Current-limiting Droop Control with Virtual Inertia and Self-Synchronization Properties

In this paper a current-limiting droop control of grid-tied inverters that introduces virtual inertia and operates without a phase locked loop unit is proposed. The proposed controller inherits a self-synchronization function and can guarantee tight bounds for the inverter frequency. In addition, using nonlinear Lyapunov theory, it is analytically proven that the inverter current never violates a given maximum value. Compared to the original current-limiting droop controller, the maximum capacity of the inverter is utilized at all times using the proposed strategy, even under grid faults. It is also proven that the proposed controller significantly reduces the resonance problem of the LCL filter. Extended simulation results are presented to verify the performance of the proposed controller under normal and faulty grid conditions

Current-limiting droop controller with fault-ride-through capability for grid-tied inverters

In this paper, the recently proposed current-limiting droop (CLD) controller for grid-connected inverters is enhanced in order to comply with the Fault-Ride-Through (FRT) requirements set by the Grid Code under grid voltage sags. The proposed version of the CLD extends the operation of the original CLD by fully utilizing the power capacity of the inverter under grid faults. It is analytically proven that during a grid fault, the inverter current increases but never violates a given maximum value. Based on this property, an FRT algorithm is proposed and embedded into the proposed control design to support the voltage of the grid. In contrast to the existing FRT algorithms that change the desired values of both the real and reactive power, the proposed method maximizes only the reactive power to support the grid voltage and the real power automatically drops due to the inherent current-limiting property. Extensive simulations are presented to compare the proposed control approach with the original CLD under a faulty grid

Mobile Application Privacy Risks : Viber Users’ De-Anonymization Using Public Data

Mobile application developers define the terms of use for the applications they develop, which users may accept or declined during installation. Application developers on the one hand seek to gain access to as many user information as possible, while users on the other hand seem to lack awareness and comprehension of privacy policies. This allows application developers to store an enormous number of personal data, sometimes even irrelevant to the application’s function. It’s also common that users choose not to alter the default settings, even when such an option is provided. In combination, the above conditions jeopardize users’ rights to privacy. In this research, we examined the Viber application to demonstrate how effortless it is to discover the identity of unknown Viber users. We chose a pseudorandom sample of 2000 cellular telephone numbers and examined if we could reveal their personal information. We designed an empirical study that compares the reported behavior with the actual behavior of Viber’s users. The results of this study show that users’ anonymity and privacy is easily deprived and information is exposed to a knowledgeable seeker. We provide guidelines addressed to both mobile application users and developers to increase privacy awareness and prevent privacy violations

AppAware: A Model for Privacy Policy Visualization for Mobile Applications

Privacy policies emerge as the main mechanism to inform users on the way their information is managed by online service providers, and still remain the dominant approach for this purpose. Literature notes that users find difficulties in understanding privacy policies because they are usually written in technical or legal language even, although most users are unfamiliar with them. These difficulties have led most users to skip reading privacy policies and blindly accept them. In an effort to address this challenge this paper presents AppWare, a multiplatform tool that intends to improve the visualization of privacy policies for mobile applications. AppWare formulates a visualized report with the permission set of an application, which is easily understandable by a common user. AppWare aims to bridge the difficulty to read privacy policies and android’s obscure permission set with a new privacy policy visualization model. To validate AppAware we conducted a survey through questionnaire aiming to evaluate AppAware in terms of installability, usability, and viability-purpose. The results demonstrate that AppAware is assessed above average by the users in all categories

Advanced Current-limiting Control of Inverter-interfaced Distributed Energy Resources to Develop Self-Protected Microgrids

In the upcoming “smart grid” era, advanced control schemes are required for inverter- interfaced DERs to guarantee stability of inverter-dominated feeders and microgrids. Nevertheless, in many of the recently proposed methods, the safe and stable operation of inverters can not be analytically guaranteed under normal and abnormal grid conditions. In this thesis, single-phase grid-connected inverters are initially considered and an enhanced Current-Limiting Droop (CLD) controller is proposed. In contrast to the original CLD, which limits the inverter current under a lower value than its maximum during faults, the proposed controller fully utilizes the inverter capacity. An inherent current limitation is proven through nonlinear ultimate boundedness theory and is shown to facilitate the operation of Fault-Ride-Through (FRT) schemes. Furthermore, conditions for asymptotic stability of the closed-loop system are derived. Additionally, a new CLD scheme is proposed, which operates without the need of a PLL and introduces a virtual inertia property to DERs. In the sequel, three-phase grid-connected inverters are investigated and a new controller in the dq-frame is proposed to deal with FRT in three-phase systems. Initially, a novel method to divide the current into its symmetrical components during unbalanced faults is proposed. Hence, based on an adaptive bounded integral controller, the proposed scheme provides voltage support to both positive and negative sequences, while ensuring the current boundedness and asymptotic stability of the closed-loop system. In the final part of this thesis, the safe and stable operation of three-phase inverter-based microgrids is investigated. Particularly, an advanced controller is proposed to deal with extreme load conditions. Through the proposed scheme, the limitation of the inverter current during transients is guaranteed, without the need of online adaptation techniques. Furthermore, the proposed approach significantly simplifies the stability analysis of microgrids, since it can be investigated through a Jacobian matrix of reduced size

Influence of fault-ride-through requirements for distributed generators on the protection coordination of an actual distribution system with reclosers

This paper analyses the existing protection scheme of a real distribution system with distributed generators, in Greece. Network protection utilizes three successive reclosers at the main trunk and fuses at the laterals. The generating units are protected by overcurrent and voltage/frequency relays. The analysis focuses on the fault-ride-through capability of the generating units and proposes the resetting of the generators and network protection relays so as to conform to the requirements imposed by distribution system operators and international standards. The proposed protection system guarantees selectivity for any short-circuits occurring inside or outside the distribution system, irrelative if the generating units are connected to the network or not. Meaningful conclusions are derived from the application of the proposed protection coordination principle

Three-phase current-limiting droop controlled inverters operating in parallel

A new current-limiting droop controller is proposed in this paper for three-phase inverters operating in parallel. Droop control is employed to ensure the proportional power sharing between the parallel inverters while an inherent current-limiting property is achieved through the control design. The current limitation is mathematically proven using nonlinear analysis of the closed-loop system which leads to the boundedness of each inverter current under a threshold value at all times. Furthermore, small-signal analysis is performed to examine the closed-loop system stability of two parallel inverters equipped with the proposed controller. The example of two parallel inverters is further exploited to validate the proposed controller through Matlab/Simulink simulation results

SRF-based current-limiting droop controller for three-phase grid-tied inverters

A nonlinear droop controller for three-phase gridconnected inverters that guarantees a rigorous current limitation and asymptotic stability for the closed-loop system is proposed in this paper. The proposed controller is designed using the synchronous reference frame (SRF) and can easily change its operation between the PQ-set mode, i.e. accurate regulation of real and reactive power to their reference values, and the droop control mode. Furthermore, nonlinear input-to-state stability theory is used to guarantee that the grid current remains limited below a given value under both normal and abnormal grid conditions (grid faults). Asymptotic stability for any equilibrium point of the closed-loop system is also analytically proven. The proposed control approach is verified through extended real-time simulation results of a three-phase inverter connected to both a normal and a faulty grid

Enhanced current-limiting droop controller for grid-connected inverters to guarantee stability and maximize power injection under grid faults

Droop controlled inverters are widely used to integrate distributed energy resources (DERs) to the smart grid and provide ancillary services (frequency and voltage support). However, during grid variations or faults, the droop control scheme should inherit a current-limiting property to protect both the inverter and the DER unit. In this brief, a novel structure of the recently developed current-limiting droop (CLD) controller is proposed to accomplish two main tasks: i) guarantee current limitation with maximum power injection during grid faults and ii) rigorously guarantee asymptotic stability of any equilibrium point in a given bounded operating range of the closed-loop system for a grid-connected inverter. Since the maximum power of the DER unit can be utilized under grid faults with the proposed enhanced CLD, then inspired by the latest fault-ride-through requirements, it is further extended to provide voltage support to a faulty grid via the maximum injection of reactive power. This is achieved by simply adjusting the reactive power reference opposed to existing control schemes which require adjustment of both the real and the reactive power. Hence, a unified current-limiting control scheme for grid-connected inverters under both normal and faulty grids with a simplified voltage support mechanism is developed and experimentally verified in this brief

Current-limiting droop controller for single-phase inverters operating in island mode

In this paper, a current-limiting droop controller with nonlinear dynamics is proposed for the stand-alone operation of single-phase inverters. The proposed controller regulates the voltage and frequency of the load depending on the real and reactive power demand, as required in modern ac micro- grids. The dynamic performance of inverters equipped with the proposed control scheme is investigated under different load conditions (linear and non-linear loads) and their current-limiting property is analytically proven to hold at all times using nonlinear ultimate boundedness theory. Then, the closed-loop stability of a single-phase inverter operating in island mode is proven for the first time using both a resistive and a constant power load. The desired controller performance is experimentally validated on a testbed consisting of a single-phase inverter connected to a linear (resistive) and a nonlinear (diode rectifier) load, where the ability of the proposed controller to operate in the droop control mode while maintaining the desired current limitation is proven under various load changes