Civil Engineering Reference
In-Depth Information
Fortunately, for wind farms with many machines, the machines can be
started in sequence to minimize this effect. For very weak system, however,
this issue can limit the number of machines that can be connected with the
grid lines. The voltage flicker severity increases as the square root of the
number of machines. The flicker caused by starting one machine varies
inversely with the fault level at the point of the grid connection, hence can
be an issue on the weaker grids.
Harmonics are generated by the power electronics employed for the soft
start of large wind turbines and for the speed control during energy produc-
ing operation. The former can be generally ignored due to the short duration
and use of the sequential starts.
The operating harmonics, however, need to be filtered out. Total harmonic
content of the wind farm is empirically found to depend on the square root
of the number of machines. Since the high voltage grid side has lower
current, designing the pulse-width-modulated (PWM) converters on the grid
side can be advantageous. The utility companies around the world are not
consistent in the way they limit the harmonics. Some limit the harmonics in
terms of the absolute current in amperes, while others set limits proportional
to the grid's short-circuit MVA at the point of interconnection.
Utilities find it convenient to meet the power-quality requirements by
limiting the total renewable power rating less than a few percent of the short-
circuit MVA of the grid at the proposed interface. The limit is generally
2 percent in developed countries and 5 percent in developing countries. This
requirement can be more restrictive than the overall power quality require-
ment imposed in accordance with the national standards.
14.8.2
Interfacing Standards
The maximum capacity the renewable power plant can install is primarily
determined from the electrical system and power quality considerations
presented in this chapter. A rough rule of thumb to control the power quality
has been to keep the renewable power plant capacity in MW less than the
grid line voltage in kV if the grid is stiff (large). On a weak grid, however,
only 10 or 20 percent of this capacity may be allowed. The regulation gen-
erally imposed on the renewable power farms these days are in terms of the
short-circuit capacity at the proposed interface site.
We recall that the major power quality issues discussed in this chapter are
as follows:
•
the acceptable voltage range on the distribution system.
•
the step load voltage.
•
the steady state voltage regulations.
•
flickers caused by the wind fluctuations.
•
the harmonics.
