3D Non-linear Transient Simulation of an SF6 550 kV 3-Phase Gas Insulated Bus

A Gas Insulated Bus (GIB) is used in power plants to transfer currents from generators to main and auxiliary transformers. The magnitudes of the transient forces between the conductors of such a bus system under short circuit conditions is one of the most important mechanical design considerations since the forces involved may cause permanent damage to the bus. Modifications to reduce the size of the GIB need to be carefully considered [1].

The combination of non-linear materials and eddy currents leads to a problem which generally cannot be solved with a steady-state eddy current solver. A transient solver is required and and inherently includes the feature that arbitrary time signals can be used for the excitation.  The CST EM STUDIO® (CST EMS) low frequency transient solver, LT, allows such a simulation to be carried out.  The fields, forces and losses on the bus bars can be calculated as a function of time. A numerical (Finite Element) solution is important for taking these effects into account as well as the accurate modelling of the geometry.

A similar device is shown in the Gas Insulated Switchgear (GIS) article in which the steady-state eddy current distribution with a magnetically linear housing material was calculated at 50 Hz. This GIS may also be simulated with the LT solver. The above design considerations and simulation requirements also apply to a GIS.

Particular strengths of the transient solver include the easy definition of field quantities to be extracted during the simulation such as fields at points and secondary quantities such as forces and losses. A state-of-the-art adaptive Runge-Kutta based time-stepping scheme is available which allows the time-step to be adjusted during the simulation. This leads to the advantage  that unecessarily small time steps are avoided without a loss in accuracy hence faster simulation times. 2nd order elements are also available for increased accuracy.

Figure 1 shows the geometry of the GIB. The housing is a non-linear steel and the conductors are made from Aluminum, assuming a conductivity of 5e7 S/m. The three phases are excited by current paths connected to the model boundaries. This model was created in CST EMS within the space of a few minutes. In practice, however, such devices are generally more complex and CAD models such as Pro/E, CATIA, STEP can be imported, modified and parameterized. The geometry in the aforementioned Gas Insulated Switchgear (GIS) was imported via the Pro/E interface.

Figure 2 shows the excitation currents applied to each phase. The excitation currents may be either taken from the in-built excitation library or defined by the user via tables or expressions. In this case, exponential-based excitations based on standard short-circuit formulae were defined (3-phase short circuit).

Figure 3 shows the cartesian components of the force on the Phase A conductor as a function of time. It should be noted that the X and Z components are equal due to the symmetrical nature of the model.

Other features available in the transient solver include the visualization of the field quantities such as the  eddy current inside the conductors as well as the magnetic field around the bars as a function of time.

This article has served to demonstrate, using a simple model, the application of the LT solver to the simulation of a heavy current, bus bar type application where interconductor forces need to be accurately calculated. The user has the opportunity to model complex imported structures or create models such as that shown in this article directly using the intuitive and easy-to-use modelling features in CST EMS.

Reference

[1] T.Takeuchi, T. Yoshizawa, Y. Kuse, and H. Hama, "3-D Nonlinear Transient Electromagnetic Analysis of Short Circuit Electromagnetic Forces in a Three-Phase Enclosure-Type Gas Insulated Bus",  IEEE TRANSACTIONS ON MAGNETICS, VOL. 36, NO. 4, JULY 2000, pp. 1754-1757