A New Method of Obtaining Insulation Co-ordination of Transformers [includes discussion]

FELLOWAIEE theseveral parts ofthetransformer insu- lation. Asaresult, there isnoonetypical shapeofinsulation strength oftrans- formers, astheshapedepends onthede- sign. TheonlIv waytoassure strength for agiven typeofvoltage stress istotest the transformer withthattypeofvoltage. Thisisillustrated by experiences with transformer failures during factory tests. Somehavefailed onlow-frecluencv tests after withstanding allothettests. Some havefailedon front-of-wave impulse tests butwillwithstand allothertests. Somehavefailed onfull-wave testsbut will withstand allother tests. The relation betweentheinsulation strengths whenthedifferent kinds oftests areapplied iscomplex, butwherever there isaconsiderable difference intheduration oftheapplied test, considerable variation canbemadeinthestrength towithstand onetestwithout affecting theinsulation strength fortheothertests.However, because tests areintertelated, frequently thegreatest savings canbe madeby changing morethanonetest. Theleast expensive transformer which issuitable foragiven application isaunit designed tomeetservice requirements withanamplemargin ofsafety butwith- outexcess strength requirements. EvYen ifitweretrtse thatbyutilizing present construction excessstrength can besecured atnoadditional cost, this con- dition mav notcontinue tobe true. Specifying onlyactualservice require- mentsencourages thedesigner touise his ingenuity in devising new structures whichwillmeettherequirements atre- ducedcost. A comparison willbemadebetween service requirements andpresent tests. Basedontheserv-ice requirements, sug- gested reduced testvalues will bederived whichwillgive asafe margin aboveserv- icerequirements. To demonstrate that thesuggested test-alues result inprac- ticable consti uction andsavings, sample transformers weremadeandtested tode- termine their volt-time curve.There- sults areshowninthecurveofFigure 6.