How to set SPD at outdoor transformer station?
Protection of Outdoor Box Transformer
Distribution of surge current
When the power supply is led from an outdoor transformer to a building equipped with lightning protection devices, GB 50057-2010 4.3.8, paragraph 4 requires that surge protectors with Class I testing should be installed at the main distribution box and distribution cabinet where the low-voltage power supply line is introduced. How to set SPD at the outdoor transformer station?
The electrical and electronic systems inside buildings equipped with lightning protection devices may be subject to ground potential counterattacks during lightning strikes (S1 damage source), as well as lightning surges from lightning strikes (S3 damage source) on outdoor box transformers and their buried lines. According to GB 50057-2010, only more severe ground potential counterattack hazards can usually be considered.
If other service facility diversion factors are not considered (or non-metallic pipelines and lines are used at the introduction point), according to the principle of resistance coupling, all lightning currents from lightning strikes on buildings are distributed between the grounding device of the building and the ground of the outdoor box transformer
According to relevant experiments, when applying a lightning current of 200kA and a lightning current of 10/350 μ s, and the grounding resistance R1=R2=30 Ω for the building and outdoor box transformer, the length of the power cable is taken as 50m, 500m, and 1000m, respectively.
In the initial stage of surge current, the distribution of lightning current is determined by the inductance of the system. In the tail stage of surge current, the rate of change of current is small, and the distribution of surge will be determined by the impedance of the system
As the length of outdoor cables increases, the impedance of the power line increases, and the lightning current entering the grounding device of the outdoor box transformer will correspondingly decrease. Therefore, the distribution of lightning current is based on the impedance distribution of the grounding path. For ease of estimation, the grounding electrode ∞ of the building's electrical equipment and the grounding electrode of the outdoor box transformer are usually divided by a 50% -50% shunt principle.
2.2 Selection of SPD
After the surge current enters the low-voltage winding of the transformer, it is discharged into the ground. The lightning current invading the low-voltage side does not cause high overvoltage on the low-voltage winding. However, the overvoltage on the low-voltage side of the transformer will undergo electromagnetic coupling between the high and low-voltage windings, and transform to the high-voltage side according to the transformer's transformation ratio, forming overvoltage on the high-voltage side. The overvoltage that is transformed from the low-voltage side to the high-voltage side by lightning strike is called positive transformation overvoltage.
Due to the much lower insulation margin of the high-voltage winding of the transformer compared to the low-voltage winding, lightning strikes often occur on the low-voltage side of the transformer, while the low-voltage side winding is not damaged, and the high-voltage side winding is damaged due to positive transformation overvoltage. Therefore, it is necessary to install SPD discharge surge current on the low-voltage side of the box transformer.
According to the distribution of surge current, the grounding device of the box transformer needs to discharge surge current equivalent to that of the building.