Civil Engineering Reference
In-Depth Information
FIGURE 11-4
DC to three-phase AC inverter circuit.
The line-commutated inverter must be connected to the AC system into
which they feed power. The design method is matured and has been exten-
sively used in the high-voltage DC transmission line inverters. Such inverters
are simple and inexpensive and can be designed in any size. The disadvan-
tage is that they act as a sink of reactive power and generate high content
of harmonics.
Poor power factor and high harmonic content in line commutated inverters
significantly degrade the quality of power at the utility interface. This prob-
lem has been recently addressed by a series of design changes in the invert-
ers. Among them is the 12-pulse inverter circuit and increased harmonic
filtering. These new design features have resulted in today's inverters oper-
ating at near unity power factor and less than 3 to 5 percent total harmonic
distortion. The quality of power at the utility interface at many modern wind
power plants exceeds that of the grid they interface.
The force-commutated inverter does not have to be supplying load and
can be free-running as an independent voltage source. The design is rela-
tively complex and expensive. The advantage is that they can be a source of
reactive power and the harmonics content is low.
11.4
Grid Interface Controls
At the utility interface, the power flow direction and magnitude depend on
the voltage magnitude and the phase relation of the site voltage with respect
to the grid voltage. The grid voltage being fixed, the site voltage must be
controlled both in magnitude and in phase in order to feed power to the
grid when available, and to draw from the grid when needed. If the inverter
is already included in the system for frequency conversion, the magnitude
