Environmental Engineering Reference
In-Depth Information
grid interface inductor
grid
DC
source
i 2
L g
micro-grid
R g
u g
S g
i g
r g
V DC
i f
i o
i 3
IGBT
bridge
u f
PWM
u
L f
i 1
L
u o
R f
S c
i
inverter
r
r f
i d
R
filter
i n duc t o r
C
harmonic
distortion
neutral
l o cal loa d s
Figure 4.1 Single-phase representation of a three-phase inverter system
grid voltage and also to facilitate the control of the real and reactive power exchanged between
the microgrid and the grid.
In this chapter the attention is paid to the microgrid voltage control, using the H repetitive
control theory developed in (Weiss and Hafele 1999). It is assumed that there is an outer
control loop to regulate the power exchanged between the microgrid and the grid by developing
appropriate reference voltages in terms of magnitude and phase shift with respect to the grid.
See Part III for more details. These reference voltages for the three phases of the microgrid
voltage are sinusoidal. It is then the task of the voltage controller to track these reference
voltages accurately so that the resulting THD is small. This controller will be subject to
disturbances which include non-sinusoidal currents, changes in the load current, changes and
distortions in the grid voltage, and changes in the DC-link voltage.
4.2 Modelling of an Inverter
It is assumed that a balanced DC link (see Part II) is present and the system can be regarded
as three independent single-phase systems shown in Figure 4.1. The filter inductor and other
inductors in the system are modelled to include two parasitic resistances: a series resistor to
model the winding resistance and a parallel resistor to model the core losses.
The local loads are represented by a single linear load in parallel with a current source i d with
harmonics. The pulse-width-modulation (PWM) block is designed such that for
V DC
2 ,
the average of the bridge output voltage u f over a switching period equals u . This makes it
possible to model the PWM block and the inverter with an average voltage approach. The
model for the PWM and inverter is thus a simple saturated unity gain, where the saturation
models the limit of the available DC-link voltage with respect to the neutral line (
| u ( t )
| <
V D 2 ).
The control objective is to maintain the microgrid voltage u o as close as possible to the
sinusoidal reference voltage u ref so that the THD of u o is small. The two circuit breakers
S c and S g appearing in Figure 4.1 are needed in the start-up and shut-down processes of the
inverter. They are assumed to be closed for controller design.
±
 
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