21-05-2013, 04:47 PM
Power Quality Experiences with Wind Power
Voltage Variations
Voltage variations on the grid are mainly caused by variations in load and power
production units. When wind power is introduced, voltage variations also emanate from the
power produced by the turbine.
This section will cover measurements and calculations of the grid voltage at the PCC at
different X/R ratios and at a different short-circuit ratios. The short-circuit ratio is defined as
the ratio between the short-circuit power of the grid at the PCC and the rated power of the
wind turbine.
Flicker
This section will cover measurements of power fluctuations during start, stop and continous
operation of wind turbines. The means by which wind turbines produce flicker and the
factors that will affect are discussed.
Continous Operation
Flicker during continous operation is caused by power fluctuations. Power fluctuations
mainly emanate from variations in the wind speed, the tower shadow effect and mechanical
properties of the wind turbine.
This section contains power measurements on different types of wind turbines during
continous operation.
Switching Operations
Switching operations will also produce flicker. Typical switching operations are the start
and shut down of wind turbines. Start, stop and switching between generators or generator
windings will cause a change in the power production. The change in the power production
will cause voltage changes at the point of common connection, PCC. These voltage changes
will in turn cause flicker. The start sequences of variable-speed wind turbines as well as
stall-regulated and pitch-controlled fixed-speed wind turbines are all different.
Harmonics
Fixed-speed wind turbines are not expected to cause significant harmonics and interharmonics.
Variable-speed wind turbines equipped with a converter produces harmonic
currents during continuous operation.
This section will focus on harmonics of variable speed wind turbines equipped with forcedcommutated
inverters.
Transients
Transients seem to occur mainly during the start and shut down of fixed-speed wind
turbines. The start-up sequence of a fixed-speed wind turbine is performed in two steps.
First, the generator is switched. To avoid a large inrush current a soft starter is used. As the
soft starter begins operating and the generator is connected to the grid the shunt capacitor
banks is switched. The shunt capacitor banks are switched directly to the grid without any
soft switching devices. As the shunt capacitor banks are connected, a large current peak
occurs.
This purpose of this section is to study transients when the shunt capacitors of wind
turbines are switched. This part will include transient measurements performed at a small
wind park consisting of two pitch-regulated wind turbines.
Frequency
The introduction of a relatively small amount of wind power into the utility grid does not
normally present interfacing or operational problems. The intermittent power production
from wind turbines is balanced by other production units.
The effect of wind power is very important in autonomous power systems. The spinning
reserve is small in an autonomous grid supplied by diesel engines. The small spinning
reserve will give rise to frequency fluctuations in case of a sudden wind rise or wind drop.
Hence, in a wind-diesel system, the voltage and frequency fluctuations will be considerably
greater than in an ordinary utility grid.
This section will contain frequency measurements from two different autonomous winddiesel
systems. One system is a conventional wind-diesel system containing of a wind farm
working in parallell with some diesel generators. The other system is a specially designed
wind-diesel system in which the diesel generator and the wind turbine work in colaberation
with each other.