Leong Kit Gan

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.

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.

Low-cost modular PV-battery microgrid emulator for testing of energy management algorithms

Within the context of microgrids, the need for a smart energy management system (EMS) has become increasingly important for users to maximise use of local energy generation and/or trade power effectively with the energy market if there is a grid connection. Many researchers have been developing algorithms to forecast the availability of renewable energy and load demands whilst optimising the energy flows within required constraints. Recently, control systems for peer-to-peer (P2P) microgrid architectures, which involve complex information and communication technologies, have also been given much attention. However, not all of these algorithms have been implemented and tested with real hardware, which may be attributed to the high cost involved and the safety concerns of a larger system. This paper describes the design, build and demonstration of a scaled down (100 W) P2P microgrid system to provide a low cost, modular, safe, portable testing environment for new EMS algorithms. The system nonetheless has realistic behaviour in terms of control interfaces, measurements and dynamics, and therefore provides a valuable insight into EMS implementation that cannot be obtained through simulations alone. In this work, three microgrid emulators were built and they can communicate with each other via TCP/IP, enabling development and demonstrations of distributed forecasting, control and optimisation algorithms.

A hybrid vanadium redox/lithium-ion energy storage system for off-grid renewable power

In off-grid renewable power systems, batteries are often used to balance the mismatch between load and electricity generation. The mismatch may be more severe in small-scale systems due to the lack of averaging effect seen in larger power systems. In the former, batteries have to cope with rapid power fluctuations whilst still delivering power to consumers. A vanadium redox flow battery (VRB) may seem to be an ideal energy storage system in this case due to its well-known durability and ease of expanding its energy capacity. However, the associated parasitic losses used for electrolyte circulation will dominate when the charge/discharge power is low, and this is particularly inefficient when no other means of energy storage is available. This work proposes the hybridisation of VRB and lithium-ion batteries (LIBs), which complement one another in terms of energy capacity, power handling capability and durability. The trade-off between the parasitic losses of VRB vs. the degradation of LIB presents an interesting optimisation problem. To investigate this, a VRB system which consists of a stack model and a mechanical model was developed before establishing an appropriate energy management system (EMS) using a conventional rule-based approach. In addition, a mixed integer linear programming technique was used to solve the multi-objective optimisation problem in order to investigate the operation of the proposed hybrid energy storage system (HESS) using realistic solar irradiance and load demand profiles. A sensitivity analysis was conducted with different weights for the two objectives. A high-fidelity physical model (Matlab/Simulink) was used to study the underlying transients and to validate the control strategies.