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The RFL 9300 Digital Current Differential Relay is a fully-
integrated relay/communications terminal, suitable for high-
speed protective relaying of overhead or underground AC
transmission lines. Lines of all lengths and all voltage levels
(with or without series/shunt compensation) are protected.
The RFL 9300 Digital Current Differential Relay is applied to
two-terminal or three-terminal lines for three-pole or single-pole
tripping.
The RFL 9300 does not require any AC line potential, and is
unaffected by possible problems associated with the AC
source, such as loss of potential due to a blown fuse, or low-
potential signals (with a poor S/N ratio).
A typical three-terminal, single channel RFL 9300 system is
shown in Figure 2. The major inputs and outputs are shown
in this diagram. A block diagram showing all inputs and out-
puts for a typical RFL 9300 relay is shown in Figure 3.
Digital Current Differential Relay
RFL 9300
The RFL 9300 measures the instantaneous value of each
phase current and residual (“ground”) current two thousand
times per second. The area under the half-cycle current wave
is determined by adding these sampled values. The dimen-
sion of this area is the “charge”, in ampere-seconds or cou-
lombs. The local and remote charges (for the same half-
cycle and for the same phase) are compared, taking poarity
into account. If the net charge differs from zero by more than
a small threshold level called the “bias”, and internal fault is
declared and breaker trip signals are issued. This technique
has been patented.
This “Charge Comparison” tripping is called “Strong Feed Trip”
or SFT. Two other tripping logic circuits operated in parallel
with SFT. These are called “Ultra-High Speed Trip” (UHS)
and “Weak Feed Trip” (WFT). UHS and WFT will be de-
scribed later in this bulletin under “AUXILIARY FUNCTIONS.”
RFL Electronics Inc. June 10, 2002
RFL 9300DCDR
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