Taming the Shrink-Swell Dragon

An Elusive Steam Generator Control Problem Is Solved at Maanshan II

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To eliminate the duplication, K was set to zero. It can be seen that the two responses in Figure 3 are mirror images of each other. Therefore, the proper amount of feedforward achieved by the set-point program of the dp controller, left the level constant. Because the LC output is the same before and after the ramp, integrated error with K  set to zero is also zero.

Nonlinear Filters

To reduce the negative effect of shrink and swell accompanying every load change, nonlinear filters were applied to the deviation of both the 1E and 3E-level controllers. They consist of three zones shown in Figure 1: the center zone has a width of ±z and a gain of kz, and the outside zones have a gain of 1.0. For the level controllers at Maanshan II, z = ±3.0% and kz = 0.4. Minor upsets produced level variations of less than ±3 %t, where kz diminishes the effects of these variations on feedwater flow, allowing the feedforward signals to do their work. Any major upset that drives the level deviation beyond 3% promotes a greater driving force to return the level within the low-gain zone where damping is heavier.

Figure 4: When the controller was left in manual too long, an offset cycle developed.

However, the proportional band of the level controllers must be tuned for stable response to a larger upset. Originally, the 3E LC was set for 60% proportional band, which was stable for deviations within the low-gain zone. Eventually, a larger upset caused the deviation to exceed 3% on both sides of set point, promoting an expanding triangular cycle. This behavior is shown in Figure 4. Increasing the proportional band to 100% dampened the cycle and eliminated any recurrence.

At one point, stability of the 1E loop was tested by placing the LC in Manual and stepping its output upward. However, the LC was left in Manual until the level exceeded the low-gain zone, and then returned to Auto. The higher gain of the level controller in this zone quickly turned the level back toward set point, but started a proportional cycle above set point. Integral action brought the cycle progressively back toward set point, and after the low-gain zone was re-entered, it dampened. This response is compared to a simple step load change in Figure 5. It did not require a re-tuning of the level controller. As a result of the WR proportional loop's help, the proportional band of the 1E LC was left at 60%, compared to the 100% setting required for the 3E LC.

Conclusion

Feedwater control must be both stable and responsive across the entire plant load. This is particularly problematic at loads below the range of steam and feedwater flowmeters, where single-element level control must be used. With the assistance of proportional feedback from a WR level measurement, the effect of inverse response was mitigated and the single-element loop response time was reduced by a factor of two.

Controlling differential-pressure across the feedwater valves by manipulating pump speed prevented unmeasured load changes from upsetting level, and programming the dp set point, as a function of power, eliminated that source of the disturbance. Interaction between the dp and feedwater flow loops under three-element level control did not surface as a problem.

Nonlinear filters reduced the effects of shrink and swell upon the level loops, but complicated tuning of the level controllers. Their proportional-band settings must be tested for stability following upsets which drive the level deviation out of the low-gain zone on both sides.

References

1. Shinskey, F. G., Process Control Systems, 4th ed., McGraw-Hill, 1996, New York, pp. 88-90.
2. "Westinghouse Plant Digital Feedwater Control System," Sec. 4.3, Feedwater I&C Maintenance Guide, Electric Power Research Institute, November 1995.
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