G. Lupo

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.

Field distribution in cable terminations from a quasi-static approximation of the Maxwell equations

A new model for the evaluation of the electric field in a cable termination realized through a nonlinear stress control tube (SCT), is presented in this paper. It is based on the electro-quasistatic approximation of the Maxwell equations: the Laplace equation describes the field in the nonconducting regions whereas a diffusion-like equation gives the field dynamics in the stress control tube. A numerical model is devised by solving the Laplace equation by finite difference and diffusion equations by the Galerkin method. It is shown that even the well-known RC transmission line model can be derived from this general approach. The underlying approximations leading to the circuital model are discussed in detail. The proposed model, in contrast with the circuital one, allows us to take into account properly the nonlinear SCT characteristics and the actual boundary conditions: in this way both spatial and temporal effects of the nonlinearity are-considered. The numerical results obtained by considering the general field approach and by using the transmission line model are compared.

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.

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.

Electric field calculation in HV cable terminations employing heat-shrinkable composites with non linear characteristics

The use of the heat shrinkable tubes has been proposed as a possible solution to the problem of controlling the longitudinal component of the electrical field at the interface between the screen and the primary insulation in HV cables. However, typical materials adopted such as carbon black-loaded polyolefins show non linear characteristics, which have to be taken into account for a proper termination design. In this paper an analysis of the electric field in a cable termination using heat-shrinkable stress-relief tube is performed on the basis of a numerical procedure in which the material is simulated with a network of non linear resistors and linear capacitors.<<ETX>>

The influence of coil parameters on the voltage distribution in a machine stator winding fed by a PWM inverter

The present paper is dedicated to the evaluation of the influence played by important design parameters like the interturn insulation thickness and the insulating material permittivity on the voltage distribution among the coils of a stator winding fed by a PWM inverter. The system, composed of a feeder cable and a stator winding is modeled and simulated by using multiconductor transmission line theory. A recently developed time-domain equivalent circuit is used to represent the multiconductor line, with losses possibly dependent on frequency, which allows to correctly describe the dielectric losses and the skin-effect in the conductors. The obtained results can offer a broad guideline to design engineers for a suitable dimensioning of the motor insulation.

Wavelet Packet Denoising of Partial Discharge Data

On-site partial discharge (PD) measurements are affected by external noise and disturbances of various nature such as interferences from radio transmissions, stochastic noise, steep front pulses from power electronics, etc. In such a hostile electromagnetic environment, the partial discharge can be completely submerged by the surrounding signals and the sensitivity and reliability of the measurement can be strongly affected. Once the signal has been recorded, a post-processing tool is needed in order to recover the PD pulses. In this paper a wavelet based technique has been used in order to extract the PD data from a noisy environment: the Wavelet Packet Transform (WPT) has been adopted, which allows a more detailed analysis of the signal with respect to the Discrete Time Wavelet Transform (DTWT). As a first step, the post-processing tool has been applied to a typical uncorrupted PD pulse which has been decomposed up to its deepest level; subsequently, an energy-based criterion in conjunction with hard-thresholding has been used in order to select a suitable mother wavelet and an adequate decomposition level; useful patterns have then been collected for the reconstruction of the signal with minimum shape distortion. Finally, the suggestions provided have been used for the extraction of numerical simulating signals, characterised by a very low signal to noise ratio (SNR), reproducing a PD pulse corrupted by external interferences and noise, in order to show the effectiveness of the proposed method.

A methodological approach for improvement of vacuum-insulated HV bushings

In this paper a methodological study is presented for the optimal design of a vacuum feedthrough to be used in HV direct current, with high electrical performances in terms of holdoff voltage. From general guidelines, widely described in the literature and relevant to simpler geometric configurations, summarized in few basic rules useful to design vacuum insulators, the authors have sketched an optimal insulator profile for a cylindrical axisymmetrical arrangement. The adopted assumptions have been validated by means of numerical tools based on the charge simulation method and the finite difference method. A prototype feedthrough has been realised, matching the constraints about the field distribution achieved theoretically with technological constraints due to the manufacturing process. High voltage experimental tests confirming the effectiveness of the suggested insulator design approach are reported.