Why is phase sequence important when connecting distributed generation to the grid?

Phase sequence refers to the order in which the voltages of the three phases in a three-phase power system reach their peak values. The standard phase sequence is typically A-B-C or 1-2-3. Maintaining the correct phase sequence is crucial when connecting distributed generation (DG) to the grid to ensure proper operation of equipment and prevent damage. If the phase sequence of the DG is incorrect relative to the grid, the voltages will not be synchronized, leading to large circulating currents. These currents can damage the DG unit, the grid equipment, and any connected loads. An incorrect phase sequence can also cause protective devices to operate incorrectly, potentially leading to widespread outages. For example, motors connected to the system may rotate in the wrong direction, causing mechanical damage. Relays designed to detect specific fault conditions may not operate correctly, compromising system protection. Therefore, before connecting any DG unit to the grid, it is essential to verify that its phase sequence matches the grid's phase sequence. This is typically done using a phase sequence indicator or a synchroscope. The indicator will show the phase sequence of both the DG unit and the grid, allowing the connection to be made only when they match. Additionally, proper commissioning procedures should include checks to verify the correct rotation of motors and the proper operation of protective relays after the DG unit is connected.