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# Grounding riddle No.7 – Neutral Grounding Design

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• #188
T.Kailash Kumar

How to design a neutral grounding resistor for 5 MW generator and what are the points to be considered before design a NGR for generator? Kindly give some design methods or articles regarding this.

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hamid

Generally for application of generator-neutral resistances (or the equivalent through a grounding transformer), the current passed through the resistor during ground faults should equal or exceed the capacitive current that would flow during a line-to-ground fault with the resistor disconnected. The upper limit of current passed through a generator neutral resistor is fixed by the desire to avoid excessive power loss, and is customarily held to 1.5 times full load generator current, or less. The ratio of Xo/Ro should preferably not exceed 1.0, including the reactance of the resistor. Cast-iron grid resistors may have power factors of 0.98 and sometimes less, which means that their reactance may be about 20 percent of their resistance, at system frequency. There are two scenarios for determining of suitable resistance. 1- Concerning of power swing and stability in low impedance grounding systems. 2- Concerning of over voltage effects and insulation problem in high impedance grounding systems. 1- Power swing and stability consideration An important consideration in resistance-grounded systems is the power loss in the resistor during line-to-ground faults. The power loss in the grounding resistor is Ir.Er or Ir2.R. Ir and Er are both in terms of normal values per phase. The power loss obtained by their product is therefore in terms of normal value per phase. Conseqcntly, if the power loss in the resistor is to be expressed in terms of total three-phase system kva rating, it must be divided by three. Thus resistor power loss in percent is R.Ir2/3, when Ir is in per unit and R in percent. The maximum power loss for this case is 89.3 percent of the system rated capacity if three times the resistance in the neutral has the same ohmic magnitude as the reactants determining the ground-fault current. If the generator reactance are lower, still higher power mill occur during grounds and may cause violent swinging of generator phase position or instability. Resistances in this region are to be avoided, and since there is always some additional resistance in the fault, it is preferable that the grounding resistors alone be sufficient to carry well beyond the peak. Effect of system size and operating voltage on size of neutral resistor to limit ground fault current to one-quarter full-load system current. The actual ohmic value of the grounding resistor required will vary widely depending upon the circuit voltage and system capacity. By

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