Utility operators work very hard to
predict demand changes across many intervals (i.e. within a given hour, daily,
weekly, monthly, etc.) in order to match supply to demand. No matter how
accurate these predictions are, the grid is characterized by short interval
variations in demand. If the momentary difference between supply and demand
exceeds a tolerable threshold, the grid may experience unfavorable power
quality events. These unfavorable events may include changes in the power
frequency, voltage sags, variation to the waveform of the AC power and presence
of harmonic currents within the power supply. If these irregularities continue
to a longer interval, noticeable brownouts or even blackouts can occur (EPRI, 2003) . All of these out of
specification power events can be very damaging to distribution equipment on
the grid itself as well as to equipment consuming the power. In the case of
blackouts, the financial impact can be exacerbated by lost productivity and
damage or loss of product because of interrupted manufacturing processes (Lineweber
& McNulty, 2001) . On various timescales energy storage,
with its highly variable output and short response times, can be used for power
regulation. An energy storage device is appropriate for regulation when it can
be quickly discharged or charged to reconcile the many momentary differences
between supply and demand.
As some energy storage technologies
are highly modular and distributable, they are particularly well suited for
voltage support and reactive power. Reactance is a localized phenomenon
experienced in AC circuits caused by equipment in the circuit that acts as a
capacitor or an inductor (i.e. equipment such as asynchronous induction motors
commonly used for air conditioning equipment) (Banta, 2012) . This equipment
causes the accumulation of electric or magnetic fields within the circuit
proximal to the equipment. This is known as reactive power, which in turn
produces an opposing electromotive force in the circuit causing the voltage and
current to come out of phase. This out of phase condition reduces the real
power, or usable power, in the circuit (Bhatia) .
The impact of this effect is represented by the circuit’s power factor:
If
a circuit has a power factor below 1, then power is being lost due to reactance,
in this case, generation equipment must provide more power to compensate in the
form of reactive power (VAR; Volt-Ampere Reactive). The problem with using
central generation to provide reactive power is that reactive power cannot be
effectively transmitted to the reactive load center over the distances common
in a traditional centralized generation power grid. Many major power outages
can be attributed, in some part, to problems with transmitting reactive power
to load centers. Placing an energy storage device at the load center, where the
most reactance occurs, can significantly reduce the required amount of reactive
power produced by central generation equipment thereby improving grid stability
(Eyer & Corey, 2010) .
Works Cited
Banta, R. L. (2012, November 23). Mechanical
Engineer, Plant Operations. (M. R. Banta, Interviewer)
Bhatia, A. (n.d.). Inductive and Capacitive
Reactance. Retrieved December 9, 2012, from CED Engineering:
http://www.cedengineering.com/upload/Inductive%20and%20Capacitive%20Reactance.pdf
EPRI. (2003). EPRI-DOE Handbook of Energy Storage
for Transmission & Distribution Applications. Washington DC: EPRI,
Palo Alto, CA, and the U.S. Department of Energy.
Eyer, J., & Corey, G. (2010). Energy Storage
for the Electricity Grid: Benefits and Market Potential Assessment Guide.
Albuquerque, New Mexico: Sandia National Laboratories.
Lineweber, D., & McNulty, S. (2001). The Cost
of Power Disturbances to Industrial & Digital Economy Companies.
Madison, WI: EPRI.
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