Page 165 - Renewable Energy Devices and System with Simulations in MATLAB and ANSYS

152 Renewable Energy Devices and Systems with Simulations in MATLAB and ANSYS ®
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distribution control are necessary to ensure a power balance. The wind energy conversion has some
simple foundations—the kinetic energy of the wind is converted into a mechanical shaft movement
and then into electrical energy through a wind turbine coupled to an electrical generator. However,
wind energy has some unavoidable constraints. Its real performance is affected by local conditions
and random nature of the wind, nearby physical obstructions, power demand profiles, and several
turbines-related factors, in addition to a possible deterioration of the performance due to aging.
A small wind energy system may be the major energy source for residential or commercial appli-
cations, or it can be part of a microgrid. All controlled sources and loads are interconnected in
a manner that enables the devices to function as dispatch centers, and noncritical loads might be
curtailed or shed during the times of energy shortfall or possible high costs of energy production.
If such a wind energy system is connected to the public distributor, it can serve as a backup system
and as a noninterruptible power supply (with storage aggregation), provides low-voltage support, or
gives surplus of energy transferred to the public network at an economical price. In order to establish
a wind-based grid-connected system, it is important to observe the following:

• Supply energy requirements of the present and future loads.
• Establish civil liabilities in case of accidents and financial losses due to shortage or low
quality of energy.
• Negotiate collective conditions to interconnect the microgrid with the public network or
with other sources of energy that are independent of wind resources.
• Establish performance criteria of power quality and reliability to reduce costs and guaran-
tee an acceptable energy supply.
Good quality wind energy systems may require extensive data processing of the electrical net-
work characteristics and analysis of how the power quality impacts the overall plant performance.
The required data for assessing real-time performance of wind turbines are limited to three real-
time variables: (1) the output power of the turbine (W), (2) the rotational speed of the turbine (rad/s),
and (3) the wind speed (m/s). Data analysis would require instrumentation to obtain four parameters:
(1) generator voltage, (2) load current, (3) distribution of the wind speed, and (4) wind turbine speed
for every machine in the field. Chapters 8 through 10 cover the details for such design.

7.2 GENERATOR SELECTION FOR SMALL-SCALE
WIND ENERGY SYSTEMS
Criteria must be established for selecting the electrical generators for small wind energy power
plants. The level of active and reactive powers for a particular application is dependent on the
variable-speed features of the generator. This analysis supports a wide set of other variables such as
voltage tolerance, frequency, speed, output power, slip factor, required source of reactive power, and
field excitation, among other parameters. In applications with variable speed, a DC link is usually
used between the generator and the load and may decouple the generator operation from the grid.
There are other factors to consider when selecting a generator for an AC system, including capacity
of the system, types of loads, availability of spare parts, voltage regulation, and cost. If several loads
are likely inductive, such as phase-controlled converters, motors, and fluorescent lights, a synchro-
nous generator could be a better choice than an induction generator (IG) for larger power applications.
IGs cannot supply on their own a high start-up surge current required for starting other motor loads
when operating in stand-alone mode. Therefore, selecting and sizing the generator is a highly techni-
cal decision and viability studies should be conducted, particularly using a power systems or power
electronics software simulator. Figure 7.1 shows how to convert the low-speed (typically 10–30 rota-
tions per minute [rpm]) high-torque turbine shaft power to electrical power. A gearbox is often used in
small wind turbines, whereas multipole generator systems are custom made for multi-megawatt solu-
tions. Between the grid and the generator, a power converter might be inserted in order to attain higher
flexibility. The most typical machines to convert the turbine shaft mechanical power into electrical