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

Abstract

The invention discloses a grid-connected inverter open-loop synchronization system suitable for a non-ideal power grid and a control method, and the method comprises the steps: carrying out the positive and negative sequence component decomposition of a common coupling point voltage dq axis component, extracting the positive sequence component, carrying out the filtering and phase compensation of a moving average filter (MAF), obtaining a system synchronization phase angle correction value, and carrying out the phase compensation of the system synchronization phase angle correction value. A grid-connected inverter switching tube of the system is controlled by adopting an open-loop synchronization technology, higher harmonic components in voltage alpha and beta signals are eliminated, the synchronization effect is improved, and the power supply quality of the system is improved. The device is simple in structure and easy to implement, alphabeta component higher harmonics in voltage can be eliminated, the synchronization effect is improved, and the electric energy quality and stability of a grid-connected inverter system are improved.

Description

Technical Field

[0001] The invention relates to an open-loop synchronization system and a control method of a grid-connected inverter suitable for a non-ideal power grid, and belongs to the technical field of power electronic inverters. Background Art

[0002] Large-scale new energy DC transmission systems present "double high" characteristics (high proportion of renewable energy and high proportion of power electronic equipment), which brings challenges to the stability of the existing power grid system. At the same time, distributed energy is developing rapidly, and grid-connected inverters as interface devices connecting new energy and the power grid have received widespread attention.

[0003] In order to maximize the system output efficiency, the inverter usually adopts a grid-following control strategy. For the grid-following grid-connected inverter system, the complexity of the traditional phase-locked loop synchronization technology makes it difficult for the inverter system to match the existing nonlinear analysis method. In order to solve the problem of grid-connected inverter operation stability, it is necessary to simplify the synchronization link structure and propose a system-level open-loop synchronization technology. However, there are only a few studies on the open-loop synchronization technology of grid-connected inverter systems. Although some literatures discuss the improvement of phase-locking performance under frequency offset, they still do not discuss the synchronization technology under non-ideal power grids such as voltage three-phase imbalance and high-order harmonic injection.

[0004] Non-ideal grid is one of the main fault types of grid-connected inverter system. It also has a very serious impact on the power quality and stability of the grid. Therefore, it is necessary to further study the system-level synchronization technology of grid-connected inverter under non-ideal grid conditions. Summary of the invention

[0005] Objective: To overcome the deficiencies in the prior art, the present invention provides an open-loop synchronization system and a control method for a grid-connected inverter suitable for a non-ideal power grid.

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

[0007] In a first aspect, an open-loop synchronization system for a grid-connected inverter applicable to a non-ideal power grid comprises:

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

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

[0010] The phase angle compensation and calculation module is used to transform the positive sequence component of the dq axis through coordinates, generate the initial value of the system open-loop synchronous phase angle after filtering through MAF, and compensate 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 power and reactive power emitted by the inverter to obtain the dq reference quantity of the grid-connected current.

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

[0013] As a preferred solution, it also includes: a grid-connected inverter, a filter circuit and an AC power grid, wherein 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] In a second aspect, a control method for an open-loop synchronous system of a grid-connected inverter applicable to a non-ideal power grid comprises 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. When it is judged to be a non-ideal grid situation, give the initial value of the phase angle.

[0016] Step 2: Calculate the d-axis positive sequence component and q-axis positive sequence component of the voltage according to the voltage and current waveforms and the initial phase angle at the common coupling point.

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

[0018] Step 4: Calculate the initial value of the open-loop synchronous phase angle according to the filtered voltage α component and β component, compensate the initial value of the open-loop synchronous phase angle, and obtain the open-loop synchronous phase angle correction value.

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

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

[0021] Step 6: Collect the voltage and current waveforms at the common coupling point of the open-loop synchronous system of the grid-connected inverter. When it is determined that it is not a non-ideal power grid situation, perform coordinate transformation on the collected voltage signal to obtain the voltage α component and β component, and calculate the synchronous phase angle through 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. According to the inverter system power, current waveform and synchronous phase angle, use PI control, coordinate transformation and pulse width modulation to obtain the driving signal of the grid-connected inverter switch tube.

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

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

[0025] Step 202: According to the voltage three-phase signal u A 、u B 、u C , calculate the α component of the voltage u α , the β component of the voltage u β , the calculation formula is as follows:

[0026]

[0027] In the formula, u A 、u B 、u C is the voltage signal V at the common coupling point collected PCC A phase, B phase, C phase voltage signal, u α is the α component of the voltage after coordinate transformation, u β is the β component of the voltage after coordinate transformation.

[0028] Step 203: According to the α component u of the voltage α , the β component of the voltage u β , calculate the voltage α component qu after phase angle shift α , the voltage β component after phase angle shift qu β , the calculation formula is as follows:

[0029]

[0030] In the formula, qu α , β They are the voltage α component and β component after phase angle shift, Where j represents the imaginary unit and π is the circumference of a circle. s represents the time corresponding to the offset phase, s represents the frequency domain unit, G(s) represents the phase angle offset transfer function, and e represents the natural constant.

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

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

[0033]

[0034] Voltage d-axis positive sequence component Voltage q-axis positive sequence component The calculation formula is as follows:

[0035]

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

[0037] As a preferred option, f is the base frequency of the power grid.

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

[0039] Step 301: voltage d-axis positive sequence component Voltage q-axis positive sequence component 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 They are the voltage α component and β component after coordinate transformation, is a given phase angle.

[0042] Step 302: Convert the voltage α component u' after coordinate transformation α , voltage β component u' after coordinate transformation β Perform MAF filtering to obtain the filtered voltage α component Filtered voltage beta component The calculation formula is as follows:

[0043]

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

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

[0046] Step 401: Based on the filtered voltage α component Filtered voltage beta component Calculate the initial value of the open-loop synchronous phase angle using the following formula:

[0047]

[0048] Where theta_OLS is the initial value of the open-loop synchronization phase angle.

[0049] Step 402: According to the initial value of the open-loop synchronization phase angle, the switch synchronization phase angle correction value is calculated using the MAF phase-frequency characteristic. The calculation formula is as follows:

[0050]

[0051] Where theta_OLS' is the open-loop synchronous phase angle correction value, Δθ is the phase angle correction value, π is the circumference, f is the grid base frequency, T w is the filter time constant, and π is the circumference of a circle.

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

[0053] Step 501: Calculate the active power and reactive power of the inverter system according to the collected voltage and current waveforms at the common coupling point. 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 are the measured three-phase voltage signals, i A 、i B、i C are the three-phase current signals collected respectively, t is the time unit, f is the base frequency of the power grid.

[0057] Step 502: P and Q are respectively compared with the power setting value P set , Q set The difference is controlled by power PI and limited to obtain the reference value I of the current d-axis component. dref , reference value of current q-axis component I qref , where the expression of power PI control is as follows:

[0058]

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

[0060] Step 503: Based on the collected 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, is the initial value of the phase angle, and T is the matrix transpose.

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

[0064] Among them, the current PI controller expression is as follows:

[0065]

[0066] In the formula, G i (s) is the current PI controller, k p2 , k i2 They are proportional coefficient and 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 Perform coordinate transformation to obtain the reference value U of the port voltage α component 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 driving signal of the switching tube of the grid-connected inverter.

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

[0069]

[0070] Where θ is the open-loop synchronous phase angle correction value theta_OLS'.

[0071] As a preferred solution, the step 6 and step 7 specifically include:

[0072] Step 601: Collect the voltage and current waveforms at the common coupling point of the grid-connected inverter open-loop synchronous system. When it is determined that the grid is not in a non-ideal state, the voltage signal V PCC , get the voltage three-phase signal u A 、u B 、u C .

[0073] Step 602: According to the voltage three-phase signal u A 、u B 、u C , calculate the α component of the voltage u α , the β component of the voltage u β , the calculation formula is as follows:

[0074]

[0075] In the formula, u A 、u B 、u C is the voltage signal V at the common coupling point collected PCC A phase, B phase, C phase voltage signal, u α is the α component of the voltage after coordinate transformation, u β is the β component of the voltage after coordinate transformation.

[0076] Step 602: α component u of voltage α , the β component of the voltage u β , calculate the synchronous phase angle, the calculation formula is as follows:

[0077]

[0078] Where theta is the synchronization phase angle.

[0079] Step 701: Calculate the active power and reactive power of the inverter system according to the collected voltage and current waveforms at the common coupling point. 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 are the measured three-phase voltage signals, i A 、i B 、i C are the three-phase current signals collected respectively, t is the time unit, f is the base frequency of the power grid.

[0083] Step 702: P and Q are respectively compared with the power setting value P set , Q set The difference is controlled by power PI and limited to obtain the reference value I of the current d-axis component. dref , reference value of current q-axis component I qref , where the expression of power PI control is as follows:

[0084]

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

[0086] Step 703: Based on the collected 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, is the initial value of the phase angle, and T is the matrix transpose.

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

[0090] Among them, the current PI controller expression is as follows:

[0091]

[0092] In the formula, G i (s) is the current PI controller, k p2 , k i2 They are proportional coefficient and 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 Perform coordinate transformation to obtain the reference value U of the port voltage α component 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 driving signal of the switching tube of the grid-connected inverter.

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

[0095]

[0096] Where θ is the synchronous phase angle theta.

[0097] Beneficial effect: The open-loop synchronization system and control method of the grid-connected inverter suitable for non-ideal power grids provided by the present invention specifically decompose the positive and negative sequence components of the dq-axis components of the common coupling point voltage, and extract the positive sequence component therein, and obtain the system synchronization phase angle correction after filtering by a moving average filter (MAF) and phase compensation, and use open-loop synchronization technology to control the system grid-connected inverter switch tube, eliminate the high-order harmonic components in the voltage αβ signal, improve the synchronization effect, and improve the system power supply quality.

[0098] The present invention provides an open-loop synchronous control method for a grid-connected inverter based on the decomposition of positive and negative sequence components. The method has a simple structure and is easy to implement. The method is beneficial to eliminating αβ component high-order harmonics in the voltage, improving the synchronization effect, and improving the power quality and stability of the grid-connected inverter system. BRIEF DESCRIPTION OF THE DRAWINGS

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

[0100] Figure 2 It is a flow chart of a control method of an open-loop synchronous system of a grid-connected inverter applicable to a non-ideal power grid according to the present invention.

[0101] Figure 3 It is the dq positive and negative sequence component decomposition control block diagram.

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

[0103] Figure 5 It is the power PI control block diagram.

[0104] Figure 6 It is the current PI decoupling control block diagram.

[0105] Figure 7 is a simulation result diagram of the control method of the present invention under the condition of unbalanced grid voltage, Figure 7 (a) is a schematic diagram of the voltage and current at the common coupling point. Figure 7 (b) is a schematic diagram of the common coupling point voltage and 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 voltage and current at the common coupling point. Figure 8 (b) is a schematic diagram of the common coupling point voltage and system synchronization phase angle after synchronization. DETAILED DESCRIPTION

[0107] The following is a clear and complete description of the technical solutions in the examples of the present invention in conjunction with the accompanying drawings in the examples of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative work are within the protection scope of the present invention.

[0108] The present invention will be further described below in conjunction with specific embodiments.

[0109] Embodiment 1:

[0110] This embodiment introduces 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] Among them, the grid-connected inverter is composed of a three-level IGBT (insulated gate bipolar transistor), the DC side is connected to a DC voltage source, 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 neutral point of the power supply is grounded.

[0112] A coordinate transformation module, used for transforming the three-phase signal into corresponding αβ components, transforming the αβ components into dq components, and transforming the dq components into the three-phase signal;

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

[0114] The phase angle compensation and calculation module is used to transform the positive sequence component of the dq axis through coordinates, filter it through MAF, generate the initial value of the system open-loop synchronous phase angle, and compensate 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 power and reactive power emitted by the inverter to obtain the dq reference quantity of the grid current;

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

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

[0118] Embodiment 2:

[0119] This embodiment introduces a control method for an open-loop synchronous system of a grid-connected inverter applicable to a non-ideal power grid. The voltage and current signals at the common coupling point are collected to calculate the power output by the grid-connected inverter. Then, the system open-loop synchronous phase angle is obtained through a coordinate transformation module and a positive and negative sequence component decomposition module. Finally, a switch tube drive signal is generated 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, the following steps are included:

[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, go to step 2; if not, perform coordinate transformation on the collected voltage signal, calculate the synchronous phase angle through the voltage αβ components, and go to step 5;

[0121] Step 2: Given the initial phase angle theta = ∫(2*π*50)dt, the synchronous phase angle is calculated by decomposing the positive and negative sequence components to decompose the voltage dq coordinate components into positive and negative sequence components, and the positive sequence component is extracted;

[0122] Step 3: Perform coordinate transformation on the positive sequence component and then pass it through MAF filtering;

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

[0124] Step 5: Calculate the power output by the inverter and obtain the switch tube drive signal through power and current PI control.

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

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

[0127] Step 202: Obtain the voltage α component u by the following formula: α , the β component of the 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 is the voltage signal V at the common coupling point collected PCC The A phase, B phase, and C phase voltage signals, iA 、i B 、i C The current signal I collected at the common coupling point PCC A phase, B phase, C phase current signal. α is the α component of the voltage after coordinate transformation, u β is the β component of the voltage after coordinate transformation. d is the d component of the current after coordinate transformation, i q is the q component of the current after coordinate transformation.

[0130] In the formula, is the initial value of the phase angle, Where f is the base frequency of 50Hz. and Respectively represent and The cosine value of Respectively represent and The sine value of .

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

[0132]

[0133] In the formula, u α 、u β They are the α and β components of the voltage signal before phase angle shift, qu α , β They are the α component and β component of the voltage signal after phase angle shift, Where f is the base frequency of the power grid, which is 50Hz here. Where j represents an imaginary unit, T s represents the time corresponding to the offset phase, here we have s represents the frequency domain unit, G(s) represents the phase shift transfer function, e represents the natural constant, and π represents the circumference of a circle.

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

[0135]

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

[0137] Specifically, Figure 3As shown, the collected voltage signal αβ components are phase rotated, and the original signal and the rotated signal are subjected to a series of operations and then coordinate transformation to obtain the corresponding dq axis components, completing the decomposition of the positive and negative sequence components. Step 3 includes the following steps:

[0138] Step 301: convert the positive sequence component extracted in step 2 into the αβ component after coordinate transformation by the following formula:

[0139]

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

[0141] like Figure 4 As shown, the MAF consists of a phase shifter, a gain and an integral component, and filters the αβ components obtained by coordinate transformation of the positive sequence component. Step 302: The αβ components u' after coordinate transformation α 、u' β After MAF filtering, the filtered αβ components are obtained.

[0142]

[0143] In the formula, G MAF (s) is the MAF transfer function, u' α 、u' β are the α and β components after coordinate transformation before filtering, are the voltage α and β components after filtering, respectively. e represents a natural constant, s represents a 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 synchronous phase angle using the following formula:

[0146]

[0147] Where theta_OLS is the initial value of the open-loop synchronization phase angle.

[0148] Step 402: Compensate the calculated initial phase angle value according to the MAF designed in step 3 according to the following formula:

[0149]

[0150] Where theta_OLS' is the open-loop synchronous phase angle correction value, which is used to generate the switch tube drive signal. Δθ is the phase angle correction value, π is the circumference of the circle, and f is the base frequency of 50Hz.

[0151] Step 5: Calculate the active power and reactive power generated by the inverter system through 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 through 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 is the measured voltage signal, i A 、i B 、i C is the collected current signal. In the formula,

[0157] Step 502: P and Q are respectively compared with the power setting value P set , Q set The difference is controlled by power PI and limited to generate the reference value I of current d and q components. dref and I qref , the power PI controller expression is as follows:

[0158]

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

[0160] like Figure 5 As shown, step 503: the reference value I of the current d and q components dref and I qrefThe d-axis and q-axis current signals obtained in step 2 are subtracted respectively, and the d-axis and q-axis reference values ​​U of the inverter port voltage are obtained through the current PI controller and the gain link. d , U q , after coordinate transformation and SVPWM (space vector pulse width modulation), the switch tube 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 current PI controller expression, k p2 , k i2 are proportional and integral coefficients respectively, and s represents the frequency domain unit. d , U q are the reference values ​​of the port voltage d-axis and q-axis, respectively, and U α , U β are the reference values ​​of the port voltage αβ respectively. When the power grid is in an ideal state, θ takes the given initial phase angle φ, otherwise it takes the open-loop synchronous phase angle correction value theta_OLS' obtained in step 4.

[0163] The present invention is based on the positive and negative sequence decomposition of the voltage dq component, and obtains the inverter synchronization phase angle by coordinate transformation of the common coupling point voltage; obtains the grid-connected inverter switch tube drive signal through power and current PI control; uses PLECS for modeling and simulation; compares the proposed synchronization method with the phase-locked loop synchronization, and finally verifies that the previously proposed synchronization technology has advantages in eliminating the common coupling point voltage harmonics when the grid voltage is unbalanced. The present invention provides a grid-connected inverter open-loop synchronization control method, which is easy to implement, is conducive to eliminating high-order harmonics in the voltage αβ components, improving the synchronization effect, and improving the power quality of the grid-connected inverter system.

[0164] Embodiment 3:

[0165] The present invention is based on the positive and negative sequence decomposition of the voltage dq components, and obtains the inverter synchronization phase angle by performing coordinate transformation on the common coupling point voltage; obtains the grid-connected inverter switch tube drive signal through power and current PI control; uses PLECS for modeling and simulation; compares the proposed synchronization method with the phase-locked loop synchronization, and finally verifies the advantages of the previously proposed synchronization technology in eliminating the common coupling point voltage harmonics when the grid voltage is unbalanced.

[0166] This example builds a grid-connected inverter open-loop synchronous control simulation model suitable for three-phase unbalanced conditions. The grid-connected control goal is to maintain the stability of the system synchronization phase angle when the grid voltage is three-phase unbalanced. The specific parameters are shown in Table 1. In this example, the grid voltage is set to occur a three-phase unbalanced fault at 1.5s. The simulation results of Example 1 are shown in Table 1. Figure 7 As shown, Figure 7 (a) is the collected common coupling point voltage V PCC and current I PCC Signal, Figure 7 (b) is V PCC And the system synchronous 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, improve the power supply quality, and achieve the control goal.

[0167] Table 1 Simulation parameters of Example 1

[0168] Fundamental frequency f 50Hz <![CDATA[DC-side power supply amplitude U dc > 730V Three-phase unbalance occurrence 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[Incoming network resistor R g > 0.5Ω

[0169] Embodiment 4:

[0170] This example builds a grid-connected inverter open-loop synchronous control simulation model suitable for harmonic injection. The grid-connected control goal is to maintain the stability of the system synchronization phase angle when high-order harmonics are injected into the grid voltage. The specific parameters are shown in Table 2. In this example, the grid voltage is set to have a harmonic injection fault at 1.5s. The simulation results of Example 2 are shown in Table 2. Figure 8 As shown, Figure 8 (a) is the collected common coupling point voltage V PCC and current I PCC Signal, Figure 8 (b) is V PCC And the system synchronous 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, indicating that the proposed open-loop synchronous system-level control method for grid-connected inverters suitable for harmonic injection conditions can effectively maintain the stability of the system synchronous phase angle. In addition, MAF can filter out the high-frequency components in the voltage signal, thereby jointly improving the system power supply quality and achieving the control target.

[0171] Table 2 Simulation parameters of 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 number 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[In-network resistor R g > 0.5Ω

[0173] The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principle of the present invention. These improvements and modifications should also be regarded as the scope of protection of the present invention.

Claims

1. An open-loop synchronous system for grid-connected inverters suitable for non-ideal power grids, characterized in that: include: A coordinate transformation module, used for transforming the three-phase signal into corresponding αβ components, transforming the αβ components into dq components, and transforming the dq components into the three-phase signal; The positive and negative sequence component decomposition module is used to perform rotation transformation on the αβ components first, and then perform positive and negative sequence decomposition to obtain the positive sequence components of the dq axis; The phase angle compensation and calculation module is used to transform the positive sequence component of the dq axis through coordinates, filter it through MAF, generate the initial value of the system open-loop synchronous phase angle, and compensate it to obtain the open-loop phase angle correction value; The power PI control module is used to perform PI control on the active power and reactive power emitted by the inverter to obtain the dq reference quantity of the grid current; The current dq decoupling control module is used to perform PI control on the grid-connected current according to the dq reference quantity of the grid-connected current and the open-loop phase angle correction value, and then perform coordinate transformation and pulse width modulation to obtain the driving signal of the grid-connected inverter switch tube.

2. The grid-connected inverter open-loop synchronization system applicable to a non-ideal power grid according to claim 1, characterized in that: 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 a filter circuit.

3. A control method for an open-loop synchronous system of a grid-connected inverter applicable to a non-ideal power grid, characterized in that: The steps include: Step 1: Collect the voltage and current waveforms at the common coupling point of the open-loop synchronous system of the grid-connected inverter. When it is judged to be a non-ideal grid situation, give the initial value of the phase angle; Step 2: Calculate the d-axis positive sequence component and q-axis positive sequence component of the voltage according to the voltage and current waveforms and the initial phase angle at the common coupling point; Step 3: Perform coordinate transformation and MAF filtering on the d-axis positive sequence component and q-axis positive sequence component of the voltage to obtain filtered voltage α component and β component; Step 4: Calculate the initial value of the open-loop synchronous phase angle according to the filtered voltage α component and β component, compensate the initial value of the open-loop synchronous phase angle, and obtain the open-loop synchronous phase angle correction value; Step 5: Calculate the inverter system power based on the voltage and current waveforms at the common coupling point. According to the inverter system power, current waveform and open-loop synchronous phase angle correction value, use PI control, coordinate transformation and pulse width modulation to obtain the driving signal of the grid-connected inverter switch tube.

4. The control method according to claim 3, characterized in that: Also includes: Step 6: Collect the voltage and current waveforms at the common coupling point of the open-loop synchronous system of the grid-connected inverter. When it is determined that the grid is not in a non-ideal state, perform coordinate transformation on the collected voltage signal to obtain the voltage α component and β component. The synchronous phase angle is calculated through 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. According to the inverter system power, current waveform and synchronous phase angle, use PI control, coordinate transformation and pulse width modulation to obtain the driving signal of the grid-connected inverter switch tube.

5. The control method according to claim 3 or 4, characterized in that: The step 2 specifically includes: Step 201: Based on the collected voltage signal V at the common coupling point PCC , get the voltage three-phase signal u A 、u B 、u C ; Step 202: According to the voltage three-phase signal u A 、u B 、u C , calculate the α component of the voltage u α , the β component of the voltage u β , the calculation formula is as follows: In the formula, u A 、u B 、u C is the voltage signal V at the common coupling point collected PCC The A phase, B phase, and C phase voltage signals, u α is the α component of the voltage after coordinate transformation, u β is the β component of the voltage after coordinate transformation; Step 203: According to the α component u of the voltage α , the β component of the voltage u β , calculate the voltage α component qu after phase angle shift α , the voltage β component after phase angle shift qu β , the calculation formula is as follows: In the formula, qu α , β They are the voltage α component and β component after phase angle shift, Where j represents the imaginary unit, π is the circumference of a circle; T s represents the time corresponding to the offset phase, s represents the frequency domain unit, G(s) represents the phase angle offset transfer function, and e represents the natural constant; Step 204: Based on the voltage component α after phase angle shift, α , the voltage β component after phase angle shift qu β Calculate the voltage α positive sequence component Voltage β positive sequence component According to the positive sequence component of voltage α Voltage β positive sequence component Calculate the voltage d-axis positive sequence component Voltage q-axis positive sequence component Among them, the voltage α positive sequence component Voltage β positive sequence component The calculation formula is as follows: Voltage d-axis positive sequence component Voltage q-axis positive sequence component The calculation formula is as follows: In the formula, is the initial value of the phase angle.

6. The control method according to claim 5, characterized in that: f is the base frequency of the power grid.

7. The control method according to claim 3 or 4, characterized in that: The step 3 specifically includes: Step 301: voltage d-axis positive sequence component Voltage q-axis positive sequence component 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 They are the voltage α component and β component after coordinate transformation, is a given phase angle; Step 302: Convert the voltage α component u' after coordinate transformation α , voltage β component u' after coordinate transformation β Perform MAF filtering to obtain the filtered voltage α component Filtered voltage beta component The calculation formula is as follows: In the formula, G MAF (s) is the MAF transfer function, are the α and β components of the filtered voltage respectively, e represents a natural constant, s represents a frequency domain unit, T w is the filter time constant.

8. The control method according to claim 3 or 4, characterized in that: The step 4 specifically includes: Step 401: Based on the filtered voltage α component Filtered voltage beta component Calculate the initial value of the open-loop synchronous phase angle using the following formula: Where theta_OLS is the initial value of the open-loop synchronization phase angle; Step 402: According to the initial value of the open-loop synchronization phase angle, the switch synchronization phase angle correction value is calculated using the MAF phase-frequency characteristic. The calculation formula is as follows: Where theta_OLS' is the open-loop synchronous phase angle correction value, Δθ is the phase angle correction value, π is the circumference, f is the grid base frequency, T w is the filter time constant, and π is the circumference of a circle.

9. The control method according to claim 3 or 4, characterized in that: The step 5 specifically includes: Step 501: Calculate the active power and reactive power of the inverter system according to the collected voltage and current waveforms at the common coupling point. 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 are the measured three-phase voltage signals, i A 、i B 、i C are the three-phase current signals collected respectively, t is the time unit, f is the base frequency of the power grid; Step 502: P and Q are respectively compared with the power setting value P set , Q set The difference is controlled by power PI and limited to obtain the reference value I of the current d-axis component. dref , reference value of current q-axis component I qref , where the expression of power PI control is as follows: In the formula, G PQ (s) is power PI control, k p1 , k i1 are the proportional coefficient and the integral coefficient respectively, and s represents the frequency domain unit; Step 503: Based on the collected 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, is the initial value of the phase angle, T is the matrix transpose; Step 504: Set the reference value I of the current d-axis component dref , reference value of current q-axis component I qref Respectively with the d-axis current signal i d , q-axis current signal i q The difference is passed through the current PI controller and the gain link to obtain the inverter port voltage d-axis reference value U d , q-axis reference value U q ; Among them, the current PI controller expression is as follows: In the formula, G i (s) is the current PI controller, k p2 , k i2 are proportional coefficient and integral coefficient respectively, and s represents the frequency domain unit; Step 505: Set the inverter port voltage d-axis reference value U d , q-axis reference value U q Perform coordinate transformation to obtain the reference value U of the port voltage α component 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 driving signal of the switch tube of the grid-connected inverter; Among them, the reference value of the port voltage α component is U α , β component reference value U β The calculation formula is as follows: Where θ is the open-loop synchronous phase angle correction value theta_OLS'.

10. The control method according to claim 4, characterized in that: The steps 6 and 7 specifically include: Step 601: Collect the voltage and current waveforms at the common coupling point of the grid-connected inverter open-loop synchronous system. When it is determined that the grid is not in a non-ideal state, the voltage signal V PCC , get the voltage three-phase signal u A 、u B 、u C ; Step 602: According to the voltage three-phase signal u A 、u B 、u C , calculate the α component u of the voltage α , the β component of the voltage u β , the calculation formula is as follows: In the formula, u A 、u B 、u C is the voltage signal V at the common coupling point collected PCC The A phase, B phase, and C phase voltage signals, u α is the α component of the voltage after coordinate transformation, u β is the β component of the voltage after coordinate transformation; Step 602: α component u of voltage α , the β component of the voltage u β , calculate the synchronous phase angle, the calculation formula is as follows: Where theta is the synchronization phase angle; Step 701: Calculate the active power and reactive power of the inverter system according to the collected voltage and current waveforms at the common coupling point. 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 are the measured three-phase voltage signals, i A 、i B 、i C are the three-phase current signals collected respectively, t is the time unit, f is the base frequency of the power grid; Step 702: P and Q are respectively compared with the power setting value P set , Q set The difference is controlled by power PI and limited to obtain the reference value I of the current d-axis component. dref , reference value of current q-axis component I qref , where the expression of power PI control is as follows: In the formula, G PQ (s) is power PI control, k p1 , k i1 are the proportional coefficient and the integral coefficient respectively, and s represents the frequency domain unit; Step 703: Based on the collected 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, is the initial value of the phase angle, T is the matrix transpose; Step 704: Set the reference value I of the current d-axis component dref , reference value of current q-axis component I qref Respectively with the d-axis current signal i d , q-axis current signal i q The difference is passed through the current PI controller and the gain link to obtain the inverter port voltage d-axis reference value U d , q-axis reference value U q ; Among them, the current PI controller expression is as follows: In the formula, G i (s) is the current PI controller, k p2 , k i2 are proportional coefficient and integral coefficient respectively, and s represents the frequency domain unit; Step 705: Set the inverter port voltage d-axis reference value U d , q-axis reference value U q Perform coordinate transformation to obtain the reference value U of the port voltage α component 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 driving signal of the switch tube of the grid-connected inverter; Among them, the reference value of the port voltage α component is U α , β component reference value U β The calculation formula is as follows: Where θ is the synchronous phase angle theta.

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