48,303 research outputs found

    Advanced control in smart microgrids

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    University of Technology, Sydney. Faculty of Engineering and Information Technology.This thesis presents various advanced control strategies in smart microgrid applications. In recent years, due to the rapid depletion of fossil fuels, increasing demand of electricity, and more strict compulsory government policies on reduction of greenhouse gas emissions, renewable energy technologies are attracting more and more attentions and various types of distributed generation (DG) sources, such as wind turbine generators and solar photovoltaic (PV) panels, are being connected to low-voltage distribution networks. Because of the intermittent nature of the renewable energy sources, it would be a good idea to connect these DG units together with energy storage units and loads to form a local micro power system, known as microgrid. This PhD thesis project aims to develop new and competitive control methods for microgrid applications. Based on a review of the state of the art of the wind power techniques, a new predictive direct control strategy of doubly fed induction generator is proposed. This method can achieve fast and smooth grid synchronization, and after grid connection, the active and reactive power can be regulated flexibly, which enables the wind power systems contributing to the grid voltage support and power quality improvement. The proposed strategy is simple and reliable, and presents excellent steady-state and dynamic performance. A new control approach using the model predictive scheme is developed for a PV system in microgrid applications. In the islanded operation, the inverter output voltage is controlled stably for the local loads. A simple synchronization scheme is introduced to achieve seamless transfer, and after being connected to the utility grid, the PV system can inject both active and reactive power into the grid flexibly within its capacity. As the capacity of DGs getting larger, the power conversion efficiency becomes more important. In order to reduce the switching loss, a multi-objective model-predictive control strategy is proposed for the control of high power converters. By revising the cost function properly, the switching frequency can be reduced considerably without deteriorating the system performance. The control strategy is simplified using a graphical algorithm to reduce the computational burden, which is very useful in practical digital implementation where high sampling frequency is required. The proposed method is very flexible and can be employed in both AC/DC and DC/AC energy conversions in microgrids. For a microgrid consisting of several DG units, various system level control methods are studied. A novel flux droop control approach is developed for parallel-connected DGs by drooping the inverter flux instead of drooping the inverter output voltage. The proposed method can achieve autonomous active and reactive power sharing with much lower frequency deviation and better transient performance than the conventional voltage droop method. Besides, it includes a direct flux control (DFC) algorithm, which avoids the use of proportional-integral (PI) controllers and PWM modulators. For a microgrid system consisting of a 20 kW PV array and a 30 kW gas microturbine, a coordinated control scheme is developed for both islanded and grid-connected operations. The experimental results from a renewable energy integration facility (REIF) laboratory confirmed the feasibility of the control strategy. The response of this microgrid under the condition of grid faults is investigated and the relevant protection mechanism is proposed. Given the intermittent nature of the renewable energy sources, and the fluctuated load profile, an appropriate solution is to use energy storage systems (ESS) to absorb the surplus energy in the periods when the power production is higher than the consumption and deliver it back in the opposite situation. In order to optimize the power flow, a model predictive control (MPC) strategy for microgrids is proposed. This method can flexibly include different constraints in the cost function, so as to smooth the gap between the power generation and consumption, and provide voltage support by compensating reactive power during grid faults

    System configuration, fault detection, location, isolation and restoration: a review on LVDC Microgrid protections

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    Low voltage direct current (LVDC) distribution has gained the significant interest of research due to the advancements in power conversion technologies. However, the use of converters has given rise to several technical issues regarding their protections and controls of such devices under faulty conditions. Post-fault behaviour of converter-fed LVDC system involves both active converter control and passive circuit transient of similar time scale, which makes the protection for LVDC distribution significantly different and more challenging than low voltage AC. These protection and operational issues have handicapped the practical applications of DC distribution. This paper presents state-of-the-art protection schemes developed for DC Microgrids. With a close look at practical limitations such as the dependency on modelling accuracy, requirement on communications and so forth, a comprehensive evaluation is carried out on those system approaches in terms of system configurations, fault detection, location, isolation and restoration

    Improving skills in rounding off the whole number

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    This study was conducted to address teaching and learning skills in rounding off a whole number. This study consisted of 15 years 4 students from the Kong Nan Chinese Primary School, Parit Raja, Johor, Malaysia. Initial survey to identify this problem was carried out by analyzing the exercise books and exercises in pre-test. Based on these analyses, a large number of students were not proficient in relevant skills. A ‘q’ technique was introduced as an approach in teaching and learning to help students master the skills of rounding whole numbers. In summary, this technique helps students to remember the sequence of processes and process in rounding numbers. A total of four sessions of teaching and learning activities that take less than an hour have been implemented specifically to help students to master this technique. Results of the implementation of these activities have shown very positive results among the students. Two post tests were carried out to see the effectiveness of techniques and the results shows that 100% of students were able to answer correctly at least three questions correctly. The t-test analysis was clearly showed the effectiveness of ‘q’ technique. This technique also indirectly helps to maintain and increase student interest in learning Mathematics. This is shown with the active involvement of students in answering questions given by the teacher

    Modular multilevel converter based LCL DC/DC converter for high power DC transmission grids

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    This paper presents a modular multilevel converter (MMC) based DC/DC converter with LCL inner circuit for HVDC transmission and DC grids. Three main design challenges are addressed. The first challenge is the use of MMCs with higher operating frequency compared to common transformer-based DC/DC converters where MMC operating frequency is limited to a few hundred hertz due to core losses. The second issue is the DC fault response. With the LCL circuit, the steady state fault current is limited to a low magnitude which is tolerable by MMC semiconductors. Mechanical DC circuit breakers can therefore be used to interrupt fault current for permanent faults and extra sub-module bypass thyristors are not necessary to protect antiparallel diodes. Thirdly, a novel controller structure is introduced with multiple coordinate frames ensuring zero local reactive power at both bridges in the whole load range. The proposed controller structure is also expandable to a DC hub with multiple ports. Detailed simulations using PSCAD/EMTDC are performed to verify the aforementioned design solutions in normal and fault conditions

    Direct usage of photovoltaic solar panels to supply a freezer motor with variable DC input voltage

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    In this paper, a single-phase photovoltaic (PV) inverter fed by a boost converter to supply a freezer motor with variable DC input is investigated. The proposed circuit has two stages. Firstly, the DC output of the PV panel that varies between 150 and 300 V will be applied to the boost converter. The boost converter will boost the input voltage to a fixed 300 V DC. Next, this voltage is supplied to the single-phase full-bridge inverter to obtain 230 V AC. In the end, The output of the inverter will feed a freezer motor. The PV panels can be stand-alone or grid-connected. The grid-connected PV is divided into two categories, such as with a transformer and without a transformer, a transformer type has galvanic isolation resulting in increasing the security and also provides no further DC current toward the grid, but it is expensive, heavy and bulky. The transformerless type holds high efficiency and it is cheaper, but it suffers from leakage current between PV and the grid. This paper proposes a stand-alone direct use of PV to supply a freezer; therefore, no grid connection will result in no leakage current between the PV and Grid. The proposed circuit has some features such as no filtering circuit at the output of the inverter, no battery in the system, DC-link instead of AC link that reduces no-loads, having a higher efficiency, and holding enough energy in the DC-link capacitor to get the motor started. The circuit uses no transformers, thus, it is cheaper and has a smaller size. In addition, the system does not require a complex pulse width modulation (PWM) technique, because the motor can operate with a pulsed waveform. The control strategy uses the PWM signal with the desired timing. With this type of square wave, the harmonics (5th and 7th) of the voltage are reduced. The experimental and simulation results are presented to verify the feasibility of the proposed strategy

    Modeling and simulation enabled UAV electrical power system design

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    With the diversity of mission capability and the associated requirement for more advanced technologies, designing modern unmanned aerial vehicle (UAV) systems is an especially challenging task. In particular, the increasing reliance on the electrical power system for delivering key aircraft functions, both electrical and mechanical, requires that a systems-approach be employed in their development. A key factor in this process is the use of modeling and simulation to inform upon critical design choices made. However, effective systems-level simulation of complex UAV power systems presents many challenges, which must be addressed to maximize the value of such methods. This paper presents the initial stages of a power system design process for a medium altitude long endurance (MALE) UAV focusing particularly on the development of three full candidate architecture models and associated technologies. The unique challenges faced in developing such a suite of models and their ultimate role in the design process is explored, with case studies presented to reinforce key points. The role of the developed models in supporting the design process is then discussed

    Voltage Multistability and Pulse Emergency Control for Distribution System with Power Flow Reversal

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    High levels of penetration of distributed generation and aggressive reactive power compensation may result in the reversal of power flows in future distribution grids. The voltage stability of these operating conditions may be very different from the more traditional power consumption regime. This paper focused on demonstration of multistability phenomenon in radial distribution systems with reversed power flow, where multiple stable equilibria co-exist at the given set of parameters. The system may experience transitions between different equilibria after being subjected to disturbances such as short-term losses of distributed generation or transient faults. Convergence to an undesirable equilibrium places the system in an emergency or \textit{in extremis} state. Traditional emergency control schemes are not capable of restoring the system if it gets entrapped in one of the low voltage equilibria. Moreover, undervoltage load shedding may have a reverse action on the system and can induce voltage collapse. We propose a novel pulse emergency control strategy that restores the system to the normal state without any interruption of power delivery. The results are validated with dynamic simulations of IEEE 1313-bus feeder performed with SystemModeler software. The dynamic models can be also used for characterization of the solution branches via a novel approach so-called the admittance homotopy power flow method.Comment: 13 pages, 22 figures. IEEE Transactions on Smart Grid 2015, in pres

    The use of real time digital simulation and hardware in the loop to de-risk novel control algorithms

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    Low power demonstrators are commonly used to validate novel control algorithms. However, the response of the demonstrator to network transients and faults is often unexplored. The importance of this work has, in the past, justified facilities such as the T45 Shore Integration Test Facility (SITF) at the Electric Ship Technology Demonstrator (ESTD). This paper presents the use of real time digital simulation and hardware in the loop to de-risk a innovative control algorithm with respect to network transients and faults. A novel feature of the study is the modelling of events at the power electronics level (time steps of circa 2 μs) and the system level (time steps of circa 50 μs)

    Determination of protection system requirements for DC UAV electrical power networks for enhanced capability and survivability

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    A growing number of designs of future Unmanned Aerial Vehicle (UAV) applications utilise dc for the primary power distribution method. Such systems typically employ large numbers of power electronic converters as interfaces for novel loads and generators. The characteristic behaviour of these systems under electrical fault conditions, and in particular their natural response, can produce particularly demanding protection requirements. Whilst a number of protection methods for multi-terminal dc networks have been proposed in literature, these are not universally applicable and will not meet the specific protection challenges associated with the aerospace domain. Through extensive analysis, this paper seeks to determine the operating requirements of protection systems for compact dc networks proposed for future UAV applications, with particular emphasis on dealing with the issues of capacitive discharge in these compact networks. The capability of existing multi-terminal dc network protection methods and technologies are then assessed against these criteria in order to determine their suitability for UAV applications. Recommendations for best protection practice are then proposed and key inhibiting research challenges are discussed
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