Haran Karmaker

Short-Circuit Analysis of Permanent-Magnet Generators

Permanent-magnet generators (PMGs) have rapidly become important in renewable energy systems, portable and standby generating systems, and in many new applications in industrial, utility, aerospace, and automotive sectors. While there has been some discussion of “fault tolerance” and fault testing of an 8-MW machine has recently been reported [1], understanding the behavior of faulted PMGs remains far from complete. This paper addresses the important case of the sudden short circuit applied to large PMG machines. It explains key differences in short-circuit behavior between the PMG and wound-field generator. The subtransient reactances and time constants of the PMG are calculated by both analytical and finite-element methods and applied to the classical circuit-theory simulation of the short-circuit fault. The finite-element method is also used to assess in detail the risk of loss of magnetization in the magnets. The complexity of the transient magnetic field requires transient nonlinear circuit-coupled finite-element analysis in three dimensions with voltage-source excitation. This paper concludes with a review of the methods of calculation and a discussion of implications for future design and application of the PMG, including factors relevant to the application of standard tests and specifications.

Comparison Between Different Design Topologies for Multi-Megawatt Direct Drive Wind Generators Using Improved Second Generation High Temperature Superconductors

Various design topologies were investigated to study the weights and costs for a direct drive wind turbine generator using improved second generation (2G) YBCO high temperature superconductors (HTS). Five different design topologies using combinations of air core, iron core, salient and non-salient configurations for both stator and rotor were considered for a generator with the nominal rating of 10 MW, 8 RPM and 3300 V. The main objective of the investigation was to minimize the HTS quantities and the reduction of total cost to achieve the same rating performance. The electromagnetic design was performed using transient finite element modeling including the improved HTS conductor characteristic properties. The mechanical design includes the support structures for both full load operation and sudden three-phase terminal fault conditions which can cause severe stresses.

Investigation and Simulation of fields in large salient-pole synchronous Machines with skewed stator slots

This paper presents the results of an investigative study performed on a large salient-pole synchronous machine with skewed stator slots. The study was carried out to aid the understanding of the phenomena related to electromagnetic field distribution, rotor damper currents and pole face iron losses in machines with skewed stator slots and an armature winding design with a fractional number of stator slots per pole. A hydrogenerator was specially instrumented to measure the variation of magnetic fields along the axial length of the machine and the induced currents in the damper bars on the rotor poles. A computational model has been developed to predict the air-gap flux density, damper currents and rotor iron losses. Measured results are compared with those predicted by simulation.

Short-circuit analysis of permanent-magnet generators

Permanent-magnet generators (PMG) have rapidly become important in renewable energy systems, portable and standby generating systems, and in many new applications in industrial, utility, aerospace and automotive sectors. While there has been some discussion of “fault tolerance,” and fault testing of an 8MW machine has recently been reported [5], understanding the behavior of faulted PMGs remains far from complete. This paper addresses the important case of the sudden short-circuit applied to large PMG machines. It explains key differences in short-circuit behavior between the PMG and wound-field generator (WFG). The subtransient reactances and time-constants of the PMG are calculated by both analytical and finite-element methods, and applied to the classical circuit-theory simulation of the short-circuit fault. The finite-element method is also used to assess in detail the risk of loss of magnetization in the magnets. The complexity of the transient magnetic field requires transient nonlinear circuit-coupled finite-element analysis in three dimensions with voltage-source excitation. The paper concludes with a review of the methods of calculation and a discussion of implications for future design and application of the PMG, including factors relevant to the application of standard tests and specifications.

Improving the starting performance of large salient-pole synchronous machines

We have carried out an engineering study with the aim of enhancing the reliability of the starting system of the salient-pole synchronous machines in a pumping-generating plant. We focused on accurate prediction of the damper bar currents, electromagnetic torque, acceleration time, and bar temperatures during the starting process. We used the time-stepping finite-element analysis (FEA) for a multiple-pole model including coupled field-circuit formulation and rotor motion. We also developed an analytical model, incorporating the equivalent circuit parameters, to simulate the starting performance of the generator/motor units. The predicted results from both the FEA model and the analytical model agree well with experimental investigations. As a result, we were able to propose a way to improve the motors reliability and allow many additional starts.

Comparison of Frequency and Time-Domain Iron and Magnet Loss Modeling Including PWM Harmonics in a PMSG for a Wind Energy Application

This paper presents the calculation of the electromagnetic losses for a 2.1-MW permanent magnet synchronous generator for wind energy application. The focus is on recognizing the significance of including the analysis of higher harmonics in the electromagnetic loss calculation. The analyzed harmonics include the ones resulting from the use of a pulse width modulation (PWM) of the voltage of the generator. The magnet losses calculated for the PWM current are several times higher than the ones calculated for the sinusoidal current. In addition, frequency-domain and time-domain models for iron loss calculation are compared. The frequency-domain model that assumes a sinusoidal variation and considers only the fundamental component of the magnetic induction in the stator core material underestimates the iron losses in the machine. Especially, when the additional losses resulting from the higher harmonics, rotational fields, and minor loops are taken into account. Finally, it is shown how the composition and thickness of the electrical steel used in the stator core of the generator influences the total core losses.

Methodologies for testing a 2 MW permanent magnet wind turbine generator

No known guide exists for testing commercial multi-megawatt size permanent magnet (PM) wind turbine generators. Recently, a project has been approved by IEEE to develop a trial use of a guide for testing PM machines. The first author of this paper is the Chair of the working group volunteering to develop this guide. The working group has started working on the guide and plans to present its work in future. The authors of this paper would like to share their experience on testing a 2 MW wind turbine generator with a PM rotor.

Use of a permeance model to predict force harmonic components and damper winding effects in salient pole synchronous machines

This paper presents a combined finite-element and analytical modelling technique for the prediction of force density harmonics in salient pole synchronous machines. The model calculates the induced currents in the damper winding cage and includes their effect on force density components in the solution. Use of a combined analytical and finite element approach reduces simulation times compared to full time-stepping finite element solutions, while including the effects of design changes on airgap force harmonics. Results of the model predictions are presented together with measured data from two different machines.

Direct drive HTS wind generator design for commercial applications

This paper describes the results of investigative studies performed for various design topologies of a direct drive HTS wind turbine generator. The investigation covers electrical, mechanical and thermal design with special considerations for design optimization for cost reduction and practical manufacturability for commercial applications. Detailed modeling and analyses are performed by taking into account the characteristic properties of the HTS and non-HTS materials. The investigations also use improved YBCO second generation (2G) HTS wire under development in a research program sponsored by the U.S. Department of Energy.

Noncontact High-Torque Magnetic Coupler for Superconducting Rotating Machines

We present the design of a noncontact high-torque magnetic coupler in the context of a high-speed (> 3600 r/min) fully superconducting rotating electric machine. The magnetic coupler is an essential component for machines where both stator and rotor are enclosed within a common cryostat. The magnetic coupler eliminates the need for rotating shaft vacuum seals, which would otherwise be needed if a conventional torque tube were used. The rotor is cooled by natural convection from the stator, similarly eliminating the need for the rotating cryogen transfer coupling used for conventional superconducting rotors. The magnetic coupler satisfies the need to transmit torque from the prime mover to the generator across a stationary cryostat boundary. We rely to an array of multiple radially spaced stages to limit axial extent and improve the torque rating of the design. We demonstrate that, while it is feasible to transmit several kilonewton meter of torque through a magnetic coupler for medium-size machines (> 10 MW), the magnetic coupler is nearly as complex as the rest of the machine. This option should be considered if known reliability issues with high-speed rotating vacuum seals and cryogen transfer couplings cannot be resolved.