IEEE Electrification Magazine - March 2014 - 69

synchronous generators (PMsgs), and fly wheels are suitable choices for increasing fault currents in microgrids.

Differential Protection Schemes
differential protection schemes are based on coupled differential directional relays that can accurately locate and
isolate faults without affecting other components in distribution systems. differential protection schemes, which
incorporate traditional protection technologies, are suited
for microgrid protections in both grid-connected and island
modes. differential protection schemes are either centralized (monitored and coordinated by central controllers) or
localized (based on local communications among relays). in
centralized schemes, central controllers monitor the
microgrid network topology and operation settings of Pds
and send tripping commands to Pds once faults are detected. centralized schemes provide more accurate results with
rather unacceptable time delays for performing the computations required by the central controllers. Localized
schemes, which are more readily adopted for industry
applications, allow for direct communication among relays
with the fastest response to faults.

Adaptive Protection Schemes
the difference between fault currents in grid-connected
and island modes would necessitate adaptive protection

schemes in microgrids. Adaptive schemes would include
two sets of relay settings, one for grid-connected and the
other for island mode. relays would select proper settings
when microgrids switch their operation modes. in island
mode, the time-oc (toc) characteristic curve of relays will
be shifted to instantaneous and/or definite-time oc settings to adapt to lower fault currents. Moreover, adaptive
protection schemes would automatically adjust the relay
settings according to the network operating state. for
example, a voltage restraint oc protection scheme would
reduce the time dials (tripping delays) of relays in cases of
large voltage depressions.

Protection Schemes Based
on Other Parameter Measurements
Voltage- or harmonic-based detection techniques are considered as alternatives once low fault currents render traditional oc protection schemes impractical in island
mode. the voltage-based protection schemes would locate
faults by detecting the "d-q" components of voltage disturbances based on the Park transformation. However, the
voltage-based detection schemes may not provide accurate fault detections, and undesirable time delays would
be introduced by additional computation and filtering.
thus, this protection scheme is usually used in combination with other fault-detection and protection schemes.

Utility Grid
12.47 kV
PCC

North
Substation

South
Substation
4.16 kV

4.16 kV

Battery
attery
LS

Vista-C
2
1
3

Wind
Wi d Stuart
St t

1
2

Loop 2

Loop 3

Loop 7

Gas-Turbine
Gas-Turbin
Synchronous
Synchronou
Generator

CTA1

Vista-B

Loop 1

Eng. 1

VanderCook

Feeder Switch-A

3
CTA2

Vista-A

2 1

Vista-D

B A

Vista-E

Feeder
eeder Switch-B

Load-Way and Load-Way PD
Loop and Loop PD

Load-Feeder and Load-Feeder PD
Substation PD

Substation Bus

Figure 2. The IIT microgrid protection system.

IEEE Electrific ation Magazine / MARCH 2 0 1 4

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