Transient voltage distribution in stator winding of electrical machine fed from a frequency converter

Standard induction motors are exposed to steep-fronted, non-sinusoidal voltages when fed from frequency converters. These wave patterns can be destructive to the insulation. The aim of the present work is to develop methods of predicting the magnitude and distribution of fast voltage within the stator winding of an electric machine fed from a frequency converter. Three methods of predicting the magnitude and distribution of fast voltages within form windings commonly used in medium and high voltage machines are described. These methods utilise some aspects of previously published works on the surge propagation studies to achieve simplification of the solution without loss of accuracy. Two of these methods are applied to the voltage calculation in random winding commonly used in low voltage machines. Multi-conductor transmission line theory forms the basis of the methods described in this work. Computation of the voltage distribution using either of these methods requires the calculation of the parameters for the slot and the end (over-hang) part of the winding. The parallel plate capacitor method, the indirect boundary integral equation method and the finite element method are the three possible methods of calculating the capacitance also described in this work. Duality existing between the magnetic and the electric field has been used for the inductance calculation. Application of these methods to the voltage calculation in the first coil from the line-end of a 6 kV induction motor is shown to be successful. From the computed and measured voltage results it is evident that the improved accuracy for the capacitance values is sufficient to give good agreement between the measured and calculated inter-turn voltages without the need to infer the presence of a surface impedance effect due to the laminated core. Application of two of these methods for the transient voltage calculation on the first coil from the terminal-end of low voltage induction motors with random windings is also shown to be successful. Comparison between the computed and the measured results shows that the turn-to-ground capacitance matrix obtained in over-hang part of the coil can be assumed the for the slot part of the coil. With this assumption modelling the first five turns in the line-end coil produce turn and coil voltage that match well with the corresponding measuring result. The methods of voltage computation described in this work should be of great help to engineers and researchers concerned with the turn strength and over-voltage protection in high and low voltage motors.reviewe

A general theory of equilibrium selection in games. Chapter 5: The solution concept

Harsanyi JC, Selten R. A general theory of equilibrium selection in games. Chapter 5: The solution concept. Working Papers. Institute of Mathematical Economics. Vol 132. Bielefeld: Center for Mathematical Economics; 1984

Improved Condition Assessment of XLPE Insulated Cables using the Isothermal Relaxation Current Technique

Well-known procedures using the isothermal relaxation current method (IRC) use an empirically-derived ageing factor (the A-factor) to estimate the condition of cables. The A-factor is calculated from plots of the product of instantaneous value of IRC and the time to that value from the start of the current, against the log of time. A-factors are computed from a standard formula using constants determined from curve fits of measured depolarization currents. From measurements of the breakdown strength of samples of real and degraded cables it has been found that A-factors do not provide reliable estimates of XLPE cable condition. However, if semi-conducting material of the cable insulation is known and cables are classified according to semi-conducting material type it is possible to get better correlation between conditions of cables as indicated by A-factors and AC breakdown voltage. This paper contains a novel analysis of result from previous researcher and also includes result from tests on other cables. The refined procedure is found to give reasonable value of A-factor for all cable tested

The effect of cable terminations on dielectric response measurements

This paper presents findings on dielectric response measurements on field aged and new medium voltage XLPE cables using a non-destructive frequency domain technique. Emphasis is laid on the laboratory and field acquired data interpretation. Results presented in this paper shows possible error being introduced into the judgement of water tree deterioration when the influence of the cable terminations is not property considered

Water Migration in Degraded XLPE Cables

During dielectric response measurements on samples of XLPE 22 kV cable that were known to have been severely-degraded in service by water trees, it was observed that the insulation of the cable appeared to degrade considerably after water-immersed cable sample were connected to the rated voltage AC supply. This paper describes results of systematic experiments made to investigate this factor further using polarisation and depolarisation current (PDC) equipment to measure the condition of cable samples. It is found that the cable condition appears to improve from poor to good condition within a few days after short circuiting of the previously-energised cable commenced. Conversely it is found that the condition of the cable appears to degenerate over time following re-application of the rated voltage to the cable samples. It is hypothesised that electrostatic forces maintain moisture in the trees during energisation and that the moisture retreats from the trees when the supply voltage is removed. The implication of this phenomenon on dielectric response measurements is discussed in the paper