Hi - I need to work further on VSI and CSI operation modes to better understand the difference between "grid following" and "grid forming" scenarios for converters. I would appreciate that somebody could recommend a paper where the basics for their design and operation were explained with certain detail. Thanks in advance!
That is a very interesting topic.
I would like to just share my understanding on the principles of VSI and CSI, in hope that it will be helpful for your project:
Firstly, I think the interesting thing about the two is the concept of Duality, just like Thevenin voltage source and Norton current source. VSI and CSI have very different characteristics, which are inter-related:
i) VSI requires constant DC-link voltage, and generates AC voltages in the form of voltage pulses; CSI requires constant DC-link current, and generates AC currents in the form of current pulses.
ii) VSI requires DC-link capacitor and AC filter inductors; CSI requires DC-link inductor and AC filter capacitors.
iii) VSI is susceptible to leg short circuit fault and requires dead time in switching upper and lower switches; CSI is susceptible to leg open-circuit fault and requires overlap time in switching upper and lower switches.
iv) VSI is a buck inverter (magnitude of ac voltage is always smaller or equal to the dc voltage) but a boost rectifier; CSI is a boost inverter but a buck rectifier.
for grid operation VSI is more common than CSI. VSI alone can work as either grid forming or grid following. For Grid forming, the VSI needs to work in Voltage Control Mode, where the output voltage of the VSI is controlled to a fixed value and frequency, to form a consistent grid. Since the voltage and frequency is fixed in this case, the current will not be directly controllable, but depends on the load connected (I = V/Z). The VSI will need to ensure that it is able to source the required current to maintain the voltage regulation, failing which, the "grid" will collapse.
For VSI operating in grid following mode, it will have to run in Current Control Mode. Instead of deciding the voltage output directly, the VSI will vary the voltage output such that the current injected to (or drawn from) the grid is controlled.
Hope this helps.
At first I thought you're asking about VSI and CSI differences (due to your main question), but according to your explanations, I think you wanna know how could you control the converter in two different conditions; "grid following" and "grid forming".
The answer is really simple, in the first type, you have to synchronize your converter with the grid, so that the frequency and other parameters match to the grid. But in the second mode, you are determining the parameters by your pre-defined settings, which is mostly useful in microgrids or off-grid utilities.
If you need more details to know or any resources, let me know.
Hi,
+
the answers abore are all correct, but there is still something in my opinion to be clarified.
In the past, CSC or Current Source Converter where converters, typically based on thyristor's technology, making use of a bulk inductance on the DC side, in order to guarantee a stable DC current. They were grid commutated converter, and they worked in two quadrant.
On the contrary, VSC or Voltage Source Converter were (and are) based on the active switches technologies (IGBT's, MOSFETs, GTO's IGCT's and so on) and they requireof a (quite big) capacitor on the DC link, in order to guarantee a stable DC voltage. They are actively commutated, and they work in 4 quadrant
Nowadays, the winnging technology is the VSC, for several reasons, including reduced losses, easyness of control, reduction of filtering requirement, quality of the produced voltages and currents , etc.
However, considering the VS converter, we can make another distinction.
We can speak about VSC controlled in current, and VSC controlled in voltage.
In this case we are no longer referring to the DC side, but we are considering the AC side.
The main eyample of a VSC current controlled is the case of a converter conneting a DC source (batttery, photovoltaics, ....) to a low impedance AC grid (or an AC grid with an infinite short circuit power). The energy transfer is done by controlling the current in the AC coupling inductances that are required to connect the converter to the grid. There are several strategies to perform this control, but this is out of the purposes of my answer. This way of working has been defined "grid following" in a previous answer.
However, if you need for exemple to supply a microgrid, for which the low impedance hypothesis is no longer consistant, it is quite impossible to perform a current control, and a control of the converter output voltage is preferred. As before, several control strategies can be implemented, but again, this is out of my purposes. This way of working has been defined "grid forming" in a previous answer
I hope this answer wasn't too boring and could help you
Cheers
Mauro
I will recommend this paper "A review of inverter topologies for single-phase grid-connected photovoltaic systems"
Article A review of inverter topologies for single-phase grid-connec...
I would recommend you to read this paper Article Control of Power Converters in AC Microgrids
Hangseng Che Can you say a few words about grid following and grid forming control of CSIs?
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