RFL 9300 RFL Electronics Inc.
August 25, 2000 3 - 18 (973) 334-3100
3.13 STUB-BUS PROTECTION
On double-breaker terminals (ring-bus or breaker-and-a-half), the stub-bus must be protected when the breakers
are closed but the line disconnect is open. This condition may occur during line maintenance. There are several
variations to this application as shown in Figure 3-17.
1. Remote terminal is single-breaker, no tapped load (2-terminal system).
2. Remote terminal is single-breaker, with tapped load (2-terminal system).
3. Remote terminal is double-breaker, with or without a tapped load (2-terminal system).
4. Remote terminals are single-breaker, no tapped load (3-terminal system).
5. Remote terminals are single-breaker, with tapped load (3-terminal system).
6. Remote terminals are double-breaker, or a combination of double breaker and single breaker, with or
without a tapped load (3-terminal system).
If a stub-bus fault occurs, the two breakers at the local station will always have to be cleared. For the condition
shown in Figure 3-17a, it doesn't matter if the remote breaker trips for a stub-bus fault at the local station. (Al-
though this would result in increased breaker maintenance). However, the remote breaker(s) cannot be allowed
to trip for the conditions shown in Figures 3-17b through 3-17f. For the conditions shown in Figures 3-17c and 3-
17f, the ring (or 1 1/2 breaker bay) at the remote station(s) must be kept intact. This is true whether there is a
tapped or untapped load at the remote station(s), and whether the disconnect at the remote station is open or
closed.
To cover all the conditions shown in Figure 3-17, the breakers at the local station must
be cleared, and the
breakers at the remote station(s) must not
be cleared. The RFL 9300 meets these requirements as follows:
1. When the disconnect switch is opened at the local station, the relay immediately enters stub-bus active
mode.
2. While in this mode all control messages received from the remote station(s) are processed by the super-
visor but are not passed to the phase controllers.
3. Permissive trip is disabled at the local station and the phase controllers are forced to enter backup
mode. While in this mode only the instantaneous overcurrent backup and inverse time-overcurrent
backup functions are available for local stub-bus protection.
4. Since the transmission line to the local station has been opened, a steady stream of WCM messages is
transmitted to the remote station(s). Transmission of all remaining control messages are blocked.
5. The remote station(s) continue to operate in normal fashion. They are not even aware that the local sta-
tion is in stub-bus mode. The steady stream of WCM messages that they are receiving enables them to
continue to provide differential protection.
Note that when the 3IO controller operates in 2-terminal mode, it requires only a single active fault detector on its
own card to trip if the remote station breaker is open. In the 2-terminal systems depicted in Figure 3-17, this will
apply at the remote stations when the local station is in stub-bus mode.
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