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Microgrid Resynchronization with Main Grid

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This example shows how you can resynchronize an islanded microgrid with the main grid by using a battery energy storage system (BESS). The model in this example comprises a medium voltage (MV) microgrid model with a battery energy storage system, a photovoltaic solar park (PV), and loads. The microgrid can operate both autonomously (islanded) or in synchronization with the main grid. In this example, the microgrid is first in islanded mode. The resynchronization function then synchronizes the microgrid to the main grid. Finally, the breaker closes to connect the microgrid to the main grid. After the resynchronization, the battery system performs a power dispatch and the loads are changed.

The model includes a microgrid controller with all the necessary functions for the resynchronization. The resynchronization operation follows the IEEE Std 2030.7 standard.

Model Overview

A substation connects the microgrid to the main grid. The loads and the PV are connected to the outgoing feeder. The model also contains a separate Operator Control Room subsystem. In a real system, the operator control room can be at the substation or somewhere distant to the microgrid.

Components in Microgrid Resynchronization to Main Grid

These are the main components of the system:

*Substation * Subsystem that connects the microgrid to the main grid. It has a connecting breaker, disconnectors, and transformers to connect the main grid to the microgrid. The substation also contains the microgrid controller and the battery energy storage system.&

Microgrid Controller

The microgrid controller receives commands and setpoints from the Operator Control Room subsystem, as well as measurements and status from other sources and loads in the microgrid. The microgrid controller has different microgrid functions. In this example, the microgrid controller comprises:

  1. PLL measurements from grid side and microgrid side

  2. Reference frequency generation for the microgrid

  3. Reference voltage generation for the microgrid

  4. Resynchronization function to provide setpoint for BESS during voltage and frequency matching

  5. Activate main breaker control command

  6. Receive setpoint and command from operator and communicate to the microgrid asset

For resynchronization, the resynchronization function matches the voltage and frequency of the microgrid to the main grid. To achieve this synchronization, the resynchronization function calculates the necessary voltage and frequency reference for the BESS system.

Microgrid Resynchronization

BESS System

The BESS system connects at the point of common coupling and installed in the substation. The BESS is controlled to achieve the voltage and frequency setpoint received from the microgrid controller for the resynchronization. After the synchronization, the Operator Control Room orders the BESS to perform a power dispatch with setpoints.

Operator Control Room

The operator control room sends all the setpoints and commands. It also plots the measured quantities and the system performance analysis.

Simulation Results

These plots show:

  1. Voltage and current of BESS.

  2. Synchronization of voltage, frequency, and phase angle.

  3. Active and reactive power output of BESS, PV, load, and main grid.

This plot shows the three phase voltage and current output of the BESS, as well as the grid current during resynchronization.

The plot shows the measured values around the resynchronization command at 1 s and the breaker closing at 4.7 s. A stable voltage and current output from BESS verifies a good resynchronization.The small grid current once the breaker closes shows a good voltage and frequency matching during resynchronization.

This plot shows resynchronization of voltage and frequency of the microgrid with the voltage and frequency of the main grid.

The quick matching of voltage, frequency, and phase angle after the resynchronization command validates the efficacy of the BESS control in resynchronization. This plot shows the active and reactive power of BESS, PV, main grid, and loads.

Stable power outputs confirm a stable resynchronization process. The uninterrupted load power verifies a smooth resynchronization of microgrid.

This plot shows the voltage and current of the microgrid loads.

Uninterrupted stable voltage and current at microgrid loads verify effective resychronization of microgrid. The plot shows the measured values around the resynchronization command at 1 s and when the breaker closes at 4.7 s.

System Performance

These plots show the results of the system performance and the impact of the resynchronization function. These performance indices include:

  1. Maximum voltage, minimum voltage, and frequency measured at relevant grid location.

  1. IEEE 2030.7 mapping on resynchronization process

  1. Improvement of key performance indices with resynchronization process

This plot shows the minimum and maximum values of voltage and frequency at different locations of the microgrid.

The resynchronization process maintains the deviations of the voltage and frequency within acceptable limits.

This plot shows the improvement in key performance indices with the resynchronization process compared to grid connection without resynchronization function.

The resynchronization process significantly improves the power capacity, load voltage, and load current fluctuations.

This plot shows the IEEE 2030.7 mapping on resynchronization process.

The resynchronization function executes all the steps from IEEE 2030.7. Grid connections without resynchronization functions cannot follow the IEEE standard.