Analysis of dc-network stability of VSC-based HVDC grids
Doktorsavhandling, 2016

This thesis presents a modelling approach for the investigation of dc-network stability in HVDC grids. It consists on dividing the system under analysis into subsystems such that their individual impact on the stability of the overall system can be studied. This principle is first applied to a point-to-point HVDC system and then to a multi-terminal HVDC system. Preliminary studies in a point-to-point HVDC system show that, under specific conditions, dc-network instabilities can occur. These are dependent on the dc-network resonance, the power flow direction and the Voltage Source Converter (VSC) control parameters. To further investigate these instabilities, the system is divided into the dc-network and the VSC subsystems. The VSC-subsystem can be interpreted as a frequency-dependent admittance whose conductance is positive or negative depending mainly on the power direction. The main characteristic of the dc-network subsystem is its resonance. Thus, if the dc-network resonance coincides with a negative conductance, there is a risk that the resonance becomes amplified, or unstable. Moreover, it has been found that decreasing the VSC ac-system strength and increasing the VSC control system delays turns the VSC conductance more negative, consequently, increasing the risk of instability. The dc-network subsystem is also studied, and it is found that high resonance peaks increase the risk of instability. A stability criterion is proposed, based on limiting the magnitude of the VSC subsystem transfer function to less than the inverse of the resonance peak around the resonance frequency. The modelling approach is also applied to a multi-terminal HVDC system, where VSC subsystems are defined together with a dc-network subsystem. The main characteristic of the dc-network subsystem is that different resonances are localized in specific terminals.For example, it has been found in a four-terminal HVDC system that one resonance can take place in terminals 1 and 2, while another resonance, with a different frequency, can take place in terminals 3 and 4. This means that, for the first resonance, the VSCs connected to terminals 1 and 2 have the strongest impact on the system stability, and analogously for the second resonance. Moreover, the resonance peak at each terminal determines the level of influence of each VSC meaning that the VSC connected to the terminal with the highest resonance peak will have the greatest impact on the system stability. On the other hand, the VSC-subsystems are, in principle, similar as the ones defined for point-to-point HVDC systems, i.e. they can be interpreted as admittances whose conductances can be negative in certain conditions. Thus, instability occurs when resonances coincides with negative conductances. Finally, simulations and tests in a real time digital simulator show the validity of the theoretical findings of this thesis.

power dependent admittance

resonance

HVDC

passivity

Multi-terminal HVDC system

VSC

dc-network stability

Hörsalsvägen 11, 412 96, Göteborg
Opponent: Assoc. Prof. Oriol Gomis-Bellmunt, Department of Electrical Engineering, Technical University of Catalonia

Författare

Gustavo Pinares

Chalmers, Energi och miljö, Elkraftteknik

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Definition of a Voltage-Source Converter dc-side admittance and its impact on dc-network stability

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Analysis of the dc Dynamics of VSC-HVDC Systems Using a Frequency Domain Approach

IEEE PES Asia-Pacific Power and Energy Engineering Conference 2013, 8-11, December 2013, Hong Kong, China,; (2013)

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Analysis and mitigation of instabilities originated from dc-side resonances in VSC-HVDC systems

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Paper i proceeding

Modeling and analysis of VSC-based HVDC systems for dc network stability studies

IEEE Transactions on Power Delivery,; Vol. 31(2016)p. 848 - 856

Artikel i vetenskaplig tidskrift

Styrkeområden

Energi

Ämneskategorier

Annan elektroteknik och elektronik

ISBN

978-91-7597-439-2

Doktorsavhandlingar vid Chalmers tekniska högskola. Ny serie: 4120

Hörsalsvägen 11, 412 96, Göteborg

Opponent: Assoc. Prof. Oriol Gomis-Bellmunt, Department of Electrical Engineering, Technical University of Catalonia