Nominal Operating Points Research Articles

Performance Comparison Between Centralized and Hierarchical Predictive Functional Control for Adaptive Cruise Control Application

This paper presents a performance comparison between two different control structures of Predictive Functional Control (PFC), namely centralized and hierarchical architectures for an Adaptive Cruise Control (ACC) system. Centralized PFC utilizes a linearized model of the vehicle longitudinal dynamics to compute the control law, while hierarchical PFC uses a simple kinematic model with inverse vehicle longitudinal dynamics. Based on the simulation results, both control structures produced a satisfactory performance. However, in the presence of disturbances such as road slopes and wind gusts, the centralized PFC becomes more sensitive and conservative than the hierarchical PFC. The main reason is that the linearization model used in centralized PFC is only effective around the selected nominal operating points. Since the decision-making process depends on the internal model performance, centralized PFC cannot compensate for this effect. Besides, it also takes a longer computation time than hierarchical PFC since more mathematical operations must be solved than the kinematic model, especially when physical and operational constraints are considered. The standard performance parameters such as Root Mean Squared Error (RMSE), settling time, and rise time are also used for the analysis. These findings can become a solid justification for using the hierarchical PFC structure in designing an ACC system for future work.

A Hybrid IPT System Implementing Misalignment Tolerance and Constant Current Output With Primary Intermediate Coil

Inductive power transfer (IPT) systems have advantages of safety, flexibility, and convenience compared to plug-in charging systems. However, in practical applications, pad misalignment is still an almost inevitable issue which may cause variation in magnetic couplings, thus affecting the power transmission and reducing the system efficiency. Therefore, IPT systems are usually expected to achieve tolerance for misalignment. To fulfill this feature, a hybrid IPT system is proposed in this article. By adopting two coils compensated by different topologies in receiver side and an additional intermediate coil in transmitter side, load-independent constant output current with improved misalignment tolerance can be achieved. In addition, when the receiver pad is very far away from the transmitter pad, the proposed system can operate safely due to the inherent limitation ability of the primary inverter current. The design process is presented to optimize the misalignment performance. Based on the experimental results of a 3.3-kW prototype, the fluctuation of the output current is less than 5% within ±300-mm misalignment in <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$y$ </tex-math></inline-formula> -axis, −30- to +60-mm misalignment in <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$z$ </tex-math></inline-formula> -axis, and −80- to +60-mm misalignment in <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$x$ </tex-math></inline-formula> -axis. The system efficiency is 96.7% at nominal operating point (NOP) and varies from 94.7% to 97.8% with variable loads and misalignment.

Multilevel inverter with active clamping diodes for energy efficiency improvement

Multilevel inverters can replace commonly used two-level inverters for three-phase electrical drives. In this lowvoltage range new wide-bandgap power switches (SiC MOSFET, GaN FET) are available. In the four-, five- and sevenlevel inverters, the majority of the power losses are caused by the threshold voltage of the clamping diodes. The new idea is the replacement of the clamping diodes by active switches, since they only have low on-resistance in the forward direction. In this case freewheeling paths of the clamping diodes would have to be actively switched. However, the control signals for the clamping switches can be generated by logic operations from the control signals for the main switches. The use of active clamping switches has significant potential to reduce semiconductor losses, but requires the development of wide-bandgap power semiconductors with reverse blocking capability. At nominal motor operation point of the 5.5 kW induction motor a loss reduction of 56 % between active clamping switches and clamping diodes is possible. With a higher number of inverter levels, the size of the motor filter can be reduced by about 70 % and also the losses can be reduced by 73 %.

Data-Driven Self-Optimizing Control: Unconstrained Optimization Problem

Controlled variable (CV) selection plays an important role in determining the performance of a process plant. Existing methods for CV selection through self-optimizing control requires linearization of rigorous models around nominal operating points. This is a very difficult task which results to large losses. This work presents a novel method for CV design. A necessary condition of optimality (NCO) was proposed to be the CV. The approach does not require the analytical expression of the NCO to be derived but is approximated through a single regression step based on data. Finite difference was used to approximate the NCO (gradient) using data; three finite difference schemes were employed for this purpose, which are forward, backward and central differences. Seven different cases with respect to number of sampling points, neighborhood points and finite difference schemes were investigated. To demonstrate the efficacy of the method in simplest way, it is applied to a hypothetical unconstrained optimization problem. The proposed method was found to have outperformed some existing approaches in many instances. A zero loss was recorded by some designed CVs. Central difference was found to be the best schemes among the three. Keywords— controlled variable, disturbance, finite difference, monotonicity, regression.

High energy-efficient electrical drive with multilevel inverter and widebandgap power semiconductors

Multilevel inverters are an alternative for electrical drives with DC link voltage between 560 and 750 V. In this voltage range new wide-bandgap power switches (SiC MOSFET, GaN FET) are available. The paper analyses three-, four-, five- and seven-level inverters. A simulation model of the drive system, including the 11 kW induction motor and motor filter is developed. By replacing IGBTs with SiC FETs, the twolevel inverter achieved a loss reduction of 59 % at 25 °C and 150 °C at nominal motor operation point. By using the five-level inverter with GaN FETs, a further loss reduction of 9 % only at low junction temperature is possible. With a higher number of inverter levels, the size of the motor filter can be reduced. With five inverter levels and 40 kHz switching frequency volume and weight can be reduced by 86 % and 78 % respectively. The overall efficiency of the drive system achieves 98.5 % at 25 °C and 98.1 % at 150 °C. Compared to the state of the art (two-level with IGBTs with 5 kHz), this is an improvement of 2.1 % at 25 °C and 2.7 % at 150 °C.

Hybrid Control Strategy for LLC Converter With Reduced Switching Frequency Range and Circulating Current for Hold-Up Time Operation

Hold-up time operation is a crucial requirement for distributed power systems and server power supplies. LLC resonant converter is widely used in these applications due to its high efficiency and power density characteristics. Conventional pulse frequency modulation (PFM)-controlled inductor-inductor-capacitor (LLC) resonant converter requires a wide switching frequency operation range to satisfy the hold-up requirement. In this article, a novel control scheme is proposed for stacked structure LLC resonant converter. During the normal operation, the frequency doubler modulation strategy is adopted to reduce the switching loss and driving loss, and the output is regulated by PFM. During the hold-up time operation, the pulsewidth modulation is adopted to increase the system voltage gain, where the switching frequency is fixed during this operation stage. For the proposed converter, the resonant tank parameters are designed at the nominal operation point, and the only design consideration for the magnetizing inductor is the zero-voltage switching for primary switches. Therefore, compared with PFM-LLC, both the switching frequency operating range and the circulating current are reduced, and high efficiency feature is obtained. Finally, a 100-W experimental prototype with input voltage range of 160-340 V was built to validate the advantages of the proposed converter.

The possibilities of cooperation between a hydrogen generator and a wind farm

In this paper, a hydrogen generator and a wind farm were taken as the research objects. The H2 generator consisted characteristics of laboratory-tested electrolyzers were determined as a function of the hydrogen mass flow. Determining the auxiliary power index of the device allowed the efficiency of the hydrogen generator to be determined as a function of hydrogen mass flow as well as the hydrogen generator relative power. The dynamic characteristics of a generator were also presented. The possibility of a given wind farm cooperating with hydrogen generators that are characterized by different powers and various efficiencies was simulated. Algorithm enables determination of hydrogen generators efficiency for devices with various performance in nominal operation point is shown. It has been shown that proper selection of the power of the hydrogen generator in relation to the power of the wind farm can ensure a high efficiency for the device.

Performance analysis of alkali metal thermoelectric converter and segmented thermoelectric generator hybrid system based on a comprehensive model

The conceptual hybrid system combined alkali metal thermoelectric converter (AMTEC) and segmented thermoelectric generator (STEG) is a promising static thermal to electric conversion system. A comprehensive model and an effective iterative numerical solution method for the AMTEC/STEG hybrid system have been developed in this paper. The comprehensive model includes the AMTEC unit model and the STEG unit model. The good agreement between the model calculation results and those provided in the published reference show the correctness and accuracy of the models. The effect of beta’’ alumina solid electrolyte (BASE) temperature and condenser temperature on the system performance, the load following characteristic and nominal operation conditions have been investigated using the comprehensive model in this paper. According to the performance analysis results, 1173 K and 653 K are selected as BASE and condenser temperatures. When system input power is 249 kW, the peak conversion efficiency can reach 34.2%. The nominal operation point has been selected according to the optimal performance parameters and load following characteristic of the hybrid system, at the nominal operation point, the system electric power, conversion efficiency, current and voltage of AMTEC and STEG can reach 84 kW, 34.2%, 165.5 A, 448.3 V, 51.7 A, 356.5 V, respectively.

Effects of the O/F Ratio on the Performance of a Low Thrust LOX/Methane Rocket Engine with an ElecPump-fed Cycle

Recently, electrically driven pump-fed cycle rocket engines (i.e., ElecPump engines) have received considerable attention. ElecPump engines are superior to pressure-fed engines and will likely replace gas generator cycle engines under a 100 kN thrust level. This study focuses on LOX/methane ElecPump engines with a low-thrust level and estimates the nominal operation point of the engine. The objective variable was set to the maximum velocity increment of the stage. The mixture ratio and combustion pressure were explored as the design variables. Thus, the optimal mixture ratio was proposed as the design variable, considering not only the engine weight but also the structural weight.

Induction Motor Design Strategy for Wide Constant Power Speed Range

This paper presents a design strategy of an induction motor (IM) for a wide constant power speed range (CPSR). The stator inductance is a key parameter that determines a CPSR, because it affects the voltage limit and the torque simultaneously. Low inductance allows a wide CPSR under a voltage limit. However, it results in torque reduction. On the other hand, if inductance is high, the opposite results take place; high torque and narrow CPSR. Therefore, there is a tradeoff between the power level and the speed range. The classical design process does not consider the CPSR, but provides a guideline to calculate core dimensions and the winding number for a nominal operation point. Thereby, the machine inductance is determined passively. This classical procedure is not appropriate for widening the CPSR. To extend the speed range under a voltage limit, a proper inductance must be chosen in the first place. In this work, a different IM design process is pursued with the objective of widening CPSR: An optimal inductance is obtained that minimizes the maximum current under a constant power condition. In the secondary part, an IM design method is proposed, which determines the winding number and the core dimension under a given inductance. Finally, the proposed analytic design is compared with the design optimized by finite-element-analysis. Based on the design results, a real IM (130 kW, 14 000 r/min) is constructed and tested. The experimental results are in good agreement with the design data.

Cascade Control of Antagonistic VSA-An Engineering Control Approach to a Bioinspired Robot Actuator.

A cascade control structure for the simultaneous position and stiffness control of antagonistic tendon-driven variable stiffness actuators (VSAs) implemented in a laboratory setup is presented in the paper. Cascade control has the ability to accelerate, additionally stabilize, and reduce oscillations, which are all extremely important in systems such as a tendon-driven compliant actuators with elastic transmission. Inner-loop controllers are closed in terms of motor positions, and outer-loop controllers in terms of actuator position and estimated stiffness. The dominant dynamics of the system (position and stiffness), composed of the mechanical part and inner loops, are identified by a closed-loop auto-regressive with exogenous input (ARX) model. The outer-loop controllers are tuned on the basis of experimentally identified transfer functions of the system in several nominal operating points for different stiffness values. After the system is identified, a controller bank is generated in which a pair of actuator position and stiffness controllers correspond to a nominal operating point and covers the area surrounding the nominal point for which it is designed. The controllers used are integral-proportional differential (I-PD) and integral-proportional (I-P) controllers, which are a variation of the PID and PI controllers with dislocated proportional and derivative gains from a direct to feedback branch that result to no overshoot for even fast reference changes (i.e., step signal), which is essential for preventing tendon slackening (meeting the pulling constraint). Analytical formulas for controller tuning based on only one parameter, λ, are also presented. Since position and stiffness loops are decoupled, it is possible to change λ for both loops independently and adjust their performance separately according to the needs. Also, the controller structure secures the smooth response without overshooting step reference or step disturbance signal, which make practical implementation possible. After all the controllers were designed, the cascade control structure for simultaneous position and stiffness control was successfully evaluated in a laboratory setup. Thus, the presented control approach is simple to implement, but with a performance that ensures a pulling constraint for tendon-driven actuators as a foundation for bioinspired antagonistic VSAs.

Controller Performance Assessment of a Photovoltaic Generator Terminated in a Current-Mode-Buck-Convertor-Load

In this paper, a comprehensive control analysis is presented, with emphasis on regulating the terminal voltage of a photovoltaic generator, interfaced with a current mode controlled buck DC-DC converter, which is, in turn terminated by a battery. The combined generator-convertor-load system possesses nonlinear behaviour, and is subject to environmental conditions, thus causing the control task complicated. Additionally, analyses of small signal equations reveal a possible unstable condition. Hence, a new method of designing a controller for worst-case scenario is presented. Additionally, results of implementing a typical linear controller (PI), which can be designed only in reference to a single nominal operation point, revealing a varied responses. Simulation and results of mathematical analysis are presented to verify the proposed method.

Adaptive incremental state space MPC for collector defocusing of a parabolic trough plant

Commercial solar plants produce energy around a nominal operating point in which the solar field outlet temperature is high and close to the thermal limit of the heat transfer fluid. The main control of the temperature is carried out by means of the fluid flow-rate that circulates through the solar field. Defocusing the collectors is normally used as a safety mechanism to avoid exceeding the thermal limit. However, in situations in which the flow is saturated, the control of defocusing the collectors becomes of vital importance and is the system in charge of controlling the solar field outlet temperature.The paper presents an Adaptive State Space Model Predictive Control strategy with an incremental formulation to control the fourth and third collector defocus angles for field outlet temperature set-point tracking at the nominal operation point, avoiding the Heat Transfer Fluid temperature from exceeding the manufacturer thermal limit (oil degradation). The state space description uses an Unscented Kalman Filter for estimating the non-measurable states. A 50 MW parabolic solar trough plant nonlinear model has been used to design and validate the strategy. Simulation results are presented showing the advantages of using the proposed strategy.

On the steady state optimization of the biogas production in a two-stage anaerobic digestion model

In this paper, we study the optimization problem of maximizing biogas production at steady state in a two-stage anaerobic digestion model, which was initially proposed in Bernard etal. (Biotechnol Bioeng 75(4):424-438, 2001). Nominal operating points, consisting of steady states where the involved microorganisms coexist, are usually referred to as desired operational conditions, in particular for maximizing biogas production. Nevertheless, we prove that under some conditions related to input substrate concentrations and microorganism biology, characterized by their growth functions, the optimal steady state can be the extinction of one of the two species. We provide some numerical examples of this situation.

Distributionally Robust Chance-Constrained Approximate AC-OPF With Wasserstein Metric

Chance constrained optimal power flow (OPF) has been recognized as a promising framework to manage the risk from variable renewable energy (VRE). In the presence of VRE uncertainties, this paper discusses a distributionally robust chance constrained approximate ac-OPF. The power flow model employed in the proposed OPF formulation combines an exact ac power flow model at the nominal operation point and an approximate linear power flow model to reflect the system response under uncertainties. The ambiguity set employed in the distributionally robust formulation is the Wasserstein ball centered at the empirical distribution. The proposed OPF model minimizes the expectation of the quadratic cost function w.r.t. the worst-case probability distribution and guarantees the chance constraints satisfied for any distribution in the ambiguity set. The whole method is data-driven in the sense that the ambiguity set is constructed from historical data without any presumption on the type of the probability distribution, and more data leads to smaller ambiguity set and less conservative strategy. Moreover, special problem structures of the proposed problem formulation are exploited to develop an efficient and scalable solution approach. Case studies are carried out on the IEEE 14 and 118 bus systems to show the accuracy and necessity of the approximate ac model and the attractive features of the distributionally robust optimization approach compared with other methods to deal with uncertainties.

Gain Scheduling Technique using MIMO Type-2 Fuzzy Logic System for LFC in Restructure Power System

In this paper, a gain scheduling technique using type-2 fuzzy logic system has been proposed for load frequency control in restructure power system. For this purpose, at first the particle swarm optimization algorithm has been employed to obtain proportional-integral-derivative controller gains at some nominal operating points; then, a multi-input multi-output type-2 fuzzy logic system is trained in order to provide a general mapping between the operating points and the related proportional-integral-derivative gains. In addition, the same particle swarm optimization algorithm has been used to train the multi-input multi-output type-2 fuzzy logic system. In the online applications, the trained type-2 fuzzy logic system is able to infer the proportional-integral-derivative gains appropriately even in the presence of noise and uncertainties. So, the type-2 fuzzy logic system is exploited due to its ability to model uncertainties which may exist in the rules and measured data. To illustrate the effectiveness of the proposed strategy, the new controller has been compared with a type-1 fuzzy gain scheduling and the proportional-integral-derivative controllers.

The Importance of Nominal Operating Point Selection in Self-Optimizing Control

A self-optimizing control (SOC) structure is when we can achieve an acceptable loss with constant setpoints for controlled variables. In the original paper (Skogestad, J. Process Control, 2000, 10 (5), 487–507), the operating point is selected first and then the controlled variables, using the optimal values for nominal disturbance as setpoints. In this work, the importance of nominal operating point selection is evaluated, showing that setpoints obtained by optimization along the disturbance region can provide a better self-optimizing structure. A simultaneous procedure is proposed where controlled variables are selected together with the nominal operating point. Despite the more expensive optimization procedure, this new policy leads to a better controlled variables set. This is corroborated by a reactor–distillation case study and a cumene unit case study where the operating point selected decreases significantly the worst-case and mean losses for the process.

Analysis, modeling and simulation of step up converter using Matlab–Simulink and simplorer

The basic configuration of step up converter usually used in photovoltaic solar systems to increase the DC voltage generated at their outputs suffers from some drawbacks just like high ripple in the output voltage, greater losses in the system and unstable dynamic behavior. To eliminate these drawbacks, this paper introduces a two-phase connection of step up converter with uncoupled smoothing reactors. Detailed analysis, simulation and control strategy have been proposed in this paper to investigate the advantages of using such connection with uncoupled reactors. This paper is intended to prove that two-phase connection with uncoupled reactors helps increasing the output power of the converter, minimizing its output ripple and making its control easier and more efficient. It also increases the converter chopping frequency and consequently decreases the size of smoothing reactors and filters used in the system. Concerning the design of such converters, it requires a long working period of time with a significant cost and specific technical tests at nominal operating points. Therefore, simulation can essentially decrease economic and development costs. Using modulation and simulation software techniques (Simplorer, Simulink, and Matlab) throughout this paper helped simulation of very fast the converter behavior and accurate determination of its dynamic characteristics. Moreover, the paper deals with modulation of voltage control technique using Matlab and Simplorer, thus regulating the converter output current and voltage. Simulation results show that this control technique provides robust output current and voltage of step up converters and is more feasible for their chopper up conversion technique.

A New Meta-Heuristic Optimization Algorithm Inspired by FIFA World Cup Competitions: Theory and Its Application in PID Designing for AVR System

This paper presents a new optimization algorithm based on human society’s intelligent contests. FIFA World Cup is an international association football competition competed by the senior men’s national teams. This contest is one of the most significant competitions among the humans in which people/teams try hard to overcome the others to earn the victory. In this competition there is only one winner which has the best position rather than the others. This paper introduces a new technique for optimization of mathematic functions based on FIFA World Cup competitions. The main difficulty of the optimization problems is that each type of them can be interpreted in a specific manner. World Cup Optimization (WCO) algorithm has a number of parameters to solve any type of problems due to defined parameters. For analyzing the system performance, it is applied on some benchmark functions. It is also applied on an optimal control problem as a practical case study to find the optimal parameters of PID controller with considering to the nominal operating points $$(K_{g}$$ , $$T_{g})$$ changes of the AVR system. The main objective of the proposed system is to minimize the steady-state error and also to improve the transient response of the AVR system by optimal PID controller. Optimal values of the PID controller which are achieved by WCO algorithm are then compared with particle swarm optimization and imperialist competitive algorithm in different situations. Finally for illustrating the system capability against the disturbance, it is applied on a generator with disturbance on it and the results are compared by the other algorithms. The simulation results show the excellence of WCO algorithm performance into the nature base and other competitive algorithms.

Monitoring of operating point and process dynamics via probabilistic slow feature analysis

Traditional multivariate statistical process monitoring (MSPM) approaches aim at detecting deviations from the routine operating condition. However, if the process remains well controlled by feedback controllers in spite of some deviations, alarms triggered in this context become no longer necessary. In this regard, slow feature analysis (SFA) has been recently applied to MSPM tasks by Shang et al. (2015), which allows for seperate distributions of both nominal operating points and dynamic behaviors. Since a poor control performance is always characterized by dynamics anomalies, one can discriminate nominal operating deviations with acceptable control performance, from real faults that deserve more attentions, according to the temporal dynamics of processes. In this work, we propose a new process monitoring scheme based upon probabilistic SFA (PSFA). Compared to deterministic SFA, its probabilistic extension takes the measurement noise into considerations and allows for missing data imputation conveniently, which is beneficial for process monitoring. Apart from generic T2 and SPE metrics for monitoring the operating point, a novel S2 statistics is considered for exclusively monitoring temporal behaviors of processes. Two case studies are provided to show the efficacy of the proposed monitoring approach.