IEEE Power & Energy Magazine - November/December 2019 - 94

GFL technology. Additional information regarding the functional requirement examples listed in the table is provided later
in this section.
The behavior as a voltage source can demand fast power
output variations from a GFM IBPS during system transients, which are caused by generation tripping, a grid voltage vector shift (see Figure 2), or a system split. These variations in ac power output will likely demand an additional
source of energy in PV and wind IBPSs. The magnitude and
duration of these power surges are important design considerations that will lead to additional costs.
Battery-based IBPSs do not require an additional source
of energy as long as the battery's response speed allows the
GFM to maintain its designed characteristics. To withstand all transients, particularly grid voltage vector shifts,
supercapacitors may be needed, which results in additional
cost implications.
Furthermore, the inrush currents needed to enertable 1. Examples of functional requirements
gize
transformers can significantly increase the current capafor GFM IBPSs and affected product aspects
compared to GFL IBPSs.
bility requirements. The ratings and characteristics of equipment energized solely by the GFM IBPSs are important
Affected Product Aspects of GFM BPSs
Compared to GFL IBPSs
design considerations.
Current contributions during faults in excess of the
Is a Higher
Is an
Are Control
IBPS rated current are commonly not required in present GFL
Examples of
Current
Energy
Algorithm
Functional
Capability
Buffer
Changes
IBPSs. An increased fault current contribution needed from
Requirements
Required?
Needed?
Needed?
GFM IBPSs is an important design consideration that will lead
for GFM IBPSs
(Hardware)
(Hardware) (Software)
to additional costs.
Potentially
Yes
Yes
Fast active
Design considerations for GFM IBPSs are affected by the
power
grid
characteristics associated with active and reactive power
variations
changes caused by voltage angle and magnitude changes,
Response to
Yes
Yes
Yes
respectively, at the point of interconnection of the IBPS.
the grid voltage
Understanding the expected range of these variations will
vector shift
reduce the likelihood of undesirable interactions.
Yes
No
Yes
Inrush currents
In today's setting, where expectations for the functionality and
Yes
No
Yes
Fault current
performance of GFM IBPSs are nonuniform across different
contributions
interconnections, manufacturers lack incentives and guidance to develop GFM capability.
Nonuniform requirements will
drive up the development cost for
I
manufacturers and, consequently,
Z
for the IBPS project developers.
VInverter
VGrid
Commonly agreed upon requirements for active and reactive power
performance, the response to small
and large disturbances, and control
VInverter
VInverter
I
I
modes are necessary to allow manuChange in
facturers to develop and maintain one
the Grid Voltage
VGrid old
I·Z
I·Z
set of products and reduce the comVGrid
VGrid new
plexity for product applications.
Ancillary services or other marketbased
approaches should be develfigure 2. The vector shift and response expected from a GFM IBPS. The GFM inoped
to
assign value to the system
verters will keep the voltage and frequency reference constant or stable, as long they
benefits
from
GFM technology, simdo not exceed their current limit. In the case of a vector shift of the grid side, the
ilarly
to
frequency
support and black
inverter voltage (VInverter) is identical before and after the change in grid voltage. Thus,
start services. In this case, the market
the voltage drop (I·Z) and inverter current (I) have to change.
battery-based IBPS launched by the Imperial Irrigation District in California uses 30 1.25-MVA inverters and can black
start motor loads and energize high-voltage transformers. This
control concept can be scaled by simply adding more inverters. Increasing the number of inverters or accepting degraded
performance, such as deeper voltage sags during motor starts,
can help to overcome the high-current rating requirements.
In general, most existing megawatt-size GFM applications
have technology of interest for bulk power systems. However,
depending on the exact requirements, the costs can be high
compared to the same-sized GFL IBPSs. The primary cost
drivers are the energy buffer, oversized equipment, and the
need for different control strategies based on user specifications.
Table 1 shows some functional requirements for GFM IBPSs
and IBPS aspects affected by these requirements, compared to

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