Patent ID: 6518771
Filing Date: 2003-02-11
Classification: H01H

Abstract:
A method of monitoring contact burnoff in a tap changer for a transformer having a multiplicity of voltage taps, said method comprising the steps of:(a) storing values of respective nominal tap voltages (US), a limiting value for the permissible contact burnoff for switching contacts and resistance contacts of the tap changer, and tap-changer-specific parameters a, b and k; (b) detecting a current tap setting of the tap changer; (c) incrementing an index n with each tap change by (c1) stepping said tap changer to a selected tap, (c2) measuring a respective load current (JL) of the selected tap, and (c3) reading out the permanently stored values for the nominal tap voltage (US) of said selected tap; (d) calculating a switching current (JSK) of the respective switching contacts and a switching current (JWK) of the respective resistance contacts in accordance with the relationships: JSK=JLParSecJWK=US+JL&CenterDot;R0sres2&CenterDot;R0wherein ParSec is a number of parallel sectors, R0 is a magnitude of a bridging resistance of the tap changer for the selected tap and sres is a resulting current distribution; (e) calculating the respective burnoff rates Ask of the respective switching contacts and Awk of the respective resistance contacts from the relationships: ASK=a&CenterDot;JSKbAWK=a&CenterDot;JWKb(f) summing up the burnoff rates (Ask) and (Awk) to obtain total volume burnoffs (GASKn)for the switching contacts and GAWKnfor the resistance contacts by the relationships: GAwkn=GAwkn-1+Awk;GAskn=GAskn-1+Ask;(g) calculating the respective burnoffs in millimeters of contact thickness for the switching contacts (GAdSKn) and for the resistance contacts (GAdWKn) over the respective contact areas F by the relationships: GAdSKn=GASKnF&CenterDot;kGAdWKn=GAWKnF&CenterDot;k;and(h) comparing the values (GAdSKn) and (GAdWKn) with the permanently stored limits and generating a report upon overstepping of the permanently stored limit or a percentage thereof.