Current controller design for inverter with LC filter at output

Thread Starter

Dan Gumeni

Joined Jun 18, 2019
8
Hi everyone! As part of school project I have to design a current controller based on a lab setup for an inverter. I've previously did something similar where I've designed the controller using the LCL filter as the plant. In that setup, I had an LC filter for the harmonics, while the second L was the inductance of the transformer between the inverter and the power grid. However, in the current project I am missing a transformer at the inverter output, so I'm stuck with only the LC transfer function as a plant. That is a problem because an LC filter does not allow for the control of the injected grid-side current (which I need to be able to do), allowing only for the control of the voltage across the capacitor. Is there a way in which this can be done? Should I take into account the impedance of the transmission line as the second L and just design like I would have an LCL filter? If I do that, should I also take into account the resistance of the line?
 

Papabravo

Joined Feb 24, 2006
13,559
Hi everyone! As part of school project I have to design a current controller based on a lab setup for an inverter. I've previously did something similar where I've designed the controller using the LCL filter as the plant. In that setup, I had an LC filter for the harmonics, while the second L was the inductance of the transformer between the inverter and the power grid. However, in the current project I am missing a transformer at the inverter output, so I'm stuck with only the LC transfer function as a plant. That is a problem because an LC filter does not allow for the control of the injected grid-side current (which I need to be able to do), allowing only for the control of the voltage across the capacitor. Is there a way in which this can be done? Should I take into account the impedance of the transmission line as the second L and just design like I would have an LCL filter? If I do that, should I also take into account the resistance of the line?
I'm not sure you can do this with passive components. I think you may need an active device which controls current, with another current or perhaps a voltage, and you probably need some kind of feedback.
 

Thread Starter

Dan Gumeni

Joined Jun 18, 2019
8
I'm not sure you can do this with passive components. I think you may need an active device which controls current, with another current or perhaps a voltage, and you probably need some kind of feedback.
Not sure I get what you mean. The current controller generates a voltage reference for the PWM modulation which is how I'm controlling the current normally. But in this case with the LC filter I don't know what exactly is the plant that I should use when I design the PI-controller of the current controller.
 

Papabravo

Joined Feb 24, 2006
13,559
As I read your original post, it created the impression that you thought it could be done with passive components. Passive components include resistors, capacitors, and inductors. Active components include transistors, and integrated circuits that perform amplification, signal processing, and logic decision making. I'm just trying to understand what is on the table here.

I'm also not sure if the techniques you are suggesting will apply to what may be a non-linear process.
 

Thread Starter

Dan Gumeni

Joined Jun 18, 2019
8
As I read your original post, it created the impression that you thought it could be done with passive components. Passive components include resistors, capacitors, and inductors. Active components include transistors, and integrated circuits that perform amplification, signal processing, and logic decision making. I'm just trying to understand what is on the table here.

I'm also not sure if the techniques you are suggesting will apply to what may be a non-linear process.
You're right, I should have included more details. It's a standard 3-phased 2-level inverter with 6 IGBTs with diodes switches, connected to the power grid. The gate signals are generated through PWM modulation. There's a current controller (PI based) that generates the voltage reference for the PWM based on a desired current value and a PLL for the grid frequency. My problem is determining the proportional and the integral gains for the PI of the current controller. In the design phase of the controller, I know you use the transfer function of the filter as a plant and you design a controller based on that. My problem here is that I only have a LC filter. Since there is no second L, there is no voltage drop between the capacitor and point of connection with the grid, so there is no current being injected there to begin with (which is what I need to control). My question is: should I consider something as my second L, so I can actually have a current passing through there, and basically design the controller for a LCL filter? And if so, should that second L be the inductance of the transmission line (since I have no transformer in this case)? And if so, should I also include the resistance of the line? I'll attach a diagram of the previous setup, where I had a LCL filter instead, where the second L was from the transformer, just so it's clearer.
 

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Papabravo

Joined Feb 24, 2006
13,559
I don't know if it would be possible to make each Li into a transformer in such a way that there would be minimal effect on the current going down the line in terms of lost power, while allowing you to measure the AC current in the line directly.
 

MrAl

Joined Jun 17, 2014
7,511
In a control system there are ways to assess if the system is controllable or not, but in any case i would want to measure the output(s) in order to generate a correction signal. I would not want to depend on the values of the passive components in order to generate the required pulse width(s).
Thus you should at the very least measure the output current(s) and use that as feedback. You may have to measure the cap voltages too.
 
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