Floating solar PV (FSPV) plant, refers to an array of photovoltaic panels on a structure that floats on a body of water, typically a reservoir or lake. America, Japan, and China have all started the floating projects. India now also has a big plan for the floating projects. In 2017, SECI has announced 10 GW plan for the floating projects. Many states are active in developing the floating projects on the surface of reservoirs, storage/irrigation ponds and lakes.
Floating projects are built above the water, quite different from the ones built on the ground or mountain, which makes challenges of the solar projects.
- 1500 V system: more benefits or risks?
The 1500 V system will reduce the safety and reliability of electrical system, while increase the voltage of DC (high DC voltage and multiple nodes are prone to generating DC arc), power induced degradation (PID), and other risks. The central inverter is not equipped with any residual current device (RCD). When the voltage is raised to 1500 V, the safety of the system performance requirements is higher. Various from AC, there is no zero-crossing point for DC, in the event of leakage, it will cause a major accident. For floating projects, the risk increased as the higher humility. The string inverter, however, has short DC circuits and long AC circuits, a zero crossing point for AC arc, as well as an RCD. Therefore, the 1500 V system of the string inverter is safer.
- How to maintain the DC combiner box?
There are fuses in the DC combiner box. Nowadays, many ground-mounted projects using no string monitoring box which make the operation and maintenance (O&M) much more difficult and energy loss even harder to detect.
- Headache of communications.
Traditional RS485 cables have a poor environmental adaptability and can be easily damaged by the stress pull caused by fluctuations on the water. When the RS485 cables are arranged together with power cables, they suffer interference. In special deployment scenarios such as wet environment, the RS485 cables are prone to damages, resulting in communication interruption. It’s difficult to locate a fault when the RS485 cable is faulty, resulting in difficulties in fault identification and rectification. Engineering of the RS485 cable is complex, causing higher cable and labor cost, especially in the floating projects.
20MW FSPV plant in Huaibei, China
- PID effect is heavier in floating projects.
Floating projects are located in high-humidity environment, causing heavier PID effect. This hush environment requires higher protect level of the components in the project. Therefore, in addition to requiring the components to have anti-PID capability, the system design also requires the inverter to have anti-PID function and higher protection capability.
- Mismatch is higher in the floating projects.
Energy loss is 1% caused by higher DC voltage and mismatch. The distance between the DC combiner box and inverter varies with the central PV system. The voltage drop (voltage difference) and lack of flexibility in inverter installation are likely to cause string mismatch. Besides, the higher the voltage, the more serious the string mismatch. The string inverter, however, can minimize the mismatch because two strings are connected to one MPPT.
2MW FSPV plant in Saga Prefacture, Japan
- Ingress protection requirement is higher for combiner box and inverters. The high temperature and humidity in India have brought challenges to the installation of electrical devices, such as the combiner box and inverter on the water surface. To ensure the long-term O&M stability, the combiner box and inverter must be protected to a higher level. But a large number of fuses are used in a combiner box for overcurrent protection. The string protection using fuses requires a large amount of preventive maintenance work. With the long-term operation, fuses are aged, resulting in fuse failures. According to statistics, the DC fuse failure rate of a power plant is significantly increased starting from the fourth year of use. Meanwhile, the traditional solutions adopt external fans for cooling. However, the protection class of fans is only IP44/54, indicating that water vapor outside can enter the box, affecting device reliability. In addition, external fans have various issues such as large noise, poor reliability, and high replacement and maintenance costs. Once a fan is faulty, the cooling capability of an inverter is greatly weakened.
FSPV plant in Singapore
Therefore, higher standards are raised for floating projects. Huawei has the total solution by string inverters to resolve the attached concerns. Huawei has offered more than 200MW floating projects in China. One of the largest floating projects, 50MW FSPV plant was commissioned in March this year, which located on Huainan, Anhui.
The unique technologies of Huawei solution are:
- Multi-MPPTs decrease mismatch loss due to DC voltage drop
Taking Huawei SUN2000-70KTL string inverter for example, each 1 MW has 88 MPPTs. The energy yield drop of one string due to shading does not affect other strings. In this way, the energy yield loss brought by string mismatch is effectively reduced and the system efficiency is improved. The simulation analysis by PVsyst suggests that the mismatch loss due to DC voltage drop in a 2 strings in 1 MPPT circuit solution is about 0.1% less than that in a 100 strings in 1 MPPT circuit solution.
440KW FSPV plant in Sumiyakiike Parkin Chikuzen, Japan
- Less DC, more AC
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. Faults on the DC side could easily cause the inverter to generate alarms and shut down. 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. The string inverter is small in size and light in weight. The inverter is prepared as a spare device onsite, facilitating installation and maintenance. The system reliability is improved and energy yield loss is reduced, earning more interests for plant investors.
- Zero-touch maintenance: no fuse requirement and IP65 protection
Huawei string inverter adopts the closed cover design. Protected to IP65, it is highly anti-corrosive and adaptable to high temperature and humidity. By removing damageable components, integrating complex functions, and removing components and operations that require maintenance, the smart PV power plant has a simple overall architecture. The smart PV power plant uses Huawei Smart PV Solution supports 25-year reliable and maintenance-free running. Huawei inverter is at 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 such as temperature, wind, and salt spray 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 maintenance-free; all of the components and 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.
FSPV plant in Aichi Prefecture, Japan
- PLCC (Power Line Carrier Communication): replace the RS485 communication cables, integrate the signal into power line, cutting workload and expense of cable routing, as well as reducing line loss on signal transmissions hence improving data transmission efficiency.
After the PLCC technology is introduced to replace the RS485 solution, the transmission rate is greatly increased from 9.6 kbit/s (a maximum transmission rate supported by RS485 cables is only 19.2 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 0.01 RMB/W. By using AC cables as channels, the PLCC technology has a high reliability. When a component is damaged, you only need to replace the faulty component, which provides excellent maintainability. Therefore, the PLCC technology can both save communication cable costs and 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, 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. Based on the high-precision string-level detection of the 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.
247.5KW FSPV plant in Fukuoka City, Japan
- Anti PID solution, virtual grounding
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 SmartACU, implement positive voltage across PV- and ground to safely prevent PID effects in the daytime, and Huawei string inverter integrated Smart Anti-PID module can inject voltage+ to PV- at night, recover power loss during daytime. This prevents + recover solution can perfectly reduce power loss by PID effects and earn more interests for plant investors.
- Higher energy yield: 2-3% more compared to central inverters
Taking 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 a particular brand, and the other adopts the SUN2000-50KTL-C1 string inverter. Both of them connected to the power grid in August, 2017. A comparison of their energy yields in October shows that Huawei inverter yields 3.22% more of the power than the other brand with a clear-cut advantage.
1MW FSPV plant in Jiangsu, China
All in all, Huawei FusionSolar Smart PV Solution perfectly fulfills the requirements for constructing floating PV plants in India, and promotes the healthy and rapid development of the country’s floating PV plant technologies.
