In this thesis, the reactive power control system of the photovoltaic (PV) inverters, provided to compensate voltage variation of point of common coupling due to reverse power flow in distribution feeders, has been evaluated and improved in terms of stability and dynamic performance. The increasing penetration of photovoltaic systems in distribution networks has imposed some challenges on the distribution network. One of the most important challenges is the overvoltage at the connection point of the PV system. Various methods have been suggested in literature to mitigate the overvoltage, including active power curtailment, grid reinforcement, on load tap changer transformers and exchanging reactive power by photovoltaic inverters. In recent years, photovoltaic inverters have been widely used to regulate the voltage of the point of common coupling with the distribution feeder due to the lower initial cost and fast dynamic response. Therefore, different control algorithms have been introduced, in form of local and centralized methods, to achieve the desired voltage regulation by exploiting the capacity of photovoltaic inverters in a distribution network. The local controllers, which usually use droop functions, are implemented on the inverter’s controller. It leads to a faster dynamic response compared to the centralized methods which require telecommunication infrastructure to transfer data. Droop function is used in current controller of the inverter to produce the reactive power reference instantaneously. In order to avoid huntingehavior in system as well as maintaining the system stability the reference reactive power of the inverter, produced by Droop function, shoulde passed through a low-pass filter. The method is presented as a particular method called delayed droop function. As the dynamic performance of the inverter’s reactive power controller is slowed down by the low-pass filter, the ability of the control system to response rapid events, such as cloud transitions, will be deteriorated. Besides, a voltage swell appears at the time of increase in PV panel’s generation. In the conventional droop characteristic, the maximum reactive power of the inverter, depending on the amount of active power as well as the nominal apparent power of the inverter, is limited in eachampling period, and the reference reactive power is calculated according to the effective voltage value in each sampling interval. According to this method, the nominal apparent power is considered as a limiter of the reactive power capacity of the inverter. Therefore, the maximum current value is dependent on the voltage changes. As a result, in conventional methods, the sensitivity of the inverter to over current is not considered. In this thesis, a novel method is presented to ensure the system stability as well as offer fast dynamics. Moreover, the proposed method not only doesn’t pose any voltage sag, but also responds to rapid voltage changes instantly. In addition, the proposed method uses a new characteristic in which the inverter sensitivity to overcurrent is taken into consideration. Therefore, although the main purpose of presenting the new method is to improve the transient state of the reactive power control system, a better voltage waveform can be achieved in the steady state as well. To demonstrate the performance of the proposed system and compare it with the conventional method, a distribution feeder with four photovoltaic systems is simulated in the MATLAB Simulink software. A detailed PV model is developed, which includes the inverter, boost converter, and MPPT algorithm, is used to achieve accurate and near-realistic results as well as to examine the effect of different components of the photovoltaic system on the stability of the control system. کلیدواژه انگلیسی: Photovoltaic system, Reactive power control, Overvoltage, droop characteristic, delayed droop