A predictive control approach for long term hydrothermal scheduling

Comment on “Short-term combined economic emission scheduling of hydrothermal power systems with cascaded reservoirs using differential evolution” by K.K. Mandal and N. Chakaborty [Energy Convers. Manage. 50 (2009) 97–104

This paper presents a case study concerning the application of model predictive control (MPC) to the long term hydrothermal scheduling of the Brazilian power system. According to MPC, the hydro and thermal generation decisions at each stage are provided by a deterministic nonlinear optimization model considering predicted inflows. The model, which is solved by interior point method, also takes into account tie line constraints between interconnected areas. In order to evaluate the performance of the approach several simulations over historical inflow scenarios were performed, and statistics about operation costs, hydro and thermal generation, power flow interchange, reservoir storage, load shortage, among others, are obtained. General results are compared to those from the stochastic model in use in Brazil and the results have shown substantial decrease in expected operation costs and load shortages, as well as an increase on water storage, both cause and effect of the better management of water resources.

This paper presents an optimization-based method for scheduling hydrothermal systems based on the Lagrangian relaxation technique. After system-wide constraints are relaxed by Lagrange multipliers, the problem is converted into the scheduling of individual units. This paper concentrates on the solution methodology for pumped-storage units. There are, many constraints limiting the operation of a pumped-storage unit, such as pond level dynamics and constraints, and discontinuous generation and pumping regions. The most challenging issue in solving pumped-storage subproblems within the Lagrangian relaxation framework is the integrated consideration of these constraints. The basic idea of the method is to relax the pond level dynamics and constraints by using another set of multipliers. The subproblem is then converted into the optimization of generation or pumping; levels for each operating state at individual hours, and the optimization of operating states across hours. The optimal generation or pumping level for a particular operating state at each hour can be obtained by optimizing a single variable function without discretizing pond levels. Dynamic programming is then used to optimize operating states across hours with only a few number of states and transitions. A subgradient algorithm is used to update the pond level Lagrangian multipliers. This method provides an efficient way to solve a class of subproblems involving continuous dynamics and constraints, discontinuous operating regions, and discrete operating states. >

Hydrothermal Optimisation by Refinement of Step-Loading Schedules

Stochastic dynamic programming has been. extensively used in the optimization of long term hydrothermal scheduling problems due to its ability to cope with the nonlinear and stochastic characteristics of such problems, and the fact that it provides a closed-loop feedback control policy. Its computational requirements, however, tend to be heavy even for systems with a small number of hydroplants, requiring some sort of modeling manipulation in order to be able to handle real systems. An alternative to closed-loop optimization is an approach that combines a deterministic optimization model with an inflow forecasting model in a partial open-loop feedback control framework. At each stage in this control policy, a forecast of the inflows during the period of planning is made, and an operational decision for the following stage is obtained by a deterministic optimization model. The present paper compares such closed-loop and partial open-loop feedback control policies in long term hydrothermal scheduling, using a single hydroplant system as a case study to focus the comparison on the feedback control performance. The comparison is made by simulation using data from historical and synthetical inflow sequences in the consideration of three different Brazilian hydroplants located in different river basins. Results have demonstrated that the performance of the partial open-loop feedback control policy is similar to that of the closed-loop control policy, and is even superior in dry streamflow periods.

Evolutionary hybrid approaches for generation scheduling in power systems

Stabilogram-diffusion analysis was used to examine how prolonged periods in microgravity affect the open-loop and closed-loop postural control mechanisms. It was hypothesized that following spaceflight: (1) the effective stochastic activity of the open-loop postural control schemes in astronauts is increased; (2) the effective stochastic activity and uncorrelated behavior, respectively, of the closed-loop postural control mechanisms in astronauts are increased; and (3) astronauts utilized open-loop postural controls schemes for shorter time intervals and smaller displacements. Four crew members and two alternates from the 14-day Spacelab Life Sciences 2 Mission were included in the study. Each subject was tested under eyes-open, quiet-standing conditions on multiple preflight and postflight days. The subjects' center-of-pressure trajectories were measured with a force platform and analyzed according to stabilogram-diffusion analysis. It was found that the effective stochastic activity of the open-loop postural control schemes in three of the four crew members was increased following spaceflight. This result is interpreted as an indication that there may be in-flight adaptations to higher-level descending postural control pathways, e.g., a postflight increase in the tonic activation of postural muscles. This change may also be the consequence of a compensatory (e.g., "stiffening") postural control strategy that is adopted by astronauts to account for general feeling of postflight unsteadiness. The crew members, as a group, did not exhibit any consistent preflight/postflight differences in the steady-state behavior of their closed-loop postural control mechanisms or in the functional interaction of their open-loop and closed-loop postural control mechanisms. These results are interpreted as indications that although there may be in-flight adaptations to the vestibular system and/or proprioceptive system, input from the visual system can compensate for such changes during undisturbed stance.

Review of hybrid evolutionary algorithms for optimizing a reservoir

The open-loop control scheme presented in part I of this paper for a dual five-level-inverter-fed induction motor (IM) drive with two dc power supplies maintains dc-link capacitorvoltage balancing and common-mode voltage (CMV) elimination throughout the operating range of the drive. The operating limitation of the proposed open-loop control scheme to take corrective action toward the existing unbalance in the dc-link-capacitor voltages is also pointed out in part I of this paper. As a solution to this, a simple closed-loop control scheme, which is based only on the switching-state redundancy, is proposed in this part of the paper. The proposed closed-loop control scheme not only prevents further unbalancing of capacitor voltages but also takes corrective actions to bring back the capacitor voltages in the balanced state. The proposed closed-loop scheme achieves dc-link capacitorvoltage balancing and elimination of CMV together in the complete modulation range, including overmodulation of up to the 24-step operation. The proposed control scheme does not affect the output fundamental voltage generated by the inverter, as it effectively utilizes only the availability of redundant switching states of the inverter, and does not call for additional power circuit hardware. The scheme is presented with the simulation studies and experimentally verified with a 1.5-kW open-end winding IM drive.

Robust optimization and stochastic programming approaches for medium-term production scheduling of a large-scale steelmaking continuous casting process under demand uncertainty

This paper presents a novel prototype three-phase current-fed PWM converter with a switched-capacitor-type resonant DC link commutation circuit operating with a new optimum PWM pattern strategy, designed with consideration for a low-pass filter. This converter can operate based on the principle of zero current soft-switching commutation. First, the steady-state operating principle of this converter with a new resonant DC link snubber circuit is described in relation to an equivalent operating circuit, together with the practical design procedure of the switched-capacitor-type resonant DC link circuit. Second, the three-phase current-fed PWM converter with the switched-capacitor-type resonant DC link circuit is discussed from a theoretical viewpoint based on a design example. An actively delayed time correction method to compensate distorted currents due to a relatively long resonant commutation time is newly implemented in the open-loop control scheme so as to obtain a new optimum PWM pattern. Finally, an experimental setup of this converter is demonstrated in a laboratory to confirm the zero current soft-switching commutation of this converter. Comparative evaluations between current-fed hard-switching PWM and soft-switching PWM converters are implemented from the viewpoint of PWM converter characteristics. © 2000 Scripta Technica, Electr Eng Jpn, 132(3): 57–67, 2000

Semi-active [formula omitted] control of vehicle suspension with magneto-rheological dampers

We present a skin-shear actuator based on the lead screw mechanism. The lead screw mechanism is simple, reliable, offers fewer components, and accommodates into compact form- factors. We show mechanical design of a single assembly unit and implement multiple units in a single handheld device. We evaluate the actuator in one instrumentation-based test and one preliminary user study. Tests show that the actuator performance matches with the open-loop control scheme when no load is placed on the actuator. The performance deteriorates with loading, particularly when quicker and high amplitude stroke are required. The user study shows that information throughput with the skin-shear is comparable to vibrations through three digits on the hand. It is shown that small compact actuators (~5g) with efficient mechanisms can render displacements (>3mm) and forces (>1N) for easily differentiating skin-shear cues.

Duchenne muscular dystrophy (DMD) is a progressive type of muscular dystrophy that causes a steady degradation of muscle fibers and leads to a progressive loss of muscle strength and functional abilities in young boys. Recent studies have shown that rehabilitative motor-assisted cycling can potentially delay loss of functional abilities in individuals with DMD. However, previous results of motor-assisted cycling for DMD participants only consider open-loop control schemes. In this paper, a motor-assisted stationary rehabilitation cycle is developed for DMD participants. A dual-objective control scheme is developed for the motor control input of the developed rehabilitative device. Specifically, a control barrier function (CBF) is designed to restrict the cadence of the rider within a user-defined desired cadence range to maximize rehabilitative efforts. A closed-loop adaptive admittance control scheme is developed to provide indirect torque tracking while the cadence of the rider is within the desired region. A Lyapunov-like stability analysis is used to guarantee asymptotic tracking of the admittance errors within the desired cadence range and uniform global asymptotic stability of the safe set.

The error characterization of soil moisture products, for example, obtained from microwave remote sensing data, is a key requirement for using these products in applications like numerical weather prediction. The error variance and root-mean-square error are among the most popular metrics: they can be estimated consistently for three datasets using triple collocation (TC) without assuming any dataset to be free of errors. This technique can account for additive and multiplicative biases; that is, it assumes that the three products are linearly related. However, its susceptibility to nonlinear relations (e.g., due to sensor saturation and scale mismatch) has not been addressed. Here, a simulation study investigates the impact of quadratic relations on the TC error estimates [also when the products are first rescaled using the nonlinear cumulative distribution function (CDF) matching technique] and on those by two novel methods. These methods—based on error-in-variables regression and probabilistic factor analysis—extend standard TC by also accounting for nonlinear relations using quadratic polynomials. The relative differences between the error estimates of the ASCAT remotely sensed product by the quadratic and the linear methods are predominantly smaller than 10% in a case study based on remotely sensed, reanalysis, and in situ measured soil moisture over the contiguous United States. Exceptions with larger discrepancies indicate that nonlinear relations can pose a challenge to traditional TC analyses, as the simulations show they can introduce biases of either sign. In such cases, the use of nonlinear methods may complement traditional approaches for the error characterization of soil moisture products.