| Three-zone overcurrent protections are widely applied in distribution systems in China. The stepping-up operation time for selectivity may result in long operation time, and the increased clearing time may lead to unexpected damage. Inverse-time overcurrent relays are often used abroad. With faults near the power sources, the operation time for backup protection may be much shorter than that of 3-zone current protection. However, compared with its definite-time counterpart, the relay setting for inverse-time overcurrent protection and its coordination with other protection schemes are complex. In recent years, with the development of integrated circuit and micro-computer, IEEE and CIGRE establish standard of inverse-time characteristics for overcurrent relays. In China, inverse time overcurrent protection has been gradually applied for power equipment and distribution lines.In this dissertation, the characteristics of inverse-time overcurrent protection were discussed, and optimization model is built for the coordinated relay setting. To avoid cross of operation curves of the main/backup relays, a critical impedance is newly proposed. Direction of short current is added as constraints for pairs of the primary/back up relays. The time intervals are included in the optimization model.Furthermore, coordination with other protection schemes e.g. zone 2 distance protection, and between overcurrent protections with instantaneous units, are studied. Influence of post-fault transient topology of parallel lines is analyzed with increased time-intervals compared with that of the single line. Traces of currents and voltages with transition resistance are shown by phasor diagram, and summarize for that.Numerical analysis for the proposed model is executed on several test systems, with reasonable results, which shows the flexibility and practicality for the operation and control of distribution systems.
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