19-10-2012, 05:38 PM
Advances in Series-Compensated Line Protection
Advances in Series-Compensated.pdf (Size: 656.49 KB / Downloads: 59)
INTRODUCTION
Transmission line series compensation increases power
transfer capability and improves power system stability.
However, series compensation increases the fault current level
and may also cause generator subsynchronous resonance. The
capacitive reactance XC is typically from 25 percent to
75 percent of the line inductive reactance XL. The authors have
also seen lines compensated to 100 percent.
Series capacitors may be installed at one or both line ends.
Line ends are typical capacitor locations, because it is
generally possible to use space available in the substation. In
turn, this reduces installation cost. Another possibility is to
install the series capacitors at some central location on the
line. Series capacitors located at the line ends create more
complex protection problems than those installed at the center
of the line.
Series capacitors require spark gaps or metal oxide
varistors (MOVs) to reduce or eliminate overvoltages across
the capacitors. Spark gaps (see Fig. 1) flash over to remove
the capacitor when the voltage exceeds a given value, but they
may not fire for low-current faults. Thus, the line protection
scheme must also perform properly with the series capacitor
still in operation.
SERIES-COMPENSATED LINE PROTECTION CHALLENGES
The line reactance change and the subharmonic-frequency
oscillations caused by the series capacitors may affect line
protective relays. Series capacitors can also generate highfrequency
transients. The analog and digital filters in
microprocessor-based relays attenuate all high-frequency
components [1]. Therefore, high-frequency transients have
very little effect on most modern relays.
ANALYSIS OF FIELD CASES
This section analyzes actual faults on series-compensated
systems. Events that result in the conditions this paper
describes are difficult to come by, as the fault must occur in
particular locations on the line. However, these events can
demonstrate that the relay is operating securely and
dependably.
The first example is from a fault on a 500 kV seriescompensated
system. There are parallel 500 kV lines, both
compensated at 70 percent with capacitors located at both line
ends. As all of the lines are compensated at the line ends on
these systems, protecting the lines can be quite challenging.
Events are captured at both line terminals on each line.
Fig. 28 shows the response of the logic designed to prevent
overreach of the Zone 1 distance element (see Section III.C).