IEEE Power & Energy Magazine - March/April 2015 - 61

System-specific technical characteristics can influence
the achievable self-sufficiency and performance of
PV storage systems considerably.

What Has Become of the German 50.2-Hz Issue?
Sometime during the years 2011 and 2012 it became obvious

the outcomes from various studies, was put in place in July 2012.

that Germany had a problem with the overfrequency pro-

It defined the process for approximately 300,000 single PV sys-

tection settings of its PV inverters installed at the low- and

tems, ranging in power from hundreds of kilowatts down to 10

medium-voltage levels. The overfrequency tripping param-

kW, that had to be retrofitted with a P(f) droop characteristic;

eter was initially set to 50.2 Hz, in compliance with the grid

the specific modifications required depended on the systems'

codes then in effect. In turn, this meant that once a grid fre-

individual commissioning dates. It was decided that the DSOs

quency of 50.2 Hz was reached, the inverters would discon-

would be responsible for the retrofitting process, whereas all

nect simultaneously from the grid (within 200 ms). During

the costs incurred were to be allocated to the end customer's

these years, approximately 13 GW of nominal PV capacity

electricity tariff: 50% to the network tariff and 50% to the Ger-

using such hazardous parameter settings was installed nation-

man Renewable Energy Sources Act (EEG) surcharge. The start-

wide. The sudden loss of this capacity during peak feed-in

ing date of the retrofitting process was summer 2013, and the

times was unlikely to be balanced out by the 3 GW of primary

planned deadline for completion is the end of 2014. The cur-

frequency control reserve that was available within the conti-

rent status of the retrofitting process reveals a slight delay com-

nental European control area.

pared with the statutory period, however. Today, every new PV

When the seriousness of the situation was recognized, a
regulation (the so-called Systemstabilitätsverordnung), based on

service providers to TSOs. This requires continually redefining the nature of ancillary services and the stakeholders involved. Against this background, existing operational
interfaces between the DSO and the TSO are currently in
the spotlight.
The operational challenges faced by 50Hertz Transmission GmbH highlight the importance of reliable PV feedin forecasts for secure and reliable power system operation.
High power gradients of up to 1.5 GW over 15 min, caused by
the fluctuating nature of PV systems and wind, are already
clearly visible within its service area and are the reason for
an increasing need for balancing power.
PV energy already regularly covers the noon peak load
and influences together with wind energy the base load
power generation. This causes a significant impact on the
price structure on the power exchange market and thereby
on the operation of conventional base load and peak load
power plants. Flexibility requirements and business models
of conventional power plants are under change.

For Further Reading
J. Appen, T. Stetz, B. Idlbi, and M. Braun, "Enabling high
amounts of PV systems in low voltage grids using storage systems," in Proc. 2014 29th EU PV Solar Energy Conf., Amsterdam, 2014, pp. 2380-2386.
march/april 2015

inverter has to come with a P(f) droop characteristic instead of
fixed threshold values to comply with current grid codes.

J. Appen, T. Stetz, M. Braun, and A. Schmiegel, "Local voltage control strategies for PV storage systems," IEEE
Trans. Smart Grid, vol. 5, no. 2, pp. 1002-1009, 2014.
T. Stetz, K. Diwold, M. Kraiczy, D. Geibel, S. Schmidt,
and M. Braun, "Techno-economic assessment of voltage control strategies in low voltage grids," IEEE Trans. Smart Grids,
vol. 5, no. 4, pp. 2125-2132, 2014.
K. Burges, M. Döring, R. Kuwahata, U. Bachmann, R.
Sikora, and M. Zirkelbach, "Operating power systems with
high photovoltaic generation during a solar eclipse, challenges and operational measures," in Proc. 4th Solar Integration
Workshop Berlin, 10-11 Nov. 2014.

Biographies
Thomas Stetz is with Fraunhofer IWES, Kassel, Germany.
Jan von Appen is with Fraunhofer IWES, Kassel, Germany.
Fabian Niedermeyer is with Fraunhofer IWES, Kassel,
Germany.
Gunter Scheibner is with 50Hertz Transmission GmbH,
Berlin, Germany.
Roman Sikora is with 50Hertz Transmission GmbH,
Berlin, Germany.
Martin Braun is with Fraunhofer IWES and the University of Kassel, Germany.
p&e
ieee power & energy magazine

Table of Contents for the Digital Edition of IEEE Power & Energy Magazine - March/April 2015