IEEE Electrification Magazine - March 2014 - 74

Vista-E
1

rip
rT

fe

ns
Tra

Loop PD

3 Load-Way PD

DER
Switch

PV

Loop 1

2

Eng. 1

DER Switches

Figure 7. The DER switches control.

Grid-Connected Mode
100

100

Island Mode
Substation

Substation
10
Time (s)

Time (s)

10
Feeder

1

0.1
Load
0.01
0.1

Feeder

1

0.1

Loop

Loop
Load

0.01
0.1

1
10
Current (kA)
(a)

1
10
Current (kA)
(b)

Figure 8. The TOC characteristics of relays in (a) grid-connected and
(b) island mode.

3

2
1

Vista-E

F2 F1

I 2a
Fault

I 2b
Fault

2
I
Vista-D
2 1
3

Vista-C
2 1
3

1b

Fault

I 1a

Fault
Vista-B
2 1
3
1

Vista-A

B A

1
2

3

Load-Way PD
Loop PD
Loop-Feeder PD (OC Relay)
Figure 9. The equidistance and weak-infeed faults in loop 1.
xx
Nondirectional OC relay: if the primary protection fails

to clear a microgrid fault or the Pcc switch does not
respond to the Mc's command to isolate a utility fault,
the Pcc switch will ultimately open in response to
the substation oc relay. this relay should have a sufficient time-delay for the operation of all protection
schemes in the load-way, loop, and loop-feeder levels.
xx
Intentional islanding and resynchronization initiated

by the MC: this operation is controlled by the Mc for
intentional islanding of the microgrid and does not
apply to any faults.

74

I E E E E l e c t r i f i c ati o n M agaz ine / MARCH 2014

in the iit, two synchronous generators are directly connected to the ss. in island mode, a synchronous generator
should remain connected, at least for a short period, to
provide sufficient fault currents for Pd operations at other
protection levels; hence, the oc relay of synchronous
generator has a longer time delay than that of the substation protection.

figure 7 shows that renewable energy resources (PV and
wind) are connected via a controlled der switch to the
load-way Pd of Vista switches. in this figure, once the relay
e3 detects the microgrid isolation, it will send a transfer
trip signal to the der switch to open. this function
prevents an uncontrollable under-/overvoltage or frequency according to ieee standard 1547, which requires der
units to be disconnected when the microgrid is deenergized. unlike renewable energy resources, the battery
storage supplies the campus load when there is a loop
fault. if a campus building or loop connected to the
battery storage is isolated, the Mc will send signals to discharge the battery storage in the building or loop. the
duration of battery storage discharge is dependent on the
available energy at the isolation instant.

Protection Coordination in the IIT Microgrid
Coordination Based on Operation Curves of Relays
figure 8 shows the toc curves of relays in grid-connected
and island modes. in this figure, load and loop curves
denote the toc curves for load-way and loop relays, which
clear the load-way and loop faults, respectively. the feeder
and substation curves denote the toc curve for the nondirectional oc relays at the loop-feeders and substation.
based on the adaptive protection scheme discussed in the
"Hierarchical Protection scheme for the Microgrid" section,
once the microgrid changes its operation to island mode,
the Mc would send a signal to the load-way and loop relays
to switch to the instantaneous/definite-time oc scheme as
shown in figure 8(b). this enables the load-way and loop
relays to respond to lower fault currents in the island mode.

Coordination Considering Special Cases in Loops
since the fault on a closed-loop iit microgrid system will
always be fed through two paths from substations, the
relay coordination for a looped distribution network differs from that of a radial network. to evaluate the protection coordination in a loop, two extreme cases are
investigated in loop 1, as shown in figure 9.
1) Equidistance loop fault: the equidistance loop faults
occur half-way between the loop-feeder breakers (e.g.,
at point 1 in figure 9). in this case, as the fault current
a
is split equally between the two paths in the loop (I 1fault
1b
I
),
and fault the loop and loop-feeder relays will detect
approximately 50% of the total fault current and the
substation relays will detect the entire fault current.



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https://www.nxtbook.com/nxtbooks/pes/electrification_december2021
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