QuickField Analysis of a Three-Phase Encapsulated Busbars System Ioan Popa * , Marian Ciontu * , Alin-Iulian Dolan * * University of Craiova/Faculty of Electrical Engineering, Craiova, Romania, ipopa@elth.ucv.ro * University of Craiova/Faculty of Electrical Engineering, Craiova, Romania, mciontu@elth.ucv.ro * University of Craiova/Faculty of Electrical Engineering, Craiova, Romania, adolan@elth.ucv.ro Abstract - In this paper, we propose an approach for the magnetic and thermal analysis of a three-phase encapsulated busbars system for high currents using QuickField software. This paper proposes a numerical model developed by coupling of the magnetic field problem with the stationary heat field problem for the geometry of a single-phase execution busbars system. The coupling of problems is realized by importing specific losses from the magnetic field problem as heat sources for thermal field problem. The magnetic field problem is also coupled to the electrical circuit. The shields are short-circuited at both ends and they are connected to the ground. For this constructive solution, in the shields occur induced currents, approximately equals to those of conductors. The analyzed constructive solution has two conductors in parallel per phase, by technological reasons, and therefore electrodynamic forces appear between them. Due to the shielding effect, the magnetic field is practically zero outside of shield and therefore the forces do not occur between phases. In the model it was taken into account the variation of electrical conductivity with the temperature. The thermal model has been validated by an analytical procedure for calculating the active conductor and shield temperatures. The global coefficient of heat transfer by convection and radiation used in thermal model was estimated using the power losses computed by magnetic model. There is a good agreement between analytically calculated temperatures and those numerically calculated. The presented model can be used for analysis, design and optimization of three-phase busbars systems in single phase execution. Keywords - encapsulated busbars, high currents, AC magnet- ics model, steady-state heat transfer model, coupled problems I. INTRODUCTION The shielded busbars systems are used to establish the connection between the generator and the transformer in a power plant. There are two types of construction [1], [2]: - Single phase execution with short-circuited shields at both ends and connected to ground (Fig. 1) or with interrupted by segments shield and connected to ground; - Three-phase execution, with a common grounded shield. For the single phase execution, the active conductor (of aluminum) is placed in a metal grounded shield (also of aluminum). For the three-phase execution all the active conductors are arranged in a common grounded shield. Internal insulation (between the active conductor and the shield) is performed in air at normal pressure or with SF 6 gas at a pressure of 3-5 bars. The construction of shielded busbars must meet the following requirements: - The elimination of the ability to produce accidental short circuits (insulation pollution etc.). - The elimination of the possibility of accidental electrocution by touching the bars under tension; - The low annual costs (return of investments and the Joule losses in the shields); - Reduction of electrodynamics forces. If the shields are short-circuited and grounded at both ends then they produce a circulation of currents ( c e I I 1 1 , c e I I 2 2 , c e I I 3 3 , Fig. 1) approximately equals and in opposite phase with corresponding currents of active conductors. This construction involves a close to zero magnetic field outside the shield and therefore the electrodynamics forces acting between phases are close to zero. This is one of the advantages of this technical solu- tion, but a disadvantage is represented by the Joule losses in the shield. In the case of single phase execution with interrupted shield but grounded, the electrodynamics forces have great values and their calculation is based on the following observations: 1) The magnetic field produced by the current of active conductor is not affected by the concentric shield. 2) The magnetic field produced by the current of the active conductor is strongly attenuated by a neighboring shield. Fig. 1. Three phase busbars system with short-circuited shields at both ends 22 _____________________________________________________________ Annals of the University of Craiova, Electrical Engineering series, No. 37, 2013; ISSN 1842-4805