X/R ratio
X/R ratio
(OP)
Hi all,
What is the X/R ratio of a typical impedance used to connect generator's neutral point to earth? If you know the Impedance (Z) and don't know the X and R, it helps if this ratio is known.
What is the X/R ratio of a typical impedance used to connect generator's neutral point to earth? If you know the Impedance (Z) and don't know the X and R, it helps if this ratio is known.






RE: X/R ratio
You could actually measure the resistance & inductance and compare that value to the listed impedance.
In the worst case, you could estimate. For instance, theoretically, each type would be completely X or R. In reality, if you needed an actual value for X/R, I would imagine something like 1000 for a reactor, and 0.001 for a resistor.
RE: X/R ratio
http://www.skm.com/support/kb/70001.htm
RE: X/R ratio
RE: X/R ratio
I was asking for a neutral grounding impedance. The link above suggested by Cuky2000 is a good one.
RE: X/R ratio
RE: X/R ratio
Neutral Grounding Resistor are normally connected to generators at the wye neutral point directly or via a grounding transformer with the purpose to limit the ground fault current to a safe level (typically below 15 A for generator rated < 15 KV) and sufficient enough to operate protective relays without damaging the generator.
Typical resistor are rated for 10 to 80 Ohms with a maximum ‘time-on’ of 10 seconds and maximum temperature rise of 760 oC per IEEE Std 32. In addition to short time rating, the standard define extended time with 610 oC temp rise and Continuous rating with a temperature rise of 380 oC.
The basic parameters to select the grounding resistor are: Line-neutral voltage, current rating, resistance and time rating.
RE: X/R ratio
What would be the X/R ratio for the above typical resistors?
I know the Ohmage value, but I don't know the X and R components.
RE: X/R ratio
RE: X/R ratio
1- small or medium kVA rating generator: NGR is usually rated to limit the generator neutral current
during a phase-to-ground fault to a value between 100 and 150 percent of the generator full-load current.
2- For large MVA generator under 15 kV: the NGR is significantly reduced if shunted across the secondary of a conventional single-phase step-down distribution transformer (typ. 25 to 100 kVA) whose primary is connected in the generator neutral circuit. The sized of the NGR is selected to limit the phase-to-ground fault to values typically from 5A to 15A.
RE: X/R ratio
RE: X/R ratio
However, operating ungrounded can lead to very high transient overvoltages if a fault develops. Therefore it is normally recommended to have grounding impedance as indicated by cuky. The grounding impedance is usually either an inductor or a resistor for case 1 and resistor for case 2.