Tension stringing and slack stringing are two distinct methods for installing conductors on overhead transmission lines. The key difference lies in how the conductor is managed during the installation process.

• Tension Stringing: This is the most common and preferred method for modern power line projects. It uses specialized equipment, like our hydraulic puller and tensioner machines, to keep the conductor elevated and clear of the ground and any obstacles throughout the entire process. This method significantly reduces the risk of damage to the conductor, insulators, and other components, while also improving safety and efficiency.

• Slack Stringing: This older method involves pulling the conductor along the ground. It is only suitable for low-voltage or low-importance lines in areas with clear rights-of-way, as the conductor is more susceptible to damage from rocks, vegetation, and other ground hazards. It is also a less safe method, especially when crossing existing infrastructure.

Properly managing sag and tension is a fundamental step in the construction of any overhead power line. The process, known as sagging, involves adjusting the conductor to the correct sag-tension ratio for a given temperature, span, and loading condition.

At Ningbo Changshi, we provide a full range of high-quality tension stringing equipment designed to make this process safe and efficient. Our equipment ensures that conductors are kept off the ground throughout the entire stringing operation and are pulled to the precise tension required by engineering specifications. This approach minimizes damage to the conductor and ensures uniform tension across all spans, which is vital for long-term reliability and safety. We offer the necessary tools, including hydraulic tensioners, conductor pullers, and other specialized overhead line tools that are used by professionals worldwide to meet these rigorous demands.

Safety is paramount in any power line construction project. Our professional equipment is designed with safety features to minimize risks, but proper procedures are equally critical. Key safety considerations and best practices include:

• Proper Equipment Inspection: Before each use, all equipment, including puller-tensioners, stringing blocks, and pulling ropes, must be thoroughly inspected for any signs of wear or damage. Our equipment is manufactured to meet rigorous quality standards, but regular checks are essential.

• Grounding and Bonding: All equipment and conductors being strung must be properly grounded to protect workers from induced voltages and static charges. Creating an equipotential work zone is a fundamental safety practice.

• Site Survey and Planning: A detailed survey of the entire transmission line route is necessary to identify potential hazards like crossing roads, energized lines, or difficult terrain. This allows for the proper placement of equipment and the use of protective measures like guard structures and insulator covers.

• Tension and Sag Control: Correct tension and sag are vital for the long-term integrity and performance of the line. The tensioning process must be carefully monitored using accurate instruments to ensure the conductor is installed according to engineering specifications.

Answer: High-voltage Alternating Current (HVAC) and High-voltage Direct Current (HVDC) are the two primary methods for transmitting electrical power. The key difference lies in the type of current used.

• HVAC is the more common method, especially for short to medium distances and within established grids. Its infrastructure is simpler and generally less expensive to build initially. However, HVAC systems experience reactive power losses and are less efficient over long distances due to factors like the skin effect and Ferranti effect.

• HVDC is a more efficient solution for long-distance bulk power transmission and for connecting asynchronous grids (grids with different frequencies). While the initial cost of converter stations is higher, HVDC lines have lower transmission losses, require fewer conductors, and can be loaded to their thermal limit without stability issues. This makes HVDC the more economical and efficient choice for very long overhead lines and submarine cables.

While both processes involve pulling cables, the equipment and methods are designed for very different environments. Overhead transmission line stringing equipment is built for heavy-duty, long-distance pulls, typically using large bullwheel puller-tensioners to maintain precise sag and tension over long spans.

In contrast, underground cable laying equipment focuses on navigating confined spaces and various ground conditions. This includes specialized pullers, reel stands, and winches designed for use in trenches, conduits, or bores. The equipment is often more compact and is engineered to handle the friction and resistance of pulling cables through the ground or underground ducts. Our comprehensive product range includes both overhead and underground solutions to meet the specific needs of these distinct project types.

When selecting a hydraulic puller-tensioner, several features are critical for ensuring safety, efficiency, and conductor integrity. At Ningbo Changshi, our hydraulic puller-tensioners incorporate these essential features:

• Infinitely Variable Speed Control: This allows for precise adjustments during the stringing process, preventing sudden jerks that could damage the conductor.

• Constant-Tension Control: This feature automatically adjusts the pulling force to maintain a steady tension, which is crucial for achieving the correct sag without overstressing the conductor.

• Safety Brake System: A spring-applied, hydraulic-released brake should engage automatically in case of a hydraulic failure, ensuring the conductor remains securely in place.

• Digital Control System: Modern machines include a digital interface to monitor and record parameters like pulling force, speed, and distance, which is vital for quality control and documentation.

• Durable Bullwheels: The bullwheels should be lined with a wear-resistant material, such as MC nylon, to protect the conductor from damage during the pull.

Stringing blocks, also known as stringing pulleys, are a fundamental accessory for overhead line stringing. Their primary purpose is to support the conductor or pulling rope as it is being pulled between two transmission towers. They are designed to:

• Reduce Friction: The smooth, rotating sheave (the wheel within the block) minimizes friction, allowing for a smoother pull and reducing the amount of force required from the puller.

• Protect the Conductor: The grooves of the sheave are designed to cradle the conductor, preventing it from being scratched, nicked, or otherwise damaged during the pull. For certain projects, like OPGW, specialized sheaves with protective linings are used to prevent damage to the delicate fiber optic core.

• Facilitate Installation: They are available in various sizes and configurations (e.g., single, two, three, or four sheaves) to accommodate different conductor types and bundle arrangements, making it easier to install multiple conductors at once.

OPGW stands for Optical Ground Wire. It is a dual-purpose cable that combines the functions of a traditional overhead ground wire with the added capability of an optical fiber cable for telecommunications.

Stringing OPGW requires a more careful approach and specialized equipment due to the sensitive nature of the fiber optic core. While standard hydraulic puller-tensioners are often used, they must be equipped with features that allow for extremely precise tension and speed control. Additionally, OPGW stringing blocks have sheaves with special linings to ensure no crushing or damage occurs to the cable's surface. A key difference is the need for puller-tensioners that can handle the OPGW's specific pulling parameters to avoid exceeding its maximum pulling tension and bending radius limits. Our OPGW stringing equipment is specifically designed to meet these stringent requirements, providing a safe and reliable solution for this high-value application.

An overhead transmission line is a complex system designed for efficient and safe power delivery. Its main components work together to ensure reliable operation. As a leading manufacturer, we provide the following:

• Conductors: These are the wires that carry the electric current. Typically made of ACSR (Aluminum Conductor Steel Reinforced), they are chosen for their excellent conductivity, lightweight nature, and high tensile strength to span long distances between towers.

• Supports: These are the structures, such as lattice towers or monopoles, that hold the conductors at a safe height above the ground. Their design is critical for withstanding the mechanical stresses from the conductors, wind, and ice.

• Insulators: Made of materials like porcelain or glass, insulators are crucial for preventing the electrical current from leaking from the conductors to the support structures. They are designed to withstand high voltages and environmental stress, ensuring the safety and efficiency of the line.

• Ground Wires: Also known as shield wires, these are positioned at the very top of the towers to protect the main conductors from lightning strikes by providing a direct path to the ground.

• Hardware and Fittings: This category includes a wide array of accessories, such as clamps, spacers, and vibration dampers, that connect the conductors to the insulators and towers, manage conductor movement, and prevent mechanical damage.

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.