to the set voltage, fixing the feedforward voltage of the excitation equation module to the set voltage to end a black-start process.

[0111] In an embodiment, the method further includes: detecting whether the renewable energy station satisfies an anti-islanding detection condition; in response to detecting that the renewable energy station satisfies the anti-islanding detection condition, performing a 75th-order reactive harmonic disturbance, and detecting in a real-time manner whether a 75th-order voltage harmonic generated by the 75th-order reactive harmonic disturbance is greater than 5%; if the 75th-order voltage harmonic is greater than 5%, performing anti-islanding protection; if the 75th-order voltage harmonic is not greater than 5%, performing a 45th-order reactive harmonic disturbance, and detecting in a real-time manner whether a 45th-order voltage harmonic generated by the 45th-order reactive harmonic disturbance is greater than 5%; if the 45th-order voltage harmonic is greater than 5%, performing the anti-islanding protection; and if the 45th-order voltage harmonic is not greater than 5%, exiting the anti-islanding detection.

[0112] For the implementation of each step in the renewable energy grid-forming control method provided in the embodiments of the present disclosure, reference may be made to the operating principle of the renewable energy grid-forming control system provided in any embodiment of the present disclosure, which will not be repeated herein.

[0113] It should be noted that the various embodiments in this specification are described in a progressive manner, each embodiment focuses on the differences from other embodiments, and the same and similar parts between the various embodiments may be referred to each other.

[0114] For the convenience of description, when describing the above system or device, the functions are divided into various modules or units and described separately. Of course, when implementing the present disclosure, the functions of each unit may be implemented in the same one or more software and / or hardware.

[0115] Through the description of the above implementations, those skilled in the art can clearly understand that the present disclosure can be implemented by means of software plus a necessary general hardware platform. Based on such an understanding, the technical solution of the present disclosure essentially or the part that contributes to the prior art can be embodied in the form of a software product. The computer software product is stored in a storage medium, such as a ROM / RAM, a magnetic disk, an optical disk, etc., and includes several instructions to cause a computer device (which may be a personal computer, a server, or a network device, etc.) to perform the method described in the various embodiments of the present disclosure or certain parts of the embodiments.

[0116] Finally, it should also be noted that in this text, relational terms such as first, second, third, and fourth, etc., are only used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Moreover, the terms "include," "comprise," or any other variation thereof are intended to cover a non-exclusive inclusion, so that a process, method, article, or device that includes a series of elements not only includes those elements but also includes other elements not explicitly listed, or also includes elements inherent to such a process, method, article, or device. Without more limitations, an element defined by the statement "includes a ..." does not exclude the existence of another identical element in the process, method, article, or device that includes the element.

[0117] The above are only preferred implementations of the present disclosure. It should be pointed out that for those of ordinary skill in the technical field, without departing from the principles of the present disclosure, several improvements and modifications may also be made, and these improvements and modifications should also be regarded as the protection scope of the present disclosure.

Claims

1. A renewable energy grid-forming control system, comprising a mechanical equation module, an excitation equation module, and a power command calculation module, whereinthe power command calculation module comprises a grid-following control module;the power command calculation module is configured to determine a measured active power and a measured reactive power;the excitation equation module is configured to, when the renewable energy grid-forming control system switches from grid-forming control to grid-following control:regulate a voltage increment of the excitation equation module based on a reference voltage and a transformed voltage until a difference between the reference voltage and the transformed voltage satisfies a first convergence condition, andreduce, after the difference between the reference voltage and the transformed voltage satisfies the first convergence condition, the voltage increment to 0 stepwise by a first step size, whereinthe transformed voltage is obtained by performing Park Transformation on a three-phase voltage of a renewable energy station controlled by the renewable energy grid-forming control system;the mechanical equation module is configured to, when the renewable energy grid-forming control system switches from the grid-forming control to the grid-following control:regulate an angular velocity increment of the mechanical equation module based on a first phase angle and a second phase angle until a difference between the first phase angle and the second phase angle satisfies a second convergence condition, andreduce, after the difference between the first phase angle and the second phase angle satisfies the second convergence condition, the angular velocity increment stepwise by a second step size to be less than or equal to a second threshold, whereinthe first phase angle is a phase angle of an output terminal of a grid-following control unit, and the second phase angle is a phase angle of an output terminal of agrid-forming control unit; andthe grid-following control module is configured to, when the renewable energy grid-forming control system switches from the grid-following control to the grid-forming control:regulate a current increment of a current loop for the grid-following control based on a difference between a reference active power and the measured active power, and a difference between a reference reactive power and the measured reactive power, until both the difference between the reference active power and the measured active power and the difference between the reference reactive power and the measured reactive power satisfy a third convergence condition, andreduce, after both the difference between the reference active power and the measured active power and the difference between the reference reactive power and the measured reactive power satisfy the third convergence condition, the current increment to 0 stepwise by a third step size.

2. The system according to claim 1, wherein the grid-following control module, when reduces the current increment to 0 stepwise by the third step size, is configured to:decrease the current increment by the third step size;determine whether the decreased current increment is less than or equal to a third threshold;set, if the decreased current increment is less than or equal to the third threshold, the current increment to be 0; andperform, if the decreased current increment is greater than the third threshold, the step of decreasing the current increment by the third step size repeatedly until the decreased current increment is less than or equal to the third threshold.

3. The system according to claim 1, further comprising a fault identification module, whereinthe fault identification module is configured to determine whether a power grid to whichthe renewable energy station is connected is in a short-circuit trigger state or a short-circuit recovery state; andthe power command calculation module is configured to perform, when the power grid is in the short-circuit trigger state or the short-circuit recovery state, power angle compensation based on the state of the power grid.

4. The system according to claim 3, further comprising a virtual admittance dynamic current-limiting module and a dynamic admittance regulation module, whereinthe dynamic admittance regulation module is configured to regulate, when the power grid is in the short-circuit recovery state, an admittance value of the virtual admittance dynamic current-limiting module to suppress a current outputted by a grid-forming converter.

5. The system according to claim 4, further comprising a harmonic (circulating current) extraction module and an auxiliary current loop module, whereinthe harmonic (circulating current) extraction module is configured to:detect, in a real-time manner, a current and a voltage at a point of common coupling of a plurality of grid-forming converters connected in parallel with each other, to obtain an Nth-order harmonic of the current and an Nth-order harmonic of the voltage;convert the Nth-order harmonic of the current into a voltage regulation quantity through a proportional integral (PI) control loop, and input the voltage regulation quantity into the virtual admittance dynamic current-limiting module to suppress a voltage loop of the virtual admittance dynamic current-limiting module; andconvert the Nth-order harmonic of the voltage into a current regulation quantity through the PI control loop, and input the current regulation quantity into the auxiliary current loop module to suppress a harmonic circulating current through the auxiliary current loop module.

6. The system according to claim 1, further comprising a voltage accumulation module and a grid-following phase-locked loop module, whereinthe power command calculation module is further configured to:close, after receiving a black-start command, a direct-current circuit breaker to perform direct-current pre-charging;close an alternating-current circuit breaker after the direct-current pre-charging is completed, and start, in response to closing the alternating-current circuit breaker, the grid-following phase-locked loop module in parallel to detect a frequency and a phase voltage peak in a real-time manner;start the mechanical equation module in a case that the measured active power is zero, and determine a control phase of the mechanical equation module; andstart the excitation equation module in a case that the measured reactive power is zero;the voltage accumulation module is configured to accumulate a feedforward voltage of the excitation equation module to a set voltage; andthe power command calculation module is further configured to: when a frequency deviation of the excitation equation module is less than 0.1Hz, and a voltage peak of the excitation equation module is greater than or equal to the set voltage, fix the feedforward voltage of the excitation equation module to the set voltage to end a black-start process.

7. The system according to claim 1, further comprising an anti-islanding module, wherein the anti-islanding module is configured to:detect whether the renewable energy station satisfies an anti-islanding detection condition;in response to detecting that the renewable energy station satisfies the anti-islanding detection condition, perform a 75th-order reactive harmonic disturbance, and detect in a real-time manner whether a 75th-order voltage harmonic generated by the 75th-order reactive harmonic disturbance is greater than 5%;if the 75th-order voltage harmonic is greater than 5%, perform anti-islanding protection;if the 75th-order voltage harmonic is not greater than 5%, perform a 45th-order reactive harmonic disturbance, and detect in a real-time manner whether a 45th-order voltageharmonic generated by the 45th-order reactive harmonic disturbance is greater than 5%;if the 45th-order voltage harmonic is greater than 5%, perform the anti-islanding protection; andif the 45th-order voltage harmonic is not greater than 5%, exit the anti-islanding detection.

8. The system according to claim 7, wherein the anti-islanding module, when detects whether the renewable energy station satisfies the anti-islanding detection condition, is configured to:detect a harmonic variation range of the renewable energy station in a real-time manner;if the harmonic variation range does not continuously exceed 2% within an anti-islanding duration, determine that the renewable energy station does not satisfy the anti-islanding detection condition;if the harmonic variation range continuously exceeds 2% within the anti-islanding duration, determine whether the renewable energy station satisfies a high / low voltage ride-through condition;if the renewable energy station satisfies the high / low voltage ride-through condition, determine that the renewable energy station does not satisfy the anti-islanding detection condition; andif the renewable energy station does not satisfy the high / low voltage ride-through condition, determine that the renewable energy station satisfies the anti-islanding detection condition.

9. A renewable energy grid-forming control method, comprising:when a renewable energy grid-forming control system switches from grid-forming control to grid-following control, regulating a voltage increment of an excitation equation module based on a reference voltage and a transformed voltage until a difference between the reference voltage and the transformed voltage satisfies a first convergence condition, and reducing, after the difference between the reference voltage and the transformed voltagesatisfies the first convergence condition, the voltage increment to 0 stepwise by a first step size, wherein the transformed voltage is obtained by performing Park Transformation on a three-phase voltage of a renewable energy station controlled by the renewable energy grid-forming control system;when the renewable energy grid-forming control system switches from the grid-forming control to the grid-following control, regulating an angular velocity increment of a mechanical equation module based on a first phase angle and a second phase angle until a difference between the first phase angle and the second phase angle satisfies a second convergence condition, and reducing, after the difference between the first phase angle and the second phase angle satisfies the second convergence condition, the angular velocity increment stepwise by a second step size to be less than or equal to a second threshold, wherein the first phase angle is a phase angle of an output terminal of a grid-following control unit, and the second phase angle is a phase angle of an output terminal of a grid-forming control unit; andwhen the renewable energy grid-forming control system switches from the grid-following control to the grid-forming control, regulating a current increment of a current loop for the grid-following control based on a difference between a reference active power and a measured active power, and a difference between a reference reactive power and a measured reactive power, until both the difference between the reference active power and the measured active power and the difference between the reference reactive power and the measured reactive power satisfy a third convergence condition, and after both the difference between the reference active power and the measured active power and the difference between the reference reactive power and the measured reactive power satisfy the third convergence condition, reducing the current increment to 0 stepwise by a third step size.

10. The method according to claim 9, wherein the reducing the current increment to 0 stepwise by the third step size comprises:decreasing the current increment by the third step size;determining whether the decreased current increment is less than or equal to a third threshold;setting, if the decreased current increment is less than or equal to the third threshold, the current increment to be 0; andperforming, if the decreased current increment is greater than the third threshold, the step of decreasing the current increment by the third step size repeatedly until the decreased current increment is less than or equal to the third threshold.

11. The method according to claim 9, further comprising:determining whether a power grid to which the renewable energy station is connected is in a short-circuit trigger state or a short-circuit recovery state; andperforming, when the power grid is in the short-circuit trigger state or the short-circuit recovery state, power angle compensation based on the state of the power grid.

12. The method according to claim 11, further comprising:regulating, when the power grid is in the short-circuit recovery state, an admittance value of a virtual admittance dynamic current-limiting module to suppress a current outputted by a grid-forming converter.

13. The method according to claim 12, further comprising:detecting, in a real-time manner, a current and a voltage at a point of common coupling of a plurality of grid-forming converters connected in parallel with each other, to obtain an Nth-order harmonic of the current and an Nth-order harmonic of the voltage;converting the Nth-order harmonic of the current into a voltage regulation quantity through a proportional integral (PI) control loop, and inputting the voltage regulation quantity into the virtual admittance dynamic current-limiting module to suppress a voltage loop of the virtual admittance dynamic current-limiting module; andconverting the Nth-order harmonic of the voltage into a current regulation quantity through the PI control loop, and inputting the current regulation quantity into an auxiliary current loop module to suppress a harmonic circulating current through the auxiliary currentloop module.

14. The method according to claim 9, further comprising:closing, after receiving a black-start command, a direct-current circuit breaker to perform direct-current pre-charging;closing an alternating-current circuit breaker after the direct-current pre-charging is completed, and in response to closing the alternating-current circuit breaker, starting a grid-following phase-locked loop module in parallel to detect a frequency and a phase voltage peak in a real-time manner;starting the mechanical equation module in a case that the measured active power is zero, and determining a control phase of the mechanical equation module;starting the excitation equation module in a case that the measured reactive power is zero;accumulating a feedforward voltage of the excitation equation module to a set voltage; andwhen a frequency deviation of the excitation equation module is less than 0.1Hz, and a voltage peak of the excitation equation module is greater than or equal to the set voltage, fixing the feedforward voltage of the excitation equation module to the set voltage to end a black-start process.

15. The method according to claim 9, further comprising:detecting whether the renewable energy station satisfies an anti-islanding detection condition;in response to detecting that the renewable energy station satisfies the anti-islanding detection condition, performing a 75th-order reactive harmonic disturbance, and detecting in a real-time manner whether a 75th-order voltage harmonic generated by the 75th-order reactive harmonic disturbance is greater than 5%;if the 75th-order voltage harmonic is greater than 5%, performing anti-islanding protection;if the 75th-order voltage harmonic is not greater than 5%, performing a 45th-order reactive harmonic disturbance, and detecting in a real-time manner whether a 45th-order voltage harmonic generated by the 45th-order reactive harmonic disturbance is greater than 5%;5        if the 45th-order voltage harmonic is greater than 5%, performing the anti-islandingprotection; andif the 45th-order voltage harmonic is not greater than 5%, exiting the anti-islanding detection.

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