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Smart grid and power quality issues 197

power electronic interfaced devices. In some microgrid design philosophies, regard-
less of the type of DER, all devices are connected via power electronics for improved
control of fault current, droop response, etc. The Consortium for Electric Reliability
Technology Solutions (CERTS) microgrid, for example, is an “all power electronic
interface,” whose potential applications include industrial parks, commercial and in-
stitutional campuses, and other locations that require uninterrupted and high PQ elec-
tricity supply [12].
From a PQ perspective, microgrids are operated in two distinctive modes: grid-con-
nected and islanded. The adverse impacts on PQ are expected not only during islanded
operation, but also during the transitions between islanded and grid-connected opera-
tions. These transitions may make some DER units to switch from voltage-controlled
mode (during islanded operation) to current controlled-mode (during grid-connected
operation), causing voltage stability problems due to the delay in detection of nonin-
tentional islanding.
In islanded mode, higher dynamics and wider ranges of interactions between
microgrid loads and available small or medium-scale DERs will result in more pro-
nounced, more frequent and longer voltage and frequency variations, This will be
further augmented by the reduced short circuit power and inertia of microgrids.
The power electronics interfaces of DER connected to the microgrid include tradi-
tional grid-commutated topologies, which might cause undesirably high levels of har-
monics, when they are connected to the power grid. Self-commutated PWM (Pulse-
Width Modulation) controlled inverters typically produce supraharmonics (waveform
distortion in the range from 2to 150 kHz) [13].
For the static performance of microgrids, the low X/R ratio of distribution line
impedance might affect the load sharing accuracy of inverters, typically resulting in
unbalances. Furthermore, harmonics and unbalances are poorly compensated with
the connection of nonlinear and unbalanced loads. For the dynamic behavior of mi-
crogrids, the voltage and frequency dependencies of load responses have to be taken
into account when choosing droop characteristics for DER units; otherwise, control-
lers may fail to ensure a proper sharing and lead to instability [14].

2.2 Power quality concerns in DC microgrids

DC microgrids have been gaining interest in academia and industry in the past couple
of years. The major advantages compared to its counterpart AC microgrid are the
simple control, reliability, and efficiency. Recently, DC microgrids have been increas-
ingly explored with the goal of more efficient integration of DER devices, including
energy storage, and nonlinear loads through the elimination of some rectification, and
power inversion or conversion stages.
The PQ issues in DC microgrids are related to differences between the ideally con-
stant voltage in a DC system and the actual voltage provided by power electronic con-
verters. Four fundamental PQ concerns have been identified in DC microgrids [15]:
1. waveform distortion in DC systems is associated with the presence of current and volt-
age oscillations (steady-state nonDC components), which can be considered similar to AC