What are Electric Power Fittings?
Electric power fittings ( also called ) are a general term for metal accessories used to connect and assemble various devices in power systems, transmit mechanical and electrical loads, or provide specific protective functions. They are indispensable basic components in transmission and distribution lines.
I. Main Classification of Electric Power Fittings
The classification of electric power fittings in China primarily follows national standards, usually divided into the following major categories based on their use and function:
Suspension Clamps
Tension Clamps
Link Fittings
Splicing Fittings
Protective Fittings
Terminal Fittings
Station Fittings / Mounting Fittings
II. Common Types and Applications of Electric Power Fittings
Let’s introduce the common types of these fittings and their specific applications in the power grid one by one.
Suspension Clamps
Function: Mainly used on straight-line towers to suspend conductors or ground wires below insulator strings, or to suspend overhead ground wires on towers. It provides a fixed suspension point for the conductor on the tower while allowing slight swinging with the wind.
Common Types:
Ship-Type Suspension Clamp: The most common type, shaped like a small boat, with an internal clamp mouth and lining (e.g., rubber lining) to protect the conductor.
Pouch-Type Suspension Clamp: Shaped like a pouch, used for conductors with smaller cross-sections.
Application Scenarios: All straight-line towers on high-voltage and ultra-high-voltage transmission lines.
Tension Clamps
Function: Mainly used on strain towers, angle towers, or terminal towers to anchor and tension conductors or ground wires, bearing the full tension of the conductor. It transfers the tension from the conductor or ground wire to the tower.
Common Types:
Bolted Tension Clamp (NLD type): Secures the conductor through bolt pressure, typically used for small to medium cross-section conductors and ground wires.
Compression Tension Clamp (NY type): Uses hydraulic or mechanical crimping to permanently join the fitting and conductor into an integral unit. It offers extremely high reliability and is used for large cross-section conductors or critical lines.
Wedge-Type Tension Clamp (NUT type): Utilizes the self-locking principle of a wedge block to grip the conductor (usually a ground wire or insulated conductor), facilitating easy installation and removal.
Application Scenarios: Start and end points of lines, angle points, and towers requiring sectional tensioning.
Link Fittings
Function: Used to connect insulator strings to towers, insulator strings to clamps, and other fittings to each other. They primarily bear mechanical loads.
Common Types:
Ball-Eye Coupling (QP type) and Socket-Clevis Coupling (WS type): Used to connect the ball and socket of insulator strings.
U-Shackle (U type): The most widely used link fitting, for connecting insulator strings to the tower’s attachment point.
Right-Angle Clevis (ZS type): Used to change the connection direction.
Extension Link (PH type): Used to increase the connection length.
Application Scenarios: Assembly of insulator strings and any location requiring interconnection between fittings.
Splicing Fittings
Function: Used to connect two conductors or ground wires to create a continuous conductive path. They must meet both mechanical strength and electrical continuity requirements.
Common Types:
Splicing Sleeve (JY type): Used for directly splicing conductors, available in hydraulic and explosive compression types.
Repair Sleeve (JYB type): Used for repairing and reinforcing damaged conductors.
Parallel Groove Clamp (JB type): Used for connecting conductors at points not under tension (e.g., jumpers), also commonly used for service drops in distribution lines.
Application Scenarios: Reconnection of broken conductors, extension of line length, jumper connections, etc.
Protective Fittings
Function: Used to protect conductors, insulator strings, etc., from damage caused by vibration, wear, corona, etc.
Common Types:
Vibration Damper (FR type): Installed on conductors or ground wires to absorb and dampen vibration energy induced by wind, preventing conductor fatigue failure.
Spacer (FJ type): Mainly used for bundled conductors (e.g., twin, quad) to maintain a fixed distance between sub-conductors, preventing conductor clashing and suppressing vibration and sub-span oscillation.
Grading Ring / Shielding Ring: Installed at both ends of insulator strings on ultra-high-voltage lines to evenly distribute the electric field, prevent corona discharge, and reduce power loss and radio interference.
Application Scenarios: Areas prone to wind-induced vibration, UHV bundled conductor lines, large crossing spans, etc.
Terminal Fittings
Function: Used to connect conductors or ground wires to terminals of substations or other electrical equipment (e.g., transformers, circuit breakers, disconnectors).
Common Types:
Copper-Aluminum Transition Terminal: Solves the problem of galvanic corrosion between copper terminals and aluminum conductors.
Bolted Terminal: Connects via bolts.
Compression Terminal: Connects via crimping, offering low contact resistance and high reliability.
Application Scenarios: Incoming and outgoing line connections for various electrical equipment within substations.
Station Fittings (Substation Fittings)
Function: Used within substations or power plants to secure and support rigid conductors like busbars and tubular busbars, and to insulate them from ground.
Common Types:
Busbar Fittings (MNG type): Used to fix rectangular or channel-shaped busbars.
Tubular Busbar Support Fittings: Used to fix and support tubular busbars, including fixed fittings, sliding fittings, terminal shielding balls, etc.
Application Scenarios: Busbar structures within substations.
III. Production Process of Electric Power Fittings
1. Raw Material Preparation and Pre-processing
The first step in production is material selection. Different materials are chosen based on the fitting’s purpose (e.g., tension clamps bearing high mechanical force, terminal fittings requiring high conductivity).
Main Materials: Include aluminum and aluminum alloys, copper and copper alloys, malleable cast iron, and carbon steel.
Pre-processing: Purchased raw materials are usually rods, tubes, or plates. They are cut into specific sized blanks using equipment like sawing machines and shearing machines according to the product design, preparing them for subsequent processing.
2. Forming Processing: Shaping the Fitting’s “Body”
This is the core step that gives the fitting its basic shape, mainly using the following processes:
Casting: This is the most common method for producing fittings with complex shapes (e.g., housings for various clamps). Molds are first created based on drawings. Molten metal (e.g., aluminum alloy, cast iron) is then poured into the molds. After cooling and solidification, a casting blank is formed. Casting can achieve complex internal structures and external shapes.
Forging and Crimping: Forging is mainly used to produce link fittings and tension clamps that need to withstand high strength. Forging machinery applies immense pressure to metal blanks, causing plastic deformation within the mold, resulting in forgings with a dense structure and excellent mechanical properties. For some compression-type clamps, specialized hydraulic crimping machines are used to permanently join the metal sleeve and the conductor together under great pressure.
Stamping: This process is mainly used to produce relatively flat parts, such as some connection yokes, bolt washers, etc. It uses punches and dies to blank, draw, and form metal sheets into the desired shape, offering high efficiency and suitability for mass production.
3. Machining: The “Precision Machining” of Elaborate Carving
Formed parts are usually just blanks, with rough surfaces and imprecise dimensions. They require machining to meet drawing specifications.
Turning, Milling, Drilling, Grinding: Use equipment like lathes, milling machines, drilling machines, and grinding machines for precision machining of blanks. Examples include turning precise threads, milling flat mating surfaces, drilling accurate mounting holes, and grinding contact surfaces to reduce contact resistance.
Deburring: This is a crucial step, requiring the removal of all sharp edges and burrs generated during machining to prevent high electric field concentration during operation, which could lead to corona discharge.
4. Heat Treatment: Optimizing Internal “Quality”
For certain parts after casting or forging (especially steel parts and some aluminum alloy parts), specialized heat treatment is performed to relieve internal stress, refine the grain structure, and improve the material’s mechanical properties (e.g., strength, toughness). This is a process involving heating, holding at a specific temperature, and controlled cooling, which can significantly enhance the fitting’s reliability and service life.
5. Surface Treatment: Bestowing the “Armor” of Anti-Corrosion
Electric power fittings are exposed long-term to outdoor environments with wind, sun, rain, snow, and frost, making anti-corrosion treatment crucial.
Hot-Dip Galvanizing: This is the most common and effective anti-corrosion method for steel fittings. The processed steel parts are immersed in molten zinc, forming a dense and firmly bonded zinc-iron alloy layer on the surface, providing excellent cathodic protection and a physical barrier.
Other Treatments: Aluminum fittings are typically anodized to form a protective film. For fittings with special appearance or protection requirements, processes like painting or powder coating may also be used.
6. Assembly and Quality Inspection: The Final “Checkpoint”
Assembly: Various processed components, such as the clamp body, pressure plate, bolts, pins, etc., are assembled into a complete product according to the design requirements.
Quality Inspection: This is the final and most important step before leaving the factory. Samples from each batch of finished products must undergo strict inspection, mainly including:
Mechanical Performance Testing: Such as breaking load tests to verify it can withstand the specified mechanical load.
Electrical Performance Testing: Such as contact resistance tests to ensure the efficiency of the conductive circuit and prevent overheating during operation.
Anti-Corrosion Performance Testing: Such as salt spray tests to evaluate the corrosion resistance of its coating or plating.
Dimensional and Visual Inspection: Ensure the product fully conforms to the design drawings.
IV. How to Evaluate High-Quality Electric Power Fittings?
Electric power fittings are indispensable connecting and supporting components in power transmission and distribution systems. Their quality is directly related to the safety and stable operation of the power grid. Breaking load tests, electrical performance tests, and anti-corrosion tests are the three core tests for evaluating the quality of electric power fittings.
Breaking Load Test: Verifying Mechanical Strength
The main purpose of the breaking load test is to verify the maximum load-bearing capacity of the fitting under static or dynamic tensile force until fracture.Test Equipment: Primarily uses horizontal tensile testing machines or universal testing machines. For some long components in electric power fittings (e.g., anchor chains, spacers), specialized horizontal tensile testing machines provide extra-long effective test spaces (up to 5 meters or even 8 meters) to meet testing requirements.
Key Standards: The testing process must strictly follow standards like GB/T 2317.1-2008 “Test methods for electric power fittings – Part 1: Mechanical tests”.
Electrical Performance Test: Ensuring Current Transmission
Electrical performance testing focuses on the performance of the fitting as part of the current path, core to evaluating its resistance and thermal stability.Contact Resistance Test: This is the most important item, typically requiring a value ≤50μΩ. The four-wire method (using a micro-ohmmeter) is commonly used to eliminate lead resistance errors for high-precision results.
Temperature Rise Test: Simulates the heating of the fitting under long-term rated current flow. Standards require that after energizing until thermal equilibrium, its temperature rise (ΔT) typically does not exceed 30K to 70K (specific values depend on the fitting type and application scenario). This test effectively identifies potential overheating risks due to poor contact.
Key Standards: IEC 61238-1:2018 is an important international standard for evaluating the electrical performance of compression-type connectors.
Anti-Corrosion Test: Assessing Environmental Durability
Anti-corrosion tests are used to assess the fitting’s ability to resist corrosion in harsh atmospheric environments (especially industrial areas, coastal areas), directly related to the equipment’s service life.Salt Spray Corrosion Test: This is the most commonly used artificial accelerated corrosion test method. It places the fitting sample in a sealed chamber and continuously sprays a certain concentration salt mist to simulate marine or de-icing salt environments. The test cycle is usually long, e.g., ≥720 hours, to observe its corrosion resistance performance.
Material and Process Selection: Research shows that ZL101A aluminum alloy has better corrosion resistance than ZL102 aluminum alloy; among anti-corrosion processes, anodizing has proven to be the most effective solution.
Key Standards: GB/T 10125 “Corrosion tests in artificial atmospheres — Salt spray tests” is the fundamental national standard for this test.
V. Practical Considerations
When performing these tests in practice, pay special attention to the following points:
Standards First: Before conducting any test, be sure to clarify the specific national, industry, or enterprise standards the product needs to follow. Test requirements vary greatly for fittings of different voltage levels (e.g., 1000kV vs. 10kV) or application environments (e.g., land vs. offshore converter stations).
Sample Representativeness: The submitted samples must represent the entire production batch. For special-shaped fittings, the clamping method needs special design, such as using hydraulic self-adapting fixtures to ensure uniform force and prevent test errors.
Comprehensive Evaluation: The three major tests are interrelated. For example, anti-corrosion coatings may affect conductivity, and mechanical deformation may damage the anti-corrosion layer. Therefore, comprehensive test results are needed to make a full assessment of the fitting’s quality.
