IEEE Electrification - March 2022 - 54

with the grid, and a partially scaled power converter connected
to the rotor circuit. The power rating of the converter
is typically near 30% of the overall generator
capacity and is defined by the variable-speed operation.
Type-4 wind turbines use a full-scale power converter,
which acts as an interface between the generator stator
windings and the grid. The Type-4 turbine can be geared or
gearless, as presented in Figure 2, where a permanentmagnet,
low-speed, synchronous generator is coupled
directly with the wind rotor without a gearbox.
Variable-speed Type-5 wind turbines use a fixedspeed
synchronous generator directly connected to the
grid and can be divided into two categories, depending
on the method used for torque conversion to achieve
constant-speed operation. A Type-Va topology using a
full-capacity-rated hydrodynamic transmission system
located between the turbine gearbox and the generator
is shown in Figure 3. This design has been used in recent
decades, but it did not achieve successful commercialization.
Any transmission failure in this design because
of torsional stresses during grid faults could cause considerable
expense. On the other hand, a powertrain solution
that embodies a hydrostatic torque reaction system
built into the turbine gearbox itself can provide considerably
lower capital and maintenance costs (Type-Vb
topology). Such a system can be rated at a small fraction
of the overall turbine capacity (5%), and it inherently
protects the main drivetrain from torsional stresses by
diverting a small amount of power into a parallel
mechanical path. This topology is depicted in Figure 4. If
a mechanical failure occurs in that subsystem, it is an
inexpensive item to replace.
As mentioned previously, Types 3 and 4 are the two
most used turbine topologies today, with many manufacturers
in different countries mass producing these multimegawatt
wind turbines for grid-scale operation. Grid
impacts and the performance of such machines in GFL
mode are well understood, and in many cases, the performance
is standardized through national and international
grid codes and standards. In contrast, for GFM operation
for Types 3 and 4, there is a knowledge gap on how to control
and operate GFM wind turbines, how to account for
this new mode of operation in the turbine design stage,
and understanding the stability, reliability, and resilience
benefits. For the Type-5 wind turbine topology in particular,
a holistic evaluation of this technology in the context
of larger power systems that shows the overall benefits
and revives industry interest is long awaited.
Variable Speed
Fixed Speed
Conventional
Synchronous
Generator
Gearbox
With Rotating
Exciter
Grid
Hydrodynamic
Transmission
Figure 3. A Type-Va wind turbine with full-capacity hydrodynamic
transmission.
Variable Speed
Torque Limiter
Pump
Fixed Speed
Grid
Gearbox With
Hydrostatic
Torque Reaction
System
Conventional
Synchronous
Generator
With Rotating
Exciter
Figure 4. A Type-Vb wind turbine with a torque limiter and recently
developed low-variable-speed system.
54
IEEE Electrification Magazine / MARCH 2022
What Does It Take for a Wind Turbine
to Become Grid Forming?
The general answer is not much, at least from an electrical
design viewpoint. Conversion to GFM operation is essentially
a control software upgrade with no need for new electrical
hardware components of the drivetrain and the power
converter in Type-3 and Type-5 wind turbines. In some
cases, however, certain modifications are needed, depending
on the design characteristics of a given turbine. The
National Renewable Energy Laboratory (NREL) has been
testing a multimegawatt Type-3 wind turbine generator in
GFM mode during 2021 with controls developed by GE. This
turbine uses the same components as those for GFL operation,
with controls redesigned to operate on programmable
f-P and V-Q droops. Since 2019, Siemens-Gamesa has been
successfully demonstrating GFM operation of the 69-MW
Type-4 Dersalloch wind power plant in Scotland. This
power plant demonstrated stable performance in GFM
mode during weeks of operation, with controls to emulate
various levels of inertia constant H.
In a GFL operation, the wind turbine converter controls
the level of injected current depending on the active and
reactive power set points with a specific phase-angle difference
from the voltage at the point of common coupling
(PCC) interconnection; therefore, to inject the desired levels
of power, the turbine controller needs to calculate the
reference current, which, in turn, requires knowledge of
the grid voltage's fundamental phasor. For this purpose, a
PLL is used to measure the phase angle of the grid voltage
at the point of interconnection. Using additional outer
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