The present invention relates to protective relays, in general, and more particularly to a programmed processor based relay for protecting power equipment from overexcitation by deriving a time-to-trip based on a selectable relationship of the ratio of voltage/hertz of the power line supplyinag energy to the power equipment.
Power equipment such as transformers, generators and the like incur heating primarily as the result of certain energy losses like copper loss, hysteresis loss and eddy current loss, for example. These losses can be directly linked to the flux density of the core of such equipment which is a measure of the excitation thereof. Flux density is a function of the integration of the winding voltage over time. It can be expressed on a per cycle basis as a function of the voltage divided by the frequency, hence the name of volts-per-hertz. Accordingly, an inverse-time volts-per-hertz relay is provided to protect such power equipment against overexcitation which may cause damage to the equipment in time. Some professional agencies, like the IEEE, for example, provide recommended overexcitation limit curves for several different types of power equipment, the curves generally prepared by working groups of the IEEE.
In operation, the overexcitation protective relay monitor the voltage of the power lines supplying energy to the power equipment being protected, computes the ratio of volts-to-hertz therefrom and utilize the recommended overexcitation limit curves to derive a time-to-trip for a detected overexcitation condition. Overexcitation protecting relays using inverse time characteristics recommended by the IEEE, for example, are being provided with all solid-state electronic components as exemplified by the paper presented at the Ninth Annual Western Protective Relay Conference in October, 1982 entitled "An Overexcitation Relay With Inverse Time Characteristics" authored by Jim Arthur et al. A typical solid state overexcitation relay for transformers has been specified by ASEA type RATUB bearing the Model No. B03-5011 (May 30, 1982).
More recently, a microprocessor-based overexcitation protective relay bearing Model No. M-0299 has been specified by Beckwith Electric Company, Inc. to provide protection to generators or transformers against overexcitation. Beckwith specifies that the trip time is programmable by a user as a descending curve of time versus volts-per-hertz. This inverse trip time curve is characterized by the user to a desired descending shape by entering the values of 6 points along the desired curve. The algorithm performs linear interpolation between the entered points to derive the time to trip.