Network integration of wind power - Wind Power Integration: Connection and System Operational Aspects - page 123

Generators far removed from the fault will see an increase in load and will tend
to slow down. The objective of protection is to remove the fault quickly so as to
prevent pole slipping between sets. Unless the fault condition is immediately
removed the generators will suffer angular instability and lose synchronism with
each other. The system will be split into separate blocks. Assuming that the fault is
rapidly cleared, all generation should be present to continue to serve load.
A fault on a super-grid system can depress the voltage over a wide area of the
network, hence creating widespread outage of WTGs. Studies in the Republic of
Ireland have shown that a severe transmission fault creates a deep voltage dis-
turbance over about one quarter of the island. Traditional synchronous generators
deliver large quantities of reactive power as fault current, essentially because the
voltage regulator attempts to restore the collapsed terminal voltage through an
increase in excitation current (see Chapter 2). Prior to about 2003, wind generators
were unable to mimic this behaviour, but the situation has changed.
The behaviour of fixed-speed and variable-speed WTGs during grid faults is
considered below.
4.6.2.1 Fixed-speed WTG fault ride-through
This type of plant absorbs more reactive power as the voltage falls: during the fault,
the depressed system voltage causes the electromagnetic torque to collapse, as may
be seen from (3.13). The WTG accelerates, slip increases and the induction
machine draws current from the system. Hence, instead of contributing to the
reactive power consumed by the fault, which helps the protection to clear the
fault quickly, the plant does the opposite, drawing reactive power from the system
and further depressing the voltage. The induction generator accelerates and
will probably have increased in speed to greater than its breakout torque position
(see Figure 3.14) when the fault is cleared. It may then be unable to supply active
power but will continue drawing reactive power. It will eventually trip from over-
speed or over-current protection. That was considered a satisfactory outcome in the
early days of wind power development: the priority was to protect the plant rather
than to support the system.
4.6.2.2 Mitigation measures
As wind generation became a significant proportion of supply, it was necessary to
require that wind farms should support the system during faults. Various flexible
AC transmission systems (FACTS) devices that can provide suitable solutions are
described in Appendix 1. A commercial device manufactured for this purpose
(DVAr) will produce three times its rated regulating output for 1 s. SVCs or
STATCOMs are also possible solutions. By maintaining nominal voltage at the
wind farm for the duration of the fault, these devices prevent the turbines drawing
massive amounts of reactive power and tripping. This leaves them online and
available to participate in the post-fault recovery. Clearly a fault close to the wind
farm will absorb MVAr from an SVC and the wind farm will trip, but this is to be
expected and other wind farms remote from the fault must cope. It is the network