2 PRODUCT DESCRIPTION
2 PRODUCT DESCRIPTION 2.1 INTRODUCTION
The L90 relay is a digital current differential relay system with an integral communications channel interface.
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, one zone of phase and ground distance protection with power swing
blocking, out-of-step tripping, line pickup, load encroachment, and POTT features is 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.
The relay uses a sampling rate of 64 samples per cycle to provide metering values and flexible oscillography.
Voltage and current metering is included as a standard feature. Additionally, currents are available as total RMS values.
Power, power factor and frequency measurements are also provided.
Diagnostic features include a sequence of records of 1024 time-tagged events. The internal clock used for time-tagging can
be synchronized with an IRIG-B signal. 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 portable computer (PC).
These tools will significantly reduce troubleshooting time and simplify report generation in the event of system faults.
A faceplate RS232 port may be used to connect a PC for programming settings and for monitoring actual values. A variety
of communications modules are available. Two rear RS485 ports are standard to allow independent access by operating
and engineering staff. All serial ports use the Modbus
RTU protocol. The RS485 ports may be connected to system com-
puters 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 environ-
ments. Another option provides two 10BaseF fiber optic ports for redundancy. The Ethernet port supports MMS/UCA2 pro-
The relay uses flash memory technology which allows field upgrading as new features are added.
The testing features can be used to verify and test settings and operations.
GE Power Management
L90 Line Differential Relay