The unified power quality conditioner（UPQC） is composed of a series active power filter and a shunt active power filter

which can comprehensively compensate the voltage quality and current quality simultaneously. However

the interaction effect of its series and shunt units must be taken into consideration. First

we established a united mathematical model of UPQC.Based on the model

the input disturbance of grid voltage and load current was estimated for each frequency component. Then

we proposed an optimal output tracking control strategy

which can make system output to track the reference values and to compensate the grid voltage and load current to normal state. Since the performance index reflects the tracking properties of the load voltage and the supply current simultaneously

the strategy can effectively eliminate the mutual influence between the series and shunt units of UPQC

and can provide a unified

coordinating power quality regulation function with good transient response characteristics. In the meantime

the strategy is realized in the three-phase abc coordinate system without need of phase-locked loop and dq transformation. The control parameter matrix is available in advance according to the Riccati equation and has no relationship with system state

which is easy for project implementation. Simulations in PSCAD/EMTDC verify that the proposed control can effectively solve the power quality issue caused by voltage sag/rise and harmonic load with superior transient response.