IEEE Power & Energy Magazine - November/December 2015 - 85

With proper incentives and market designs, all flexibility
options can be deployed to minimize the overall costs
of a clean, reliable power system.

unchanged. When governor controls are added to utilityscale solar PV plants, the systemwide frequency response is
substantially improved [Figure 4(b)].

Frequency and Voltage Ride-Through
To date, wind turbines are the only generators required to
ride through disturbances. Early wind machines used simple
induction generators that provided no dynamic grid support
in the event of a power system disturbance. These turbines
focused on energy capture rather than grid support. In fact,
because they had response characteristics different from
conventional synchronous generators, they were required to
disconnect if frequency or voltage deviated from nominal to
prevent them from making the situation worse. As wind penetration increased, the loss of significant amounts of wind generation during a disturbance became a reliability concern, and
utilities wanted to impose ride-through requirements on wind
turbines. Wind-turbine technology had advanced, and the
wind industry supported ride-through requirements, but they
wanted them to be standardized to facilitate product design.
With considerable input from both the wind industry and the
power system industry, the Federal Energy Regulatory Commission addressed these concerns in 2005 with Order 661A,
which requires all new wind turbines to ride through lowvoltage events rather than disconnecting. This order also set
power factor design criteria and supervisory control and data
acquisition requirements for wind generators.
The North American Electric Reliability Corporation
considered applying the same requirement to all new generators when it drafted the new standard PRC-024-1, Generator Frequency and Voltage Protective Relay Settings.
During several balloting processes conducted by the North
American Electric Reliability Corporation, the industry
repeatedly defeated the inclusion of an actual ride-through
performance requirement. Instead, the standard requires
only that the primary protective relays not be set to trip
the generator within the "no trip zone" of voltage and frequency curves. Conventional generators do not need to ride
through an event, but wind turbines must still meet the
requirement imposed by the Federal Energy Regulatory
Commission to ride through disturbances. Wind turbines
are also required to provide voltage support if needed by
the power system.
Ride-through requirements for distributed PV systems are
also evolving. Utilities require distributed PV systems to disconnect when voltage or frequency is out of bounds. This is
november/december 2015

based on safety concerns to prevent the PV system from energizing a portion of the grid that is supposed to be de-energized
(anti-islanding). As with wind, the requirement was reasonable when there was not much PV generation on the power
system, but it is not viable with thousands of megawatts of
generation that might disconnect during a disturbance. Fortunately, anti-islanding technologies have improved, and safety
concerns can be addressed while having distributed PV ride
through disturbances. IEEE 1547, Standard for Interconnecting Distributed Resources with Electric Power Systems,
is being modified to allow PV ride-through. California and
Hawaii are requiring ride-through capabilities from new distributed PV installations.

Power System of the Future
Increasing amounts of wind and PVs, which are largely connected to the electricity grid by nonsynchronous power electronic converters, is part of a bigger trend in the evolution of
power systems. This trend also includes increased levels of
power electronics embedded in loads (e.g., modern electronic
loads) and transmission (e.g., high-voltage dc transmission). The
more distributed nature of wind, PVs, and other forms of generation (e.g., combined heat and power) along with more active
consumer participation are all contributing to a dramatic shift in
the nature and characteristics of the future electricity grid. These
changes create the need for more physical flexibility that can be
sourced from many different assets on the electricity grid. (See
"Additional Sources of Flexibility.")
The island of Ireland is an interesting example in which
the increase in nonsynchronous generation is necessitating
the development of both innovative wind power plant controls and holistic solutions. These solutions include advanced
wind turbine controls (i.e., synthetic inertia), fast demand
response, and synchronous generators capable of riding
through larger frequency swings all within the regulatory
and market framework. ERCOT also found that fast demand
response is more effective than the response from conventional generator governors in stabilizing the power system
after a major disturbance.
As the system continues to evolve toward higher levels
of VG, this market framework must provide the appropriate
signals to incentivize sufficient flexibility in both the operational and investment time horizons. Not only is a sufficient
level of capacity required to meet future demand, but the
nature of this capacity is fundamentally different than it was
in the past because of the need for flexibility. This issue is
ieee power & energy magazine

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Table of Contents for the Digital Edition of IEEE Power & Energy Magazine - November/December 2015

IEEE Power & Energy Magazine - November/December 2015 - Cover1
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