The project, which started producing power in 2010, became the world's biggest offshore project when it was completed later that year. It is located in the outer Thames estuary off the Kent coast.
Vattenfall said the project was running at reduced capacity while the fault was being fixed. The work is expected to be completed by February.
The project consists of 100 Vestas 3MW turbines. It will be surpassed as the world's largest offshore project by the 504MW Greater Gabbard project, which is due for completion this year.
The project first produced power from turbine number C08 on Sunday 16 May in wind speeds of 10-13 metres per second.