Steffen Grossmann

Model to assess the reliability of electrical joints

Electrical joints can be weak points in the electric supply system. Based on theoretical and experimental investigation, a mathematical model is presented which describes the resistance characteristic of joints depending on parameters such as conductor material, load, design, assembly quality and its surroundings. Furthermore, using the example of busbar joints, a possibility of how limiting values for the joint resistance can be determine using a mathematical model.

Flashover behaviour of insulators with inhomogeneous temperature distribution in gas insulated systems under DC voltage stress

Due to the increasing need for long-distance energy transmission and space saving installations, the focus is more and more shifted to DC operated gas insulated systems. Reaching the steady state of the electrical field distribution after energizing, the resistive field is controlled by conductivities of insulating materials and SF6. The heating of the inner conductor due to operating currents causes an inhomogeneous temperature distribution within the insulating material, influencing local conductivity. Hence, the location of the highest electrical field under DC stress is shifted with respect to the situation under AC or impulse voltage stress. Consequently, the flashover behaviour of insulators depends on the kind of the electrical stress (AC or DC). In this paper, a CFD model was developed based on experiments to precalculate the temperature distribution at various electrode temperatures. Following up, time-dependent dielectric calculations show the changing electrical field distribution. After reaching the steady state, its inverted character with a shifted high field region can be seen. Experimental results of long-term DC voltage tests confirm this general behaviour.

Influence of gas temperature on the breakdown voltage in gas-insulated systems

In the context of increasing transmission capacities during the last years, the investigation of thermal effects, especially for gas-insulated systems, is required. Higher operating currents decrease the gas density in the vicinity of the heated conductor significantly. Hence, the insulation strength of the system is reduced. Dimensioning for different load scenarios requires a comprehensive knowledge of the temperature influence on the insulation performance of AC and DC installations. This paper introduces an algorithm to predict the temperature-dependent breakdown voltage utilizing the streamer criterion and proves its functionality by experimental investigations. Necessary input parameters, like the electrical field strength and the gas density, can be gained from numerical calculations. The insulation strength of the investigated arrangement at a conductor temperature of 100 °C is decreased by (10 … 15) % in comparison to a conductor temperature of 20 °C. The accordance between measurement and calculation proves the quality of the model.

Validity of the voltage-temperature relation for contact elements in high power applications

With the voltage-temperature (V-T) relation, established by Kohlrausch in 1900, the electric potential distribution in an arbitrarily shaped, thermally insulated, current carrying conductor can be used to calculate the associated temperature distribution. An important application is the determination of the maximum temperature in electric contacts. However, researchers found deviations from the V-T relation for large contact systems when heat dissipation to the environment via radiation and convection becomes significant. The present investigation aims to verify the applicability of the V-T relation to typical spring-loaded contact elements for high power applications as for instance plug-in connections in highvoltage switchgear. A connector with a variable cooling system was continuously loaded with currents up to 6.7 kA to create a multitude of thermally symmetric and asymmetric steady-state operating points. Contact element temperatures were measured and compared to calculations with the V-T relation. When the load current varies over time, the spatially distributed heat capacity of the contact element and the adjacent conductors is charged or discharged to reach the electric-thermal steady-state for which the V-T relation applies. The transient temperature distribution may differ significantly, which is demonstrated by numerical calculations.

Basic investigations on joints with cylindrical aluminum conductors made by press- and shrink-fit for high-current devices

Electrical joints with cylindrical aluminum conductors are often used in Gas Insulated Switchgears (GIS) and Gas Insulated Lines (GIL). Separable connections have to be used for dealing with relative movement between the conductor and the encapsulation initiated by thermal expansion on one end of the conductor. On the other end of the conductor permanent connections can be used with the intention to reduce the costs and operating power losses. Therefore, technologies like the press- and the shrink-fit are suitable. In order to use those technologies for electrical joints proper joining parameters have to be determined from baseline investigations and the long-term behavior of the joints has to be investigated. This paper presents and compares these two joining technologies, press- and shrink-fit. Joints are made with both technologies and the influence of different parameters like surface properties and forms on the joint resistance are being investigated. The behavior of the joint resistance over time is determined from oven-experiments at a temperature of 105 °C. Furthermore a comparison between the cylindrical contacting of the shrink-fit and the flat contacting like it can be found at bus bar joints is made with focus on the contact resistance.

Experimental verification of convective heat transfer computations for gas insulated switchgear

Temperature rise of gas insulated switchgear (GIS) has to be limited according to the demands of the IEC 62271 standard. Computational methods for temperature rise are essential during the design of GIS. The heat in GIS is mainly transferred by convection, as the used insulating gas sulfur hexafluoride (SF6) has a high cooling capacity that further increases as the gas is pressurized. The dimensionless Rayleigh number indicates the intensity in which the flow is driven. If a critical Rayleigh number is exceeded, the flow transits from the laminar to the turbulent regime and the heat transfer coefficient increases steeply. Computational fluid dynamics (CFD) is a useful tool to compute heat transfer and temperature rise of a power device while considering the actual geometry, but modeling assumptions have to be made to cope for turbulence. Experimental investigations are carried out to verify the computed heat transfer of natural convection flows. Also, a Rayleigh number region for the transition of the flow regime in an annulus is determined. The investigations are performed for air and the Rayleigh number is varied by using different filling pressures. This practice allows the investigation of flows for Rayleigh numbers varying over three orders of magnitude. An oil mist is used to show the flow patterns, which allows the flow regime to be assessed by visual investigations. The influence of several modeling assumptions on the accuracy of computed heat transfer coefficients is compiled.

Influence of space charges on the field transition in gas-insulated DC systems

DC operated gas-insulated systems (GIS) play a key role in satisfying recent requirements of energy transmission. After energising, the initial electrostatic field is changing into the electric currents field, which is determined by conductivity processes based on charge carriers. Experimental results show the temperature-dependent effects of the changed field distribution under long-term DC stress and identify clear differences to calculations, which are solely based on conventional RC models. This work proves experimentally, what was exclusively calculated theoretically in literature so far: additional charge carriers, especially negative ions, accelerate the field transition after energising with DC.

Nonlinear dielectric behaviour of insulating oil under HVDC stress as a result of ion drift

The paper presents two nonlinear dielectric effects from literature in mineral insulating oil, when stressed with DC voltage. In the first part it is shown how current time measurements fit Kerr field strength measurements. In the second part, the nonlinear current time effect is noted for polarity reversal. Both effects are caused by oil intrinsic charge carriers and their special dielectric properties. The current-time-behaviour and the nonlinear field strength distribution are calculated solving the Poisson-Nernst-Planck-equation time dependently.

Experimental investigations to the joint resistance of bolted substation and transmission line connectors and its conformity to test standards

SummaryThis paper summarizes the experimental investigations on joint resistance and current distribution of connections with straight connectors. Different sizes (2–4 covers) and types of contact surface (as cast, riffled) are reviewed. Connectors with 2 covers have the lowest joint resistance whereas connectors with 4 covers do have the best performance factor. The kind of contact surface does not affect the initial joint resistance. To compare these two conductor groove shapes long term tests are necessary. Calculations using a resistance network prove the measured initial joint resistances. The current distribution in the connector body is influenced insignificantly by the joint resistance. The more covers a connector possesses the longer is the estimated lifetime.

Thermal dimensioning of an explosion protected residual current operated circuit-breaker with overcurrent protection by the thermal network method

Residual current operated circuit-breakers with overcurrent protection (RCBOs) prevent individuals and devices from harmful residual currents and protect the connected cables from over-currents. If the RCBO is used in explosive atmospheres, an additional enclosure is necessary to protect against dangerous explosions. To ensure safe operation and functionality, the enclosure must be thermally designed so that the temperatures of the RCBO and the surfaces of the enclosure do not exceed the specified limit. The experimental proof, that all limit temperatures are not exceeded, is expensive and time-consuming. Instead the temperature distribution can be computed using thermal models. The thermal network method (TNM) computes steady-state temperature distributions faster than other numerical methods. To thermally optimize a design of a new enclosure, individual thermal network models for the RCBO and the enclosure have been set up, verified and interconnected. Using the interconnected thermal network, several parameter studies have been conducted. By the means of the parameter studies changes for an optimized thermal design were proposed. Due to the fast computations of the TNM, the optimization process is very efficient and changes of the design could already be proposed after a few days.