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68-71 Selection of grid-connected inverter

Author: Source: Datetime: 2016-12-22 17:05:55
4.1.4 Selection of grid-on solar inverters

Grid-on inverter is a key component of PV grid-on power generation system, which converts DC power into AC power, which is the current source following the grid frequency and voltage changes. DC-DC converter link adjusts the working point of PV array to track the maximum operating point; DC-AC inverter (DC-DC converter) is used to control the output voltage of DC-DC converter. The variable link mainly causes the output current to be in phase with the grid voltage, and obtains the power factor.

For large, large-scale photovoltaic power plants generally use centralized photovoltaic grid-connected inverter. Inverter configuration selection, in addition to the entire photovoltaic power plant according to the technical indicators and refer to the manufacturer to provide the product manual to determine, but also focus on the following technical indicators,

(1) Rated output power
The rated output power represents the ability of the inverter to supply power to the load or to the grid. The choice of inverter should first consider the power of the PV array to meet the maximum load of equipment under the power requirements • When the electrical equipment to pure resistive load-based or power factor greater than 0.9 - generally use the inverter rated output power ratio Electrical equipment, the total power of 10% to 15%. Grid-connected inverter rated output power and solar power ratio is generally 90%

(2) The output voltage adjustment performance
Output Voltage Regulation Performance is characterized as the voltage stability of the inverter output voltage. General inverter are given when the DC input voltage within the allowable fluctuations in the range of changes in the inverter output AC voltage fluctuations in the percentage of deviation, that is, voltage regulation. Good voltage regulation of the inverter should be ≤ 3%.

(3) 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, which are defined as follows.
Maximum efficiency θmax: the maximum efficiency achievable by the inverter.
European efficiency θeuro: Calculated according to the weighting formula at different power point efficiencies.
California efficiency θcec: to consider the impact of DC voltage on the efficiency of the average again.
MPPT efficiency θmppt: the maximum power point that the inverter tracking accuracy.
At present, the advanced level: θmax> 96.5%, θmppt> 99%.

(4) Protection
In addition to the consideration of voltage / current (over / under voltage, over / under frequency, anti islanding, low voltage ride-through, short circuit protection, reverse power protection) in grid-connected inverter selection, The total harmonic distortion rate is small, in order to minimize the interference on the grid.

(5) Start performance

The selected inverter shall be capable of reliable start-up at rated load. High-performance inverter can be done several times at full load start without damaging the power switching devices and other circuits.

For large-scale photovoltaic power plants, usually we select 250kW, 500kW centralized grid-connected inverter. 10MW class and even larger capacity photovoltaic power station, if possible, should choose more power inverter, such as stand-alone power up to 1MW and above the centralized grid-connected inverter, so the cost-effective than the higher. At present, domestic market-oriented inverter, generally has the following characteristics:

• High efficiency IGBT and power modules are used to reduce system loss and improve system efficiency.

• Use all-fiber drive, reliable to avoid false triggering and greatly reduce the electromagnetic interference on the system, thereby enhancing the stability and reliability of the machine.

• Re-optimize the structure and circuit design, reduce system components, reduce system cost, improve the system's cooling efficiency, and enhance the stability of the system.

• Using the new intelligent vector control technology, can suppress the impact of three-phase unbalance on the system, and at the same time improve the utilization of DC voltage, expand the system's DC voltage input range.

• Adopting international popular touch screen technology to design new intelligent man-machine interface, which greatly increases the system parameters of monitoring. The graphical interface is designed by ergonomics so that users can grasp the overall information of the system in time, and enhance data collection and storage function. Records all historical parameters, faults and events within the last 100 days and can be easily exported, providing the basis for further data processing.

• Increased protection against direct current, overcurrent, undervoltage, frequency fault, AC overvoltage, AC over-voltage, AC over-voltage, over-voltage, over-voltage, over-voltage protection Undervoltage, IMP fault, temperature fault, communication failure, the most comprehensive fault diagnosis and detection.

With a variety of advanced communication methods, RS485 / GPRS / Ethernet and other communication interfaces and accessories, even if the power plant is located in remote areas, but also through a variety of networks in a timely manner informed of system health.

• After several upgrades of the system monitoring software, can adapt to multi-language Windows platform, integrated environmental monitoring system, simple interface, parameter-rich, easy to operate.

• Group control function designed for PV power plants, which can monitor the weather changes and decide the turn-off or turn-on of multiple inverters according to the real-time information. The test results show that the group control can effectively improve the system efficiency by 1% 2%, thus giving users more revenue.

• With low voltage through, reactive power, active adjustment and other functions (optional).

The system circuit and control algorithm, using the international authority simulation software (SABER, PSPICE, MATLAB) for strict simulation and calculation, all parameters are optimized design results, the whole laboratory and on-site environment Humidity, temperature) of the harsh test, and according to the test results of the second optimization of the system in order to achieve optimal performance.

• Improve the domestic service system, a strong after-sales service capabilities, quick response, post-operation and maintenance costs low.
• Power-frequency isolation transformer to isolate the PV array from the grid.

• With DC input manual cut-off switch, AC power grid manual disconnect switch, emergency stop operation switch.

• User-friendly LCD interface, which can display real-time operating data, real-time fault data, historical fault data (more than 50), total power generation data, historical power generation (by the key operation), LCD (RS485) or Ethernet (Ethernet) remote communication interface, which RS485 Modbus communication protocol, Ethernet interface supports TCP / IP protocol, support dynamic (DHCP) or static IP address.


Now the domestic advanced level of grid-connected inverter is introduced as an example. The photo, performance and characteristics of the new 500KW centralized grid-connected PV inverter with integrated DC cabinet are shown as follows. The circuit block diagram, technical index and efficiency curve are as follows (See Figure 4-19).


Domestic large-scale centralized PV power plant, and more to lMWp power generation unit in parallel combination, the choice of two 500kW inverter grid • Application example shown follow diagram.

Domestic-advanced-level-of-grid-connected-inverter-introduction


500kW inverter performance and characteristics
• Low voltage ride-through (LVRT)
• New IGBT module with high conversion efficiency
• Advanced MPPT tracking algorithm, the maximum power point tracking accuracy greater than 99%
• Maximum conversion efficiency of 98.2%
• Wide DC voltage input range, the output power continuously adjustable
• Adjustable reactive power, power factor range from -0.95 (lead) to +0.95 (hysteresis)
• Perfect protection system to make the inverter more reliable
• Pure sine wave output, small current harmonics, no pollution to the power grid, no impact
• Multi-language LCD display and a variety of communication interfaces
* Accurate output power calculation
• Adapt to high altitude and low temperature cold regions
• Integrated DC cabinet
〇 Received certificate
  • Golden Sun certification
 * Low voltage through LVRT certification
• Germany TiiV certification

Circuit frame

circuit-diagram

Technical indicators

Parameter  GS-CENTRAL500K3TLC   
DC side parameters
Maximum DC voltage 880Vdc
Maximum power voltage tracking range 500〜820Vdc
Maximum DC power 550kWp
Maximum input current 1200A
Maximum number of input channels 8
AC side parameters
Rated output power 500kW
Rated grid voltage 3∅,315V
Allow the grid voltage 280 〜350Vac
Rated grid frequency 50Hz/60Hz
Allowed grid frequency 47 〜51.5Hz/57 〜61.5Hz
Total current waveform distortion rate <3%(rated power)
Power factor -0.95(Ahead)〜+0.95(Hysteresis)
System
Maximum efficiency 98.2%
European efficiency 98.0%
Degree of protection IP20(indoor)
Night since power consumption <150W
Allowable ambient temperature -25 〜+45*0
cooling method wind cooled
Permissible relative humidity 15%〜95%.No condensation
Allow the highest elevation 6000m(More than 3000m to be derated use)
Display and communication
Display Touch screen
Standard communication RS485/RS232
Optional communication Ethernet / GPRS
Mechanical parameter:
Width x height x depth 1600mmx2000mmx800mn
weight 1680kg


Efficiency curve

Figure-4-19-New-500kW-grid-on-inverter-and-performance-characteristics

                                    500kW grid-on inverter and its performance characteristics

Figure-4-20-grid-on-inverter-seletion

                                                            Grid-on inverters selection

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