IEEE Electrification - March 2022 - 63

Type-5 Promise and Challenges
At first glance (see Figure 16), Type-5
wind power can meet all the stability
challenges in a conceptually new (but
old, from a physics standpoint) way by
keeping the grid largely synchronous
while being able to integrate high
shares of variable generation. If this
sounds too good to be true, it probably
is. There are challenges, of course.
One main challenge of Type-5 turbines
is pole slipping, which can happen
between the synchronous generator and the grid,
resulting in the flow of synchronizing power, which reverses
twice every slip cycle. Pole slipping can occur after faults
that cause a prolonged loss-of-load or loss-of-generation
excitation. The pulsating torques produced during pole slipping
can expose the shaft to excessive oscillatory shocks. If
it is not addressed properly, a pole slip can result in serious
damage to both the generator and the gearbox. If pole slipping
is detected, the generator must be disconnected from
the grid as soon as possible.
In large conventional power plants interconnected
with the power transmission system, the probability of a
pole slip is low because there are many levels of protection
against it. In particular, transmission-level protection
systems ensure fault durations of less than approximately
0.1 s, which prevents generator rotor-angle excursions of a
pole-slip magnitude. With larger numbers of smaller distributed
synchronous generators, a potentially damaging
The pulsating
torques produced
during pole slipping
can expose the shaft
to excessive
oscillatory shocks.
situation with pole slipping can be
avoided if proper countermeasures
are implemented. In a Type-5 wind
turbine, a prolonged voltage fault
could cause significant rotor-angle
excursions due to imbalance between
the mechanical and electromagnetic
torques driving the machine out of
synchronism. One example of such
an event was recorded on the Orkney
Islands in Scotland because of a 0.3-s
voltage fault on a 33-kV network. A
Type-5 Windflow 500-kW wind turbine operating along
with total of 10 MW of power-converter-based wind turbines
was exposed to high-frequency voltage transients,
causing a pole slip in the generator. The following protective
measures can be employed in Type-5 turbines to prevent
this from occurring:
x The use of TLG technology to protect the gearbox and
the generator from the aforementioned torque transients
(noting that in the aforesaid instance, the
torque limiter behaved as a sacrificial element, which
was appropriate given that it is significantly less
expensive than the gearbox or generator)
x Improved gearbox designs
x The use of blade pitch control and/or generator frequency
sensing to reduce the mechanical torque during
low-voltage events
x If a pole slip happens, the use of pole-slip protection
in the form of admittance relays to disconnect the
Type-5 Plant
Large Synchronous
Generator
SG
Grid
WindSG = Electrical Equivalent Substitute to a Large Conventional Generator
* Variable-Speed Operation Without Power Electronics and With
No Impact on Grid Stability or Protection
* Same Grid Strength
* Same Synchronizing Torque
* Same Grid Services
* Real Inertia
* Same Transient Performance
* Same Grid Forming
* Same Black Start
* Same Voltage
Figure 16. A Type-5 promise.
IEEE Electrification Magazine / MARCH 2022
63
Grid
Variable
Speed
Fixed
Speed
IEEE Electrification - March 2022

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