Unearthing of VFD Transformer Star Point

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nssudhakar1968 Participant Join Date: Dec 2009 Posts: 3	Unearthing of VFD Transformer Star Point 09/24/2013 7:53 AM
	A 300 KW, 690Volt six pulse VFD had been provided with a Dry type input transformer of 800 KVA, 6600 Volts / 690 Volts, the primary being Delta and Secondary Star connected. Here the star point is left unearthed. Why is it so? Can any one suggest the possible reasons?
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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: 32175 Good Answers: 839	#1 Re: Unearthing of VFD transformer star point 09/24/2013 8:51 AM
	Not without knowing the use of the building in which it is installed, no. __________________ "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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1capybara Power-User Join Date: Mar 2012 Location: the sandbox Posts: 341 Good Answers: 6	#2 Re: Unearthing of VFD transformer star point 09/24/2013 9:01 AM
	if the VFD requires a neutral, how do you intend to provide its neutral? from this floating star point? this is - imho - installer error. __________________ BSEE but always learning
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LAA_Lucke Guru Join Date: Feb 2010 Posts: 1013 Good Answers: 36	#7 In reply to #2 Re: Unearthing of VFD transformer star point 09/25/2013 5:32 AM
	Yes he can use the star point as neutral! it is the neutral point here! (That is if the VFD requires a Neutral ! not necessary so).
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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: 32175 Good Answers: 839	#3 Re: Unearthing of VFD transformer star point 09/24/2013 9:09 AM
	Is there a qualified Electrician dealing with this locally or is it all home-brew stuff? __________________ "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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JRaef Guru Join Date: Sep 2006 Location: California, USA, where the Godless live next door to God. Posts: 4665 Good Answers: 804	#4 Re: Unearthing of VFD transformer star point 09/24/2013 9:49 AM
	Generally it would be a bad idea to leave the star point unearthed. You want the phase-to-earth voltage to be as low as possible to give any common mode noise an easy path to earth. In addition, most VFD front-ends include MOVs and/or common mode capacitors that come from the factory with an earth reference connection in star form. If your secondary of the transformer is floating, those devices attempt to become the earthing point of the star for the entire transformer, so the first fault burns them out. Most VFD mfrs will give you instructions on how to remove the earthing connection, which prevents that, but leaves the drive exposed to common mode noise. But without knowing all of the details of what is involved in your installation, we don't know if there was some legitimate reason for doing this. Sometimes, for instance if there is a neutral resistance connection elsewhere to limit earth fault energy, this must be done. We just don't know. __________________ All I every really wanted to be, was... A LUMBERJACK!.
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GW Guru Join Date: Oct 2006 Location: Surrey BC Canada Posts: 1571 Good Answers: 42	#6 In reply to #4 Re: Unearthing of VFD transformer star point 09/25/2013 2:31 AM
	GA. We used to use ungrounded Y secondaries with DC drives to allow for fault tolerance of one phase to ground. We originally provided some rather simple pilot lights Connected referenced to ground, so if a phase did ground 2 lights where bright and one dim. Code here does not allow that any more. It has a strong preference for hard grounding -especially in residential and commercial applications. In industrial applications where full time electrical personnel are present they may opt for high resistance grounding, with ground fault detection and ground fault protection systems. This allows for orderly planned shutdown to clear the fault - imagine a molten ladle of steel - you tolerate the fault until it is safe to shut down the line. As you state, the manufacturer may give you the option of removing the Y connected protection.
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1capybara Power-User Join Date: Mar 2012 Location: the sandbox Posts: 341 Good Answers: 6	#5 Re: Unearthing of VFD Transformer Star Point 09/25/2013 12:11 AM
	And if the neutral isnt earthed elsewhere its voltage will float because it isnt referenced to earth __________________ BSEE but always learning
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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: 32175 Good Answers: 839	#8 Re: Unearthing of VFD Transformer Star Point 03/21/2024 8:23 AM
	Are there any drawings? Are there commissioning records? Are there periodic inspection and testing records? Who did them? Those are the people to ask. __________________ "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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1capybara (2); GW (1); JRaef (1); LAA_Lucke (1); PWSlack (3)

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