روش تشخیص جزیره ای بر مبنای یک رویکرد جدید برای زاویه فاز ولتاژ مبدل های قدرت ثابت Islanding detection method based on a new approach to voltage phase angle of constant power inverters

The distributed generation (DG) can be defined as electric generation facilities connected to an area of an electric power system (EPS) through point of common coupling (PCC), which is located nearby local loads [1]. In recent years, the penetration of DGs in power systems is increasing [2, 3]. The connections of DGs to utility system have some protection issues such as islanding. The unintentional islanding refers to the condition that one or more DGs and some loads are disconnected from the rest of main power system while the loads are supplied by DGs in the isolated part of the power [4, 5]. The islanding situation imposes some considerable problems into power systems such as power quality problems (frequency and voltage deviations), safety hazards to network personnel, overload condition, adverse effects on system’s protection and reconnecting problems [6–۸]. Thus, island must be detected by islanding detection methods immediately. Various methods are proposed for the islanding detection by researchers. Islanding detection methods could be categorised as communication and local detection methods. Moreover, local detection methods can be considered as passive and active methods [7, 9]. The passive methods detect island by monitoring the changes of parameters such as frequency deviation or voltage variation at the PCC. Not only are they both simple and low-priced to implement, but also they have no considerable adverse effects on the power system and DGs operation. If the power mismatch between loads and DG is small, the deviation of parameters will not go beyond the threshold. Therefore, the passive methods cannot detect island condition in a reasonable time because they have a large non-detection zone. Thus, the only deviation of system parameters cannot be enough criteria for islanding detection [10–۱۲]. The over or under voltage and frequency protection, phase jump detection and rate of change of frequency and power are some of typical passive [13–۱۵]. In active methods, a small disturbance is injected into the power system by DG through the PCC to create changes in the system parameters. In fact, in the grid-connected mode, the small disturbance cannot create considerable variations in the power system parameters such as voltage or frequency because the DG parameters are dictated by the power system. However, when an island is formed, the small disturbance can create an enough variation in the island parameters. Hence, active methods have a smaller non-detection zone compared with the passive methods [16, 17], but due to the disturbance injection, they have unfavourable impacts on the power quality. Active methods are usually applied to DGs more than other methods despite their disadvantages imposed on power quality. They do not need costly communication infrastructure and, also, their detection accuracy in better than that of passive methods [18]. Some of active methods that have been recently proposed are slip-mode frequency shift (SMS), Sandia frequency shift (SFS), negative-sequence current injection [19–۲۱], robust anti-islanding algorithm [22] and voltage positive feedback for voltage source inverter [23–۲۶]. This paper proposes a new active detection approach with insignificant negative impacts on the power quality. The implementation of this method does not require any considerable changes in structure of inverter and does not cause any problem for DG and the power system during load switching. In the proposed method, the difference between the DG instantaneous voltage phase angle (VPA) and the nominal VPA, which is constant, will be added to the input angle of abc-to-dqo transformation block [derived from phase-locked loop (PLL) block]. In the islanding condition, the instantaneous VPA will change; thus, the error amplitude (difference) will be grown. In the grid-connected mode, the error value is insignificant because instantaneous VPA is close to the nominal VPA; therefore, this method will not negatively affect the system parameters. In fact, there are no significant adverse effects on the power quality during grid connected mode of power systems. Despite insignificant problems of power quality, the islanding detection time is reasonable in this method. The rest of paper is structured as follows. Section 2 presents the power system model and DG control system. In Section 3, the intended method is described. Section 4 provides simulation proofs for evaluation.