GE L30 Instruction Manual page 256

5.6 GROUPED ELEMENTS
c) STUB BUS
PATH: SETTINGS GROUPED ELEMENTS
STUB BUS
The stub bus element protects for faults between two breakers in a breaker-and-a-half or ring bus configuration when the
line disconnect switch is open. At the same time, if the line is still energized through the remote terminal(s), differential pro-
tection is still required (the line may still need to be energized because there is a tapped load on a two terminal line or
because the line is a three terminal line with two of the terminals still connected). Correct operation for this condition is
achieved by the local relay sending zero current values to the remote end(s) so that a local bus fault does not result in trip-
ping the line. At the local end, the differential element is disabled and stub bus protection is provided by a user-selected
overcurrent element. If there is a line fault, the remote end(s) will trip on differential but local differential function and DTT
signal (if enabled) to the local end, will be blocked by the stub bus logic allowing the local breakers to remain closed.
• STUB BUS FUNCTION: There are three requirements for stub bus operation: the element must be enabled, an indica-
5
tion that the line disconnect is open, and the
ods of setting the stub bus trigger and thus setting up stub bus operation:
1. If
STUB BUS TRIGGER
causing zero currents to be transmitted to remote end(s) and DTT receipt from remote end(s) to be permanently
blocked. An overcurrent element, blocked by disconnect switch closed, provides protection for the local bus.
2. An alternate method is to set
ment. The instantaneous overcurrent element must operate quickly enough to pick up the
operand, disable the local differential, and send zero currents to the other terminal(s). If the bus minimum fault cur-
rent is above five times the instantaneous overcurrent pickup, tests have confirmed that the
operand always pick up correctly for a stub bus fault and prevents tripping of the remote terminal. If minimum stub
bus fault current is below this value, then method 1 should be used. Note also that correct testing of stub bus oper-
ation, when this method is used, requires sudden injection of a fault currents above five times instantaneous over-
current pickup. The assigned current element should be mapped to appropriate output contact(s) to trip the stub
bus breakers. It should be blocked unless disconnect is open. To prevent 87L tripping from remote L30 relays still
protecting the line, the auxiliary contact of line disconnect switch (logic "1" when line switch is open) should be
assigned to block the local 87L function by using the
• STUB BUS DISCONNECT: Selects a FlexLogic™ operand to represent the open state of auxiliary contact of line dis-
connect switch (logic "1" when line disconnect switch is open). If necessary, simple logic representing not only line dis-
connect switch but also the closed state of the breakers can be created with FlexLogic™ and assigned to this setting.
• STUB BUS TRIGGER: Selects a FlexLogic™ operand that causes the
line disconnect is open. It can be set either to "On" or to an instantaneous overcurrent element (see above). If the
instantaneous overcurrent used for the stub bus protection is set with a time delay, then
the associated instantaneous overcurrent
cover the stub between CTs of the associated breakers and disconnect switch.
5-120
SETTING GROUP 1(6)
STUB BUS FUNCTION:
Disabled
STUB BUS DISCONNECT:
MESSAGE
Off
STUB BUS TRIGGER:
MESSAGE
Off
STUB BUS TARGET:
MESSAGE
Self-reset
STUB BUS EVENTS:
MESSAGE
Disabled
STUB BUS TRIGGER
is "On", the
STUB BUS OPERATE
STUB BUS TRIGGER
pickup
L30 Line Current Differential System
LINE DIFFERENTIAL ELEMENTS
Range: Disabled, Enabled
Range: FlexLogic™ operand
Range: FlexLogic™ operand
Range: Self-reset, Latched, Disabled
Range: Disabled, Enabled
setting is set as indicated below. There are two meth-
operand picks up as soon as the disconnect switch opens,
to be the pickup of an assigned instantaneous overcurrent ele-
CURRENT DIFF BLOCK
STUB BUS OPERATE
operand. The source assigned for the current of this element must
5 SETTINGS
STUB BUS
STUB BUS OPERATE
STUB BUS OPERATE
setting.
operand to pick up if the
should use
STUB BUS TRIGGER
GE Multilin