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

The use of arc-suppression coils in power systems with open-delta regulators

Arc-suppression coil systems are able to improve high voltage system reliability and safety. If these systems are used in high voltage power distribution networks which incorporate open-delta regulators, dangerous over-voltages can occur. Because the open-delta regulators increase the line to earth voltage in two phases only, out of balance line to earth capacitance currents flow through the arc-suppression coil. It is shown that under certain conditions the resultant system voltages can reach dangerous levels. It is recommended that a detailed analysis be carried out before arc-suppression coils and open-delta regulators are installed in the same power system

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

Magnetic flux density distribution in axial flux machine cores

[Abstract]: A three-dimensional analytical model is presented to evaluate the magnetic flux density distribution in the core of an axial flux machine. The model predicts significantly higher flux density near the outer radius of the core than that at the inner radius. This has been confirmed by experimental test results. The model also predicts the presence of a curvature-related radial component in the magnetic flux density distribution. It is argued that if it can be established that eddy currents, induced by the tendency for the flux to flow radially, have sufficient shielding effect, then radial flux can be ignored and two-dimensional modelling can be use