While designed a grid connected PV system, the inverter is considered as the heart of the system and selecting an appropriate inverter whose voltage range is matched with the string voltage of the selected modules become very important. I few consider an inverter of 500 KW capacity, the first thing comes is that the inverter will be allowed for a typical overloading up to 120% and according no of the inverter should be selected. Hence, if you look at the following example of a 5 MW grid conned plant in out PVSYST analysis, 10 inverter of SMA make are selected. This helps in optimizing the cost as it will be unreasonable to install yet another inverter in the system. The installation of the 10th inverter will lead to under loading of the all the inverters and the inverter will perform in the MPPT range during the morning and evening out of operation when solar radiations are low. It may also be noted that inverter output is defined based on the STC and its output need to correct for its operating temperature. Following graph shows the characteristic curve of a SMA inverter and highlights the output variation w.r.t temperature as well as irradiance. The voltage range of the inverter selected is 430 – 820V.
The next step is to do the string sizing. String sizing is the no of modules that we will connect in series and parallel before connecting them to the inverter. Series and parallel combination of module in the string will determine the voltage and ampere input to the inverter. In the present case for a 10 inverter the voltage range is 430-820 and we have selected 11 no of modules in series to meet the input voltage requirement of the inverter.
It is important to carry out a simulation of the selected configuration which will result in annual energy generation and performance ration of the proposed configuration. In this case we analyze the system design PVSYST simulation software and a typical 5MW plant delivers 8300 MWH/ year bases and PR is 74.9%.
The same plant of 5 MW was also analyzed for the system sizing through string inverters of SMA 10 KW capacity each and the total 460 no of inverter were selected. This new configuration has 8 no of module in the series and 2604 in parallel which frame the part of the string conned with inverter. A simulation was conducted for the saying capacity with string inverter and it was found that annual energy generation in string inverter is 8318MWh/yr.
Dr. Sanjay Vashishtha and Er. Gaurav Kumar Sharma
First Green Consulting Pvt Ltd.