Transient Stability Analysis of Islanded AC Microgrids with a Significant Share of Virtual Synchronous Generators

Chen, Yuan; Xie, Peilin; Yang, Dan; Xiao, Xiangning

doi:10.3390/en11010044

Cited by 21 publications

(15 citation statements)

References 18 publications

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“…They are normally connected to the main grid via one point only [1,2], and they are using their own (intelligent) complex approach due to its dependency on frequency of the output power, which is an additional degree of freedom in the system that requires high level analysis. In [26], an outstanding analysis in the same VSG dependent environment, swinging from Synchronous Generator (SG) to inverter-dominant environment, reaches the conclusion that there is "an unstable region for inverter-dominated islanded microgrid" and that "the identified unstable region provides an important basis for the parameter tuning of VSG operating in an island microgrid". The paper emphasizes the complexity of having mixed SG and IbER or VSG-driven IbER.…”

Section: Introductionmentioning

confidence: 99%

“…Thus, when the microgrid operates in an isolated mode, only a minor mechanical inertia is available. The most popular countermeasure is the use of virtual synchronous generator (VSG) control techniques implemented into inverter-based energy resources (IbER) in frequency controlled microgrids [25][26][27], while participation to the power balancing mechanisms is also supported through frequency-based droop control of a selected part of the distributed generation.This effort of simulating rotational inertia behavior with the inverters is pursued as well because scholars consider that this mechanism is still needed when rotational machines and inverters coexist. Thus, scholars approaching the hybrid configurations of rotational machines and inverter-based energy resources consider implementing the latest, so called Virtual Synchronous Generator/Machine (VSG/VSM) functionality.…”

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On the Electrostatic Inertia in Microgrids with Inverter-Based Generation Only—An Analysis on Dynamic Stability

et al. 2019

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Microgrids are about to change the architecture and the operation principles of the future power systems towards smartness and resiliency. Power electronics technologies are key enablers for novel solutions. In this paper we analyze the benefits of a "microgrid by design" architecture (MDA), using a solid-state transformer (SST) as a low-voltage grid-former and inverter-based generation only. In this context, the microgrid stability is maintained with the help of "electrostatic energy inertia" that can be provided by the capacitor connected to the DC busbar behind the SST inverter topology. This happens in a natural way, alike the mechanical inertia in power systems with synchronous machines, however without depending on frequency and without the need of a rotational inertia. This type of microgrid always operates (both fully connected to the main grid or in islanding mode) with all the necessary mechanisms needed to maintain the microgrid stable-no matter of the perturbations in the upstream of the point of common coupling (PCC). In the case of microgrids with inverter-based generation only (including the energy storage systems), there is no mechanical inertia and different stability mechanisms need to be applied compared to the stability principle of the classical power systems. Our proposed mechanism differentiates from the recently proposed stability assessments of microgrids based on virtual synchronous generators from the control theory perspective. This paper is a continuation of our previous work where the MDA was first introduced. The use-cases and scenarios are based on realistic and yet reasonable complexities, by coupling the disturbance magnitude with the voltage stability limit in power grids. The paper finds meaningful disturbances to test the electrostatic energy inertia at the boundaries of grid stability, as guidance to understand the range of voltage variation for extreme conditions. The results show that in microgrids with inverter-based generation only and passive loads (RLC type) the operation is no longer frequency dependent. The energy of the DC busbar capacitor as electrostatic energy inertia of the MDA has a role similar to that of the rotational machines in classical grids in terms of maintaining dynamic stability, however impacting two different types of stability.Energies 2019, 12, 3274 2 of 23 operation and balancing management system [3]. One of the first attempts in defining the microgrid concept was done by the Consortium for Electric Reliability Technology Solutions program (CERTS) in their report from October 2003 [4,5]. Here, a microgrid was defined as a group of micro-generators and storage-units able to separate and isolate itself from the main grid with little or no disruption to connected loads. The distributed energy resources (DER) that play the role of the generation system in the microgrid are often interfaced through power electronic inverters, forming an inverter-based microgrid [6].Inverter-interfaced energy resources behave completely different compared to synchrono...

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On the Electrostatic Inertia in Microgrids with Inverter-Based Generation Only—An Analysis on Dynamic Stability

et al. 2019

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“…The load frequency control (LFC) scheme that represents the short-term behavior of frequency and power of the SCHPS is depicted in Figure 1b. That scheme is derived from the swing equation of a synchronous generator (1) [19]. In this equation, that considers small deviations of power and rotor speed, the inertial characteristics of the synchronous generators are described by the equivalent constant of inertia, H T .…”

Section: Introductionmentioning

confidence: 99%

Proposals for Enhancing Frequency Control in Weak and Isolated Power Systems: Application to the Wind-Diesel Power System of San Cristobal Island-Ecuador

Ochoa

Martínez

2018

Energies

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Wind-diesel hybridization has been emerging as common practice for electricity generation in many isolated power systems due to its reliability and its contribution in mitigating environmental issues. However, the weakness of these kind of power systems (due to their small inertia) makes the frequency regulation difficult, particularly under high wind conditions, since part of the synchronous generation has to be set offline for ensuring a suitable tracking of the power demand. This reduces the power system's ability to absorb wind power variations, leading to pronounced grid frequency fluctuations under normal operating conditions. This paper proposes some corrective actions aimed at enhancing the frequency control capability in weak and isolated power systems: a procedure for evaluating the system stability margin intended for readjusting the diesel-generator control gains, a new wind power curtailment strategy, and an inertial control algorithm implemented in the wind turbines. These proposals are tested in the San Cristobal (Galapagos Islands-Ecuador) hybrid wind-diesel power system, in which many power outages caused by frequency relays tripping were reported during the windiest season. The proposals benefits have been tested in a simulation environment by considering actual operating conditions based on measurement data recorded at the island.

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“…However, these methods lead to more complex information needed for decision making by microgrid control system. Hou [26], Zhang [27], and Chang [28] proposed improved virtual synchronous generator control and neural network adaptive control algorithms, which can simultaneously improve the efficiency and stability. In summary, many papers have suggested improvements for conventional droop control, but at the same time have influenced microgrid power quality, or introduced communication mechanism which cause stability problems.…”

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A Reactive Power-Voltage Control Strategy of an AC Microgrid Based on Adaptive Virtual Impedance

et al. 2019

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As an effective carrier of distributed generation, a microgrid is an effective way to ensure that distributed power can be reasonably utilized. However, due to the property of line impedance and other factors in a microgrid, reactive power supplied by distributed generation units cannot be shared rationally. To efficiently improve reactive power sharing, this paper proposes a reactive power-voltage control strategy based on adaptive virtual impedance. This method changes the voltage reference value by adding an adaptive term based on the traditional virtual impedance. Meanwhile, a voltage recovery mechanism was used to compensate the decline of distributed generation (DG) output voltage in the process. MATLAB/Simulink simulations and experimental results show that the proposed controller can effectively improve the steady state performance of the active and reactive power sharing. Finally, the feasibility and effectiveness of the proposed control strategy were verified.Energies 2019, 12, 3057 2 of 15 this problem, various schemes based on conventional droop control have been proposed by domestic and overseas researchers [9][10][11][12][13][14][15][16][17]. Tuladhar et al. [9] proposed to overlay the harmonic signal onto the reference fundamental voltage, which can change the magnitude of fundamental voltage according to the harmonic power generated by DG units. However, this proposal results in output voltage distortion and power quality reduction, and the harmonic wave is enhanced by the inductive circuit of the microgrid. Zhang et al.[10] utilized feeder parameters. Proper power-sharing is achieved through modifying the reactive droop parameter. However, line resistance values are difficult to obtain accurately. The global variable of bus voltage and the integral element is introduced in Sao [11] and Zhong [12], loads can be accurately shared in a steady state, but bus voltage measurement requires communication over long distances. De Brabandere [13] proposed that a decoupling matrix is established according to the resistance and inductive feeder parameters, controlling active and reactive power respectively. However, feeder parameter errors are prone to appear, which affect power-sharing. Li [14] and Ma [15] proposed a virtual impedance equivalent method to a simultaneous increase of the output impedance of each DG unit, which is introduced to reduce circular current. In Wei [16] and Yu [17], system stability is enhanced while circular current is reduced by creating a virtual resistance or impedance. However, the virtual control methods mentioned above intensify line voltage drop and lower output voltage of the DG unit; therefore, the power quality cannot be guaranteed.Another method of solving the problem is to combine communicational and non-communicational droop control methods together. He and Li [18] adopted the disturbance thought, utilized reactive information caused by feeder parameter differences, and used active disturbance for reactive droop control through simultaneous communication to reach reaso...

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Transient Stability Analysis of Islanded AC Microgrids with a Significant Share of Virtual Synchronous Generators

Cited by 21 publications

References 18 publications

On the Electrostatic Inertia in Microgrids with Inverter-Based Generation Only—An Analysis on Dynamic Stability

On the Electrostatic Inertia in Microgrids with Inverter-Based Generation Only—An Analysis on Dynamic Stability

Proposals for Enhancing Frequency Control in Weak and Isolated Power Systems: Application to the Wind-Diesel Power System of San Cristobal Island-Ecuador

A Reactive Power-Voltage Control Strategy of an AC Microgrid Based on Adaptive Virtual Impedance

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