1. General principles

In order to ensure the safety of high-voltage electrical appliances under normal operation, maintenance, short circuit and overvoltage conditions, high-voltage electrical appliances should be selected according to the following conditions:

(1) according to the normal working conditions including voltage, current, frequency, mechanical load, etc.

② Short-term withstand current, peak withstand current, turn-on and turn-off current are selected according to short-circuit conditions;

③ Selection according to environmental conditions including temperature, humidity, altitude and earthquake;

(4) according to the ability to withstand overvoltage, including insulation level, etc.

⑤ According to the different characteristics of various types of high-voltage electrical appliances, including the operation performance of switches, the coordination of protective characteristics of fuses, the load and accurate grade of transformers, etc.

2. Select high-voltage electrical appliances according to normal working conditions.

(1) according to the working voltage selection of high-voltage electrical appliances, its rated voltage should be consistent with the nominal voltage of the system in the loop, and its allowable maximum working voltage Umax should not be less than the maximum operating voltage Uy of the loop, that is, Umax≥ Uy.

(2) AccordIng to the working current, the rated current in of hIgh-voltage electrical apparatus should not be less than the continuous working current ig of the circuit under various possible operation modes, that is, In≥ Ig.

3. Select high-voltage electrical appliances according to short-circuit stability conditions.

① General requirements for short-circuit stability check

② Thermal effect of short-circuit current

③ short circuit stability check

4. Choose high-voltage electrical appliances according to environmental conditions

(1) choose the environmental temperature of electrical appliances.

② Select the environmental humidity of electrical appliances.

③ The influence of high altitude on high voltage electrical appliances.

(4) the influence of earthquake on high-voltage electrical appliances.

5. Insulation level of high-voltage electrical appliances

6. Choose according to the different characteristics of various high-voltage electrical appliances.

① Selection of rated short-circuit breaking current

② Selection of rated short-circuit closing current

③ Selection of rated operation sequence

④ Selection of rated out-of-step breaking current

⑤ Rated out-of-phase ground fault current, developing fault current and selection of breaking current.

⑥ Selection of rated near-field fault breaking current

⑦ Selection of rated line charging interrupting current, rated cable charging interrupting current, rated capacitor bank interrupting current, rated capacitor bank closing inrush current, rated induction motor interrupting current, rated no-load transformer interrupting current, rated reactor interrupting current, etc. The above-mentioned breaking current and closing current are not specified in the standard, but they should be selected according to the test data given by the manufacturer in use.