Knowledge of compressor system behaviour during a trip is essential to obtain reliable operation. Experience at a gas treatment plant has shown that the export compressors (43 MW) may enter the unstable area of the performance characteristics and suffer from surge and rotating stall in situations with voltage drops in the electricity grid.

A detailed dynamic simulation model has been established to analyse the impact of different mechanical, process and control tuning aspects. The electric motor driven compressor, the protection valves and piping have been modelled and dynamic analyses performed to reveal the transient response.

The main challenges are related to the compressor system/process design and to the compressor and variable speed drive (VSD) response during power dip or compressor trip. Based on experiences detailed sensitivity analyses have been performed, covering the impact of: compressor discharge volumes, performance characteristics — head rise to surge, dimensions of the recycle piping as well as the sizing, characteristics and trip signal delay of the recycle protection valve(s).

This paper reports experience from analyses of the export compressor and driver behaviour during the first 3 seconds of an electric power/voltage dip. It is partly based on earlier tests and dynamic simulations performed for the facility, and partly on new technology challenges and opportunities. The main objective has been to study the compressor system sensitivity related to variation in available driver power during power dip and its impact on the transient rundown trajectory and recovery phase.

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