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1 February 2007

Letting Off Steam

Researchers present a modification of a conventional feed water three-element control strategy to solve problems of drum level control

By Miguel A Delgadillo and María A. Hernández

Research studies show there is interest in solving the instability problem of level control in power plant steam generators by designing drum level phenomena models to establish a control law for a feed water control system. However, a model-based control strategy may be difficult for control engineers to understand, especially those with skills acquired in field experiences with proportional plus integral plus derivative controller, or PID, loops but with no mastering in modern control strategies. Now there is a modified conventional feed water three-element control strategy that is easy to understand and solves some problematic situations of drum level control.

The new strategy can lead to better performance, especially at full load (generated electric energy) with the supplementary fire (duct burners or after burners) in operation where the feed water control valve is near its full opening. Such a strategy avoids, as much as possible, the saturation of the control signal to the valve, eliminating the permanent oscillation of the controlled variable (drum level). The control system behavior also shows good stability from start-up to full load, including the transition from one-to-three elements operating mode and vice versa, as well as the duct burners entry. It shows similar stability during the stop of the heat recovery steam generator (HRSG) from full load with duct fire in operation to the HRSG out of service.

Due to the software blocks communication system of the maker, a transition band of the steam flow signal as transition signal assures a three-to-one or one-to-three element bump less transfer operation. Laboratory tests simulated the proposed control strategy with a simplified dynamic process model before putting the control system into operation on site. The tests will show real process runs and control-tune parameters.

The case of Dos Bocas Veracruz

The feed water system of the HRSG of the combined cycle power plant at Dos Bocas Veracruz included a conventional three-element feed water control system operating with electric analog components. A vertical steam drum makes the level control with less capacitance (volume per unit of level), and then the level dynamics are more sensitive than that of the horizontal drums.

Modernizing the control equipment by substituting the initially installed analog equipment with a digital one made it necessary to translate the control strategy into a new digital environment. The available tools in the digital medium allowed us to program a more complicated strategy to improve the control performance. However, we need now to deal with time execution of fieldbus elements that introduce dead time due to time consumption and its synchronization time of each element.

Controlling drum level

The problem of controlling the drum level involves the operating conditions of the drum and the deaerator from start-up to full load. Since there is another HRSG connected by a main header, the influences on each other add an additional element to account for the control strategy and tuning.

Due to the rangeability problem in pressure drop, measurement elements in the feed water, and steam flows, we traditionally designed the control strategy to operate at the start-up of the HRSG in the mode of one element, that is, with the drum level signal as a feedback control signal on a single input/single output loop.

In order to assure reliable flow signals, we chose the transition point from one-to-three element mode in a gap of 10-16% of steam range measurement of the steam flow, in this case between 50,000 to 80,000 pounds per hour (pph).

A critical operative situation appears when we reach full load and the feed water control valve is working near full opening; frequently, this introduces an oscillatory behavior in all control variables of master and slave controllers. This is because of the limited capacity of the feed water valve to control situations where disturbances need to respond beyond the valve capacity.

Another problem that easily appears is when natural frequency of the master and slave controllers is similar. Then oscillations behavior is present in all control variables.

New control strategy

We must program implantation in modern commercial control equipment. This gives good stability of the control system performance from start-up to full load, including the duct burner start-up and burner trip. It includes smooth manual-to-auto and auto-to-manual mode transfer. This is the same for one-to-three and three-to-one element transfer mode at any operating point. The feed water control system must live in harmony with the deareator level and pressure control loops. The control system must stand the start-up and shutdown of the other HRSG connected by a steam header.

To accomplish these objectives, we added to the conventional three-element control strategy a variable proportional gain as function of the level error in the master controller and a variable proportional gain as function of the steam flow in the slave controller.

The variable proportional gain in the master controller provides good stability when the level is near its set point, giving a low gain. As the absolute level error increases, the proportional gain also increases until it reaches a limited value.

We proposed the variable proportional gain of the slave controller as a function of steam flow, and it remains constant until the steam flow reaches 380,000 pph. From this value, the proportional gain increases linearly until the steam flow reaches 390,000 pph. Beyond this steam flow value, the proportional gain is constant. The reason to increase this gain at loads near the maximum load generation is to give a stronger control signal response to the feed water valve. This avoids, as much as possible, the saturation of the control valve position, and thus avoids the oscillatory behavior in all control variables of the feed water system.

In order to have a friendly operator interface, we designed a control strategy so the master controller faceplate was the only place to perform manual/auto and auto/manual transfer operation. This way the operator would manipulate the control valve from this faceplate without having to see the slave controller or operate it at any time. Then the slave controller is always in auto, or it is tracking to the master controller output signal for a good return to automatic mode of the master controller.

Due to the programming tool, it takes time to execute dialog between programming blocks in the control equipment. To transfer from one-to-three element operation mode, we proposed to do it within a transfer band in order to assure this transition dialog has finished and the transition is complete without fear of a reverse action in the steam flow interrupting the transfer operation.

Then, below 50,000 pph of steam, one-element mode is always in operation. For steam flow above 80,000 pph, we assure the three-element control mode. However when the steam flow is going upward, one element mode is in operation until the steam flow exceeds 80,000 pph. When the steam flow is going downward, it maintains the three-element node until the steam flow reaches 50,000 pph.

We introduced another security action with a programming logic to preclude 1/3 or 3/1 element transfer operation interruption by deactivating the auto to manual, and vice versa, transfer operation.

Process simulation

We designed a simplified model to test the control strategy before implanting it as a final version on site running with the real process. We developed the model as a program running inside the control equipment and connected it internally with the control strategy. 

Since the load and steam flow are closely related, we used the main steam throttle valve opening to simulate load changes and to perform tests along the entire load range. Once the steam flow reaches 80,000 pph, the control strategy transfers automatically to the three-element feed water flow without disturbance. While steam flow is ascending, the control system is able to keep all related drum variables under acceptable control margin.

When the system reaches full load and the control system output signal to the feed water valve falls into a cycling behavior with saturation of valve opening (the valve opening reaching its maximum value, 100% ), the proportional gain of the slave controller characterizes with the steam flow. So when this flow is near to full load, any change in process conditions may require a valve opening beyond the valve capacity. Therefore, it reaches a 100% opening and introduces a cycling behavior to the control system. This consequently increases the proportional gain within a steam-flow range, thus reacting a little stronger when the steam flow is near to the corresponding full load with the after burners (duct burners) at maximum heat flow.

Former strategy results

Before we implanted the variable gain strategy, we put in operation a digital conventional three-element strategy. With the performance near the full load, instabilities could force the operator to transfer the controller to manual mode when the drum level falls near 20 inches and the other control variables are in oscillatory behavior.

Using process modeling and simulation test in the laboratory let us mend errors before putting the control strategy in operation on site. Moreover, these tests let us have a set of initial tuning parameters to put the control equipment in operation with the real process.

The availability of powerful digital equipment let us program more complicated control strategies. And the variable proportional gain as a function of the error in this job proved to be useful to improve the performance of conventional feed water three-element control strategy.

Another advantage of variable gains is to adjust tuning parameters as a function of the operating point as it was applied to the slave controller where variable proportional gain is a function of the steam flow with satisfactory results improving the performance at full load.

About the Authors

Miguel A Delgadillo and María A. Hernández are researchers at Gerencia de Control e Instrumentación Instituto de Investigaciones Eléctricas Cuernavaca, Mor.