Multi-Physics Analysis And Control Of Heavy Current Busbar Bridge Noise

The low voltage and heavy current busbar bridge is a new type of transmission facility, which is widely applied to transmitting current and power in power plants, large and medium-sized enterprises as well as transformer substations. The electric performance and reliability of busbar bridge systems have gained a good market and brought considerable economic benefits. However, the working noise of such facilities is remarkable, especially when the power consumption increases.On-the-spot investigations show that the working noise is mainly composed of electromagnetic noise and ventilating noise, which have three possible generating sources: the vibration of busbars and steel boxes caused by the alternating electromagnetic force; the aeolian vibration caused by air convection when the temperature inside the bridge rises; the vibration of the magnetic components of the busbar bridge system caused by the magnetostrictive effect.In order to understand the underlying mechanism, energy sources and propagation paths of the working noise, an analysis of the electromagnetic field for busbar bridge systems is required, so that the magnetic field distribution and the electromagnetic force on busbars and steel boxes can be obtained. The analysis is the basis of studying the working noise and it also plays an important role in optimizing the structure of busbar bridge systems. To study the causes and propagation paths of the noise of busbar bridge system requires knowledge as diverse as Electromagnetics, Mechanics, and Acoustics. In this paper, steps for analyzing the noise field outside the bridge are establised. In the first place, edge finite element method is used to calculate the distribution of electromagnetic force in the system. Then, node electromagnetic forces are applied as "body force" loads in the subsequent vibration analysis. Afterwards, node displacements are used as boundary condition to solve acoustic problem.Three-dimensional simulation models are established according to the solid one. In vibration analysis, busbars as well as epoxy resin insulators are meshed with solid hexahedral elements and the rest are meshed with shell elements due to the small thickness of steel plates. At the same time, the finite element models used in electromagnetic and vibration analysis are not exactly the same. In electromagnetic field analysis, it is necessary to build finite element model for the air inside or outside the bridge, whereas this is not needed in structural vibration analysis. The boundary element model is created by taking out the surface of the structural vibration finite element model.Electromagnetic force is in direct proportion to the operating currents. This is because the skin effect and proximity effect will strengthen with the increase of operating current. The node forces on left and upper box plates are much bigger than that of the right and lower ones. With the increase of current, the noise of the system rises accordingly. Numerous DOFs of the system lead to the intensive distribution of natural frequency, so the mode shapes changed obviously when the natural frequency has small variation. The relative deformation of the steel box is much bigger than that of other parts because the box is made of large and thin plates, whose stiffness is small. Therefore, resonance occurs more easily on the box. More noise is generated by box plates than by busbars because the stiffness of these large-thin plates is much smaller than that of busbars. The sound pressure level is of symmetric distribution along z direction, and the sound field is stronger in the middle of the bridge. The sound pressure level of right side plates is much higher than that of the left side-plates because the distribution of electromagnetic force on side plates of both sides is asymmetric. In vertical direction the sound field beneath the busbar bridge is much stronger than that of the above.Box plates are the strongest radiated noise source. The vibrations of these plates come from the vibration of box plates themselves caused by electromagnetic force and from the vibration that is transferred from busbars to box plates through epoxy resin insulators. Hence, the noise can be reduced in two aspects. On one hand, improve the distribution of eddy current field in the system. On the other hand, change the structural form and geometric parameters of the main acoustic radiation components.A series of experiments have been carried out in the laboratory in order to verify the reliability of the simulation models and the validity of the noise reduction methods. Five experimental schemes are worked out at last, and they are: (1) experiment for the original model; (2) experiment for the model of moving all busbars upward by 30 mm; (3) experiment for the model of increasing vents on box plates; (4) experiment for the model of pressing grooves on box plates; (5) experiment for the model of riveting L-shaped corner plates on the box.These proposed methods could reduce the noise to some degree. Comprehensively evaluating these methods, we can conclude that the method of moving all three-phase busbars upwards by 30 mm is not a very good one because of the consequent system mode change. Among these methods, the method of increasing vents on box plates has a satisfying noise reduction effect, for the A-weighted sound level can be reduced almost 7 dB compared with the original design, and people in the laboratory can feel the reduction of the noise obviously. This method will not add process, and the general structure of the busbar bridge system has few changes. At the same time, this method can also improve the ventilation of the system, and therefore it is one of the advisable methods to reduce the noise.The paper proposed an effective method to analyze the radiated noise of busbar bridge system. That is using the edge finite element method to calculate three-dimensional eddy current field and the combined finite element and boundary element method to solve the radiated sound field. Through the comparison between calculated results and measured ones, the reliability of the calculation is validated. Based on the analysis of vibration and sound field, the noise reduction methods are put forward, and numerical simulations and experiments study are performed on every improving method. The study shows that the improving methods can reduce the noise at different levels, among which the method of increasing vents on box plates is a better way to reduce it. The proposed methods can be applied in other type busbar equipments.