Page 222 - Wind Energy Handbook

196 WIND-TURBINE PERFORMANCE

4.7.5 Power measurement

As it is the net power which is of interest the power transducer should be located
downstream of any auxiliary loads. It is generally assumed that the wind turbine
will be operating at a nominally fixed speed. For variable speed operation, the IEA
indicated that the rotor speed must also be measured to enable changes in kinetic
energy to be calculated and compensated for, and this should be done to within 1
percent of the nominal rotor speed. No such prescription is included in the
international standard.
Generally the electrical output will be three-phase, 50=60 Hz with voltage in the
range 380–415 V. The recommended approaches are the ‘3 watt meter’ method and
the ‘2 watt meter’ method where no neutral connection exists. Both of these take
account of load imbalance between the phases. The IEC standard refers to IEC
60688 and recommends a transducer of class 0.5 or better (which means a maximum
error of 0.5 percent at rated power); the current transformers, and voltage transfor-
mers if used, should reach the equivalent standard (IEC 60044-1 and 60186
respectively). Usually these transducers will have an analogue output.
Alternative power measurement equipment, such as kWh meters equipped to
produce pulse outputs, can be used provided an equivalent accuracy can be estab-
lished. Whatever transducer is chosen a calibration should be obtained. The
transducers must be able to cope with the power range 50 percent to 200 percent
of the turbine rated capacity.

4.7.6 Wind-turbine status

At least one output should be measured which indicates the operational status of
the wind turbine system. MEASNET make clear that this should not be a sensor
showing whether the turbine is connected to the grid, but rather showing that the
turbine is available. This should be used to determine the time periods for which
the measured power data should be selected for performance analysis.

4.7.7 Data acquisition system

An automatic digital data acquisition system capable of taking analogue signals
(and pulse train inputs where appropriate) should be used. Raw data from all
channels should be stored and preferably the system should be able to collect data
continuously over the measurement period. Commercially available equipment
now enables quite sophisticated data logging systems to be built around a standard
micro-computer. A typical arrangement is shown in Figure 4.21. Some data proces-
sing such as the application of calibrations and averaging can be done on-line with
more complex analysis left to be done later. It is required that the resolution of the
data acquisition system does not reduce the accuracy of the data collected, indeed
measurement uncertainty should be minimal compared to the sensors used. Care
should also be taken to ensure that the signals are free from spurious noise.