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News; Photovoltaic power generation system

Published: Thursday 19 May, 2016

With the promulgation of distributed photovoltaic new deal, distributed photovoltaic power generation system has attracted the attention of the industry. In practical applications, distributed photovoltaic power generation system can be divided into grid and off grid type. Grid type is the system of direct current into alternating current, incorporated into the power grid, off grid PV system is self occupied, local consumptive and flexible application. In actual use, off grid PV system everywhere, such as solar street lamp, traffic signal lamp, agricultural deinsectization lamp, especially in some network coverage in remote mountainous areas, from the important role of grid photovoltaic power generation system is highlighted. Here, it mainly introduces the composition of the off grid photovoltaic power generation system. So what are the parts of the photovoltaic power generation system? What are the functions of the photovoltaic power generation system?
Light is irradiated to the PV array, light energy is changed into electric energy, PV array output current due to the environmental impact. Therefore, it is unstable, after DC-DC converter transforms it into a stable current, to load to the battery, charging the battery, battery to supply power to load. If it is connected to the sale of electricity, do not need batteries, but through the grid connected inverter, the DC current into AC current, and to the power grid for sale. In other words, off grid photovoltaic power generation system must be used to storage battery, and the grid is not necessarily required.
Control system of PV array output voltage and current of real-time sampling and judge of photovoltaic power generation system is working in the maximum power point, then according to the tracking algorithm, changing the PWM signal duty ratio, and control the output voltage of the photovoltaic array make the working point to the maximum power point approximation. In the battery charging and discharging control module, when the battery voltage is charged or discharged to a certain set value, it will automatically shut down or open.
PV array module
Photovoltaic power generation system based on the principle of photoelectric effect of the photovoltaic array components will be directly converted into electricity. Photovoltaic battery monomer is used for the smallest unit of photoelectric conversion, a monomer to produce a voltage is about 0.45V, current work about 20~25mA/cm2, photovoltaic battery monomer for series and parallel package, it becomes a photovoltaic cell array assembly.
When when subjected to light irradiation of the solar battery is connected with a load, the current flowing through the load, and on both ends of load terminal voltage is established, then solar cell can use below the solar battery load characteristic curve to represent. It shows that the relationship between the output voltage and the output current and the output power of the photovoltaic cell is the relationship between the output voltage and the output power of the photovoltaic cell, and the I-V characteristics and P-V characteristics. It can be seen from the figure, the characteristic curve of photovoltaic power generation system with strong nonlinear, constant current source is neither a constant voltage source. From its P-V characteristic curve, it can be seen that the output power of the solar power is similar to that of an opening downward. The power of the parabola vertex corresponds to the maximum power point of the P-V curve of the sunshine intensity, the corresponding voltage is called the maximum power point voltage. In order to improve the conversion efficiency of photovoltaic power generation system, it is necessary to keep the system running in the vicinity of the maximum power point of the P-V curve.
Several important technical parameters of photovoltaic cell array:
(1) short circuit current (Isc): the maximum output current at a given intensity and temperature.
(2) open circuit voltage (Voc): maximum output voltage at a given intensity and temperature.
(3) maximum power point current (Im): the current at a given intensity and temperature at the maximum power point.
(4) maximum power point voltage (Um): the voltage at the maximum power point in a given day and temperature.
5) maximum power point (Pm): the maximum power output of the solar cell array in a given day and temperature.
DC-DC converter
The electric energy which is issued by the photovoltaic cell board is the direct current electric energy which changes with the weather, temperature, load and so on, and the power quality and the performance of the electric energy is very poor, so it is difficult to directly supply the load to use. A converter, which is composed of a power electronic device, which is a DC-DC converter, which is suitable for the control and transformation of the power supply, becomes the power supply load or the power grid which is suitable for the load. The basic function of power electronic converter is to convert a fixed power into another form of power output, so as to meet the requirements of different load. It is the key component of photovoltaic power generation system, and generally have several functions: maximum power point tracking, battery charging, PID automatic control, DC power boost or buck and inverter.
The relationship between the output voltage and the input voltage of the DC-DC converter is realized by controlling the on-off time of the switch, and the control signal can be completed by the PWM signal. The main working principle is to keep the on-off period (T) constant, regulating the duration of the switch to control the voltage. Duty cycle of D for PWM signals. According to the different forms of input and output, the power electronic converter can be divided into four categories, namely, AC-DC converter, DC-AC converter, DC-DC converter and AC-AC converter. The DC-DC converter is used in the off grid photovoltaic power generation system.
DC-DC converter and its working principle is controlled by adjusting the switch, a constant DC voltage into another a (fixed or adjustable) DC voltage, wherein the diode freewheeling, LC circuit is used to filter. The relationship between the output voltage and the input voltage of the DC-DC converter is realized by controlling the on-off time of the switch, and the control signal can be completed by the PWM signal. The main working principle is to keep the on-off period (T) constant, regulating the duration of the switch to control the voltage. Duty cycle of D for PWM signals. According to the different forms of input and output, the power electronic converter can be divided into four categories, namely, AC-DC converter, DC-AC converter, DC-DC converter and AC-AC converter. The DC-DC converter is used in the off grid photovoltaic power generation system.
DC-DC converter and its working principle is controlled by adjusting the switch, a constant DC voltage into another a (fixed or adjustable) DC voltage, wherein the diode freewheeling, LC circuit is used to filter. DC-DC conversion circuit can be divided into many types, from the point of view can be divided into: step-up, step-down, lifting pressure and cook.
Buck converter (BuckConverter) a kind of output voltage is equal to or less than input voltage of the single non isolated DC-DC converter; main structure of the conversion circuit of buck boost converter (Buck-BoostConverter) by the PWM controller and a transformer or two independent inductance combination, can produce a stable output voltage. When the input voltage is higher than the target voltage, the switching circuit is reduced; when the input voltage falls below the target voltage, the system can adjust the working cycle, makes the conversion circuit and boosting operation is carried out; and rise pressure converter BoostConverter output voltage higher than the input voltage of the single non isolated DC DC converter, the power electronic devices and components and buck converter is the same, the difference between the two is only different circuit topologies.

Battery
Energy storage device is essential in the stand-alone photovoltaic power generation system. Now optional storage can there are many methods, such as capacitor energy storage, flywheel energy storage, superconducting magnetic energy storage, but from a convenient, reliable, price and other factors to consider, most large and medium-sized PV system use maintenance free lead-acid battery as system of the storage device.
But used lead-acid batteries, also have shortcomings, it is relatively expensive, initial investment to be able to occupy the entire power system of 1 / 4 to 1 / 2, and the battery is the relatively weak link, so if not properly managed, the battery failure in advance, increase the operating cost of the whole system.
Photovoltaic control module
The PV module is controlled by a single chip microcomputer as the control center, which provides the best charging current and voltage, and is fast, stable and efficient. And in the process of charging the battery to reduce the loss, as far as possible to extend the battery life, while protecting the battery from over charging and over discharge hazards. If the user is a DC load. Through the solar controller can provide stable DC (due to the impact of external factors such as weather, solar array from the DC voltage and current is not very stable) for the load, but also through the sensor control circuit (photo, voice, etc.) to achieve full automatic light switch function.
SCM's primary work is current acquisition circuit and a voltage acquisition circuit to the acquisition of current and voltage comparison operation, the duty and the MPPT algorithm to adjust the PWM empty than D, the photovoltaic array assembly work at the maximum power point.
Off grid inverter
Residential use of off grid photovoltaic power generation system as part of the load is AC load, it is also required from the grid type inverter, the PV module to the DC power into AC load to use. There is no big difference between the PV grid connected inverter and the grid connected inverter in the main circuit structure, the main difference is that the grid connected inverter needs to be considered. Is the same frequency; in phase; anti island and other special circumstances of the ability to control. PV off grid inverter does not need to consider these factors.
In order to improve the overall performance of the off grid photovoltaic power generation system and ensure the long-term stable operation of the power station, the performance index of the inverter is very important.
Application of off grid photovoltaic power generation system:
Off grid photovoltaic power generation system is widely used in remote mountainous areas, non power areas, islands, communication base stations and street lights and other applications.
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