Line Differential Protection 1

[Qingwei]

Hi:
I am a freshman of line protection.Now I am researching in the line differential protection.In many papers they all involved the line charging current.Just because I don't have the experience of operation.Can you tell me what the effect of line charging current in current differential protection?And how the manufacturers deal with this problem?

With best regards.


Qingwei

 

[jbartos]

Suggestion: Visit

http://www.selinc.com/techpprs/6135.pdf

etc. for more info

 

[DiscoP]

If the relay you use has no line charging current compensation, then the protection will see this as differential current. You need to check that the line charging current will not operate the protection. If it does, then you need to adjust the pick-up setting.

ALSTOM's P543 and P544 relays, you can enter the line capacitance as a setting, then the relay will compensate for the line charging current.

 

[521AB]

Line charging current compensation is based, as far as I know, on voltage measurement.
You enter the relay the capacitive line data (XC1, XC0 i.e. positive sequence line reactance and zero sequence capacitive line reactance.
Out of XC1 and XC0 the relay calculates the phase-phase and the phase-earth capacitances.
Out of the measured voltage UL1, UL2 and UL3 (phase to earth, for each phase) the relay calculates the charging current in each phase (ic = C dV / dt !!!).
The relay then sends on the other side of the line the measured current minus the calculated charging current (or minus 50% of the calculated charging current, it depends on what the other relay at the other line-end does).
In practice the 2 relays work on the "net" line curent. This allows you to have more sensitive settings for line differential protection.
ABB REL 561 is a relay I know has this feature.
Remember you need voltage measurement!
In case of fuse failure (lost of VT signal), the relay might trip if the charging current generates a high differential current. The relay should be able to switch from "compensated setting" to "non compensated settings" if a proper signal is activated (by fuse failure detection for instance..)
I hope I didn't confuse you.

 

[Qingwei]

First I thank for everyone who replied my question.I know the shortcoming of the compensation based the voltage measurement.So I want to know whether any new method is found to threat this problem.


With best regard.