Table of Contents
Why NGR is not used in LV system?
Low Resistance Grounding is used for large electrical systems where there is a high investment in capital equipment or prolonged loss of service of equipment has a significant economic impact and it is not commonly used in low voltage systems because the limited ground fault current is too low to reliably operate …
How do you size an NGR?
The value of the NGR is calculated as Ir =5 x (I1+I2+I3) and the set point of the ground-fault relays on the feeders is 20\% of Ir. If a ground fault occurs on Feeder 3, then Feeders 1 and 2 will each see their charging currents I1 and I2 flow through the current transformer.
Why is the neutral ground resistor used in medium voltage systems?
Optimal neutral ground resistor rating of the medium voltage systems in power generating stations. Neutral grounding resistors (NGRs) are used to protect insulation breakdown in faulty electrical equipment. These faults are caused by transient over-voltages produced by arcing ground on ungrounded systems.
How does neutral grounding affect fault calculation?
The Neutral Grounding Resistor will limit the ground fault to a maximum of 5 to 10 A. Once the current rating is determined, the Resistance or Ohmic Value of the resistor is calculated by dividing the Line to Neutral Voltage by the Current Rating. i.e. for a 480 V System Neutral Grounding Resistor rated at 5 A.
How is NGR value calculated?
How do you select NGR?
The following factor should be considered before selecting the NGR resistance value.
- The fault current should be kept in mind while deciding on the NGR value.
- The system capacitance should be taken care, the NGR should discharge the capacitor during the earth fault.
- System voltage should be considered.
Why neutral grounding is necessary?
Solidly Neutral Grounding slightly reduces the problem of transient over voltages found on the ungrounded system and provided path for the ground fault current is in the range of 25 to 100\% of the system three phase fault current..
What are the advantages of neutral grounding?
Advantages of neutral grounding:
- Voltages of the healthy phases do not exceed line to ground voltages i.e. they remain nearly constant.
- The high voltages due to arcing grounds are eliminated.
- The protective relays can be used to provide protection against earth faults.
Why neutral grounding is required?
Importance of Neutral Grounding The neutral points of transformers, generators and rotating machinery to the earth ground network provides a reference point of zero volts. This protective measure offers many advantages over an ungrounded system, like: Reduced magnitude of transient over voltages.