Omar Jasim

Investigation of induction machine phase open circuit faults using a simplified equivalent circuit model

An induction motor model based on the per-phase equivalent circuit is used to simulate operation with an open circuit fault. The model considers both spatial field harmonics as well as saturation effects to correctly model the motors behaviour under faulty conditions, where the non-linearities may produce problematic torque pulsations due to the unbalanced nature of the winding distribution. A model is presented which takes into account the most prominent nonlinearities, however keeping simulation times low in order to enable the development of fault tolerant control strategies. In addition, this paper presents a study of the behaviour of an induction motor drive with a phase open circuit fault. A new fault remedial control strategy for this type of fault will also be described. Experimental tests on an instrumented vector controlled rig have been used to verify simulation results.

Induction Motor parameters identification using Genetic Algorithms for varying flux levels

This paper describes a novel approach for identifying induction motor electrical parameters in function of flux levels based on experimental transient measurements from a vector controlled induction motor (I.M.) drive and using an off line genetic algorithm (GA) routine with a linear machine model. The evaluation of the electrical motor parameters is achieved by minimizing the error between experimental and simulation model responses. An accurate and fast estimation of the electrical motor parameters is performed by running a number of optimizations using experimental tests taken under different operating conditions (flux level). Results are verified through a comparison of speed, torque and line current responses between the experimental IM drive and a Matlab - Simulink model.

Operation of an induction motor with an open circuit fault by controlling the zero sequence voltage

This paper presents a fault ride through method for use when open circuit winding faults appear on an induction motor drive. A feed forward compensation term is introduced into the zero sequence component of the dq reference voltages which considerably reduces current and torque ripple in the faulted motor drive. If the machines “neutral point” voltage can be modulated, then a reasonably high speed can be achieved before field-weakening control is required. The paper describes the application of the control method to a delta-connected machine with an open winding fault, with an unmodified inverter drive. The proposed scheme has been verified by experimental results.

A simplified model for induction machines with faults to aid the development of fault tolerant drives

A new mathematical model of a three-phase induction machine, suitable for the simulation of machine behaviour under fault conditions is presented. The model employs a simple state space equivalent circuit based model of the induction machine, which is enhanced to include space harmonics and main flux saturation effects. The model has been improved by including a variation of machine inductances with rotor and flux position, and creates a simulation with reasonable accuracy and fast computation time. For healthy and fault conditions, comparisons show that the simulated saturation and space harmonic frequencies and magnitudes match those obtained from experiment. This model can be used to develop and optimise control strategies for fault ride through.

Advanced control method for VSC-HVDC systems connected to weak grids

This paper proposes a novel control method for modular multilevel converter (MMC) based high voltage direct current (HVDC) power transmission system connected to a very weak or strong AC system under balanced or unbalanced operating conditions. The control concept is developed based on no need for phase-locked loop (PLL) where the voltage source converter (VSC) is controlled directly in ABC frame. The proposed control scheme regulates the instantaneous active and reactive power independently at the Point of Common coupling (PCC). A Multi-Input-Multi-Output (MIMO) state space model has been derived in ABC frame for the MMC converter. An enhanced outer loop is presented for achieving a smooth transition between weak and strong AC network by providing reactive power support to the AC voltage at PCC and exchanging maximum amount of active power within the converter rating limits. Moreover, the controller is designed to tolerate for computational delays. The effectiveness of the proposed control is validated by simulations results.

The series bridge converter (SBC): a hybrid modular multilevel converter for HVDC applications

This paper presents a novel hybrid modular multilevel voltage source converter suitable for HVDC applications. It has the advantages of other modular multilevel topologies and can be made more compact making it attractive for offshore stations and city infeed applications. The Operating principle of the converter and internal energy management are discussed with simulation results from a scaled medium voltage demonstrator presented to validate the concepts.

ESBC: An enhanced modular multilevel converter with H-bridge front end

This paper presents the Enhanced Series Bridge Converter (ESBC), a hybrid modular multilevel converter with H-bridge front end suitable for high power grid applications. It retains the advantages of other modular multilevel topologies while offering compact structure, making it attractive for offshore stations, back-back HVDC stations, and city centre infeeds. The structure, operating principles and energy management of the converter are discussed. Simulation results from a scaled down medium voltage demonstrator are presented to validate the concept.

Improved EPLL performance using multiple sinusoidal signal integrators for grid-connected VSC system synchronisation

This paper presents a novel synchronisation technique, which is based on the structural modification of the conventional Enhanced Phase-Locked Loop (EPLL) by introducing Multiple Sinusoidal Signal Integrator (MSSI) modules. The proposed MSSI-EPLL proves to overcome the weakness of the EPLL by employing adaptive MSSI modules in both amplitude and frequency loops to effectively eliminate low order harmonics under distorted AC grid condition, without sacrificing the dynamic performance. The test bench setup also considers the application of a single-to-ground fault at the Point of Common Coupling (PCC) and a step change in AC system frequency. For these test scenarios, the EPLL suffers noticeable periodic ripples during the dynamic process due to phase and amplitude jumps, with slower response time and steady oscillations during frequency change. But, these drawbacks are significantly mitigated by the MSSI-EPLL, as proven mathematically and confirmed through simulation studies.

The series bridge converter (SBC): Design of a compact modular multilevel converter for grid applications

This paper presents a novel hybrid modular multilevel voltage source converter suitable for grid applications. The proposed converter retains the advantages of other modular multilevel topologies and can be made more compact making it attractive for offshore stations and other footprint critical applications like city in feeds. In this paper, the basic operating principle and design criteria for the converter implementation are presented. The submodule capacitor requirements which have significant influence on the size of a converter station are also evaluated and compared to the MMC. The performance of the converter is supported by simulation results from a representative medium voltage scaled demonstrator.