Environmental Engineering Reference
In-Depth Information
Both techniques rely on there being a signifi cant change in power fl ow within the island when
the mains is lost. This will naturally cause (a) the generator to accelerate or decelerate
(detected by ROCOF) and (b) a change in the generator power factor (detected by vector
shift). Unfortunately, there is no guarantee that there will be a signifi cant change in power
fl ow and so neither technique proves to be perfect in practice.
6.5.3 Loss - of - mains Protection for Inverters
An inverter that is intended for grid connection will not normally be designed to island. It
will be designed to convert all the power available at a given time from the generating source
(PV for example). It will not be designed to supply a constant AC voltage to a variable load.
Islanding is only possible if the load demand exactly matches the source supply, a very
unlikely scenario. Nonetheless, the designs of such inverters usually include explicit loss-of-
mains protection, indeed connection standards such at the UK's Engineering Recommenda-
tion G83/1 [2] can require this.
6.6 Fault Ride - through
Prior to about 2001, all wind turbines were required to shut down immediately on the occur-
rence of any disturbance on the grid. This was primarily to reduce any risk of islanding. It
was a sensible requirement in systems with very low wind power penetrations. Wind turbines
were fi tted with loss-of-mains protection relays triggered by any of undervoltage, overvoltage,
underfrequency, overfrequency, rate of change of frequency (ROCOF) and vector shift
incidents.
If such a protection relay is set to trip reliably on a genuine loss-of-mains, then it will also
be prone to nuisance tripping when there is merely a transient disturbance on the mains. This
raises the possibility of
block tripping
where a widespread transient disturbance across the
network could cause a large number of loss-of-mains relays to trip simultaneously. Block trip-
ping is highly undesirable in the context of controlling the voltage and frequency of the overall
power system. The concern is that, following a disturbance on the grid, such as the tripping of
a large generator, a substantial block of wind power would also trip off, which would aggravate
the shortage of generation, making matters considerably worse for frequency control. Because
of this, new regulations are now being drafted and introduced, particularly for very large wind
farms that are to be connected to the transmission network. These regulations will require that
wind turbines do not trip unnecessarily, but rather that they will ride through transient distur-
bances and to stay connected and resume generation as soon at the fault is cleared.
The fi rst requirement would be for wind turbines not to be tripped off the grid during local
faults that result in PCC voltage reductions, as is the practice at present. Wind turbines fi tted
with controllers capable of such behaviour have been tested and shown to be capable of riding
through a fault and re-establishing full power immediately after the end of the fault. It is
expected that increasingly sophisticated control functions are likely to be demanded from
wind and other renewable energy generators as penetrations increase. Even with small but
numerous embedded renewable generators in the future, it may eventually become important
to coordinate their response both to steady state and transient network conditions. At present
no such arrangements are in place.
