Network integration of wind power - Wind Power Integration: Connection and System Operational Aspects - page 124

impedance between the wind farm and the fault that allows an SVC to support the
local voltage.

4.6.2.3 DFIG WTG fault ride-through
Until about 2003, DFIGs, like fixed-speed WTGs, were unable to ride through faults
causing the wind farm terminal voltage to fall below about 70% of the nominal
voltage. This is because of the power requirements of the rotor. Effectively the
power electronics self-protected through operation of the 'crowbar' (Figure 3.18)
and the turbine tripped as soon as 20 ms after the fault. The machine was then
incapable of serving load following fault clearance.
The challenge of achieving fault ride-through with DFIGs is to supply reactive
power to the system while protecting the WTG gearbox from mechanical shocks
and the power electronics of the DC link from over-currents. The following strategy
goes some way to achieving these objectives:

the WTG stator is disconnected from the grid on fault detection ●

the WTG pitch angle is adjusted to reduce turbine power to zero ●

the DC link capacitance is used in conjunction with the system-side converter
to provide reactive power - essentially behaving as a STATCOM

●

the turbine speed is adjusted to the pre-fault value ●

when the fault is cleared, the WTG stator is re-connected to the grid ●

In this way significant reactive power and close to zero active power can be
supplied during the fault, helping to support system voltage and thus allowing ride-
through down to 15% of nominal terminal voltage for a short period. There is a
problem reported that the period of depression of active power over such a large
amount of generation taken together with a relatively slow return to active power
delivery (because the turbines are no longer optimally set to extract wind energy)
has the potential to create instability for very high penetrations of wind power. Put
another way, although the wind turbines ride through the fault, the energy gap
during the fault and in the recovery period threatens the system.

4.6.2.4 Fully converted variable-speed WTG fault ride-through
The fully converted variable-speed configuration described in Section 3.6.2 lends
itself to successful fault ride-through. The reduced voltage at the wind farm term-
inals will result in decreased active power export. Excess power will therefore be
generated by the turbine during the fault. However, the DC link capacitor may be
used to absorb this excess power. Meanwhile, the grid-side converter can supply
reactive power to the system during the fault. Essentially the situation is similar to
any DC link, which can provide support to the system under stress while isolating
the adjacent system from the disturbance.

4.6.2.5 Grid codes
Some TSOs claim that, to allow a significant penetration of wind turbines, a fault
ride-through down to 5% or even 0% of nominal voltage is required for a short
period. The closer to the fault, the lower will be the voltage until the fault is