Environmental Engineering Reference
In-Depth Information
e.g. 20 kV or regional standard voltage. The common connection point with the
network operator may be on high-voltage level, which implies a grid transformer
transforming from medium to high voltage level.
The apparent grid short-circuit power at the connection point is an important pa-
rameter in a project to erect a WES or a wind park, limiting the permissible power
rating of the generators to be paralleled. In regions of rural character the distribu-
tion system, designed for consumer needs, may require power line reinforcement to
allow economically reasonable WES installations at a chosen connecting point.
The voltage deviation occurring at the connecting point due to WES operation is
determined by the short-circuit apparent power, together with the generator proper-
ties and the load conditions. The utilities set an upper limit for permissible voltage
deviations in view of disturbing other grid-connected consumers.
The ratio of the short-circuit power and rated apparent WES or wind park power
is called the connection factor; it is a criterion for acceptable rating of the generators
to be installed in a wind park.
Usual grid regulation codes require that the voltage variation due to WES must
not exceed 2% of nominal voltage at the connection point. Consequently, the max-
imum apparent power rating of the generator (or generators) S eV in dependence of
the apparent short-circuit power S kV is required to comply with:
S eV = P nG
λ
S kV
k
p 10 min
(7.1)
·
50
Reduction factor k in (7.1) takes account of rush-currents expected when con-
necting the system:
k = 1 for synchronous generators using a soft synchronizing method, and for
converter coupled generators;
k = 4 for asynchronous generators, when switched on at 95% up to 105% of
synchronous speed, without current limiting measures;
k = I a / I N for asynchronous generators, when started as motors from the grid;
k = 8 in case I a is unknown.
Asynchronous generators for direct coupling with the grid which are in use with
ratings up to 2000 kW are switched on by thyristor soft starting devices (repre-
senting phase-controlled inverters) are considered k
2. Systems above ratings of
1.500 kW are mostly coupled by means of converters, so that k
1.
Generally a switching-current factor is specified to take account of voltage vari-
ations due to switching actions:
k max = I max / I nG
(7.2)
A simple model to calculate voltage deviations at the connection point V is shown
in Fig. 7.1a, where the grid is represented by an infinite bus and a short-circuit
impedance. U B is the infinite bus voltage, Z k the impedance described by resistive
and inductive component in series
Z k = R k + j
ω
L k ;
ψ k = atan(
ω
L k / R k )
(7.3)
 
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