Power Cable Insulation Research Articles

Synergy of the future: high voltage insulated power cables and railway‐highway structures

The rationalisation of the territorial resources leads to consider carefully the possibilities of using motorway and railway infrastructures (existing or planned ones) for cable line installation within them. The full compatibility of such synergy ‘transport – electrical power transmission’ involves different branches of engineering. The study provides an overview of the research undertaken for a fully reliable use of this combination. The manuscript aims at highlighting a procedural approach that should be followed in order to analyse all the key elements involved in the safe operation of power transmission lines hosted in transport infrastructures. First, the geometrical compatibility between different types of power transmission technologies, (i.e. gas insulated lines and high-voltage direct/alternating current insulated cable lines), and transport infrastructures (i.e. motorway galleries, railway galleries and railway prospection tunnels) is considered. Subsequently, the behaviour and magnitude of the magnetic field generated by the different power transmission technologies inside the transport infrastructures are presented. Moreover, the study analyses the effects of a phase-to-screen short circuit that could occur inside the hosting facility. Finally, an analysis of the reliability of the synergy between power transmission systems and transport infrastructures is discussed.

Polypropylene based thermoplastic polymers for potential recyclable HVDC cable insulation applications

Thermoplastic materials are highly desirable for power cable insulation application because of their recyclability and ease of processing. They are particularly suitable for high voltage direct current (HVDC) cable insulation because of the absence of byproducts during cable production, which may result in the reduction of undesirable space charge accumulation and the degassing cost. Polypropylene based polymers may have the potential for recyclable thermoplastic cable insulation application because of their high melting temperatures and adjustable mechanical and electrical properties. This work aimed at evaluating the potential of isotactic polypropylenes, propylene-ethylene block copolymers and random copolymers for thermoplastic cable insulation application. We investigated the melting and crystallization behavior, crystal structure, supramolecular structure, mechanical properties, space charge distribution and DC breakdown strength of six different polymers (two isotactic polypropylenes, two block copolymers and two random copolymers). A comprehensive analysis of these parameters indicates that the propylene-ethylene binary random copolymers are more potentially suitable for thermoplastic cable insulation application.

Designing and Implementation a Lab Testing Method for Power Cables Insulation Resistance According with STAS 10411-89, SR EN ISO/CEI/17025/2005

Insulation resistance measurement is one of the most important tests required by standards and regulations in terms of electrical safety. Why these tests are is to prevent possible accidents caused by electric shock, damage to equipment or outbreak of fire in normal operating conditions of electrical cables. The insulation resistance experiment refers to the testing of electrical cable insulation, which has a measured resistance that must be below the imposed regulations. Using a microcontroller system data regarding the insulation resistance of the power cables is acquired and with SCADA software the test results are displayed.

Application and Analysis on Frequency Conversion Series Resonance Technology in 220kV Cable Withstand Voltage Testing

AC withstand voltage test is the most severe, the most direct and the most effective test method of judging the XLPE Power Cable insulation Conditioning, It is of decisive significance in determining weather the Power Cable is measured up. Traditional test transformer is bulky, it requires high-capacity of power supply when it is used in AC withstand voltage test of high-capacity Power Cable. This paper adopted the Frequency Conversion Series Resonance Technology in 220kV Cable Withstand Voltage test program and accomplished the test successfully, Greatly reduced the capacity of the power test equipment, it also helps to promote the test quality and the work efficiency, which proved the feasibility and effectiveness in the long-distance high-voltage XLPE cables grade AC voltage test with the Frequency Conversion Series Resonance Technology.

Diagnosis of Cable Insulation Defects Based on ANFIS–PCA Method

Partial discharge (PD) detection is an effective method to detect the condition of power cable insulation. Since different modes of cable defect have different discharge characteristics, the PD detection method is required to be able to collect PD pulse and identify the type of the defect. In this paper, the statistical parameters of partial discharge are extracted and an ANFIS-PCA-based method for diagnosing cable insulation defects is proposed. The ANFIS system is applied to pattern classification of partial discharge. The experimental results show that the method proposed in this paper can classify and identify the various discharge patterns effectively.

Polyethylene/polyhedral oligomeric silsesquioxanes composites: Dielectric, thermal and rheological properties

Composites of low-density polyethylene (LDPE) containing 1 wt% and 5 wt% polyhedral oligomeric silsesquioxanes (POSS) of different substituents (methyl, isobutyl, isooctyl) were obtained by extrusion. The composites' potential application in high-voltage (HV) power cable insulation was investigated by broadband dielectric spectroscopy, progressive-stress breakdown tests, and surface partial discharge tests. Thermal conductivity and viscoelastic characteristic were also examined as they play an important role in both material property and manufacture. Additional measurements using scanning electron microscopy and differential scanning calorimetry were carried out to obtain information for possible performance explanations. The results showed that POSS could enhance the erosion resistance and slightly improve the thermal conductivity of LDPE without compromising its dielectric breakdown strength or unfavorably increasing its dielectric permittivity or viscosity.

Highly Insulating Polyethylene Blends for High-Voltage Direct-Current Power Cables

The insulation of state-of-the-art extruded high-voltage direct-current (HVDC) power cables is composed of cross-linked low-density polyethylene. Driven by the search for sustainable energy solutions, concepts that improve the ability to withstand high electrical fields and, ultimately, the power transmission efficiency are in high demand. The performance of a HVDC insulation material is limited by its residual electrical conductivity. Here, we demonstrate that the addition of small amounts of high-density polyethylene (HDPE) to a low-density polyethylene (LDPE) resin results in a drastic reduction in DC conductivity. An HDPE content as low as 1 wt % is found to introduce a small population of thicker crystalline lamellae, which are finely distributed throughout the material. The change in nanostructure correlates with a decrease in DC conductivity by approximately 1 order of magnitude to about 10-15 S m-1 at high electric fields of 30 and 40 kV mm-1 and elevated temperature of 70 °C. This work opens up an alternative design concept for the insulation of HVDC power cables.

Online Loss Parameters Diagnosis Device Development of Power Cable Insulation

In southern China, power cable lines often running in water, and the state of XLPE insulation is difficult to detection effectively. This paper studied loss parameters online diagnosis technology of power cable insulation. An online loss parameters diagnosis device was development, which consists of current comparator, voltage controlled current source (VCCS), standard capacitor, GPRS wireless transmission system, control system and the data processing unit. 108 cables were detected using this device, and it was found out that one cable appeared to middle aging and eight cables appeared to slight aging. The butterfly water trees were observed in middle aging cable, which prove the effectiveness of online testing in the loss current measurement.

Experimental investigation and signal processing techniques for degradation assessment of XLPE and PVC/B materials under thermal aging

Operating conditions subject power cables insulation to a variety of stresses. Among them, the thermal constraint is considered as one of the most severe factors causing irreversible damage to the electrical insulation. In light of this, accelerated thermal aging experiment was performed on cross-linked polyethylene (XLPE) and Polyvinyl chloride class B (PVC/B) samples. This paper deals with an experimental study based on the measurement of the AC current through the insulation bulk. First, the waveform in addition to the RMS value of the current were analyzed at different aging stages and for five AC applied voltages ranging from 3 to 12 kV. Fast Fourier Transform (FFT) spectral analysis is employed to assess harmonic contents of the current signal and evaluate its total harmonic distortion (THD). Further in-depth analysis was proposed by decomposing the current into several time-frequency bands using the Discrete Wavelet Transform (DWT) method, then Standard deviation-Multi Resolution Analysis (STD-MRA) representation is adopted for the sake of determining the details that correlate with the degradation level of the both studied materials. The obtained results show that our experimental-signal processing techniques methodology could be an efficient diagnostic tool for monitoring the degradation process of insulation materials used in extruded power cables.

Effect of space charge layers on the electric field enhancement at the physical interfaces in power cable insulation

This paper intends to investigate the effect of two vented water trees w 1 and w 2 growing simultaneously from both conductor and insulation screens of medium voltage cable in the presence or not of space charge, on the electric field distribution. +12 kV DC voltage has been applied. Such simulation has been implemented under Comsol Multiphysics® environment using the finite element method. Particular interest is assigned to the effect of the polarity, density and layers thickness of the space charge as well as to the behavior of this charge initially accumulated at water trees tips. Note that for the polarity influence, the four polarities configurations related to the space charge accumulated at w 1 and w 2 have been considered. As results, the electric field enhancement at the semiconductor layers especially close to the water tree defect is strongly dependent not only upon the relative permittivity distribution inside water trees, but also on the amount, density and layers thickness of the space charge. In the absence of space charge, we show that, with regard to the inhomogeneous (linear and parabolic) distribution of the permittivity, the homogeneous one causes much more local enhancement of electric field at the trees tips. Moreover, the electric field distribution depends on the geometrical parameters (length and width) and shape of trees; the spherical form engenders a higher electric field than elliptical one. In addition, the association of the space charge could lead to the initiation of the electric trees even at lower voltages. Furthermore, the initiation of electrical trees from the physical interfaces is more probable in the case of opposite polarities especially for negative-positive configuration. Such electrical trees start not only from the tip but also from the water tree root (interface of treed region, un-treed one and semiconductor) which is particularly more vulnerable than inside.

Trend adjusted lifetime monitoring of underground power cable

Insulated power cables are the most critical devices used in electrical network for urban areas. Also, lifespan knowledge of these devices is the important information for decision-makers to make their investment planning. This study has presented a practical method for estimating the remaining lifetime of a power cable. The validation of the method has been tested under two different cases for 154kV XLPE insulated underground power cable system. Power cable thermal network has been modeled for a short-duration transient according to electrical analogy. Lifetime monitoring has been carried out by considering loss of life trend depending on the load raising an annual basis. The results show that the accuracy of the remaining service life estimation is related to receiving data with high sample-rate and determining loss of life trend curve belonging to the power cable. The realistic estimations are carried out using trend-adjusted exponential smoothing.

Bayesian Inference of Power System Insulation Reliability in the Presence of Voltage Harmonics

This article illustrates a Bayesian inference methodology for the parametric evaluation of the reliability of power system components in the presence of voltage harmonics. The methodology starts from an experimentally-based life model for solid insulation under distorted voltage, which belongs to the class of the popular Inverse Power Model. Thereafter, a novel approach is developed for building the prior distribution of the unknown life model parameters, described by a joint Normal distribution, that implies a Lognormal model for the hazard rate and the Mean Time To Failure. The applicability and efficiency of the proposed Bayesian methodology is checked by means of a broad amount of Monte Carlo simulations performed for the insulation of power capacitors and cables subjected to harmonic voltages. The results prove the satisfactory behavior of the Bayesian methodology especially for quite small samples; this is an important outcome, since – as well known in the case of real power systems - small samples are a common practical case.

Mechanical response of high density polyethylene to gamma radiation from a Cobalt-60 irradiator

The response of High Density Polyethylene (HDPE) to gamma irradiation is important for a variety of applications, from the durability of hip replacements after gamma sterilization to the degradation of power cable insulation that guides the licensing and regulation of nuclear power plants. HDPE samples are irradiated with up to 58.8 kGy from a Cobalt-60 gamma irradiator, and mechanical properties are examined using models assuming exponential behavior. Increasing the radiation dose led to increases to the ultimate strength and the Rockwell hardness with a corresponding reduction in the maximum elongation at ultimate strength, supporting the hypothesis that the samples increased their strength and brittleness.

PEA System Modeling and Signal Processing for Measurement of Volume Charge Distributions in Thin Dielectric Films

This paper discusses our effort to develop advanced pulsed electroacoustic (PEA) measurement system capabilities that incorporate: 1) improved signal processing tools for increased signal/noise ratios and 2) integrated PEA modeling tools. In addition, we emphasize the state-of-the-art system electronic components, integrated environmental controls, and sensor improvements required to achieve high spatial resolution while maintaining reasonable temporal resolution for both ambient and in vacuo measurements of thin dielectrics charged using electron beam injection, which is most applicable for spacecraft charging tests. PEA measurement systems provide an important tool to investigate the spatial extent and dynamic evolution of embedded charge distributions in thin dielectric materials. This knowledge has important applications in spacecraft industries, as well as for semiconductors, high-power electronic devices, high-voltage dc power cable insulation, and high-energy and plasma physics apparatus. The emphasis of this paper is on improved signal processing methods and integrated PEA modeling tools.

On-line monitoring method for long distance power cable insulation

There are cross-linked metal sheath and voltage drop in the long distance power cable which make difference between the short and the long distance power cables on on-line insulation monitoring technology. In order to solve the problem, two terminal synchronous tanδ measurement as a new method of on-line insulation monitoring had been put forward in the paper. The working principle and theoretical basis of calculation were given. The feasibility of the method was demonstrated when there was synchronization error in the system. The Matlab software was used to simulate the circuit of three-phase cable and to calculate tanδ of cable insulation. The results show that the load current will not influence on tanδ measurement if choose the average of vector sum of voltage at the two ends of the cable. And the harmonic and frequency fluctuation had little influence on the methods. The methods were proved to be validity and feasibility in the paper. It can be obtained that the methods were suitable for all kinds of cable insulation monitoring whether the metal sheath of three-phase cable was cross-linked or not.

Physico-chemical study of thermally aged EPDM used in power cables insulation

Ethylene-propylene-diene monomer (EPDM) rubber used as power cables insulation was exposed to a thermal aging in oven for different temperatures ranging from 80 to 140 °C. Attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy, differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) were carried out to identify the main structure changes of the material before and after thermal aging. It is found that for pristine EPDM specimens and those subjected to thermal aging at 80 and 100 °C, there was no influence of these aging temperatures on the chemical structure of the material. On the other hand, an increase in oxidation with exposure time was observed for high temperatures (120 and 140 °C). Oxygenated species such as C–O– C, C=O and O–C=O groups were formed. The DSC analysis shows that the crystallinity of the EPDM remains very low. The thermal stability of EPDM, assessed by TGA, showed that although volatile gases migrated from the sample, the studied EPDM keeps high thermal stability. Note that thermal aging effect is not very significant on the material bulk while it is accentuated on its surface. Quantitatively, the carbonyl index was determined since the carbonyl groups are usually the most oxidation products on thermooxidative degradation of cross-linked EPDM.

Accelerated aging study on 15 kV XLPE and EPR cables insulation caused by switching impulses

Cross-linked polyethylene (XLPE) and Ethylene Propylene Rubber (EPR) are the common polymeric materials used for power cable insulation in the USA. While in service, the main threats to the cable insulation include electrical stress, thermal stress, and mechanical stress. The electrical stress, from either operation voltages or abnormal over-voltages during transients, has negative impacts to the power cables insulation. The excessive electrical stress will trigger the aging of insulation materials, which will lead to the failures of cables, resulting in outages of power systems. The aging phenomena of polymeric power cables have been extensively studied during the past decades. Although the study of aging mechanisms was done on molded samples, little is known about the aging phenomena of the XLPE and EPR cables by switching impulses. In this experiment, the 15 kV XLPE and EPR cable samples were aged up to 10,000 switching impulses. The study helped to assess the reliability of the XLPE and EPR cables serving in electrical power systems.

Modelling of aged cavities for partial discharge in power cable insulation

In this study, the ageing process of cavities in solid insulation of power cables is divided into three stages, and for each stage a circuit model is proposed. Each of these circuit models preserves the basic characteristics of the cavity under the circumstances of the corresponding stage and provides the expected behaviour of generated partial discharge (PD) pulses. This set of models simulate cavity wall effects, the non-uniformity of internal electrical field and charge deployed on the wall surfaces with considering the existence of electronegative gases according to the requirements of the three defined ageing stages. Finally, the simulation results, in the forms of phase resolved PD patterns are presented showing good agreement with the published experimental measurements drawn from accelerated ageing process setups.

Extending Replacement Intervals of Elastomeric Components by Evaluating Samples Removed From Service

Elastomers play an essential role in pressure vessels as seals, hoses, gaskets, diaphragms, liners, and other critical components. Polymeric materials used for nonpressure applications such as electric power cable insulation have received more attention, with respect to the effects of aging and life management, than critical elastomeric pressure vessel components such as seals. Performance of these seals depends upon the effects of both time- and event-dependent aging. Understanding and addressing aging and degradation are fundamental to effective preventative maintenance and life cycle management programs. Proper management of elastomers is necessary for performance requirements and for cost containment. In one of the case studies presented, synthetic seals in engines were quite costly to change on the suggested OEM (original equipment manufacturer) maintenance schedule, where replacement intervals were based on assumed operating conditions applicable to other industries. When the actual service for these engines is as emergency diesel generators (EDGs, e.g., in nuclear power plants), the use is primarily in stand-by mode, and the usage is far different than that of the manufacturer's target application. In this investigation, the assessment of pressure seal performance was based on situation-specific operation and environmental parameters. This resulted in much longer replacement intervals. In another case study, alternate materials were identified that are expected to improve performance over the original material.

Partial Discharge Pulse Propagation and Attenuation in Power Cable Insulation: Simulation in Finite Element Method

Travelling wave method is commonly used for partial discharge (PD) location since the past. However, for a long cable, the attenuation of PD pulse propagation could be significant. This is due to more energy loss during the propagation of pulse, which can affect the sensitivity of the PD source location within cable insulation. In this work, PD pulse propagation along cable insulation from the defect site was modelled using finite element analysis (FEA). The model was used to simulate the electric field distribution in the cable insulation due to the PD pulse propagation and to calculate the attenuation constant. The effect of cable length and PD source location in the cable insulation on the attenuation constant was also studied. Through this work, a better understanding of PD pulse propagation and attenuation constant using the FEA model geometry that has been developed can be attained.