Comparative Study of Simulation of Temperature Rise in Ring Main Unit

Summary

This paper addresses the temperature rise problem of the ring main unit by establishing a simplified model and conducting a comparative analysis in two temperature field-solving modules.

Abstract

The Ring Main Unit (RMU) is a critical device in power distribution systems used for connecting and distributing electricity. However, due to its compact internal structure and high current load, heat dissipation issues are particularly prominent. To address this problem, this study innovatively proposes a simplified RMU model, employing finite element simulation methods to accurately solve for the ohmic losses of conductors under actual operating conditions and obtain ohmic loss data for various components. This is the first in-depth investigation of the RMU's temperature rise problem using such a comprehensive approach. Subsequently, the temperature field was solved using two different temperature field analysis modules, with a detailed comparison and analysis of the simulation results to identify similarities, differences, and trends in temperature distribution. The results indicate that the temperature field solution model, which considers convective heat transfer, is more accurate and aligns with actual operating conditions. This research provides an innovative approach and practical solutions for the design and optimization of RMUs. Future research can further explore multiphysics coupling analysis methods to address structural design and mandatory validation issues for high and ultra-high voltage RMUs and other electrical equipment, thereby providing important insights for engineering design.

Introduction

The ring main unit is a group of high-voltage switchgear mounted in a steel metal cabinet or made of assembled spaced ring network power supply unit of electrical equipment. The overall structure of the load switch and conductive circuit consists of the conductive circuit, which includes a number of components comprising the main core of the ring unit. However, due to its compact internal structure, the ring main unit faces challenges in heat dissipation. This can lead to thermal deformation and aging when operating for extended periods in high-temperature environments. These issues not only affect the service life of the unit but also impact its insulating properties, posing safety risks. In particular, equipment damage and electrical accidents become more likely, posing significant safety hazards.

Within different research areas, scholars have conducted a series of studies on the temperature rise of overhead line switchgear and analyzed various factors affecting the temperature distribution¹. In Polykrati et al.², a mathematical model for the estimation of the temperature rise of components installed on the distribution network during a short-circuit fault is presented. The model was applied to the common disconnecting switches of the network, and the characteristics of the results were plotted according to the different forms of the asymmetrical part of the short-circuit current waveform and the initial value of the short-circuit DC current component. Guan et al., on the other hand, have taken into account the contact resistance and electromagnetic repulsion by building an equivalent contact bridge to simulate the contact interface and further analyzed the electromagnetic-thermal coupling field and temperature rise experiment³. In addition, the researchers investigated the temperature field and thermal stress distribution of the dynamic and static contacts inside the ring main unit by finite element simulation, which provided a basis for the study of circuit breaker life⁴. Finally, Mueller et al. have focused on the geometrical characteristics of heat sinks and evaluated the effects of material selection, total surface area, temperature uniformity, and maximum surface temperature on thermal performance⁵. These studies provide valuable insights and methods to improve switchgear performance and reliability, reduce temperature rise, and extend equipment life. Wang et al. proposed a MiNET Deep Learning Model (MDLM) in the UPIOT environment with the purpose of detecting fault diagnosis of electrical ring cabinets, which was validated to have an identification accuracy of 99.1%, which is significantly higher than that of other methods⁶. Lei et al. studied the thermal performance of a GIS busbar in a steady state using the magneto-fluid-thermal coupling analysis method, thereby optimizing the conductor and tank diameter based on the temperature rise simulation results⁷. Ouerdani et al. used the RMU temperature rise simulation model to determine the temperature rise at critical locations inside it, thereby fixing the duration of the maximum overload for the components inside the RMU accordingly⁸. Zheng et al. described a conventional rectangular busbar in a model of high-current switchgear by building a two-dimensional model and applying the finite element method (FEM) for electromagnetic field calculations. It enabled them to obtain the distribution of bus conductor current density and power loss. An irregular busbar was designed after considering the effects of proximity effect and skin effect. This irregular busbar design improved the performance of conventional rectangular busbar⁹.

As for the aspect of using the icepak simulation, Wang et al. carried out a temperature rise simulation through vortex field, airflow field, and temperature field theories and found that the temperature rise of the ring main unit was more serious under natural convection. They successfully reduced the temperature rise level by adding forced air cooling and making improvements to the internal contact structure¹⁰. Zhu et al.¹¹ used the icepak to simulate a thermal model in order to compare the effect of the presence of thermal vias on the PCB and the presence of heat sinks on the temperature of the power devices. Finally, the theoretical analysis is compared with the simulation results to verify the correctness of the theoretical analysis. Mao et al.¹² studied the temperature and internal airflow distribution under summer operating conditions by thermal simulation based on the CAE software in the icepak simulation. The problem of how to improve the cooling efficiency and control the temperature rise of multiple silver-plated contacts is given, and the temperature and internal airflow contours captured in the simulation will lay the foundation for the design of the cooling scheme for the six silver-plated contacts mounted in the sealing unit. Conversely, in the use of a steady-state thermal module, Zhang¹³ Modeling methods are discussed for solving the thermal network of a high-pressure bushing using an alternative transient procedure. Test and simulation results are in good agreement with the thermal steady state and transient states of the bushing. The transient results are then used to evaluate the bushing overload capacity. Vaimann et al.¹⁴ developed and analyzed an analytical thermal model of a synchronous reluctance motor for predicting the temperature of its different components and the set total parameter thermal network.

With the continuous advancement of research on electrical equipment such as ring main units, conventional temperature rise tests, and production methods are relatively inefficient. Therefore, by utilizing finite element technology combined with offline tests, not only the design cost issues are addressed, but adjustments and optimizations can be promptly made to real-world problems based on simulations. Based on the research progress mentioned above, the use of ANSYS Icepak and Steady-state thermal coupling for comparative analysis is rarely mentioned. Therefore, the protocol describes the mechanism research of finite elements, uses numerical and morphological combinations to establish a finite element temperature rise simulation model for the enclosure, and discusses the finite element temperature rise simulation model based on the results of the two analytical modules by comparing the results of the two simulation modules. Through the comparison between the two simulation modules, we will get the characteristics of the temperature rise trend of the ring main unit and find the most applicable method so as to provide the necessary basis and research ideas for a strategy to mitigate the temperature rise of the ring main unit.

Protocol

1. Model

NOTE: Due to the complex structure of the ring main unit (Figure 1A), an online design software was chosen to simplify the operation of the ring main unit.

1. Modelling simplification
   1. Partially simplify the model, preserving the air box section of the RMU while removing or simplifying other components such as insulating shafts, fastening bolts, nuts, sealing components, and pressure support brackets. The simplified version is shown in (Figure 1B).
      1. In the process of simplifying the 630A type ring main unit, remove the insulated shaft connecting the circuit breaker room with the instrument box and many fixed bolts and nuts. Take out the sealing parts and pressure-retaining bracket and connect the static contacts of the isolated static beam with the lower branch busbar under the premise of ensuring that the whole setup has the same conductive current and only the vacuum circuit breaker, the circuit breaker fixing plate, and the static contacts and the vacuum circuit breaker are retained.
      2. Retain only the vacuum circuit breaker, circuit breaker fixing plate, static contact, and vacuum circuit breaker blocking plate. Overall, remove bolts and gaskets from the model, fill the holes after removing the bolts with solids, reduce the number of mesh parts, and optimize the irregular shapes of the parts. Remove instruments for panel operation, mounting plates, brackets, and other operating parts, such as instrument boxes, that have no effect on the temperature rise simulation process.
      3. Remove the insulated housings of some components can be disregarded under simulation as they have little effect on the simulation results. In addition, grounding switches that have no effect on the use of the equipment during normal operation remove them, and retain the circuit breaker room for simulation.
   2. To delete any section, simply select it and click on the Delete option.

2. Eddy field solution

1. Pre-processing settings
   NOTE: The Eddy current field emulation is the basis for performing the temperature field solution, which requires the subsequent analysis of the solved heat source as a load on the temperature field.
   1. Refer to the equipment documentation for the ring main unit and the relevant manuals to gather information on the physical properties and parameters of each component of the ring main unit. Set the physical attributes and parameters of the ring main unit components in Maxwell based on the information obtained, as detailed in Table 1.
   2. Set the eddy current field load current at 630 A with a frequency of 50 Hz. In the Maxwell software, select One Side of the Upper and Lower Outgoing Arms, enter the excitation module, and set the current magnitude to 630 A. In the solution settings section, choose a frequency of 50 Hz.
      NOTE: In a ring main unit's conductive circuit, the pathway formed by all the components from the upper outlet arm to the lower outlet arm is known as the phase sequence. Therefore, in this paper, the phases A, B, and C are arranged from left to right.
   3. The material parameters of the components of the ring main unit are shown in Table 2.
   4. Direct the current through the outgoing line arms, flexible connections, busbars, circuit breakers, static contact support busbars, and branch busbars for each phase. The aim is to realize a current path that allows the components to complete the load.
   5. Utilize Maxwell's adaptive meshing to complete grid control for the model. Use the Maxwell adaptive mesh partitioning method for larger components and the local mesh refinement for smaller internal components.
      NOTE: Maxwell can continuously enhance grid precision during the solving process, eliminating the need to click Mesh Operations for additional mesh partitioning.
   6. Set the solution step size. Click on Analysis in the model tree, open the Solve Step settings, and set the Maximum Number of Passes to 10. Keep other settings at their default values without making any changes.
2. Principle of Eddy current field calculation^15,16.
   1. Use Maxwell's first equation, which describes the action of charge on the generation of an electric field¹⁷.
           (1)
      where ρ represents the charge density; ε₀ represents the vacuum dielectric constant.
   2. Use Maxwell's second equation, which describes the relationship between a changing magnetic field and an electric field and the effect of a magnetic field on the motion of a charge.
           (2)
      where  represents the magnetic field strength. This equation describes that a varying magnetic field produces a vortex electric field, i.e., the spin of the vortex electric field is equal to the negative of the rate of change of the magnetic field with time.
   3. Use Maxwell's third equation, which describes the effect of magnetic charge on the production of a magnetic field.
           (3)
      This equation describes the magnetic field produced by a magnetic charge as being passive, i.e., there are no monopoles in the magnetic field.
   4. Use Maxwell's fourth equation, which describes the relationship between a varying electric field and a magnetic field and the effect of an electric current on a magnetic field.
           (4)
      where represents the current density and μ₀ represents the vacuum permeability. This equation describes that a varying electric field produces a vortex magnetic field, i.e., the spin of the vortex magnetic field is equal to the sum of the current density and the rate of change of the electric field with time.
   5. Based on the above equations, use the Maxwell 3D using Eddy current solver module to solve the ohmic loss generated by the conductive circuit in the RMU, which provides a heat source for the subsequent thermal simulation analysis. Its mathematical expression is given as¹⁸
           (5)
      where σ denotes the conductivity of the conducting loop material; J is the current density in the loop.
3. Calculation results
   1. Click on the Maxwell 3D option in the interface and open the validation check to review all settings for errors. If there are no errors, proceed to click Analyze All to initiate the solving process.
   2. Utilize Maxwell's post-processing Calculator to compute and plot the ohmic losses in the Eddy current field of the ring main unit, as shown in Table 3.

3. Temperature field solution

NOTE: For comparative purposes, divide the temperature field into Icepak and steady state thermal. Set up and solve each separately to achieve a comparative analysis.

1. Icepak model setup
   1. Set the material properties as follows: designate all circuit solid materials as Cu-Pure, with surfaces using Cu-polished-surface. For the panel components, select Aluminum6061-T6 material, with a surface coating of Paint-AL surface with an emissivity of 0.35. See Table 4 for details. Right-click on the Selected Component, click Edit and then go to Properties to set the material for both the surface and solid materials.
   2. Select the model and click on Set in the Edit menu, then choose Multilevel Meshing Level to adjust the mesh settings. Set the external cabinet to a mesh level of 2, and all boundaries to a mesh level of 2. For all other components, set the mesh level to 3. Finally, open Mesh control and click Generate to create the mesh.
   3. To ensure the accuracy and efficiency of the simulation regardless of the grid size, validation of grid independence is necessary. Import the geometric model of the temperature field enclosure, established using the design software for meshing.
   4. As depicted in Figure 2, the polarization curves of the four grid sets are well-aligned. At a working voltage of 0.5 V, the current densities for the four grid sets are 2.357 A/cm², 2.358 A/cm², 2.356 A/cm², and 2.454 A/cm², respectively, with the error between the maximum and current densities being less than 1%. To balance efficiency and accuracy, determine the grid size which comes to be 987924.
2. Solution setup
   1. Set the directions of the solution domain Cabinet to Opening.
   2. In the software, select Problem Step. Under Basic Parameters, check the Surface-to-Surface Radiation Model, choose Zero Equation for Turbulent Flow Regime, select the Gravity option for Natural Convection, and set the ambient temperature to 20 °C.
   3. In the File settings, choose Volumetric Heat Losses for EM Mapping and select All Objects Shown to complete the loss settings.
3. Temperature field calculation
   1. In icepak, apply three main conservation equations for energy: mass conservation equation, momentum conservation equation, and energy conservation equation. Specifically, use the momentum conservation equation, which is as follows¹⁹:
           (6)
      Energy Conservation Equation:
           (7)
      Mass conservation equations:
           (8)
      Energy transfer equation for heat transfer from a solid heat source:
           (9)
      ρ represents the density of the fluid; v represents the flow velocity vector; T represents the temperature; p is the pressure; τ is the viscous force on the surface of the micro metabolite; κ is the heat transfer coefficient; S_h is the body heat source; h is the specific enthalpy of the fluid and F is the body force of the micro metabolite.
      NOTE: The results of the temperature field calculations are shown in Figure 3A and Figure 4A.
4. Steady-state thermal model setup
   1. Maintain the material properties as per Table 3 in the material settings. Generate the ohmic losses resulting from the eddy current field simulation analysis in the Steady State Thermal module by clicking on Thermal Load generation.
   2. Click on the Convective Temperature Value and set it to 20 °C, with a convective coefficient of 5 (W/m²°C) applied to the inner walls of the cabinet, components, and external cabinet. Apply the settings and generate. Set the output to solve for temperature by clicking on Solve > Output Results.
      NOTE: The main temperature field governing equations in the steady-state thermal temperature field calculation^20,21,22 principle is usually derived from the law of heat conduction (Fourier's law of heat conduction). In the one-dimensional case, the temperature field heat transfer equation can be expressed as²⁰:
           (10)
      In this equation, T represents the temperature inside the object, t is time, x is spatial coordinates, and α is the thermal diffusivity. This equation describes the variation of temperature with respect to time and space, where the right side expresses the relationship between heat conduction rate and temperature gradient. In a more general three-dimensional scenario, the heat conduction equation for the temperature field can be expressed in the following form:
           (11)
      ρ represents the density of the object, c is the specific heat capacity, K is the thermal conductivity and Q is the heat source term within the volume. This equation describes the variation of the temperature field, influenced by heat conduction, heat sources, and thermal capacitance.
   3. The temperature field calculation results are shown in Figure 3. Compare the temperature values summarized in Table 5 and Table 6.

Results

Based on the data in Table 3, the following conclusions can be drawn: The overall losses for Phases A, B, and C are relatively similar. Specifically, the total losses for Phase A are 16.063 W/m³, Phase B is 16.12 W/m³, and Phase C is 19.57 W/m³. The locations with higher losses may be at the connections of various components. This is mainly because contact resistance and conductor resistance typically exist at these connection points. When current passes through these connections, signific...

Discussion

This paper is a comparative simulation analysis of the temperature rise of the ring cabinet based on engineering modeling software and finite element software, and the most suitable solution for the actual temperature rise situation is analyzed by two finite element temperature field solution modules. Thermal management is also described in Icoz²³ as a critical and essential component in maintaining the high efficiency and reliability of electronic components. The significance of conducting a comp...

Materials

Name	Company	Catalog Number	Comments
Air	/	/	Conventional gases
Aluminum	/	/	Alloy Materials
Copper	/	/	Alloy Materials
Icepak	ANSYS company	ANSYS 2021R1	A CFD thermal simulation software
PC hosting	/	12th Generation Intel(R) Core(TM) i5-13500F CPU	Host computer equipment
SolidWorks	Subsidiary of Dassault Systemes	SolidWorks2021	An engineering software drawing tool
Steady-state thermal	ANSYS company	ANSYS 2021R1	A thermal simulation solution tool