IEEE Electrification Magazine - March 2014 - 71

operation flowing through the cables in each loop is fewer
than 0.5 kA. thus, the fault current magnitude in island
mode (case 1 in table 1) is adequate to trigger oc Pds.
Moreover, loops 1-3 are directly connected to ns while
loops 4-7 are connected to the ss, so the utility grid contributes more to fault currents through ns for faults located in loops 1-3 than those located in
loops 4-7 as shown in table 1. similarly, the utility grid would provide higher
currents through the ss to faults
located in loops 4-7 than those located in loops 1-3.

Localized Differential Protection
Scheme Implemented by
Communication-Assisted
Directional OC Relays

in figure 3, the microgrid protection is divided into four
levels: the load-way level, loop level, loop-feeder level, and
microgrid level. the protection devices and operation rules
in each level are summarized in table 2. the detailed
protection strategy of each level will be discussed next in
this section.

Microgrids are
introduced to
address the issues
with the economics
and the resilience
of power delivery
systems.

the differential protection scheme can
be centralized or localized. the iit
microgrid uses the localized differential
protection scheme as the centralized
control suffers from time delays. the
Vista switches installed within the loops are equipped with
communication-assisted directional oc relays. the two
relays at each end of a loop section are equipped with optical
fiber communication, which facilitates fast (2 ms) and highly
reliable communication capability between them.

Adaptive Relay Settings for Grid-Connected
and Island Modes
As the fault current level shown in table 1 reduced in
island mode, the communication-assisted oc relays in the
iit microgrid can change the settings upon receiving the
islanding signal from the master controller (Mc). the
relays switch to instantaneous/definite-time oc protection scheme, considering the fault current levels in island
mode, to ensure a fast response to lower fault currents.

Loop Structure to Facilitate Hierarchical
Protection and Increase Service Reliability

Protection Strategy

Load-Way Protection

the green blocks in figure 2 denote
load-way directional oc digital relays,
which clear load-way faults. figure 4
shows a section of loop 1 with Vista
switches b, c, and d. in this figure, the
black dashed arrow indicates the positive flow direction on each loop Pd.
the red, bold arrow denotes the fault
current direction. in this figure, the
load-way protection and its backup
protection are shown. in figure 4(a),
once a fault at the load-way of Vista
switch c is cleared by the load-way Pd
(c3), the campus building connected to the Vista switch c
will be isolated. if the breaker at c3 fails, relay c3 will
immediately send transfer trip signals to c1 and c2, which
are mounted on the same Vista switch. figure 4(b) shows
the pair of loop Pds in Vista c (c1 and c2), which provide
backup protections for c3. should this backup protection
operate, the buildings connected to Vista c will be isolated.
the load-way Pds can be programmed up to four shots of
automatic reclosing. the load-shedding and other control
schemes could also be implemented on the load-way protection level based on under/overvoltage and under/overfrequency functions of these relays.

Loop Protection
the communication-assisted digital directional relays
(blue blocks) in figure 2 are implemented on the loop-way
of Vista switches to provide differential protections at loop
levels. figure 5 shows the same section shown in figure 4
to present the loop protection schemes when faults occur
in a loop section between Vista switches c and d. in this
figure, there is a communication channel between coupled relays on both sides of each cable segment (c1 and

the loop structure provides an uninterrupted electricity supply when one section of the loop is isolated because of a
fault. the iit microgrid features a hierarchical protection
structure to enhance the protection reliability. the loop Pds
in figure 2 not only clear the faults within
the loop without any interruption to loads
but also provide backup protection to the
TABLE 1. The Fault Currents (rms) at the IIT Microgrid.
load-way Pds in case a load-way fault
Fault
Case 1 :
Case 2 :
occurs. if the load-way and loop-level proLocation
Islanded
Mode
Grid-Connected
Mode
tection fails to clear the faults within a loop,
PCC at NS
PCC at SS
the entire loop is isolated quickly by respeca
b
a
b
tive loop-feeder relays; thus, an uninterLoops 1-3
12 kA ~ 15 kA
8 kAa ~ 9 kAb
3 kA ~ 4 kA
rupted and reliable supply to other healthy
Loops 4-7
12 kAa~ 15 kAb
8 kAa~ 9 kAb
loops is ensured. the hierarchical protecaFault current is lower when the fault is close to the middle point of the loop.
tion strategy at the iit microgrid is presentbFault current is higher when the fault is close to the loop feeder. (All currents are calculated for single-phase to ground fault.)
ed in the "Protection strategy" section.

IEEE Electrific ation Magazine / MARCH 2 0 1 4

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