Modeling of Compressor Characterisics and Active Surge Control
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In this thesis, the compressor characteristics, being representations of the compressor pressure ratio as a function of the gas ﬂow through the compressor, have been studied. Three diﬀerent types of representations of the compressor characteristics are presented, implemented and tested with respect to simulation friendliness and eﬀectiveness. These are based on a physical model, a 4th-order polynomial approximation method and a table lookup method. In addition, two diﬀerent types of active surge controllers have been critically reviewed, i.e. the Close Coupled Valve (CCV) and the Drive Torque Actuation, and subsequently implemented and tested in SIMULINK. Based on the tests carried out on the compressor characteristics it is concluded that the 4th-order polynomial approximation method works best in an online environment. On the other hand, the table lookup method provides better representation of the actual data, but the method is somewhat slower compared to the other one. The usefulness of the physical model is limited, but together with parameter identiﬁcation its applicability can be extended. Moreover, the active surge controllers have proved to be mathematically stable and shown to perform adequately. However, both face a problem with regard to measurement delay. Based on simulations and other considerations, the indications are that the drive torque actuation is the most promising solution for active surge control and should thus be the focus for further investigations. A possible solution for the measurement delay problem is to use a state observer. As a part of the thesis, two state observers have been implemented and tested, but with limited success.