The SG125HV Virtual Central Solution
1. What is the virtual central solution?
Central inverters and string inverters have been widely used as mainstreams in utility power plants and distributed power plants. Many may ask: which one is better? Actually, this is a false proposition because each has its merits. Historically, central inverters have been used in utility-scale projects. The larger unit power leads to less equipment, which can cut system costs down and make installation and operation easy. By contrast, the string inverters are mainly utilized in complex terrain due to smaller unit power and more MPPT’s.
With increasingly enhanced voltage grade and per-unit power for solar systems, Sungrowis proud to introduce its “Virtual Central Solution” firstly. Specifically, multiple string inverters close to medium-voltage transformer are installed centrally to cut down system costs and improve convenience of maintenance and operation. The virtual central solution can be adapted to varied application scenarios,achieving advantages of both string and central inverters.
Multiple string inverters combined to create a “virtual central inverter” and it would operate as follows:
● Each of the inverters is considered a power module for the virtual central inverter with the communication and control information being received at the same time.
● The response time for all inverters in the group will be essentially identical.
● The controller receives monitoring information from each inverter so there is no reduction in data on the individual inverter performance.
● Single failures are reported, but do not affect the operation of the remaining inverters.
● Individual inverters can be removed/replaced without having to shut down the system.
● All utility required functions such as dynamic power factor control and power curtailment can be implemented as if it was a central inverter.
The Virtual Central Solution utilizes the SG125HV string inverter. The converged strings are connected to the DC input side of the SG125HV and all of inverters close to medium-voltage transformers are installed centrally. The AC output side of the SG125HV is connected to the low voltage side of the transformer through the LV cabinet.
Fig-1 Virtual Central Solution Sketch
The unit capacity ranging from 1MW to 5MW can be flexibly customized according to users’ demands by taking the SG125HV virtual central solution. For instance, 20inverters can form a 2.5MW block.
2. Why the SG125HV?
The 1500V string inverter SG125HV unveiled by Sungrow in 2016 has the highest unit power up to now in inverter industry, reaching up to 125kW, making the virtual central solution possible.
Fig-2 SG125HV String Inverter
Apart from the virtual central solution, the SG125HVitself has excellent features such as high efficiency ensuring high plant yield. In addition, a well-designed cooling system enables it to work stably with full power operation in temperatures up to50°Cwithoutderating. Ithas a single DC input and the DC/AC ratio can reach up to 1.5. In addition, the lightweight design makes it easy to install. The above features make the SG125HV more competitive among the virtual central solutions.
3. The Virtual Central Solution creates more value
Saved system cost and less cable loss
In general, string inverters are evenly distributed throughout PV arrays and are far away from MV transformers. This reduces the length of DC cables but requires longer AC cables. String inverters in the Virtual Central Solution, on the contrary, are installedclose to MV transformer and connected with longer DC cables and shorter AC cables. This difference is illustrated in the below diagram.
Fig-3 Virtual Central Solution and Traditional Distributed Solution
It is obvious that the Virtual Central Solution requires shorter AC cables than the traditional distributed design, saving cabling costs as in the figure above. Besides, given the low AC voltage and higher DC voltage,the Virtual Central Solution also features less cable loss at the same power rate benefited from higher DC voltage and shorter AC cables.
The system cost is further reduced by the increased unit power of the SG125HV inverter. While major string inverters are rated at around 60kW and have to be connected with an AC combiner before connecting to the LV cabinet of the transformer, the SG125HV can be directly connected to the LV cabinet without an AC combiner. Thus, SG125HV enables a more system design by reducing over 50% of system equipment comparing with the same capacity system with 60kW inverters.
The centralized installation of the Virtual Central Solution also saves cost and time for installation. Its input & output ports are close to each other and simplify the cable connection work.
Another benefit brought by this design is its convenience for O&M. When one inverter fails, it can be replaced quickly by an on-site spare resulting in less yield loss, as is the case for all string inverter plants.
The SG125HV virtual central solution makes it possible to design solar systems more flexibly through combining advantages of central and string inverters. Continually cutting down initial investment of solar systems and O&M costs and enhancing yields are developing trends in solar industry. The SG125HV is expected to be widely used in the future large, utility scale PV plants considering the value it brings to users.