A method for controlling voltage at a point of regulation on a system of windfarms, including a plurality of local windfarms with each windfarm including a plurality of individual wind turbine generators, a collector system for each local windfarm including a collector transformer and network of conductors joining individual wind turbine generators at a point of common connection, and at least one transmission line providing a connection to a grid for the plurality of local windfarms, wherein voltage control at the point of regulation is subject to constraints on maintaining designated system parameters within specified tolerance, the method comprising: generating, by a system controller for the windfarm system control system, a reactive power command (854, 855, 856) for local windfarm controllers for each of the local wind farms (810, 815, 820), adapted to control voltage at the voltage regulation point to a designated reference value subject to at least one additional constraint on an operating parameter (850, 863) for the system of windfarms; calculating a total reactive power command for the system of windfarms; assigning reactive power commands to individual local windfarms according to the reactive power capability for the individual local windfarm controller; determining when a designated operating parameter (850, 863) for the system of windfarms for at least one additional designated constraint reference point on the system has exceeded a designated limit; restoring the designated operating parameter (850, 863) within the designated limit for the at least one additional designated constraint reference point; and wherein the designated operating parameter (850, 863) for at least one additional constraint point is voltage on a collector bus, the method comprising: measuring voltages on a collector bus for the at least one additional constraint point for each of the plurality of local windfarms (810, 815, 820); determining when the voltage on any collector bus exceeds a designated value; determining the reactive load capability (850, 851, 852) for each of the plurality of local windfarms (810, 815, 820); identifying a collector bus with voltage within the designated value and with available reactive power capability. transferring reactive load (850, 851, 852) to the collector bus with voltage within the designated value and with available reactive power load capability; and shifting the regulation point for the system of windfarms to the limiting collector bus when unable to otherwise restore voltage on the collector bus. generating, by a system controller for the windfarm system control system, a reactive power command (854, 855, 856) for local windfarm controllers for each of the local wind farms (810, 815, 820), adapted to control voltage at the voltage regulation point to a designated reference value subject to at least one additional constraint on an operating parameter (850, 863) for the system of windfarms; calculating a total reactive power command for the system of windfarms; assigning reactive power commands to individual local windfarms according to the reactive power capability for the individual local windfarm controller; determining when a designated operating parameter (850, 863) for the system of windfarms for at least one additional designated constraint reference point on the system has exceeded a designated limit; restoring the designated operating parameter (850, 863) within the designated limit for the at least one additional designated constraint reference point; and wherein the designated operating parameter (850, 863) for at least one additional constraint point is voltage on a collector bus, the method comprising: measuring voltages on a collector bus for the at least one additional constraint point for each of the plurality of local windfarms (810, 815, 820); determining when the voltage on any collector bus exceeds a designated value; determining the reactive load capability (850, 851, 852) for each of the plurality of local windfarms (810, 815, 820); identifying a collector bus with voltage within the designated value and with available reactive power capability. transferring reactive load (850, 851, 852) to the collector bus with voltage within the designated value and with available reactive power load capability; and shifting the regulation point for the system of windfarms to the limiting collector bus when unable to otherwise restore voltage on the collector bus. measuring voltages on a collector bus for the at least one additional constraint point for each of the plurality of local windfarms (810, 815, 820); determining when the voltage on any collector bus exceeds a designated value; determining the reactive load capability (850, 851, 852) for each of the plurality of local windfarms (810, 815, 820); identifying a collector bus with voltage within the designated value and with available reactive power capability. transferring reactive load (850, 851, 852) to the collector bus with voltage within the designated value and with available reactive power load capability; and shifting the regulation point for the system of windfarms to the limiting collector bus when unable to otherwise restore voltage on the collector bus.