Conventional Cascade Control Research Articles

Direct Predictive Voltage Control for Grid-Connected Permanent Magnet Synchronous Generator System

In this paper, a direct predictive voltage control (DPVC) is proposed for the grid-connected permanent magnet synchronous generator (PMSG) system. In the proposed strategy, the dc-link voltage regulation and power regulation terms are merged into one cost function, thereby eliminating the conventional cascade control structure. The strategy selects the voltage vector that not only will generate a dc-link voltage closer to the setpoint at instant <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$k+2$</tex-math></inline-formula> , but will generate an active power that can further reduce that voltage error for the future instant. To cope with the mismatch between the actual and nominal parameters, Kalman filter has been added to compensate for the steady-state error. Besides, to enhance the dynamic performance and to save the effort of parameter tuning, the weighting factor is further eliminated from the cost function by sorting the terms into two groups and independently evaluating each group in turn. To fully validate the effectiveness of the proposed strategy, the experimental test waveforms of the DPVC strategy with and without weighting factor have been presented and compared with that of the conventional PI-MPC strategy in various testing conditions.

An LSTM-PSO-Based Predictive Strategy and Its Application in the Main Steam Temperature Control in Power Plants

We proposed an LSTM-PSO predictive controller strategy to overcome the large delay and strong disturbance during main steam temperature control. First, the long short-term memory (LSTM) network model was developed to forecast the future trend of mainstream temperature, and the deviation between the model output and the expected value after feedback correction was minimized by the particle swarm optimization (PSO) algorithm. Then, the LSTM-PSO predictive controller was taken as the primary controller to maintain the steam temperature of the coal-fired boiler. A comparative simulation with conventional PID cascade control and dynamic matrix controller strategies was given, and the test calculations indicate that the adjustment time was shortened and the overshoot was reduced. Finally, the LSTM – PSO predictive control strategy was deployed into the actual boiler operation process of a 300 MW coal-fired power plant, within 0.5 °C steady-state error and 2 °C dynamic error.

An Adaptive Backstepping Sliding-Mode Control for Improving Position Tracking of a Permanent-Magnet Synchronous Motor With a Nonlinear Disturbance Observer

To provide reliable position control for a permanent-magnet synchronous motor (PMSM) under conditions of lumped disturbances such as external load torque fluctuation and system parameter variation, an adaptive backstepping sliding-mode control (ABSMC) with a nonlinear disturbance observer (NDO) is proposed. An ABSMC is a non-cascade technique that employs a position-current single-loop control structure rather than a conventional cascade control structure for vector control of the PMSM. This method uses Lyapunov theory to design a control law to ensure the motor position reaches a desired value in a finite period of time and consequently achieves a rapid transient response. The proposed ABSMC incorporates an adaptive convergence gain to avoid a large overshoot under point-to-point position command. The NDO with a nonlinear observer gain is proposed to estimate unknown disturbances and provide feed-forward compensation for the ABSMC, improving robustness and reducing steady-state position error. The proposed methods are applied to an industrial motor drive to confirm their validity in real-world operating environments.

A Systematic Control Design Method with Active Damping Control in Voltage Source Converters

This paper proposes a systematic control design method for active damping control of grid-connected voltage source converters (VSCs). The proposed control method considers the conventional cascaded control loops and improves them by including additional states feedback-based active damping. In such a way, all control gains are lumped into one control gain matrix based on the proposed formulation. The lumping of all control gains into one matrix leads to a linear optimization problem, so different techniques can be used to calculate control gains. This work calculates them by using a simple but effective optimal control theorem as a noteworthy feature. The proposed control method can overcome the challenges of designing multiple control loops, evaluating wide time scale dynamics, and tuning required control parameters. Moreover, direct relationships between the proposed tuning parameters and system well-known stability and performance indicators such as maximum damping factor, minimum damping ratio, and the control efforts are identified, providing good physical insight. Finally, the proposed control structure and optimal gain calculations ensure power converter robustness against uncertainties in the grid’s short-circuit ratio (SCR) and different operating-point conditions. When the grid’s SCR changes from 10 (strong grid condition) to 1 (ultra-weak grid condition), the system under the proposed control method maintains good stability margins and simultaneously provides a fast dynamic response by facilitating the implementation of a high-bandwidth phase-locked loop (PLL). The performance of the proposed control strategy was investigated analytically and practically by conducting eigenvalue analysis, simulations, and experiments.

DC Drive Adaptive Speed Controller Based on Hyperstability Theory

The scope of this research is to develop a hyperstable adaptive control system of a direct current (DC) drive speed for effective load torque attenuation. The proposed speed controller is based on the model reference adaptive control framework and integrated into the conventional DC drive cascade control system. Its main features are as follows: (1) the boundedness of the control action signal, as well as the armature current control loop non-stationarities, are taken into consideration with the help of the reference model hedging technique; (2) its inputs include only measurable signals, thus there is no need to use any kind of state estimators; (3) it attenuates the disturbances, which are matched with its control action signal, particularly, the inertia moment non-stationarity and load torque. The asymptotic hyperstability of the obtained DC drive control system is proven with the help of Lyapunov’s theorems and Popov’s criterion. The numerical experiments corroborate the obtained results. They include the demonstration of disadvantages of the conventional cascade control system under conditions of the drive parameters’ non-stationarity and advantages of the proposed solution for different disturbance types and amplitudes.

Genetic Algorithm Applied to State-Feedback Control Design of Grid and Circulating Current in Modular Multilevel Converters

This paper discusses the application of a genetic algorithm (GA) to control system design for Modular Multilevel Converters (MMCs). In particular, genetic algorithm is used to compute the gains of a state-feedback controller for multi-input/multi-output (MIMO) plant model. This GA-optimized state-feedback controller is used to control both grid and circulating current of the MMC. This assures that the two currents’ input-coupled dynamics are managed using a MIMO strategy. A detailed MATLAB®/Simulink® model of a three-phase MMC is further used to validate the proposed control technique. Different simulations show that the GA-optimized state-feedback controller outperforms the conventional cascaded control.

A Full State-Variable Direct Predictive Control for Islanded Microgrids With Parallel Converters

In this work, we propose a high-quality control solution for islanded microgrids with multiparallel power converters; it uses a full state-variable direct model predictive control (FSV-DMPC) and has a simple structure. Unlike the conventional cascaded control loops, the proposed FSV-DMPC solution tracks the optimal reference generated by a robust droop loop using a unified cost function. This proposal enables the FSV-DMPC to be inserted into the entire control framework with plug-and-play capability; it is robust to parameter variations while also guaranteeing dynamics and stability. We conduct a deep analysis of the proposed approach, taking into account both the characteristics of the solution and the bounded stability of the system. Through comprehensive comparative studies with a classical double-loop linear controller, we validate that our solution achieves superior output voltage regulation during the load transients in terms of voltage error and settling time. Meanwhile, similar steady-state performances are accomplished for both methods. Finally, we verify our approach experimentally in different scenarios through a lab-constructed microgrid test bench. Experimental data confirm that the proposed approach achieves excellent steady-state and transient performances and obtains accurate load sharing.

Energy Saving Quadrotor Control for Field Inspections

This article presents a control strategy to improve the energy efficiency of a quadrotor for field inspections. A power consumption analytical model for a quadrotor is constructed, which indicates that the power consumption of a quadrotor is correlated with the roll and pitch angles. Based on that, a cascaded bounded control framework is developed to regulate the roll and pitch angles within the predefined energy-efficient ranges, while a successful inspection trajectory tracking can be achieved simultaneously. Experimental studies are carried out to prove the effectiveness of the proposed method through a mock-up scenario of wind farm inspection. Compared to the conventional cascaded control framework, the proposed energy saving control strategy is capable of saving 5% energy for the same inspection assignment under wind disturbance.

Dual PIO-Based Controller Design for Robustness Improvement of a Magnetic Levitation System

Magnetic levitation systems have a high potential in industrial applications. However, their nonlinear and open-loop unstable nature combined with susceptibility to system parameter uncertainties and external disturbances poses a challenging control problem. As a result, most control techniques applied to these systems are advanced but complex. This paper proposes a proportional–integral observer (PIO)-based control scheme still using a simple conventional cascade controller to enhance robustness. Design of the controller is presented along with computer simulations to verify that the controller is capable of maintaining nominal performance even under such severe conditions. In addition, the performance of the observer’s compensatory action is analyzed according to the singular perturbation theory.

Impact of converter control strategy on low- and high-frequency resonance interactions in power-electronic dominated systems

The transient stability of power-electronic dominated systems such as wind farms and photo-voltaic power plants depends on the control settings of the involved converters, the grid resonances and the system’s operating conditions. This paper focuses on investigating the impact of converter control algorithms on possible interactions that could endanger the overall stability of these kinds of systems. Mainly, two control approaches namely the conventional cascaded control structure, consisting of an inner current-control loop and various outer-loop controls, and the immediate voltage control structure which generates the converter voltage directly from the power and voltage controllers without necessarily including a current-control loop, are compared. The latter control structure falls in the class of grid-forming control strategies and the impact of different variants of this control structure will also be investigated. For this purpose, the ac-side input admittance of a grid-connected converter is derived for the two approaches and passivity-based frequency-domain stability analysis is used to assess their impact on system stability. Finally, the analytical findings are verified through time-domain simulations and recommendations to increase system stability are given accordingly.

An empirical Study Investigation of Task Allocation Process Barriers in the Context of Offshore Software Development Outsourcing: An Organization Size Based Analysis

This research presented a holistic approach in determining the trade-off optimized Proportional-Integral-Derivative (PID) tunings for both servo and regulatory controls of the cascade control loop by using Genetic Algorithm (GA). Performance of GA-based PID tunings was significantly compared with the IMC-based single loop tunings and conventional cascade control tunings. GA-based PID tunings eliminated the complicated mathematic calculations in obtaining the correlation PID tuning values and also reduce the dependency on engineering knowledge, experience, and skills. The performance of transient and steady-state responses was compared through time domain specification, performance index, and process response curve. It is concluded that the GA-based PID tunings for the cascade control loop had produced the best result for both servo and regulatory control objectives, which is eventually determined.

Full-State Feedback Control of Back-to-Back Converters Based on Differential and Common Power Concepts

The back-to-back (BTB) converter is one of the most popular converter topologies for the control of electrical machines, power transmission systems, and power quality applications. The conventional cascade control structure is commonly used due to its simple design procedure and reliable operation. However, the bandwidth of the dc-link voltage controller has to be limited in order to avoid instability issues and be restrictive in high-performance applications. This paper presents a differential- and common-current (power)-based state-feedback control for BTB converters. This controller features a fast control of active and reactive powers, and a stiff regulation of the dc-link voltage. A theoretical analysis of the proposed controller along with a strategy for current limiting is presented. Its robustness was tested against variations of the filter parameters, the dc capacitance, the grid inductance, and the grid voltage values. The controller was experimentally validated under nominal operation, voltage sags, and connected to a weak grid by using a 15-kVA BTB converter in a laboratory test network.

Model Predictive Voltage and Power Control of Islanded PV-Battery Microgrids With Washout-Filter-Based Power Sharing Strategy

This paper proposes a new control strategy of microgrids for the improved voltage quality. In the existing control techniques, the droop control is commonly adopted as a decentralized power sharing method at the cost of voltage deviations. Besides, the conventional cascaded control featuring relatively slow dynamic response shows difficulties in handling the fluctuation of renewable energy outputs, leading to further voltage quality deterioration. In this paper, an advanced model predictive power control strategy by considering the battery constraints is proposed for the bidirectional dc–dc converters to smooth the solar photovoltaic (PV) outputs and stabilize the dc-bus voltages. A model predictive voltage control scheme taking into account the voltage changing trend is then developed to control the distributed inverters to improve the output ac voltages. Furthermore, a washout-filter-based power sharing approach with the plug-and-play capability is adopted to achieve a proper load sharing among parallel inverters and mitigate the voltage deviation. The proposed control strategy is numerically simulated in MATLAB/Simulink and experimentally verified by hardware-in-the-loop tests under the condition of fluctuating PV outputs and variable power demands. (This paper is accompanied by a video showing the hardware-in-the-loop test.)

A novel sliding mode single-loop speed control method based on disturbance observer for permanent magnet synchronous motor drives

A novel speed control method based on sliding mode control and disturbance observer is studied for permanent magnet synchronous motor drives. Different from the conventional speed and current cascade control structure in the field-oriented vector control, the new controller adopts the single-loop control structure, in which the speed and quadrate axes current controllers are combined together. First, a multiple-surface sliding mode controller is designed for the speed control system of permanent magnet synchronous motor. Although the sliding mode controller has the strong robustness for the matched disturbance in the system, it still cannot deal with mismatched disturbance effectively, such as external load disturbance and some parameter variations. Thus, the disturbance observer is introduced to estimate the disturbance in the motor, which is designed by combining the proposed sliding mode controller. Finally, the effectiveness is tested under various conditions by both simulation and experiment. The results show that the designed controller has the fast transient response and robustness under different operating conditions.

Position Control Improvement of Permanent Magnet Motor Using Model Predictive Control

Fast and accurate transient response is the main requirement in electric machine position control. Conventional cascade control structure has sluggish response due to the limitation of inner control loop bandwidth. In this paper, in order to decrease the Permanent Magnet Synchronous Motor (PMSM) transient response time it can be used reference model using feed-forward signals. In this structure, feed-forward signals generated by simplified model of permanent magnet synchronous motor. In this paper, feed-forward signals generated are emplyed in model predictive control; which are combined with conventional cascade control structure. Using this approach, a fast transient response and satisfactory tracking ability will be guaranteed. The proposed method is compared with the model reference method and conventional cascade structure. Simulation results showed a good performance of proposed method related to both methods. Verification of simulation results were carried out by experimental results

Gain‐scheduled constrained state feedback control of DC–DC buck power converter

A gain-scheduled constrained state feedback controller (SFC) for a SiC MOSFET direct current (DC)-DC buck power converter with a non-synchronous control strategy is presented. First, the synthesis process of a gain-scheduled SFC for a control system with a state-dependent parameter is proposed. Next, constraints of important states and control variables are a posteriori introduced into control algorithm to provide satisfactory dynamic behaviour and safe operation of the plant. The boundary values of control signals which provide permissible values of the future state-space variables are derived from the models of power converters that take into account two modes of operation (i.e. charging and discharging mode). The novelty of the proposed approach lies in efficient control algorithm that utilises the gain-scheduled SFC with the parameter-dependent constraint handling method. The designed control algorithm is extensively tested through simulation and experiments at a switching frequency of 36 kHz. The proposed solution is further compared with a conventional cascade control structure based on two proportional integral controllers for voltage and current loops as well as with a non-constrained state feedback control structure to illustrate its performance and low computational complexity.

Digital fixed-frequency hysteresis current control of a BLDC motor applied for aerospace electrically powered actuators

In the conventional cascade control structure of aerospace electrically powered actuators, the current (or electromagnetic torque) loop plays a critical role in realizing a rapid response for a digitally controlled BrushLess Direct Current (BLDC) motor. Hysteresis Current Control (HCC) is an effective method in improving the performance of current control for a BLDC motor. Nevertheless, the varying modulating frequency in the traditional HCC causes severe problems on the safety of power devices and the electromagnetic compatibility design. A triangular carrier-based fixed-frequency HCC strategy is expanded by relaxing the constraints on the rising and descending rates of the winding current to advance the capability of HCC to realize fixed-frequency modulation in the steady state. Based on that, a new flexible-bound-size quasi-fixed-frequency HCC is proposed, and the range feasible to realize fixed-frequency modulation control can cover the entire running process in the steady state. Meanwhile, a corresponding digital control strategy is designed, and four digitalization rules are proposed to extend the capacity to achieve fixed-frequency modulation control to the unsteady working state, that is, a novel fixed-frequency modulation is realized. Simulation and experimental results prove the effectiveness of this improved fixed-frequency HCC strategy.

Feedback Linearization-Based Current Control Strategy for Modular Multilevel Converters

Modular multilevel converters (MMCs) are multi-input multi-output (MIMO) nonlinear systems. The control systems for MMCs are required to simultaneously achieve multiple control objectives, e.g., output current regulation, submodule capacitor voltage control, and circulating ripple currents suppression. Existing cascaded control strategies for MMCs achieve those control objectives with relatively complex controllers, and the controller parameter design is normally difficult for such nonlinear systems with highly coupled states. In view of this, a feedback linearization-based current control strategy is proposed for an MMC system in this paper. The nonlinear state function model of the MMC is presented and transformed to a linearized and decoupled form with the help of the input-output feedback linearization technique. Based on the linearized system, simple linear controllers are employed to regulate the output and inner differential currents of the MMC, which significantly reduces the difficulty in controller design. The stability of the proposed control strategy is analyzed. The experimental verification results show that, compared to the conventional cascaded control strategies for MMCs, the proposed feedback linearization control strategy is able to achieve improved steady-state and dynamic performances. The robustness of the proposed control strategy against parametric uncertainties is experimentally investigated.

Variable Cascade Control Structure for Tubular Reactors

AbstractThe application of a variable control structure for tubular reactors, based on multiple temperature measurements, is explored. The proposed structure allows the controller to adapt to temperature error variations along the tubular reactor. Controllers can be designed from simple input‐output first‐order dynamical models obtained from step responses and are coupled with a dynamic law for assigning a variable weight to conventional feedback cascade control loops. The simulation results show that the variable structure based on linear proportional‐integral controllers enhances the performance and robustness properties of conventional cascade schemes for controlling the outlet reactor concentration. Major advantages are shown in the face of a variety of concentration and temperature feed disturbances.

Minimum Entropy-Based Cascade Control for Governing Hydroelectric Turbines

In this paper, an improved cascade control strategy is presented for hydroturbine speed governors. Different from traditional proportional-integral-derivative (PID) control and model predictive control (MPC) strategies, the performance index of the outer controller is constructed by integrating the entropy and mean value of the tracking error with the constraints on control energy. The inner controller is implemented by a proportional controller. Compared with the conventional PID-P and MPC-P cascade control methods, the proposed cascade control strategy can effectively decrease fluctuations of hydro-turbine speed under non-Gaussian disturbance conditions in practical hydropower plants. Simulation results show the advantages of the proposed cascade control method.