How to calculate power distribution

In modern architecture and industrial production, the safe and stable operation of power system is very important. As an important part of power system, the design and calculation of power distribution is particularly critical. Reasonable calculation of power distribution can not only ensure the normal operation of equipment, but also effectively avoid potential safety hazards such as overload and short circuit, and improve energy utilization efficiency. This paper will briefly introduce the basic calculation methods and steps of power distribution.

I. Overview of Power Distribution

Power distribution mainly refers to the system that provides electric energy for motor, mechanical equipment, production line and other power loads. Different from lighting distribution, power distribution usually involves large current and power, so the selection requirements for lines, switches and protection devices are more stringent.

Second, the basic content of power distribution calculation

The calculation of power distribution mainly includes the following aspects:

1. Load calculation
2. Cable selection and current carrying capacity verification
3. Selection of circuit breaker or fuse
4. Short-circuit current calculation
5. Voltage drop verification

# 1. Load calculation

Load calculation is the basis of the whole distribution system design. Usually, the need coefficient method is used for calculation:

$$ P_c = K_x imes P_e $$

Among them:
-$P_c$: calculated power (kW)
-$K_x$: demand coefficient (selected by looking up the table according to the equipment type and usage)
-$P_e$: sum of rated power of equipment (kW)

Through load calculation, the capacity of distribution lines and the capacity requirements of transformers can be preliminarily determined.

# 2. Cable Selection and Ampacity Verification

Cables should be selected according to the calculated current and meet the requirements of long-term allowable current carrying capacity. The formula for calculating current is as follows:

$$ I_c = frac{P_c}{sqrt{3} cdot U cdot cosvarphi} $$

Among them:
-$I_c$: calculated current (a)
-$U$: line voltage (v)
-$cosvarphi$: power factor (usually 0.8~0.85)

Then, according to the calculated current look-up table, select the appropriate cable cross-sectional area, and check whether the current carrying capacity under the actual laying conditions meets the requirements.

# 3. Selection of circuit breaker or fuse

The rated current of the circuit breaker should be greater than or equal to the calculated current of the line, and at the same time, it should be able to avoid the starting current of the motor. General motor circuit breaker setting value can be according to:

$$ I_{set} = 1.2 sim 1.5I_{max} $$

Where $I_{max}$ is the maximum working current of the line.

# 4. Short-circuit current calculation

Short-circuit current is the basis for selecting protection equipment and checking the dynamic and thermal stability of equipment. The calculation of short-circuit current is complicated, and factors such as system impedance, transformer parameters and line length need to be considered. It can be obtained by look-up table method or software-aided calculation.

# 5. Voltage Drop Verification

In order to ensure the normal operation of the equipment, the voltage drop at the end of the line should not exceed the allowable value (usually 5%). The voltage drop calculation formula is:

$$ Delta U = sqrt{3} cdot I_c cdot (R cosvarphi + X sinvarphi) cdot L $$

Among them:
-$R$, $X$: resistance and reactance per unit length (ω/km)
-$L$: line length (km)

If the voltage drop exceeds the allowable value, the cable cross-sectional area should be increased or the power supply distance should be shortened.

Third, summary

Accurate calculation of power distribution is the premise to ensure the safe and reliable operation of power system. In the design process, various factors should be comprehensively considered in combination with the actual engineering requirements to ensure reasonable selection, reliable protection, energy saving and high efficiency. At the same time, with the development of electrical design software, many calculations can be completed by professional software (such as ETAP, SKM, etc.) to improve efficiency and accuracy.

In a word, it is very important for electrical engineers to master the basic calculation method of power distribution, which is the basis of realizing scientific distribution and intelligent management.