IEEE Power & Energy Magazine - July/August 2016 - 41

imposed by the growing presence of intermittent wind and
solar generation in the generation mix. at the distribution
level, in addition to concerns related to privacy and cybersecurity, recommendations focused on the need for a transition
to dynamic pricing for end consumers, promotion of innovation in network design and management, data availability
and ownership, and tariff reforms to support distribution
network cost recovery.
only a few years later the perceived challenges facing
the power system are substantially different. The change is
mostly due to the irruption of distributed energy resources
(DeRs), a diverse array of technologies including gas-fired
distributed generation, solar photovoltaics (PVs), small- and
medium-sized wind farms, electric vehicles, energy storage,
and demand-side management. These DeRs, unlike traditional centralized generating units, are characterized by their
small capacities (several kilowatts to several megawatts), their
diverse nature (generation, storage, responsive demand, or
any combination thereof), and their connection to electricity
distribution grids. DeRs, if properly integrated, may have the
potential to deliver not only the valuable electricity services
that are traditionally provided by centralized generating units
but also new services enabled by their distributed nature.
It is now apparent that DeRs can significantly affect the
design, operation, organization, and regulation of the distribution network. It is also widely admitted that distribution regulation has to be modified substantially to prevent
these DeRs from driving serious economic distortions. The
extent to which DeRs will affect all the activities of the
power sector-from the future generation mix to how voltage is controlled at the distribution level to the expansion
of the capacity of the transmission network-is a matter of
debate today. This article focuses on the impacts of DeRs
on the design, operation, and regulation of the transmission
network. The discussion herein has broader implications
regarding how to understand power systems, and electricity
networks in particular, of the future.

The End of the
"Trickling Down" Paradigm
The first key message of this article is that the proliferation
of DeRs in many power systems has invalidated the "trickling down" paradigm and demands a fresh look at how the
distribution and transmission networks have to be designed,
operated, and regulated. By the trickling down paradigm,
one must understand the traditional image of a power system, where power flows from the large power plants to the
end consumers, trickling down from the highest voltage levels to the lowest ones.
We have been living under the trickling down paradigm
for more than a century. large power plants inject power into
the meshed transmission network via step-up transformers;
that power, after traveling major distances in this network,
is brought by step-down transformers in substations to lower
voltages into the meshed high-voltage distribution network.
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next, other transformers in other smaller substations bring
the power to lower voltages in the medium-voltage grid,
which typically has a meshed design but it is operated radially. finally, the so-called distribution transformers, located
in special-purpose poles in the streets or hidden in large
buildings or underground, bring power to the consumers
located nearby. This traditional power system perspective is
represented in figure 1.
The mental representation that we have of the power system conditions our way of thinking. But this mental image
is shattered when we learn, for instance, that on a sunny day
in germany in august 2015, solar PVs-located at different
voltage levels in the distribution network-produced 24 gW
(plus another 18 gW of wind), out of the total demand of
55 gW. germany set a world record for solar power production with 25.8 gW produced at midday on 20-21 april 2015,
a record that will surely be broken since the federal government has set a target of 66 gW of installed solar PV capacity
by 2030 and a goal of 80% of electricity from renewable
sources by 2050.
The deployment of solar PVs in germany, as well as in
many other countries, teaches us another important lesson.
according to a study by the fraunhofer Institute for solar
energy systems, over 98% of germany's more than 1 million PV power plants are connected to the low- and mediumvoltage grids, where most consumption is located. PV plants
of over 1 MW installed capacity account for only 15% of the
total PV capacity in germany. The best solar resources in
germany are located in the south. It is therefore possible to
have the majority of the electricity generation in a country
located at the lower voltage levels of the distribution network
of some part of the country, therefore creating substantial
power flows with large potential impacts at all voltage levels.
Indeed, these flows could impact a country's transmission
system and interconnections with neighboring systems.
What can be considered "distributed" generation? In
some countries, solar PV generation is mostly deployed at
the utility scale, in solar plants of diverse sizes. a few of
these utility-scale plants exceed 500 MW in capacity, and
they are connected to the transmission grid. The same can
be said of other technologies that are termed distributed but
can be installed in sizes small enough to fit at a residence to
sizes comparable to large conventional power plants. This is
the case of storage, diesel or gas-fired turbines, or combinedheat-and-power systems. Despite the availability of domestic-scale wind power, utility-scale wind has dominated. Most
wind generation happens in wind farms, which also range
in size-from a few megawatts to several hundreds-and
in the voltage level at which they connect to the grid. The
best wind resources for electricity production in germany
are located mostly in the north, and many of the grid farms
are also connected at distribution voltages. other energy
resources mostly belong to low and sometimes also medium
voltage, like electric vehicles, or demand response at residential or commercial levels. In conclusion, analogously to
july/august 2016

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