Abstract
The power system is going through a change in its very foundations. More and more power converters are being integrated into the electric grid to interface renewable energy resources and in high-voltage direct-current (HVDC) transmission systems. This article presents a discussion on the stability of power systems when HVDC transmission systems based on modular multilevel converters (MMC) are connected in grid-forming (GFM) mode to the legacy power system using concepts of energy functions and Lyapunov stability theory and considering aspects of the interoperability between GFM converter technologies. As a base for the stability analysis, we review the main GFM converter technologies (droop and virtual synchronous machine), highlighting their differences. Then, we present a model using the center-of-inertia formulation for a multi-machine/multi-GFM converter power system representing a close future scenario of power systems where GFM converters might adopt different technologies. To illustrate the theoretical Lyapunov-based stability analysis, simulations performed in Matlab/Simulink showed the behavior of a 12-bus test system during a frequency disturbance that originated from the sudden connection of a load. To reflect the interoperability of different GFM technologies and the power system, scenarios with one single GFM technology and a scenario with mixed technologies were investigated. For the test system considered, the frequency response with fewer oscillations and a higher frequency nadir was obtained when all GFM converters were operated as VSMs that have a higher inertial response contribution.
Highlights
To face the pressing climate crisis and to counteract its effects, all sectors responsible for the emission of greenhouse gases must make significant changes
This system was conceived upon a modification of the IEEE 9 bus test system by exchanging one synchronous generator by an modular multilevel converters (MMC)-high-voltage direct-current (HVDC) and adding an extra bus with power generation from an HVDC system
The MMC-HVDCs considered in this work were modeled as MMC converters connected to an ideal DC voltage source
Summary
To face the pressing climate crisis and to counteract its effects, all sectors responsible for the emission of greenhouse gases must make significant changes. Most VSC-based systems commissioned around the world are controlled according to a grid-following (GFL) strategy This operation strategy consists in synchronizing the voltage output of the converter with the frequency, amplitude, and phase angle of the grid voltage [16,17]. A simple GFL control strategy to inject active and reactive power to the main grid was required for the grid-connected operation mode for power converters where a PLL structure was used to guarantee that the converter is synchronised with the main power network In this case, the frequency and voltage output of the converters are governed by the main network, which can be represented most of the time as an infinite bus (from the point of view of the converters). Robust control techniques are required to maintain a suitable operation, regarding the complexity of the power system [39,40]
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