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Ashknaz Oraee

University of Cambridge, Department of Engineering, PhD student

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Supervisors: Dr Richard McMahon
Phone: +44 1223 748328
Address: Electrical Engineering Division, CAPE Building, 9 JJ Thomson Avenue, Cambridge, CB3 0FA, United Kingdom

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Mohammad Nasser Hashemnia

Islamic Azad University Of Mashhad

Sharif University of Technology

University of Zaragoza

Mohammad Ali Saket

University of British Columbia

rahab abderrezak

University of Technology Sydney

Ayman Abdel-khalik

Sultan Qaboos University

Hamma Lakhdar El-Oued University

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Papers by Ashknaz Oraee

Effects of Rotor Winding Structure on the BDFM Equivalent Circuit Parameters

The brushless doubly fed machine (BDFM) is an alternative to the doubly fed induction generator, ... more The brushless doubly fed machine (BDFM) is an alternative to the doubly fed induction generator, widely used in wind turbines without use of brush gear and slip ring. Rotor design is important for achieving an optimal multi-MW BDFM. To date, nested-loop rotors have been exclusively used in various BDFMs, although they may not be ideal for larger machines. This paper studies, both analytically and experimentally, different rotors for the D160 and D400 frame size BDFMs. An optimization on the rotor equivalent circuit parameters is carried out and verified with different parameter extraction methods. A comparison of the performance of the suggested rotors is also presented.

Converter Rating Optimization for a Brushless Doubly fed Induction Generator

This article studies the converter rating requirement of a Brushless doubly-fed induction generat... more This article studies the converter rating requirement of a Brushless doubly-fed induction generator (DFIG) for wind turbine applications. Practical constraints such as the generator torque speed requirement, reactive power management and grid low-voltage ride-through (LVRT) are considered. Experimental data have been used to obtain a realistic system model of a Brushless DFIG in a wind turbine. The study shows that there is a minimum converter rating, dependant on operating point, but this may not be achievable without machine derating. The use of a capacitor bank enables the converter ratings to be reduced without compromising the machine’s performance. The effect of the machine’s leakage inductance on the interplay of LVRT, achievable system power factor, converter rating and losses is investigated.

A Study of Converter Rating for a Brushless DFIG Wind Turbine

This paper studies the converter rating requirement of a Brushless Doubly-Fed Induction Generator... more This paper studies the converter rating requirement of a Brushless Doubly-Fed Induction Generator for wind turbine applications by considering practical constraints such as generator torque-speed requirement, reactive power management and grid low-voltage ride-through (LVRT). Practical data have been used to obtain a realistic system model of a Brushless DFIG wind turbine using steady-state and dynamic models. A converter rating optimization is performed based on the given constraints. The converter current and voltage requirements are examined and the resulting inverter rating is compared to optimization algorithm results. In addition, the effects of rotor leakage inductance on LVRT performance and hence converter rating is investigated.

Performance analysis and testing of a 250 kW medium-speed brushless doubly-fed induction generator

This study presents the performance analysis and testing of a 250 kW medium-speed brushless doubl... more This study presents the performance analysis and testing of a 250 kW medium-speed brushless doubly-fed induction generator (DFIG), and its associated power electronics and control systems. The experimental tests confirm the design, and show the system's steady-state and dynamic performance and grid low-voltage ride-through capability. The medium-speed brushless DFIG in combination with a simplified two-stage gearbox promises a low-cost low-maintenance and reliable drivetrain for wind turbine applications.

Dynamic modeling of a wind turbine with brushless doubly fed induction generator

Brushless Doubly Fed Induction Generator (BDFIG) has been recently proposed to be used in variabl... more Brushless Doubly Fed Induction Generator (BDFIG) has been recently proposed to be used in variable speed wind turbines. This paper intends to investigate the influence of certain model simplifications to obtain a reduced-order model of a wind turbine with BDFIG suitable for transient studies. To achieve this goal, small signal analysis is performed for a recently manufactured 250 kVA BDFIG to obtain system modes and their participation factors. Identifying highly damped modes, influences of neglecting dynamics of their participated states are studied through time-domain simulation in MATLAB/Simulink.

Reduction of the torque ripple in Brushless Doubly-Fed Machine

… , Proceedings of the …, Jan 1, 2011

In this paper, a new structure for the rotor of the Brushless Doubly-Fed Machine (BDFM) is introd... more In this paper, a new structure for the rotor of the Brushless Doubly-Fed Machine (BDFM) is introduced to reduce the machine's torque ripple. The ripple has a considerable effect on the machine start up. At the first section of the paper, analytical expressions for the air gap permeance distribution, the air gap MMF distribution, cogging torque and back EMF are presented. Then, the effects of skewing the rotor loops are analyzed. Finally, the relation between non-skewed quantities and their values in skewed coordinate are introduced.

Magnetic Field Harmonic Analysis in brushless doubly fed machine

… , Proceedings of the …, Jan 1, 2011

In this paper, harmonic sources in a BDFM are studied, and a method for specifying rotor and stat... more In this paper, harmonic sources in a BDFM are studied, and a method for specifying rotor and stator space harmonic components is established. A criterion for evaluating different rotor designs is introduced, based on space harmonics. Then the equivalent space and time harmonic circuits are obtained, using basic equivalent circuit. Finally, the effects of these harmonics on the torque-speed characteristic are studied.

A survey on rotor faults in brushless doubly fed machines

… , Proceedings of the …, Jan 1, 2011

The cause of about 10% of faults in induction machines is related to the rotor part. In this pape... more The cause of about 10% of faults in induction machines is related to the rotor part. In this paper, an analytical method for general modeling of brushless doubly-fed machines (BDFMs) with broken rotor loops and end ring is presented. The approach is based on calculation of resistance and inductance matrices through the winding function of the machine, so the assumption of sinusoidal MMF is not needed. It is also shown that a fault in a loop can lead to faults in other parts of the rotor. To validate the method a simulation in MATLAB/Simulink is performed based on the D180-frame prototype BDFM.

Eddy current losses in brushless doubly-fed machines

… , Proceedings of the …, Jan 1, 2011

The application of the brushless doubly-fed machine (BDFM) is in wind power generation in harsh c... more The application of the brushless doubly-fed machine (BDFM) is in wind power generation in harsh conditions, where reliability is important. In this paper, eddy current losses in a BDFM in four categories: a) the rotor classic eddy current losses, b) stator classic eddy current losses, c) rotor excessive eddy current losses, and d) stator excessive eddy current losses, are calculated for single frequency of the rotor and double frequency of the stator magnetic fields.

Effects of Rotor Winding Structure on the BDFM Equivalent Circuit Parameters

The brushless doubly fed machine (BDFM) is an alternative to the doubly fed induction generator, ... more The brushless doubly fed machine (BDFM) is an alternative to the doubly fed induction generator, widely used in wind turbines without use of brush gear and slip ring. Rotor design is important for achieving an optimal multi-MW BDFM. To date, nested-loop rotors have been exclusively used in various BDFMs, although they may not be ideal for larger machines. This paper studies, both analytically and experimentally, different rotors for the D160 and D400 frame size BDFMs. An optimization on the rotor equivalent circuit parameters is carried out and verified with different parameter extraction methods. A comparison of the performance of the suggested rotors is also presented.

Converter Rating Optimization for a Brushless Doubly fed Induction Generator

This article studies the converter rating requirement of a Brushless doubly-fed induction generat... more This article studies the converter rating requirement of a Brushless doubly-fed induction generator (DFIG) for wind turbine applications. Practical constraints such as the generator torque speed requirement, reactive power management and grid low-voltage ride-through (LVRT) are considered. Experimental data have been used to obtain a realistic system model of a Brushless DFIG in a wind turbine. The study shows that there is a minimum converter rating, dependant on operating point, but this may not be achievable without machine derating. The use of a capacitor bank enables the converter ratings to be reduced without compromising the machine’s performance. The effect of the machine’s leakage inductance on the interplay of LVRT, achievable system power factor, converter rating and losses is investigated.

A Study of Converter Rating for a Brushless DFIG Wind Turbine

This paper studies the converter rating requirement of a Brushless Doubly-Fed Induction Generator... more This paper studies the converter rating requirement of a Brushless Doubly-Fed Induction Generator for wind turbine applications by considering practical constraints such as generator torque-speed requirement, reactive power management and grid low-voltage ride-through (LVRT). Practical data have been used to obtain a realistic system model of a Brushless DFIG wind turbine using steady-state and dynamic models. A converter rating optimization is performed based on the given constraints. The converter current and voltage requirements are examined and the resulting inverter rating is compared to optimization algorithm results. In addition, the effects of rotor leakage inductance on LVRT performance and hence converter rating is investigated.

Performance analysis and testing of a 250 kW medium-speed brushless doubly-fed induction generator

This study presents the performance analysis and testing of a 250 kW medium-speed brushless doubl... more This study presents the performance analysis and testing of a 250 kW medium-speed brushless doubly-fed induction generator (DFIG), and its associated power electronics and control systems. The experimental tests confirm the design, and show the system's steady-state and dynamic performance and grid low-voltage ride-through capability. The medium-speed brushless DFIG in combination with a simplified two-stage gearbox promises a low-cost low-maintenance and reliable drivetrain for wind turbine applications.

Dynamic modeling of a wind turbine with brushless doubly fed induction generator

Brushless Doubly Fed Induction Generator (BDFIG) has been recently proposed to be used in variabl... more Brushless Doubly Fed Induction Generator (BDFIG) has been recently proposed to be used in variable speed wind turbines. This paper intends to investigate the influence of certain model simplifications to obtain a reduced-order model of a wind turbine with BDFIG suitable for transient studies. To achieve this goal, small signal analysis is performed for a recently manufactured 250 kVA BDFIG to obtain system modes and their participation factors. Identifying highly damped modes, influences of neglecting dynamics of their participated states are studied through time-domain simulation in MATLAB/Simulink.

Reduction of the torque ripple in Brushless Doubly-Fed Machine

… , Proceedings of the …, Jan 1, 2011

In this paper, a new structure for the rotor of the Brushless Doubly-Fed Machine (BDFM) is introd... more In this paper, a new structure for the rotor of the Brushless Doubly-Fed Machine (BDFM) is introduced to reduce the machine's torque ripple. The ripple has a considerable effect on the machine start up. At the first section of the paper, analytical expressions for the air gap permeance distribution, the air gap MMF distribution, cogging torque and back EMF are presented. Then, the effects of skewing the rotor loops are analyzed. Finally, the relation between non-skewed quantities and their values in skewed coordinate are introduced.

Magnetic Field Harmonic Analysis in brushless doubly fed machine

… , Proceedings of the …, Jan 1, 2011

In this paper, harmonic sources in a BDFM are studied, and a method for specifying rotor and stat... more In this paper, harmonic sources in a BDFM are studied, and a method for specifying rotor and stator space harmonic components is established. A criterion for evaluating different rotor designs is introduced, based on space harmonics. Then the equivalent space and time harmonic circuits are obtained, using basic equivalent circuit. Finally, the effects of these harmonics on the torque-speed characteristic are studied.

A survey on rotor faults in brushless doubly fed machines

… , Proceedings of the …, Jan 1, 2011

The cause of about 10% of faults in induction machines is related to the rotor part. In this pape... more The cause of about 10% of faults in induction machines is related to the rotor part. In this paper, an analytical method for general modeling of brushless doubly-fed machines (BDFMs) with broken rotor loops and end ring is presented. The approach is based on calculation of resistance and inductance matrices through the winding function of the machine, so the assumption of sinusoidal MMF is not needed. It is also shown that a fault in a loop can lead to faults in other parts of the rotor. To validate the method a simulation in MATLAB/Simulink is performed based on the D180-frame prototype BDFM.

Eddy current losses in brushless doubly-fed machines

… , Proceedings of the …, Jan 1, 2011

The application of the brushless doubly-fed machine (BDFM) is in wind power generation in harsh c... more The application of the brushless doubly-fed machine (BDFM) is in wind power generation in harsh conditions, where reliability is important. In this paper, eddy current losses in a BDFM in four categories: a) the rotor classic eddy current losses, b) stator classic eddy current losses, c) rotor excessive eddy current losses, and d) stator excessive eddy current losses, are calculated for single frequency of the rotor and double frequency of the stator magnetic fields.

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