Civil Engineering Reference
In-Depth Information
Z
w
= source impedance of the wind farm.
Z
l
= impedance of the interconnecting line from the wind farm to
the grid.
then, after connecting the proposed wind capacity with the grid, the com-
bined equivalent Thevenin's network would be as shown in (b), where:
11 1
ZZZZ
t
=+
+
(14-21)
i
w
l
where Z
t
= total combined source impedance of the two systems.
The combined short circuit MVA at the point of the interconnection is then:
Related MVA Capacity
per unit
MVA
=
(14-22)
SC
Z
t
The higher the short-circuit MVA, the stiffer the network. Certain mini-
mum grid stiffness in relation to the renewable power capacity is required
to maintain the power quality of the resulting network. This consideration
limits the total wind capacity that can be added at a site under consideration.
The wind capacity exceeding that limit may be a difficult proposal to sell to
the company in charge of the grid.
Not only the magnitude of Z
t
is important, the resistance R and the reac-
tance X components of Z
t
have their individual importance. The fundamental
circuit theory dictates that the real and reactive power in any electrical
network must be separately maintained in balance. Therefore, the real and
reactive components of the wind generator impedance would impact the
network, more so in a weaker grid. The R/X ratio is often found to be 0.5,
which is generally satisfactory for fixed-speed wind generators. Lower R/X
ratio may pose another limitation on designing the system.
The fault current decays exponentially as e
-R/X
. Low R/X ratio causes the
fault currents to linger on, making the fault protection relaying more difficult.
The voltage regulation (variation from zero to full load on the wind farm)
is yet another design consideration that is impacted by the R/X ratio. The
estimated voltage regulation of the wind farm must be compared with the
contractual limit with the utility. In doing so, the actual continuous maxi-
mum load the wind farm can deliver must be taken into account, not the
nominal rating which can be much higher. The acceptable voltage regulation
in industrial countries is typically 5 percent, and 7 percent in developing
countries.
Starting the wind turbine as the induction motor causes an inrush of
current from the grid, causing a sudden voltage drop for a few seconds. This
can be intolerable by the grid line. Most countries limit this transient voltage
dip to 2 to 5 percent, the higher value prevailing in the developing countries.
