The invention is a new approach for coordinating volt-var control (VVC) between sub-transmission and distribution systems through optimal reactive power dispatch of distributed energy resources (DERs) that are aggregated as virtual power plants (VPPs). At the sub-transmission level, shunt devices and the reactive power provided by the VPPs are coordinated and optimised using VVC algorithm with weighted sum of multiple objects that include minimising voltage deviations from desirable levels at load buses, minimising loses, minimising solar curtailment, minimising demand response (DR) usage and minimising mechanical switching of shunt elements. The algorithm runs every five minutes and is solved using the AC optimal power flow technique. At the distribution level, each VPP runs a distribution VVC algorithm to dispatch reactive power from DERs. The goal of the VPP reactive power control is to meet sub-transmission service requirements while satisfying all the constraints at distribution side. Each VPP updates its reactive power capability every five minutes to allow the sub-transmission controller to formulate the optimisation problem for the next dispatch interval. The proposed tool is simulated on a Duke Energy Carolina system to demonstrate the capability of providing voltage support by dispatching reactive power of DERs as a VPP.