POWER February 2020 - 41

PLANT DESIGN
2. Voltage curve at the network connection
point with fault ride-through. Courtesy: Voith
Turbo
ers the hydraulically pressurized slipping
torque limiter, which also transmits torque
from the shaft to the hub with a frictional
connection via a press fit (Figure 1). However,
the press fit in this case is produced
hydraulically using pressurized oil. The
selected applied oil pressure makes it
possible for the two coupling halves to
slip when the set torque is exceeded but
not release until the slip angle reaches between
30 degrees and 120 degrees.
System Modeling
The calculation of the transient torques
at the shafts of turbine-generator sets
requires consideration of the overall system
for electrical energy infeed. Essential
system levels in this consideration
are power grid, disturbances on the grid,
transformer, grid connection, and synchronous
generator. Each system level
is described by a comprehensive system
of differential equations. The experts involved
intervene to some extent in the
power plant structure on a system-specific
and location-specific basis.
Depending on the fault event considered,
different sensitivity must be acShaft
line design
Typical design
limit
Limiting
element
Tripping of the
torque limiter
Additional
requirement due
to FRT
Design process -
step 1
Design process -
step 2
Conclusion of the
design process
Hydraulically pressurized
slipping torque
limiting coupling
3.0-4.0 p.u.
Shaft coupling
1.5-3.0 p.u.
FRT inherently
possible
System calculation for
optimizing of the limiter
Calculation for faulty
synchronization and
short circuits
Adaptation of the limiter
according to steps 1
and 2 (such as 1.5 or
2.0 p.u.)
3. Maximum torque load depending on the coupling type. Courtesy: Voith Turbo
4. Load on the shaft line components in the event of malsynchronization. Courtesy: Voith Turbo
5. Load on the shaft line components in the event of a two-phase terminal short circuit from no
load. Courtesy: Voith Turbo
corded to different levels. The shaft line/
generator level is deterministic for the
torque loads. System-specific data for the
respective power plant is absolutely necShear-pin
torque limiting
coupling
3.0-4.0 p.u.
Shearing element
2.0-3.0 p.u.
Delayed tripping,
Increased shaft line
load
Determining the limiter
by system calculation
with FRT
Calculation for faulty
synchronization and
short circuits
Adaptation of the shaft
line components for
elevated limits (such as
3.5-4.5 p.u.)
N/A
N/A
FRT does not
determine the design
N/A
Calculation for faulty
synchronization and
short circuits
No further measures
for the FRT
requirement
Table 1. Design process taking fault ride-through (FRT) into account with torque limits per-unit
(p.u.). Courtesy: Voith Turbo
February 2020 | POWER
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Rigid disc-pack
coupling
6.0-11.0 p.u.
essary. The same applies to the grid connection
conditions and the transformer. A
comparable precision in control is necessary
in selected studies; often, standard
controller structures are sufficient.
Network Faults and Other
Malfunctions
As was previously mentioned, due to
the EU regulation, there is now an FRT
requirement for conventional power
plants in the industrial power range. The
requirement indicates that a short circuit
in the vicinity of the power plant cannot
lead to disconnection of the power plant
from the system, referred to as fault ridethrough.
In the worst case, this results
in a voltage curve at the network infeed
point corresponding to Figure 2.
In terms of the load on the mechanical
shaft line, this fault case corresponds
approximately to a three-phase terminal
short circuit with subsequent successful
grid-reclosing. The fault ride-through is
therefore not a mechanical worst-case
situation for the shaft line. For a shaft line
design philosophy that operates with a
torque limiter, such as a shear-pin cou41

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POWER February 2020

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