Control Parameters

Prime movers can be controlled in a variety of ways, depending on the requirements of the specific application. Control options can be categorized into one of the following general areas:

• Speed control

• Prime mover parameters

• Drive equipment parameters

• Process parameters

Regardless of the application, speed and/or load control are required. Speed control is the primary control loop for most prime movers. Speed control is used, for example, in mechanical drive applications where output power depends directly on running speed.

In most electric generation applications, however, it is desirable to control the prime mover for load rather than speed. When the generator is feeding electricity into a grid system, the prime mover rotational speed is locked to the frequency of the grid once its generator has been synchronized to the grid and the breaker is closed. However, the load (kW) can be controlled by raising or lowering the speed reference to vary the energy input to the prime mover. With speed held constant, a change in energy input will produce a corresponding change in output power.

In generator applications, the governor system may include an automatic synchronizer to adjust prime mover speed to match bus frequency and adjust generator phase to match bus phase and provide voltage matching. Import/export controls allow the governor to regulate the flow of power between the utility and the connected system and to limit output so that it does not exceed the generator rating.

Driven equipment and process control parameters can include pressure, flow, temperature, level, or speed. Gas turbines frequently are controlled on the basis of exhaust temperature. A second example is the control of a reciprocating engine- or turbine-driven compressor pumping gas through a pipeline, where pipeline pressure may be the controlling parameter.

When based on driven equipment or process parameters, prime mover control may be accomplished in one of two ways:

1. Adjusting the actuator output signal can control parameters that are directly related to prime mover rotational speed. Typically, this would be limited to process parameters such as pressure or flow. Figure 13-7 illustrates this process for a steam turbine. As shown, the process control and the speed control functions both reside within the prime mover control. Each receives an input signal, compares it with the reference set point, and generates a corrective signal. The two controllers then compete for control of the actuator through a low-signal selector (LSS). The lowest corrective signal is selected and used to position the energy-input valve.

2. A remote speed set point can be used to control the prime mover. As shown in Figure 13-8, a process Fig. 13-8 Remote Process Control.

With centrifugal compressors, the advantage of variable speed operation is not as significant. Although they can operate at variable speed, the turn-down range is limited because a minimum mass flow is required to prevent surge or back-flow in the compressor. In such cases, it may be advantageous to maintain full-load design speed or to use speed control over a more limited operating range.

For large steam turbine systems, where most or all of the steam generation is dedicated to the turbine, both turbine and boiler must be controlled by the same system. Since the boiler cannot respond rapidly, the steam turbine valves modulate for initial load response at constant throttle pressure. The control system then acts to restore the proper rate of fuel flow to the boiler.

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