1. Aim and introduction

The aim of the project is to build a low-voltage cabinet panel and analyse the thermodynamics of the system. A high-current LV busbar system should be considered as a multiconductor transmission line. It is used for the transmission and distribution of electrical power and it is commonly deployed in the buildings, such as: public garages, farm buildings, logistic centers, hospitals, hotels and shopping malls. Busbar systems are characterized by distributed network architecture due to the fact that they create a link between the transformer, the LV switchboard and the floor distribution cabinets along a building. Busbars are optimized for high-current transmission and not designed for transmitting communication signals [1].

2. Objectives of the project

The objectives of the project is:-

• Build the model as per the provided information.
• Mesh the model with non-conformal meshing.
• Solve the problem for natural convection.

3. Model Setup

The schematic of the low-voltage cabinet as provided is shown in Fig. 1. The cabinet consists of 8 circuit breakers (CBs) connected through busbars of the specified dimensions. Liberty has been taken to dimension the components for which no data has been provided.

Fig. 1. Cabinet and panel schematic.

A cabinet of dimensions 3.5m × 3m × 2.5m is built with a panel dimension of 1.6m × 2m × 0.7m. The dimensions of the busbars is as provided in Fig. 2. Using this data, a cabinet and panel enclosure with the determined CBs and busbars are constructed as can be seen in Fig. 3.

Fig. 2. Data provided for the dimensions and rating of the components.

Fig. 3. Created geometry in Icepak.

4. Methodology

The created geometries of the CBs and busbars have to be provided with material properties.

Object	Surface material	Solid material
CBs	Paint – non-metallic	Fe-pure
Busbars	Cu – polished surface	Cu-pure

The busbars carry current of different magnitudes. The heating of these objects depend on their length, hence Joule heating is turned on for busbars as seen in Fig. 4. A current magnitude is provided for Joule heating for each of the busbars according to the table provided in Fig. 2. The longest length through which the current flows has also been provided for power calculation.

Fig. 4. Defining properties of the busbars.

The CBs are provided with the total power emitted value as can be seen in Fig. 5

Fig. 5. Defining properties of the CBs.

An assembly is created to combine all the block objects that include the CBs and busbars with the intention to mesh them separately. This is required to perform non-conformal meshing to reduce the cell count and eliminate mesh bleeding. Slack values are provided for the assembly in each direction. To check if the created assembly would run into a conflict during meshing, an automatic case check tool option provided under the Macros toolbar is used. As can be seen in Fig. 6, a case check for the current assembly does not result in any conflict.

Fig. 6. Automatic case checking to find conflicts due to assembly creation and meshing.

The next step, and the most important one of all, is to create a suitable mesh for the geometry. To create a non-conformal mesh, the created assembly is provided with per-object meshing parameters to enhance the number of nodes in each direction. The generated mesh has 9.9 million nodes. The visual quality is as can be seen in Fig. 7. There is no mesh generated for min y and min z, as the cabinet is on the ground and back surface is on the cabinet wall.

Fig. 7. The generated mesh on a cut plane.

The quantitative analysis of the mesh reveals the following values:-

• Face alignment = 1
• Skewness = 1
• Volume
  • Range = 3.72e-11 m³ to 3.26e-06 m³

The qualitative and quantitative analysis of the mesh indicates the quality of the mesh is good to capture the physics of the flow.

Now as the mesh is set up, the problem setup has to be provided. For this, the problem setup wizard is used to indicate the nature of the problem. As the flow is buoyancy driven, natural convection is the method of heat transfer analysis. The operating density and the acceleration due to gravity is set. A zero equation turbulence model is used to model turbulence and radiation heat transfer is included. The model is set to solve for a steady state solution. Monitor points are set up on the CBs and busbars to monitor convergence. BCGSTAB stabilization method is provided for pressure and temperature in case of natural convection problems. The model can be saved now and then the solution is started.

5. Results and Discussion

After the solution has converged, it is loaded back into Icepak for post processing. Object faces and cut planes are used to visualize the data generated from the solution on the model.

Fig. 8. Temperature contours on the object face.

Fig. 8 shows the temperature distribution over the busbar and CB assembly. As is expected, the highest temperature is at the CB IG with a value of 81.2°C. Heat can be seen dissipating from the IG to other busbars and CBs with a falling gradient towards min Y.

Fig. 9 shows the circulation of the fluid particles within the enclosure around the CBs and busbars. The air gets heated up and is then replaced by cold air due to buoyancy thereby effectively cooling the entire cabinet. The highest velocity attained by the fluid particles is 26.6 m/s and before it flows out of the enclosure.

Fig. 9. Particle trace with speed as the scalar variable.

Fig. 10. Pressure contours on a cut plane along Z axis.

Fig. 10 shows the pressure distribution on a cut plane along Z axis. The pressure is the lowest at locations of high velocity and hence highest pressures is observed near the walls. Higher appreciable pressure drops can be seen in cases of forced convection cooling.

Fig. 11 presents a report of the total heat flow from each of the circuit breakers. The heat loss from each of the element corresponds to the power rating provided. The highest heat flow as indicated is from IG, which receives 231 W of power to be dissipated as heat. The heat dissipated from the bus bars is of far lesser magnitudes than that of the CBs, because it emanates from the Joule heating of these elements supplied with a current.

Fig. 11. Summary report of the heat flow through the CBs.

6. Conclusions

A model of a low-voltage CB and busbar panel is created. The cabinet is cooled by natural convection, where the flow enters through a grille placed at the bottom of the cabinet and then leaves from the grille at the top. The geometry is meshed using non-conformal methods and is then solved for a buoyancy driven natural convective steady state solution. The results generated form the solution is plotted and the physics of the flow explained.

References

[1]	Z. Hasirci, I. Cavdar, N. Suljanovic and A. Mujcic , “Investigation of current variation effect on PLC channel characteristics of low-voltage high power busbar systems,” in IEEE PES Innovative Smart Grid Technologies, Europe, 2014.