Understanding L60 Oscillography; Overview - GE L60 Instruction Manual

UNDERSTANDING L60 OSCILLOGRAPHY

8.5 Understanding L60 oscillography

8.5.1 Overview

The L60 oscillography feature is a powerful tool for tuning, commissioning, and troubleshooting. It also helps to understand
the theory of phase-comparison and how the L60 relay incorporates standard analog phase comparison principles. The
L60 oscillography allows observation of not only AC waveforms and 87PC operate signals, but all details of composite
signal forming, fault detector operation, input and output processing, squares forming, coincidence detection, and
integration of the signal. All currents are processed per CT breaker on breaker-and-a-half applications (applies to
composite signal, fault detectors, and so on).
The phase comparison operating current is either mixed from all three phase currents into one composite quantity using
the I_2 – K × I_1 formula, or it is just 3I_0. In contrast to phase current waveforms, where raw samples are captured and
displayed, the operating current is digitally filtered with the DC component and harmonics removed. During no-fault
8
conditions, the operating current is relatively small and dictated mostly by the load positive sequence current (FDL and
FDH detectors drop off). However, the
the negative half of the power cycle are present in oscillography once the operating current is greater than 0.02 pu. When
the operating current (or
87PC BRK1
(and
BRK1 FDL BRK2 FDL
transmitting squares on the positive (
preparing to process the phase comparison algorithms according to the selected schemes and setting values.
The next step is to adjust pulses according to channel asymmetry and channel delay. The received pulse is adjusted in
accordance to the
, and so on) are also delayed to properly align with a local pulse. The local aligned signals (
ALIGNED
) are derived from either one CT current or from two CTs current and are delayed as per channel delay setting.
ALIGNED
Even when FDL and FDH operate, the scheme does not produce until the
the FDH delayed by the channel delay until received signal arrives. The scheme is now ready to produce a trip.
8-14
Figure 8-10: Main L60 oscillography signals
local pulses at the positive half of the power cycle and the
POS
current for two-breaker applications) exceeds the FDL pickup setting, the
87PC BRK2
for two breaker applications) flags are asserted, indicating a fault condition and thus initiating
) and negative (
Tx POS
setting. If this setting is quite high, then the adjusted signals (
CHANNEL ASYMMETRY
CHAPTER 8: APPLICATION OF SETTINGS
) halves of the sinewave. At this moment, the logic is
Tx NEG
flag is asserted, which represents
FDH ALIGNED
L60 LINE PHASE COMPARISON SYSTEM – INSTRUCTION MANUAL
local pulses at
NEG
,
RX1P ALIGNED RX1N
and
POS ALIGNED NEG