A photovoltaic grid-connected inverter to improve power quality control method, device and medium

Abstract

The present application is a method, device and medium for improving power quality control of photovoltaic grid-connected inverters, which belongs to the technical field of grid-connected converters and provides a solution: detecting the inverter-side current, the voltage at the common connection point and the grid-side current at the common connection point; calculating the voltage amplitude and phase angle of the common connection point, as well as the inverter-side current amplitude and phase angle; generating a power positive-sequence reference current; extracting real-time harmonic current, negative-sequence current and zero-sequence current, and calculating the amplitude and phase angle of the negative-sequence current, and the peak values ​​of the zero-sequence and harmonic currents; allocating coefficients of the negative-sequence current, the zero-sequence current and the harmonic current and multiplying the coefficients of the three with their respective corresponding current values ​​and then adding them to obtain a compensation reference current; calculating whether the sum of the power positive-sequence reference current and the compensation reference current is greater than the inverter current limit, if so, the compensation current limit is applied, otherwise, no compensation is required, thereby realizing efficient utilization of the inverter capacity.

Description

Technical Field

[0001] The present invention belongs to the technical field of grid-connected converters, and relates to a control method, a device and a medium for improving power quality of a photovoltaic grid-connected inverter. Background Art

[0002] Renewable energy generation represented by photovoltaics is growing rapidly, especially distributed photovoltaic power generation. Nonlinear and unbalanced loads such as electric vehicles are increasing day by day. Distributed power sources and loads are continuously connected, and the power quality of distribution networks has dropped sharply. The harmonics and unbalanced content of the power grid have increased, which seriously affects the stability of the power grid, increases the power grid loss, and interferes with the normal operation of loads. Since the circuit topology of photovoltaic inverters is similar to that of active filters, the filtering function can be realized while generating electricity by changing the inverter control algorithm. In addition, distributed photovoltaics are scattered in the distribution network. The rational use of these hardware resources can achieve large-scale regional governance and effectively improve the power quality of the current distribution network.

[0003] To this end, some literature has proposed to extract the harmonics and unbalanced components from the grid current by sampling it, superimpose the active component of photovoltaic power generation, and use this as the reference current of the inverter, so that the inverter can provide harmonics and unbalanced currents to improve the power quality of the grid. However, this method does not take into account that the main task of the inverter in actual engineering is still to generate electricity, and the capacity of the inverter is limited. When the inverter is close to full load, the inverter is still used to control harmonics, which may cause its current to exceed the designed safe operating range and cause damage to the inverter. For this reason, some literature has proposed power quality optimization algorithms that meet the inverter capacity limit. These optimization algorithms construct a multi-objective optimization function for power quality, and then solve it through an intelligent algorithm to obtain a compensation current reference, thereby achieving inverter power quality control. However, these methods assume that the inverter is an ideal model and its output can accurately track the input reference current. However, due to the delay, limited bandwidth and poor tracking ability of high-order harmonics of the actual inverter, the compensation effect obtained by modeling calculation is often unable to be achieved. Therefore, there is a waste of inverter capacity. In addition, these methods need to manually determine the weights of various power quality indicators when constructing the objective function, which is subjective to a certain extent, resulting in the objective function having a better effect only under certain working conditions and the reflected results are not comprehensive. Summary of the invention

[0004] In view of the existing technical defects, the purpose of the present invention is to provide a control method for improving the power quality of a photovoltaic grid-connected inverter, aiming to solve the problems of low capacity utilization, unsatisfactory compensation effect and strong subjectivity in the design of optimal parameters of the existing power quality optimization control.

[0005] To achieve the above object, the present invention provides a method for improving power quality control of a photovoltaic grid-connected inverter, comprising:

[0006] Detecting the inverter-side current, the voltage and the grid-side current of the common connection point; wherein the current flowing into the inverter-side current is positive, and the current flowing out of the grid-side current is positive;

[0007] Based on the detected inverter-side current and voltage at the common connection point, the common connection point voltage amplitude and phase angle, as well as the inverter-side current amplitude and phase angle are calculated;

[0008] Generate a power positive sequence reference current based on the voltage amplitude and phase angle of the common connection point and the power command;

[0009] Based on the detected grid-side current at the common connection point, the grid-side current waveform distortion rate and three-phase imbalance are calculated; and the real-time harmonic current, negative-sequence current, and zero-sequence current are extracted from the detected grid-side current at the common connection point, and the amplitude and phase angle of the negative-sequence current, as well as the peak values ​​of the zero-sequence and harmonic currents are calculated;

[0010] Based on the current amplitude and phase angle on the inverter side, the amplitude and phase angle of the negative sequence current on the grid side, and the peak values ​​of the zero sequence current and harmonic current, the limiting coefficients of the negative sequence current, zero sequence current and harmonic current are allocated and multiplied by their respective corresponding current values ​​and then added to obtain the compensation reference current;

[0011] Get the remaining capacity of the inverter, set the limiting coefficients of negative sequence current, zero sequence current and harmonic current to 1, calculate whether the sum of the power positive sequence reference current and the compensation reference current is greater than the inverter current limiting, if so, the compensation current limiting is performed, otherwise, there is no need to perform the compensation current limiting;

[0012] The compensation current limiting is specifically as follows: using data envelopment analysis, by inputting the grid current waveform distortion rate, three-phase imbalance and the remaining capacity of the inverter, multiple groups of efficiency values ​​are output, and the result with the largest efficiency value and the limiting coefficients of the negative-sequence current, zero-sequence current and harmonic current corresponding to the efficiency value are selected as the compensation result to achieve efficient utilization of the inverter capacity.

[0013] The application scenario of this control inverter is that the photovoltaic inverter is connected to the grid through the common connection point, and the common connection point is also connected to the local load. By detecting the inverter side current of the common connection point as the feedback value of the control loop, the inverter output current is controlled, the common connection point voltage is detected and phase-locked, and the grid voltage phase angle is obtained as the inverter power reference phase angle to realize the inverter to transmit power to the grid. The inverter side current of the common connection point is detected and the positive sequence fundamental current amplitude and phase angle in the current are calculated for the purpose of subsequent calculation of negative sequence current capacity. The amplitude and phase angle calculation adopts fast Fourier transform, and the voltage phase angle of 0 degrees is used as the reference phase angle.

[0014] Furthermore, the power positive sequence reference current is calculated through the voltage amplitude and phase angle of the common connection point and the inverter grid-connected power value, and its expression is:

[0015]

[0016] Where P and Q are the inverter power commands, V m and I m are the amplitudes of voltage and current phasors respectively; and are the phases corresponding to voltage and current respectively; u d is the projection of voltage in the dq coordinate system; i ref,d and i ref,q They are the d-axis reference value and q-axis reference value of the power positive sequence reference current in the dq coordinate system respectively. The power positive sequence reference current is obtained by performing dq inverse transformation. i pq .

[0017] Furthermore, the compensation reference current expression is:

[0018]

[0019]

[0020] In the formula, i iq To compensate the reference current, i 1. i 2. i 0 are the calculated positive sequence, negative sequence and zero sequence currents, i h is the harmonic current; k 2. k 0. k h They are the limiting coefficients of negative sequence current, zero sequence current and harmonic current respectively; i L It is the difference between the detected inverter side current and the grid side current. All currents are three-phase currents in the abc coordinate system.

[0021] Furthermore, the grid-side current is decomposed into positive-sequence, negative-sequence, and zero-sequence components by the symmetrical component method;

[0022] The positive and negative sequence components are transformed from the three-phase coordinate system to the two-phase stationary coordinate system through Clark transformation;

[0023] An orthogonal generator based on second-order generalized integral is used to separate the fundamental wave and generate orthogonal signals;

[0024] Then, the positive sequence current and negative sequence current in the three-phase abc coordinate system are obtained by Clark inverse transformation, and the zero sequence current is the three-phase load current. i L The average value is used to calculate the harmonic current.

[0025] The expression is:

[0026]

[0027] In the formula , subscripts 1 and 2 represent positive sequence and negative sequence respectively.

[0028] The zero-sequence current is:

[0029]

[0030] The three-phase coordinate system is transformed into a two-phase stationary coordinate system by Clark, and the positive sequence and negative sequence components can be expressed as:

[0031]

[0032] In the formula , which is a 90° lag operation. It can be seen that to separate the positive and negative sequence components, it is necessary to first obtain the orthogonal signal of the input signal.

[0033] The orthogonal generator based on the second-order generalized integral is used to generate a 90° phase angle offset based on the sinusoidal signal. This method can also filter out the high-order harmonics in the signal. The transfer function expression is:

[0034]

[0035] The amplitude-frequency characteristics and phase-frequency characteristics are:

[0036]

[0037] Where D(s) is the transfer function between the input signal i(s) and the output signal i'(s), is the resonant frequency, k is the system gain, Q(s) is the transfer function of the input signal i(s) and the output signal qi'(s), qi'(s) and i'(s) are a set of orthogonal signals, |D| and |Q| are the amplitude-frequency characteristics of the transfer functions D(s) and Q(s), ∠D and ∠Q are the phase-frequency characteristics of the transfer functions D(s) and Q(s), ω is the frequency, and s represents the dependent variable of the transfer function.

[0038] Output signal Always lags behind the signal 90°, therefore, the resonant frequency If it is set to the fundamental frequency of the power grid, only the fundamental frequency component can pass through the system without loss, and the harmonic components in the signal will be greatly attenuated, thereby achieving the goal of separating the fundamental wave and generating orthogonal signals.

[0039] The positive-sequence and negative-sequence components in the separated two-phase stationary coordinate system are subjected to Clark inverse transformation to obtain the positive-sequence and negative-sequence currents in the grid-side current, and the negative-sequence current is subjected to fast Fourier transformation to obtain its amplitude and phase angle.

[0040] Furthermore, the data envelopment analysis method is expressed as:

[0041]

[0042] In the formula, θ l Representatives of the surveyed l The efficiency value of a decision-making unit, λ j For the j The combination coefficient of the decision making unit, S — and S + are the slack variables for the input indicators and the residual variables for the output indicators; X lj For the l The decision-making unit j An input indicator, Y lj For the l The decision-making unit j In this model, the power quality data corresponding to each set of limiting coefficients is a decision unit. Each decision unit has an input indicator, namely the inverter input capacity, and two output indicators, namely the grid current waveform distortion rate and the grid current three-phase imbalance. The efficiency value of each decision unit is finally calculated based on the input and output.

[0043] The algorithm makes a vertical comparison of the actual effects of the inverter in improving power quality under different parameters, selects the relatively optimal operating parameters, objectively and truly reflects the inverter's management capabilities, and improves the inverter capacity utilization rate.

[0044] Another object of the present invention is to provide an inverter control device using the photovoltaic grid-connected inverter to improve power quality control method, comprising:

[0045] A detection module, used to detect the inverter-side current, the voltage and the grid-side current of the common connection point; wherein the current flowing into the inverter-side current of the common connection point is positive, and the current flowing out of the grid-side current of the common connection point is positive;

[0046] A current and voltage calculation module, used for calculating the common connection point voltage amplitude and phase angle, and the inverter side current amplitude and phase angle based on the detected common connection point inverter side current and common connection point voltage;

[0047] A generating module, used for generating a power positive sequence reference current based on a voltage amplitude and a phase angle of a common connection point and a power command;

[0048] A comprehensive calculation module is used to calculate the grid-side current waveform distortion rate and three-phase imbalance based on the detected grid-side current at the common connection point; and to extract the real-time harmonic current, negative-sequence current, and zero-sequence current from the detected grid-side current at the common connection point, and calculate the amplitude and phase angle of the negative-sequence current, as well as the peak values ​​of the zero-sequence and harmonic currents;

[0049] A construction module is used to allocate coefficients of negative sequence current, zero sequence current and harmonic current based on the current amplitude and phase angle of the inverter side, the amplitude and phase angle of the negative sequence current of the grid side, and the peak values ​​of the zero sequence current and harmonic current, and multiply the three with their respective corresponding current values ​​and then add them to obtain a compensation reference current;

[0050] The judgment module is used to obtain the remaining capacity of the inverter, set the limiting coefficients of the negative sequence current, zero sequence current and harmonic current to 1, and calculate whether the sum of the power positive sequence reference current and the compensation reference current is greater than the inverter current limiting. If so, the compensation current limiting is performed, otherwise, there is no need to perform the compensation current limiting;

[0051] The compensation current limitation is specifically as follows: data envelopment analysis is used to input the grid current waveform distortion rate, three-phase imbalance and the remaining capacity of the inverter, and multiple groups of efficiency values ​​are output. The result with the largest efficiency value and the coefficients of the negative-sequence current, zero-sequence current and harmonic current corresponding to the efficiency value are selected as the compensation result to achieve efficient utilization of the inverter capacity.

[0052] A computer-readable storage medium stores a program, which, when executed by a processor, implements the above-mentioned photovoltaic grid-connected inverter power quality improvement control method.

[0053] Beneficial effects:

[0054] 1. The present application provides a smarter, more reliable and more efficient way to manage grid-connected inverters, which can significantly improve the performance of the inverter and the power quality of the grid, especially when the power quality of the grid deteriorates; improve the inverter capacity utilization rate: the proposed capacity demand calculation model considering the phase difference of positive and negative sequence currents is more accurate, thereby increasing the manageable capacity of the inverter and improving the inverter capacity utilization rate.

[0055] 2. By detecting and calculating the amplitude and phase angle of the negative-sequence current to be compensated and the given amplitude and phase angle of the power reference current, the influence of the positive and negative sequence phase difference is considered when calculating the negative-sequence capacity demand, and the capacity demand for compensating the negative-sequence current is accurately calculated, so that the inverter capacity utilization rate is improved, the implementation difficulty is greatly reduced, and the power quality improvement effect is better.

[0056] 3. There is no need to subjectively allocate the capacity of the inverter to improve the power quality. The power quality under multiple groups of different operating parameters is measured online in real time under the current operating conditions. The optimal operating parameters under the current conditions are obtained through super-efficiency data envelopment analysis, which can better adapt to the operating status of the power grid. It has high flexibility and implicitly considers the impact of factors such as delay, hardware performance, and control error on the compensation effect based on online measurement data. Compared with the ideal model data, it has better engineering value. BRIEF DESCRIPTION OF THE DRAWINGS

[0057] Figure 1 is a flow chart of a method for controlling power quality improvement of a photovoltaic grid-connected inverter according to Embodiment 1;

[0058] Figure 2 is a diagram of the main circuit and control structure of the system of Example 1;

[0059] Figure 3 is a structural diagram of harmonic unbalanced current detection in Example 1;

[0060] Figure 4 is a flow chart of generating a current limiting coefficient in Embodiment 1;

[0061] Figure 5 is a flow chart of current limiting coefficient optimization in Example 1;

[0062] Figure 6 It is a simulation result diagram of the photovoltaic grid-connected inverter disclosed in the present invention for improving power quality control when the power quality of the power grid deteriorates under the condition of sufficient capacity in Example 1;

[0063] Figure 7 It is a simulation result diagram of the photovoltaic grid-connected inverter disclosed in the present invention for improving power quality control when the power quality of the power grid deteriorates under the condition of insufficient capacity in Example 1. DETAILED DESCRIPTION

[0064] In order to make the purpose, technical solution and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0065] In view of the problems existing in the prior art, the present invention provides a method and system for improving power quality control of a photovoltaic grid-connected inverter. The present invention is described in detail below in conjunction with the accompanying drawings.

[0066] In distributed photovoltaic power generation systems, photovoltaic power sources and loads are often connected to the grid at the same time. Since the local load contains nonlinear unbalanced loads, harmonics and unbalanced current components will be generated. This part of the current will pass through the line impedance Z g The harmonics and unbalanced currents flow into the power grid, causing the power quality of the PCC point and the power grid along the line to deteriorate, threatening the safe and stable operation of the power grid and the load. The control proposed in the present invention utilizes the idle capacity of the photovoltaic inverter to actively transmit the required harmonics and unbalanced currents to the local load, avoiding the harmonic unbalanced currents from flowing into the main power grid, thereby achieving local governance and greatly reducing the impact on the power quality of the power grid.

[0067] like Figure 1 As shown, the photovoltaic grid-connected inverter power quality improvement control method provided by the embodiment of the present invention includes:

[0068] Detecting the inverter-side current, the voltage and the grid-side current of the common connection point; wherein the current flowing into the inverter-side current is positive, and the current flowing out of the grid-side current is positive;

[0069] Based on the detected inverter-side current and voltage at the common connection point, the common connection point voltage amplitude and phase angle, as well as the inverter-side current amplitude and phase angle are calculated;

[0070] Generate a power positive sequence reference current based on the voltage amplitude and phase angle of the common connection point and the power command;

[0071] Based on the detected grid-side current at the common connection point, the grid-side current waveform distortion rate and three-phase imbalance are calculated; and the real-time harmonic current, negative-sequence current, and zero-sequence current are extracted from the detected grid-side current at the common connection point, and the amplitude and phase angle of the negative-sequence current, as well as the peak values ​​of the zero-sequence and harmonic currents are calculated;

[0072] Based on the current amplitude and phase angle on the inverter side, the amplitude and phase angle of the negative sequence current on the grid side, and the peak values ​​of the zero sequence current and harmonic current, the limiting coefficients of the negative sequence current, zero sequence current and harmonic current are allocated and multiplied by their respective corresponding current values ​​and then added to obtain the compensation reference current;

[0073] Get the remaining capacity of the inverter, set the limiting coefficients of negative sequence current, zero sequence current and harmonic current to 1, calculate whether the sum of the power positive sequence reference current and the compensation reference current is greater than the inverter current limiting, if so, the compensation current limiting is performed, otherwise, there is no need to perform the compensation current limiting;

[0074] The compensation current limiting is specifically as follows: using data envelopment analysis, by inputting the grid current waveform distortion rate, three-phase imbalance and the remaining capacity of the inverter, multiple groups of efficiency values ​​are output, and the result with the largest efficiency value and the limiting coefficients of the negative-sequence current, zero-sequence current and harmonic current corresponding to the efficiency value are selected as the compensation result to achieve efficient utilization of the inverter capacity.

[0075] When the grid current waveform distortion rate or three-phase imbalance exceeds the grid requirements, it indicates that the nonlinear unbalanced load has increased, so the inverter power quality management function is activated. Since the inverter terminal voltage is generally a constant value, the capacity limit in this invention is equivalent to the limitation of the current amplitude; considering that the inverter has occupied a part of the capacity for power transmission, it is necessary to calculate whether the inverter capacity is sufficient. If it is sufficient, there is no need to limit the amplitude. If it is insufficient, the harmonic unbalanced current used to manage the power quality is further limited. The management goal is transformed into using limited capacity to improve the power quality as much as possible. The core lies in selecting the limiting proportional coefficient; multiple groups of limiting coefficients are calculated, and then the reference command current is synthesized to monitor the power quality management effect after management. The limiting proportional coefficient with the best management effect is selected through the super-efficiency data envelopment analysis method and put into operation, and finally the purpose of improving the power quality is achieved.

[0076] The main circuit and control structure of the system provided by the embodiment of the present invention are shown in FIG. Figure 2 As shown, the main circuit includes a three-phase three-wire photovoltaic inverter, a local load, and a power grid, which are connected to a common connection point (PCC). The data that the control system needs to sample includes the inverter side current at the common connection point. i abc , common connection point voltage u abc , grid-side current at the point of public connection i gabc , the fundamental positive sequence reference current is calculated through the voltage amplitude and phase angle of the common connection point and the inverter grid-connected power value, and its expression is:

[0077]

[0078] Where P and Q are the inverter power commands, V m and I m are the amplitudes of voltage and current phasors respectively; and are the phases corresponding to voltage and current respectively; u d is the projection of voltage in the dq coordinate system; i ref,d and i ref,qThey are the d-axis reference value and q-axis reference value of the power positive sequence reference current in the dq coordinate system respectively. The power positive sequence reference current is obtained by performing dq inverse transformation. i pq .

[0079] By monitoring the grid current i gabc The power quality can be used to determine whether the current power grid needs to improve its power quality. Under normal operation, the inverter does not perform power quality management. When the power management does not meet the requirements of the power grid, the power quality management is started and the extracted harmonic unbalanced current is connected.

[0080] Harmonic unbalanced current detection link such as Figure 3 As shown, the inverter side current i abc and grid-side current i gabc After subtracting, we get the load current i L , after Clark transformation, it is converted to a two-phase stationary coordinate system, and a second-order generalized integral-based orthogonal generator (SOGI-QSG) is used to generate a 90° phase angle offset based on a sinusoidal signal. This method can also filter out high-order harmonics in the signal, and its transfer function expression is:

[0081]

[0082] The amplitude-frequency characteristics and phase-frequency characteristics are:

[0083]

[0084] Where D(s) is the transfer function between the input signal i(s) and the output signal i'(s), is the resonant frequency, k is the system gain, Q(s) is the transfer function of the input signal i(s) and the output signal qi'(s), qi'(s) and i'(s) are a set of orthogonal signals, |D| and |Q| are the amplitude-frequency characteristics of the transfer functions D(s) and Q(s), ∠D and ∠Q are the phase-frequency characteristics of the transfer functions D(s) and Q(s), and ω is the frequency.

[0085] The resonant frequency If it is set to the grid fundamental frequency, then only the fundamental frequency component can pass through the system without loss, and the harmonic components in the signal will be greatly attenuated, thus achieving the goal of separating the fundamental and generating orthogonal signals. In the two-phase stationary coordinate system, the positive sequence and negative sequence components can be expressed as:

[0086]

[0087] In the formula , is a 90° lag operation, Figure 3 In T 1 is the matrix of the above formula, so the fundamental positive sequence and negative sequence current components in the two-phase stationary coordinate system are obtained, and then the positive sequence current in the three-phase abc coordinate system is obtained by Clark inverse transformation. i 1 and negative sequence current i 2. If the inverter is a three-phase four-wire inverter, the zero-sequence current can be controlled. To detect the zero-sequence current, it is only necessary to measure the three-phase load current. i L The average value can be obtained, and its expression is:

[0088]

[0089] Further, the harmonic current can be obtained as:

[0090]

[0091] In the formula, i L is the difference between the current flowing into the PCC and the current flowing out, i 1. i 2. i 0 are the positive sequence, negative sequence, and zero sequence currents calculated above, and all currents are three-phase currents in the abc coordinate system, thereby realizing the extraction of harmonics and unbalanced currents to be controlled. i iq To compensate the reference current, i 1. i 2. i 0 are the calculated positive sequence, negative sequence and zero sequence currents, i h is the harmonic current; k 2. k 0. k h They are the limiting coefficients of negative sequence current, zero sequence current and harmonic current respectively; i L It is the difference between the detected inverter side current and the grid side current. All currents are three-phase currents in the abc coordinate system.

[0092] The current limiting coefficient generation process is as follows: Figure 4 As shown, the control capacity demand is estimated based on the extracted negative sequence, zero sequence and harmonic currents. Since the inverter terminal voltage is generally a constant value, the capacity limitation is equivalent to the limitation of the current amplitude in this invention. i h and i 0 contains currents of multiple frequencies, and the waveform is non-sinusoidal. According to the maximum margin, the capacity requirement is taken as the peak value within one power frequency cycle, which are recorded as I h ,I 0; and i pq and i 2 are both fundamental components. Considering the influence of current phase, the capacity demand calculated after the superposition of the two is less than the capacity demand calculated separately. Therefore, the capacity demand is calculated based on the accurate model after the superposition of the two. Taking phase A as an example, the power reference current of phase A and the reference current for improving power quality are:

[0093]

[0094] In the formula, A , B They are i pq and i 2, the power transmission capacity is I pq = A , the maximum negative sequence capacity requirement is I 2= B , i pq and i 2 The current after superposition is:

[0095]

[0096] The superimposed amplitude Similarly, the amplitude of the reference current of phase B and phase C is obtained, and the maximum amplitude after the three-phase superposition is taken as M , then the current for negative sequence control is obtained i The exact capacity requirement of 2 is I 2a = M - A Assume that the capacity limit of the inverter for power quality management is I r (remaining capacity) to determine whether the power quality management meets the inverter capacity limit, that is, I 2a +I 0 +I h ≤ I r If satisfied, full compensation can be selected, set k 2. k 0. k h are all 1, k 2. k 0. k h They are i 2. i 0. i hThe limiting coefficient is ; otherwise, k 2. k 0. k h Set the value from 0 to 1 to k 2 I 2a + k 0 I 0 + k h I h ≤ I r , that is, to limit the compensation current component, in order to maximize the use of capacity, the limiting coefficient is taken so that k 2 I 2a + k 0 I 0 + k h I h = I r , then the reference current for improving power quality is:

[0097]

[0098] At this time, the calculation expression of the compensation coefficient is:

[0099]

[0100] Change in steps k 2. k 0. k h , the calculation of the change step size is as follows Figure 4 As shown, first determine the negative sequence compensation capacity boundary I r2 , and then obtain the range of change of each limiting coefficient, and then obtain the change step size according to the required number of limiting coefficient combinations n, and then generate n groups of limiting coefficients according to the step size. Therefore, this method generates compensation coefficient groups according to specific working conditions. These combinations contain different compensation strategies, so the optimal solution selected has a better governance effect. In theory, the more combinations there are, the greater the probability of better compensation effect. However, since the compensation effect measurement corresponding to each group of compensation coefficients requires a certain amount of time, the more combinations there are, the more total time required for measurement, so generally 10 to 20 groups of data are sufficient.

[0101] The current limiting coefficient selection process is as follows: Figure 5As shown, first take the first set of limiting coefficients to limit the compensation reference current. After 0.1 seconds, the system is basically stable. At this time, sample the grid current, calculate the waveform distortion rate (THD) and three-phase imbalance. The waveform distortion rate mainly reflects the control effect of harmonic current, and the three-phase imbalance mainly reflects the control effect of negative sequence current and zero sequence current. Record these two indicators as input for evaluating this set of limiting coefficients. Then select the next set of limiting coefficients to put into operation and record the power quality indicators until all the data of the n+1 set of coefficients are obtained. Using the super-efficiency data envelopment analysis method, the mathematical model is as follows:

[0102]

[0103] In the formula, θ l Representatives of the surveyed l The efficiency value of a decision-making unit, λ j For the j The combination coefficient of the decision making unit, S — and S + are the slack variables for the input indicators and the residual variables for the output indicators; X lj For the l The decision-making unit j An input indicator, Y lj For the l The decision-making unit j In this model, the power quality data corresponding to each set of limiting coefficients is a decision unit. Each decision unit has an input indicator, namely the inverter input capacity, and two output indicators, namely the grid current waveform distortion rate and the grid current three-phase imbalance. The efficiency value of each decision unit is finally calculated based on the input and output.

[0104] The algorithm makes a vertical comparison of the actual effects of the inverter in improving power quality under different parameters, selects the relatively optimal operating parameters, objectively and truly reflects the inverter's management capabilities, and improves the inverter capacity utilization rate.

[0105] The expressions of current waveform distortion rate and three-phase unbalance of grid current are as follows:

[0106]

[0107] In the formula, I h is the hth harmonic current (RMS value), I 1 is the fundamental current (RMS value), I 2 is the negative sequence current (RMS value); THDi is the current waveform distortion rate, is the three-phase unbalance of the grid current.

[0108] It should be noted that the embodiments of the present invention can be implemented by hardware, software, or a combination of software and hardware. The hardware part can be implemented using dedicated logic; the software part can be stored in a memory and executed by an appropriate instruction execution system, such as a microprocessor or dedicated design hardware. A person of ordinary skill in the art will appreciate that the above-mentioned devices and methods can be implemented using computer executable instructions and / or contained in a processor control code, such as a carrier medium such as a disk, CD or DVD-ROM, a programmable memory such as a read-only memory (firmware), or a data carrier such as an optical or electronic signal carrier. Such code is provided. In addition, in software such as MATLAB, RTLAB, dSPACE, Typhoon, etc., the control method described in the present invention can be implemented by control modeling.

[0109] The parameters are shown in Table 1. Figure 6 (a) in the figure is the grid current and inverter current waveform. Figure 6 (b) in the figure is the grid current waveform distortion rate and three-phase imbalance. The inverter starts to perform power quality management at 0.2s. It can be seen that after the management, the three-phase imbalance of the grid current is significantly reduced, and the harmonics are also greatly reduced. By observing the inverter current, it is not difficult to see that the harmonic unbalance current reduced by the grid is provided by the inverter to achieve the control target.

[0110] Since the available control capacity is set to 10A, it is necessary to limit the harmonic unbalanced current. After the inverter detects the power quality control signal, it calculates the limiting coefficient according to the available capacity and capacity demand as shown in Table 2. The power quality control starts at 0.2s. The negative sequence current and harmonic current in the grid current are as follows: Figure 7 As shown in (a), the grid current THD and three-phase imbalance change as follows Figure 7 As shown in (b), it can be seen that the power quality of the grid current changes accordingly with every 0.1s change in the limit coefficient, which illustrates the impact of the limit coefficient on power quality management. After the compensation effect of all limit coefficient combinations is determined, the data required for data envelopment analysis is obtained: input X, output Y1, Y2, as shown in Table 3. Further, the management effect of each group of limit coefficients is obtained through super-efficiency data envelopment analysis, that is, the efficiency column in Table 3. The seventh group of limit coefficients with the highest efficiency is selected for power quality management, which verifies the superiority of the algorithm.

[0111] Table 1

[0112]

[0113] Table 2

[0114]

[0115] Table 3

[0116]

[0117] Example 2

[0118] A device using embodiment 1, comprising:

[0119] A detection module, used to detect the inverter-side current, the voltage and the grid-side current of the common connection point; wherein the current flowing into the inverter-side current of the common connection point is positive, and the current flowing out of the grid-side current of the common connection point is positive;

[0120] A current and voltage calculation module, used for calculating the common connection point voltage amplitude and phase angle, and the inverter side current amplitude and phase angle based on the detected common connection point inverter side current and common connection point voltage;

[0121] A generating module, used for generating a power positive sequence reference current based on a voltage amplitude and a phase angle of a common connection point and a power command;

[0122] A comprehensive calculation module is used to calculate the grid-side current waveform distortion rate and three-phase imbalance based on the detected grid-side current at the common connection point; and to extract the real-time harmonic current, negative-sequence current, and zero-sequence current from the detected grid-side current at the common connection point, and calculate the amplitude and phase angle of the negative-sequence current, as well as the peak values ​​of the zero-sequence and harmonic currents;

[0123] A construction module is used to allocate coefficients of negative sequence current, zero sequence current and harmonic current based on the current amplitude and phase angle of the inverter side, the amplitude and phase angle of the negative sequence current of the grid side, and the peak values ​​of the zero sequence current and harmonic current, and multiply the coefficients of the three with their respective corresponding current values ​​and then add them up to obtain a compensation reference current;

[0124] The judgment module is used to obtain the remaining capacity of the inverter, set the limiting coefficients of the negative sequence current, zero sequence current and harmonic current to 1, and calculate whether the sum of the power positive sequence reference current and the compensation reference current is greater than the inverter current limiting. If so, the compensation current limiting is performed, otherwise, there is no need to perform the compensation current limiting;

[0125] The compensation current limitation is specifically as follows: data envelopment analysis is used to input the grid current waveform distortion rate, three-phase imbalance and the remaining capacity of the inverter, and multiple groups of efficiency values ​​are output. The result with the largest efficiency value and the coefficients of the negative-sequence current, zero-sequence current and harmonic current corresponding to the efficiency value are selected as the compensation result to achieve efficient utilization of the inverter capacity.

[0126] Example 3

[0127] A computer-readable storage medium stores a program, which, when executed by a processor, implements the photovoltaic grid-connected inverter power quality improvement control method of embodiment 1.

[0128] The device and its modules of the present invention can be implemented by hardware circuits such as very large-scale integrated circuits or gate arrays, semiconductors such as logic chips, transistors, etc., or programmable hardware devices such as field programmable gate arrays, programmable logic devices, etc., and can also be implemented by software executed by various types of processors, or by a combination of the above hardware circuits and software, such as firmware.

[0129] The above description is only a specific implementation mode of the present invention, but the protection scope of the present invention is not limited thereto. Any modifications, equivalent substitutions and improvements made by any technician familiar with the technical field within the technical scope disclosed by the present invention and within the spirit and principle of the present invention should be covered by the protection scope of the present invention.

Claims

1. A method for improving power quality control of a photovoltaic grid-connected inverter, characterized in that: include: Detecting the inverter-side current, the voltage and the grid-side current of the common connection point; wherein the current flowing into the inverter-side current is positive, and the current flowing out of the grid-side current is positive; Based on the detected inverter-side current and voltage at the common connection point, the common connection point voltage amplitude and phase angle, as well as the inverter-side current amplitude and phase angle are calculated; Based on the voltage amplitude and phase angle of the common connection point and the power command, the power positive sequence reference current is generated; the expression of the power positive sequence reference current is: Where P and Q are the inverter power commands, V m and I m are the amplitudes of voltage and current phasors respectively; and are the phases corresponding to voltage and current respectively; u d is the projection of voltage in the dq coordinate system; i ref,d and i ref,q They are the d-axis reference value and q-axis reference value of the power positive sequence reference current in the dq coordinate system respectively. The power positive sequence reference current is obtained by performing dq inverse transformation. i pq ; Based on the detected grid-side current at the common connection point, the grid-side current waveform distortion rate and three-phase imbalance are calculated; and the real-time harmonic current, negative-sequence current, and zero-sequence current are extracted from the detected grid-side current at the common connection point, and the amplitude and phase angle of the negative-sequence current, as well as the peak values ​​of the zero-sequence and harmonic currents are calculated; Based on the current amplitude and phase angle on the inverter side, the amplitude and phase angle of the negative sequence current on the grid side, and the peak values ​​of the zero sequence current and harmonic current, the limiting coefficients of the negative sequence current, zero sequence current and harmonic current are allocated and multiplied by their respective corresponding current values ​​and then added to obtain the compensation reference current; the compensation reference current expression is: In the formula, i iq To compensate the reference current, i 1. i 2. i 0 are the calculated positive sequence, negative sequence and zero sequence currents, i h is the harmonic current; k 2. k 0. k h They are the limiting coefficients of negative sequence current, zero sequence current and harmonic current respectively; i L is the difference between the detected inverter-side current and the grid-side current. All currents are three-phase currents in the abc coordinate system; Get the remaining capacity of the inverter, set the limiting coefficients of negative sequence current, zero sequence current and harmonic current to 1, calculate whether the sum of the power positive sequence reference current and the compensation reference current is greater than the inverter current limiting, if so, the compensation current limiting is performed, otherwise, there is no need to perform the compensation current limiting; The compensation current limiting is specifically as follows: using data envelopment analysis, by inputting the grid current waveform distortion rate, three-phase imbalance and the remaining capacity of the inverter, multiple groups of efficiency values ​​are output, and the result with the largest efficiency value and the limiting coefficients of the negative-sequence current, zero-sequence current and harmonic current corresponding to the efficiency value are selected as the compensation result to achieve efficient utilization of the inverter capacity.

2. The photovoltaic grid-connected inverter power quality control method as claimed in claim 1, characterized in that: The grid-side current is decomposed by the symmetrical component method to obtain positive sequence, negative sequence and zero sequence components; The positive and negative sequence components are transformed from the three-phase coordinate system to the two-phase stationary coordinate system through Clark transformation; An orthogonal generator based on second-order generalized integral is used to separate the fundamental wave and generate orthogonal signals; The positive sequence current and negative sequence current in the three-phase abc coordinate system are then obtained through Clark inverse transformation. The zero sequence current is the average value of the three-phase load current, and the harmonic current is calculated.

3. The photovoltaic grid-connected inverter power quality improvement control method according to claim 2, characterized in that: Orthogonal generator expression based on second-order generalized integral: The amplitude-frequency characteristics and phase-frequency characteristics are: Where D(s) is the transfer function between the input signal i(s) and the output signal i'(s), is the resonant frequency, k is the system gain, Q(s) is the transfer function of the input signal i(s) and the output signal qi'(s), qi'(s) and i'(s) are a set of orthogonal signals, |D| and |Q| are the amplitude-frequency characteristics of the transfer functions D(s) and Q(s), ∠D and ∠Q are the phase-frequency characteristics of the transfer functions D(s) and Q(s), ω is the frequency, and s represents the dependent variable of the transfer function.

4. The photovoltaic grid-connected inverter power quality improvement control method according to claim 1, characterized in that: The expression of data envelopment analysis is: In the formula, θ l Representatives of the surveyed l The efficiency value of a decision-making unit, λ j For the j The combination coefficient of the decision making unit, S — and S + are the slack variables for the input indicators and the residual variables for the output indicators; X lj For the l The decision-making unit j An input indicator, Y lj For the l The decision-making unit j In this model, the power quality data corresponding to each set of limiting coefficients is a decision unit. Each decision unit has an input indicator, namely the inverter input capacity, and two output indicators, namely the grid current waveform distortion rate and the grid current three-phase imbalance. The efficiency value of each decision unit is finally calculated based on the input and output.

5. A device for improving power quality control of a photovoltaic grid-connected inverter using the method described in any one of claims 1 to 4, characterized in that: include: A detection module, used to detect the inverter-side current, the voltage and the grid-side current of the common connection point; wherein the current flowing into the inverter-side current of the common connection point is positive, and the current flowing out of the grid-side current of the common connection point is positive; A current and voltage calculation module, used for calculating the common connection point voltage amplitude and phase angle, and the inverter side current amplitude and phase angle based on the detected common connection point inverter side current and common connection point voltage; A generating module, used for generating a power positive sequence reference current based on a voltage amplitude and a phase angle of a common connection point and a power command; A comprehensive calculation module is used to calculate the grid-side current waveform distortion rate and three-phase imbalance based on the detected grid-side current at the common connection point; and to extract the real-time harmonic current, negative-sequence current, and zero-sequence current from the detected grid-side current at the common connection point, and calculate the amplitude and phase angle of the negative-sequence current, as well as the peak values ​​of the zero-sequence and harmonic currents; A construction module is used to allocate limiting coefficients of negative sequence current, zero sequence current and harmonic current based on the current amplitude and phase angle of the inverter side, the amplitude and phase angle of the negative sequence current of the grid side, and the peak values ​​of the zero sequence current and harmonic current, and multiply the three with their respective corresponding current values ​​and then add them to obtain a compensation reference current; The judgment module is used to obtain the remaining capacity of the inverter, set the limiting coefficients of the negative sequence current, zero sequence current and harmonic current to 1, and calculate whether the sum of the power positive sequence reference current and the compensation reference current is greater than the inverter current limiting. If so, the compensation current limiting is performed, otherwise, there is no need to perform the compensation current limiting; The compensation current limitation is specifically as follows: data envelopment analysis is used to input the grid current waveform distortion rate, three-phase imbalance and the remaining capacity of the inverter, and multiple groups of efficiency values ​​are output. The result with the largest efficiency value and the coefficients of the negative-sequence current, zero-sequence current and harmonic current corresponding to the efficiency value are selected as the compensation result to achieve efficient utilization of the inverter capacity.

6. A computer-readable storage medium, characterized in that: A program is stored thereon, and when the program is executed by a processor, the method for improving power quality control of a photovoltaic grid-connected inverter as described in any one of claims 1 to 4 is implemented.

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