Environmental Engineering Reference
In-Depth Information
Block diagram of a current-controlled VSI with the H repetitive current controller in the
Figure 3.1
natural frame
which has a gain slightly higher than 1 and a phase lead at the fundamental frequency. It is
introduced to compensate the phase shift and gain attenuation caused by the computational
delay, PWM modulation, the inverter bridge and the LC filter. It also attenuates the harmonics in
the feed-forwarded grid voltages and improves the dynamics during grid voltage fluctuations
(Timbus et al. 2009). Such a structure is capable of coping with unbalanced grid voltages
and voltage sags within the range given by the nature, and waveform quality requirements.
This filter could be designed analytically but here it is chosen by trial-and-error according to
the principles just mentioned. Moreover, it does not affect the independence of each phase.
Once the circuit breaker is closed, there should be no current exchanged with the grid until
the current references are changed to be non-zero. When the inverter generates power, the
repetitive current controller makes appropriate contributions on top of the feed-forwarded grid
voltages. Although the controller is a current controller, the output of the inverter is still a
voltage around the grid voltage. Hence, local loads can be connected. When the grid is down,
the grid voltages ( u ga , u gb and u gc ) fed to the filter F ( s ) can be replaced with rated three-phase
voltages and the current references I d and I q can be set as 0. This allows the local loads to
work properly, although in an open-loop manner. This is different from the case without a
feed-forwarded voltage added to the output of the current controller. It is possible to introduce
a voltage loop to improve the local load voltage; see Chapter 6.
3.2 Controller Design
In this section, the current controller is designed based on the H and repetitive control
techniques. The main objective of the H repetitive current controller is to inject a clean and
balanced current to the grid in the presence of grid voltage distortion. The block diagram of the
H repetitive current control scheme is shown in Figure 3.2, where P is the transfer function
of the control plant, C is the stabilising compensator to be designed and M is the transfer
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