What is Sag and Tention in the transmission line?
When electrical power is transmitted over long distances, it is often carried by a conductor, which is a metal wire or cable that is suspended between two poles or towers. The height at which the conductor is suspended above the ground is called ground clearance. It is important to maintain a certain amount of ground clearance for safety reasons, such as to prevent people or animals from coming into contact with the conductor.
The sag of the conductor is the amount that it dips or droops between the two poles or towers that support it. This is important to consider because the sag of the conductor can affect the tension in the wire, which is the force that is applied to it. If the tension in the conductor is too high, it could break, which would interrupt the transmission of electrical power.
The span is the distance between the two poles or towers that support the conductor. The sag of the conductor and the span are related because the longer the span, the more the conductor will sag due to the weight of the wire and the forces acting on it.
When a conductor is suspended between two poles or towers, the weight of the conductor is distributed along the length of the wire. If the weight is uniformly distributed, it is assumed that the shape of the conductor will be a parabola when it is freely suspended. This means that the conductor will dip down in the middle and then curve back up towards the supports. The amount of sag, or droop, in the conductor increases as the length of the span, or distance between the supports, increases.
There are two main methods used to calculate the sag and tension in a conductor:
1. Parabolic method
2. Catenary method.
The parabolic method is used for small spans, typically up to 300 meters. In this method, it is assumed that the shape of the conductor is a perfect parabola, and the sag and tension can be calculated using mathematical formulas based on this assumption.
The catenary method is used for larger spans, such as when a conductor needs to cross a river or other obstacle. In this method, it is assumed that the shape of the conductor is a perfect catenary, which is a curve that is formed when a flexible cord or cable hangs freely from two points. The sag and tension in the conductor can be calculated using mathematical formulas based on this assumption.
Factors affecting the sag:
- Conductor weight: The sag of a conductor is directly proportional to its weight. This means that as the weight of the conductor increases, the sag also increases. The weight of the conductor can be increased due to ice loading, which occurs when ice accumulates on the conductor. The additional weight of the ice causes the conductor to sag more.
- Span: The sag of a conductor is also directly proportional to the square of the span length. This means that as the span length increases, the sag also increases at a faster rate. For example, if the span length is doubled, the sag will increase by a factor of four.
- Tension: The sag of a conductor is inversely proportional to the tension in the conductor. This means that as the tension in the conductor increases, the sag decreases. Higher tension increases the stress in the insulators and supporting structures, which can cause them to fail if the tension becomes too high.
- Wind: Wind can cause the sag of a conductor to increase, but only in the inclined direction. This means that if the wind is blowing in a direction that is not perpendicular to the conductor, it can cause the conductor to sag more in the direction that the wind is blowing.
- Temperature: The sag of a conductor is also affected by temperature. At low temperatures, the sag is reduced due to the decrease in the conductor's coefficient of expansion. At higher temperatures, the sag increases due to the increase in the conductor's coefficient of expansion.