Ridethrough during input power transients and dropouts

A primary function of an ac power source or power conditioner is the ability to sustain output power during short interruptions of input power. This is called ridethrough. As its name suggests, ridethrough is a converters ability to "ride through" these short glitches without being affected or affecting the load in any way. The manner in which a converter's capacity for ridethrough is generally characterized is by the length time that a total absence of input power can be tolerated without causing any anomaly in the output. Typically, it is measured in units of line frequency periods, that is, so many cycles of input power. The standard, measured at maximum output power, is one or, sometimes, two cycles. For 60 Hz line frequency this equates to 17ms and 34ms, respectively.

For a power converter to do this, it must have some capacity for energy storage, to sustain the load during input power interruption. A bulk storage capacitor can provide a small battery of energy to maintain output power. These work on a similar basis to ordinary batteries, except that they store much less energy and require no maintenance. Capacitors are internal to the converter.

A related measure of a converter’s energy storage is holdup. Holdup is a measure of how long a converter can "hold up" the output in the face of a permanent interruption of input power. The difference from ridethrough is that, in the case of holdup, input power is not coming back soon enough to avoid a converter reset and eminent interruption of power to the load. Holdup times are longer than ridethrough times because there is a period, after the ridethrough interval, when the converter’s internal energy store is so depleted that a reset and soft-start is initiated, although there is some small amount still left that is delivered to the output. This amount is only a few percent of the total energy store. The purpose of the reset and soft start is often to avoid output overshoot and excessive inrush currents upon reapplication of input power.

Much longer ridethrough times can be achieved with conventional batteries. Lead-Acid battery technology is cost effective and the one most used. Sometimes the batteries far outweigh and outsize the converter itself. Ridethrough capacity is limited only by battery size. Power converters that employ batteries for the purpose of ridethrough are called Uninterruptible Power Sources or UPS systems. By then however, the attribute is no longer called ridethrough, but simply “Battery Life”.