IEEE Power & Energy Magazine - November/December 2017 - 69

Because the VSM is a voltage source, during the
closest faults, some intervention is needed to protect
the solid-state devices from overcurrent.
Summary and Predictions
Compared to traditional generation, converter-connected
devices are limited in maximum current due to the use of solidstate power electronics. They are also limited in terms of active
power response because of application-specific limits of power
and energy availability at the dc bus, particularly for renewable
energy sources. However, if these constraints are managed,
it is possible to provide stable converter-dominated-or even
converter-only-power systems serving dynamic real-world
customer loads that have unbalanced and nonlinear components. To accomplish this, at least a certain proportion of the
converters needs to be grid forming, acting as voltage sources
(not current sources), mitigating power quality, and providing
active power as the loads demand.
To enable seamless, dynamic power sharing between converters, conventional frequency-power set points and droop
slopes can be configured with low update rates commensurate with conventional network practice. Depending on the
grid-forming converter-controller variety, it may offer VSM
functionality with inertial support or VSM0H functionality
that provides fast-acting droop response but no inertia. It is
entirely feasible to operate a converter-only network with
zero inertia and bounded frequency-nadir excursions, this
having been demonstrated in simulation and practical experiments. However, there is insufficient industry experience to
establish the exact proportion of inverters that need to be
grid forming to create viable systems. Converting existing
commercial installations to become grid forming will be
impractical or impossible for much of the installed base.
Because so many existing grid-connected devices and
machines expect frequency to change relatively slowly, sudden removal of all inertia from an existing network would
be impracticable, with, for example, unpredictable consequences for protection relays as well as large power transients for directly connected pumps and ac machine loads.
As noted in the opening, the evolution of the system will
likely occupy a continuum toward lower levels of synchronous resources. Whatever strategies are adapted must be
capable of working with some synchronous machines.
More likely, in the near to medium term, we will require
a managed balance of inertia-providing and noninertia-providing devices, the choice for each device made considering
the properties of the connected energy source. However, in the
long term, a more holistic system-planning approach is needed.
Band-aid solutions applied in the interim by system operators
worldwide to maintain reliability may inadvertently limit further integration of converter-connected devices. Defining the
november/december 2017

grid-forming function as an essential reliability service may
attract new grid-forming inverters to the market but may also
provide additional incentives for existing synchronous generation. Requiring grid-forming converter control from new converter-connected devices could be another route.
Still, it will take time until a sufficient number of these
is installed. More research is needed to bridge that implementation gap between available grid-forming converter
technology and integration of this technology into a power
system with its existing energy resources, protection systems, interconnection requirements, energy and ancillary
services, market structures, and other attributes.

For Further Reading
M. Yu, A. J. Roscoe, A. Dys´ko, C. D. Booth, R. Ierna, J. Zhu,
and H. Erdal, "Instantaneous penetration level limits of nonsynchronous devices in the British power system," IET Renewable Power Generation, 2016. doi: 10.1049/iet-rpg.2016.0352.
L. Weifeng, D. Pengwei, and L. Ning, "Probabilisticbased available transfer capability assessment considering
existing and future wind generation resources," IEEE Trans.
Smart Grid, 2017.
D. Ramasubramanian, Z. Yu, R. Ayyanar, V. Vittal, and
J. Undrill, "Converter model for representing converter interfaced generation in large scale grid simulations," IEEE
Trans. Power Syst., vol. 32, no. 1, pp. 765-773, Jan. 2017.
J. Matevosyan and P. Du, "Inertia: Basic concept and impact on ERCOT grid," in Proc. 15th Int. Workshop LargeScale Integration of Wind Power into Power Systems, Vienna,
Austria, Nov. 2016.
B. Kroposki, B. Johnson, Y. Zhang, V. Gevorgian, P. Denholm. B-M. Hodge, and B. Hannegan, "Achieving a 100%
renewable grid," IEEE Power Energy Mag., vol, 15, no, 2,
pp, 61-73, Mar./Apr. 2017.

Biographies
Thomas Ackermann is with Energynautics GmbH, Darmstadt, Germany.
Thibault Prevost is with RTE, France.
Vijay Vittal is with Arizona State University, Tempe.
Andrew J. Roscoe is with the University of Strathclyde,
Glasgow, United Kingdom.
Julia Matevosyan is with the Electric Reliability Council
of Texas, Taylor.
Nicholas Miller is with GE's Energy Consulting Group,
Schenectady, New York.
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