Wind Power Generation - Renewable Energy: Technology, Economics and Environment - page 319

Grid connection. With regard to the connection of wind energy converters to the
public power grid or any isolated power grid, direct and indirect grid connections
are distinguished /7-6/; for both types asynchronous and synchronous generators
are suitable (Table 7.1).
− For direct connection to an invariable frequency power grid, as it is the case for
public European supply, synchronous generators turn at a constant number of
revolutions and asynchronous generators at an almost constant number of revo-
lutions according to the grid frequency (Fig. 7.12). Due to the inevitable "hard"
connection, particularly in case of synchronous generators, high dynamic stress
may be exerted onto the power train (hub, shaft, gearbox and generator rotor).
This is why in most cases asynchronous generators are applied for direct grid
connection.

Table 7.1 Comparison of direct and indirect grid connection with regard to the applied
generator type (see /7-6/)

Synchronous generator Asynchronous generator

Direct
grid
connec-
tion

n G = f
constant number of revolu-
tions; hard grid connection

n G = ( 1 - s) f; 0 ≥ s ≥ - 0.01
slightly declining number of revolutions
that decreases with increasing converter
size; simple grid synchronisation; reactive
power consumer; relatively hard grid con-
nection

0.8 f ≤ n G ≤ 1.2 f
variable number of revolutions; squirrel
cage induction machines via direct current
intermediate circuit or direct converter (re-
active power consumer); slip ring induc-
tion machine via dynamic slip control,
oversynchronous static Kraemer system
(both reactive power consumers) or double
loaded asynchronous generator with direct
stator and indirect rotor connections (e.g.
via direct current intermediate circuit) (re-
active power generation); soft grid connec-
tion
n G generator number of revolutions; s slip (deviation from nominal number of revolu-
tions); f grid frequency.
− Indirect grid connection converters may be connected via a direct current in-
termediate circuit, which allows for the operation of wind energy converters at
a variable number of revolutions and generates alternate current at variable fre-
quencies. Current is first converted to direct current by a rectifier and subse-
quently reconverted into alternate current by an inverted rectifier to match the
voltage and frequency specifications of the power grid. This allows for opti-
mum aerodynamic operation of the rotor within a revolution range from 50 to
120 % of the nominal number of revolutions (Fig. 7.12). The variable number

Indirect
grid
connec-
tion

0.5 f ≤ n G ≤ 1,2 f
variable number of revolu-
tions; grid connection via a
rectifier with connected in-
verted rectifier (i.e. intermedi-
ate direct current circuit or
direct converter); soft grid
connection