Taming the Shrink-Swell Dragon

An Elusive Steam Generator Control Problem Is Solved at Maanshan II

By Greg Shinskey

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This scheme was added as shown to the 1E system of Maanshan Unit II. Although published information refers to the WR level input as "feedforward,"2 actually it is not" it is a proportional feedback input, because changing the FRV position changes WR level. In essence, this scheme combines proportional control of WR level with PI control of NR level.

With WR gain K set at zero, a 15-min. period of NR level loop oscillation was observed on Unit II, with a proportional-band setting of 60% and an integral time of 8 minutes. These settings were approximately optimum, as the step load response of the loop was a symmetrical (bell-shaped) curve followed by a small overshoot and satisfactory damping, as shown in Figure 2. These features are characteristic of a response curve having a minimum Integrated Absolute Error (IAE).

The load change was simulated from a steady state at zero deviation by placing the LC in Manual, stepping its output by an acceptable increment, and then transferring immediately back to Auto. This placed the controller output apart from the steady plant load by that increment, requiring it to integrate back to its last steady-state value, which matched the load. An increment in controller output in one direction is equivalent to an increment of the same size in load in the other direction.

Take careful note though. To test a liquid-level loop by stepping its set point is a mistake. First, most level loops operate at constant set point all the time, so set-point response is meaningless. Secondly, the steady-state output of a level controller matches the load and is not a function of its set point. Following a step in set point, the LC output will return to its previous steady-state value. This does not require the controller to integrate, as a load change does, and produces a response curve having an integrated error of zero, and hence an area overshoot of 100%. By contrast, the overshoot resulting from a load change is determined by the integral setting of the controller.

Read Also: PID World Tour--The Final Performance

From the load change simulation came the conclusion that the WR gain K should be set to 5, because that was the amplitude ratio of the NR to WR cycles observed in Unit I. This had an immediate stabilizing effect on NR level. As a result of this success, K was then increased to 8, 10 and finally 12, where it remained. This reduced the loop oscillation period by a factor of two, and allowed a commensurate 4-min. reduction in the integral time of the NR controller. This was the fastest and most stable 1E response ever observed at the plant. The NR proportional band was left unchanged at 60%.

Differential-pressure Control

Feedwater flow is a product of FRV opening and the square-root of its pressure drop dp. The pressure drop in this plant is regulated by manipulating the speed of the feedwater pumps. This loop is especially important to 1E control, because any variation in valve dp changes feedwater flow, requiring the NR LC to operate the valve by integrating the resulting level deviation. Therefore the dp controller should be tightly tuned. At Maanshan II its proportional band was reduced to 50%, where a lightly-damped oscillation broke out; it was then increased to 75% and left there.

The dp loop gain was expected to change with the number of pumps n being manipulated by the controller, and so the proportional band of the controller was multiplied by n. However, this left the loop overdamped with three pumps running, indicating this was overly cautious. The expected interaction between the dp controller and the three parallel feedwater flow controllers, however, did not create any problems.

The set point of the dp controller at Unit II was programmed to be proportional to plant power generation. This conserves pump horsepower, which varies directly with the product of feedwater flow and FRV dp. A properly executed program keeps the steady-state opening of the FRV approximately constant with generated power.

Figure 1 shows a neutron flux signal used as a feedforward input to the 1E control system. This was implemented in Unit II, along with the dp set-point program. During a rampdown from 14 to 7 power, the NR level was driven down about 12%, where it stayed until the ramp was ended. A constant deviation during a load ramp is common to feedback loops without feedforward (Figure 3), because the controller must integrate a constant deviation in order to ramp its output. However, without feedforward, the level should have risen during the rampdown, as more water was being delivered to the SG than the falling load required. So the low level during the ramp indicated that too much feedforward was actually being applied.

The gain K of the flux feedforward input should be set at m/, where m is the output of the NR level controller. Accordingly, the LC output at the initial and final steady-states were compared, and found to be the same! The reason for this is the programming of dp with power"all of the load change was transferred from the FRV to the dp loop. In other words, the reduction in feedwater flow required to match the plant load was achieved by reducing the dp across the valve, instead of closing it. As a result, the feedforward input from neutron flux actually duplicated what the dp set-point program had accomplished.

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