Estimation and Analysis of Electric Speed ​​Algorithm: Process and Precautions

1. The current on the low-voltage side of the transformer is 1.5 times the capacity (1.44 times when accurate);

2. The three-phase short-circuit current of the low-voltage outlet of the transformer is 150 times the capacity divided by the percentage of the short-circuit impedance of the transformer (accurately 144 times);

3. The rated current of the 10kV side of the transformer is 6% of capacity (5.8% when accurate), and the rated current of 6kV side is 10% of capacity (9.6% when accurate);

4. The rated current of the 0.4kV three-phase compensation capacitor is 1.5 times of the capacity (1.44 times when accurate);

5, 380V three-phase motor rated current is capacity and 2 times;

6. Under normal circumstances, the low-voltage side power factor is compensated to 0.95, and the capacitor capacity is about 1/3~2/5 of the transformer capacity. It is generally used in places where it is used, especially in places with large capacity such as electric resistance furnaces or gas discharge lamps.

7. Copper discharge flow estimation table

Copper row cross section S (mm2), allowable current carrying capacity I (A), copper row cross section S (mm2), allowable current carrying capacity (IA)

S≤100 I=3.5S;

200<S≤600 I=2.5S

100<S≤200 I=3S;

S>600 I=2S

Note: The smaller the copper strip thickness, the larger the current carrying capacity per mm2.

8. Calculation of rated and starting current protection of three-phase 380V cage motor

The rated current amperage is twice the capacity of kilowatts, and its starting current is about 12~14 times of the capacity. For the direct-start motor, the short-circuit of the short-circuit molded case switch is 17~24 times its capacity; for light load For motors that do not start frequently, the fuse current is 2.5 to 3 times the rated current of the motor; for heavy-duty starter motors, the fuse current is 4 to 5 times their rated current.

9. How to choose the capacity of low-voltage compensation capacitors? How to choose the protection fuses?

In general, the total kilometer of compensation is kilovolt-amperes of transformer capacity and 30%~40%, and the current amperage of single-capacity current is 1.5 times of the capacity of the capacitor. The fuse current of the protection fuse is not less than the rated current of the capacitor. 1.5 times.

Estimated example:

A 1250kVA transformer, short circuit impedance is 6%, voltage 0.4kV rated current Ie=1.5*1250=1875A; low voltage short circuit current Ik=1250*150/6=31250=31.25kA; high voltage 10kV side current I, I= 1250*6%=75A;

The low-voltage capacitor compensation capacity is approximately Q=1250*1/3=420kVar.

Therefore, the high voltage current transformer is selected 75/5;

Low-voltage current transformer 2000/5,2500/5;

The circuit breaker can select the flow breaking capacity to be no less than 35kA;

Two compensation capacitor cabinets, one 200kVar and one 220kVar.

Note: If the high-voltage current transformer is not stable, the ratio can be increased or the current transformer can be selected.