8 THEORY OF OPERATION
Initialization in our peer to peer architecture is done independently at each terminal. Relays can be turned on in any order
with the power system either energized or de-energized. Synchronization and protection functions are accomplished auto-
matically whenever enough information is available.
After a relay completes other initialization tasks such as resetting of buffer pointers and determining relay settings, initial
values are computed for any state variables in the loop filters or the protection functions. The relay starts its clock at the
nominal power system frequency. Phaselet information is computed and transmitted.
Outgoing messages over a given channel are treated in the same way as during the channel recovery process. The
special start-up message is sent each time containing only a single time step value.
When incoming messages begin arriving over a channel, that channel is placed in service and the loop filters are
started up for that channel.
Whenever the total clock uncertainty is less than a fixed threshold, the phase locking filter is declared locked and differ-
ential protection is enabled.
The average total channel delay in each direction is not critical, provided the total round trip delay is less than 4 power sys-
tem cycles. The jitter is important, and should be less than plus or minus 130 microseconds in each direction. The effect of
a difference in the average delay between one direction and the other depends on the number of terminals. In the case of a
2 or 3 terminal system, the difference is not critical, and can even vary with time. In the case of a 4 or more terminal system,
variation in the difference limits the sensitivity of the system.
The allowable margin of 130 microseconds jitter includes jitter in servicing the interrupt generated by an incoming mes-
sage. For both incoming and outgoing messages, the important parameter is the jitter between when the time stamp is
read and when the message begins to go out or to come in.
The quality of the crystal driving the clock and software sampling is not critical, because of the compensation provided
by the phase and frequency tracking algorithm, unless it is desired to perform under or over frequency protection.
From the point of view of current differential protection only, the important parameter is the rate of drift of crystal fre-
quency, which should be less than 100 parts per million per minute.
A 6 Mhz clock with a 16 bit hardware counter is adequate, provided the method is used for achieving the 32 bit resolu-
tion that is described in this document.
An 8 bit time stamp is adequate provided time stamp messages are exchanged once per cycle.
A 4 bit message sequence number is adequate.
Channel asymmetry (the difference in the transmitting and receiving paths channel delay) cannot be higher than 1 to 1.5
GE's adaptive elliptical restraint characteristic is a good approximation to the cumulative effects of various sources of error
in determining phasors. Sources of error include power system noise, transients, line charging current, current sensor gain,
phase and saturation error, clock error, and asynchronous sampling. Errors that can be controlled are driven to zero by the
system. For errors that cannot be controlled, the master computes the covariance matrix for each source of error for each
phase. A total covariance matrix is computed for each phase by adding the matrices from each source.
The system computes the covariance matrix for errors caused by power system noise, harmonics, and transients. These
errors arise because power system currents are not always exactly sinusoidal. The intensity of these errors varies with
time, growing during fault conditions, switching operations, or load variations, for example. The system treats these errors
as a Gaussian distribution in the real and in the imaginary part of each phasor, with a standard deviation that is estimated
from the sum of the squares of the differences between the data samples and the sine function that is used to fit them. This
error has a spectrum of frequencies. Current transformer saturation is included with noise and transient error.
GE Power Management
8.1.16 HARDWARE AND COMMUNICATION REQUIREMENTS
8.1.17 ON-LINE ESTIMATE OF MEASUREMENT ERRORS
L90 Line Differential Relay