IEEE Power & Energy Magazine - November/December 2019 - 30

In response to customer demands for cleaner energy, Xcel Energy
has made a commitment to reduce carbon emissions by 80%
compared to 2005 levels across all of its service territories by 2030.

New technology, digitization, and data-driven business
models will unlock new sources for flexibility. An important role for the system operator is to create a digital framework, promote the free exchange of data, remove obstacles
for new technologies, and digitize market processes. This is
expected to bring substantial gains by increasing the use of
the transmission system while maintaining system security.
Sector coupling between electricity and heating, transportation, and the gas systems will allow for efficient energy
storage and system balancing.

Wind Plants Provide Reserves
at Xcel Energy
Xcel Energy has improved reliability and reduced costs to
customers while steadily integrating ever-higher penetrations of renewable generation on its 7-GW Colorado system.
In 2010, Xcel's Energy Management System enacted stateof-the-art practices that enabled 4-s AGC at wind facilities
in Colorado.
In 2009, when VERs reached a penetration level of approximately 10% of annual energy consumption, Xcel Energy
found it could not accommodate additional VERs without
regular curtailment. At any moment, a system has some
amount of maximum VER headroom, which can be defined
by the total demand (plus net interchange) minus the dispatch
set points of the units that must be online to provide reliability
services (typically minimum generation level plus downward
reserve requirements). When potential VER output exceeded
the maximum VER headroom, system dispatchers had to
manually curtail wind plants to balance the system. Having
wind generators provide real-time balancing services reduced
the amount of wind-generation curtailments and fossil-fuel
costs by allowing fossil-fueled units to be dispatched at minimum generation. A few days of testing resulted in such a large
reduction in wind-generation curtailment that AGC control of
wind became the preferred curtailment mode. The AGC also
improved reliability by enabling fast-acting wind generation
to meet the balancing authority area's regulation needs.
Xcel Energy also found that a relatively low level of
renewable generation curtailment was a cost-effective integration tool. For example, during the last three years, VERs
satisfied approximately one-quarter of Xcel's Colorado
energy demand, while only about 3% of the potential VER
output was curtailed. In other words, the company's Colorado system has been able to accept 97% of all renewable
generation at a 25% penetration level. This is an important
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ieee power & energy magazine

observation for other grids, as it is often more economical to
curtail some energy rather than design a system with large
amounts of storage, for example, to reduce all curtailment.
The increasing magnitude of VER variability has challenged the system's ability to economically dispatch its generation portfolio while maintaining system reliability. Xcel
Energy carries extra regulation to address the increase in
short-term variability. In addition, Xcel holds a 30-min flex
reserve, which is sufficient to cover the largest potential
renewable generation ramps as a function of real-time VER
output. Appropriate volumes of regulation and the 30-min
flex reserve are determined by studying historic VER variability and projecting the additional variability of future
VER plants. The company has also determined the volume
of curtailed VER generation that can dependably provide upregulation and 10-min spinning reserves. This enables the use
of curtailed VER generation to provide some of the spinning
reserve requirements. In this way, some reliability concerns
associated with the inherent variability of very high VER
penetrations are addressed by the VERs providing essential
reliability services. If curtailment is necessary to preserve
reliability, it should be used for productive purposes, such as
contributing to reserves and controlling ramp rates.
In response to customer demands for cleaner energy, Xcel
Energy has made a commitment to reduce carbon emissions
by 80% compared to 2005 levels across all of its service territories by 2030, and it aspires to serve customers with 100%
carbon-free electric generation by 2050. Increasing VER
penetration is clearly one of the ways that the organization
plans to achieve these goals. Increasing levels of curtailment will naturally occur as more VER capacity is built, and
Xcel Energy has realized that curtailed VERs are valuable
resources for minimizing operating costs and improving
system reliability.

Dynamic Reserve Requirements
at ERCOT
The RTO for most of Texas, ERCOT, is an islanded grid with
limited dc ties to Mexico and the Eastern Interconnection.
ERCOT has 22 GW of installed wind nameplate capacity and
approximately 1,700 MW of utility-scale PV capacity. The
interconnection queue indicates that VERs, especially PVs,
will grow quickly. This has led ERCOT to continuously revise
its generation interconnection requirements, improve forecasting performance, update IBR impact studies, and fine-tune
study methods for determining ancillary service requirements.
november/december 2019

IEEE Power & Energy Magazine - November/December 2019

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