Burnt 525kW 3.3kV Auto Transformer

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r4roty Active Contributor Join Date: Jun 2010 Location: zaria nigeria Posts: 12	Burnt 525kW 3.3kV Auto Transformer 07/01/2010 2:23 PM
	How do i prevent a re-occurence of a 525kw auto transformer burn-out when the start button is accidentally depressed in an attempt to stop a 400kw motor running on full load?This control panel has 3no sf6 400A Rollarcs as main , star & cross-over contactors. Initially ,me thinks since there is no phase rotation and the back emf at full speed is zero,(neglecting IaRa losses) motor line voltage should be equal to line voltage =3.3kv hence re-inserting the auto xfmer by pushing the start button will only result in a momentary drop in speed in accordance with the % voltage tap setting of the auto xfmer.during transition and thereafter crawls back to nominal speed after transition.Nominally, motor is drawing 62A at 3300v and running at 1470rpm.Auto transformer impulse withstand is 75kv.Transition time for motor start-up is set at 6secs. Is this proposition correct? or there is a control wiring defect inside the MCC ? r4rotry Zaria Nigeria __________________ r4roty
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kvsridhar Guru Join Date: Sep 2009 Location: Temporarily at Ashburn, VA Posts: 2744 Good Answers: 164	#1 Re: Burnt 525kW 3.3kV Auto Transformer 07/02/2010 12:07 AM
	If you are using a closed transition (Korndorfer) starter, and if the control wiring is something like this, you should have had no issue, since the Start contactor should not have switched on, if you press the start button, since the NC contact of the Run contactor would prevent the auto-transfomer coming online. However, if the 1 and 2 contactors are allowed to switch on with Run contactor On, the Autotransformer will be short-circuited, and is likely to burn. Please check your control circuit and let me know...thanks. __________________ Nothing worthwhile can ever be taught, it can only be learnt.
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mountk2 Power-User Join Date: Jun 2010 Posts: 348 Good Answers: 10	#2 Re: Burnt 525kW 3.3kV Auto Transformer 07/02/2010 1:00 AM
	There seems to be a defect in control wiring. Review control diagram and inspect the wiring to trace out the interlock. __________________ Knowledge sharing is a WIN-WIN situation
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Dave K. Active Contributor Join Date: Mar 2010 Location: Texas Posts: 22 Good Answers: 1	#3 Re: Burnt 525kW 3.3kV Auto Transformer 07/02/2010 1:13 PM
	I can't help with your suggestion other than check to see if there is an unused contact on the main motor run contactor. If there is then wire the start button through it such that if the motor is running the START button circuit is open and if the motor if off then the START button will then work properly. Dave
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Anonymous Poster	#4 Re: Burnt 525kW 3.3kV Auto Transformer 07/05/2010 9:00 AM
	A flip-down plexiglass cover over the start button and stop button is a simple way to physically prevent accidentally hitting thewrong button, but interlocks should be designed to prevent this electrically.
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PWSlack Guru Join Date: Jan 2007 Location: In the bothy, 7 chains down the line from Dodman's Lane level crossing, in the nation formerly known as Great Britain. Kettle's on. Posts: 31998 Good Answers: 837	#5 Re: Burnt 525kW 3.3kV Auto Transformer 01/05/2025 5:06 AM
	<...prevent a re-occurence...> comes about by getting the installation checked over by a qualified Electrical Engineer, re-assessing the design, replacing the <...burn-out...> parts, carrying out alterations to protect the replacement <...auto transformer...> with (an) appropriate protective device(s), and getting that individual to recommission the installation. __________________ "Did you get my e-mail?" - "The biggest problem in communication is the illusion that it has taken place" - George Bernard Shaw, 1856
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Anonymous Poster (1); Dave K. (1); kvsridhar (1); mountk2 (1); PWSlack (1)

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