What Is a Power Transmission Tower?

Transmission towers are the support points for overhead lines. One circuit on the transmission tower is a single-circuit transmission tower, and two circuits on the transmission tower are double-circuit transmission towers. Single circuit refers to a circuit where a load has one power supply; dual circuit refers to a circuit where a load has two power supplies. Generally, enterprises with high requirements for power supply reliability, or important substations in the region, use double-circuit power supply. This can protect one power supply from power failure for another reason and the other power supply can continue to supply power. High and small users often use a single power supply.

The transmission tower is a high-rise structure, which is very sensitive to slope deformation and has high requirements for uneven settlement of the foundation. The common structural forms of transmission tower foundations are independent foundations, enlarged foundations, and pile foundations. The structural forms of transmission towers are mainly steel structures. Conventional transmission towers and basic structures are difficult to adapt to the ground movement and deformation of the mine's goaf, which may cause the transmission towers to deflect or even capsize.

At present, domestic

For a long time, the materials of transmission towers in China mainly consist of Q235 and Q345 hot-rolled angle steels. Compared with advanced countries in the world, the steel used in transmission towers in our country has a single material, low strength value, and small choice of materials. With the continuous growth of China's power demand, at the same time, due to the shortage of land resources and the improvement of environmental protection requirements in China, the problems of route selection and the demolition of houses along the line have become increasingly serious. Large-capacity, high-voltage transmission lines have developed rapidly and appeared. Multi-circuit lines on the same tower, and AC 750, 1000kV and DC ± 800kV transmission lines with higher voltage levels. All these make the iron tower larger, and the design load of the tower is also getting larger.

1. Wind vibration load of ultra (extra) high voltage transmission tower

The height of UHV transmission towers increases, the flexibility of the structure increases, and the vibration effect caused by pulsating wind increases. It is necessary to properly consider the wind vibration load in the structure design to enhance the wind resistance of the structure.

At present, the tower structure design introduces wind load adjustment coefficient (the coefficient of wind vibration) to consider this dynamic effect, but the selection of the coefficient should meet the structural safety and economic requirements of ultra (U) high-voltage transmission towers.

Wind load is the main variable load of transmission towers. It can be regarded as two parts: the average wind that is far from the natural frequency of the structure, a static property, and the pulsating wind that is close to the natural frequency of the structure, and has dynamic and random properties. The natural vibration frequency will cause forced vibration of the structure.

Transmission towers are high-rise structures, with low natural frequency and close to the pulsating wind frequency, which are prone to resonance, produce large displacements, and cause damage to the structure. Therefore, the dynamic effects of wind loads need to be considered in the structural design to enhance the wind resistance of the structure.

2. Safety evaluation of iron tower structure during capacity expansion of transmission line

Tower safety assessment is an important part of transmission lines, and tower component corrosion is one of the main forms of tower damage, which often results in deterioration of material properties and strength, which affects the bearing capacity of the tower structure and affects structural safety.

Corrosion of tower components often leads to a decrease in the mechanical properties and strength of the component materials, affects the bearing capacity of the tower structure, and threatens the safe and stable operation of the transmission line. The relationship between the mechanical properties of the rusted components of the tower and the different degrees of corrosion was analyzed, and the safety evaluation criteria and corresponding treatments and countermeasures of the tower were proposed.

3. Safety evaluation of rusted iron tower

Safety According to the role of components in the structure of the tower, tower components can be divided into three types: main material, inclined material and auxiliary material. The safety assessment is mainly the main material and the inclined material, and the auxiliary material should consider the complete connection of its components.

The steel materials commonly used in transmission tower structures in China are Q235, Q345, and Q390. The current design standards use material yield strength as the standard strength, and the structure is safe when the structural stress is less than the design strength. When the member's cross-section corrosion causes the component stress to reach the material yield strength, the component yields. Damage, structural strength failure, and unsafe condition.

The safety assessment criteria and countermeasures and measures for rusted iron towers are as follows.

1)-Grade components: No damage, no damage required, no need to deal with.

2) Secondary components: the cross-section loss rate is less than 10%, and necessary reinforcement and repair measures need to be taken.

3) Three-level components: The cross-section loss rate is greater than 10%, which is a dangerous component. It is recommended to remove and replace.

4.Troubleshooting and treatment of steel anchor of steel tower tension clamp

The 220kV double-split conductor of the iron tower suddenly slipped out under normal operating conditions, and the sag of the conductor dropped sharply, which seriously endangered the safe operation of the power grid, and then resumed power transmission after a power outage. The cause of the failure was confirmed: the construction process of the tension clamp does not meet the requirements of the relevant construction process regulations, the aluminum pipe is negative, the position of the mold compression joint is greatly deviated, and the steel anchor and the tension clamp aluminum pipe are not connected as a whole. Under the long-term load and micro-vibration of the wire, the steel core of the wire is broken, which causes the tension clamp steel anchor to be pulled out.

5. Reliability of iron towers for ultra (extra) high voltage transmission lines

As the direct support structure of the transmission line, the reliability of the tower structure is related to the safety of the entire line. A reasonable approach is to compare the reliability probability or reliability index of the structural members, or to establish a considerable safety factor method through certain derivation. Compare. The variable loads of transmission line tower components include wind load, ice load, ground line load, etc. Among them, the high wind load is the live wind load which can change the load, which is the main consideration in the design.

For the basic components of transmission towers, axial compression members can be used to determine the reliability setting level of the transmission towers.

The meteorological load recurrence period of China's 500kV ordinary lines is 30 years, level 2 reliability level. After considering 1.2 times the wind load adjustment factor, its reliability setting level reaches the level 4 structural importance once in 100 years of the US guidelines. Reliability level: Compared with European standards, the reliability level of China's 500kV lines is equivalent to the level 2 reliability level once in 150 years of European standards, which indicates that China's 500kV transmission lines are safe compared with similar foreign standards. ^[3]