Research on simulated devices for Solar photovoltaic grid-connected generation system

On the standpoint of energy conservation and emission reduction, one device simulated photovoltaic grid-connected generation system based on SPWM was designed in the paper. And DC/AC inverter could transduce efficiently direct current to alternating current. The MCU(Micro-Control-Unit), in this system could achieve the control method for maximum-power-point and tracking for frequency and phase. Moreover, the MCU could implement PWM (Plus-Width Modulating) through programming. The system showed clearly the whole photovoltaic grid-connected generation system using simulated methods


Introduction
With the rapid development of modern science and technology, the depletion of petrochemical energy and the impact on the environment are becoming increasingly serious, looking for renewable clean energy has become increasingly urgent global research topics.At present, the solar energy inexhaustible clean energy has been favored.The radiant energy of the sun can be converted into heat, which is a common use of solar energy.Photovoltaic power generation is the use of the energy directly into electrical energy the sun radiation, that is to say, light radiation can be converted into electrical energy by solar cells, after energy storage, control and transformation, which become direct or alternating current energy that can be used directly by people.The so-called grid connected, simply speaking, when voltage, phase, phase sequence and frequency are same, the principle of parallel operation can be achieved [1,2] .

System structure and analysis
As shown in Figure 1  voltage and frequency adjustment [3,4] .

system detection circuit
The design of the detection circuit is very important, and it directly affects the accuracy of the data acquisition by the microcontroller.The system has five detection circuits, which respectively realize the functions of frequency tracking,

frequency detection circuit
As shown in Figure 4 for the frequency detection circuit, the detected AC input voltage, through the differential amplifier circuit the which consists of integrated operational amplifier The current detection circuit is that a small resistor is connected in the current detecting circuit, such as milli ohm resistance, using the same differential amplifying circuit, the voltage signals at both ends of the ohm resistor are detected, adjusted and input to the microcontroller, after calculation of microcomputer, corresponding to calculate the current value.
As shown in Figure 6, the current detection circuit.The operational amplifier uses the independent operational amplifier of TL082, which can avoid mutual interference between the sampling circuits, so that the output signal is accurate and anti-interference is strong.
It is worth noting that the sampling circuit must be repeatedly debugged, to make the amplitude, stability and smoothness of the waveform determine, only in this way can the detection signal be input to the microcontroller to ensure the accuracy of the signal.

Conclusion
The device is a simulation device, which uses the voltage source to simulate the photovoltaic solar cell, so some places need to be perfected.For example, the actual maximum power point tracking (MPPT) is more complex, need to consider many factors of light intensity, temperature, humidity.but in the simulation device, Just focusing on simulating the intensity of the light (such as when the maximum output power is reached, the load resistance value is equal to the resistance value in the power supply), and other factors is ignored.We are still actively studying and have achieved more new results.

Fig. 1
photovoltaic grid connected analog device.The voltage U s generated by the solar panel, through the DC-AC

Figure 3
Figure 3 shows the PV inverter simulation device and main filter circuit, inverter main circuit adopts the single-phase voltage type inverter circuit, two bridge arm consisting of four IGBT IGBT, Infineon produced by IKA15N60T, the IGBT with anti parallel diode, the main circuit connection more simple.In addition, the maximum emitter voltage of the IGBT is 600V, the maximum collector current is 15A, and the maximum junction temperature is 175 DEG C, which meets the application requirements.Through the drive circuit of the 4 wave SPWM wave drive to its grid, to achieve the DC/AC

Fig. 3 3 . 1
Fig. 3 inverter and filter main circuit of photovoltaic grid connected analog device maximum power point tracking, under voltage protection, overcurrent protection and phase sampling .The frequency tracking detecting circuit transforms the sine wave into the rectangular wave of the same frequency, input the rectangular wave to the microcontroller, and the time difference is obtained by the zero crossing of the detecting waveform, Thus, the frequency of the voltage signal is obtained, and frequency tracking is realized.The sampling circuit of maximum power point tracking is based on the features of maximum power output, When the load equivalent resistance is equal to the load internal resistance, the power output power is the maximum.use the matching method of the sampling voltage values, If the supply voltage 60V is constant (output voltage of solar panels),after the load is connected, the full bridge type inverter voltage regulation is used, in order to make the load equivalent resistance equal to the internal resistance of the power supply, then the Load obtain the output voltage U d , should be equal to half of the power supply voltage, to realize the function of maximum power point tracking (MPPT).MATEC Web of Conferences 139, 00138 (2017)