Maintaining OHTLs presents several challenges, particularly as infrastructure ages and weather events become more severe. The main issues include:

• Environmental Damage: Harsh weather conditions like high winds, ice, and lightning can cause significant physical damage to towers and conductors.

• Corrosion and Deterioration: Over time, components like conductors and hardware can corrode, particularly in coastal or polluted environments, reducing their structural integrity.

• Vegetation Management: Keeping trees and other vegetation clear of the lines is an ongoing task to prevent outages and maintain safety clearances.

• Aging Infrastructure: Many older transmission lines require frequent inspection and potential replacement to ensure they meet modern safety and reliability standards.

These challenges are typically addressed through regular, proactive maintenance programs, including visual inspections, aerial patrols (often using drones), and modern condition monitoring technologies that can detect faults and deterioration early. At Ningbo Changshi, we design our equipment with durability and ease of maintenance in mind to help our clients overcome these challenges.

While underground cable systems have their own advantages, OHTLs are generally the preferred method for long-distance, high-voltage power transmission due to a number of factors:

• Cost-Effectiveness: OHTLs have significantly lower construction and installation costs compared to underground cables, which require extensive trenching and more complex insulation.

• Easier Maintenance and Fault Detection: When a fault occurs on an OHTL, it is typically easier and quicker to locate and repair because the line is openly accessible. Faults in underground cables, however, can be difficult to pinpoint, leading to longer outage times.

• High Capacity and Scalability: Overhead lines can be designed to carry very high voltages and large amounts of power, and they can be more easily upgraded or expanded to meet growing demand.

Despite being more vulnerable to weather, the overall benefits in terms of cost, maintenance, and capacity make OHTL the backbone of most national power grids.

The primary types of conductors used in overhead transmission lines are designed to balance electrical conductivity, mechanical strength, and cost-effectiveness. The most common types include:

• AAC (All-Aluminum Conductor): Made solely of aluminum, these are lightweight and have good conductivity, but are often used for distribution rather than high-voltage transmission due to their lower strength.

• AAAC (All-Aluminum Alloy Conductor): Made from high-strength aluminum alloys, these conductors offer better strength-to-weight ratio and improved sag characteristics compared to AAC.

• ACSR (Aluminum Conductor Steel Reinforced): This is the most widely used type for overhead transmission lines. It consists of a central core of steel strands for high tensile strength, surrounded by layers of aluminum wires for excellent conductivity. This combination allows for long spans with less sag, making it ideal for high-voltage applications.

• ACSS (Aluminum Conductor Steel Supported): A newer type of conductor where the steel core supports the entire mechanical load. This allows the aluminum strands to operate at higher temperatures without losing strength, resulting in lower sag and a higher current-carrying capacity (ampacity).

Conductor sag refers to the vertical dip or curve that an overhead conductor makes between two support points (towers or poles). It's a critical factor in OHTL design for several reasons:

• Safety Clearance: Sag determines the minimum vertical distance between the conductor and the ground, as well as other objects like buildings, roads, and trees. Maintaining adequate sag is essential to prevent flashovers, electrical hazards, and ensure public safety.

• Conductor Tension: Sag is inversely proportional to conductor tension. A tighter wire (less sag) has higher tension, which can increase the risk of mechanical failure or breaking, especially under heavy loads from ice or wind.

• Environmental Factors: Sag changes with temperature, ice loading, and wind pressure. We must perform careful sag-tension calculations to ensure the line operates safely under all potential weather conditions, preventing the conductors from coming into contact with each other or the ground.

The corona effect is a phenomenon of partial electrical discharge that occurs when the electric field strength at the surface of a conductor exceeds the dielectric strength of the surrounding air. This ionization of the air produces a faint bluish glow, hissing sound, power loss, and radio interference.

To mitigate the corona effect, we implement several design strategies:

• Larger Conductor Diameter: A larger diameter conductor reduces the electric field gradient at its surface, thus minimizing the likelihood of corona discharge.

• Bundled Conductors: For very high-voltage lines (typically 220 kV and above), multiple smaller conductors are used in a bundle, which effectively increases the overall equivalent diameter of the conductor, significantly reducing the electric field and corona losses.

• Smooth Surface: Conductor defects, scratches, or moisture on the surface can increase the local electric field. Using conductors with a smooth, clean surface helps in reducing this effect.

While the terms are often used interchangeably by the general public, in the electric power industry, a transmission line is a specific type of power line. The term power line is a broad category that includes all lines that carry electricity, from the generating station all the way to a customer's home.

The key distinction lies in function, voltage, and location:

• Transmission Lines: These are the "highways" of the electrical grid. They are designed for the bulk movement of electricity at very high voltages (typically 115 kV and up) over long distances, such as from a power plant to a substation. They are often supported by tall, imposing steel towers and are uninsulated, relying on air for safety clearance. Our company supplies the specialized tools and equipment needed to string and maintain these high-strength conductors.

• Distribution Lines: These are the "local roads" of the grid. They carry electricity at lower voltages (typically below 69 kV) from substations to individual communities, businesses, and homes. They are usually supported by shorter wooden or concrete poles and are often insulated because they are closer to the ground and public areas.

In summary, a transmission line is a power line, but not all power lines are transmission lines. Power lines also include distribution lines and sub-transmission lines.

The primary purpose of a transmission power line is to efficiently transport large amounts of electricity at very high voltages from a power generating source, such as a power plant or solar farm, to a substation. By using high voltage for long-distance transport, we significantly reduce power loss due to electrical resistance. This ensures that the majority of the electricity generated is delivered to where it is needed. Our company provides the specialized equipment and tools essential for the construction and maintenance of these critical infrastructure projects.

An overhead transmission power line consists of three primary components:

• Conductors: These are the actual wires that carry the electrical current. They are typically made of steel-reinforced aluminum (ACSR) to balance strength and conductivity.

• Insulators: Made from materials like ceramic or glass, these components isolate the conductors from the support structures to prevent the electrical current from flowing to the towers or ground.

• Support Structures: These are the towers or poles (made of wood, steel, or concrete) that hold the conductors and insulators at a safe height above the ground.

Zone protection is a fundamental concept in power system protection where the entire electrical grid is divided into specific areas or "zones." Each zone is monitored by a dedicated set of protective relays. When a fault (e.g., a short circuit) occurs within a particular zone, the relays for that zone act quickly to isolate the faulty section by tripping the associated circuit breakers. The goal is to limit the impact of the fault to the smallest possible area, ensuring the rest of the power system remains operational. Our specialized tools for power line construction and maintenance are crucial for building and servicing the infrastructure that makes this protection possible.

Several schemes are used to provide fast and reliable protection for transmission lines. The most common include:

• Distance Protection: This is a widely used method where relays measure the impedance of the line to determine the distance to the fault. Relays are set with multiple "zones of reach" to provide both primary and backup protection. For example, a Mho relay is commonly used for long transmission lines due to its immunity to power swings.

• Differential Protection: This scheme compares the current entering and leaving a protected zone. Under normal conditions, these currents are balanced. An imbalance indicates a fault within the zone, triggering the relay to act.

• Pilot Protection: This scheme uses communication channels (like fiber optics, microwave, or power line carriers) to exchange information between the relays at both ends of a line, enabling high-speed tripping for faults anywhere on the line.

Our company's equipment is used by professionals who install and maintain the components required for these advanced protection systems.

Zero-sequence impedance (Z₀) is a value that represents the opposition a transmission line presents to the flow of zero-sequence current. In a balanced three-phase system, the sum of the currents is zero, so there is no zero-sequence current. However, during an unbalanced fault—such as a single-phase-to-ground fault—a zero-sequence current flows through the ground and back through the line. The zero-sequence impedance is crucial because it directly influences the magnitude of these fault currents.

Its importance lies in fault analysis and protective relaying. Accurate zero-sequence impedance values are essential for calculating fault currents and designing protective relay settings to ensure a ground fault is detected and isolated quickly and reliably.

For a transmission line, the positive-sequence impedance (Z₁) and negative-sequence impedance (Z₂) are typically considered equal. They represent the opposition to the flow of balanced three-phase currents.

The zero-sequence impedance (Z₀) is fundamentally different because the three-phase currents are in-phase and return through the ground and ground wires. This creates a different magnetic field pattern and, as a result, Z₀ is usually 2 to 3.5 times greater than Z₁ and Z₂. This difference is critical for understanding and calculating asymmetrical fault currents.

The most common entry-level requirement is a high school diploma or equivalent. However, employers highly value candidates with a basic knowledge of algebra and trigonometry. Most aspiring linemen complete a technical program or an apprenticeship, which can last from one to four years. These programs provide essential hands-on training in electrical theory, safety procedures, and equipment operation. A Commercial Driver's License (CDL) is also often required to operate heavy vehicles and equipment.

A lineman's day is physically demanding and often begins with a safety briefing and equipment check. They may perform routine maintenance, such as inspecting and replacing damaged poles, wires, and transformers. A significant part of the job involves responding to emergency outages, particularly after severe weather events. This requires working long hours in challenging conditions to restore power to communities. The work is both crucial and rewarding, as linemen are often the first on the scene to help people in difficult situations.

A 132 kV overhead transmission line consists of four main parts: conductors, insulators, cross-arms, and the tower structure. Conductors, such as ACSR (Aluminum Conductor Steel Reinforced), transmit the electrical energy. Insulators, typically made of porcelain or polymer, prevent the electricity from flowing to the tower and the ground. Cross-arms provide the support for the insulators and conductors. The tower structure itself, which can be made of steel lattice, wood, or concrete, provides the physical support for the entire line.

Designing 400 kV lines presents unique challenges compared to lower voltage systems. Our focus is on providing equipment that addresses these issues. Key considerations include:

• Electromagnetic Field (EMF) Mitigation: As 400 kV lines generate strong electromagnetic fields, design must ensure they comply with international safety standards (e.g., ICNIRP). This often involves specific conductor arrangements and larger right-of-ways.

• Insulation Coordination: Protecting the line from lightning and switching overvoltages is crucial. This requires carefully selecting and positioning high-quality insulators and surge arresters to prevent flashovers.

• Conductor Configuration: Using bundled conductors (e.g., two or more conductors per phase) is common at this voltage to reduce corona effect, which can cause power loss and radio interference.

• Structural Integrity: Towers must be designed to withstand extreme environmental conditions, including high winds and ice loading. Fatigue analysis is also a critical part of ensuring long-term reliability.

The design of a 132 kV line involves balancing electrical, mechanical, and environmental factors. Key considerations include:

• Electrical Clearance: Ensuring safe distances between conductors, and between conductors and the ground, especially with factors like wind and temperature changes. This is critical for preventing flashovers.

• Conductor Sag: The amount a conductor sags between two support structures. This must be carefully calculated to maintain minimum ground clearance under various conditions, including high temperatures and ice loading.

• Insulation Coordination: Selecting appropriate insulators and surge arresters to withstand lightning and switching overvoltages, which are a major concern at this voltage level.

• Tower Structure: Designing towers that can withstand the mechanical loads from the conductors, wind pressure, and other environmental stresses.

• Environmental Impact: Considering the line's impact on the environment, including land use and visual aesthetics.

A 400 kV overhead transmission line project requires a wide range of specialized equipment. As a leading manufacturer, we provide the following:

• Conductors: Bundled conductors, such as ACSR (Aluminum Conductor Steel Reinforced), are essential for efficient power transmission.

• Insulators and Hardware: High-strength insulators, including porcelain or polymer types, along with associated hardware like clamps, tension sets, and suspension assemblies.

• Tower Structures: The physical support for the line, often large steel lattice towers designed to handle significant mechanical loads.

• Stringing Equipment: Specialized hydraulic pullers, tensioners, and conductor carts are used to safely and precisely install the conductors. For 400 kV, this equipment must be robust enough for multi-bundled conductors.

Transmission line losses are a fundamental part of electrical systems and are mainly caused by the physical properties of the conductors and the nature of the current flowing through them.

• Resistive (I²R) Losses: This is the most significant type of loss, where electrical energy is converted into heat as the current flows through the resistance of the conductor. The loss is proportional to the square of the current, which is why transmitting power at higher voltages and lower currents is more efficient.

• Corona Losses: At high voltages (e.g., 400 kV), the electric field around the conductors can ionize the surrounding air, resulting in a visible glow, hissing sound, and energy loss. This is a key reason for using bundled conductors to reduce the electric field gradient.

• Dielectric and Leakage Losses: These losses are minor but occur due to the insulating materials and air around the conductors. Leakage current can flow from the conductors to the ground, especially in humid conditions.

Reducing losses is crucial for improving grid efficiency and managing costs. As a manufacturer of power line equipment, we focus on providing tools and materials that help minimize these losses. Key methods include:

• High-Voltage Transmission: The most effective way to reduce losses is to transmit power at higher voltages, which lowers the current for a given amount of power. This is why EHV (Extra-High Voltage) lines like 400 kV are used for long-distance transmission.

• Conductor Optimization: Using conductors with a larger cross-sectional area and low resistance materials (e.g., aluminum conductors with steel reinforcement) directly reduces I²R losses. Our stringing equipment is designed to handle these larger and often bundled conductors.

• Power Factor Correction: Maintaining a high power factor reduces the reactive current in the line, which in turn minimizes overall current flow and I²R losses. This is often achieved using shunt capacitors or more advanced Flexible AC Transmission Systems (FACTS).

• Regular Maintenance and Modernization: Regularly inspecting and maintaining lines, as well as using modern equipment and smart grid technologies, can identify and fix issues like loose connections or faulty components that contribute to losses.