IEEE Electrification Magazine - March 2014 - 72

TABLE 2. The Protection Devices and Operation Rules at Each Protection Level.
Protection Level

Protection Devices and Operation Rules in Grid-Connected and Island Modes

Load-way
protection

DOCa digital relay with adaptive relay setting (responding to lower fault current in island
mode):
-Operates only in load-way faults (DOC and autoreclosing).

Loop
protection

DOCa digital relay with adaptive relay setting:
-Operates in loop faults [primary and backup permissive overreach transfer trip (POTT)
schemes]
-Backup protection for load-way protection.

Loop-feeder
protection

OCb relay:
-Operates to isolate the faulted loop only when the load-way and loop protections have
failed within the loop.

Microgrid-level
protection

OCb relay and PCC switch:
In grid-connected mode:
-Unintentional islanding operation due to external fault or disturbance
based on the signal from the MC
-OCb relay (backup protection for the entire microgrid)
-Intentional islanding operation based on the islanding command from the MC.
In island mode:
-Resynchronization initiated by a command from the MC.

aDOC: directional OC.

bOC: nondirectional OC.

d2) for facilitating the permissive overreaching transfer
trip (Pott) protection scheme.
in figure 5(a), when a fault occurs in a segment within
a loop, relays within the loop will sense the fault current.
the relays sensing the positive directional fault current
(d2, c1, and b1) will send permissive signals to their coupled relays (c1, d2, and c2), respectively. in this case, only
the coupled relays d2 and c1 will clear the fault because
each would sense a positive fault current and receive
permissive signals. the tripping criterion is because both coupled relays
detect positive directional fault current, which indicates that the fault
has occurred within the loop section
located between them. figure 5(a)
shows that once d2 and c1 cleared
the fault, the loop is converted into
two parallel feeders and the loop
structure is changed to a radial network without any load interruptions.
the backup protection operates if a
Pott scheme fails (e.g., breaker at c1
fails). in figure 5(b), once the breaker
failure is detected, c1 will send a
transfer trip signal to c2 to open the
breaker at c2. Here, an outage will
occur in buildings fed by Vista switch c while other load
points in the same loop will not be affected. directional

relays eliminate traditional coordination time intervals for
primary protections and facilitate an accurate fault-locating
scheme within a loop. the load-way and loop relays are
high-speed digital relays with an operating time of fewer
than 1 cycle (exclusive of the relay's time delay and the
breaker operating time). thus, very high-speed and accurate protections are implemented at the load-way and loop
protection levels of the iit microgrid.

Loop-Feeder Protection
table 2 shows that the loop-feeder protection is the upper level of the loop
protection, which acts as a backup protection for the entire loop. When a
fault in the load-way or loop results in
the failure of load-way and loop protection schemes (e.g., loss of communication between Pds or any Pd
failure), the loop on outage will be isolated by the backup protection provided by loop-feeder Pds. Loop-feeder
relays (purple blocks in figure 2) are
nondirectional oc relays, which are
set to be slower than load-way and
loop relays and faster than substation
relays. the proposed setting prevents
nuisance tripping and ensures that faulted loops are isolated before substation relays get to operate.

A key feature
of the microgrid
operation includes
its seamless
islanding from the
utility grid and ability
to be self-controlled
in island mode.

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

Table of Contents for the Digital Edition of IEEE Electrification Magazine - March 2014