relay differential protection - finding slope? Knee? 2

[meeko]

Hi, I am trying to test a relay (GE multilin 745) for diferential protection. I am able to confirm it picks up on differential with a setting a of .3 X CT(5) - and picks up at 1.55 for both W1 and W2 abc.
My problem is trying to find the slope of the relay.

I then apply 5A to W1 and 2A to W2. the relay has picked up or "tripped" here as the differential current is exceeded that of the relay. When I increase current to W2 to 2.22A the relay resets, and due to the differentia current is not great enough for a trip operation. When i increase current again to W2 to 7.5A the relay will pick up or trip again, now the differential current has exceeded the settings of the relay.

From here what do i do to find the slope of the relay, and the knee point?

i know the formula for slope is---> slope= diff/restraint.


But i am losttt, what do i do from here??? and no idea how to find the knee point

 

[DTR2011]

from the manual slope=(Id/Ir)× 100%.

From me:
algebra (slope definition)....y=mx+b, with "m" being the slope. A slope is defined from two points on a line (X1, Y1)-(X2,Y2). Test at two points to obtain the first slope. Do the calculations to confirm. Repeat for slope #2.

I would suggest the following as testing on slope #2 involves higher currents and I don't know what you are using for test equipment. Be sure to limit the times of your injection so as to not smoke the relay.

to find the kneepoint (or point where slope #1 changes to slope #2), confirm you have slope #1 correct first (in the linear region). Do the same for slope #2. Once you feel comfortable with these values. From here, test from slope #2, going down (right to left on the slope line) do the tests at smaller and smaller increments (check the relay specs). You will then find a point where there is the change between the slopes. This is the kneepoint value. You will want to make a spreadsheet to keep track of and calculate the test values and results.

Things could get murky for you if you are trying to test a delta wye (star) transformer and are limited to three test currents. You have to account for the phase angle shift, and phase/line current calculations. Again a spreadsheet helps, or your test set software may do the calculations for you.

If you are trying to get a "feel" for how to test this, you may consider changing the configuration settings to something simple, like making the transformer a wye-wye, with a simple ratio of 10:1 (138kV/13.8 kV) with similar CT ratios (400:1/4000:1), or something like that. This allows for testing with two currents. I would consider this a "practice run" only and once you feel comfortable with the concept, change your settings back to what is intended for the real application and go from there. Some will argue that testing this way is "good enough", including many manufacturers. If your intention is to actually learn something (and actually test the settings) I suggest you complete the testing with the real settings that are issued for the relay.

Hope this helps.

http://www.gedigitalenergy.com/products/manuals/745/745man-ab.pdf

pg 258 gives the GE procedure.

 

[meeko]

thanks for the quick reply! getting a hang of this now