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Representing a major technology toward the proliferation of sustainable energy resources, droop controlled microgrids have attracted numerous modeling approaches with a view to improving their operation and applicability. Acknowledging the low inertia of distributed generators and the network characteristics at the distribution level, this paper proposes a reduced order model for inter-inverter oscillations in autonomous droop controlled microgrids. At first, a simplified equivalent network on the basis of Kron reduction is suggested as an effective topology for the study of inter-inverter oscillating currents. The focus shifts toward low-frequency modes following the detailed small signal model and its simplification according to the singular perturbation theory with multiple time scales. Assuming accurate and instantaneous tracking of the voltage by the inner controllers on their local reference frame, the quasi steady state study results in a simple closed-loop model. This model is successfully benchmarked against the detailed model and other germane models over the full range of operational damping ratios. [ABSTRACT FROM AUTHOR]

This paper introduces a new technique to characterize the ac grids stability connected to multiterminal high-voltage direct current networks. The proposed method identifies the maximum active powers that converters can exchange with an ac network without losing the transient stability using trajectory sensitivity analysis. Therefore, the generators' rotor angles variations due to various converters active powers exchange are measured and normalized. Consequently, the norm of each trajectory vector is used as an indicator of the system proximity to instability. To enhance the accuracy of the method for online stability assessment applications, the steady-state initial conditions of the sensitivity network variables are precisely obtained using the Newton shooting method. For this purpose, the detailed differential and algebraic models representing the dynamics of the ac and dc networks have been formulated and analytically solved simultaneously with the sensitivity network. Comprehensive simulation studies on the IEEE 68-bus benchmark system are carried out using PSCAD/EMTDC for validation purposes. Results show that the modified approach provides more accurate results compared to the normal trajectory sensitivity analysis with less time compared to multiple time-domain simulations. [ABSTRACT FROM PUBLISHER]

Microgrids are becoming popular because of the rise of distributed energy resources (DERs). The quest for efficient utilization of DERs resulted in the development of hybrid dc/ac microgrids, which consist of independent dc and ac subgrids. Controlling the power exchange across hybrid microgrids is an important aspect in maximizing the benefits. There are numerous techniques proposed in the literature to control the power flow; however, most of these techniques use proportional-integral controllers which are difficult to tune and exhibit slow response. To eliminate the drawback of existing control solutions, this letter proposes a novel strategy to exchange active power among dc and ac microgrids. The proposed control strategy is based on the hill climbing algorithm that uses perturbations of power angle δ and observes the corresponding changes in the active power. An average model of the hybrid microgrid is first developed for the evaluation of the proposed algorithm. The proposed algorithm is then applied to verify its effectiveness for achieving sufficient power exchange and enhancing the dynamic response. The model is implemented and tested using MATLAB/Simulink. Moreover, the proposed control strategy is experimentally validated using a real-time simulator, OPAL-RT. [ABSTRACT FROM PUBLISHER]

This paper introduces an efficient algorithm to adaptively determine the droop coefficients of generating units in hybrid ac–dc networks. These generating units include conventional synchronous generators and multiterminal high-voltage direct-current (MTDC) converters. The proposed algorithm relies mainly on trajectory sensitivity to reschedule power generation according to the new calculated droop coefficients to ensure and/or system stability margin for set of credible contingencies with different load conditions. The Newton shooting method is used to find the new steady-state values for the systems when load is changed. MTDC converters are modeled in such a way to emulate the inertia behavior of the synchronous generators. This virtual emulation is achieved by providing two additional control layers that link and control the converter powers through inertia constants similar to synchronous generators. Also, an expression for generated powers controlled by the droop coefficients is developed to be utilized in the algorithm. Finally, comprehensive simulation studies on the IEEE 68-bus benchmark system are carried out using PSCAD/EMTDC interfaced with MATLAB to validate the proposed algorithm. [ABSTRACT FROM AUTHOR]

The low cost of diesel generators have led to their increased presence in hybrid microgrids. The presence of low-inertia devices in such microgrids makes the stability of hybrid microgrids a pressing concern. This paper analyzes the impact of equal and unequal active power sharing conditions on the dynamic stability of a hybrid microgrid. The sensitivity of low-frequency eigenvalues to changes in active power droop gains is studied and a “domain of stability” region is proposed to assess the stability of hybrid microgrids under different power sharing conditions. The different operating regions within the domain of stability are defined. Based on the sensitivity analysis and the domain of stability, the optimal droop gain for the diesel generator to improve the stability of the microgrid is determined. Time-domain simulations are carried out in the MATLAB/Simulink software environment and are used to validate the small-signal stability analysis results. [ABSTRACT FROM AUTHOR]

This paper proposes the notion of “critical cluster” in inverter based droop controlled microgrids, thus representing the neighborhood of the distributed generation units that determines the small signal stability margin of the entire system. The clustering starts from the lowest impedance electrical connection between distributed generators, termed as “critical line”. At first, the systematic study of few similar microgrids by means of the eigenvalue analysis of their small signal models reveals the correlation between the location of their dominant low frequency modes and the individual connections between neighboring inverters. Second, the sensitivity analysis of active power droop gain with respect to network parameters confirms the previous findings and shows the dominating effect of the critical line's impedance. Simulations and small signal analysis studies assess the impact of the choice of various interconnection points when two individual droop controlled islanded microgrids are connected to form a single microgrid. At the same time, simplified models on the basis of critical clusters allow the effective approximation of the stability margin corresponding to the original systems. The results reveal that different interconnection points lead to strong variations in the performance of the coupled microgrid both in terms of transient response and small signal stability. [ABSTRACT FROM PUBLISHER]

New grid codes impose fault ride through (FRT) requirements on large doubly fed induction generator (DFIG)-based wind parks connected to transmission systems in order to reduce the loss of huge generation power due to temporary faults. Recently, the focus is set toward the development of different wind turbine technologies to enhance the FRT capability of wind parks with no consideration for transmission system protection schemes. This paper proposes a new communication-based dual time–current–voltage (Dual-TCV) tripping characteristic for directional overcurrent relays (DOCRs) that considers the FRT capability of wind parks by taking fast fault isolation actions in transmission systems. The protection coordination problem is formulated and solved to determine the optimal relay tripping settings. The proposed approach is tested on the IEEE 24-bus transmission system with up to eight DFIG-based wind parks. The outcome of this study reveals that protection schemes based on DOCRs governed by the proposed Dual-TCV tripping characteristic ensures fast fault isolations that significantly enhances the FRT operation of wind parks in adherence to grid code requirements. The transient analysis verifies the superior performance of the proposed protection approach in enhancing the FRT operation of wind parks. [ABSTRACT FROM PUBLISHER]

This paper investigates a special class of dynamic power system problem, namely subsynchronous resonance (SSR) resulted from a series-compensated network connecting doubly-fed induction generator (DFIG) based wind farms. A novel two-degree-of-freedom (2DOF) control strategy combined with a damping control loop is designed and analyzed for enhancing the system stability and alleviates the SSR that may arise due to the induction generator effect (IGE). The proposed control strategy is tested at different operating conditions of series compensation levels and low wind speeds to ensure the system stability. The doubly-fed induction generator based wind farms without the proposed control strategy leads to overall system instability during high series compensation and low wind speeds. Hence, the mitigation of the SSR and damping enhancement are critical to the entire power system stability. A reliable way of analyzing the system and designing effective control strategies against SSR based on the eigenvalue analysis and impedance based stability criterion is deployed. Moreover, analytical explanations have been elaborated to verify the procedure of the controller design. Fault ride-though capability has also been investigated with the proposed control strategy that is flexible to be integrated with the FRT schemes so as to assist the wind farm in mitigating the SSR during the fault recovery stage. Finally, time domain simulations are carried out to demonstrate the effectiveness of the proposed control strategy for mitigating the SSR and damping power system oscillations. [ABSTRACT FROM PUBLISHER]