How does the use of dynamic voltage restorers (DVRs) improve power quality in microgrids by addressing voltage disturbances?

Dynamic Voltage Restorers (DVRs) significantly improve power quality in microgrids by actively compensating for voltage disturbances such as sags, swells, and voltage unbalance, thereby protecting sensitive loads and enhancing overall system stability. A DVR is a series-connected power electronic device installed between the source and the sensitive load. It operates by injecting a voltage waveform into the system to compensate for any voltage deviations, maintaining a constant and stable voltage at the load terminals. Here’s how DVRs address specific voltage disturbances: Voltage Sags: When a voltage sag occurs (a temporary reduction in voltage), the DVR injects a voltage in series with the line voltage. This injected voltage is precisely controlled in magnitude and phase angle to restore the load voltage to its nominal value. The DVR draws the required power from an energy storage system (ESS), such as batteries or supercapacitors, or from the upstream grid. By injecting the compensating voltage, the DVR effectively isolates the load from the voltage sag, preventing it from affecting the load's operation. Voltage Swells: When a voltage swell occurs (a temporary increase in voltage), the DVR absorbs the excess voltage by injecting a voltage with opposite polarity. This effectively reduces the voltage at the load terminals, maintaining a stable voltage level. As with voltage sags, the DVR relies on its energy storage system or the upstream grid to handle the excess power. Voltage Unbalance: DVRs can also compensate for voltage unbalance, where the voltages on the three phases are unequal. The DVR injects different voltages on each phase to balance the load voltages, ensuring that the sensitive loads receive a balanced power supply. Harmonic Distortion: While primarily designed for voltage sag and swell compensation, some advanced DVRs can also mitigate harmonic distortion by injecting compensating harmonic voltages to cancel out the harmonic currents drawn by nonlinear loads. In summary, DVRs enhance power quality by: Maintaining a constant voltage at the load terminals, regardless of disturbances on the source side. Providing fast response to voltage disturbances, typically within a few milliseconds. Compensating for voltage sags, swells, and voltage unbalance. Improving the reliability and performance of sensitive equipment. As an example, consider a microgrid supplying power to a critical industrial process that is sensitive to voltage sags. The microgrid experiences frequent voltage sags due to motor starting or faults on the distribution system. Installing a DVR at the point of connection of the industrial load will protect the process from these voltage sags, ensuring continuous operation and preventing costly downtime. Therefore, DVRs are valuable tools for improving power quality in microgrids by actively mitigating voltage disturbances and ensuring a stable power supply to sensitive loads.