C. Petrarca

Multiconductor transmission line analysis of steep-front surges in machine windings

The numerical evaluation of the electrical stress in the line-end coil of the stator winding of a medium voltage motor fed by a pulsed width modulated (PWM) inverter seems to be indispensable for a rational design of the machine. In order to fulfil such a task, the system, composed of a feeder cable and a stator winding, is modelled and simulated by using multi-conductor transmission line theory. The model can take into account the main phenomena occurring along the lines, i.e. the propagation and the reflection, together with the time dispersion introduced by the losses, eventually dependent on the frequency. The multi-conductor transmission line is solved in the time domain by adopting a technique based on the perturbation theory of the spectrum of symmetric matrices, which sensibly decreases the computational effort with respect to the analysis in the frequency domain. Furthermore, an accurate calculation of the characteristic matrices, which contain the cross-sectional information of the line, is performed by means of a FEM package, so taking into account the effective field distribution in the region of interest. The influence of the accurate evaluation of the capacitance and inductance matrices is considered by comparing the numerical results of the proposed model with those obtained by a simple equivalent circuit, frequently adopted in the literature. In order to validate the proposed model, the simulated results are compared with experimental data.

Analysis of the voltage distribution in a motor stator winding subjected to steep-fronted surge voltages by means of a multiconductor lossy transmission line model

In this paper, the effect of steep-fronted voltage waveshapes infringing on a pulse-width-modulated (PWM) inverter fed induction motor is studied. The system, composed of a feeder cable and a stator winding, is modeled and simulated by using multiconductor transmission line theory in order to predict the voltage distribution among the coils of the stator winding. A recently developed time-domain equivalent circuit is used; it allows one to correctly describe the dielectric losses and the skin-effect in the conductors. The relationship among the voltage distribution inside the electrical insulation and parameters like the rise time of the applied voltage, the cable length, and the distributed losses is deeply discussed. Good agreement has been found among experimental and numerical results.

A Galerkin model to study the field distribution in electrical components employing nonlinear stress grading materials

An effective model is presented for the evaluation of the electric field dynamics inside electrical components, using nonlinear stress grading materials. The model, implemented in a numerical procedure, permits the solution of the Laplace equation and the diffusion equation by adopting the Galerkin method. Two-dimensional domains, even of very complex shapes and the finite thickness of the grading materials are properly taken into account, allowing an accurate evaluation of the electric field distribution and a sound understanding of the influence of the different types of nonlinearities on the stress grading efficiency. The proposed technique has been applied to study the field distributions inside a cable termination equipped with a stress control tube and in a suspension cap-and-pin glass insulator covered with an anti-corona layer. Numerical results for sinusoidal power frequency and standard impulse voltages elucidate the different role of resistive and capacitive contributions in determining the overall potential maps.

EM fields associated with lightning channels: on the effect of tortuosity and branching

Usually the electric and magnetic fields associated with lightning have been computed by assuming the lightning current to be contained in a straight vertical channel of negligible cross section above a flat perfectly conducting plane. Such a model, which does not take into account that real lightning is characterized by tortuosity and branching, is not able to justify the fine structure of the fields radiated by lightning discharges whose time-domain behavior exhibits a jagged shape with remarkable spectral content in several bands of practical interest. In this work the effect of channel tortuosity and branching is investigated by adopting a suitable numerical technique. The discharge channel has been regarded as a fractal antenna whose associated EM field has been evaluated by superimposing the contribution of the single line radiators composing the whole channel. Such a field has been compared with that generated by a simple dipole antenna in order to study the influence of the fractal nature of the channel on the generated EM fields. The relationship between the fractal dimension of the discharge channel and the fractal dimension of the generated time domain EM fields has been considered and the influence played on such a relationship by the distance between EM source and observation point has also been studied by analyzing the fields evaluated at far and close distances.

Lightning electromagnetic fields and induced voltages: Influence of channel tortuosity

Models for calculation of lightning induced overvoltages usually assume a straight and vertical lightning channel. However, it is well known that the lightning path is tortuous on scales ranging from 1 m to 1 km. In this paper the tortuosity effect is analyzed for both lightning-generated electromagnetic fields and induced voltages. For a schematic representation of tortuous lightning channel, it is shown that at close and intermediate ranges the predominant effect is due to the inclination of the lowest channel segment; only for fields at relatively far ranges the overall tortuosity effect becomes appreciable.

EM fields generated by lightning channels with arbitrary location and slope

It is well known that lightning discharges follow a tortuous path; therefore, a general technique able to evaluate the electromagnetic (EM) fields associated with discharge currents flowing into tortuous channels seems to be worthy of consideration. Two techniques have been adopted to find the EM field generated by a current pulse traveling along a single line radiator with arbitrary slope and location above the ground. The first one employs the Fraunhofer approximation, which can provide useful information only on distant radiated fields. The second technique is exact, but applies only to the case of a velocity of propagation v of the current pulse equal to c (velocity of light). Even this solution is indeed inadequate for our purposes since v<c, as evidenced in the literature. In this paper, we evaluate the EM field associated to an arbitrarily oriented radiator without making any mathematical approximation in order to obtain closed-form solutions for the fields. A schematic square-pulse representation of the current and charge distributions along the discharge channels are adopted. The fields due to arbitrary time-varying sources are obtained adopting a suitable convolution integration.

Electrical properties of different composite materials for stress relief in HV cable accessories

In order to ensure optimal electrical performances of high voltage cable accessories an important aspect which determine both the steady-state long-term and impulse performances of the components, resides on the appropriate selection of the grading material. The use of a simple circuital approach, and significant control parameters, allow the evaluation of suitable limits for the values of the resistivity and permittivity of the adopted composite material which should guarantee a reliable operation of the components. In order to supply this significant information and compare the performances of thermosets and rubber based composites, typically adopted in these applications, the electrical properties of carbon black-loaded polyolefins and EPDM materials are analysed for a broad range of applied field and temperature.

Analysis of ultrawide-band detected partial discharges by means of a multiresolution digital signal-processing method

Abstract Ultrawide-band detection of partial discharges (PDs) mainly aims at recording the true shape of a PD current signal. It is thus possible to achieve more useful information than that provided by other detection techniques about sources and causes of both PD current signals and the physical processes taking place. However, external interference, reflections, oscillations and stray elements in the measuring circuit may distort the signal being recorded; hence, the information extracted from it might sometimes be meaningless. In the paper, a digital signal-processing method for reliable analysis of ultrawide-band detected PD current signals is proposed. The method, exploiting the multiresolution approach peculiar to the Wavelet Transform, is capable of setting the detected signal free from most disturbances affecting its true shape, thus ensuring the significance of the measurements carried out on the signal itself. After a brief remark about the main features of the Wavelet Transform and its multiresolution approach, the proposed method is described in detail. Experimental results obtained on actual PD current signals are then given in order to highlight the method’s reliability and effectiveness.

Classification of partial discharges for DC equipment

Computer aided partial discharge detection, recognition and classification at AC voltage has become common practice in recent years. Characteristic discharge phase distributions have been found, thus providing the possibility to recognise and assess possible defects in HV apparatus. For diagnostic purposes, fingerprints from these distributions can be derived and compared to those previously ascertained and stored in a data-bank. For DC voltage a similar technique can be used when the phase angle is replaced by the time between discharges. Successful results have been obtained by employing this method on several artificial defects.

Calculation of Voltages Induced on Overhead Conductors by Nonvertical Lightning Channels

Effects of lightning channel tortuosity and tilt on lightning electric fields and voltages induced on overhead conductors are examined. Overall inclination of the bottom few hundred meters of the lightning channel can appreciably change both the ground-level vertical electric held at distances less than 1 km or so and induced voltage peak relative to the vertical-channel assumption. Smaller-scale tortuosity is responsible for the fine structure of held and induced voltage waveforms. This fine structure can be pronounced at some hundreds of meters and beyond, but is insignificant at shorter ranges.