Wind Power Generation - Renewable Energy: Technology, Economics and Environment - page 334

for isolated operation facilitating the control of current, voltage and reactive
power. From the current viewpoint, high voltage direct current transmission re-
sults cost-efficient from a distance of approximately 60 km onwards.

7.2.6 Grid-independent applications

The majority of all individual wind turbines and wind parks are connected to the
power grid. However, there is also a multitude of grid-independent and wind-
diesel systems designed for small power grids which are outlined throughout the
following sections.

Wind-battery systems. In most cases, grid-independent wind battery systems are
small capacity wind turbines which, coupled with a battery and a battery charge
regulator, supply one or several power consumers. The nominal capacities of the
majority of these systems stretch from a few 100 W up to several 100 kW. Par-
ticularly in developing countries, where sites often offer both sufficient solar ra-
diation and above-average wind conditions, the described wind-battery systems
may additionally be combined with one or several photovoltaic modules (wind-
battery-photovoltaic systems).
Wind-battery systems are generally provided with smaller wind energy con-
verters than those outlined in the preceding sections. Their nominal capacities
usually amount to several kilowatt, wind turbines of rotor diameters up to 6 m
with nominal capacities ranging from 10 to 20 kW are referred to as small wind
energy converters /7-10/; however, this term is also applied for converters with
rotor diameters of approximately 16 m and nominal capacities of up to 100 kW
/7-5/. These small converters encompass fast converters provided with 2 to 3 rotor
blades as well as slow converters of numerous rotor blades. Within this range
there are also more converters equipped with vertical axes (Savonius type or H
rotor).
The following framework conditions should be ensured for technically and
economically sensible application /7-11/.
• The wind speed should exceed 6 m/s at least on 200 days per year.

• Maximum wind lull period must not exceed 48 h.

• If possible, the lower edge of the rotor should be positioned at least 10 m

above the highest obstacle in the surrounding area.

• Secure starting torque at about 4.5 m/s (i.e. smooth running gearbox or no

gearbox, special electronic starting control).

• Generator capacity must not exceed 1.5 times the maximum supplied capacity.

• Noise emissions must not exceed 60 dB at a distance of 20 m from residential

areas and 45 dB for installation on vessels.

• There should be no considerable vibration resonance from idle mode up to the

operating point of the converter, tower and guying, if provided.