A Utility Perspective of Wind Energy - Wind Turbine Technology: Fundamental Concepts of Wind Turbine Engineering - page 735

problem through integrated resource planning or by connecting a wind power plant into the
network at a voltage sufficiently high to ensure that any fluctuations are easily absorbed by
the “pool” into which they are fed. Secondly, line loading could become a problem in rural
areas. Individual wind turbines could be connected to single phase lines potentially causing
phase imbalances. Larger wind plants avoid this problem by being connected at substations.
Thirdly, there is the safety issue of controlling small turbines and disconnecting them
in the event that grid connection is lost. Turbine controls are needed to prevent “islanding”
where disbursed wind turbines could continue to operate after a fault caused disconnection to
the grid. Several turbines could possibly self excite without regulating voltage and frequency
control from the grid. These issues are avoided by safety shut-down systems built into the
turbine controls. In addition, utilities can require a mandatory lock out of isolated wind tur-
bines to prevent accidental re-energization of distribution lines that may be under repair.
Any negative effects on the transmission system become more pronounced on a so-called
weak grid , where long lower-voltage distribution lines are used to connect into the higher-
voltage system (33 kV or higher). It is important to carefully consider large concentrations
of wind power in areas where the grid is weak. To avoid this problem, wind turbines are con-
nected to a higher-voltage line. As a very rough guide, it has been suggested that wind power
capacity in megawatts should not exceed the network voltage in kilovolts, but on a weak grid
only a tenth of this capacity may be allowable.
Wind turbines can potentially to provide power to the grid during “black outs” or other
emergency conditions. In the event of major grid disruptions wind turbines working with
other generation, with voltage and frequency regulating capabilities, can help to provide
emergency power during so called “black starts” when central control and full generating
capacity may not be available. Interest in distributed generation, grid security and optional
operating configurations has increased since recent terrorist attacks.
Electric Power Quality
Electric power quality has been precisely defined by the International Electrotechnical
Commission (IEC) and the North American Reliability Council (NARC). These organiza-
tions aim to ensure that frequency and voltage remain within specifications. A key aspect of
this effort is to accommodate the load changes through automatic generation control. This
produces several benefits in power quality. One of these is to insure that any light flicker
caused by voltage changes does not cause undue annoyance to the utility customers. Wind
power plants are able to meet these requirements, with proper interface design.
The IEC and NARC define how often a voltage change of a given magnitude can occur,
as shown by the chart in Figure 13-5 [Ewart et al. 1978]. This graph is read by following the
bold “normal operations” line until it intersects with the desired diagonal line titled “ percent
Excursion,” interpolating as necessary. The abscissa of this intersection, read on the bottom
scale, gives the minimum allowable time for the excursion to take place. For example, a 1 per-
cent excursion is marked by the point A and must not occur more often than every 0.03 minutes.
An excursion of 3 percent -- which would produce a flicker that is quite noticeable -- is marked
B and must not occur more often than once every minute. Although a 1 percent excursion
would not be detectable in lighting, this standard is also designed to ensure that fluctuations do
not upset electronic equipment.
In addition to industry standards, most utilities have local reliability standards. These
specify limits for disconnection times that must not be exceeded. For example, a 99.9 percent
security standard means that a consumer must not be cut off from the electrical supply for
more than 0.1 percent x 8,760 or 8.6 hours per year, on average. To meet these standards,
utilities consider system interaction issues at both regional and local levels. In general, most