WIND ENERGY SYSTEMS - Cd3wd

Chapter 5—Electrical Network 5–40
Similarly,
I 2 =0.6(151.38/ − 78.93 o )=90.83/ − 78.93 o
A
From Eq. 70 the input current to the autotransformer is
I L = V 1I 1
= (76.2)(93.35/ − 57.92o )
=56.01/ − 57.92 o A
V L 127
This is only a little more than twice the rated running current of the motor, hence should not cause
substantial voltage fluctuations.
The total motor losses will be
P loss = 3[(0.6)(57.07)]2 +3(93.35) 2 (0.30) + 3(90.83) 2 (0.14) = 11, 340 W
120
which is (0.6) 2 = 0.36 of the losses during the full voltage start. This is still an order of magnitude
greater than the operating losses at full load and would result in damage to the motor if it does not
startwithin10to20seconds.
The total starting torque is
T m = 90(90.83)2 (0.14)
π(1200)(1)
=27.57 N · m/rad
which is about 46 percent of rated torque. If this torque is not adequate to start the motor load, then
the autotransformer taps can be changed to provide a larger starting voltage of perhaps 0.7 or 0.8
times rated voltage, at the expense of larger line currents.
7 CAPACITY CREDIT
Wind generators connected to the utility grid obviously function in the role of fuel savers.
Their value as a fuel saver may be quite adequate to justify their deployment, especially in
utilities that depend heavily on oil fired generating plants. The value to a utility may be
increased, however, if the utility could defer building some conventional generating plants
because of the wind turbines presence on the grid. Wind generators would have to have some
effective load carrying capability in order to receive such a capacity credit.
Some may feel that since the wind may not blow at the time of the yearly peak load
that the utility is forced to build generation equipment to meet the load without considering
the wind, in which case the wind cannot receive a capacity credit. This is not a consistent
argument because any generating plant may be unavailable at the time of the peak load, due
to equipment failure. The lack of wind is no different in its effect than an equipment failure,
and can be treated in a standard mathematical fashion to determine the effective capacity of
the wind generator.
Wind Energy Systems by Dr. Gary L. Johnson November 21, 2001