Gas Turbines' Sensitivity to Critical Speeds

581 WordsFeb 21, 20182 Pages
Gas turbines are very sensitive to critical speeds affecting their low pressure blades. These critical speeds may be close to the rated operating speed leaving a small margin on the allowed frequency range before reaching a protective Changeover. Typically a protective instantaneous low-speed Changeover on gas turbines may be set at 96% of the nominal system frequency. Furthermore, system generation and stability are at risk as the frequency drops. This is specially the case for a thermal generation plant where power output mostly depends on motor-driven auxiliary loads, such as boiler feed water pumps, coal pulverizing, and draft fans. The drop in system frequency instigates a rapid fall of power output to the auxiliary loads, causing further reduction of the energy input to the turbine generator. This sequence of events further deteriorates the system frequency endangering the entire plant stability. To halt the drop in frequency, it is necessary to intentionally, and automatically disconnect a portion of the load equal to or greater than the generation deficiency in order to achieve balanced power economics while maintaining system stability. Automated load shedding systems are necessary for industrial power systems since sudden disturbances can plunge a system into a hazardous state much faster than an operator can react. These automated schemes must be designed and implemented to possess in-depth knowledge of system operating parameters and must rely on time sensitive

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