IEEE Power & Energy Magazine - November/December 2019 - 93

GFM capability relies on the existence of an energy buffer
(battery storage, headroom on wind or PV IBPSs, supercapacitors,
or a combination of these, depending on the application).
Commercially available GFL IBPSs are not currently made
with a comparable overcurrent capability, even though it is
technologically possible.

Contingency Frequency Control During
Islanding Conditions
In islanded scenarios with a large share of IBPSs, additional
energy storage or deliberate headroom on IBPSs might be
required to provide sufficient contingency frequency control.
However, GFL IBPSs need sufficient system strength to provide a stable frequency response. To maintain the required
system strength, additional SCs may be needed.
A related question is whether the combined capability of
SCs and GFL IBPSs in controlling voltage and frequency
can emulate the capabilities traditionally provided by a SG
or if using GFM inverters is essential. While many operators share these concerns, limited studies indicate that GFM
IBPSs can help to mitigate these issues; additional research
is required to confirm this.

Initiate/Support System Restoration
GFL IBPSs have a limited ability to support system restoration
since they require a minimum system strength for stable operation. Conversely, GFM IBPSs could be used in the early stages
of system restoration, including acting as black start units, provided they are equipped with a sufficient energy buffer.

Accurate and Fast Simulation Models
Electromagnetic transient (EMT) models may be required
in an inverter-dominated power system to overcome issues
associated with accuracy and the appropriateness of phasordomain models. The Australian Electricity Market Operator has developed full EMT network models for three of its
five regions. The Electric Reliability Council of Texas and
National Grid in Great Britain are also using EMT models for
the parts of their networks with a high penetration of IBPSs.
A consequence of providing an in-depth level of detail and
accuracy is that EMT simulations are computationally intensive and, therefore, impractical for control-room applications
such as dynamic security assessment tools. Significantly
faster simulations with the accuracy of the EMT models are
required and are the subject of ongoing research.

Manufacturer's Perspective
The majority of IBPSs connected to bulk power systems
are based on GFL technology. During the last two decades,
manufacturers have focused on pushing the limits of this
november/december 2019

technology, successfully developing grid-support capabilities (fault ride through, voltage control, frequency control,
weak grid operation, and other similar features) for the vast
majority of applications. However, the limits of GFL technology are being approached in areas such as Ireland, Texas,
and South Australia.
Akin to developing any manufactured product, IBPS producers depend on a robust market for their products. The
eventuality that a power system could not accept the connection of additional IBPSs due to reliability constraints would
severely impact manufacturers. Hence, producers are interested in and open to incorporating new features if they are
necessary to secure power system operation.
Due to intense competition, IBPS manufacturers generally only consider developing new products and capabilities
if sufficient incentives and market value exist, for example,
based on grid code requirements or market incentives. To
identify system needs and ensure the availability of solutions, the following fundamental rules should be respected.
✔ Technical requirements need to be transparently discussed and proven.
✔ Technical features critical for the overall operational
security, and cannot be delivered as ancillary services,
should be defined as minimum interconnection requirements for all generators (e.g., fault ride-through
capability and active power-frequency control).
✔ Ancillary services should be acquired on a least-cost
basis. This applies to conventional services such as
load and generation balancing, voltage support, and
black start, as well as potential new services such as
FFR and damping.
IBPSs presently operate in maximum power point (MPP)
tracking mode to harvest the most energy from a variable
energy resource and maximize the commercial value of a
project. An energy buffer is not required for such an operation
and typically not included with present technology. However,
GFM capability relies on the existence of an energy buffer
(battery storage, headroom on wind or PV IBPSs, supercapacitors, or a combination of these, depending on the application). Depending on the size and capabilities, the economics
associated with an energy buffer could be prohibitive. The
GFM capability could also result in extended times of operation outside of MPP mode for PV and wind IBPSs.
GFM IBPSs based on megawatt-sized battery systems
are already available on the market and being used in several
actual projects, some of which allow for the parallel operation of several inverters performing GFM functions. The
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IEEE Power & Energy Magazine - November/December 2019

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