The future power systems will be more automated and disaggregated by connecting distributed energy resources. The coordination between power generators and demand needs information transfer through a communication infrastructure. The development of Virtual Power Plant (VPP), has become prominent technology to provide the flexibility required in future power systems with a high penetration level of renewables. The flexibility that a VPP can provide will mainly depend on the development of hierarchical control methods for the VPP. The hierarchical control method for a VPP is able to simplify the complexity of heterogeneous network assets in terms of active and reactive power controls at the PCC. The virtue of hierarchical control lays in the division of control works at different control layers according to the timescale of target services; thus, this conceptual control scheme enables a group of sources to provide a wide range of ancillary services.