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Aleksander Polajner, Roman Tomažič, Jure Praznik

Diagnostic and analytics centre in ELES

With reorganization in 2013, ELES decided to introduce an effective management of its fixed assets. In order to meet strategic goals, a series of important activities were performed, including the establishment of a Diagnostic and analytics center.
The main goals of the project are an effective support to asset management, support to maintenance activities, and implementation of investments and spatial planning. Very important is also life cycles evaluation of our assets, a reliable determination of remaining lifetime and modern approach to condition monitoring of HV equipment.
Focus  in the presentation will be on the implemented technical information systems and planned upgrades, with which we wish to achieve our goals.

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S. Pack, J. Plesch

Transient voltage profile in high voltage grids in Austria

The voltage profile in high voltage grids in Austria operates during the steady state with a sinusoidal voltage profile. The nominal sinusoidal voltage profile may be influenced by switching operations, atmospheric discharges or system perturbations. For the investigation of varying voltage profiles, field measurements of the line-to-ground voltage are performed. For the analysis and evaluation of transient voltages, the measurements in different parts of the Austrian high voltage grids were performed.
Measurements were performed in one of the largest substation in Austria. The substation includes two air-insulated busbars with the nominal voltage levels of 220 kV and 380 kV, which are connected with a three phase, three winding transformers. To measure the transient voltages with a very high accuracy, three resistive-capacitive voltage dividers (RC-dividers) were installed next to the tower arrangement of an incoming 220 kV overhead lines. Because of the very wide frequency range between DC signals up to transient signals with a frequency content of about 1 MHz, RC dividers were used as a most accurate reference for high system voltages. In two additional nodes of the substation the transient voltages were measured at the secondary terminal box of the installed inductive voltage transformers. The nominal voltage level of the voltage transformers are 220 kV and 380 kV.
The behaviour of switching operations of the circuit breakers at the tertiary winding of a three phase, three winding transformer (400/230/30 kV) was measured with capacitive voltage dividers. At the tertiary side of the transformer a 100 MVA shunt reactors is connected. The field measurements of the transient voltages were performed in two different constructed substations (A and B). The connection between the transformer and the shunt reactor in substation A is realised with energy cables and in substation B with an air-insulated busbar. Due to the structure of both substations a different behaviour of the line-to-ground voltages in case of switching operations were evaluated.
To record the transient stress in high accuracy, a digital memory oscilloscope was used. By analysing the measured transient voltage waves or high frequency voltage signals, the peak values, stress duration time and the frequency spectrum were evaluated.

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Zvonko Bregar, Leon Valenčič

Leveling the production from renewable resources

According to the global warming theory, the man made anthropogenic emissions of gases are the main cause for the warming of the global climate. The European Commission thus foresees in its directives a drastic and swift move away from the usage of fossil fuels for electricity generation towards the usage of renewables. But the electricity from renewables is intermittent and cannot be predicted well: the generating time usually does not apply to the time when electricity is actually needed. The solution is seen in various types of energy storage (pumping storage plants, batteries, methanisation etc.), in smart networks and in the pan-European transmission systems bindings. This paper considers 500 MW of photovoltaics in Slovenia and determines the required volume of electricity storage. To achieve average power (approx. 10% of installed capacity) one needs to have 50 regulating basins of the existing Avče pumping storage plant. An alternative can be the leveling to approximately 60% of the average power, for which five times less regulation basins are required, however, according to the installed capacity and investment costs, this solution is at least by a factor of 10 worse than the solution involving classical system generation units.

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Božidar Filipović-Grčić, Ivo Uglešić, Srećko Bojić

Analysis of transient recovery voltage on SF6 circuit breakers when switching unloaded 400 kV transmission lines - comparison between EMTP-RV simulations and on-site measurements

This paper presents analysis of transient recovery voltage (TRV) on SF6 circuit breakers (CBs) when switching unloaded 400 kV transmission lines. On site testing of circuit breakers was performed when switching unloaded 400 kV transmission lines in case of the following configurations of 400 kV transmission network:
. Test configuration 1: testing of CBs in 400 kV line bay when switching 230 km long unloaded transmission line;
. Test configuration 2: testing of CBs in 400 kV line bay when switching simultaneously two unloaded transmission lines 230 km and 167,8 km long.
Switching was performed with and without grading capacitors connected in parallel to CB breaking chambers. Both test configurations were modeled in EMTP-RV software and calculation results were compared to on-site measurements.

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Ambrož Božiček, Leopold Herman, Boštjan Blažič, Igor Papič

Testing of the industrial controller of the Static Var Compensator using a real-time digital simulator

The introduction of the power-electronic devices into medium and high-voltage networks represents a key milestone in the process of the modernization of high-power devices. The technology development increased the complexity of the solid-state high-power devices that are controlled by modern controllers, which provide stable and highly dynamic operation, offering high operational flexibility.

In the past, scaled analogue electrical models of networks and devices were an indispensable element during testing of the controllers. Scaled models offered relatively accurate electrical representation of real devices but were quite inflexible by means of changing of the system topology and were often damaged due to a malfunction of the controller under test.

The major breakthrough in testing of controllers was achieved by the fast development of digital computers, which now offers high precision modeling and accurate real-time simulations of electrical networks. These simulators enable also the exchange of analog and digital signals between the simulation environment (software) and the real device under test (controller, protective relays, etc.), thereby opening new possibilities for the integration of real devices into digital simulations.

The paper presents the development and testing of an industrial controller for the static var compensator (SVC). The main tasks were the development of control algorithms, testing of the protection schemes, controller debugging, testing the operation using different models of network or loads etc. The work was carried out with the use of the actual SVC controller, while the power network was simulated within the real-time digital simulator (RTDS).

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M. Walch, R. Woschitz

Investigations on a combined instrument transformer under several operation conditions

In the century of the energy-revolution through Europe, system operators have to overcome new challenges. Due to the integration of wind and solar power plants and the fluctuation of energy production, a reliable, capable and stable grid operation is becoming more important. New projects in the energy industry, like new connection lines, have a time-consuming approval process. During the realization of investments, the power demand is increasing constantly. Hence, the reliability of the installed electrical high voltage equipment has to be ensured. Due to the combination of voltage and current instrument transformers in one housing, several effects were observed in the past and may connected with higher ambient temperature and several longer heat periods in Central Europe. The combined instrument transformer has to be investigated with respect to the current generated heat losses and regarding to a potential heat input from the connected mountings.
In the case of the combined instrument transformer, the manufacturer assures a reliable operation at 40°C ambient temperature and an operating current from 1575 A up to 3150 A, whereat the rated current is 1200 A in parallel connection. The research program includes several current and voltage endurance. During the current endurance test, the moisture in oil content and the current generated heat losses are in the main focus. Furthermore in the cyclical current endurance test, the breathing of the instrument transformer has been investigated. The breathing of the equipment may be a potential input for moisture from the environment.
Accompanying diagnosis measurement, like moisture in oil, dissolved-gas-analysis, dielectric loss factor and partial discharge measurement has been performed. Through the dielectric loss factor measurement, a high loss factor was observed. Also the PD-measurement showed a potential boundary surface issue, which could be referred to higher moisture in oil content or to higher content of glue between the paper layers. Additionally the clamp connections of the primary winding are under investigation. The focus of the investigations is laid on the temperature behaviour of combined instrument transformers, which get stressed by an artificial generated ambient temperature of 40°C.

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A. Pirker , R. Woschitz , H. Steuerer, M. Hesse

Method to evaluate the live time extension of medium voltage cables

In many Austrian distribution grids PE-insulated medium voltage cables installed in the 1980's are still in service. Failures caused by water-trees in the insulation are well known and reported. For grid operators this raises the question of whether to replace the cables to maintain the reliability of the grid.
Another possibility instead of replacing the cables is to extend the service life of PE-insulated cables using a treatment to refit the insulation. This treatment uses a silicon based fluid which is pressed into the insulation of the cable to inactivate existing water trees. Initially cables were treated using a fluid containing phenylmethyl-dimethyloxysilane components. These components often caused accelerated corrosion of aluminium conductors because of chemical reactions in the past. Due to this fact a couple of years ago the fluid was improved using alkoxylane to prevent corrosion. The effectiveness of this improved method for life time extension of medium voltage cables has been investigated in a cooperation of the Austrian Netz Burgenland Strom GmbH, the German UtilX Europe GmbH and the Institute of High Voltage Engineering and System Performance, Graz University of Technology. Sections of 20 kV cables were used for investigating this insulation refit. An impulse voltage step test has been carried out on these and on refitted samples to evaluate the effect of the treatment.

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Maddalena Schembari

Voltage and reactive power control for maximum utilization of a GW-size EHVAC offshore wind farm interconnection

The paper deals with the steady-state operation of a radial EHVAC cable transmission system with multiple power injection points, e.g. offshore wind farms, and variable shunt compensation. Generator voltages and variable compensation settings are determined by means of a power flow-based algorithm, aiming at symmetrical current magnitude profiles along individual cable lines (CLs): the proposed operation minimizes Joule losses and maximizes cable line (CL) exploitation. Network code constraints on the reactive power exchanged with the bulk power system are also considered. Successful application to a hypothetical 199 km long, 400 kV-50 Hz cable system collecting up to 2 GW from several OWFs is presented, showing that the proposed operation ensures at least 97% transmission efficiency at full load, with near-unity generator power factor values. Moreover, possible constraints due to sending end voltage rise at full load suggest the implementation of voltage control at the network connection point.

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A.Contin, A.Piccolo

Off-Line PD Tests for Quality Assessment of Insulation of AC Rotating Machines

PD testing has become, for utilities and OEMs, a key tool for the condition assessment of the insulation of high voltage rotating machines. PDs can be measured off-line on sample stator bars/coils or on fully assembled stators, while also monitored on operating machines(on-line testing and monitoring). Even though PD testing is actually widely applied throughout a broad range of motors and generators (from 300kW to 1600 MW) there isn't a clear agreement about how to deal with data interpretation. Traditionally PD interpretation has been made by PD amplitude evaluation, but this method has shown weaknesses because of the variability of results given by the relevant chosen instrument. As PDs can be described as high frequency pulses, their amplitude is strongly influenced by the type of sensor and by the acquisition bandwidth (IEC60270 or wide bandwidth) of the acquisition system.
To overcome this problem calibration has been applied to try to get uniform results out of different PD system, but however this method has shown limits when measuring with different bandwidths. Moreover it has been proved that for a reliable analysis, amplitudes shall be associated to each type of PD activity. In order to overcome the before mentioned limits, actually Phase Resolved PD pattern (PRPD) shape analysis is the only method that allows to go through a reliable insulation assessment independently from the type of sensor and acquisition unit.
The following paper describes the application of PRPD patterns shape analysis, when applied to a set stator coils, manufactured with different manufacturing technology. By using this method, it's possible to get consistent results even when a different PD system are used for the analysis thus allowing the comparison of the test results and a reliable insulation condition assessment.

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Rok Leskovec

The use of synchronous machines for reactive power compensation in the grid

Synchronous machines represent the most important transformer of mechanical energy in the electric power and are at the same time the most important factor in regulating voltage or managing the flows of reactive power. Most commonly, they perform both roles at the same time (generation of real and reactive power). The cases where the synchronous machine is preserved also after the generation of real power are still uneconomic (closure of a power plant).

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M. Babuder, M. Hrast, A. Polajner, J. Majcen

Specific design features of the variable shunt reactors technology

The authors report on the initial experience in conceiving the Technical Specification of the variable shunt reactor to be installed in the Slovenian Power Transmission Network. Dilemmas in selecting basic design features and comments on the appropriate choice of most important material characteristics and constituent functional parts are described.

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Stane Vižintin, Boris Žitnik, Borut Vertačnik

Dielectric strength depending on different parameters and corresponding insulating air distances

Dielectric endurance between the two electrodes in the air depends on many parameters. Physical speaking, the air can withstand a certain maximum electric-field strength. If it is exceeded the ionization takes place and the arc accurs. For higher voltages, therefore we need to provide longer air distances, which of course the designers of electric power switchgears take into account. Minimum distances with a certain safety factor for some general conditions are laid down in regulations and standards. If we want to shorten the distances to an optimal value, we need to carefully analyze the conditions. Even regarding voltages there are significant differences in withstand capability according to the shape and duration. A special case is a DC voltage with its polarity. The electric field in the air is generally non-homogeneous and depends on the electrode shape. Sometimes we have to consider a variable air distance. There are also atmospheric conditions, pollution, altitude, water and ice. This paper describes physical background of the flashover and arc formation and how individual parameters affect it. Some practical examples in the electric power system are shown.

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Tim Gradnik

Development and role of on-line monitoring systems in transformer condition assessment

An increasing number of sensors and devices, algorithms and on-line monitoring systems for monitoring operational or diagnostically important parameters of modern power transformers are available on the market today. These tools can, if properly applied, effectively supplement conventional diagnostic methods, improving responsiveness and reliability of transformer condition assessment.
Typical diagnostic assessment of transformer condition, despite the number of devices and available on-line monitoring methods, still largely bases on results of physical-chemical analyses and electrical measurements. The main reasons for such "conservative" approach are encompassed by many factors: complexity of the ageing functional and related testing methods in power transformers. Further, sensors and on-line monitoring systems often tend to be less reliable and accurate than expected for diagnostic purposes. Finally, as frequency of the transformer faults is relatively small, limited feedback on the operational reliability of the monitoring systems is available, slowing down the technological evolution of these systems.
In this contribution, a briefing of experiences with use of on-line monitoring systems in Slovenia is presented, illustrating the problematics presented above. On the basis of experience, exchange of information inside the CIGRE Working Group A2.44 "Intelligent Condition Monitoring" and literature overview, guidelines for efficient use of sensors and comprehensive integration of monitoring systems dataflow with conventional diagnostic results are given.

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Jurij Kurnik, Jože Špiler

The flexibility of operations and the requirements of the grid for existing and new nuclear power plants

Nuclear power plants are in general understood as very inflexible energy generation units, due to low fuel costs, they mostly operate as the units for the generation of band energy, however, in practice, they are also capable of flexible operations, following the burden and active cooperation in system functions of the electrical grid quality maintenance. The paper presents the European Utility Requirements for new light line nuclear power plants with regard to the network, the flexibility of new nuclear power plants and the cases of following the burden of the existing nuclear power plants in Europe.


Mladen Iglič, Boris Žitnik, Rado Ferlič

Overvoltages in 110 kV Cable in cases of Cable-OHL and Cable-GIS connection

Overhead lines of the 110 kV grid are being built more frequently in combination with shorter cable connections, especially at the connections in the GIS network. At each transition of the OL-cable-GIS, overvoltage occurs due to the change in the surge impedance, and the overvoltage endangers the insulation of devices. Several factors impact the overvoltage quantity. These factors are: amplitude and the inclination of the lightning wave, the type and the height of the overhead line tower, the implementation of earthing connections, the resistance of earthing, the length and intersection of the cable etc. The optimum implementation of the insulation coordination is possible by using computer simulations (EMTP-RV). Based on the implemented simulation model of the considered network, such a solution of overvoltage protection is selected to effectively protect the insulation of built in devices.

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Simon Podkoritnik, Miha Bečan, Boštjan Barl

Use of various measures for reduction of overvoltages regarding the operation of 2x400 kV barrel shaped OTL and equipment selection during the preliminary project design

The operational reliability of the overhead transmission line is also determined on the basis of overvoltages which can occur in the system. The frequency of isolation failure in 400 kV system is studied statistically and deterministically according to lightning and switching overvoltages which according to the Insulation Coordination standard IEC 60071-1:2006 represent the main insulation load. The 2×400 kV barrel-shaped OTL is one of the most widespread forms of dual-system overhead transmission lines. In our attempts to provide greater reliability of overhead transmission lines, we achieve this by installing additional protection conductors and overvoltage arresters. Considering various measures to increase operational reliability of the overhead transmission line, we have to choose the proper equipment already during the preliminary project design.

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Anton Urankar, Saša Jamšek, Klemen Dragaš, Miran Marinšek

Investment plan of the electrical energy transmission system operator

ELES, d.o.o. Company, which is an electrical energy network system operator, provides for safe, reliable and continuous transmission of electrical energy. The company's work is mainly divided in five areas. One of them is the Division for asset and project management. One of its tasks is to ensure the long-term quality of means enabling the consumers and other stakeholders timely, reliable and cost-efficient services for ensuring their needs. We introduce, implement and permanently improve the optimum asset management system with systematic, coordinated activities and best practices. Efficient asset management comprises the quality, risk management as well as the benefits and costs in the total life cycle of assets.
The strategic orientations of the Division for asset and project management are:
- the introduction of the model: the owner of the assets, asset manager and service provider,
- asset management in the life cycle by considering risk management,
- support tools management,
- competence development.
In April 2015, based on the requirements of the Energy Act (EZ-1) and the general act regulating this area, the Division for asset and project management prepare an Investment plan for the electrical energy transmission system operator from 2016 to 2018. The investment plan also reveals the sources of financing and the distribution of investments by priorities. The plan also states and determines the criteria for the definition of the prioritised investments. The Portfolio Prioritisation Model (PPM) was used to determine the order of investments by priorities. This model determines the priority order of investments with consideration of the reduction of risks to invested means for each investment and by considering business factors. Business factors are immediately connected with the goals of ELES company.

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Miha Bečan, Simon Podkoritnik, Bojan Volk

Technical solutions for implementation of surge arresters on 110 kV OTL

In 2014 in Slovenia, line surge arresters (LSA) were installed on the HV overhead line for the first time. A total of 212 LSA and 30 surge arrester monitoring system were installed on the Vrtojba- Sežana section, from tower 29 to tower 138 of the two-system 110 kV overhead transmission line (OTL) between Gorica and Divača. The paper presents technical solutions involving the protection of overhead lines with LSA. It also presents various approaches in LSA distribution along the OTL and in the selection of LSA parameters.

The paper also provides the reasons for the necessary comprehensive monitoring of the LSA condition and the their efficiency analysis. The efficiency of the installed arresters is quite an interesting topic, since the work group C4.39, which operates within the CIGRE, currently deals with this topic. The need for greater reliability of overhead lines determines the number of LSA which need to be efficiently distributed along the line, i.e. by knowing the costs of line outages.

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Vladimir Djurica, Garsia Kosinac

The 'Wire Explosion' experiment as a demonstration of utilising electrical energy from natural phenomena in the atmosphere

The GEN Svet energije is an interactive multimedia centre that offers visitors comprehensive and professional information on the meaning of energy and its everyday use, the electrical energy generation technologies and its economic, social and environmental aspects. By presenting classical technologies for electrical energy generation in Slovenia, we often face the questions of visitors on the possibility of generating the same energy in alternative ways. A very frequent idea is connected to the development of technology that would enable the storage and exploitation of electrical energy from natural phenomena in the atmosphere. This thought was also the starting points for implementing a new experiment at Svet energije, which would clearly display the physical background and the possibility of realising such an idea. The Milan Vidmar Electric Power Research Institute assisted us in designing the idea and the model, since they have implemented the 'Wire Explosion' experiment.

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Breda Cestnik, Karol Grabner

Options for reduction of electromagnetic fields in high-voltage facility environment

For electric power facilities of nominal voltage above 1 kV slovenian legislation provides for verification of conformity to the limit values of electric and magnetic fields. In order to limit any possible exceeding the limit value of electric field strength (E) and magnetic flux density (B), and in general, it is important to know of the possibilities of a reduction in E and B.
The presentation reveals some broad options for reduction of the electric and magnetic fields in the environment of high-voltage electric power facilities. Illustrative examples showing the effect of a field reduction are provided. Some of the measures reduce both the E and B and the other are specific (only for E or only for B).

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Maša Majkić, Rudi Vončina

The importance of environmental risks assessemnt in asset management

Operators have been reporting environmental load for more than 20 years. During this time, the relationship between operators, individuals and various public and civil stakeholders has changed considerably. This is reflected mostly by the activities related to the establishment of ecological networks and other protected areas. Therefore, operators need to update the methods of monitoring and reporting, without perceiveing that as their obligation but as a part of a stable business process that requires up-to-date adjusting.
The lifetime of transmission lines and related facilities for electricity trasmission is very long. In that time, the principles of environmental and nature protection may have been established. In case the infrastructure was built before the enforcement of the environmental and nature protection rules, potential problems may occur in maintenance or upgrading phase, when the proposed alternatives of a project "hit" the area with an increased protection regime. Thus, relocation of planned facilities or removal of existing facilities from sensitive areas or change in technology that usually exceeds the limits of economic eligibility, may be required.
In the case of renovation or upgrade of the existing power line or planning a new one, it is recommended that environmental risks are promptly identified and taken into account. Special attention should be given to identifying environmental constraints (taking into account the characteristics of the intervention as well as the development potential of the plan area). Environmental risk assessment requires identification of all present environmental eg. legal, civil trends and Asset management must suitably incorporate them into the business process.
Sensitivity models built for each environmental component and accounting for their attributes can help by identify and evaluate the potential risk for the intervention plan. Such an approach could be used to objectively evaluate the sensitive areas. It is also important to ensure continuous monitoring on the field and identification of the potential points of conflict in the area. The latter can be achieved through identification and appropriate consideration of people's interests for the specific locations, i.e.economic interests, spatial development of the region, the attitude of the local population to their living environment regarding the planned intervention. This enables constant adjustment of the necessary activities considering all the possible situations and appropriate involvement of all relevant stakeholders in the process of reducing the level of risk.

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