IEEE Electrification Magazine - March 2015 - 18

signals. Another major challenge is the cost allocation of
infrastructure investment among key market participants.
For example, to integrate large amounts of renewable generation into the grid, transmission and distribution
upgrades are needed in most cases. How to fund these
upgrades has been debated for years, and there does not
seem to be a clear solution.
In addition to the economic challenges, there are many
technical and operational challenges as well. By the laws
of physics, the fundamental challenge of power systems is
the balance of supply and demand while complying with
system performance and reliability standards. Electricity
cannot yet be stored as easily and affordably as gasoline,
but we are getting there. Therefore, generation and consumption have to be balanced in real time. Most of the
world uses ac synchronous systems, which require all
generators to operate at the same
voltage, frequency, and phase angle
to within an allowable margin.
Compared with a small system in
which generation and load are colocated and the number of nodes in the
network is modest, it is much harder
and more complex to balance a large
interconnected power system in
which hundreds of large generators
are connected through thousands of
miles of transmission lines to serve
millions of people. The locations, network topologies, and dynamics and
variability of generation and load are
all contributing factors that determine how supply and demand are
balanced within a specific service
area as well as in the entire power
system. In the United States, regional
system operators and independent
system operators have the most challenging job of balancing tens to hundreds of gigawatts of electric power-second
by second, minute by minute, and hour by hour-in a geographic area that usually covers multiple states. Some of
their major tasks include generator control and dispatch,
frequency and voltage regulation, load forecasting and unit
commitment, system protection and recovery, and metering and account settlement.
The centralized ac power system model is neither flexible nor resilient. The generation plants are not flexible
because of their sheer size and the time it takes to ramp up
and down. The smaller gas-fired peaker units have faster
ramp rates, but they do not run all the time because of
higher operation costs. (The current natural gas production
boom and low price have made gas-fired power plants
more competitive. However, it remains to be seen whether
the low gas price is sustainable.) The load is not flexible
either. Most of the electric loads today are passive, meaning
that they are unaware of the generation supply situation

and the electricity pricing. For example, on a hot summer
day, the air-conditioning systems will operate at full duty
cycle while the power system is stressed to its limit. The
dynamic pricing response is only limited to those loads that
are retrofitted with demand response control systems.
In the distribution system, reverse power flow is a
major challenge as the penetration of distributed renewable generation such as rooftop PV increases. The current
ac power system has been designed and optimized for
power flow in one direction: from central generation to
end consumers. It is anticipated that at higher penetration, distributed wind, solar, and other small generation
can exceed power consumption needs at certain interconnection points, and, therefore, reverse power flows
can occur in the direction from consumers back to the
distribution grid and possibly back into the bulk power
system. Grid planners and operators
need to make sure that the protection equipment recognizes bidirectional power flows and that equipment ratings and settings properly
account for the total fault currents
coming from all sources. Another
key challenge in the distribution grid
is the management of variability
from wind and solar generation so
that it does not cause voltage variations to exceed the American
National Standards Institute limits.
Without proper management, the
variability can cause excessive operation and premature failure of
voltage-regulation equipment such
as load tap changers, voltage regulators, and capacitor banks. A third
challenge is the complexity involved
in the coordinated system operation
through the integration of sensors, power-flow controllers, communications, supervisory control and data
acquisition (SCADA), and operation software such as Distribution Management System (DMS). The integration of
information technology (IT) and operation technology
(OT) are becoming more and more important for grid
operation, but the existing solutions tend to be quite
complex and expensive.
These technical and operational challenges are particularly difficult for the rural electric system because of its
long distribution feeders and distance from the transmission lines. When renewable generation plants are built in
rural areas, they tend to be large due to the availability of
land. As a result, reverse power flow, voltage regulation,
and wear and tear of equipment are all real problems for
the relatively weak rural electric grid. Reconfiguring and
upgrading the system hardware and software adds significant cost, and the integration of IT and OT is technically
nontrivial.

The costly system
upkeep, lack of
flexibility, and
reliance on largely
fossil fuel-based
power generation
are some of the
reasons that the
existing systems
are not sustainable.

I E E E E l e c t r i f i c ati o n M agaz ine / marCh 2015

Table of Contents for the Digital Edition of IEEE Electrification Magazine - March 2015