Commercial Wind Turbine Systems and Applications - Wind Turbine Technology: Fundamental Concepts of Wind Turbine Engineering - page 253

Wind system sizing and integration are more of a problem on small grids than on region-
al networks where power quality is negligibly affected by even large numbers of turbines.
On a small grid, the degree of penetration often varies seasonally as a function of both the
wind resource and the electrical demand. Wind turbines sized for modest penetration dur-
ing peak summer loads will have a high penetration during the off season. This may require
dumping of excess power that might otherwise disturb grid stability, or it may require the use
of a fully controllable wind turbine, or the shutting down of other generators.
Limitations on Diesel Fuel Savings
One factor that affects maximum wind penetration on diesel-powered grids is how much
the output of the diesel generators can be reduced safely and economically [Oei 986, Qi
993]. Diesel engines operate inefficiently at partial load, and their operating and mainte-
nance costs may increase when they do not run at nearly full capacity. This inefficiency is
caused by incomplete combustion, engine friction, generator excitation current losses, and
losses in pumps, fans, and other auxiliary equipment. Partial load, suboptimal operation of
diesels can cause excess smoke and result in carbon buildup in the exhaust system and stack
fires. Therefore, fuel savings from reduced demand on a diesel generator are limited because
of the machine's reduced performance under partial load.
Ideally, there should be a method to restrict partial-load operations, or to operate the grid
with one or more diesels shut down but left in quick-start standby mode with oil heaters, to
provide quick response to load changes while maximizing fuel savings and economic benefit
to the system. Having different size diesels, an operator can dispatch the appropriate size en-
gine to match the expected load and available wind energy. Operators should avoid frequent
and damaging stop-start cycles of the engine as wind speed varies.
Grid Load Management
Load management is important in the successful integration of wind turbines into a small
grid. Because power is expensive and often in short supply on small grids, users (unlike their
counterparts on large grids) are accustomed to limiting their consumption to essential needs.
Nonessential loads are either not met or delayed until the time of day when rates are lower.
The effectiveness of wind power can be increased by regulating the grid load (a so-called
smart grid ) to reduce the degree of wind penetration. If nonessential loads are switched on
as wind generation increases, the penetration will remain relatively constant, and it will be
easier to maintain power quality. For example, the dump circuit illustrated in Figure 4-3
could direct excess wind power into a lower-priority heating load or dissipate it through a re-
sistance load bank. With this load-management approach, fixed costs of operating the entire
grid do not change, but fuel costs decrease as more wind energy becomes available.
Load management can be as simple as an on-off switch. On Caribbean islands it is
common to connect a single diesel and wind turbine to charge batteries, run refrigeration,
make ice, and provide light. During the night the entire system is shut down. These are
intermittent-duty systems.