The turbines in a hydroelectric dam station would probably go for a long time, but the connection to the actual grid may not be so robust. Wires. Transformers. Lubes. Coolants. Connectors. There's always something that can break or run out. The systems are usually made to make this work simple, but there still has to be someone who checks the readings, and replace stuff or some critical bit will stop the electricity from working correctly.