Floating solar PV (FSPV) plant refers to an array of solar PV panels mounted on a structure that floats on a body of water, typically a reservoir or a lake. Like America, Japan, and China, India has also started working on the development of floating projects. In 2017, Solar Energy Corporation of India announced a 10GW plan for developing floating projects, which is being endorsed by various proactive states. Floating solar projects are built above the water, and hence, their design and construction is quite different and far more challenging from the ones built on the ground or rooftop. Some of these challenges are highlighted below:
Risks of the 1,500V solar PV system
The 1,500V solar PV system reduces the safety and reliability of the electrical system, by increasing the voltage of direct current, power induced degradation (PID), and other risks. For floating projects, the risk is increased on account of higher humidity. Unlike a central inverter, a string inverter has a zero-crossing point for AC arc, as well as an RCD, which makes the 1,500 V solar system combined with string inverter much safer.
Maintenance of DC combiner box
Absence of string monitoring box in ground-mounted projects, makes the operation and maintenance (O&M) of DC combiner boxes that have numerous fuses, much more difficult and energy loss even harder to detect.
Traditional RS485 cables have a poor environmental adaptability and can be easily damaged by continuous movement of water, resulting in communication interruption, and difficulty in fault detection and rectification. RS485 cable development involves complex engineering with higher cable and labor cost, especially in floating solar projects.
PID effect is heavier in floating projects
Floating projects are located in high-humidity environment, causing greater PID effect, and this harsh environment entails higher levels of protection in the project. Therefore, the system design requires the inverter to have anti-PID function and higher protection capability.
Mismatch is higher in the floating projects
Energy loss caused by higher DC voltage and mismatch is around 1 per cent. The distance between the DC combiner box and inverter varies in a central PV system, and the voltage drop due to this distance and lack of flexibility in inverter installation are likely to cause string mismatch, which increases with voltage. A string inverter can minimize the mismatch because two strings are connected to one maximum power point tracker (MPPT).
Higher ingress protection requirement
High temperature and humidity conditions in India lead to challenges in the installation of electrical devices, such as the combiner box and inverter on water surface. A combiner box has a large number of fuses for over-current protection, which requires a large amount of preventive maintenance work.
Therefore, floating projects require higher standards for components as well as design. Huawei has an integrated solution to resolve the attached concerns, through its string inverters. The company has offered solutions for more than 200MW of floating projects in China, including one of the largest floating projects of 50 MW, which was commissioned in March 2018 on Huainan in Anhui.
The unique features offered by the Huawei solution are:
Multi-MPPTs decrease mismatch loss due to DC voltage drop
We take the case of Huawei SUN2000-70KTL string inverter for example, where each 1 MW has 88 MPPTs. Here the drop in energy yield of one string due to shading does not affect other strings, and hence, the energy loss brought by string mismatch is effectively reduced and the system efficiency is improved. Studies suggest that the mismatch loss due to DC voltage drop in two strings with one MPPT circuit solution is about 0.1 per cent less than that with 100 strings and one MPPT circuit solution.
Reduced DC faults
Huawei inverter can be installed close to PV modules, which leads to shorter DC cable and prevents DC arcing. Short circuits or grounding faults are likely to generate electricity leakage and electricity shocks, and such faults on the DC side could easily cause the inverter to generate alarms and shutdown. Supposing a 1 MW central inverter shuts down due to a fault on the DC side, the energy yield loss is far more than the loss of a 70 kW string inverter. A string inverter is prepared as a spare device onsite, facilitating installation and maintenance, and this improves the system reliability.
The smart PV power plant uses Huawei Smart PV Solution and supports 25-year reliable and maintenance-free running. Huawei inverter is in the IP65 protection class and supports isolation between internal and external environments, so that components are running in a stable environment, and impacts caused by external environments on the component lifetime are reduced; the system is not equipped with any damageable components, or any components that need to be replaced periodically such as fuses and fans, thereby achieving a maintenance-free system; all of the components and the entire system adopt a 25-year reliability design and lifetime simulation, and also pass strict verification tests. Therefore, system components do not need to be replaced within the entire lifecycle and can reliably and economically operate.
PLCC (Power Line Carrier Communication)
PLCC has replaced the RS485 communication cables, and it integrates the signals into power line, cutting workload and expense of cable routing, as well as reducing line loss on signal transmissions, hence, improving data transmission efficiency. With the introduction of the PLCC technology, the transmission rate is greatly increased from 9.6 kbit/s to 200 kbit/s. In terms of construction, the PLCC technology uses AC cables as channels without additional cable arrangement, saving costs on communication cables and engineering. By using AC cables as channels, the PLCC technology has a high reliability. Therefore, the PLCC technology can save communication cable costs as well as improve the reliability.
Unique IV curve scanning
The complex onsite environment of floating plants poses challenges to troubleshooting and fault-locating. The inspection outside the PV plant can be performed on a boat, but the inspection inside PV arrays can only be conducted on foot, which is inefficient and difficult. Huawei cutting-edge Smart I-V Curve Diagnosis locates problematic PV strings and analyzes the root cause almost instantly by using Huawei big data technologies such as data mining and pattern recognition, thereby reducing testing costs compared to traditional methods. In most centralized power plants, it is hard to detect string faults, and the loss of electric energy yield cannot be compensated. However, based on the high-precision string-level detection of the Huawei’s smart PV controller, the system can detect faults in a timely manner. By analyzing databases, the system can accurately locate a specific faulty device and propose handling suggestions based on preset measures and O&M experience.
Anti PID solution
The power degradation of PV modules brought by PID greatly impacts the return on investment. Huawei patent Smart Anti-PID technology provides Anti-PID module in Smart ACU, implementing positive voltage across PV system and ground to safely prevent PID effects in the daytime, and Huawei string inverter integrated Smart Anti-PID module can inject voltage at night, recover power loss of daytime. This prevention and recovery solution can perfectly reduce power loss by PID effects and earn more interests for plant investors.
Higher energy yield
Taking the case of a floating PV plant in Jiangsu, China for example, two 1 MW sub-arrays with the same structured layout and tilt are selected. One adopts the 500KTL central inverter of Company S, and the other adopts the SUN2000-50KTL-C1 string inverter. Both of them were connected to the power grid in August, 2017. A comparison of their energy yields in October shows that Huawei string inverter yields 3.22 per cent more energy than the other brand’s central inverter.