Civil Engineering Reference
In-Depth Information
with the value computed from the measured speeds of the wind and the
rotor. The turbine speed is accordingly changed to assure maximum power
production at all times.
13.1
Interface Requirements
Both the wind and the pv systems interface the grid at the output terminals
of the synchronizing breaker at the output end of the inverter. The power
flows in either direction depending on the site voltage at the breaker termi-
nals. The fundamental requirements on the site voltage for interfacing with
the grid are as follows:
•
the voltage magnitude and phase must equal to that required for
the desired magnitude and direction of the power flow. The voltage
is controlled by the transformer turn ratio and/or the recti-
fier/inverter firing angle in a closed-loop control system.
•
the frequency must be exactly equal to that of the grid, or else the
system will not work. To meet the exacting frequency requirement,
the only effective means is to use the utility frequency as a reference
for the inverter switching frequency.
•
in the wind system, the synchronous generators of the grid system
supply magnetizing current for the induction generator.
The interface and control issues are similar in many ways between both
the pv and the wind systems. The wind system, however, is more involved
since the electrical generator and the turbine with large inertia introduce
certain dynamic issues not applicable in the static pv system. Moreover, wind
plants generally have much greater power capacity than the pv plants. For
example, many wind plants that have been already installed around the
world have capacity in tens of MW each. The newer wind plants in the
hundreds of MW capacity are being installed and more are planned.
13.2
Synchronizing with Grid
The synchronizing breaker in
Figures 13-1
and
13-5
has internal voltage and
phase angle sensors to monitor the site and grid voltages and signal the
correct instant for closing the breaker. As a part of the automatic protection
circuit, any attempt to close the breaker at an incorrect instant is rejected by
the breaker. Four conditions which must be satisfied before the synchroniz-
ing switch will permit the closure are as follows:
