POWER December 2015 - 54

SOLAR POWER
proportionately, and at an equal rate as the
system ramps down, and that has a dampening
effect on voltage fluctuation. "
Smart Inverters
Colavito said it's becoming common for large
commercial solar PV systems to operate at
fixed PFs down to 0.95, sometimes as low as
0.9, and Standard Solar has begun taking this
into account with its system design (Figure
2). Nearly all smart inverters on the market
now are capable of this kind of operation, and
string inverters used on larger systems up to 5
MW and 10 MW are adding the capability.
" The manufacturers have done a really
good job of integrating a lot of really advanced
smart integration features into their
inverters, which allows them to be more flexible,
and it's built into the inverters whether
you require it or not, " he said. " We're now at
a stage where it's a function of the utilities
getting the ability to access and work with
these smart inverters. "
Right now, these systems are capable of
a lot more functions than utilities are calling
up. Colavito likened the situation to that of a
teenager being given a Lamborghini for his
first car and leaving it in second gear.
But making non-unity work has some
tradeoffs. By definition, this sort of operation
has more apparent power than real power.
That means the system has to be designed
around that parameter.
" If your inverter power is listed in kVA
[reactive power], and you're operating at
non-unity so that kW is not equal to kVA, "
Colavito said, " then my actual kW is going
to be less. So if my inverter set is good for 2
MVA, and I'm running at 0.95, I'm not going
to get 2 MW of power output. "
Because of that, solar PV system engineers
have to know ahead of time what utilities
expect from them.
" If we understand what they want to do in
terms of curtailment, power factor, or ramprate
control at the beginning, it's not really
a problem for us, " he said. " It is a problem
when it's late in the game, we have a plan
in place, and now you want to introduce all
this, because it will have a financial impact
on the project performance. If we plan for it
up front, we can build in extra inverter capacity
for a pretty small amount. "
That means project planners and utilities
have to start working together very early on
in the process, he said, both for proper engineering
and to educate the utility about what
the system is capable of.
As PV panel prices continue to fall, however,
Colavito sees another method of PV
grid integration on the horizon, one that turns
some traditional power generation conventions
on their head: Substantially oversizing
54
2. Lack of unity. Like Standard Solar's 2.5-MW solar photovoltaic plant in Cecil County, Md.
(now owned by TerraForm Power), more large commercial plants are being designed to operate at
non-unity power factors in order to dampen voltage fluctuations. Courtesy: Standard Solar
the system relative to its nameplate capacity.
" I think rather than using energy storage
for grid integration, we're going to be using
more really high DC-to-AC ratios, really large
amounts of PV input to smaller AC capacities,
so that fluctuation is clipped off by the inverter,
resulting in more of a trapezoidal output. "
While that means some generation is lost,
it results in a much flatter, higher-quality
output from the grid standpoint and much
less fluctuation, he said. The lost generation
in general is often less valuable than the
flat generation below the cutoff point. Current
systems are designed at about 1.3 to 1.5
DC-to-AC ratios, but Colavito expects that to
rise to 1.8 or even 2.0 before much longer.
The newest inverters are being designed to
handle these higher ratios, he said. As panel
costs fall, they become a smaller and smaller
component of the overall project costs, making
this additional capacity less of a financial
issue, especially since most of the soft costs
are not affected by higher ratios.
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Storage Has a Role
Still, energy storage seems destined to play a
role in solar PV load management, but it may
not be behind the meter. Daniel Girard, EPC
director for S&C Electric Co., said his company
is seeing more and more utilities looking
at community energy storage-placing
storage batteries in neighborhoods to help
manage output from distributed PV systems.
Community storage, he explained, can
help with volt/VAR control or essentially
serve as an uninterruptible power supply
(Figure 3). For commercial systems, though
he agreed smart inverters are an important
development, Girard said they've seen clients
combine oversizing PV plants with
storage to be able to send power out over a
longer period.
" We have a client right now, who is being
restricted to under 3 MW that they can
produce, so they're going to put in energy
storage with their PV so that they can sell
power at night and in the morning. They want
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POWER December 2015

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