Harmonic current calculation method and device, equipment and storage medium

Abstract

The invention discloses a harmonic current calculation method and device, equipment and a storage medium. The method comprises the following steps: acquiring power grid parameters after a photovoltaic inverter is connected to a power grid; performing rotation transformation on the power grid parameters to obtain a voltage harmonic phasor under a rotating coordinate system; an equivalent control circuit of the photovoltaic inverter is constructed, a current dq-axis component matrix of a PCC node is obtained based on the equivalent control circuit, and the PCC node is a coupling common point of the photovoltaic inverter and a power grid; the current dq-axis component matrix of the PCC node is converted into a current harmonic phasor matrix with a voltage harmonic phasor as an excitation condition; and performing decomposition calculation on the current harmonic phasor matrix to obtain harmonic current. And calculating the harmonic voltage to obtain a harmonic current, so as to evaluate the operation possibility of the photovoltaic inverter under the distorted power grid under different conditions through the harmonic current.

Description

Technical Field

[0001] The present invention relates to the technical field of power systems, and in particular to a method and device for calculating harmonic currents, equipment and a storage medium. Background Art

[0002] Photovoltaic inverters are widely used in power systems. Photovoltaic inverters convert the widely varying DC voltage generated by photovoltaic solar panels into AC power of the mains frequency, and feed the converted electric energy back to the power system or the microgrid for local consumption. Photovoltaic inverters play a vital role in solar grid-connected power generation systems and are one of the core components of the system.

[0003] When the access point of the photovoltaic inverter is far away from the main grid, due to the presence of a large number of nonlinear loads in the grid and the large equivalent impedance of the end line of the grid, the grid voltage at the common coupling point between the photovoltaic inverter and the grid will be distorted, and the background harmonic voltage will be superimposed on the ideal sine. Compared with the access to the ideal grid, when the photovoltaic inverter is connected to such a grid with poor power supply quality, additional current distortion will be generated due to grid harmonics. This additional current distortion is superimposed on the original industrial frequency sinusoidal current, causing the output power quality of the photovoltaic inverter to deteriorate, and will cause the current peak value to change, which is easy to trigger the overcurrent protection action and cause disconnection from the grid. Summary of the invention

[0004] Based on this, it is necessary to propose a calculation method and device, equipment and storage medium for harmonic current to obtain accurate harmonic current, and then provide directional suggestions for control loop design, so as to effectively reduce harmonic current, ensure the normal operation of equipment and avoid related accidents.

[0005] To achieve the above object, the present application provides a first aspect of a method for calculating harmonic currents, the method comprising:

[0006] Obtain grid parameters after the photovoltaic inverter is connected to the grid;

[0007] Performing harmonic decomposition and rotation transformation on the power grid parameters to obtain voltage harmonic phasors in a rotating coordinate system;

[0008] Constructing an equivalent control circuit of the photovoltaic inverter, and obtaining a current dq-axis component matrix of a PCC node based on the equivalent control circuit, wherein the PCC node is a coupling common point between the photovoltaic inverter and the power grid;

[0009] Converting the current dq-axis component matrix of the PCC node into a current harmonic phasor matrix using the voltage harmonic phasor as an excitation condition;

[0010] The current harmonic phasor matrix is ​​decomposed and calculated to obtain the harmonic current.

[0011] Furthermore, the grid parameters at least include the instantaneous value of the phase voltage of the grid, and the harmonic decomposition and rotation transformation of the grid parameters are performed to obtain the voltage harmonic phasor in the rotating coordinate system, specifically including:

[0012] Performing harmonic decomposition and rotation transformation on the phase voltage of the power grid to obtain dq-axis components of the harmonic voltage of the power grid in a rotating coordinate system;

[0013] The dq-axis components of the harmonic voltage are converted into dq-axis component phasors of the harmonic voltage in a rotating coordinate system.

[0014] Further, the harmonic decomposition and rotation transformation of the phase voltage of the power grid to obtain the dq-axis components of the harmonic voltage of the power grid in the rotating coordinate system specifically includes:

[0015] Performing harmonic decomposition on the phase voltage of the power grid to obtain three-phase harmonic voltage in the phase voltage of the power grid;

[0016] The three-phase harmonic voltage of the power grid is expressed as:

[0017]

[0018] In the formula, u ga-h ,u gb-h ,u gc-h are the harmonic voltages of the three phases A, B, and C of the power grid, k is the harmonic order, U k-1 is the k-1th negative sequence harmonic voltage, ω 0 is the grid fundamental angular velocity, t is the time variable, θ k-1 Indicates the phase angle of the k-1th negative sequence harmonic voltage, U k+1 is the k+1th positive sequence harmonic voltage, θ k+1 Represents the phase angle of the k+1th positive sequence harmonic voltage;

[0019] Rotating and transforming the three-phase harmonic voltage of the power grid to obtain dq-axis components of the voltage harmonics of the power grid in a rotating coordinate system;

[0020] The dq-axis components of the voltage harmonics of the power grid are expressed as:

[0021]

[0022] In the formula, u gd-h 、u gq-h are the d-axis component and q-axis component of the harmonic voltage in the rotating coordinate system respectively; ω 0 is the grid fundamental angular velocity, t is the time variable, u ga-h ,u gb-h ,u gc-hare the harmonic voltages of the three phases A, B and C of the power grid respectively.

[0023] Furthermore, the dq-axis component phasor of the harmonic voltage in the rotating coordinate system is expressed as:

[0024]

[0025] In the formula, is the d-axis component phasor of the kth harmonic voltage in the rotating coordinate system, is the q-axis component phasor of the kth harmonic voltage in the rotating coordinate system, U gdk is the d-axis component of the kth harmonic voltage in the rotating coordinate system, θ gdk is the phase angle of the d-axis component of the kth harmonic voltage in the rotating coordinate system, U k-1 is the k-1th negative sequence harmonic voltage, U k+1 is the k+1th positive sequence harmonic voltage, θ k+1 represents the phase angle of the k+1th positive sequence harmonic voltage, θ k-1 Represents the phase angle of the k-1th voltage harmonic.

[0026] Furthermore, the current dq-axis component matrix of the PCC node is obtained based on the equivalent control circuit, specifically including:

[0027] Obtaining a transfer function from the output current of the photovoltaic inverter to the PCC node voltage based on the equivalent control circuit;

[0028] Calculating a current dq-axis component matrix of the PCC node according to the transfer function, the dq-axis components of the harmonic voltage of the power grid and the impedance preset in the equivalent control circuit;

[0029] The current dq-axis component matrix expression of the PCC node is:

[0030]

[0031] Where s is the Laplace domain, I d is the d-axis component of the current at the PCC node, I q is the q-axis component of the current at the PCC node, Fe is the transfer function from the PV inverter output current to the PCC node voltage, and Z gdq is the preset impedance, U gd is the d-axis component of the harmonic voltage of the power grid, U gq is the q-axis component of the harmonic voltage of the power grid.

[0032] Furthermore, the decomposing and calculating the current harmonic phasor matrix to obtain the harmonic current specifically includes:

[0033] Decomposing the current harmonic phasor matrix into negative-sequence current harmonic components and positive-sequence current harmonic components in a three-phase stationary coordinate system;

[0034] The harmonic current is calculated according to the negative-sequence current harmonic component and the positive-sequence current harmonic component.

[0035] Further, the harmonic current is calculated according to the negative sequence current harmonic component and the positive sequence current harmonic component, specifically including:

[0036] Substituting the negative-sequence current harmonic component and the positive-sequence current harmonic component into the harmonic current time domain expression to calculate the harmonic current;

[0037] Among them, the time domain expression of the harmonic current is:

[0038]

[0039] In the formula, t is the time variable, i a-h 、i b-h 、i c-h are the harmonic currents of the three phases A, B and C of the power grid, k is the harmonic order, I k-1 is the k-1th negative sequence harmonic current, I k+1 is the k+1th positive sequence harmonic current, ω 0 is the grid fundamental angular velocity, is the phase angle of the k-1th negative sequence harmonic current in the three-phase stationary coordinate system, is the phase angle of the k+1th positive-sequence harmonic current in the three-phase stationary coordinate system.

[0040] To achieve the above-mentioned object, the second aspect of the present application provides a harmonic current calculation device, the device comprising: a parameter acquisition unit, an equivalent control unit and a decomposition calculation unit;

[0041] The parameter acquisition unit is used to acquire grid parameters after the photovoltaic inverter is connected to the grid;

[0042] Performing harmonic decomposition and rotation transformation on the power grid parameters to obtain voltage harmonic phasors in a rotating coordinate system;

[0043] The equivalent control unit is used to construct an equivalent control circuit of the photovoltaic inverter, and obtain a current dq-axis component matrix of a PCC node based on the equivalent control circuit, wherein the PCC node is a coupling common point between the photovoltaic inverter and the power grid;

[0044] The decomposition calculation unit is used to convert the current dq-axis component matrix of the PCC node into a current harmonic phasor matrix using the voltage harmonic phasor as an excitation condition;

[0045] The current harmonic phasor matrix is ​​decomposed and calculated to obtain the harmonic current.

[0046] To achieve the above-mentioned purpose, the third aspect of the present application provides a computer-readable storage medium storing a computer program, characterized in that when the computer program is executed by a processor, the processor executes the steps of the method described in the first aspect.

[0047] To achieve the above-mentioned purpose, the fourth aspect of the present application provides a computer device, including a memory and a processor, characterized in that the memory stores a computer program, and when the computer program is executed by the processor, the processor executes the steps of the method described in the first aspect.

[0048] The embodiments of the present invention have the following beneficial effects:

[0049] The embodiment of the present invention proposes a method for calculating harmonic current, which includes: obtaining grid parameters after a photovoltaic inverter is connected to the grid; performing a rotation transformation on the grid parameters to obtain voltage harmonic phasors in a rotating coordinate system; constructing an equivalent control circuit of the photovoltaic inverter, and obtaining a current dq-axis component matrix of a PCC node based on the equivalent control circuit, wherein the PCC node is a coupling common point between the photovoltaic inverter and the grid; converting the current dq-axis component matrix of the PCC node into a current harmonic phasor matrix with voltage harmonic phasors as excitation conditions; and decomposing and calculating the current harmonic phasor matrix to obtain harmonic current. Harmonic current is obtained based on harmonic voltage calculation, so as to evaluate the possibility of the photovoltaic inverter operating under distorted grids under different conditions through harmonic current. BRIEF DESCRIPTION OF THE DRAWINGS

[0050] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0051] in:

[0052] Figure 1 Schematic diagram of a flow chart of a method for calculating harmonic current in an embodiment of the present invention;

[0053] Figure 2 A schematic diagram of a system in which a photovoltaic inverter is connected to a non-ideal power grid containing background harmonics in an embodiment of the present invention;

[0054] Figure 3 This is a block diagram of an equivalent control circuit in an embodiment of the present invention;

[0055] Figure 4This is an overall control block diagram of a photovoltaic inverter after considering grid impedance in an embodiment of the present invention;

[0056] Figure 5 Grid voltage waveform and photovoltaic inverter current waveform diagram of an embodiment of the present invention;

[0057] Figure 6 is a structural block diagram of a harmonic current calculation device in an embodiment of the present invention;

[0058] Figure 7 1 is a diagram showing the internal structure of a computer device in an embodiment of the present invention. DETAILED DESCRIPTION

[0059] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments 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 ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0060] When a large number of nonlinear devices are connected to the grid through a common coupling point, the grid is no longer considered an ideal grid, but rather an inductive grid. Harmonic distortion generated by various loads in the grid, especially nonlinear loads, and unbalanced current flowing into the non-ideal grid will increase the voltage harmonic content, causing the PV inverter to generate current harmonics when connected to the grid due to the decrease in power quality provided by the grid and the increase in harmonics, further deteriorating the power supply quality and increasing the burden on components.

[0061] In order to reduce the impact of current harmonics on power supply quality, the feasibility of photovoltaic inverters operating under different distorted grid conditions can be evaluated based on harmonic currents, which can help control parameter setting and main circuit design margin optimization to improve power supply quality. Therefore, it is very important to obtain accurate harmonic currents.

[0062] The present invention provides a method for calculating harmonic current, which can be found in Figure 1 , Figure 1 : is a flow chart of a method for calculating harmonic current in an embodiment of the present invention, the method comprising:

[0063] Step 110, obtaining grid parameters after the photovoltaic inverter is connected to the grid.

[0064] In one embodiment of the present invention, there are background harmonics in the grid voltage, see Figure 2 , Figure 2The schematic diagram of the system in which the photovoltaic inverter is connected to a non-ideal power grid containing background harmonics in an embodiment of the present invention is shown. The photovoltaic inverter is based on the currently commonly used three-phase bridge voltage source DC-AC converter topology, and the photovoltaic inverter grid-connected control adopts conventional voltage vector-oriented direct current control. Figure 2 in, u a ,u b ,u c Represents the instantaneous value of the output phase voltage of the photovoltaic inverter; i a ,i b ,i c Indicates the inductor current of the photovoltaic inverter, that is, the instantaneous value of the grid-connected phase current at the PCC node. The PCC node is the coupling common point between the photovoltaic inverter and the grid; u ga ,u gb ,u gc Indicates the instantaneous value of the grid phase voltage, which contains harmonic components; u pa ,u pb ,u pc Indicates the instantaneous value of the phase voltage of the PCC node; L represents the inductance of the photovoltaic inverter filter inductance, R represents the parasitic resistance of the photovoltaic inverter filter inductance, L g Indicates the inductance value of the grid side, R g Indicates the equivalent resistance value of the grid-side inductance, U dc Represents the DC bus voltage of the photovoltaic inverter. Since the bus capacitance is large, it can play an energy buffering role. Therefore, when constructing the harmonic current calculation model, it can be considered that U dc Keep it constant.

[0065] In an embodiment of the present invention, the grid parameters may include parameters such as the instantaneous value of the output phase voltage of the photovoltaic inverter, the instantaneous value of the grid-connected phase current at the PCC point, the instantaneous value of the phase voltage of the PCC node, the instantaneous value of the grid phase voltage, the photovoltaic inverter filter inductance value, the grid-side inductance value, the parasitic resistance value of the photovoltaic inverter filter inductance, the equivalent resistance value of the grid-side inductance and the photovoltaic inverter DC bus voltage.

[0066] Step 120, performing harmonic decomposition and rotation transformation on the grid parameters to obtain voltage harmonic phasors in a rotating coordinate system.

[0067] In the embodiment of the present invention, the obtained power grid parameters are subjected to rotation transformation and can be converted into corresponding variables in the dq rotating coordinate system, for example, the power grid voltage U g Rotation transformation to get u gd 、u gq ,u gd 、u gq They represent the d-axis component and q-axis component of the grid voltage in the rotating coordinate system respectively. The photovoltaic inverter inductor current i is transformed by rotation to obtain i d 、i q,i d 、i q They represent the d-axis component and q-axis component of the grid current in the rotating coordinate system, and the instantaneous voltage value u at the PCC node p Rotation transformation to get u pd 、u pq ,u pd 、u pq They respectively represent the d-axis component and q-axis component of the PCC node voltage, and so on.

[0068] Since the grid voltage contains harmonic components, after obtaining the instantaneous value of the grid phase voltage, the instantaneous value of the grid phase voltage can be harmonically decomposed to obtain the harmonic voltage, and then the harmonic voltage can be rotationally transformed to obtain the harmonic voltage phasor in the rotating coordinate system.

[0069] Step 130, constructing an equivalent control circuit of the photovoltaic inverter, and obtaining a current dq-axis component matrix of the PCC node based on the equivalent control circuit, wherein the PCC node is a coupling common point between the photovoltaic inverter and the power grid.

[0070] In an embodiment of the present invention, an equivalent control circuit of a photovoltaic inverter is constructed, and a calculation model of harmonic current is constructed based on the equivalent control circuit to obtain an expression of a current dq-axis component matrix of a PCC node.

[0071] Step 140: convert the current dq-axis component matrix of the PCC node into a current harmonic phasor matrix using the voltage harmonic phasor as an excitation condition.

[0072] The obtained voltage harmonic phasor is substituted into the expression of the current dq-axis component matrix of the PCC node to obtain the current harmonic phasor matrix of the PCC node.

[0073] Step 150: Decompose and calculate the current harmonic phasor matrix to obtain the harmonic current.

[0074] After obtaining the current harmonic phasor, the current harmonic phasor is decomposed into harmonic currents in a stationary coordinate system.

[0075] The embodiment of the present invention calculates the harmonic current by calculating the harmonic current based on the harmonic voltage, so as to provide directional suggestions for the control loop design through quantitative calculation, thereby effectively reducing the harmonic current, increasing the load capacity, reducing the risk of impact on the power grid, ensuring the normal operation of the equipment, and avoiding related accidents.

[0076] In one embodiment of the present invention, the grid parameters at least include the instantaneous value of the grid phase voltage. Step 120, performing harmonic decomposition and rotation transformation on the grid parameters to obtain the voltage harmonic phasor in the rotating coordinate system, specifically includes:

[0077] Step 121. Perform harmonic decomposition and rotation transformation on the phase voltage of the power grid to obtain the dq-axis components of the harmonic voltage of the power grid in the rotating coordinate system.

[0078] Since the three-phase three-wire power grid has no neutral line and no zero-sequence current, the zero-sequence part of the phase voltage can be ignored. The zero-sequence voltages in the output phase voltages of the three-phase three-wire photovoltaic inverter can cancel each other out. It can be considered that the output phase voltages of the photovoltaic inverter do not contain zero-sequence voltages, and there is theoretically no zero-sequence current path in the three-phase three-wire power system, which further reduces the influence of zero-sequence voltage harmonics. Therefore, in the embodiment of the present invention, the influence of zero sequence is ignored, and only positive-sequence harmonics and negative-sequence harmonics are considered. The harmonic voltage contained in the grid voltage can be obtained by harmonic decomposition of the grid phase voltage, and finally the harmonic voltage is subjected to rotation transformation to obtain the dq-axis components of the harmonic voltage of the grid in the rotating coordinate system.

[0079] Specifically, Step 121, the positive sequence harmonics and negative sequence harmonics of the phase voltage of the power grid are decomposed and rotated to obtain the dq axis components of the harmonic voltage of the power grid in the rotating coordinate system, which specifically includes:

[0080] Step 1211, perform harmonic decomposition on the phase voltage of the power grid to obtain the three-phase harmonic voltage in the phase voltage of the power grid.

[0081] In the time domain, the phase voltage u of the three-phase three-wire power grid is ga 、u gb ,u gc The harmonic voltage u contained in ga-h ,u gb-h ,u gc-h They can be defined as follows:

[0082]

[0083] In the formula, u ga-h ,u gb-h ,u gc-h are the harmonic voltages of the three phases A, B, and C of the power grid, k is the harmonic order, U k-1 is the k-1th negative sequence harmonic voltage, ω 0 is the grid fundamental angular velocity, t is the time variable, θ k-1 Indicates the phase angle of the k-1th negative sequence harmonic voltage, U k+1 is the k+1th positive sequence harmonic voltage, θ k+1 Represents the phase angle of the k+1th positive-sequence harmonic voltage.

[0084] Step 1212, perform rotation transformation on the three-phase harmonic voltage of the power grid to obtain the dq-axis components of the voltage harmonics of the power grid in the rotating coordinate system.

[0085] In the embodiment of the present invention, the three-phase harmonic voltage is subjected to rotation transformation to obtain the dq axis component of the harmonic voltage, that is, the three-phase variable is converted into two variables, which can simplify the calculation. In addition, because the grid-connected control algorithm adopted by the photovoltaic inverter is direct current control based on dq transformation, the dq axis component converted into the harmonic voltage is consistent with the actual object, which can improve the credibility of the model.

[0086] Among them, the dq-axis components of the voltage harmonics of the power grid are expressed as:

[0087]

[0088] In the formula, u gd-h 、u gq-h are the d-axis component and q-axis component of the harmonic voltage in the rotating coordinate system respectively; ω 0 is the grid fundamental angular velocity, t is the time variable, u ga-h ,u gb-h ,u gc-h They are the harmonic voltages of the three phases A, B and C of the power grid respectively.

[0089] Step 122, convert the dq axis components of the harmonic voltage into the dq axis components phasors of the harmonic voltage in the rotating coordinate system.

[0090] In one embodiment of the present invention, the dq-axis component phasor of the harmonic voltage in the rotating coordinate system is expressed as:

[0091]

[0092] In the formula, is the d-axis component phasor of the kth harmonic voltage in the rotating coordinate system, is the q-axis component phasor of the kth harmonic voltage in the rotating coordinate system, U gdk is the d-axis component of the kth harmonic voltage in the rotating coordinate system, θ gdk is the phase angle of the d-axis component of the kth harmonic voltage in the rotating coordinate system, U k-1 is the k-1th negative sequence harmonic voltage, U k+1 is the k+1th positive sequence harmonic voltage, θ k+1 represents the phase angle of the k+1th positive sequence harmonic voltage, θ k-1 Represents the phase angle of the k-1th voltage harmonic.

[0093] In one embodiment of the present invention, step 130, obtaining the current dq-axis component matrix of the PCC node based on the equivalent control circuit, specifically includes:

[0094] Step 131. Based on the equivalent control circuit, the transfer function from the PV inverter output current to the PCC node voltage is obtained.

[0095] In the embodiment of the present invention, the equivalent control circuit block diagram can refer to Figure 3 In the rotating coordinate system, the transfer function matrix expression G of the photovoltaic inverter inductor current and output voltage is 0 It can be expressed as:

[0096]

[0097] Where s represents the Laplace operator, I 2x2 represents the second-order unit matrix, A and B represent the state matrix and input matrix respectively, R represents the parasitic resistance of the photovoltaic inverter filter inductor, L represents the inductance of the photovoltaic inverter filter inductor, ω 0 Indicates the grid fundamental wave angular velocity (i.e. 2π×grid fundamental wave frequency 50Hz);

[0098] Among them, the specific expressions of matrices A and B are:

[0099]

[0100] In the embodiment of the present invention, the PCC node voltage feedforward term transfer function matrix expression is: It can be expressed as:

[0101]

[0102] In the formula, represents the steady-state value of the d-axis component of the PCC node voltage in the rotating coordinate system, G PI-PLL It is a phase-locked loop proportional integral regulator, and its expression is:

[0103]

[0104] In the formula, k p-PLL Indicates the proportional coefficient of the built-in proportional-integral regulator of the phase-locked loop, k i-PLL It indicates the integral coefficient of the proportional-integral regulator built into the phase-locked loop.

[0105] In an embodiment of the present invention, a conversion matrix for estimating an output voltage error and a current error caused by a phase-locked loop angle error is determined.

[0106] Among them, the output voltage error conversion matrix expression caused by the phase-locked loop angle error is It can be expressed as:

[0107]

[0108] In the formula, Represents the steady-state value of the d-axis component of the PV inverter output voltage, Represents the steady-state value of the q-axis component of the PV inverter output voltage.

[0109] According to the transfer function matrix of the output voltage difference caused by the phase-locked loop angle error and the voltage feedforward term in equations (6) and (8), the intermediate variable is defined as for:

[0110]

[0111] Among them, the conversion matrix expression for estimating the dq axis current error caused by the phase-locked loop angle error is It can be expressed as:

[0112]

[0113] In the formula, and They respectively represent the steady-state values ​​of the d-axis and q-axis components of the photovoltaic inverter inductor current.

[0114] In the embodiment of the present invention, the proportional-integral regulator control law in the current loop can be expressed as a matrix G pi-CRT :

[0115]

[0116] In the formula, k p-CRT Indicates the proportional coefficient of the current loop corresponding to the proportional-integral regulator, k i-CRT Indicates the integral coefficient of the proportional-integral regulator corresponding to the current loop.

[0117] In the embodiment of the present invention, the feedforward decoupling matrix G of the dq axis current components DE It is expressed as:

[0118]

[0119] Combining equation (11) and equation (12), we can define the intermediate variable matrix G c for:

[0120]

[0121] In the embodiment of the present invention, the output voltage of the photovoltaic inverter is limited by the switching cycle and has a certain hysteresis effect. The delay characteristic can be approximately expressed as G by a first-order low-pass filter link. VSC :

[0122]

[0123] Where, T s Represents the switching cycle of the PV inverter.

[0124] Combining the above formulas (4)-(14) and Figure 2The equivalent control block diagram of the photovoltaic inverter is shown in Figure 1, and the inductor current is defined as positive when it flows into the grid. The transfer function F(s) from the photovoltaic inverter output current to the PCC node voltage can be obtained:

[0125]

[0126] Step 132, calculate the current dq axis component matrix of the PCC node according to the transfer function, the dq axis components of the harmonic voltage of the power grid and the preset impedance in the equivalent control circuit.

[0127] In one embodiment of the present invention, the grid impedance is considered, see Figure 4 , Figure 4 The overall control block diagram of the photovoltaic inverter after considering the grid impedance in the embodiment of the present invention. Generally, the grid is resistive and inductive, and its dq domain impedance Z gdq It can be expressed in matrix form as:

[0128]

[0129] In the formula, Z gdd The d-axis-d-axis component of the grid impedance represents the change in the grid d-axis voltage caused by the grid unit d-axis current. gdq It represents the d-axis-q-axis component of the grid impedance, and characterizes the change in the grid q-axis voltage caused by the grid unit d-axis current. gqd It represents the q-axis-d-axis component of the grid impedance, and characterizes the change in the grid d-axis voltage caused by the unit q-axis current of the grid. gqq It represents the q-axis-q-axis component of the grid impedance, and characterizes the change in the grid q-axis voltage caused by the unit q-axis current of the grid.

[0130] After considering the grid inductive reactance, combined with Figure 4 In the control block diagram, in the s domain (Laplace domain), the d-axis component U of the grid voltage can be gd(s) and the q-axis component U gq(s) Obtain the d-axis component I of the PCC node current of the photovoltaic inverter d(s) and the q-axis component I q(s) , written in matrix form, can be expressed as the current dq axis component matrix expression of the PCC node:

[0131]

[0132] Where s is the Laplace domain, I d is the d-axis component of the current at the PCC node, I q is the q-axis component of the current at the PCC node, Fe is the transfer function from the PV inverter output current to the PCC node voltage, and Z gdq is the preset impedance, U gdis the d-axis component of the harmonic voltage of the power grid, U gq is the q-axis component of the harmonic voltage of the power grid.

[0133] In one embodiment of the present invention, to simplify the formula, an intermediate variable G may be defined as 1(s) :

[0134]

[0135] Based on this, the matrix expression of the current dq axis component of the PCC node can be simplified as:

[0136]

[0137] The current dq axis component matrix expression obtained in equation (19) is converted into the current harmonic phasor matrix form with the voltage harmonic phasor as the excitation condition:

[0138]

[0139] In the formula, I dk is the kth current harmonic d-axis component in the rotating coordinate system, I qk is the kth current harmonic q-axis component in the rotating coordinate system, θ dk2 is the phase angle of the d-axis component of the kth current harmonic in the rotating coordinate system, θ qk2 is the phase angle of the q-axis component of the k-th current harmonic in the rotating coordinate system, and the Laplace operator s=kw0×j, where j represents an imaginary unit.

[0140] In one embodiment of the present invention, step 150, decomposing and calculating the current harmonic phasor matrix to obtain the harmonic current, specifically includes:

[0141] Step 151. Decompose the current harmonic phasor matrix into negative-sequence current harmonic components and positive-sequence circuit harmonic components in the three-phase stationary coordinate system.

[0142] The current harmonic phasor matrix in equation (20) is decomposed into negative-sequence harmonic and positive-sequence current harmonic components in the three-phase stationary coordinate system, which can be expressed in phasor form as follows:

[0143]

[0144] In the formula, I k-1 ,I k+1 They correspond to the k-1th and k+1th current harmonics in the stationary coordinate system, respectively. and They correspond to the phase angles of the k-1th and k+1th current harmonics in the stationary coordinate system. The k-1th and k+1th current harmonics in the stationary coordinate system are obtained by formula (21), that is, the negative sequence current harmonic component and the positive sequence harmonic component in the stationary coordinate system are obtained.

[0145] Step 152, calculate the harmonic current based on the negative sequence current harmonic component and the positive sequence current harmonic component.

[0146] After the negative sequence current harmonic component and the positive sequence current harmonic component are obtained in the stationary coordinate system, the harmonic current can be calculated based on the negative sequence current harmonic component and the positive sequence current harmonic component.

[0147] Specifically, the negative-sequence current harmonic component and the positive-sequence current harmonic component are substituted into the harmonic current time domain expression to calculate the harmonic current.

[0148] Among them, the time domain expression of harmonic current is:

[0149]

[0150] In the formula, t is the time variable, i a-h 、i b-h 、i c-h are the harmonic currents of the three phases A, B, and C of the power grid, k is the harmonic order, I k-1 is the k-1th negative sequence harmonic current, I k+1 is the k+1th positive sequence harmonic current, ω 0 is the grid fundamental angular velocity, is the phase angle of the k-1th negative sequence harmonic current in the three-phase stationary coordinate system, is the phase angle of the k+1th positive-sequence harmonic current in the three-phase stationary coordinate system.

[0151] The present invention constructs a calculation model for harmonic current, calculates the harmonic voltage according to the grid parameters, and calculates the harmonic current based on the harmonic voltage, so as to provide directional suggestions for the control loop design through the harmonic current, thereby effectively reducing the harmonic current, increasing the load capacity, reducing the risk of impact on the grid, ensuring the normal operation of the equipment, and avoiding related accidents.

[0152] In one embodiment of the present invention, the calculation method of harmonic current may specifically be:

[0153] Step 1: Set the required grid parameters and define the basic electrical physical quantities. As shown in Table 1, set the harmonic order k, the k-1th grid harmonic voltage effective value U k-1 , the effective value of the k+1th grid harmonic voltage U k+1 , the k-1th grid voltage harmonic phase angle θ k-1 , the k+1th grid voltage harmonic phase angle θ k+1 , PV inverter inductance L, grid inductance L g , PV inverter inductor parasitic resistance R, grid resistance R g , steady-state current of photovoltaic inverter Photovoltaic inverter steady-state current Photovoltaic inverter switching cycle T s , the d-axis component of the steady-state voltage of the PV inverter PCC node Steady-state value of d-axis component of photovoltaic inverter output voltage Steady-state value of the q-axis component of the photovoltaic inverter output voltage Current loop control parameter k p-c , current loop control parameter k i-c , phase-locked loop control parameter k p-PLL , phase-locked loop control parameter k i-PLL , grid fundamental frequency ω 0 , effective value of grid fundamental voltage.

[0154] Table 1 Basic electrical physical quantities

[0155]

[0156]

[0157] According to the above table, the harmonic order k=6, the three-phase voltage contains five negative sequence and seven positive sequence harmonics. According to formula (1) and the parameters in the above table, the time domain expression of the three-phase harmonic voltage of the power grid is defined, that is, the initial condition is (unit Ford V):

[0158]

[0159] Step 2: Substituting the above values ​​into equation (3), the phasor form of the kth (k=6) voltage harmonic in the dq coordinate system can be obtained:

[0160] U gdk That is U gd6 =4j,U gqk That is U gq6 =16;

[0161] Substituting the above values ​​into equation (18), we can get the intermediate variable matrix G 1 The value of:

[0162]

[0163] Step 3: According to formula (19), the phasor form of the kth (k=6) current harmonic can be obtained as:

[0164] I d6 =0.0137-0.0681j

[0165] I q6 = -0.2951 - 0.0723j (22)

[0166] Step 4: According to formula (20), the phasor form of the k-1th negative sequence harmonic and the k+1th positive sequence current harmonic components in the three-phase stationary coordinate system is calculated as follows:

[0167] Right now

[0168] Right now

[0169] From this we can get I k-1 , I k+1 Effective value and phase angle, I k-1 That is, I5 = 0.1172; I k+1 That is, I7 = 0.1866, Right now Right now

[0170] Based on the above, referring to formula (21), the final time domain expression of harmonic current can be obtained as follows (unit: ampere A):

[0171]

[0172] Then, the theoretical calculated value of phase current harmonic distortion rate is 3.12%.

[0173] Step 5: Simulate and verify the method. Use the same data to build a photovoltaic inverter simulation model, inject 5th negative sequence and 7th positive sequence harmonics into the three-phase power grid, and observe the photovoltaic inverter current waveform with the current flowing into the power grid as the positive direction. Figure 5 , Figure 5 The grid voltage waveform and photovoltaic inverter current waveform of the embodiment of the present invention are shown in Figure 1. The frequency domain Fourier analysis is performed on the waveform, and the effective value of the 5th harmonic current is 0.1097A, and the phase angle is 14.20° ahead of the 5th harmonic voltage in the grid voltage. The effective value of the 7th harmonic current is 0.1931A, and the phase angle is 12.95° ahead of the 7th harmonic voltage in the grid voltage. The theoretical calculated value of the phase current harmonic distortion rate is approximately equal to 3.14%. It can be seen that the theoretical calculation results are basically consistent with the simulation results, so the correctness of the calculation method and steps of the harmonic current of the embodiment of the present invention can be verified.

[0174] The present invention also provides a harmonic current calculation device, which can be referred to Figure 6 , Figure 6 6 is a structural block diagram of a harmonic current calculation device in an embodiment of the present invention. The device includes: a parameter acquisition unit 601 , an equivalent control unit 602 and a decomposition calculation unit 603 .

[0175] The parameter acquisition unit 601 is used to acquire grid parameters after the photovoltaic inverter is connected to the grid.

[0176] The grid parameters are transformed by rotation to obtain the voltage harmonic phasors in the rotating coordinate system.

[0177] The equivalent control unit 602 is used to construct an equivalent control circuit of the photovoltaic inverter, and obtain the current dq-axis component matrix of the PCC node based on the equivalent control circuit, wherein the PCC node is a coupling common point between the photovoltaic inverter and the power grid.

[0178] The decomposition calculation unit 603 is used to convert the current dq axis component matrix of the PCC node into a current harmonic phasor matrix using the voltage harmonic phasor as an excitation condition.

[0179] The harmonic current is obtained by decomposing and calculating the current harmonic phasor matrix.

[0180] The harmonic current calculation device of the present invention calculates the harmonic current by calculating the harmonic current based on the harmonic voltage, so as to provide directional suggestions for the control loop design through quantitative calculation, thereby effectively reducing the harmonic current, increasing the load capacity, reducing the risk of impact on the power grid, ensuring the normal operation of the equipment, and avoiding related accidents.

[0181] Figure 7 FIG. 1 shows an internal structure diagram of a computer device in one embodiment of the present invention. The computer device may be a terminal or a system. Figure 7 As shown, the computer device includes a processor, a memory and a network interface connected via a system bus. The memory includes a non-volatile storage medium and an internal memory. The non-volatile storage medium of the computer device stores an operating system and may also store a computer program. When the computer program is executed by the processor, the processor may implement each step in the above method embodiment. The internal memory may also store a computer program. When the computer program is executed by the processor, the processor may implement each step in the above method embodiment. Those skilled in the art will understand that Figure 7 The structure shown in the figure is only a block diagram of a part of the structure related to the solution of the present application, and does not constitute a limitation on the computer device to which the solution of the present application is applied. The specific computer device may include more or fewer components than those shown in the figure, or combine certain components, or have a different arrangement of components.

[0182] In one embodiment, a computer device is provided, including a memory and a processor, wherein the memory stores a computer program, and when the computer program is executed by the processor, the processor executes each step in the above method embodiment.

[0183] In one embodiment, a computer-readable storage medium is proposed, which stores a computer program. When the computer program is executed by a processor, the processor executes each step in the above method embodiment. A person of ordinary skill in the art can understand that all or part of the processes in the above embodiment method can be completed by instructing the relevant hardware through a computer program. The program can be stored in a non-volatile computer-readable storage medium, and the program can include the processes of the embodiments of the above methods when executed. Among them, any reference to memory, storage, database or other media used in the embodiments provided in this application may include non-volatile and / or volatile memory. Non-volatile memory may include read-only memory (ROM), programmable ROM (PROM), electrically programmable ROM (EPROM), electrically erasable programmable ROM (EEPROM) or flash memory. Volatile memory may include random access memory (RAM) or external cache memory. By way of illustration and not limitation, RAM is available in many forms, such as static RAM (SRAM), dynamic RAM (DRAM), synchronous DRAM (SDRAM), double data rate SDRAM (DDRSDRAM), enhanced SDRAM (ESDRAM), synchronous link (Synchlink) DRAM (SLDRAM), memory bus (Rambus) direct RAM (RDRAM), direct memory bus dynamic RAM (DRDRAM), and memory bus dynamic RAM (RDRAM), etc.

[0184] The technical features of the above embodiments may be combined arbitrarily. To make the description concise, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0185] The above embodiments only express several implementation methods of the present application, and the descriptions thereof are relatively specific and detailed, but they cannot be understood as limiting the scope of the present application. It should be pointed out that, for a person of ordinary skill in the art, several modifications and improvements can be made without departing from the concept of the present application, and these all belong to the protection scope of the present application. Therefore, the protection scope of the present application shall be subject to the attached claims.

Claims

1. A method for calculating harmonic current, characterized in that: The method comprises: Obtain grid parameters after the photovoltaic inverter is connected to the grid; Performing harmonic decomposition and rotation transformation on the power grid parameters to obtain voltage harmonic phasors in a rotating coordinate system; Constructing an equivalent control circuit of the photovoltaic inverter, and obtaining a current dq-axis component matrix of a PCC node based on the equivalent control circuit, wherein the PCC node is a coupling common point between the photovoltaic inverter and the power grid; Converting the current dq-axis component matrix of the PCC node into a current harmonic phasor matrix using the voltage harmonic phasor as an excitation condition; The current harmonic phasor matrix is ​​decomposed and calculated to obtain the harmonic current.

2. The method according to claim 1, characterized in that The grid parameters at least include the instantaneous value of the phase voltage of the grid, and the harmonic decomposition and rotation transformation of the grid parameters to obtain the voltage harmonic phasor in the rotating coordinate system specifically includes: Performing harmonic decomposition and rotation transformation on the phase voltage of the power grid to obtain dq-axis components of the harmonic voltage of the power grid in a rotating coordinate system; The dq-axis components of the harmonic voltage are converted into dq-axis component phasors of the harmonic voltage in a rotating coordinate system.

3. The method according to claim 2, characterized in that The harmonic decomposition and rotation transformation of the phase voltage of the power grid to obtain the dq-axis components of the harmonic voltage of the power grid in the rotating coordinate system specifically includes: Performing harmonic decomposition on the phase voltage of the power grid to obtain three-phase harmonic voltage in the phase voltage of the power grid; The three-phase harmonic voltage of the power grid is expressed as: In the formula, u ga-h ,u gb-h ,u gc-h are the harmonic voltages of the three phases A, B, and C of the power grid, k is the harmonic order, U k-1 is the k-1th negative sequence harmonic voltage, ω0 is the grid fundamental angular velocity, t is the time variable, θ k-1 Indicates the phase angle of the k-1th negative sequence harmonic voltage, U k+1 is the k+1th positive sequence harmonic voltage, θ k+1 Represents the phase angle of the k+1th positive sequence harmonic voltage; Rotating and transforming the three-phase harmonic voltage of the power grid to obtain dq-axis components of the voltage harmonics of the power grid in a rotating coordinate system; The dq-axis components of the voltage harmonics of the power grid are expressed as: In the formula, u gd-h 、u gq-h are the d-axis component and q-axis component of the harmonic voltage in the rotating coordinate system respectively; ω0 is the fundamental angular velocity of the power grid, t is the time variable, u ga-h ,u gb-h ,u gc-h are the harmonic voltages of the three phases A, B and C of the power grid respectively.

4. The method according to claim 2, characterized in that The dq-axis component phasor of the harmonic voltage in the rotating coordinate system is expressed as: In the formula, is the d-axis component phasor of the kth harmonic voltage in the rotating coordinate system, is the q-axis component phasor of the kth harmonic voltage in the rotating coordinate system, U gdk is the d-axis component of the kth harmonic voltage in the rotating coordinate system, θ gdk is the phase angle of the d-axis component of the kth harmonic voltage in the rotating coordinate system, U k-1 is the k-1th negative sequence harmonic voltage, U k+1 is the k+1th positive sequence harmonic voltage, θ k+1 represents the phase angle of the k+1th positive sequence harmonic voltage, θ k-1 Represents the phase angle of the k-1th voltage harmonic.

5. The method according to claim 2, characterized in that The obtaining of the current dq-axis component matrix of the PCC node based on the equivalent control circuit specifically includes: Obtaining a transfer function from the output current of the photovoltaic inverter to the PCC node voltage based on the equivalent control circuit; Calculating a current dq-axis component matrix of the PCC node according to the transfer function, the dq-axis components of the harmonic voltage of the power grid and the impedance preset in the equivalent control circuit; The current dq-axis component matrix expression of the PCC node is: Where s is the Laplace domain, I d is the d-axis component of the current at the PCC node, I q is the q-axis component of the current at the PCC node, Fe is the transfer function from the PV inverter output current to the PCC node voltage, and Z gdq is the preset impedance, U gd is the d-axis component of the harmonic voltage of the power grid, U gq is the q-axis component of the harmonic voltage of the power grid.

6. The method according to claim 1, characterized in that Decomposing and calculating the current harmonic phasor matrix to obtain the harmonic current specifically includes: Decomposing the current harmonic phasor matrix into negative-sequence current harmonic components and positive-sequence current harmonic components in a three-phase stationary coordinate system; The harmonic current is calculated according to the negative-sequence current harmonic component and the positive-sequence current harmonic component.

7. The method according to claim 6, characterized in that The calculating the harmonic current according to the negative-sequence current harmonic component and the positive-sequence current harmonic component specifically includes: Substituting the negative-sequence current harmonic component and the positive-sequence current harmonic component into the harmonic current time domain expression to calculate the harmonic current; Among them, the time domain expression of the harmonic current is: In the formula, t is the time variable, i a-h 、i b-h 、i c-h are the harmonic currents of the three phases A, B and C of the power grid, k is the harmonic order, I k-1 is the k-1th negative sequence harmonic current, I k+1 is the k+1th positive sequence harmonic current, ω0 is the grid fundamental angular velocity, is the phase angle of the k-1th negative sequence harmonic current in the three-phase stationary coordinate system, is the phase angle of the k+1th positive-sequence harmonic current in the three-phase stationary coordinate system.

8. A harmonic current calculation device, characterized in that: The device comprises: a parameter acquisition unit, an equivalent control unit and a decomposition calculation unit; The parameter acquisition unit is used to acquire grid parameters after the photovoltaic inverter is connected to the grid; Performing harmonic decomposition and rotation transformation on the power grid parameters to obtain voltage harmonic phasors in a rotating coordinate system; The equivalent control unit is used to construct an equivalent control circuit of the photovoltaic inverter, and obtain a current dq-axis component matrix of a PCC node based on the equivalent control circuit, wherein the PCC node is a coupling common point between the photovoltaic inverter and the power grid; The decomposition calculation unit is used to convert the current dq-axis component matrix of the PCC node into a current harmonic phasor matrix using the voltage harmonic phasor as an excitation condition; The current harmonic phasor matrix is ​​decomposed and calculated to obtain the harmonic current.

9. A computer-readable storage medium storing a computer program, characterized in that: When the computer program is executed by a processor, the processor is caused to perform the steps of the method according to any one of claims 1 to 7.

10. A computer device comprising a memory and a processor, characterized in that: The memory stores a computer program, and when the computer program is executed by the processor, the processor is caused to perform the steps of the method according to any one of claims 1 to 7.

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