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23-26 Solar PV Power Generator System Configuration and Components

Author: Source: Datetime: 2016-12-26 09:47:13
Photovoltaic system components

For stand-alone PV system, the composition of Figure 2-3. The grid-connected photovoltaic system is shown in Figure 2-4.
grid-on photovoltaic system component diagram
① solar cell phalanx square phased array is the main components of photovoltaic systems, which will receive the solar energy can be directly converted into direct current. In order to meet the requirements of high voltage and high power generation, the square array is connected in series and parallel by several solar cell modules (PV modules), anti-reflection and bypass diodes, lightning protection DC combiner boxes and cables. Through a certain mechanical way Fixed combination. The square brackets must have sufficient rigidity, strength to wind, snow, etc., and can be firmly installed on the appropriate basis.

Installed on the ground more high-power photovoltaic power plants, often need a lot of square, according to the appropriate spacing further string, parallel connection to the inverter input converters. The arrangement of PV arrays is called photovoltaic arrays.

In the building grid-connected photovoltaic system, if the residential or building design to consider the square of the installation orientation and the angle of the angle, etc., and bored buried anchor bolt and other fixed components, then the solar array Is much more convenient, and can reduce the power loss.

Building grid-connected PV system and the outstanding features of the advantages of combining with the building, there are two main forms.

The combination of building and photovoltaic system is a preliminary combination of the two, ready-made flat-panel solar modules installed in the roof of the building, etc., leading through the inverter and control devices connected to the grid, photovoltaic systems and power grid in parallel to the residential Users) power supply, redundant power to the grid feedback, less than access to electricity from the grid.

The combination of building and PV modules is a further development of the combination of photovoltaic devices and building materials. The external walls of buildings are generally used paint, mosaic and other materials, in order to beautiful, and some even use expensive glass curtain wall, and its function is to protect the internal and decorative role. If the roof, sun walls, sun visors, and even windows and other materials used to replace photovoltaic devices, you can do both, both as a building materials and decorative materials, but also power.

PV building integrated system, to design a good cooling ventilation, because the solar module power generation efficiency with its surface temperature rise of the decline. Good ventilation cooling, the component can reduce the surface temperature of about 15 °, the power output increased by 8% or more.

② inverter Inverter as a key component of photovoltaic systems is to load or power grid to provide high-quality power of the fundamental protection. In order to achieve reliable operation of photovoltaic systems, high-efficiency output, the inverter must meet the following requirements.

A. Can output voltage stability of the AC. Whether it is input voltage fluctuation, or load changes, it must reach a certain voltage stability accuracy, the static state is generally 2% of the soil.
B. Can output AC power with stable frequency. Requirements of the exchange of electrical energy to achieve a certain frequency stability accuracy, static state is generally 0.5% soil.
C. The output voltage and its frequency can be adjusted within a certain range. The adjustable output voltage range is ± 5% and the output frequency range is ± 2Hz.
D. Has a certain overload capacity, - can generally overload 125% ~ 150%. When the overload 150%, should be able to last 3s; overload 125%, should be able to last 1min and above.
E, the output voltage waveform with harmonic components should be as small as possible. General output waveform distortion rate should be controlled to within 7%, in order to help reduce the size of the filter.
F. With short circuit, overload, overheating, over voltage, under voltage protection and alarm functions.
G. Start smooth, start-up current is small, stable and reliable operation.
H. Commutation loss is small, high inverter efficiency, generally 85% to 95%.
I. Has a fast dynamic response.

Inverter according to the mode of operation, can be divided into independent operation inverter and grid-connected inverter. Stand-alone Inverters are used in stand-alone solar photovoltaic systems to power independent loads. Grid-connected inverters are used for grid-connected solar photovoltaic power generation systems, which feed the power into the grid. Inverter according to the output waveform can be divided into square wave inverter and sine wave inverter. Square wave inverter, the circuit is simple, low cost, but the harmonic components, generally used for several hundred watts or less and the harmonic requirements of the system is not high. Sine wave inverter, the cost is higher, but can be applied to a variety of loads. In fact, sine wave inverter has long been the mainstream.

Grid-on inverter is mainly composed of inverter and grid-on  protector, as shown in Figure 2-5.
grid-on inverter configuration ( insulation transformer mode )
Inverter part, its function is to use high-power transistors to DC high-speed cutting, and converted to AC; b. Control part of the electronic circuit, its function is to control the inverter part; protection Part, but also by the electronic circuit, and its function is in the inverter internal fault protection from the role.

Grid protector is a safety device, mainly used for frequency fluctuation, over voltage and power outages, etc. monitoring. If problems are detected, the inverter should be stopped in time to disconnect the PV system from the grid to ensure safety. It is usually installed in the inverter, but there are separate settings.

In China, grid-connected PV inverters often include MPPT (Maximum Power Point Tracking). The output of the solar cell matrix changes with the solar irradiance and the surface temperature of the solar cell matrix. Therefore, it is necessary to track the operating point of the solar cell matrix and control it so that the square matrix is always at the maximum output to obtain the maximum power output. This will use technology. The DC voltage of the grid-connected inverter is changed every time, and the output power of the solar cell is measured and compared with the last time. The DC voltage of the grid-connected inverter is always changed in the direction of increasing the power.

On the grid inverter circuit has entered the application of the main grid frequency transformer insulation, high-frequency transformer insulation and transformer-free way 3 kinds.

A.Grid frequency transformer insulation, the use of pulse width modulation (PWM) inverter to generate the exchange frequency of the grid, and the use of power frequency transformer for insulation and transformer. It has a good anti-lightning and eliminate spikes of performance. But because of the use of grid frequency transformer, which is more cumbersome.
B. High-frequency transformer insulation, small size, light weight, but the circuit is more complex.
C. No transformer mode, small size, light weight, low cost, high reliability, but there is no insulation between the grid.

In addition to the first method, the latter two methods have the function of detecting the DC current output, further improving the safety. The transformerless approach, due to its advantages in cost, size, weight and efficiency, is widely used today. The booster boosts the DC voltage of the solar cell matrix to the voltage required by the transformerless inverter. The inverter changes DC to AC. The controller has the function of a network protection relay and has a manual switch required for networking to isolate the inverter from the grid in the event of an abnormality (see Figure 2-6).
non-transformer mode grid-on inverter circuit
③ other system balance related components, including energy storage devices, controllers, AC power distribution cabinets, monitoring equipment, transformers, the relevant chapters of this book are described in detail, and therefore not repeat them.

(2) the configuration of the photovoltaic system

Configuration refers to the PV system of various components of the capacity design and performance parameters of the consideration, in order to achieve the best match. This is the photovoltaic power generation system project in the implementation of an important issue can not be ignored. System components of the considerations, not only for the pursuit of the system the most efficient, most generating capacity, but also consider the best cost-effective, and photovoltaic systems from up to 20 to 25 years of life cycle to consider the technical economics of systems engineering.

For example, in a grid-connected PV system, the power capacity of the inverter is selected. In addition to considering many performance parameter adaptations, it is also important to note that the efficiency of the inverter is reduced at low power. The choice of the ideal inverter capacity should be based on the solar panel output power to consider, which in turn depends on the installation location of the latitude. It is known that the ratioP1/P0 of the inverter rated power (P1) to the nominal power of the solar panel (P0) is usually less than one. In Europe, the ratio recommended by professionals is 0.65 to 0.8 for Nordic countries and 0.75 to 0.9 for Central European countries, and 0.85 to 1.0 for countries in Southern Europe. In China, the ratio of 0.9 to 1.0, the specific ratio between the installation location and other appropriate to determine.

However, in the independent photovoltaic power generation system, the main consideration should be to meet the load power production needs, while power supply reliability and economy is also important. Therefore, the inverter capacity considerations, mainly from the inverter output to meet the load requirements. As a typical stand-alone photovoltaic power plant demonstration project, now the normal operation of power supply for nearly 20 years of China's Tibet Lake 25kW sector photovoltaic power plant configuration is described below.

The geographical coordinates of Shuanghu PV power station are 89 ° E, 33.5 ° N and 5100 m above sea level. The local climate has obvious plateau characteristics, annual sunshine hours up to 3000h, solar annual total radiation in 7000MJ / m2 above, and the annual distribution of more balanced. Construction of photovoltaic power station is mainly used to solve the lighting, watching TV and other residents living electricity, taking into account public utilities (schools, hospitals, railway stations, banks, government offices, etc.) electricity load total power 29.2kW,

According to the special circumstances of Shuanghu and the forecast of local electricity load, Shuanghu photovoltaic power station should be built into independent operation of photovoltaic power generation system, with the appropriate capacity of diesel generator as a backup power to enable in emergency situations. The power station is composed of a solar battery array, a storage battery group, a DC control system, an inverter, a rectifier charging system, a diesel generator set, power supply electricity lines and related housing civil facilities. In accordance with the requirements and conditions given, according to the basic principles and guiding principles of power plant design, the optimal design and calculation, the power plant parts of the main performance parameters are as follows.

The nominal power of the solar cell                                       25kWp
Storage battery group                                                          300V / 1600Ah
Inverter                                                                              30kVA, 380V, 50Hz three-phase positive mysterious wave output
DC control system AC distribution system capacity                 30kW, 300V shunt input control
AC power distribution system                                                180kVA, 220V / 380V three-phase four-wire two-way output
Rectifier charging system                                                       75kW, DC 300 ~ 500V adjustable
Diesel Generating Set                                                             50kW (or 120kW)

The overall configuration of the PV system is shown in Figure 2-7.
The overall configuration of the solar power system
Photovoltaic power plants in sunny weather conditions, the sun light to the solar array, the solar cells to the sun's energy into electrical energy, through the DC control system to charge the battery pack. Need to use electricity, the battery through the DC control system to the inverter power. Inverter will be converted to the usual DC frequency and voltage of AC, AC and then by the distribution system and transmission lines, the AC will be sent to the user home to the load power supply. When the battery discharge excessive or due to other causes of voltage is too low, you can start a backup diesel generator, the rectifier charging device to charge the battery pack to ensure that the system through the inverter normal power supply. In the case that the system can not be powered by the inverter, such as inverter damage, line and equipment failure and maintenance, etc., diesel generator as an emergency power supply through the AC power distribution system and transmission lines directly to the user power supply.

In the overall technical design, fully consider the future expansion of the needs and power supply reliability requirements, the performance parameters of all parts are left margin. DC control system, AC power distribution system and distribution lines are two working design, leaving the output / input interface, in order to access the first two inverters. Obviously, here the system configuration, P1/P0 value is selected as 1.2, and grid-connected PV system considerations are completely different.

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