Comparison between unipolar and bipolar single phase grid-connected inverters for PV applications

An inverter is essential for the interfacing of photovoltaic panels with the AC network. There are many possible inverter topologies and inverter switching schemes and each one will have its own relative advantages and disadvantages. Efficiency and output current distortion are two important factors governing the choice of inverter system. In this paper, it is argued that current controlled inverters offer significant advantages from the point of view of minimisation of current distortion. Two inverter switching strategies are explored in detail. These are the unipolar current controlled inverter and the bipolar current controlled inverter. With respect to low frequency distortion, previously published works provide theoretical arguments in favour of bipolar switching. On the other hand it has also been argued that the unipolar switched inverter offers reduced switching losses and generates less EMI. On efficiency grounds, it appears that the unipolar switched inverter has an advantage. However, experimental results presented in this paper show that the level of low frequency current distortion in the unipolar switched inverter is such that it can only comply with Australian Standard 4777.2 above a minimum output current. On the other hand it is shown that at the same current levels bipolar switching results in reduced low frequency harmonics

Modeling of the solid rotor induction motor

Conventionally the rotors of cage type induction motors are laminated. There is also the possibility of using a solid rotor made from magnetic steel. This option offers advantages associated with ease of construction and reduced material costs. There are two main versions of solid rotor construction. The simpler version is essentially a steel cylinder without a cage or end-rings. A solid steel rotor with an embedded aluminum or copper cage constitutes the other version. There has been very little published work on the first version and, to the author's knowledge, there has not been anything reported on the second version. In this paper an equivalent circuit model is developed for the solid rotor induction motor. The model allows analysis of both rotor versions. It highlights the operational advantages and disadvantages of solid rotor construction

Curvature-related eddy-current losses in laminated axial flux machine cores

In this paper we present an axiperiodic quasi-static model to evaluate the magnetic flux density distribution and power loss due to curvature related radial flux in the laminated core of axial flux machines. It is shown that the relatively low effective permeability in the radial direction and the shielding effect of induced eddy currents result in negligible radial flux density compared to the peak flux densities in the axial and circumferential directions. This justifies the assumption of zero radial flux which simplifies electromagnetic modelling of axial flux machine cores. The model predicts that power loss due to curvature related radial flux is insignificant compared to normal eddy current loss if the core permeability, core conductivity and number of poles are sufficiently high. A laboratory technique is proposed for the practical detection of power loss due to curvature related radial flux

Grid frequency support from inverter connected generation

This paper presents a case study to demonstrate impact of current fleet of inverter connected generation (roof top photovoltaics and utility scale grid following inverters) that have been allowed to connect to power networks with no obligations to respond to system under frequency events. In this study, a grid forming inverter with droop control and grid following inverter connected to a modified IEEE nine-bus test system have been used to show system impact when inverters do not provide system support, and benefits when such inverters provide system support for under frequency events. The case studies performed show that there is potential for inverter connected generation to support transmission grid frequency subject to renewable resource availability or when backed up by energy storage facilities. There is need for grid codes to be reviewed further to put more obligation on intermittent inverter connected generation to provide more grid frequency support functionality, particularly for under frequency events which are the most common in power systems in comparison to over frequency events

Feasibility Analysis of Implementing Hybrid Powered Electric Vehicle Charging Stations in Sarawak

— The transportation sector in Sarawak completely depends on fossil fuel which produces a high quantity of greenhouse gases. A suitable design of charging stations for electric vehicles (EVs) equipped with grid-integrated renewable energy resources (RERs) can help in addressing this issue. This paper proposes to enhance the execution requirements of the hybridpowered electric vehicle charging stations (EVCSs) in Sarawak. A generalized approach for modelling a renewable energy-based hybrid microgrid equipped with EVCS is presented in detail. Four types of microgrid configurations with biomass and solar photovoltaic (PV) systems have been studied to find the optimal size of each component feasible for EVCS. Each design of the hybridpowered EVCS has been analyzed in terms of economic and environmental viability using the climate data with associated monetary data. The analysis shows that the cost of lowering emission to zero is directly proportional to the total net present cost (RM 259,088) when using PV microgrid-powered EVCS. The outcome of this paper provides insight for policymakers on the technical and financial benefits of EVCS deployment. It also promotes the industry of Plug-in Electric Vehicles (PEVs) in Malaysia

Internationalisation of Technology and Society course at University of Southern Queensland

There is growing interest among engineering educators in the question of internationalisation of engineering courses. In this paper the authors first present an analysis of the factors that may cause course designers to internationalise their course material. It is then demonstrated how those factors have influenced the design and delivery of one of the courses offered by the faculty of Engineering and Surveying at the University of Southern Queensland (USQ). Details of the student profiles and general student feedback on the course are also included. The proportion of off-campus students from South East Asia is very significant. Technology and Society is a one-semester course that deals with a range of topics in the management of the interface between engineers and the rest of society. It meets the requirement from Engineers Australia that engineering graduates, among other things, should have an understanding of social, cultural, economic and environmental responsibilities of the professional engineer in both a local and global context. All engineering students at USQ take it, normally in the second year of their program. As USQ offers this course both in Australia and overseas, it is important to make the course as international as possible. The paper argues that that whilst some of the factors lead unquestionably to globalisation of the curriculum, there are others that would lead to localisation of the curriculum. It reviews the process of internationalisation of the course and concludes with a summary of the authors’ experiences with internationalisation so far and some predictions on likely scenarios and outcomes

Ultra-capacitor assisted battery storage for remote area power supplies: a case study

Remote areas power supplies (RAPS), with their crucial requirement for good inverter input voltage regulation, differ markedly from other applications such as uninterruptible power supplies (UPS), electric vehicles (EVs) and hybrid electric vehicles (HEVs). Also, the overall cost of remote area power supplies is significantly affected by the life expectancy and the cost of associated battery storage. Ultra-capacitors can be used to smooth battery current demand. A smoother current demand results in a steadier inverter input voltage. There are also claims that a smoother current demand profile improves the life expectancy of lead acid batteries. This case study aims to quantify the potential benefit of ultra-capacitor assisted battery storage. It is shown that significant benefits are possible if ultracapacitors are used together with lead-acid or nickel iron batteries

DC offset elimination in a single-phase grid-connected photovoltaic system

[Abstract]: Australian Standard AS 4777.2-2005, section 4.9 imposes limits on DC injection into the AC network by grid connected inverters. One way to ensure that this requirement is met is to use a power transformer as interface between the output of the inverter and the AC network. But this adds costs, mass, volume and power losses. It is, therefore, an advantage to design the inverter system so that zero DC offset is guaranteed at its output. Ideally no DC would be expected at the output of the inverter. In practice, however, in the absence of special measures, a small amount of DC is present because of circuit component imperfections. Techniques that have been proposed so far for the elimination of the DC offset current are based on the sensing of the DC offset voltage at the output of the inverter. The output of the sensor is used to drive a feedback system designed to control operation of the inverter so that the DC offset is eliminated. The focus of this paper is on the mathematical modeling of a recently proposed dc offset sensor and dc offset control system. Experimental validation of the model is presented. It is demonstrated that while satisfactory performance is achievable the technique has some serious disadvantages

Distortion in single phase current controlled PV inverters for grid connection

Unipolar switched single-phase inverters experience lower switching losses and cause less EMI when compared with bipolar switched inverters. However, these benefits come at the expense of increased distortion in the current waveform near the voltage zero-crossing. Moreover, it is demonstrated in this paper that switching delay causes generation of low frequency harmonics in the current waveform. In spite of that, unipolar switched inverters can be designed to operate within harmonic limits specified by the Australian Standard 4777.2

Sub-domain scaling for finite element analysis of electrical machines

Finite element modelling of geometries with large dimensional differences between adjacent sub-domains or domains which have poor aspect ratios can impose significant demands on meshing algorithms and computational resources. A simple sub-domain scaling technique for electromagnetic analysis of electrical machines by finite element analysis is proposed. The technique can be used to improve aspect ratios and is useful in modelling electrical machines with small air gap lengths