Environmental Engineering Reference
In-Depth Information
the uneven order components in the ac side current and even order components in
the dc side voltage, see 4.3.2.1. A dual performance is exhibited by the voltage
source inverter (VSI), when sinusoidal ac side current and constant dc side voltage
is assumed; consequent harmonics appear in the ac voltage and in the dc current,
see e.g. 4.3.3.4.
To describe the small-signal behaviour of a semiconductor device in terms of
network theory, the hybrid model may be used, as for a transistor, linking input and
output sinusoidal quantities:
U 1
I 2
= h 11
I 1
U 2
h 12
(6.48)
h 21
h 22
This equation corresponds with the circuit model in Fig. 6.7, the parameters of
which are explained as follows:
h 11 = short-circuit input impedance;
h 12 = open-circuit voltage reaction;
h 21 = short-circuit current gain;
h 22 = open-circuit output admittance.
An intermediate circuit VSI constitutes a time-variant non-linear system. Under
certain conditions the model of Fig. 6.7 may be extended to include both fundamen-
tals and dc and ac side harmonics. This requires a sophisticated analysis, such as
described in [San06].
In the case of an ac-dc inverter, the ac side input mesh, with terminal voltage
U 1 , consists of a ohmic-inductive RL series impedance and voltage source coupled
with output voltage U 2 . The output mesh contains a capacitive C admittance and a
current source coupled with the input current.
The complete converter is then modelled as in Fig. 6.8 [Ack05]. Let U 1 be the
generator terminal voltage (in case of fully fed system) or the rotor terminal voltage
(in case of doubly fed asynchronous generator), and U 2 the grid side terminal volt-
age. The intermediate dc circuit is modelled by the two source currents I dc , 1 , I dc , 1
and the capacitance C .
In most simulations, however, constant dc (capacitor) voltage is assumed, and the
inverter is modelled by a transfer factor only, neglecting losses and energy storage
properties. Furthermore, only ac quantities of fundamental frequency and constant
dc quantities are regarded. In this case, the VSI model is simplified to a real transfer
function dependent only on the modulation factor, see 4.3.3.
Dc-dc converters are mostly modelled by the averaging method [Nirg01].
Fig. 6.7 Four-pole equivalent
circuit model in hybrid form
 
Search WWH ::




Custom Search