Calculating line impedance for distance relaying

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rian0201

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Yes, it is because in feeds changes the impedance seen by the relay compared to what it is supposed to see (the settings you had provided).

If it is just a radial line then it is okay.

You also have to check for minimum faults, high impedance faults, overloading and out of step if any..


Sent from Mars
 
mbrooke

mbrooke

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rian0201 said:
Yes, it is because in feeds changes the impedance seen by the relay compared to what it is supposed to see (the settings you had provided).
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Even if the line is broken with breakers for the gen instead of hooking directly to the line? (assuming yes). How would I take such into account? What confuses me is even if I account for the gen, the impedance will be different when the unit is off vs being on.


If it is just a radial line then it is okay.
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Makes sense.


You also have to check for minimum faults,
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This would be the lowest short circuit current under a number of anticipated contingencies + generator outages?



high impedance faults, overloading and out of step if any..


Sent from Mars
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This is taken care of my load flow studies, and should the line overload, a inverse time current curve takes care of conductor damage (or NESC sag distance limits, which ever comes first).
 
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Bugman1400

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mbrooke said:
Id be interested. :cool: Great info btw. I found this on converting primary ohms into secondary ohms:

http://www.relaytech.com/files/34789987.htm


Infeeding... is it correct I do not have to worry about it unless I have a generator directly attached to the line?
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Any source will cause infeed.....it could be a ground source. For example, if you have a xfmr that is tapped off the line and it is g-wye on the highside; it will cause zero-seq current to flow from the middle of the line towards a ground fault.
 
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Bugman1400

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I've never seen line impedance measured for modeling. The model is what helps you determine what settings go in the relay. I don't think relays can measure the line impedance for you unless you use PMUs. Relays can measure voltage and current and determine the impedance it sees for a fault. It then uses that measured impedance and compares it to the your line impedance setting to determine the approximate distance to the fault.
 
mbrooke

mbrooke

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Bugman1400 said:
Any source will cause infeed.....it could be a ground source. For example, if you have a xfmr that is tapped off the line and it is g-wye on the highside; it will cause zero-seq current to flow from the middle of the line towards a ground fault.
Click to expand...

I see, and makes sense. But lets say I have a generator mid line. How would this change the way I program the relay.
 
mbrooke

mbrooke

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Bugman1400 said:
I've never seen line impedance measured for modeling. The model is what helps you determine what settings go in the relay. I don't think relays can measure the line impedance for you unless you use PMUs. Relays can measure voltage and current and determine the impedance it sees for a fault. It then uses that measured impedance and compares it to the your line impedance setting to determine the approximate distance to the fault.
Click to expand...


So in a nut shell, the programmed impedance is the reference value to which the measured impedance is compared?
 
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Bugman1400

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mbrooke said:
I see, and makes sense. But lets say I have a generator mid line. How would this change the way I program the relay.
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You typically program the relay (Zone 2) to account for the infeed on the mid-line generator. The generator could huge or small. Either way, it should be modeled. Then, you are able to plot faults and see just how much infeed there is relative to what your line relay "sees". Failing to account for the infeed could result in severely under protecting the entire line.
 
mbrooke

mbrooke

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Bugman1400 said:
Yep!
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Gets a lot more clearer as I understand the concept now, and to be honest its rather ingenious this came out some 80 years ago yet its the basis of all relaying around the world even in the most advanced systems.


A bit off topic. But in straight bus applications you are putting the VTs for step distance on the bus, right? Theres a debate going on about putting them on the line instead.
 
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Bugman1400

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mbrooke said:
Well the fear is if you loose a bus VT, you take the step distance protection out for all lines (24 relays).
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I have never seen a set of 3PH line VTs for a straight bus application. It would be much cheaper and more reliable to provide a backup up set. In the E-M days, a voltage balance relay would be used to switch between the two sets of bus VTs. Typically, LOP is alarmed on via SCADA. I would expect that whatever would cause a bus VT fuse to pop would cause ALL the line VTs to do the same.
 
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rian0201

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Bugman1400 said:
I've never seen line impedance measured for modeling. The model is what helps you determine what settings go in the relay. I don't think relays can measure the line impedance for you unless you use PMUs. Relays can measure voltage and current and determine the impedance it sees for a fault. It then uses that measured impedance and compares it to the your line impedance setting to determine the approximate distance to the fault.
Click to expand...

There is an instrument from omicron for line impedance measurement, in my place it is mandatory to measure rather than calculation..
 
mbrooke

mbrooke

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Bugman1400 said:
I have never seen a set of 3PH line VTs for a straight bus application. It would be much cheaper and more reliable to provide a backup up set. In the E-M days, a voltage balance relay would be used to switch between the two sets of bus VTs. Typically, LOP is alarmed on via SCADA. I would expect that whatever would cause a bus VT fuse to pop would cause ALL the line VTs to do the same.
Click to expand...


But if the relay is responding to loss of voltage it could be due to short circuit on the secondary wiring which would then clear the other when the faulted circuit is switched over? Just curious how they did it.


How much does a VT run for, $50,000?


LOP for feature readers:
http://www.pes-psrc.org/Reports/Loss_of_AC_Voltage_Considerations_for_Line_Protection.pdf
 
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