Given the existential threat[1]
of climate change, our electric grid needs to shift away from carbon fuel
resources to a renewable power resource. Traditional hydrocarbon based
generation systems are based on rotational kinetic energy that are synchronized
with the frequency of the power grid. This synchronous provide rotational
inertial energy to the power grid that can withstand small voltage or frequency
fluctuation in the power grid providing a foundational characteristic of a
stable power grid. Replacing these Synchronous generation system with renewable
energy systems such as PV, wind, or storage that are Inverter Based Resources
(IBRs) reduces the rotational inertia in the system thereby reducing the
ability of the power grid to stabilize.
Traditional IBRs are “Grid Following
(GFL) Inverters” depend of the grid frequency to synchronize it output. These
GFL IBRs output current that is
synchronized to grid. On the otherhand the Grid Forming (GFM) Inverters can
make its own voltage waveform which helps to maintain system voltage. GFM IBRs
can provide very fast responses to the disturbances in the power frequency to
help maintain the frequency of the power grid thus providing increased inertia
to the grid. These GFMs can also provide blackstart capabilities to the grid.
As the grid becomes more
saturated with IBRs, there is need to include more GFMs IBRs. A new metric –
“voltage forming ratio”- quantifies the how saturated the power grid with IBRs.
This ratio is calculated by dividing the Output of Inverter Based Resources
(IBR) by Total Generation capacity. There is a liner decrease in stability of
the system with penetration of GFL IBRs, however to keep the grid stable, the
GFL should be a serious concern around 60 percent penetration. These depends on
the system characteristics, types of disturbances, and the location of the
grid. GFM can increase stability of grid in all these scenarios of high
penetration of IBRs.
The GFM IBRs are an emerging
technology that has yet to see mass adoption. While GFM IBRs have demonstrated
their potential they are challenges of standardization before they replace GFL
IBRs. The capabilities and the functionality have not been standardized. The
vendors and manufacturers need to work on the interoperability in order to
increase adoption of GFMs.
To work on this issues of
interoperability, group of industry, government, and researchers have created a
group UNIFI (https://unificonsortium.org) that is seeking to address the challenges of integrating GFM IBRs in
the grid. By developing uniform specifications and technical requirements to
cover GFMs for all IBR applications, it will address the challenges of
interconnection, integration and interoperability of these systems.
The proposed specifications for
GFMs can be divided into two categories- 1) Requirements during Normal Grid
conditions, and 2) Requirements Outside of Normal Conditions. Under normal
conditions, the GFMs can change its output based on the grid conditions and
dispatch energy, but also provide damping to the voltage to stabilize frequency
and thereby increasing the strength of the grid. During operations outside of
normal conditions, the GFMs can provide ride through by injecting current
during and after a voltage sag to aid in voltage recovery. During asymmetrical
faults, the GFMs can maintain a balanced internal voltage. In case of the
abnormal frequency, the GFMs can aid in the frequency recovery and stability.
Other features can include islanding, black start, regulating harmonics and
others.
As these capabilities are
standardized, then it will make it easy for the GFM IBRs to have mass
commercialization thus enabling the transition to 100% renewable energy to
power our grid.
[1] https://www.un.org/sg/en/content/sg/statement/2018-09-10/secretary-generals-remarks-climate-change-delivered
No comments:
Post a Comment