Grid-connected inverter open-loop synchronization system and control method suitable for non-ideal power grid

Abstract

The application discloses a grid-connected inverter open-loop synchronization system and a control method suitable for a non-ideal power grid, performs positive and negative sequence component decomposition on dq axis components of a point of common coupling voltage, extracts positive sequence components, performs Moving Average Filter (MAF) filtering and phase compensation on the positive sequence components, obtains a system synchronization phase angle correction amount, controls switching tubes of a grid-connected inverter of the system by using an open-loop synchronization technology, eliminates high-order harmonic components in voltage alpha-beta signals, improves synchronization effect, and improves power supply quality of the system. The application has a simple structure, is easy to implement, is beneficial to eliminating high-order harmonic components of alpha-beta components in voltage, improving synchronization effect, and improving power quality and stability of the grid-connected inverter system.

Description

Technical Field

[0001] This invention relates to an open-loop synchronization system and control method for grid-connected inverters suitable for non-ideal power grids, belonging to the field of power electronic inverter technology. Background Technology

[0002] Large-scale renewable energy DC transmission systems exhibit "dual high" characteristics (high proportion of renewable energy and high proportion of power electronic equipment), posing challenges to the stability of existing power grid systems. Meanwhile, distributed energy resources are developing rapidly, and grid-connected inverters, as interface devices connecting renewable energy sources and the power grid, are receiving widespread attention.

[0003] To maximize system output efficiency, inverters typically employ grid-connected control strategies. For grid-connected inverter systems, the inherent complexity of traditional phase-locked loop (PLL) synchronization technology makes it difficult to match existing nonlinear analysis methods. To address the stability issues of grid-connected inverter operation, it is necessary to simplify the synchronization loop structure and propose system-level open-loop synchronization technology. However, currently, only a limited number of studies focus on open-loop synchronization technology for grid-connected inverter systems. While some literature discusses improvements in PLL performance under frequency offset conditions, it still lacks discussion on synchronization techniques for non-ideal grids such as three-phase voltage imbalance and high-order harmonic injection.

[0004] Non-ideal power grids are one of the main types of faults in grid-connected inverter systems, and their impact on power quality and stability is also very serious. Therefore, it is necessary to further study the system-level synchronization technology of grid-connected inverters under non-ideal power grid conditions. Summary of the Invention

[0005] Objective: In order to overcome the shortcomings of the existing technology, the present invention provides an open-loop synchronization system and control method for grid-connected inverters suitable for non-ideal power grids.

[0006] Technical solution: To solve the above technical problems, the technical solution adopted by the present invention is as follows:

[0007] Firstly, an open-loop synchronization system for grid-connected inverters suitable for non-ideal power grids includes:

[0008] The coordinate transformation module is used to transform the three-phase signal into the corresponding αβ component, convert the αβ component into the dq component, and convert the dq component into the three-phase signal.

[0009] The positive and negative order component decomposition module is used to first perform a rotation transformation on the αβ components, and then perform positive and negative order decomposition to obtain the positive order components of the dq axis.

[0010] The phase angle compensation and calculation module is used to transform the positive sequence components of the dq axis, filter them through MAF, generate the initial value of the open-loop synchronization phase angle of the system, and compensate for it to obtain the open-loop phase angle correction value.

[0011] The power PI control module is used to perform PI control on the active and reactive power generated by the inverter to obtain the dq reference value of the grid current.

[0012] The current dq decoupling control module is used to perform PI control on the grid current based on the dq reference value of the grid current and the open-loop phase angle correction value, and then perform coordinate transformation and pulse width modulation to obtain the drive signal of the grid-connected inverter switching transistor.

[0013] As a preferred embodiment, it also includes: a grid-connected inverter, a filter circuit, and an AC power grid. The DC side of the grid-connected inverter is connected to a DC voltage source, and the AC side is connected to the AC power grid via the filter circuit.

[0014] Secondly, a control method for an open-loop synchronization system of a grid-connected inverter suitable for a non-ideal power grid includes the following steps:

[0015] Step 1: Collect the voltage and current waveforms at the common coupling point of the open-loop synchronous system of the grid-connected inverter. If the grid condition is determined to be non-ideal, give the initial value of the phase angle.

[0016] Step 2: Based on the voltage and current waveforms and initial phase angle at the common coupling point, calculate the positive d-sequence component and positive q-sequence component of the voltage.

[0017] Step 3: Perform coordinate transformation and MAF filtering on the positive sequence components of the voltage along the d-axis and q-axis to obtain the filtered voltage α and β components.

[0018] Step 4: Calculate the initial value of the open-loop synchronization phase angle based on the filtered voltage α and β components, compensate for the initial value of the open-loop synchronization phase angle, and obtain the corrected value of the open-loop synchronization phase angle.

[0019] Step 5: Calculate the inverter system power based on the voltage and current waveforms at the common coupling point. Based on the inverter system power, current waveforms, and open-loop synchronous phase angle correction value, use PI control, coordinate transformation, and pulse width modulation to obtain the drive signal for the grid-connected inverter switching transistors.

[0020] As a preferred option, it also includes:

[0021] Step 6: Collect the voltage and current waveforms at the common coupling point of the open-loop synchronization system of the grid-connected inverter. If it is determined that the grid is not ideal, perform coordinate transformation on the collected voltage signal to obtain the voltage α component and β component. Calculate the synchronization phase angle using the voltage α component and β component.

[0022] Step 7: Calculate the inverter system power based on the voltage and current waveforms at the common coupling point. Based on the inverter system power, current waveforms, and synchronization phase angle, use PI control, coordinate transformation, and pulse width modulation to obtain the drive signal for the grid-connected inverter switching transistors.

[0023] As a preferred embodiment, step 2 specifically includes:

[0024] Step 201: Based on the collected voltage signal V at the common coupling point PCC The three-phase voltage signal u is obtained. A u B u C .

[0025] Step 202: Based on the three-phase voltage signal u A u B u C Calculate the α component u of the voltage. α The β component of voltage u β The calculation formula is as follows:

[0026]

[0027] In the formula, u A u B u C The voltage signal V at the common coupling point was acquired. PCC The voltage signals of phase A, phase B, and phase C, u α The α component and u of the voltage after coordinate transformation β This represents the β component of the voltage after coordinate transformation.

[0028] Step 203: Based on the α component u of the voltage α The β component of voltage u β Calculate the voltage α component qu after phase angle shift. α The voltage β component qu after phase angle shift β The calculation formula is as follows:

[0029]

[0030] In the formula, qu α qu β These are the voltage α and β components after phase angle shift, respectively. In the formula, j represents the imaginary unit, and π is the value of pi. s G(s) represents the time corresponding to the phase offset, s represents the frequency domain unit, G(s) represents the phase offset transfer function, and e represents the natural constant.

[0031] Step 204: Based on the voltage α component qu after phase angle shift αThe voltage β component qu after phase angle shift β Calculate the positive sequence component of voltage α Voltage β positive sequence component According to the positive sequence component of voltage α Voltage β positive sequence component Calculate the positive sequence component of the voltage d-axis Positive sequence component of voltage q-axis

[0032] Wherein, the positive sequence component of voltage α Voltage β positive sequence component The calculation formula is as follows:

[0033]

[0034] Positive sequence component of voltage d-axis Positive sequence component of voltage q-axis The calculation formula is as follows:

[0035]

[0036] In the formula, This is the initial value of the phase angle.

[0037] As the preferred solution f is the base frequency of the power grid.

[0038] As a preferred embodiment, step 3 specifically includes:

[0039] Step 301: For the positive sequence component of the voltage d-axis Positive sequence component of voltage q-axis Perform coordinate transformation to obtain the voltage α component u' after coordinate transformation. α Voltage β component u' after coordinate transformation β The calculation formula is as follows:

[0040]

[0041] In the formula, u' α 、u' β These are the voltage α and β components after coordinate transformation, respectively. Given the phase angle.

[0042] Step 302: Transform the voltage α component u' α Voltage β component u' after coordinate transformation β MAF filtering is performed to obtain the filtered voltage α component. Filtered voltage β component The calculation formula is as follows:

[0043]

[0044] In the formula, G MAF (s) is the MAF transfer function. These represent the α and β components of the filtered voltage, respectively, where e represents the natural constant, s represents the frequency domain unit, and T... w is the filter time constant.

[0045] As a preferred embodiment, step 4 specifically includes:

[0046] Step 401: Based on the filtered voltage α component Filtered voltage β component The initial value of the open-loop synchronization phase angle is calculated using the following formula:

[0047]

[0048] In the formula, theta_OLS is the initial value of the open-loop synchronous phase angle.

[0049] Step 402: Based on the initial value of the open-loop synchronization phase angle, the switching synchronization phase angle correction value is calculated using the MAF phase frequency characteristics. The calculation formula is as follows:

[0050]

[0051] In the formula, theta_OLS' is the open-loop synchronous phase angle correction value, Δθ is the phase angle correction amount, π is pi, f is the grid fundamental frequency, and T w π is the filter time constant, and π is the mathematical constant pi.

[0052] As a preferred embodiment, step 5 specifically includes:

[0053] Step 501: Based on the collected voltage and current waveforms at the common coupling point, calculate the active and reactive power of the inverter system. The calculation formula is as follows:

[0054] P = u A (t)i A (t)+u B (t)i B (t)+u C (t)i C (t)

[0055]

[0056] In the formula, P is the active power generated by the inverter system, Q is the reactive power generated by the inverter system, and u A u B u C The measured three-phase voltage signals are i, ... A i Bi C The three-phase current signals were collected separately, with t as the unit of time. f is taken as the base frequency of the power grid.

[0057] Step 502: Compare P and Q with the power setting value P respectively. set Q set The difference is calculated, and after passing the difference through power PI control and limiting, the reference value I of the current d-axis component is obtained. dref Reference value I of the q-axis component of the current qref The expression for power PI control is as follows:

[0058]

[0059] In the formula, G PQ (s) represents power PI control, k p1 k i1 These are the proportional coefficient and integral coefficient, respectively, and s represents the frequency domain unit.

[0060] Step 503: Based on the acquired three-phase current signal i A i B i C Calculate the d-axis current signal i d q-axis current signal i q The calculation formula is as follows:

[0061]

[0062] In the formula, Let T be the initial value of the phase angle, and T be the matrix transpose.

[0063] Step 504: Set the reference value I of the d-axis component of the current. dref Reference value I of the q-axis component of the current qref respectively with d-axis current signal i d q-axis current signal i q The difference is calculated, and then passed through a current PI controller and a gain circuit to obtain the d-axis reference value U of the inverter port voltage. d q-axis reference value U q .

[0064] The expression for the current PI controller is as follows:

[0065]

[0066] In the formula, G i (s) is a current PI controller, k p2 k i2 These are the proportional coefficient and the integral coefficient, respectively, and s represents the frequency domain unit.

[0067] Step 505: Set the inverter port voltage d-axis reference value U d q-axis reference value U q By performing a coordinate transformation, the reference value U of the port voltage α component is obtained. α β component reference value U β According to the reference value U of the port voltage α component α β component reference value U β Pulse width modulation is used to obtain the drive signal for the switching transistors of the grid-connected inverter.

[0068] Among them, the reference value U of the port voltage α component α β component reference value U β The calculation formula is as follows:

[0069]

[0070] In the formula, θ is taken as the open-loop synchronous phase angle correction value theta_OLS'.

[0071] As a preferred embodiment, steps 6 and 7 specifically include:

[0072] Step 601: Acquire the voltage and current waveforms at the common coupling point of the grid-connected inverter's open-loop synchronous system. If it is determined that the grid situation is not ideal, then based on the acquired voltage signal V at the common coupling point... PCC The three-phase voltage signal u is obtained. A u B u C .

[0073] Step 602: Based on the three-phase voltage signal u A u B u C Calculate the α component u of the voltage. α The β component of voltage u β The calculation formula is as follows:

[0074]

[0075] In the formula, u A u B u C The voltage signal V at the common coupling point was acquired. PCC The voltage signals of phase A, phase B, and phase C, u α The α component and u of the voltage after coordinate transformation β This represents the β component of the voltage after coordinate transformation.

[0076] Step 602: The α component of voltage u α The β component of voltage u β The synchronization phase angle is calculated using the following formula:

[0077]

[0078] In the formula, theta is the synchronization phase angle.

[0079] Step 701: Based on the collected voltage and current waveforms at the common coupling point, calculate the active and reactive power of the inverter system. The calculation formula is as follows:

[0080] P = u A (t)i A (t)+u B (t)i B (t)+u C (t)i C (t)

[0081]

[0082] In the formula, P is the active power generated by the inverter system, Q is the reactive power generated by the inverter system, and u A u B u C The measured three-phase voltage signals are i, ... A i B i C The three-phase current signals were collected separately, with t as the unit of time. f is taken as the base frequency of the power grid.

[0083] Step 702: Compare P and Q with the power setting value P respectively. set Q set The difference is calculated, and after passing the difference through power PI control and limiting, the reference value I of the current d-axis component is obtained. dref Reference value I of the q-axis component of the current qref The expression for power PI control is as follows:

[0084]

[0085] In the formula, G PQ (s) represents power PI control, k p1 k i1 These are the proportional coefficient and integral coefficient, respectively, and s represents the frequency domain unit.

[0086] Step 703: Based on the acquired three-phase current signal i A i B i C Calculate the d-axis current signal i d q-axis current signal i q The calculation formula is as follows:

[0087]

[0088] In the formula, Let T be the initial value of the phase angle, and T be the matrix transpose.

[0089] Step 704: Set the reference value I of the d-axis component of the current. dref Reference value I of the q-axis component of the current qref respectively with d-axis current signal i d q-axis current signal i q The difference is calculated, and then passed through a current PI controller and a gain circuit to obtain the d-axis reference value U of the inverter port voltage. d q-axis reference value U q .

[0090] The expression for the current PI controller is as follows:

[0091]

[0092] In the formula, G i (s) is a current PI controller, k p2 k i2 These are the proportional coefficient and the integral coefficient, respectively, and s represents the frequency domain unit.

[0093] Step 705: Set the inverter port voltage d-axis reference value U d q-axis reference value U q By performing a coordinate transformation, the reference value U of the port voltage α component is obtained. α β component reference value U β According to the reference value U of the port voltage α component α β component reference value U β Pulse width modulation is used to obtain the drive signal for the switching transistors of the grid-connected inverter.

[0094] Among them, the reference value U of the port voltage α component α β component reference value U β The calculation formula is as follows:

[0095]

[0096] In the formula, θ is taken as the synchronous phase angle theta.

[0097] Beneficial effects: The open-loop synchronization system and control method for grid-connected inverters applicable to non-ideal power grids provided by this invention specifically involves decomposing the dq-axis components of the common coupling point voltage into positive and negative sequence components, extracting the positive sequence component, filtering it with a moving average filter (MAF) and performing phase compensation to obtain the system synchronization phase angle correction, and using open-loop synchronization technology to control the switching transistors of the grid-connected inverter, eliminating high-order harmonic components in the voltage αβ signal, improving the synchronization effect, and enhancing the power supply quality of the system.

[0098] This invention provides an open-loop synchronization control method for grid-connected inverters based on positive and negative sequence component decomposition. The method has a simple structure, is easy to implement, and helps to eliminate high-order harmonics of the αβ components in the voltage, improve the synchronization effect, and enhance the power quality and stability of the grid-connected inverter system. Attached Figure Description

[0099] Figure 1 This is a schematic diagram of the framework of an open-loop synchronization system with a grid-connected inverter.

[0100] Figure 2 This is a flowchart illustrating the control method of the open-loop synchronization system of a grid-connected inverter applicable to a non-ideal power grid according to the present invention.

[0101] Figure 3 This is the control block diagram for the positive and negative order component decomposition of dq.

[0102] Figure 4 It is a block diagram of coordinate transformation and MAF structure control.

[0103] Figure 5 This is a block diagram of a power PI control.

[0104] Figure 6 This is a block diagram of the current PI decoupling control.

[0105] Figure 7 This is a simulation result diagram of the control method of the present invention under the condition of power grid voltage imbalance. Figure 7 (a) is a schematic diagram of the collected voltage and current at the common coupling point. Figure 7 (b) is a schematic diagram of the common coupling point voltage and the system synchronization phase angle after synchronization.

[0106] Figure 8 This is a simulation result diagram of the control method of the present invention under high-order harmonic injection. Figure 8 (a) is a schematic diagram of the collected voltage and current at the common coupling point. Figure 8 (b) is a schematic diagram of the common coupling point voltage and the system synchronization phase angle after synchronization. Detailed Implementation

[0107] The technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the protection scope of the present invention.

[0108] The present invention will be further described below with reference to specific embodiments.

[0109] Example 1:

[0110] This embodiment describes an open-loop synchronization system for grid-connected inverters suitable for non-ideal power grids, such as... Figure 1 As shown, it includes a grid-connected inverter, a filter circuit, an AC power grid, a coordinate transformation module, a positive and negative sequence component decomposition module, a phase angle compensation and calculation module, a power calculation module, a power PI control module, and a current dq decoupling control module.

[0111] The grid-connected inverter is composed of three-level IGBTs (insulated gate bipolar transistors). The DC side is connected to a DC voltage source, and the AC side is connected to the AC grid through a filter circuit. The AC grid voltage is represented by a three-phase AC voltage source, and the power supply neutral point is grounded.

[0112] The coordinate transformation module is used to transform three-phase signals into corresponding αβ components, convert αβ components into dq components, and convert dq components into three-phase signals.

[0113] The positive and negative order component decomposition module is used to first perform a rotation transformation on the αβ components, and then perform positive and negative order decomposition to obtain the positive order components of the dq axis;

[0114] The phase angle compensation and calculation module is used to transform the positive sequence components of the dq axis into coordinates, filter them through MAF, generate the initial value of the open-loop synchronization phase angle of the system, and compensate for it to obtain the open-loop phase angle correction value.

[0115] The power PI control module is used to perform PI control on the active and reactive power generated by the inverter to obtain the dq reference value of the grid current.

[0116] The current dq decoupling control module is used to perform PI control on the grid current based on the dq reference value of the grid current and the open-loop phase angle correction value, and then perform coordinate transformation and pulse width modulation to obtain the drive signal of the grid-connected inverter switching transistor.

[0117] The αβ components represent the values ​​on the α-axis and β-axis in a two-phase stationary coordinate system, while dq represents the values ​​on the d-axis (DC component) and q-axis (AC component) in a two-phase rotating coordinate system.

[0118] Example 2:

[0119] This embodiment introduces a control method for an open-loop synchronous system of a grid-connected inverter suitable for non-ideal power grids. It acquires the voltage and current signals at the point of common coupling, calculates the power output of the grid-connected inverter, and then obtains the open-loop synchronous phase angle of the system through a coordinate transformation module and a positive and negative sequence component decomposition module. Finally, it generates switching transistor drive signals through a power and current PI control module and SVPWM (Space Vector Pulse Width Modulation) technology to complete the control of the grid-connected inverter system. Figure 2 As shown, it includes the following steps:

[0120] Step 1: Collect the voltage and current waveforms at the common coupling point of the open-loop synchronous system of the grid-connected inverter, and determine whether it is a non-ideal grid situation. If so, proceed to Step 2; if not, perform coordinate transformation on the collected voltage signal, calculate the synchronous phase angle through the voltage αβ components, and proceed to Step 5.

[0121] Step 2: Given the initial phase angle theta=∫(2*π*50)dt, calculate the positive and negative sequence components of the synchronous phase angle to the voltage dq coordinate components, and extract the positive sequence components.

[0122] Step 3: Perform coordinate transformation on the positive sequence components, and then apply MAF filtering;

[0123] Step 4: Calculate the initial value of the open-loop phase angle using the MAF output, and then compensate for it to obtain the corrected value of the open-loop synchronization phase angle;

[0124] Step 5: Calculate the power output of the inverter and obtain the drive signal for the switching transistor through PI control of power and current.

[0125] Specifically, step 2 includes the following steps:

[0126] Step 201: Based on the voltage signal V of the common coupling point acquired in Step 1 PCC and current signal I PCC .

[0127] Step 202: Obtain the α component u of the voltage using the following formula. α The β component of voltage u β The d-component of the current i d The q component of the current i q :

[0128]

[0129] In the formula, u A u B u C The voltage signal V at the common coupling point was acquired. PCC The voltage signals of phase A, phase B, and phase C, iA i B i C The current signal I at the common coupling point was acquired. PCC The current signals of phases A, B, and C. α The α component and u of the voltage after coordinate transformation β Let be the β component of the voltage after coordinate transformation. d Let i be the d-component of the current after coordinate transformation. q Let q be the current component after coordinate transformation.

[0130] In the formula, Given the initial value of the phase angle, we have Where f is taken as the fundamental frequency of 50Hz. and Represent and The cosine value, Represent and The sine value.

[0131] Step 203: The voltage αβ components are shifted in phase angle through the transfer function in the following formula, which can be achieved through the delay module in the simulation.

[0132]

[0133] In the formula, u α u β These are the α and β components of the voltage signal before the phase angle shift, respectively. α qu β These are the α and β components of the voltage signal after phase angle shift, respectively. In the formula, f is the base frequency of the power grid, which is taken as 50Hz here. j represents the imaginary unit, T... s This indicates the time corresponding to the offset phase, and here we have... s represents the frequency domain unit, G(s) represents the phase offset transfer function, e represents the natural constant, and π is the mathematical constant pi.

[0134] Step 204: The α and β components of the voltage signal after phase angle shift are used to obtain the positive sequence components of the d-axis and q-axis using the following formula:

[0135]

[0136] In the formula, These are the positive-sequence components of voltage α and β, respectively. These are the positive sequence components of the voltage along the d-axis and q-axis, respectively. This is the initial value of the phase angle.

[0137] Specifically, such as Figure 3As shown, the phase angle of the acquired voltage signal αβ component is rotated. After a series of operations, the original signal and the rotated signal are transformed by coordinate transformation to obtain the corresponding dq axis components, thus completing the positive and negative sequence component decomposition. Step 3 includes the following steps:

[0138] Step 301: Convert the positive-order components extracted in Step 2 into coordinate-transformed αβ components using the following formula:

[0139]

[0140] In the formula, These are the extracted positive sequence components of the voltage along the d-axis and q-axis, respectively, u' α 、u' β These are the voltage α and β components after coordinate transformation, respectively. This is the initial value of the phase angle.

[0141] like Figure 4 As shown, the MAF consists of a phase-shifting stage, a gain stage, and an integrator stage, and filters the αβ components obtained from the coordinate transformation of the positive-sequence components. Step 302: αβ component u' after coordinate transformation α 、u' β The filtered αβ component is obtained after MAF filtering.

[0142]

[0143] In the formula, G MAF (s) is the MAF transfer function, u' α 、u' β These are the α and β components after coordinate transformation before filtering. These are the α and β components of the filtered voltage, respectively. e represents the natural constant, s represents the frequency domain unit, and T... w is the filter time constant.

[0144] Specifically, step 4 includes the following steps:

[0145] Step 401: Calculate the initial value of the system's synchronization phase angle using the following formula:

[0146]

[0147] In the formula, theta_OLS is the initial value of the open-loop synchronous phase angle.

[0148] Step 402: Based on the MAF designed in Step 3, compensate for the calculated initial phase angle value using the following formula:

[0149]

[0150] In the formula, theta_OLS' is the open-loop synchronous phase angle correction value, used to generate the switching transistor drive signal. Δθ is the phase angle correction amount, π is pi, and f is taken as the fundamental frequency of 50Hz.

[0151] Step 5: Calculate the active and reactive power generated by the inverter system using the voltage and current signals obtained in Step 1, and then obtain the reference signal of the current dq component at the common coupling point through the power PI control loop.

[0152] Specifically, step 5 includes the following steps:

[0153] Step 501: Calculate the active power and reactive power of the inverter system using the voltage and current signals collected in Step 1 using the following formula.

[0154] P = u A (t)i A (t)+u B (t)i B (t)+u C (t)i C (t)

[0155]

[0156] In the formula, P is the active power generated by the inverter system, Q is the reactive power generated by the inverter system, and u A u B u C For the measured voltage signal, i A i B i C The acquired current signal. In the formula,

[0157] Step 502: Compare P and Q with the power setting value P respectively. set Q set The difference is calculated, and after passing the difference through power PI control and limiting, reference values ​​I for the current d and q components are generated. dref and I qref The expression for the power PI controller is as follows:

[0158]

[0159] In the formula, G PQ (s) is the expression for the power PI controller, k p1 k i1 These are the proportional coefficient and integral coefficient, respectively, and s represents the frequency domain unit.

[0160] like Figure 5 As shown, step 503: reference values ​​I for the d and q components of the current. dref and I qrefThe difference between the current signals obtained in step 2 and the current signals on the d and q axes is calculated, and the reference values ​​U of the inverter port voltage on the d and q axes are obtained through a current PI controller and a gain circuit. d U q After coordinate transformation and SVPWM (Space Vector Pulse Width Modulation), the switching transistor drive signal is obtained. The current PI controller and coordinate transformation expressions are as follows:

[0161]

[0162] In the formula, G i (s) is the expression for the current PI controller, k p2 k i2 These are the proportional and integral coefficients, respectively, and s represents the frequency domain unit. U d U q These are the reference values ​​for the port voltage on the d-axis and q-axis, respectively. α U β These are the reference values ​​for the port voltages α and β, respectively. When the power grid is in an ideal state, θ takes the initial value of the given phase angle φ; otherwise, it takes the open-loop synchronous phase angle correction value theta_OLS' obtained in step 4.

[0163] This invention is based on the positive and negative sequence decomposition of the voltage dq components. By performing coordinate transformation on the voltage at the common coupling point, the synchronization phase angle of the inverter is obtained. The drive signals for the grid-connected inverter switching transistors are obtained through power and current PI control. Modeling and simulation are performed using PLECS. The proposed synchronization method is compared with phase-locked loop (PLL) synchronization, ultimately verifying the advantages of the previously proposed synchronization technology in eliminating harmonics at the common coupling point voltage when the grid voltage is unbalanced. This invention provides an open-loop synchronization control method for grid-connected inverters, which is easy to implement, helps eliminate high-order harmonics in the voltage αβ components, improves synchronization performance, and enhances the power quality of the grid-connected inverter system.

[0164] Example 3:

[0165] This invention is based on the positive and negative sequence decomposition of the voltage dq components. By performing coordinate transformation on the voltage at the common coupling point, the synchronization phase angle of the inverter is obtained. The drive signal of the grid-connected inverter switching transistor is obtained through power and current PI control. PLECS is used for modeling and simulation. The proposed synchronization method is compared with phase-locked loop synchronization, and the advantages of the previously proposed synchronization technology in eliminating harmonics of the voltage at the common coupling point when the grid voltage is unbalanced are finally verified.

[0166] This example establishes a simulation model for open-loop synchronous control of a grid-connected inverter suitable for three-phase unbalanced conditions. The grid-connected control objective is to maintain the stability of the system's synchronous phase angle when a three-phase voltage imbalance occurs in the grid. Specific parameters are shown in Table 1. In this example, the grid voltage is set to experience a three-phase unbalanced fault at 1.5s. The simulation results of Example 1 are as follows: Figure 7 As shown, Figure 7 (a) is the collected voltage V at the common coupling point. PCC and current I PCC Signal, Figure 7 (b) is V PCC Synchronize with the system phase angle waveform, from Figure 7 It can be seen that when a fault occurs in the system using this control method, the synchronization phase angle remains unchanged without distortion. This indicates that the proposed open-loop synchronization system-level control method for grid-connected inverters suitable for three-phase unbalanced conditions can effectively maintain the stability of the system synchronization phase angle, avoid its distortion, improve the system synchronization effect, enhance power supply quality, and achieve the control objective.

[0167] Table 1 Simulation parameters for Example 1

[0168] Fundamental frequency f 50Hz <![CDATA[DC side power supply amplitude U dc > 730V Three-phase imbalance occurs at time t 1.5s <![CDATA[MAF time constant T w > 0.0005 <![CDATA[Filter inductor L vf > 2.8mH Filter capacitor C 25μF <![CDATA[Capacitor in series with resistor R C > 1.5Ω <![CDATA[Incoming network inductor L g > 2mH <![CDATA[Network entry resistor R g > 0.5Ω

[0169] Example 4:

[0170] This example establishes a simulation model for open-loop synchronous control of a grid-connected inverter suitable for harmonic injection. The grid-connected control objective is to maintain the stability of the system's synchronous phase angle when high-order harmonics are injected into the grid voltage. Specific parameters are shown in Table 2. In this example, the grid voltage is set to experience a harmonic injection fault at 1.5s. The simulation results of Example 2 are as follows: Figure 8 As shown, Figure 8 (a) is the collected voltage V at the common coupling point. PCC and current I PCC Signal, Figure 8 (b) is V PCC Synchronize with the system phase angle waveform, from Figure 8 It can be seen that when a fault occurs in the system using this control method, the synchronous phase angle remains unchanged without distortion. This indicates that the proposed open-loop synchronous system-level control method for grid-connected inverters, which is suitable for harmonic injection, can effectively maintain the stability of the system's synchronous phase angle. In addition, MAF can filter out high-frequency components in the voltage signal, thereby jointly improving the system's power supply quality and achieving the control objective.

[0171] Table 2 Simulation parameters for Example 2

[0172] Fundamental frequency f 50Hz <![CDATA[DC side power supply amplitude U dc > 730V High-order harmonic injection occurrence time t 1.5s Injected harmonic order k 5 Injected harmonic amplitude 155.5V <![CDATA[MAF time constant T w > 0.008 <![CDATA[Filter inductor L vf > 1mH Filter capacitor C 25μF <![CDATA[Capacitor in series with resistor R C > 1.5Ω <![CDATA[Incoming network inductor L g > 1mH <![CDATA[Network entry resistor R g > 0.5Ω

[0173] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A control method for an open-loop synchronization system of a grid-connected inverter suitable for a non-ideal power grid, characterized in that: Includes the following steps: Step 1: Collect the voltage and current waveforms at the common coupling point of the open-loop synchronous system of the grid-connected inverter. If the grid condition is determined to be non-ideal, set the initial phase angle value. Step 2: Based on the voltage and current waveforms and initial phase angle at the common coupling point, calculate the positive d-sequence component and positive q-sequence component of the voltage. Step 3: Perform coordinate transformation and MAF filtering on the positive sequence components of the voltage along the d-axis and q-axis to obtain the filtered voltage α and β components. Step 4: Calculate the initial value of the open-loop synchronization phase angle based on the filtered voltage α and β components, compensate for the initial value of the open-loop synchronization phase angle, and obtain the corrected value of the open-loop synchronization phase angle. Step 4 specifically includes: Step 401: Based on the filtered voltage α component Filtered voltage β component The initial value of the open-loop synchronization phase angle is calculated using the following formula: In the formula, theta_OLS is the initial value of the open-loop synchronization phase angle; Step 402: Based on the initial value of the open-loop synchronization phase angle, the switching synchronization phase angle correction value is calculated using the MAF phase frequency characteristics. The calculation formula is as follows: In the formula, theta-OLS' is the open-loop synchronous phase angle correction value, Δθ is the phase angle correction amount, π is pi, f is the grid fundamental frequency, and T w The filter time constant; Step 5: Calculate the inverter system power based on the voltage and current waveforms at the common coupling point. Based on the inverter system power, current waveforms, and open-loop synchronous phase angle correction value, use PI control, coordinate transformation, and pulse width modulation to obtain the drive signal for the grid-connected inverter switching transistors.

2. The control method according to claim 1, characterized in that: Also includes: Step 6: Collect the voltage and current waveforms at the common coupling point of the open-loop synchronization system of the grid-connected inverter. If it is determined that the grid is not a non-ideal grid, perform coordinate transformation on the collected voltage signal to obtain the voltage α component and β component. Calculate the synchronization phase angle using the voltage α component and β component. Step 7: Calculate the inverter system power based on the voltage and current waveforms at the common coupling point. Based on the inverter system power, current waveforms, and synchronization phase angle, use PI control, coordinate transformation, and pulse width modulation to obtain the drive signal for the grid-connected inverter switching transistors.

3. The control method according to claim 1 or 2, characterized in that: Step 2 specifically includes: Step 201: Based on the collected voltage signal V at the common coupling point PCC The three-phase voltage signal u is obtained. A u B u C ; Step 202: Based on the three-phase voltage signal u A u B u C Calculate the α component u of the voltage. α The β component of voltage u β The calculation formula is as follows: In the formula, u A u B u C The voltage signal V at the common coupling point was acquired. PCC Phase A, Phase B Phase C voltage signal, u α The α component and u of the voltage after coordinate transformation β The β component of the voltage after coordinate transformation; Step 203: Based on the α component u of the voltage α The β component of voltage u β Calculate the voltage α component qu after phase angle shift. α The voltage β component qu after phase angle shift β The calculation formula is as follows: In the formula, qu α qu β These are the voltage α and β components after phase angle shift, respectively. In the formula, j represents the imaginary unit, and π is the mathematical constant pi; T s The time corresponding to the phase offset is represented by s, where s represents the frequency domain unit, G(s) represents the phase offset transfer function, and e represents the natural constant. Step 204: Based on the voltage α component qu after phase angle shift α The voltage β component qu after phase angle shift β Calculate the positive sequence component of voltage α Voltage β positive sequence component According to the positive sequence component of voltage α Voltage β positive sequence component Calculate the positive sequence component of the voltage d-axis Positive sequence component of voltage q-axis Wherein, the positive sequence component of voltage α Voltage β positive sequence component The calculation formula is as follows: Positive sequence component of voltage d-axis Positive sequence component of voltage q-axis The calculation formula is as follows: In the formula, This is the initial value of the phase angle.

4. The control method according to claim 3, characterized in that: f is the base frequency of the power grid.

5. The control method according to claim 1 or 2, characterized in that: Step 3 specifically includes: Step 301: For the positive sequence component of the voltage d-axis Positive sequence component of voltage q-axis Perform coordinate transformation to obtain the voltage α component u' after coordinate transformation. α Voltage β component u' after coordinate transformation β The calculation formula is as follows: In the formula, u' α 、u' β These are the voltage α and β components after coordinate transformation, respectively. Given the phase angle; Step 302: Transform the voltage α component u' α Voltage β component u' after coordinate transformation β MAF filtering is performed to obtain the filtered voltage α component. Filtered voltage β component The calculation formula is as follows: In the formula, G MAF (s) is the MAF transfer function. These represent the α and β components of the filtered voltage, respectively, where e represents the natural constant, s represents the frequency domain unit, and T... w is the filter time constant.

6. The control method according to claim 1 or 2, characterized in that: Step 5 specifically includes: Step 501: Based on the collected voltage and current waveforms at the common coupling point, calculate the active and reactive power of the inverter system. The calculation formula is as follows: P=u A (t)i A (t)+u B (t)i B (t)+u C (t)i C (t) In the formula, P is the active power generated by the inverter system, Q is the reactive power generated by the inverter system, and u A u B u C The measured three-phase voltage signals are i, ... A i B i C The three-phase current signals were collected separately, with t as the unit of time. f is taken as the base frequency of the power grid; Step 502: Compare P and Q with the power setting value P respectively. set Q set The difference is calculated, and after passing the difference through power PI control and limiting, the reference value I of the current d-axis component is obtained. dref Reference value I of the q-axis component of the current qref The expression for power PI control is as follows: In the formula, G PQ (s) represents power PI control, k p1 k i1 These are the proportional coefficient and the integral coefficient, respectively, and s represents the frequency domain unit; Step 503: Based on the acquired three-phase current signal i A i B i C Calculate the d-axis current signal i d q-axis current signal i q The calculation formula is as follows: In the formula, Here, T is the initial value of the phase angle, and T is the matrix transpose. Step 504: Set the reference value I of the d-axis component of the current. dref Reference value I of the q-axis component of the current qref respectively with d-axis current signal i d q-axis current signal i q The difference is calculated, and then passed through a current PI controller and a gain circuit to obtain the d-axis reference value U of the inverter port voltage. d q-axis reference value U q ; The expression for the current PI controller is as follows: In the formula, G i (s) is a current PI controller, k p2 k i2 These are the proportional coefficient and the integral coefficient, respectively, with s representing the frequency domain unit; Step 505: Set the inverter port voltage d-axis reference value U d q-axis reference value U q By performing a coordinate transformation, the reference value U of the port voltage α component is obtained. α β component reference value U β According to the reference value U of the port voltage α component α β component reference value U β Pulse width modulation is used to obtain the drive signal for the switching transistors of the grid-connected inverter; Among them, the reference value U of the port voltage α component α β component reference value U β The calculation formula is as follows: In the formula, θ is taken as the open-loop synchronous phase angle correction value theta-OLS'.

7. The control method according to claim 2, characterized in that: Steps 6 and 7 specifically include: Step 601: Acquire the voltage and current waveforms at the common coupling point of the grid-connected inverter's open-loop synchronous system. If it is determined that the grid situation is not ideal, then based on the acquired voltage signal V at the common coupling point... PCC The three-phase voltage signal u is obtained. A u B u C ; Step 602: Based on the three-phase voltage signal u A u B u C Calculate the α component u of the voltage. α The β component of voltage u β The calculation formula is as follows: In the formula, u A u B u C The voltage signal V at the common coupling point was acquired. PCC Phase A, Phase B Phase C voltage signal, u α The α component and u of the voltage after coordinate transformation β The β component of the voltage after coordinate transformation; Step 603: The α component u of the voltage α The β component of voltage u β The synchronization phase angle is calculated using the following formula: In the formula, theta is the synchronization phase angle; Step 701: Based on the collected voltage and current waveforms at the common coupling point, calculate the active and reactive power of the inverter system. The calculation formula is as follows: P=u A (t)i A (t)+u B (t)i B (t)+u C (t)i C (t); In the formula, P is the active power generated by the inverter system, Q is the reactive power generated by the inverter system, and u A u B u C The measured three-phase voltage signals are i, ... A i B i C The three-phase current signals were collected separately, with t as the unit of time. f is taken as the base frequency of the power grid; Step 702: Compare P and Q with the power setting value P respectively. set Q set The difference is calculated, and after passing the difference through power PI control and limiting, the reference value I of the current d-axis component is obtained. dref Reference value I of the q-axis component of the current qref The expression for power PI control is as follows: In the formula, G PQ (s) represents power PI control, k p1 k i1 These are the proportional coefficient and the integral coefficient, respectively, and s represents the frequency domain unit; Step 703: Based on the acquired three-phase current signal i A i B i C Calculate the d-axis current signal i d q-axis current signal i q The calculation formula is as follows: In the formula, Here, T is the initial value of the phase angle, and T is the matrix transpose. Step 704: Set the reference value I of the d-axis component of the current. dref Reference value I of the q-axis component of the current qref respectively with d-axis current signal i d q-axis current signal i q The difference is calculated, and then passed through a current PI controller and a gain circuit to obtain the d-axis reference value U of the inverter port voltage. d q-axis reference value U q ; The expression for the current PI controller is as follows: In the formula, G i (s) is a current PI controller, k p2 k i2 These are the proportional coefficient and the integral coefficient, respectively, with s representing the frequency domain unit; Step 705: Set the inverter port voltage d-axis reference value U d q-axis reference value U q By performing a coordinate transformation, the reference value U of the port voltage α component is obtained. α β component reference value U β According to the reference value U of the port voltage α component α β component reference value U β Pulse width modulation is used to obtain the drive signal for the switching transistors of the grid-connected inverter; Among them, the reference value U of the port voltage α component α β component reference value U β The calculation formula is as follows: In the formula, θ is taken as the synchronous phase angle theta.

8. An open-loop synchronization system for a grid-connected inverter suitable for a non-ideal power grid, implementing the control method according to any one of claims 1 to 7, characterized in that: include: The coordinate transformation module is used to transform three-phase signals into corresponding αβ components, convert αβ components into dq components, and convert dq components into three-phase signals. The positive and negative order component decomposition module is used to first perform a rotation transformation on the αβ components, and then perform positive and negative order decomposition to obtain the positive order components of the dq axis; The phase angle compensation and calculation module is used to transform the positive sequence components of the dq axis into coordinates, filter them through MAF, generate the initial value of the open-loop synchronization phase angle of the system, and compensate for it to obtain the open-loop phase angle correction value. The power PI control module is used to perform PI control on the active and reactive power generated by the inverter to obtain the dq reference value of the grid current. The current dq decoupling control module is used to perform PI control on the grid current based on the dq reference value of the grid current and the open-loop phase angle correction value, and then perform coordinate transformation and pulse width modulation to obtain the drive signal of the grid-connected inverter switching transistor.

9. The grid-connected inverter open-loop synchronization system for non-ideal power grids according to claim 8, characterized in that: Also includes: The grid-connected inverter consists of a filter circuit and an AC power grid. The DC side of the grid-connected inverter is connected to a DC voltage source, and the AC side is connected to the AC power grid via the filter circuit.

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