X/R ratio - Doesn't it change from point to point in the system? 1

[bdn2004]

X/R ratio... Isn't this the reactance/resistance at a certain point in the power system? Like the Thevinin reactance / Thevenin resistance ?
If so, why doesn't power analysis software calculate this value and give it to you throughout the system ? just like it does the various short circuit values?

As far as I can tell our power analysis software allows you to put this value in from the Utility - and that's it.

 

[HamburgerHelper]

It does change as you move around the system. I don't know what software you are using but in the software I have used, when you apply a fault it gives you the fault current and the X/R ratio among other things. You will see the biggest changes when you change voltage or are near an auto transformer.

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If you can't explain it to a six year old, you don't understand it yourself.

 

[dpc]

Yes it changes. If you run a short circuit calculation at a particular bus, the results should include the X/R ratio at the fault point. Components like transformers, generators, etc, must have X/R ratio (or actual X and R) defined in the stored data.

 

[cuky2000]

X/R ratio vary at each location in the network.
For example, near generating stations: 15-40, T. Line: 5-20. Installation near transformers and reactors have tendency to increase.

Please notice the following:
1) HV circuit breaker interrupting capacity are tested at X/R=17. Above that derating factor should be applied.
2) For disconnect switch, X/R has minimun or none impact on the switch rating.
3) Sequence X/R also change with the location in the network

 

[bdn2004]

Is there a way to physically measure or meter over time the X/R ratio at certain points in the system? Or can that be done with a calculation of voltage, currents and power factor?

I have discovered how to find this value in the power analysis software, but the software is relying on the mathematical model we’ve created. As with anything - that’s just an approximation of reality. Did you model everything accurately? Are your motors oversized? Does the voltage vary? etc, etc With something that effects the numbers so drastically there would be something to assure us we’re using the right value.

 

[davidbeach]

Sure, you can measure it. All you need is a bolted fault. The model is much more convenient. If you’re working with an overhead utility system you can get enough faults to spot check the model. If you’re working with an industrial system that’s all insulated conductor the faults may be too few to do much verification. Load current alone isn’t going to let you know.

 

[waross]

A system is comprise of a number of resistances and reactances in series. Combined they determine the X/R ratio. A fault shorts out part of the system and the system X/R ratio is determined by the ratio of the Xs and Rs left in the circuit.

Bill
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"Why not the best?"
Jimmy Carter

 

[cuky2000]

Measure the phase angle between the current and voltage (θ) or the power factor = cos (θ). then X/R = tan (θ)

 

[waross]

That will give the combined X/R ratio of the system and the load. During a fault the load is typically shorted out of the circuit.
The X/R ratio of the part of the system still in the circuit up to the point of the fault is the issue under discussion.

Bill
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"Why not the best?"
Jimmy Carter

 

[HamburgerHelper]

Load that can impact your X/R ratio is contribution due to large motors. Motors contribute fault current during faults. I have seen oscillography showing this but it was never enough for anyone to care. I live in a very industrialized area so I would have thought this would come up if it mattered. I have never seen transient studies ,which would take into account the decaying motor contribution, ever carried out to set relaying inside of refineries. When do you care about motor contribution?

------------------------------------------------------------------------------------------
If you can't explain it to a six year old, you don't understand it yourself.

 

[davidbeach]

Motor contribution affects withstand but is so short lived that it doesn’t affect relay coordination.

 

[dpc]

Motor contribution affects withstand but is so short lived that it doesn’t affect relay coordination.

Generally true, but can be an issue for sensitive relays, especially instantaneous trips. I am aware of one case where directional OC relays in a refinery substation misoperated due to motor contributions to a utility side fault.

 

[waross]

Was that a synchronous motor dpc?
I'm asking here, not telling, but I was under the impression that the contribution from an induction motor was at a frequency equal to twice the slip frequency below the line frequency. That may be around 57 Hz for an induction motor initially and then the frequency dropping as the motor speed dropped.
This would produce a beat frequency of 6 Hz so that about every 6Th cycle the motor contribution would add to the grid contribution and about every midway point, or every 6th cycle offset by 3 cycles the motor contribution would subtract from the grid contribution.
I am not disputing that the every 6th cycle peak could trip a sensitive relay and I recognize the increase in the magnetic forces caused by the motor contribution.
Is this understanding fairly accurate?

Bill
--------------------
"Why not the best?"
Jimmy Carter