Instantaneous Elements; Instantaneous Element Error During L90 Synchronization - GE L90 Instruction Manual

10.8 INSTANTANEOUS ELEMENTS

10.8INSTANTANEOUS ELEMENTS
As explained in the Theory of Operation chapter, two or three L90 relays are synchronized to each other and to system fre-
quency to provide digital differential protection and accurate measurements for other protection and control functions.
When an L90 system is starting up, the relays adjust their frequency aggressively to bring all relays into synchronization
with the system quickly. The tracking frequency can differ from nominal (or system frequency) by a few Hertz, especially
during the first second of synchronization. The 87L function is blocked during synchronization; therefore, the difference
between system frequency and relay sampling frequency does not affect 87L function. However, instantaneous elements
have additional error caused by the sensitivity of Fourier phasor estimation to the difference between signal frequency and
tracking frequency.
To secure instantaneous element operation, it is recommended either to use FlexLogic™ as shown below to block the
instantaneous elements during synchronization, or to use a different setting group with more conservative pickup for this
brief interval.
Figure 10–5: FLEXLOGIC™ TO BLOCK INSTANTANEOUS ELEMENT DURING 87L STARTUP
The elements must be treated selectively. If, for example, the phase undervoltage setting includes margin sufficient to
accommodate the maximum additional error on startup, blocking or delay are not needed for phase undervoltage. Similarly,
if the phase instantaneous overcurrent setting has sufficient margin, blocking is not needed. Note that significant zero-
sequence and negative-sequence current or voltage error will not appear during L90 startup, therefore all elements using
these quantities are safe.
The table below indicates the maximum error and recommended block durations for different elements.
ELEMENT
Phase undervoltage
Phase instantaneous overcurrent
Ground distance zone 1
Phase distance zone 1
10
10-26

10.8.1 INSTANTANEOUS ELEMENT ERROR DURING L90 SYNCHRONIZATION

MAXIMUM ERROR ON STARTUP,
(OPERATE SIGNAL VS. SETTING)
18%
9%
7%
4%
L90 Line Current Differential System
10 APPLICATION OF SETTINGS
RECOMMENDED BLOCK
DURATION
0.7 seconds
0.5 seconds
Not needed
Not needed
GE Multilin