Drawing on system voltages obtained from phasor measurement units (PMUs) and wide-area communication network, a secondary voltage control formulation is developed, with particular reference to a power system having SVCs and STATCOMs. Based on the system load-flow equations linearised about the operating point defined by measured nodal voltages, the control procedure is formulated in terms of constrained optimisation with a quadratic objective function and a set of linear constraints derived from operating limits of reactive-power compensation systems and generators. The variables in the optimisation are the reference voltages of SVCs, STATCOMs and synchronous generators. The problem formulated is solved by standard quadratic programming method which is well-established and efficient in terms of computing time requirement in the context of online application. The procedure developed is applicable to any changes in loads and/or system configuration. Representative results of voltage control co-ordination are presented in the paper.