This chapter introduces the characteristics of grid-connected inverter
The grid-connected inverter is a key component of the solar photovoltaic grid-connected power generation system. It inverts DC power into AC power, which is a current source that follows changes in grid frequency and voltage. The current grid-connected inverter products on the market are mainly DC-DC and DC-AC two-stage energy conversion structures: the DC-DC conversion link adjusts the working point of the photovoltaic array to track the maximum working point; DC-AC inverse The variable link mainly makes the output current and the grid voltage have the same phase and obtains the power factor at the same time.
For large and super large photovoltaic power plants, centralized photovoltaic grid-connected inverters are generally used. The configuration and selection of the inverter should be determined according to the various technical indicators of the entire photovoltaic power station and refer to the product manual provided by the manufacturer, but also focus on the following technical indicators.
(1) Rated output power
The rated output power indicates the ability of the inverter to supply power to the load or the grid. When selecting an inverter, the power of the photovoltaic array should be considered first to meet the electrical power requirements of the equipment under the maximum load. When the electrical equipment is mainly pure resistive load or the power factor is greater than 0.9, the rated output power of the inverter is generally selected to be 10% to 15% larger than the total power of the electrical equipment. The ratio of the rated output power of the grid-connected inverter to the power of the solar battery is generally 90%.
(2) Adjustability of output voltage
The regulation performance of the output voltage characterizes the regulation of the inverter output voltage. Generally, the inverter gives the percentage of deviation of the output AC voltage fluctuation of the inverter when the DC input voltage changes within the allowable fluctuation range, that is, the voltage adjustment rate. The voltage regulation rate of the inverter with good performance should be ≤3%.
(3) Overall machine efficiency
The efficiency of the whole machine represents the size of the power loss of the inverter itself. Inverter efficiency is also divided into maximum efficiency, European efficiency (weighted efficiency), California efficiency, and MPPT efficiency. They are defined as follows.
(4) Protection function
When selecting the grid-connected inverter, in addition to the protection functions of over/under voltage, over/under frequency, anti-islanding effect, low voltage ride through, short circuit protection, reverse power protection, etc., its voltage (current) should be considered at the same time The total harmonic distortion rate is small to minimize the interference to the power grid.
(5) Start-up performance
The inverter selected should be able to ensure reliable startup under rated load. The high-performance inverter can achieve continuous full-load starting multiple times without damaging the power switching devices and other circuits.
For large-scale photovoltaic power plants, 250kW, 500kW centralized grid-connected inverters, 10MW or even larger capacity photovoltaic power plants are usually selected. If possible, larger power inverters should be selected, such as a single unit with a power of 1MW and above. Type grid-connected inverter. This is more cost-effective.
Currently, centralized inverters on the market in China generally have the following characteristics:
·Using new high-efficiency IGBTs and power modules to reduce system losses and improve system efficiency.
·Using all-optical fiber drive, reliably avoid false triggering and greatly reduce the influence of electromagnetic interference on the system. Thereby enhancing the stability and reliability of the whole machine.
·Re-optimized structure and circuit design. Reduce system components, reduce system costs, and improve system heat dissipation efficiency. Enhance system stability.
·Using new intelligent vector control technology. It can suppress the impact of the three-phase unbalance on the system, and at the same time improve the utilization rate of the DC voltage, and expand the DC voltage input range of the system.
·Using internationally popular touch screen technology. Design a new type of intelligent man-machine interface. Significantly increase the monitoring system parameters; the graphical interface is ergonomically designed to facilitate users to grasp the overall information of the system in time, and the data collection and storage functions are enhanced. It can record all historical parameters, faults and events within the last 100 days. It is convenient to export and provides a basis for further data processing.
·Enhanced protection function. Compared with ordinary inverters, DC ground fault protection is added. Emergency stop button and on/off knob provide dual protection. The system has DC overvoltage, DC undervoltage, frequency failure, AC overvoltage, and AC The most comprehensive fault judgment and detection for undervoltage, IMP fault, temperature fault, communication fault, etc.
·With a variety of advanced communication methods, RS485/GPRS/Ethernet and other communication interfaces and accessories, even if the power station is located in a remote location, the system operation status can be obtained in time through various networks.
·The system monitoring software that has been upgraded many times can adapt to the multilingual Windows platform, integrated environment monitoring system, simple interface, rich parameters, and easy to operate.
·The group control function specially designed for photovoltaic power stations can monitor the weather changes in real time, and determine the shutdown or opening of multiple inverters based on real-time information; the test results show that the group control can effectively improve the system efficiency by 1%~2 %, thereby bringing more benefits to users.
·With low voltage ride through, reactive power, active power adjustment and other functions (optional).
·The circuit and control algorithm of the system use international authoritative simulation software (SABER, PSPICE, MATLAB) for rigorous simulation and calculation. All parameters are the results of multiple optimization designs. Humidity, temperature) are strictly tested, and the system is optimized for the second time according to the test results to achieve the best performance.
·Perfect domestic after-sales service system, strong after-sales service capability, quick response, low cost of operation and maintenance in the later period.
·Power frequency isolation transformer to realize mutual isolation between photovoltaic array and power grid.
·With DC input manual disconnect switch, AC grid manual disconnect switch, emergency stop operation switch.
·Humanized LCD liquid crystal interface, through key operation, the liquid crystal display (LCD) can clearly display real-time operating data, real-time fault data, historical fault data (more than 50), total power generation data, historical power generation (press Monthly and yearly query) data; can provide RS485 or Ethernet (Ethernet) remote communication interface, among which RS485 follows Modbus communication protocol, Ethernet interface supports TCP/IP protocol, and supports dynamic (DHCP) or static acquisition of IP address.