[[abstract]]© 1995 Institute of Electrical and Electronics Engineers - Stability analysis programs are a primary tool used by power system planning and operating engineers to predict the response of the system to various disturbances. Important conclusions and decisions are made based on the results of stability studies. This paper presents a theoretical foundation of direct methods for both network-reduction and network-preserving power system models. In addition to an overview, new results are offered. A systematic procedure of constructing energy functions for both network-reduction and network-preserving power system models is proposed. An advanced method, called the BCU method, of computing the controlling unstable equilibrium point is presented along with its theoretical foundation. Numerical solution algorithms capable of supporting online applications of direct methods are provided. Practical demonstrations of using direct methods and the BCU method for online transient stability assessments on two power systems are described. Further possible improvements, enhancements and other applications of direct methods are outlined[[department]]電機工程學

Microgrid is widely accepted as an effective mean of integrating various distributed energy resources (DERs) through their interface converters to provide electric power of high quality and reliability. These distributed resources interface converters can operate in an autonomous fashion without any communication for enhanced system reliability and reduced complexity. Conventionally, the real power-frequency droop control and the reactive power-voltage magnitude droop are adopted as the decentralized control strategies in these DERs interface converters for the autonomous power sharing operations. However, the reactive power sharing of $Q\hbox{--}V$ droop control often deteriorates due to its dependence on the line impedances. In this paper, a $Q\hbox{--}\dot{V}$ droop control method with $\dot{V}$ restoration mechanism is proposed to improve reactive power sharing. Its operation principle and control method are explained and analyzed. Simulation and experimental results are presented to validate the effectiveness of the proposed method.

As the penetration of renewable energy sources is increasing in the AC micro-grid, the stability of the closed-loop system has raised a major concern since conventional distributed interface converters (DICs) used in the AC micro-grid do not have a rotating mass, and hence low inertia. High penetration of DIC-based micro-grid may result in poor frequency and voltage response during large disturbance. In order to overcome this difficulty, the virtual synchronous generator (VSGs) was proposed recently in which the DIC mimics conventional synchronous generators (SGs) by designing proper parameters of the SG into each local droop control mechanism of the DIC. Meanwhile, due to the recent advances of distributed control, the concept of consensus-based control can be applied to study this droop control problem of VSGs. One important feature of this consensus-based control is that it can be implemented on each local DIC with communications among their neighboring DICs. In contrast to most existing secondary control schemes, no central controller is required. Under this framework, if DICs are redesigned as VSGs, the frequency and voltage of each DIC can be restored to their pre-specified values obtained from the steady-state analysis. In addition, the proper real and reactive power sharing still can be achieved according to the nominal rating of each DIC. The stability of the closed-loop system is ensured by the transient energy function under certain mild conditions. Numerical experiments of a 14-bus/6-DIC micro-grid system on real-time simulators are performed to validate the effectiveness of the proposed control mechanism.

The task of autonomous power sharing in a micro-grid is usually achieved by decentralized droop control on individual interface power converters, which may suffer from the dependence on output line impedances. Inaccurate reactive power sharing will occur under strongly non-uniform line impedances. Due to inherently distributed and heterogeneous nature of the micro-grid, it becomes an ideal platform for applications of consensus control algorithms. In this paper, the consensus-based droop control with sparse communication network is proposed to overcome the drawback of existing droop control methods. In particular, when line impedances of the power grid are either lossy with the uniform R/X ratio or even pure resistive, the consensus droop control is still an effective method for autonomous real and reactive power sharing. In addition, closed-loop system stability of the proposed consensus-based droop control method can be ensured by the energy function approach under certain conditions. Real-time simulations of two micro-grid systems are studied to validate the feasibility of the proposed consensus-based droop control method.

In this paper, a novel transformer-less adjustable voltage quadrupler dc-dc converter with high-voltage transfer gain and reduced semiconductor voltage stress is proposed. The proposed topology utilizes input-parallel output-series configuration for providing a much higher voltage gain without adopting an extreme large duty cycle. The proposed converter cannot only achieve high step-up voltage gain with reduced component count but also reduce the voltage stress of both active switches and diodes. This will allow one to choose lower voltage rating MOSFETs and diodes to reduce both switching and conduction losses. In addition, due to the charge balance of the blocking capacitor, the converter features automatic uniform current sharing characteristic of the two interleaved phases for voltage boosting mode without adding extra circuitry or complex control methods. The operation principle and steady analysis as well as a comparison with other recent existing high step-up topologies are presented. Finally, some simulation and experimental results are also presented to demonstrate the effectiveness of the proposed converter.

This paper provides a survey of studying complex network theory for modern smart grid applications. A brief overview of complex network theory will be explored first. Topological characteristics, statistic characteristics, such as self-organized criticality and critically slow down, and dynamical characteristics, including synchronizations, consensus control, and pinning control, will be briefly addressed. Then, we will illustrate how complex network theory can be applied to modern smart grids in structural vulnerability assessment, cascading blackouts, grid synchronization, network reconfigurations, distributed droop control, pinning control for micro-grid autonomous operations, and effective grid expansions. Some emerging topics and future perspectives are also addressed.