Surge protection on 27kV ungrounded delta system
Surge protection on 27kV ungrounded delta system
(OP)
I'm involved in a project where the existing plant distribution is 27kV with a 5kV standby generator. The plant receives power from the utility’s 27kV grounded wye distribution system. The step up transformer between the 27kV bus and the 5kV bus has a delta ungrounded primary, (27kV), and grounded wye secondary (5kV). The original distribution system had MOV surge arrestors on the primary of every 27kV transformer on site. After completion of the generator addition, the plant experienced the failure of most of the MOV surge arrestors that were connected to the plant distribution system when the plant was operated on generator. All MOV surge arrestors were then removed from the 27kV distribution system. I did a little research and found that the MCOV ratings required for 27kV class MOV surge arrestors were not high enough for a ungrounded delta system.
I believe that the step up transformer between the 27kV system and the 5kV generator should have had a wye primary. This would have avoided damage to the MOV arrestors. I have discussed this system with an engineer familiar with, but not directly involved with the generator installation. He maintains there was a reason for this transformer to have the delta primary… but cannot recall the reason. Does anyone know of a reason for installing this transformer with a delta ungrounded primary? I’m starting to think it was installed in error and should have been a wye, or grounded delta, since I do not know of any reason to validate the ungrounded delta primary. Does anyone have some experience on a similar issue?
I believe that the step up transformer between the 27kV system and the 5kV generator should have had a wye primary. This would have avoided damage to the MOV arrestors. I have discussed this system with an engineer familiar with, but not directly involved with the generator installation. He maintains there was a reason for this transformer to have the delta primary… but cannot recall the reason. Does anyone know of a reason for installing this transformer with a delta ungrounded primary? I’m starting to think it was installed in error and should have been a wye, or grounded delta, since I do not know of any reason to validate the ungrounded delta primary. Does anyone have some experience on a similar issue?






RE: Surge protection on 27kV ungrounded delta system
I would call the primary of a GSU to be the generator side even though it is the low side as power will always flow from low to high side. Probably best just to refer to high and low side. Because of this uncertainty in terminology the whole problem may have come about from someone calling for a delta primary meaning low side and the transformer being ordered by someone who took primary to be high side.
RE: Surge protection on 27kV ungrounded delta system
RE: Surge protection on 27kV ungrounded delta system
RE: Surge protection on 27kV ungrounded delta system
RE: Surge protection on 27kV ungrounded delta system
RE: Surge protection on 27kV ungrounded delta system
BTW... most 23.8kV terminations in the plant are 25kV class 200a load break elbows, in order to get a load break elbow with the proper MCOV rating i would have to go to a 35kV class load break elbow... which is GREAT!, except for the fact that 35kV load break MOV will not fit a 25kV bushing!... woe is me... what's next?!... LOL! I won't even go into the use of 15kV terminations on 25kV class cable!