directional protection

[mustafaABB]

Hi every body
how can set the angle for the directional over current protection , I mean some times we seee angle 90 or 60 or 45 or 30 what is the criteria for selecting the above angles for the directional over cuurnt and even for directional earth fault.
i heard from friend he told me that the power transformer when being solidly earthed the angle should be 60 .
can you please explain .


Thanks for all

 

[gvasiliou]

Hi,
Have a look in this pdf, page 12 of 30, chapter 9.14.2
I think you will find the answer there ;-)

http://www.fecime.org/referencias/npag/chap9-122-151.pdf

 

[mustafaABB]

thank you sir
i have this file but it is still difficult to understand why in some networks we use 90 degree or use 60 or use 45 degree and other .

 

[DTR2011]

If you search a bit, it also has to do with the voltage level. Higher voltages tend to have angles closer to 90 deg. It has to due with the construction of the transmission line, or if it is a cable it is difference as well.

I don't have the reference handy to quote - sorry.

 

[mustafaABB]

thank you for your help i will read the manual and keep in touch in order to discuss more .

Thank you

 

[spectrohelioscope]

MustafaABB,

Just to let you know that in the past, a lot of old Electromechanical relays out there were not set using the calculated line impedance angle, and this was done in purpose.

Some of the reasons that the protection engineers did this was to add more resistive coverage on the relay protected zone just using a lower angle but still calculating and maintaining the same protected reach. This is related to the calculated/estimated dynamic caracteristics of the relays during a fault conditions, etc...

A good practice that I have seen for the distance elements is that you will use the calculated reach at the line impedance angle and divide this with the Cosine of the differente between the line angle and the desire lower angle (60,45,30) that is been used to add more resistive coverage when tilting the protective impedance zone to a lower angle.

This will give you a higher number that represents the same reactive reach that you calculate with the line impedance angle, but now calculated for 60,45 or 30 degrees tilt.

I hope this helps,

Thanks.

LAP

 

[DiscoP]

This is all true for a distance relay.

The npag link is to some useful information about directional oc angles. Also worth noting, that the angle depends on which voltage is connected, and it's polarity relative to the ct. The npag will talk in terms of Vbc being used to directionalise Ia. Sometimes a different connection is used, which greatly affects the MTA required.

 

[mustafaABB]

thanks for all
really it was very nice and valuable information for the directional protection.