IEEE Electrification Magazine - March 2014 - 97

to represent the convergence of values a microgrid can offer.
although simplistic, Figure 1
shows how a microgrid provides
value to a customer. We start with a
customer who has a peak average
load as well as a mission-critical load.
The mission-critical load is essential
and needs the highest level of reliability. Since there is a difference
between the customer's peak load
and their mission-critical load, we
can assume that this grid meets our
first criteria of a microgrid by having
multiple interconnected loads.

efficiency Through demand
Reductions
The bottom left corner of Figure 1
shows a customer with no DG. This
customer relies completely on the utility to support his or her load. The load
supported by the utility is shown in
gray. This is the vast majority of electricity users today. as we move to the
right in Figure 1, we can see the first
benefit of a microgrid: to improve efficiency. efficiency uses customers' own
sources of generation to reduce their
overall load and, in turn, reduce their
electric bills. as you can see, the DG
cannot meet the customer's missioncritical load, let alone his or her peak
load. But it does help save real dollars
by reducing his or her electric bills,
which is shown in the green shaded
areas. The fact that the customer has a
source of DG also meets our second
criteria for qualification as a microgrid.

Mission-Critical Reliability
as the customer connects additional
DG resources, we see their demand
on the utility continue to reduce.
More importantly, at value point 2 in
the figure, the customer's DG is now
capable of supporting his or her
most critical loads, which we will call
the mission-critical load. The second
major value of the microgrid is the
reliability improvements that are
achieved when customers generate
enough power locally to support part,
or all, of their load.

This ability is crucial in the fourth
criteria of a microgrid, islanding. In the
event that utility power is lost, a
microgrid must be able to support at
least its critical loads with its available
DG. at value point 2 in Figure 1, we can
see that if the utility source (the gray
part) goes away, the customer has
enough on-site DG to keep the missioncritical loads running. This improves
the overall reliability of his or her power
system if utility power is lost and also
allows the customer to reduce his or
her demand when connected to the
grid even if utility power is available.
This brings up an important concept in microgrids, which is time. There
is a reason we do not show time in this
figure. as we have visited and built
microgrids around the world, one of the
often missed concepts is the difference
between power and energy. Microgrids
are often analyzed in terms of power,
looking at the loads and available DG in
terms of kilowatts or megawatts. Most
people see it as a logic equation:

quality issues that could affect the
load. Would you consider this a
microgrid? Would a building with rooftop solar panels alone be considered a
microgrid? Most microgrid customers
would say no, as these systems would
not support the reliability of the loads
for any usable period of time.
every microgrid customer we have
met wants to improve both the efficiency and the reliability of his or her
power system. They want the DG they
invest in to reduce their electricity
demand and support at least a portion
of their load for an effective period of
time. Depending on the customer, the
typical expectation starts around 6 h
and can extend to days, weeks, or
even months. Therefore, we often use
the equation below, which focuses on
energy, not power, as the determining
factor for a microgrid.

IF (average Power Demand ÷ DG
Power) > 0
Then Microgrid.

With this equation, as the number
gets closer to zero, the overall reliability of the microgrid improves because
the DG resources can support the customers' peak load for longer periods
of time. The farther the result is above
one, the less likely the microgrid can
support the customer's average load
for any significant period of time.
Table 1 illustrates some examples.
any value shaded red would be able
to keep the customer operating for fewer
than 1 h and probably only be useful for
temporary outages or power quality
issues. When the total DG energy equals
that total energy of the consumer, it
means the customer is supported for
1 h, which is colored yellow because this
is typically not long enough to satisfy
any reliability improvement needs.
Where we start to get a microgrid that
can last for a reasonable amount of
time is in the green areas.

This equation is great when you
want to talk about the efficiency
improvements a microgrid can provide. as the result of the equation gets
closer to one, the customers become
less reliant on the utility, in turn
reducing their demand costs. Between
one and zero, customers can generate
more power than they use, a concept
we will cover a little later.
The challenge is that this equation
focuses only on the efficiency of the
system and does not factor in how
long the DG used in the equation will
last, which is called its energy. You can
have entire buildings with megawatts
of load supported for a few seconds by
an uninterruptable power system.
These systems use dozens, hundreds,
and sometimes rooms full of batteries
to support critical loads for short periods of time in the event that utility
power is lost or there are power
	

IF 0 < (average customer MWh ÷
DG MWh) < 1
Then Microgrid That Will last.

Total Reliability
as we move to the right in Figure 1, we
start to reach a tipping point. Between
IEEE Electrific ation Magazine / MARCH 2 0 1 4

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