How does a cascade control loop improve control stability in a biomass power plant?

A cascade control loop improves control stability in a biomass power plant by addressing disturbances closer to their source and reducing the lag time associated with traditional single-loop control. It uses two control loops: a primary (outer) loop and a secondary (inner) loop. The primary loop controls the main process variable (e.g., steam temperature or pressure). The output of the primary controller becomes the setpoint for the secondary controller. The secondary loop controls a process variable that directly affects the primary variable and is susceptible to disturbances (e.g., fuel feed rate or air flow rate). For example, consider controlling steam temperature in a superheater. The primary loop would measure the steam temperature and adjust the setpoint for the secondary loop, which controls the fuel feed rate to the boiler. If a disturbance occurs, such as a change in fuel quality or heating value, the secondary loop detects and corrects for it before it significantly impacts the steam temperature. The secondary loop quickly adjusts the fuel feed rate to maintain the desired heat input. This reduces the impact of the disturbance on the steam temperature, improving the stability of the primary loop. The key advantage of cascade control is its ability to quickly respond to disturbances and reduce their propagation through the system. By controlling the intermediate process variable (fuel feed rate), the secondary loop effectively isolates the primary loop from the effects of the disturbance, resulting in more stable and precise control of the steam temperature.