Transient load

Thermal power plant components operated at high temperatures (>500°C) and pressures, such as superheater headers, steamline sections and Y-junctions, deserve great attention for both operation safety and plant availability concerns. In particular, during plant operation transients -startups, shutdowns or load transients - the above components may undergo high rates of temperature / pressure variations and, consequently, non-negligible time-dependent stresses which, in turn, may locally destabilize existing cracks and cause the release of acoustic emission. 

Steam headers and steamline sections may undergo high rates of temperature / pressure variations during plant operation transients - startups, shutdowns or load transients - and are... 

Incidentally, whenever you undertake corrective action and things get worse, you are actually still very close to identifying the cause. Also, we should remember that certain problems depend on timing. So in our case above, the rate at which we applied the load transients was important. 

Blog Entry 2 You are right. I did not have any answer to my problem of load transient while using 267xT-Adj device—until I saw this reply. Because I simply followed your software tool, which has no mention about Cinx. But after introducing Cinx the output is stable under all load conditions. Thanks. 

Gemmen R.S., Johnson C.D., 2005, Effect of load transient on SOFC operation - Current reversal on loss of load. Journal of Power Sources 144, 152-164. 

Smith T.P., Haynes C.L., Wepfer W.J., Liese E.A., Tucker D. (2006) Hardware based simulation of a fuel cell turbine hybrid response to imposed fuel cell load transients. In Proceedings of the ASME International Mechanical Engineering Conference and Exhibition, Chicago, IL, IMECE2006-13978. 

Fig. 5.1 Idealized representation of the transient change in fiber and matrix stress that occurs during the isothermal tensile creep and creep recovery of a fiber-reinforced ceramic (the loading and unloading transients have been exaggerated for clarity). It is assumed that the fibers have a much higher creep resistance than the matrix. The matrix stress reaches a maximum at the end of the initial loading transient. After full application of the creep load, the matrix stress relaxes and the fiber stress increases. Upon specimen unloading, elastic contraction of the composite occurs, followed by a time-dependent decrease in fiber stress and increase in matrix stress. Overall, creep tends to increase the difference in stress between constituents and recovery tends to minimize the difference in stress. After Wu and Holmes.15...

Fig. 5.2 Comparison of creep behavior and time-dependent change in fiber and matrix stress predicted using a 1-D concentric cylinder model (ROM model) (solid lines) and a 2-D finite element analysis (dashed lines). In both approaches it was assumed that a unidirectional creep specimen was instantaneously loaded parallel to the fibers to a constant creep stress. The analyses, which assumed a creep temperature of 1200°C, were conducted assuming 40 vol.% SCS-6 SiC fibers in a hot-pressed SijN4 matrix. The constituents were assumed to undergo steady-state creep only, with perfect interfacial bonding. For the FEM analysis, Poisson s ratio was 0.17 for the fibers and 0.27 for the matrix, (a) Total composite strain (axial), (b) composite creep rate, and (c) transient redistribution in axial stress in the fibers and matrix (the initial loading transient has been ignored). Although the fibers and matrix were assumed to exhibit only steady-state creep behavior, the transient redistribution in stress gives rise to the transient creep response shown in parts (a) and (b). After Wu et al 1...

Whenever we start-up, or subject the converter to sudden line/load transients, the current no longer stays at the steady value it has under normal operation (i.e. when delivering the required maximum rated load current). For example, if we suddenly short the output the control circuitry in an effort to regulate the output may momentarily expand the duty cycle to the highest permissible value (as set by the controller). We then are no longer in steady state, and so under the increased on-time voltseconds, the current ramps up progressively, and can reach the set current limit. 

Suppose we suddenly increase the load current of a converter from 4 A to 5 A. This is a step load and is essentially a nonrepetitive stimulus. But by writing all the transfer functions in terms of s rather than just as a function of jco, we have created the framework for analyzing the response to such disturbances too. We will need to map the stimulus into the s-plane with the help of the Laplace transform, multiply it by the appropriate transfer function, and that will give us the response in the s-plane. We then apply the inverse Laplace transform and get the response with respect to time. This was the procedure symbolically indicated in Figure 7-3, and that is what we need to follow here too. However, we will not perform the detailed analysis for arbitrary load transients here, but simply provide the key equations required to do so. 

The complete EVA-II / ADAM-II system was operated for a total time of 10,150 hours including steady state conditions at both full and partial load, transient procedures, and also special test situations such as tube blockage [28] (see also section B.I.). 

Appendix 5A presents the design bases for sizing the overpressurization protection system. The loss of load transient which is used to size the primary safety valves is not intended to be used as a design transient for any other NSSS equipment. 

F. Provide sufficient steam volume to allow acceptance of the insurge resulting from any loss of load transient without liquid or two-phase flow reaching the primary safety valve nozzles. 

The total spray flows shall be sufficient to keep the pressure below the reactor trip setpoint during an insurge of water during the Maneuvering and Load Follow and Loss of Load transients. 

A. At the onset of the loss-of-load transient, the reactor coolant and main steam systems are at maximum rated output plus a two percent uncertainty. By choosing the highest possible power output, the heatup rate of the primary loop is maximized along with the rate of pressurization. 

The principal natural phenomena that influence transient operation are the temperature coefficients of the moderator and fuel and the buildup or depletion of certain fission products. Reactivity balancing may occur through the effects of natural phenomena or the operation of the reactor control system using the RCCs or chemical "shim." A change in the temperature of moderator or fuel (e.g., due to an increase or decrease in steam demand) will add or remove reactivity (respectively) and cause the power level to change (increase or decrease, respectively) xmtil the reactivity change is balanced out. RCC assemblies are used to follow fairly large load transients, such as load-follow operation, and for startup and shutdown. 

UPS system and standby generator All source transients no load transients All All... 

Shielded isolation transformer Most source transients no load transients None None... 

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