Stability analysis of a grid-connected VSC controlled by SPC

In the near future a large part of traditional generation based on conventional synchronous machines (SM) will be replaced by renewable generation based on voltage source converters (VSC). In this sense, power system operators have begun to demand VSC-based power plants be able to participate in the frequency and voltage regulation, and are also interested in services like inertia emulation and damping of power oscillation, functions that today are carried out by large synchronous generators. Therefore, several studies have suggested new ways to control voltage source converters, that try to emulate the behavior of synchronous generators and are known generically as Virtual Synchronous Machines. The synchronous power controller (SPC) is a flexible solution that emulates the classical swing equation of a synchronous machine and improves its response. The SPC inherits the advantages of conventional synchronous generators, while it fixes many of its drawbacks. In this work, a sensitivity analysis of a VSC connected to the grid and controlled by SPC is performed. In this sense, a non-linear mathematical model of the system is first developed. This non-linear model is then linearized, obtaining a linear model from which the eigenvalues and sensitivities of the system to some relevant parameters are calculated. Finally, time-domain simulations are performed to confirm the results of the sensitivity analysis.Postprint (author's final draft

Coherency of synchronous generators

The linearized model of a synchronous generator, known in the literature as the Heffron-Phillips model, is extended to apply to a general power system with an arbitrary number of generators, and which takes into account network resistances. In this model a group of constants, called the M coefficients are developed which relate the torque, E(,q)(\u27\u27), and terminal voltage of each machine to the rotor angles and E(,q)(\u27\u27)\u27s of the various machines. In this investigation these constants are used to determine and analyze the coherency of synchronous generators;Using the M coefficients and generator parameters the system A matrix is formed. The coupling between the system inertial frequencies and the exciter frequencies is investigated using modal analysis. By partitioning the A matrix, two subsets, called the inertial matrix and the exciter matrix, are formed. The modal frequencies obtained from the partitioned matrices compare quite favorably with those obtained from the complete A matrix. This suggests decoupling the system\u27s inertial and exciter frequencies into independent groups;The Ml coefficients, a subset of the M coefficients, are contained in the inertial matrix. These coefficients are used to develop a technique for establishing inertial coherency that does not require time solutions or eigenvectors. It determines the tendency of coherent machines to swing together with a minimum amount of computation;Dynamic equivalents, formed when two generators are inertially coherent but with exciter modes and mode shapes that show no indication of coherency, are dealt with. A method of eliminating the proper exciter mode for the formation of an exciter equivalent is developed using the constraints of coherency. This provided a criterion by which other methods of exciter reduction can be judged. This modal reduction technique is advantageously incorporated into the more conventional exciter reduction method;The techniques developed in this research are applied to a 4-machine test system. The M coefficients and A matrices are calculated for a number of different cases. Inertial coherency is established and the improved method of exciter reduction is shown to be effective

“Grid”-Less Power Systems: A Vision for Future Structure of Power Networks

This paper proposes a new paradigm in the structure of power systems to facilitate the large scale move to renewables-based distributed generation necessary to help decarbonize the current electricity networks. Since the design of the incumbent power system topologies is to control large synchronous generators, critical control metrics degrade as the penetration of converter-based units increases. Specifically, the reduction in short circuit level, phase angle movement, and rate of change of frequency limit the wider adoption of converter-based units. This paper proposes structural changes and control that inherently solve such critical performance issues through physically decoupling all synchronous generators from the network. A set of back-to-back AC/DC/AC converters controlled by a universal virtual synchronous machine-based control algorithm, introduced in the paper, allows the repurposing of existing plant to enable the integration of more converter-based units. Despite being physically disconnected, this new structure/control still benefits from inertial capacities of synchronous generators to suppress the oscillations caused by disturbances. Moreover, the method enables further exploitation of synchronous generators as energy storage mechanisms. PSCAD/EMTDC simulations demonstrate the advantages of the proposed structure and control system in different normal and abnormal scenarios

Power quality improvement by pre-computed modulated field current for synchronous generators

Although power quality aspects of electrical machines have been extensively studied and investigated for a large number of years, room for improvement still exists in the field of classic, wound-field, synchronous generators. This paper proposes an innovative method of power quality improvement for single-phase synchronous generators in which the usual DC field current is replaced by a calculated current waveform. The optimised field current waveform is designed in such a way that harmonics created by the machine geometry and the winding configuration are significantly reduced

Improving Virtual Synchronous Generator Control in Microgrids using Fuzzy Logic Control

Virtual synchronous generators (VSG) are designed to mimic the inertia and damping characteristics of synchronous generators (SG), which can improve the frequency response of a microgrid. Unlike synchronous generators whose inertia and damping are restricted by the physical characteristics of the SG, VSG parameters can be more flexibly controlled to adapt to different disturbances. This paper therefore proposes a fuzzy logic controller designed to adaptively set the parameters of the VSG during a frequency event to ensure an improved frequency nadir and rate of change of frequency (ROCOF) response. The proposed control method is implemented and tested on the power inverter for the battery energy storage system of the Banshee Microgrid Feeder 2 test case system using MATLAB/SIMULINK. The effectiveness of the adaptive control scheme is validated by comparing its performance with a constant parameter VSG, a virtual inertia only fuzzy controller, and an inertial-less inverter control

Nonlinear Analysis of an Improved Swing Equation

In this paper, we investigate the properties of an improved swing equation model for synchronous generators. This model is derived by omitting the main simplifying assumption of the conventional swing equation, and requires a novel analysis for the stability and frequency regulation. We consider two scenarios. First we study the case that a synchronous generator is connected to a constant load. Second, we inspect the case of the single machine connected to an infinite bus. Simulations verify the results

A Study of the Impact of Reduced Inertia in Power Systems

Inertia in power systems plays an important role in maintaining the stability and reliability of the system by counteracting changes in frequency. However, the traditional sources of synchronous generation are being displaced by renewable resources, which often have no inherent inertia. This paper investigates the impact of reduced system inertia on several aspects of the dynamic stability of power systems, such as angular stability, primary frequency response, and oscillatory modes. This study is performed on a large-scale 2000 bus synthetic Texas model by selectively replacing synchronous generators with inverter-based generation resources. This paper also compares the analysis results obtained by the above-mentioned inertia-reduction approach of renewable integration with another approach in which the inertia constant of all synchronous generators is decreased. This paper demonstrates that only reducing the inertia of all synchronous generators in a system does not provide an accurate analysis of the challenges associated with the reduced system inertia caused by renewable integration