Shielded Power Cable Research Articles

Comparison of On-Line Partial Discharge Detection Techniques for High Voltage Power Cable Joints and Terminations

Cable joints and terminations play a vital role in providing dependable electrical connection, mechanical support and physical safeguard. These provide electrical stress control to shielded power cables. Despite their usefulness, they suffer from partial discharges (PD) because of enhanced voltage stress, moisture ingress and poor workmanship (during installation). Therefore, it is necessary to undertake on-line PD detection to determine their state. Some of these techniques are capacitive coupler (CC); acoustic emission (AE); and high frequency current transducer (HFCT). This article presents a literature review of these techniques based on the cost, availability, and applicability. The comparative analysis is also provided on location of their sensors, quantification and detection ability. The CC technique involves the quantification of the coupler sensor input measured in mV/pC by realizing the time of flight between two sensors for which the results are used to estimate location of the PD. Meanwhile, the AE technique has an advantage of high immunity to electrical noise, with a caveat that acoustic signals are highly attenuated within the cable joints. Additionally, the combination of the acoustic sensors and PD electrical couplers can be used to discriminate PDs from electrical noise. The HFCT has two methods – with demonising (HFCT-WiD) and without demonising (HFCT-WoD). The HFCT-WiD technique can significantly reduce the sensor’s detection sensitivity due to its high value of noise to signal ratio (NSR). Comparatively, the HFCT has the best results of quantification and detection ability for PDs among all three techniques investigated. However, in places where electrical noise is severe, PD activities may be detected effectively with AE technique. Further work is needed to statistically map these methods and establish their correlation with experimental data.

Inclusion of frequency dependences into prediction model of conducted electromagnetic emissions for a VFD motor system

This paper presents a quantitative prediction model of conducted electromagnetic emissions (EME) for a variable frequency drive (VFD) motor system based on the macro-modeling approach. This model relies on the effective representations of the frequency dependent characteristics of the parasitic behaviors between converter arms and heatsink, of shielded power cable, and of motor windings. The frequency dependent performances between converter arms and heatsink, and of the motor windings are obtained by processing measurement data. Then the vector fitting method (VFM) with passivity enforcement and circuit synthesis method are adopted to obtain the corresponding equivalent circuits. The equivalent circuit implementation of shielded power cable is achieved by applying a node-to-node admittance functions (NAFs) model, which considers the propagation phenomenon and frequency dependent losses accurately. The conducted EME model is compatible with commonly used transient circuit solvers since it only includes constant circuit elements. The conducted emissions of the VFD system are then evaluated by analyzing the common mode (CM) and differential mode (DM) voltages and currents.

Transient Disturbances Based Non-Intrusive Ageing Condition Assessment for Cross-Bonded Cables

The cross-bonding is the most common shield/sheath-bonding system now in use on single-conductor shielded power cables for long distance power transmission. But the complex bonding brings great challenges to the assessment of cable ageing conditions. This paper proposes a non-intrusive ageing assessment method using the transient disturbances originating from the system. The cable relative permittivities, instead of the traditional dielectric loss angle, were extracted from the responses of the transient disturbances to character the ageing state more sensitively. The decoupling analysis of the cross-bonded cable and the corresponding relative permittivity identification model were investigated. Since the transient disturbances appear naturally and frequently, a continuous tracking of the cable ageing conditions can be established without interferences to the power system. Moreover, only the magnitudes rather than the phase angles of the cable currents/voltages are required in this method, which effectively simplifies the functions of the signal acquisition devices. The proposed method provides a new idea to assess the ageing conditions of the cross-bonded cables, and has the potential to be applied in industry.

Frequency-Dependent Multi-Conductor Transmission Line Model for Shielded Power Cables Considering Geometrical Dissymmetry

In this paper, a broadband equivalent node-to-node admittance functions (NAFs) model for shielded power cables are presented aiming at the direct time-domain simulation. The finite-element method is employed to obtain the frequency-dependent per-unit-length resistances, self- and mutual-inductances, and the self- and mutual-capacitances of the power cables. The matrix rational approximations are then applied to obtain the rational model (in pole-residue form) of the terminal admittance matrix of the multi-conductor transmission line (MTL) model. Next, the numerical stability of the rational model is guaranteed by the terminal (port) passivity enforcement technique. The circuit representation of the equivalent NAFs of the full cable with the length of interest can be synthesized based on the pole-residue description. It is validated by comparing the frequency response with the one computed by applying the original MTL model.

Influence of motor cable on common-mode currents in an inverter-fed motor drive system

Induction motor drive systems fed by cables are widely used in industrial applications. However, high-frequency switching of power devices will cause common-mode (CM) voltages during operation, leading to serious CM currents in the motor drive systems. CM currents through the cables and motors in the drive systems can cause electromagnetic interference (EMI) with the surrounding electronic equipment and shorten the life of induction motors. Therefore, it is necessary to analyze the CM currents in motor drive systems. In this paper, high-frequency models of unshielded and shielded power cables are formulated. The frequency-dependent effects and mutual inductances of the cables are taken into account. The power cable parameters are extracted by the finite element method and validated by measurements. High-frequency models of induction motors and inverters are introduced from existing works. The CM currents at the motor and inverter terminals are obtained, and the influence of the cable length and cable type on the CM currents is analyzed. There is a good agreement between the experimental results and the CM currents predicted by the proposed models.

Radiated EMI Modeling and Performance Analysis of a PWM PMSM Drive System Based on Field-Circuit Coupled FEM

The radiated electromagnetic interference (EMI) behavior may have a negative impact on the machining accuracy of grinding machines. Therefore, this paper focuses on the radiated EMI generated by an ultra-precision pulsewidth modulation permanent-magnet synchronous motor (PMSM) drive system. The excitation for the radiated EMI model, the common-mode (CM) ground current at the output terminal of the converter, is obtained by simulating its conducted EMI model. The reasonable simplified FE models for the converter frame, the shielded power cable, and the motor frame are established. In addition, an equivalent circuit macro model of the CM impedance of the three-phase PMSM windings is obtained by using impedance measurements, the vector fitting method, and the fast residue perturbation method. Finally, the 3-D time harmonic field-circuit coupled finite-element method is employed to evaluate the radiated EMI.

RF-Properties of Single Shielded Power Cable Connectors

In this paper, the correlation between surface transfer impedance and magnetic coupling attenuation of a single shielded power cable in line injection measurements is derived and discussed. Furthermore, magnetic coupling attenuation is calculated, taking into account the influence of cable connectors. Finally, based on the results of this paper, a simple substitution approach by means of line injection measurements to determine the properties of shielded power cable connectors in the RF range is presented.

High Frequency Signal Propagation in Solid Dielectric Tape Shielded Power Cables

High frequency signal propagation in shielded power cable is important to many cable diagnostic techniques including time domain reflectometry, partial discharge location, etc. The signal propagation characteristics, including electromagnetic propagation velocity, cable characteristic impedance, and attenuation are well studied in power cables with concentric neutral wires or solid metallic shield. However, high frequency signal propagation in tape shielded cable is more complicated as conduction at the tape laps tends to decrease with cable aging, which forces more of shield current to spiral down the tape shield. The cable inductance increases with the fraction of the shield current in circumferential direction, which affects all the signal propagation characteristics. High frequency signal propagation characteristics of tape shielded power cable were studied using a custom manufactured 15 kV cable sample which has a PET tape intercalated with the copper tape shield to eliminate conduction at the overlap, which simulates the effects of corrosion in aged tape shielded cables. The inductance of the cable sample was computed as a function of fraction of shield current following a spiraling path along the cable, as were the signal propagation velocity, and the cable characteristic impedance, all of which are in reasonable agreements with measurements. Calculations and measurements of high frequency attenuation indicate that large eddy current and “transformer effect” losses in the ground shield semicon dominate once a substantial fraction of shield current spirals down the tape.

Computation of PD calibration reflectograms in shielded power cable

The basis for computing PD calibration reflectograms in shielded power cable is presented. The computed reflectograms include the effects of cable attenuation, initial pulse width, cable length, and PD detection bandwidth. The authors have developed a program based on the algorithms presented in this paper which they are making freely available to the technical community.

A new approach to the statistical enlargement law for comparing the breakdown performances of power cables - part 2: application

A new theoretical approach to the application of the enlargement law for assessing the breakdown performances of shielded power cables was illustrated in the companion paper (Part 1) of this series. In the present paper, practical case-studies of the proposed theory are presented. The so-called crossing quantities of two cable lines, i.e. crossing length, crossing dielectric strength and crossing breakdown voltage, are analyzed on the basis of the results of dielectric strength tests performed on cable models insulated with different commercial compounds. In particular, the application is relevant to mini-cable samples realized with two ethylene-propylene rubber (EPR) compounds, and with two cross-linked polyethylene (XLPE) compounds, all subjected to standard lightning impulse dielectric strength tests. The estimation of the crossing quantities, performed for various power cable voltage ratings, shows that the crossing length - if any - is a strong function of the cable size and voltage rating. Thus the choice of the best compound should be based on a comparison between the value of crossing length and the typical lengths of full-size cables. Furthermore, since the crossing quantities are strongly affected by the values of the Weibull parameters estimated from the dielectric strength tests, a sensitivity analysis of such quantities to the uncertainty in the estimation of the Weibull parameters is also carried out.

High Frequency Loss From Neutral Wire-Shield Interaction of Shielded Power Cable

The conductor and ground shield layers of shielded distribution cable provide a smooth interface between conductors and insulation. Their semiconducting nature causes attenuation of high frequencies which propagate down the cable, as the high frequency displacement current flows through the shield layers and, in the case of concentric neutral cable, in a circumferential direction through the ground shield to a neutral wire. Such attenuation is relevant to partial discharge detection and the use of shielded power cable for wideband communications. This paper discusses the high frequency attenuation in concentric neutral shielded power cable and develops an analytic approximation for predicting high frequency attenuation which is compared with finite element computations and experimental data.

Manufacturing and performance criteria for medium voltage power cable semiconducting shields [Feature article

The subject of semiconducting shield (screen) layers for shielded power cable is interesting as a result of the many technical requirements that must be met simultaneously and that will be described in this article. This contribution is timely in that the IEEE Insulated Conductors Committee (ICC) has a Working Group developing a Guide on cable shields. One purpose of this article is to show that the carbon black is not necessarily the dominant issue in the formulation of shield compounds. Processing and compatibility with the overall cable system is as, or more, important to the shield properties and stability in service than the carbon black used. Through held experience, one commercial conductor shield compound used during the 1990s was unstable in the field as a result of high sensitivity to shear, which resulted in the shield layer going essentially nonconductive in service. Another purpose of this contribution is to show that shield layers are not mix and match. The shield layer must be suitable for the process and dielectric with which it is used. Shields that are designed for use with XLPE dielectric are often not suitable for use with EPR dielectric, and vice versa. Likewise, the appropriate shield depends on the configuration of the extrusion line on which the cable is manufactured.

Dispersion and PD detection in shielded power cable [Feature article

Dispersion, the variation in propagation velocity with frequency, is one of those phenomena that people in the field of partial discharge (PD) detection have talked about since at least the early 1980s, but which is lacking a formal treatment in the literature. When one of the authors recently published an analytic theory of PD propagation in shielded power cable, both reviewers said "dispersion, dispersion". They believed that dispersion was important but gave no evidence for that assertion. In fact, as shown below, dispersion in shielded power cable has little if any impact on measurement of PD magnitude because the Fourier components of the pulse attenuate to insignificance before they can disperse to a degree that would cause appreciable change in the peak pulse amplitude or integral of the pulse waveform. However, dispersion does cause some distortion of the pulse shape that has implications for PD location, as it has a second-order effect on the timing of the peak PD amplitude relative to other pulses in the pulse train caused by multiple reflections from the ends of the cable. A Gaussian PD pulse in the time domain will have a Fourier spectrum that is also Gaussian in the frequency domain. If the Fourier components in the frequency domain propagate down the cable at differing velocities, the waveform to which they add will vary as a function of distance propagated, and the energy in the PD pulse is likely to spread out in time, which would have an effect on wide band PD detection no matter what the means of detection.

Partial Discharge Pulse Propagation in Shielded Power Cable and Implications for Detection Sensitivity

Boggs and Stone (1982) defined the fundamental limits to the electrical detection of corona and partial discharge (PD), i.e., wideband detection of a PD-induced pulse in the presence of thermal noise. This paper treated the effect of frequency-dependent attenuation in shielded power cable in that context. However, most of the plots in that paper were the result of numerical computations. In the same year, Stone and Boggs set out a theory for high-frequency attenuation of shielded power cable. They showed good agreement between attenuation predicted from measured material properties and measured, high-frequency attenuation of shielded power cable. Since 1982, measurements of high-frequency cable attenuation have been reported by a number of authors for a variety of cables. In addition, software tools have become available that facilitate an analytic solution for the parameters of interest. This article summarizes the theory for PD propagation in shielded power cable for both symmetric (Gaussian) and asymmetric PD-pulse waveforms, based on the assumption that the attenuation constant (dB/m or Nepers/m) of the cable is proportional to frequency. This appears to be the most complete possible analytic exposition of PD attenuation in shielded-power cable, which has obvious applications to field PD measurements of such cable.

Analytic Solutions for Pulse Propagation in Shielded Power Cable for Symmetric and Asymmetric PD Pulses

Shielded power cable can be considered as a lossy transmission line, the high frequency attenuation of which causes the PD pulse amplitude, as well as the pulse energy, to decrease as a function of distance propagated. We develop analytical expressions which characterize PD pulse propagation in a shielded power cable for both symmetric and asymmetric PD pulse waveforms for an attenuation constant which increases linearly with frequency.

High frequency properties of shielded power cable. 3. Loss from neutral wire-shield interaction

In this paper, we describe the phenomenon and the basis for an analytic approximation for this loss, which we then compare with the loss as computed using finite-element analysis. The mathematical solution for the loss is too complicated to be included here. As a result, the full mathematical treatment of the problem has been submitted for publication. The discussion here will be restricted to a qualitative description of the phenomenon and graphical quantification with comparison to finite-element computations and measured data

Partial Discharge Pulse Propagation in Shielded Power Cable and Implications for Detection Sensitivity

Partial Discharge Pulse Propagation in Shielded Power Cable and Implications for Detection Sensitivity

High frequency properties of shielded power cable. Part 2: sources of error in measuring shield dielectric properties

This paper provides a theoretical context for the types of errors that are likely to occur during the measurement of high frequency cable shield properties. The paper considers a RC dielectric system as it relates to measuring the dielectric properties of cable shield materials.

High frequency properties of shielded power cable - part 1: overview of mechanisms

This paper provides an overview of the mechanisms of high frequency (HF) loss in shielded power cables and discusses the challenges in the measurements of cable material properties at HF.

"Applied" science [ICEA medium voltage power cable specifications

The Insulated Cable Engineers Association (ICEA) has issued new standards, published by the National Electric Manufacturing Association (NEMA) for solid-dielectric insulated power cables. These new are: ICEA S-95-658-1999/NEMA WC70, Standard for Nonshielded Power Cables Rated 2000 Volts or Less for the Distribution of Electrical Energy; ICEA S-96-659-1999/NEMA WC 71, Nonshielded Power Cables Rated 2001-5000 Volts for the Distribution of Electrical Energy; and ICEA S-93-639-2000/NEMA WC 74, 5-46 kV Shielded Power Cable for Use in the Transmission and Distribution of Electric Energy. These are written as application standards rather than the insulation material-based standards of the past and have been updated to reflect the latest conductor constructions, insulations, and jacket materials used to manufacture wires and cables. The advantage of this approach is that all insulations suitable for use on a specific product can be contained in a single document. This article provides information on the existing NEMA/ICEA and ANSI/ICEA standards, the changes to these standards, and information on the new standards.