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2. Plant Dynamics Code Development 2.1. Compressor Surge Control One of the goals for current year for S-CO2 cycle control strategy development was an introduction of compressors surge protection systems for the two compressors with corresponding equipment and control logic. Usually [6], the surge control system consists of the compressor bypass (recirculation) line with an integrated cooler. When an approach to surge is detected by the compressor control system, the bypass line valve is opened such that part of the flow is recirculated from the high-pressure compressor outlet to the low-pressure compressor inlet. This partial flow recirculation causes the compressor outlet pressure to decrease and the compressor inlet pressure to increase resulting in lower pressure ratio across the compressor. Low pressure ratio will result in increased flow rate through the compressor such that the compressor operating point moves away from low-flow high-pressure-ratio surge conditions. Since the flow recirculation brings hotter flow from the compressor outlet to the colder compressor inlet, an additional cooler is installed in the recirculation line to avoid otherwise uncontrolled temperature increase in the compressor. It is noted that the initiation of the compressor surge protection system starts with a detection of the approach to surge. Thus, a reliable surge prediction criterion is required. Several criteria have been proposed [7] for centrifugal compressors, such as conditions where the pressure ratio-versus-flow rate curve reaches its maximum point or some empirical blade stall criteria. However, there are no commonly accepted surge criteria mostly due to the lack of experiment data, since experiment verification of surge requires operation of a compressor under surge conditions which would damage a compressor. For these reasons, a combination of several surge conditions is used in the Plant Dynamics Code. The surge flow is defined as a maximum of flow rates at which either pressure curve reaches its maximum or a blade stall criterion is satisfied. It is unknown how applicable such empirical correlations are to the supercritical conditions. The results obtained so far (reported previously in [1] and further in this report) show that unstable compressor operation, similar to that expected under surge conditions, is sometimes calculated even at flow rates above the surge limit based on the above mentioned criteria. An additional effort has therefore been made to investigate the cause of the instability before a reliable surge control system can be developed for S-CO2 cycle compressors. 2.1.1. Stability Issues During the simulation of different transients with the ANL Plant Dynamics Code, unexpected cycle behavior was sometimes calculated. An example of such unstable behavior is demonstrated in Figure 1 during a load reduction at 3 %/min rate from 100 % to 0 % load with subsequent load increase back to 100 %. At about 3500 seconds, some parameters start to oscillate leading to a very small time step defined by the code 8PDF Image | Supercritical Carbon Dioxide Cycle Control Analysis
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