Figure 5
The APC-based unit control concept for main steam pressure control Figure 5 can be differentiated from the conventional unit control approach in several important ways:
- The APC is able to fully stabilize the non-self-stable loop. Therefore, it is not necessary to use the steam turbine to help stabilize steam pressure. The stabilization structure, which modifies the turbine load set point as a function of the pressure control deviation, can be omitted. As a result, the electrical load will follow its set point with a very high accuracy.
- The APC can be used as a full-pressure controller without encountering any stability issues. Dynamic tracking of the pressure set point in case of load changes or frequency disturbances is not necessary. For this reason, determination of the corresponding model parameters is no longer required. In comparison to the conventional control concept, the commissioning time for an APC-based controller is reduced drastically.
- There is no need to perform a dynamic feed-forward calculation for the required overtiring. Only a static feed-forward calculation of the boiler load set point as a function of the steady-state unit load is performed. The required overtiring is determined automatically by the APC, which uses the pressure control deviation as an input value. Eliminating the testing required to determine these model parameters results in much lower commissioning and testing costs.
- Only a boiler time constant and steam storage time constant are required by the APC. The APC is very robust in terms of these parameters so that a highly accurate measurement is less important.
APC Control Delivers Tighter, Faster Performance
In Figure 6, the control performance during steady state operation can be seen. Compare this level of performance with that shown in Figure 3 and it’s obvious that the main steam pressure is as stable as it was before, and the pressure control deviations are quite similar. However, with the APC there was no modification of the turbine set point as a function of the pressure control deviation, the electrical load followed its set point with a very high accuracy, and the corresponding control deviation was practically zero. From a load control standpoint, control performance was enhanced compared to the former control structure.
Figure 7 shows the control deviation for a load increase at a rate of approx. 4.5% per minute. Again, there was virtually no control deviation for the electrical load. Control performance, which was already very good before Figure 4, was further improved.
The APC not only stabilized the main steam pressure but also handled the pressure increase experienced because the unit was operated in sliding pressure mode, as the higher load set point requires a higher main steam pressure. Comparing the test data with that in Figure 4 shows that the pressure increase was performed slower than before. However, this was not a question of the different control concepts used but a question of how quickly the commissioning engineer wanted the pressure set point to increase. When the APC structure was commissioned, it was decided to perform pressure changes slowly to allow smooth boiler operation.
Figure 8 illustrates how the unit responded when the load set point was increased and then reduced. The load change rate was approximately 5% per minute. The APC was able to quickly stabilize pressure, and the load followed its set point with a very high accuracy.
The APC not only stabilized the main steam pressure but also handled the pressure increase experienced because the unit was operated in sliding pressure mode, as the higher load set point requires a higher main steam pressure. Comparing the test data with that in Figure 4 shows that the pressure increase was performed slower than before. However, this was not a question of the different control concepts used but a question of how quickly the commissioning engineer wanted the pressure set point to increase. When the APC structure was commissioned, it was decided to perform pressure changes slowly to allow smooth boiler operation.
Figure 8 illustrates how the unit responded when the load set point was increased and then reduced. The load change rate was approximately 5% per minute. The APC was able to quickly stabilize pressure, and the load followed its set point with a very high accuracy.
Figure 6
Figure 7
Figure 8
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BIG THANKS TO MR/ Dipl-Ing. Bernd Lamb is a project engineer in the Process Optimization group of Siemens Energy’s Fossil Power Generation Division
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