IEEE Std C37.243 pdf free download

IEEE Std C37.243 pdf free download.Application of Digital Line Current Differential Relays Using Digital Communication.
A disadvantage of the current differential scheme is the cost associated with the requirement for communications between the relays on all terminals of the protected line. The communications medium is typically dedicated fiber or T1(E1 )/SONET(SDH) digital communications networks links; however, other options ranging from twisted wire pairs to Ethernet networks are applied or available. The requirement for communications between the relays at all terminals of the protected line gives rise to issues in dealing with how to handle loss of communications during service. Also, since the scheme is sensitive to differences in current entering and leaving the protected line, any tapped load must be measured; otherwise it must be compensated for when setting the sensitivity of the fault detection elements. Another disadvantage of the current differential scheme is that if the protected line is sectionalized (i.e., open switch on the line creating two line sections), the scheme must be disabled or must be able to recognize that a stub bus condition exists in order to prevent tripping of the non-faulted line section.
Aside from the general comparisons to other schemes above, the subclauscs below detail some specific advantages and disadvantages of the current differential scheme versus other common forms of line protection.
4.2.1 Line current differential compared to step distance protection
Without pilot communications, simple distance relaying cannot provide instantaneous clearing of all line faults without risk of misoperation due to overreach. Settings for step distance protection must make an allowance for errors in line impedance calculations, system modeling assumptions, and instrument transformer tolerances. The instantaneous tripping elements must be set to operate for a portion of the line that is less than 100%. Time delayed tripping is provided for the remainder of the line sections. In comparison, line current differential protection provides instantaneous tripping for 100% of the line length and should not operate for out of zone faults.
Step distance protection is also susceptible to unwanted operation during high load conditions and must be studied for the security of the scheme during power swings. High load flows through the protected line arc not a concern when current differential schemes are applied as the current summations for through current result in zero net difference into and out of the line.
For lines with long tapped load elements, the reach of step distance zones on the line may be affected by the need to protect the tapped portion of the line. This is due to the effects of infeed for faults on the tapped portions of the line. Also, on lines with three or more terminals, infeed could make coordination of step distance very dift’icult depending on the strength of the various sources. As long as reliable communications exists between all line terminals, lines protected with current differential schemes will still have high speed, selective clearing for 100% of the line regardless of line lengths, source strengths, or number of terminals.
Line current differential protection requires only line current values, thus PTs are not required for the scheme (although potentials may be used for line charging compensation). Loss of potential in a step distance scheme must be dealt with by defaulting protective elements into certain states making them susceptible to miscoordination, or by disabling elements reducing the amount of protection in effect on the line.IEEE Std C37.243 pdf download.