How to reasonably choose inverters with different system voltages
With the increasing pursuit of the power generation efficiency of photovoltaic power stations, the voltage on the DC side of photovoltaic power stations has also been gradually increased. The highest DC voltage of the early 600V inverter has been replaced by the 1000V voltage, and has even been increased to 1100V, 1500V or even higher voltage. There is no doubt that the use of higher high voltage on the DC side of the photovoltaic system will result in lower line loss and higher system efficiency. Therefore, the current 1500V system solution has become the first choice for large-scale photovoltaic power plants.
The 1500V system is suitable for large-scale photovoltaic power stations, especially medium- and high-voltage grid-connected photovoltaic power stations. Higher DC voltage will not only greatly reduce the line loss and improve the efficiency of the DC side, the inverter with high voltage DC input is more suitable for designing inverters with higher AC voltage output, which effectively increases the AC output voltage level and reduces the AC side line It reduces the winding loss of the low-voltage side of the transformer, and improves the efficiency of the step-up transformer and the overall system efficiency of the photovoltaic power station. Therefore, high-voltage DC systems such as 1500V are very suitable for applications in large-scale photovoltaic power generation systems.
However, the excessively high DC voltage system also puts forward higher withstand voltage requirements and challenges to other equipment in the system. The 1500V system has higher requirements for system safety due to its high DC voltage characteristics, requiring the withstand voltage performance of inverters, photovoltaic modules, combiner boxes and other equipment, especially the insulation and withstand voltage of cables, switches, connectors, etc. It is required to reach the corresponding or even higher index of 1500V. It will also increase the cost of power station construction and operation invisibly.
In addition, system safety issues will also become prominent. HVDC is more likely to cause problems such as arcs and fires in the case of poor insulation and poor contact. When there are problems such as loose joints and damage to cable insulation, the impact of failures will be greater. The risk of electric shock and leakage accidents is greater, and the damage caused will be more serious.
Although the 1500V high voltage DC system improves the system efficiency by increasing the DC voltage, it will also bring greater risk of component short-board effect mismatch due to the series connection of more component strings in a component string, and the components are unbalanced. The probability that the attenuation will affect the entire string will also increase.
The 1500V high voltage DC system will cause the hot spots and PID attenuation of the components to become more serious. The main cause of PID attenuation is that the high voltage difference between the photovoltaic cell and the frame in the module caused by the high voltage of the photovoltaic module string induces the attenuation of the photovoltaic cell power generation efficiency. The PID problem has been widely manifested in 1000V systems. When the system voltage is further increased to 1500V, this problem will become more prominent.
With the promotion of various distributed photovoltaic power stations and the implementation of the county-wide promotion plan, more and more small and medium-sized photovoltaic power stations have been built in various places. Can household photovoltaics or photovoltaic power stations installed on the roofs of various buildings use 1500V or higher DC voltage solutions?
We believe that most of the distributed photovoltaic power stations are household photovoltaics or photovoltaic power stations installed on the roofs of various buildings. They are closer to people’s daily life. They are usually connected to the user-side grid and have a lower voltage. In addition to pursuing the power generation efficiency of photovoltaic power stations, the more important thing is safety, and system solutions below 1100V should be adopted as far as possible. High-efficiency inverters with low starting voltage and wide MPPT range should be selected.