This invention relates to a control system and a method for increasing the operational flexibility of a steam turbine for variations in ambient temperature and/or condenser cooling capability. These variations impact the steam turbine by changing the exhaust or backpressure of the system.
Steam turbines accept high pressure, high temperature steam which expands through stationary and moving rows of nozzles and blades (“buckets”) to convert heat energy into mechanical (rotational) energy. Combining a steam turbine with an electrical generator allows electric energy to be produced. FIG. 1 illustrates a representative steam turbine power plant. As shown in FIG. 1, a steam turbine T drives an electrical generator G through the turning of a rotor R on which blades or buckets B are mounted. The turbine is typically comprised of a series of stages S1-Sn with stage Sn being the last stage of the turbine. Steam flow to the turbine is through a control valve V and steam is directed at the buckets through nozzles or diaphragms N. A coolant (air or circulating water) is provided through a condenser C. Those skilled in the art know it is common practice to include both a gas turbine with a steam turbine to create a combined unit. Such a configuration has very high efficiency because steam for the steam turbine is generated from the thermal energy in the gas turbine exhaust.
Steam turbines are either condensing or non-condensing. For condensing steam turbines, a recommended exhaust pressure is established by the design of the last stages of the turbine, and the ability of a condenser C to accept exhaust heat energy. The cooling capability of the condenser can be a limiting factor if it results in the system being unable to achieve maximum steam expansion in turbine T. Such a limitation is particularly acute on hot days (for air condensers), or for periods when there is insufficient cooling water (for water-cooled condensers). Usually these are the same times when electric power demand is greatest, and the selling price of electricity the highest, so the limitations are most pronounced during these times. Further, limited cooling capacity results in higher backpressures which may force generating plants to reduce their electricity output until backpressure levels return to within acceptable limits.
Plant operators are often challenged to operate at higher than recommended backpressures during peak demand times because of the power generation demands. However, sustained operation at higher than recommended backpressures will result in blade response that significantly increases the probability of a high cycle fatigue failure due to aeromechanical instabilities. Even short-term operation at higher than normal backpressures can result in irreversible, cumulative blade fatigue that may necessitate taking the turbine out of service for repair. A typical backpressure range for a steam turbine power plant is approximately 1.0 to 3.0 inches Hg when using a water-cooled condenser C. For installations with air condensers, this range increases to 3.0 to 5.5 inches Hg. For a nearly constant steam flow, the backpressure can increase to twice these levels on days when limited cooling or high ambient temperatures are experienced.
Since operations at higher backpressures results in a steam turbine performing outside its design capabilities, a feedback system is provided within turbine T to prevent its operation in unsafe conditions. The feedback comprises a combination of alarm indications and trip set points which result in taking turbine T “off line”. Specifically, as exhaust or backpressure increases, the incidence angle of steam flow significantly deviates from an optimum angle. This produces flow separation within the turbine which results in high bucket excitation, vibratory response, and potential bucket failure. Currently, trip set points are based upon static backpressure measured in accordance with established general rules of operation. The protective features prevent aeromechanic instabilities such as blade stall, flutter, and buffeting.
Control systems for steam turbines currently use fixed set points on backpressure to protect from aeromechanic instabilities. The recommended set points are based upon turbine class, blade design, and blade size. A drawback of such control systems is that set points for alarm and tripping are based solely upon static pressure levels; and as such, tend to be conservative. Turbine exhaust pressure limitations minimize the possibility of failures at high blade or bucket cycles which are induced by flow disturbance or aeromechanical instabilities. FIG. 2 provides a schematic of a representative control system S. Here, set points are commonly a function of exhaust flow velocity and typical values vary from 4 to 10 inches of Hg, vacuum. An alarm is initiated as these limits are approached to warn an operator to take appropriate action to lower the backpressure. The operator can, for example, reduce the load or increase condenser cooling. If unabated, however, further increases in backpressure (by between 1 to 3 inches Hg) results in the protective system tripping the steam turbine and taking it off line. FIG. 3 illustrates a commonly applied control schedule.
There is currently a need for steam turbines that can operate over a wide range of backpressures, particularly, at high backpressures. The problem is to increase allowable operating range for a condensing-type steam turbine through an improved means of providing backpressure protection. It is also important to provide aeromechanical protection to the steam turbine blades without unduly restricting the turbine's operational capabilities.