ABB REL301 Instruction Manual page 12

The REL 302/302 software which does the sampling has 8 states; these states corre-
spond to the sampling rate (8 samples per cycle). Movement from state to state is con-
trolled by a timer. The timer is loaded with a state time at the beginning of the
state.The code executed within a state should be completed before the timer expires.
The software then waits for the timer to time out.
The fundamental frequency components are extracted from the samples (each cycle)
and converted to voltage and current phasor values using a Fourier notch-filter algo-
rithm. An additional dc-offset correction algorithm reduces overreach errors from de-
caying exponential transients. 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 compensation 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 in-
put samples are used to compute the current and voltage vectors and rms values. No
dc offset compensation is performed. High-set instantaneous overcurrent and Zone1
distance unit tests are executed. This will speed up tripping by as much as one cycle
for high current, close-in faults.
Instantaneous overcurrent, inverse time overcurrent protection, and out-of-step
blocking are also conducted during the fault mode and background mode.
For Zone2 and Zone3 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 background mode.
1.4.3 Unique Characteristics of REL 301/302
A unique characteristic of the REL 301/302 system is its phase selection principle. It
determines 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 infor-
mation, the fault type can be clearly identified and the actual distance to the fault can
be estimated.
High-resistance ground-fault detection is available in REL 301/302. Sensitive direc-
tional pilot tripping is activated through an FDOG Timer (FDGT). The pilot distance
unit is always active and can have the priority for tripping.
Load-loss tripping entails high-speed, essentially simultaneous clearing at both ter-
minals 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 Zone1 relays at one or both terminals. This causes direct tripping of the local
breaker without the need for any information from the remote terminal. The remote
terminal recognizes the loss of load-current in the unfaulted phase(s) as evidence of
tripping of the remote breaker. This, coupled with Zone2 distance or directional over-
(10/94)
I.L. 40-386.3
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