The circuit files for this case are in these two links:circuit_files.zip circuit_files.tar.gz
As this case is an extension of the previous case (click here), the system conditions are fairly similar. This includes the three-phase source, the three-phase load and the Voltage Source Converter (VSC) which remain identical. The change appears in the filter found in the file comp_filter.csv. In the previous case, a simple inductor filter was used. The disadvantage of using this filter is the large amount of switching ripple found in the current injected into the grid. This case uses an inductor-capacitor-inductor (LCL) filter. This results in a much better attenuation of the switching harmonic current produced by the VSC and therefore the current injected into the grid has significantly reduced harmonics.
The control algorithm has not changed. The PLL remains identical to the previous case. The current control has been modified to a certain extent to account for the higher order filter. The LC filter will result in resonance and control has to damp this resonance. Various strategies have been used - in this case, active damping of the grid current is used to reduce oscillations. However, the active damping control has not been fine tuned and therefore, the grid current will still contain oscillations.
To begin with let us consider an inverter inductor of 500 micro Henry and a grid inductor of 500 micro Henry while the filter capacitor between them to be 10 micro Farad. Let us examine the effect of the filter by plotting the inverter output current and the grid current of phase a together in the same plot.
Until the inverter is connected to the grid at 0.1s, the currents are completely different. The inverter switching has not been blocked which is why the inverter output current is non-zero. However, the disconnect switches are off which is why the grid current is zero. After 0.1s, the currents show the effect of the filter.
From the above figure, the red waveform is the inverter output current. This waveform contains a signficant amount of switching frequency harmonics. However, the current injected into the grid can be seen to have a much better waveform. The grid current still has some switching frequency content. The filter capacitor provides a bypass path for the switching ripple current in the inverter output current as the capacitor has a lower impedance for higher frequencies. The attenuation provided by the LCL filter will depend on the parameters of the two inductors and the capacitor chosen. Smaller inductors will be better as this will reduce the size and bulk of the filter.
The next figure shows the tracking performance of the compensator.
The compensator provides the entire load current. The blue waveform is the phase a load voltage scaled down by a factor of 5. The red waveform is the grid current which is zero except for some switching ripple content. This shows that the current tracking performance of the converter is fairly good.
Let us now examine the effect of increasing the two inductors to 1 milli Henry while increasing the filter capacitor to 50 micro Farad. Below is the effect of the filter. The filter results in a significantly smoother current. However, the oscillations produced by the LCL filter have not been effectively damped and can be seen in the current injected into the grid. This is because the damping co-efficients used are the same as the previous case and have not been redesigned for the new filter parameters.
The tracking performance of the compensator is shown below.
The plot below shows the effect of the oscillations in the grid current. As can be seen the oscillations are not negligible and are of lower frequency which implies that they will not be acceptable. Therefore, fine tuning the damping on the grid current is of importance and is the main challenge in a converter with a LCL filter.
As a final comparison, let us further increase the filter capacitor to 100 micro Farad while keeping the inductors at 1 milli Henry. Below is the filter performance.
Below is the plot showing the tracking performance.
Below is the plot showing the three-phase currents injected into the grid.
This case shows how a filter can be designed for a grid connected converter. This case shows how simulations can be used to design circuit parameters.