The Neutral Grounding Resistors are generally employed in the LV and MV distribution networks to protect power transformers and generators from damaging currents in the event of earth fault. This protection system represents the best choice in terms of fast response to localize the fault and consequently to reduce and prevent the overheating and mechanical stress on the conductors.
Alternative protection systems like insulated neutral or direct earthing neutral have several and significant disadvantages such as dangerous transient over-voltages and high difficulties to localize the ground fault in the insulated systems or damaging fault currents with possible arc blast in the earthing neutral systems.
By inserting the Neutral Grounding Resistors with appropriate characteristics between the neutral point of the transformer and the ground it is possible to limit the ground fault current for a short time to allow a quick localization of the fault but, at the same time, it does not damage the equipment.
The advantages of this solution are:
to limit the damages to the electrical equipment, such as switchgear, transformers, cable and rotating machine and to prevent the extension of the fault with benefit also for the safety of the personnel
to protect the conductors and the system equipments from overheating and mechanical stresses
In many low and medium voltage systems, the system neutral may not be available. This is specifically true on Delta and ungrounded Wye Connected systems. To be able to ground these systems, grounding transformer (Zigzag or Wye-Delta type) can be used to create a neutral, which in turn can be connected to ground either directly, or more commonly, through a Neutral Grounding Resistor. These combinations are known as artificial neutrals and in each of them is possible to install a Neutral Grounding resistor.
The resistors must absorb a great energy for very short time, few seconds, and as components of the security system they must assure a high reliability. For this reason it is fundamental design them optimizing the weight of active material (stainless steel) of the resistor elements according to the maximum acceptable heating overtemperatures.
Each PEW Neutral Grounding Resistor is designed and manufactured to meet the technical specifications issued by the customer.