Alstom MiCOM P846, Technical Manual

Page: 110 / 298

P846/EN AP/D Application Notes
(AP) 6-4
MiCOM P846
2. APPLICATION OF INDIVIDUAL PROTECTION FUNCTIONS
The following sections detail the individual protection functions in addition to where and how
they may be applied.
2.1 Open line detector function
Traditional methods for detecting an open line condition are the following:
1. Trip signals coming from line protection relays:
It consists in monitoring all protection signals likely to order the opening of the line. It
allows anticipating the line opening. For the other events such as manual commands,
one of the two other methods is possible.
2. Auxiliary contacts associated with breakers and disconnecting switches:
For tie lines that are subject to very low or reversing power flows, the detecting
method can be based on direct monitoring of breakers and disconnecting switches
positions as provided by their auxiliary contacts. This technique is however expensive
in terms of installation and maintenance. Moreover, auxiliary contacts are not totally
reliable especially those associated with disconnecting switches.
3. Low current measurements:
For transmission lines with significant and unidirectional power flows, line-opening
events can be detected by low current measurement. To be efficient, it requires high
sensitivity current measurements. However, even in this case, it can be applied only
for local end detection since, for an opening event at the remote end, the line charging
currents are misleading the low current detectors. Moreover, this method can be
misleading during low or reverse flow conditions.
These three methods are usable for local detection only.
The techniques implemented in the P846 relay are an improvement to these traditional
methods since they enable line opening events to be detected at both local and remote ends
using only the line voltages and currents. As discussed in Operation Section paragraph 1.3,
its multi-criteria fuzzy-logic algorithm detects open line events under steady-state and
transient conditions.
AP
2.2 Application
The DLO can be applied to transmission lines from 120 kV to 1100kV and is suitable in
applications where shunt and/or series compensation are applied to the line. It is a device
made for detecting the status (open or closed) of a radial (no mid-tap) transmission line.
Table 1 lists the characteristics of transmission lines from 120kV to 1100kV. For each level is
given the SIL Base Power, the line charging in MVar per km, in p.u. per km and in km per
p.u.. The Base Power rating of the line refers to the surge impedance loading of the line
which is the power delivered by the line to a purely resistive load equal to the line's surge
impedance. It is computed by the following expression:
2
l l
C
kV
SIL
Z
−
=
(MW)
The results showed in Table 1 are interesting in that the line charging is nearly a constant
value in the SIL based per-unit system, i.e. we have about 780 km per p.u. of line charging
whatever the voltage rating. This means that we don’t need to adapt the relay settings to a
specific voltage level, since the measurements are self-adaptive to the transmission line
characteristics through the proper selection of the base values.