CN108983861A - Optimal power point-tracking method when generation part shading grid-connected based on photovoltaic multi-electrical level inverter - Google Patents

Abstract

The invention discloses an optimal power point tracking method based on photovoltaic multi-level inverter grid-connected when partial shading occurs. The method includes: The power supply measures the voltage and current; calculates the total output power; changes the reference voltage on the DC side of the H-bridge, and compares and calculates the optimal output power; the corresponding voltage value is the reference voltage on the DC side of the H-bridge. Partial shading is divided into two groups. In the balanced state, the traditional method can be used to optimize; in the unbalanced state, according to the principle of constant cosine distance, the power output voltage of the unshaded unit is found on the characteristic circle, and the shaded unit is calculated. The power supply output reference voltage, so as to calculate the minimum value of the voltage output by OPPT. The system will operate stably at this minimum voltage value and can quickly return to normal conditions after partial shading ends. According to the characteristics of the topological structure, the present invention realizes the stable transition and rapid recovery of the system during partial shading.

Description

Optimal power point tracking for local shading based on photovoltaic multilevel inverters connected to the grid tracking method

technical field

The present invention relates to the technical field of photovoltaic multilevel inverters, in particular to an Optimum Power Point Tracking (OPPT) method based on photovoltaic multilevel inverter grid-connected and partial shading.

Background technique

Many scholars have devoted themselves to maximum power point tracking when partial shading occurs in photovoltaic arrays, and have proposed many optimization algorithms. MPPT under the traditional topology needs to consider the specific output power of each photovoltaic panel in the system. Therefore, when a photovoltaic panel is partially shaded, its output power will decrease. For the total output power, the original The power corresponding to the voltage level is no longer the maximum power point MPP, and the local MPP will change from one to two.

If a large number of photovoltaic panels are connected in series and parallel, the probability of multiple peaks will increase greatly, and the complexity of finding the global MPP will also increase exponentially. For all photovoltaic inverter grid-connected systems, adjusting the output voltage of MPPT will take some time for the system to reach a new stable state, so the corresponding power cannot be obtained immediately. Therefore, algorithms such as particle swarm optimization (PSO), group search optimization algorithm (GSO), etc. that need to obtain a large amount of sample data and then filter out the optimal value will consume a lot of time and are not suitable for actual operation. Even if reinforcement learning (RL) is used later to obtain MPP, the complexity of the algorithm itself is not low. There are also many dynamic optimization algorithms, which are dedicated to adjusting the search step size, shortening the time to reach the maximum power point and avoiding falling into local optimum, but multiple voltage adjustments with large step size will also make the system unstable, and It is also extended when the system reaches stability. The optimization of these algorithms is dedicated to reducing the number of searches, and did not delve into the reasons for the emergence of multi-peaks and how to reduce the peaks.

Contents of the invention

The purpose of the present invention is to solve the above-mentioned defects in the prior art, and to provide an optimal power point tracking method based on photovoltaic multi-level inverter grid-connected when partial shading occurs. According to the characteristics of the topology, the method realizes The stable transition and rapid recovery of the system when partial shading is ensured.

The purpose of the present invention can be achieved by taking the following technical solutions:

An optimal power point tracking method based on photovoltaic multi-level inverter grid-connected when local shading occurs, the optimal power point tracking method includes the following steps:

S1. Collect the photovoltaic power supply output voltage _vpvn and output current _ipvn of each cascaded unit;

S2. Calculate the total output power of the photovoltaic power supply where N represents the number of cascade units;

S3. Determine whether shading occurs. If shading does not occur, determine the range of reference voltage variation, and search for the maximum power point through the MPPT control method as the optimal solution;

S4. If shading occurs, judge whether the shading degree of each cascaded unit is the same. If the shading degree is the same, search for the maximum power point as the optimal solution through the MPPT control method;

S5. If the shading degree is different, that is, unbalanced partial shading occurs, only one unit is considered to be shading, and the value is found on the characteristic circle, and the cosine distance is constant to find the corresponding value of the shading unit Output voltage reference value;

S6. Calculate the total output voltage reference value as the output of the OPPT.

Further, the step S2 includes:

S201. Perform preprocessing on the collected voltage and current data strings of the photovoltaic power supply according to the sampling density, and select a window of an appropriate size to divide the data strings;

S202. Filter out the most representative data in each small window to represent the value of this time period.

Further, the method for filtering out the most representative data in each small window in the step S202 is as follows:

Select the middle value of the window or the average value of the window as the most representative data.

Further, the step S3 includes:

S301. Determine the variation range of the reference voltage, and use the minimum inverter voltage required for grid connection as the lower limit of the OPPT output voltage;

S302. Comparing the before and after changes of the photovoltaic power supply voltage in the basic unit, and judging whether shading occurs;

S303. Assuming that when partial shading occurs, the light intensity of the series photovoltaic panels in each cascaded unit is the same, that is, there is always only one maximum power point. Based on this assumption, change the shading degree of photovoltaic power sources in all units, and calculate Use the MPPT search method to determine the upper limit of the OPPT output voltage;

S304. By changing the light intensity of all basic units at the same time, it is determined that under a certain light intensity, there is a maximum output power. When the light intensity decreases, the output power also decreases; when the light intensity increases, the output power also increases. decreases, this maximum power point is denoted as P _m (I _m , V _m ).

Further, the process of step S4 is as follows:

When balanced partial shading occurs, the shading conditions of photovoltaic power sources in each cascaded unit are the same. Consider the number of possible peaks of photovoltaics connected in series and parallel in the unit and the voltage range of each peak to determine the maximum possible voltage range. The voltage interval where the power point is located is searched in this voltage interval by using the MPPT search method, and the local optimal value found is used as the global optimal value.

Further, the MPPT search method is a perturbation and observation method.

Further, the step S5 includes:

S501. Select a cascade unit to conduct a shading experiment, and only change its light intensity, while allowing other cascade units to be at the most suitable light intensity;

S502. According to the different light intensity of the shading unit, the output voltage of each cascaded unit is also different. The output voltage of the unshaded voltage will automatically rise and rise to a specific value. Set the light intensity change step size and measure the corresponding Voltage value;

S503, fitting the above-mentioned voltage values to obtain a fitting function, which is a circle whose center is C(-N·I _m , V _m ), and whose radius is (N+1)I _m . The point P _m (I _m , V m ) _m ) as the maximum power point, recorded as the reference point, define the light intensity corresponding to the light intensity lower than P _m as the partial shading of the photovoltaic panel, connect the origin O and P _m , the straight line That is, the variation range of the reference voltage of the shading unit;

S504. When unbalanced partial shading occurs, find the point corresponding to the voltage value of the non-shading unit on the circle whose center is C, draw a straight line parallel to the y-axis through this point, and The intersection point is the output voltage reference value of the shading unit.

Compared with the prior art, the present invention has the following advantages and effects:

(1) The present invention proposes an OPPT controller. Under this topology, there is only one OPPT controller, and the most traditional MPPT control method is supported for balanced or unbalanced partial shading.

(2) When unbalanced partial shading occurs, it is found that the voltage variation law of the unshaded unit is a special circle, and the cosine distance is proposed to select the lowest value of the OPPT output voltage, and the system can be stabilized only by operating at the lowest value transition.

Description of drawings

Fig. 1 is actual topological structure and control algorithm schematic diagram of the present invention, is research object with the topological structure of 4 cascade units;

Fig. 2 is the relationship diagram of inverter DC side output power and output voltage of the present invention;

Fig. 3 is a schematic diagram of different situations of voltage, current and power that may occur when three series photovoltaic panels in a certain stage have partial shading in the 4 cascaded units of the present invention; wherein, Fig. 3 (a) shows that partial shading occurs There are 3 MPPs when it is overcast, Figure 3(b) shows that there are 2 MPPs when partial shading occurs, and Figure 3(c) shows that there is only one MPP when partial shading occurs;

Figure 4 is a diagram of the voltage range of the peak value of a single-stage photovoltaic array under different light intensities, that is, under different output currents;

Figure 5 is a diagram of the power change of a cascaded unit when partial shading occurs, and the output power balance of each stage is achieved by controlling the voltage and current. It is known that the output power of each stage is P ₀ during normal operation, and partial shading occurs When , the output power of the shading unit drops to P ₁ ', and the shading unit reaches its own maximum power point P ₁ by controlling the voltage and current;

Figure 6 is a schematic diagram of the rising law of the voltage of the unshaded unit when single-stage partial shading occurs, where Figure 6(a) defines the circle as a characteristic circle, changing the light intensity of the shading unit, the voltage will change along the characteristic circle, Figure 6(b) introduces the cosine distance to determine the reference value of the output voltage of the shading unit;

Figure 7 is a schematic diagram of the measured values and reference values of the output voltages at all levels when the inverter unit is partially shaded under the grid-connected topology of 4 cascaded units; Figure 7(a) is a schematic diagram of the output voltage of the photovoltaic power supply of the shaded unit, Figure 7(b) is a schematic diagram of the output voltage of the photovoltaic power supply of the unshaded unit;

Figure 8 is a schematic diagram of the measured values of the grid voltage and current when the inverter unit is partially shaded under the grid-connected topology of 4 cascaded units;

Figure 9 is a schematic diagram of the output voltage of the inverter under the grid-connected topology of 4 cascaded units, where Figure 9(a) is a schematic diagram of the measured value of the inverter output voltage when a certain unit is partially shaded, and Figure 9(b ) shows the schematic diagram of the measured value of the output voltage of the inverter under normal light;

Fig. 10 is a flow chart of the optimal power point tracking method when local shading occurs based on photovoltaic multilevel inverter grid-connected disclosed by the present invention.

Detailed ways

In order to make the purpose, 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 in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments It is a part of embodiments of the present invention, but not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by persons of ordinary skill in the art without making creative efforts belong to the protection scope of the present invention.

Example

The following is a brief introduction to OPPT.

OPPT, Optimum Power Point Tracking (OPPT) is referred to as the OPPT algorithm. The difference between OPPT and the traditional Maximum Power Point Tracking (Maximum Power Point Tracking, MPPT) is that it is also looking for the maximum power point. OPPT selects the global optimal value under the constraints of actual conditions. This solution is not necessarily the global maximum power point. The characteristic of this algorithm is that it does not need to spend a lot of resources to find the global optimal value with a small power increment.

As shown in Figure 1, this topology is a switch tube-diode cascaded multi-level inverter, which is composed of front-end and back-stage converters connected in series, and the front-end converter consists of N switch tubes- The diode units are cascaded, specifically, a fully controlled power electronic switch S _1n and an uncontrolled diode D _1n are connected in series.

The post-stage converter is a single-phase full-bridge inverter circuit, specifically a single-phase full-bridge inverter circuit composed of four fully-controlled power electronic switches, and the fully-controlled power electronic switches S ₂₁ and S ₂₃ are connected in series Bridge arm 1 is obtained, S ₂₂ and S ₂₄ are connected in series to form bridge arm 2, and the output of the inverter is connected to the fifth-order RLC filter.

In this embodiment, N is selected as 4, that is, the number of cascaded units is 4. In each cascade unit, the photovoltaic power supply is connected in series by three photovoltaic panels of the model Trina Solar TSM-250PA05.08. The filter is a fifth-order LC filter, and its symmetrical topology enables bidirectional flow of energy. The power grid parameters are 110V, 60Hz, and two loads 1 and 2 are connected.

The entire control process includes: OPPT collects the voltage value and current value of the photovoltaic power supply in the cascaded unit of the inverter DC side, the reference voltage of the inverter DC side calculated by OPPT; collects the grid voltage, and obtains the current voltage through the PLL phase-locked loop Frequency, through Park Transformation, the collected grid voltage and current, together with the reference voltage output by OPPT, are sent to the PWM generator, and the final output is a square wave with a sinusoidal duty cycle to control the DC side. Switch tubes in the cascode unit.

Wherein, the flow process of OPPT control is as shown in accompanying drawing 10, comprises the following steps:

S1. Collect the photovoltaic power supply output voltage _vpvn and output current _ipvn of each cascaded unit;

S2. Calculate the total output power of the photovoltaic power supply where N represents the number of cascade units;

The step S2 process is as follows:

S201. Perform preprocessing on the collected voltage and current data strings of the photovoltaic power supply according to the sampling density, and select a window of an appropriate size to divide the data strings;

S202. Filter out the most representative data in each small window to represent the value of this time period. For simplicity, the middle value is generally selected.

According to the sampling error and the fluctuation of the system, the window size can be properly selected, and the average value can be used instead. Usually, when the system is relatively stable, no preprocessing is required.

S3. Determine whether shading occurs. If shading does not occur, determine the range of reference voltage variation, and search for the maximum power point as the optimal solution through the traditional MPPT control method;

The step S3 process is as follows:

S301. Determine the variation range of the reference voltage, and use the minimum inverter voltage required for grid connection as the lower limit of the OPPT output voltage;

S302. Comparing the before and after changes of the photovoltaic power supply voltage in the basic unit, and judging whether shading occurs;

S303. Assume that when partial shading occurs, the light intensity of the series-connected photovoltaic panels in each cascaded unit is the same, that is, there is always only one maximum power point. Based on this assumption, change the shading degree of photovoltaic power sources in all units, and calculate Use the traditional MPPT search method, such as the perturbation and observation method, to determine the upper limit of the OPPT output voltage.

S304. By simultaneously changing the light intensity of all the basic units, it can be determined that there is a maximum output power under a certain light intensity. When the light intensity decreases, the output power also decreases; when the light intensity increases, the output power also decreases. This maximum power point is denoted P _m (I _m , V _m ).

Among them, to determine the variation range of the reference voltage, the voltage and current output by the photovoltaic array in all units can be measured by changing the light intensity of each cascaded unit, and the output power can be calculated, and the relationship between voltage and power can be plotted and fitted.

There are three photovoltaic panels connected in series in each cascade unit. When the maximum power is obtained at the same voltage value, the three photovoltaic panels have the same light intensity to obtain the maximum power. Therefore, when changing the light intensity of the cascaded units to calculate the output power range, only the same light intensity of the three photovoltaic panels is considered.

As shown in Figure 2, the horizontal curve means that under a certain light intensity, the OPPT output voltage is changed, the output power of the DC side is measured, and the voltage-power curve is drawn. In the figure, 4 light intensities are selected for simulation, and 4 voltage-power curves are obtained. It can be seen from the figure that the selection of OPPT voltage has upper and lower limits. The vertical curve indicates where the maximum power point occurs under different light intensities. It can be seen from the figure that the light intensity is not directly proportional to the maximum power, but there is a most suitable light intensity, and the maximum power point under this light intensity is P _m . None of the following studies exceed this voltage level.

S4. If shading occurs, determine whether the shading degree of each cascade unit is the same, if the shading degree is the same, the optimization method is the same as S3;

This step is specifically as follows: when balanced partial shading occurs, the shading conditions of the photovoltaic power sources in each cascaded unit are the same. Consider the number of possible peaks of photovoltaics connected in series and parallel in the unit and the voltage range of each peak, and combine with step S301 to determine the voltage range of the maximum power point within the feasible voltage range, and then use the disturbance observation method to search within this range. The local optimum found is used as the global optimum.

In this topology, it is assumed that the parameters of the three photovoltaic panels of each cascaded unit are the same, and the output current is changed by changing their respective light intensity. The three photovoltaic panels have different peak numbers under different light intensities (output currents), and the MPP may fall on any peak. As shown in Figure 3(a), Figure 3(b), and Figure 3(c), this is one of the relationships among voltage, current and power when partial shading occurs.

Assuming three peaks occur, then:

1. The widths of the three voltage intervals [0, v ₁ ], [v ₁ , v ₂ ] and [v ₂ , v ₃ ] are equal;

2. The current in the three voltage ranges is constant;

3. The three local optimal values are marked as ①, ② and ③ respectively;

①, ② and ③ correspond to [0, v ₁ ], [v ₁ , v ₂ ] and [v ₂ , v ₃ ].

If the power of the maximum power point of each peak is the same, the critical condition is

I ₁ ·V ₁ =I ₂ ·V ₂ =I ₃ ·V ₃ (1)

Wherein, I ₁ , I ₂ and I ₃ represent the magnitudes of currents corresponding to the three voltage ranges respectively, and there are I ₁ =aI, I ₂ =bI, I ₃ =cI; V ₁ =1.5V ₂ =3V ₃ .

So you can get

aIV ₁ =bIV ₂ =cIV ₃ (2)

Wherein, a=b+x, b=c+y, c=z.

So it can be concluded that:

According to the relationship between the above currents, it can be quickly judged which voltage range the maximum power point falls in. So that when partial shading occurs, the initial value of the search voltage can be quickly adjusted.

In practical applications, as shown in Figure 3, the number of peaks is not the same. For this reason, we specially mark it, as shown in Figure 3(a), when there is only one peak, its MPP is marked as ①; as shown in Figure 3(b), when there are two peaks, the MPP of the high voltage level The MPP is marked as ①, and the low voltage level is marked as ②. As shown in Figure 3(c), when two peaks appear, the MPP of the high voltage level is marked as ①, the MPP of the medium voltage level is marked as ②, and the low voltage level is marked as ③.

Set the light intensity of the three photovoltaic panels from complete darkness to full brightness (1000W/m ² ), and according to the corresponding output current of the three photovoltaic panels and the MPP intervals ①, ② and ③, the MPP voltage interval distribution map can be drawn. As shown in Figure 4, the three coordinate axes respectively represent the output currents of the three photovoltaic panels, and the points in the middle with three colors and three shapes represent three voltage level intervals. Among them, when the MPP falls in the ① interval, it is represented as a square point, in the ② interval, it is represented as a circle point, and in the ③ interval, it is represented as a triangle point. Since it was previously assumed that the three photovoltaic panels are exactly the same, the three coordinate axes are symmetrical in rotation, forming a cube. It can be seen that the square point occupies the vast majority and is located in the middle of the entire cube figure; the round point is located at the three vertices away from the cube origin; the triangle point is located at the three vertices close to the cube origin.

The actual situation corresponding to Figure 4 is: when the degree of partial shading of the three photovoltaic panels is relatively similar, the MPP will always fall in the ① interval; when one of the photovoltaic panels has relatively serious partial shading, the MPP will fall in the ② Interval; when two of the photovoltaic panels suffer severe local shading, the MPP will fall in the ③ interval. Therefore, we can quickly find out which range the MPP falls in according to the current relationship after shading, and adjust the voltage.

For a grid-connected system, the initial value of the MPPT voltage will always be set higher for smooth grid-connection, so the voltage search is actually a transition from high voltage to low voltage, so the MPP falling in the ① range can always use the traditional method turn up. When it falls in the range of ② and ③, it is necessary to perform a voltage adjustment, and then use the traditional method to search.

It is worth noting that when the MPP appears in the ② and ③ intervals, the voltage levels are too low to meet the minimum voltage requirements of the inverter DC side, and at this time the global optimal power value is different from the local optimal value in the ① interval The difference is not much, so no voltage adjustment is needed or necessary.

During grid-connected operation, when changing the reference value of the DC side voltage of the inverter, that is, changing the MPPT output voltage reference value, it takes a certain amount of time for the grid to reach stability. Therefore, in actual operation, the algorithm that requires multiple large steps to modify the voltage value is not applicable. The advantage of using the traditional method is that it can simplify the amount of calculation and facilitate the porting of the code to the chip. And this power point becomes the optimal power point OPP.

S5. If the shading degree is different, that is, unbalanced partial shading occurs, only one unit is considered to be shading, and the value is found on the characteristic circle, and the cosine distance is constant to find the corresponding value of the shading unit Output voltage reference value;

The step S5 process is as follows:

S501. Select a cascade unit to perform a shading experiment, and only change its light intensity, so that other cascade units are at the most suitable light intensity.

S502. According to the different light intensity of the shading unit, the output voltage of each cascaded unit is also different. The output voltage of the unshaded voltage will automatically rise and rise to a certain value. Set the light intensity change step size and measure the corresponding voltage value.

S503. Fit these voltage values to obtain a fitting function. This is a circle whose center is C(-N·I _m , V _m ), and whose radius is (N+1)I _m . The point P _m (I _m , V m ) _m ) as the maximum power point, recorded as the reference point. The light intensity corresponding to the light intensity lower than P _m is defined as partial shading of the photovoltaic panel. Connecting the origins O and P _m , the straight line That is, the variation range of the reference voltage of the shading unit.

S504. When unbalanced partial shading occurs, find the point corresponding to the voltage value of the non-shading unit on the circle C, draw a straight line parallel to the y-axis through this point, and The intersection point is the output voltage reference value of the shading unit.

This topology has an obvious disadvantage. For each cascaded unit, the output power must be balanced for normal operation, otherwise the system will collapse. According to the voltage-power characteristic of photovoltaic panels, it is a convex function, which means that the same power value can correspond to two voltage values. That is, these units can still be kept in balance through voltage regulation. As shown in Figure 5, the original output power of each stage is P ₀ . When a unit is partially shaded, its power curve will drop. In order to balance the power, the output voltage of the unshaded unit must be increased. The result of MPPT search is to hope that the shading unit can output the maximum power, that is, the power change is P ₀ →P ₁ '→P ₁ . Therefore, the output voltage of the voltage shading unit can be controlled from V ₀ →V ₁ ′→V ₁ , and the output voltage of the unshaded unit can be controlled from V ₀ →V ₂ ′→V ₂ . In actual operation, due to the limitation of the cascaded topology, the current is clamped, and the actual output current of the shading unit cannot be measured. In other words, the maximum power of the shading unit cannot be obtained directly. However, its voltage change law follows the above conclusions, so the voltage can be measured and adjusted to achieve power balance. The control voltage only needs to control the output voltage of the MPPT.

When partial shading occurs, the output voltage of the shading unit will automatically drop, and the output voltage of the unshaded unit will automatically increase. It can be seen by observation that the output voltage of the unshaded unit will rise to a certain value and maintain stable operation at this voltage value. By changing the light intensity of the shading unit, the size of this specific value under different light conditions can be measured.

When testing the characteristics of photovoltaic panels, change the light intensity from 250W/m ² to 1000W/m ² . When the light intensity is 750W/m ² , the output power of the photovoltaic panel is the maximum, increasing or decreasing the light intensity will reduce the output power of the photovoltaic panel. Therefore, the corresponding voltage under this light intensity is V _max , and the corresponding current is I _max . This point is denoted as the maximum power point P _max (I _m , V _m ).

Change the light intensity of the shading unit from 250W/m ² to 750W/m ² , and obtain a set of specific values of the output voltage of the unshaded unit. Through fitting, it can be known that these specific values are distributed in a circle centered at (-4I _m , V _m ), on a characteristic circle with a radius of 5I _m , the fitting curve is shown in Figure 6(a), the abscissa represents the current, and the ordinate represents the voltage.

When partial shading occurs, the voltage of the unshaded unit is a constant value, so calculating the output voltage of the OPPT is also calculating the power supply output reference voltage of the shading unit. As shown in Figure 6(b), when partial shading occurs, the voltage of the unshaded unit rises to point B. Due to the current clamp, the voltage of the shaded unit can only be in a straight line parallel to the y-axis change. In order to keep the selected voltage value close to the characteristics of P _max (I _m ,V _m ), a cosine distance is introduced: connect the origin O and P _max (I _m ,V _m ) and intersect the circle at point A, then and The point of intersection is point C. The cosine distance between point C and point A is 1, that is, the cosine distance cos∠BOC between point B and point C is equal to the distance cos∠BOA between point B and point A. The point C is the desired output voltage reference value of the shading unit. Add up the voltage reference value of each unit, which is the lowest value of the output voltage of OPPT. When the same degree of partial shading occurs, the system will operate stably at the lowest value of the OPPT output voltage. When different degrees of partial shading occur, according to the measured current value, in Select the corresponding voltage value. In particular, since the minimum value of the OPPT output voltage cannot be lower than the minimum value of the DC side voltage of the inverter, when the minimum value of the OPPT is too small, it will operate according to the minimum voltage requirement of the DC side of the inverter.

After simulation verification, it is proved that when a certain unit is partially shaded, the system can operate stably if the reference voltage is selected according to the rules described in Figure 6(b). The method of selecting the reference voltage is not unique, and determining the reference voltage of the shading unit by cosine distance is only one of the most convenient and feasible methods.

The control effect is shown in FIG. 7 , for the reference value of the output voltage of the four cascaded units, a total reference value is given. After averaging to each level, the predicted values are shown in Figures 7(a) and 7(b). In Figure 7(a), the photovoltaic voltage of the shading unit drops sharply, and in Figure 7(b), the voltage of the unshaded unit rises, and its variation follows Figure 5. . In order to ensure the stability of the voltage on the DC side of the voltage inverter, it is necessary to control the output voltage of the power supply in the unit and select an appropriate operating voltage. Although the unshaded unit is not operating at the maximum power point in the traditional sense at this time, its output power will not be reduced too much, and the power balance of each cascaded unit can be maintained.

When the light changes suddenly, the voltage and current changes on the grid side are shown in Figure 8. The voltage on the grid side is basically unaffected, and the current will change steadily until it reaches a new stable value. In practice, the illumination changes continuously and slowly, so the impact on the system is smaller and the change process will be more stable.

When the power supply of each unit is unbalanced, although it is a pathological operation, it can eliminate the voltage spike caused by the discharge of the inductor in the filter during inversion. Figure 9(a) shows the measured value of the inverter output voltage when a unit is partially shaded, and Figure 9(b) shows the measured value of the inverter output voltage under normal light. It can be seen that the voltage in Figure 9(a) has no spikes. Therefore, it can be considered to add a low-power power supply to eliminate the spike.

To sum up, in the present invention, the series connection of the front-stage cascaded switch tube-diode converter and the rear-stage inverter circuit can well realize a multi-level ladder wave that is approximately sinusoidal, and simplifies the topology of the power unit system. Complexity disperses a large number of series and parallel connections of photovoltaic panels, reduces the occurrence of multiple peaks of power, and reduces the number of searches. In the present invention, the number of MPPT controllers is reduced by exploring the change law of voltage and power when partial shading occurs in the whole unit and when partial shading occurs in one unit, and the number of MPPT controllers is simplified from one level to one MPPT controller, regardless of the number of cascades. There is one and only one controller, and this improved MPPT controller is called OPPT controller.

S6. Calculate the total output voltage reference value as the output of the OPPT.

The above-mentioned embodiment is a preferred embodiment of the present invention, but the embodiment of the present invention is not limited by the above-mentioned embodiment, and any other changes, modifications, substitutions, combinations, Simplifications should be equivalent replacement methods, and all are included in the protection scope of the present invention.

Claims (7)

1. optimal power point-tracking method when a kind of generation part shading grid-connected based on photovoltaic multi-electrical level inverter, feature exist In the optimal power point-tracking method includes the following steps:

S1, the photo-voltaic power supply output voltage v for acquiring each concatenation unit_pvnWith output electric current i_pvn；

S2, the total output power of photo-voltaic power supply is calculatedWherein N indicates the quantity of concatenation unit；

S3, judge whether to shade, if do not shaded, determine reference voltage variation range, pass through MPPT control method Maximum power point is searched for as optimal solution；

S4, if it happens shade, judge whether each concatenation unit shading degree is identical, if shading degree is identical, passes through MPPT control method searches for maximum power point as optimal solution；

If S5, shading degree are different, that is, unbalanced part shading occurs, only considers that one and only one unit hides Yin finds the value in characteristic circle, constant by COS distance, finds the corresponding output voltage reference value of shading unit；

S6, total output voltage reference value, the output as OPPT are calculated.

2. it is according to claim 1 based on photovoltaic multi-electrical level inverter it is grid-connected generation part shading when optimal power point with Track method, which is characterized in that the step S2 includes:

S201, it is pre-processed according to voltage and current serial data of the sampling density to the photo-voltaic power supply of acquisition, selects suitable size Window divide serial data；

S202, the value that most representational data are used to represent the period is filtered out in each wicket.

3. it is according to claim 1 based on photovoltaic multi-electrical level inverter it is grid-connected generation part shading when optimal power point with Track method, which is characterized in that the method for filtering out most representational data in each wicket in the step S202 It is as follows:

The median of selected window or the average value of selected window are used as most representational data.

4. it is according to claim 1 based on photovoltaic multi-electrical level inverter it is grid-connected generation part shading when optimal power point with Track method, which is characterized in that the step S3 includes:

S301, determine reference voltage variation range, will it is grid-connected needed for minimum inverter voltage as OPPT output voltage lower limit；

S302, the front and back variation for comparing photo-voltaic power supply voltage in basic unit, judge whether to shade；

When part shading occurs for S303, hypothesis, the intensity of illumination of the tandem photovoltaic plate in each concatenation unit is the same, i.e., most High-power point always only one, based on this it is assumed that changing the shading degree of photo-voltaic power supply in all units, calculateOPPT output voltage limit is determined with MPPT method for searching；

S304, the intensity of illumination by changing simultaneously all basic units are determined under some intensity of illumination, are had maximum defeated Power out, when intensity of illumination reduces, output power also reduces therewith；When intensity of illumination increases, output power also reduces, this A maximum power point is denoted as P_m(I_m,V_m)。

5. it is according to claim 1 based on photovoltaic multi-electrical level inverter it is grid-connected generation part shading when optimal power point with Track method, which is characterized in that the step S4 process is as follows:

When the part shading balanced, the photo-voltaic power supply shading situation in each concatenation unit is identical, considers that string is simultaneously in unit Voltage range where peak value number that the photovoltaic of connection is likely to occur and each peak value, determines maximum work in feasible voltage range Voltage range where rate point, then searched in the voltage range with MPPT method for searching, the local optimum found is as complete Office's optimal value.

6. optimal power when generation part shading grid-connected based on photovoltaic multi-electrical level inverter according to claim 4 or 5 Point-tracking method, which is characterized in that the MPPT method for searching is perturbation observation method.

7. it is according to claim 1 based on photovoltaic multi-electrical level inverter it is grid-connected generation part shading when optimal power point with Track method, which is characterized in that the step S5 includes:

S501, choose a concatenation unit carry out shading experiment, only change its intensity of illumination, and allow other concatenation units all in Most suitable intensity of illumination；

S502, the intensity of illumination difference according to shading unit, the output voltage of each concatenation unit is also different, and do not shade voltage Output voltage can rise automatically, and rise to a particular value, and setting intensity of illumination changes step-length, measure corresponding voltage Value；

S503, the above-mentioned voltage value of fitting, obtain fitting function, this is that a center of circle is C (- NI_m,V_m), radius is (N+1) I_m Circle, point P_m(I_m,V_m) it is used as maximum power point, it is denoted as reference point, intensity of illumination is defined and is lower than P_mCorresponding intensity of illumination is light It lies prostrate plate and part shading occurs, connect origin O and P_m, straight lineThe as variation range of shading elements reference voltage；

S504, when uneven part shading occurs, find the corresponding point of non-shading cell voltage value on the circle for being C in the center of circle, It crosses the point and makees straight line parallel in y-axis, withIntersection point be the shading unit output voltage reference value.