Load rejection

Momentary overvoltages due to a sudden load rejection, which may overspeed the generator and develop higher voltages or... 

Load rejection The load side intenupter. feeding a large load at the far end, trips... 

More than two overvoltages occurring at the same instant such as a load rejection associated wdth ground and phase faults. 

Limiting the system voltage swing during a load rejection or off-peak periods, and protect it from overvoltages... 

SaturiUioii of power transformers as a result of periodic overloading and load rejections. 

While the former would stress the generator windings, the latter may cause a voltage swing during a load rejection or load fluctuation and result in a line outage. These features, if not controlled, may render the system unstable. Overvoltages must therefore be controlled within an acceptable limit. Table 24..3 prescribes one such limit. 

A study of various systems has revealed that the load angle for an uncompensated line should be maintained at about 30° only. This means that an uncompensated line may be loaded to just nearly half its steady-state level to retain a high level of stability during load fluctuations, particularly during light loads or load rejections, switching of large inductive loads or any type of minor or major line fault. 

Adding shunt capacitors would also reduce Zq but would raise the electrical line length hence it is not considered. Moreover, on EHVs, the charging shunt capacitances, Cq, as such require compensation during light loads or load rejections to limit the voltage rise (regulation) at the far end or the midpoint. Hence no additional shunt compensation is recommended. 

I Shunt reactors These are provided as shown in Figure 24.23 to compensate for the distributed lumped capacitances, C , on EHV networks and also to limit temporary overvoltages caused during a load rejection, followed by a ground fault or a phase fault within the prescribed steady-state voltage limits, as noted in Table 24.3. They ab.sorb reactive power to offset the charging power demand of EHV lines (Table 24.2, column 9). The selection of a reactor can be made on the basis of the duty it has to perform and the compensation required. Some of the different types of reactors and their characteristics are described in Chapter 27. 

Note Similarly, load lines can be drawn on a load rejection, or the generator or the line outages. 

The job of most control loops in a chemical process is one of regulation or load rejection, i.e., holding the controlled variable at its setpoint in the face of load... 

Cascade control was discussed quahtatively in Sec, 8.2. There is a secondary (or "slave ) loop and a primary (or master ) loop. Both load rejection and performance can sometimes be improved by using cascade control. 

Unfortunately much of Ais interaction analysis work has clouded the issue of how to design an effective control system for a multivariable process. In most process control applications the problem is not setpoint responses but load responses. We want a sy stem that holds the process at the desired values in the face of load disturbances. Interaction is therefore not necessarily bad, and in fact in some systems it helps in rejecting the effects of load disturbances. Niederlinski [AIChE J 1971, Vol 17, p. 1261) showed in an early paper that the use of decouplers made the load rejection worse. 

As pointed out earlier, the problem with pairing on the basis of avoiding interaction is that interaction is not necessarily a bad thing. Therefore, the use of the RGA to decide how to pair variables is not an effective tool for process control applications. Likewise the use of the RGA to decide what control structure (choice of manipulated and controlled variables) is best is not effective. What is important is the ability of the control system to keep the process at setpoint in the face of load disturbances. Thus, load rejection is the most important criterion on which to make the decision of what variables to pair, and what controller stmcture is best. 

Usually the INA is a very conservative measure of stability. Compensators are found by trial and error to reshape the INA plots so that the circles are small and the system is diagonally dominant. The INA method strives for the elimination of interaction among the loops and therefore has limited usefulness in process control where load rejection is the most important question. 

Other choices of decouplers are also possible. However, since decoupling may degrade the load rejection capability of the system, the use of decouplers is not recommended except in those cases where setpoint changes are the major disturbances. 

In most chemical processes the principal control problem is load rejection. We want a control system that can keep the controlled variables at or near their setpoints in the face of load disturbances. Thus the closedloop regulator transfer function is the most important. 

Before an efficient control system can be designed it is necessary to consider how all sections of the control loop will behave under the influence of variations in load (i.e. what are the load rejection properties of the system ) and/or its set point (i.e. what are the set point following characteristics ). This requires experimental investigation, or a time-dependent mathematical analysis (i.e. in the unsteady state), or both. Although each section of the loop will necessitate a separate analysis, there... 

FIGURE 2.7 Voltage swell due to step load rejection. The nominal 480-V generator bus experienced a rise to 541 V that lasted for approximately 18 cycles. 

EXAMPLE 4.1. Consider the two blending systems shown in Fig. 4.8. The flow rate or composition of stream 1 is the disturbance. The flow rate of stream 2 is the manipulated variable. In Fig. 4.8a the sensor is located after the tank, and therefore the dynamic lag of the tank is included in the feedback control loop. In Fig. 4.8h the sensor is located at the inlet of the tank. The process lag is now very small since the tank is not inside the loop. The control performance in part h, in terms of speed of response and load rejection, would be better than the performance in part a. In addition, the tank now acts as a filter to average out any fluctuations in composition. ... 

Cascade control was discussed qualitatively in Section 4.2. It employs two control loops the secondary (or slave ) loop receives its setpoint from the primary (or master ) loop. Cascade control is used to improve load rejection and performance by decreasing closedloop time constants. 

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