How to calculate the voltage drop in a power line?

The voltage drop in power lines is a critical indicator of the efficiency of electrical energy transmission, directly affecting the stable operation of the power system and the normal functioning of electrical equipment. The calculation of voltage drop is of great significance for the design, operation, and maintenance of power systems, as well as for the control of power quality. Below is a detailed introduction to the methods of calculating voltage drop, expanded to meet the requirements of professional electrical terminology.

Line Voltage Loss Calculation Constants

Calculation of Voltage Drop in Three-Phase Lines with Concentrated Loads

The formula for calculating voltage drop is:
ΔU=(P⋅R+Q⋅X)
The percentage of voltage drop is:
ΔU%=[Ue / ΔU]×100%
Where:
Ue is the rated voltage of the line (kV);
P is the active power transmitted by the line (kW);
Q is the reactive power transmitted by the line (kVAR);
R is the resistance of the line (ohms), calculated as ( R = R0 ⋅ L), where ( r0 ) is the resistance per unit length of the line (ohms/km), and ( L ) is the length of the line (km);
X is the inductive reactance of the line (ohms), calculated as ( X = X0 ⋅ L ), where ( X0 ) is the inductive reactance per unit length of the line (ohms/km), typically ranging from 0.35 to 0.4 ohms/km for general overhead lines.

The values of ( r0 ) and ( X0 ) can be found in electrical engineering handbooks. The resistance and reactance of overhead lines depend on factors such as the material, length, and cross-sectional area of the line.

Calculation of Voltage Drop in Low Voltage Overhead Lines

For low voltage overhead lines, the calculation of voltage drop can be simplified as:
ΔU=M−C⋅S
or
ΔU=Ue⋅ΔU%
The percentage of voltage drop is:
ΔU%=[(M−C⋅S)/Ue]×100%
Where:
M is the load moment (kW·m), calculated as ( M = P ⋅ L );
C is the constant for voltage drop calculation, which can be found in specific tables;
S is the cross-sectional area of the conductor (mm²).

Calculation of Voltage Drop in Single-Phase 220 Volt Lighting Lines

For single-phase 220 volt lighting lines, the percentage of voltage drop is calculated as:

or

Where:
γ is the electrical conductivity (m/(ohm·mm²)), calculated as ( γ= 1/ρ ), where ρ is the resistivity (ohm·mm²/m);
r0 is the resistance per unit length of the line (ohms/km), calculated as ( r0 =1 / γ⋅S ).

When calculating voltage drop, it is necessary to consider factors such as the material, length, cross-sectional area, and load characteristics of the line. Additionally, practical calculations must account for the operating state, ambient temperature, and conductor arrangement, all of which can impact voltage drop.

In the design and maintenance of power systems, precise calculation of voltage drop allows for the rational selection of conductor cross-sectional areas, optimization of line layouts, and the adoption of appropriate compensation measures, thereby improving the efficiency of power transmission and ensuring the safe and stable operation of the power system. Moreover, voltage drop calculations are an important basis for evaluating the operating state of lines and formulating line renovation plans.

In summary, the calculation of voltage drop is a fundamental skill that electrical engineering professionals must master. It not only concerns the economic operation of the power system but also directly affects the power quality experienced by users. Therefore, in practical work, voltage drop calculations should be conducted in strict accordance with relevant standards and specifications to ensure the reliability and economy of the power system.