Digital adaptive protection system for a distribution network

Pao Yue-kong Library Electronic Theses Database

Digital adaptive protection system for a distribution network


Author: Lo, Kai-on
Title: Digital adaptive protection system for a distribution network
Degree: M.Sc.
Year: 1995
Subject: Protective relays
Electric power systems -- Protection
Electric transformers
Hong Kong Polytechnic University -- Dissertations
Department: Dept. of Electrical Engineering
Pages: iv, 98 leaves : ill. (some col.) ; 30 cm
Language: English
InnoPac Record:
Abstract: This dissertation presents the computer programs for simulating two adaptive protection systems which do not require real time remote communications : adaptive duo bias transformer protection and adaptive IDMTL overcurrent feeder protection. The purpose is to improve the sensitivity of the traditional protection systems while not impairing the stability of the systems. In the transformer protection system, change in tap position causes the voltage ratio to deviate from the nominal value. This creates an apparent differential current in the current transformer secondary circuits. In the program, voltage ratio at different taps are obtained from the decoded binary bits of the tap position indicator. The output current of the transformer referred to the input side of the transformer is then determined from the voltage ratio. Finally, the actual differential current is calculated from the measured differential current and the referred output current. The program releases the relay setting while the tap is being changed to avoid tripping caused by the circulation current. The program changes to non-adaptive mode when the tap indication circuit is found defective. In the IDMTL overcurrent feeder protection system, the program measures the loading of the feeder and adjusts the pick-up setting continuously. With the output from a current summator for the parallel feeders and the circuit breakers' status from the SCADA system, the program determines whether a large sudden increase in feeder current is caused by the switch-off of the parallel feeder. With the output from a sequence converter which gives the unbalance content of the phase currents of the feeder to be protected, the program considers a "small" overcurrent as a high impedance fault if current unbalance exceeds the set limit. The program have been tested by experimental data and by simulation, the performance are proved to be satisfactory.

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