Power output control method of small power photovoltaic grid connected power generation under complex illumination

Due to the irregular change of lighting conditions, there is a large error in the control of photovoltaic grid connected power output. Therefore, this paper proposes proposes a power output control method for low-power photovoltaic grid-connected power generation considering complex illumination. Based on the analysis of the basic characteristics of photovoltaic grid connected power generation, the output characteristics of photovoltaic array under complex lighting conditions are studied. Combined with its variation characteristics, based on fuzzy control strategy, the output of photovoltaic grid connected energy storage system is adjusted by filter control with variable time constant, and economic operation and stable operation are taken as constraints. The test results show that the design method has a high degree of fit between the control of photovoltaic grid connected power output and the actual results, and the error is within a reasonable range, which is obviously due to the two groups of comparison methods.


INTRODUCTION
The traditional primary energy such as coal, oil and natural gas has been used by human beings for a long time, but the energy on our earth is actually very limited and can not be consumed by human beings without limit, and will be exhausted one day [1][2] . There is only one feasible way: vigorously develop new energy. Since the end of last century, countries all over the world have paid more and more attention to the development and utilization of new energy [3][4][5] . Solar energy is a green and clean alternative energy. However, today's solar photovoltaic power generation still has some defects, such as low efficiency, high power generation cost and high on grid electricity price. These defects lead to the inability of large-scale production of solar photovoltaic power generation and limit its large-scale promotion and utilization, which is greatly constrained in economic benefits [6] . Therefore, taking measures to greatly improve the utilization rate of light energy and reduce the power generation cost is the top priority of photovoltaic power generation research. It is assumed that when the light intensity of the sunlight perpendicular to the surface of the silicon wafer and the temperature of the silicon wafer are constant, the output volt ampere characteristics of the solar cell are theoretically determined for the same photovoltaic module, and there is a maximum power value when the photovoltaic panel is not aged and covered. Therefore, in order to maximize the use of solar energy and reduce the power generation cost, it is necessary to make the photovoltaic cell run near the maximum power output node as far as possible, for example, by adjusting the equivalent impedance of the load. At the same time, it will also affect the stable output of power, and even make the power curve fluctuate seriously. If no measures are taken, the photovoltaic module will enter an unstable working state. Photovoltaic power generation is a new power generation mode that directly converts solar radiation energy into electric IOP Publishing doi:10.1088/1742-6596/2195/1/012052 2 energy by using the photovoltaic effect of solar cell semiconductor materials. However, photovoltaic power generation is random and intermittent, which will affect the quality of power and aggravate the fluctuation of power grid. Equipped with certain energy storage in the light energy system, it can effectively suppress power fluctuation and improve the safety and stability of power grid operation. Therefore, the smooth control of the output power of the optical energy motor unit has become the focus of research.

2.1.Analysis of basic properties of photovoltaic cells
The basic model of photovoltaic cell based on light energy is shown as Figure 1.  On this basis, the photovoltaic cell model based on MATLAB / Simulink is constructed.

STUDY ON OUTPUT CHARACTERISTICS OF PHOTOVOLTAIC ARRAY UNDER
Thus, the series array in

Filtering control of variable time constant
Because of the dynamic change of illumination condition, the fixed filter can not meet the output requirement. Therefore, the variable time constant is added in this paper, and the filter time constant is adjusted at the same time to keep the SOC of the energy storage system in a certain range, so as to achieve the goal of smoothing the output and taking into account the life of the storage battery. The control structure is shown in Figure 2. Similarly, when the energy storage battery is in the low position and the battery is in the state of discharge compensation, reducing the time constant s T can effectively prevent the SOC from exceeding the lower limit. Similarly, when the energy storage battery is in the low position and the battery is in the state of discharge compensation, reducing the time constant AA can effectively prevent the SOC from exceeding the lower limit.

4.2.Control constraints
On the basis of the above, but also on the photovoltaic grid-connected power output control constraints, this paper mainly from the following two aspects.

Photovoltaic grid-connected economic operation
In the process of battery operation, large charging and discharging current and over charging and over discharging will affect the battery life, so it is necessary to make the battery work in a certain bound range. To this end, we must first control the current and voltage.
(1) Current constraint conditions, when the charging and discharging current of the battery exceeds the limit, the service life of the battery will be affected. (2) Voltage constraints, when the battery voltage is lower or higher than a certain value will affect the battery life.  Under the condition of economic operation, the control strategy of energy storage system searches for fuzzy rules according to the SOC status of storage battery, PV power forecast value, load forecast value and l P exchange status of current PV power interconnection line, and achieves the control of battery charging and discharging to achieve the goal of economic operation. The fuzzy rules table is shown in Table 1. In the table, l P for "+" means positive power status: photovoltaic grid-connected to the distribution network to sell electricity; For "-" means negative power status: means photovoltaic grid-connected distribution network to buy electricity.

Photovoltaic grid-connected stable operation
On the premise of guaranteeing the service life of the battery, the fuzzy control of the storage battery is designed according to the power fluctuation and the continuous power supply capacity of the system. Based on the current SOC status of the battery, the predicted photovoltaic power and the predicted load value, the system can run stably by querying the fuzzy rule table. The fuzzy rules table is shown in Table 2. On this basis, to ensure the reliability of the control.

4.3.Fuzzy control output
On the basis of the above, the output of defuzzy control is the process of converting the fuzzy quantity of fuzzy inference and decision output into the definite control quantity, which corresponds to the fuzzy quantity of the system. According to the symmetry of the output fuzzy set in this paper, the output defuzzy quantity is obtained by weighted average method. The calculation formula is shown in formula (10).

5.1.Experimental environment design
Using Matlab/Simulink to build 20 photovoltaic power plant models and active power control models with the rated power of 1MW, and through the simulation of the photovoltaic power plant output power control to verify the distribution strategy proposed in this paper, the control model is shown in Figure 3, in simulation, it is assumed that each photovoltaic inverter is in normal state.  As can be seen from the figure 4, the set power of the photovoltaic power plant is lower than the predicted power of the photovoltaic power plant. When the set value is constant, the actual output power of the PV plant can ensure the stable output of the set power. When the set power is changed, the PV plant can complete the tracking of the new set power in a short time.

CONCLUSION
With the rapid development of solar photovoltaic power generation technology in the world, more and more large scale photovoltaic power plants begin to be connected to the power grid, and the stability of