By their nature and function, power distribution networks are often subject to incidental flaws. It is therefore important to install protection systems. This protection depends on the treatment of neutral. They are 3 methods of neutral earthing :
Neutral isolated (Fig. 1) – This technique induces significant risks in the event of one phase grounding. It is used only on small networks at low voltage.
- Direct grounding of the neutral (Fig. 2) – The neutral system is directly connected to the earth. In such case, the short circuit current can reach 20 to 30 times the rated current.
- Neutral grounding with limitation of fault current (Fig. 3) – This technique consists in inserting an impedance between neutral and earth. There are 3 types of impedances earthing:
1. Earthing through resistor
2. Earthing through reactance
3. Suppression coil (Petersen coil)
The neutral earthing resistor is the most common, economical and sufficient solution. It is used when the neutral of the supply transformer is available and its own impedance is not enough to limit the fault current. Our Neutral grounding resistors, supplied all over the world, are known for their solidity, reliability, performance and low maintenance requirement.
CALCULATION OF NEUTRAL EARTHING RESISTORS
Three parameters are required to specify the earthing resistor:
* T (in seconds) during which is present the default
* U (V) : Rated voltage line to line voltage or system voltage (kV)
* If (A) : current through the resistor during the fault
Resistance value :
The rise in the resistor temperature will be provided by the relation :
As per IEEE-32 for neutral earthing resistor made from stainless steel resistance material, the allowable temperature rise for 10s, 30s, or 60s time rating is 760°C, 610°C for extended time rating and 385°C for continous rating.