Grid-connected and off-grid switching synchronous control method based on double rotating coordinate systems

Abstract

The invention discloses a grid-connected and off-grid switching synchronous control method based on a dual-rotating coordinate system, which is used for grid voltage and phase synchronization during grid connection and off-grid, and comprises the following steps: acquiring three-phase grid voltage, performing Clark transformation, and calculating a phase angle of a grid voltage signal; calculating a phase angle increment and an angular frequency and an output frequency of a power grid fundamental wave; calculating a phase angle step length of the internal rotating coordinate system and a phase angle step length in a grid-connected mode; and calculating a grid-connected phase angle # imgabs0 # of the power grid under a double-coordinate system formed by a power grid phase rotating coordinate system and an internal rotating coordinate system. The three-phase power grid voltage is subjected to Clark transformation, the second-order generalized integral is applied, the voltage # imgabs 1 # and the voltage # imgabs 2 # in a static coordinate are subjected to phase shift by 90 degrees, then the positive sequence and the negative sequence of the voltage are separated, interference of a negative sequence component on phase estimation is avoided, and the phase locking accuracy is improved. And calculating a phase angle # imgabs3 # under a phase rotating coordinate system and an internal rotating coordinate system of the power grid, so that a faster and smaller-lag phase locking method is realized, and the consistency of phase switching and frequency switching during grid connection and grid disconnection of the power grid is ensured.

Description

Technical Field

[0001] The present invention relates to the technical field of microgrid control, and particularly relates to a grid-connected and islanded switching synchronization control method based on a double rotating coordinate system. Background Art

[0002] As the proportion of new energy power generation increases year by year, the requirements for the grid connection of energy storage systems are getting higher and higher. For grid-following energy storage systems, the basis for achieving efficient grid connection is the accurate tracking of the grid frequency and phase. The traditional technical solution is to use the method of a three-phase phase-locked loop (SRF-PLL) based on the d, q synchronous rotating coordinate system, and use a PI controller to make the q-axis output voltage zero to achieve the same output phase as the grid. However, due to the introduction of the PI controller, there is a phase lag, resulting in a large current impact during grid connection.

[0003] In addition, when switching from the grid-connected state to the islanded state, the system switches from the current source mode to the voltage source mode. How to ensure the consistency of the phase and frequency during the two-state switching is also one of the key technologies for grid-connected and islanded switching. Summary of the Invention

[0004] In view of this, it is necessary to provide a grid-connected and islanded switching synchronization control method based on a double rotating coordinate system that can ensure the consistency of phase and frequency.

[0005] A grid-connected and islanded switching synchronization control method based on a double rotating coordinate system, used for synchronizing the grid voltage and phase during grid connection and islanding. The specific steps include: Step 1, collect the three-phase grid voltage and perform Clark transformation to calculate the phase angle of the grid voltage signal; Step 2, calculate the phase angle increment, the angular frequency of the grid fundamental wave, and the output frequency; Step 3, calculate the phase angle step of the output voltage of the converter module in the islanded mode; Step 4, calculate the estimated grid phase angle in the double coordinate system formed by the grid phase rotating coordinate system and the internal rotating coordinate system .

[0006] Preferably, in step 1, the specific steps of collecting the three-phase grid voltage and performing Clark transformation to calculate the phase angle of the grid voltage include: Step 1.1, the digital signal processing module of the converter samples the voltage signals of the three-phase grid to obtain the current grid voltage value, and the sampling period is the carrier period ; Step 1.2, perform Clark transformation on the three-phase grid voltage signals to obtain the orthogonal signals in the stationary coordinate system , , and the conversion formula is as shown in formula (1): (1); Wherein, U a , U b , U c are the three-phase grid voltages; Step 1.3, obtaining two sets of orthogonal signals and , and by using a double second-order generalized integral; Step 1.4, separating the positive-sequence voltage components and of the three-phase voltage in the stationary coordinate system through positive- and negative-sequence extraction, as shown in Equation (2): (2); Wherein, , represents a phase angle difference of 90 degrees in the time domain; Step 1.5, calculating the current instantaneous phase angle of the power grid by using the four-quadrant arctangent function atan2(y, x), as shown in Equation (3): (3).

[0007] Preferably, the specific steps of the second step, calculating the phase angle increment, the angular frequency of the power grid fundamental wave, and the output frequency, include: Step 2.1, calculating the increment of the phase angle of the power grid fundamental wave within adjacent sampling periods, as shown in Equation (4): (4); Wherein, is the current phase angle of the power grid fundamental wave, is the phase angle of the power grid fundamental wave in the previous control period; Step 2.2, calculating the angular frequency of the power grid fundamental wave, as shown in Equation (5): (5); Wherein, , is the carrier frequency of the DSP; Step 2.3, calculating the output frequency of the power grid fundamental wave, as shown in Equation (6): (6).

[0008] Preferably, the specific steps of the third step, calculating the phase angle step of the output voltage of the converter module in the off-grid mode, include: In the off-grid mode, the converter module is transformed from a current source module into a voltage source module, and the phase step of the output voltage depends on the fundamental frequency and the carrier frequency of the DSP , as shown in Equation (7): (7).

[0009] Preferably, in step four, under the double coordinate system formed by the grid phase rotation coordinate system and the internal rotation coordinate system, calculate the grid connection phase angle The specific steps include: Step 4.1, calculate the phase angle in the grid phase rotation coordinate system; Step 4.2, calculate the phase angle in the internal rotation coordinate system; Step 4.3, synchronize the phase angle in the grid phase rotation coordinate system with the phase angle in the internal rotation coordinate system to obtain the estimated grid phase angle .

[0010] Preferably, in step 4.1, the specific steps to calculate the phase angle in the grid phase rotation coordinate system include: Step 4.1.1, estimate the grid phase angle ; Step 4.1.2, calculate the difference between the phase angle of the fundamental wave period and the estimated grid phase angle , and use this difference as the input value of the PI controller; Step 4.1.3, use the increment of the phase angle of the grid fundamental wave in adjacent sampling periods as the phase angle step in the grid connection mode, and limit the amplitude of the phase angle step signal in the grid connection mode through a low-pass filter; Step 4.1.4, in the off-grid mode, use the phase step of the output voltage as the phase angle step signal in the off-grid mode; Step 4.1.5, calculate the sum of the output value of the PI controller and the limited phase angle step signal value in the grid connection mode or the phase step of the output voltage in the off-grid mode, and obtain the phase angle in the grid phase rotation coordinate system after amplitude limiting; Step 4.1.6, feedback the phase angle calculated in step 4.1.5 to the subtracter in step 4.1.2 and the adder in step 4.1.5 to correct the phase angle 。

[0011] Preferably, the PI controller is used to compensate for the phase difference caused by phase accumulation, and the output value of the PI controller is limited between [-30, +30] degrees.

[0012] Preferably, in step 4.2, calculate the phase angle in the internal rotating coordinate system The specific steps include: Step 4.2.1, establish an internal rotating coordinate system. When the power grid is normally powered, the internal rotating coordinate system uses the phase angle step and the coefficient The product of dynamically adjusts the phase step, and after limiting, the phase angle in the internal rotating coordinate system is obtained, and when the phase angle in the power grid phase rotating coordinate system crosses the power frequency period, make the phase angle in the internal rotating coordinate system and the phase angle in the power grid phase rotating coordinate system synchronized; Step 4.2.2, in the off-grid mode, the phase angle adjustment coefficient is 1, and the phase step is used to dynamically adjust the phase step by multiplying it with the coefficient 1.

[0013] Preferably, the determination method for the phase angle in the power grid phase rotating coordinate system in step 4.2.1 to cross the power frequency period is: the phase angle in the power grid phase rotating coordinate system obtained in the current calculation period, and the phase angle in the power grid phase rotating coordinate system in the previous calculation period, when, it is considered that the phase angle has crossed the power frequency period.

[0014] Preferably, in step 4.3, synchronize the phase angle in the power grid phase rotating coordinate system with the phase angle in the internal rotating coordinate system to obtain the estimated power grid phase angle The specific steps include: Step 4.3.1, when the power grid is detected, through measure 1 and measure 2, make the phase angle in the power grid phase rotating coordinate system and the phase angle in the internal rotating coordinate system synchronized, as the estimated power grid phase angle ; Among them, measure 1, when the phase angle in the power grid phase rotating coordinate system crosses the power frequency period, the phase angle in the internal rotating coordinate system and the phase angle in the power grid phase rotating coordinate system are synchronized; measure 2, in the internal rotating coordinate system, the phase step By a coefficient associated with the grid output frequency to dynamically adjust the phase step size; Step 4.3.2, when the grid loss is detected, the input phase angle step sizes of both the grid phase rotation coordinate system and the internal rotation coordinate system are , and the phase angle under the internal rotation coordinate system is synchronized with the phase angle under the grid phase rotation coordinate system and used as the estimated grid phase angle

[0015] In the above grid-connected and off-grid switching synchronization control method based on the double rotation coordinate system, by performing Clark transformation on the three-phase grid voltage and using the second-order generalized integral, the voltage and in the stationary coordinate system are phase-shifted by 90 degrees, and then the positive and negative sequences of the voltage are separated, avoiding the interference of the negative sequence component on the phase estimation and improving the accuracy of phase locking. Calculate the phase angles under the grid phase rotation coordinate system and the internal rotation coordinate system, and implement a phase-locking method with faster speed and smaller lag to ensure the consistency of the switching phase and frequency when the grid is connected and disconnected. The algorithm of the present invention is simple, easy to implement, low in cost, and convenient for popularization. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1 is a flowchart of the grid-connected and off-grid switching synchronization control method based on the double rotation coordinate system according to an embodiment of the present invention.

[0017] Figure 2 is a signal processing structure block diagram of Step 1 in the grid-connected and off-grid switching synchronization control method based on the double rotation coordinate system according to an embodiment of the present invention.

[0018] Figure 3 is a structure block diagram of the SOGI module in Step 1 of the grid-connected and off-grid switching synchronization control method based on the double rotation coordinate system according to an embodiment of the present invention.

[0019] Figure 4 is a structure block diagram of the grid phase angle estimation in the grid phase rotation coordinate system of the grid-connected and off-grid switching synchronization control method based on the double rotation coordinate system according to an embodiment of the present invention.

[0020] Figure 5 is a structure block diagram of the grid phase angle estimation in the internal synchronous rotation coordinate system of the grid-connected and off-grid switching synchronization control method based on the double rotation coordinate system according to an embodiment of the present invention.

[0021] Figure 6 is an application structure block diagram of an embodiment of the present invention.

[0022] Figure 7 is an oscilloscope display graph 1 when the grid is connected and disconnected according to an embodiment of the present invention.

[0023] Figure 8 It is the second oscilloscope display pattern when the grid-connected inverter switches to off-grid in the embodiment of the present invention.

[0024] Figure 9 It is the third oscilloscope display pattern when the grid-connected inverter switches to off-grid in the embodiment of the present invention. Detailed implementation manners

[0025] Taking the grid-connected and off-grid switching synchronization control method based on the double rotating coordinate system as an example in this embodiment, the present invention will be described in detail below in combination with specific embodiments and drawings.

[0026] Please refer to Figure 1 , which shows a grid-connected and off-grid switching synchronization control method provided by an embodiment of the present invention. The specific steps are as follows: Step 1: The DSP of the converter module samples the three-phase grid voltage to obtain the magnitude of the current grid voltage value, and the sampling period is the carrier period .

[0027] Step 2: Use the Clark transformation to convert the three-phase voltage signals into orthogonal signals in the stationary coordinate system , , and the conversion formula is: (1); Among them, U a , U b , U c are the three-phase voltages. The specific operation is as in Figure 2 step 2.

[0028] Step 3: Use the double second-order generalized integral to obtain two sets of orthogonal signals and , and . The structure of the SOGI module is as shown in Figure 2 . The whole operation is as in Figure 2 step 3.

[0029] Specifically, the double second-order generalized integrator (DSOGI) generates orthogonal signals (α-β coordinate system) through two parallel second-order generalized integrators (SOGI) to achieve a 90° phase shift of the fundamental component and harmonic suppression. Its transfer function exhibits a band-pass filtering characteristic, with the center frequency being the grid fundamental frequency, which can effectively filter out non-fundamental components. The structure diagram of the SOGI module is as shown in Figure 3 .

[0030] Step 4: Then, through the positive sequence extraction module, the positive sequence voltage components of the three-phase voltages in the stationary coordinate system are separated and , such as Figure 2 step 4 in

[0031] (2); wherein, , represents a phase angle difference of 90 degrees in the time domain.

[0032] Step 5, use the four-quadrant arctangent function to calculate the current phase angle of the power grid , such as Figure 2 step 5 in

[0033] Specifically, when (x, y) falls in the first quadrant, the output range of atan2(y, x) is ; when (x, y) falls in the second quadrant, the output range of atan2(y, x) is ; when (x, y) falls in the third quadrant, the output range of atan2(y, x) is ; when (x, y) falls in the fourth quadrant, the output range of atan2(y, x) is .

[0034] That is, (3).

[0035] Step 6, calculate the angular frequency. From the phase angle obtained in Step 5, based on the phase angle difference between the previous and the current times and the sampling time interval, the angular frequency can be calculated.

[0036] (4); (5); (6); wherein, is the carrier frequency.

[0037] Step 7, calculate the output frequency of the fundamental wave of the power grid .

[0038] (7).

[0039] Step 8, calculate the step size of the internal rotating coordinate system .

[0040] Since in the off-grid mode, the module no longer follows the power grid frequency, that is, the output frequency in the off-grid mode is fixed and does not need to follow the dynamic changes of the large power grid. So, in the off-grid mode, the converter module is converted from the current source mode to the voltage source mode. Therefore, the phase angle step size of the output voltage It is determined by the fundamental wave frequency and the carrier frequency of the DSP and the phase angle step in the off-grid mode is a fixed value: (8); The phase angle step in the grid-connected mode is : (9).

[0041] In steps 1 to 8, all the basic values required for calculation in the double coordinate system are obtained. According to these calculation results, the phase-locked of the present invention is divided into two parts. One part is the grid phase rotating coordinate system, and its structure is as Figure 4 shown, and the other part is the internal rotating coordinate system, and its structure is as Figure 5 shown.

[0042] is the estimated value of the grid phase, which is subtracted from the reference phase angle , and the deviation is input into the PI controller. Then, the output of the PI controller is limited to obtain the adjusted result of the phase angle, and then added to the increment of the reference phase angle. The estimated phase angle is limited to within the range of , and finally is obtained.

[0043] The PI controller is a proportional-integral controller. The control of the PI loop in the block diagram is to compensate for the phase difference caused by phase accumulation, and its output is limited between [-30, 30] degrees.

[0044] In the grid-connected state, the internal total rotating coordinate system detects the output (grid) frequency in real time and divides it by the given frequency. The ratio of the two is used as the adjustment coefficient of the internal phase angle; in the off-grid mode, this coefficient is 1.

[0045] is the internal phase angle, and its magnitude is limited to within the range of , is the grid fundamental wave frequency, such as 50 Hz.

[0046] As described above, the double coordinate system phase angle synchronization mechanism is: in is the internal rotating coordinate system phase angle. When it is detected that the grid phase crosses the power frequency period, is synchronized with so that in the state without the grid, the phase when switching to the off-grid mode is the same as the phase when the grid exists.

[0047] The method for judging the crossing of the power frequency period is that the estimated phase angle , and the phase angle estimated in the previous calculation period When it is the case, it is considered that the phase angle has crossed the power frequency synchronization.

[0048] The grid connection and disconnection signal is a dry contact signal provided by the Static Transfer Switch (STS). When the STS detects the loss of the power grid, the STS cuts off the three-phase AC input, and at the same time sends out the grid connection and disconnection dry contact signal. When the DSP receives this signal, it switches the phase estimation to the off-grid mode.

[0049] When the STS detects the restoration of the power grid, the STS sends the phase of the power grid and the amplitude of the power grid voltage to the PCS module, and the PCS pre-starts the grid connection control so that the phase and amplitude of the PCS meet the following two conditions: (1) Adjust to adjust the phase angle step When the estimated phase angle The difference from the phase angle detected by the STS is between -0.02777 and 0.02777 rad; (2) Adjust the amplitude of the inverter voltage so that the amplitudes of the three-phase inverter voltages are all less than 50 V from the amplitude of the power grid voltage; After meeting conditions (1) and (2), the PCS module can send a grid connection command to the STS module, and at the same time the PCS module can switch to the grid connection mode.

[0050] As Figure 6 The energy storage system shown, in which the output of the photovoltaic system is connected to the maximum power point tracking module MPPT, the output of the MPPT is DC-coupled with the battery to provide DC voltage input for the PCS module. The PCS system consists of an AC / DC bidirectional converter, an AC contactor, and a static transfer switch (STS). The output of the PCS is connected to the load, and the STS is connected to the power grid.

[0051] The waveform diagrams during grid connection and disconnection are as shown in Figure 7 , Figure 8 and Figure 9 shown. Figure 7 Shown are the various operating stages of the module, including from shutdown (section A) to soft start during grid connection startup (section B), then to stable grid connection operation (section C), then followed by the process of switching from grid connection to off-grid (section D), and finally the off-grid stable operation stage (section E). Figure 8 and Figure 9 Are enlarged views of section D. Figure 8 In it, the power grid voltages (U a , U b , U c ) do not overlap with the inverter voltages (U inva , U invb , U invc ). Figure 9Move the grid voltage and the inverter voltage to the same horizontal position. From Figure 8 it can be seen that the time from grid-connected to off-grid until the voltage is stable is about 4 ms; from Figure 9 it can be seen that after switching from the grid-connected mode to the off-grid mode, the amplitudes of the three-phase voltages do not change.

[0052] In the above synchronous control method for grid-connected and off-grid switching based on the double rotating coordinate system, through the Clark transformation of the three-phase grid voltage and the use of the second-order generalized integral, the voltage in the stationary coordinate system and are phase-shifted by 90 degrees, and then the positive and negative sequences of the voltage are separated, avoiding the interference of the negative sequence component on the phase estimation and improving the accuracy of phase locking. Calculate the phase angles in the grid phase rotating coordinate system and the internal rotating coordinate system to implement a faster and less lagging phase locking method, ensuring the consistency of the switching phase and frequency when the grid is connected and disconnected. The algorithm of the present invention is simple, easy to implement, low in cost, and convenient for popularization.

[0053] It should be noted that the above are only the preferred embodiments of the present invention and are not used to limit the present invention. For those skilled in the art, the present invention can have various modifications and changes. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. A method for synchronous control of grid-connected and off-grid switching based on a dual rotating coordinate system, which is used for grid voltage and phase synchronization when connected to the grid and off-grid, characterized in that: The specific steps include: Step 1: collect the three-phase grid voltage and perform Clark transformation to calculate the phase angle of the grid voltage signal; Step 2, calculating the phase angle increment, the angular frequency of the grid fundamental wave and the output frequency; Step 3, calculating the phase angle step of the internal rotating coordinate system and the phase angle step in the grid-connected mode; Step 4: Calculate the grid connection phase angle in the dual coordinate system formed by the grid phase rotation coordinate system and the internal rotation coordinate system. .

2. The method for synchronous control of on-grid and off-grid switching based on a dual rotating coordinate system according to claim 1, characterized in that: The specific steps of collecting the three-phase grid voltage and performing Clark transformation to calculate the phase angle of the grid voltage in step 1 include: Step 1.1: The digital signal processing module of the converter samples the voltage signal of the three-phase grid to obtain the voltage value of the current grid. The sampling period is the carrier period. ; Step 1.2: Perform Clark transformation on the three-phase grid voltage signal to obtain mutually orthogonal signals in the stationary coordinate system. , , the conversion formula is shown in formula (1): （1）； Among them, U a , U b , U c is the three-phase grid voltage; Step 1.3, use the biquadric generalized integral to obtain two sets of mutually orthogonal signals and , and ; Step 1.4: Separate the positive sequence voltage of the three-phase voltage in the stationary coordinate system by extracting the positive and negative sequence. and , as shown in formula (2): （2）； in, , Indicates a 90-degree phase angle difference in the time domain; Step 1.5, use the four-quadrant inverse tangent function atan2(y, x) to calculate the current instantaneous phase angle of the power grid , as shown in formula (3): （3）。 3. The method for synchronous control of on-grid and off-grid switching based on a dual rotating coordinate system according to claim 2, characterized in that: The specific steps of step 2, calculating the phase angle increment, the angular frequency of the grid fundamental wave and the output frequency, include: Step 2.1, calculate the increment of the phase angle of the grid fundamental wave in adjacent sampling periods , as shown in formula (4): （4）； in, is the current phase angle of the grid fundamental wave, is the phase angle of the previous control cycle of the grid fundamental wave; Step 2.2, calculate the angular frequency of the grid fundamental wave , as shown in formula (5): （5）； in, , is the carrier frequency of DSP; Step 2.3, calculate the output frequency of the grid fundamental wave , as shown in formula (6): （6）。 4. The method for synchronous control of on-grid and off-grid switching based on a dual rotating coordinate system according to claim 3, characterized in that: The specific steps of step 3, calculating the phase angle step of the output voltage of the converter module in the off-grid mode, include: In off-grid mode, the converter module is converted from a current source module to a voltage source module, and the phase step of the output voltage is Depends on fundamental frequency and the carrier frequency of the DSP , as shown in formula (7): （7）。 5. The method for synchronous control of on-grid and off-grid switching based on a dual rotating coordinate system according to claim 4, characterized in that: Step 4: Calculate the grid connection phase angle in the dual coordinate system formed by the grid phase rotation coordinate system and the internal rotation coordinate system. The specific steps include: Step 4.1: Calculate the phase angle in the grid phase rotation coordinate system ; Step 4.2, calculate the phase angle in the internal rotating coordinate system ; Step 4.3: Rotate the grid phase by the phase angle in the coordinate system The phase angle with the internal rotating coordinate system Synchronize and get the estimated grid phase angle .

6. The method for synchronous control of on-grid and off-grid switching based on a dual rotating coordinate system according to claim 5, characterized in that: Step 4.1: Calculate the phase angle in the grid phase rotation coordinate system The specific steps include: Step 4.1.1, estimate the grid phase angle ; Step 4.1.2, calculate the phase angle of the fundamental period With the estimated grid phase angle The difference is used as the input value of the PI controller; Step 4.1.3, the increment of the phase angle of the grid fundamental wave in adjacent sampling periods As the phase angle step length in the grid-connected mode, the phase angle step length signal in the grid-connected mode is limited by a low-pass filter; Step 4.1.4, in off-grid mode, the phase step of the output voltage As the phase angle step signal in off-grid mode; Step 4.1.5, calculate the output value of the PI controller and the phase angle step signal value after limiting in the grid-connected mode or the phase step of the output voltage in the off-grid mode The phase angle in the grid phase rotation coordinate system is obtained by limiting the amplitude ; Step 4.1.6: Substitute the phase angle calculated in step 4.1.5 into Feedback to the subtractor in step 4.1.2 and the adder in step 4.1.5 to correct the phase angle in the grid phase rotation coordinate system .

7. The method for synchronous control of on-grid and off-grid switching based on a dual rotating coordinate system according to claim 6, characterized in that: The PI controller is used to compensate for the phase difference caused by phase accumulation, and the output value of the PI controller is limited to between [-30, +30] degrees.

8. The method for synchronous control of on-grid and off-grid switching based on a dual rotating coordinate system according to claim 6, characterized in that: Step 4.2, calculate the phase angle in the internal rotating coordinate system The specific steps include: Step 4.2.1, establish the internal rotating coordinate system. When the power grid is powered normally, the internal rotating coordinate system uses the phase angle step With coefficient The product of dynamically adjusts the phase step size, and the phase angle in the internal rotating coordinate system is obtained after limiting. , and the phase angle in the grid phase rotation coordinate system When crossing the power frequency cycle, the phase angle in the internal rotating coordinate system Phase angle in the coordinate system rotated with the grid phase synchronous; Step 4.2.2, in off-grid mode, the phase angle adjustment coefficient is 1, and the phase angle step length is used The product with the coefficient 1 dynamically adjusts the phase step size.

9. The method for synchronous control of on-grid and off-grid switching based on a dual rotating coordinate system according to claim 8, characterized in that: The phase angle of the grid phase rotation coordinate system in step 4.2.1 The method for judging the cross-frequency cycle is as follows: the phase angle in the grid phase rotation coordinate system obtained in the current calculation cycle , and the phase angle of the power grid phase rotation coordinate system in the previous calculation cycle is When , it is considered that the phase angle crosses the power frequency synchronization.

10. The method for synchronous control of on-grid and off-grid switching based on a dual rotating coordinate system according to claim 8, characterized in that: Step 4.3: Rotate the grid phase by the phase angle in the coordinate system The phase angle with the internal rotating coordinate system Synchronize and get the estimated grid phase angle The specific steps include: Step 4.3.1: When the existence of the power grid is detected, measures 1 and 2 are used to rotate the phase angle of the power grid in the coordinate system. The phase angle with the internal rotating coordinate system Synchronization, as the estimated grid phase angle ; Among them, measure 1, the phase angle in the grid phase rotation coordinate system When crossing the power frequency cycle, the phase angle in the internal rotating coordinate system Phase angle in the coordinate system rotated with the grid phase Synchronization; Measure 2, phase angle step in internal rotating coordinate system Pass coefficient Associated with the grid output frequency to dynamically adjust the phase step size; Step 4.3.2: When the power grid is lost, the input phase angle step of the power grid phase rotation coordinate system and the internal rotation coordinate system is , the phase angle in the internal rotating coordinate system Phase angle in the coordinate system rotated with the grid phase After synchronization, the estimated grid phase angle .

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