In microgrid planning, topological design is a critical concern for ensuring certain features such as high reliability in islanded operation. This paper proposes a graph partitioning and integer programming integrated methodology for the optimal loop-based microgrid topology planning while considering the distributed energy resources in the microgrid. The proposed methodology is applied to a microgrid test system and the planning results are discussed. The results demonstrate that the proposed planning methodology is able to accurately and efficiently determine an optimal loop structure for microgrids, and exhibit the potentials for applying the proposed planning methodology in practical microgrid applications.

Optimal Planning of Loop-Based Microgrid Topology

Challenging wind power projects can be confronted by very low short circuit ratio (SCR <; 4) and sometimes even extremely low ratio (SCR <; 2). Wind power plants (WPP) feeding in their power by means of voltage source converters (VSC) have the potential to be operated in those high impedance grids to a certain extent if they are appropriately controlled. The physical boundaries are well known and referred to as voltage collapse and loss of angle stability (generic: loss of stability, LOS). Measurements and simulations indicate the potential of advanced wind turbines (WT) based on VSC to approach the ultimate physical LOS limits very effectively. This could provide additional opportunities for wind power projects at sites, which may so far not be utilized without grid reinforcement.

Control of wind power plants utilizing voltage source converter in high impedance grids

The ongoing development of renewable energy and microgrid technologies has gradually transformed the conventional energy infrastructure and upgraded it into a modernized system with more distributed generation and localized energy storage options. Compared with power grids utilizing synchronous generation, inverter-based networks cannot physically provide large amounts of inertia, which means that more advanced and extensive studies regarding stability considerations are required for such systems. Therefore, appropriate analytical methods are needed for the voltage stability analysis of renewable-dominated power systems, which incorporate a large number of inverters and distributed energy sources. This paper provides a comprehensive literature review of voltage stability analyses of power systems with high levels of renewable energy penetration. A series of generalized evaluation schemes and improvement methods relating to the voltage stability of power systems integrated with various distributed energy resources are discussed. The existing voltage stability analysis methods and corresponding simulation verification models for microgrids are also reviewed in a systematic manner. The traditional and improved voltage stability analysis methods are reviewed according to the microgrid operation mode, the types of distributed generators, and the microgrid configurations. Moreover, the voltage stability indices, which play a crucial role in voltage stability assessments, are critically evaluated in terms of the applicable conditions. The associated modeling and simulation techniques are also presented and discussed. This contribution presents guidelines for voltage stability analysis and instability mitigation methods for modern renewable-rich power systems.

https://www.mdpi.com/2673-4826/3/1/6/pdf?version=1645272873

Analytical Methods of Voltage Stability in Renewable Dominated Power Systems: A Review

The unfolding-type flyback inverter operating in Discontinuous Conduction Mode (DCM) has been previously investigated as a low cost solution for a photovoltaic AC module application. The aim of the present work is to improve the efficiency of the scheme by adopting Continuous Conduction Mode (CCM) operation. The paper presents the control modeling and analysis of the flyback-CCM inverter and discusses its control challenges. These include 1) the presence of a right-half plane (RHP) zero in the direct output current control transfer function, 2) the inevitable DCM operation when the instantaneous AC voltage is low, and 3) issues associated with the large variation in the power level. An indirect average current control scheme to avoid the problem of the RHP zero has been proposed and its effectiveness verified experimentally. The results show an improvement of 7.4% in the weighted California efficiency over the benchmark flyback-DCM scheme for a 200W PV panel. The total harmonics distortion and output power factor satisfy IEC61727. Practical implementation problems faced in the sensing of current are also addressed.

A low cost high efficiency inverter for photovoltaic AC module application

The real-time transient stability detection and emergency control technology based on wide area response has become a hot research area in power system stability studies. Several different technologies have been proposed, but lots of problems remain to be solved before they can be applied in practice. A wide area measurement system (WAMS) based test platform is developed for transient stability detection and control. The design as well as main function modules of the platform are introduced. In addition, three generator power angle prediction methods and six response based transient instability detection technologies are given. Results of engineering application demonstrate that the developed test platform can provide a real-time operation environment, which can effectively compare and analyze the validity and practicability of these transient stability detection technologies. Based on the measured perturbed trajectories from actual power systems or the Real-Time Digital Simulators (RTDS), the platform can realize the assessment and visual result presentation of various responses from different transient instability detection technologies. The test platform can be applied to different power systems and it is convenient to embed new transient instability detection modules. Meanwhile some deficiencies and shortcomings in engineering application are pointed out and corresponding suggestions are given. In conclusion, the hardware and software structure, function modulus and engineering applications are presented. The application in actual power systems shows that it has a good application perspective.

/pdf/development-of-a-wams-based-test-platform-for-power-system-jp5mfh5nrm.pdf

Development of a WAMS based test platform for power system real time transient stability detection and control

In this paper, a software package of finite element (FE) analysis for induction machines is presented. The back-EMF in windings is expressed by magnetic vector potential, which is used to couple with transient finite element analysis. The equations of drive circuit and rotor bars are described by loop circuit equations. The pre-processing and post-processing of this package are developed based on AutoCAD. The FE model and some machine parameters can be input by friendly graphical user interfaces. The calculated magnetic field distribution and transient results can be shown in AutoCAD or graphical interfaces.

Power system islanding based on multilevel reduced graph partitioning algorithm

The instability detection method of a power system by directly analyzing the transient response of the electric power system is not only useful for stopping the time domain simulation of system trajectory to reduce CPU time for preventive control program, but also can be used to trigger transient stability emergency control and system splitting based on wide area measurement system Reliability, speediness and computational simplicity are of primary importance in developing online instability detection method With regard to these requirements, a novel method for instability detection based on generator angle and angular velocities is proposed in this paper System instability or out-of-step condition is detected by the characteristics concave or convex of the critical machine trajectory of the post-fault system The generalized transient energy function method is applied to analysis reliability and robustness of the proposed method The proposed method was successfully tested on several different power systems varying in size from Single-Machine-Infinite-Bus system and IEEE 50 generator systems The method is shown to be a potential tool for initiating online emergency control (7 pages)

Study on power system transient instability detection based on wide area measurement system

The re-closing time of a transmission line has a distinct influence on power system transient stability. There is an optimal re-closing time (ORT) for all transmission line faults, no matter the instantaneous fault or permanent one. Based on the transient energy function (TEF) theory with classical models considered, the ORT can be calculated accurately. This paper mainly focuses attention on the validity and practicability of ORT obtained from this method. After analyzing the characteristic and influencing factors of the ORT, the setting strategy of re-closing time for transmission line is presented. And it will conduct simulations with a complex real system of China North Grid. Simulation studies show that this technique can significantly increase the security margin and damp the oscillation of the power system. It is indicated that the variation of ORT of a certain transmission line is insensitive to the influencing factors: the fault location, fault clearing time and pre-fault power flow. Finally, the setting scheme is drawn to optimize the re-closing time of the key transmission lines in the network under current technological conditions.

Optimizing the re-closing time to improve the transmission capacity of power system

In this paper, a method is proposed for fast and accurate online detection of transient instability based on real-time measurement. The cluster of critical machines is determined and refreshed at each time. Determination of the irrevocable loss of synchronism is made based on the geometric characteristic of critical machines trajectory. Unlike the previous extended equal-area criterion method, it is unnecessary to calculate the unstable equilibrium point. The speed of algorithm is fast enough to prevent system from transient instability. Simulations are carried out on the IEEE 50 generator test system and a real power system. The results indicate that the proposed method has the features of reliability and effectiveness in identifying the loss of synchronism with inter-area mode.

Power system transient instability detection algorithm based on real time measurement

Self-adaptive system islanding refers to the separation of an interconnected power system into electrically isolated islands automatically based on the identification of unstable mode. A fast searching algorithm for self-adaptive power system islanding, which is called multilevel reduced graph partitioning (MLRGP) algorithm, is presented in this paper. The aim of this algorithm is that the generation load imbalance in each island is as small as possible. The algorithm computes a reasonable islanding strategy of power system G=(V, E) in O(|E|) time. A C++ program is developed based on the algorithm. The verification of the islanding program is proven with simulations on a practical 212-bus power system. It costs only about 20 ms to get a reasonable islanding strategy on an ordinary PC, which satisfies the speed demand of self-adaptive system islanding.

A Novel Fast Searching Algorithm for Power System Self-Adaptive Islanding

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