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Unbalanced loads in islanded inverter-based microgrids induce voltage and current imbalances, posing significant challenges to the operation of critical equipment and sensitive loads. To address this issue, conventional circuit breakers, strategically placed between microgrids, play a key role in fault isolation during unbalanced conditions. However, they exhibit weaknesses in effectively addressing power quality issues, frequency variations, and power supply interruptions to specific loads. To solve these limitations, this article introduces the new concept of a negative-sequence virtual circuit breaker. Microgrid inverters are responsible for implementing this concept, which consists of eliminating negative-sequence voltages and currents at the point of-common-coupling between microgrids without altering positive-sequence voltages and currents. This last issue allows the entire electrical system to operate on a single frequency, guaranteeing the loads’ supply. The compensators responsible for implementing the proposed concept are unilateral bandpass filters with complex coefficients. The primary advantage of these compensators over conventional integral compensators is their ability to offer plug-and-play operation. The effectiveness of the proposed concept has been verified through experimental tests in a laboratory setup.

Microgrids (MGs) are exposed to voltage quality deterioration due to the presence of voltage unbalance. To deal with this problem, existing solutions based on distributed generation (DG) units interfaced by power electronics offer two type of strategies for voltage unbalance compensation depending on whether the compensation is performed at one remote node or at multiple local nodes. The first type is limited to a single node, and the second type is limited to apply at the DG units output (locally). This article presents a multinodal control scheme where DGs can compensate for voltage unbalance at multiple remote nodes of the MG, thus overcoming both state-of-the-art strategies limitations. In particular, negative-sequence voltage is eliminated at as many remote nodes as the number of available DG's. A systematic approach for the multiple-input/multiple-output (MIMO) nature of the problem is presented covering three aspects. First, a square MIMO control strategy is established and a feasibility test is derived to assess whether the problem can be solved. Second, the cross-coupling interaction between the multiple controllers is minimized by optimally selecting, which DGs will contribute to mitigate the remote unbalances. Third, stability and transient dynamics are analyzed. Laboratory experimental results corroborate the control performance.

The presence of unbalanced loads in power systems creates voltage imbalances that perturb the operation of sensitive equipment like induction motors, power electronics converters, and adjustable speed drives. To address this issue, this article presents a novel control scheme to perform remote negative-sequence voltage fair compensation at any node of a grid-forming inverter-based islanded ac microgrid. The fair compensation reduces the voltage imbalance of the remote node by adjusting the negative-sequence output voltage of the inverters according to a predefined objective. For example, the goal could be to equalize the inverters’ negative-sequence output voltage. Alternatively, the aim could be to reduce this voltage at some inverters to perturb less the local loads at the expense of increasing it in other inverters. Therefore, the proposed controller achieves two control objectives: negative-sequence voltage reduction at a remote node and fair compensation through the inverters running this service. The controller also features inherent plug-and-play and parallel operation. Unlike state-of-the-art controllers on negative-sequence voltage compensation, the control scheme operation is accomplished by only broadcasting the remote node voltage measurement, thus significantly reducing the communication control data. The article presents the large-signal control design and stability analysis, and the performance in a laboratory microgrid with selected experimental results.