Pattern purchased the share in the project owned by US developer AEI Energy for $45.25 million, plus AEI's share of the project debt.
The US-based operator now holds a 70% stake in the project — up from 31.5% — with the remaining 30% owned by Chilean mining group Antofagasta Minerals.
El Arrayán was commissioned earlier this month and is the largest operational development in South America.
It uses 50 Siemens SWT-2.3-101 turbines and was built over three years by Skanska Chile.
Most of the power produced by the project will be supplied to mining company Minera Los Pelambres, a subsidiary of Antofagasta.