IEEE Power & Energy Magazine - November/December 2019 - 90

penetration increases, the controllers must respond more
robustly to a system with faster dynamics.
Currently, for secure operation, some synchronous areas
(e.g., Ireland, Texas, and South Australia) frequently limit the
output of IBPSs or require sufficient must-run SGs. Installing
synchronous condensers (SCs) to provide the characteristics
necessary to support reliable operation with very high IBPS penetration has also been pursued, since maintaining a sufficient
number of SGs online is often difficult and expensive. In the
long run, operational constraints and the need for additional
investments into SCs could significantly affect further development of IBPSs.
In recent years, the research community has pursued
the concept of grid-forming (GFM) IBPS technology as an
alternate robust IBPS controller. However, constructing an
exact definition of a GFM IBPS is complex, as the characteristics are still being shaped in concert with the changing
needs of power systems around the world. For the purpose of this article, a GFM IBPS broadly refers to an IBPS
that is capable of supporting the operation of an ac
power system under normal, disturbed, and emergency
conditions without having to rely on services from SGs
or SCs. This includes system conditions when 100% of the
electricity demand is supplied from IBPSs and situations with very low IBPS penetration as well as transitions
between the two. More specifically, it would be desirable
for a GFM IBPS to have the following functionality:
1) Under normal (small-signal) conditions, it behaves as
an ac voltage source (the voltage behind impedance)
while respecting its internal physical limitations. The
control and associated settings of this voltage source
should be designed depending on the power system
to which it is connected.
2) It works autonomously if it is
110
isolated from the bulk power
100
system.
90
3) Under transient conditions, it be80
haves as described in 1) but may
70
temporarily fall into a specific
60
operation regime to respect its
50
own limits. However, as soon as
40
the limits are not at risk of be30
ing violated, it must return to the
20
behavior described in 1).
10
4) Similar to select SGs contracted
0
to provide black start services at
present, some GFM IBPSs are
expected to have a sufficient
energy buffer (battery storage,
possibly coupled with a supercapacitor) to initiate system
restoration after a blackout,
while others should be capable
of supporting the grid-restorafigure 1. A 2016 forecast of the highest hourly penetration levels of IBPSs in Europe
tion process.
by 2025.
Austria
Bosnia and Herzegovina
Belgium
Bulgaria
Switzerland
Czech Republic
Germany
Denmark
Estonia
Spain
Finland
France
Great Britain
Greece
Croatia
Hungary
Ireland
Italy
Lithuania
Latvia
Montenegro
Macedonia
Northern Ireland
The Netherlands
Norway
Poland
Portugal
Romania
Serbia
Sweden
Slovenia
Slovakia

(%)

EirGrid in Ireland and National Grid in Great Britain have
shown that these challenges increase dramatically when
IBPSs serve more than 65% of the system load.
Although large synchronous areas such as continental Europe (CE) may not reach percentages as high in
the next 10 years, parts of these synchronous areas (such as
Germany or Denmark) are already experiencing situations
when IBPSs serve a significant portion of their local load,
in some cases in excess of 100%. The reliable operation of
such systems currently depends on support from the rest of
the synchronous area. Following the occurrence of a lowprobability, high-consequence event such as system separation (e.g., a 4 November 2006 event in CE), these smaller
parts of the larger synchronous area require the ability to
avoid a total collapse. As depicted in Figure 1, the forecast
from 2016 shows that the highest IBPS penetration level in
eight of 33 European countries would reach 100% of the load
by 2025. These instantaneous penetration levels could typically be three to five times higher than the annual average
penetration levels.
A large presence of online SGs inherently slows the overall
system dynamic changes, which allows present grid-following
(GFL) IBPSs (that have fast and rigid controllers) to accurately
track the angle of the grid voltage and inject current at the correct phase angle and frequency. However, as IBPSs replace
SGs, system dynamic changes become faster, resulting in the
rigid fast inverter controllers potentially failing to adequately
synchronize with the system. Based on control theory, only
an even faster controller can track a fast-moving reference.
At these fast response timescales and with rigid control, even
a small perturbation can result in significant consequences;
these will be discussed in subsequent sections. Thus, as IBPS

ieee power & energy magazine

november/december 2019

IEEE Power & Energy Magazine - November/December 2019

Table of Contents for the Digital Edition of IEEE Power & Energy Magazine - November/December 2019