WORLD-GENERATION MAY/JUNE 2016
6
The construction of small solar farms is
running ahead of grid integration rules in
many areas, and that can be a problem for
utilities. In states where there is nothing
like California’s trendsetting Rule 21 in
place to oversee the process – and FERC
rules do not apply – utilities are finding the
best way to manage this solar flood is to
“smarten” their transmission and distribu-
tion grids.
By embracing the smart grid, utilities
are not only mitigating the technical diffi-
culties associated with integrating solar
farms, they are also putting themselves in a
position to benefit from the enhanced sta-
bility and reliability renewable generation
can provide. On a broader level, investment
in a smarter grid can directly benefit rate-
payers – through the use of the most eco-
nomic technologies, designs, and operating
practices – while also helping states meet
their renewable portfolio standard goals.
YES, SOLAR IS DIFFERENT
Until very recently, when distributed
generation was added to distribution sys-
tems, it was fossil-fueled, synchronous, and
exhibited familiar electrical characteristics.
Not so with today’s solar inverter-based
generation.
When presented with applications for
the integration of small solar farms, utility
engineers are finding they must deal with
an entirely new set of issues, not the least
of which is whether they want to control or
just monitor these solar facilities.
It’s important to mention that utilities
often lack control over when and where
solar farms are sited. As such, distributed
generators are often clustered, resulting in
a higher-than-average penetration on indi-
vidual distribution feeders. This means that
the connection of solar farms to the distri-
bution grid can have a massive impact on
existing equipment, especially distribution
transformers – an impact that can be
obscured in the rush to go green.
MINIMIZINGTHE IMPACT
The integration process generally
begins when the solar developer submits an
interconnection study describing the proj-
ect in detail. This report will include gener-
ation and control equipment as well as
interconnection points, whether such equip-
ment is “certified,” and whether the genera-
tor will connect using the utility’s equip-
ment (cables, transformers, switches, etc.)
– even if that equipment is behind the
meter. If the developer intends to use utility
equipment, an “added-facilities” contract
may be required, and that could give the
utility additional control over the project. At
that point, the utility begins its own due dili-
gence. This includes analyzing the site and
modeling various generating scenarios to
determine the impacts on their grid.
Once the interconnection study has
been analyzed and modeling completed,
utility engineers can begin to answer initial
questions. As noted earlier, one of the first
things utilities need to consider when inte-
grating small solar farms is not just how
they are going to connect to them but also
how they are going to isolate them when
necessary. Many times power from renew-
ables will go to one or more transformers
before it’s distributed. It’s far better to
aggregate the power and then bring it onto
the network through a single transformer,
so it can be controlled and isolated more
effectively. If you don’t do it this way, you’ll
have a lot of issues with power factor and
power quality.
Further, if the utility can affect the
choice of inverters used on small solar
farms, it’s better to use string inverters and
aggregate power at one node. String invert-
er technology has improved dramatically in
the last four or five years.
Next, it is critical for utility engineers to
determine – among other things – whether
their distribution transformers, feeders, and
other equipment have sufficient capacity to
accommodate the additional generation
from these small solar farms.
Make no mistake, integration issues
increase with the size of the solar farm. If
we’re dealing with a 1 MW farm, the whole
network is affected, and utilities should be
concerned with system-wide protection
schemes, coordination, SCADA, etc. But
smaller farms, those adding a few kW of
power to the grid, will generally not cause
significant disruptions.
The ideal approach is to modularize the
solar farm, building it in increments, and
adding string inverters as you go along to
get the voltage you want.
But no matter how the solar farm is
developed, one of the biggest challenges is
the high learning curve utilities face when
they take over. How is it going to respond
to loads? How will the utility handle inter-
mittency? How much to reduce power out-
put and for how long? When to take or
dump power? All these things need to be
discussed and planned.
ENSURING INTEROPERABILITY
Interoperability is the ability of grid
components to communicate to one another
through common protocols and standards-
based application program interfaces (API).
When it comes to integrating DER, new
systems and components must be interop-
erable – not only with each other but also
with legacy systems and components.
Ideally, utilities should be able to integrate
DER – including solar, wind, and energy
storage – in varying sizes, in numerous
PERSPECTIVE
INTEGRATING SMALL SOLAR FARMS
BY GARY RACKLIFFE,ABB
Vice President
Smart Grids
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