Stub Bus Application; Stub Bus Implementation; Figure 60: Stub Bus Arrangement - GE P642 Technical Manual

P64x
5.9

STUB BUS APPLICATION

When a winding isolator is open (e.g. for maintenance purposes), a section of energized line, called the stub, could
be left unprotected. In the diagram below, we see that the HV winding disconnector is open and there is a fault in
the stub zone, which needs to be protected against.
V00696

Figure 60: Stub Bus arrangement

We do not want the standard differential protection or low impedance REF protection to operate, as this may have
repercussions on the rest of the system. In this case, Stub Bus protection should be used, when available. Stub Bus
protection is achieved by using the isolator status , applying this to an opto-input and combining it with a phase
overcurrent element (linked to the relevant winding) to provide a trip signal for the relevant circuit breakers.
5.9.1

STUB BUS IMPLEMENTATION

When the protection is used in one and half breaker busbar topology and the disconnector of any of the three
windings of the transformer is open, the differential and low impedance REF elements related to this winding are
affected.
The differential and low impedance REF protection elements should not trip for a stub bus fault and should be
blocked on a per winding basis.
5.9.1.1 STUB BUS ACTIVATION
The Stub Bus active signals HV StubBus Act, LV StubBus Act and TV StubBus Act are asserted when the
appropriate conditions are met to activate Stub Bus for the relevant winding. A fast under current DDB signal,
available on a per winding basis along with the isolator status, should be used to assert the respective Stub Bus
active signal. The fast under current DDB will become asserted if the current measured falls below the I<Current
Set of the relevant CBF element. For the fast undercurrent DDBs, I<Current Set represents a percentage of the CT
secondary nominal current (In). CBF does not need to be activated for the fast under current DDB to assert.
5.9.1.2 STUB BUS DIFFERENTIAL BLOCKING
When a Stub Bus active signal is asserted for a winding, the current of the respective winding is not considered in
the calculation of the differential and bias currents. The differential trip is not blocked however, and should be
blocked on the winding with Stub Bus active only (it should remain unblocked on the other windings). For example,
if a fault occurs within the differential zone and the HV Stub Bus is active, the differential element should only trip
the breakers connected to the LV and TV sides of the transformer. In this case, blocking of the HV winding
differential trip in can be achieved via the PSL logic.
Low impedance REF protection is blocked internally on a per winding basis by the fixed scheme logic. For example,
if HV StubBus Act is activated, then HV low impedance REF is blocked.
In applications where the protection is per differential scheme, the Stub Bus protection cannot be used since the
differential protection trip will be blocked each time any Stub Bus is activated.
P64x-TM-EN-1.3
HV
Isolator
open signal
Chapter 6 - Transformer Differential Protection
TV
P645
LV
137