Environmental Engineering Reference
In-Depth Information
7.4.3 I NVERTERS
There are a number of inverters on the market; however, there are only a few manufacturers of
inverters for wind turbines. Inverters need to be designed for the much different inputs of wind tur-
bines, as inverters for PV have less stringent operating requirements. There are inverters for hybrid
systems where power is taken from the battery storage. For wind turbines with permanent magnet
alternators, the output is rectified to DC and the inverter converts that to the constant voltage and
frequency of the grid. Since the wind turbine is controlled mechanically, the inverter controls the
electrical aspect, synchronization of phase and power transfer. Some wind systems use battery stor-
age before the inverter, which means a different design for the inverter. The early inverters used
short-length, square wave pulses with proper timing on the cycle to input power to the grid. The
square wave pulses add harmonics to the output. Later inverters have been improved and have effi-
ciencies over 90% under 75% load with 2% harmonic distortion. At low winds and loads, inverter
efficiencies will be less.
The field test of a wind turbine (permanent magnet alternator, three-phase, 10 kW) connected
to the grid through an inverter (single-phase, 10 kW) indicated a problem with the inverter in wind
speeds of 13 m/s and greater [14]. Less power was delivered because the inverter entered a pause
mode, and if the pause mode happens too many times within a certain time period, the inverter quits
functioning and has to be reset manually.
The main safety function of the inverter is to disconnect the wind turbine from the utility line
when there is a fault on the utility line; otherwise, there would be hot wires, since the wind generator
is still operating. There needs to be a disconnect (may be fused) between the wind turbine and the
inverter, and a fused disconnect between the inverter and the utility grid.
7.5 LIGHTNING
Lightning is always a problem for electronics, especially for wind turbines connected to the grid,
as lighting strikes on the grid will send spikes a long ways. A wind turbine is generally the tall-
est lightning rod around, so lightning protection, a path to ground, is imperative. Manufacturers'
instructions on grounding and number and connection of copper rods (size and length) must be
implemented, plus all other measures for lightning protection of controllers and inverters, from
varistors to blow-out cans. Even then lightening can still cause problems with damage to controllers,
electrical systems, blades, and generators. Apart from lightning current, the induced electromag-
netic fields may damage the pitch control systems inside the hub. Damage due to lightning is the
most costly repair, as replacement of blades and generators may require a crane.
A 1995 German study estimated that 80% of wind turbine insurance claims paid for damages
that were caused by lightning [15]. Mean annual thunderstorm days and lightning flash density show
those regions of the United States with the greatest risk from lightning for wind turbines. Lightning
was monitored at wind farms in the Turbine Verification Project [16] by collecting data on direct
strikes on wind turbines and utility line surges. The estimated average number of strikes per turbine
per year ranged from 0.04 for California to 0.43 in Nebraska. The information also includes the cost
of the repair [17]. Lightning protection for wind turbines has improved; however, lightning is capri-
cious, and sometimes even the best protection is not sufficient. Blades should have internal lightning
conductors running all the way to the tips of the blades. One example of lightning protection added
after installation was that due to surges on the utility line damaging the controller. The solution was
an underground copper grid, which connected all the guy wires plus the turbine tower.
7.6 RESISTANCE DUMP LOAD
Also, if a wind turbine uses resistive loads for overspeed control, the resistors have to be outside.
During loss of load and high winds at the AEI Wind Test Center, the resistors, which were inside the
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