Transmission, Distribution, and Renewable Energy Generation Power Equipment

Aging and Life Extension Techniques, Second Edition

Bella H. Chudnovsky

• Publisher: CRC Press
• Year: 2017
• Print ISBN:
  9781498754750
• eBook ISBN:
  9781351648820
• Topic: Technology & Engineering
• Subtopic: Energy
• Index: Technology & Engineering

The revised edition presents, extends, and updates a thorough analysis of the factors that cause and accelerate the aging of conductive and insulating materials of which transmission and distribution electrical apparatus is made. New sections in the second edition summarize the issues of the aging, reliability, and safety of electrical apparatus, as well as supporting equipment in the field of generating renewable energy (solar, wind, tide, and wave power). When exposed to atmospheric corrosive gases and fluids, contaminants, high and low temperatures, vibrations, and other internal and external impacts, these systems deteriorate; eventually the ability of the apparatus to function properly is destroyed. In the modern world of "green energy", the equipment providing clean, electrical energy needs to be properly maintained in order to prevent premature failure. The book's purpose is to help find the proper ways to slow down the aging of electrical apparatus, improve its performance, and extend the life of power generation, transmission, and distribution equipment.

Section I

Transmission and Distribution Electrical Equipment

1	Electrical Contacts Overheating, Wear, and Erosion

1.1 ELECTRICAL CONTACTS

Electrical contacts provide electrical connections, and the primary purpose of an electrical contact is to provide the passage of electrical current across the contact interface. There are four categories in contact design: make–break contacts, sliding contacts, fixed contacts, and demountable contacts.

1.1.1 CLASSIFICATION OF ELECTRICAL CONTACTS

1.1.1.1 Make–Break Contacts

Make–break contacts are designed either to maintain the connection between the conducting materials or break apart during overload or short circuit.

Make–break contacts come in several types, which depend on the level of power load. Higher current types of make–break contacts are almost always made out of copper, due to its high conductivity. Smaller make–break contacts are often made of copper on the inside and a silver alloy on the outside. Different interrupting medium (compressed air, oil, SF₆, vacuum, etc.) is used for insulation and arc quenching. Separable and breaking contacts control the electric circuit in plug connectors and circuit breakers (CBs), and are used for periodical closing and opening of an electrical circuit in different contactors, switches, relays, and so on.

Make–break contacts include stationary contact and moving contact.

Stationary contacts provide the permanent joint and also are divided into two categories: clamped (bolted, screwed, and wrapped) and nonseparable (welded, soldered, and glued). In moving contacts, at least one contact member is rigidly or elastically connected to the moving part of the device. The contacts may be tipped with an arc-resistant material to resist erosion from the high-power arc, and the surfaces may be plated with silver to improve conductivity. Moving contacts are also of several kinds: separable, breaking, and sliding.

1.1.1.2 Sliding or Rolling Contacts

Sliding contacts are made of the conductors that slide over each other without separation. The sliding contacts are used in electrical machines, current pickoffs of transport and lifting machines, where they commutate currents of high and moderate intensity. Sliding contacts are used also in radio-electronic devices, in control and automatic systems operating on low-current level [1].

Sliding or rolling contact design is based on a different concept than make–break contact. These contacts never actually break contact with one another during relative movement which is breaking a flow of electricity. These contacts will move fast during this movement, and therefore must be extremely durable. Sliding contacts provide very little resistance, so they work quickly and accurately.

As a rule, low-current sliding contacts should combine high wear resistance and low friction with high stability of the contact voltage drop. To meet these requirements, contacts with good oxidation and wear resistances made of gold, platinum, rhodium, and palladium alloys are widely used. In addition to high cost of these components, they are not always suitable to meet the service requirements such as vibration stability, impact resistance, and reliability under severe weather and extreme conditions (e.g., low-current sliding contacts of ships and airplanes).

1.1.1.3 Fixed Contacts

Fixed contacts or connections must be clamped or bolted together at the beginning of their use and remain that way for years without being moved. They are used to maintain the overall integrity of the component of which they are a part. These contacts are not stopping or starting electrical flow. Fixed contacts include a wide range of bolted and crimped (clamped) contacts. Terms crimp and clamp are often used to describe the method used to attach a connector to coaxial cable. With a crimp connector, a ring is crimped around the outer conductor (shield) to a slotted or knurled stem. A clamp connector uses a V-shaped wedge ring to secure the outer conductor (shield) to the connector body.

Crimped joints employ the ultimate extreme force of contact making, causing the metal to flow, and make a permanent connection. These types of joints are very attractive for permanent connections due to the trouble-free nature of these joints, and the simplicity and rapidity of the crimping operation. Bolted or crimped contacts are used in interrupting chambers to secure and to maintain the integrity of the electrical component. Bolting is used because it is cheap and convenient.

A clamped joint avoids the reduction in cross section caused by drilling to insert bolts, and gives a more uniform distribution of the contact force, making the contact more efficient and hence running cooler. Clamped contacts are made by mechanically joining conductors directly using intermediate parts, specifically, clamps. These contacts may be assembled or disassembled without damaging the joint integrity. The simplest case of a clamped contact is the joint of two massive conductors with flat contact surfaces, such as busbars.

1.1.1.4 Demountable (Detachable) Contacts

Demountable (detachable) contacts are used to detach a breaker unit from an electrical network, which allows the breaker to be repaired or maintained. This type of electrical contacts is usually found in medium-voltage (MV) metalclad CBs. It helps in taking the breaker off the network by easily sliding it off the busbars for maintenance purposes, which should be done off load.

These contacts, like the make–break contacts, may be carrying high currents at high voltages (HVs; e.g., HV isolators or high or MV fuse contacts). They have to carry current reliably for long periods, without overheating or loss of contact, but do not control whether the current continues to flow through the breaker, like a make–break contact.

Demountable (detachable) contacts are not subjected to the stress of arcing; hence, they do not get the inherent cleaning action associated with it. They are frequently designed to have some frictional action on closing to remove superficial oxide or corrosion films which might impede contact. These contacts have a high contact force but not so high as the contact force in a bolted contact because of the excessive mechanical wear which would be caused when separating the contacts.

Demountable contacts are almost always attached and detached while there is no electricity running through the breaker. Copper and its alloys are the most frequently used materials for the bulk of demountable contacts.

1.1.2 PARAMETERS OF ELECTRICAL CONTACTS AFFECTING THE PERFORMANCE

In all types of electrical contacts, the conducting materials join together and the performance of electrical contact often depends on quality of the joint and conditions of the materials in mating parts. Additionally, the mechanical ability to break the electrical current efficiently during an overload or a short circuit is a very important factor in make–break and sliding contacts.

In automotive application, the electrical contact reliability is an essential issue as the cars experience severe operating conditions such as the engine vibration, operating temperature, and outdoor environmental conditions. The performance of electrical systems and devices depends on the reliability of electrical contacts; therefore, a thorough understanding of what may cause the failure in electrical contacts is extremely important.

To improve a performance and extend, the life of electrical contact and of electrical equipment in whole, multiple parameters and conditions should be considered and analyzed. The reliability and longevity of electrical contact depend on many factors determined by design (electrical and mechanical), and may be enhanced by improving environmental and service conditions [2].

Electrical and mechanical designs play an important role for the contacts, which functions is to interrupt an electrical current, to provide a required function during the life of equipment.

In electrical design the following parameters should be considered:

• Voltage (circuit voltage, voltage at the contacts during circuit interruption)

• Current (amperes, direct or alternating current, frequency)

• Type of load (inductive, capacitive, resistive, motor load, and overload requirements)

• Contact protection and arc suppression

In mechanical design to provide proper contact when the electricity is flowing or should be interrupted, the following factors should be considered:

• Contact force

• Frequency of operation

• Speed of opening and closing

• Wipe or slide between contacts on closure, or butt type closure

• Chatter during opening, or bounce during closing

• Contact gap when fully open

It is important to take into account the method of operating contacts: mechanical, electromagnetic, thermostatic, or manual.

Environmental conditions should always be considered for all types of electrical contacts to control the quality of metallic surfaces connected together in the contact whether they are in open air or sealed. It is important to take into account a composition of atmosphere in...

Table of contents

• Cover
• Half Title
• Title Page
• Copyright Page
• Table of Contents
• Preface
• Author
• Acronyms
• SECTION I Transmission and Distribution Electrical Equipment
• SECTION II Renewable Energy Equipment challenges
• SECTION III Testing, Monitoring, and Diagnostics of Electrical Equipment
• Index