I.L. 40-386.4
1-5
of the checks are broken into small parcels, so that the whole complement of tasks is performed
over a one-cycle period (eight passes through the loop). Some checks are performed more than
once per cycle (e.g. critical timers).
The REL301/302 sampling software has 8 states; these states correspond to the sampling rate
(8 samples per cycle). Movement from state to state is controlled by a timer. The timer is loaded
with a state time at the beginning of the state.The code executed within a state must be com-
pleted before the timer expires. The software then waits for the timer to time out. If the timer
expires before the code has completed execution, a time out error results, blocking relay trip-
ping.
The fundamental frequency components are extracted from the samples (each cycle) and con-
verted to voltage and current phasor values using a Fourier notch-filter algorithm. During the
process, the sum of squares of the inputs are accumulated to provide rms values of current and
voltage. The Fourier coefficients and sums are calculated for computing the phase angles. The
sum of squares and the sums of the Fourier coefficients are updated for each sample, using the
information from the previous seven samples, to provide a full cycle of data.
1.4.2
Fault Mode and Restricted Fault Tests
Upon entry into the fault mode, the sums of the Fourier coefficients and sum of squares from
the background mode are stored. New sums are obtained, using fault data, to which offset com-
pensation has been applied.
To speed up tripping for severe faults, restricted fault testing is implemented. The last half cycle
of background mode input samples and the first half cycle of fault mode input samples are used
to compute the current and voltage vectors and rms values. No dc offset compensation is per-
formed. High-set instantaneous overcurrent and Zone-1 distance unit tests are executed. Re-
stricted fault testing can speed up tripping by as much as one cycle for high current, close-in
faults, up to approximately 50% of the setting reach.
Instantaneous overcurrent, inverse time overcurrent protection, and out-of-step blocking are
also conducted during the fault mode and background mode.
For Zone-2 and Zone-3 faults, impedance computation and checking will continue throughout
the specified time delay. The impedance calculation will be performed once every cycle, in the
fault mode and then continued in the background mode.
1.4.3
Unique Characteristics of REL301/302
A unique characteristic of the REL301/302 system is its phase selection principle. It deter-
mines the sum of positive and negative sequence currents for each phase by a novel method
which excludes the influence of pre-fault load current. From this information, the fault type can
be clearly identified and the actual distance to the fault can be estimated using a calculation
based on the selected fault type.
High-resistance ground-fault detection is available in REL301/302. Sensitive directional pilot
tripping is activated through an FDOG Timer. The pilot ground distance unit is always active
and can have the priority for tripping dependent on the FDOG Timer setting.
Load-loss tripping entails high-speed, essentially simultaneous clearing at both terminals of
a transmission line for all fault types, except three-phase, without the need of a pilot channel.
Any fault location on the protected circuit will be within the reach of the Zone-1 relays at one or
both terminals. This causes direct tripping of the local breaker without the need for any informa-
