Influence of the Digital Filter Transient Characteristic on the Behavior of Microprocessor Current Protection

In microprocessor current protection, instantaneous, amplitude, current, and average values can be used as current information parameters. They are determined as a result of the appropriate processing of the digital filter output signal. In unsteady modes, the specified parameters depend on the transient characteristic of the filter. Therefore, the behavior of the microprocessor current protection in case of damage is largely determined by the specified characteristic of the digital filter. Non-recursive Fourier filters and their varieties are most widely used in existing defenses. The main disadvantage of these filters is their relatively low speed. Based on them, filters with higher dynamic properties are implemented, in which frequency properties are preserved. Based on the similarity of the transient characteristic, the following varieties are distinguished: filters with a monotonic transient characteristic; filters with an aperiodic transient characteristic; filters with an oscillatory transient characteristic. The effect of these filters on the behavior of a micro-processor current protection containing the main high-speed and backup slow-acting stages has been investigated. As a result of the mentioned investigations, it has been shown that digital filters with a monotonous transient characteristic provide stable functioning and selective action of protection steps in case of damage in the main and adjacent sections. Filters with an aperiodic transient characteristic contribute to the non-selective action of the high-speed stage in case of short circuits at the beginning of the adjacent section. Filters with an oscillatory transient characteristic also contribute to the non-selective operation of the high-speed stage and can cause its unstable operation at short-circuit currents close in values to the operating and return currents. The slow-acting protection stage functions steadily and selectively for all types of transient characteristics of digital filters.

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