Since the neighboring wind farms are under similar weather conditions

their outputs are strongly correlated. Referring to conventional planning methods

wind farm outputs are supposed to be mutually independent without considering the influence of wind power correlation. Such methods are not suitable for future power system with high proportion of renewable energy. This paper proposes a generation and transmission co-planning method considering wind power correlation and discusses its influence. First

the Copula theory is applied to describe the correlation of wind power outputs. Then

a wind power probability model is developed based on the fuzzy C-means clustering model of correlated wind power. Further

the economical co-planning model is developed

which is further solved by the generic algorithm. Finally

the influence of wind power correlation in different system scales is analyzed via numerical results from the Garver 6-bus and South Brazil 46-bus systems. The results show that the wind power correlation has strong influence on the local generation and transmission expansion planning

while weak on the global planning. In order to reduce the adverse impact of wind power convergence

the transmission line connecting integration points with strongly correlated wind farms should be avoided. With increased transmission investment cost and the reduced operation cost

the influence of wind power correlation on the total cost is not obvious.