IEEE Power & Energy Magazine - November/December 2017 - 56

The VPP is also used to remotely control a large num-
ber of production units at the same time. When Germany has
too much supply, prices on the intraday or day-ahead market
may be negative. This signals the trader to curtail production
(to avoid losing money). The VPP software disaggregates
the required portfolio level reduction to individual wind
and solar plants, taking into account current and forecast out-
put; see Figure 8.
In European energy markets, there are many possibilities
to generate income from renewables. The most important
markets are the intraday and spot markets, where most wind
and solar energy is traded. After the large-scale introduction
of wind and solar power into the intraday market in Germany
in 2012, this market option became very popular, as it was
increasingly essential for traders to compensate forecast devi-
ations in the very short term. In addition, the market rules for
intraday and day-ahead trading have been modified for wind
and solar, e.g., by introducing 15-min products (instead of
1 h) to manage the steep diurnal ramps of solar power.
One new modification in the German energy market is that
wind power can participate in the market for tertiary reserve
power (a 15-min reserve product that must respond within
5 min). The German transmission system operator (TSO) agreed
that renewables should participate in ancillary services and
set up additional prequalification rules for wind plants. The
first plants have already been certified and have entered the
regulation market, with solar to follow. Due to the decrease in
prices for secondary (frequency control products with a 30-s
response) and tertiary reserves, the main focus now is pri-
mary reserves (i.e., PFR). The processes to prequalify plants,
bid into the regulation markets, and manage the activation of
reserve power by the grid operator are covered by the VPP.
The result is that now approximately 90% of wind and
approximately 20% of the solar in Germany are balanced
by different traders and the remainder by the TSO (via

Germany's Renewable Energy Law). These third-party aggre-
gators essentially manage the forecasting, communications/
control, and balancing needs of many of the DERs, alleviating
the TSOs' need to manage them.

North American Response
to DER Impacts on Reliability

Power (MW)

For several years, the North American Electric Reliability
Corporation (NERC) has been studying the effects resulting
from renewable and variable generation resources replac-
ing traditional generation on the bulk power system (BPS).
While large, synchronous generators have traditionally pro-
vided essential reliability services, such as inertia, frequency
response, and voltage support, these inverter-based (or asyn-
chronous) resources do not provide these services unless they
are equipped with the proper digital controls. NERC's Essen-
tial Reliability Services Task Force, now the Essential Reli-
ability Services Working Group (ERSWG), concluded that
DERs are a growing resource that would affect the planning,
operation, and reliability of the BPS. The DER Task Force
(DERTF) was formed to conduct a preliminary assessment on
the impact of DERs, addressing planning models and studies,
operational impacts, data collection, reliability metrics, and
coordination with IEEE standard 1547, NERC Reliability
Standards, and the NERC Reliability Functional Model.
To ensure reliable operation, system modeling is critical to
BPS operations and planning for both normal operations and
disturbances. All components of the system must be represented
in the models-either directly or in an aggregated way-with
sufficient fidelity to enable the model to provide meaningful and
accurate simulations of actual system performance. A modular
approach to represent DERs in BPS studies, with some level
of data validation, may ensure accurate representation of the
resources for the specific BPS study type. While dynamic mod-
els for different DER technologies are available, limited exist-
ing knowledge and experience of
modeling DERs in system planning
studies and operating with higher
penetration DER levels will require
Meteorological Production
future collaborative research, knowl-
edge exchange, and learning.
Even though both load and DERs
are BTM, the modeling for each of
Curtailment Schedule
these respective network elements
Real-Time Measurement
requires a different set of data. As
DER penetration levels increase,
the classical transmission model of
distribution system load (netted gen-
eration and load) is no longer valid;
the unique characteristics of DERs
must be modeled separately. Data
figure 8. The dispatch of a large portfolio of wind farms due to negative prices
for DER modeling and verification
on intraday market. The curtailment schedule shows the 15-min dispatch schedule
purposes must be collected, and the
entered by the trader; the meteorological production illustrates the potential power
industry should determine the level
output due to the wind conditions; and the real-time measurement shows the real
of granularity needed.
power output based on real-time data.

56

ieee power & energy magazine

november/december 2017



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