| Earthing transformer is used in medium-sized point insulation of the three-phase power system, which is used |
| for providing a man-made neutral point for such system. The neutral point can be connected to the ground directly, |
| or can be connected to ground through reactor, resistor or extinction coil. |
| Medium-sized point insulation system lead out man-made medium-size point with the purpose as follows: |
| 1) Overhead lines are easily affected by the live cloud, dust, hail, fog and rain and the induction gradually |
| electrostatic charge will appear, by which the line-to-ground voltage can be changed. With the induction charge |
| gathering, the line and the associated equipment will result in a high suspension potential to ground, which will |
| damage insulation of the line and the associated equipment. If the neutral point man-made connected to earth, the |
| induction electrostatic charge can be transmitted to the earth, which will avoid the damage to the line and the |
| associated equipment. |
| 2) In the neutral point insulation system, high-voltage shock exists generally. If the neutral point connected |
| to the ground, the high-voltage shock can be declined to the minimum extent. |
| 3) When single phase ground fault occurs in neutral point insulation system, usually in the form of arc light |
| grounding,means intermittent electric arc appears. Meanwhile no fault line-to-ground voltage will be line voltage, |
| line-to-ground capacitance current will increase to 1.732 times of the original, and the capacitance current of |
| fault point is a relatively 1.732 times, so the capacitance current of fault point will be 3 times of the normal |
| line-to-ground capacitance current. The capacitance of point can be compensated by the inductor current through |
| the extinction coil, by which the intermittent arc will be put out automatically soon. |
| 4) Medium-sized point insulation system lead out man-made medium-size point and connect to the ground (not |
| through extinction arc), mating with automatic protection device, can isolate the fault part from the no-fault |
| part. |
| |
