X/R ratio 2

[Deansharafi]

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.

 

[DanDel]

It would depend on the nature of the impedance; usually it is designed as resistive(unadulterated resistor) or reactive(inductor). In each case, the actual X and R values could be retreived from the manufacturer.
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.

 

[cuky2000]

See the enclose X/R graph for generators similar to one published on Ansi/IEEE C37.010

http://www.skm.com/support/kb/70001.htm

 

[DanDel]

Deansharafi, are you asking for the X/R ratio for a neutral-grounding impedance, or for the generator itself?

 

[Deansharafi]

DanDel,
I was asking for a neutral grounding impedance. The link above suggested by Cuky2000 is a good one.

 

[alehman]

Cuky's link is for impedances of typical generators, not grounding impedances. For generators >5kV, the grounding impedance is commonly sized such that ground fault current is slighly above the capacitive charging current of the system (usually a few amps).

 

[cuky2000]

I do not see clear the relation between the X/R and the neutral grounding resistor value.

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.

 

[Deansharafi]

""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.""
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.

 

[alehman]

X is not commonly specified for such resistors because it is so small in comparison to R (at power frequencies) as to be negligable for most purposes. This may require a call to a resistor mfg.

 

[cuky2000]

The common relation between the X/R ratio and the neutral grounding resistor (NGR) is the short circuit current level. Fault calculation requires the X/R ratio not only for the generator, but also all system components such as transformer(s), motor(s), etc. The NGR is selected in accordance with the SC level as follow:

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.

 

[Deansharafi]

If the generator is not grounded at all, then it will eliminate phase to ground faults, however phase faults are still damaging the generator and there is no way to reduce them. Is the practice of not grounding generator nutral point only aimed at eliminating ground faults, or there is another reasons as well?

 

[alehman]

Operating with the neutral ungrounded will minimize (not eliminate) ground fault current. There is a small amount of ground fault current due to capacitance in the system.

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.