Instantaneous Elements; Instantaneous Element Error During L30 Synchronization - GE L30 Instruction Manual

9.4 INSTANTANEOUS ELEMENTS

9 APPLICATION OF SETTINGS
9.4INSTANTANEOUS ELEMENTS

9.4.1 INSTANTANEOUS ELEMENT ERROR DURING L30 SYNCHRONIZATION

Two or three L30 relays are synchronized to each other and to system frequency to provide line current differential protec-
tion and accurate measurements for other protection and control elements. When an L30 system starts up, the relays
adjust their tracking frequency aggressively to bring all relays into synchronization within the system quickly. The tracking
frequency can differ from the nominal (or system frequency) by a few hertz, especially during the first second of synchroni-
zation, with one relay having tracking frequency above system frequency and another below system frequency. The same
re-synchronization process happens if a channel fails for a long time, leading to phase and frequency lock loop (PFLL) fail-
ure and then the channel is restored and relays are re-synchronized to each other again.
The 87L element is blocked during synchronization; therefore, the difference between system frequency and relay sam-
pling frequency does not affect 87L element itself. However, instantaneous protection elements can experience additional
error caused by the difference between signal frequency and tracking frequency.
The following curves demonstrate the maximum error with tracking frequency both above and below system frequency. At
time t=0 s, the channel is restored and in approximately 100 ms required to check channel reliability and to receive valid
timestamps, while the re-synchronization process starts. As can be observed, instantaneous current, voltage, and imped-
ance elements experience accuracy error, especially during first 100 to 330 ms during the re-synchronization process after
either system start up or recovering from the communications failure. Because relay tracking frequency can be above or
below system frequency randomly, consider the worst error depending on the element type. For example, for overvoltage or
overcurrent, the worst case happens when error causes the measured quantity to appear above system value (positive
error), which occurs when relay tracking frequency is below system frequency. For undervoltage or impedance, the worst
case happens when error causes the measured quantity to appear below system value (negative error), which occurs when
relay tracking frequency is above system frequency.
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9-10 L30 Line Current Differential System GE Multilin