Structural composition of solar photovoltaic power generation system

Structural composition of solar photovoltaic power generation system

The solar photovoltaic power generation system is mainly composed of solar cell arrays, controllers, inverters, energy storage equipment and auxiliary equipment. The main equipment is as follows.

The previous article introduced the solar cell array, and this article will introduce the other components of the solar photovoltaic power generation system.

(1) Controller. The controller is mainly used to control the charge and discharge of the battery. According to the different circuit modes, the controller can be divided into parallel type, series type, pulse width modulation type, multi-channel control type and intelligent control type. Different controllers have different structures and functions, but their basic principles are the same. Figure 1 shows the principle block diagram of the most basic photovoltaic control circuit. In the figure, the controller mainly controls switch 1 and switch 2, which are respectively the charge control switch and the discharge control switch. When the switch 1 is closed, the solar battery charges the battery. When the battery is overcharged, the switch 1 cuts off the charging circuit in time and stops supplying power to the battery. When the battery voltage is lower than the set protection mode, switch 1 automatically resumes charging the battery. When switch 2 is closed, the battery supplies power to the load. When the battery is over-discharged, the switch 2 disconnects the discharge circuit in time, the battery stops supplying power to the load, and so on. Switches 1 and 2 can be electronic switches such as power switching devices, or mechanical switches such as ordinary relays. Usually the controller is also equipped with anti-reverse charging diodes and battery protection diodes and other protective measures. Some intelligent controllers also have information collection, storage and communication functions. Generally, the rated working voltage of the controller is 12V or 24V, and the medium and high-power controllers also have 48, 110, 220, 500V, etc.

Structural composition of solar photovoltaic power generation system
Figure 1 – Block diagram of photovoltaic control circuit

17Figure 1 – Block diagram of photovoltaic control circuit

(2) Inverter. In addition to the conversion function of inverting direct current into alternating current, the inverter should also have the function of maximizing the performance of photovoltaic cells and protecting the system from failure. According to the operation mode, inverters can be divided into off-grid operation inverters and grid-connected inverters. Off-grid inverters are used in independent solar cell power generation systems to supply power to independent loads. Its function is relatively simple, mainly including automatic operation control and various protection functions, such as overcurrent, overheating, overload, short circuit, lightning strike, abnormal output, internal fault, grounding, reverse connection, etc. In a large-scale photovoltaic power generation system, the output power is usually merged into the grid for use by other residential users, and grid-connected inverters are used in this process. In addition to the general functions of off-grid inverters, grid-connected inverters should also have: maximum power tracking control MPPT, islanding detection, automatic voltage adjustment, DC component detection, etc. There are many performance indicators, such as rated output voltage, current, frequency, capacity and overload capacity, voltage stability, waveform distortion, power factor, output efficiency, DC component, harmonics and waveform distortion, voltage unbalance, noise, etc. For specific standards, please refer to Q/GDW617-2011 “Technical Regulations for Connecting Photovoltaic Power Stations to the Grid”.

At present, the design of grid-connected inverter structure mainly focuses on two-stage inverters that adopt DC/DC and DC/AC two-stage energy conversion and single-stage inverters that adopt one-stage energy conversion.

(3) Energy storage equipment. So far, there are many ways of energy storage, and the main energy storage method used for photovoltaic power generation systems is battery storage. Considering electrical performance, size, quality, life, maintainability, safety, reusability, and economy, lead-acid batteries are the most widely used in photovoltaic systems. Lithium iron phosphate batteries have also been used in some microgrid systems due to their long cycle life, good safety performance, and temperature characteristics, and are expected to become a strong competitor of lead-acid batteries within a few years. Sodium-sulfur batteries and flow batteries are regarded as emerging, high-efficiency and large-capacity electric energy storage batteries with broad development prospects.

(4) Auxiliary equipment. In addition to the above-mentioned main equipment, solar photovoltaic power generation system should also include some auxiliary equipment, such as combiner box, AC power distribution cabinet, etc.

The combiner box can be divided into two types: ordinary type and intelligent type. The ordinary type mainly has basic functions such as lightning protection and convergence, and the main equipment configured includes DC fuses, DC circuit breakers, lightning protection devices and other components. On the basis of the ordinary type, the intelligent combiner box adds intelligent monitoring modules and other components, which can monitor the operation of the photovoltaic array in real time, determine the faulty photovoltaic tandem, locate and give an alarm, and send an alarm message to the monitoring system through the communication interface of the device.

The AC power distribution cabinet provides the output interface for the inverter, configures the output circuit breaker to connect to the grid, and realizes the monitoring and display of the inverter’s output power and equipment protection. Its internal equipment mainly includes AC circuit breakers, AC lightning protectors, metering energy meters (with communication interfaces), voltage and current meters, power quality analyzers, etc.