POWER February 2020 - 40

PLANT DESIGN
Fault Ride-Through Using
Slip-Capable Torque-Limiting
Couplings
As more intermittent renewable energy resources are added to the power grid,
generator connectivity during a grid fault becomes ever more important to mitigate
the risk of cascading blackouts. An innovative coupling design may help
plants safely achieve fault ride-through requirements.
Todd Lehman and
David Bergström
M
ost people are aware that renewable
energy resources are not
a consistent source of power.
Wind and photovoltaic systems, in particular,
cannot be counted on to respond
to an event that triggers demand within
the grid. Potential issues can be stabilized
in several ways including synchronous
condensers, static VAR compensators,
and battery storage, but there will
continue to be a need for base spinning
reserve sources of electrical production
to maintain grid power and stability.
Grid instability in the U.S. is not yet a
common issue because the U.S. baseload
generation model is still centralized
and, at a generating level, that provides
protection from rolling blackouts or cascading
power failures. However, the
country's
electrical power
generation
model is changing with regulatory and
economic drivers moving generation
more toward decentralization with the
constant closure of baseload coal, nuclear,
and natural gas power generating
stations. As electrical power generation
decentralization
continues, more and
more microgrids develop, and they lack
the spinning reserve of energy to cover
times of high power demand. Therefore,
it becomes more important to maintain
the connectivity of all generation sources
without interruption.
In Europe, the process of electrical
grid decentralization has progressed to
the point that generator connectivity to
the grid must be maintained during a
grid fault for a mandated period of time
in order to mitigate the risk of cascading
blackouts. This threat to the European
power grid has been addressed by Commission
Regulation (EU) 2016/631. Even
though this regulation substantially targets
wind turbines and photovoltaic sys40
tems,
it still has considerable influence
on conventional power plant turbinegenerator
sets. As a result, turbine-generator
systems are required to maintain
connection through low-voltage events,
short circuit conditions, and breaker
reclosures. This grid fault ride-through
(FRT) requirement can create serious
shaft line mechanical issues for the components
of a turbine-generator. These issues
occur because of the torques that
are developed by the generator during a
power system grid fault situation.
Previously, disconnection of the turbine
from the generator during a grid or malsynchronization
event was allowed to occur
via a shaft line torque-limiting device
that would release as soon as the set
torque was exceeded. The torque limiter
release allowed for the separation of the
rotating masses of the turbine and generator,
protecting them and allowing them
to shut down safely. With the resetting of
the torque limiter and the restarting of the
turbine-generator system, power generation
could once more take place.
This release of the torque limiter to
protect the shaft line is no longer allowed
so that blackouts caused by chain reaction
events can be avoided. The function
of the torque limiter must now be to
maintain the mechanical connection of
the turbine to the generator but slip to
trim off the torque peaks. These peaks
are generated during the mandated time
of sustained connection and remain until
the breakers are allowed to react. Then,
when the breakers re-close while the
generator is still operating, the torque
limiter will have to provide the same
torque limitation. It will slip to accommodate
any malsynchronization between
the grid and the operating phase angle of
the generator.
Present-day turbine-generator sets are
designed using three essential philosowww.powermag.com
phies,
and the philosophy implemented in
each case is reflected in the shaft line coupling
design. The system, consisting of the
electrical power system, the system fault,
electrical machine, shaft line, and controller,
has been modeled as a whole, and the
transient torque along the shaft line was
calculated. The influence of each coupling
type within the system has been analyzed,
taking into account the current EU regulation
and the shaft line coupling design.
Coupling Types in Turbine-Generator
Sets
Three different shaft line coupling designs
will be considered in relation to the
new EU regulation.
The first shaft line coupling design
considers a rigid, sustainable version
that uses a disc-pack connection coupling
where torque is transmitted from
the shaft to the hub via a press fit. The
connection coupling is selected to withstand
infrequently occurring faults, such
as gen-set malsynchronization or short
circuit events.
The second shaft line design considcoupling
ers
a shear-pin
design. The
shear pins break above the set torque,
and torque transmission is no longer
possible. Once the shear pins are replaced,
torque can again be transmitted
and power generated.
The third
shaft
line design consid1.
Structure of a hydraulically pressurized slipping
torque limiter able to continue transmitting
torque in slip mode. Courtesy: Voith Turbo
POWER | February 2020
http://www.powermag.com
POWER February 2020

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