IEEE Power & Energy Magazine - May/June 2014 - 34

Solutions for intelligent distribution controls
that provide necessary coordination between many devices,
including distributed PV, are evolving.

between small distributed applications and large utilityscale plants.
the integration of large-scale Pv plants in the transmission system can follow the successful model already established by wind integration, with the consequential impact
of variability treated in the same manner. at the distribution level, locally high penetrations of connected Pv capacity can be very disruptive to operations. Power variability
due to intermittent cloud shading of Pvs, in itself, is not of
concern at the distribution level because energy balance is
achieved on a much wider basis at the transmission level.
however, the consequential impact of power variability is
voltage variation that can cause premature failure of utility
voltage-regulating equipment and power quality degradation
for all customers served by the distribution system.
While energy storage is often discussed as a mitigating approach, voltage variations can, in most cases, be
much more economically addressed using reactive power.
Dynamic reactive devices, such as static synchronous compensators (statcom) and static var compensators (svcs)
can be applied to mitigate voltage variations at the feeder
level and cover the temporal range of Pv variability that
cannot be mitigated by mechanically switched voltage regulators. ieee standard 1547 has until recently prevented Pvs
from participating in providing mitigation of these problems.
recent modifications to the standard have opened the door
for advanced inverters to use their reactive power capability
to help mitigate voltage variations caused by Pvs. solutions
for intelligent distribution controls that provide necessary
coordination between many devices, including distributed
Pvs, are evolving and are expected to help manage this
emerging challenge that faces the grid.

Dynamic Reactive Power Sources
the growth in wind and solar power and DG and the retirement of coal plants and other large aging central-station
generation plants will have an unintended consequence on
the performance of the transmission system. today, many
of the oldest thermal units are located near large urban load
centers. these units, which may be retired or displaced in
the near future, often provide essential voltage support and
needed short-circuit strength. this dynamic support is critical to maintain a strong and stiff voltage for the stability of
the grid during and after disturbances such as the loss of
a major transmission line. unlike active power (watts), the
need for and the provision of reactive power (vars) is highly
34

ieee power & energy magazine

locational. since utility-scale wind and solar plants tend to
be built far from load centers, the reactive power produced
on a remote windy plain or out in the sunny desert is of little
value to maintaining voltage in urban load centers.
historically, nearly all electricity transmitted through
the grid was delivered via synchronous generators equipped
with excitation systems. in contrast, wind and solar use
asynchronous generating technologies that contribute little
to short-circuit strength. Wind and solar energy can provide the necessary dynamic reactive power to the grid to
support voltage for normal operating conditions, but these
asynchronous generators do not create the same level of voltage stiffness during deep grid disturbances as conventional
synchronous generators. in addition to loss of dynamic reactive capability near load centers, there is growing evidence
that the aggregate load on the grid is becoming less "grid
friendly." modern electronic loads, air conditioning, and
computers can all increase the requirement for dynamic
reactive support. the retirement of conventional generators
and the displacement of remaining generators with wind and
solar power could alter the present systems' capabilities to
manage disturbances on the grid.
Generation retirements are typically announced fewer
than two years before the planned retirement date, making
the lead time for needed grid reinforcements short and transmission solutions impractical. For many voltage problems,
shunt capacitors are a relatively inexpensive approach and
can be installed quickly. however, shunt capacitors cannot
regulate voltage dynamically due to the discrete switching
necessary for operation. Power electronics, such as svcs,
have been used successfully for many years to meet dynamic
voltage regulation requirements but require a stiff grid voltage that is created by nearby generation. more advanced
power electronic devices such as statcom can provide
improved performance in a weaker grid, but in a very weak
grid they still have limited ability to stabilize voltage during a disturbance. the most robust and often the only viable option is synchronous condensers, which replicate the
dynamic reactive power capability of a conventional power
plant without the capability of generating power for the grid.
an emerging trend in north america is the conversion of
retired generation to synchronous condensers. this involves
removing the turbine and operating the synchronous generator to produce only reactive power. this is often a very
attractive approach from both a system performance and
economic perspective.
may/june 2014



Table of Contents for the Digital Edition of IEEE Power & Energy Magazine - May/June 2014

IEEE Power & Energy Magazine - May/June 2014 - Cover1
IEEE Power & Energy Magazine - May/June 2014 - Cover2
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IEEE Power & Energy Magazine - May/June 2014 - Cover3
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