Contact and Long-Term Behavior of Current-Carrying Connections in Electrical Power Engineering

Currently, the electrical power supply system, consisting of generators, transmission and distribution networks, and consumers, is undergoing a fundamental change. A new structure of few central and many decentralized electrical power generation plants, coupled with increasing volatility of feed-in power due to the use of renewable energy sources, is facing an increasing demand for electrical power, for example, due to the electrification of mobility. Electrical power is increasingly being transported over greater distances between producers and consumers, and existing operating resources are correspondingly heavily burdened. The requirements for availability, safety, resilience, and cost of modern electrical installations have significantly increased. Examples of this are switching and distribution systems, but also new devices such as charging stations for or the power on-board network in electric vehicles. In addition to the safe operation of the electrical power supply network through stable switching states, there must be no failures in the transmission and distribution network or between individual plant components.

A current-carrying connection has both an electrical and a mechanical function. Electrically, a current-carrying contact must be established between the conductors. Mechanically, the conductors should either be firmly connected (e.g., compression connection) or movably joined (e.g., sliding connection). When connecting the conductors, a reproducible and low contact resistance should be achieved, which does not increase unacceptably during the operating time [1, 2]. This ensures the galvanic coupling between the conductors via electrical contacts and a current can be carried permanently. In medium and high voltage technology, the design of the connection must also be such that no unacceptable partial discharges occur. The required lifetime of current-carrying connections in electrical energy technology is usually several decades. Stable contact and long-term behavior are therefore prerequisites for safe and reliable operation of all components and systems.

In electrical power engineering, metals are primarily used as conductors. The specific electrical resistance ρ or the electrical conductivity κ of metals depend on the freely moving electrons in the lattice (represented by ρ₀). The electrons are scattered, and thus the specific electrical resistivity ρ is increased, by temperature-induced lattice vibrations (ρ_T) and lattice disturbances, such as defects, dislocations, grain boundaries, or foreign atoms (ρ_G) (Eq. (3.1)) [1]. The movement of the electrons transports energy and thus also heat. This means that metallic conductors with a low specific electrical resistance ρ also have a high thermal conductivity λ. This indirect proportionality is described by the Wiedemann-Franz-Lorenz law, taking into account the thermodynamic temperature T and Lorenz number L (Eq. (3.2)). In the temperature range in which electrical power engineering systems are operated, it is the case that the movement of electrons dominates energy transport. At very low temperatures near absolute zero, lattice vibration is the dominant mechanism of energy transport that must be taken into account in thermal conductivity [1]. The mechanical properties of pure metals are altered by cold forming or alloying. This increases the number of lattice disturbances and thus the specific electrical resistance due to greater scattering of electrons. This allows the mechanical and electrical properties to be set according to specific applications. As a result, the properties of the contact materials must not change impermissibly during a long operating period and at maximum operating temperature, as this can strongly negatively affect the contact and long-term behavior of current-carrying connections. The behavior of conductor and coating materials is therefore of fundamental importance and will be examined in more detail below (Fig. 3.1).

The current-carrying transition of two current-carrying conductors is referred to as an electrical contact [1]. Depending on the geometry of the conductors, point, line, or plane contacts are created (Fig. 4.1). The technical implementation of an electrical contact is an electrical connection, which is also called current-carrying connection.

Current-carrying connections age depending on operating and environmental conditions. Aging increases the contact and connection resistance, thereby increasing the power loss and the temperature of the connection. Currently, five mechanisms are known that determine aging depending on the type of connection and the joining elements used (Fig. 5.1). In current-carrying connections with contact partners made of the same conductor materials, the force reduction, chemical reactions and at sufficiently high current density, electromigration dominate the long-term behavior. In bimetallic connections, interdiffusion must also be taken into account. If relative movements, caused by thermal expansions or during switching and plugging processes, are possible between the contact partners, friction wear must be considered.

The approach described in this chapter aims to translate the requirements for designing current-carrying connections for operation in electrical energy systems into a constructive design and instructions for assembly of the connection in six clearly defined steps (Fig. 6.1).

The function of a current-carrying connection in electrical energy technology is demonstrated in standardized procedures under uniform requirements. For almost all test procedures, no proof of lifetime is provided, even if numerous test procedures are sometimes referred to as aging tests. The purpose of testing the function of current-carrying connections is therefore more to meet minimum requirements for current-carrying connections with the same application area. Accordingly, the electrical function of the respective connection is demonstrated application-specifically. The basis for this are reproducible load and uniform evaluation criteria, with which current-carrying connections for an application area but especially from different manufacturers can be comparably tested. The loads of the respective application in operation are the basis for testing the connection. In addition, accelerated aging is often sought in the test compared to the rated operation with the aim of not activating additional aging mechanisms or to force or suppress these inappropriately [2]. Internationally, depending on the type of connection and the corresponding application area, numerous procedures for testing the electrical function of current-carrying connections are standardized. The following chapter therefore first considers the influencing factors in the test procedures and their basic mode of operation with regard to the activated aging mechanisms. In Sect. 7.2, tests according to valid standards are presented as examples and in Sect. 7.3, framework conditions for development-accompanying tests are introduced.

The contents of the book are the result of the authors' many years of work at the Chair for High Voltage and High Current Engineering at the Dresden University of Technology. We see ourselves as the spokespersons for a research group that has been active in this field for more than 40 years. The high quality of the research is reflected in the numerous qualification works, which in turn represent a unique treasure trove of knowledge. The essential findings of this unique feature in the research at the chair are summarized in this book. It is intended to provide students, engineers, experts, and interested parties with a basis for utilizing and preserving the knowledge about the contact and long-term behavior of current-carrying connections.