Zijad Bajramovic

Very fast electromagnetic transients in air insulated substations and gas insulated substations due to disconnector switching

Very fast electromagnetic transients (VFT) in air insulated substations (AIS) and gas insulated substations (GIS) caused by switching operation of disconnectors are investigated. Results obtained from very extensive field tests and from digital simulations as well as a comparison between field tests and calculation results of VFT in AIS and GIS due to disconnector switching are presented. Field tests of disconnector switching in 220 kV air insulated substations Grabovica and Kakanj as well as test circuit of 110 kV GIS section are performed. Digital simulation of VFT for different models of power network are performed using EMTP-ATP.

Application of finite element method in calculation of large and complex grounding systems

Grounding systems have the most important role in safety of staff and substation equipment and therefore they require detailed analysis of relevant parameters. This paper discusses the possibility of application of software packages based on finite element method (FEM), when conducting analysis of grounding systems. Firstly the paper describes a mathematical model with assumptions on which is based. Complex grounding grids buried in both single-layer and double-layer soil where analyzed. In order to verify the presented model, calculation results are compared with results of various analytical and numerical methods, as well as with measured results. Finally to show the practical application of this approach, an example of a real grounding grid is calculated.

Experimental results of ELF electric and magnetic fields of electric power systems in Bosnia and Herzegovina

Investigations of electric and magnetic fields on 400 kV transmission lines Tuzla-Visegrad, Sarajevo 10-Sarajevo 20, Tuzla-Ugljevik and 220 kV transmission line Kakanj-Zenica are performed. These transmission lines are important objects for operation of electric power system of Bosnia and Herzegovina. In the paper measurements of extra low frequency (ELF) electric and magnetic fields (EMF) of single 220 and 400 kV overhead transmission lines with flat conductor arrangement but with different horizontal distances between phases and different heights above ground are presented. These electric and magnetic field measurements are also compared with calculations results performed with two software programs (one commercially available and one developed by authors on the base of a solver). Also, in the paper the parameters influencing electric and magnetic fields measurements are investigated.

Influence of Resistance of Electric Arc on Transients Due to Disconnector Switching in Air-Insulated Substations

In this paper, switching overvoltages due to disconnector switching in air-insulated substations (AISs) are presented. Measurements of these switching overvoltages were performed in the AIS Grabovica (Hydro Power Plant-HPP Grabovica) on River Neretva and the AIS Kakanj (Thermo Power Plant-TPP Kakanj). These power plants are important objects for operation of Bosnia and Herzegovinas electric power system. Investigations of operating of air-disconnector-type centre break was performed in order to determine switching-overvoltage levels on primary and secondary circuits that can lead to relay tripping in AIS Grabovica and AIS Kakanj. During operations of disconnector (synchronization or disconnecting of generator from network), malfunctions of signaling devices and burning of supply units of protection relays appeared. At the same time, sparking between primary terminals of the instrument current transformer occurred. The influence of resistance of electric arc between the contacts of the disconnector is analyzed. Computer simulations by means of alternative transients program-electromagnetic transients program are performed. Differences between the measured and the calculated values were under 4.52% for overvoltage factor and under 2.56% for overvoltage wave frequency. Comparison of the transient computer simulations with the field measurements showed that calculations could be used for the assessment of the transient overvoltages caused by disconnector switching.

Computer Simulation of Transients Due to Disconnector Switching in Air-Insulated Substations

Switching overvoltages due to disconnector switching in air-insulated substations (AIS) are presented. Analysis of shape and levels of switching overvoltages by means of experimental and computing investigations on 220 kV switchyards of Thermo Power Plant (TPP) Kakanj are presented. Computer simulations by means of EMTP-ATP [1] are performed.

Automatic Compensation Coil-Petersen Coil in Distribution Grid

The introductory part of the paper gives the basic theoretical considerations of the ways of neutral point treatment in distribution grids with special reference to the method of neutral grounding of distribution transformers by a compensation coil. The second part gives the analysis of the use of compensation coil as a future choice of neutral point grounding on the particular distribution grid TS 35/10 kV Bjelasnica. Also, the paper has conducted analysis of the available data and techno-economic analysis of neutral points grounding of transformers in TS 35/10 kV Bjelasnica in case of transformer neutral grounding by low ohmic resistor and in case of resonant neutral point grounding. In addition to the analysis of the (available) measured values, the adequate dynamic models of distribution grids are made by using the EMTP software package and calculations for neutral grounding by the compensation coil are done, as well as in the case of isolated grids, and a comparison of the results of ground fault currents was made.

Electric Field Calculation on Surface of High-Voltage Transmission Line Conductors

This paper presents a calculation of the value of the electric field on the surface of the conductors and in the immediate vicinity of the high voltage transmission line of 400 kV level. For the calculation of the values of the electric field, the CSM method is used. Verifying the established model was carried out by comparing the calculation results with results of measurements of the electric field at a height of 1 m above ground level. Measurements of the value of electric field lines below the height of 1 m above ground were carried out over the years in Bosnia and Herzegovina. The measurements and calculations were carried out on 400 kV lines of standard dimensions and reduced dimensions. Using the established model, values of the electric field on the surface of the conductors and in their immediate vicinity were calculated. The calculated values of the electric fields are used to determine the corona onset voltage gradients.

Calculation of the corona onset voltage gradient under variable atmospheric correction factors

Corona phenomena on conductors are of importance for high voltage transmission lines. In this paper a calculation of the corona onset voltage gradient of AC conductors has been performed for taking into account the effects of atmospheric conditions. Corona characteristic on overhead transmission lines (OHTL) are influenced by a several factors. The conductor surface roughness and atmospheric conditions have a direct influence on the corona onset voltage gradient. Atmospheric conditions have the largest influence on the value of the air density. The air density is function of the ambient temperature and pressure. Ambient temperature at any given location is subject to daily and seasonal variable. The transmission line crossing a high humidity area and thus humidity should be taken into account in evaluating of the corona onset voltage gradient. Increased voltages in the 400 kV electric power network of Bosnia and Herzegovina are increasing the value of the voltage gradient and higher power losses due to AC corona. Therefore, it is important to determine the value of the voltage gradient on conductor surface as well as corona onset voltage gradient to accurate determined power losses due to AC corona.

A New Concept in the Design and Implementation of the Grounding Transmission Lines

Lightning overvoltage cause the majority of outages in electrical transmission lines, mainly through a back flashover in the transmission system insulation. Key factors in reducing the number of back flashovers are: selection of route in design power lines, grounding, and increase of withstand insulation voltage and installation of metal oxide surge arresters along the transmission line. Reduction of grounding impedance is one of the most efficient ways to prevent the occurrence of back flashovers. The main issues with grounding in Bosnia and Herzegovina and the region are the wide range of specific resistivity in different soil types (10–10,000 Ωm), relatively high isokeraunic levels and high voltage lightning currents (in excess of 150 kA). The paper outlines the main features of own experimental research for reducing back flashovers by lightning current.

Impact of Distributed Generation on Power Quality in Medium Voltage Distribution Networks

This paper provides an analysis of the impact of mini hydropower plant “Kordici” on the 20 kV distribution network of Bugojno area in terms of power quality. Therefore, measurement and evaluation of the quality of the supplied power was carried out at the point of common coupling between the mini hydropower plant and the distribution network. Measurement was carried out for the period from March 7 to March 18, 2016 and the data were evaluated in accordance with EN 50160 standard.