Network integration of wind power - Wind Power Integration: Connection and System Operational Aspects - page 135

4.9.4 Wind farm protection
Most wind farm protection is a developer's internal matter. However, a prudent
network operator will seek assurance that the protection design, installation, com-
missioning and maintenance are such that the grid is not threatened. One issue
remains unresolved.
Embedded generation was protected traditionally by some form of islanding
detection. In the UK codes of practice, referred to as G59 or G75, were applied. In
essence a view was taken that embedded generation should not remain connected to
customer load in the absence of connection to the main grid - a condition known as
'loss of mains'. Disconnection from the grid was detected fundamentally by Rate of
Change of Frequency (RoCoF) or Rate of Change of Angle (RoCoA) relay elements
(Jenkins et al. , 2000). A standard G59 relay emerged which embodied RoCoF
or RoCoA and over-voltage, under-voltage, over-frequency, under-frequency and
other features. RoCoF tends to be set at about 0.5 Hz/s. The difficulty is that a
severe disturbance, especially on a smaller system, may cause RoCoF relays to
operate, hence removing all embedded generation. This was not a problem with low
levels of penetration. However, as penetration grows, there is the potential for loss
of a major system infeed to be accompanied by a further loss of generation due to
mal-operation of loss of mains protection on embedded plant. The emerging grid
codes require wind farms to respond in a manner similar to modern traditional
generation, controlling voltage and frequency. If RoCoF or RoCoA cannot be
allowed to trip the wind farm, and the other elements have to be set wide enough to
allow a satisfactory operational range, there is no obvious way of detecting
islanding. Some countries have sought to monitor the immediate feeder circuits
using unit protection, but this fails to detect a more remote cause of islanding. The
implications for system operation of spurious embedded generation tripping will be
discussed further in Chapter 5.
A possible philosophy is that, even though the absolute output of a wind farm
is unpredictable, if it is controlling frequency and voltage, it may be re-considered
as safe for operation with customers. When the customer load passes outside the
range for which it has controllability, it will trip in any case, thus self-islanding. If
an emergency trip needs a remote re-set, the wind farm will remain islanded. The
more difficult issue is network re-closure. If this is automatic, a re-closure may
result in a mal-synchronisation shock to the wind farm and a dramatic transient
over-flux in transformers. It would seem that unless network connectivity and wind
farm status can be incorporated into a real-time model controlling reclosure, the
problem remains unsolved.
Another approach is to continue to trip the wind farms but to use a different
principle. Research is ongoing to determine whether a measured change in network
harmonics would give rise to a way forward. It has also been suggested that tripping
the capacitors associated with induction turbines would allow discrimination
between dynamic events and loss of grid (O'Kane et al. , 1999).