Electric power converter systems are used to transform and/or condition electrical power in a variety of applications.  For example, electrical power converter systems may transform AC power from a power grid to a form suitable for a standalone application (e.g., powering an electric motor, lights, electric heater, household or commercial equipment, telecommunications equipment, computing equipment, and so forth).  Another example standalone application is an uninterruptible power supply (UPS).  UPS systems have become useful as backup power supplies for hospitals, financial institutions, industrial sites, telecommunications, and the like during interruptions of a public three-phase power supply grid.  Increasingly, domestic homeowners also rely on UPS systems to supplement and/or replace power from the public power supply grid during failures. 
UPS systems typically incorporate some type of electrical power converter or other power transformation device/subsystem.  An electrical power converter system may comprise one or more subsystems such as a DC/AC inverter, DC/DC converter, and/or AC/DC rectifier.  Typically, electrical power converter systems include additional circuitry and/or programs for controlling the various subsystems, and for performing switching, filtering, noise and transient suppression, and device protection. 
By way of example and historical explanation, power converter systems initially were built for specific applications.  One early type of power converter was specifically designed for inverting direct current, constant voltage sources (e.g., batteries) to alternating current outputs (e.g., for operation of AC motors).  Converters of this type are termed DC/AC “inverters,” and they have taken the form of, for example, transformers interconnecting a DC power supply with a plurality of logic control switches to generate the necessary alternating current waveform.  A rectifier is another type of power converter for converting alternating current to direct current (AC/DC).  Rectifiers have proven themselves useful for adapting household 110-volt alternating current to 12-volt direct current for operation of battery-powered appliances.  Devices of this type have been as simple as a step-down transformer connected to a diode bridge and a smoothing capacitor for full-wave rectification.  Direct current to direct current (DC/DC) converters have been provided for conditioning direct current power from a variable power source (e.g., a wind-driven direct current motor, photovoltaic panel or the like) for charging a battery or array of batteries. 
Uninterruptible power supplies have been developed that permit power to be converted from a direct current power supply to a three-phase AC load in the event of a failure of the AC grid, and for recharging the DC power supply from the AC grid through the same apparatus when the AC grid is not in a failure mode.  In other implementations, a UPS system transforms AC power from the AC grid into DC power, and then transforms the DC power into AC power, which is then provided to the AC load, instead of having the AC load directly powered by the AC grid.  In the event of a failure of the AC grid, such a UPS system can continue to power the AC load by obtaining DC power from an alternate source (such as a battery) and then transforming that DC power into AC power. 
In many situations, the AC load needs to be provided with AC power having a frequency (e.g., cycles per second) that matches the frequency of the AC grid.  One reason to match frequency is that many devices comprising the AC load or devices coupled thereto have internal clocks or other components whose timing/cycles are based on the frequency provided by the AC grid.  If the frequencies do not match, then such clocks can lose significant accuracy in their timing, ranging from a loss (or gain) of several seconds to even hours over a period of time. 
In implementations where an AC load is directly powered from an AC grid, such frequency matching issues are less of a concern.  That is, while the AC grid frequency may vary at times, the AC grid frequency always adjusts itself to average 60 Hz, for example, and can therefore be relied upon to provide accurate cycles. 
However, in implementations where the AC load is powered from a UPS rather than directly from the AC grid, the AC power provided by the UPS to the AC load may not necessarily match the AC grid frequency.  In these situations, a frequency difference of 1% or greater, for example, from the AC grid frequency can cause noticeable cycle errors or other timing irregularities at the AC load.  While oscillator tolerance in an internal microcontroller/microprocessor of the power inverter (or other power converter device) of the UPS can reduce cycle error, such reduction or other reliance on an internal clock of the microcontroller/microprocessor of the power inverter is not sufficient to guarantee substantially error-free cycles. 
Clearly, minimum cycle error is needed by certain loads.  Various institutions that rely heavily on computer data processing (such as financial institutions and telecommunication service providers) have little tolerance for significant cycle errors.