Considering the rising penetration of diverse types of renewable generating units (RGUs) in modern electricity networks, the active and reactive power management and the coordination of RGUs with different operating states can be intricate. Specifically, the RGUs may not be always allowed to operate with their maximum generation capacity, as they are committed to provide reactive power support to the grid during both normal and emergency conditions. In that case, the optimal control setpoints can be assigned to the RGUs, such that the voltages can be maintained within an acceptable range, the power losses can be minimized and the inherent uncertainties of renewables can be addressed. In this paper, a power management system is designed for the optimal allocation of the active and reactive power flow through the network, and to provide the voltage management locally, considering the service provider's requirements. Particularly, a combined local/centralized control system is implemented at substation levels to monitor and modify the voltage control setpoints of RGUs in real-time, which can improve the voltage profile in the local networks, minimize the power losses and reduce network congestions by allowing the optimal active power transfer. Based on the simulation results, when the proposed combined local-centralized strategy is implemented, the voltages can be improved for all buses, the congestion constraints of the power lines can be maintained and the power losses can be significantly minimized.