Imbalance Compensation | Evaluating Microgrid Control with Simscape Electrical, Part 5

Here we can see a visual representation of a balanced system on the left and an unbalanced system on the right. In an unbalanced system we can see variations in wave form magnitude, phase, or both. Unbalanced operation is undesirable. And we need a good way to analyze system and balance so we can assess the severity of an imbalance and also to drive appropriate control mechanisms to address an imbalance.

The way that we can effectively analyze system imbalance is through symmetrical components. Symmetrical components are used to express an unbalanced three-phase system as a combination of three balanced systems. Those three balanced systems are referred to as positive sequence, negative sequence, and zero sequence. We're not going to cover the math of symmetrical components here. Plenty of textbooks are available that will cover that. It's better to show you what they are, using animation and MATLAB.

Here's a visualization of symmetrical components for a balanced system. Let me orient you around what we're seeing, and we'll go column by column. The column on the left shows the sequence components. At the top is the positive sequence. The middle is the negative sequence. And the bottom is the zero sequence. Note that for a balanced system, only positive sequence exists and the positive sequence is equal to the original vectors.

The middle column is where we add the sequence vectors to reconstruct the original vectors. At the top is phase A reconstruction. The middle is phase B reconstruction. And the bottom is phase C reconstruction. Note that because negative sequence and zero sequence are zero, then all we see in this middle column are the positive sequence vectors.

The final column is our original victors, with phase A at the top, phase B in the middle, and phase C at the bottom. Note that the reconstructed vectors match the original vectors. So the middle column matches the right column.

Now, let's look at an unbalanced system. You can see in our left column that we now have a negative sequence. Also note that positive sequence is reduced in magnitude. As this example is more ground or neutral path, we've got no zero sequence. In the middle column, we add the sequence vectors to reconstruct the original vectors. So in the middle, you can see phase A we're adding the positive and negative phase A vectors, as A1 and A2. We add those together to get our original vector. We do the same for phase B and phase C.

As you can see, those reconstructed vectors match our original vectors. Also note the original vectors are very unbalanced in this particular case.

So one way we can use sequence data is to provide a control input to correct system imbalance. Patrice, over to you.

Yes, OK. In this scenario, the microgrid is connected to the utility grid. And the microgrid is operating in grid following mode. A single phase load is connected on the grid side, causing an imbalance in the microgrid voltage that manifests itself as negative signals. This compromised the supply to the microgrid load. The BESS switches to negative seconds compensation in order to mitigate the microgrid voltage imbalance and, hence, provide a balanced power supply to the microgrid load.

Here is a measurement of negative sequence at the microgrid load. Negative sequence composition is activated at one second. You can see that the negative sequence is then reduced. Here we see that microgrid voltage before. And after negative sequence compensation is activated. You can see that after compensation is activated, the voltage magnitude are equalized.