Ch. Weindl

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.

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.

Comparison of different calculation methods for a power semiconductor based interphase power flow controller

In this paper a new dynamic interphase power flow controller (DIPFC) calculated in state space is described and compared with a fundamental-component based model. The analytic calculation in state space is based on a transformation into space-phasor and zero-sequence components. Accordingly the entire system is divided into orthogonal networks for each switching status. The resulting state-space descriptions can be coupled into a linear inhomogeneous boundary value problem utilizing the periodic switching sequence in steady-state. The solution can be transformed back into natural coordinates. Therefore the entire system and all time functions can be calculated analytically. Finally the analytic method is verified using a fundamental-component model calculating the fundamental impedance of the switched inductance by an evaluation of the Fourier coefficient integral.

Development of an analytic state-space description of power systems including thyristor controlled series compensation

The paper describes a method to create an analytic state-space description of power systems including thyristor controlled series compensated lines. It utilizes the periodicity of all state variables in steady-state operating mode. The system representation is based on four physically conceivable and periodic switching states which can be distinguished in stationary operation mode. The entire system, consisting of the surrounding three-phase network, the thyristor controlled reactor and the optionally switched compensation capacitors is represented in complex space-phasor networks for each of the possible switching states. In this way, the resulting state-space description considers the eigenvalues and switching sequence of the complete network structure. The respective linear independent circuits can be coupled into linear inhomogeneous boundary value problems for each operating mode which can be solved as a whole. Therefore the time functions of all state variables are obtained by a closed loop calculation.

Analytic state-space description of networks including static VAr compensators

In this paper an approach for the calculation of networks, including current converter bridges is generalized to systems including static VAr compensators (SVCs). It utilizes the periodicity of all state variables in steady-state operating mode again. The entire system, consisting of the entire 3-phase network, the thyristor controlled reactor, the optionally switched capacitors and filter circuits is represented in complex space phasor components for each switching status. The resulting state space description considers the eigenvalues and switching sequence of the complete network structure. The respective linear independent circuits are coupled into a linear inhomogeneous boundary value problem which can be solved as a whole. Therefore the time functions of all state variables are obtained analytically. By further signal processing the harmonic load, power flows, reactive compensation factors and stationary resonance conditions can be calculated.