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In practice, the load-curve and distributed generation (DG) penetration level determines the power imbalance level that a network can experience if islanding occurs. Therefore, with the prior knowledge of load-curve and DG penetration level, the setpoint of rate-of-change-of-frequency (ROCOF) relays can be adjusted so as to make them suitable for a real network. This paper first investigates the subcritical power imbalance region of ROCOF relays through analytical formulation followed by extensive simulation study in order to establish the maximum boundary limit of ROCOF's nondetection zone (NDZ) under all possible deficit/excess of active and/or reactive power imbalance scenarios. Second, ROCOF's reliability (assessed by detection rate and false alarm rate) is expressed analytically and then, validated numerically by simulating a test network of Australia in MATLAB and OPAL-RT real-time digital simulation platform. Finally, ROCOF's performance is assessed through receiver operating characteristics curves and a detailed reliability study under variable setpoints and detection time of the relays; the assessment considers the number of islanding events associated with the time-wise percentage of power imbalance level computed from the net load demand and variable DG penetration in a real network. All these test results demonstrate a clear operational guideline for ROCOF relay. [ABSTRACT FROM AUTHOR]

This paper presents a new islanding detection method for synchronous distributed generation (DG). The proposed method is developed based on the combination of rate of change of exciter voltage (RCEV) and open–close circuit breaker (OCCB) at a DG connection point. Whenever the RCEV parameter is not sufficient enough to detect an islanding condition efficiently and correctly, the OCCB strategy will be triggered. The proposed OCCB strategy sends a switching command to one phase of the three-phase circuit breaker (CB), which connects the DG to the grid. In this way, the CB will be immediately opened and then closed. This switching condition will cause a variation in the value of RCEV. This variation is small for the conditions when DG operates in parallel with the main grid, whereas it will be large during the occurrence of islanding. The implementation of the proposed strategy is cost-effective because it only uses the CB at the DG output and does not require any extra equipment. The proposed approach is tested on two different systems including a hypothetical power system and the IEEE 33-bus test system in MATLAB/Simulink environment. The simulation results show that the proposed method eliminates the nondetection zone and also reveals high accuracy compared with previously introduced methods. Moreover, it has been proved that the performance of the proposed approach is not affected by DG capacity, DG location, the number of DG units, as well as network configuration. [ABSTRACT FROM AUTHOR]

Islanding detection is critical to the safety of grid-tied inverters. This paper presents an impedance-based analysis of the active frequency drift (AFD) islanding detection method. To study this method, the output impedance of a grid-tied inverter is modeled. The Zqq of the inverter impedance features a negative incremental resistor, whose magnitude is equal to the dc impedance magnitude of the local load defined by the test standard. With a large value of feedforward gain $N$ in the AFD method, the phase of Zqq is shown to drop below 180°. Under an islanding condition, due to a lack of phase margin, the inverter system becomes unstable with a frequency drift away from its steady state. The islanding condition is then identified by a frequency detection unit. Experimental results verify the analysis. [ABSTRACT FROM AUTHOR]

Islanding detection is a critical protection issue, as conventional protection schemes such as vector surge (VS) and rate of change of frequency relays do not guarantee islanding detection for all network conditions. Integration of multiple distributed generation (DG) units of different sizes and technologies into distribution grids makes this issue even more critical. This paper presents a comprehensive analysis of the effectiveness of a new method for islanding detection in DG networks. The proposed method, which is based on multiple features and support vector machine (SVM) classification, has the potential to overcome the limitations of conventional protection schemes. The multifeature-based SVM technique utilizes a set of features generated from numerous set of offline dynamic events simulated under different network contingencies, operating conditions, and power imbalance levels. Parameters (such as voltage, frequency, and rotor angle) showing distinguishable variation during the formation of islanding are selected as features for classification of the events. Features associated with different islanding and nonislanding events are used to train the SVM. The trained SVM is tested on a typical distribution network containing multiple DG units. Simulation results indicate that the proposed method can work effectively with high degree of accuracy under different network contingencies and critical levels of power imbalance that may exist during islanding. [ABSTRACT FROM AUTHOR]