RFL 9300 RFL Electronics Inc.
October 29, 2001 2 - 13 (973) 334-3100
In 3-terminal systems the analog output of each RFL 93B ACT I/O is passed to an RFL 93B PC Phase Control-
ler Module. The RFL 93B PC uses an equivalent 10-bit serial A/D converted to convert the analog signal to a
digital value. The converted value is compared with a sample at least 1-cycle old and the difference or “control”
CT current value is recorded. The integral of all control CT current values recorded during a half-cycle is com-
puted when the half-cycle-ending zero-crossing time-tag is recorded.
Note - In a 2-terminal system the control CT current value is a true CT current value while in 3-terminal systems
the control CT current value is a transient value.
In both 2- and 3- terminal systems the integral is converted to an equivalent ampere rms value, and stored in an
on-board buffer along with the half-cycle’s start and finish time-tags. A bit indicating polarity is also stored. Inte-
gral values representing the positive half-cycles of the phase A, B, and C currents and the negative half-cycles
of the 3I0 current, are truncated to six bits. The truncated value is transmitted to the remote station as a CCD
message. Transmitting the currents to the remote station in this manner greatly reduces the communications
channel throughput requirements without compromising system protection. If all recorded samples or both the
positive and negative half-cycle integral values were transmitted, the throughput requirements would be much
greater. This communication technique has been proven through extensive field and laboratory tests.
2.7.1 WEAK CURRENT MESSAGE - WCM
For three terminal systems the integral sum of the control CT current samples is continuously calculated over
the latest 8-millisecond interval. If this sum is <0.5A rms for a minimum of 16ms WCM transmission to the re-
mote station is enabled. In this case the WCM conveys no information on breaker status. This message is es-
sential for 3-terminal systems since a control message must be received from both remote stations to enable
permissive trip. This is a change from the way CCS operates in two terminal mode, as there is no control mes-
sage transmitted if the control CT current is <0.5A rms, unless the breaker is open.
If the relay is configured for 2-terminal operation the WCM represents an open breaker at the sending station.
However if the relay is configured for 3-terminal operation a phase controller could be sensing a constant strong
true CT current and still be sending a WCM since the transient value is <0.5A rms.
It is important to recognize this difference in order to understand CCS 3-terminal operation.
The RFL 9300 will not store a CCD message in the local on-board buffer or transmit it to the remote station if
either of the following occur:
1. The CT current recorded during a given half-cycle interval is less than 0.5 A
rms
.
2. The duration of the half-cycle is less than 6 ms.
The RFL 9300 executes the following trip algorithms during each 0.5-ms system clock interval.
1. Strong-feed trip (SFT).
2. Weak-feed trip (WFT).
3. Switch-into-fault (SIF) with communications.
4. Ultra high-speed trip (UHST).
5. Open Conductor.
The SFT, WFT, SIF, and UHST algorithms depend on a comparison of local and remote current waveforms. For
these algorithms, a knowledge of event timing is critical. When each current value arrives from the remote sta-
tion, the RFL 9300 time-tags it. Each algorithm uses different formulas to adjust the received time-tag.
In 2-terminal systems the adjusted received time-tags are used to select or “nest” with the locally stored half-
cycle integral that should be compared with the received CCD or WCM control signal value when executing the
SFT algorithm.
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