The use of standby power supplies containing a battery and an inverter for maintaining A.C. electrical power to computers during failure of the power lines voltage is well known. It is highly desirable that the changeover operation be carried out as quickly as possible in order to avoid the loss of the contents of volatile memory. Ideally, such changeover systems should be very quick-acting, typically in the range of one millisecond or so, since the filter capacitors of the onboard power supply of the computer are not designed to supply power for more than a time of the order of one-quarter of the line voltage waveform period, i.e., 3 or 5 milliseconds or so.
In addition to the requirement for rapid reaction time for achieving the changeover from line power to battery power, there remains the problem of providing a power line voltage fault detector which will give a rapid and unambiguous reaction to power line abnormalities mandating system changeover from normal power line operation ("line mode") to battery-powered operation ("battery mode"). In addition to total power line voltage failure, such abnormalities also include transient and relatively long-term line overvoltage and undervoltage conditions as well.
A great variety of voltage comparator circuits exist which respond to variations in the peak value of the power line voltage waveform to detect such conditions, and to initiate system changeover in response thereto. To the applicant's knowledge, none of these systems is configured to respond immediately to one mode of power line voltage failure of significant statistical likelihood, namely, complete failure of the power line voltage at or in the immediate vicinity of an instantaneously zero line voltage condition, i.e., at an axis crossing. There remains a need for a power line fault detector which will not only respond to more conventional out-of-range conditions, but which will also respond immediately to total failure of the power line voltage at an axis crossing.
Finally, there is a need for a voltage abnormality detection circuit which will respond instantaneously to a catastrophic in-phase rise in the power line voltage. One particularly dangerous condition can be created by accidental breaking of the neutral line in a Y-connected three-phase system feeding a local power distribution network. The result of such line interruption and the resulting imbalance causes the power line voltage in the various phases to undergo a gross continuing phase-coherent increase in power line voltage amplitude. Conventional comparators may readily respond in sufficient time to cope with such changes, provided that the circuit interruption occurs at or near the peak amplitude of the normal power line voltage. If, on the other hand, such an interruption occurs in the vicinity of an axis crossing, then thereafter, and in particular in the next quadrant, the sine wave is characterized by a grossly increased amplitude, and during the time this wave form is rising from zero to a value which will trigger a conventional comparator, the rate of rise of the line voltage during the first portion of the rise is so high as to cause damage to electrical systems. Such accidents are not unknown, and frequently result in permanent damage to a great number of electrical accessories connected to the power lines at that time. Accordingly, there is a need for a voltage abnormality detection circuit which will guard against such situations.