What methods can be used to mitigate circulating currents in parallel inverters operating within a microgrid?

Circulating currents in parallel inverters operating within a microgrid can lead to increased losses, reduced efficiency, and potential equipment damage. These currents arise due to slight differences in the inverter's output voltages, impedances, or control parameters. Several methods can be used to mitigate circulating currents: 1. Accurate Synchronization: Precise synchronization of the inverter output voltages is essential. This involves ensuring that the voltage magnitudes, frequencies, and phase angles of all parallel inverters are closely matched. Phase-locked loops (PLLs) are commonly used to synchronize the inverters to a common reference voltage or frequency. Reducing synchronization errors minimizes voltage differences that drive circulating currents. 2. Virtual Impedance Control: Implementing virtual impedance control can help to equalize the voltage drops across the inverters' output impedances. This technique involves adding a virtual impedance to the inverter's control loop, which simulates the effect of an actual impedance. By adjusting the virtual impedances of the inverters, the voltage drops can be made more uniform, reducing circulating currents. 3. Droop Control: Proper droop control settings can help to share the load proportionally among the inverters. Droop control involves adjusting the inverter's output voltage or frequency based on its output power. By properly tuning the droop gains, the inverters can be made to share the load more evenly, reducing circulating currents. 4. Master-Slave Control: In a master-slave control configuration, one inverter is designated as the master and controls the voltage and frequency of the microgrid, while the other inverters (slaves) follow the master's voltage and frequency. This approach can help to eliminate circulating currents by ensuring that all inverters are operating at the same voltage and frequency. However, the master inverter becomes a single point of failure. 5. Average Current Sharing: Implementing average current sharing control can help to equalize the current drawn from each inverter. This technique involves measuring the current drawn from each inverter and adjusting the inverter's output voltage or frequency to ensure that all inverters are drawing the same amount of current. 6. Common AC Bus Impedance Reduction: Minimizing the impedance of the common AC bus to which the inverters are connected can also help to reduce circulating currents. This can be achieved by using larger conductor sizes or by reducing the length of the connections between the inverters and the bus. 7. PWM Modulation Techniques: Advanced Pulse Width Modulation (PWM) techniques can be employed to minimize harmonic distortion, a contributing factor to circulating currents. Techniques like Space Vector Modulation (SVM) can improve the quality of the output waveform. For example, if two inverters have slightly different output voltage magnitudes, circulating currents will flow between them. Virtual impedance control can be used to add a virtual impedance to the inverter with the higher voltage, reducing its output voltage and minimizing the voltage difference. Combining several of these methods often yields the best results in minimizing circulating currents and optimizing microgrid performance.