Jonathan Shek

A time series approach to design of a permanent magnet synchronous generator for a direct-drive wind turbine

The design of a permanent magnet generator for a wind turbine is studied. The best generator is chosen from a range of designs based on minimizing material cost and energy loss. The main loss mechanisms in an air-cored machine are copper and eddy current losses. The energy lost for each design is evaluated in the steady-state and then in the time domain so as to better take into account the thermal behavior of the machines.

Damper Windings in Induction Machines for Reduction of Unbalanced Magnetic Pull and Bearing Wear

In large induction machines (such as cage large induction motor pumps in the petro-chemical industry and cage or wound-rotor induction generators in wind turbines) reliability and longevity is advantageous. This is particularly relevant to wind turbine generators which can be inaccessible. There will be some degree of tolerance and wear that will lead to low-level rotor eccentricity. For a pm-pole pair machine there will be pm±1 pole pair flux waves set up by the eccentricity which will generate unbalanced magnetic pull (UMP), as well as additional higher space harmonics. This paper addresses the use of stator damper windings to reduce the side-band flux waves and hence attenuate the UMP. Examples are put forward in terms of a 10 pole cage-rotor machine with static or dynamic rotor eccentricity and then extended to use a 4 pole machine wound-rotor machine. A tested analytical model is developed to include these damper windings and the wound rotor; they are shown to reduce the UMP, particularly in a wound-rotor machine. The simulations here are in terms of a wound-rotor machine but this can be extended for DFIG operation.

Unbalanced Magnetic Pull in Cage Induction Machines for Fixed-Speed Renewable Energy Generators

The paper addresses unbalanced magnet pull (UMP) due to rotor eccentricity in cage induction generators used in wind turbines. The rationale for the work is to investigate ways to reduce bearing wear. The paper studies both static and dynamic rotor eccentricity with axial uniformity and also non-uniformity (e.g., due to a misplaced bearing). The damping action of the cage is discussed and a suitable analytical model used to calculate the UMP. Both sub- and super-synchronous UMP is investigated. It is found that the characteristic for the UMP varies depending on whether there is static or dynamic eccentricity. A use of additional damper windings is also investigated and it is found that these can help substantially reduce the UMP when the machine is lightly loaded with dynamic eccentricity. A previously investigated 10-pole machine is used in the simulations. The analytical model has previously been validated for this. The range of simulation covers both motoring and generating modes (±10% slip). This, together with the simulation of the UMP damping windings, represents a good contribution to the literature since they have not been previously addressed.

The Development of an Indexing Method for the Comparison of Unbalanced Magnetic Pull in Electrical Machines

Unbalanced magnetic pull in cylindrical electrical machines has been studied by several authors but it can be hard to quantify and measure. This paper reviews some of the work related to UMP calculation and measurement and develops the concept of a UMP index in order to allow comparative studies. This is because the UMP can vary greatly in terms of magnitude depending on the type and topology of the machine and also the loading. Methods for measurement of UMP are also discussed and results put forward from a UMP measurement rig.

Modelling and experimentation of grid-forming inverters for standalone hybrid wind-battery systems

This paper presents a methodology to model and develop a standalone hybrid wind-battery system in the laboratory environment to reduce experimentation cost, which would otherwise be costly in the field. A full-scale system is established using off-the-shelf components. The development time is expedited using this approach. In addition, it is believed that a full-scale system in the laboratory presents a more controlled test environment and its performance is close to the real system. The steady-state and dynamic analysis of a standalone hybrid wind-battery system are presented, from both a modelling and an experimental perspective. Three single-phase grid-forming inverters and a fixed speed wind turbine are used as a platform for case studies. The grid-forming inverters adopt droop control method which allows parallel operation of several grid-forming sources. Droop control-based inverters are known as independent and autonomous due to the elimination of intercommunication links among distributed converters. Moreover, the adopted fixed speed wind turbine employs a squirrel cage induction generator which is well known for its robustness, high reliability, simple operation and low maintenance. The simplicity and robustness of these selected components minimise the problems faced by the remote communities where technical assistance is limited. The results show a good correlation between the modelling, the experimental measurements, and the field tested results.

Synchronisation control and operation of microgrids for rural/island applications

A microgrid has been recognised as a potential solution for decentralised systems in supplying electricity to remote areas. Typically, a microgrid operates in parallel with the utility grid. However, this work investigates the synchronisation and connection of two or more microgrids. The work specifically looks at scenarios where connection to a utility grid is complex due to technical and economical limitations, such as in rural areas. Nevertheless, these microgrids are able to connect to a utility grid in the future. A microgrid system model consisting of a wind turbine, photovoltaic (PV) array, direct current (DC) storage, voltage source inverter (VSI) control and load demand will be used for the case study. The proposed design of a phase-locked loop (PLL), main controller (MC) and synchronisation controller (SC) will be laid out. These components are utilised to achieve successful connection between microgrids. Finally, power flow studies and system performances are discussed. The success of the proposed algorithm and studies validate the microgrid system as a sustainable energy system.

Damper windings in induction machines for reduction of unbalanced magnetic pull and bearing wear

In large induction machines (such as cage large induction motor pumps in the petro-chemical industry and cage or wound-rotor induction generators in wind turbines) reliability and longevity is advantageous. This is particularly relevant to wind turbine generators which can be inaccessible. There will be some degree of tolerance and wear that will lead to low-level rotor eccentricity. For a p m -pole pair machine there will be p m ±1 pole pair flux waves set up by the eccentricity which will generate unbalanced magnetic pull (UMP), as well as additional higher space harmonics. This paper addresses the use of stator damper windings to reduce the side-band flux waves and hence attenuate the UMP. Examples are put forward in terms of a 10 pole cage-rotor machine with static or dynamic rotor eccentricity and then extended to use a 4 pole machine wound-rotor machine. A tested analytical model is developed to include these damper windings and the wound rotor; they are shown to reduce the UMP, particularly in a wound-rotor machine. The simulations here are in terms of a wound-rotor machine but this can be extended for DFIG operation.

Modeling and Characterization of Downwind Tower Shadow Effects Using a Wind Turbine Emulator

This paper presents the modeling and characterization of the tower shadow effects using a wind turbine emulator in a laboratory environment. In particular, the downwind wind turbines are considered here as their tower shadow effects are more significant compared to the upwind counterpart. Simulation and experimental results have shown that the wind speed deficit due to this nonideal effect is significant. In addition, the tower shadow effects occur typically two to three times per revolution, depending on the number of blades. The modeling of the tower shadow profiles for tubular and four-leg tower configurations is presented. Typically, these towers are used in small wind turbine applications. The tower shadow profiles are emulated experimentally using a wind turbine emulator with its characteristics being explained. The limitations of emulating the tower shadow effects using a wind turbine emulator are demonstrated through the frequency response test performed in this study. In this study, the wind turbine emulator is connected to an isolated grid which is formed by three single-phase inverters. Finally, this paper concludes with a sensitivity analysis of the power oscillations for different widths and magnitudes of the tower shadow profile.

Analysis of Tower Shadow Effects on Battery Lifetime in Standalone Hybrid Wind-Diesel-Battery Systems

In a standalone hybrid wind-diesel-battery system, the battery lifetime is often optimistically over-predicted by hybrid system designers and battery manufacturers. As a result, battery replacement takes place more often than required. One of the reasons is due to the underestimation of battery wear from the short charge–discharge cycles, otherwise known as microcycles. A microcycle takes place when the power generation closely matches the load demand. The detrimental effect of microcycles on a battery-based standalone hybrid renewable energy system was previously investigated, however, only from the perspective of short-term renewable energy fluctuations. This research paper provides a new insight on the battery lifetime reduction, resulting from microcycles that are generated through the tower shadow phenomena. Downwind wind turbines are considered here, as their tower shadow effects are more significant compared to the upwind counterpart. This paper briefly presents the modeling of tower shadow profiles for both tubular and four-leg tower configurations. Experimental results have shown that the microcycles due to this nonideal effect are significant in a battery-based standalone hybrid system. The quantification of the battery lifetime reduction due to this effect is demonstrated. Finally, the paper concludes with a sensitivity analysis of the battery lifetime reduction for different tower configurations, operating at different load conditions.

Comparison of permanent magnet synchronous and induction generator for a tidal current conversion system with onshore converters

Tidal current conversion systems are moving towards commercialisation. Tidal energy developers are looking to optimise their systems by testing all the available options and taking advantage of the experience from the wind energy industry. The key focus of this paper is to compare an induction generator with a permanent magnet synchronous generator in a tidal current conversion system with onshore converters. The architecture of a tidal system with onshore converters is an option for tidal sites with small distances to shore as previous research has shown. In order to investigate the two generator technologies full resource-to-grid models in MATLAB/Simulink are developed. The analysis of these models compares generator efficiency, energy capture, losses at each stage, the cost and the maintenance for each system. Results show that the tidal system with PMSG is more efficient and generates fewer losses to transmit power onshore. In addition, since both systems tested are using a gearbox, the size, cost and maintenance of the PMSG are comparable to the reliable and cost-effective option of SCIG.