Green power conversion of solar photovoltaic power generation system

The application of grid-connected solar photovoltaic power generation system began in the early 1980s. The United States, Japan, Germany, and Italy all made efforts to this end. At that time, large-scale photovoltaic grid-connected power plants were built, ranging in size from 100 kW to 1 MW. And they are all experimental power plants invested by the government. However, the test results were not very satisfactory. Because the solar cells were expensive at the time, it was difficult for the power company to accept. 

The market that the developed countries have mainly developed in recent years is the rooftop grid-connected power generation system. The reason is that the power grid distribution in developed countries is very dense, grid-connected power generation does not use batteries, and the electricity cost of peak power consumption in the grid is high. The electricity price of photovoltaic power generation in areas with good sunlight is close to the commodity price (estimated to enter the cost from 2000 to 2005). The calculation period), it is predicted that the roof grid-connected power generation system will be widely applied after 10 years. 

The grid-connected solar photovoltaic system consists of a photovoltaic cell array, a controller, and a grid-connected inverter. The grid-connected inverter directly feeds the electrical energy into the public grid without passing through the storage of the battery. Since the electric energy is directly input into the power grid, the battery is dispensed with, the process of storing and releasing the battery is saved, the energy loss is reduced, the space occupied by the system and the system investment and maintenance are saved, and the cost is reduced; on the other hand, the power generation capacity can be made very well. Large and can guarantee the reliability of the power supply of the electrical equipment. However, since the inverter output is connected in parallel with the power grid, it is necessary to maintain the consistency of the electrical characteristics of the two sets of power supply voltage, phase, frequency, etc., otherwise the two sets of power supplies may be charged and discharged with each other, causing internal consumption and instability of the entire power supply system. 

The main component of the solar grid-connected photovoltaic power generation system is the inverter or power conditioner (PCU). The PCU converts the direct current generated by the solar photovoltaic power generation system into a standard alternating current that meets the requirements of the power sector. When the power department stops supplying power or public When the grid is faulty, the PCU will automatically cut off the power. The grid connection between the AC output of the grid-connected photovoltaic power generation system and the public grid is set. When the grid-connected photovoltaic power generation system exceeds the actual amount of power required by the system load, the excess power is transmitted to the public grid. When the energy output by the solar photovoltaic system is less than the actual amount of power required by the system load, the power required by the system load can be supplemented by the public grid. At the same time, it is also necessary to ensure that the solar photovoltaic system will not feed the electrical energy to the public power grid during the failure or maintenance of the public power grid, so that the system can operate stably and reliably. Solar grid-connected power generation is the development direction of solar photovoltaic power generation, representing the potential energy utilization technology of the 21st century. 

In the late 1980s, Japanese scholar S.Nonaka and others pioneered the development of a current source solar array grid-connected inverter. This grid-connected inverter is well adapted to the characteristics of solar modules. Better performance. However, due to the use of the current source inverter main circuit, the main circuit and control are complicated, so it has not been well developed. Since the 1990s, with the development of power electronics and control technology, voltage-type PWM inverter flow technology has become more mature. Due to its superior bidirectional power converter and current control performance, this technology is directly applied to the grid-connected power generation of solar arrays, and the sinusoidal current characteristics of the grid side are obtained, realizing the 'green' power conversion. The single-phase voltage type solar cell array grid-connected control system is shown in FIG. 

When the grid is running, the control system controls the DC side voltage Vd of the solar array, and the control system feeds the grid under the excitation of the solar array. As seen from Figure 1, the grid-connected inverter system consists of a grid-connected transformer T, an AC inductor L, a power tube (T1 to T4), a DC storage capacitor C, a microprocessor control system, and a solar array PV. The sinusoidal control process of the grid side during grid-connected operation is as follows: 


First, the DC reference voltage Vd* is compared with the feedback voltage Vd. The error voltage signal ΔVd, ΔVd is regulated by the voltage and the current regulation signal is Im*. The phase is obtained by the unit sine wave signal sinωt synchronized with the grid voltage, and the two are multiplied by the sinusoidal current signal iN*. After being controlled by the current regulator, the PWM mode generator outputs a control signal to force the output current to track the input current. When iN is inverted from VN, electrical energy will be fed from the solar array to the grid.