Converter soft start method based on mode seamless switching

Abstract

The invention relates to a converter soft start method based on mode seamless switching, which belongs to the technical field of converter soft start, and comprises the following steps: a converter adopts a network following / network construction hybrid control structure; after the converter receives a starting instruction, voltage and frequency are automatically constructed in a network construction control mode, and a filter capacitor is pre-charged; performing amplitude and phase angle pre-synchronization on the voltage of the output port of the converter and the voltage of the power grid based on a pre-synchronization strategy; and switching the converter from a network construction control mode to a network following control mode to complete soft start. According to the method, the pre-charging requirement of the alternating current capacitor is met, and soft-start smooth grid-connected operation of the converter is realized through a pre-synchronization strategy.

Description

Technical Field

[0001] The present invention belongs to the technical field of converter soft starting, and in particular relates to a converter soft starting method based on seamless mode switching. Background Art

[0002] As the proportion of clean energy such as wind and solar power in the power system continues to increase, modern power systems are moving towards a high proportion of renewable energy. As a key device for grid-connected power generation using renewable energy, the control methods of converters have attracted extensive attention and research. Existing converter control methods can be divided into grid-following (GFL) control and grid-forming (GFM) control. Grid-following control has good current control capabilities, but also has poor voltage and frequency support capabilities and is prone to instability in weak grids. Compared with grid-following control, grid-forming control supports the grid and enhances system stability, but grid-forming control has slow power regulation speed and poor stability in strong grids. Therefore, grid-forming control and grid-following control can complement each other to adapt to large variations in grid strength.

[0003] Currently, there has been considerable research on the switching of converters between following and forming grid modes. For example, existing converter following and forming grid switching methods can operate at the same operating point by adjusting reference values ​​using virtual current and virtual voltage, enabling smooth and rapid switching when switching control modes. Other following and forming grid switching control methods, devices, and control systems calculate the grid-connected short-circuit ratio of a new energy station as a following and forming grid switching signal. Their pre-synchronization control allows both following and forming grid control to pre-synchronize their respective power angles, voltages, and currents, thereby improving the grid connection stability and adaptability of new energy stations. Most existing research applies following and forming grid switching strategies to scenarios where converters adapt to changes in grid strength. In particular, this mode switching method has certain advantages during the soft-start process of a following converter.

[0004] Generally, to prevent the transient current surge during startup from damaging the internal components of the energy storage converter, pre-charging of these components, such as the busbar capacitors, is required before switching to achieve soft-start. Traditional converters utilize a soft-start circuit using an industrial frequency isolation transformer and soft-start resistors. This pre-charging circuit, constructed for high-power converters, consists of a three-phase low-voltage circuit breaker, a three-phase low-voltage contactor, a three-phase step-up transformer, a single-phase diode bridge rectifier, and a current-limiting resistor. The charging logic is designed to avoid inrush currents on the switching devices. Alternatively, capacitor pre-charging circuits, comprised of current-limiting resistors and thyristors, or current-limiting resistors and diodes, accelerate charging. However, these soft-start circuits require additional resistors and related switching relays, increasing converter space and manufacturing costs.

[0005] For this reason, many scholars have proposed a soft starter method of converter with simpler structure, faster pre-charge speed and lower manufacturing cost. For example, by controlling a smaller number of relay switches and current limiting resistors, while pre-charging the DC bus capacitor and AC filter capacitor of the converter, the pre-charge speed of the capacitor is greatly accelerated and the pre-charge cost is reduced; by using limited components to form the soft start circuit of the bus AC or DC of the energy storage converter, the same set of soft start resistors is used in the AC or DC soft start process of the energy storage converter, which effectively reduces the impact current generated by the energy storage converter and also effectively reduces the space and manufacturing cost of the soft start circuit module of the energy storage converter. However, the additional design of the soft start circuit still has problems such as pre-charge resistance heating and voltage drop loss, which has certain limitations. SUMMARY

[0006] The purpose of the present application is to provide a converter soft start method based on seamless switching of follow / grid control, to meet the pre-charging requirements of AC capacitor, and to realize smooth grid-connected operation of the converter through pre-synchronization strategy.

[0007] In order to achieve the above purpose, the technical scheme of the present application provides a converter soft start method based on mode seamless switching, which comprises the following steps:

[0008] The converter adopts a hybrid control structure of follow grid control / grid control;

[0009] When the converter receives a start instruction, the grid control mode is used to automatically build voltage and frequency for pre-charging the filter capacitor;

[0010] At the same time, based on the pre-synchronization strategy, the amplitude and phase angle of the converter output port voltage and the grid voltage are pre-synchronized;

[0011] After the capacitor pre-charge and converter pre-synchronization are completed, the converter is switched from the grid control mode to the follow grid control mode to complete the soft start.

[0012] Preferably, the steps include: sampling the three-phase output voltage u abc and the three-phase output current i abc , obtaining the output voltage u d , u q in the synchronous rotating coordinate system and the output current i d , i q in the synchronous rotating coordinate system; wherein the subscripts d and q represent the d-axis and q-axis of the synchronous rotating coordinate system respectively, d is the active axis and q is the reactive axis.

[0013] Preferably, the follow grid control structure includes a phase-locked loop, a power outer loop and a current inner loop, the power outer loop adjusts the output power according to the active and reactive instruction values, and the current inner loop ensures that the actual output current is consistent with the reference current.

[0014] Preferably, the network control mode is as shown in formula (1) to formula (6):

[0015] θ GFL =∫ω1dt (1)

[0016]

[0017] Where θ GFL ,ω1,i dref1 / i qref1 、e dref_GFL / e qref_GFL Indicates the phase angle, actual angular frequency, output dq axis reference current value and output modulation wave dq axis voltage value of the converter in the grid-following control state; k pPLL and k iPLL Respectively represent the proportional coefficient and integral coefficient in the phase-locked loop PI coefficient; P ref and Q ref are active power command value and reactive power command value respectively; L f Represents the filter inductance.

[0018] Preferably, the phase-locked loop is responsible for tracking the phase information of the grid voltage, the power outer loop adjusts the output power according to the active and reactive power command values, and the current inner loop ensures that the actual output current is consistent with the reference current.

[0019] Preferably, the network control structure includes an active loop, a reactive loop, a voltage outer loop and a current inner loop, and simulates the operating characteristics of a synchronous generator by adjusting the amplitude and frequency of the output voltage.

[0020] Preferably, according to the transformation from the three-phase coordinate system to the stationary two-phase coordinate system, the instantaneous output active power P0 and the instantaneous output reactive power Q0 of the new energy converter are calculated, as shown in formula (7):

[0021]

[0022] Where u α 、u β is the output voltage in the stationary two-phase coordinate system, i α 、i β Represents the output current in a stationary two-phase coordinate system.

[0023] Preferably, the network control mode is as shown in formula (8) to formula (14):

[0024] θ GFM =∫ω2dt (8)

[0025]

[0026] udref =U ref +K q (Q ref -Q0) (10)

[0027]

[0028] Where θ GFM ,ω2,ω0,i dref2 / i qref2 、e dref_GFM / e qref_GFM Indicates the phase angle, actual angular frequency, rated angular frequency, output dq axis reference current value and output modulation wave dq axis voltage value of the converter in the grid control state; u dref U is the d-axis output reference voltage; ref Indicates the rated amplitude of the output voltage; C f is the filter capacitor; D p is the damping adjustment coefficient; J is the inertia coefficient; K q is the reactive-voltage droop coefficient.

[0029] Preferably, the pre-synchronization strategy includes calculating the amplitude difference and phase angle difference between the converter output voltage and the grid voltage, and achieving pre-synchronization of the amplitude and phase angle through corresponding controllers.

[0030] Preferably, the grid voltage u grid and the converter output voltage u abc After passing through the low-pass filter, the difference between the amplitude and phase angle of the two is calculated using equations (15) to (18), and the corresponding controller is used to realize the pre-synchronization of the converter output port voltage with the grid voltage amplitude and phase angle;

[0031]

[0032] ΔU=U grid -U pcs (17)

[0033]

[0034] Where u ga 、u gb 、u gc They are the three-phase voltages of the grid connection points abc; u 0a 、u 0b 、u 0c The three-phase voltages abc are output by the converter respectively; U pcs 、U grid are the amplitudes of the converter output port voltage and the grid voltage at the grid connection point; ΔU and Δθ are the amplitude difference and phase angle difference between the converter output port voltage and the grid voltage, respectively.

[0035] In summary, the present invention has the following beneficial technical effects:

[0036] The complementary advantages of grid-following and grid-forming converters make switching between grid-following and grid-forming control modes highly valuable. Compared to traditional converter soft-start methods, this approach pre-charges AC capacitors based on the automatic voltage and frequency buildup of the grid-forming converter, eliminating the need for additional switches and current-limiting resistors in the soft-start circuit. This allows converter soft-start to be implemented without changing the converter control structure.

[0037] The existing technology mainly relies on the combination of control switch equipment and current-limiting resistors to pre-charge the bus capacitor to reduce the impact of the inrush current. However, this method brings a series of problems such as pre-charging resistor heating, voltage drop loss, increased converter space, and high manufacturing cost. In response to these shortcomings, the present invention automatically constructs voltage and frequency through a network control mode to achieve pre-charging of the AC capacitor, and on this basis, uses a pre-synchronization strategy to accurately pre-synchronize the amplitude and phase angle of the converter output port voltage with the grid voltage. When the capacitor charging process is completed and the pre-synchronization is completed, the converter can smoothly switch to the grid control mode and achieve grid-connected operation.

[0038] The present invention can play an important role in reducing the impact of capacitor surge current on switching devices during converter startup, thereby extending the service life of the equipment and improving the reliability of the system. In addition, the present invention provides an effective solution for seamless switching between follow-up and grid-building control modes based on pre-synchronization, so as to achieve smooth grid connection with converter soft start, further reduce the impact of converter closing current on the grid, and effectively avoid grid voltage fluctuations and system instability problems that may be caused by startup surge current. During the entire grid connection process, the output power of the converter can transition smoothly, ensuring the continuity and reliability of grid operation.

[0039] Since there is no need to add additional switching devices and pre-charging resistors, the entire soft start process of the present invention can be achieved without increasing the space occupied by the converter, thereby significantly reducing manufacturing costs and control complexity. At the same time, the seamless switching mechanism between the grid-following and grid-building control modes adopted by the present invention not only improves the control flexibility of the converter, but also provides a wide range of applicability for different application scenarios. Whether it is an energy storage converter or other types of converters, the present invention can achieve the goals of smooth grid connection and soft start by optimizing the control strategy, making an important contribution to the technological progress of the converter industry. BRIEF DESCRIPTION OF THE DRAWINGS

[0040] Figure 1 Schematic diagram of the converter structure with hybrid control of following / building network in a converter soft starting method based on seamless mode switching of the present invention;

[0041] Figure 2 A pre-synchronization control structure diagram for network construction control in the soft start method of the converter based on mode seamless switching of the application;

[0042] Figure 3 A follow / network construction seamless switching soft start flowchart in the soft start method of the converter based on mode seamless switching of the application;

[0043] Figure 4 A simulation model related parameter diagram in the soft start method of the converter based on mode seamless switching of the application;

[0044] Figure 5 A converter direct start simulation result diagram in the soft start method of the converter based on mode seamless switching of the application;

[0045] Figure 6 A converter soft start simulation result diagram in the soft start method of the converter based on mode seamless switching of the application. DETAILED DESCRIPTION

[0046] The technical solutions in the embodiments of the application will be clearly and completely described below with reference to the drawings in the embodiments of the application. Obviously, the described embodiments are only part of the embodiments of the application, rather than all the embodiments of the application. Based on the embodiments in the application, all other embodiments obtained by those skilled in the art without creative efforts fall within the protection scope of the application.

[0047] The existing converter soft start method mainly precharges the bus capacitor by controlling the combination of switching devices and current limiting resistors to reduce the impact of the impact current. To solve the problems of the traditional converter soft start method, such as precharge resistor heating, voltage drop loss, converter space increase, and high manufacturing cost, the application provides a soft start method of a converter based on mode seamless switching. The method automatically constructs a voltage and frequency AC capacitor for precharging through network construction control mode, and pre-synchronizes the amplitude and phase angle of the converter output port voltage and the grid voltage based on a pre-synchronization strategy. After the capacitor charging is completed and the pre-synchronization is completed, the method switches to follow-network control and operates in parallel. The method reduces the impact of the capacitor impact current on the switching device, further reduces the impact of the converter closing current on the grid, and realizes the soft start and smooth parallel operation of the follow-network converter.

[0048] The embodiments of the application disclose a soft start method of a converter based on mode seamless switching, which aims to realize filter capacitor precharging and follow-network smooth parallel operation by complementary advantages of follow-network control and network construction control. The method achieves the target by controlling follow / network seamless switching without increasing the structure of the converter. The specific steps of the method are as follows:

[0049] (1) Converter / grid hybrid control structure

[0050] The control structure design of the converter is the basis for realizing the soft start function. In this embodiment, the converter adopts a hybrid control structure of following / building a network. Figure 1 As shown, the three-phase output voltage u of the sampling converter abc And the three-phase output current i abc , and perform coordinate changes respectively to obtain the output voltage u in the synchronous rotating coordinate system d ,u q And the output current i in the synchronous rotating coordinate system d ,i q Wherein, the subscripts d and q represent the d-axis and q-axis of the synchronously rotating coordinate system, respectively. d is the active axis, and q is the reactive axis. This coordinate transformation helps simplify the complexity of the control algorithm and achieve precise control of the converter's output power.

[0051] The control structure of grid-following control is divided into a phase-locked loop, a power outer loop, and a current inner loop. Its control mode is shown in equations (1) to (6):

[0052] θ GFL =∫ω1dt (1)

[0053]

[0054] Where θ GFL ,ω1,i dref1 / i qref1 、e dref_GFL / e qref_GFL Indicates the phase angle, actual angular frequency, output dq axis reference current value and output modulation wave dq axis voltage value of the converter in the grid-following control state; k pPLL and k iPLL Respectively represent the proportional coefficient and integral coefficient in the phase-locked loop PI coefficient; P ref and Q ref are active power command value and reactive power command value respectively; L f Represents the filter inductance.

[0055] The phase-locked loop is responsible for tracking the phase information of the grid voltage, the power outer loop adjusts the output power according to the active and reactive power command values, and the current inner loop ensures that the actual output current is consistent with the reference current. In particular, through the current reference value I dvirREF and I qvirREF Affects i d and i q The dynamic characteristics of the system are equivalent to introducing active damping into the system, thereby enhancing the stability of the system.

[0056] like Figure 1As shown, according to the transformation from the three-phase coordinate system to the stationary two-phase coordinate system, the instantaneous output active power P0 and instantaneous output reactive power Q0 of the new energy converter are calculated, as shown in formula (7):

[0057]

[0058] Where u α ,u β is the output voltage in the stationary two-phase coordinate system, i α ,i β Represents the output current in a stationary two-phase coordinate system.

[0059] The control structure of network control is divided into active loop, reactive loop, voltage outer loop and current inner loop. Taking virtual synchronous machine control as an example, its control mode is shown in Equation (8) to Equation (14):

[0060] θ GFM =∫ω2dt (8)

[0061]

[0062] u dref =U ref +K q (Q ref -Q0) (10)

[0063]

[0064] Where θ GFM ,ω2,ω0,i dref2 / i qref2 、e dref_GFM / e qref_GFM Indicates the phase angle, actual angular frequency, rated angular frequency, output dq axis reference current value and output modulation wave dq axis voltage value of the converter in the grid control state; u dref U is the d-axis output reference voltage; ref Indicates the rated amplitude of the output voltage; C f is the filter capacitor; D p is the damping adjustment coefficient; J is the inertia coefficient; K q is the reactive-voltage droop coefficient.

[0065] Among them, the voltage deviation Δu d , Δu q Affect u respectively d ,u q dynamics, thereby adjusting the impedance characteristics of the simulated synchronous machine.

[0066] The core of network control is to simulate the operating characteristics of synchronous generators by adjusting the output voltage amplitude and frequency, thereby effectively supporting the power grid. In this embodiment, network control is not limited to virtual synchronous machine control but can also include various network control structures such as droop control, virtual oscillator control, and matching control.

[0067] (2) Pre-synchronization structure

[0068] When the converter receives the start command, it automatically establishes voltage and frequency using the grid-building control mode to pre-charge the AC capacitors. Simultaneously, the converter output voltage is pre-synchronized with the grid voltage in terms of amplitude and phase angle based on a pre-synchronization strategy. Figure 2 The figure shows the pre-synchronization module of the grid control. grid and the converter output voltage u abc After passing through the low-pass filter, the difference in amplitude and phase angle between the two is calculated using equations (15) to (18), and the corresponding controller is used to pre-synchronize the converter output port voltage with the grid voltage amplitude and phase angle. This process ensures that the converter voltage amplitude and phase angle are highly consistent with the grid voltage at the moment of grid connection, thereby avoiding the generation of grid-connected inrush current.

[0069]

[0070] ΔU=U grid -U pcs (17)

[0071]

[0072] Where u ga 、u gb 、u gc They are the three-phase voltages of the grid connection points abc; u 0a 、u 0b 、u 0c The three-phase voltages abc are output by the converter respectively; U pcs 、U grid are the amplitudes of the converter output port voltage and the grid voltage at the grid connection point; ΔU and Δθ are the amplitude difference and phase angle difference between the converter output port voltage and the grid voltage, respectively.

[0073] (3) Soft start method process for seamless switching between the network and the building network

[0074] After receiving the start signal, the converter first enters the grid control mode and waits for the filter capacitor pre-charging to be completed and synchronization to be completed. Then, switch 1, switch 2, and switch 3 are switched from port 2 to port 1 respectively, switching the converter from the grid control mode to the grid-following control mode. The soft start process is completed, and then the AC circuit breaker can be closed and normal operation can be carried out. This process realizes the seamless switching of the converter control mode, ensuring that the converter can smoothly connect to the grid and operate normally. The process of the grid-following / grid-following switching soft start method is as follows: Figure 3 shown.

[0075] In order to verify that the soft start method of the present invention has a certain improvement effect on reducing the converter starting impact current, the present invention designed a converter soft start simulation experiment. Figure 4 The converter parameters used in the experiment are provided, including rated active power of 1250 kW, rated AC line voltage of 690 V, rated DC voltage of 1300 V, and rated angular frequency of 100 π rad / s.

[0076] right Figure 1 The simulation verification experiment of a 1250kW energy storage converter grid-connected system is shown. Figure 5 The simulation results of the direct start of the converter without soft start function are shown, showing the large capacitor surge current and the impact on the grid voltage. Figure 5 (a) and Figure 5 As shown in (b), the converter starts directly after the start delay time t0. Since the capacitor is not pre-charged, a large capacitor surge current will occur after the AC circuit breaker is closed. This capacitor surge current will cause a certain impact on the grid voltage, such as Figure 5 (c) As shown, this leads to system instability and equipment damage.

[0077] In contrast, Figure 6 The simulation results of the proposed soft-start method are presented. After the startup delay, from t0 = 0.2s to t1 = 2.3s, the converter automatically establishes voltage and frequency based on the grid-connected control mode to pre-charge the capacitors. Simultaneously, the pre-synchronization module is activated to synchronize the output voltage with the grid voltage in amplitude and phase angle. After pre-charging and pre-synchronization are complete, the AC circuit breaker is closed, and the converter smoothly switches to normal operation in the grid-connected control mode. Throughout the soft-start process, the generation of capacitor current surges is effectively avoided, enabling smooth grid connection of the converter and reducing the startup surge current from the converter to the grid.

[0078] In the present invention, pre-charging of the filter capacitor is achieved through the grid-forming control mode, eliminating the large current impact generated by direct charging of the capacitor; the pre-synchronization module ensures that the amplitude and phase angle of the converter output voltage are highly consistent with the grid voltage, avoiding the impact current at the moment of grid connection; the seamless switching mechanism of following / forming the grid realizes the smooth transition of the converter control mode, ensuring the stable operation of the grid; the entire soft-start process does not require additional switching devices and pre-charging resistors, reducing manufacturing costs and control complexity.

[0079] The proposed converter soft-start method achieves smooth grid connection and soft-start of the converter by optimizing the control strategy without changing the converter's basic structure. Simulation results fully validate the effectiveness of this method, demonstrating that it can significantly reduce the impact of converter starting current on the power grid, improve system stability and reliability, and has promising application prospects and promotional value.

[0080] The soft-start method for the converter with seamless switching between the grid and the grid adopted by the present invention does not require changing the converter control structure. Instead, it realizes pre-charging of the filter capacitor by switching the converter's own control mode, and realizes smooth grid connection based on the soft-start strategy. Compared with the traditional soft-start device, the present invention avoids the problems of complex control, space occupation, voltage drop loss and high manufacturing cost caused by the additional addition of switching devices and pre-charging resistors. In order to verify the effectiveness of this method, the present invention designed a simulation test experiment for the soft start of the energy storage converter. The experimental results show that the method proposed by the present invention can effectively reduce the capacitor impact current and realize smooth grid connection of the converter, thereby reducing the impact of the converter starting current on the grid.

[0081] The converter / network hybrid control structure diagram mentioned in the present invention is not limited to Figure 1 The control structure shown can also be other grid-following and grid-forming control structures well known in the industry. The converter in the examples mentioned in the present invention is not limited to a 1250kW energy storage converter; it can be a converter of various power levels well known in the industry. Furthermore, the voltage and current loop controller and the pre-synchronized voltage amplitude and phase controller mentioned in the present invention are not limited to PI controllers; they can also be virtual impedance or other control links.

[0082] Finally, it should be noted that the above is only a preferred embodiment of the present invention and is not intended to limit the present invention. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art can still modify the technical solutions described in the aforementioned embodiments or make equivalent substitutions for some of the technical features therein. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A converter soft start method based on seamless mode switching, characterized in that: The following steps are involved: The converter adopts a hybrid control structure of following the grid / building the grid; When the converter receives the start command, it automatically builds voltage and frequency using the grid control mode to pre-charge the filter capacitor; At the same time, the amplitude and phase angle of the converter output port voltage and the grid voltage are pre-synchronized based on the pre-synchronization strategy; After the capacitor pre-charging and converter pre-synchronization are completed, the converter is switched from the grid-forming control mode to the grid-following control mode to complete the soft start.

2. A converter soft start method based on seamless mode switching according to claim 1, characterized in that: The steps include: sampling the three-phase output voltage u of the converter abc And the three-phase output current i abc , obtain the output voltage u in the synchronous rotating coordinate system d 、u q And the output current i in the synchronous rotating coordinate system d 、i q ; Among them, the subscript d and subscript q represent the d-axis and q-axis of the synchronous rotating coordinate system respectively, d is the active axis and q is the reactive axis.

3. The converter soft start method based on seamless mode switching according to claim 2, characterized in that: The grid-following control structure includes a phase-locked loop, a power outer loop and a current inner loop. The power outer loop adjusts the output power according to the active and reactive command values, and the current inner loop ensures that the actual output current is consistent with the reference current.

4. The converter soft start method based on seamless mode switching according to claim 3, characterized in that: The control modes are shown in equations (1) to (6): i GFL =∫ω1dt (1) Where θ GFL ,ω1,i dref1 / i qref1 、e dref_GFL / e qref_GFL Indicates the phase angle, actual angular frequency, output dq axis reference current value and output modulation wave dq axis voltage value of the converter in the grid-following control state; k pPLL and k iPLL Respectively represent the proportional coefficient and integral coefficient in the phase-locked loop PI coefficient; P ref and Q ref are active power command value and reactive power command value respectively; L f Represents the filter inductance.

5. The converter soft start method based on seamless mode switching according to claim 4, characterized in that: The phase-locked loop is responsible for tracking the phase information of the grid voltage, the power outer loop adjusts the output power according to the active and reactive command values, and the current inner loop ensures that the actual output current is consistent with the reference current.

6. The converter soft start method based on seamless mode switching according to claim 5, characterized in that: The network control structure includes an active loop, a reactive loop, a voltage outer loop and a current inner loop. It simulates the operating characteristics of a synchronous generator by adjusting the amplitude and frequency of the output voltage.

7. The converter soft start method based on seamless mode switching according to claim 6, characterized in that: According to the transformation from the three-phase coordinate system to the stationary two-phase coordinate system, the instantaneous output active power P0 and instantaneous output reactive power Q0 of the new energy converter are calculated, as shown in formula (7): Where u α 、u β is the output voltage in the stationary two-phase coordinate system, i α 、i β Represents the output current in a stationary two-phase coordinate system.

8. The converter soft start method based on seamless mode switching according to claim 7, characterized in that: The control modes are shown in equations (8) to (14): i GFM =∫ω2dt (8) at dref =U ref +K q (Q ref -Q0) (10) Where θ GFM ,ω2,ω0,i dref2 / i qref2 、e dref_GFM / e qref_GFM Indicates the phase angle, actual angular frequency, rated angular frequency, output dq axis reference current value and output modulation wave dq axis voltage value of the converter in the grid control state; u dref U is the d-axis output reference voltage; ref Indicates the rated amplitude of the output voltage; C f is the filter capacitor; D p is the damping adjustment coefficient; J is the inertia coefficient; K q is the reactive-voltage droop coefficient.

9. The converter soft starting method based on seamless mode switching according to claim 8, characterized in that: The pre-synchronization strategy includes calculating the amplitude difference and phase angle difference between the converter output voltage and the grid voltage, and achieving pre-synchronization of the amplitude and phase angle through the corresponding controller.

10. The converter soft start method based on seamless mode switching according to claim 9, characterized in that: Grid voltage u at the grid connection point grid and the converter output voltage u abc After passing through the low-pass filter, the difference between the amplitude and phase angle of the two is calculated using equations (15) to (18), and the corresponding controller is used to realize the pre-synchronization of the converter output port voltage with the grid voltage amplitude and phase angle; ΔU=U grid -U pcs (17) Where u ga 、u gb 、u gc They are the three-phase voltages of the grid connection points abc; u 0a 、u 0b 、u 0c The three-phase voltages abc are output by the converter respectively; U pcs 、U grid are the voltage at the converter output port and the grid voltage amplitude at the grid connection point respectively; ΔU and Δθ are the amplitude difference and phase angle difference between the converter output port voltage and the grid voltage, respectively.

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