IEEE Power & Energy Magazine - January/February 2015 - 73

Innovative tools for the future coordinated and stable operation
of the pan-European electricity system have been researched
and developed by two projects.
important for power system studies. further, not all generators
send their breaker positions to the SCaDa system.
a comprehensive survey for all electricity producers is
planned to collect the appropriate data for the online estimator. But generators missing from the estimator must be
estimated in some way. work is ongoing to determine how
best to take into account the missing generators.
the nordic countries are also working together to determine how best to share the results between the tSos so that
the data can be used to analyze frequency disturbances after
the event. estimated inertia values may then be verifiable.

Impact on Frequency
It has long been assumed that calculating the inertia present
for a disturbance, given the type of disturbance, is a simple
matter of taking a data record, looking at the rate of change
of frequency (roCof), and finding the resulting inertia.
this is a simplification, however, and has been shown not
to give results consistent with the results from simulations.
Issues that complicate the inertia calculation are:
✔✔ oscillations superimposed on the system frequency
✔✔ the voltage dependence of loads
✔✔ the frequency dependence of loads
✔✔ the differences among data recorded in different
locations.
the upshot is that the effect of large disturbances on system roCof is not determined solely by the amount of inertia
present in the system. efforts are being made to find methods
that can take these effects into account. Such methods may
include the use of pmus to measure the frequency in different parts of the nordic system and the development of algorithms that can highlight the effect of inertia. methods being
studied include simple filtering, curve fitting, system identification, and calculations based on localization of inertia.
the roCof may affect protection systems in wind turbines, but the lowest system frequency (frequency nadir) in
the nordic system is also of interest. many protection systems are frequency-activated and should only be necessary
in the case of an impending system collapse. If activated
only by the loss of one production unit, unnecessary widespread load shedding may result. It is therefore imperative to
ensure that the system frequency always stays above 49.0 hz.
the frequency nadir is determined not only by the inertia
in the system but also by the speed of the fCr. an inspection of a selection of machines in the system revealed that
the fCr response does not seem to react to imbalances in the
way that has been specified. until a comprehensive survey is
january/february 2015

performed, modeling of the control action of the production
units is required to estimate the frequency nadir after the loss
of the largest control unit. this is the subject of ongoing work.

The Impact of Future Production and
Consumption on Inertia and Frequency
to meet renewable energy targets, it is expected that more
wind power will be installed in the power system, both
within the nordic region and in continental europe. During
low-load situations, high wind speeds in europe are expected
to lead to scenarios where wind power generation displaces
conventional generation in the nordic region, resulting in
low amounts of system inertia.
Various scenarios will be studied to understand the scale of
the actions that need to be taken to maintain the operational
safety of the power system. these scenarios include:
✔✔ the impact of different levels of hydro power
✔✔ the effect of phasing out nuclear power
✔✔ the impact of increased development of wind power
plants and other reS.
actions taken early on may include limiting the production
of the largest generation units or requiring a certain amount of
inertia to be online at all times. these are not, however, solutions to the long-term problem of dimensioning the system to
accommodate large amounts of renewable energy. Synthetic
inertia may need to be a requirement for future wind power
plants. this involves having the frequency converters in wind
turbines release their kinetic energy in much the same way
conventional generating units do. furthermore, hVdc connections could be used to transfer inertial response from one synchronous system to another. more extensive measures, such as
new markets for inertia, may also be required.

Conclusion
the growing share of electricity generation from intermittent reS as well as increasing market-based cross-border
flows and the related physical flows are giving rise to uncertainties in transmission network operation. In the mainland
synchronous area of central europe, the difference between
actual physical flows and the market exchanges can be very
substantial due to large installations of renewable energy
generation, such as wind and photovoltaic. remedial actions
were identified by previous smart grid studies undertaken
within fp6 in operational risk assessment, flow control by
phase shifter coordination, and operational flexibility measures for this area. In coming years, tSos will seek to manage power flows within the capability of existing networks
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Table of Contents for the Digital Edition of IEEE Power & Energy Magazine - January/February 2015