Page 295 - Alternative Energy Systems in Building Design

WIND POWER MANAGEMENT 269

must be overcome. The following are a number of measures used to regulate and
stabilize wind energy production.

GRID COMPATIBILITY

In general, wind turbine generators designed for electrical power production require
reactive power for excitation. However, the reactive power shifts current and voltage
phase angles, which results in a loss of power production. The cosine of the shifted
angle, which is referred to as the power factor p, is a multiplier that varies from 0 to 1.
Electrical power output from generators is expressed as E = V (voltage) × I (current) ×
cos(p). It is at a maximum when p = 0 because cos(0) = 1, which make E = V × I. Thus,
at the zero angle [cos(0)], the unity power factor is achieved. In order to reach the unity
power factor, the output of the generators is connected to large banks of capacitors that
reduce the phase angle to achieve this goal. Since various types of wind turbine genera-
tors have unique power-output performance profiles, they could create transmission grid
disturbances. In order to resolve this problem, manufacturers of wind turbines make use
of extensive modeling of dynamic electromechanical characteristics with each wind
farm. The modeling allows transmission-system operators to control the power output of
the generators, ensuring predictable and stable power-output performance.
Unlike steam or hydroelectric power turbine-driven synchronous generators, wind
turbines incorporate power-factor-correction capacitors along with electronic control
of circuitry, which stabilizes power-output resonance. Wind turbines, referred to as
doubly fed machines, deploy solid-state converters between the turbine generator and
the collector system, making them suitable for grid interconnection. As a rule, grid
transmission providers’ supply wind farm developers with a specific grid code that
specifies power factor, frequency stability, and dynamic characteristics of the wind
farm turbines during a system fault.

CAPACITY FACTOR

In view of the fact that wind speed is never constant, a wind farm’s annual energy
production cannot match the generator’s expected yearly nameplate ratings output. As
a result, a multiplier known as the capacity factor is used to adjust the total annual
hourly power production. The multiplier is the ratio of actual power productivity in a
year and its theoretical maximum.
Typical capacity factors of wind turbines range from 20 to 40 percent. Upper values
represent maximum power production for most favorable installation sites. As an
example, a 2-MW turbine with a capacity factor of 30 percent will produce only
0.30 × 2 × 24 × 365 = 5256 MW per year as opposed to 17,520 MW.
The capacity factor essentially accounts for power-production limits that are inherent
properties of wind. Capacity factors for other types of power plants, such as gas and
hydroelectric power-generation systems, reflect the amount of downtime required for
maintenance. For example, most nuclear plants that run full time at maximum output
capacity have a capacity factor of 90–95 percent.