POWER December 2012 - 49

RENEWABLES
these ramps can challenge system operators,
particularly if " down " ramps occur as
demand increases, and vice versa. These
challenges can be intensified if the rest
of the bulk power system lacks sufficient
generating resources that can be dispatched
quickly, whether that takes the form of a
pumped hydro facility or a fast-start gasfired
machine.
Aidan Tuohy, senior project engineer
with EPRI, told POWER that multiple issues
must be addressed to successfully integrate
large amounts of distributed energy
resources.
First is the need for system flexibility
as intermittent resources such as wind
and solar ramp up and down. Flexibility
to accommodate these fluctuations can be
accomplished by dispatching generation
resources such as combined cycle and combustion
turbine units, pumped hydro storage
resources, and even battery storage.
Second, forecasting tools are necessary
to help system operators anticipate the
amount of intermittent resource that may
be available. Although accurate day-ahead
forecasts can be difficult for solar and wind
resources, even an hours-ahead forecast can
help operators more efficiently utilize the
generation system. For example, the Electric
Reliability Council of Texas (ERCOT),
among other balancing authorities, has integrated
forecasting methods into its market
operations, and accurate forecasting is proving
important for Xcel Energy's Colorado
operations, where more than 50% of overall
generation has been provided by wind a
number of times.
Third, grid stability issues have grown
in importance over the past 20 years as
diminishing amounts of inertia from conventional
power plants have impeded
somewhat the grid's ability to offer frequency
response. Tuohy speculated that the
grid's frequency response ability has been
declining and could be due in part to market
design issues in which generators are
sometimes disincentivized from providing
frequency response services. The problem
may be exacerbated as inverter-based energy
from solar and wind resources displaces
system inertia provided by conventional
power plants. That's because while large
conventional generators spin at 60 Hz,
inverter-based power sources don't. While
wind can provide what Tuohy called " emulated
inertia, " the result is a decline in the
system's naturally occurring inertia, which
may be a factor in grid stability.
Fourth, because most distributed PV
systems are connected to the grid using
standards spelled out under IEEE 1547
guidelines to prevent islanding, it can be
circle 23 on reader service card
December 2012 | POWER
www.powermag.com
47
difficult to control system ride-throughs
when voltages drop. Tuohy said efforts are
under way to reconcile IEEE standard with
bulk system needs.
Fifth, wind resources in particular can
increase conventional generator cycling,
which can have long-term effects on operations
and maintenance costs, outages, and
lifetime efficiency.
Market Rules and Regulations
Operational issues are only one part of the
story. It's sometimes easy to forget just how
regulated and controlled the electric power
market is. Both state and federal regulators
guide power market functions through policy
mandates and incentives. Deregulation
took hold in around half of the states during
its high water mark in the late 1990s and led
to a patchwork quilt of regulatory structures.
Public policy makers-whether elected or
appointed-apply a sometimes bewildering
array of incentives, directives, and BandAids
to achieve a variety of goals.
http://www.powermag.com
POWER December 2012

Table of Contents for the Digital Edition of POWER December 2012
