Grid Synchronization | Evaluating Microgrid Control with Simscape Electrical, Part 4

So what I have here is a simple animation of two voltages, either side of a connector, which in this case is closed. If the voltages are of equal magnitude, frequency, and phase, then no current flows through the connector. However, if we start to phase shift the microgrid voltage relative to the utility grid voltage, then current starts flowing through the connector. The larger the phase shift, the larger the current.

Let's imagine we have this phase shift, but the connector is open. If we were then to close the connector, we would get a current in rush, which is undesirable. What we need to do, prior to connection, is bring the two voltages into alignment, in terms of magnitude, frequency, and phase. Once we have the alignment, we can close the connector with minimal current in rush.

Grid synchronization control is the process of measuring utility grid voltage and actively aligning the microgrid voltage magnitude, frequency, and phase with the utility grid voltage, prior to connection. And we'll consider a specific scenario. Patrice.

Yes. In this scenario, the microgrid is initially connected to the utility grid and operating in grid-following mode. The utility grid is then disconnected and the microgrid switches to grid forming mode. A grid synchronization process then begins to align the microgrid voltage with the utility grid voltage. Once algement reaches a defined threshold, then reconnection with the utility grid is made.

To run the scenario, we double click on TestParam block and select Test 2 from the dropdown. This will configure the control system and loading and generation condition appropriately, prior to us running the simulation. Here, we see the simulated system response for this scenario. At the top we see the microgrid frequency and the utility grid frequency. Note that the utility grid frequency is maintained at 60 hertz, while the microgrid frequency is subject to more variation.

In the model, we see the voltage phase difference between the microgrid voltage and the utility grid voltage. When the microgrid is islanded, we see that the phase difference increases, as we are no longer synchronized. At two second, a synchronization signal is sent to the BESS, at which point it begins adjusting the phase of the microgrid voltage, in order to reduce the phase difference. At five second, the phase difference is within tolerance and the breaker is closed.

Now, let's look at the voltage found in more detail. On the left, we see the phase shift between microgrid and utility grid voltage, following grid disconnection. On the right, we see that the phase of the voltages have been aligned, prior to connection at 0.5 seconds. Note that there is some discrepancy in voltage magnitudes, but this isn't a normal situation. The key is to get the match within a certain tolerance, in order to minimize current in-rush before connection is made.

Here, we see the current flowing in the connection between the microgrid and utility grid, following resynchronization. Note, there is a trendgen but it is significantly reduced from the trendgen that would occur if a synchronization procedure was not performed.