Reactive controls

The Smith dead-time compensator is designed to aUow the controUer to be tuned as tightly as it would be if there were no dead time, without the concern for cycling and stabUity. Therefore, the controUer can exert more reactive control. The dead-time compensator utilizes a two-part model of the process, ie, Gp, which models the portion of the process without dead time, and exp — sTp,pj ), which models the dead time. As seen from Figure 18b, the feedback signal is composed of the sum of the model (without dead time) and the error in the overaU model Gpj exp — sTppj )), ie, C —. Using... 

Optimizing power transfer through reactive control 24/792... 

In LT systems reactive control is provided to improve the load p.f. and hence its load-carrying capacity, as discussed in Chapter 23. This is achieved by offsetting the inductive content of the load ctirrcnt at the receiving or the consumer end by the use of shunt capacitors and hence support the system by reducing line losses and improving its active load current (/ cos 0) carrying capacity. 

In the following we consider the case of i transmission line, 132 kV and above, being more typical and complex for the purpose of reactive control. Based on this, it would be easier to apply appropriate reactive control to a distribution network and large inductive loads such as an arc or induction furnace. 

This product can be reduced by reducing X, using series capacitance with a reactance X, which will reduce to - Xq. This is where reactive control plays a major role. By meticulous reactive power manage-ment, the Ferranti effect can be controlled and the electrical line length increased to the desired level. It is a different matter that the electrieal length of the line cannot be raised infinitely, for reasons of stability, as discussed later. 

To optimize this power transfer through reactive control let us study equation (24.10) for the parameters that can be varied to achieve this objective. The active power transfer will depend upon the following factors ... 

A line can be theoretically loaded up to these levels. But at these levels, during a load variation, the far-end voltage may swing far beyond the desirable limits of 5% and the system may not remain stable. With the use of reactive control it is possible to transfer power at the optimum level (Pnias) hd yet maintain the far-end (or midpoint in symmetrical lines) voltage near to and also to have a near-flat voltage profile. 

Reactive control can alter the line length ( f LC) to the level at which the system will have the least possible swings. It is evident from these curves that an uncompensated line of a much shorter length may not be able, to transfer even its natural load (Pq) successfully. This is due to the steeply drooping characteristics of the voltage profile at about this load point, which may subject the... 

To decide on the best reactive control one should choose the most appropriate electrical line length from the load characteristics drawn already. Choose the one that can transmit the optimum power, and then compensate this to obtain the required line length. For the 400 kV, 50 Hz system considered, we can choose a radial line with an electrical line length of 200-250 km. The compensation is provided so that the P ,aj point, which lies far from the natural power transmission point Pq, shifts within a stable region, i.e. near the Pq region. Then from equation (24.8),... 

Above we have dealt primarily with the technical aspects of reactive controls. For commercial implications, see Lakervi and Holmes. 

The series capacitors tire connected in series with the power lines to provide reactive control to an individual load or to a power distribution or transmission system. They are therefore switched with the pow er lines and are thus permanently connected devices. 

However, power systems that cater to almost fixed loads at a time and whose variations occur only at specific times of the day may not reejuire it fast response. In such cases, it is possible to provide manual switching methods which will give enough time between two switchings. Manual switching, how ever, has certain shortcomings, due to the human factor such as its accuracy and diligence, as noted above. The recommended practice is therefore to select fast reactive controls as noted below. 

Reactive control is also possible through synchronous condensers. As they rotate, the rotor stores kinetic energy which tends to absorb sudden Huctuations in the supply system, such as sudden loadings. They are. however, sluggish in operation and very expensive compared to thyristor controls. Their rotating masses add inertia, contribute to the transient oscillations and add to the fault level of the system. All these factors render them less suitable for such applications. Their application is therefore gradually disappearing. 

Since reactive controls are normally meant for large to very large installations, the practice so far has been to use thyristors only for such applications. With the advent of IGBTs. smaller installations can now be switched through IGBTs. 

When the reactive control is not automatic, then during offpeak periods (such as during the night) it is important that some of the banks are dropped manually to avoid an overcompensation and a consequent overvoltage. 

In this part the author provides all relevant aspects of a reactive control and carries out an exhaustive analysis of a system for the most appropriate control. Harmonic effects and inductive interferences as well as use of filter and blocking circuits are covered. Capacitor switching currents and surges and methods of dealing with these are also described. 

The low-power-density, low enrichment reactor core uses soluble boron and burnable poisons for shutdown and fuel bumup reactivity control. Low worth grey rods provide load following. A heavy uranium flywheel extends the pump coastdown to allow for emergency action during loss-of-flow transients. 

Boric acid [B(OH)3] is employed in primary coolant systems as a soluble, core reactivity controlling agent (moderator). It has a high capture cross-section for neutrons and is typically present to the extent of perhaps 300 to 1,000 ppm (down from perhaps 500 to 2,500 ppm 25 years ago), depending on nuclear reactor plant design and the equilibrium concentration reached with lithium hydroxide. However, boric acid may be present to a maximum extent of 1,200 ppm product in hot power nuclear operations. 

As model studies and radical reactivity control have improved, the so-called cascade (or domino) reactions have emerged as a very powerful method for natural product synthesis, since they offer a unique route to prepare complex backbones from appropriately designed but quite simple precursors. A few selected reactions will be presented here. 

At one extreme diffusivity may be so low that chemical reaction takes place only at suface active sites. In that case p is equal to the fraction of active sites on the surface of the catalyst. Such a polymer-supported phase transfer catalyst would have extremely low activity. At the other extreme when diffusion is much faster than chemical reaction p = 1. In that case the observed reaction rate equals the intrinsic reaction rate. Between the extremes a combination of intraparticle diffusion rates and intrinsic rates controls the observed reaction rates as shown in Fig. 2, which profiles the reactant concentration as a function of distance from the center of a spherical catalyst particle located at the right axis, When both diffusion and intrinsic reactivity control overall reaction rates, there is a gradient of reactant concentration from CAS at the surface, to a lower concentration at the center of the particle. The reactant is consumed as it diffuses into the particle. With diffusional limitations the active sites nearest the surface have the highest turnover numbers. The overall process of simultaneous diffusion and chemical reaction in a spherical particle has been described mathematically for the cases of ion exchange catalysis,63 65) and catalysis by enzymes immobilized in gels 66-67). Many experimental parameters influence the balance between intraparticle diffusional and intrinsic reactivity control of reaction rates with polymer-supported phase transfer catalysts, as shown in Fig. 1. 

Reviews have already been published by J. H. Fendler on Polymerized Surfactant Vesicles 91 92,931 which refer to Novel Membrane Mimetic Systems , synthetic strategies leading to them and their characterization and potential utilization in various areas such as solar energy conversion and reactivity control. It is the intend of this appendix to bring the reader up to date on the state of the art of polymerized liposomes. 

Reactivity Control. The movable boron-carbide control rods are sufficient to provide reactivity control from the cold shutdown condition to the full-load condition. Supplementary reactivity control in the form of solid burnable poison is used only to provide reactivity compensation for fuel burnup or depletion effects. The movable control rod system is capable of bringing the reactor to the subcritieal when the reactor is an ambient temperature (cold), zero power, zero xenon, and with the strongest control rod fully withdrawn from the core. In order to provide greater assurance that this condition can be met in the operating reactor, the core is designed to obtain a reactivity of less than 0.99, or a 1% margin on the stuck rod condition. See Fig. 7. 

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