The Ultracapacitor Uninterruptible Power Supply (U-UPS) provides short-term back-up power and can mitigate process disruptions from frequency or voltage dips based on ultra-fast detection of grid events and successive injection of active or reactive power. The Ultracapacitor Uninterruptible Power Supply can provide transition power up to 30 seconds until a longer-term back-up source like diesel generator is up and running, thus replacing lead-acid batteries with their limited life expectancy.