Induction motor control: multivariable analysis and effective decentralized control of stator currents for high performance applications

Adequate control of the stator currents is a fundamental requirement for several high-performance induction motor (IM) control schemes. In this context, classical linear controllers remain widely employed due to their simplicity and success in industrial applications. However, the models and methods commonly used for control design lack valuable information –which is fundamental to guarantee robustness and high performance. Following this line, the design and existence of linear fixed controllers is examined using individual channel analysis and design. The studies here presented aim to establish guidelines for the design of simple (time-invariant, low order, stable, minimum-phase and decentralized), yet robust and highperformance linear controllers. Such characteristics ease the implementation task and are well suited for engineering applications, making the resulting controllers a good alternative for the stator currents control required for high-performance IM schemes; e.g., field oriented, passivity-based and intelligent control. Illustrative examples are presented to demonstrate the analysis and controller design of an IM, with results validated in a real-time experimental platform. It is shown that it is possible to completely decouple the stator currents subsystem without the use of additional decoupling elements

Fast frequency support control in the GB power system using VSC-HVDC technology

A fast frequency support control scheme for voltage source converter based high-voltage direct-current (HVDC) links has been designed, simulated and experimentally implemented and validated. The effectiveness of the proposed scheme has been tested on simplified GB power system models with both averaged and switched converter models. System performance has been initially assessed using different software simulation platforms (PSCAD and MATLAB/Simulink). System validation has been carried out using an experimental test-rig. It is shown that simulation and experimental results agree on well when the fast frequency support provision is enabled. For completeness, the effectiveness of the control scheme has been tested for two contingency scenarios: (i) when a high-voltage alternating-current interlink in parallel with the HVDC link is disconnected, and (ii) for a substantial increase in system load

Criterion for the electrical resonance stability of offshore wind power plants connected through HVDC links

Electrical resonances may compromise the stability of HVDC-connected Offshore Wind Power Plants (OWPPs). In particular, an offshore HVDC converter can reduce the damping of an OWPP at low frequency series resonances, leading to system instability. The interaction between offshore HVDC converter control and electrical resonances of offshore grids is analyzed in this paper. An impedance-based representation of an OWPP is used to analyze the effect that offshore converters have on the resonant frequency of the offshore grid and on system stability. The positive-net-damping criterion, originally proposed for subsynchronous analysis, has been adapted to determine the stability of the HVDC-connected OWPP. The reformulated criterion enables the net-damping of the electrical series resonance to be evaluated and establishes a clear relationship between electrical resonances of the HVDC-connected OWPPs and stability. The criterion is theoretically justified, with analytical expressions for low frequency series resonances being obtained and stability conditions defined based on the total damping of the OWPP. Examples are used to show the influence that HVDC converter control parameters and the OWPP configuration have on stability. A root locus analysis and time-domain simulations in PSCAD/EMTDC are presented to verify the stability conditions

Input-output signal selection for damping of power system oscillations using wind power plants

AbstractDuring the last years wind power has emerged as one of the most important sources in the power generation share. Due to stringent Grid Code requirements, wind power plants (WPPs) should provide ancillary services such as fault ride-through and damping of power system oscillations to resemble conventional generation. Through an adequate selection of input–output signal pairs, WPPs can be effectively used to provide electromechanical oscillations damping. In this paper, different analysis techniques considering both controllability and observability measures and input–output interactions are compared and critically examined. Recommendations are drawn to select the best signal pairs available from WPPs to contribute to power oscillations damping. Control system design approaches including single-input single-output and multivariable control are considered. The recommendation of analysis techniques is justified through the tools usage in a test system including a WPP

Flux-torque cross-coupling analysis of FOC schemes: Novel perturbation rejection characteristics

Field oriented control (FOC) is one of the most successful control schemes for electrical machines. In this article new properties of FOC schemes for induction motors (IMs) are revealed by studying the cross-coupling of the flux-torque subsystem. Through the use of frequency-based multivariable tools, it is shown that FOC has intrinsic stator currents disturbance rejection properties due to the existence of a transmission zero in the flux-torque subsystem. These properties can be exploited in order to select appropriate feedback loop configurations. One of the major drawbacks of FOC schemes is their high sensitivity to slip angular velocity perturbations. These perturbations are related to variations of the rotor time constant, which are known to be problematic for IM control. In this regard, the effect that slip angular velocity perturbations have over the newly found perturbation rejection properties is also studied. In particular, although perturbation rejection is maintained, deviations to the equilibrium point are induced; this introduces difficulties for simultaneous flux and torque control. The existence of equilibrium point issues when flux and torque are simultaneously controlled is documented for the first time in this article

Optimal energy management system using biogeography based optimization for grid-connected MVDC microgrid with photovoltaic, hydrogen system, electric vehicles and Z-source converters

Currently, the technology associated with charging stations for electric vehicles (EV) needs to be studied and improved to further encourage its implementation. This paper presents a new energy management system (EMS) based on a Biogeography-Based Optimization (BBO) algorithm for a hybrid EV charging station with a configuration that integrates Z-source converters (ZSC) into medium voltage direct current (MVDC) grids. The EMS uses the evolutionary BBO algorithm to optimize a fitness function defining the equivalent hydrogen consumption/generation. The charging station consists of a photovoltaic (PV) system, a local grid connection, two fast charging units and two energy storage systems (ESS), a battery energy storage (BES) and a complete hydrogen system with fuel cell (FC), electrolyzer (LZ) and hydrogen tank. Through the use of the BBO algorithm, the EMS manages the energy flow among the components to keep the power balance in the system, reducing the equivalent hydrogen consumption and optimizing the equivalent hydrogen generation. The EMS and the configuration of the charging station based on ZSCs are the main contributions of the paper. The behaviour of the EMS is demonstrated with three EV connected to the charging station under different conditions of sun irradiance. In addition, the proposed EMS is compared with a simpler EMS for the optimal management of ESS in hybrid configurations. The simulation results show that the proposed EMS achieves a notable improvement in the equivalent hydrogen consumption/generation with respect to the simpler EMS. Thanks to the proposed configuration, the output voltage of the components can be upgraded to MVDC, while reducing the number of power converters compared with other configurations without ZSC.This work was partially supported by Spain's Ministerio de Ciencia, Innovaci ' on y Universidades (MCIU), Agencia Estatal de Investigaci ' on (AEI) and Fondo Europeo de Desarrollo Regional (FEDER) Uni ' on Europea (UE) (grant number RTI2018-095720-B-C32), by the Federal Center for Technological Education of Minas Gerais, Brazil (process number 23062-010087/2017-51) and by the National Council of Technological and Scientific Development (CNPq-Brazil)

The multivariable structure function as an extension of the RGA matrix: relationship and advantages

It is common practice to specify the performance of control design tasks in terms of an output response to a given input. In spite of a greater complexity, this is also the case for multivariable plants, where for clarity of performance specification and design remains desirable to consider the inputs and outputs in pairs. Regardless of the structure and internal coupling of the plant, it is convenient to establish if decentralized control is capable of meeting design specifications: the control structure will be easy to implement, economic (less programming burden upon implementation), and may provide further physical insight. In line with this, the analysis and design of decentralized controllers using the relative gain array (RGA) and the multivariable structure function (MSF) are presented for the general multivariable case. It is demonstrated that the RGA matrix can be expressed in terms of the MSF. Moreover, it is shown that the correct interpretation of the MSF offers significative advantages over the RGA matrix analysis. While the RGA offers insight about the adequate pairing of input-output signals in a multivariable system, the MSF, besides providing this information, plays a crucial role in the design of stabilizing controllers (and their requirements) and the subsequent robustness and performance assessment of the closed loop control system. Theoretical results are drawn for a general n×n plant, with examples from electrical power systems and laboratory tank processes included to illustrate key concepts

Structural robustness assessment of electric machine applications using individual channel analysis and design

Adequate control of three-phase machines, such as induction motors -IMs- and synchronous generators, is of paramount importance for the electric power industry. These are multivariable, non-linear systems. In this paper, the individual channel analysis and design framework is used to formally demonstrate that the electrical subsystems of the IM and of the permanent magnet SG, due to their inherent structural robustness, are the multivariable equivalent to stable, minimum-phase, single-input single-output systems. As a cnsequence, an adequate performance and robustness may be achieved through fixed, stable, minimum-phase, diagonal controllers –justifying the widespread use of control schemes based on fixed, classical linear controllers such as PI

In-depth cross-coupling analysis in high-performance induction motor control

High-performance field oriented control (FOC) of induction motors (IMs) relies on the accurate control of their electrical dynamics. In particular, perfect decoupling control of the stator currents should be ideally achieved for a FOC scheme to be efficient. However, the decoupling effectiveness afforded by most stator currents controllers may be influenced not only by the parameters and the operating condition, but also by the specific controller structure and the adopted coordinate system. A measure to assess decoupling effectiveness is non-existent in the IM control literature. To bridge this gap, an in-depth analysis of the cross-coupling inherent characteristics of the electrical subsystem of IMs under different well-known control structures is presented in this paper. Specifically, four control strategies previously studied and experimentally validated in the literature are critically assessed in this work: 1) stationary frame proportional-integral (PI) control, 2) synchronous frame PI control, 3) synchronous frame PI control with decoupling networks, and 4) improved stationary frame diagonal control. The decoupling capabilities of controllers in stationary and synchronous coordinates are examined, with a detailed insight on the role of decoupling methods. The analysis is performed in the frequency domain under the framework of individual channel analysis and design (ICAD). By application of ICAD, the decoupling effectiveness of FOC schemes is clearly exposed and quantified, with an assessment of the effects of parametric uncertainty being carried out for completeness. The effect of the inverter dynamics over cross-coupling is also treated using digital simulations. The results are useful to determine the conditions in which each control strategy has either advantages or disadvantages. Additionally, it is possible to determine the effect of several operating parameters over the stator currents cross-coupling such as nominal flux and torque level