Year: 2021
Type: dissertation
Abstract: The share of photovoltaic systems has greatly increased in electricity generation in recent years. As the prices of solar power plant components have decreased lately, companies need to find ways to achieve savings. By connecting central inverters in parallel to a same winding of a transformer, a cheaper regular transformer can be used instead of a transformer having two separate windings on the inverter side. However, this parallel connection creates a path for circulating current to flow in the system which produces additional losses and stresses vulnerable system components. In this thesis, the circulating current in the parallel-connected system is analyzed. A simulation model is used to obtain the common-mode voltage produced by the two inverters and the equivalent circuit of the system is used to calculate the circulating current from the voltage. Laboratory measurements are conducted to verify the accuracy of the calculations and define if the parallel connection is feasible to use with the existing system components. The inverters used in this thesis are large modular 1500 V 5 MW central inverters both having four identical power sections. These inverters are connected to the same MV transformer so that they share a galvanic connection thus allowing circulating current to flow in the system. It was found that the calculated circulating current reaches high magnitude from less than 10 A rms to above 100 A rms depending on the system DC voltage level and number of power sections in the inverters. The laboratory measurements showed that the circulating current in the system is higher than the calculated circulating current. The calculated circulating current is lower as the simulation model and the equivalent circuit do not take account the non-idealities and resonance behaviour of the system components. It can be concluded that it is not feasible to use the parallel connection with the components currently in the system due to the high circulating current, and that the phenomenon should be further investigated.
Source: Aaltodoc (Aalto University)
Author: Lind, Jaakko
Language: English
Subfield: Electrical and Electronic Engineering
Field: Engineering
Domain: Physical Sciences
Open Access status: green
OpenAlex ID: W7124228376