05-05-2011, 03:15 PM
DC System Technologies for Large Scale Integration of Wind Energy Systems with Electricity Grids
Abstract:
The ever increasing development and availability of power electronic systems is the underpinning technology that enables large scale integration of wind generation plants with the electricity grid. As the size and power capacity of the wind turbine continues to increase, so is the need to place these significantly large structures at off-shore locations. DC grids and associated power transmission technologies provide opportunities for cost reduction and electricity grid impact minimization as the bulk power is concentrated at single point of entry. As a result, planning, optimization and impact can be studied and carefully controlled minimizing the risk of the investment as well as power system stability issues. This paper discusses the key technologies associated with DC grids for offshore wind farm applications.
Keywords: wind energy; wind turbine; wind farm layout; High Voltage Direct Current (HVDC) power transmission; circuit breakers; DC cable; interconnection requirement; regulation
1. Introduction
Over the last twenty years, wind energy has become the fastest developing renewable energy technology [1]. The size of wind turbines has increased from a few tens of kWs in the 1980’s to multi-MW size today [1]. With the development of wind turbine and power electronics technologies, large scale wind farms of hundreds of MWs of power are being developed in many countries around the world. More and more of these farms are proposed to be located offshore, due to large space
OPEN ACCESS
Energies 2010, 3 1304
requirements [2]. Compared with the onshore wind farms, the offshore wind farms have access to
significantly better wind energy resources and hence offer larger energy generating capability.
Meanwhile as the power capacity of the offshore wind farms increases, the adequacy of the wind
farm electrical system design becomes critical. The overall purpose of designing the electrical system
is to collect the energy from individual wind turbines, transmit it to the shore and convert it to
appropriate voltage level to enable electricity grid interconnection. Electrical systems layouts aim to
maximize the overall energy generation.
The ever increasing penetration level of wind power with the electricity grid also presents many
challenges to modern power systems. In the past, it was common practice to disconnect a wind farm
from the grid in the event of a network fault. However, as the generation capacity of wind farms has
increased significantly and will continue to do so, the regulation of the technical management of the
grid is necessary. New power electronic technologies must meet the technical requirements so that the
large amounts of electricity generated by the wind can be injected into the grid at varying power level
and from a range of wind farm locations [1].
A large number of wind farms necessitate the development of DC transmission and distribution
technologies. DC technologies offer a number of advantages such as low power losses, no connection
distance limitation, no resonance etc. They also have major disadvantages concerning control and
switching actions. High voltage alternating current (HVAC) subsea transmission schemes maybe used
for offshore wind farms. However due to the limitation of the transmission distances, high voltage
direct current (HVDC) transmission schemes are preferred. As a consequence, subsea power cables
and DC circuit breakers (CBs) are a critical component of an HVDC transmission system used in any
offshore electrical power scheme [3,4]. Considering the interconnection of offshore wind farms,
technologies of DC transmission cables and CBs need to be understood.
This paper is organized as follows. Section 2 provides a discussion of existing offshore wind farm
power generation schemes. Section 3 describes different electrical system configurations for offshore
wind farms. Section 4 summarizes the existing interconnection requirements which mainly include
active power, frequency, reactive power voltage quality and fault ride-through requirement. The
Section also discusses in detail the operation and control methods of large wind farms in case of
electricity grid disturbances and faults. The technical issues associated with DC cables and DC (CBs)
are discussed in Section 5. Finally, conclusions are summarized in Section
2. Offshore Wind Farm Electricity Grid Interconnection
The main stages of large scale grid integrated wind energy systems are illustrated in Figure 1. These
stages include wind turbines, power electronic converters, transmission cables and CBs. The electrical
collector system interconnects the wind turbines in the farm and connects them to the single collection
point. For a small wind plant, the collection point lies in the basement of the tower. For a large wind
plant, the collecting point can be part of an offshore substation or multiple collecting points might
be used
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