1. Introduction
There is much interest in renewable energy due to concern over
the environment, and wind is considered to be one of the most
promising renewable energy sources. One of the reasons is that
wind is an infinite and free source of energy with no harmful waste
products. A wind turbine converts the kinetic energy from the wind
into mechanical energy. It is then converted into electricity, which
is sent to a power grid. There are two basic configurations, vertical
and horizontal-axis wind turbines. This paper is concerned with
horizontal-axis wind turbines, having three blades [1]. The yaw
mechanism, which is responsible for orientating the turbine towards
the wind, is ignored in this paper.
The power converter is one of the most vulnerable components
of a wind turbine. When the converter of an offshore wind turbine
develops a fault, it could be difficult to repair due to accessibility
problems, e.g. as a result of bad weather, etc. Normally, a wind
turbine is equipped with a dedicated full envelope controller that
regulates its operation. In this paper, a collective control approach
that allows a cluster of (5e10) wind turbines to share a single
converter (hence a single controller), which could be located in a
place where it is more accessible away from the turbines, is proposed.
Maintaining a dedicated power converter for an individual
turbine and placing each of them away from the turbines (i.e. for
improved accessibility) would be significantly more expensive, and,
therefore, a single converter is shared between all the turbines in a
cluster. The resulting simplified turbines are constant-speed stallregulated
[2] machines with standard asynchronous generators.
Constant-speed and stall-regulated turbines are known to be more
reliable than variable-speed and pitch-regulated turbines, respectively.
Each cluster is connected by a mini-AC grid (or network),
whose frequency can be varied through a centralised ACeDCeAC
power converter.
A number of clusters with its own dedicated mini-grid would be
linked to constitute an offshore wind farm, which could subsequently
be interconnected with an onshore wind farm through an
appropriate transmission system. Various types of transmission
system can be found in the literature, including the ones that
exploit the high-voltage direct current (HVDC) [3e5], but this topic
is not discussed in this paper.