A. Mohd Ariffin

Comparing simulation results and experimental measurements of electroluminescence phenomenon in dielectric materials

Under continuous application of high electrical stresses, the insulation system of an underground power cable is subjected to long-term degradation that can eventually cause premature breakdown of the cable. This is as a result of electronic charges being injected into the dielectric material when intense electric field is applied. These charges will then interact with polymer molecules; causing even more charges to accumulate within the vicinity of the material and thus further deterioration. The interaction of the charges can lead to the dissipation of energy in the form of photons; a process known as electroluminescence (EL). In addition to space charge probing, the measurement of EL has becoming increasingly prominent in understanding the mechanisms that contribute to the early electrical ageing of an insulating material. This paper attempts to simulate EL behaviour based on the underlying hypotheses of charge injection and bipolar recombination processes that take place within a polymer. A detailed comparison between the computation results and the experimental measurements carried out by different researchers will be discussed in order to evaluate the mechanisms responsible for this phenomenon.

Analysis of wave propagation in Time Domain Reflectometry circuit simulation model

Time Domain Reflectometry (TDR) has been commonly used for testing and diagnosis of faults along a transmission line. It involves the sending of an electrical pulse along a cable and using an oscilloscope to observe the reflected pulses. In this paper, the experimental set up based on Time Domain Reflectometry technique is developed using PSpice circuit simulation software. From the PSpice circuit simulation, the incident and reflected pulses at various nodes are obtained. In order to determine the wave propagation in a Test cable, the actual reflected pulse from the Test cable need to be evaluated. However, in practice the actual reflected pulses cannot be obtained directly from the Test cable but must be measured from the oscilloscope. Therefore, in this paper, the relationship between the actual reflected pulses from the Test cable to the reflected pulses measured from an oscilloscope is established by using the electrical circuit analysis and transmission line theory. Moreover, the characteristic impedance of the Test cable can be computed accurately from this relationship.

Wave propagation characteristics of polymeric underground cables

Knowledge on wave propagation characteristics of both ideal and degraded XLPE cables due to water treeing are the important basics to examine the healthiness of a cable through localization of faults due to water treeing in cables using time domain reflectometry (TDR) measurement technique. In this study, both ideal and degraded XLPE cables are modelled and simulated using MATLAB. The same program is also used to simulate four other cables in different geometries to investigate whether cable geometry influences the results. All results are compiled and compared to determine the wave propagation characteristics of water treeing in XLPE cables.

Analysis of high frequency wave propagation characteristics in medium voltage XLPE cable model

Wave propagation characteristics of underground medium voltage cables are usually determined using the theory of natural modes of propagation. There are three important parameters involved in wave propagation along a cable which are the length of the cable, load condition, and propagation characteristics of the cable. Indeed, the last parameter is the most difficult one to be determined and normally should be modeled. Propagation losses occur in XLPE power cables as pulses propagate through the cable. Since different types of cable have different component parameters, the wave propagation characteristics such as attenuation and propagation speed will also vary accordingly. As such, the wave propagation characteristics for different cable types with their cable geometry are studied and analyzed using a developed MATLAB simulation model.

Simulation of Electroluminescence using a Bipolar Recombination Model

A dynamic bipolar charge recombination model originally developed to explain charge injection and electroluminescent (EL) behaviour of pin-plane polymeric resin samples under ac stress has been adapted to allow the simulation of EL from polyethylene films sandwiched in a plane-plane electrode system. The model is described and obtained results compared with experimental measurements. Comparison reveals that there is good agreement between the model and experimental data, indicating that under ac stress and in this case EL may be entirely due to charge injection and bipolar recombination.

Application of time domain reflectometry technique in detecting water tree degradation within polymeric-insulated cable

Polymeric-insulated power cables are often subjected to multiple sources of degradation. Generally, the main cause for electrical breakdown in this type of cable insulation is usually due to the microscopic impurities and defects located in the bulk, or even at the interfaces of the material. When the dielectric is subjected to a high electrical stress, imperfections such as protrusions, contaminants and microvoids, will all act as points where the electric field is enhanced; increasing the likelihood that degradation processes will be initiated. The intensification of electric field within the insulating material can cause localized discharge to occur continuously, and thus tree-like channels can be developed in the long-run. This paper attempts to investigate whether the existence of water tree region can be detected within polymeric-insulated cables, and the proposed method for the detection mechanism is the time domain reflectometry (TDR). When water trees are present within an insulation system, the characteristic impedance of the material also changes so this can cause reflection of signal propagating along the cable. It was found that there is a difference in TDR signals between un-degraded cable and water tree degraded cable. It is hoped that the difference in these time domain signals can actually assist in determining the location where the presence of water trees can be considered as significant.

Determining the Occurrence of Electroluminescence in Polymeric Materials with respect to the Applied Alternating Electrical Stress

The emission of low level of light known as the electroluminescence (EL) has been used extensively to study charge injection and recombination mechanisms at the metal-polymer interface. This phenomenon has also been used as a probe to study early electrical ageing in high voltage cable insulation system since it can be associated with the energy dissipation of mobile and trapped charges in the molecules of the dielectric medium. Nowadays many works have been undertaken in order to understand the nature of EL in polymeric materials and its relationship with electrical ageing. This paper attempts to lay down the basic theory of the mechanism accountable for the phenomenon and relate it with some experimental observations made on typical polymers; namely low-density polyethylene (LDPE) and polyethylene naphthalate (PEN).

Simulation modeling of polarization and depolarization current analysis for underground cable insulation

High voltages, when applied continuously to an underground cable, leads to the deterioration of its insulation. Gradual yet permanent changes are made to its dielectric properties, caused by electronic charges created within the insulation material; affecting its ability to withstand high electrical stresses. Over an extended period of time, this continual insulation degradation plays a major role in its eventual cable breakdown. Therefore the monitoring of insulation condition is essential to power utilities, to avoid costly disruptions. One of the techniques developed to assess the insulation condition of underground cables is the polarization and depolarization current (PDC) analysis. This technique, executed by measuring the insulations polarization and depolarization currents, is commonly used to assess degradation in the transformer insulation. However it can also be used to monitor the degradation within cable insulation to a certain degree. This paper attempts to describe how PDC can be used to determine the condition of cable insulation via simulation modeling, developed based on the PDC theory for an ideal cable. The simulation results are then compared with experimental PDC measurements to determine the severity of insulation degradation.

Comparing simulation modelling and measurement results of polarization/depolarization current analysis on various underground cable insulation systems

Continuous application of high voltages in an underground cable can cause slow deterioration of its insulation system. Electronic charges can be generated within the medium of the material and this can lead to permanent change of its dielectric properties to withstand high electrical stresses. The long-term degradation of underground cable insulation plays an important role in its eventual breakdown event. One of the ways to assess the insulation condition of an underground cable is by measuring its polarization and depolarization currents (PDC). This technique has been widely used in assessing the performance of transformer insulation system, but can also be used to monitor the degradation within a cable to some extent. The paper aims to describe thoroughly the principle mechanism of PDC and how it can be used to monitor the condition of cable insulation. A simulation model has been developed based on theoretical conjectures of PDC in an ideal cable. The simulation results were then compared with PDC experimental measurements using new unused cables. In addition to this, simulated PDC currents for cables that are subjected to various degrees of degradation were also analyzed in order to deduce severity categorization for replacement exercise of in-service cables. By comparing the simulated data with the experimental results, it is hoped that a clearer understanding on the PDC mechanisms within underground cable insulation can be achieved.

Distribution of electric field in medium voltage cable joint geometry

Cable joint is used to connect different sections of cable because a cable section is limited to a certain length. The design of a cable joint mainly depends on the cable type, the applied voltage and the cores. These factors contribute to the way of how electric field stress is distributed at the cable joint. If there are defects exist within the cable joint insulation material, the electric field at that region is altered. The alteration may cause electrical discharges to occur within the defects if the electric field magnitude is larger than the breakdown strength at the defect sites. Therefore, this paper investigates the electric field distribution in a medium voltage cable joint in the presence of defects. The investigation was done through modelling a medium voltage (MV) cable joint using finite element analysis (FEA) software. Several parameters such as the defect size and location, insulation material dielectric constant and insulation thickness have been studied of their effects on the electric field distribution at the cable joint. The results obtained may be able to help in the designing of cable joint structures which can reduce the electric field stress.