This article outlines a generalized reduced-order modeling approach for assessing the transient synchronization stability of multiple grid-tied converter systems operating in grid-feeding mode and ...having heterogeneous characteristics. The analytical approach utilizes the nonlinear quasi-static dynamics of the phase-locked loop (PLL) for obtaining the reduced-order models (ROM) of multiple heterogeneous grid-feeding converters having same and different point of synchronization (POS). This is a significant contribution in contrast to the prior effort where reduced-order modeling has been carried out only for homogeneous grid-tied multi-converter systems. In addition to this, an innovative methodology has been proposed in this article for analyzing the transient synchronization stability of the multi-converter systems leveraging the Lyapunov's direct method. The Lyapunov functions are constructively synthesized by invoking an assumption that the reduced-order models are conservative in nature. Following this, the well-known Lyapunov's second theorem is applied for deriving the sufficient conditions required for obtaining the transient stability boundary of the heterogeneous grid-feeding converter systems. Numerical simulations are performed in MATLAB 2017a platform to demonstrate that the ROM are highly accurate and computationally efficient. Besides, the efficacy of the proposed methodology for analyzing the transient synchronization stability of heterogeneous grid-feeding multi-converter systems is verified numerically.
Large scale renewable energy systems interfaced with voltage source converters (VSCs) experience several challenging issues when integrated to weak grids. One of the specific issue i.e. control loop ...interaction of vector-controlled VSC under high grid impedance leads to several instability mechanism ultimately resulting in unwanted tripping of the VSC unit. As a countermeasure, a unique nonlinear adaptive stabilizing control (NASC) technique has been proposed in this paper to enhance the dynamic stability of the VSC during such scenarios. A small-signal model is developed first to examine the instability mechanism of VSC connected to weak grids under abnormal operating conditions and varying set points of the power controller. The dominant modes are identified from the analysis and a stabilizing controller is developed based on adaptive dynamic surface control philosophy for enhancing its stability. The proposed controller has been proved to be uniformly ultimately bounded using Lyapunov analysis. Besides, the stabilizing mechanism of VSC is analyzed with the proposed controller by performing small-signal stability analysis of the entire system. The efficacy of the proposed control scheme is demonstrated by performing numerical simulations in MATLAB 2017a and conducting experimental validations in controller-hardware-in-the Loop (CHIL) platform.
The incorporation of ancillary services on distributed generators (DGs) at utility level has imposed several challenges on the existing distribution networks. To address one of such problem, this ...paper discusses the impact of ancillary services, mainly the voltage ride through (VRT) capability and active power curtailment control (APCC) of DGs on their anti-islanding protection scheme (AIPS). In this study, different types of load models (viz. ZIP, induction motor (IM) and composite loads) have been considered and non-detection zone (NDZ) characteristics are obtained for a modified IEEE 1547/ UL 1741 test system for analyzing the problem. Further demonstration of this issue has been carried out by conducting time domain simulations in PSCAD/EMTDC platform on modified IEEE 1547 and CIGRE low voltage benchmark test system having composite loads at each of the load buses. Finally, an appropriate mitigation framework has been proposed by incorporating a hybrid islanding detection technique to enable simultaneous implementation of VRT, APCC and AIPS in DGs. The online implementation of the proposed framework is realized in MATLAB/Simulink platform and real-time simulations are performed in OPAL-RT (OP5600) test bed on the modified IEEE 1547/ UL 1741 test system for validating its efficacy.
The impact of dc-bus voltage control (DVC) on transient synchronization stability of grid-tied inverters operating in grid-feeding mode has been assessed in this brief. A nonlinear quasi-static model ...magnifying the dynamics of phase-locked loop (PLL) and DVC of grid-tied inverter has been developed first. Following this, the steady-state and transient stability boundaries of the model are analytically characterized and numerically evaluated. The proposed model and the stability assessment method eliminates the inaccuracies associated with the previously developed model which considers the dynamics of PLL alone.
This letter proposes a new current saturation strategy (CSS) for grid-forming (GFM) inverters to comply with the existing low-voltage ride-through (LVRT) capability requirements. The proposed control ...philosophy enables the GFM inverters to follow one of the standardized grid codes by limiting the output current during LVRT via a new control parameter, the power factor angle (PFA). First, a nonlinear mathematical model capturing the full-order averaged dynamics of a GFM inverter embedded with current limiting control has been developed. Following this, the proposed CSS has been analytically evaluated by developing a simplified equivalent circuit model. Finally, numerical and experimental results are presented to verify the accuracy of the CSS for assessing the large-signal stability of a GFM inverter subjected to severe symmetrical grid faults.
This brief presents a theoretical approach to investigate the cause of marginal stability/instability of an islanded system following unintentional islanding event when DGs are enabled with P(f) and ...Q(V) regulation. Nondetection zone being a traditional cause as per previous investigations, a small-signal model of the standard UL 1741 anti-islanding test system is developed in this brief under islanded condition to illustrate the problem of undamped oscillations. Dynamic model validation has also been performed to verify the correctness of the developed linearized state space model. The eigenvalue analysis of the model successfully illustrates the reason behind sustained oscillations of the system under loss of mains scenario. Finally, time-domain simulations are performed in PSCAD/EMTDC platform to demonstrate the effect of such instability/marginal stability of power island.
Distributed generators are required to be integrated with sophisticated anti-islanding protection system (AIPS). However, most conventional AIPS fails to detect unintentional islanding event during ...perfectly matched loading conditions. To overcome this issue, in this article, a hybrid islanding detection algorithm has been proposed based on the rate of change of kinetic energy and root-mean-square value of absolute frequency deviation at the point of common coupling (PCC). The methodology presents a significant contribution prior to the state-of-the-art methods in terms of the disturbance injection mechanism through the q -axis current control loop of the grid-tied inverter. The method activates the disturbance signal injection mechanism only when an unintentional islanding event is suspected based on monitoring certain parameters at the PCC. This mechanism has been proposed to have zero nondetection zone, very fast operation, straightforward implementation, and significantly less impact on the power quality and stability of the power electronics system. The performance of the proposed method has been evaluated by conducting 200+ islanding and nonislanding case studies in simulation as well as an experimental platform. The proposed method is robust, accurate, and fast enough to detect an islanding event cleverly with a detection time as small as 120 ms.
The conventional over/under voltage, over/under frequency based anti-islanding protection scheme presents significant nondetection zone (NDZ) under critical loading conditions of distribution ...networks. To overcome this challenge, a unique hybrid technique has been proposed in this article for the anti-islanding protection of distributed generators (DGs). The algorithm requires the injection of an active oscillatory disturbance signal of very small magnitude through the current control loop along the direct axis of the synchronously rotating reference frame of the converter. Small signal stability analysis of the system is carried out to analyze the effect of such active signal injection having different frequencies. The anti-islanding protection algorithm first involves the superimposition of d-axis voltage. Thereafter, two novel indexes are proposed based on which the trip signal logic is developed for the protection scheme. The methodology has been found to detect an unintentional islanding scenario within 90 ms from the initiation instant. The efficacy of the proposed hybrid anti-islanding protection scheme is tested under various abnormal operating conditions by performing simulations on the CIGRE LV test system. Experimental validation of the proposed methodology has also been carried out in Controller Hardware-in-the-Loop (CHIL) platform using Typhoon HIL 602+ and Speedgoat baseline real-time target machine.
This article presents a unique design method for an adaptive neural network based backstepping-like control (ANNBC) scheme. The technique is employed for synthesizing the primary controller for ...inverter interfaced distributed generators (IIDGs) integrated to an autonomous distribution network. Further, an optimal distributed secondary control framework is developed for a multiple IIDGs-based autonomous distribution network. The secondary controller facilitates optimal gain selection to regulate the frequency of the system and voltage of the critical bus to their desired set points. The framework also achieves accurate real and reactive power sharing among the IIDGs according to their power ratings. The novel design procedure of the proposed control framework takes into account the entire system dynamics of the IIDG including the uncertain terms (viz., load current and network dynamics) and is completely independent of the system parameters information. Suitable update laws are designed for estimating the unknown weights of the neural network and the uncertain system parameters. Lyapunov analysis is used to show that the tracking errors and parameter estimation errors are uniformly ultimately bounded. Finally, case studies are conducted on a typical autonomous distribution network having a single and multiple IIDGs modeled in MATLAB/Simulink platform.
Security of various cyber-physical systems is a major concern for researchers worldwide. Nowadays, microgrids also form such cyber-physical system to achieve a number of objectives such as enhanced ...frequency regulation, economic active power sharing, proper reactive power sharing, etc. Communication links are essential in such microgrids to ensure bilateral flow of data to be fed to the secondary controllers of the distributed generators integrated to the power network. This article, therefore, studies the impact of various kinds of cyberattacks viz., false data injection, denial-of-service, and replay attack on the performance of these communication enabled secondary controllers having potential vulnerabilities. Additionally, it presents a unique framework to ensure system stability when communication links are subjected to such attacks. The analysis has been carried out in a microgrid operating in islanded mode where the controller area network (CAN) bus communication network is utilized to form the cyber-physical system. Simulations are performed in MATLAB/Simulink platform and real-time evaluation of the proposed framework has been carried out in a test bed having OPAL-RT (OP5600) simulator and physical CAN devices to form the communication links.
