Steam Turbine Trip Mitigation
University of Cincinnati, Cincinnati, Ohio
The University of Cincinnati, Central Utilities Plant operates a 21-megawatt steam turbine that provides power to the university campus and affiliated hospital facilities. The steam turbine was experiencing frequent “phantom” trips. These nuisance shutdowns resulted in unnecessary demand charges from the local electric provider at significant cost to the university. ENTRUST Solutions Group was contracted to perform a thorough review of the steam turbine trip logic, make recommendations to improve the trip reliability, and implement the recommended PLC logic changes.
The steam turbine trip logic’s detailed review revealed 93 conditions could initiate a steam turbine trip, with 73 initiated by high turbine or generator temperature, faulty RTDs, or high vibration indication. A single point of failure initiated almost all trips with no time delay.
The PLC trip logic was modified to utilize the existing instrumentation to add redundant detection of trip conditions with a short time delay for a steam turbine trip to be initiated. The trip logic was modified to require that both RTDs indicate high temperature to initiate a steam turbine trip where duplex RTDs existed. The trip logic using simplex RTDs was modified to include RTD signal failure detection as a condition to initiate a steam turbine trip. The faulty RTD trip conditions were changed to alarm only conditions. The vibration trip logic was modified to require that both the “X” and “Y” vibration probes needed to indicate high vibration to initiate a steam turbine trip.
By optimizing the steam turbine trip logic using redundant measurements, the total number of conditions that could initiate a steam turbine trip was reduced from 93 to 42 while still providing the same trip protection level. The steam turbine has now operated 60+ days without any phantom trips occurring. The system’s improved reliability significantly reduces the unnecessary utility demand charges and has resulted in substantial cost savings to the university.