Neutral grounding resistors

The Neutral  Grounding Resistors are generally employed in medium and low voltage (MV and LV) 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 equipments.

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 TELEMA Neutral Grounding Resistor is designed and manufactured to meet the technical specifications issued by the customer.

The main features and characteristics of the product are:

• resistor elements in stamped stainless steel grids or edgewound technology
  
• rating currents up to 10 kA
• insulation level from 1 kV to 132 kV
• protection class from IP00, for indoor installations, to complete IP55 solutions for outdoor applications
• painted enclosures or as alternative mild galvanized, stainless steel

Many optional features/equipments can be associated with the Neutral Grounding Resistors:

• Terminal box for I/O cables
  
• Terminal box for additional devices
• Zigzag grounding transformer, dry type, air cooled
• Three phase transformer Wye / Delta type
• Current and/or Voltage transformers
• Load-break or load-less load switches
• Vacuum contactors
• Surge arrestor
• Space heater