CN112821443B - A photovoltaic grid-connected inverter and its modulation method - Google Patents

Abstract

The invention discloses a photovoltaic grid-connected inverter and a modulation method thereof. By obtaining the current and voltage of a public grid, the current and voltage are both instantaneous current and instantaneous voltage; the instantaneous current and instantaneous voltage in one day are calculated in seconds as The unit records and obtains the median instantaneous current and median instantaneous voltage in one day; one day is 24H and its starting time is 00:00; the current and voltage of the photovoltaic grid connected to the grid during adjustment are the median instantaneous current and the median instantaneous voltage; specifically, adjust the current and voltage when the photovoltaic grid is connected to the grid by the modulation method; obtain the time nodes of the median instantaneous current and the median instantaneous voltage in the next week, and count the time with the highest number of occurrences The node is marked as the access time; when the photovoltaic grid is connected to the public grid during the access time, when the photovoltaic grid is integrated into the public network, it is necessary to connect to the public grid at the most suitable time node to avoid damage to the public grid.

Description

A photovoltaic grid-connected inverter and its modulation method

technical field

The invention belongs to the field of photovoltaic grid-connected inverters, and relates to photovoltaic grid-connected technology, in particular to a photovoltaic grid-connected inverter and a modulation method thereof.

Background technique

In the distributed power generation system with solar energy as input, the grid-connected inverter has the function of converting direct current into alternating current that meets the requirements of commercial power and connecting it to the grid. In traditional grid-connected systems, isolated and non-isolated grid-connected inverters with output transformers are often used to generate electrical isolation between the power grid and the DC side to ensure personal safety. At the same time, it can also provide voltage matching and grid-connected current DC component suppression. High-frequency transformers add size, weight, and cost, while high-frequency transformers complicate control and reduce efficiency. With its advantages of high efficiency, small size and low cost, non-isolated grid-connected inverters have become the mainstream of grid-connected inverter applications in the field of small and medium power generation. However, because the non-isolated grid-connected inverter does not have a transformer, it does not have the electrical isolation function of the transformer, which brings a new problem, that is, the leakage current problem. The leakage current is essentially a common-mode current, which is caused by the parasitic capacitance between the DC power supply and the ground, forming a common-mode current loop formed by the parasitic capacitance, the DC power supply, the AC filter, and the grid impedance. Changes in the common-mode voltage create large common-mode currents on parasitic capacitances. Because the existence of ground leakage current, that is, common mode current, will reduce the efficiency of the system, damage the quality of output power, increase the electromagnetic interference of the system, pose a threat to the human body, and form a potential safety hazard. Moreover, too much leakage current to the ground will cause saturation of the AC filter, reduce the filtering effect, and may also cause damage to the grid-connected inverter. Therefore, for the opposite type, that is, the transformerless non-isolated grid-connected inverter, the peak value of the leakage current to the ground should be less than 300mA in German VDE-0126-1-1. If the specified range is exceeded, the leakage current monitoring unit shall disconnect the grid-connected inverter from the grid within 0.3s. In order to resist the leakage current of the non-isolated grid-connected inverter, the change of the common mode voltage should be kept as small as possible. If the common-mode voltage can be guaranteed to be a certain value, the common-mode current can be basically eliminated, that is, the power device adopts PWM control so that the sum of the voltages on the negative terminals of the DC power supply at both ends of the inverter output is constant.

SUMMARY OF THE INVENTION

The purpose of the present invention is to provide a photovoltaic grid-connected inverter and a modulation method thereof, which are used to solve the problem of reducing the harmonic content by changing the vector sequence, thereby optimizing the algorithm of SVPWM.

The object of the present invention can be realized through the following technical solutions:

A photovoltaic grid-connected inverter, comprising a photovoltaic monitoring module, an auxiliary circuit module, a control circuit, a DC/DC module, an inverter module and a grid-connected module;

The photovoltaic monitoring module is used to detect the photovoltaic group and control the connection mode of the photovoltaic group;

The auxiliary circuit module is used to provide voltage to the electronic components on the control circuit;

The control circuit provides switching signals for the photovoltaic grid-connected inverter, and ensures the safe operation of the photovoltaic grid-connected inverter;

The DC/DC module is used to convert the DC voltage generated by the photovoltaic group into the DC voltage required by the inverter part;

The inverter module inverts the electric energy emitted by the photovoltaic array into alternating current with the same frequency and phase as the grid voltage; the power electronic device is controlled by the switching signal sent by the control circuit in the form of pulse width modulation, so as to realize switching on and off, Thereby, the direct current generated by DC/DC is inverted into three-phase alternating current;

The grid-connected module is used to integrate the photovoltaic grid that meets the power consumption requirements into the public grid, and specifically includes the following steps:

Step 1: obtaining the current and voltage of the public power grid, wherein the current and voltage are both instantaneous current and instantaneous voltage;

Step 2: Record the instantaneous current and instantaneous voltage in one day in seconds, and obtain the median instantaneous current and median instantaneous voltage in one day; one day is specifically 24H and its start time is 00:00;

Step 3: The current and voltage of the photovoltaic grid connected to the grid during adjustment are the median instantaneous current and the median instantaneous voltage; specifically, the current and voltage of the photovoltaic grid connected to the grid are adjusted by a modulation method;

Step 4: Obtain the time node of the median instantaneous current and the median instantaneous voltage in the next week, and count the time node with the highest number of occurrences, and mark it as the access time;

Step 5: Connect the photovoltaic grid to the public grid within the access time.

Further, the photovoltaic monitoring module is used to detect the photovoltaic group and control the connection mode of the photovoltaic group. Specifically, the real-time voltage and real-time current of the photovoltaic grid are obtained, and the current and voltage of the storage battery are obtained at the same time. When the current of the storage battery is lower than the loss value, the photovoltaic panels in the photovoltaic grid are connected to be connected in parallel with each other;

When the voltage of the storage battery is lower than the loss value, the photovoltaic panels in the photovoltaic grid are connected to be connected in series with each other.

A modulation method of a photovoltaic grid-connected inverter, the modulation method is specifically: the CPU is initialized, the user register is initialized; at this time, the PWM interrupt is started, at this time, the PWM system starts self-checking, and after the self-checking is normal, the grid is connected relay, and start the phase lock;

When waiting for an interrupt, load the PWM interrupt program and start the interrupt protection. The MPPT algorithm is used to convert the coordinates of the relevant power, and the PI algorithm is used to perform the coordinate return conversion, and the SVPWM algorithm is used for interrupt recovery.

Further, the output mode of the PWM is a center-aligned mode, the highest output precision of the PWM is 2 clock cycles, and the modulo value of the PWM is half of the count value corresponding to the output period; by the formula PWM count modulo value×PWM The PWM cycle is obtained by clock cycle×2; the PWM pulse width is obtained by the formula PWM count value×PWM clock cycle×2; and a dead time is set in the PWM.

获得三项静止参考坐标系，其中，i_a、i_b、i_c为逆变器输出电流，u_a、u_b、u_c为逆变器输出电压，e_ga、e_gb、e_gc为网侧相电压，L为逆变器输出电流，R为逆变器与网侧之间的等效电阻；Further, the use of the MPPT algorithm to convert the coordinates of the relevant electricity is specifically, through the matrix

Obtain three stationary reference coordinate systems, where i _a , _ib , and ic are the output current of the inverter, u _a , _ub , and uc are the output voltage of the inverter, and _{e ga} , _{e gb} , and e _gc are the network side-phase voltage, L is the output current of the inverter, R is the equivalent resistance between the inverter and the grid side;

此时选择d-q同步旋转坐标系的初始参考轴，则得出

消除由网侧谐波引起的输出电流失真,通过调节转换后的d、q轴电流即可调节输出有功和无功功率，控制q轴电流为零，逆变输出电流与网侧电压同频同相，功率因数此时为1。The matrix is transformed into the dq coordinate system through the Clarke change and Park change, so as to convert the inverter output current, inverter output voltage, grid-side phase voltage, inverter output current, and the relationship between the inverter and the grid side, etc. The effective resistance is converted into a direct current, so as to achieve an ideal state. Specifically, the Clarke change and Park change are,

At this time, the initial reference axis of the dq synchronous rotation coordinate system is selected, and the result is

Eliminate the output current distortion caused by the harmonics on the grid side, adjust the output active and reactive power by adjusting the converted d and q-axis currents, control the q-axis current to zero, and the inverter output current and grid-side voltage have the same frequency and phase , the power factor is 1 at this time.

其中，u为输出量，e为误差信号，K_p为PI算法中的比例调节系数、K_i为PI算法中积分调节系数；Further, the PI algorithm is specifically

Among them, u is the output quantity, e is the error signal, K _p is the proportional adjustment coefficient in the PI algorithm, and K _i is the integral adjustment coefficient in the PI algorithm;

Let T represent the sampling period, then the n-1th PI output is

Further, described SVPWM algorithm is specifically, reads the sampling signal of voltage and current, calculates the size and direction of the next target vector, and judges the sector where the target vector is located, selects the synthetic vector according to the information of the sector where it is located, calculates. The action time of the synthesized vector is loaded with the PWM count value, and the narrow pulse is deleted at the same time. After judging the sector position of the target vector, it can continue to judge whether the target vector is in the first half area or the second half area of the sector according to the rotation angle. In the first half area, The second vector order is used, and in the second half, the first vector order is used.

Further, the first vector sequence is specifically, U ₀ →U ₁ →U ₂ →U ₇ →U ₇ →U ₂ →U ₁ →U ₀ ; the second vector sequence is specifically, U ₇ →U ₂ →U ₁ →U ₀ →U ₁ →U ₂ →U ₇ ;

The U ₀ , U ₁ , U ₂ , and U ₇ are all space voltage vectors.

Further, the method for judging the sector where the target vector is located is a boundary condition method.

Compared with the prior art, the beneficial effects of the present invention are:

(1) By obtaining the current and voltage of the public power grid, wherein the current and voltage are both instantaneous current and instantaneous voltage; the instantaneous current and instantaneous voltage within a day are recorded in units of seconds, and the median value within a day is obtained Instantaneous current and median instantaneous voltage; one day is 24H and its start time is 00:00; the current and voltage of the grid-connected photovoltaic grid during adjustment are median instantaneous current and median instantaneous voltage; specifically, through modulation The method adjusts the current and voltage when the photovoltaic grid is connected to the grid; obtains the time nodes of the median instantaneous current and median instantaneous voltage in the next week, and counts the time nodes with the highest number of occurrences, and marks them as the access time; The photovoltaic grid is connected to the public grid within the time of entry, so that when the photovoltaic grid is integrated into the public network, it needs to be connected to the public grid at the most suitable time node to avoid damage to the public grid;

(2) By obtaining the real-time voltage and real-time current of the photovoltaic grid, and simultaneously obtaining the current and voltage of the storage battery, when the current of the storage battery is lower than the loss value, the photovoltaic panels in the photovoltaic grid are connected in parallel with each other; When the voltage of the battery is lower than the loss value, the photovoltaic panels in the photovoltaic grid are connected to each other in series; the real-time voltage and real-time current of the photovoltaic grid can be preliminarily adjusted in real time to avoid damage to the components in the photovoltaic grid;

(3) Initialize through the CPU and initialize the user register; at this time, start the PWM interrupt, and the PWM system starts self-checking. After the self-checking is normal, the grid-connected relay is turned on, and phase locking is started; when waiting for the interrupt, load the PWM interrupt program, and start the interrupt protection, use the MPPT algorithm to convert the coordinates of the relevant power, and use the PI algorithm to convert the coordinates back, and use the SVPWM algorithm to perform interrupt recovery, which can achieve a detailed theoretical analysis of the SVPWM control strategy. Through in-depth analysis, it is found that the essential difference between it and PWM lies in the distribution of zero vectors. Therefore, a lot of research work has been done on zero vectors. Finally, a vector sequence that is different from traditional SVPWM is proposed. After simulation, it is verified that This method has the advantage of reducing harmonics.

Description of drawings

In order to explain the embodiments of the present invention or the technical solutions in the prior art more clearly, the following briefly introduces the accompanying drawings that need to be used in the description of the embodiments or the prior art. Obviously, the accompanying drawings in the following description are only These are some embodiments of the present invention. For those of ordinary skill in the art, other drawings can also be obtained according to these drawings without creative efforts.

Fig. 1 is the principle block diagram of the photovoltaic grid-connected inverter of the present invention;

2 is a schematic diagram of the modulation method of the present invention;

Fig. 3 is the output power diagram of the photovoltaic array of the present invention;

Fig. 4 is the PV curve diagram of the volt array of the present invention;

FIG. 5 is a space voltage vector diagram of the present invention.

Detailed ways

In order to make the purposes, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments These are some embodiments of the present invention, but not all of them. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the protection scope of the present invention.

Accordingly, the detailed descriptions of embodiments of the invention provided in the following drawings are not intended to limit the scope of the invention as claimed, but are merely representative of selected embodiments of the invention.

As shown in FIG. 1, a photovoltaic grid-connected inverter is characterized in that it includes a photovoltaic monitoring module, an auxiliary circuit module, a control circuit, a DC/DC module, an inverter module and a grid-connected module;

The photovoltaic grid-connected power generation system is a power conversion system that converts solar energy into electrical energy through photovoltaic devices, and then uses an inverter to integrate the electrical energy into the grid. The grid-connected photovoltaic inverter system is mainly composed of photovoltaic arrays, inverters, controllers and protection units designed according to product requirements. The inverter system realizes DC/AC conversion, and the controller with DSP as the core can track the voltage on the grid side. Frequency and phase, the voltage output by the inverter is controlled by software to be in the same frequency and phase, and the maximum power point tracking technology can also be used to track the maximum power point of the photovoltaic array when it is working, so that the system is not affected by weather and light, and can run at the maximum power point. power status. The photovoltaic monitoring module is used to detect the photovoltaic group and control the connection mode of the photovoltaic group; the auxiliary circuit module is used to provide voltage to the electronic components on the control circuit;

Specifically, the control circuit provides switching signals for the photovoltaic grid-connected inverter, and ensures the safe operation of the photovoltaic grid-connected inverter;

The DC/DC module is used to convert the DC voltage generated by the photovoltaic group into the DC voltage required by the inverter part;

The inverter module inverts the electric energy emitted by the photovoltaic array into alternating current with the same frequency and phase as the grid voltage; the power electronic devices are controlled by the switching signal sent by the control circuit in the form of pulse width modulation to realize the opening and closing, so that the The direct current generated by DC/DC is inverted into three-phase alternating current;

The grid-connected module is used to integrate the photovoltaic grid that meets the power consumption requirements into the public grid, which includes the following steps:

Step 1: obtaining the current and voltage of the public power grid, wherein the current and the voltage are both instantaneous current and instantaneous voltage;

Step 2: Record the instantaneous current and instantaneous voltage in one day in seconds, and obtain the median instantaneous current and median instantaneous voltage in one day; one day is specifically 24H and its start time is 00:00;

Step 3: The current and voltage of the photovoltaic grid connected to the grid during adjustment are the median instantaneous current and the median instantaneous voltage; specifically, the current and voltage of the photovoltaic grid connected to the grid are adjusted by a modulation method;

Step 4: Obtain the time node of the median instantaneous current and the median instantaneous voltage in the next week, and count the time node with the highest number of occurrences, and mark it as the access time;

Step 5: Connect the photovoltaic grid to the public grid within the access time.

In the specific implementation of the present invention, the grid connection of the photovoltaic power grid to the public power grid also needs to be realized;

The output frequency and phase must be adjusted; if the main grid is lost, the inverter must be shut down immediately; the output current can synchronously follow the system voltage (current source) The output current waveform is good, the waveform distortion and frequency fluctuation are lower than the threshold value The injection current is the same as the system voltage Phase, achieve power factor of 1; suppress reactive power exchange, low loss, reduce potential overvoltage In the event of a fault (loss of mains voltage, short circuit or insulation damage), the inverter must be able to automatically de-load from the mains ; The normal operation of the inverter main grid and photovoltaic grid-connected power generation system should not be affected by the reference voltage signal from the main grid, and the input terminal resistance should be able to adapt to the actual operating characteristics of the photovoltaic power generation system (such as maximum power point tracking operation. method) In order to ensure the normal operation of the photovoltaic power generation system, the voltage fluctuation should be low enough (<3%) in the case of overvoltage (such as low temperature open-loop operation), the photovoltaic power generation system can still operate normally under overload conditions, in order to limit the input power at Within a given range, the inverter needs to automatically modulate the operating point in the direction of the open-circuit voltage in the photovoltaic power generation characteristic curve; this situation mainly occurs when the rated power of the inverter is lower than the rated power of the photovoltaic power generation system, and under strong sunlight, Overload rarely occurs, and the photovoltaic power generation system should supply power to the inverter without loss of public grid power. Even in low sunlight intensity, the inverter should be able to maintain stable operation; modern photovoltaic power generation systems can supply power to the public grid at an intensity of 50Wm2; and even when the inverter is input at 10% of its rated power, It can also ensure 90% conversion efficiency. The input and output interfaces must be installed with similar equipment to suppress transient overvoltage (such as lightning inrush current). The electromagnetic compatibility must meet the ENS014 standard; the equipment has low noise and is suitable for operation in residential buildings.

Further, the photovoltaic monitoring module is used to detect the photovoltaic group and control the connection mode of the photovoltaic group, specifically, to obtain the real-time voltage and real-time current of the photovoltaic power grid, and to obtain the current and voltage of the storage battery at the same time. When the current of the battery is lower than the loss value, the connection of the photovoltaic panels in the photovoltaic grid is converted into parallel with each other;

When the voltage of the storage battery is lower than the loss value, the photovoltaic panels in the photovoltaic grid are connected to be connected in series with each other.

As shown in Figure 2, a modulation method of a photovoltaic grid-connected inverter, the modulation method is as follows: the CPU is initialized, the user register is initialized; at this time, the PWM interrupt is started, and the PWM system starts self-checking. After the self-checking is normal, the Turn on the grid-connected relay and start the phase lock;

When waiting for an interrupt, load the PWM interrupt program and start the interrupt protection. The MPPT algorithm is used to convert the coordinates of the relevant power, and the PI algorithm is used to perform the coordinate return conversion, and the SVPWM algorithm is used for interrupt recovery.

In the specific implementation of the present invention, the output mode of PWM is center-aligned, the highest output precision of PWM is 2 clock cycles, and the modulo value of PWM is half of the count value corresponding to the output period; by formula PWM count modulo value × PWM The PWM period is obtained by the clock cycle × 2; the PWM pulse width is obtained by the formula PWM count value × PWM clock cycle × 2; and a dead time is provided in the PWM, and when the present invention is implemented, the dead time is 2 μm;

获得三项静止参考坐标系，其中，i_a、i_b、i_c为逆变器输出电流，u_a、u_b、u_c为逆变器输出电压，e_ga、e_gb、e_gc为网侧相电压，L为逆变器输出电流，R为逆变器与网侧之间的等效电阻；In the specific implementation of the present invention, using the MPPT algorithm to convert the coordinates of the relevant electric quantity is as follows: through the matrix

Obtain three stationary reference coordinate systems, where i _a , _ib , and ic are the output current of the inverter, u _a , _ub , and uc are the output voltage of the inverter, and _{e ga} , _{e gb} , and e _gc are the network side-phase voltage, L is the output current of the inverter, R is the equivalent resistance between the inverter and the grid side;

在d-q坐标系下，三相逆变器的有功功率和无功功率分别为：

适当选择d-q同步旋转坐标系的初始参考轴，如d轴与电网电动势重合，即e_gd＝V(V为网侧电压峰值)，q轴的分量为零，即e_gq＝0,则稳态下有功功率取决于d轴电流，无功取决于q轴电流，则上式改写成，

采用这样的选择方式，可以看出忽略了旋转坐标系下网侧电压中的零序谐波，能够消除由网侧谐波引起的输出电流失真，其本质上为一种扰动前馈补偿控制方法。通过调节转换后的d、q轴电流即可调节输出有功和无功功率，控制q轴电流为零，逆变输出电流与网侧电压同频同相，所以此时功率因数为1；The matrix is transformed into the dq coordinate system through the Clarke change and Park change, so as to convert the inverter output current, inverter output voltage, grid-side phase voltage, inverter output current, and the relationship between the inverter and the grid side, etc. The effective resistance is converted into a direct current, so as to achieve an ideal state. Specifically, the Clarke change and Park change are,

In the dq coordinate system, the active power and reactive power of the three-phase inverter are:

Properly select the initial reference axis of the dq synchronous rotating coordinate system, such as the d axis coincides with the grid electromotive force, that is, e _gd = V (V is the peak voltage of the grid side), and the component of the q axis is zero, that is, e _gq = 0, then the steady state The lower active power depends on the d-axis current, and the reactive power depends on the q-axis current, then the above formula is rewritten as,

Using this selection method, it can be seen that the zero-sequence harmonics in the grid-side voltage in the rotating coordinate system are ignored, and the output current distortion caused by the grid-side harmonics can be eliminated, which is essentially a disturbance feedforward compensation control method . By adjusting the converted d and q-axis currents, the output active and reactive power can be adjusted, the q-axis current is controlled to be zero, and the inverter output current is in the same frequency and phase as the grid-side voltage, so the power factor is 1 at this time;

其中，u为输出量，e为误差信号，K_p为PI算法中的比例调节系数、K_i为PI算法中积分调节系数；将T表示采样周期，则第n-1次的PI输出为

In the specific implementation of the present invention, the PI algorithm compensates the current error, and the formula is

Among them, u is the output quantity, e is the error signal, K _p is the proportional adjustment coefficient in the PI algorithm, and K _i is the integral adjustment coefficient in the PI algorithm; if T is the sampling period, the n-1th PI output is

In the specific implementation of the present invention, the SVPWM algorithm is specifically as follows: reading the sampling signals of voltage and current, calculating the size and direction of the next target vector, judging the sector where the target vector is located, and selecting a composite vector according to the information of the sector where it is located , calculate the action time of the composite vector and load the PWM count value, and delete the narrow pulse at the same time. After judging the sector position of the target vector, you can continue to judge whether the target vector is in the first half area or the second half area of the sector according to the rotation angle. area, use the second vector order, in the second half area, use the first vector order The first vector order is specifically, U ₀ →U ₁ →U ₂ →U ₇ →U ₇ →U ₂ →U ₁ →U ₀ ; The second vector sequence is specifically, U ₇ → U ₂ → U ₁ → U ₀ → U ₁ → U ₂ → U ₇ ; as shown in FIG. 5 , U ₀ , U ₁ , U ₂ , and U ₇ are all spaces voltage vector;

Further, the method for judging the sector where the target vector is located is the boundary condition method;

The output characteristics of photovoltaic cell arrays are strongly nonlinear and have a unique maximum power point under certain temperature and sunshine intensity. When the array works at this point, it can output the maximum power under the current temperature and sunshine conditions. All photovoltaic power generation systems hope that the solar cell array can output as much power as possible under the same sunshine and temperature conditions. The grid-connected inverter must continuously adjust the reference voltage to achieve the maximum power point tracking, and modulate the pulse. A wide way to maximize the power; as shown in Figure 3, the output power of the photovoltaic array has a maximum point P _max , and the output voltage corresponding to this point is U _max . When the operating voltage of the photovoltaic array is less than the maximum power point voltage U _max , The array output power increases with the rise of the solar cell terminal voltage; when the array operating voltage is greater than the maximum power point voltage _Umax , the array output power decreases with the rise of the battery terminal voltage. The idea of MPPT is essentially a self-optimization process, that is, by controlling the array terminal voltage U _max , the array can output maximum power without being affected by sunlight and temperature. When the intersection of the load characteristics and the photovoltaic array characteristics is to the left of the U _max corresponding to the maximum power point, the MPPT will control the operating voltage of the photovoltaic array to increase; and when the intersection is to the right of the corresponding voltage U _max of the maximum power point of the array, the MPPT will Control the voltage drop of the photovoltaic array. To do this, the solar array must implement maximum power point tracking control to continuously obtain maximum power output under any insolation. At present, the MPPT algorithms commonly used in photovoltaic power generation systems are self-optimization methods, including constant voltage method, disturbance observation method, incremental conductance method 2, intermittent scanning method and intelligent control method. Tracking requires the use of digital processors, and the maximum power point tracking technology is still under continuous development and improvement.

因此在算法中判断功率对电压的导数，若

表示当前功率未到功率最大点，则递增电压：若

表示当前功率点在最大点的右部，则递减电压；在导数等于零或非常接近于零的时候，电压保持不变即可。The present invention adopts the incremental conductance method, as shown in Fig. 4, combined with the characteristics of the PV curve of the photovoltaic array, it is not difficult to find that at the maximum output power point, the derivative of the power to the voltage is zero, that is,

Therefore, the derivative of power to voltage is judged in the algorithm, if

Indicates that the current power has not reached the maximum power point, then increase the voltage: if

Indicates that the current power point is on the right of the maximum point, then the voltage is decreased; when the derivative is equal to zero or very close to zero, the voltage can remain unchanged.

因此，可以通过

的符号来判断电压的增加或减小。and

Therefore, by

to judge the increase or decrease of the voltage.

The advantage of using this method is that when the light intensity of the photovoltaic array is constant, the system can be relatively stable at the maximum power point; when the light intensity changes, the system output can follow the change smoothly, and the generated voltage disturbance range is small, and The response speed is fast, and it can adapt to the situation of rapid changes in light intensity.

The above formulas are calculated by removing the dimension and taking its numerical value. The formula is a formula obtained by collecting a large amount of data and performing software simulation to obtain the latest real situation. The preset parameters in the formula are set by those skilled in the art according to the actual situation.

Furthermore, the modulation method of the photovoltaic grid-connected inverter is stored in the processor, and the processor is an integrated circuit chip with signal processing capability. In the implementation process, each step of the modulation method can be completed by an integrated logic circuit of hardware in the processor or instructions in the form of software. The above-mentioned processor may be a general-purpose processor, including a central processing unit (Central Processing Unit, referred to as CPU), a network processor (Network Processor, referred to as NP), etc.; may also be a digital signal processor (Digital Signal Processing, referred to as DSP), Application Specific Integrated Circuit (ASIC for short), Field-Programmable Gate Array (FPGA for short) or other programmable logic devices, discrete gate or transistor logic devices, and discrete hardware components. Various methods and steps of the invention in the embodiments of the present invention can be realized or executed. A general purpose processor may be a microprocessor or the processor may be any conventional processor or the like. The steps of the method invented in combination with the embodiments of the present invention can be directly embodied as executed by a hardware decoding processor, or executed by a combination of hardware and software modules in the decoding processor. The software modules may be located in random access memory, flash memory, read-only memory, programmable read-only memory or electrically erasable programmable memory, registers and other storage media mature in the art. The storage medium is located in the memory, and the processor reads the information in the memory, and completes the steps of the modulation method of the embodiment in combination with its hardware.

In the embodiments provided by the present invention, it should be understood that the disclosed apparatus, apparatus and method may be implemented in other manners. For example, the device embodiments described above are only illustrative. For example, the division of the modules is only a logical function division, and there may be other division methods in actual implementation; the modules described as separate components may be They may or may not be physically separated, and components displayed as modules may or may not be physical units, that is, they may be located in one place, or may be distributed to multiple network units. Some or all of the modules may be selected according to actual needs to achieve the purpose of the method in this embodiment.

It will also be apparent to those skilled in the art that the present invention is not limited to the details of the above-described exemplary embodiments, but that the present invention may be embodied in other specific forms without departing from the spirit or essential characteristics of the present invention.

Therefore, the embodiments are to be regarded in all respects as illustrative and not restrictive, and the scope of the invention is to be defined by the appended claims rather than the foregoing description, which are therefore intended to fall within the scope of the claims. All changes within the meaning and range of the equivalents of , are included in the present invention. Any reference signs in the claims shall not be construed as limiting the involved claim.

Furthermore, it is clear that the word "comprising" does not exclude other units or steps and the singular does not exclude the plural. Several units or means recited in the system claims can also be realized by one unit or means by means of software or hardware. Second-class terms are used to denote names and do not denote any particular order.

Finally, it should be noted that the above embodiments are only used to illustrate the technical methods of the present invention and not to limit them. Although the present invention has been described in detail with reference to the preferred embodiments, those of ordinary skill in the art should understand that the technical methods of the present invention can be Modifications or equivalent substitutions can be made without departing from the spirit and scope of the technical method of the present invention.

Claims (3)

1. A photovoltaic grid-connected inverter is characterized by comprising a photovoltaic monitoring module, an auxiliary circuit module, a control circuit, a DC/DC module, an inversion module and a grid-connected module;

the photovoltaic monitoring module is used for detecting the photovoltaic group and controlling the connection mode of the photovoltaic group;

the auxiliary circuit module is used for providing voltage for electronic components on the control circuit;

the control circuit provides a switching signal for the photovoltaic grid-connected inverter and ensures the safe operation of the photovoltaic grid-connected inverter;

the DC/DC module is used for converting direct-current voltage generated by the photovoltaic group into direct-current voltage required by an inversion part;

the inversion module inverts the electric energy generated by the photovoltaic array into alternating current with the same frequency and phase as the voltage of the power grid; the power electronic device is controlled by a switching signal sent by a control circuit in a pulse width modulation mode to realize switching on and switching off, so that direct current generated by DC/DC is inverted into three-phase alternating current;

the grid-connected module is used for merging a photovoltaic power grid meeting the power utilization requirement into a public power grid, and specifically comprises the following steps:

the method comprises the following steps: acquiring current and voltage of a public power grid, wherein the current and the voltage are instantaneous current and instantaneous voltage;

step two: recording instantaneous current and instantaneous voltage in one day according to a unit of second, and obtaining median instantaneous current and median instantaneous voltage in one day; wherein one day is 24H and the starting time is 00: 00;

step three: adjusting the current and voltage of the grid connection of the photovoltaic power grid into a middle instantaneous current and a middle instantaneous voltage; specifically, the current and the voltage of the photovoltaic power grid during grid connection are adjusted through a modulation method;

step four: acquiring a time node of the occurrence of the middle instantaneous current and the middle instantaneous voltage in a future week, counting the time node with the highest occurrence frequency, and marking as access time;

step five: connecting the photovoltaic power grid to a public power grid within access time;

the photovoltaic monitoring module is used for detecting a photovoltaic group and controlling the connection mode of the photovoltaic group, specifically, acquiring real-time voltage and real-time current of a photovoltaic power grid, and simultaneously acquiring current and voltage of a storage battery, and when the current of the storage battery is lower than a loss value, connecting and converting photovoltaic panels in the photovoltaic power grid into mutual parallel connection;

when the voltage of the storage battery is lower than a loss value, connecting and converting photovoltaic panels in the photovoltaic power grid into series connection;

the modulation method specifically comprises the steps that a CPU is initialized, and a user register is initialized; starting PWM interruption at the moment, starting self-checking of a PWM system at the moment, switching on a grid-connected relay after the self-checking is normal, and starting phase locking;

when waiting for interruption, loading a PWM interruption program, starting interruption protection, converting the coordinates of related electric quantity by adopting an MPPT algorithm, performing coordinate return conversion by adopting a PI algorithm, and performing interruption recovery by adopting an SVPWM algorithm;

the output mode of the PWM is a center alignment mode, the highest output precision of the PWM is 2 clock cycles, and the modulus of the PWM is half of the count value corresponding to the output cycle; calculating a PWM period by a formula PWM counting module value multiplied by a PWM clock period multiplied by 2; calculating PWM pulse width by a formula PWM counting value multiplied by PWM clock period multiplied by 2; and the PWM is internally provided with dead time;

the conversion of the coordinates of the related electric quantity by adopting the MPPT algorithm is specifically realized by a matrix

Three stationary reference coordinate systems are obtained, wherein i _a 、i _b 、i _c For the inverter output current u _a 、u _b 、u _c For the inverter output voltage, e _ga 、e _gb 、e _gc The voltage is the voltage of the grid side, L is the output current of the inverter, and R is the equivalent resistance between the inverter and the grid side;

the matrix is converted into a d-q coordinate system after Clarke change and Park change, so that the output current of the inverter, the output voltage of the inverter, the phase voltage of a network side, the output current of the inverter and the equivalent resistance between the inverter and the network side are converted into direct current quantity, an ideal state is achieved, specifically, Clarke change and Park change are changed,

at this time, the initial reference axis of the d-q synchronous rotation coordinate system is selected, and then the result is obtained

Eliminating output current distortion caused by network side harmonic waves, adjusting output active power and reactive power by adjusting converted d-axis current and q-axis current, controlling the q-axis current to be zero, inverting the output current to be in the same frequency and phase with network side voltage, and then setting a power factor to be 1;

the PI algorithm is specifically

Where u is the output, e is the error signal, K _p For proportional adjustment coefficient, K, in PI algorithm _i Adjusting coefficients for integral in a PI algorithm;

if T represents the sampling period, the PI output of the n-1 th time is

The SVPWM algorithm is specifically characterized in that sampling signals of voltage and current are read, the size and the direction of the next target vector are calculated, the sector where the target vector is located is judged, a synthetic vector is selected according to the information of the sector where the target vector is located, the action time of the synthetic vector is calculated, PWM counting values are loaded, narrow pulses are deleted at the same time, after the position of the sector of the target vector is judged, whether the target vector is in the first half area or the second half area of the sector is continuously judged according to the rotation angle, the second vector sequence is used in the first half area, and the first vector sequence is used in the second half area.

2. The pv grid-connected inverter as claimed in claim 1, wherein in the modulation method, the first vector sequence is U ₀ →U ₁ →U ₂ →U ₇ →U ₇ →U ₂ →U ₁ →U ₀ (ii) a The second vector order is specifically, U ₇ →U ₂ →U ₁ →U ₀ →U ₁ →U ₂ →U ₇ ；

Wherein said U is ₀ 、U ₁ 、U ₂ 、U ₇ Are all space voltage vectors.

3. The grid-connected photovoltaic inverter according to claim 2, wherein the method for determining the sector where the target vector is located is a boundary condition method.

Images