M. Vasak

University of Montenegro, Titograd, Podgorica, Montenegro

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Publications (36)14.67 Total impact

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    ABSTRACT: Renewable energy sources bring to fore the significance of power electronics and corresponding control. The topic of wind turbine fault-tolerant control has emerged as a promising line of research for improving their market competence. The paper presents summarized control of wind turbine converter with a fault-tolerant control extension for generator electromechanical faults, based on a proper modulation of the stator magnetic flux. The modulation also introduces hard periodic oscillations in the stator currents and generator output power. Simulation results obtained with a Matlab-Simplorer co-simulation and detailed power converter model show that DC-link capacitor and corresponding control algorithm act like an energy buffer that suppress the influence of generator stator oscillations on currents passed to the grid and maintain the high quality of power production. Following from the results, the generator and grid side controls are justified to be considered as two independent problems.
    7th IET international conference on Power Electronics, Machines and Drives, Manchester, United Kingdom; 04/2014
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    ABSTRACT: Increase of wind turbine availability is one of the main goals in wind energy research trends. The paper concerns with avoiding wind turbine shut- down under generator stator insulation degradation. We propose a fault-tolerant control that modulates the machine magnetic flux and reallocates the electrical stress from faulty to healthy generator stator parts, and extracts the maximum available power in the faulty operation under a strict condition of stopping further fault propagation. The algorithm is set for the case of 700 kW wind turbine with direct-drive permanent magnet synchronous generator but can be applied for any other type of generator as well. Proposed method is suitable for a wide range of generator operation and fault conditions, from early stage of fault-development to inter-turn short circuit. The method is also conceived as a modular extension to the conventional wind turbine control system. Simulation results are obtained and possible wind turbine operating area under different stage of fault is mapped.
    European Wind Energy Association 2014 Annual Event, Barcelona, Spain; 03/2014
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    ABSTRACT: Electrical machines in the high-voltage class are usually designed with open stator slots. This wide open slots cause an increase of higher order harmonics, vibrations, noise, and temperature; thus, the machine efficiency is decreased. To counteract this disadvantage, magnetic slot wedges are applied. Due to the impact of high magnetic and mechanical forces, these wedges can fall out and may cause further serious damages. Up to now, reliable detection methods for single missing slot wedges are coupled with a disassembling of parts of the machine. In this paper, a method is investigated which provides the possibility of detection, based on the measurement of electrical terminal quantities only.
    IEEE Transactions on Industry Applications 01/2014; 50(2):1161-1167. · 1.67 Impact Factor
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    ABSTRACT: Before applying current-signature-analysis-based monitoring methods, it is necessary to thoroughly analyze the existence of the various harmonics on healthy machines. As such an analysis is only done in very few papers, the objective of this paper is to make a clear and rigorous characterization and classification of the harmonics present in a healthy cage rotor induction motor spectrum as a starting point for diagnosis. Magnetomotive force space harmonics, slot permeance harmonics, and saturation of main magnetic flux path through the virtual air-gap permeance variation are taken into analytical consideration. General rules are introduced giving a connection between the number of stator slots, rotor bars, and pole pairs and the existence of rotor slot harmonics as well as saturation-related harmonics in the current spectrum. For certain combinations of stator and rotor slots, saturation-related harmonics are shown to be most prominent in motors with a pole pair number of two or more. A comparison of predicted and measured current harmonics is given for several motors with different numbers of pole pairs, stator slots, and rotor bars.
    IEEE Transactions on Industrial Electronics 09/2013; 60(9):4025-4033. · 6.50 Impact Factor
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    ABSTRACT: This paper concerns with wind turbine generator fault-tolerant control that avoids system shut-down and enables safe operation with less than nominal power production. We focus on generator stator isolation inter-turn fault that can be characterized before triggering the safety device and applied for any type of generator used in wind turbines. A low complexity model predictive tracking controller is proposed to achieve very accurate flux modulation that prevents the fault propagation while power delivery under fault is deteriorated as less as possible compared to healthy machine conditions. Presented fault-tolerant control strategy is developed taking into account its modular implementation and installation in available control systems of existing wind turbines to extend their life cycle and energy production. Simulation results for the case of a 700 kW wind turbine are presented.
    Control Conference (ECC), 2013 European; 01/2013
  • M. Vasak, A. Martincevic
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    ABSTRACT: Approximately 42% of total energy usage today is spent on the heating of the human environment. Energy consumption can be significantly reduced by proper design of the heating control system - performed studies suggest that in average about 12% reduction of energy usage can be achieved based on control system improvement. This paper describes an efficient control of a family house heating system in its very common configuration. We use model predictive control to fully take into account the prediction of disturbances (room temperatures), reference values of power per rooms and physical limitations of actuators. This will result in an optimal flow rate and temperature of medium required to meet all the demands. In this paper we will first present the model of a heating system, and from it derive model predictive control and verify it through simulation.
    Information & Communication Technology Electronics & Microelectronics (MIPRO), 2013 36th International Convention on; 01/2013
  • M. Vasak, A. Starcic
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    ABSTRACT: The largest energy consuming subsystem in a building is heating, ventilation and air conditioning (HVAC) system. It is thus essential to optimize the behavior of that subsystem in order to achieve energy savings. In this work focus is on efficient energy usage in a sample zone system consisting of two coupled zones. Ventilation, heating and cooling are done by air handling unit (AHU) and variable air volume (VAV) boxes. Optimization of the system means finding optimal control sequences for AHU and VAV boxes. The model of the coupled zones is linear, while the models of the AHU and VAV boxes are nonlinear. In order to find a suboptimal solution, which will keep zone temperature in given boundaries, while handling ventilation requirements, linear programming and linear zone system model are used to find appropriate cooling/heating powers, and then those powers are used to obtain control sequence for AHU and VAV boxes.
    Information & Communication Technology Electronics & Microelectronics (MIPRO), 2013 36th International Convention on; 01/2013
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    ABSTRACT: Electrical machines in the high voltage class are usually designed with open stator slots. This wide open slots cause an increase of higher order harmonics, vibrations, noise and temperature, thus the machines efficiency is decreased. To counteract this disadvantage magnetic slot wedges are applied. Due to the impact of high magnetic and mechanical forces these wedges can fall out and may cause further serious damages. Up to now reliable detection methods for single missing slot wedges are coupled with a disassembling of parts of the machine. In this paper an method is investigated which provides the possibility of detection, based on the measurement of electrical terminal quantities only.
    ECCE2012, Raleigh, North Carolina, USA; 09/2012
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    ABSTRACT: Faults of wind turbine generator electromechanical parts are common and very expensive. This paper introduces a fault-tolerant control scheme for variable-speed variable-pitch wind turbines that can be applied to any type of generator. We focus on generator stator isolation inter-turn fault that can be characterized before triggering the safety device. A simple extension of the conventional control structure is proposed that prevents the fault propagation while power delivery under fault is deteriorated as less as possible compared to healthy machine conditions. Presented fault-tolerant control strategy is developed taking into account its modular implementation and installation in available control systems of existing wind turbines to extend their life cycle and energy production. Simulation results for the case of a 700 kW wind turbine are presented.
    12th International Workshop on Advanced Motion Control, Sarajevo, Bosnia and Herzegovina; 03/2012
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    ABSTRACT: Faults of wind turbine generator electromechanical parts are common and very expensive. This paper introduces a fault-tolerant control scheme for variable-speed variable-pitch wind turbines that can be applied to any type of generator. We focus on generator stator isolation inter-turn fault that can be characterized before triggering the safety device. A simple extension of the conventional control structure is proposed that prevents the fault propagation while power delivery under fault is deteriorated as less as possible compared to healthy machine conditions. Presented fault-tolerant control strategy is developed taking into account its modular implementation and installation in available control systems of existing wind turbines to extend their life cycle and energy production. Simulation results for the case of a 700 kW wind turbine are presented.
    Automatika – Journal for Control, Measurement, Electronics, Computing and Communications. 01/2012; 54(1).
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    ABSTRACT: Modern electric machines are required to have the best possible dynamic performances. In induction machines this is achieved by control strategies that are applied with respect to the flux in the air gap and therefore they require precise information on flux position. This paper proposes an observer with autotuning capability that uses the unscented Kalman filter algorithm for providing on-line estimation of states and parameters of the fundamental wave model of the machine. The algorithm uses power converter reference values of stator voltages, measured stator currents and rotor speed as inputs. Such observer provides accurate estimates of flux position and fundamental stator currents required for e.g. field-oriented control, taking into account machine parameters variability. Design procedure of the observer is presented and both simulation and experimental results are provided.
    Power Electronics, Electrical Drives, Automation and Motion (SPEEDAM), 2012 International Symposium on; 01/2012
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    ABSTRACT: The winding system of high voltage machines is usually composed of pre-formed coils. To facilitate the winding fitting process stator slots are usually wide opened. These wide opened slots are known to cause disturbances of the magnetic field distribution. Thus losses are increased and machine's efficiency is reduced. A common way to counteract this drawback is given by placing magnetic slot wedges in the slots. During operation the wedges are exposed to high magnetic and mechanical forces. As a consequence wedges can get loose and finally fall out into the air-gap. State-of-the-art missing slot wedge detection techniques deal with the drawback that the machine must be disassembled, what is usually very time consuming. In this paper a method is investigated which provides the possibility of detecting missing magnetic slot wedges based only on measurement of electrical quantities and without machine disassembling. The method is based on exploitation of machine reaction on transient voltage excitation. The resulting current response contains information on machine's magnetic state. This information is composed of several machine asymmetries including the fault (missing wedge) induced asymmetry. A specific signal processing chain provides a distinct separation of all asymmetry components and delivers a high sensitive fault indicator. Measurements for several fault cases are presented and discussed. A sensitivity analysis shows the high accuracy of the method and the ability to detect even partially missing slot wedges.
    IECON 2012 - 38th Annual Conference on IEEE Industrial Electronics Society; 01/2012
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    ABSTRACT: A method for dynamic modelling of induction machine with a doubly slotted air gap is proposed and implemented for the case of a cage induction motor. The described method is easily extensible to wound rotor machines. A numerical description of the air gap permeance is provided that takes into account a slotted stator and rotor structure as well as their mutual, time and space dependant positions as a function of rotor rotation. The multiple coupled circuit model approach is used with the modified winding function in order to calculate the inductance of all motor windings. The developed model is general in nature and could be used for the analysis of different dynamic regimes of induction machine, particularly different combinations of stator and rotor slot numbers. Model validation is provided by stator current spectrum analysis of a standard four pole induction motor with S=36 and R=32 slots. The experimental results presented clearly support these findings.
    Electrical Machines (ICEM), 2012 XXth International Conference on; 01/2012
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    ABSTRACT: Modern electrical AC drives with best available performances are based on the so-called fundamental wave machine model approaches. This paper introduces an upgrade of fundamental wave model approach by respecting both inherent and fault-induced deviations of machine flux from its fundamental component. A field-oriented control scheme for an asymmetric induction machine is presented. The algorithm is based on observing newly introduced flux-angle-based variations in the transient leakage inductance due to the asymmetry. A simple extension of the conventional rotor field-oriented control structure is proposed that takes into account detected variations and improves machine performance in the asymmetry conditions. Detection and characterization of newly formed modulation in transient leakage inductance are performed by employing an unscented Kalman filter. Simulation results for the case of a 5.5 kW induction machine are presented.
    Power Electronics and Motion Control Conference (EPE/PEMC), 2012 15th International; 01/2012
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    ABSTRACT: Inverter fed induction machine drives have become one of the most important part in industrial applications. The devices are usually operated near or at the rated vales. Additionally high dynamic operations, overload cycles and the fast switching inverter are putting additional stress to all machines components. The stator winding system is one of the most critical components. Faults in the winding system of a machine can cause a dramatic damage due to their fast developing behavior. An early detection of winding faults can reduce these consequences. In this paper a method is presented to achieve reliable fault detection on stator winding faults focused on application at higher speed levels where immediate detection is imperative. Higher inverter modulation indexes provide the possibility to estimate the transient leakage inductance in each PWM modulation cycle. This parameter contains the information on machine asymmetries. Specific signal processing steps provide a high sensitive fault indicator.
    Electrical Machines (ICEM), 2012 XXth International Conference on; 01/2012
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    ABSTRACT: The paper analyzes the stator current spectrum of a healthy squirrel cage induction machine. The knowledge of stator current spectrum of the healthy cage rotor induction machine is a starting point for diagnosis of different faulty regimes using a noninvasive diagnostic technique known as motor current signature analysis. Magnetomotive force (MMF) space harmonics, slot permeance harmonics as well as saturation of main magnetic flux path through the virtual air gap permeance variation are taken into analytical consideration. Air gap flux density waves were obtained by multiplying the corresponding MMF waves with air gap permeance waves. General rules which give the connection between the number of rotor bars and the existence of rotor slot harmonics in the stator current spectrum are derived, too. Their appearance as well as magnitude depends on the corresponding air gap flux density wave pole pair number. Predicted spectral components of the stator current are experimentally verified on two laboratory motors with different number of rotor bars.
    Diagnostics for Electric Machines, Power Electronics & Drives (SDEMPED), 2011 IEEE International Symposium on; 10/2011
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    ABSTRACT: Wind turbines are usually installed on remote locations and in order to increase their economic competence malfunctions should be reduced and prevented. Faults of wind turbine generator electromechanical parts are common and very expensive. This paper proposes a fault-tolerant control strategy for variable-speed variable-pitch wind turbines in case of identified and characterized squirrel-cage generator rotor bar defect. An upgrade of the torque control loop with flux-angle-based torque modulation is proposed. In order to avoid or to postpone generator cage defects, usage of pitch controller in the low wind speed region is introduced. Presented fault-tolerant control strategy is developed taking into account its modular implementation and installation in available control systems of existing wind turbines to extend their life cycle and energy production. Simulation results for the case of a 700 kW wind turbine and the identified rotor bar fault are presented.
    17th International Conference on Electrical Drives and Power Electronics, Stara Lesna, Slovakia; 09/2011
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    ABSTRACT: Inverter operation significantly reduces sensitivity of rotor bar detection methods based on current signature analysis (CSA). Considering the high frequency and transient electrical properties of the machine a very sensitive detection was proven possible by different authors recently. These methods apply high frequency or transient voltage signals to the machine terminals using the inverter and measure the current reaction. For on-line detection however these high frequency and transient electrical properties are usually superposed with inherent asymmetries like slotting or spatial saturation. To ensure a fault detection in all operating states and to keep sensitivity high the flux/load dependencies of these inherent asymmetries have to be identified during a pre commissioning procedure using for example neural network approach. After this procedure an accurate compensation is possible in all operating states. In this paper a measurement and signal processing chain is described to directly separate the different inherent asymmetries from the fault induced asymmetry without pre commissioning. This signal processing chain is based on repetitive data sampling during normal operation of the drive and applying a set of Fourier transforms. Measurements on a special manufactured machine are presented. The cage of the test rotor allows a single replacement of each individual bar thus enabling to investigate the effect of even partially broken bars. Measurement results prove the efficiency of the proposed method to accurately eliminate the inherent asymmetries caused by slotting and spatial saturation in various points of operation.
    SDEMPED 2011, Bologna; 09/2011
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    ABSTRACT: A fact is that wind energy is both green and expensive energy. In order to increase its economic competence, wind turbine faults should be reduced and prevented. In wind turbines faults most commonly occur in the gearbox and in the generator system components like power converter or generator electromechanical parts. This paper proposes a fault-tolerant control strategy for variable-speed variable-pitch wind turbines in case of identified and characterized generator electromechanical faults like broken rotor bar or winding inter-turn fault. In particular we propose an upgrade of the torque control loop with flux-angle-based torque modulation. Usage of pitch controller in the low wind speed region is also proposed to intentionally reduce power capture in order to avoid or to postpone the system fault development. Presented fault-tolerant control techniques are developed considering their easy implementation and installation in available control systems of existing wind turbines to extend their life cycle and energy production. Simulation results for the case of 700 kW direct-drive wind turbine and the identified stator winding fault are presented.
    Industrial Electronics (ISIE), 2011 IEEE International Symposium on; 07/2011
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    ABSTRACT: Multi-mass drives are in principle low-damped oscillatory systems. Controllers for such systems must be designed with care, especially when safety constraints or equipment constraints need to be respected during their operation. Recently a robust protective control technique based on invariant sets was introduced and experimentally verified for such applications. The idea is to correct the control input to the drive as to avoid the multi-mass drive constraints violation in the future. In this paper we merge Linear Quadratic Regulator (LQR) with the correction procedure and exploit convexity of the raised control problem to significantly reduce the required on-line computational power and memory requirements and to broaden the applicability of the control scheme to multi-input systems.
    Industrial Electronics (ISIE), 2011 IEEE International Symposium on; 07/2011