How do I simulate weak grids for on-grid inverters?

I am currently simulating an on-grid three-level NPC inverter, and I would like to verify my controller under weak grid conditions, the questions are:

1. Which block should I use to simulate weak grid conditions?
2. Are there any specific blocks or methods to specifically add more harmonic distortion to specific harmonics? For example, my controller is based on PR controller and is able to attenuate the 3rd, 5th and 7th harmonic. I want to simulate an increase the grid distrubance on the 3rd harmonic, so i can test out the PR's capabilities.

Anybody who is familiar with this simulation or topic, please provide some help, I would be very thankful!

The image only shows the Inverter section, on the DC side consists of PV arrays and a battery.

Answer 1

[Samhitha]

Hi @bieron surya,

1. To simulate a weak grid in Simulink, the approach involves increasing the impedance between the grid and the inverter. This can be achieved by placing a Three-Phase Series RLC Branch block between the grid source and the inverter terminals.
2. Setting higher values for resistance and inductance will show the behaviour of a weak grid, where the system is more prone to voltage dips and less able to handle sudden load changes.
3. To introduce specific harmonics like the 3rd harmonic into the system, a Three-Phase Programmable Voltage Source can be used. In its parameters, under the "Harmonics" tab, it is possible to define multiple harmonic components.
4. For the 3rd harmonic, the harmonic order should be set to 3, and the amplitude can be specified as a percentage of the fundamental voltage (typically between 5% to 10%). A phase angle can also be specified depending on the desired waveform shape.

For more details, you can refer to the following documentation:

https://www.mathworks.com/help/sps/powersys/ref/threephaseprogrammablevoltagesource.html

[bieron surya]

Thank you very much for the suggestions! This is really helpful!

So putting three-phase series RLC branch between the grid source and inverter terminals can decrease the SCR ratio for weak grids, correct?

[Samhitha]

Hi @bieron surya,

Yes, placing a Three-Phase Series RLC Branch between the grid source and inverter increases the grid impedance, effectively reducing the Short Circuit Ratio (SCR). This models a weak grid condition accurately in simulations.

[Sukanta Roy]

Hi, Thank you for your work. I have some question, can you help sharing your view.

1. how you can calculate the interconnection R+jX for strong grid and weak grid?

2. Also, using Pi-line is difficult. So if you use RL branch, it is also adding resonance into the system, how you are handling it while designignt he LCL or LC filter at the output of inverter?