PF control application in a cascade control scheme. The control applications would communicate among each other to provide integrated control and protection of the entire train.
Advanced GT closed-loop to power turbine speed (NPT) minimum governor After passing all start permissives, a two-shaft GT enters the startup sequence where it passes through two phases: Open-loop control: during the initial phase of a startup, the fuel demand is calculated using open-loop control techniques. This phase typically consists of ignition, flameproof, fuel control valve (FCV) warmup and FCV ramp sub-phases. Closed-loop control: after the open-loop control phase, the GT applies closed-loop control to ramp up high-pressure rotor speed (NHP) and then NPT. This phase
Figure 1. Train in shutdown.
Figure 2. Train start.
May 2021 48 HYDROCARBON ENGINEERING
depends on the startup goal and typically consists of NHP warmup, NHP acceleration, NHP loading, NPT warmup, NPT control, and NPT loading sub-phases. The startup sequence quickly ramps the local SP through NHP and NPT pre-configured critical zones. This minimises the turbine’s operating time at these critical speeds. For a two-shaft GT, normally either NPT or NHP loop is selected as the main control loop PV, thus determining which process variable will be controlled when no limiting conditions exist. That loop then calculates a proportional integral derivative (PID) bidirectional response to deviations of its PV from a local or remote SP. The remaining loops operate as limiting control loops that protect the GT against excessive speeds such as NPT, pressures such as compressor discharge pressure (CDP), temperatures such as exhaust gas temperature (EGT), flameout, critical speed operation, and axial compressor surge. If only one limiting loop is active, its PID response is selected. If multiple limits are exceeded, the GT controller selects the high limiting loop with the lowest or most-negative proportional plus derivative response and adds the lowest or most-negative differential integral from any active loop. For safe operation, limiting loops have more priority than the main PV control loop.
EGT limiting loop The EGT is the temperature at the GG (gas generator) discharge. In multi-shaft applications, EGT is also called T4 and the power turbine exhaust temperature is referred to as T6. While the combustion chamber temperature (T3) imposes a more direct limitation on the turbine’s power output than EGT does, the latter is frequently the only one of the two that is measured. The combustion temperature is then indirectly limited
