This study investigates the use of photovoltaic systems to regulate the voltage amplitude of electric power networks and to analyze their impact on the stability of Line Voltage Unbalance Factor (LVUF). It will be shown that instability of the LVUF can occur when single-phase domestic photovoltaic systems equipped with a voltage controller are connected to the three-phase low voltage network. The degradation of the LVUF is one of the problems to be avoided in three-phase electrical networks. To address this problem, this paper presents a complementary algorithm to be applied on the existing controller of the photovoltaics system. This algorithm calculates the optimum reference voltage for the controller, to keep the voltage amplitude within the permitted range, and at the same time prevent the degradation of the LVUF.