Wind farms are an option to provide reactive power while contributing to the control of voltage levels. However, the energy supplied to the power system by wind farms varies according to wind speed. In addition, the electrical system needs more reactive energy support at certain times of the day due to the different electricity consumption patterns of the population. In recent years, several studies have found a correlation between wind generation and electricity demand due to climatic factors. This work presents a new methodology to establish the required values of reactive power from wind turbines to maintain voltage levels within operating standards, considering the correlation between wind generation and electricity demand. The methodology uses the T-Copula transform to correlate the probability distribution curves for the expected values of wind generation and load. The result is a random sample to be inserted into an optimal power flow to determine the transmission systems operating points. From the results of the optimal power flow, a confidence interval is calculated for the injected reactive power values. Such an interval can assist operators in reactive power management one day ahead of operations. The proposed methodology was tested using a 140-bus Spanish existing power network to analyze the values of reactive power injected by wind generators when considering the correlation between generation and demand. The results demonstrate that, when this correlation is considered, the reactive power injection intervals increment more than 0.05 per-unit for some periods and one more system bus. This information can direct operators’ attention to properly define the controllers’ reactive power range at wind turbines’ connection points.