I. Mladenovic

Determination of the characteristic life time of paper-insulated MV-cables based on a partial discharge and tan(δ) diagnosis

Monitoring the condition of electrical equipment insulation by means of partial discharge measurement and analysis is an effective diagnostic tool for the prediction of service failures. In this paper an accelerated aging system specialized for paper insulated lead covered (PILC) cables, with a highly sensitive and selective PD-detection/localization and tan(delta) measurement is presented. It facilitates the monitoring of the entire aging process and the later development of sophisticated diagnostic criteria based on the physical aging process.

Dependencies of the PD- and tan(δ)-characteristics on the temperature and ageing status of MV PILC cables

Diagnostic tools and methods for the state evaluation of MV (medium voltage) and HV (high voltage) power cables are mostly based on single key parameters like the partial discharge (PD) levels (inception voltage and intensity) or the values of the dissipation factor tan(δ). The main goals are the determination of the functionality and actual condition of the equipment. Additionally, the prediction of the rest life-time of power components could help to improve the investment planning and the maintenance strategy. In the field of cables and especially in case of PILC (Paper Insulated Lead Covered) cables, the requirements on the measurement technology and diagnostic systems are rather complex. In many cases the attention is directed to the methodology and the accuracy of the measurement equipment itself or to the value or spectrum of the measurement frequencies. However, the reliability of these methods is also very dependent on the experience of the test engineers and the knowledge and interpretation of all measured parameters. Therefore, besides the electrical parameters, in particular the temperature and humidity during the measurement, the equipments dimensions, cable lengths and noise levels besides others must be taken into account. In this article, the principal dependencies of two major electrical criteria, which are the tan(δ)- and the PD-characteristics on the cable temperature and the cable age or the operational history of the cables, are presented. The strong correlation on e.g. the temperature makes clear, that the interpretation of a single criteria and a subsequent prognoses of the cable status are nearly impossible without further information. The derived results would be a subject of an immense scattering depending on seasonal influences, the load status during the disconnection of the analyzed lines, time constants, cable length, etc.

Determination of the environmental conditions for the accelerated ageing of MV-PILC cables

The accelerated artificial ageing of dielectric materials requires a specially designed laboratory setup and well defined environmental and technical conditions in order to obtain a suitable and realistic ageing factor. The ageing factor (AF) is to be chosen in a way that faults in at least a great number of the test samples happen within the expected project duration. Otherwise, the time schedule of the accelerated ageing process must enable a good extrapolation to the ageing scenario under real load conditions. By a defined pre-test schedule and through a monitoring of specific parameters like the partial discharges (PD) and tan(d) of the cable samples or the fault rate, the best suitable operating point of the artificial ageing systems is determined. In order to define the environmental and technical conditions, a pre-test has been done. This, together with its main results is described in this paper, as well as the principles of he realized accelerated ageing system specialized for paper insulated lead covered (PILC) cables, with a highly sensitive and selective PD-detection/localization and tan(d) measurement. It is shown that it facilitates the monitoring of the entire ageing process and therefore the later development of sophisticated diagnostic criteria based on the physical ageing process.

ICAAS — Integrated system for lasting accelerated aging of MV cables, data monitoring and acquisition

A reliable knowledge of the network components condition is of primary importance for creating an optimal investment and maintenance plan for electrical distribution networks. A commonly used tool for this purpose is a partial discharge measurement and analysis. It enables a good overview over the electrical equipment condition and in this way also a prediction of possible service failures. In the case of paper insulated lead cover (PILC) cables, there are still no established criteria for assuming the time to the next failure. Therefore, an accelerated aging system assigned to this cable type was developed, built up and tested. It facilitates a highly sensitive and selective PD-detection and accurate tan(ô)-measurements. Furthermore, by the constant monitoring of the entire aging process, a knowledge data-base is formed up, and the later development of sophisticated diagnostic criteria based on the physical aging process is made possible. In this paper the basic principles and the functionality of a novel system for the accelerated ageing of MV cables called ICAAS (Integrated Cable Accelerated Ageing System) are presented.

Artificial aging and diagnostic measurements on medium-voltage, paper-insulated, lead-covered cables

Present and future condition-based maintenance methods and preventive maintenance strategies will require knowledge of the present condition of the equipment. For cable networks in gen eral, and for PILC cables in particular, our understanding of the physical phenomena that occur inside cable insulation systems during operation, and their influence on the remaining service lifetime, is still fragmentary. The ICAAS system was developed primarily to facilitate an artificial aging program on MV PILC cables, based on tan δ, return voltage, and PD measurements. An extensive database was built up over a period of two years. Further results will be presented in due course. It should be noted that the ICAAS is fairly freely configu rable so that, with a little effort, it could be applied to other cable types, cable garnitures, sleeves, and terminations. It could also be used to study specific cable defects introduced during manu facture or for the general verification of production quality.

Comparison of parametric partial discharge and dissipation factor characteristics of MV PILC cables

Within a long-lasting project, differently pre-aged, as well as brand new medium voltage (MV) paper insulated lead covered cables (PILC) have been artificially aged. Hereby, the dissipation factor and the partial discharges (PD) have been regularly monitored over the complete lifetime of the accelerated aged samples. Additionally, measurement cycles with varied temperatures and voltages have been accomplished in regular time intervals. All measurements were carried out with the network frequency of 50 Hz. In this paper, the results of parametrical studies over a set of predefined voltages (0.4·times nominal voltage up to 2.2·times) and cable temperatures (15°C to 85°C), accomplished under defined and monitored environmental conditions, will be presented and discussed. Additionally, possible correlations of the PD activity and its intensity on the functional characteristic of tan(δ) will be analyzed by the use of three-dimensional parametrical comparison studies.

Development of the partial discharges inception voltage for different sets of pre-aged PILC cable samples

Diagnostics and the rest life time estimation of the electrical equipment play an important role in the power network investment and maintenance planning. In order to develop a reliable correlation between the partial discharges and the time to the next failure of the MV PILC cables, a special system called ICAAS (Integrated Cable Accelerated Aging System) for the artificial accelerated aging was developed. The aging system includes highly sensitive and selective PD-detection and tan(δ)-measurement units. During the entire aging process, a knowledge data-base will be formed up, by the constant monitoring of all relevant aging parameters and measured values. The functionality of the ICAAS was confirmed in a pre-test and the most suitable ageing and physical parameters for the main aging experiment, which should last for two years in continuity, were determined. Therefore two different sets of cable samples were used in the pre-test. Group one was made up of samples of brand new cables, in a second group nine 20 years old cables were aged, including cables with an existing and defined partial discharge level. In this paper the results, by meaning of partial discharge activities and environmental influence of the pre-test will be shown and discussed. The physical processes that appear in the cable insulation of PILC cables will be discussed, and in this way the limitations in the electrical aging parameters.

Development of the p-factor in an accelerated ageing experiment of the MV PILC cables

Insulation materials deteriorate with time due to numerous operational and environmental influences. Owing to thermal and electrical ageing, moistening, etc., several chemical and physical deterioration processes in the insulation materials arise, which result in the decrease of the electrical strength and influence dielectric processes and characteristics like conduction processes and the polarization of the insulation. By using the return voltage method (RVM) it is possible to determine probable decrease in electrical strength without damaging. This method has been shown as very appropriate particularly in the case of cables with impregnated paper insulation. MV PILC (medium voltage paper insulated lead covered) cables and its remaining time of life are in the focus of an international research project for which purpose an automated ageing system called ICAAS (Integrated Cable Accelerated Ageing System) was developed. The RVM method was applied on specially selected samples of the entire test field with an average operation age of 20, 40 and 60 years. All Cables in the test field will be artificially and continuously aged in the ICAAS for around two years. In this article the results of the initial return voltage measurements are presented and discussed. The measurements were repeated in defined time intervals, and the influences of the ambient temperature and the air humidity were monitored. The return voltage curves under different conditions and corresponding p-factors were determined and shown.

Empiric Approach for Criteria Determination of Remaining Lifetime Estimation of MV PILC Cables

© 2012 Mladenovic and Weindl, licensee InTech. This is an open access chapter distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Empiric Approach for Criteria Determination of Remaining Lifetime Estimation of MV PILC Cables

Effects of MV PILC cables ageing process on the diagnostic parameters: P-factor and tanδ

During field operation, power equipment is exposed to numerous impacts (especially thermal and electrical stress), which result in a continuous deterioration of its insulation material. The grade of the deterioration can be weighted by different diagnostic measurements. Within an accelerated ageing experiment on paper insulated lead covered (PILC) cables, several diagnostic parameters have been measured regularly during the ageing process of ca. 2 years. The cable samples are PILC cables (20 kV, 150 mm2) of 13 m length, which are differently pre-aged (from brand new, up to cables that have been in operation for 60 years). In this article aging effects of this insulation system on dissipation factor and parameters of return voltage curve as well as on p-factor will be shown and discussed.