IEEE Electrification Magazine - March 2014 - 77
v_vista-C
4.0
0.0
-4.0
-4.0
v_vista-D
4.0
0.0
Fault Is
Cleared
by
Fault Is
POTT Applied
(D2-C1)
-4.0
(kA)
30
i_fault
30
0
(kA)
x
(kA)
(kA)
20
10
0
-10
-20
20
10
0
-10
-20
20
10
0
-10
-20
(kA)
(kA)
20
10
0
-10
-20
i_relay-C1
20
10
0
-10
-20
(kA)
(kA)
(kA)
20
10
0
-10
-20
i_fault
Fault Is Fed
Through
Vista C
-30
-30
20
10
0
-10
-20
Fault Is
Isolated
by Backup
(D2-C2)
-4.0
(kA)
Fault Is
Applied
v_vista-D
(kV)
(kV)
4.0
v_vista-C
(kV)
(kV)
4.0
20
10
0
-10
-20
i_relay-D2
i_grid
i_gen
2.90
3.00
3.10
3.20
x
3.30
i_relay-C1
D2 Tripped;
C1 Failed
i_relay-D2
i_grid
i_gen
2.90
3.00
3.10
3.20
3.30
(b)
(a)
Figure 13. The simulation results when P SG = 4 MW in case A2. (a) The fault is cleared by the POTT scheme. (b) The fault is isolated by the
backup loop PDs once the POTT scheme has failed.
loop 1 (see figure 2). figure 10 shows the instantaneous
simulation results for the faulted phase. Here, v_vista-C is
the voltage at Vista c, and i_fault, i_relayC1 and i_relay-C2 are the fault current
and relay c1 and relay c2 currents. in
figure 10(a), the fault is cleared by the
load-way Pd installed at Vista c. in this
figure, the fault current of the load-way
of Vista c is about 13 kA (rms), which
triggers the load-way relay to clear the
fault in less than 0.1 s. this time includes
the time-oc delay and operating times
of the relay and breaker. in figure 10(b), it
is assumed that the load-way Pd has
failed to clear the fault and the fault is
cleared by the backup protection provided by the two loop Pds at Vista c. in this
figure, the fault clearing time is about
0.16 s, which is the total operating time
of the primary and backup protections.
figure 10(b) shows the current for the
two loop Pds (c1 and c2). both scenarios
result in building outages at Vista c;
however, the duration of the fault is much shorter when
the fault is cleared by the load-way Pd.
figure 11 shows the results when
the same fault has occurred at t = 3 s
and the synchronous generator supplied 4 MW of the campus load
according to the tertiary control signal. the synchronous generator will
also contribute to the fault current.
Here, i_grid and i_gen are the currents of the utility grid and the synchronous generator. figure 11(a) and
(b) shows the scenarios in which the
faults are cleared by load-way Pd
and the backup loop Pds. in these
figures, the utility grid contributes
about 12 kA (rms), and the synchronous generator contributes 3 kA
(rms) to the total fault current.
Differential
protection schemes
are based on
coupled differential
directional relays
that can accurately
locate and isolate
faults without
affecting other
components in
distribution systems.
A2: Loop Fault
this case shows how the Pott
scheme and its backup protection
IEEE Electrific ation Magazine / MARCH 2 0 1 4
77
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