Environmental Engineering Reference
In-Depth Information
As noted in Table 12-3, the curve-fit cyclic rate is 2,583 cycles per hour, slightly more
than the design rate that does not include all bins or very small loads. The design rotational
speed of the Mod-2 wind turbine is 17 rpm, so the overall cyclic rate is approximately 2.5
cycles per rotor revolution, or
2.5P
. This is a reasonable rate for a two-bladed turbine.
Example 2: Exponential Distribution of Blade Flatwise Loads
A heavily-instrumented Micon 65/13 115-kW wind turbine with SERI 8m blades was
the primary test turbine in a program to collect long-term inflow and structural test data
[called
LIST
, Sutherland 2002]. This turbine has a fixed-pitch, 3-bladed rotor located up-
wind of a tubular tower. A 20-hour set of blade cyclic load data was collected during this
program, and cycles in this set were counted by the rainflow method. The count was then
sorted into bins 0.5-kN-m wide and converted to 1-hr rates. The flatwise cyclic load results
are illustrated in Figure 12-9.
In this figure we note that the cycle count continued to increase as the blade loads de-
creased, unlike the Mod-2 blade design spectrum in Figure 12-8. This distribution with many
cycles of small loads is best fit with an
exponential equation
, as follows:
n
= 4, 550 exp[-0.3836
L
F
]
(12-3a)
PDF
=
n
/0.5
(12-3b)
10,000.00
1,000.00
PDF
=
n
/0.5 cycles/hr/kN-m
100.00
10.00
Exponential Curve-Fit
n
cycles/hr/bin
1.00
0.10
Measured Spectrum: 20 hr; 462,600 Cycles
0.01
0
5
10
15
20
25
30
35
Cyclic Flatwise Blade Load,
L
F
(kN-m)
Figure 12-9. Curve-fitting of a Micon 65/13 wind turbine blade flatwise cyclic load
spectrum with an exponential distribution, and calculation of the related probability
density function.
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