With the increasing maturity of photovoltaic power generation technology, the reduction of power plant construction costs and the improvement of the subsidy system, the capacity of photovoltaic power plants has grown from the first few kilowatts to the current tens of megawatts or even several hundred megawatts, in order to comply with the development of photovoltaic power plants. The trend, more powerful centralized grid-connected inverters were quickly introduced to the market, and have been very good market applications.
According to statistics from the world's most authoritative photovoltaic inverter industry research institutes as of December 2013, among the photovoltaic power plants with a capacity of more than 5MW, approximately 2% of the world's power plants have adopted string-type solutions. The highest proportion in Germany is only 12%, while in China where PV is developing rapidly, it is less than 1%. In the United States, ground power stations with a capacity of more than 5MW reach 2.3 GW, and the proportion of accesses using string-type schemes is also less than 1%. In India, this proportion is even lower. India, as an emerging photovoltaic market, has more than 5MW of photovoltaic ground power stations. 580MW, almost all of the above 5MW photovoltaic ground power stations adopt centralized grid-connected inverters.
In summary, the acceptance of centralized inverters in large-scale ground power stations is extremely high. Mr Pradah, SMA's Key Account Manager, also fully agrees to use centralized grid-connected inverters in large-scale ground power plants above 5MW. He said: “In large-scale photovoltaic power plants, especially those with a capacity greater than 5MW, the best The solution is a centralized inverter. SMA has been promoting and applying this solution worldwide and has received good customer response."
With string string inverters applied to large-scale ground power stations above 5MW in recent years, the photovoltaic industry has caused controversy over the application plan. Therefore, string string solutions have such a huge advantage compared to centralized solutions. ?
1. Will the application of string-type grid-connected inverters bring about an increase in revenue?
Photovoltaic power station construction as a business investment, investment return rate is the focus of attention of all parties, regardless of the use of any program, allowing customers to maximize the investment rate of return is the perfect solution. Will the use of string-type grid-connected inverters for large-scale ground power stations really bring more benefits to customers? Specific analysis is as follows:
This comparison data comes from the actual upload data of a power station inverter. The orange label part is a brand string inverter, and the blue label part is a three brand centralized inverter. In order to ensure the fairness of the comparison data, this comparison combines the output power of the string, the degree of attenuation of the components, and the AC-DC side line loss and other factors, and then obtains the actual value data.
a. String output power
The array string test array input power was 1.02 MW, and the centralized test array input power was 1.04 MW, which increased the power generation by 2% for the string test array for fairness.
b. Component attenuation
The centralized power generation array assembly was put into operation in 2011, and the attenuation was serious. The string inverter inverter power generation array module was put into operation in 2013, and the attenuation of the components was much lower than that of the centralized power generation array. After actual measurement, the average attenuation of the centralized test array component reaches 3.13% in two years, and the final data must be taken into consideration for component attenuation.
c. AC and DC line losses
Combining the respective networking features of the string solution and the centralized solution, the line loss on the AC side of the string inverter is larger, the DC line loss of the centralized inverter is larger, and the power generation data uploaded by the string inverter is larger. Does not include AC line loss. According to the analysis, the cable loss accounted for 1%, so the string test array needs to deduct this part of the power generation.
Taking into account the above three factors, the final test data is as follows:
From the above data comparison, it can be concluded that the average output of the string inverter is 1.418% lower than that of the A manufacturer; the average output of the string inverter is 2.174% lower than that of the B manufacturer; and compared with the C manufacturer The string inverter output is only 1.0% higher. After calculating the average power generation, it is concluded that the string inverter is 0.864% lower than that of the centralized inverter. In summary, the application of a string inverter in a large ground power station cannot bring benefits to customers.
Second, does the string inverter meet the requirements of the equipment of large-scale ground power station?
(1) Zero voltage ride through protection issues
According to GB/T 19964-2012 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 zero, when the voltage drops to Below curve 1, allows the inverter to be cut out of the grid.
First of all, according to the string-type inverter networking method, there is no high-frequency carrier synchronization among the inverters in the string-type solution, and it is impossible to solve the problem of the parallel circulating current between the inverters. Second, in this scheme, the impedance of the inverter line far from the box change is greater. In addition, because the AC side of the string solution adopts the multi-machine parallel mode, multiple inverters cannot uniformly output the phases of the voltage and the current when the grid voltage drops. The above reasons can seriously affect the inverter's determination of zero voltage ride through faults and process control.
In the GB/Z 19964-2005 standard implementation phase, the centralized grid-connected inverter not only passes the laboratory low-voltage ride-through test, but also passes the on-site low-voltage ride-through test. It can be seen that the test in the laboratory only shows that a single device can pass through. Functionality, but field tests illustrate the ability of the inverter to respond to grid faults under actual operating conditions. In order to prove that the inverter can deal with grid faults in the actual working conditions in the field, the zero voltage ride through test in the future will inevitably increase the field test link, but the string inverter can pass the test of the field test this is a problem.
(2) Problems with anti-island protection
The so-called islanding effect refers to the fact that when some lines of the power grid lose power due to faults or maintenance, the connected grid-connected power generation devices connected to the power line continue to supply power, and together with the surrounding loads constitute a phenomenon of self-sustained power supply.
Although the requirement for inverter protection against islanding is relatively low in large-scale ground power stations, the GB/T 19964-2012 standard still requires the power station to have anti-islanding protection equipment. Usually, the inverter adopts active and passive dual anti-islanding protection. In order to ensure that in any case the inverter can reliably disconnect from the grid. Active protection usually uses a small disturbance signal injected into the grid to determine whether the power is lost by detecting the feedback signal. Passive protection usually uses the method of detecting output voltage, frequency, and phase to determine the state of islanding.
The string inverters are directly connected in parallel on the AC side of a large ground power station. Because active protection uses the method of injecting distorted signals, it cannot be used in multi-machine parallel systems. Therefore, it is impossible to implement active protection in island protection and there are application risks. If a resonating island is generated, it will pose a safety threat to the line maintenance personnel and cause damage to the electrical equipment, which will seriously affect the operation safety of the power station. The AC output side of the centralized inverter does not need to be connected, but directly connects to the double-split winding transformer. It is fully capable of performing both active and passive island protection at the same time and has higher reliability.
(3) Support grid dispatch
For large-scale ground power stations, it is a common demand to support grid dispatch, and to control the inverter to issue specific values ​​of reactive power or active power. Regardless of whether the string inverter or the centralized inverter adopts RS485 as the communication interface, the response speed is relatively slow, and the centralized scheme only has 2 inverters per megawatt, and the scheduling is convenient, but for the string inverter For each megawatt of the device, up to 40 inverters need to be scheduled, which is very complicated and is not conducive to remote scheduling and management of the power plant.
(4) PID effect suppression strategy
The negative ground of the inverter is currently recognized as the most reliable solution to suppress the PID effect. For the string inverter, the virtual negative ground circuit is usually used to suppress the PID effect. For example, a virtual circuit failure group string inverter cannot guarantee the suppression of the PID effect, which is far worse than the solid negative grounding reliability. The centralized inverter uses the GFDI (PV Ground-Fault Detector Interrupter) solution, that is, the inverter monitors the impedance of the PV+ to the ground in real time. If the impedance of the PV+ to ground is lower than the threshold, the inverter immediately stops the alarm and shuts down the safety hazard. . The GFDI consists of breaking devices and sensors to ensure the reliability of the negative ground and the safety of operation.
(5) Breaking device protection issues
In large ground power stations, fault protection is very important for the power station. Whether software protection or hardware protection is used, the inverter is required to be able to operate reliably in the event of a fault and to protect the safety of the power plant. However, for a string inverter with a DC switch (not a breaker) on the DC side, if a DC-side ground fault occurs, the DC switch does not have a breaking capacity and the DC-side fault cannot be cut off. The lack of hardware protection features.
Third, can string inverters improve the maintenance efficiency of the power station?
(1) Problems with the standby inverter
According to the calculation of 100MW power plant, a total of 4,000 string inverters are needed. The number of spare inverters that the manufacturer needs to provide is 10, which is only 0.25%. However, due to the large number of string inverter components and inverter topology For reasons such as complexity, the failure rate is much higher than 0.25%. Secondly, large ground power stations are located in deserts and the Gobi, and logistics is underdeveloped. Even though the string manufacturer promises to send a fault-recovery inverter back to the manufacturer during the warranty period, customers need to pass the inverter outside the warranty period. Logistics transportation to the designated address requires the customer to transport a single 55kg grid-connected inverter to the logistics receiving point, which undoubtedly increases the workload of the customer.
(2) The problem of field replacement
Large-scale ground power stations usually occupy a large area. Some power stations have poor on-site 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 inverters directly to the fault point for replacement, which takes time and effort and affects the maintenance efficiency of the power station.
Centralized grid-connected inverters adopt modular design of devices, and the main devices can be quickly replaced by plugging and unplugging. After fault location, the maintenance time does not exceed 20 minutes. The entire maintenance process is maintained by professional after-sales service personnel. No owner involvement is required. In addition, centralized inverter manufacturers have established after-sales service centers and spare parts management centers in the cities near power plants to ensure the after-sales service and spare parts supply at the project site.
(3) Problems with maintenance costs
The maintenance method of the string inverter inverter replacement is destined to be much higher than the centralized inverter maintenance cost, especially after the warranty period, the string inverter maintenance costs will account for the power station operating costs Not small. Analyzed from the perspective of the life of components, the switching power supply and aluminum electrolytic capacitor in the inverter have the shortest lifespan, which ranges from 5 years to 8 years. For the string inverter, the whole machine needs to be replaced, and the centralized inverter is required. The device is maintained by replacing the faulty module, and only the switching power supply needs to be replaced (the centralized inverter does not need to be replaced within the operating life of the metal film capacitor), and the maintenance cost is low. In addition, professional after-sales personnel can provide more thoughtful services for customers and are more suitable for the maintenance of large-scale ground power stations.
In addition, centralized inverters made in China are also used in many foreign projects, taking the example of a class of inverter manufacturers such as Sun Power and TBEA. Solar Power Centralized Inverter was successfully applied to Italy's Puglia 120MW photovoltaic grid-connected power station project, Italian 12MW project in Sardinia, 5MW project in Tenerife, Spain, etc.; TBEA centralized grid-connected inverter successfully won the bid of 100MW in Pakistan. Photovoltaic power plant project, Algeria 90MW photovoltaic power station project, etc. The foreign brands SMA and Bonfiglioli also applied centralized inverters to non-local projects such as SMA Kalkbult, SouthAfrica 75MW project, Kagoshima, Japan 70MW project, Adelanto Solar PowerPlant 10.4MW project, etc.; Bonfiglioli Thornton USA 2.3MW Project, Golmud 200MW Project in China, Karadzhalovo 60MW Project in Bulgaria, and Siwogunga Project in Tamil Nadu, India. It can be seen that capable inverter manufacturers can guarantee the stable operation of grid-connected inverters around the world, and provide reliable photovoltaic grid-connected power generation equipment to customers around the world.
Fourth, summary
Centralized inverters have undergone 12 years of development and currently occupy the vast majority of the global photovoltaic market, becoming the most mainstream solution for large-scale terrestrial power stations, while the majority of string inverters are still used in distributed power plants and small scale. Ground station. According to authoritative statistics, it is predicted that by 2018, global high-power inverters will still account for 80% of system shipments, and photovoltaic power generation equipment will continue to maintain a reasonable application development trend.
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