Wind Energy Systems - Wind Energy Systems for Electric Power Generation - page 102

Fig. 5.22 Concept of an autonomous island system with renewable energy generation
Fig. 5.22. Two wind energy generators, one with a synchronous machine, the other
with an induction machine, are combined with a solar generating system. Central
part of the system is a management and compensation unit which the authors of
[Sou01] called an “electronic synchronous machine” because of its ability to control
active and reactive power. To bridge short-time peak loads and wind weakness, e.g.
for 10 min, a battery storage is provided which is coupled with the grid via a self-
controled converter. The device is responsible for frequency and voltage control. It
is completed by an active filter to reduce harmonics.
5.4.2 Stand-Alone Systems
5.4.2.1 Small Systems in the kW Range
The market for wind energy systems (WES) with electric output in stand-alone con-
figuration is currently growing. Often a combination with other regenerative sources
such as photovoltaic systems (PVS) is intended. The range of ratings considered is
from below 1 kW up to around 20 kW. Low investment cost is a criterion especially
in this area. Small turbines are normally equipped with fixed blades. Effort and cost
for maintenance should be low, in view of suitability for rural or less developed ar-
eas. Safe operation properties is indispensable, considering operation by unskilled
persons in residential areas.
Most stand-alone systems are for low-voltage ac output. They are mostly equip-
ped with an energy storage device, generally a lead-acid battery of suitable capacity.
The design of small size wind systems should realize:
- low cut-in speed,
- simple speed control to run with best-point power coefficient up to rated power,