According to related investigations, as of December 2013, about 2% of the world's power plants have adopted string-type solutions to access solar power plants with a capacity of more than 5MW. This proportion is only 12% in Germany, but less than 1% in China where solar energy is developing rapidly. In the United States, ground power plants with a capacity of more than 5 MW reach 2.3 GW, and the proportion of accesses using a string-type scheme is also less than 1%. In India, this proportion is even lower. India, as an emerging solar market, has more than 5 MW of photovoltaic ground power stations. 580MW, almost all of the 5MW or more solar ground power stations use centralized grid-connected inverters. In summary, the acceptance of centralized inverters in large-scale ground power stations is extremely high.
IHS statistics: Application of solar power station inverters over 5MW
In recent years, string inverters have also begun to be used in large-scale ground power plants above 5MW. This has caused the solar industry to argue about these two application solutions. So, which of the string solutions and centralized solutions is better? What are their respective advantages?
I. About income:
Analysis of measured data of a power station
Note: Taking into account the unusual power generation due to shutdown or other reasons on the June 8th string array, the day's data was excluded.
Through the consideration of the actual data of a power station inverter, considering the three factors of the output power of the string, the degree of component attenuation, and the AC/DC line loss, the string inverters are lower than those of the A and B manufacturers. Above 1%, compared with C manufacturers, it is only 1% higher. After the average, the string inverter is 0.864% lower than that of the centralized inverter.
Second, on the requirements of large-scale ground power station equipment functions:
(1) Zero voltage ride through protection issues
According to GB/T19964-2012 requirements for low voltage ride through faults, the inverter must have zero voltage ride through capability, requiring the inverter to maintain 0.15s on-grid operation when the grid voltage drops to 0, when the voltage drops to the curve Below 1 allows the inverter to be cut out of the grid.
Zero voltage ride-through capability requirements for photovoltaic power stations:
Disadvantages of string inverters: Limitation of networking methods - There is no high-frequency carrier synchronization between inverters, which does not solve the problem of parallel circulation of the inverters; the impedance of the inverter line at the far end of the box is larger Multi-machine parallel mode - multiple inverters will not be able to unify the phases of output voltage and current when the grid electricity industry falls.
Centralized grid-connected inverters: Both can pass low-voltage traverse tests in the laboratory and in the field.
(2) Anti-island protection
Islanding effect: When some power lines are out of power due to faults or maintenance, the grid-connected power generation devices connected to the power-off lines continue to supply power, and together with the surrounding loads constitute a phenomenon of self-sufficient islanding. The GB/T19964-2012 standard requires the power station to have anti-islanding protection equipment. Normally, the inverter adopts active and passive double anti-island protection to ensure that the inverter can reliably disconnect the power grid in any case. Active protection is usually used to inject small interference signals into the power grid, and determine whether power is lost by detecting feedback signals. Passive protection usually uses the method of detecting output voltage, frequency, and phase to determine the state of islanding.
String inverters: The AC side is directly connected in parallel, and the method of injecting distorted signals due to active protection cannot be used in a multi-machine parallel system, and active protection in island protection cannot be implemented.
——Application risk: The formation of a resonating island will pose a safety threat to the line maintenance personnel, cause damage to electrical equipment, and seriously affect the operational safety of the power station.
Centralized Inverter: The AC output does not need to be connected directly to the double-split winding transformer, and it is active and actively isolated.
(3) Support grid dispatch
The two points in common: All adopt RS485 as the communication interface, and the response speed is correspondingly slow.
String inverters: The scheduling of 40 inverters per megawatt is not conducive to remote scheduling and management of the power plant;
Centralized inverter: It is more convenient to schedule only 2 inverters per megawatt.
(4) PID effect suppression strategy
Currently recognized as the most reliable solution to the PID effect: the negative ground of the inverter
String inverters: A virtual negative ground circuit is used to suppress PID effects. For example, a virtual circuit failure group string inverter cannot guarantee the suppression of PID effects, which is far worse than the solid negative grounding reliability.
Centralized Inverter: Insulation resistance monitoring + GFDI (PV Ground-Fault Detector Interrupter, consisting of breaking device and sensor) scheme, ie the inverter instantly monitors the impedance of PV+ to ground. When the impedance of PV+ to ground is lower than the threshold, the inverter will immediately stop the alarm.
(5) Breaking Device Protection
String inverters: DC switches are used instead of circuit breakers on the DC side. DC switches do not have a breaking capability when a ground fault occurs on the DC side. Therefore, fault points cannot be cut and hardware protection functions are lost.
Third, on maintenance efficiency:
(1) Backup inverter
String inverters: If 100 MW power plants are used, the string inverters will require 4,000 units, while the manufacturers provide only 10 units, and the ratio is only 0.25%. However, the number of its components and components, inverter topology complex and other reasons, the failure rate is much higher than 0.25%. Most of the large-scale ground power stations are located in the desert and the Gobi, and their logistics and transportation have become more realistic issues.
(2) On-site replacement
String inverters: Large ground power stations usually occupy a large area. Some power stations have poor road conditions and special topography, causing inconvenience in on-site maintenance. Especially in mountainous and hilly power stations, the on-site road conditions are poor, and the operation and maintenance personnel cannot transport the string inverter directly to the fault point for replacement.
Centralized grid-connected inverters: Most components are designed in components, and the main devices can be quickly replaced by plugging and unplugging. After fault location, the maintenance time does not exceed 20 minutes. In addition, centralized inverter manufacturers have established after-sales service centers and spare parts management centers in the cities near the power station.
(3) Maintenance costs
String inverters: The maintenance cost of the whole machine replacement is higher. After the warranty period expires, the maintenance cost will be more than the operating cost of the power station.
Centralized inverter: The maintenance cost of replacing faulty components is low.
By comparing the above aspects, we may see that the advantages of a centralized inverter are far greater than that of a string inverter. But in the future, the development of string inverters will not be better. Whether these disadvantages will become advantages, and successful counterattacks on centralized inverters are all possible.
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