On Monday, March 2, 1998 at 10:00:00 AM UTC+2, Craig Palmer wrote:
My company is involved with the electrical design and installation for
diesel and gas power stations up to 20MW in size. Recently, we have
been using a locally manufactured expoxy encapsulated11kV/110V 50VA and
100VA three phase PT for generator voltage sensing and protection.
We have begun to see a number of faults with these PT's where a HV
protection fuse blows (100mA) without any LV protection activating. The locals on site have checked the PT and are satisfied to put it back into service where it operates for anything up to a further 3 hours before
blowing apart the HV windings.
Since beginning to investigate other suppliers, we have found that
no-one we can find in Australia will guarantee their three phase PT's in
this voltage range and VA rating for use in power generation. Is our
only option to go three single phase units?
The question has also been raised by us as to the value of providing HV fusing for these PT's? Generally the smallest 11kV fuse we can obtain
does nothing to protect the windings of the PT. Most people I have
spoken to suggest that you may as well just directly connect the PT to
the bus.
I welcome anyone to comment on any of the above.
Regards,
Craig Palmer
Electrical Engineer
HI
Just as additional info. If your PT'S is stared and earthed and the supply transformer is also earthed via a neutral point or through a NECR you have the problem that when a earth fault occurs then the circulating currents cause the PT and fuses to blow.
Regards
Dino
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