ELECTRICIANS MEET

Bay of Plenty Beacon, Volume 10, Issue 7, 5 August 1946, Page 3

ELECTRICIANS MEET

WHAKATANE INSTITUTE ■ s TRIP TO OPOTIKI On Thursday, July 25 the Whakatane Branch of the Association made the trip to Opotiki with the object of soliciting the membership of the Opotiki wiremen and others eligible to become members of the Institute and to hear a lecture by Mr F. R. Combes, Superintending Engineer to the Paper Mills and vice-president of the Association.

Making an early start from Whakatane the meeting commenced at the Bay of Plenty Power Board’s Office in Opotiki at 7.30 p.m. There were present 16 members from Whakatane, and 11 posible members from the Opotiki si'de. Mr Lysaght, President, thanked the Patron, Mr J. D. Dicker, Engineer-Manager of the Power Board, for the facilities provided and expressed pleasure that so many had been able to attend.. After the minutes and correspondence had been attended to, Mr Combes proceeded with his lecture entitled “Alternator Protection” with special reference to the failure at the Whakatane Paper Mills of a 3125 K.V.A. 3,300 volt 3 phase 50 cycle 3,000 R.P.M. Alternator at 3.45 a.m. on the morning of January 14 this year.

The appearances were that a soldered joint of an end turn carrying approximately 546 amps had heated up until the solder melted and, running down, burnt the and so caused either a short, between turns or phases, which had developed into a 3 phase short including a short to earth.

The relays, according to ihe indicators, operated correctly but unfortunately the insulation of the end turns was mostly linen tape and wooden details, and these caught fire, and the intensity can be gaged from the fact that 48 hours later the machine was still hot. Although a very short period is taken to shut down and make the alternator dead electrically, in that time immense damage can be done when it is Realised that the temperature of the fault would be between 2500 degrees C. and 3500 C., and the short circuit would deliver into the fault about 60,000 K.V.A. i.e. 30,000 K.V.A. per machine assuming an impedence of about 8 per cent, which is normal for turbo-alternators. The rotor, that is the moving part of the machine under discussion weighs 4 tons 18 cwt. so it can be seen that the time for it to come to rest is considerable and the force draught caused by the fan on the rotor revolving at 3000 r.p.m. would have terrific effect on a fire once started. The damage done was confined to one end but this, of course, meant a complete rewind. This was carried out and the machine put back into service on May 8 „ A series of large-sized photographs were handed round showing the extent of the damage immediately after the fire and the repaired job. From this point Mr Combes went on to deal with a description of various methods of protection adopted for alternators and illustrated his lecture with a series of largescale diagrams covering more particularly Alternator protection as in (1) Merz Price balanced voltage. (2) Merz Price balanced current (considerable improvement on (1). (3) Self Balancing Protection. It should be noted that any faults outside the Alternator should not be able to trip the Alternator, but should be cleared by protective gear as near the fault as possible. (4) Core Balance System. This system is not always effective between phases. (5) Reyrollee between turn protection.

(6) McColl Biased Beam (largely used in England).

(7) McColl Differential Protection. (8) ASEA Differential Protection (in use at Paper Mills). It is now usual to have far more complicated protection and manufacturers will go to almost any extent to protect large generators. Mr Combes’ lecture was followed by a question period during which Mr Dicker raised the question of the danger of open circuiting the secondary side of a current transformer, and Mr Combes at his suggestion emphasised the damage done under such circumstances, to the metal of the core, the danger of burn out, and also the high voltage which may be built up with the possibility of a fatal accident.

Mr Dicker thanking the members for the honour in appointing him patron, stressed the benefits to be derived by all from such an Association and promised them every assistance. He believed that it was a live body and here to stay, and within the limit of the rules, its scope should be as wide as possible.

In closing, Mr Dicker stated he would acquaint the Board with the Institute’s objects and possibilities and felt sure that they in turn, would assist wherever possible. Mr Combes was thanked by acclamation for his lecture and the formal portion of the meeting -was closed. During supper many details were discussed, and at 10.30 p.m. the Whakatane branch started for home after a most enjoyable and instructive evening.