Electrical Damping Assessment and Stability Considerations for a Highly Electrified Liquefied Natural Gas Plant

Abstract

In recent years, the Oil & Gas industry has been subjected to a progressive electrification process aiming to comply with global environmental requirements on CO2 emissions reduction. High-power electric motors fed by Variable Frequency Drives (VFDs) have replaced gas turbines as drivers for gas compression applications. In Liquefied Natural Gas (LNG) plants, unexpected downturns could be experienced in case of high torsional vibrations of power generations units. These torsional vibrations derive from the interaction among turbine-generator (TG) units and VFDs and are known as Sub-Synchronous Torsional Interactions (SSTIs). SSTIs can lead to instability when the overall electromechanical system lacks sufficient damping. In this scenario, electrical damping assessment is fundamental in order to ensure stability and reliable operation of an LNG plant. Negative electrical damping is strictly related to the negative incremental resistance behavior of the power converters and it is influenced by the converter’s control system. In this paper, a real case study based on Thyristor Variable Frequency Drives (TVFDs) is considered. Ad hoc dynamic models of the power converters and of the TG unit are developed and combined in order to provide an accurate estimation of the electrical damping. It is demonstrated that the electrical damping is affected by variations of the main control system parameters and how the use of a simplified model instead of an ad hoc model can impact the stability evaluation.

Electrical Damping Assessment and Sensitivity Analysis of a Liquefied Natural Gas Plant: Experimental Validation

“…Besides the impact of the Power System Stabilizer (PSS) on the SSR damping has been previously demonstrated in [11][12][13][14]. Aware of these considerations and starting from the combined electromechanical model developed in [1], the same authors present in this paper a more accurate model providing the electrical damping assessment. Differently from the model presented in [1], both the control systems of the VFD and of the TG unit are included in the overall model.…”

“…Up-to-date Liquefied Natural Gas (LNG) plants are highly electrified and based on high power electrical motors supplied by Variable Frequency Drives (VFDs). The interaction among the VFDs and the Turbine-Generator (TG) units may cause torsional vibrations known as Sub-Synchronous Torsional Interactions (SSTIs) [1][2][3]. Over the last years some SSTI phenomena have been experienced on site in the LNG plants by the authors.…”

“…In [1] a connection between the power electronics analysis and the power systems analysis is established and a complete model of a LNG plant is provided combining the dynamic model of the power conversion stage and the TG electromechanical model. The sole theoretical model allows electrical damping assessment in the case of a basic LNG plant configuration with one TG unit and one VFD.…”

“…The sole theoretical model allows electrical damping assessment in the case of a basic LNG plant configuration with one TG unit and one VFD. However the analysis provided in [1] lacks a deep investigation about the impact of the synchronization system design on the electrical damping estimation. In the same model the design of the TG controller is not considered.…”

“…Aware of these considerations and starting from the combined electromechanical model developed in [1], the same authors present in this paper a more accurate model providing the electrical damping assessment. Differently from the model presented in [1], both the control systems of the VFD and of the TG unit are included in the overall model. It allows one to detect which control parameters have more impact on the generator units' electrical damping.…”