ABB ACS800-67, System Description And Start-Up Manual

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32 System description
Grid fault ride-trough capability
Although the advantage of doubly-fed concept is that the size of the wind turbine converter
is significantly smaller than full-power converter, the drawback is that the rotor-side
converter is a vulnerable part of the system. It has a restricted overcurrent limit and it
needs special attention especially during faults in the grid. When faults occur and cause
eg, voltage dips or sags, the magnetic flux in the generator can not change
instantaneously. As a result sudden change in the stator, supply voltage is followed by a
large change in the generator currents. The converter responds to the change of rotor
currents so that the rotor currents are maintained as required by the rotor-side converter
control.
Since the output voltages and currents of the rotor-side converter are limited, the rotor-
side converter may not be able to maintain the rotor currents within given limits during
severe grid faults and thus it must be protected. Wind turbine converter can be equipped
with hardware-based protection device, a crowbar. The crowbar is used to protect the
converter in case of unexpected electrical power network failure. There are two types of
crowbars, a passive crowbar that does not allow the grid fault ride-through function, and an
active crowbar that allows to operate through pre-determined electrical power network
failure without tripping (grid fault ride-through operation).
The crowbar consists of the crowbar unit and a high power resistor. The active crowbar is
controlled by the rotor-side converter control firmware, and in case of failure, it can protect
the converter independently. The crowbar is triggered if DC link voltage is too high or
alternatively if the rotor current is too high.
Grid codes typically require that the wind turbine must remain connected to the grid under
different kinds of grid failure events (eg, voltage dip/sag, short interruption, swell etc). It is
very common that the wind turbine
•
has to stay connected to the power system for a certain period
•
may not take power from the power system
•
produces capacitive reactive current.
Wind turbine converter can be equipped with a DC chopper for DC link power dissipation.
The DC chopper may be needed if grid fault ride-through or high swell threshold is
required. The DC chopper is connected to the DC link and it operates independently
always when DC link voltage rises above its triggering level. A diagram of the wind turbine
system with the DC chopper is shown below.