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This paper proposes a decentralized nonlinear autoadaptive controller for reducing system losses by the optimal management of the reactive power supplied by the inverters of photovoltaic (PV) units. This ancillary service can be furnished on the base of standard needs or on voluntary basis. The control design is based on an optimization procedure involving the sensitivity theory in conjunction with the Lyapunov function and provides control laws acting as references of the PV inverter local controller. PV inverters must operate in a decoupled manner in order to provide the reactive power imposed by the control law and to transfer the active power produced by PV modules. The experiences and results conducted in an indoor laboratory as well as on an actual distribution network managed by ENEL Distribuzione S.p.A. demonstrate its effectiveness in reducing system losses.Index Terms-Ancillary service, decentralized controller, Lyapunov function, photovoltaic (PV) generation, power electronic interface, renewable sources, sensitivity theory, system losses.NOMENCLATURE P TR , V TR , ϑ TR Total active power generation and the voltage and phase angle vectors at substation transformers. V , ϑVoltage magnitude and phase vectors at all buses. V min , V max Minimum and maximum voltage vectors at all buses. P L , Q L Active and reactive power demands at each load bus. P PV , Q PV Vectors of active and reactive powers supplied by all photovoltaic (PV) inverters. S min , S max Vectors of minimum and maximum apparent power capacities of all PV inverters. Y ij e jϕ ij Generic element of the nodal admittance matrix of the network, where Y ij and ϕ ij are the corresponding magnitude and the phase angle, respectively. y ij Admittance of the branch connecting the buses i and j.

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