8 THEORY OF OPERATION
8 THEORY OF OPERATION 8.1 OVERVIEW
All differential techniques rely on the fact that under normal conditions, the sum of the currents entering each phase of a
transmission line from all connected terminals is equal to the charging current for that phase. Beyond the fundamental dif-
ferential principle, the three most important technical considerations are; data consolidation, restraint characteristic, and
sampling synchronization. The L90 uses new and unique concepts in these areas.
Data consolidation refers to the extraction of appropriate parameters to be transmitted from raw samples of transmission
line phase currents. By employing data consolidation, a balance is achieved between transient response and bandwidth
requirements. Consolidation is possible along two dimensions: time and phases. Time consolidation consists of combining
a time sequence of samples to reduce the required bandwidth. Phase consolidation consists of combining information from
three phases and neutral. Although phase consolidation is possible, it is generally not employed in digital schemes,
because it is desired to detect which phase is faulted. The L90 relay transmits data for all three phases.
Time consolidation reduces communications bandwidth requirements. Time consolidation also improves security by elimi-
nating the possibility of falsely interpreting a single corrupted data sample as a fault.
The L90 relay system uses a new consolidation technique called "phaselets". Phaselets are partial sums of the terms
involved in a complete phasor computation. The use of phaselets in the L90 design improves the transient response perfor-
mance without increasing the bandwidth requirements.
Phaselets themselves are not the same as phasors, but they can be combined into phasors over any time window that is
aligned with an integral number of phaselets (see PHASELET COMPUTATION section for details). The number of phase-
lets that must be transmitted per cycle per phase is the number of samples per cycle divided by the number of samples per
phaselet. The L90 design uses 64 samples per cycle and 32 samples per phaselet, leading to a phaselet communication
bandwidth requirement of 2 phaselets per cycle. Two phaselets per cycle fits comfortably within a communications band-
width of 64 Kbaud, and can be used to detect faults within a half cycle plus channel delay.
The second major technical consideration is the restraint characteristic, which is the decision boundary between situations
that are declared to be a fault and those that are not. The L90 uses an innovative new adaptive decision process based on
an on-line computation of the sources of measurement error. In this adaptive approach, the restraint region is an ellipse
with variable major axis, minor axis, and orientation. Parameters of the ellipse vary with time to make best use of the accu-
racy of current measurements.
The third major element of L90 design is sampling synchronization. In order for a differential scheme to work, the data
being compared must be taken at the same time. This creates a challenge when data is taken at remote locations.
The GE approach to clock synchronization relies upon distributed synchronization. Distributed synchronization is accom-
plished by synchronizing the clocks to each other rather than to a master clock. Clocks are phase synchronized to each
other and frequency synchronized to the power system frequency. Each relay compares the phase of its clock to the phase
of the other clocks and compares the frequency of its clock to the power system frequency and makes appropriate adjust-
ments. As long as there are enough channels operating to provide protection, the clocks will be synchronized.
The L90 system uses a peer to peer architecture in which the relays at every terminal are identical. Each relay computes
differential current and clocks are synchronized to each other in a distributed fashion. The peer to peer architecture is
based on two main concepts that reduce the dependence of the system on the communication channels: replication of pro-
tection and distributed synchronization.
Replication of protection means that each relay is designed to be able to provide protection for the entire system, and does
so whenever it has enough information. Thus a relay provides protection whenever it is able to communicate directly with
all other relays. For a multi-terminal system, the degree of replication is determined by the extent of communication inter-
connection. If there is a channel between every pair of relays, every relay provides protection. If channels are not provided
between every pair of relays, only those relays that are connected to all other relays provide protection.
Each L90 relay measures three phase currents 64 times per cycle. Ground current is derived from phase currents at each
relay. Synchronization in sampling is maintained throughout the system via the distributed synchronization technique.
GE Power Management
L90 Line Differential Relay