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It is known that the choice of gain ( K ) in the Sandia frequency shift (SFS) scheme has direct impacts on the stability of a system with grid-connected distributed generations (DGs). In this paper, a scheduled perturbation technique is proposed to reduce the stability impact of K . In the proposed technique, chopping fraction (cf) is used to compensate for reduction in the value of K , where higher cf values are used to achieve zero nondetection zone (NDZ) under low gain SFS. It is shown by analysis that theoretical reduction of NDZ can be always achieved for a nonzero value of cf. Simulations for single- and multi-DGs systems are performed to verify the analytical analysis. It is shown that an appropriate design of scheduled signal duty cycle ( d ) is of critical importance to realize the proposed reduction in NDZ. While close synchronization of perturbation signals for multi-DG system is required, a delay of 0.33 s is shown to be tolerated for a two-DG system. Synchronization can be achieved either through locally synchronized timers or by limited communication among DGs. The proposed technique provides an attractive option for systems with high DG penetration by reducing the negative impact of K on stability. [ABSTRACT FROM PUBLISHER]

Islanding detection is important to ensure the reliability and safety of distributed generation (DG). In this paper, a new active islanding detection method (IDM) is proposed, and it depends on individually estimating an overall transient stiffness measure for any multi-DG system to establish a clear separation between prior- and post-islanding stiffness. For the multi-DG system to avoid spectrum overlapping, each of its DGs is required to perturb at distinct frequencies. By using this concept of perturbation separation, the proposed technique can be applied to multi-DG systems without requiring any communication among the DGs. Simulation results show that the proposed technique is scalable and robust against different loading conditions and variations of grid stiffness levels as well as with respect to the number of connected DGs and different types of DG controllers. It is also shown that the proposed technique can successfully distinguish islanding conditions from other disturbances that may occur in power system networks. [ABSTRACT FROM AUTHOR]

In this paper, a new islanding detection method (IDM) is proposed to dynamically estimate islanding occurrence. The proposed dynamic estimators estimate amplitudes and phase angles of the current injected by the grid at the point of common coupling with the distributed generation (DG) in addition to the DG's bus voltage. A distributed two-level algorithm is proposed to detect an islanding condition for single and multi-DG configurations. Analytical design and transient analysis are carried out for the islanding detection problem to determine the nondetection zone (NDZ) of the proposed islanding detection algorithm. A local low-frequency meshed communication network is sufficient to achieve distributed islanding detection capability for a general multi-DG network with negligible NDZ. It is shown through simulations that the proposed IDM can successfully distinguish an islanding condition from other disturbances that may occur in power system networks. [ABSTRACT FROM PUBLISHER]

Although active islanding detection techniques have smaller nondetection zones than passive techniques, active methods could degrade the system power quality and are not as simple and easy to implement as passive methods. The islanding detection strategy, proposed in this paper, combines the advantages of both active and passive islanding detection methods. The distributed-generation (DG) interface was designed so that the DG maintains stable operation while being grid connected and loses its stability once islanded. Thus, the over/undervoltage and over/underfrequency protection method would be sufficient to detect islanding. The main advantage of the proposed technique is that it relies on a simple approach for islanding detection and has negligible nondetection zone. The system was simulated on PSCAD/EMTDC and simulation results are presented to highlight the effectiveness of the proposed technique. [ABSTRACT FROM AUTHOR]

In previous literature, constant RLC loads were assumed to impose, on the islanding detection method, the hardest detectable case. For this reason, distributed-generation (DG) islanding studies are usually analyzed and performed by using constant RLC loads. In this paper, different types of loads are taken into account by modeling the load's voltage and frequency dependence. The performance of the over/undervoltage and over/underfrequency protection (OVP/UVP and OFP/UFP) method is examined for the different load models and the results are compared with the constant RLC load case. The analysis is conducted on a constant power-controlled DG designed to operate at unity power factor. A generalized formula, for calculating the non-detection zone of the OVP/UVP and OFP/UFP method, is derived in terms of the load's voltage and frequency dependence parameters. The analysis is further extended on a constant current-controlled DG interface. [ABSTRACT FROM AUTHOR]

Islanding detection of Distributed Generation (DG) is considered as one of the most important aspects when interconnecting DGs to the distribution system. With the increasing penetration and reliance of the distribution systems on DGs, new interface control strategies are being proposed. Aside from its main task of supplying active power, the DG could provide voltage support, improve the power factor, or mitigate other power quality problems. This paper examines the impact of the interface control strategy of inverter based DGs on islanding detection. The Non-detective Zone (NDZ) for over/under voltage and over/under frequency is derived analytically for each interface control and validated by simulation. [ABSTRACT FROM AUTHOR]

Abstract-The Sandia frequency-shift (SFS) method is one of the active islanding detection methods that possesses small nondetection zones (NDZs). The effectiveness of the SFS depends to a great extent on its design parameters (cf[subo] and k). In this letter, a formula is derived to aid protection engineers to determine the optimal parameter setting for the SFS islanding detection method. It is concluded that the parameter (k) has a significant effect on the NDZ and a modified version of the SFS is proposed that relies only on this parameter. [ABSTRACT FROM AUTHOR]