2 PRODUCT DESCRIPTION
2 PRODUCT DESCRIPTION 2.1INTRODUCTION
The L90 Line Current Differential System is a digital current differential relay system with an integral communications chan-
The L90 is intended to provide complete protection for transmission lines of any voltage level. Both three phase and single
phase tripping schemes are available. Models of the L90 are available for application on both two and three terminal lines.
The L90 uses per phase differential at 64 kbps transmitting two phaselets per cycle. The current differential scheme is
based on innovative patented techniques developed by GE. The L90 algorithms are based on the Fourier transform–
phaselet approach and an adaptive statistical restraint. The restraint is similar to a traditional percentage differential
scheme, but is adaptive based on relay measurements. When used with a 64 kbps channel, the innovative phaselets
approach yields an operating time of 1.0 to 1.5 cycles (typical). The adaptive statistical restraint approach provides both
more sensitive and more accurate fault sensing. This allows the L90 to detect relatively higher impedance single line to
ground faults that existing systems may not. The basic current differential element operates on current input only. Long
lines with significant capacitance can benefit from charging current compensation if terminal voltage measurements are
applied to the relay. The voltage input is also used for some protection and monitoring features such as directional ele-
ments, fault locator, metering, and distance backup.
The L90 is designed to operate over different communications links with various degrees of noise encountered in power
systems and communications environments. Since correct operation of the relay is completely dependent on data received
from the remote end, special attention must be paid to information validation. The L90 incorporates a high degree of secu-
rity by using a 32-bit CRC (cyclic redundancy code) inter-relay communications packet.
In addition to current differential protection, the relay provides multiple backup protection for phase and ground faults. For
overcurrent protection, the time overcurrent curves may be selected from a selection of standard curve shapes or a custom
FlexCurve™ for optimum co-ordination. Additionally, three zones of phase and ground distance protection with power
swing blocking, out-of-step tripping, line pickup, load encroachment, and permissive overreaching transfer trip (POTT) fea-
tures are included.
The L90 incorporates charging current compensation for applications on very long transmission lines without loss of sensi-
tivity. The line capacitive current is removed from the terminal phasors.
For breaker-and-a-half or ring applications, the L90 design provides secure operation during external faults with possible
Voltage, current, and power metering is built into the relay as a standard feature. Current parameters are available as total
waveform RMS magnitude, or as fundamental frequency only RMS magnitude and angle (phasor).
Table 2–1: DEVICE NUMBERS AND FUNCTIONS
Wattmetric Zero-Sequence Directional
Adaptive Fault Detector
(sensitive current disturbance detector)
Ground Instantaneous Overcurrent
Neutral Instantaneous Overcurrent
Phase Instantaneous Overcurrent
Negative Sequence Instantaneous Overcurrent
Ground Time Overcurrent
L90 Line Current Differential System
Neutral Time Overcurrent
Phase Time Overcurrent
Negative Sequence Time Overcurrent
AC Circuit Breaker
Neutral Directional Overcurrent
Phase Directional Overcurrent
Negative Sequence Directional Overcurrent
Power Swing Blocking
Segregated Line Current Differential