Load Disturbance Rejection Research Articles

Multivariable Correlation-based Tuning for Load Disturbance Rejection

Abstract In many industrial processes, the setpoint signals present few changes and variations in the output are due mainly to the disturbances that enter the closed-loop and may be understood as load disturbances. Even though this is a common problem, the response to load disturbances is a topic not well covered in the direct data-driven control literature. This work seeks to fill that gap presenting a direct data-driven method to tune a multivariable controller in order to achieve certain load disturbance response described by some reference model. This work extends a previous one dealing with the monovariable case and uses the same correlation approach employed before in the Correlation-based Tuning (CbT).

Modelling and multi-loop selective control of industrial coal pulveriser

Coal-based power generation continues to be a key player in power generation. Pulverisation of coal is an important, but an energy intensive process. Efficient control of the pulveriser can improve the efficiency of the power plant, which otherwise lies in the range of 30-40%, a rather low value. This paper discusses various computational results and their analysis for the case study carried out at ESSAR thermal power plant, Hazira, Gujarat, India. It proposes more accurate mathematical model for a pulveriser and a 3PI (proportional integral) with selective control for improved performance. The model is validated with real plant data and is observed to capture system dynamics more effectively than the models existing in the literature. It is observed that the new model with proposed control has better load disturbance rejection properties and is more robust to parametric variations compared to the control strategy employed in the industry.

An Ellipse-Optimized Composite Backstepping Control Strategy for a Point-of-Load Inverter Under Load Disturbance in the Shipboard Power System

The shipboard power system (SPS) is expected to be configured as a hybrid microgrid (MG) to realize redundant bus design and high penetration of renewables (REs). Point-of-load (POL) inverters in SPSs play a critical role to feed for sensitive ac load. To realize voltage-oriented control objective under load disturbance due to load uncertainty or nonlinearity, this paper proposes an ellipse-optimized composite back-stepping control strategy for a POL inverter. Firstly, the backstepping algorithm is harnessed to derive the pseudo-inductor-current-loop reference and decoupled switching functions in d-q frame, which can rigorously guarantee the global large-signal stability of the system. A Kalman filter is designed to estimate the load current and feedforward it to the backstepping controller for load disturbance rejection, saving three current sensors. The controller gains are quantitatively selected to achieve the optimal system damping and maximized dynamic response, which can be intuitively interpreted via an ellipse-based strategy from a geometrical point of view. Rigorous stability proof and robustness analysis of the system is also provided. Finally, simulation and experimental results verify the effectiveness of the proposed control scheme.

Modelling and multi-loop selective control of industrial coal pulveriser

Coal-based power generation continues to be a key player in power generation. Pulverisation of coal is an important, but an energy intensive process. Efficient control of the pulveriser can improve the efficiency of the power plant, which otherwise lies in the range of 30-40%, a rather low value. This paper discusses various computational results and their analysis for the case study carried out at ESSAR thermal power plant, Hazira, Gujarat, India. It proposes more accurate mathematical model for a pulveriser and a 3PI (proportional integral) with selective control for improved performance. The model is validated with real plant data and is observed to capture system dynamics more effectively than the models existing in the literature. It is observed that the new model with proposed control has better load disturbance rejection properties and is more robust to parametric variations compared to the control strategy employed in the industry.

Control System Loop-Shaping as a Mathematical Optimization Problem: An Ensemble of Models

This paper proposes an extension to the established loop-shaping method, where the loop-shaping method is modeled mathematically and solved as a mathematical optimization program. In this process, a novel cost function is proposed and employed as the objective of optimization in the controller tuning problem. This cost function evaluates different characteristics of the system in the frequency domain, such as the stability margins and the operability bandwidth. These characteristics reflect the quality of load disturbance rejection, set-point tracking, and operability bandwidth of the control system. Consequently, the proposed technique circumvents running a time-domain simulation completely. More importantly, the proposed technique can be used with any controller structure. The generic cost function can be customized to fit any control objective and application. This is conducted in the second half of this paper, with the aim of obtaining a system with near-optimum time-domain performance with respect to the integral of time times absolute error (ITAE) criterion. At the same time, the control system obtained with the customized cost function has better operability bandwidth than that of a system optimized in the time-domain for the ITAE criterion. The proposed optimization model is analyzed for two plant models: first order and second order plus dead-time (FOPDT), (SOPDT), respectively.

Gain Scheduled Multivariable control of a Continuous Stirred Tank Reactor

Most chemical reactors, representing Multi-Input Multi-Output (MIMO) systems, are highly nonlinear and require complex control than single-input single-output (SISO) systems. In the present work, the system is linearized around different operating points. As linear design strategies are intended to work in linear regions, they are employed to evaluate local controller parameters. Decentralized controllers using constant detuning factor and decoupling controllers using static decouplers are designed. A gain scheduler is developed to deal with changes in operating conditions. The performances are evaluated by simulating the nonlinear equations of the system. Decentralized controllers cannot mitigate the interactions due to its structure possessing single loop. It is exhibited that the decoupling controllers offer better control for tracking of desired set point and rejection of load disturbances than those of decentralized controllers. The superiority of the suggested controller has been portrayed by performing simulation study on a Continuous Stirred Tank Reactor (CSTR).

A Modified Cascade Control Strategy for Tobacco Re-Drying Moisture Control Process With Large Delay-Time

A novel series structure is proposed based on cascade control for dominating a class of unstable processes with large delay-times. The modified structure consists mainly of one controller that exists in the inner loop, and the inner loop controller is devised on the basis of internal model control (IMC) principles. The outer loop setpoint tracking controller and the inner loop load disturbance rejection controller are designed according to the H2 optimal performance. At the same time, the tuning parameters are selected through the experience approach. Moreover, a suitable example and real value are recommended for the inner loop controller based on a wide-ranging simulation, and the results are implemented to certify the effectiveness of the proposed method. Simultaneously, the proposed method is applied to in a real-world tobacco production line, and the test results prove that the above method is significant in terms of rapidity, robustness and residual errors.

Explicit Model-Based Real PID Tuning for Efficient Load Disturbance Rejection

In the process control, many PID loops are primarily devoted to rejecting load disturbances, and some of them are crucial for the quality of the overall plant operation. In such a scenario, automatic tuning is highly desired. However, load disturbance rejection calls for strong feedback up to quite high frequencies with respect to the dominant plant dynamics, on which most tuning rules are centered. As such it is difficult for a rule to yield good and, above all, uniform results in the face of all the various process structures it can be confronted with. In this paper, we propose an explicit model-based PID tuning rule specifically targeted at the problem just evidenced. The rule minimizes the magnitude of the nominal disturbance-to-output frequency response, at the same time preventing that magnitude to exhibit a peak or a plateau around its maximum. This characteristic, together with tuning the PID derivative filter, leads to sharp disturbance rejection without incurring in an excessive control sensitivity to high-frequency measurement noise and mitigates the problems caused by heterogeneous process dynamics. The proposed approach is assessed by comparing the rule with selected counterparts, on a literature benchmark with different process structures. A laboratory experiment is finally presented to show that our rule can withstand real-world operating conditions.

New hybrid‐microgrid topology using a bidirectional interleaved converter as a robust power interface operating in grid‐connected and islanded modes

This study presents a new microgrid topology that uses a bidirectional interleaved converter performing a power interface between DC buses in a hybrid microgrid allowing for both grid-connected and islanded modes. The authors propose a new control strategy and controllers' design method aiming at achieving a high-performance dynamic response regarding the converter load and generation disturbance rejection capability. In the grid-connected mode, the interleaved converter operates in the buck mode providing a high-power-quality DC microgrid voltage. In the islanded mode, the interleaved converter operates in the boost mode and it is responsible for regulating the DC link of the back-to-back converter that connects the main grid to the AC microgrid. A detailed mathematical model is presented to obtain a MIMO system that takes into account the system's disturbances to analyze both stability margins and disturbance-rejection response. Simulations of the proposed topology are carried out in PSCAD/EMTDC in a microgrid operating in grid-connected and islanded operation modes. Experimental results are provided in order to validate the proposed control tuning method.

Real Time Simulation of Modified Bias Based Load Disturbance Rejection Controller for Frequency Regulation of Islanded Micro-Grid

Abstract The paper presents a real time modelling of self-reliant isolated grid comprising both governable and ungovernable sources. The frequency of this system must be maintained to its desired value by reducing its deviation occurred when there is a mismatch between the load demand and the actual power generation. In order to achieve it, the actual power output of the micro-grid is required to regulate. Therefore, an attempt is made in this paper to design a modified bias (MB) with LDR (load disturbance rejection) based PID (proportional integral derivative) Controller and it can also be called droop based controller (DBC). Simulation of the micro-grid system with proposed MB-LDR based control scheme is successfully done in Real Time Simulation platform using the digital simulator of OPAL-RT OP4510. Simulation results of the system for different conditions are presented and analysis is given in details. Thereafter, comparative performance of the results obtained using proposed MB-LDR based controller with that of LDR and classical controllers is made. Results show that the proposed MB-LDR based controller gives better response as compare to other two methods in terms of peak transient deviation, settling time and number of oscillations.

Torque Ripple Minimization and Control of a Permanent Magnet Synchronous Motor Using Multiobjective Extremum Seeking

In this paper, a multiobjective extremum-seeking (MOES) approach is proposed for torque control of a permanent magnet synchronous motor and minimization of its torque ripple. The latter aspect is important in human–machine interface applications such as haptic interfaces requiring smooth torque profiles at slow speeds. The proposed MOES scheme combines an adaptive iterative learning control method with an adaptive proportional-integral (PI) controller, which makes the system less sensitive to load disturbances and improves the control performance for torque regulation during transient events. Experiments are performed on a proof-of-concept exercise machine that generates desired torque profiles and mechanical impedance based on user's preference. The performance of the proposed controller is further compared with a recently proposed adaptive PI controller. The experimental results validate the effectiveness of the proposed controller in terms of torque ripple suppression, steady-state and transient performance, and load disturbance rejection.

Direct Synthesis Based PID Controller Design for Time-Delayed Unstable Processes in Parallel Control Structure

In this paper, a proportional–integral–derivative controller design for unstable time-delay system in the parallel control structure is proposed. The parallel control structure has two separate controllers for the set-point tracking and the load disturbance rejection. The two proposed controllers are designed through direct synthesis approach by selecting the separate closed-loop reference model for the set-point and the load disturbance response. The proposed design methods are applicable for both the low-order and the high-order unstable processes without model reduction in the higher-order process. The effectiveness of the proposed methods is demonstrated through simulation of different examples and comparison with the recently reported methods in the literature.

Novel tuning rules for PIDC and PID load frequency controllers considering robustness and sensitivity to measurement noise

A novel method for designing Load Frequency Controllers (LFC) is proposed in the present paper. Proportional-Integral-Derivative controllers with and without serial compensator – PID and PIDC – are considered in contexts of single-area and multi-area power systems. The proposed design methodology focuses on achieving efficient load disturbance rejection, while simultaneously preserving an adequate level of robustness with respect to measurement noise and model uncertainties. Explicit tuning formulas are provided for single-area power systems with reheated, non-reheated and hydro turbines, characterized by two free parameters. Dependence of integral performance measures (IAE and ITAE), as well as TVd for evaluating control effort, and sensitivity indices (Ms, Mp and Mn) in terms of these parameters is analyzed, and the associated performance-robustness trade-off is elaborated in detail. The proposed method is extended to multi-area power systems in a straightforward manner using decentralized PIDC/PID tuning. A comparative numerical study is performed involving several LFC techniques recently reported in literature.

Gain scheduling design based on active disturbance rejection control for thermal power plant under full operating conditions

To integrate more renewable energy into the power grid, thermal power plants have to accelerate the speed of power output and extend their operating ranges, and this can result in great challenges for their safe operations and even safety accidents. To this end, a gain scheduling design based on active disturbance rejection control (ADRC) is proposed for thermal power plants under full operating conditions. The urgency of the proposed control strategy is illustrated by analyzing the control difficulties of coordinated control systems. Then the scheduling parameter selection and the linear switching method for the proposed control strategy are analyzed. Moreover, the qualitative stability analysis based on the Kharitonov theorem and the quantitative calculation of stability regions are carried out to ensure the stability of the closed-loop system. Simulations of the power tracking under different load tracking rates and disturbance rejection under the coal quality variation are carried out. Simulation results show that the tracking and disturbance rejection performance in both power output and throttle pressure loops has been improved simultaneously compared with the regular ADRC and the traditional proportional-integral control strategies. Based on the verified superiority, the proposed gain scheduling design based on ADRC shows a promising potential in industrial applications.

Rejection of varying-frequency periodic load disturbances in wind-turbines through active disturbance rejection-based control

Wind turbines are complex mechatronic systems subject to different kinds of disturbances. Among all disturbance sources, those from periodic nature are the most relevant and important for maintaining the structural health of the entire wind turbine. However, periodic disturbances in wind turbines, mainly given by wind shear, tower shadow and rotational sampling of turbulence, have a variable fundamental frequency that changes as function of the turbine angular speed. In consequence, conventional controllers have issues to mitigate such varying-frequency periodic loads. This paper proposes two individual pitch control schemes to attenuate the main periodic load components of blade flap-wise and hub yaw/tilt-wise bending moments in horizontal-axis wind turbines under varying-frequency conditions. Both proposals are founded on the paradigm of Active Disturbance Rejection Control (ADRC): the former is formulated in the time-domain using a Linear-Time-Invariant (LTI) standard controller, while the latter is designed in the angular position domain using a Linear-Position-Invariant (LPI) approach. Several realistic simulations with different wind turbulence intensities and average wind speeds are performed using a 5 MW three-bladed horizontal-axis wind turbine implemented in the FAST code by NREL. Simulation results of the proposed control schemes show improved load reductions compared with other control techniques.

Current Sharing Control for Parallel DC–DC Buck Converters Based on Finite-Time Control Technique

The problem of current sharing controller for parallel dc–dc buck converter system is investigated in this paper. Specifically, to achieve the goal of sharing control and improve the systems dynamic performance, a new finite-time current sharing control algorithm is designed and employed. Under the proposed control algorithm, rigorous proofs show that not only the output voltage of the converter system can reach the desired reference voltage in a finite time, but also the objective of current sharing can be achieved within almost the same time. In addition, when the external load is time-varying and unknown, a finite-time load estimator is given to handle the load variation, and an adaptive finite-time current sharing control algorithm is subsequently developed. Experimental results are presented to verify the effectiveness of the proposed method, and to show its advantages over some traditional control algorithms. It is shown that a faster convergence rate and a better load disturbance rejection performance can be yielded by the present algorithm. Finally, it is shown that the main results can be extended to the case of $n$ parallel dc–dc buck converters.

Set Point Tracking and Load Disturbance Rejection with PID and I-PD Controllers in Different Zones of Barrel Heating System

Set Point Tracking and Load Disturbance Rejection with PID and I-PD Controllers in Different Zones of Barrel Heating System

A Disturbance Rejection Control Strategy for Droop-Controlled Inverter Based on Super-Twisting Algorithm

In distributed generation systems, multiple inverters paralleled operation is essential for the power requirement of local loads. To achieve the reasonable power distribution among the inverters, droop control is an effective method for the paralleled operation systems without the control interconnection. In general, the multi-loop control structure is usually utilized to regulate the drooped voltage across local loads. However, by using the traditional multi-loop controller, such as proportional-integral and proportional-resonant controllers, the dynamic process of load variation or mode transfer is slow, even the voltage overshoot or drop may occur. To address the above issues, a compact control structure-based super-twisting algorithm has been proposed in this paper. Benefitting from the designed sliding mode controllers, the dynamic response, the load disturbance rejection, and reference tracking performance can be improved. In addition, the robustness to parameters mismatch can also be achieved by using the proposed control method. The comparative simulation and experimental results validate the correctness and feasibility of the proposed method.

PI/PID Control Design Based on a Fractional-Order Model for the Process

Abstract In this paper we propose a new tuning rule called FOMRoT for integer PID and PI controllers. Based on a fractional order model, the devised tuning rule aims at providing a good performance in both set-point tracking and load disturbance rejection tasks, with a constraint on the maximum sensitivity. The comparison with other tuning rules which also deal with these trade-offs shows the effectiveness of the tuning rule, which can be applied with processes with different self-regulated dynamics without the need to change the structure of the considered model.

Modified disturbance observer-based control for stable multivariate processes with multiple time delays

In this study, the authors propose a modified disturbance observer-based (DOB) control strategy for input load disturbance rejection, in cases of stable multivariate processes with multiple time delays. Unlike the existing multivariate disturbance observer-based (MDOB) control strategies using approximate inverse model to estimate external disturbances, the proposed MDOB control strategy instead designs a V canonical structure, which can estimate the external disturbances rapidly and exactly. Based on the V canonical structure, the controllers for disturbance rejection are designed analytically by minimizing 2-norm of the outputs caused by disturbances. The tunable parameters of the controllers are determined by considering both the control performance and robustness. Simulation studies demonstrate the effectiveness of the proposed method.