WIND ENERGY SYSTEMS - Cd3wd

Chapter 9—Wind Power Plants 9–30
be prevented for long term operation. However, reverse power may be acceptable or even
necessary during some operating conditions, so considerable sophistication may be required.
If the turbines need to be started by utility power, as in the case of Darrieus turbines, then
reverse power will flow during the starting cycle. Also, depending on the length and energy
requirements of the turbine shut-down and start-up cycles, it may be justified to allow reverse
power flow during wind lulls if the average power flow over a 10 or 30 minute period is toward
the utility.
Another relay which would be required for the main circuit breaker at least would be one
that detects reverse phase or the loss of one phase of the three-phase system. Actual reversed
phase sequence would be unlikely after the windfarm electrical system is once correctly wired,
but the loss of one phase is not uncommon, caused either by a broken line or the failure of a
circuit breaker to reclose properly. Induction generators would try to support the voltage on
the lost phase, with possible heavy fault currents.
A synchronism check relay prevents a circuit breaker from closing if the windfarm generators
are out of phase with the utility. It would not be required during normal startup
conditions with induction generators since these would not have a voltage present at the time
of connection to the utility. However, if the utility should have a circuit breaker opened elsewhere
on the system, perhaps due to lightning, which recloses after a few tenths of a second,
the induction generator voltages will not have had time to decay to zero, and will most probably
be out of phase. The resulting high currents and torques could easily damage both the
generators and the turbines. The safest approach would be to do a complete shutdown of the
windfarm when utility power is lost for any reason, and then initiate a standard startup. With
more experience, and fast acting solid state controls, it may be possible to add capacitance
and local resistive load at the windfarm to maintain voltage, frequency, and phase so that
automatic reclosing of the windfarm into the utility would be feasible.
Other relays may be considered for specific protection of devices. A differential relay may
be used on the main transformer to detect differences between input and output, which would
indicate an internal fault. It could also be used on the generators, but may be difficult to
justify economically in the windfarm setting. A ground overcurrent relay may be used to
detect large currents flowing in the ground connection of a wye connected transformer, which
would indicate certain types of system failure. Other relays could be used as well.
9 WINDFARM COSTS
We have considered the costs of the electrical equipment necessary to connect wind turbines
to the utility grid, hopefully in enough detail to illustrate the process. Prices of electrical
equipment can change substantially over short periods of time, and labor costs vary with the
part of the country, the remoteness of the site, and the site terrain, so the actual figures used
for examples will not be accurate in most situations. One should always call suppliers and
contractors to get current prices.
Wind Energy Systems by Dr. Gary L. Johnson November 21, 2001