IEEE Power & Energy Magazine - November/December 2019 - 95

With a very high penetration of GFL IBPSs that rely on fast
rigid controls to inject current into the grid, the stability of the
power system cannot be assured. As the number of online SGs
decreases, the impact of electromechanical dynamics becomes
less pronounced and the faster electrical dynamics dominate.
Chief among the fast control loops that result in instability is
the phase-locked loop losing synchronization with the network. Among other studies, this phenomenon has been demonstrated in studies on a 36-node model of the Great Britain
network for 2030 (Figure 3) and a futuristic 100-node model of
a portion of a North American utility's system (Figure 4). The
latter study also shows that one GFM IBPS can ensure small
signal stability of the isolated system upon its islanding from
the main grid.
Due to their voltage-source behavior, GFM IBPSs provide an immediate step response and inherently adapt to
grid changes compared to the slower response of GFL IBPSs
(Figure 5). Additional control loops on the GFL IBPS cannot
achieve this behavior, as they need to measure system variables (voltage, current, frequency) to react accurately, which
inherently slows the response. For example, the FFR from
GFL IBPSs can help improve system stability only if there
is a sufficient number of SGs connected to the grid maintaining a minimum inertia level and ensuring an acceptable
RoCoF in the first 100 ms after a generator trip. Studies at
the University of Strathclyde have shown that when this condition is not met, the FFR from the GFL IBPS can be detrimental to stability (Figure 6).
Several research projects have investigated GFM controls and proposed various technical solutions that meet the
november/december 2019

8
Active Power

4
pu

Research Perspective

requirements stated in the beginning of this article. National
Grid and the University of Strathclyde have demonstrated
that all the studied scenarios could be stabilized, even in
an extreme case representing a system split with 93% IBPS
penetration and very high power transfers, upon replacing
a portion of the GFL IBPS (10-30%) with GFM controls.
The European Commission-funded MIGRATE project also
demonstrated the stability benefits of GFM controls on the
Irish transmission system. While the exact required percentage of GFM IBPSs depends on the characteristics of the
system being evaluated, research has shown that generally
10-30% of the total IBPSs is adequate.
Project teams from the Electric Power Research Institute
(EPRI) and Arizona State University, EPRI and Washington
State University, the University of Strathclyde and National
Grid, and MIGRATE have investigated the following GFM
control conditions:
✔ a virtual synchronous machine (VSM) that emulates
the beneficial behavior of an SG
✔ a VSM with zero inertia, which is similar to an SG
without inertia
✔ frequency droop, which recreates the link between load
and generation imbalance and frequency deviation
✔ angle droop, which directly links the load and generation
imbalance to a deviation of the terminal voltage angle

Voltage Magnitude

0
-4
Reactive Power

-8
-12

0

1

2

3

4

5

s
(a)
8

Active Power
Voltage Magnitude

4
pu

most effectively decides how to manage the cost of development and deployment. Another incentive is allowing developers
to access network locations with the lowest SCR that cannot be
securely served by GFL technology.
The key is for manufacturers, grid operators, and policy makers to maintain a dialog on conditions under which
GFM technology is needed and its expected performance.
Manufacturers then can develop new capabilities, balancing performance objectives and costs most efficiently.
Manufacturers also need to closely work with grid operators
and developers to understand how to best leverage the new
capability in different situations, whether it be through
simulation or operational practice. This iterative dialogue is
crucial for optimizing the effectiveness of any additional
technical capabilities and especially for managing the cost
of GFM technology deployment.
There is no one-size-fits-all solution to achieve the desired
grid-support capabilities; there are many ways with a wide
variation in costs. Once the physical needs of a power system with a very high penetration of IBPSs are clear and a
technology-agnostic description of the desired performance
exists, industry has the ability to provide solutions that fit the
need, one of which may be a GFM IBPS.

0
-4
-8
-12

Reactive Power

0

0.5

1

1.5

2

2.5

s
(b)

figure 3. The simulation results for the Great Britain network
in 2030. The response of SG in southeast Scotland for a
marginally stable case with (a) 70% IBPSs penetration versus
a marginally unstable case with (b) 71% IBPSs penetration.
ieee power & energy magazine

95
IEEE Power & Energy Magazine - November/December 2019

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