IEEE Power & Energy Magazine - November/December 2019 - 91

Unlike synchronous machines that act as a source of system
strength, GFL IBPSs do not contribute to system strength but,
rather, have the overall effect of reducing it.
This article attempts to describe the GFM functionality
needed for secure grid operation with a high penetration of
IBPSs without being overly prescriptive as to how to achieve
it. The functionality is also subject to the physical limitations
of an inverter, such as short-term current-carrying capability
and the availability of an energy buffer. The necessity for
and amount of these capabilities must be determined based
on specific system requirements that must be confirmed
by simulations.
These functionalities are required in addition to the capabilities of the existing GFL technology, which are typically
mandated by grid codes. These requirements include operating
in a stable and coordinated manner with other IBPSs as well
as SGs, not causing adverse control system interactions, and
maintaining industry-standard characteristics such as faultride-through capability, fault current injection to support grid
voltage, and active power-frequency control.
Some of the functionalities that may be expected from
GFM IBPSs may not be provided, even by SGs in current
power systems. However, this does not preclude one from
recognizing the capabilities that could be harnessed from
IBPSs of the future and may be necessary under high IBPS
penetration levels.
To achieve a high penetration of IBPSs, it is not enough to
resolve the operational issues mentioned previously. Broader
system resiliency must be considered, including resource
adequacy and reserve availability and managing uncertainty
due to weather-dependent generation. This article focuses on
system security from an SO's perspective, in particular, stability, while operating at a high penetration of IBPSs. It also
discusses GFM IBPSs as one possible solutions, including
the manufacturers' perspective along with a summary of
ongoing research.

Operators' Perspective
This section describes eight key challenges encountered by
SOs in synchronous areas with a high penetration of IBPSs.
These challenges are expected to worsen as the IBPS penetration level increases, unless adequate system-specific solutions
are developed and implemented. Existing practices adopted
by some SOs to address these challenges include maintaining
a sufficient amount of SGs, constraining the total output of
IBPSs, or applying constraints to reduce the largest credible
contingency. Other solutions, such as installing SCs, GFM
IBPSs, or a combination of both, were discussed in recent
years. To date, however, no grid code mandates a GFM capability, although a draft of such a requirement was considered
november/december 2019

in Great Britain in 2018 and with revised proposals expected
in 2019.

System Strength
Sufficient system strength needs to be maintained at all times,
under both normal and contingency system conditions, and
has traditionally been represented by the fault level available
at a specific node in the power system in relation to the rating
of an IBPS connecting at this node or short circuit ratio (SCR).
More recently, SOs started to use versions of an aggregated
SCR, recognizing that electrically close IBPSs have a cumulative effect on the system strength of that entire part of the grid.
Historically, system strength issues have been associated with
the connection of IBPSs electrically remote from SGs. In several jurisdictions, a significant increase in IBPS penetration,
along with a decline in the number of online SGs, have caused
system strength issues to affect the entire power system rather
than only some remote parts.
Present GFL IBPSs are designed to operate in a stable
manner down to a defined minimum system strength level.
The loss of multiple network elements can result in a decline
in system strength, compared to normal conditions. The automatic disconnection of IBPSs or a significant power runback
is sometimes used due to the inherent inability of GFL IBPSs
to maintain stability under reduced system strength. In areas
with a significant concentration of IBPSs, the occurrence of
multiple outages could result in concurrent power reductions
of several GWs of IBPSs, which is significantly larger than
the largest credible contingency.
Unlike synchronous machines that act as a source of
system strength, GFL IBPSs do not contribute to system
strength but, rather, have the overall effect of reducing it.
In some cases, SCs are installed to mitigate this inherent
limitation of GFL IBPSs. The expectation of SOs is that
the connection of IBPSs should not result in reduced system
strength below the levels required for existing power plants
and the overall power system. A GFM IBPS has the potential
to achieve this goal.

Inertia and Rate of Change of Frequency
There is a strong interrelation between system strength and
inertia since currently both are provided by synchronous
machines. Presently, synchronous inertia must be maintained
at all times for islanded or dc-connected power systems such
as in Great Britain, Ireland, Texas, and Tasmania. A minimum level of overall synchronous inertia is needed for two
reasons. The first is to reduce the initial rate of change of
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IEEE Power & Energy Magazine - November/December 2019

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