IEEE Electrification Magazine - June 2016 - 38

reactive power-flow control and balance. Frequency
and phase regulation is more complex in small ac
microgrids than in the large traditional ac grid.
xx
In microgrids connected to the ac grid, fast disconnection is a requirement to maintain the microgrid and
all the loads operational during and after a grid fault.
This results in the need for a costly, complex, and
expensive solid-state switch between the ac grid and
a microgrid like the one in Figure 2. In developed
countries with reliable ac grids, these devices are still
required, although they are used very few times during the lifetime of the equipment.
xx
The island detection in ac microgrids can be more
complex as multiple elements have to be connected
to a bus that can be an island or not an island at different times. Active anti-islanding detection methods
may have to be centralized or shared among several
devices using complex algorithms.

Figure 1. Vanadium redox flow batteries are a perfect fit for
microgrid applications.

Benefits of dc Microgrids

Figure 2. The 13.8-kV static transfer switch used in the Santa Rita
Jail ac microgrid.

Energy Flow

Motor
Figure 3. An ac-powered motor drive with a rectifier front end.

several technical challenges that are difficult and/or
costly to resolve.
xx
The large inertia of the machines used in large-scale
ac power generation provides stability to the ac grid.
AC microgrids are mainly based on distributed generation connected through inverters. The small inertia
of the system results in additional complications and
control challenges to keep the system stable.
xx
In addition to voltage regulation, an ac system requires
frequency and phase angle control to achieve real and

38

I E E E E l e c t r i f i c ati o n M agaz ine / j un e 2016

It is hard to argue against the fact that local distribution
systems would greatly benefit from using dc rather than
ac as an interconnection bus. Some of the facts validating
this statement follow.
xx
Most renewable sources produce dc power, and most
modern loads-commercialized as ac loads such as
LED lights, consumer electronics, and motor drives-
are provided with a front-end rectifier while the load
itself is served from an internal dc bus and can
therefore be connected more efficiently to a dc bus
by eliminating the front end and its losses. For
example, large rectifier/inverter motor drives commonly used in intensive manufacturing plants use
topologies like the one in Figure 3. These drives
could save up to 50% of their losses if powered
directly from a dc bus.
xx
Many classic ac generation resources using rotating
electromechanical machines can operate more efficiently if they are connected though variable-frequency
power converters instead of being forced to operate
connected to a fixed-frequency ac bus.
xx
Electric energy-storage devices, fundamental components for the operation of microgrids, are also dc in
nature. Among the different battery storage technologies, flow batteries have characteristics that are very
attractive to microgrids, such as deep discharge capability without affecting cycle life, fast response, decoupled power and energy ratings, and no cell-to-cell
equalization needs. However, these batteries have
limitations to supplying high voltages without
increasing the cost of the system, and therefore would
need a dc-dc in front of the dc-ac converter in ac
applications. These battery technologies that are
already provided with an internal dc-dc converter
would be easily integrated to a fixed dc bus with
reduced cost and increased efficiency.



Table of Contents for the Digital Edition of IEEE Electrification Magazine - June 2016

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