The loss in the transmission line is called transmission loss or line loss, which refers to the power loss on the line resistance when the load current passes through the line.

Energy loss caused by transmission of electric energy through transmission lines, referred to as line loss. In addition to the transmission lines of electric energy, there are transformers and other transmission and transformation equipment in the power network, which will also produce power loss. The sum of these power losses (including line loss) is called network loss. Line loss is caused by the loss of active power in power transmission, which is mainly composed of the following three parts.

The loss of active power caused by the current flowing through the conductor with resistance is the main part of the line loss. In the formula, P and Q are the active power, reactive power and current flowing through the line respectively; U is the voltage measured at the same point with P and Q on the route; R is the resistance of the line, which is related to the cross-section of the conductor, the material of the conductor and the length of the line. Active power loss caused by voltage in the line and leakage of insulation between lines and between lines.

Transmission loss calculation:

The voltage quality is an important quality indicator of the power supply system. If the voltage supplied to the client exceeds its allowable range, the normal operation of the customer's electrical equipment will be affected, and in serious cases, the electrical equipment will be damaged, causing losses to the customer. Therefore, it is an important task for power supply enterprises to strengthen the voltage management to provide qualified power for customers. The voltage in the power grid fluctuates with the change of load. The country has stipulated the allowable voltage fluctuation range under different voltage levels. Guodianong (1999) No. 652 Document clearly stipulates the rural power voltage:

1. The allowable fluctuation range of distribution line voltage is ± 7% of the standard voltage.

2. The allowable fluctuation range of low-voltage line to home voltage is ± 10% of the standard voltage.

Voltage loss refers to the algebraic difference between the starting voltage and the terminal voltage of the line, which is caused by line resistance and reactance. Reactance (inductive reactance) is caused by the high variable magnetic field generated around the conductor through AC current. The reactance XO (/km) per kilometer length of various overhead lines can be obtained by calculation or looking up relevant data.

The three-phase line is only connected to a three-phase line with a concentrated load when the line is not terminated. Set the line current as, the line resistance R, the reactance X, the starting and ending voltages of the line U1 and U2, and the power factor of the load c0s. Voltage drop △ ù=ù 1 - △ ù 2=Z; Voltage loss is the algebraic difference △ U=△ U1 - △ U2 between U1 and U2.