Virtual synchronous machine control for doubly-fed induction machine
based wind energy conversion systems [long version]
Abstract
Facing the climate crisis, more and more renewable inverter-based
resources (IBRs), such as wind energy conversion systems (WSs), replace
grid-connected synchronous machines (SMs). SMs inherently provide
balanced and sinusoidal grid voltages but also power system inertia,
whereas standard grid-following (GFL) control of IBRs leads to
decreasing power system inertia. This paper proposes grid-forming (GFM)
control for doubly-fed induction machine (DFIM) based wind energy
conversion systems (WSs) based on the virtual synchronous machine (VSM)
concept with the following extensions: (i) Maximum power power tracking
(MPPT) compensation for accurate inertia emulation, (ii) feedforward
torque control (FTC) for fast power reference tracking, (iii) de-rated
operation or reference power point tracking (RPPT) to provide additional
power reserves, (iv) dynamic droop power saturation control to avoid
excessive power overshoots (as available power reserves are taken into
account), and (v) grid voltage control utilizing reactive power from
both, DFIM stator and rotor-side back-to-back inverter. The modeling and
control of the DFIM-based WSs is shown in detail and the grid model is
based on the IEEE 9-bus test system. Comprehensive simulation results
show that the GFM control enables unlimited power penetration of IBRs
and that the grid frequency dynamics of VSM-based power systems can be
interpreted and handled analogously to SM-based power systems. During
normal operation, compared to existing VSM control without FTC, the
proposed VSM control with FTC increases the wind energy yield, i. e.
typical MPPT or RPPT performance is achieved, similar to the performance
of GFL control. For high power penetration of IBRs, the proposed VSM
control enables stable operation due to its inertia emulation or GFM
capability, whereas GFL control tends to instability. During faults and
system splits, the VSM provides higher power system damping than a real
SM due to (i) virtual/internal damping that can be adapted by software
(hence almost independently from the hardware, whereas the response of
SM damper windings depends on the physical machine design and cannot be
altered during operation), and due to (ii) faster droop control which
adapts the virtual turbine power without the mechanical delays that
dominate real turbine dynamics.