2 PRODUCT DESCRIPTION
2 PRODUCT DESCRIPTION 2.1INTRODUCTION
The L90 Line Differential Relay is a digital current differential relay system with an integral communications channel inter-
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 2 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 exist-
ing 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 elements, 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 POTT features 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).
Diagnostic features include a sequence of records capable of storing 1024 time-tagged events. The internal clock used for
time-tagging can be synchronized with an IRIG-B signal or via the SNTP protocol over the Ethernet port. This precise time
stamping allows the sequence of events to be determined throughout the system. Events can also be programmed (via
FlexLogic™ equations) to trigger oscillography data capture which may be set to record the measured parameters before
and after the event for viewing on a personal computer (PC). These tools significantly reduce troubleshooting time and sim-
plify report generation in the event of a system fault.
A faceplate RS232 port may be used to connect to a PC for the programming of settings and the monitoring of actual val-
ues. A variety of communications modules are available. Two rear RS485 ports allow independent access by operating and
engineering staff. All serial ports use the Modbus
RTU protocol. The RS485 ports may be connected to system computers
with baud rates up to 115.2 kbps. The RS232 port has a fixed baud rate of 19.2 kbps. Optional communications modules
include a 10BaseF Ethernet interface which can be used to provide fast, reliable communications in noisy environments.
Another option provides two 10BaseF fiber optic ports for redundancy. The Ethernet port supports IEC 61850, Modbus
TCP, and TFTP protocols, and allows access to the relay via any standard web browser (L90 web pages). The IEC 60870-
5-104 protocol is supported on the Ethernet port. DNP 3.0 and IEC 60870-5-104 cannot be enabled at the same time.
The L90 IEDs use flash memory technology which allows field upgrading as new features are added. The following Single
Line Diagram illustrates the relay functionality using ANSI (American National Standards Institute) device numbers.
L90 Line Differential Relay