1.The impact of part of the shadow
Some shadows are divided into hard shadows and soft shadows. Soft shadow refers to some shadow problems caused by dust, fallen leaves, clouds, snow, shadows, tall buildings, etc. The hard shadow components of the hard shadow are aging due to different orientation, angle, parameters, and some battery components. The problem of loss caused by other reasons, these collectively referred to as part of the shadow issues.
In actual engineering, some shadow problems cannot be avoided. Even if the installation procedures are correct, the solar irradiation received by the photovoltaic array will be affected by shadow obstruction in some periods of the day or some days within a year. China has a wealth of solar radiation resources and vast buildings that can be available for the development of photovoltaic building integrated systems. However, due to the impact of some shadow problems, the promotion of photovoltaic power generation systems is restricted to some extent, especially for small -scale enabled small -scale small -scale small -scale small models. There are great restrictions on photovoltaic power generation systems. Therefore, it has greater practical value to solve part of the shadow problems in the photovoltaic power generation system.
The existence of some shadow problems will seriously affect the photovoltaic power generation system. When the external environment changes, the characteristics of photovoltaic components also change accordingly. When the intensity of the photovoltaic component is reduced, the short -circuit current of photovoltaic components will decrease, and the changes in the opening voltage are not great. Therefore, compared with the photovoltaic components that are not affected by shadows, the photovoltaic components are not affected by shadows. The maximum power point current drops sharply. This will cause the photovoltaic components to connect to the maximum power points from their respectives, and the overall output power will be severely reduced. Figure 7.1 is a diagram of the two photovoltaic components in series. A diode is connected in parallel at both ends of each photovoltaic component. The PV1 light is normal and PV2 is covered by shadows. Figure 7.2 (A) is the I-V characteristic curve of the two photovoltaic components. It can be seen from the figure that when the working current of the series of the links is greater than the short-circuit current of PV2, the PV2 will work in the second quadrant, that is, the negative pressure area, that is, the negative pressure area. As shown in Figure 7.2 (b), point C and D, at this time the photovoltaic component changes from a power generation unit to a load unit. If the negative pressure continues to increase, and the photovoltaic component does not have parallel to the two -polar pipe, the current flowing through the photovoltaic component has risen sharply, and the loss power consumption power increases sharply. If this situation continues for a long time, it is possible to form hot spots and damage photovoltaic components. In order to prevent this situation, it is generally connected to the cross -pole diode at both ends of the photovoltaic component. In this way, when there is negative pressure at both ends of the photovoltaic component, the cross -pole diode is turned on, the current flows from the bypass diode, and the portable photovoltaic component side is placed. Voltage, thereby avoiding damage to photovoltaic components in thermal spots. However, after the parallel diode, multiple power peak points will appear in the P-V curve of the system, as shown in Figure 7.2 (b), so the traditional maximum power point tracking technology may be tracked to the local maximum power point (such as point B). This makes the maximum power point tracking failure [139].
From the above analysis, some shadows are mainly manifested in the following aspects.
① All photovoltaic components connected in series are deviated from the maximum power point, causing the output power of photovoltaic components to be severely reduced, so the conversion efficiency of the entire photovoltaic power generation system is reduced a lot.
② When the photovoltaic component does not have a parallel wiping diode, if the voltage end of the photovoltaic component drops to the negative value, it will be transformed from the power supply to the load. Therefore, the heat spots effect may cause damage to the photovoltaic component, which will cause the system reliability to decrease.
③ When the photovoltaic component is equipped with bypass diode and the cross -rotor diode is turned on, the series voltage of all photovoltaic components will decrease. When the number of cross -pole diodes of the drive, the entire photovoltaic power generation system may be forced to stop due to failure to work properly. At the same time, when the light is severely uneven or the cross-rotor diodes are turned on, the P-V curve of the photovoltaic road will appear multiple great value points, and the ordinary MPPT algorithm fails.
It can be seen from the above analysis of some shadow issues that research shadow problems is of great significance. Therefore, how to overcome some shadow issues of photovoltaic power generation systems is a hot spot in Chinese scholars in China in recent years, and it has made some breakthrough progress.
2.Research status of some shadow problems
In recent years, some research progress has been made in the modeling of photovoltaic batteries, skewers and joint output characteristics, and reasonable selection of photovoltaic components under shadow conditions. Literature [140] Study the changes in the output characteristics of different macro and micro structure layouts under the shading conditions of the photovoltaic arrays, and use the switch tube with the diode to achieve the co -connection of photovoltaic components Essence Literature [141] Considering the two -dimensional pipeline equivalent circuit model of the photovoltaic battery metastic element of the avalanches, and the mathematical model of photovoltaic components was established. From the perspective of photovoltaic components, it shows that multiple tandem battery elements are in parallel to a diode. Reduce the impact of shadows. Literature [142] Using the theory of support vector machines, the mathematical model method of optical array in local shadow conditions is established. The literature [143] focused on the lack of matching of photovoltaic array caused by random shadows, comparing the array loss ability of different series and parallel mode.
For the solution of some shadow problems, the current mainly includes two aspects: one is from the perspective of the MPPT method to study the new MPPT algorithm based on multiplayer; the other is from the perspective of the circuit to study the appropriate circuit top Using the conventional MPPT algorithm can achieve the maximum power point tracking well.
1) New MPPT algorithm based on multiple peak values
Under some shadow conditions, due to the addition of the diode of parallel wing, the P-V feature curve of the photovoltaic branch road will appear. Therefore, the conventional MPPT algorithm may converge at the local maximum power point, which will make the MPPT algorithm fail. Therefore, you must find a new MPPT algorithm to avoid its convergence at the local maximum power point, thereby achieving the true maximum power point tracking.
Literature [144] introduces a composite MPPT method combined with conventional algorithms. This algorithm is divided into two stages of the first stage through online measurement opening voltage and short -circuit current, and the optimal working current and optimal working voltage of the photovoltaic array are similar to its short -circuit current and the opening voltage. The optimal load line. In the second stage, the conventional incremental conductive method is used to achieve accurate positioning. By comparing the data saved in the first stage, it is necessary to determine whether the algorithm is tracked to the real maximum power point, so as to avoid converging to the local maximum power point. Literature [145] The optimal working current corresponding to the maximum power point through the P-I curve of the photovoltaic array is scanned by cyclical, and the photovoltaic array work is at the optimal working current by controlling the corresponding power electronic converter. Literature [146] proposes a MPPT method suitable for multi-peak value based on the P-V curve scanning. This method determines the optimal working voltage of the photovoltaic array by scanning the P-V curve, thereby achieving maximum power point tracking. Literature [147] Apply Fibonacci search method to the maximum power point tracking. The literature [148] studied the MPPT method based on state space. These two methods are also applicable to the multi -peak characteristics. Literature [149] studied the interval positioning of the maximum power point of the derivative equivalent area and the method of seeking the sampling point of the guidance, and proposed the maximum power point tracking method on the basis of the interval positioning method. Literature [150] Based on the fault diagnosis, the photovoltaic array is divided into fault support and no fault support, and the laws of multi -pole value point are analyzed. On the basis of this Value point maximum power point tracking control strategy. Literature [151] Aimed at the non -linearity of the output characteristics of photovoltaic components and affected by the external environment, it discussed the maximum power point method of the genetic neural network to track the photovoltaic array, which predicts the effect of predicting the maximum maximum power tracking.
The new MPPT algorithm introduced above can be tracked to the real maximum power point, but the disadvantages of these methods cannot make up for the system power loss caused by the shadow problem.
2) Solution based on circuit topology structure
In recent years, the “distributed maximum power point tracking” proposed by the academic community (DISTRIBUTED MPPT (DMPPT)) is from the perspective of circuit topology. The electronic electronic converter is based on this basis to form a photovoltaic power generation system. This method allows each photovoltaic component to work at the maximum power point of each photovoltaic component under the influence of some shadows. Another solution is to connect a power compensation unit parallel to each photovoltaic component. By providing a compensation current to the photovoltaic component affected by shadows, the P-V curve of the photovoltaic array presents the single peak characteristics.
(1) DC component circuit
The DC component circuit is also known as DC Mic (Module Integrated Converter), including parallel DC component circuits and series DC component circuits [152]. For series DC component circuits, each photovoltaic component has an independent DC/DC converter, which can independently track the maximum power point [153-157], as shown in Figure 7.3 (a). The US TIGO Energy Corporation and Israel SOLAREDGE Technology Co., Ltd. have launched some solar power optimizers. The appearance of the product is shown in Figure 7.4 (a) and (b). The peak efficiency of the new power optimizer is 99.5%, and the weighted efficiency is 98.9%. This type of optimizer can emit more electricity in shadow and no shadow environment. It can adjust the current of each photovoltaic component to eliminate any form of loss. It is easy Sex and implementation of component -level photovoltaic system monitoring.
The series DC component circuit shares a inverter device. Each DC component works in real time to track the maximum power point of their respective photovoltaic components. The circuit itself must have power loss, so it will inevitably reduce the efficiency of the entire photovoltaic power generation system. Because the circuit is connected in series, the circuit of each DC component cannot be inserted, and their output current is the same. Because the photovoltaic component parameters cannot be completely consistent, when there is no shadow impact, the output current of the DC component circuit that is the first to reach the maximum power point will be possible to become a public output current. Other DC component circuits can only work at the quasi -maximum power point state. ; When some shadows exist, the output current affected by the shadow will affect the public output current, resulting in other DC components connected in the series will also work at the maximum power point. At the same time, if a certain or certain DC component circuit fails, the performance of the entire system may decline.
The parallel DC component circuit is shown in Figure 7.3 (b). A single DC component circuit must have a voltage function, which requires a large voltage gain. Therefore, compared with the series DC component circuit, the design of the parallel DC component circuit must choose a high -gain circuit topology, so there is a certain difficulty [158.159]. At the same time, the parallel DC component circuit public DC bus also has similar problems to the series DC component circuit, that is, the real -time work of each DC component, and the circuit itself has loss; when there is no shadow effect, there is also a competitive problem of DC bus voltage; When there is a shadow impact, the output voltage affected by the shadow is affected by the output voltage of the DC component circuit will affect the public DC bus voltage, and it also affects other DC component circuits to work at the maximum power point state.
(2) AC component circuit
AC component circuit (also known as photovoltaic miniature inverters) is to directly connect small inverters with photovoltaic components, and the communication side is connected to a low -voltage power grid to achieve maximum power point tracking of each photovoltaic component to effectively avoid shadow problems [160 ]. The AC component circuit includes industrial frequency isolation and high -frequency isolation types, as shown in Figure 7.5 (a) and (b), and can also be divided into current source type and voltage source. Direction of development. The power level of this module is between 100 and 500W, and this system is considered one of the important development directions of future grid -connected inverters. According to the latest research report of the IHS, the number of micro -inverters in 2013 exceeded 500MW for the first time. In 2017, the shipping volume of global photovoltaic micro -inverters will increase to 2.1GW. At present, micro -inverters are very popular in the US market, especially the residential market, and the penetration rate has exceeded 40%. Organizing photovoltaic components with integrated miniature inverters are called AC components. Solarbridge and Enecsys have launched a series of strategic cooperation with photovoltaic component suppliers to supply AC components.
